Low oxygen eddies in the eastern tropical North Atlantic

Low oxygen eddies in the eastern tropical North Atlantic: Implications for N2O cycling

Nature,

24 May, 2017

Abstract

Nitrous oxide (N2O) is a climate relevant trace gas, and its production in the ocean generally increases under suboxic conditions. The Atlantic Ocean is well ventilated, and unlike the major oxygen minimum zones (OMZ) of the Pacific and Indian Oceans, dissolved oxygen and N2O concentrations in the Atlantic OMZ are relatively high and low, respectively. This study, however, demonstrates that recently discovered low oxygen eddies in the eastern tropical North Atlantic (ETNA) can produce N2O concentrations much higher (up to 115 nmol L−1) than those previously reported for the Atlantic Ocean, and which are within the range of the highest concentrations found in the open-ocean OMZs of the Pacific and Indian Oceans. N2O isotope and isotopomer signatures, as well as molecular genetic results, also point towards a major shift in the N2O cycling pathway in the core of the low oxygen eddy discussed here, and we report the first evidence for potential N2O cycling via the denitrification pathway in the open Atlantic Ocean. Finally, we consider the implications of low oxygen eddies for bulk, upper water column N2O at the regional scale, and point out the possible need for a reevaluation of how we view N2O cycling in the ETNA.

Introduction

Nitrous oxide (N2O) is an important climate-relevant trace gas and the oceans are thought to contribute approximately 35% of all natural sources to the atmosphere1. In the troposphere N2O acts as a greenhouse gas and has a global warming potential which is ~300 times that of CO2 over 100 year time-scales2. Due to its relative chemical stability, N2O also survives transport to the stratosphere where it undergoes photochemical reactions that destroy ozone3. In the oceans, N2O is produced via the nitrification and denitrification pathways. During nitrification, N2O can be produced as a by-product during ammonia oxidation (AO), or through nitrifier-denitrification whereby AO organisms reduce nitrite (NO2−) to N2O. In oxygenated waters, nitrification-N2O yields (i.e. those arising from either AO or nitrifier-denitrification) are small, however, under low DO concentrations nitrification-N2O yields may increase substantially4, 5. As DO concentrations approach anoxic conditions, denitrification can also be ‘turned on’, and although it can both produce and consume N2O, net denitrification yields up to 2% have been observed6, 7.

Due to the sensitivity of N2O production to low oxygen conditions, the greatest oceanic accumulations, and likely the largest fluxes to the atmosphere, occur in the vicinity of suboxic and anoxic oxygen minimum zones (OMZs), such as those found in the Arabian Sea and the eastern tropical Pacific8,9,10. In comparison, the more ventilated Atlantic Ocean, with higher oxygen concentrations11, 12, has lower N2O production and concentrations13, 14. Here, we demonstrate for the first time, however, that recently discovered low oxygen mesoscale eddies in the otherwise oxygenated tropical North Atlantic15, can induce substantial increases in N2O production and cause shifts in the N2O cycling pathways.

The OMZ in the North Atlantic Ocean is rather well ventilated, and lowest DO concentrations are around 40 µmol kg−1 11, 12. Recently, however, coherent mesoscale cyclonic eddies (CE) and anticyclonic mode water eddies (ACME) in the eastern tropical North Atlantic (ETNA), which form off the coast of west Africa along topographical features such as headlands, and then propagate westwards past the Cape Verde Islands16, have been shown to create extremely low DO concentrations (as low as ~2 µmol kg−1)15. The low DO concentrations inside the eddy have the potential to have important implications for biogeochemical processes, including N2O cycling. Until recently, however, these potential implications have not been studied, as observations have been opportunistic and most have originated from moored and glider based sensors at the Cape Verde Ocean Observatory (CVOO; Fig. 1). In early 2014, however, a dedicated multi-disciplinary shipboard survey of one of these eddies (hereinafter referred to as ‘suboxic eddy’) was conducted. This survey allowed us to investigate how N2O cycling may be impacted by low oxygen eddies in the ETNA (sampling parameters and stations are outlined in the Methods section). The results from this work not only demonstrate the potential importance of low oxygen eddies as a source of N2O, they also provide insights into how N2O cycling in the ETNA may respond to future DO decreases.