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 delta O-18 and delta D of water, delta C-13 of dissolved inorganic carbon (DIC), salinity and dissolved oxygen (DO) in waters from 37 stations during April and July of 2008. Seafloor delta O-18 values resemble typical Gulf of Mexico seawater (approximate to 1.1%) while surface waters values are substantially lower (e.g., < -2.0%) due to mixing with river-sourced freshwater. Salinity-delta O-18 regressions for 2008 surface waters show the delta O-18 of discharge to average -6.8% in April and -5.1% in July. Salinity-delta D regressions show the delta 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 delta C-13(DIC) 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 delta C-13(DIC) and DO. The delta C-13(DIC) of subpycnocline water ranges from -2.3 to 0.3%, with lower values reflecting increased respiration. The inshore (10 m depth) delta C-13(DIC)-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 delta C-13(DIC)-DO relationships suggest that the delta C-13 of biogenic carbonates may provide a valuable tool for paleo-redox studies in this region. (c) 2012 Elsevier Ltd. All rights reserved.