Species Sorting Through Preferential Survival Of Aragonite Biota

Geological Processes and Evolution - 6

alright... enough about those Deccan Basalts.

A juicy paper on skeletal mineralogy, sea water chemistry and evolution in the December issue of Geology:

Controls on carbonate skeletal mineralogy: Global CO2 evolution and mass extinctions -  Andrey Yu. Zhuravlev and Rachel A. Wood

There has been cyclical shifts in sea-water composition through the Phanerozoic known as Aragonite seas and Calcite seas wherein inorganic aragonite and low-Mg calcite (cements, ooids) respectively preferentially precipitated. That is mimicked by skeletal mineralogy but with a catch. And that is that sea-water chemistry strongly influences the choice of skeletal mineralogy only for those groups that are newly acquiring the ability to build skeletons.  Once skeletonization is acquired most organic groups do not change their skeletal mineralogy through subsequent sea-water compositional changes.

Doing a survey of changes in skeletal mineralogy over time  Zhuravlev and Wood found out that the cyclical shifts in carbonate mineralogy is superposed on a longer shift towards aragonite, both in inorganic phases as well as a skeletal mineral. This they attribute to a gradual decrease in the pCO2 of sea-water (ultimately due to an atmospheric decrease) over the Phanerozoic. As pCO2 decreases the amount of dissolved carbonate ions (CO3) increases. An increased saturation of CO3 increases the supply of this ion to crystal nucleation sites. Such a rapid precipitation environment favors the growth of aragonite over calcite for reasons to do with the atomic structure of aragonite which is constructed more rapidly especially along the C crystallographic axis.

Besides CO3 saturation, ions like SO4 and Mg inhibit the rapid growth of trigonal calcite by poisoning lattice sites for CO3 and Ca. If SO4 and Mg are present in low quantities then in low CO3 saturation states, calcite is favored.  If SO4 and Mg are present in large quantities in sea-water they interfere with the growth of the calcite structure and amplify the kinetic advantage aragonite has over calcite in CO3 saturated waters.

That's not the only story though. The increase in aragonite skeletal groups was not gradual but step-wise, coinciding with mass extinctions. The end -Devonian, end-Permian and the end-Cretaceous mass extinctions ( low pCO2 , high SO4..?) were especially severe on low-Mg calcite skeletal groups wiping them out preferentially.

Figure below shows the step-wise proportional increase in aragonite groups as well as inorganic aragonite (histograms) over the Phanerozoic. Dotted lines are mass extinctions.

This favored the proliferation of aragonite biota which occupied and radiated in the ecological niches emptied by the extinction of calcite biota.

...at this point my posts usually begin to drift towards evolution... and so...

Evolution occurs through a variety of mechanisms. Lord Tennyson's evocative "nature red in tooth and claw" is the one that occupies popular imagination. But that mechanism of evolution - natural selection - as Darwin later explained, works through competition between individuals within a species. Besides natural selection, random genetic drift also is a common mechanism of change acting at the level of the individual or the gene.

But evolution can occur through processes acting at one hierarchical level above the individual i.e at the level of the group or species through mechanisms such as species sorting. In this case the change is driven through the differential extinction and/or reproduction of species that share certain traits. A process of sorting is taking place at the level of species. Certain types of species are surviving while others are going extinct.

Species that precipitate low-Mg calcite or aragonite skeletons are a good example. Evolution is occurring not through a direct competition for resources between calcite skeleton individuals and aragonite skeleton individuals but by conditions preferentially favoring the precipitation of aragonite over calcite.

This is because the ability to precipitate either aragonite or calcite is a property shared by every individual in either of these groups. The variation for evolution to work on comes from differences not between individuals within a group but by differences at a level higher than the individual i.e. the group. Differences between groups such as aragonite mineralogy versus calcite mineralogy determine the differential survival of entire groups.

Adaptive complexity is best explained by natural selection acting at the level of the individual. But not all patterns of life are explainable this way. The history of life has seen large-scale biotic turnovers wherein one group of organisms fade away and another become more common. Many such turnovers coincide with mass extinctions.

It is not always clear and easy to explain why certain groups became extinct while others survived and later prospered. Species sorting can occur for a variety of reasons....like...small size...wider geographic range...higher fecundity...all group properties that may help one type of group do better and survive tough ecological conditions compared to another group...

...or maybe in the case of marine groups that precipitate carbonate skeletons....mineral kinetics.

See: Geological processes and evolution

Environmental Impact Assessments In India Are Becoming A Joke

...time for another rant on the Environmental Impact Assessment ( EIA) process (which is required for clearing large developmental projects) in India.

What do you do to get a green signal for a development project in a bio-diverse wetland frequented by migratory birds?

Answer: Do the EIA in the summer months..when the wetlands are naturally drier and there are no winter migratory birds! Then file a report saying that there is no ecological value to this area...go ahead and build your project!

According to Dr Asad R. Rahmani of Bombay Natural History Society and Prof Asha Rajvanshi of the Wildlife Institute of India, that is what is happening in the case of the Naupada swamps in coastal Andhra Pradesh where an EIA report filed neglected to do a multi-year survey and instead considered only the period of 3 months from March to May.

How ludicrous.....how absurd can the process get?!!

The problem begins with how the state classifies the land. This piece of wetland is considered a "wasteland" by the district authorities. The hired consultants use that as a baseline and make no attempt to seriously evaluate the ecosystem.

That's how increasingly the EIA system seems to work...terms of reference and baselines are purposely kept narrow, limited and ill-considered and consultants with a past history of saying what the government wants to hear are favored. By relegating the EIA to a side-show with no real powers to halt a project or make its design conditional to the EIA finding,  a culture of shoddy science, incompetence and sheer dishonesty is being encouraged.

Beyond popular articles in mainstream media the EIA is coming under some more serious scrutiny in science publications as well. In the January 25 2010 issue of Current Science Devendra Kumar Agrawal, Mahendra S. Lodhi and Shradha Panwar do a case study of the potential impact of planned hydroelectric projects in the Alaknanda catchment in Uttarakhand and conclude that the EIA process is inadequately equipped to evaluate the region wide ecological impact of these projects. The EIA considers each project as separate and its larger impact on adjoining areas is not included in the scope of the EIA study. This piecemeal approach does not consider cascading and other non-tangible effects of these projects.

The Ministry of Environment and Forest which has the final say on these reports and projects is applying different clearance standards for different issues.

Here is what the Minister of Environment and Forest Mr. Jairam Ramesh said recently about the decision to withhold introduction of Bt Bringal (Times of India February 10 2010):

"It is my duty to adopt a cautious, precautionary principle based approach till such time independent scientific studies establish to the satisfaction of both the public and the professional the safety of the product".

Fine words..Mr Minister..but how about extending the same principles ...caution, precautionary principle based approach...independent science...satisfaction of both the public and the professional...to the EIA process?

Why has the EIA process been rendered so toothless that a project can begin even before the relevant EIA is completed..?

You Mr. Minister are overlooking and presiding over a grotesque perversion of science and the consequent loss of valuable biodiversity, ecological services and the livelihoods of people who depend on them.

India is a signatory to the Ramsar Agreement... a commitment to protect valuable wetlands.

Don't be a mute spectator.

Buddhist Rock Cut Monuments And Deccan Lava Flows

During my recent field trip to the Konkan west coast one of the geologists leading the trip mentioned that all the famous Buddhist rock cut monuments situated in the Deccan volcanic province were excavated in lavas of the Bushe Formation.

I had never given any thought to this before. If you look at the various famous Buddhist rock monuments...Ellora, Ajanta, Bhaja, Karla, Elephanta ...they do occupy  similar looking basalt slopes and escarpments. Those are the lavas belonging to the Bushe Formation confirmed the geologist.

The Deccan Basalt lava pile is subdivided into hierarchical geological units - each corresponding to an eruptive episode - based on their geochemical similarities. Here is the basic chemical stratigraphy:

The Bushe Formation is quite thick and made up of compound lava flows. That means that eruptive episodes were made up of blobs of lava units that spread and overlapped and coalesced with other lava units to form a compound unit, forming a humpy undulating lava pile in the process.

Like a overlapping stack of pancakes...like this one..with the Bushe Formation for comparison..

So why did Buddhist monks and artisans choose these lava flows out of the many rock slopes and escarpments from other lava flows available..?

My quick speculations:

1) East of the Western Ghat escarpment is the Deccan plateau which is dissected into spurs of Deccan Traps and flat valleys. The Bushe Formation is well exposed in these spurs forming slopes and thick rock escarpments.

2) The locations of the caves in this part of the Deccan where the Bushe Formation is prominent are along ancient trade routes. Patronage from sympathetic merchants and rulers provided sustenance for the monks and artisans.

3) The Bushe Formation occupies a convenient middle tier in the multi-storey Deccan landscape.

Landscape evolution has transformed the Deccan lava pile in to a series of steep slopes and escarpments. The Bushe Formation especially the escarpments are situated above forested slopes which grade into the valley floors. That would have made Bushe escarpments ideal locations... isolated enough from the roads and villages to suit the Buddhist monastic lifestyle but still accessible via the forested slopes. Many of these caves also acted as resting stops for traders and merchants and so making them not too inaccessible by going too far up the landscape might have been a consideration.

Here is a picture of the Bhaja caves near Pune...occupying a steep rock slope.

Formations younger than Bushe form the upper story escarpments of the Deccan landscape. Access to these may have been too tough to negotiate.

4) The workability of the rock material.

Maybe there is a difference in the way rock material of the Bushe gives way to hammers and chisels compared with other especially the younger lavas. The Bushe has been described as a blocky kind of a lava while the younger lava which also form escarpments have been described as having splintery properties that may not give way to form the rounded and blocky shapes the artisans wanted.

I don't know. This is a problem that some experimental Archaeology can throw light on.

These rock-cut monuments are a wonder. If you are in the area don't miss out visiting them.

Tafoni Occurs In Basalts Too

First things first. Go over to Michael Welland's blog - Through the Sandglass and congratulate him. His superb book Sand:The Never-Ending Story has been awarded the John Burroughs medal for excellence in natural history writing.

Sometime back Michael wrote a couple of posts on Tafoni, the beautiful enigmatic natural rock weathering sculptures found commonly in sandstones but also in other types of lithologies. The images that stuck with me from his posts were of desert landscapes and sandstone sculptures....so I got a pleasant surprise when I came across Tafoni in basalt on my recent field trip to the west coast.

Tafoni are ellipsoidal..bowl shaped natural rock cavities...go here to learn more.

They commonly occur in locations where salt weathering is intense. Water percolates through natural cracks in rocks and precipitate salts. These crystallizing salts exert pressure on the surrounding rock and little by little the host rock gives way to form holes and pits. Besides salt weathering the natural variation in hardness in rocks masses may also promote this differential weathering.

The location was just south of the town of Alibag ( Alibag incorrectly shown...it is directly west of the annotation... on the coast), near Korlai fishing village.

There are wave cut basalt benches exposed at low tide which are intruded by dikes. These basalt flows are studded with secondary silica and zeolites mineralization and ...Tafoni.

Here are some examples:

A close up of the beautiful honeycomb structure

Pits and closely spaced joints..

Associated silica and zeolite geodes

Pits...large and small..

Cluster of Tafoni on vertical and inclined surfaces..separated by regions which lack Tafoni..

Besides being located in the intertidal zone where constant wetting and drying and salt weathering is strong..there may be a couple of other reasons why Tafoni is localized to these flows.

First, these flows are not one homogenous rock mass. They are compound flows...i.e. made up of smaller flow units...small and big blobs of lava that overlap, interfinger and coalesce to form a larger compound flow representing one macro eruptive episode. That means there are flow unit scale variations in cooling rates and vesicular content that makes individual flow units unequally suseptible to weathering.

And second,  is the presence of dikes. Not directly..but these dike swarms are concentrated in regions where the crust has undergone extension...leaving the rocks riddled with closely spaces fractures.

That makes them vulnerable to dissolution through circulating water and chemical weathering.

The Plumbing Beneath The Deccan Volcanic Province

All that lava had to have come from somewhere.

A faculty friend invited me for field work leading up to an international conference on dike swarms of the Deccan Volcanic Province. There are thousands of dikes intruding the Deccan Basalts and many of them feeder to the lavas but they can be grouped into three broad clusters.

A roughly east-west trending cluster parallel to the Narmada-Tapi-Satpura rift which is a very ancient rift zone active intermittently since the Proterozoic. A roughly N-S trending cluster parallel to the West Coast rift which likely originated through the rifting of India with Madagascar around 90 mya., and a not so well oriented cluster in the region northeast of Mumbai.

The map below shows these dike clusters. NTDS is the Narmada Tapi Dike Swarm...WCDS is the West Coast Dike Swarm......WGE is the Western Ghat Escarpment.

 Source: Building a Continental Flood Basalt Province

Chemical affinities of these various dike swarms with the Deccan lava formations suggest that the east-west trending dike complexes likely were feeders to the older and middle part of the lava pile, while the ones along the west coast and northeast of Mumbai likely fed the younger lava formations. Lots of dikes though show no affinities and may represent feeders to now eroded lava flows or may not be feeders but intrusives that terminated within the lava pile.

Matching dike swarms to lava formations is important since it can tell us about which regional stress fields were active and influential in controlling volcanism during a particular eruptive phase and secondly how far did the lavas flow from the source vents. These kind of studies suggest that the feeders dike systems were not always situated close to the formations they have an affinity with, indicating that lavas flowed for tens to a hundred odd km away from the source vents.

I missed the first part of the field work which took place around the cluster in the northern part i.e. north of Dhule in the E-W dike zone. But you can get an idea of the immensity of these dike swarms in the image below which is around Nandurbar north of Dhule.

...I  jumped on the bus for the second part which was along the west coast south of Mumbai in the area between Alibag and Murud.  Here the dikes fell broadly into two compositional categories. An older tholeiitic composition stage and likely feeders to the younger Deccan formations and a later alkaline stage younger than the youngest of the flood basalts represented by lamprophyres, nephelinites and phonolites. These represent post Deccan intrusives.

Overall these dikes were smaller in extent that the east-west trending cluster but more varied in composition. The locations were spectacular, exposed along wave cut benches, accessible only at low tides. Here are some examples:

Exposed along the beach at Korlai village south of Alibag

Same beach..notice the reddened margins which define the contact of the dike with the host rock

..like a serpent..

.. exposed at Murud village further south...

..you can see this Lamprophyre dike continuing into the overlying flow..

 
The trip was great fun. I have not been involved really with the Deccan Basalts since my M.S. days, so I got to learn quite a bit about the advances in the field and features which I had always seen on the wayside but not given much thought to made more sense to me. 

The most important part of the trip was off course the group lunches.. Yummy coastal food under a coconut plantation.

I will be posting more pictures and stuff about this trip in the weeks to come.