The marine nitrogen cycle over the past 165 million years

Abstract

Nitrogen and phosphorus are the two macro-nutrients that limit biological productivity in the ocean. N is supplied by the biological fixation of N2 from a virtually inexhaustible atmospheric source whereas the supply of P depends on geological processes. However, N2 fixation can be limited by micro-nutrients, especially iron. To address the feedbacks between the N-cycle and productivity on geological timescales, we present a record of N and C isotopes over the past 165 Ma in marine sediments. The N isotope composition of the deep ocean averages 3.2‰, and while relatively stable for most of this time, did range between -2 and +9‰. In comparison, exported production in distal areas of the ocean over the last 90 Myr averages 7.0‰, and ranges between +3 and +11‰, with the greatest difference from deepwater occurring when vertical mixing was limited. These significant multi-million-year time scale fluctuations result from interactions between plate tectonics and ocean biogeochemistry. We further present a coupled climate model that shows that winds that induce upwelling change due to continental meander. Importantly, the response of N cycling to changes in upwelling differs as those centers have moved in response to continent configuration. Upwelling induced productivity along low latitude east-west orientated Tethyan coastlines was elevated throughout much of the late Jurassic to Eocene. As Tethys closed, changing N dynamics are recorded by high δ15N, arising from increased N losses from the water column compared to burial in sediments. As upwelling increased again in the late Miocene, albeit weaker than before, along north-south coasts with narrow shelves, and ocean fronts became more pronounced, another abrupt increase in δ15N occurred. The implications on N-fixation is that meridional overturning circulation has separated areas of N-fixation and upwelling, leading to limitation by N in the modern ocean, unlike the situation of Tethys. However, limitation of N-fixation by Fe in the vast Panthalassic Ocean led to the most severe N-starvation.