Abstract
The role of ocean acidification in the end-Permian mass extinction is highly controversial with conflicting hypotheses relating to its timing and extent. Observations and experiments on living molluscs demonstrate that those inhabiting acidic settings exhibit characteristic morphological deformities and disordered shell ultrastructures. These deformities should be recognisable in the fossil record, and provide a robust palaeo-proxy for severe ocean acidification. Here, we use fossils of originally aragonitic invertebrates to test whether ocean acidification occurred during the Permian–Triassic transition. Our results show that we can reject a hypothesised worldwide basal Triassic ocean acidification event owing to the absence of deformities and repair marks on bivalves and gastropods from the Triassic Hindeodus parvus Conodont Zone. We could not, however, utilise this proxy to test the role of a hypothesised acidification event just prior to and/or during the mass extinction event. If ocean acidification did develop during the mass extinction event, then it most likely only occurred in the latest Permian, and was not severe enough to impact calcification.
Highlights
The role of ocean acidification in the end-Permian mass extinction is highly controversial with conflicting hypotheses relating to its timing and extent
Analyses of δ11B from brachiopod calcite indicate instead that a decrease in pH started with the mass extinction event and continued into the basal Triassic, in agreement with earth-system modelling[13], but this new data is confounded with potential issues of diagenetic alteration
Prior to the mass extinction event, assemblages are dominated by the organophoshatic brachiopod Lingularia freboldi and sponge spicules from a glauconitic sandstone, which records a species turnover to Lingularia sp. and the presence of abundant siliceous sponge spicules across the mass extinction event
Summary
The role of ocean acidification in the end-Permian mass extinction is highly controversial with conflicting hypotheses relating to its timing and extent. Unlike ocean deoxygenation, which becomes more severe with a progressive rise in temperature, carbonate undersaturation can only develop if the uptake of C O2 into the oceans is relatively rapid, and subsequently can only persist for a few tens of thousands of y ears[2] This has made understanding the role, timing, duration and extent of ocean acidification in past hyperthermals problematic, especially because geoscientists lack a robust and widely accepted proxy for ocean acidification that can be applied beyond the Cenozoic. Ocean acidification and carbonate undersaturation in the basal Triassic are not, supported by the sedimentological record of the H. parvus Zone, e.g., the widespread development of anachronistic carbonate facies[14], which indicates conditions that were at least saturated, rather than undersaturated, and supports the bulk rock δ11B results This means that there are three different hypotheses for the timing of ocean acidification during the Permian–Triassic. Changes that permanently alter the preservable parts of invertebrates, such as the shell, should be readily recorded in the fossil record; these include reduced growth rates, smaller sizes, increased porosity, shell degradation, disordered calcite fibre ornamentation, morphological deformities, increased dissolution, and shell repair 15–23
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