Calibration of hydroclimate proxies in freshwater bivalve shells from Central and West Africa

Abstract

Freshwater bivalve shell oxygen and carbon stable isotope ratios (δ 18 O, δ 13 C) may act as recorders of hydroclimate (e.g., precipitation−evaporation balance, discharge) and aquatic biogeochemistry. We investigate the potential of these hydroclimate proxies measured along the growth axis of shells collected from the Oubangui River (Bangui, Central African Republic) and the Niger River (Niamey, Niger). Biweekly water samples and in−situ measurements collected over several years, along with daily discharge data from both sites allowed a direct comparison with proxies recorded in the shells. Data from a total of 14 unionid shells, including three species (Chambardia wissmanni, Aspatharia dahomeyensis, and A. chaiziana), confirmed that shells precipitate carbonate in oxygen isotope equilibrium with ambient water. Because water temperature variations were small, shell δ 18 O values (δ 18 O shell ) also accurately record the seasonality and the range observed in water δ 18 O (δ 18 O w ) values when calculated using an average temperature. Calculated δ 18 O w values were in good agreement over the entire record of measured δ 18 O w values, thus δ 18 O shell records can be reliably used to reconstruct past δ 18 O w values. Discharge and δ 18 O w values from both rivers fit a logarithmic relationship, which was used to attempt reconstruction of past hydrological conditions, after calculating δ 18 O w values from δ 18 O shell values.A comparison with measured discharge data suggests that for the two rivers considered, δ 18 O shell data are good proxies for recording discharge conditions during low(er) discharge levels, but that high discharge values cannot be accurately reconstructed due to the large scatter in the discharge−δ 18 O w relationship. Moreover, periods of bivalve shell growth cessation due to high turbidity or air exposure should be taken into account. While δ 13 C values of dissolved inorganic carbon in both rivers showed clear seasonality and correlated well with discharge, most of the shells analyzed did not record these variations adequately, likely due to the complication of vital effects including the variable contribution of metabolic CO 2 . Thus, tropical African unionid δ 18 O shell values can be used to reconstruct δ 18 O w values with high confidence to provide insight on past hydroclimate such as precipitation−evaporation balance and periods of low discharge.

Seasonal oxygen isotope variations in freshwater bivalve shells as recorders of Amazonian rivers hydrogeochemistry

“…water temperature) changes [34]. In freshwater unionid bivalve shells that lived in lake or floodplain lake, growth decreases or cessations were linked to high water turbidity related to high discharge events [22,34]. In the two Brazilian A. trapesialis specimens presented here, the lowest δ 18 OS values, supposedly characteristics of the wet season (see below), occurred at the time of growth line deposition.…”

“…It has been shown in many bivalve species that shell growth patterns can be used to give an age to the shell as well as to reconstruct environmental parameter (e.g. water temperature) changes [34]. In freshwater unionid bivalve shells that lived in lake or floodplain lake, growth decreases or cessations were linked to high water turbidity related to high discharge events [22,34].…”

Freshwater pearl mussels as a stream water stable isotope recorder

“…They also used shell δ 18 O of Unio pictorum and Unio tumidus as a proxy of historical discharge of the river Meuse in the Netherlands. Kelemen et al () have documented the potential for shell δ 18 O values in three species of unionid shells ( Chambardia wissmanni , Aspatharia dahomeyensis , and Aspatharia chaiziana ) to reconstruct past δ 18 O signatures in stream water of the Oubangui and Niger Rivers in Central and West Africa.…”

“…Despite the advent of field deployable laser spectrometers (Berman, Gupta, Gabrielli, Garland, & McDonnell, 2009) and the deployment of compact environmental laboratories in the field (Floury et al, 2017;Von Freyberg, Studer, & Kirchner, 2017), collection of such data remains prohibitively labour and time intensive. Here, we build on prior work where oxygen stable isotope ratios (δ 18 O) obtained from mechanically drilled freshwater bivalve shell material have been extensively used for palaeoenvironmental reconstructions (e.g., Helama & Nielsen, 2008;Versteegh, Troelstra, Vonhof, & Kroon, 2009;Versteegh, Vonhof, Troelstra, Kaandorp, & Kroon, 2010), including hydroclimate variables (Kelemen et al, 2017). We show a proof of concept of a new approach to stream signal reconstruction based on freshwater mussels, specifically the freshwater pearl mussel Margaritifera (Linnaeus, 1758;Bauer, 1987).…”

Freshwater pearl mussels from northern Sweden serve as long-term, high-resolution stream water isotope recorders

“…One underutilized approach for hydroclimate reconstruction is the use of freshwater mussels as natural stream water stable isotope recorders (Dettman et al, 1999;Kelemen et al, 2017;Pfister et al, 2018Pfister et al, , 2019. Their shells can provide seasonally to annually resolved, chronologically precisely constrained records of environmental changes in the form of variable increment widths (which refers to the distance between subsequent growth lines) and geochemical properties (e.g., Nyström et al, 1996;Schöne et al, 2005a;Geist et al, 2005;Black et al, 2010;Schöne and Krause, 2016;Geeza et al, 2019Geeza et al, , 2020Kelemen et al, 2019). In particular, similar to marine (Epstein et al, 1953;Mook and Vogel, 1968;Killingley and Berger, 1979) and other freshwater bivalves (Dettman et al, 1999;Kaandorp et al, 2003;Versteegh et al, 2009;Kelemen et al, 2017;Pfister et al, 2019), Margaritifera margaritifera forms its shell near equilibrium with the oxygen isotope composition of the ambient water (δ 18 O w ) (Pfister et al, 2018;Schöne et al, 2005a).…”