Skip to main content
eScholarship
Open Access Publications from the University of California

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

  • Author(s): Lemieux, Sydnie Lynn
  • Advisor(s): Treude, Tina
  • et al.
Abstract

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.

Main Content
Current View