Texas Brazos River Flow and Mussel Growth Reconstructions Using Stable Oxygen, Hydrogen, and Carbon Isotopes and Trace Elements
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The interaction between drought and river regulation is monitored to better understand river flow mixing, evaporation, and surface-groundwater exchange in changing regional climates and in increasingly regulated waterways. I compared Brazos River stable isotope (δ^18O and δD) and electrical conductivity values with reservoir, creek, and aquifer samples in the Brazos watershed, the largest watershed in Texas. Shells from two common species of Brazos River mussel, Amblema plicata and Cyrtonaias tampicoensis, were serially-sampled in the inner and outer shell layers for δ^18O, δ^13C, and trace elements to examine the isotopic and chemical signatures of the 2011-2014 drought. Predicted aragonite δ^18O for the 2012-13 study interval has an irregular pattern that complicates development of growth chronologies in modern shells. To circumvent this problem, clumped isotope (Δ47) temperature measurements were used for interpreting segments of shell growth chronologies. To characterize the influence that biological and environmental variables have on shell chemistry, one specimen from each of the above two mussel species were studied using paired isotope-trace element analyses and cathodoluminescence. The Brazos River Alluvium Aquifer and the Lake Whitney reservoir, both on the main river channel, represent water source endmembers of dilute runoff water and evaporated saline water, respectively. The difference between river and precipitation Δ^18O, or Δ^18ORIV-PPT, a measurement of degree of evaporation, ranged from 0.9‰ for a small creek, to 2.7‰ for the Brazos River, to at least 3.7‰ in Lake Whitney. Δ^18O values and trends were similar in coeval shell transects, indicating that δ^18O is a valid chronometer when calibrated, although all shell had winter growth cessations. Δ^13C trends were similar between shells, suggesting strong environmental control influenced by upstream dam releases. The shell isotope chronologies can be used to reconstruct variation in river discharge, flow source, and salinity. Shell δ^13C, Sr/Ca, and Mn/Ca generally covaried in the shell regions sampled, and shell δ^13C is thought to be controlled by upstream dam releases based on previous work. Relationships between Sr/Ca and temperature are consistent with temperature-paced metabolic control on shell Sr/Ca as in other studies.
Van Plantinga, Alexander (2015). Texas Brazos River Flow and Mussel Growth Reconstructions Using Stable Oxygen, Hydrogen, and Carbon Isotopes and Trace Elements. Doctoral dissertation, Texas A & M University. Available electronically from