Freshwater bivalve shells as hydrologic archives in the Congo Basin

Abstract

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 longterm 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 (d 18 O w ) in its d 18 O shell record. Discharge data showed an inverse relationship with d 18 O w values, which was well described with a logarithmic fit. An exception was the Kasai River, where the d 18 O w record shows an additional peak occurring during the high discharge period, which renders the discharge-d 18 O 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 d 18 O w amplitude, which was reflected as low d 18 O shell variability. The Congo and Kasai rivers had Q max /Q min ratios~2 to 2.5, while the Oubangui showed a much higher Q max /Q min (~19). Shells correspondingly showed a large d 18 O shell range (amplitude between 2.4 and 5.0‰) for individual Oubangui shells, and lower amplitude for other sites (1.0 to 2.2‰). 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 d 18 O w values. Shells with wide range of d 18 O values reflect high seasonal variability in rivers, while shells with lower d 18 O amplitude correspond to sites with more steady river conditions over the year. Our study illustrates that fossil shell d 18 O values could indicate Q max /Q min values in ancient African river systems.

Assessing the Suitability of Native Mollusk Shells for Sclerochronochemistry Studies: An Analysis of Eight Persian Gulf Species

“…Because of the key role of ancient shells and shell fossils in the reconstruction of past climate conditions, they have long been a matter of interest to geological Vernal et al 1992;Geary et al 1992;Goodwin et al 2001;Ullmann et al 2017;Killam & Clapham 2018;Scholz et al 2020;Morán et al 2021), archaeological studies (Quitmyer et al 1997;Leng & Lewis 2016). Indeed, one of the main motivations for the study of bivalves (Müller et al 2017;Huyghe et al 2020;Das et al 2021;Kelemen et al 2021;), gastropods (Cespuglio et al 1999;García-Escárzaga et al 2019;Irie & Suzuki 2020), and extinct species such as belemnites Urey et al 1951;Alberti et al 2021) is to gain insight into climatic conditions.…”

Investigating the Structure and Composition of Freshwater Oyster Adhesive and Shell

“…Etheria elliptica produce a hard shell to protect themselves from predators, and an adhesive to allow them to maintain their position in the high energy environment in which they live. Despite showing promise as paleoclimate archives [3,4,5,6], little is known about the adhesive the organisms produce during settlement nor has there been a systematic review of the shell structure. A combination of x-ray photoemission electron microscopy (X-PEEM), scanning electron microscopy (SEM), and microhardness testing data on the adhesive and shell show the mineralized portion of the shell (prismatic, nacre, and growth layers) consists entirely of aragonite, as does the adhesive (Figure 1).…”