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