Stability of the marine nitrogen cycle over the past 165 million years

Nitrogen and phosphorus are the two macro-nutrients that limit biological productivity in the ocean. While the supply of P depends on geological processes, N is biologically supplied from an inexhaustible atmospheric source, but can be limited by micro-nutrients, especially iron. Here we present a record of N and C isotopes over the past 165 Ma in marine sediments to address feedbacks between the N-cycle and productivity. Over most of the last 165 Myr, the fixed N averaged +3.2‰, (-2 and +9‰), but higher in distal areas of the ocean due to limited vertical mixing. Using an isotope box model and a coupled climate model we show that this is caused by winds that induce upwelling changing due to continental meander. Upwelling along low latitude east-west orientated Tethyan coastlines results in low δ¹⁵N, while upwelling along narrow N-S coastlines as it does today, results in high δ¹⁵N due to denitrification.