Cycling of dissolved and particulate organic matter in the open ocean

Abstract

Radiocarbon (Δ 14 C), δ 13 C, bulk carbon and organic constituent concentration measurements are presented for dissolved and particulate carbon pools from the North Central Pacific Ocean (NCP) and the Sargasso Sea (SS). We operationally define three overlapping pools of dissolved organic carbon (DOC): (1) DOC that is oxidizable by UV radiation (DOC uv ); (2) “extra” DOC measured by Co/CoO flow‐through high‐temperature catalytic oxidation (DOC Ft‐htc ), which also has low Δ 14 C values like DOC uv (Bauer et al., 1992a); and (3) a potential residual DOC fraction that is the difference between DOC measured by discrete‐injection high‐temperature catalytic oxidation (DOC htc ) and DOC Ft‐htc , and which has unknown Δ 14 C signature. The distribution of a large fraction of DOC appears to be controlled by circulation of deep ocean waters between major oceans. The DOC in the SS is slightly younger than would be expected if circulation was the sole process controlling DOC cycling. We propose that there is more bomb 14 C in the deep SS DOC to account for this difference. The Δ 14 C values of suspended, and to a lesser extent sinking particulate organic carbon (POC), decrease with depth, with the suspended POC displaying a much steeper gradient in the SS than in the NCP. These data reflect the incorporation of low‐activity organic matter into the POC pool, possibly through incorporation of DOC by physical adsorption and/or biological heterotrophy.