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Benthic biogeochemical processes related to water column oxygen deficiency in the Santa Monica Basin, North East Pacific Ocean


Mid-water oceanic depths in coastal upwelling regions often contain oxygen deficient waters, called oxygen minimum zones (OMZs). These hypoxic (< 60 �M O2) conditions can have adverse effects on ecosystems, as many organisms cannot survive in low O2 conditions. This study investigated the OMZ in the topographically isolated Santa Monica Basin (SMB), California, a recipient of high nutrient input. The last survey of this area ~35 years ago, reported a pervious 350 year expansion of the SMB OMZ. In order to assess the OMZ since the last evaluation, sediment cores from 12 stations were retrieved by a multicorer from O2-ventilated (>60 μM O2) to near-anoxia (~4 μM O2) regions along two depth-transects ranging from water depths between 71 and 907 m. The sediment porewater and supernatant water of the cores were analyzed for sulfate (SO42-), nitrate (NO3-), phosphate (PO43-), ammonium (NH4+), total sulfide, dissolved iron (Fe (II)), total alkalinity (TA) and bacterial sulfate reduction. The two deepest stations (907 and 893 meters, ~5 �M O2) exhibited down-core accumulation of NH4+ and TA, while also displaying enhanced rates of sulfate reduction close to the sediment surface; these patterns are all evidence of low oxygen conditions in the overlying water column. Shallower stations upslope (starting at 777 m water depth) featured increasing signs of bioturbation and bioirrigation effects in the geochemical profiles of NH4+, TA, PO43- and Fe (II). Low sulfate reduction rates (areal rates range from 0.13–0.86 mmol m-2 d-1) were detected at all stations. These results were compared with data separate from this thesis, including: 210Pb lamination analyses, the presence and activity of macrofauna at the seafloor, and iron speciation analyses. According to the stations sampled, we could not identify a definite spreading or reduction of the OMZ at the seafloor since the last survey of the SMB was done ~35 years ago.

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