Model study of organic carbon attenuation and oxygen mass transfer in persistent aggregate layers in the deep sea

A time series study from 1989 to 2017 indicates that, increasingly, carbon export to abyssal sediments in the California Current Ecosystem (CCE), 220 km west of the central California coast (Sta. M), occurs as rapidly sinking pulses of particulate organic carbon (POC). Nearly continuous sediment trap collections confirm that POC export to 3400 m increased significantly after 2011 with carbon attenuation (as fraction of POC remineralized) ranging from a low of 57% to a high of 77% during periods of pulsed flux. All of the major pulse events for the period resulted in the delivery of detrital aggregates that covered part or all of the sediment surface as an organic carbon rich layer at ~4000 m depth. However, the magnitude of the measured Sediment Community Oxygen Consumption (SCOC) did not increase proportionally to the organic carbon inventory change. Here, a model of oxygen consumption, informed by the time series data at Sta. M in the CCE, suggests that aggregate flocs constitute a major source for benthic carbon and a barrier to mass transport of oxygen, leading to reduced carbon attenuation in surface sediments. The correlation of POC delivery with aggregate coverage shows that the majority of POC delivery during pulse events is likely to consume all of the oxygen within the aggregates. Results indicate that an increasing fraction of POC reaches the abyssal sediments at Sta. M under conditions that support increased net burial of carbon.