Controls on iron, manganese and intermediate oxidation state sulfur compounds in the Cariaco Basin

Oxygen, hydrogen sulfide, thiosulfate, sulfite, iron and manganese were measured at three stations in the Cariaco Basin in January and May 2005 to investigate the spatial and temporal variability of the chemistry in the basin as a part of the CARIACO time-series. Sulfide and oxygen profiles at the oxic/anoxic interface differed significantly between the three stations and suboxic zone thickness changed with both space and time. Higher inventories of H₂S at a station on the broad sill, and a suboxic zone which varied from 5 to 50 m at this site (most variable among the three stations), may be related to a higher source of sulfide in shallower waters coupled with the effect of an intrusion between the two cruises, causing a large portion of the high sulfide inventory to be oxidized and form thiosulfate and sulfite. Thiosulfate and sulfite concentrations were similar to those previously observed, but, in contrast to previous work, generally exhibited a strong minimum within the suboxic zone. This is consistent with enhanced biological consumption or chemical oxidation/reduction within the suboxic zone following intermediate oxidation state sulfur compound formation. The concentrations of dissolved iron and manganese in the anoxic portion of the basin were ∼ 50% higher during January 2005 compared to samples from historical data and from May 2004 or May 2005. The source of the metals appeared to be related to high rainfall during late 2004 and early 2005 along the Venezuelan coast which may have resulted in delivery of a large amount of terrigenous material to the system. Saturation calculations suggest that metals were removed between January and May by mineral precipitation, possibly as FeS and CaMn(CO₃)₂ for Fe and Mn, respectively. Intrusions of oxygenated water and lateral advection of particulate matter from the coast are important processes in providing oxidant to the suboxic zone.