FRESHWATER MUSSEL SHELLS AS ARCHIVES OF PAST AFRICAN RIVER BIOGEOCHEMISTRY

The Oubangui River, the largest tributary of the Congo River System, is in danger of impacts from climate and land use change due to growing populations in the watershed. This study uses water chemistry data collected fortnightly over one year (March 2010-2011) to calibrate shells of the freshwater mussel Chambardia wissmanni as archives of environmental and climate change. Shells of various sizes were collected in March 2011 from the Oubangui River at Bangui, Central African Republic. Peak river discharge occurs from August to November (Q>5000 m³s^-1) corresponding with lower δ¹⁸O water values (-0.3 to -2.3‰) and lower δ¹³C values in dissolved inorganic carbon (DIC, -12.9 to -15.7‰). Conversely, baseflow (Q<1000 m³s^-1) is characterized by more ¹⁸O enriched water values (+0.6 to +1.6‰) and ¹³C enriched DIC (-10.3 to -7.8‰). Three shells were sampled for δ¹⁸O and δ¹³C using a New Wave Micromill. Samples were taken every 0.8 to 1 mm covering ~27 mm of the two smaller shells and ~5 mm of the larger shell. All shells have a similar range in δ¹⁸O and δ¹³C values, and present similar patterns. Temperature and δ¹⁸O water values were used to construct model shell δ¹⁸O values based on published paleo-temperature equations. Shell data did not match modeled data well, which indicates large growth hiatuses, or isotopic disequilibrium. Nevertheless, considering data from different shells correlate well, environmental data are most likely being recorded in these shells. Shell δ¹³C values were more negative compared to expected shell values, indicting incorporation of metabolic carbon in the shells, as has been noted in many bivalve species. Contrary to previous studies, however, size did not have an affect on shell δ¹³C values. Trace element data (e.g., Sr/Ca, Mg/Ca, Ba/Ca) are currently being analyzed in these shells and will hopefully shed light on their annual growth patterns. Once the chronology is understood, environmental proxies can be calibrated in these shells.