Abstract
Severe paleoclimatic change during the Toarcian (Early Jurassic) oceanic anoxic event (OAE) was characterized by a negative δ 13 C excursion, increased weathering, higher seawater temperatures, oceanic deoxygenation, and mass extinction. We present abundance and size data (n ≈ 36,000) for the two dominant epifaunal bivalve species from the Toarcian OAE, Yorkshire, UK. We statistically correlate the biotic data with geochemical proxies for environmental change and show that our results are comparable with changes in present-day ecosystems affected by hypoxia. Bositra radiata dominated during declining oxygen levels immediately before the Toarcian OAE sensu stricto, and shell size doubled when δ 13 C org was decreasing, indicating a connection with primary productivity. Small Pseudomytiloides dubius dominated during the Toarcian OAE and varied sharply in abundance, indicating that it was highly opportunistic. P. dubius shell size is strongly related to Mo concentration, [Mo]; this indicates a relationship between size and N 2 -fi xing primary productivity via [Mo] limitation. A secondary factor contributing to small shell size was lower oxygen levels. After the Toarcian OAE diversity increased, P. dubius was less abundant and shell size doubled, indicating that bivalve populations were less limited by resources and conditions were more favorable. Size frequency distributions show that both Toarcian species had short life spans, rapid generation times, high recruitment, and high juvenile mortality. The opportunist Mulinia lateralis is a present-day analog for P. dubius. This research provides a case study for the long-term impacts of deoxygenation upon marine ecosystems, including that being observed today.
Highlights
IntroductionA key impact of increasing atmospheric pCO2 and eutrophication is the development of oceanic hypoxia (with oxygen content of 1%–30% of saturation, compared to anoxia, the absence of oxygen) (Diaz and Rosenberg, 2008)
A key impact of increasing atmospheric pCO2 and eutrophication is the development of oceanic hypoxia (Diaz and Rosenberg, 2008)
The aims of this study were to (1) quantify the size and abundance of the only two bivalve species [Bositra radiata (Goldfuss) and Pseudomytiloides dubius (Sowerby)] that occurred in significant numbers during the Toarcian oceanic anoxic event (OAE), (2) correlate these biotic data with geochemical proxies for environmental changes, and (3) compare the results with present-day benthic communities
Summary
A key impact of increasing atmospheric pCO2 and eutrophication is the development of oceanic hypoxia (with oxygen content of 1%–30% of saturation, compared to anoxia, the absence of oxygen) (Diaz and Rosenberg, 2008). Quantitative high-resolution, long-term studies of ecological responses to oceanic deoxygenation from the geological record and present day are sparse (e.g., Falkowski et al, 2011). In present-day oceans, the impacts of hypoxia are confounded by multiple synergistic anthropogenic pressures, such as marine pollution and overfishing. Investigations of the biotic effects of oceanic deoxygenation from the geological record have the advantage that they are driven entirely by natural causes and not influenced by anthropogenic factors, and the results can show how marine biota might respond to increasing deoxygenation on millennial time scales. This study quantifies, at 1000 yr resolution, the relationships between geochemical proxies for environmental change during the Toarcian (Early Jurassic) oceanic anoxic event (OAE) (ca. 183 Ma) and the size and abundance of bivalves
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