Friday, February 7, 2020

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?


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.

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