Observations of the Effect of Non-steady State Injections of Oxygen Into Anoxic Waters of the Cariaco Basin, Venezuela

Traditionally, the Cariaco Basin has been considered to be a classic example of an anoxic basin where transport of organic rich material from the surface layers to depth, restricted vertical mixing, and anaerobic diagenesis dominate. Many studies explicitly or implicitly assume that distributions of chemical species and microbial activity change relatively gradually and that sediment properties solely reflect processes in the overlying water. However the CARIACO time series has repeatedly obtained evidence that intruding oxygenated water must be extremely important in controlling both water chemistry and microbial activity. In May 2008 repeated cruises took place over a period of weeks which clearly demonstrated that relatively large volumes of oxygenated water had recently intruded to depths of at least 300 m over a period of days to weeks. We saw clear evidence of deep oxygen maxima, minima in methane and sulfide concentrations below the oxygen/sulfide interface and apparent perturbations in other measured parameters including nutrients, sulfur species and microbial activity. Data suggest, among other things, that microbial activity lags production of sulfur intermediates like elemental sulfur, implying that chemoautotrophic bacteria in the system may use elemental S. We will present data on changes in hydrography, nutrient distributions, microbial rates and other variables resulting from the intrusions and which allow us to estimate the zone of influence of this intrusion. Ultimately, properties (such as N and S chemistry) of sediments in such systems could be affected by changes in chemistry of the system caused by lateral injections of oxidants and other material from outside the basin.