Clumped Isotope Verification of δ18O-Based Freshwater Mussel Shell Growth Chronology for a High-Resolution Climate and River Discharge Record

Abstract:

Isotope profiles in freshwater mussel shells can be used to reconstruct climate, water source, and river discharge, but problems arise from variable water temperature and δ¹⁸O. To resolve this complexity and expand the application of isotope sclerochronology to the study of past river systems, we measured δ¹⁸O and Δ₄₇ in two common freshwater mussel species from the Brazos River in Texas. To compare the environmental record with the shell record and develop a sclerochronology, weekly water temperature and δ¹⁸O data were collected from the Brazos River near College Station from January 2012 to August 2013. The river data reveal complex, irregular patterns for predicted aragonite δ¹⁸O. Comparing δ¹⁸O profiles from micromilled transects (70-200 µm increments) of coeval shell growth within and between shells yielded consistent patterns. Shell δ¹⁸O can be accurately matched to predicted δ¹⁸O, providing a chronology of shell growth. However, without a water temperature and δ¹⁸O record, interpreting a sclerochronology would be impossible. Shell Δ₄₇ can potentially provide a seasonal chronology to verify the δ¹⁸O sclerochronology, which would be invaluable for the use of δ¹⁸O sclerochronology in historical and ancient shells. For Δ₄₇ analyses, samples were taken at 0.5 mm resolution in presumed seasonal dark and light growth bands. Clumped temperatures range between 21 and 35 ± 4˚C (Henkes et al., 2013) and track the river temperature record, supporting the interpreted shell δ¹⁸O chronology. Shell Δ₄₇-calculated water δ¹⁸O values range from -1.2 to 1.5 ± 0.9‰ and match river δ¹⁸O. High-resolution shell δ¹⁸O profiles combined with Δ₄₇ temperatures can reconstruct a weekly history of water δ¹⁸O, and with the observed river discharge vs. water δ¹⁸O relation, produce a qualitative record of river discharge. These analytical techniques applied to a historical Brazos River mussel shell collected prior to dam construction reveal weekly records of natural environmental variability.