Sedimentary sources of old high molecular weight dissolved organic carbon from the ocean margin benthic nepheloid layer

Abstract

Average 14C ages of dissolved organic carbon (DOC) in the ocean are 3–6,000 years, and are influenced by old DOC from continental margins. However, sources of DOC from terrestrial, autochthonous, and sedimentary organic carbon seem to be too young to be responsible for the old DOC observed in the ocean. Since colloidal organic carbon (COC, i.e., high molecular weight DOC), which is chemically very similar to that of bulk DOC, can be effectively isolated from seawater using cross-flow ultrafiltration, it can hold clues to sources and pathways of DOC turnover in the ocean. Radiocarbon measurements on COC in the water column and benthic nepheloid layer (BNL) from two continental margin areas (the Middle Atlantic Bight and the Gulf of Mexico) and controlled laboratory experiments were carried out to study sources of old DOC in the ocean margin areas. Vertical distributions of suspended particulate matter (SPM), particulate organic carbon (POC), nitrogen (PON), and DOC in the water column and bottom waters near the sediment-water interface all demonstrate a well developed benthic nepheloid layer in both ocean margin areas. Δ 14C values of COC isolated from the BNL (ranging from −362 ± 4‰ to −456 ± 3‰) were distinctly lower than their counterparts in surface waters (with a range of >0‰ to −210 ± 5‰). In other words, COC from the BNL was much older than COC from the overlying water column. These results, together with strong concentration gradients of SPM, POC, PON, and DOC, suggest a sedimentary source for organic carbon species and possibly for old COC as well in BNL waters. This is confirmed by the results from controlled laboratory experiments which showed that COC isolated from sediment resuspension had Δ 14C values between −260‰ and −352‰, which are significantly lower than those of bulk sediment (−87 ± 6‰) or resuspended particles (−138 ± 8‰). The heterogeneity of Δ 14C signatures in bulk SOC thus points to a preferential release of old organic components from sediment resuspension, which can be the transport mechanism of the old benthic COC observed in ocean margin areas. Old COC from continental margin nepheloid layers may thus be a potential source of old DOC to the deep ocean.