Nitrogen isotope evidence for expanded ocean suboxia in the early Cenozoic

The million-year variability of the marine nitrogen cycle is poorly understood. Before 57 million years (Ma) ago, the 15N/14N ratio (d15N) of foraminifera shell-bound organic matter from three sediment cores was high, indicating expanded water column suboxia and denitrification. Between 57 and 50 Ma ago, d15N declined by 13 to 16 per mil in the North Pacific and by 3 to 8 per mil in the Atlantic.The decline preceded global cooling and appears to have coincided with the early stages of the Asia-India collision.Warm, salty intermediate-depthwater forming along the Tethys Seamargins may have caused the expanded suboxia, ending with the collision. From 50 to 35 Ma ago, d15N was lower than modern values, suggesting widespread sedimentary denitrification on broad continental shelves. d15N rose at 35 Ma ago, as ice sheets grew, sea level fell, and continental shelves narrowed.