Constraining the drivers of barium isotope composition in marine barite: Insights from Pliocene-Holocene Eastern Mediterranean sapropels

Abstract

The stable isotopic composition of barium in marine barite (δ138/134Babarite) has been proposed as a proxy for reconstructing past Ba cycling in the oceanic water column. However, whether δ138/134Babarite can also be a proxy for export productivity remains uncertain, particularly in semi-enclosed basins such as the Eastern Mediterranean (EMed). Here we present the first record of δ138/134Babarite extracted from EMed Pliocene to Holocene sapropels from six Ocean Drilling Program (ODP) Sites. Despite strong variability in Ba concentrations (∼400–4000 ppm) and organic carbon content (∼3–25 %)—reflecting varying export productivity levels—the δ138/134Babarite values show limited variation (ranging between 0.02 to −0.16 ‰), even during periods of exceptionally high inferred export productivity (e.g., during late Pleistocene sapropel S5). Consistent δ138/134Babarite values are also observed in sapropel S1 sites located at a wide range of water depths (∼900–3600 m). Well-preserved pelagic barite crystal textures (observed under scanning electron microscopy) and barite sulphur isotope composition similar to seawater (∼21 ‰; Paytan et al., 2004) in the sapropels, suggest that neither partial dissolution in bottom waters nor postdepositional Ba ion-exchange significantly altered the δ138/134Babarite signal. These results suggest that fluctuations in marine productivity had a limited impact on the dissolved Ba pool and its isotopic composition in the EMed upper water column during sapropel deposition. Instead, the EMed δ138/134Ba was likely controlled primarily by the Ba isotope composition of Atlantic Water inflow and local dissolved Ba sources, such as riverine and groundwater inputs, rather than by productivity-driven barite fluxes. This finding highlights that δ138/134Babarite in sedimentary records should not be interpreted as a straightforward proxy for export productivity, particularly in semi-restricted basins