Paper No. 9
Presentation Time: 4:00 PM

FRESHWATER MUSSEL SHELLS (CHAMBARDIA WISSMANNI) AS ARCHIVES OF PAST AFRICAN RIVER BIOGEOCHEMISTRY


GRANIERO, Lauren E., Geology, Texas A&M University, College Station, TX 77840, GILLIKIN, David P., Department of Geology, Union College, 807 Union St, Schenectady, NY 12308, BOUILLON, Steven, Deptartment of Earth & Environmental Sciences, Katholieke Universiteit Leuven, Celestijnenlaan 200 E, Leuven, 3001, Belgium and YAMBÉLÉ, Athanase, Service de l'Agrométéorologie et de Climatologie, Direction de la Météorologie Nationale, Bangui, B.P. 941, Central African Republic, l_graniero@tamu.edu

Freshwater mussel shells have the potential to record environmental conditions in rivers around the world. This study uses water chemistry data collected fortnightly over one year (March 2010-2011) from the Oubangui River at Bangui (Central African Republic) to complete the first ever calibration study of an African freshwater mussel species (Chambardia wissmanni). Shell δ18O values correlate with modeled δ18Os suggesting shells are precipitated in isotopic equilibrium. However, growth ceases or slows for > 5 months during the rising limb of the hydrograph which coincides with high total suspended sediment (TSM) load. δ18Ow values are largely controlled by precipitation and evaporation, so are related to discharge (Q). Thus, if there is a strong correlation between δ18Ow values and Q, δ18Os values should track Q. A linear relationship proved more accurate than a logarithmic relationship, but still overestimated both peak Q (calculated ≈ 11,000 m3s-1, actual ≈ 8,500 m3s-1) and base flow (calculated ≈ 2,600 m3s-1, actual ≈ 750 m3s-1). δ13Cs values were more negative compared to expected shell values calculated using published fractionation factors between aragonite and HCO3-, indicating metabolic carbon (CM) is incorporated in the shells, as has been noted in many bivalve species. A mixing model using the end-member δ13Cs values of HCO3- and soft tissues (as a proxy of CM) suggests that yearly CM incorporation varies from 0 to 29 % with high values corresponding to high TSM during the rising limb of the hydrograph. This supports that C. wissmanni filtration is impaired by high discharge, increased turbidity and high TSM, since these shells incorporate more CM during this time. Living and archived C. wissmanni shells have the potential to provide a high-resolution record of past African river biogeochemistry and to detect large scale changes in discharge, which may be applicable to other tropical rivers.