Stable isotope characterization of hypoxia-susceptible waters on the Louisiana shelf: Tracing freshwater discharge and benthic respiration

To examine the sources of freshwater and carbon cycling associated with Louisiana shelf hypoxia, we measured δ18O and δD of water, δ13C of dissolved inorganic carbon (DIC), salinity and dissolved oxygen (DO) in waters from 37 stations during April and July of 2008. Seafloor δ18O values resemble typical Gulf of Mexico seawater (≈1.1%) while surface waters values are substantially lower (e.g., <−2.0%) due to mixing with river-sourced freshwater. Salinity-δ18O regressions for 2008 surface waters show the δ18O of discharge to average −6.8% in April and −5.1% in July. Salinity-δD regressions show the δD of discharge to be −38% in April and −28% in July. Together these regressions suggest Mississippi discharge was the dominant freshwater source in the study area in April followed by a shift to nearly total Atchafalaya discharge during July, a trend that coincides with summer coastal current reversals. The δ13CDIC of July surface water varies from −5.0 to 1.2% and correlates with salinity indicating mixing of seawater and river water. April surface water shows no relationship with salinity because of the influence of primary productivity, which enriches certain waters in δ13CDIC and DO. The δ13CDIC of sub-pycnocline water ranges from −2.3 to 0.3%, with lower values reflecting increased respiration. The inshore (10 m depth) δ13CDIC-DO relationship yields a lower y-intercept relative to offshore (20 m depth) bottom waters, possibly indicating a terrestrial source of OC being respired. Mass balance estimations of respired OC do not have the accuracy to quantify any difference between nearshore and offshore locations. Regardless, the δ13CDIC-DO relationships suggest that the δ13C of biogenic carbonates may provide a valuable tool for paleo-redox studies in this region.