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.

 

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.

Plate Tectonics And Coral Reef Distribution And Diversity

Plate tectonics drive tropical reef biodiversity dynamics- Fabien Leprieur, Patrice Descombes, Theo Gaboriau, Peter F. Cowman, Valeriano Parravicini, Michel Kulbicki, Carlos J. Melian, Charles N. de Santana, Christian Heine, David Mouillot, David R. Bellwood & Loıc Pellissier

Interesting work!

Tropical coral reefs require sunlit shallow sea water to thrive. This is provided by coastlines and continental shelf areas. Plate motions has shifted continents. The geography of shallow sea habitats in which corals thrive has accordingly shifted. This study reconstructs this dynamic over the past 140 million years of the breakup of Gondwanaland and the movement of continents since. It is summarized beautifully in this graphic.

Source: Leprieur et al 2016

The optimum locations of coral habitats moved as the configuration of the Tethyan ocean changed. The fossil coral record shows that coral diversity was maximum in the Western Tethys in the Eocene (55 -33 ma), then shifted to the Arabian Peninsula and Western Indian Ocean during the Late Eocene - Oligocene (37-15 ma). Finally as the Tethys Ocean closed due to the India Asia collision, coral biodiversity hotsposts shifted since the middle Miocene to the Indo- Australian Archipelago.

There is a further step. The study models species diversification based on the distribution of estimated paleo-bathymetry and the residence time of habitat (both controlled by plate tectonics). The reasoning is that the frequency of evolution of new species (biodiversity) depends on the distribution of optimal habitat and how long these habitats remain in place. Large long lasting shallow continental shelves will offer an opportunity for populations of ancestral species to disperse over broad areas and diverge into new species. Tectonically complex areas like western Tethys (in the Eocene) and Indo-Australian Archipelogo in Pliocene-Quaternary have sea floor topography and shelf configurations which fragments habitats based on changes in water depth, current and wave strength and direction and nutrient availability. Populations get split and isolated in such regions leading to genetic divergence and speciation. This is mimicked well by the simulations.  Over time, rich diversity can evolve in such long lasting optimas by the process of dispersal and isolation. The locus of evolution of biodiversity can then shift as plate motions move ideal habitats across the globe. New species can arise from parapatric speciation (adjacent to the range of the ancestor) or sympatric speciation (within the range of the ancestor). The study finds that for either modes of speciation, their simulation matches the observed distribution of fossil and extant coral diversity.

The graphic below captures the comparison between fossil coral diversity and simulated coral diversity. Eocene, Miocene and Quaternary observed diversity (left side)  matches the model results (right side).

Source: Leprieur et al 2016

The scientists caution that local fluctuations like sea water temperature and acidity and ecological complexity will also play a role in evolution of  diversity and call for further validation of their work. But overall, its a nice demonstration of the role of plate tectonics in controlling coral habitats and diversity.

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.

My Rather Tenuous Connection With The Mars Rover Project Scientist

I have no connection with the Mars Rover program :)...but this @geosociety tweet a few days ago caught my eye:

@geosociety Fellow John Grotzinger, JPL geologist on Mars Curiosity rover mission, in LA Times. http://lat.ms/QzG81W  MT @earthmagazine

John Grotzinger was profiled in an article in the LA Times. He is project scientist for the Mars mission and in charge of directing the earth science effort to glean information about the geology of Mars. Here is what the article says about his work-

For much of his post-PhD career, the geologist kept his feet planted firmly on Earth. He combed ancient sedimentary rocks for signs of early life. He took trips around the globe, family in tow, to collect 550-million-year old specimens in Namibia and Oman.

What it left out was that Prof. Grotzinger is a carbonate sedimentologist. So.. I guess I can claim that I share an academic kinship with him :)

I am quite familiar with his work in carbonates. When I was working on my PhD in the mid 1990's he was already a faculty at MIT. His PhD research on Proterozoic carbonates of the Northwest Territories in Canada was directed by J. Fred Read at Virginia Polytechnic. During several GSA meetings I did get an opportunity to listen to his presentation on various aspects of Proterozoic carbonate platform evolution. He later moved to Cal Tech and JPL in Pasadena, California.

For long, carbonate sedimentologists gave much more attention to Phanerozoic carbonates and less attention to Proterozoic carbonate deposits. There was an economic incentive in that. Many Phanerozoic carbonate basins host prolific oil and gas deposits. The origin, growth and architecture of Phanerozoic carbonate sedimentary platforms,  a term for depositional basins in which hundreds to thousands of feet of calcium carbonate sediments accumulate, was studied quite intensely and we gained a very detailed understanding of these systems. All this work ultimately helps exploration geologists make reasonable predictions on the location and thicknesses of strata best suited to be oil reservoirs.

A Buried Devonian Manhattan Made Of Calcium Carbonate

I'm continuing with the theme of carbonate reservoir rocks - and for good reason. I found this gem of a story about coral reef oil reservoirs from the Devonian carbonate depositional basins of Alberta, Canada in the comments thread of a post on the Oil Drum -

RockyMtnGuy writes:

Well, yes, carbonate reservoirs do test your wits. The ones in Alberta are particularly difficult to deal with. They are just like the girl with the curl right, in the middle of her forehead: when they are good, they are very good, but when they are bad, they are horrid.

A classic was the Rumsey Reef, which they found not too far from where I grew up.

In 1982, Gulf Canada Resources discovered a small pinnacle, called the Rumsey Reef, just to leeward of the Stettler-Fenn-Big Valley reefs. It produced over 3.7 million barrels of high gravity oil from one well; 90% was recovered during the first three years. During that period it flowed 3,000-4,000 bbls/day. A decade later, Gulf explorers, Lemon and Taylor (1993) presented a paper with the wistful title, “The Rumsey Leduc Pinnacle Reef: Where are the Rest?”

The Rumsey Reef is an oil field about the size and shape of a New York skyscraper, just full of oil waiting to be sucked out. Based on the geological history of the area, there must be thousands of similar reefs out there, but they just don't show up on seismic.

Probably most of the remaining Leduc pinnacle reefs in central Alberta, and we have measured many of them, are physically too small to be adequately resolved by reconnaissance seismic exploration, whether 2D or 3D. Our measurements suggest pinnacle reefs, similar to Rumsey in size, are pillars of coral growing between 550' to 700' high from the Cooking Lake carbonate platform. They appear to range from 70' to 225' in diameter.

Wow! .. where does one start?

The Rumsey Reef is an oil field about the size and shape of a New York skyscraper, just full of oil waiting to be sucked out. 

Today most of the Devonian strata of Alberta is in the subsurface, buried underneath younger sediment.

But imagine a sea floor 400 million years ago, where thousands of self assembled towers were being built by organisms scavenging calcium and carbonate ions from sea water to form skeletons made up of the mineral aragonite or calcite. A  living breathing city of underwater skyscrapers hundreds of feet tall, extending in narrow zones tens of kilometers in length, along the edges of depositional basins, where the shallow sea bed suddenly slope into an abyss. The Devonian Alberta basin was a flat shallow water area that gave way along steeper slopes possibly due to faulting activity to deeper waters. These are classic settings for a pinnacle reef to form.

The schematic figure below shows the different settings where reefs form. The patch reefs shaped like towers are the pinnacle reefs. They are called patch reefs because they occur as isolated communities.

Source

A Pinnacle reef is a type of reef wherein the structure is a shape of a pinnacle or tower.  Reefs are biologically constructed structures that rise from the sea floor to form mounds. They are made up of the skeletons of organisms that prefer sunlight depths. Ordinarily, in shallow seas, the upward growth of reefs stop when the reef organisms start getting exposed to the atmosphere at low tides. The reef then grows laterally forming a broad structure.

In certain settings though, a rapidly subsiding sea floor creates space, maintaining a certain water depth, and the reef grows upwards maintaining its colonies in the optimal sunlight zone. Such conditions are present around volcanic islands. As volcanoes become dormant and erode and subside, reefs nucleate along their flanks and grow upwards as pinnacles. This situation is observed today along many tropical volcanic chains in the Pacific ocean.

Another environment for the formation of pinnacle reefs occurs when rapid sea level rise creates sufficient water depth. Pioneer colonies of reef building organisms tolerant of greater water depths may initiate the building of a bio-structure. As the reef grows into shallower water, a different species assemblage may become more common. The communities that build pinnacle reefs may change as the towers grow through different water depths.

The image below is of the coast of Belize. You can see a long north south trending white barrier reef in the center of the image. To the right is the open Caribbean sea. To the left are relatively deeper and quieter lagoons, environments where pinnacle reefs would likely grow. Some of the white isolated patches you see may be pinnacle reefs. This is just to give you an idea of the setting for pinnacle reefs, but I can't say for sure whether those patches are pinnacle reefs from this particular image.

Many of these Holocene reefs in Belize and the West Indies islands have grown on a Pleistocene limestone substrate (figure to the left, SEPM strata.org), which during the last sea level fall was weathered into an undulating topography known as karst. In early Holocene, sea level rose and flooded this limestone substrate. Reefs that took root in the depressions i.e. greater water depths, grew upwards to form pinnacles. In Alberta, the situation was different. The basin was an extensive shallow sea and the sea bed was broken into deeper and shallower areas by faults. My guess is that the Alberta basin experienced many episodes of sea level rise in the Devonian, and reef communities growing along the edges and flanks of fault blocks repeatedly grew upwards to form pinnacles.

Today, the dominant reef building organism are species of scleractinian corals, a group of coral organisms which have a symbiotic live in relationship with algae. In the Devonian, the community structure of reefs was different. There were corals present, but they predominantly belonged to two now extinct groups known as tabulate corals and rugose corals. Along with corals were stromatoporoids, an important coral like colonial organism, also now extinct. And there was a supporting cast of algae and molluscs.

The reef building activity in the Alberta basin lasted hundreds of thousands of years. It was not a continuous process. Sea level falls would have interrupted reef formation. A subsequent sea level rise would have allowed reef builders like stromatoporoids to colonize older reef substrates and resume the construction of these enormous towers. Each reef building episode may have lasted a few tens of thousands of years.

Eventually, water depths become considerably deeper, stopping reef growth. Sedimentary conditions changed and buried the reef under layers of mud or fine grained sediment. The reef itself, because it is built by organisms having branching structures is quite porous. Besides this primary porosity, the Alberta basin reefs underwent extensive alteration during burial. The minerals aragonite and calcite were replaced in patches by dolomite. This created more porosity as the replacement dissolved the original minerals and a denser dolomite occupied the space. Oil then migrated into the open pores and got trapped because the reef is capped by fine impervious material. Many of these living towers got transformed over time into a reservoir rock,  a buried skyscraper full of oil.

Regarding the Alberta basin, all these reefs are in the subsurface, and although they are invisible to seismic surveys, new exploration methods using  telluric currents have successfully identified more pinnacle reefs. Many of them will turn out to be prolific oil reservoirs.

But I can't get that 400 million year old underwater Manhattan of calcium carbonate out of my mind!

Matt Ridley On Ocean Acidification and Coral Calcification

Matt Ridley (author of Genome and the Red Queen) on his blog The Rational Optimist clarifies recent thinking about ocean acidification and its predicted impact on coral reefs. It is a strong defense of recent observations and experiments that indicate that the degree of ocean acidification expected through global warming does not pose a catastrophic threat to corals.

Some additional thoughts I had.

Many coral species in experimental conditions show enhanced calcification rates at higher pCO2. That may happen because dissolving more CO2 in water increases the bicarbonate content of sea-water among other dissolved inorganic carbon species providing a source for CO3 to bond with Ca to form the skeletal material.

This is the representation:

CO_{2 (aq)} + H₂O H₂CO₃ HCO₃^- + H⁺ CO₃^2- + 2 H⁺

So as pH increases so does the bicarbonate (HCO3) content of sea-water.


Just a reminder that decreasing ocean pH and calling it acidification does not mean that the oceans will becoming technically an acid, that is having pH below 7. In fact they will becoming less alkaline from the current pH of 8.1 to about 7.9 or so projected several decades on. This is within the range of variation in pH of seawater inside the coral body cavity and at the interface of the tissue and calcification sites. In other words organisms have already built in mechanisms to handle fluctuations in pH within certain limits.

Off course if pH keeps dropping, at some point the dynamics will change and calcification will be affected. That is because at the site of calcification bicarbonate (HCO3) is stripped of its proton (H+) and the carbonate used to bond with Ca. The proton has to be transported away from this site of lower pH (higher H+ activity) into the coral body cavity and beyond which is at relatively higher pH (lower H+ activity). If sea-water pH is lowered so will eventually the pH in the coral body cavity as there is an easy exchange between the two. The gradient in pH between the body cavity and the calcification site will decrease and that will hinder the transport of H+ away from the calcification site. With H+ building up at that site it becomes harder to strip more protons from HCO3 to form CO3. What that pH range is and whether such pH conditions will ever be reached in the natural oceans due to global warming is not very well understood.

Still there are other dangers to corals from global warming and the most important seems to be coral bleaching caused by the expulsion of symbiotic algae. This may happen as the temperature of water rises causing some internal mechanism within the coral to expel algae or via invasion of the coral by other parasitic microbes that may expel the algae.

Coral metabolism is tightly coupled to the health of this photosynthetic algae and CO2 released through coral metabolism contributes according to some estimates about 70% of the carbon for building skeletons. The other 30% comes from the sea-water HCO3 I mentioned in the earlier para. So removing the algae and degrading the metabolism of the coral would mean cutting off a significant supply of carbon for skeletons.

Again the effect of all these parameters have been demonstrated in experiments that have run from a few tens of hours to a few weeks. For example how important this metabolic source of carbon is in the long run and whether different coral species may be able to harvest more and more carbon directly from sea-water bicarbonate in case the supply of metabolic carbon is shut off is an interesting question.

Likely there will be coral species who will have flexible mechanisms to harvesting carbon. They will flourish and others who have no physiological flexibility might die off.

Nature is resilient due to its variability. The community structure of coral reefs may change over the next few hundred years. But corals as a group will live on.

Meanwhile there are many local dangers to coral reefs in the form of overfishing and destructive fishing methods, turbidity due to increased coastal runoff, nutrient overdose leading to algal blooms that have choke off the corals. As Matt Ridley's article reminds us, these have the potential of wiping out many coral reefs faster than global warming might.  

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.