Stable and clumped isotope sclerochronologies of mussels from the Brazos River, Texas (USA): Environmental and ecologic proxy
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2018 Elsevier B.V. Oxygen isotope sclerochronology with mollusk shells can provide vital environmental and ecologic information, but is complicated in regulated, subtropical rivers by irregular river water 18O and consequently irregular shell 18O patterns. We performed multi-proxy analyses (18O, 13C, clumped isotopes) from serially-sampled freshwater mussels in two specimens each of Amblema plicata and Cyrtonaias tampicoensis from the Brazos River, Texas to age specimens and test whether they accurately record environmental conditions such as river discharge and water source. Oxygen isotopic measurements are similar to predicted aragonite 18O values based on temperature and water 18O, but the record is complex and irregular. To better resolve the chronologies, we performed clumped isotope analyses on select shell intervals. Clumped isotope temperatures (T(47)) ranged from 19 to 36 C. Summer T(47) values were 35 C higher than measured temperatures, suggesting an offset in paleothermometer calibration. Chronologies based on 47 and 18O reveal specimen ages of 34 years and winter growth cessation (or dramatic slowing) in both C. tampicoensis and A. plicata, highlighting thermal limitations on growth and potential biases in environmental reconstructions. Compared with modern shells, historical shells collected before intensive damming showed lower 18O values yet similar clumped temperatures, suggesting enhanced evaporation in modern river water due to impoundment and/or the 201112 drought. Carbon isotope trends were similar between conspecific shells, but differed between species, with substantially higher average values for A. plicata shells (9) than C. tampicoensis shells (12). The correlation between shell 13C values and the release of 13C-enriched waters from upstream reservoirs demonstrates that 13C can be used as a proxy for dam release in regulated rivers.
