Darwin's Atolls - Beneath The Idyll

 I've expanded on the link I posted last week on Darwin's Atolls. 

A beautiful theory has been undone by ugly facts...well, let's just say facts.

How do coral atolls, those shimmering ring shaped islands set against the blue tropical ocean, form? Charles Darwin had pondered this question on the H.M.S. Beagle as she sailed across the Pacific and Indian Oceans in the early 1830s. He famously reasoned that coral colonies begin growing in the shallow waters surrounding oceanic volcanoes. Eventually the volcanoes sink into the ocean, while the coral keep growing upwards. The central area where the volcano existed becomes a deep lagoon, surrounding by a ring of coral reefs. 

 But he just assumed that the present day corals atolls are growing on a volcanic foundation.

Actually, most are not. Tropical region atolls rest on an earlier generation of coral and limestone. These in turn have grown on an even earlier layer of coral growth and so on through the past few million years.

Darwin at that time didn't know that the climate over the past 2.6 million years had shifted periodically between glacial and inter-glacial phases resulting in sea level changes. The story begins in the Pliocene around 5 million years ago. A long period of sea level stability resulted in reefs and other calcium carbonate sediments accumulating on the shallow waters above sea floor ridges. Thick deposits formed flat topped carbonate banks. The earth's climate began to change around 2.6 million years ago as polar and high latitude ice sheets expanded and then withdrew. As a result, sea level started falling and rising cyclically. 

These repeated sea-level fluctuations amplified around half a million years ago. When sea level fell, rain water dissolved the exposed carbonate flats to form an uneven karst topography comprising a depressed bowl with an elevated rim. Subsequently, when sea level rose, new coral growth began in the optimal water depths above this jagged limestone rim. With every sea level fall and rise, erosion and new coral growth accentuated the relief between the central depression which became the lagoon and the rim which was now made up of towering stacked coral reefs. The modern looking atoll evolved in this manner.

A recent detailed study by Andre Droxler and Stephan Jorry compiling decades of research on the Pacific Ocean and Indian Ocean reefs demonstrates this elegantly.

The image above shows a portion of the Chagos atolls in the Indian Ocean. Notice how the rings of coral reefs have formed on a submerged flat 'mesa' or table land. These are the vestiges of the carbonate banks that formed during the Pliocene and later became the foundation for the modern atolls.

Great scientists get it wrong frequently too. Darwin erred in his too loyal adherence to the principle, 'Present Is The Key To The Past'.

He observed that some volcanic islands have coral colonies growing around them. He extrapolated this condition to the past and theorized that atolls began as reefs fringing volcanic islands. He couldn't imagine how otherwise corals could grow in the middle of the ocean without them having a shallow water foundation to colonize.

Uniformitarianism has been a very helpful paradigm in understanding many aspects of the past, but geologists have learned to apply it with caution. During the long 4.5 billion year history of our planet, critical combinations of atmosphere and ocean compositions and continental configurations coupled with the evolving biosphere have resulted in unique geologic products and processes which have no modern analogue.

Darwin's atolls offer us some lessons about our future too. Ten thousand years of relative climate stability has allowed civilization to flower, but has also lulled us into a dangerous complacency about nature and its permanence. The idyll of these atolls is ephemeral. Their very foundations are testimony to rapid environmental change and great dyings of marine ecosystems. Such changes await us in the future too, now hastened by our own agencies.

Several hundreds of years from now our descendants may well be looking on at a planet where many of Darwin's atolls have disappeared under the sea and our forests and croplands diminished by fire and dust. Will it be possible to modulate this coming change? For that, we must heed the signals from our geological past. Our present behavior will be the key to our future.

A more detailed write up about Andre Droxler and Stephan Jorry's work has been posted on the Rain to Rainforest Media website - If Darwin only knew: His brilliant theory of atoll formation had a fatal flaw.

Readings: Darwin's Atolls, Pre Toba Humans, Herbivore Diets And Ecology

Posting some interesting readings:

1) A beautiful theory has been undone by ugly facts. How do coral atolls, those shimmering ring shaped islands set against the blue ocean, form? Charles Darwin had famously reasoned that coral colonies begin growing on the slopes of volcanoes. Eventually the volcanoes sink into the ocean, while the coral keep growing upwards. The central area where the volcano existed becomes a deep lagoon, surrounding by a ring of coral reefs. But he just assumed that the present day corals atolls are growing on a volcanic foundation. Actually, most are not. Tropical region reefs and atolls rest on an earlier generation of coral and limestone. These in turn have grown on an even earlier layer of coral growth and so on through the past few million years. 

Darwin at that time didn't know that the climate over the past 2-3 million years had shifted periodically between glacial and inter-glacial phases resulting in sea level changes, and how these repeated sea-level fluctuations can create environments where corals grow during a sea level rise or later dissolve during a sea level fall to form a karst landscape. This jagged uneven surface in turn becomes the foundation for a new generation of corals. A new detailed study of the Pacific Ocean and Indian Ocean reefs demonstrates this elegantly.

Paper- The Origin of Modern Atolls: Challenging Darwin's Deeply Ingrained Theory.

Write up - Darwin's theory about coral reef atolls is fatally flawed.

2) Did Homo sapiens enter India prior to the devastating Toba eruption that took place about seventy four thousand years ago or after? This question is of interest in elucidating the timelines and dispersal routes of our species from Africa. Homo sapiens had reached Australia by around 60,000 years ago with India being one obvious migration path.  There are no skeletal human fossils from this time period in India and stone tools have been variously interpreted as belonging either to Homo sapiens or an earlier archaic human. There were few accurately dated sites from the time period of 80,000 years ago to 50,000 years ago. Now, some new work from the Son Valley, Madhya Pradesh, shows long term human occupation in north India from pre Toba eruption times. The layers containing stone tools span from about 79,000 years ago to 65,000 thousand years ago. The tools resemble those from the Middle Stone Age of Africa, Arabia and Australia and are interpreted to have been the handiwork of Homo sapiens. 

Paper- Human occupation of northern India spans the Toba super-eruption ~74,000 years ago.

3) The ecologic context of the evolution of our genus Homo is of great interest. A recent study focuses on using carbon isotopes to tease out dietary shifts in herbivore fauna living in East Africa in the late Pliocene to Early Pleistocene. Analysis of herbivore teeth from 3.6 million years ago to 1.05 million years ago reveals a shift from C3 derived food (woody vegetation) to C4 derived food (grasses), first around 2.7 million years ago and again later around 2.1 million years ago.  Woodlands were giving way to more open savanna, a change that coincides with the evolution of Paranthropus and Homo. 

Paper- Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia.

Write up - Researchers use fossilized teeth to reveal dietary shifts in ancient herbivores and hominins.

 

Readings: Measuring Sea Levels, Human Evolution, Elements

 Some interesting articles from the past few weeks:

1) What is global mean sea level? What is relative sea level? Is sea level rising or falling along the India coastline? Science writer Shreya Dasgupta explains how scientists measure sea level change with special reference to the Indian coast.

The Surprisingly Difficult Task of Measuring Sea-Level Rise Around India

2) A long thought human ancestor that turns out to be a contemporary.. a cousin perhaps. Multiple human species coexisted across African landscapes in the Mid Pleistocene, around 300,000 to 200,000 years ago, just when skeletal features that we recognize as 'modern' were evolving.  New fossil finds supplemented by genetics is enriching our understanding of human origins. Fine summary article by Katarina Zimmer.

Genetics Steps In to Help Tell the Story of Human Origins

3) What is an element? From Lavoisier to Mendelev to recent times, Philip Ball traces the contentious issue of pinning down what exactly an element is? 

What is an element?

Field Photo: Sea Cliffs And Holocene Sea Level Highstand, India West Coast

All along India's coast there are indicators that 4000-6000 years ago sea level was higher than the present level, oscillating between 1-4 meters above present high tide level at different times. Since then, the sea has gradually receded to its present level. As a result, we can observe stranded beach ridges, cemented beach rock and dunes a few hundred meters inland of the present high tide mark. And we can see erosional notches on sea cliffs marking the past high tide level.

I saw these erosional notches in the sea cliffs exposed along the coast near Harnai village in Konkan.

The satellite image shows the location of the sea cliffs.

The picture below shows a sea cliff with an erosional notch (arrow) about 1.5 meters above the high tide level. This is at the Fattegad Fort near Harnai village. Also, notice the rocky platform that has formed at the current tidal level.

This notch can be traced all along the line of sea cliffs in the area. You can see it very clearly on this cliff, a little north of the previous location.

And the picture below shows a close up of the notch. Sea level must have held steady at this level for a few hundred years to have formed such a distinct erosional feature.

Why was sea level higher in the past? It has to do with the ice age and the end of the last glacial phase. The earth has been in the grips of an ice age for the past 2.6 million years. Conditions have cyclically fluctuated between colder glacial phases and warmer interglacial periods. During glacial phases,  growth of polar ice caps results in a sea level fall. During warmer interglacial phases, polar ice caps melt and raise sea levels. The last glacial phase lasted between 115 - 12 thousand  years ago.  During this time the sea level was as much as 100 meters lower than today. Large swaths of the continental shelf was land then. The earth then moved into a warmer interglacial phase. As a result of melting polar ice, the sea has been rising steadily for the past 10-11 thousand years, flooding the continental shelf, and culminating in a sea level highstand (maximum) about 4000-6000 years ago. This maximum was about 1-4 meters above the present sea level.

There is evidence scattered all along India's west and east coast (and all over the world) of this Holocene sea level high. For example, there are tidal flat deposits about 1 meter above present sea level along the Porbundar coast in Gujarat. Shells collected from these deposits give an age of about 4000 thousand years. Exposed reefs from Mithapur in Jamnagar district in Gujarat give an age of about 2100 years. Oyster reefs exposed along Saurashtra coast about 2 meters above present sea level are about 2500-3000 years old.

To the south, in Madh Island (Mumbai) and along Konkan coast, there are layers of hardened sand and pebbles, locally known as 'Karal', which occur 2-4 meters above present sea level. These sediments once formed a pebbly beach.  At Kelshi village in Konkan, there are fossil beach ridges a few hundred meters inland of the present high tide mark. I saw these on my recent visit. In central Kerala 4000 year old beach ridges occur 3 km inland, indicating that sea level was 3-4 meters higher then.  Along the east coast, there are 4000-6000 year old beach ridges along the Krishna-Godavari coastline. These ridges become younger towards the coast, indicating that the sea has been receding since about 4 thousand years ago.  Along the Baruva-Gandavaram coast in  Andhra Pradesh, sea cliffs have preserved a succession of erosional notches at 4.7 m, 2.3 m and 1.8 m above sea level.

All these features indicate sea level peaked about 4000-6000 thousand years ago and has been falling in fits and starts since. The exact mechanism for this sea level fall is not well understood.

Scientists have put together data form various localities to come up with a composite sea level curve for the Holocene. The curve below has been drawn up using data from Gujarat and shows sea level rising throughout early and mid Holocene. The late Holocene has seen a lowering of seas.

Source: U.B Mathur et.al. 2004

This lowering has now been reversed and the seas are rising again globally, this time induced by anthropogenic global warming as continental glaciers melt and the ocean water expands as it gets warmer. It is estimated that sea level will rise between 0.5 - 1 meter by 2100. In centuries to come, the extent of sea level rise will depend on future warming trends and the extent of melting of the Greenland and Antarctica ice sheets. If significant portions of these ice sheets melt, sea level will rise by several meters in the next few hundred to couple of thousand years.

And what about the flat rocky platforms seen in the tidal zone below the sea cliffs? How do they form? The main mechanism is cliff retreat. Waves pound at cliffs cutting a notch at the bottom of the cliff. Undercutting of the cliff eventually causes collapse of the rock face and a retreat. Over time, as the cliff retreats, a broad sea facing platform develops. This is further acted upon by weathering.  The likely process involves "water layer leveling" combined with wave erosion. Water layer leveling means the lowering and leveling of the rock surface due to physical and chemical weathering by the action of sea water. Standing pools of water and the continuous wetting and drying conditions in the intertidal zone act to weaken the rock and create a loose surface layer which is then removed by wave action, generating a flat surface.

The picture below shows a wide intertidal rocky platform from near Murud on India's west coast.

India's Konkan coast is beautiful and has interesting geology too. Do visit if you can.

References:

1) Falling Late Holocene Sea-Level Along The Indian Coast- U.B. Mathur, D.K. Pandey, Tej Bahadur 2004

2) Quaternary Sea Level Changes Along Indian Coast - S.S Mehr 1992

3) Evidence of Late Holocene shoreline progradation in the coast of Kerala, South India obtained from OSL dating of palaeo-beach ridges - Linto Alappat et.al. 2015

Rhizome Structures Of Early Plants And Their Impact On Paleosols And Landscapes

From time to time it is instructive to move away from the subject of animal evolution that does tend to dominate media reports. From a sedimentology perspective, plant evolution too has played an extremely important role in shaping sediment composition and fabric, fluvial architecture and the structure of our landscape:

Belowground rhizomes in paleosols: The hidden half of an Early Devonian vascular plant- Jinzhuang Xue et.al. 2016

The colonization of terrestrial environments by rooted vascular plants had far-reaching impacts on the Earth system. However, the belowground structures of early vascular plants are rarely documented, and thus the plant−soil interactions in early terrestrial ecosystems are poorly understood. Here we report the earliest rooted paleosols (fossil soils) in Asia from Early Devonian deposits of Yunnan, China. Plant traces are extensive within the soil and occur as complex network-like structures, which are interpreted as representing long-lived, belowground rhizomes of the basal lycopsid Drepanophycus. The rhizomes produced large clones and helped the plant survive frequent sediment burial in well-drained soils within a seasonal wet−dry climate zone. Rhizome networks contributed to the accumulation and pedogenesis of floodplain sediments and increased the soil stabilizing effects of early plants. Predating the appearance of trees with deep roots in the Middle Devonian, plant rhizomes have long functioned in the belowground soil ecosystem. This study presents strong, direct evidence for plant−soil interactions at an early stage of vascular plant radiation. Soil stabilization by complex rhizome systems was apparently widespread, and contributed to landscape modification at an earlier time than had been appreciated.

My interest in this subject is a little tangential. It deals with how to recognize unconformities and disconformities in the field in carbonate sequences. During episodes of sea level falls, marine basins covered by layers of calcium carbonates shells and skeletons get exposed to atmospheric elements. Plants colonize this exposed surface and their root systems physically disrupt the layers of sediment. Rain water and organic acids released by plants dissolve sediments creating pore spaces. The disruption may be clearly visible as solution pits and collapse structures... a karst topography...

I have written a detailed post about this topic and so I won't repeat the lecture over here except to put up this image of a karst developing on Pleistocene limestones from South Florida. Notice how chemical dissolution and the action of roots have caused collapse pits on the limestone surface - 

Land Plants And Expression Of Disconformities in Limestone Sequences

Do read..

Which Are Older? Lakshadweep Islands Or Andaman Nicobar Islands?

A friend asked me this question:

Which formed first, Andamans or Lakshadweep?

My answer was-

Lakshadweep islands, as a system of living coral reefs, lagoons and sparkling shell sand beaches, is Holocene in age (past 12 thousand  years). These coral communities rest on earlier Pleistocene reefs. So, the history of exposed reefs and atolls is a Quaternary Period phenomenon going back several hundred thousand years. Periodic polar ice cap growth and melting drove sea level fluctuations, resulting in  episodic shallow seas and vertical coral growth and reef building. Below these Pleistocene and Holocene corals lie earlier Cenozoic carbonate sediments (Source 1, 2 ) . These sediments were deposited in a subtidal marine setting, with reefs and sand shoal type environments prevailing from time to time.

We are not sure whether vertical coral growth during deposition of these earlier carbonate sequences created coral islands. It is possible that during this long Cenozoic history, there may have been episodic appearance of islands. Coral island systems and small sand shoals, environments lasting for thousands of years, would have developed due to vigorous coral growth and a static sea level, before being submerged again as sea level rose and drowned them.

And what lies below? All this Eocene to Pleistocene  (56 million to 2.5 million years) sediment sequence has been deposited on top of a Palaeocene-Eocene  (66 million to 56 million years) volcanic basement. This basement is the northern part of the Chagos-Laccadive ridge, formed when the Indian plate rode over a hot area of the mantle known as the Reunion hotspot. Below the lava is Indian Precambrian continental crust. The foundation of the Chagos-Laccadive ridge is therefore a rifted sliver of continental crust separated from the west coast shelf margin during India's separation from Africa.

The map below summarizes the setting of the Chagos-Laccadive ridge with respect to the Indian shelf margin. 

Source: Deepwater West Coast India - Pre-Basalt and Other Mesozoic Petroleum Plays: Glyn Roberts et al. 2010

Regarding Andamans.. This island chain are the central part of the Burma-Sunda-Java subduction complex in which an accretionary prism and deep sea turbidite deposits are exposed. This means the islands are made up of marine sediment and oceanic crust of a subducting slab (oceanic Indian plate) which got scraped off and plastered on to the overriding plate (oceanic South East Asian plate).

A tectonic cross section of the Andaman subduction complex is shown below.

 Source:  Mud volcanoes show gas hydrates potential in India's Andaman Islands-  Vignesh Ayyadurai et. al. 2015

Sediment and volcanic material and mafic igneous oceanic crust making up the Andaman chain may have started appearing above sea level from Eocene times (~50 million years ago).  Eocene sediment of the Mithakari Group contains detritus derived from earlier Late Cretaceous -Early Eocene ophiolites (slices of oceanic crust). This indicates that ophiolite blocks were thrust up and were exposed above sea level and were being eroded.  Such accretionary prism settings and forearc basins are cannibalistic, in that, the older deposits are emplaced above sea level and become a source of sediment for younger sequences. As tectonic plates continue to push against each other, these younger sequences in turn are moved upwards along thrust faults and become exposed above sea level. Certainly, by Pliocene times (5 million years to 2.5 million years ago), there would have been a large enough island chain.

I guess to the best of my knowledge the answer is that, although in the Lakshasdweep area, coral reef and atoll environments may be emerged above sea level episodically over the past tens of millions of years, as permanent land the Andamans are older.

One misconception I have encountered regarding Lakshadweep is that the Chagos-Laccadive ridge is a southerly extension of the Aravalli mountain chain.

This is not correct.

As I mentioned above, the basement of the ridge is likely Precambrain continental crust  which rifted apart from the southerly west coast margin of India. So, the continental crust making up the ridge would have been part of the Southern Granulite Terrain and western Dharwar craton (craton- earliest formed pieces of continental crust going back more than 3 billion years ago) of south India. The Aravalli craton and the Southern Granulite Terrain / Dharwar craton were two distinct cratonic blocks which collided and sutured by early -mid Proterozoic times (2.5 billion to 1 billion years ago). The Chagos-Laccadive ridge is oriented NNW-SSE parallel to the Indian west coast shelf margin and the Dharwar structural trends.  Post rifting, as the Indian western margin moved over the Renunion hot spot, volcanism covered this basement with lava, enhancing the ridge structure. The Chagos-Laccadive-Maldive ridge is a hotspot trail which marks the movement of the Indian plate above the Reunion hotspot.

One can imagine extending in an arcuate line the Aravalli mountain trend south to connect with the Chagos-Laccadive ridge.

Source:  The Central India Tectonic Zone: A geophysical perspective on continental amalgamation along a Mesoproterozoic suture-  K. Naganjaneyulu and M. Santosh 2010

But these were two different pieces of continental crust in the Archean. In the above figure the Dharwar and the Bhandara Cratons form a South Indian crustal block, while the Bundelkhand and Aravalli Fold Belt form the North Indian crustal block.  The Aravalli mountains terminate north of the Central Indian Tectonic Zone (shown by roughly east-west trending fault lines). This is the suture zone between the North Indian and South Indian crustal blocks.

In Search Of Early Humans In India

I attended a talk yesterday by Prof. S. N Rajguru on the evidence for early hominins in India. This was part of the C. Meenakshi Memorial Lecture series hosted by the Bhandarkar Oriental Research Institute in Pune.

Prof. Rajguru is an acknowledged expert on Quaternary geology and paleoclimates of the Indian subcontinent and the talk followed the contours of his expertise.

Here is the abstract which was given to us:

In Search of an "Early Man" (>2.5 Ma yrs to around 50 Ka yrs) in India

Recent progress in absolute dating methods, in understanding palaeo-landscape of India in light of behavior of monsoonal rainfall in the last 10 million years (Ma), fluctuations in sea level on 7500 km, long coastline of India and the rise of Himalayas and Tibetan plateau from 1000 m during the Late Miocene (around 15 Ma yrs ago) to around 3000 m during early Pleistocene(~1 Ma yrs ago) have added new information on chronology and environment of "Early Man" (hominin) in India.

It is now know that the Indian monsoon is at least 10 Ma old and was fairly strong till 2 Ma yrs. It started fluctuating (strong, moderate, weak) during the Pleistocene (2.5 Ma-10 Ka). The Indian monsoon was relatively strong to moderate during early Pleistocene (2.5 Ma - 0.7 Ma), moderate to weak during middle Pleistocene (0.7 Ma- 130 Ka) and weak during the late Pleistocene (130 Ka- 10 Ka).

Owing to changes in the strength of monsoonal rainfall the landscape of India also responded dramatically. The sea level went down by 100 to 150 m below the present sea level around 18 Ka BP (before present) and was high around 125 Ka BP by 5 to 7 m in tectonically stable part of coastal India. The peninsular rivers also responded to climatic changes in terms of strong erosion and excess deposition. The Himalaya was affected by tectonic movements and by glaical and interglacial climates which were part of global climatic changes during the Pleistocene.

These environmental changes did affect the biological world including 'hominins' in India.  It appears that early man was present in the foothills of the Himalayas in NW India around 2.6 Ma yrs BP and around 1.7 Ma BP in the coastal parts near Chennai in Tamil  Nadu. These two important discoveries in the form of stone tools and bones with cut marks made by early man raise doubts about the 'out of Africa' migration of early man in Asia during the early Pleistocene.

We are not certain who was the maker of stone tools like choppers around 2.6 Ma yrs or little  earlier in the foothills of the Himalayas near Chandigarh. Most likely he belongs to "Australopithicus" group of hominin. On the other hand 'Homo erectus' was responsible for making Acheulian (Lower Paleolithic) artifacts, like handaxes, cleavers, etc., around Chennai during the early Pleistocene (~ 1.7 Ma).

There is a long gap between 'Homo erectus' (~1.7 Ma, with brain capacity of 800 to 1100 cc) and modern man or Homo sapien sapiens (with brain capacity of 1100-1300cc) who had his origin in Africa 200,000 yrs BP.

In Indian context, there are large number of Lower Paleolithic sites well preserved in varieties of environment during the middle Pleistocene (0.7 Ma- 130 Ka). There is a cultural change in the form of stone tools consisting of small size scrapers, points, choppers, etc., made on flakes removed from cryptocrystalline minerals like chert and chalcedony. It is as yet not clear whether the maker of Middle Pleistocene artifacts was 'Modern Man' who arrived in India via West Asia, or he still belongs to advanced form of ' Homo erectus'. Excepting a single fossil skull of hominin, dated to around 250 Ka yrs in the Central Narmada Basin, we do not have any other human fossil data of early late Pleistocene (~ 100 Ka BP).  Thus, the search for early man, though shrouded in scientific controversy, will continue in future in old Himalayas, warm peninsula and in humid coastal strip of India.

One quibble is the comment that evidence of putative 2.6 million year old stone tools from the Siwalik foothills of the Himalaya raises doubt about the 'Out of Africa' migration of early humans into Asia in early Pleistocene. I really don't see how this is so. Unless you are claiming that bipedalism and hominins originated in Asia, a position for which there is not an iota of evidence, all this discovery does (and the finding is still being debated) is deepen the timeline of the earliest migration of hominins from Africa into Asia.
I came across the same argument in a more detailed article about the Siwalik stone tools and scratch marks on bovine bones discovery in Outlook magazine. That article went even further and stated that this 2.6 million year old tools and bone marking may tell us "when we started looking and behaving more like Homo sapiens rather than apes" Huh!... How so? How does an unconfirmed find of possible Early Pleistocene tools in India  signal the evolution  of Homo sapiens from ape like ancestors? The mere presence of these archaic tools can't be a signal. If so, the same reasoning applies to the Pleistocene tool record in Europe and China too. So, such a broad overreaching claim explains nothing.  It is just sensational journalism by Outlook magazine.

I don't want to dwell on this issue too much since it was not the thrust of Prof.  Rajguru's talk. Instead, he gave quite an engrossing presentation with lots of pictures of the various field sites where stone tools ranging in age from 1.7 million years to 50 thousand years or so have been found. He spoke in detail about the paleo-environments and it is clear that humans in India occupied a wide range of landscapes and ecology during the Pleistocene. There were some awesome sites in Ladakh in the Himalayas, a few sites along the Gujarat coast associated with mid-late Pleistocene aeolian carbonate sand made up of Miliolite shells (great cross bedding), and a very interesting site closer to Pune. This was in a laterite cave in a sea cliff overlooking the Arabian sea near the town of Guhaghar. Tools provisionally dated to about a hundred thousand years old or so have been found there.

What did come out of this talk was some of the limitations faced by researchers in India. The first is the lack of a skeletal record of humans. We have just a few skeletal fragments from the Narmada valley. I asked Prof. Rajguru about this lack of bones. He suggested that the African record is richer in bones because of preservation in volcanic ash and fine river muds and sands. In contrast, most of the sites where stone tools have been found in India are surface sites where the preservation potential of skeletal material is poor. That does point the way to a future program of more focused search for bones in the Pleistocene sediments across India. The second problem is establishing absolute chronology. Dating methods are very expensive and until recently Indian researchers could afford to date only the occasional sample. Or, they had to rely on collaboration with western (and Australian) researchers. Hopefully this will change in the future.

Prof. Rajguru is a field geologist. He must be in this seventies now but his enthusiasm for field work is still undiminished. He kept stressing the importance of understanding the geology, stratigraphy and paleo-ecology of human habitation sites to fully understand the significance and variability of human occupation in India in terms of past climates and landscapes. Even without skeletal material one can make a useful contribution toward understanding human evolution....

having said that.. we do need to find more bones!

Balmy Shores And Icy Wastes- Carbonates Overlying Proterozoic Glacial Deposits

This problem was articulated quite well in a review article more than 20 years ago in 1993: Balmy Shores and Icy Wastes: the paradox of carbonates associated with glacial deposits in Neoproterozoic times. The  thinking has been that the Neoproterozoic must have seen very abrupt changes in climate from a "Snowball" earth with glaciers spreading to low latitudes, to a much warmer climate,  more amenable for widespread carbonate deposition. A variety of explanations for the origin of these carbonates were proposed depending upon their stratigraphic relationships with glacial deposits. There are carbonates interlayered with glacial deposits. Some layers based on their being composed of intraclasts (particles of eroded older limestones) were interpreted as detrital limestones. In other cases of very fine grained rock, it was thought that glaciers grinding and eroding older Proterozoic rock would have delivered fine rock flour to lakes and seas. This  reactive rock flour then recrystallized into a secondary carbonate layer.

In other instances, there is a "cap" carbonate, i.e. a layer overlying a glacial deposit. These cap carbonates vary in the environments of  deposition they represent. They are supratidal  to intertidal  dolomites, or shallow  subtidal limestones to deeper water limestones. They are primary precipitates, meaning the calcium carbonate crystals precipitated out of sea water or in the case of dolomites, an initial aragonite or calcite mineralogy was replaced by the mineral dolomite. Their presence overlying glacial deposits indicate an end to glacial conditions and marking a sudden shift of climate to warmer times. The sea water was thought to be supersaturated in calcium and magnesium ions, but the reason for this has not been well understood.

A new study (T. M. Gernon et al 2016) on these cap carbonates now points the finger to an increase in alkalinity of sea water due to the alteration of volcanic glass being formed at sea floor spreading centers due to the breakup of the Rodinia supercontinent in the Neoproterozic-

During Neoproterozoic Snowball Earth glaciations, the oceans gained massive amounts of alkalinity, culminating in the deposition of massive cap carbonates on deglaciation. Changes in terrestrial runoff associated with both breakup of the Rodinia supercontinent and deglaciation can explain some, but not all of the requisite changes in ocean chemistry. Submarine volcanism along shallow ridges formed during supercontinent breakup results in the formation of large volumes of glassy hyaloclastite, which readily alters to palagonite. Here we estimate fluxes of calcium, magnesium, phosphorus, silica and bicarbonate associated with these shallow-ridge processes, and argue that extensive submarine volcanism during the breakup of Rodinia made an important contribution to changes in ocean chemistry during Snowball Earth glaciations. We use Monte Carlo simulations to show that widespread hyaloclastite alteration under near-global sea-ice cover could lead to Ca2+ and Mg2+ supersaturation over the course of the glaciation that is sufficient to explain the volume of cap carbonates deposited. Furthermore, our conservative estimates of phosphorus release are sufficient to explain the observed P:Fe ratios in sedimentary iron formations from this time. This large phosphorus release may have fuelled primary productivity, which in turn would have contributed to atmospheric O2 rises that followed Snowball Earth episodes.

One broader picture that is emerging is that the time period between 1000 and 550 million years ago was one of the most remarkable times in the history of our planet. On one side, from the origin of the earth, lies 3 billion years of a microbial biosphere, and on the other, the extraordinary diversification of large complex multicellular life. The 400 odd million years of the Neoproterozoic created the conditions for this transformation. The infographic below summarizes the changes in the geo-biosphere during Neoproterozoic  times.

Source: Butterfield N.J. 2015

There were changes in continental configuration due to plate tectonics (breakup of supercontinent Rodinia ~ 850 mya), extreme icehouse conditions in the Cryogenian with two major glacial phases, the Sturtian and the Marinoan, lasting tens of millions of years  and sea water chemical changes with increase in ocean oxygenation to crucial threshold levels needed for metabolically demanding activity, major perturbations in organic and inorganic carbon cycles and widespread deposition of phosphate on the sea floor. A confluence of these events not just led to the formation of geologically unusual deposits but also influenced the evolution of complex multicellular life. That in turn completely changed sea floor sediment fabric and chemistry, further providing new ecologic niches for evolutionary innovations. Engineered by animal activity, the construction of a new world began.

Coral Reefs, Atolls And Sea Level Rise

Will coral reefs and atolls (coral islands) be able to keep pace with the current and projected sea level rise and remain geologically stable in the coming decades and centuries? Will atolls in  the Pacific and Indian Oceans remain habitable?

Regarding  the first question,  I came  across a couple of recent  studies that suggest that reef growth in the Pacific, Indian and Caribbean seas has historically and in the geological  past been able to keep pace with sea level rise of magnitudes equal to or even greater than the current rate of change of sea level.

In a recent issue of  Geology, P.S Kench and colleagues study six time slices of shoreline position of the Funafuti Atoll in the tropical Pacific Ocean and find out that there has been no loss of  island due to erosion by sea level rise. This part of the Pacific has experienced some of the highest measured rates of  sea level rise amounting to about 5 mm per year over the past 60 years. Their analysis showed that reef islands in this group shifted their size, shape and positions in response to sea level rise.

What could be happening? Coral reefs are prolific producers of carbonate skeletal material. As sea level rises, corals grow upwards and outwards from established communities keeping pace with the sea level rise so as to remain in the optimum water depth range. Wave energy keeps breaking down corals and produce carbonate sand which then gets redistributed and deposited in adjacent areas including island beaches. Corals thus form a renewable supply of sediment that balances sediment lost to erosion. Thus coral islands, although may change in shape and position due to changes in depositional locus will not experience any net loss of land.

Studies which go back in geological time also seem to confirm that coral reefs have often extraordinary growth rates that they can sustain for centuries and may keep up with extremely rapid episodes of sea level rise. In a special issue of Sedimentology ( Feb 2015 Open Access) on carbonate response to sea level change, Gilbert F. Camoin and Jody M. Webster document very rapid coral growth rates  using age constrained fossil coral reefs from Barbados in the Caribbean Sea and from atolls in the Pacific and Indian Oceans.

Their results show that following the melting of the global ice caps beginning around twenty thousand years ago, coral reefs kept pace with high rates of sea level rise amounting to 6-10 mm per year and astonishingly in places like Tahiti, for periods of a few  centuries, amounting to 45 mm per year. This very high rate dated to 14.65 k to 14.3 k corresponds to a Melt Water Pulse i.e. an accelerated rise in sea level due to collapse of portions of the ice sheet. Healthy reef growth means a steady supply of sediment to replenish coral island beaches, thus maintaining geological stability through periods of sea level rise.

This suggests that many coral atolls will not simply vanish beneath the waves as sea level rise in the coming centuries, although they will change their shape and positions. The other danger besides sea level rise is the changing chemistry of sea water and other biological changes that might harm coral growth. Sea water acidification may slow down the capacity of corals to build calcium carbonate skeletons, although again, studies on the impact of changing pH on coral growth have shown mixed results, with ill effects on some coral species in some locations, while others seem to have sufficient internal buffering capacity to maintain normal growth patterns. Increasing sea water temperature may also result in a) expulsion of symbiotic algae that corals depend on, thus slowing down their growth and/or b) infection by parasites that might harm the coral animal. So, there is still much to worry about the health of coral ecosystems as the earth warms and ocean temperatures rise.

Now to the second question - will coral atolls remain habitable? Habitations on these islands are built on a foundation of dead coral communities and sand which are not going to be lifted up in response to sea level rise. Although the fringing living reef communities will supply sediment to these islands, powerful storms and high tides will still pose problems. Reefs don't form water tight sea walls around these atolls and tidal surges will bring sea water further inland.

Another problem is the impact of sea level rise on groundwater. Many of these island  communities rely on a thin fresh water aquifer for their water supply. The foundation of these islands is porous Pleistocene limestone. Holocene coral communities and sand is piled up on this Pleistocene foundation to build the island. The fresh water aquifer usually occurs in this Holocene sediment. The pores and fractures in the Pleistocene limestone below the fresh water lens is filled with sea water. The contact between the fresh water aquifer and the underlying sea water aquifer is called the Thurber Discontinuity. The graphic below shows a typical cross section and hydrogeology of a coral atoll.

 Source: Bailey et. al. 2010 adapted from Ayers, J.F.; Vacher, H.L. Hydrogeology of an atoll island: A conceptual model from detailed study of a Micronesian example. Ground Water 1986, 24, 2-15

What will be the impact of sea level rise on this fresh water lens. This is an active area of study and early results seem to suggest a variety of outcomes with small fresh water lenses further diminishing while larger ones persisting. This is a complex topic with a variety of controlling parameters like amount of eustatic sea level rise, island size and shape and island topography which will channel the extent of storm wave washover. As sea level rises over the next few decades and centuries, especially on coral atolls which are experiencing erosion and loss of land, the danger of salinization of the fresh water lens is a real possibility, which will make living on these islands a difficult proposition.

Assessment Of India Coastal Erosion

Some data points on changes in India's coastline due to erosion and accretion over 15 years (1989-91 to 2004-2006)

Assessment of coastal erosion along the Indian coast on 1 : 25,000 scale using satellite data of 1989–1991 and 2004–2006 time frames 

The  long  stretch  of  coastline  on  either  side  of the Indian  peninsula  is  subjected  to  varied  coastal  processes  and  anthropogenic  pressures,  which  makes  the coast vulnerable to erosion. There is no systematic inventory  of  shoreline  changes  occurring  along  the  entire  Indian coast  on 1:25,000  scale, which is required for  planning  measures  to  be  taken  up  for  protecting the coast at the national level. It is in this context that shoreline  change  mapping  on  1:25,000  scale  for  the entire Indian coast based on multi-date satellite data in GIS  environment  has  been  carried  out  for  1989–1991 and  2004–2006  time  frame.  The present  communication discusses  salient  observations  and  results  from  the shoreline   change inventory.   The   results   show   that 3829 km (45.5%) of the coast is under erosion, 3004 km (35.7%)  is  getting accreted, while  1581 km (18.8%)  of the coast is more or less stable in nature. Highest percentage  of  shoreline  under  erosion  is  in the Nicobar Islands (88.7), while the percentage of accreting coastline  is  highest for  Tamil  Nadu  (62.3)  and Goa  has the highest   percentage   of   stable   shoreline   (52.4).   The analysis shows that the Indian coast has lost a net area of  about  73 sq.km  during  1989-1991  and  2004–2006 time  frame.   In Tamil Nadu,   a   net  area   of  about 25.45 sq.km has increased due to accretion, while along the Nicobar Islands  about  93.95 sq. km  is  lost  due  to erosion.  The  inventory  has  been  used  to  prepare a Shoreline Change Atlas of the Indian Coast.

Background geological processes keep reshaping coastlines, but this short time frame assessment seems to have captured several anthropogenic disturbances. And one big natural event- the 2004 Indian Ocean tsunami appears to have caused considerable erosion in the Andaman and Nicobar Islands.

How Are Diagenetic Studies Useful In Understanding Sedimentary Basin History

I dusted of my PhD dissertation last week for two reasons. A friend insisted that she wanted to see my research.. and then this paper in the Journal of Sedimentary Research (behind paywall):

Diagenetic Evolution of Selected Parasequences Across A Carbonate Platform: Late Paleozoic, Tengiz Reservoir, Kazakhstan by J. A. D. Dickson and J. A. M. Kenter

The work is eerily similar to what I did for my PhD which was carrying out a detailed study of cementation patterns in Middle and Late Ordovician carbonate parasequences from the southern Appalachians.

Dickson and Kenter use petrographic techniques along with cathodoluminescence to tease apart the cementation sequence and pore space modification of the carbonate rocks. Hydrocarbon reservoir quality depends in part on how reaction of sediment with water either dissolves material to create pore space or precipitates cements to modify pore space. So, understanding the timing of these events in the context of the burial history of the sediment pile on a basin wide scale can help geologists predict reservoir quality.

Ok, so what are Parasequences?

Geologist Robert Young On Rebuilding After Superstorm Sandy

The complicated decision on whether to and how much to rebuilt coastlines after destruction from a major storm lies at the intersection of geology, climate change, sea level rise, preserving communities and livelihoods, and the economics of insurance risk and the real estate market.

Geologist Robert Young, director of the Program for the Study of Developed Shorelines and professor of coastal geology at Western Carolina University speaks passionately and authoritatively about these issues in a talk hosted by Tom Ashbrook on OnPointRadio.

As he says; The map of the coastal U.S. 50 years on will look very different from now and we need a national plan to get from here to there. Its good to hear a geologist at the center of this discussion.

Listen: After Big Storms: Rebuild Or No?

The Dolomite Problem - Peeking Under The Hood

A recent paper in Geology addresses one of the most enduring problems in sedimentary geology- the origin of the mineral dolomite. One can extend their findings to answering the origin of massive dolomite carbonate sequences that recur throughout earth history. Massive dolomite means that most of the rock is made up of the mineral dolomite and such dolomite strata can be hundreds of feet thick, representing deposition over millions of years.  The paper  examines one specific mechanism of sedimentary dolomite formation. Microbial activity in shallow water depths has long been suspected to induce dolomite precipitation and the paper brings out the specifics of the bacterial micro-environment that might aid dolomite to precipitate directly from sea water as pore filling cement or replace existing calcite or aragonite.

Such microbial ecosystems exist today in very restricted settings such as hypersaline lakes and supratidal flats and whether such microbial induced dolomite can explain the thick dolomite sequences which were deposited in more varied environments is a question that still needs more attention.

David Bressan on Scientific American blog has written two posts on the history of research on dolomites and in reference to the paper in Geology on the question of the microbial origin of dolomite using the Triassic Dolomite sequences of the Alps as an example. A considerable part of this sequence is made up of thinly laminated strata. The laminae have been interpreted as structures arising from sediment being trapped or precipitated between bacterial sheets. Based on these microtextures there is case being built up that much of the Triassic dolomite, especially that deposited in earlier phase in the Norian stage is likely to be of microbial origin i.e. induced by microbial activity.

First a few SEM images that show a spatial link between microbial filaments and incipient dolomite crystals.

Milankovitch Cycles and Carbonate Deposition

Over at Open Mind, Tamino has written a series of excellent posts on Milankovitch climate cycles and how they influence the growth and decay of ice sheets.

Milankovitch cycles are orbital cycles which result in periodic fluctuations in the amount of solar energy received by the earth. Three types of cyclical changes in the earth's movement around the sun, eccentricity, obliquity and precession are said to determine climatic cycles on earth.

Read the posts (Part 1, Part 2, Part 3, Part 4).

These climate cycles are of great interest in my field of carbonate sedimentology. Ancient shallow water carbonate basins are often made up of  exquisitely layered strata with the same sediment type recurring at intervals throughout the sequence. The origin of this rhythmic deposits have been a subject of great debate and discussion.

One explanation has been that Milankovitch cycles causes growth and decay of ice sheets. This periodic locking in and release of sea water by ice sheets causes fluctuations in sea level resulting in changing water depths and cyclical deposition of sediment. A great many carbonate sequences have been explained as being deposited under the influence of sea level changes caused by such orbital forcing. The image below is one of the most famous examples of Milankovitch forced cycles.. the Triassic dolomite platform which makes up the famous dolomite mountains in Italy.

Source: The Latemar Controversy

The other popular explanation and which may well be correct in many instances as well is that shallow marine carbonate systems generate cyclicity by self organization i.e in these systems there is an inbuilt interplay between water depth, currents and sediment production and deposition. The locales of sediment production and deposition keep shifting in response to local thresholds resulting in the same conditions appearing and disappearing from any one particular region. The result is cyclical carbonate strata which are hard to distinguish from orbitally forced cyclical sediment.

This debate over the generation of carbonate cyclicity continues.. but the interest in not just academic. There are differences in the three dimensional geometry and internal structure of sediment bodies built up by orbitally forced cyclicity versus those built up by local thresholds.

Cycle thickness, lateral continuity of facies, the extent of sea level fall and the duration of exposure to fresh water are some of the differences between the two systems that will influence the pattern and movement of fluids in the basin which may result in differences in the patterns of porosity and permeability, two properties of great economic importance.

A petroleum geologist looking at a hydrocarbon lined pore space in a rock chip or thin section and wondering whether this porosity extends all across the basin will have to think big.. of large scale facies patterns, of cycles upon cycles, of fluid flow, of sea levels rising and falling, of continental configurations and paleo-geography... and even the movement of the earth across the heavens..

That's what makes geology such a fascinating subject of study..

Map Of Red Sea Corals

From my Nature News Feed- A great map of Red Sea coral reefs.

Credit: Gwilym Rowlands

This is a fine example of habitat mapping done on a resolution finer than previous examples and focusing not just on living corals but taking a historical perspective and exploring how sea-level change, climate and weathering influenced the substrate conditions forcing new coral growth to adapt and shift their locus.

Diagenetic History Of The Great Barrier Reef Of Australia

That giant organism is slowly giving up its deepest secrets.

From the November 2009 issue of Sedimentology : The Great Barrier Reef: a 700 000 year diagenetic history - Colin J. R. Braithwaite and Lucien F. Montaggioni

Most people rightly tend to think of the Great Barrier Reef as a living wonder. But the current living ecosystem, the coral communities and associated faunal and floral assemblages have been built on a foundation of a community of dead corals. And that ancient community when it was living grew on a foundation on an earlier community and so on.. for hundreds of thousands of years through many episodes of community growth and decline and growth again.

A borehole recovered sediments from Ribbon Reef 5 northwestern Australia. Analysis shows ten such coral reef growth episodes, ten sedimentary depositional units beginning around seven hundred thousand years ago. These episodic events correspond to major sea level changes driven by Pleistocene glacial growth (sea level fall) and melting (sea level rise). Coral reef building stopped when sea level fell and resumed when sea-level rose again.

This study delves into the mineralogical and geochemical transformations (Diagenesis) that occur in these reefs when sea-level fell and exposed the sea bed and the coral communities to fresh water.

As is the case with diagenetic research a lot of the base data is collected from micro scale analysis and this study does a detailed job. Cement mineralogy, (chemical precipitates) morphology, habits and associations are carefully documented. Textural features such as solution fabrics and pore space types are noted. The overall conclusion is that during sea-level fall meteoric (fresh water) aquifers developed in the exposed layers and there was considerable amounts of reaction of the coral skeleton with fresh water leading to destruction of minerals like aragonite and high Mg calcite and precipitation of low Mg calcite.

Based on these diagenetic textures distinct vadose zone (soil profile above the water table) and phreatic zone (below water table) demarcations are recognized in these ancient aquifers and some units show evidence of the water table being mobile i.e. migration up and down likely due to changing rainfall conditions.

That's the micro-level stuff and its a well characterized in this study.

My interest in diagenesis tends to make me look at the larger basin scale picture and I found much to think about in this paper. Diagenesis occurs during the establishment of large scale hydrologic systems in sediments and rocks. The scale and geometry of these hydrologic systems controls the extent and 3-D shape of alteration of the sediment.

Such thinking in terms of diagenetic reaction fronts, shapes and volumes is very important because these altered volumes of sediment may due to their properties of porosity and permeability act later in the sediment history as pathways and reservoirs for hydrocarbons and other economically important minerals like copper, lead and zinc.

Although not looked at from such an economic angle this study does recognize the larger controls on the development of hydrologic systems during sea-level falls.

One important diagenetic pattern revealed was that the fresh water reactions that took place cannot be tied to any particular emergent event although aquifer formation likely affected only the youngest deposition unit. This conclusion is based on the observation that there is no superimposition of diagenetic textures and cements.

This is a bit tricky. Say during a sea-level fall a particular depositional unit experiences meteoric diagenesis and then sea-level rises and drowns that unit. A new coral community will grow on that surface and form a younger depositional unit. Later sea-level falls again. Another fresh water aquifer develops. If it is thick enough it will affect not only the youngest sediment unit but the ones underlying it. So the older depositional units might then contain evidence of more than one generation of diagenetic products, a superimposition of events.

That kind of evidence of superimposition is lacking in these reefal units, leading the authors to conclude that the meteoric aquifers that formed were thin and did not penetrate deeper into the sequence.

The major reason for this is the lay of the land and the location of the reefal bodies in the context of the Australian shelf.  The Barrier Reef is located some distance away from the Australian mainland and away from any continental relief. During sea-level falls the undulating topography of the reef, made up of mounds and depressions would have meant that the reefs would have been exposed as a chain of islands.

The depth to which groundwater circulates in these exposed islands i.e. the thickness of fresh water lenses under these islands is governed by the width of the islands, the hydrologic conductivity of the sediment and the amount of recharge. These relationships observed in several Holocene islands suggest a meteoric lens thickness of just 1% of island width.

This contrasts with carbonates sequences which accumulate as bodies fringing continental land masses with coastal relief. Here during sea level falls, the hinterland relief provides a stronger hydraulic head for fresh water to flow towards and into the exposed sea bed and circulate deeper into the sediment layers, generating diagenetic reaction zones which cut across depositional units. 

One consequence of the thin meteoric lenses that episodically developed during the Barrier Reef history is that much of the sediment remained only partially altered. This is indicated not just by the sporadic presence of mineral aragonite which usually dissolves in fresh water, but by a cross plot of the oxygen and carbon isotopes of coral skeletal material in which the overall pattern of fresh water sediment interaction is brought out beautifully.

Here is the theory. Meteoric water is enriched in the lighter isotopes compared with sea water and compared with marine carbonate sediment. Imagine a marine carbonate sediment body reacting with meteoric (fresh) water. Marine carbonate is the dominant reservoir of carbon relative to meteoric water (means there is much more dissolved carbon in marine water than in fresh water) and thus the carbon composition of pore fluids quickly reaches a state of equilibrium with dissolving marine carbonate during water-rock interaction. So any product precipitated from this pore fluid will have the signature of marine carbon. In contrast, meteoric water is the main oxygen reservoir, and thus the oxygen composition of pore fluids with change towards a marine signature only after prolonged interaction with the isotopically heavier marine carbonate.

A pattern of relatively invariant oxygen and variable carbon is thus indicative of less water rock interaction during diagenesis.

That's what the bulk of data points of the key units in the above graph is pointing to. Here units 1, 9 and 2 are mostly marine and are not altered much by meteoric diagenesis. In the  rest of the units the bulk of the samples show a constrained oxygen isotope signal (compared to total range of 0 - 9.5 parts per thousand) and a very variable carbon isotope signal. The depleted but constrained oxygen isotope values suggest that the pore fluid composition were not impacted much by marine oxygen implying limited reaction with rock as does the variable carbon which would have been quickly acquired a uniform marine signature if water rock reactions had been prolonged.

All this is because of thin discontinuous meteoric lenses and thick vadose profiles where diagenesis is concentrated in pockets and along very thin reaction fronts leaving other pockets of the sediment body unaltered.

Enjoyed the paper. It's a good mix of micro-scale detailed work on cements and geochemistry but done in a way that allows you to draw a larger picture of the controls that landscape, platform geometry and hydrologic systems exert on diagenesis.

BRAITHWAITE, C., & MONTAGGIONI, L. (2009). The Great Barrier Reef: a 700 000 year diagenetic history Sedimentology, 56 (6), 1591-1622 DOI: 10.1111/j.1365-3091.2008.00982.x

Indian Groundwater Extraction May Be Contributing To Sea Level Rise

I don't know what to make of this calculation which I picked up in a New Scientist story. A few weeks ago there was a study using NASA's Grace satellite measurements that showed an increase in groundwater extraction from North Indian aquifers.

A second study on these satellite measurements asserts that the groundwater loss amounts to about 54 cubic km per year over a time period of 2002 and 2008. A lot of this extracted groundwater ends up in the sea and could be contributing to raising sea-levels by 0.16 millimeters every year, about 5% of the total sea level rise. That is about the same as contributed by runoff from melting Alaskan glaciers the authors conclude.

I don't have access to the full paper so I don't know the details of the calculation but here is what the scientists have to take into account:

Part of the groundwater extracted will be taken up by plants and remain there over the life of the plant and make its way into the food chain.
Part of it will be lost through the plants through evapo-transpiration.
Part of the water will remain in soil adhering to clay and sand particles.
Part of it will make its way back to the aquifer.
Part of it will make its way through the soil to local streams and eventually to the sea.
Part of it will be lost by direct evaporation. That evaporated water (and the water lost by evapo-transpiration) will fall as rain and part of it will infiltrate as groundwater and part of it will be surficial runoff into streams and eventually into the sea.

Just giving you  something to think about what happens to extracted groundwater.

Maldives Reef Sea Level Curves: What About Palk Strait....And Ram Sethu?

A paper in the May issue of Geology by P.S. Kench, S.G. Smithers, R.F. McLean, and S.L. Nichol works out the Holocene sea level history of the Maldives:

Holocene reef growth in the Maldives: Evidence of a mid-Holocene sea-level highstand in the central Indian Ocean

Radiometrically calibrated ages from three reef cores are used to develop a Holocene reef growth chronostratigraphy and sea-level history in the Maldives, central Indian Ocean. Last interglacial reef (U-series age 122 ± 7 ka) was encountered at 14.1 m below mean sea level. An age of ca. 8100 calibrated (cal) yr B.P. immediately overlying this Pleistocene surface records the initiation of Holocene reef growth. Massive in situ corals occur throughout the cores and the consistency of the three age-depth plots indicate that the reef grew steadily between 8100 and 6500 cal yr B.P., and at a decreasing rate for the next 2 k.y. The position of modern sea level was first achieved ca. 4500 cal yr B.P. and sea level reached at least 0.50 ± 1 m higher from 4000 to 2100 cal yr B.P. before falling to present level. Emergent fossil microatolls provide evidence of this higher sea level.

Results are significant to two long-standing issues relating to Maldivian sea level history. First, the ambiguity of a late Holocene highstand has been resolved with clear evidence of its existence reported here. Second, the uncertainty of the regional pattern of sea-level change in the central Indian Ocean has been clarified, the Maldivian results broadly agreeing with island records in the eastern, rather than western Indian Ocean. Our results provide the first field evidence confirming geophysical model projections of a highstand 4–2 k.y. ago in the central Indian Ocean, though the observed level (+0.50 ± 0.1 m) is lower than that projected.

As sea level rises coral reefs grow upwards. They thus track the rising sea level. Using the relationship between depth at which corals of a particular age are found the researchers calculated that following the end of the last glaciation, sea level rose uninterrupted at a faster rate between 8100-6500 years ago and then at a slower rate between 6000-2000 years ago. There has been a minor drop of around 0.5 metres since. This drop is indicted by emergent coral atolls i.e corals which are now exposed above sea level and are dead. That would mean that in the past sea level was higher.

How can one be sure that sea-level rose continuously? Could it have been interrupted by a sea-level drop? The Holocene corals don't show any evidence of any significant sea-level drop. Discoloration and recrystallization - signs of meteoric diagenesis- are absent and the mineralogy of the corals is 95%-99% aragonite, which again indicates no contact with fresh water. Aragonite is stable in sea-water but will alter to calcite during fresh water incursions.

So this use of coral reef growth as a proxy for sea level change is a common technique in constructing local sea level curves. How local are these curves? It would be hard to extrapolate the Maldives curve as a general statement on the Indian ocean since the Maldives are a relatively isolated island chain. Fortunately we have more data. The paper refers to previous work done in the eastern Indian ocean at Cocos island and on the western Indian ocean sites of Reunion and Mauritius. The eastern sites are in agreement with the Maldives sea-level curves while in the western sites evidence of the late Holocene highstand and later drop in the form of emergent atolls and reefs is not present. But for most of the Holocene they concur with the Maldives.

Look at the figure below for geographic orientation.


This Holocene sea-level curve is beginning to look like it applies to a wide regional extent of the central and northern Indian ocean.

As my post title indicates what does this say about the Palk straits, which is a narrow body of water separating the coasts of south India and Sri Lanka. Well, as it happens the Holocene sea-level history of the straits is of huge interest not just to geologists but to ...say... about 900 million plus Hindus.

The Indian epic Ramayan tells the story of king Ram who crossed the Palk Straits with his army to rescue his wife Sita held prisoner by emperor Ravana of Lanka. Legend has it that the sea-level was low and had exposed most of the Palk strait sea bed. Ram's army build a causeway (Ram Sethu) over the remaining stretches of water and walked across into Sri Lanka (why didn't they take a boat?....beats me!). Supporters say they can see the causeway from satellite images and there is physical evidence too. I have dispelled the many myths regarding this bridge in this post and here.

The Ramayan like many other Indian epics like the Mahabharata existed and was passed down generations as an oral narrative recording Indian society beginning maybe as early as 1000 B.C. (3000 B.P.) until it was compiled as a written document sometime early A.D. So the epic contains early enough memories for us to speculate whether the legend of a low sea-level could have a physical basis and whether a late Holocene sea -level drop could have exposed the Palk strait sea bed.

The central / north Indian ocean sea level curve (Maldives, Cocos, Reunion) says no sea level drop of a few meters in the late Holocene or for that matter the entire Holocene. But basins like the Palk strait which are attached to large continents may not have the same sea-level history as recorded on isolated islands. Tectonic uplift or subsidence and patterns of sedimentation, especially influx of large amounts of clastic sediment from the continental mainland may result in a local sea -level history different from surrounding regions.

Has that happened in the Palk strait? Unfortunately the kind of detailed studies that this paper represents using corals have not been done. Overall the Holocene history looks similar to the Maldives. As the last glaciation ended an exposed Pliocene-Pleistocene surface was flooded by rising sea-levels beginning early Holocene (8000-9000 B.P.) Coral growth was initiated and continues until present day. Like the Maldives there are emergent coral atolls and coral-sand islands (Rameshwaram). The younger exposed corals are dated to around 2600 B.P. which speak to a late Holocene highstand and then a slight drop in sea-level of about one meter or so.

There are geologists and geo-archaeologists who won't agree with this at all. They will bring up the same objections I outlined above regarding the reliability of extrapolating of sea-level curves. I have spoken to some of them and they insist and there are "findings" that show that Holocene sea-level rise and coral growth in the Palk Strait was interrupted by a sea-level drop of several meters sometime several thousand years ago and that would have exposed most of the sea-bed between India and Sri Lanka. I hope they publish these findings and make their data available for others to review.

Fundamentalists engage in a most literal interpretation of scripture. But amongst the many fantasies, exaggerations and story telling there may also be a record of some real event. A late Holocene sea-level drop in the Palk strait is not out of the question, despite what the Maldives sea-level curve is telling us. This is a small basin adjoining a large continent and it may have a peculiar history of its own.

I would love to see this north Indian ocean Holocene puzzle solved.

Land Plants And Expression Of Disconformities in Limestone Sequences

Geological Processes and Evolution - 2

A paper in the January issue of Geology looks at the influence of land plants on the detrital sedimentary cycle.

Evidence from Middle Ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation -

Atmospheric CO2 has decreased by about 500 times from Archean times to recent. So rain water composition has become less acidic over time. But groundwater does not show such a temporal decrease in acidity. In fact groundwater may be more acidic on average in younger geological periods compared with the Archean. This is because of the evolution of land plants by Silurian times which started contributing organic acids to groundwater. The amount of organic acid from the plant system easily offset any decrease in rain water carbonic acid from decreasing atmospheric CO2.

The authors identify a geologic time window - Mid-Late Proterozoic to Late Ordovician when groundwaters were more alkaline than the time period before and after. This is because a) a spurt of continental crust formation by mid-late Proterozoic started consuming CO2 in weathering reactions and b) this stable continental crust provided a substrate for accumulation of carbonate sediments, another important CO2 sink. By early Paleozoic, atmospheric CO2 had dropped compared to the Archean (it was still about 10 times more than the recent ~ 300 ppm). And land plants had not yet evolved.

The paper in Geology demonstrates using the geochemistry of mid-Ordovician paleosols that the groundwater was alkaline in composition. There is some interesting geochemistry for those into such things like element mobility. Other studies in the past have focussed on the detrital composition of sediments and have come to the same conclusions. For example there is an abundance of detrital illite and K feldspar in the mud and sand fractions of early Paleozoic sediments as compared with younger sediments from similar provenance. In acidic groundwater K is mobile and is taken up by chelation into land plants or is transported away in solution. So diagenetic illite is more common in post-Paleozoic sediments.

I have often run into the influence of land plants on sediments from a different perspective, one that involves identification of exposure surfaces in carbonate sequences. Post Silurian during sea-level drops the exposed sea bed will get colonized by land plants. The release of organic acids leads to acidic groundwater and rapid dissolution of the carbonate sediment. Add to that is the physical disruption of sediment by the action of roots and burrowing fauna. All this leads to the development of a highly irregular, pitted surfaces with collapse structures, caverns and infill breccias and a deep chalky weathering profile often with Fe hydroxide stains and crusts. The image below is of a Pleistocene skeletal limestone from a mid shelf facies setting exposed in south Florida for the past several tens of thousands of years.


Notice the karst like profile already developing and penetrating the sequence. If this surface is overlain by sediments in the future the disconformity would be easy to spot in the field.

No such luck in the mid-late Ordovician. Look at this mid-late Ordovician disconformity in the image below. This is the top of a depositional cycle, a parasequence boundary in stratigraphic jargon. The facies setting is similar to the Pleistocene example above.


The climate too has been interpreted to be warm and seasonally moist. The contact just above the coin is an exposure surface. It is a planar surface with a few small pits but overall there is no evidence of the kind of disruptive features you see underneath younger disconformities.

This cycle is part of a cyclic sequence. I could never get a handle on the individual cycle frequency but using regional stratigraphic markers and conodont biostratigraphy a rough estimate is that surface was exposed for tens of thousands of years during Richmondian sea level falls. There is plenty of microscopic evidence of exposure in the form of preserved vadose diagenetic fabrics, but in the field it looks just like a bedding plane. In the late Ordovician there was a land plant cover but it was mostly of the bryophyte grade i.e. mosses and liverworts. These plants don't have disruptive root systems and don't release the massive quantities of organic acids for penetrative weathering profiles to develop.

There are other controls in carbonate systems that can form karstic surfaces in the absence of land plants. One is the facies setting. Supratidal and sabkha facies with dolomite and evaporite minerals dissolve quickly in carbonate under saturated groundwater and these facies often develop the kind of disruptive weathering profiles you associated with a land plant cover.

And the other control is time. Very long lived exposure of limestone, surfaces that are exposed to meteoric condition for tens of millions of years will eventually succumb and form distinctive karst. The image below is of the Knox unconformity between lower Ordovician passive margin platform carbonates and a mid Ordovician foreland basin sequence.


The Knox surface developed during a major sea-level fall that lasted at least 10 million years. A mature karst terrain developed across facies tracts. Sinkhole and caverns fills and intraformational breccias can be seen to depths of 250 m beneath the unconformity.

There have been a couple of recent studies that make connections between evolution and geological processes. One dealt with an increase in diversity in mineral species after the evolution of photosynthesis. The other pointed out a connection between sea water chemistry, evolution of animals and skeletal mineralogy.

Geology may influence evolution. The paper points out that the early radiation of land plants likely took place in alkaline groundwaters. That may explain the affinity for alkali shown by many land plant groups. Subsequently as plant life took hold over terra firma, evolution influenced geology through the transition to acidic groundwater, detrital and diagenetic sediment composition, element mobility and soil composition and characteristics of disconformities.

See: Geological Processes and Evolution