Controls on temporal variability of the geochemistry of the deep Cariaco Basin

Studies of the Cariaco Basin on the continental shelf of Venezuela, as a pa rt of the Carbon Retention In A Colored Ocean (CARIACO) program, have revea led that the chemistry of the deeper waters of the system is more variable than previously believed. Small oxygen maxima have been observed on a numbe r of occasions at depths where oxygen was previously absent, suggesting the occurrence of intrusions of oxygenated water into the region of the oxic/a noxic interface (250-300m). Apparently because of these events, the oxic/an oxic interface deepened by about 100 during the period of our observations. We also observed a dramatic decrease in H2S concentrations at all depths b elow the oxic/anoxic interface during this same period. Bottom waters, for example, had an H2S concentration of about 75 muM in November 1995, but sin ce November 1997, concentrations in bottom water have not exceeded 55 muM. Water of sufficient density to sink to the bottom of the Basin has been obs erved on one occasion at sill depth just north of the eastern sill. However , based on a simple box model, the decrease in deep-water sulfide does not appear to be due to intrusion of oxygenated water alone, as concentrations of other measured species, and of hydrographic parameters, have remained co nstant with time. Instead, we postulate that an earthquake that took place in July 1997 resulted in a turbidity current that transported large quantit ies of coastal sediment containing oxidized iron into the deep waters of th e basin. If the final products of reaction were elemental sulfur and iron s ulfide, the sediment associated with the oxidized iron would have produced a turbidite layer about 10 cm thick. Previous earthquakes have produced tur bidites of similar thickness. (C) 2001 Elsevier Science Ltd. All rights res erved.