Description: The continental slope of India is exposed to an intense perennial oxygen minimum zone (OMZ) supporting pelagic denitrification. Sediments that are presently in contact with the lower boundary of the denitrification zone indicate marked changes in the intermediate and bottom waters ventilation of OMZ during the past 9,500 years. The δ15N of sediment suggests that the OMZ waters were less ventilated during the early Holocene (between 9.5 and 8.5 ka BP) resulting in intensified denitrifying conditions with an average δ15N value of 7.8‰, while at the same time stable Mo isotope composition (average δ98Mo of -0.02‰) indicates that the bottom waters that were in contact with the sediments were better oxygenated. By the mid-Holocene OMZ became more oxygenated suppressing denitrification (average δ15N of 6.2‰), while bottom waters gradually became less oxygenated (average δ98Mo of 1.7‰). The mid-Holocene reduction in denitrification coincided with a global decrease in atmospheric N2O as inferred from ice core records, which is consistent with a decreased contribution from the Arabian Sea. Since ~5.5 ka BP OMZ waters have again been undergoing progressive deoxygenation accompanied by increasing denitrification.

Description: The correlation between concentrations of dissolved barium (dBa) and silicon (dSi) in the modern ocean supports the use of Ba as a paleoceanographic proxy. However, the mechanisms behind their linkage and the exact processes controlling oceanic Ba cycling remain enigmatic. To discern the extent to which this association arises from biogeochemical processes versus physical mixing, we examine the behavior of Ba and Si at the Congo River-dominated Southeast Atlantic margin where active biological processes and large boundary inputs override the large-scale ocean circulation. Here we present the first combined measurements of dissolved stable Ba (δ138Ba) and Si (δ30Si) isotopes as well as Ba and Si fluxes estimated based on 228Ra from the Congo River mouth to the northern Angola Basin. In the surface waters, river-borne particle desorption or dissolution and shelf inputs lead to non-conservative additions of both dBa and dSi to the Congo-shelf-zone, with the Ba flux increasing more strongly than that of Si across the shelf. In the epipelagic and mesopelagic layers, Ba and Si are decoupled likely due to different depths of in situ barite precipitation and biogenic silica production. In the deep waters of the northern Angola Basin, we observe large enrichment of dBa, likely originating from high benthic inputs from the Congo deep-sea fan sediments. Our results reveal different mechanisms controlling the biogeochemical cycling of Ba and Si and highlight a strong margin influence on marine Ba cycling. Their close association across the global ocean must therefore mainly be a consequence of the large-scale ocean circulation. Key Points Stronger enrichment of dissolved barium (dBa) than silicon (dSi) observed in the shelf-zone of the Congo plume Diatom silica production has negligible effect on dissolved Ba isotopic compositions in large river plumes Strong dBa enrichment (up to 24 nM) in the deep water of the northern Angola Basin likely originates from high benthic inputs

Description: Stable barium isotopes are a potential proxy for riverine inputs into the ocean that reflect monsoon variability and climate change. However, dissolved Ba isotope (δ138BaDBa) geochemistry in river estuaries, a dynamic land to ocean transition zone, has rarely been systematically examined to date. Here, we show that significant Ba isotope fractionation occurs at near-zero salinities in the Yangtze and Pearl River Estuary, whereas conservative mixing dominates δ138BaDBa distributions beyond low salinities, which are well predicted by an ion exchange model. Elevated δ138BaDBa in the river endmember results from preferential removal of light Ba isotopes by adsorption to fluvial particles. Subsequently, δ138BaDBa rapidly drops to minimum signatures at increased salinities indicating particle desorption of isotopically light Ba. Nevertheless, the apparently conservative δ138BaDBa-salinity relationship beyond the low-salinity minimum in both estuaries provides a modern calibration for using Ba isotopes as a proxy for paleosalinity and river water inputs into the ocean.

Description: 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 (δ138Bapw) 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 δ138Bapw 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 (138ε) of −0.60 ± 0.10‰ 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 δ138Bapw values of +0.5 to +0.7‰ 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. Key Points We present a data set of dissolved stable Ba isotopic compositions in surface sediment pore waters of a large river estuary Pore water Ba isotope values generally decrease with increasing sediment depth, reflecting post-depositional Ba isotope fractionation A diffusion-reaction model predicts the distribution and fractionation of stable Ba isotopes in the sediment pore waters well