Graduate Student -- Sigman Lab
Department of Geosciences
Nitrogen isotope evidence for expanded ocean suboxia in the early Cenozoic
The million-year variability of the marine nitrogen cycle is poorly known. Between 65 and 57 million years ago (Ma), the 15N/14N (δ15N) of foraminifera shell-bound organic matter from three sediment cores was high, indicating expanded water column suboxia and denitrification. Between 57 and 50 Ma, δ15N declined by 13-16‰ in the North Pacific and 3-8‰ in the Atlantic. The decline preceded global cooling, instead appearing to coincide with the early Asia-India collision. Warm, salty intermediate water forming along the Tethys Sea margins may have caused the expanded suboxia by lowering the preformed oxygen content of intermediate waters, ending with the collision. From 50 to 35 Ma, δ15N was lower than modern, suggesting widespread sedimentary denitrification on broad continental shelves. δ15N rose at 35 Ma likely due to decreasing sedimentary denitrification as Antarctic ice sheets expanded, sea level fell, and continental shelves narrowed.