Trace elements in mussel shells from the Brazos River, Texas: environmental and biological control

Abstract. In sclerochronology, understanding the drivers of shell chemistry is necessary in order to use shells to reconstruct environmental conditions. We measured the Mg, Ca, Sr, Ba, and Mn contents in water samples and in the shells of two freshwater mussels (Amblema plicata and Cyrtonaias tampicoensis) from the Brazos River, Texas to test their reliability as environmental archives. Shells were analyzed along growth increments using age models established with stable and clumped isotopes. Shells were also examined with cathodoluminescence (CL) microscopy to map Mn / Ca distribution patterns. Sr / Ca correlated with Mn / Ca, while Mg / Ca and Ba / Ca showed no clear trends. Mn / Ca correlated inversely with the log of river discharge. Because dissolved and inorganic particulate sources of manganese are low during low flow, peak Mn / Ca values may come from elevated feeding or metabolic rates. Shell Mn / Ca values were used to reconstruct river discharge patterns, which, to our knowledge, has previously only been performed with shell chemistry using oxygen isotopes.