Particles size distribution and carbon flux across the Arabian Sea Oxygen Minimum Zone

Abstract

The goal of the Arabian Sea section of the TARA oceans expedition was to study Large Particulate Matter (LPM > 100 µm) distributions and possible impact of associated mid-water biological processes on vertical carbon export through the Oxygen Minimum Zone (OMZ) of this region. We found that spatial patterns in LPM distribution resulted 5 from the timing and location of surface phytoplankton bloom, lateral transport, microbial processes in the core of the OMZ, and zooplankton activity at the lower oxycline. Indeed , satellite-derived net primary production maps showed that the northern stations of the transect were under the influence of a previous major bloom event while, the most southern stations were in a more oligotrophic situation. Lagrangian simulations 10 of particle transport showed that deep particles of the northern stations could originate from the surface bloom while the southern stations could be considered as driven by 1-D vertical processes. In the first 200 m of the OMZ core, minima in nitrate concentrations and the Intermediate Nepheloid Layer (INL) coincided with high concentrations of 100 µm < LPM < 200 µm. These particles could correspond to colonies of bacteria 15 or detritus produced by anaerobic microbial activity. However, the calculated carbon flux through this layer was not affected. Vertical profiles of carbon flux indicate low flux attenuation in the OMZ, with a Martin model b exponent value of 0.22. At the lower oxycline, a deep nepheloid layer was associated to an increase of carbon flux and an increase in mesozooplankton abundance. Zooplankton feeding on un-mineralized sink-20 ing particles in the OMZ is proposed as a mechanism for the observed deep particle aggregation. These results suggest that OMZ may be regions of enhanced carbon flux to the deep sea relative to non-OMZ regions.