Ammonium “nutrient capacitor” model for δ15N signatures associated with marine anoxic events

Abstract

Geochemical records of ancient periods of warm climate can be useful to help understand the looming effects of modern anthropogenic warming, including changes to biogeochemical nutrient cycles. Stable nitrogen isotope compositions of marine sediments archive the balance of processes in the global nitrogen cycle. However, the unusual isotopic signals of Mesozoic oceanic anoxic events (OAEs) remain enigmatic, thus hindering our understanding of nitrogen cycle processes and dynamics under conditions of ocean deoxygenation. Here, we present an ammonium "nutrient capacitor" model of the water-column nitrogen cycle to explain the anomalously negative isotopic compositions seen in Mesozoic OAE sediments. Our model applies isotopic inferences derived from high-resolution records of Lake Kivu sediments to show how periodic chemocline overturning of redox-stratified water columns during Mesozoic OAEs may have delivered ammonium to the photic zone in excess of primary producer requirements. Smoothed, stochastic sampling of the changing fluxes within the nitrogen cycle across these events can simulate OAE nitrogen isotope records.