Stable Barium Isotope Fractionation in Pore Waters of Estuarine Sediments

The development of stable barium (Ba) isotope measurements provides a novel tool to investigate the geochemical cycling of Ba in the ocean and its sediments. In sediment pore waters, gradients of dissolved Ba concentrations result from various diagenetic processes. The distribution and fractionation of Ba isotopes in the pore waters are expected to further improve our understanding of these early diagenetic control mechanisms. Here, we present pore water profiles of dissolved stable Ba isotopic signatures (delta Ba-138(pw)) from shallow water sediments covering the entire Pearl River Estuary (PRE) in Southern China. We find pronounced depth-dependent Ba isotope variations generally showing a shift from heavy to light delta Ba-138(pw) signatures from the sediment surface down to 15 cm depth. These gradients are well reproduced by a diffusion-reaction model, which generates an apparent fractionation factor ((138)epsilon) of -0.60 +/- 0.10 parts per thousand pointing to preferential removal of low-mass Ba isotopes from the pore water during solution-solid phase interactions. Consequently, the combined diagenetic processes induce the highest delta Ba-138(pw) values of +0.5 to +0.7 parts per thousand in the pore waters of the topmost sediment layer. Although the detrital fraction dominates the Ba content in the PRE surface sediments, the determined gradients of pore water Ba isotopes, together with concentration variations of Ba and other redox-sensitive elements such as manganese (Mn), show that non-detrital excess Ba carriers including Mn oxides and authigenic barite clearly affect the post-depositional Ba dynamics. Stable Ba isotopes are thus a potentially powerful tracer of Ba geochemistry during early sediment diagenesis in estuarine depositional environments. Plain Language Summary Dissolved barium (Ba) concentrations change with depth in sediment pore waters because of various chemical reactions involving the formation and dissolving of manganese (Mn) and barite (BaSO4) minerals. The development of stable Ba isotope measurements provides a novel tool to better understand these reactions in sediment pore waters. Here, we measured the dissolved stable Ba isotopic compositions in sediment pore waters near the bottom of the Pearl River Estuary, China. Pore water Ba isotopic signatures generally decrease from the sediment surface downward. The change of Ba concentrations and isotopic compositions measured in the field is well reproduced by a chemical model including diffusion and mineral formation/dissolving processes. This change in pore water Ba isotopes results from a combination of different processes including Ba sticking to sediment particles formed during the cycling of Mn minerals and the formation of BaSO4 minerals, which preferentially remove lighter Ba isotopes from the pore water into the solids. This is significant because it allows pore water Ba isotopes to be used to trace chemical reactions during Ba cycling in freshwater and marine sediments.