| Abstract: | Hypoxia, with dissolved oxygen levels < 1.4 ml L-1, is a recurring summer feature of Louisiana shelf bottom waters. Stable isotope characterization (delta^18O and delta D) of surface waters over the hypoxic zone shows a shift of dominant river influence from the Mississippi River during April to the Atchafalaya in July. Carbon isotopes of dissolved inorganic carbon ([delta]^13CDIC) in bottom waters reveal the respiration of terrestrial organic carbon (OC) at inshore localities of 10 m depth and the respiration of marine OC at depths equal to and greater than 20 m. [delta]^18O and [delta]^13C profiles of Louisiana shelf Conus shells collected in 1972 show no evidence for summer hypoxia. Comparison with modern Conus records reveal a [delta]^13CDIC reduction during the last four decades associated with intrusion of ^13C-depleted fossil fuel CO2. Summer [delta]^13C reductions in Texas shelf Pteria shells may imply dissolved oxygen (DO) was reduced by [approximately equals]0.7 ml L-1, although this may be attributed to influence of Brazos River discharge on shell [delta]^18O and [delta]^13C. Foraminifera fauna measured in age-calibrated sediments from the Texas shelf reveal a low oxygen conditions on between 1960 and modern sediments. From 1950 to 1960, fauna indicate oxygenated bottom waters. Contemporaneous increases of foraminifera [delta]^13Cand [delta]^18O suggest this event is associated with severe drought (the Little Dust Bowl). The synchronicity of these data suggests a link between Brazos River discharge and shelf hypoxia. |
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