Debate persists about the causes of the Cretaceous-Paleogene mass extinction. The bolide impact at Chicxulub left a globally distributed iridium anomaly at the primary extinction horizon that is coincident with the rapid extinction of both terrestrial and marine organisms. However, some experts propose that eruption of the Deccan Traps Large Igneous Province caused the extinction. Although Large Igneous Province volcanism commonly correspond with mass extinctions, questions remain about the link between Deccan volcanism and the Cretaceous-Paleogene extinction Ocean Acidification which encompasses transient decreases in pH, and carbonate mineral saturation states resulting from the large and rapid injection of CO₂, into the atmosphere-ocean system, may have caused extinctions throughout Earth history. The Deccan Traps volcanism released large quantities of CO₂, which likely caused warming and Ocean Acidification on a Global scale. Sedimentological indicators, such as reduced carbonate weight percent and increased planktic Foraminiferal fragmentation, suggest eruption of the Deccan Traps forced Ocean Acidification before the Cretaceous-Paleogene boundary. The bolide impact may have caused lesser and transient Ocean Acidification. During and following Ocean Acidification, dissolution of seafloor carbonate and biological compensation restore balance by neutralizing acidity and elevating alkalinity. Over longer time scales, silicate weathering plays a similar role.
In a paper published in the journal Geology on 28 October 2019, Benjamin Linzmeier, Andrew Jacobson, Bradley Sageman, Matthew Hurtgen, and Meagan Ankney of the Department of Earth and Planetary Sciences at Northwestern University, Sierra Petersen of the Earth and Environmental Sciences Department at the University of Michigan, Thomas Tobin of the Department of Geological Sciences at the University of Alabama, and Gabriella Kitch and Jiuyuan Wang, also of the Department of Earth and Planetary Sciences at Northwestern University, describe the results of a study which used calcium isotope ratios from Mollusc shells as a proxy for ocean acidification before and across the Cretaceous-Paleogene boundary.
The calcium isotope system offers a valuable proxy for detecting OA in deep time, as the ration of different stable isotopes in seawater and carbonate sediment are sensitive to the balance between weathering inputs and carbonate output, carbonate mineralogy, and changes in isotopic fractionation during primary carbonate mineral production. To determine if Deccan volcanism or the bolide impact perturbed ocean carbonate chemistry, Linzmeir et al. measured the ratios of isotopes of calcium in ths shells of aragonitic Molluscs from Seymour Island, Antarctica, that span the Latest Cambrian to earliest Palaeocene interval.
Map of Seymour Island, Antarctica, with fossil collection localities and example shells of Lahillia (left) and Cucullaea (right) analysed in this study. Green coloured zones are informal subdivisions of the López de Bertodano Formation. Stratigraphic positioning of fossils shown on map was done by plane projection, which corresponds well to fossils measured in stratigraphic distance to the Cretaceous-Paleogene boundary. Linzmeir et al. (2019).
The López de Bertodano Formation was deposited in the James Ross Basin, in an open ocean–facing shelf environment with water depths near 150 m. The formation consists of siliciclastic clays and silts with interspersed sand beds and carbonate concretions. Sedimentation rates were high (10–30 cm per thousand years). Linzmeir et al. used magnetostratigraphic reversals (the Earth reverses its magnetic polarity at irregular intervals, something which is recorded in some iron-rich rocks; since these reversals are identical around the globe, they can be dated from sections with the best established chronologies, and those dates be applied in less well constrained sites) and the Cretaceous-Paleogene boundary to date their section, and compare it to other deposits of similar age elsewhere, and the Deccan eruptions.
The analyses included samples from 23 horizons. Most shells were from Bivalves of the genera Lahillia (26 specimens) and Cucullaea (9 specimens). Two Amberleya gastropods and four samples of carbonate cement from sediment attached to shells were also measured. Sampling of shells combined multiple years of growth and averaged potential seasonal variation in calcium isotope ratios. Lahillia and Cucullaea were shallow infauna that have previously been shown to have recorded oxygen isotope levels from seawater rather than pore water so shell calcium isotope ratios most likely reflects a seawater source. Amberleya was a slow motile epifaunal surface deposit–feeding Gastropod with similar characteristics.
Elemental and isotopic analyses were performed at Northwestern University. Elemental analyses by inductively coupled plasma–optical emission spectrometry have an uncertainty of ± 5%. Calcium isotope ratios were measured using a high-precision ⁴³Ca-⁴³Ca double-spike thermal ionization mass spectrometry technique. Results are relative to the Ocean Scientific International Ltd. seawater standard. Radiogenic strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were also analysed by thermal ionization mass spectrometry.
Samples yielded a range of calcium isotope levels, with carbonate cement having much higher values than shells. Changes through stratigraphy also exist. Calculated temperatures do not correlate with calcium isotope variations for 13 shells with paired analyses. Different Molluscs from the same or closely spaced horizons yielded similar calcium isotope ratios, suggesting no species-specific 'vital effects.' Variations in calcium isotope ratios coincide with excursions in sedimentological indicators of carbonate saturation from globally distributed locations, which presumably reflect fluctuations in seawater pH and (CO₃²⁻) forced by volcanic CO² emissions.
Diagenetic alteration of aragonitic Mollusc shells can increase the level of strontium relative to calcium and change oxygen and strontium isotope ratios, even when original mineralogy is mostly preserved. The three shells showing the highest strontium levels also have lower different calcium isotope ratios than shells with lower strontium levels from the same horizons, suggesting that diagenesis of aragonitic Mollusc shells also to affects calcium isotope ratios differently compared to bulk carbonate sediments and microfossils. Linzmeir et al. excluded these shells with anomalously high strontium from further interpretation.
Heavier calcium ions tend to precipitate out of the water column more rapidly, forming part of carbonate deposits on the seafloor. When the sea becomes acidified, these carbonate deposits are attacked, releasing the ions back into the seawater. This means that when the sea becomes acidified rapidly, there will be a spike in the level of heavier calcium isotopes in the water column, which can then be incorporated into the shells of marine organisms. Therefore if the shells of Molluscs from a section all show an increase in the ratio of heavier calcium isotopes at the same time, then this is likely to represent an increase in ocean acidification. Molluscs are known to preferentially incorporate higher levels of heavier isotopes in their shells, probably as a biproduct of the way their intracellular channels delivers ions to the extrapallial fluid, from which the shell precipitates, with the effect that they tend to give a stronger signal than other shell-forming animals, although the exact mechanism for this is unknown.
The high-precision calcium isotope record through the Cretaceous-Palaeocene mass extinction displays considerable complexity driven by the response of biocalcifiers to volcanic CO₂ outgassing. In particular, the short time scales of carbonate saturation state variation point to biotic compensation rather than chemical compensation as the dampening mechanism to Ocean Acidification. The first positive excursion (rise in heavier calcium isotopes) corresponds to deep-sea warming and may indicate reduced saturation from CO₂ outgassing that was independent of the size of Deccan flows. The first negative excursion lags increased flow size but coincides with increased carbonate export, and a local extinction identified by statistical analysis of fossil occurrences. The minor extinction may relate to water depth change. The second positive excursion indicates a return to lower saturation due to either biocalcification recovery or renewed Ocean Acidification. Volcanic CO₂ inputs may have prolonged this excursion and stressed biocalcifiers, eventually causing increased carbonate saturation and thereby the negative excursion that begins below the Cretaceous-Palaeocene boundary, In total, Linzmeir et al.'s data suggests that Deccan volcanism perturbed ocean carbonate chemistry before the Cretaceous-Palaeocene boundary and further support the hypothesis that the combined effect of Deccan volcanism and the Chicxulub impact may have been necessary to drive the Cretaceous-Palaeocene mass extinction.
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