Rise in the ¹⁵N-to-¹⁴N Ratio of Otolith-bound Organic Matter Associated with Late Cretaceous Cooling
Abstract
The nitrogen isotopes (δ15N) of the organic matter preserved in fossil otoliths are a promising tool for reconstructing past changes in environmental conditions and trophic level. We analyzed the δ15N of fossil otolith-bound organic matter in Late Cretaceous Eutawichthys maastrichtiensis, E. zideki and Pterothrissus sp. otoliths from three different localities along the U.S. east coast. Despite morphological signs of alteration of the otolith biomineral material, there is no significant difference in otolith-bound organic matter δ15N between well-preserved and poorly preserved otoliths. Both genera showed a ~4‰ increase in otolith-bound organic matter δ15N from Campanian (10.89 ± 1.02‰ in Eutawichthys spp. and 11.08 ± 0.79‰ in Pterothrissus sp.) to Maastrichtian (14.87 ± 1.25‰ and 14.78 ± 0.80‰). The similarity of the change in two separate genera argues against a trophic level change. Moreover, the proximity of the deposits and other geographic considerations argue against the locations of the host sedimentary deposits as the cause. Rather, the N isotopic change is best interpreted as an environmental (i.e., “baseline”) signal at the regional scale or greater. The lower δ15N in the Campanian may be a consequence of its warmer climate, consistent with warming-correlated declines in foraminifera-bound δ15N during the Cenozoic. More specifically, our otolith data are consistent with the foraminifera-based finding that warmer conditions are associated with a lower global rate of water column denitrification. This interpretation is speculative, and δ15N measurements from other fossil types and other regions are called for. Nevertheless, the large, coherent signal observed in this first study suggests that the nitrogen isotopes of fossil otoliths hold important information about the ocean and marine ecosystems over the Earth history. Show more
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publishedExternal links
Book title
Goldschmidt2022 AbstractsPublisher
European Association of GeochemistryEvent
Organisational unit
03775 - Haug, Gerald H. / Haug, Gerald H.
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