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Abstract:
The global environment was greatly perturbed at the Cretaceous/Paleogene (K/Pg) boundary. The ocean carbon cycle was apparently significantly affected. For example, a global collapse in the surface-to-deep ocean carbon isotopic (δ13C) gradient has been interpreted as a weakening in biological export production in response to the mass extinction. To evaluate existing hypotheses on ocean biogeochemistry changes, additional information on ocean nutrient cycles is needed. To provide observational constraints on K/Pg changes in the ocean nitrogen cycle, we measured the nitrogen isotopes (δ15N) of mixed-species planktonic foraminifera-bound organic matter across the K/Pg boundary (73-55 Ma) at South Atlantic DSDP Site 525A. Based on other Cenozoic foraminifera-bound δ15N data, we tentatively interpret the observed K/Pg-associated changes as reflecting the rate of water column denitrification, a process restricted to ocean suboxic zones. A decrease in δ15N occurred at around 200 thousand years prior to the K/Pg boundary (66.21 Ma). The timing of this δ15N excursion aligns with a decrease in foraminiferal δ18O at Site 525A, which may reflect warming in response to a major pulse of Deccan volcanism at this time. Lower δ15N during this interval suggests contracted ocean suboxia. Across the K/Pg boundary, δ15N increased and remained relatively elevated for about three million years. The post-K/Pg elevation in δ15N and its apparent return to baseline appears to correspond with a period of low planktonic foraminiferal δ13C, which has been proposed to reflected weakening in biological export production. Elevated δ15N might reflect stronger denitrification in the water column, suggesting larger suboxic zones during this post-K/Pg period. At first inspection, this result appears to be inconsistent with previous inferences of a weakening in the biological pump. However, the potential for overlapping effects from multiple post-K/Pg changes, including in climate, requires careful consideration.
