Biogeochemical response to widespread anoxia in the past ocean

Abstract

Oxygen is a key element for life on earth. Oxygen concentrations in the ocean vary greatly in space and time. These changes are regulated by various physical and biogeochemical processes, such as primary productivity, sea surface temperatures and ocean circulation. In the geological past, several periods of widespread anoxia have been identified. These are typically accompanied with major perturbations of the cycles of carbon, phosphorus (P) and nitrogen (N). These are registered in the geological records and can be used to study Earth’s past environmental conditions. Our knowledge of the biogeochemical response to long-term deoxygenation in the ocean is still limited. This study focuses on the biogeochemistry in the ocean during a well-demarcated anoxic event that occurred in the mid-Cretaceous (94 Ma ago), when atmospheric CO2 levels were higher than at present. This event lasted approximately 550 kys and is termed Oceanic Anoxic Event 2 (OAE2). Most available geological records for OAE2 are from the North Atlantic, which during the mid-Cretaceous was a semi-enclosed deep basin (i.e. proto-North Atlantic) with a restricted connection to the Pacific and Tethys Ocean. In this research, a multi-box ocean model describing the cycles of water, carbon, oxygen, N and P of the proto-North Atlantic is built to better understand the key mechanisms involved in the development of widespread anoxia during OAE2. Because our knowledge of spatial variability in bottom-water conditions in the northern open ocean of the proto-North Atlantic is limited, proxy data from several deep-sea sites in the northern proto-North Atlantic were collected. Proxy data strongly suggest that, during OAE2, bottom waters in the entire deep proto-North Atlantic were anoxic and that the ocean circulation in the basin was restricted. Moreover, the N isotopic composition (δ15N) of organic matter buried in sediments in samples treated with acid led to selective removal of N compounds and thus should not be used to describe N dynamics in past environments. A compilation of published and new δ15N from samples that have not been treated with acid demonstrates that δ15N values for OAE2 are most negative in the open ocean (although values for OAE2 are never lower than -3 ‰). Our model results are in good agreement with observations, showing severe anoxia/euxinia in the open ocean and coastal waters of the southern proto-North Atlantic and strong oxygen depletion along the north and north-west coast. Our results indicate that high primary productivity and N2-fixation led to widespread anoxia in the proto-North Atlantic during OAE2. All P sources are a requirement for sustaining such primary productivity. Model results suggest that low-oxygen concentrations in the Pacific Ocean and reduced ocean circulation are needed for the development of anoxia in the deep northern proto-North Atlantic. In addition, ammonium accumulated due to limited nitrification and became the dominant recycled N nutrient. Model results also show significant regional differences in N dynamics, with the open ocean acting as a major source and sink of N, whereas the coastal ocean mainly acted as a source of N for non-diazotrophic primary productivity.