Ocean Deoxygenation Conference | Kiel 2018

The functioning of Oxygen Minimum Zones in the Pacific remains debated. In particular, the fate of the OM is unclear owing to competing processes (preservation versus microbial activity). Recent high-vertical resolution in situ observations off Peru reveal an unexpected biogeochemical structuring in the oxygen minimum zone, potentially associated with a specific vertical particles distribution and ecosystem niches, impacting the vertical transfer of both energy and essential elements. Within the Suboxic Layer (<20 mol kg-1, SL), we found Intercalated Oxygen Maxima (IOM) less than 10 mol kg-1 above background and less than 50 m in vertical extent. Typically, IOMs are found in the SL with lower vertical density gradient, regionally or within the SL of one station. CTD attached ADCP of 300 kHz (LADCP) showed: echo maxima persisted throughout the day within the SL associated with these IOMs and sometimes without IOM, echo maxima persisted at the upper and below the lower oxycline throughout the day. LADCP also showed echo maxima that migrated diurnally, and were located within the stratified, no-IOM depth of the SL during the day. These daytime echo maxima did not overlap the strong echo maxima associated with the lower oxycline. Beam attenuation (Cp) showed a maximum close to the upper oxycline. In the depth range of the stratified no-IOM part of the SL, the Cp decreased approximately exponentially. Cp displayed no profile pattern associated with the IOMs. Cp profiles showed no apparent relation with LADCP echo profiles as might be expected if Cp is a proxy of particulate organic matter and LADCP echo a proxy of zooplankton.