Sea Water Chemistry Triggers For Evolution Of Biomineralization

Geological Processes and Evolution #20

The bulk of the shells and skeletons of marine creatures are built out of aragonite or high-Mg calcite (> 4 mole% MgCO3) or low-Mg calcite. These three calcium carbonate minerals, along with dolomite (calcium magnesium carbonate), also occur as marine cements, i.e., they are precipitated from sea water as mineral grains in the open spaces between shell particles, resulting in loose sediment getting bound in to hard rock.

I came across this paper by Rachel Wood and colleagues from 2017 on the link between sea water chemistry and the evolution of biomineralization as evidenced in the limestone strata from Siberia. The time period is from 545 million years ago to 500 million years ago, a span in which early animals began secreting calcium carbonate skeletons. What were the main triggers for this evolutionary change?

Abstract:

The trigger for biomineralization of metazoans in the terminal Ediacaran, ca. 550 Ma, has been suggested to be the rise of oxygenation or an increase in seawater Ca concentration, but geochemical and fossil data have not been fully integrated to demonstrate cause and effect. Here we combine the record of macrofossils with early marine carbonate cement distribution within a relative depth framework for terminal Ediacaran to Cambrian successions on the eastern Siberian Platform, Russia, to interrogate the evolution of seawater chemistry and biotic response. Prior to ca. 545 Ma, the presence of early marine ferroan dolomite cement suggests dominantly ferruginous anoxic “aragonite-dolomite seas”, with a very shallow oxic chemocline that supported mainly soft-bodied macrobiota. After ca. 545 Ma, marine cements changed to aragonite and/or high-Mg calcite, and this coincides with the appearance of widespread aragonite and high-Mg calcite skeletal metazoans, suggesting a profound change in seawater chemistry to “aragonite seas” with a deeper chemocline. By early Cambrian Stage 3, the first marine low-Mg calcite cements appear, coincident with the first low-Mg calcite metazoan skeletons, suggesting a further shift to “calcite seas”. We suggest that this evolution of seawater chemistry was caused by enhanced continental denudation that increased the input of Ca into oceans so progressively lowering Mg/Ca, which, combined with more widespread oxic conditions, facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy.

Dolomite abundance through geologic time shows a positive correlation with periods of ocean anoxia. One reason could be that sulphate reducing bacteria which thrive in anoxic environments remove dissolved sulphate which interferes with dolomite formation. A 'shallow oxic chemocline' means that only the shallows were oxygen rich, while deeper water were oxygen poor or anoxic. These conditions changed after about 545 million years ago with increasing oxygen in even deeper waters thus increasing habitat suitable for the evolution and spread of oxygen demanding animals. Sponges may have played an important role in the ventilation of the water column by actively removing suspended organic matter during filter feeding, thus making more oxygen available to be transferred to deeper waters.

The terms "aragonite-dolomite seas", "aragonite seas" and "calcite seas" refer to geologic time-bound conditions facilitating the precipitation of marine cements of that mineralogy. Excessive magnesium is a hindrance to formation of calcite and a lowering of Mg/Ca meant a shift from "aragonite seas" to "calcite seas". From Cambrian to recent times, periodic swings in Mg/Ca of sea water has caused either aragonite or calcite to become the dominant marine precipitate.  

It is notable that the mineralogy of skeletons when they first evolve in a particular animal group seems to be determined by the prevailing sea water chemistry. Animal groups like the molluscs which acquired the ability to biomineralize during 'aragonite-high Mg calcite seas' of the late Ediacaran -Early Cambrian (550-520 million years ago) used these minerals to build their skeletons. Later in the Paleozoic, sea water chemistry changed to favor the precipitation of low Mg calcite. Animal groups like the trilobites, echinoderms, brachiopods and tabulate corals that first evolved skeletons during this time period (Early Mid Cambrian to Ordovician, ~520-450 million years ago) began using low-Mg calcite as their shell mineral.

The graphic shows the first appearance of carbonate skeletal groups with their inferred primary mineralogy plotted against the temporal distribution of aragonite and calcite seas (inferred from marine cements).

Source: Susannah M. Porter 2010: Calcite and aragonite seas and the de novo acquisition of carbonate skeletons.

Interestingly, once acquired, animals did not switch their shell mineralogy to match subsequent changes in sea water chemistry. Most aragonite shell secreting animals retained this mineralogy during later 'calcite seas' (e.g. Ordovician to early Permian and Jurassic-Cretaceous) and vice versa ('aragonite seas'- Permian-Triassic, Cenozoic). A wholesale change in skeletal mineralogy may require too many evolutionary steps and would be physiologically demanding. Conserving mineralogy even during changing ambient conditions is likely an evolutionary trade off.

One question remains unanswered. There is evidence as early as 560 million years ago of soft bodied animals making tracks and burrows on the sea floor. If sea water chemistry then was conducive for the precipitation of early dolomite, why didn't at least some early animal groups make skeletons out of dolomite? Perhaps the answer lies in mineral kinetics. Dolomite is slow to precipitate. Its atomic structure is made up of layers of calcium carbonate alternating with layers of magnesium carbonate. This is more difficult to build than the relatively simpler structures of aragonite and calcite which are made up of only calcium carbonate with a few magnesium ions substituting for calcium.

In latest Ediacaran-early Cambrian times, as oxygen levels rose and animal diversity increased, ecologic interactions became more complex. The rise of predators and predator-prey arms races would have favored the evolution of a protective shell that could be assembled rapidly. Faster precipitating minerals like aragonite and calcite became the fixed construction material.

Open Access.

Articles: Herculaneum, Magma Ascent, Early Human Migration, Indian Cheetah

Some interesting articles on a variety of topics that I came across in the past few weeks.

1) What Really Happened at Herculaneum?

This off course refers to the violent eruption of Mount Vesuvias in 79 A.D.  A new study analyses the way bone and soft tissue react to extreme heat and proposes that the people found dead at Herculaneum did not vaporize but died of asphyxiation.

2)  The long wait and rapid rise of deep magma.

Magma can reside in deep chambers at the boundary between the crust and mantle for thousands of years before rising to the surface rapidly in a matter of a few days.

3) Neanderthal Genes Hint at Much Earlier Human Migration From Africa.

It was thought that 60,000 years ago modern humans migrated out of Africa and interbred with Neanderthals beginning around 40,000 years ago. As a result all non-Africans carry some Neanderthal DNA. A new DNA analysis technique now suggests that an earlier wave of humans migrated out of Africa some 200,000 years ago and interbred with Neanderthals. Their descendants back migrated to Africa carrying with them the legacy of this earlier mating. As a result, Africans too carry a genetic legacy of Neanderthals.

4) Introduce the cheetah, with caution and guidelines.

There is a proposal to introduce the African cheetah into the Indian landscape. Neha Sinha argues that a grasslands policy needs to be put in place first.
 

Sedimentary Structures: Building Stones of Badami, Aihole And Pattadakal Temples

Is this sandstone slab in its original geological orientation (as when the sedimentary layers were deposited) or is it upside down? I'll answer this a little later, but first some background.

I recently visited the Chalukya style temples and rock cut monuments at Aihole, Pattadakal and Badami (6th -8th CE) in northern Karnataka and noticed some great sedimentary structures in the building stones. The term sedimentary structures refers to the shape and form sedimentary layers get sculpted into by the action of waves, currents, tides and wind during deposition of the sediment. The size of the deposited sedimentary particles and the orientation of layers are a reflection of both the vigor of the currents and waves and the direction of flow of water or wind.  

These monuments are made up of Neoproterozoic age (900-800 million year old) sandstones. Geologists have recognized using detailed sedimentological analysis that the sandstones formed mostly in a large braided river system that flowed in a northwesterly direction.

Between  roughly 1800 -800 million years ago, over the course of a billion years, the Indian continental crust sagged due to various tectonic forces to form several long lasting sedimentary basins. The Kaladgi Basin in which the Badami area sandstones were deposited is one such basin. The paleogeographic reconstruction below shows the position of the Indian continent at about one billion years ago and the location of the various sedimentary basins within it.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

Much of this deposition took place in inland or epeiric seas that flooded the Indian continent. During intervals of sea level fall, rivers carved valleys and deposited coarse sediment. The Badami Cave sandstones are river deposits of the Kaladgi Basin. The stratigraphic column shows various sedimentary deposits of the Kaladgi Basin and their inferred environments of deposition.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

The Badami braided river system was receiving sediment eroded from Archean age (>2.5 billion year old) rocks situated SE of the basin. These were granites, granodiorites, and low to medium grade metamorphic  rocks of the Dharwar craton (a large block of stable old continental crust). 

Land plants did not exist then. Weathered debris was moved quickly by surface flow into streams. Large sediment load, moving by traction i.e. by rolling and sliding on the stream bed, repeatedly choked the channels, forcing bifurcation of streams and formation of braids. Very broad braided rivers formed since there were no plants to stabilize banks.  The Badami sandstones (Cave Temple Formation) are technically known as arenites. This term indicates that the rock is made up of mostly coarse sand with very little finer sized mud. Accumulation of mostly coarser sand size and pebbly particles reflects a locale of repeated high discharges and vigorous currents which winnowed away the finer sized mud.  The braided river shown below as an example is from the Canterbury Plains of New Zealand.

 Source: Braided Rivers: What's the Story?

The Badami rocks preserve a record of  various subenvironments of this paleo-river. Picture shows channel and bar deposits in outcrop.

Source: Mukhopadhyay et. al. 2018; Stratigraphic Evolution and Architecture of the Terrestrial Succession at the Base of the Neoproterozoic Badami Group, Karnataka, India.

As river channels episodically migrated sideways and the basin floor subsided to accommodate more sediment, channel deposits and adjacent sand bars got stacked to form thick 'multi-story' sandstones. Each bed tells a story of a discrete depositional episode.

The arrangement of sand layers within each bed tells us about the subenvironments in which it formed and the energy and direction of water flow during deposition. I came across many types of these internal structures. I recognized tabular cross beds, trough cross beds, planar lamination and rippled beds. Water (or wind) can move & shape sand into piles or waves. Sand grains roll along the direction of flow, then avalanche down the steeper side (lee side) of the wave forming a layer inclined (cross) to the orientation of the main sand body. Successive avalanches form a set of cross beds. The graphic shows the formation of a set of cross beds.

 Source: Dr. Diane M Burns in Teaching Sedimentary Geology in the 21st Century.

Here is an example of cross beds from near the town of Badami.

And this one is from a building stone from Pattadakal temple.
 

Such cross beds were built by sediment avalanching on the lee side of migrating sand bars during high flow.

This picture show trough cross bedding from near the Badami cave complex. These represent the internal structure of migrating sinuous sand dunes on a channel floor. 

See this elegant explanation by Dawn Sumner, a sedimentologist at the University of California at Davis,  of how trough cross beds form.



Email subscribers who may not be able to see the embedded video, click on this link: Trough Cross Bedding Video.

And here is a beautiful example of trough cross bedding found in a Pattadakal temple building stone.

This is planar lamination on a slab at Pattadakal. The bed is constructed of parallel layers of coarse sand. It is interpreted to have been deposited in a high flow regime from sheets of water flowing over mid channel sand bars.
 

Ripples on a slab at Pattadakal. This is a rare preservation of a bedding surface showing rippled sand. Erosion usually cuts off the wavy upper part. These ripples indicate migration of small sand waves in a quieter flow regime on the channel floor.

Remember, cross beds are the inclined layers that form on the lee side of a ripple or wave or dune. Here are small cross sets on the floor of Aihole rock cut temple! These represent the cross beds formed by migration of small ripples. The ripples themselves have been eroded away. Arrows indicate the direction of water flow and cross bed accretion as ripples migrated.

Okay, let's go back to my first question. Is the slab I showed in the picture geologically upside down?

Yes it is. But how to tell?

As sand avalanches down the lee slope it forms a tail at the toe of the slope resulting in cross beds which become tangential to the floor. In picture the cross beds are tangential towards the top of slab i.e. that is actually the base.

Lets see at how the cross bed contact with the top and bottom bedding plane looks in an outcrop. Here is the original depositional orientation of cross beds manifest in this outcrop near Badami caves. They show a tail or tangential contact of the cross beds with the base. Since top of cross beds are not usually preserved they show a high angle contact truncated by upper bedding plane.

This slab is upside down too! Notice again the tangential contact of the cross beds (white arrow) is towards the top, which means that must have been the base. Yellow arrow points to high angle contact with the upper bedding surface. 

Towards the top of the exposed section of sandstone around Badami I came across some truly impressive examples of cross bedding. These particular exposures were on the crags opposite the four main Badami temples. There is a narrow passage past the archaeological museum and a short climb to the top. Take a look at these beauties!

These large cross beds reminded me of the inclined beds of wind blown sand dunes. Is it possible that abandoned sand bars were sculpted by wind in to big dunes? Or does this upper level sandstone represent, as a recent study suggests, the beginning of a marine incursion in to the basin? In this scenario, deposition of sand took place in high-energy shallow waters near the shore. These cross beds represent large migrating sand waves which were eventually shaped in to beach ridges and tidal bars.

The outcrops and building stones of these monuments mostly record the processes within the Badami braided paleo-river. 900 million yrs ago a complex of channels and bars, quieter pools and rippled sand beds existed where these temples stand today.

Do visit Aihole, Pattadakal and Badami and gaze at its splendid architecture and sculptures. But spare some time to appreciate the magnificent record of our natural history that these monuments preserve. 

Quiz- Is this slab upside down or in its true depositional orientation? 😉

Until next time....

Glacial Saraswati?: New Data On An Old Question

Did a perennial glacial fed river flow through the Indus Civilization region of what is now Haryana and Rajasthan? Previous work on the fluvial history of this region had indicated that a distributary of the glacially sourced Sutlej was flowing through a network of paleo-channels buried under the river now known as the Ghaggar until around 8,000 years ago. The Sutlej distributary system then died out, turning that river course into a smaller monsoon fed channel system.

For a more detailed history of research on this topic you can follow this link - Ghaggar /Saraswati Posts.

Recently, in November 2019, Anirban Chatterjee and colleagues published new data on deposits of grey sand in the subsurface of the Ghaggar channel and adjacent floodplains. The youngest of these deposits are 4, 500 years old. Geochemical fingerprinting points to High Himalayan granites and gneisses as their source. This likely extends the glacial phase of the Ghaggar to more recent times, until about the beginning of the urbanization of the Indus Valley Civilization (IVC).

Here is the abstract:

The legendary river Saraswati of Indian mythology has often been hypothesized to be an ancient perennial channel of the seasonal river Ghaggar that flowed through the heartland of the Bronze Age Harappan civilization in north-western India. Despite the discovery of abundant settlements along a major paleo-channel of the Ghaggar, many believed that the Harappans depended solely on monsoonal rains, because no proof existed for the river’s uninterrupted flow during the zenith of the civilization. Here, we present unequivocal evidence for the Ghaggar’s perennial past by studying temporal changes of sediment provenance along a 300 km stretch of the river basin. This is achieved using 40Ar/39Ar ages of detrital muscovite and Sr-Nd isotopic ratios of siliciclastic sediment in fluvial sequences, dated by radiocarbon and luminescence methods. We establish that during 80-20 ka and 9-4.5 ka the river was perennial and was receiving sediments from the Higher and Lesser Himalayas. The latter phase can be attributed to the reactivation of the river by the distributaries of the Sutlej. This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks. The timing of the eventual decline of the river, which led to the collapse of the civilization, approximately coincides with the commencement of the Meghalayan Stage.

The geological work looks to be sound. The data on sediment fingerprinting overlaps with what we know about High Himalayan geochemical signatures and present day Sutlej sand composition.

I do want to comment on another sentence from the abstract (emphasis mine)-

"This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks"

Saraswati is the name given to this river by the Vedic people. Correlation of the river's perennial phase between 9,000-4,500 years ago with Saraswati is valid only if  you can demonstrate that the Vedic people were inhabitants of this region from before 4,500 yrs ago. Geological studies cannot establish this. A combination of archeology, linguistics (cracking the Indus script would be nice!) and genetics will eventually answer that. The other scenario is that the Vedic people could have migrated into this region much later and began venerating a smaller monsoonal Ghaggar as Saraswati. Work by Liviu Giosan and colleagues suggests that stronger monsoons over the Siwaliks sustained sufficient flow in the old channels of the Ghaggar until the late IVC period (~1800-1600 B.C).

When did this river come to be called the Saraswati is still an open question.

Two recent genetics papers using ancient DNA recovered from the IVC site of Rakhigarhi and from Central Asia argue that people from the Pontic-Caspian steppes migrated into South Asia between 2000 -1500 B.C. bringing with them the Indo-Iranian branch of the Indo-European language family. These would presumably be the Vedic people.

Read these papers. They are interesting.

1) On the existence of a perennial river in the Harappan heartland.
2) The formation of human populations in South and Central Asia.
3) An Ancient Harappan Genome Lacks Ancestry from Steppe Pastoralists or Iranian Farmers.

Readings: Erectus SE Asia, Devonian Fossil Forest, Archean Iron Formations

Some selected readings:

1) New dates of Homo erectus from Ngandong Java shows late surviving populations until 117,000 to 108,000 years ago. A short clean summary by Razib Khan on SE Asian hominin diversity.

Southeast Asia during the Eemian was a hominin paradise.

Paper: Last appearance of Homo erectus at Ngandong, Java, 117,000–108,000 years ago.

2) Exquisite preservation of one of the earliest forests from the Mid Devonian ( ~385 million years ago) of New York containing a modern looking root system.

Paper - Mid-Devonian Archaeopteris Roots Signal Revolutionary Change in Earliest Fossil Forests.

Write up : The World’s Oldest Forest Has 385-Million-Year-Old Tree Roots.

3) Before around 2.3 billion years ago there was very little oxygen in the atmosphere. This was a time before the evolutionary invention of oxygenic photosynthesis wherein bacteria harvest electrons from H2O and release oxygen as a byproduct. Instead, during this time another photosynthesis pathway known as photoferrotrophy was prevalent. Here, bacteria use light and ferrous iron (Fe+2) to fix CO2 as biomass, releasing ferric iron (Fe+3) as byproduct. This ferric iron then accumulated to form large iron deposits. But these deposits lack organic matter. How to explain this if the iron was being produced from a biomass? Scientists point to a role of silica. At that time the oceans were saturated in free silica. Experimental work shows that in the presence of free silica cell surfaces repel iron hydroxides, thus creating a source of organic matter free iron deposits. This organic matter then was acted upon by methane producing microbes. The methane released kept the temperature of the earth warmer than it would have been under a dim early sun.

Fascinating story of the feedback between geology and evolution.

Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans.