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The million-year variability of the marine nitrogen cycle is poorly understood. Before 57 million years (Ma) ago, the ¹⁵N/¹⁴N ratio (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 ago, δ¹⁵N 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-depth water forming along the Tethys Sea margins may have caused the expanded suboxia, ending with the collision. From 50 to 35 Ma ago, δ¹⁵N was lower than modern values, suggesting widespread sedimentary denitrification on broad continental shelves. δ¹⁵N rose at 35 Ma ago, as ice sheets grew, sea level fell, and continental shelves narrowed.