Barium Isotopic Compositions of Modern Brachiopod Shells: Implications for Seawater δ138Ba Reconstruction

Barium isotopic compositions (δ138Ba) in marine carbonates hold critical information on marine export productivity or water-mass mixing processes. Although coral skeletons and associated carbonates are regarded as reliable archives for recording seawater barium isotope variations, their application to deep-time studies is complicated by diagenetic alteration. In contrast, brachiopod shells, which are composed of low-magnesium calcite and are more resistant to diagenetic alteration, have been widely used to reconstruct past seawater conditions. However, whether brachiopod shells can faithfully record seawater δ138Ba values has not been investigated, limiting their application in paleoenvironmental reconstruction. Here, we present δ138Ba data for 34 modern brachiopod specimens belonging to six species collected from the southwestern and northern New Zealand margin, as well as the southeastern Australian margin. The δ138Ba values of brachiopod shells range from 0.13‰ to 0.69‰, with an average of 0.43‰ ( n = 34, SD = 0.12). Brachiopod shells from different locations display significantly different δ138Ba values, with samples from Goat Island exhibiting the highest δ138Ba values (mean = 0.55‰, n = 6, SD = 0.09) and those from Western Port exhibiting the lowest values (mean = 0.16‰, n = 3, SD = 0.02). No significant interspecific differences are observed at the same site, indicating that vital effects exert little influence on brachiopod δ138Ba values. This observations suggests that brachiopod δ138Ba values are primarily controlled by environmental rather than biological factors. To further constrain the factors controlling brachiopod δ138Ba values, we explore their relationships with environmental parameters and shell chemistry. No significant correlations are observed between δ138Ba values and seawater temperature or rainfall, suggesting that these factors do not exert a primary control on brachiopod δ138Ba values. Instead, the δ138Ba values are strongly correlated with salinity (r = －0.88, p < 0.001) and chlorophyll a (r = －0.87, p < 0.001), indicating that the pronounced differences in brachiopod δ138Ba values among sites are more strongly controlled by site-specific hydrographic and geochemical conditions. Therefore, we propose that brachiopod Ba isotope compositions across sites are governed by terrestrial inputs and oceanic water-mass mixing. Brachiopods from Western Port, a semi-enclosed bay, yield the lowest δ138Ba values, likely reflecting the influence of terrestrial inputs. Goat Island in northern New Zealand is directly connected to the Pacific, and the δ138Ba values of brachiopods from this location closely resemble North Pacific surface seawater values (δ138Ba = 0.53‰). Bass Strait, Doubtful Sound, and Dusky Sound are influenced by the Antarctic Circumpolar Current and East Australian Current, where the Southern Ocean and Pacific water masses mix, thus, the δ138Ba values of brachiopods from these three sites (0.42‰ for Bass Strait, 0.46‰ for Doubtful Sound, and 0.34‰ for Dusky Sound, respectively) fall between those of the Southern Ocean (δ138Ba = 0.32‰) and Pacific surface waters. Collectively, our study demonstrates that brachiopod shells primarily reflect local seawater δ138Ba and related hydrographic conditions, and therefore have strong potential as archives for reconstructing past seawater δ138Ba values.