Thursday, October 13, 2016

Sea Water Chemistry And Shell Mineralogy: Tales Of Mesozoic Bivalves

Years ago when I was in the second year of college, I along with friends, went for a fossil collection tour to the town of Ariyalur in South India. Rocks of Cretaceous age outcrop all around, and these strata have now become one of the most famous fossil localities in India.

We collected ammonites, echinoids, plant leaf impressions on clay and bivalves... lots and lots of bivalves.. In the picture below are the remains of my collection of molluscs. On the top left is an oyster with a clam clinging on to one of its valves. Bottom left is another oyster with its jagged valve margin. In the middle is a largish clam and to the right is an oyster whose layered shell structure is clearly visible.

I have some photomicrographs too taken from thin sections given to me by a friend.

In the above image the foliated shell microstructure of a piece of a bivalve can be clearly seen in cross polarized light.

And in the image below, a coarser prismatic crystal structure of a shell fragment is visible in the center of the image.

Most molluscs groups (including bivalves) in today's tropical seas built their skeletons using the CaCO3 polymorph aragonite. I say tropical seas, because molluscs with calcite skeletons are more common in temperate waters, such as for example in the marine communities living on the continental shelf of the southern coasts of Australia. In the Cretaceous seas though, even at tropical latitudes, calcite bivalves were common. In this apparent puzzle lies a very interesting story of climate change, sea floor spreading, changing sea water chemistry, the evolutionary decline and success of different bivalve groups during the Mesozoic, the emergence of bivalve reefs and the localization of hydrocarbon reservoirs.

Geologists over the years have noticed that the primary mineralogy of abiotic calcium carbonate precipitates such as ooids and marine cements has oscillated through geologic time between phases where they are made up of aragonite versus phases where they are made up of low Mg calcite. "Aragonite Seas" persisted from the late Neoproterozoic to the Early Cambrian, from Permian to Late Triassic, and from middle Cenozoic to Recent. "Calcite Seas" persisted from Middle Cambrian to Late Carboniferous (Pennsylvanian) and from early Jurassic to Middle Cenozoic.

Nature is messy. These aragonite and calcite phases are not water tight compartments. Aragonite ooids have been reported from the Ordovician and calcite ooids from Triassic. The change from aragonite to calcite during the switch from aragonite to calcite seas is not abrupt. Wolfgang Kessling in a recent review reminds us that seas are fuzzy in their latitudinal and temporal distribution of aragonite and calcite and  local conditions can buck general patterns.There is also a need for a more rigorous quantification of the temporal distribution of aragonite and calcite abiotic precipitates

The control over this oscillating abiotic mineralogy is the Mg/Ca ratio of seawater. A higher Mg/Ca (higher than 2) ratio inhibits calcite precipitation (Mg ions interfere with the construction of calcite atomic structure), while lower Mg/Ca (less than 2) ratio favors the precipitation of calcite since it is less soluble than aragonite.

Why does sea water Mg/Ca oscillate through time? The driver seems to be rates of sea floor spreading and formation of new mafic oceanic crust. During such times, the alteration of oceanic crust by reaction with sea water forms Mg rich hydrated alumino silicate minerals like Serpentine. This process withdraws Mg from sea water. Beginning early Jurassic, the breakup of super-continental blocks created a global system of mid oceanic ridges and enhanced the rates of ocean crust formation. This sea floor spreading persisted through the Jurassic and Cretaceous lowering  the Mg/Ca ratio of sea water and ushering in the "calcite seas".

Other parameters from time to time also affect the mineralogy of abiotic calcium carbonate deposits. In the Early-Mid Triassic, upwelling currents brought sulphate ions from deeper waters onto continental shelves. Sulphate suppresses calcite precipitation. By late Triassic times though, conditions changed. CO2 release by exhalations from the Central Atlantic Magmatic Province and from gas hydrates increased the amount of dissolved CO2 in sea water. This increased the solubility of aragonite suppressing its precipitation in the time interval bracketing the Late Triassic mass extinction and the Triassic-Jurassic transition.A long term trend of lowering of sea water Mg/Ca then resulted in the persistance of calcite as the favored CaCO3 mineral throughout the rest of the Mesozoic.

How has the changing sea water chemistry impacted the mineralogy of calcium carbonate skeletons of marine organisms? There are broad patterns of shell mineralogy acquisition and maintenance through the history of life which I will mention a little later. But since this post is on Mesozoic bivalves let me point to a survey by Michael Hautmann on shell mineralogy trends in Mesozoic epifaunal bivalves which tell a very interesting story of geological processes and evolutionary feedbacks.

Hautmann surveyed several families of epifaunal bivalves across the transition from the Triassic "aragonite seas" to the Jurassic and Cretaceous "calcite seas".  He choose to study mineralogical variation within families because that variation more likely reflected a cause and effect with changing sea water composition. That is because other factors that may influence shell mineralogy such as ecology, life habits and soft tissue organization remained the same.  He also studied only epifaunal taxa (living on the sea floor) as against infaunal taxa (living in the sediment column). Epifaunal taxa are in contact with normal sea water as against infaunal taxa which may encounter highly modified pore waters from time to time.

The survey found:

1) The Inoceramidae which initially had acquired a calcite skeleton in the Middle Paleozoic remained a marginal group until they began to diversify with the onset of the "calcite seas in the Jurassic

2) The Megalodointoidea had initially evolved an aragonite skeleton. They suffered a severe decline in diversity by late Triassic and across the Triassic -Jurassic boundary. The few remaining lineages maintained an aragonite skeleton in the Jurassic "calcite seas", but their diversity remained low.

3) The Ostreidae,Gryphaeidae, Pectinidae, Plicatulidae, and Buchiidae are all clam and oyster bivalve families. They are bimineralic. They have an outer skeletal layers composed of calcite and a middle and inner layers made up of aragonite. From this initial state inferred from preserved Triassic fossils,  taxa within these families at the beginning of Jurassic replaced the middle and inner aragonite layers with calcite.

One common theme that emerged is that thick shelled taxa more commonly shifted from aragonite to calcite. Thin shelled families like the Arcoidia maintained their aragonite shells throughout the Mesozoic. This suggests that as conditions became more conducive for calcite precipitation, maintaining a thick aragonite shell may have become energetically expensive. As a result there was selective pressure to reduce the amount of aragonite in the shell. Conditions at the Triassic-Jurassic transition may have facilitated this switch in mineralogy. Atmosphere CO2 levels and dissolved aqueous CO2 levels were high enough for sea water to be undersaturated with respect to both calcite and aragonite for a short interval at the Triassic-Jurassic boundary. Such undersaturated conditions would have favored the incorporation of calcite in the shell since it is the least soluble of the CaCO3 polymorphs and hence requires less energy to secrete than aragonite.

4)  Hippuritoidea (rudists) bivalves evolved from a surviving lineage of the Megalodontoidea by late Jurassic and Early Cretaceous. These were initially aragonite in composition. However, by late Jurassic times they had evolved an outer shell layer of prismatic low Mg calcite. This acquisition appears to have triggered a great expansion in their diversity. Rudist families with largely calcitic shells replaced families with shells that with either bimineralic or made up of aragonite. By Campanian times (Late Cretaceous) they reached a peak diversity of 300 species in the Tethyian oceans.

Rudists had weird shapes. Late Jurassic forms were made up of two tube or box shaped valves. By Cretaceous weirder forms had evolved. (image source: David Jablonski 1996). There were corkscrew and flattened forms. There were conical forms with one valve flattened and which served as a lid while the other became an inverted cone. These conical bivalves lived with the long inverted cone buried in a muddy substrate in close proximity to each other forming enormous dense colonies on the shallow sea floor. They were so prolific that rudist colonies displaced Scleractenian corals as the chief reef builders of the Mid-Late Cretaceous.   Because the long inverted cone became a large cavity after the death of the organism, these rudist beds upon burial had plenty of preserved porosity. Many of the most prolific hydrocarbon reservoirs of the Mid-Late Cretaceous (e.g. Gulf of Mexico) are hosted in thick rudist reefs.

In summary, many bivalve families were affected by the changing sea water condition through the Mesozoic. Selective pressure to maintain metabolic efficiency of carbonate secretion resulted in a switch to the energetically less costly calcite, especially in thick shelled bivalves. Groups like the Megalodontoidea that were conservative and maintained their original mineralogy suffered declines in their diversity.

What about the rest of the invertebrate marine taxa that secrete calcium carbonate skeletons? Do they also show a similar adaptability and a switch between aragonite shells and calcite shells as conditions change?

Susannah Porter presented a survey of the mineralogy of 40 animal taxa from the time when they first evolved skeletons over the past 550 million years or so beginning earliest Cambrian.

Source: Porter 2010

The data showed that out of the 37 taxa whose mineralogy is known with confidence, 25 acquired the mineralogy which matched the sea water chemistry of that time. Only two taxa appeared with a non-matching mineralogy. Ten taxa first acquired skeletons at a time when the sea water chemistry is not well constrained. This suggests that the prevailing sea water chemistry has had a strong influence of skeletal mineralogy, but only its first acquisition, although it remains to be seen whether the apparent fit presented by Porter will remain strong with better constraints on sea water chemistry. The data also show that most taxa has since not changed mineralogy even as sea water chemistry oscillated between "aragonite seas" and "calcite seas". This suggests that mineralogy of the shell is evolutionarily constrained and that the costs of maintaining a mineralogy not favored by sea water is less than the cost of switching mineralogies.

Other studies, such as that by J.G. Carter and colleagues indicate some instances of de novo acquisition of calcite mineralogy during aragonite seas. They argue that there is no evidence of a strong relationship between shell mineralogy and changing sea water chemistry. This may be so in certain cases. But changing sea water chemistry might still play a role in the long term success of a lineage. The argument for this is that the initial acquisition of calcite may have occurred for other reasons. Even in an aragonite sea, ecologic conditions and life habits may have created selection pressures for a different constructional morphology built using a mineral phase that provided better structural stability. Later, when sea water chemistry changed to a calcite conducive state, these calcite shell groups opportunistically diversified.

Despite these few examples of mineralogy switches, either coinciding or not coinciding with changes in sea water chemistry, it does appear that most taxa do not switch the mineralogy that they first acquired.

Why should this be so?

Some studies of shell matrix proteins involved in mineral secretion show that most of these proteins are polymorph specific. Since a large number of proteins are involved, a switch to a different mineralogy may require the evolution of a functionally different suite of proteins. This constraint may explain the conservation of first acquired mineralogy in most taxa.

How were the many families of Mesozoic bivalves able to switch their mineralogy in response to changing sea water chemistry? Perhaps because many of them were already bimineralic and the expansion of calcite into previously aragonite layers of the same shell did not require extensive evolutionary changes?.. I am only speculating.

Finally mass extinctions has influenced the relative proportions of aragonite and calcite skeletal marine communities. Aragonite taxa increased in abundance after the Permian-Triassic mass extinction. This extinction might have selectively culled a greater proportion of calcite skeletal groups. Subsequently, as I have mentioned before, aragonite skeletal groups declined across the Late Triassic mass extinction, probably due to high pCO2 values favoring the precipitation of less soluble calcite skeletons. And later in the Cenozoic, aragonite groups again increased after the Cretaceous-Paleogene mass extinction. The famous calcitic rudists for example went extinct during this environmental catastrophe.

Andrey Zhuravlev and Rachel Wood have argued that there appears to be a broad trend of increasing aragonite biota through the Phanerozoic. Replacement of calcite taxa by aragonite taxa occurred episodically at mass extinctions. They have proposed that this punctuated pattern of aragonite increase reflects a long term decrease in atmospheric carbon dioxide partial pressure (pCO2).  Aragonite precipitation is sensitive to pCO2 levels. Lower pCO2 levels increase the carbonate saturation state of oceans and favor the formation of aragonite.

Further, these scientists suggest that changes in pCO2 levels take place over shorter times spans of a few thousands of years. A coincidence of pCO2 perturbations with mass extinctions may exert greater influence over the selective decline and success of calcite and aragonite groups. Seawater Mg/Ca ratios on the other hand change very slowly as a response to varied rates of sea floor spreading. They may set the background state. The two parameters i.e. Mg/Ca ratio and mass extinctions operated at different rates and explain two different aspects of skeletal mineralogy trends.
During times of aragonite and calcite seas (Mg/Ca ratios) different taxa acquired skeletal mineralogies that matched the sea water chemistry of that time. But the origin of new mineralized groups is not clustered at the transition between these periods. Rather,  millions of years separated the origin of different groups. For example in the calcite seas from Early Cambrian to Late Carboniferous, new calcite groups appear at different times in the Cambrian, Ordovician, Silurian and Devonian. Similarly, although aragonite seas appeared by early Permian, several new aragonite taxa first appeared much later in the Triassic.

On the other hand, changes in the proportions of aragonite and calcite skeletal groups across mass extinctions took place relatively rapidly over the course of a few tens to hundreds of thousands of years. This did not occur because the same group switched mineralogies during the crisis. Nor do taxa with favorable mineralogy originate necessarily just after the mass extinction. Mineralogic proportions change by preferential removal of a particular skeletal mineralogy and subsequent post extinction diversification of the surviving fauna.

If changes in pCO2 levels is an important driver of changes in carbonate mineralogy, what will the current episode of global warming and increasing pCO2 do to marine aragonite versus calcite biotas?

And how good really is the fit between changes in pCO2 and mass extinctions?  Clearly, refinements to our understanding on this subject are needed. Many a PhD awaits to be written on this fascinating interplay between geological processes, mass extinctions and trends in abiotic and skeletal carbonate mineralogy.

This has been a fun post to research and write. Great things happen when you dust off those fossils long ago collected and hidden away in the recesses of your home.

Hautmann, M. (2006). Shell mineralogical trends in epifaunal Mesozoic bivalves and their relationship to seawater chemistry and atmospheric carbon dioxide concentration Facies, 52 (3), 417-433 DOI: 10.1007/s10347-005-0029-x

Kiessling, W. (2015). RESEARCH FOCUS: Fuzzy seas Geology, 43 (2), 191-192 DOI: 10.1130/focus022015.1

PORTER, S. (2010). Calcite and aragonite seas and the de novo acquisition of carbonate skeletons Geobiology, 8 (4), 256-277 DOI: 10.1111/j.1472-4669.2010.00246.x

Zhuravlev, A., & Wood, R. (2009). Controls on carbonate skeletal mineralogy: Global CO2 evolution and mass extinctions Geology, 37 (12), 1123-1126 DOI: 10.1130/G30204A.1