Stable isotope characterization of hypoxia-susceptible waters on the Louisiana shelf: Tracing freshwater discharge and benthic respiration
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abstract
To examine the sources of freshwater and carbon cycling associated with Louisiana shelf hypoxia, we measured 18O and D of water, 13C of dissolved inorganic carbon (DIC), salinity and dissolved oxygen (DO) in waters from 37 stations during April and July of 2008. Seafloor 18O 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-18O regressions for 2008 surface waters show the 18O of discharge to average -6.8 in April and -5.1 in July. Salinity-D regressions show the D 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 13CDIC 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 13CDIC and DO. The 13CDIC of sub-pycnocline water ranges from -2.3 to 0.3, with lower values reflecting increased respiration. The inshore (10m depth) 13CDIC-DO relationship yields a lower y-intercept relative to offshore (20m 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 13CDIC-DO relationships suggest that the 13C of biogenic carbonates may provide a valuable tool for paleo-redox studies in this region. 2012 Elsevier Ltd.
