100 Years of benthic foraminiferal history on the inner Texas shelf inferred from fauna and stable isotopes: Preliminary results from two cores

Coastal regions, such as the Texas-Louisiana shelf, are subject to seasonal hypoxia that strongly depends on the magnitude of freshwater discharge from local and regional river systems. We have determined benthic foraminiferal fauna and isotopic compositions in two Pb-210 dated box cores (BR4 and BR5) to examine the evidence for nearshore hypoxia and freshwater discharge on the Texas shelf during the last 100 years. The Pb-210 chronologies of both cores reveal sedimentation rates of 0.2 and 0.1 cm yr(-1), translating to similar to 60 and similar to 90 year records. The fauna of both cores were almost exclusively composed of Ammonia parkinsoniana and Elphidium excavatum, indicating euryhaline ambient waters. The Ammonia-Elphidium (A-E) index, a qualitative measure of low oxygen conditions, shows an increase from values between 20 and 50 to near 100 in both cores, suggesting low oxygen conditions between 1960 and the core top. Between 1950 and 1960 (9-10 cm), low A-E values in BR4 coincide with high delta O-18 and delta C-13 values greater than 0 parts per thousand and -1 parts per thousand respectively. This event corresponds to severe drought (the Texas Drought of Record) over the Brazos River drainage basin and considerably reduced river discharge from 1948 to 1957. High A-E values prior to this event imply low-oxygen conditions were prevalent prior to anthropogenic exacerbation of Louisiana shelf hypoxia and at least since the dredging of a new Brazos River delta in 1929. Elphidium excavatum delta C-13 values are very low (-4 parts per thousand) and indicative of significant vital effect. The delta C-13 values of A. parkinsoniana average -3 parts per thousand and exhibit little variability, most likely reflecting pore waters influenced by aerobic and anaerobic respiration. The association of lowered Brazos River discharge with more oxygenated shelf bottom waters suggests Brazos River discharge and shelf hypoxia are linked, but the influence of Mississippi-Atchafalaya discharge can also contribute to shelf stratification. (C) 2012 Elsevier Ltd. All rights reserved.