AGU Fall Meeting location:San Francicso, CA date:14-18 December 2015
The bivalve Etheria elliptica abundantly occurs in the majority of African river basins. We investigate its potential use for the reconstruction of ambient water chemistry and climate by means of stable oxygen isotope ratios in specimens from the Congo river (Kisangani), the Oubangui river (Bangui) and Victoria Nile (Jinja). Unlike other common African bivalve species, E. elliptica contains distinct organic rich growth increments with lunar periodicity, providing a moon-monthly time frame for sequential microchemistry. However, cavities in the shell complicate age reading and little is known about the continuity of these growth increments. We set up a comparative study between different techniques to visualize and enhance growth features, and find that staining with Mutvei’s solution and confocal fluorescence microscopy perform equally well. Despite the presence of cavities, growth lines can generally be followed from umbo to shell margin, corroborating their use as temporal anchor points. Preliminary isotope results of two micro-sampled Congo river specimens show18Oshell values varying intra-annually between -1.9 and -3.8 ‰ (VPDB), in line with a predicted range of -2.1 to -4.1 ‰ based on bimonthly18Owater and T monitoring. Reconstructed intra-annual 18Owater variability from18Oshell values and observed T correlates with discharge, reflecting rainfall and runoff variability in the upstream catchment area. In two Victoria Nile specimens, collected 20 km downstream from Lake Victoria, 18Oshell values are high and relatively constant, varying between +1.8 and +3.2 ‰. Enrichment of 18Oshell is consistent with heavy rainfall signatures and elevated surface evaporation in Lake Victoria. These first results suggest that E. elliptica is well-suited for the reconstruction of African climate and hydrology at a continental scale.