A nitrogen isotopic shift in fish otolith-bound organic matter during the Late Cretaceous

The nitrogen isotopes of the organic matter preserved in fossil fish otoliths (ear stones) are a promising tool for reconstructing past environmental changes. We analyzed the ¹⁵N/¹⁴N ratio (δ¹⁵N) of fossil otolith-bound organic matter in Late Cretaceous fish otoliths (of Eutawichthys maastrichtiensis, Eutawichthys zideki and Pterothrissus sp.) from three deposits along the US east coast, with two of Campanian (83.6 to 77.9 Ma) and one Maastrichtian (72.1 to 66 Ma) age. δ¹⁵N and N content were insensitive to cleaning protocol and the preservation state of otolith morphological features, and N content differences among taxa were consistent across deposits, pointing to a fossil-native origin for the organic matter. All three species showed an increase in otolith-bound organic matter δ¹⁵N of ~4‰ from Campanian to Maastrichtian. As to its cause, the similar change in distinct genera argues against changing trophic level, and modern field data argue against the different locations of the sedimentary deposits. Rather, the lower δ¹⁵N in the Campanian is best interpreted as an environmental signal at the regional scale or greater, and it may be a consequence of the warmer global climate. A similar decrease has been observed in foraminifera-bound δ¹⁵N during warm periods of the Cenozoic, reflecting decreased water column denitrification and thus contraction of the ocean's oxygen deficient zones (ODZs) under warm conditions. The same δ¹⁵N-climate correlation in Cretaceous otoliths raises the prospect of an ODZ-to-climate relationship that has been consistent over the last ~80 My, applying before and after the end-Cretaceous mass extinction and spanning changes in continental configuration.