Fundamentally different global marine nitrogen cycling in response to severe ocean deoxygenation

Abstract

The present-day marine nitrogen (N) cycle is strongly regulated by biology. Deficiencies in the availability of fixed and readily bioavailable nitrogen relative to phosphate (P) in the surface ocean are largely corrected by the activity of diazotrophs. This feedback system, termed the “nitrostat,” is thought to have provided close regulation of fixed-N speciation and inventory relative to P since the Proterozoic. In contrast, during intervals of intense deoxygenation such as Cretaceous ocean anoxic event (OAE) 2, a few regional sedimentary δ15N records hint at the existence of a different mode of marine N cycling in which ammonium plays a major role in regulating export production. However, the global-scale dynamics during this time remain unknown. Here, using an Earth System model and taking the example of OAE 2, we provide insights into the global marine nitrogen cycle under severe ocean deoxygenation. Specifically, we find that the ocean can exhibit fundamental transitions in the species of nitrogen dominating the fixed-N inventory––from nitrate (NO3−) to ammonium (NH4+)––and that as this transition occurs, the inventory can partially collapse relative to P due to progressive spatial decoupling between the loci of NH4+ oxidation, NO3− reduction, and nitrogen fixation. This finding is relatively independent of the specific state of ocean circulation and is consistent with nitrogen isotope and redox proxy data. The substantive reduction in the ocean fixed-N inventory at an intermediate state of deoxygenation may represent a biogeochemical vulnerability with potential implications for past and future (warmer) oceans.

Vitamin B 12 -dependent biosynthesis ties amplified 2-methylhopanoid production during oceanic anoxic events to nitrification

“…We, therefore, postulate that ocean oxygenation and circulation play a major role via their control on the balance between aerobic and anaerobic nitrogen cycling processes. Maintenance of sufficiently high concentrations and fluxes of nitrite would require both elevated total ocean nutrient concentrations and overturning circulation vigorous enough to promote oxidative nitrogen cycling (i.e., OAE-like conditions) (90). Upwelling of ammonium-rich deep water into the oxic zone, as suggested previously for the Cretaceous Atlantic Ocean (90-93), could have fueled nitrification and consequently, the proliferation of Nitrobacter spp.…”

“…Importantly, biomarker evidence and nitrogen cycle modeling suggest intensification of multiple nitrogen cycle processes (nitrogen fixation, nitrification, denitrification) during anoxic events (10,74,90,97,100). However, proliferation of Nitrobacter and other NOB during OAEs may have exerted a feedback on the nitrogen cycle by limiting the intensification of fixed nitrogen loss via anammox.…”

“…4). Global transects of biomarker distributions could further be used to investigate the spatial and temporal decoupling of aerobic and anaerobic nitrogen cycling processes predicted from nitrogen cycle modeling (90). Regardless, nitrifier ecotypes coexist in a continuum, implying that nitrifiers such as Nitrospina, Nitrospira, Nitrococcus, Nitrosomonas, and Nitrosococcus spp.…”

Biotic and Isotopic Vestiges of Oligotrophy on Continental Shelves During Oceanic Anoxic Event 2

“…It was assumed by Higgins et al (2012) that the entire 15 N-enriched fixed N pool was exhausted by denitrification and/or annamox, and was thus evacuated as N 2 into the atmosphere in the open ocean. However, on continental margins of the proto-North Atlantic, where oxygen-deficient conditions were not as extreme during OAE2 (see van Helmond et al, 2014van Helmond et al, , 2015, δ 15 N org values are higher (<+3.5‰; Ruvalcaba Baroni et al, 2015), which could result from productivity dominated and driven by 15 N-enriched nitrate (Naafs et al, 2019).…”

On the use of models in understanding the rise of complex life

“…Consequently, [PO 4 ] is decoupled significantly above the nitrogen levels determining productivity and P burial in a more anoxic ocean-at around double the levels that would make it proximately limiting. Recent GENIE results suggest the decoupling of P above N could be even greater [66]. Extending such a spatial model to predict δ 15 N distributions could provide a test of this scenario against available data.…”