Ocean Oxygenation and the End-Cretaceous Mass Extinction
Le résumé fourni par la source
The Cretaceous/Paleogene (K/Pg) mass extinction significantly perturbed the marine environment and the global carbon cycle. We provide new constraints on ocean oxygen and biogeochemical changes across the K/Pg boundary, combining records of foraminifera-bound organic matter nitrogen isotopes (FB-δ15N) and simulations from a biogeochemical model. We report FB-δ15N records from DSDP Site 525A, ODP Sites 1049C and 1050C in Atlantic and ODP Sites 1209 and 1210 in Pacific. We find a decrease in FB-δ15N occurred around 200 kyr prior to the K/Pg boundary, aligning with decreased foraminiferal δ18Ο indicating warming that may be due to Deccan Trap volcanism. The pattern of FB-δ15N decline across the records points to a reduction in water column denitrification indicating an ODZ contraction. This response of ODZ contraction to warming is consistent with our prior findings from the Cretaceous and Cenozoic, and the model experiments offer a circulation-based mechanism. In contrast, across the K/Pg boundary, FB-δ15N increases immediately and significantly in both Atlantic and Pacific oceans, suggesting ODZ expansion in the Pacific and ODZ development in the Atlantic. We interpret the ODZ expansion to be the consequence of smaller sinking particle sizes after the mass extinction, slowing sinking rates, shoaling the depth of particle remineralization, and intensifying the oxygen consumption in the shallow subsurface where ODZs are most prone to develop. From the model simulations, we find that the existence of post-K/Pg ODZs requires that the biological pump could not have declined significantly in response to the K/Pg event. This finding opposes the suggestion of a weakened biological pump after the K/Pg mass extinction, which was partly based on the apparent collapse of the ocean's vertical carbon isotope gradient.
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