A marine sink for chlorine in natural organic matter

Abstract

Chloride is abundant in oceans, but is relatively unreactive. Spectroscopic imaging reveals the presence of a chloride sink in organochlorine compounds that can be produced abiotically or by phytoplankton. Chloride—the most abundant ion in sea water1—affects ocean salinity, and thereby seawater density and ocean circulation. Its lack of reactivity gives it an extremely long residence time2. Other halogens are known to be incorporated into marine organic matter3,4,5. However, evidence of similar transformations of seawater chloride is lacking, aside from emissions of volatile organochlorine by marine algae6,7,8. Here we report high organochlorine concentrations from 180 to 700 mg kg−1 in natural particulate organic matter that settled into sediment traps at depths between 800 and 3,200 m in the Arabian Sea, taken between 1994 and 1995. X-ray spectromicroscopic imaging of chlorine bonding reveals that this organochlorine exists primarily in concentrated aliphatic forms consistent with lipid chlorination, along with a more diffuse aromatic fraction. High aliphatic organochlorine in particulate material from cultured phytoplankton suggests that primary production is a source of chlorinated organic matter. We also found that particulate algal detritus can act as an organic substrate for abiotic reactions involving Fe2+, H2O2 or light that incorporate chlorine into organic matter at levels up to several grams per kilogram. We conclude that transformations of marine chloride to non-volatile organochlorine through biological and abiotic pathways represent an oceanic sink for this relatively unreactive element.