Do Oceanic Hypoxic Regions Act as Barriers for Sinking Particles? A Case Study in the Eastern Tropical North Atlantic

Rafael Rasse,Giorgio Dall'Olmo

doi:10.1029/2019gb006305

Abstract

AbstractIn oxygen minimum zones (OMZs), the attenuation rates of particulate organic carbon (POC) fluxes of large particles are known to be reduced, thus increasing the efficiency with which the biological carbon pump (BCP) transfers carbon to the abyss. The BCP efficiency is expected to further increase if OMZs expand. However, little is known about how the POC fluxes of small particles—a significant component of the BCP—are attenuated inside OMZs. In this study, data collected by two BGC‐Argo floats deployed in the hypoxic OMZ of the eastern tropical North Atlantic were used to estimate net instantaneous fluxes of POC via small particle during 3 years. This information was analyzed together with meteorological data and published POC fluxes of large particles and allowed us to conclude that (1) major pulses of surface‐derived small particles toward the OMZ interior coincided with seasonal changes in wind stress and precipitation; (2) a permanent layer of small particles, presumably linked to microbial communities, was found in the upper section of the OMZ which might play a key role attenuating POC fluxes; and (3) fluxes of large particles were attenuated less efficiently inside this poorly oxygenated region than above it, while attenuation of small‐particle fluxes were equivalent or significantly higher inside the OMZ. These results highlight that more information about the processes controlling the fluxes of small and large particles in hypoxic OMZs is needed to better understand the impact of hypoxic OMZs on the BCP efficiency.

Highlights

The biological carbon pump (BCP) is a set of mechanisms that removes atmospheric carbon dioxide by exporting particulate organic carbon (POC) from the upper ocean toward its interior (Volk & Hoffert, 1985)
Data collected by two BGC‐Argo floats deployed in the hypoxic oxygen minimum zones (OMZs) of the eastern tropical North Atlantic were used to estimate net instantaneous fluxes of POC via small particle during 3 years
ΔT values were calculated by using published absolute POC fluxes measured by drifting sediment traps and mostly due to large particles in the region sampled by float 6901175 between March and April 2014 (Engel et al, 2017, Figures 9, 10)

Summary

Introduction

The biological carbon pump (BCP) is a set of mechanisms that removes atmospheric carbon dioxide by exporting particulate organic carbon (POC) from the upper ocean toward its interior (Volk & Hoffert, 1985). OMZs are water masses with low oxygen levels located at intermediate depths, which represent ~5% of the oceans volume (e.g., 200–1000 m, O2 < 60 μmol kg−1, Deutsch et al, 2011; Paulmier & Ruiz‐Pino, 2009) These regions appear to reduce POC remineralization rates allowing POC to reach deeper waters relative to oxygenated areas (O2 > 120 μmol kg−1, Cavan et al, 2017, Devol & Hartnett, 2001; Engel et al, 2017; Martin et al, 1987; Roullier et al, 2014; Van Mooy et al, 2002). This hypothesis is based on measurements of the attenuation rates of POC fluxes mostly due to large sinking particles because these are the predominant fraction of particles collected by conventional sediment traps (e.g., >100 μm; Arístegui et al, 2009; Buesseler et al, 2007; Devol & Hartnett, 2001; Engel et al, 2017; Gardner et al, 1985; Martin et al, 1987; Trull et al, 2008; Van Mooy et al, 2002)

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