Reconstructing central African hydroclimate over the past century using freshwater bivalve shell geochemistry

Abstract

Centennial records of climatic and hydrological data are scarce on the African continent. Freshwater bivalve shells have the potential to record climate-related proxies, from which reconstructions of river discharge (Q) or water isotope variations over long time periods would be possible. The Oubangui River, the largest right-bank tributary of the Congo River, is one of the few African rivers for which long-term Q records are available. This, together with the availability of museum-archived shells, makes it an ideal location to study changes in hydroclimate in central Africa over the past century and to validate the potential of proxies recorded in freshwater shells. We analysed the carbon and oxygen isotope composition (δ13C, δ18O) across the growth axis of museum-archived (collected between 1891 and ∼ 1952) and contemporary (live-collected in 2011 and 2013) Chambardia wissmanni shells from the Oubangui River, covering sections of the past ∼ 130 years. Shell isotopes show a clear baseline shift. Both δ13C and δ18O exhibit pronounced cyclicity in recent and historical shells, but recent shells showed a much wider range in δ18O values and a narrower range in δ13C values compared to historical specimens. In the Oubangui River, high Q is represented as low water δ18O (δ18Ow) values, while low Q is represented as high δ18Ow values, and these correlations are expected to be expressed in shell δ18O (δ18Oshell) values similarly. The historical δ18Oshell values covered only the lower part of the range measured in recent shells, which suggests a major change in the low-flow conditions of the Oubangui River between the 1950s and 2010s. Reconstructed Q values, based on the logarithmic relationship established between recent water δ18O values and measured Q, tended to overestimate the low flow values in the past, suggesting a different δ18Ow–Q relationship in the earlier part of the 20th century. Thus, while the freshwater shell δ18O patterns are consistent with the documented long-term Q changes in the Oubangui, the shells show that the most pronounced changes in hydroclimate over the past century are expressed in low-flow sections of the hydrograph and likely result from a combination of changes in the rainfall regime, aquifer recharge, and/or vegetation cover in the upper catchment. These results are consistent with the suggestion that dry periods in the upper Congo basin have become more extreme in recent times and highlight that freshwater shells could offer a valuable archive to study recent changes in catchments where no long-term empirical hydrological or climatological data are available.