Freshwater bivalve shells as hydrologic archives in the Congo Basin

We test the applicability of bivalve shell oxygen isotope composition to reconstruct hydrological dynamics in four riverine sites in the Congo River basin. Twenty-three specimens from the Unionoida order were collected from locations where long-term discharge data are available, and in situ measurements and water samples were collected over several years. Due to the highly variable (species-specific) shell morphology, various sampling techniques were used to analyze the shell sections; however, every specimen recorded the seasonality of the host water oxygen stable isotope composition (delta O-18(w)) in its delta O-18(shell) record. Discharge data showed an inverse relationship with delta O-18(w) values, which was well described with a logarithmic fit. An exception was the Kasai River, where the delta O-18(w) record shows an additional peak occurring during the high discharge period, which renders the discharge-delta O-18(w) relationship more complex than in the other systems investigated. Low ratios of maximum to minimum discharge (Q(max)/Q(min)) were found to result in a low delta O-18(w) amplitude, which was reflected as low delta(18)O(shell )variability. The Congo and Kasai rivers had Q(max)/Q(min) ratios similar to 2 to 2.5, while the Oubangui showed a much higher Q(max)/Q(min) (similar to 19). Shells correspondingly showed a large delta O-18(shell) range (amplitude between 2.4 and 5.0 parts per thousand) for individual Oubangui shells, and lower amplitude for other sites (1.0 to 2.2 parts per thousand). Thus, shells have a high resolving power to be used to record hydrological variability, since long-term changes in precipitation pattern, discharge, land-use change, or other hydrological changes have an influence on delta O-18(w), values. Shells with wide range of delta O-18 values reflect high seasonal variability in rivers, while shells with lower delta O-18 amplitude correspond to sites with more steady river conditions over the year. Our study illustrates that fossil shell delta O-18 values could indicate Q(max)/Q(min) values in ancient African river systems. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study evaluates the applicability of bivalve shell oxygen isotope composition in reconstructing hydrological dynamics in the Congo River basin. The findings indicate the potential of using fossil shell delta O-18 values to infer Q(max)/Q(min) values in ancient African river systems, with shells reflecting both seasonal variability and stability in different river conditions.