Interstitial dissolved organic carbon (DOC) concentrations within sinking marine aggregates and their potential contribution to carbon flux

Abstract

Accurate estimates of the quantity of organic carbon sedimenting to the sea floor are important in evaluating the rate at which carbon is sequestered in the deep sea and the impact of the ocean on the global carbon cycle. However, extensive studies quantifying marine sedimentation over the past decades have considered only the particulate fraction of sinking material. Dissolved organic carbon (DOC) carried along in the interstices of sinking marine snow, the particles that comprise the bulk of particle flux throughout most of the ocean, has not been included previously. Empirical measurements of the interstitial DOC concentrations of individual aggregates of marine snow from coastal California and Washington revealed high values ranging from 8.9 to 140 mg L−1 that were significantly correlated with aggregate size, decreasing as aggregate size increased. Solubilization of particulate matter within aggregates by associated bacteria and reduced diffusion rates due to the fractal geometry of aggregates help maintain these high interstitial concentrations against diffusive processes. Although interstitial DOC concentrations were one to two orders of magnitude higher than ambient DOC concentrations in the surrounding seawater, the cumulative interstitial DOC in aggregates contributed <2.5% to total DOC in the water column. However, DOC comprised up to 31% of the total organic carbon in aggregates, averaging about 20%, indicating that previous measurements of sedimenting carbon in the ocean that have included only the particulate fraction have significantly and systematically underestimated the vertical flux of organic carbon.