Description: In order to understand the hydrothermal input of barium (Ba) to the ocean interior and the changes of Ba in the hydrothermal plume, we measured dissolved (〈0.2 μm) and particulate (〉0.2 μm) Ba concentrations, as well as stable isotopes, in the non-buoyant hydrothermal plume at the Rainbow hydrothermal vent field on the Mid-Atlantic Ridge. Samples were taken during a GEOTRACES compliant cruise (M176/2) onboard the German R/V Meteor from September to October 2021. Particulate samples were first leached to measure labile components and then digested to measure refractory components. Our investigation covered the plume with a high spatial resolution, extending from a distance of 200 m to 60 km away from the Rainbow hydrothermal vent field.

Description: To better understand the association between barium (Ba) and silicon (Si) at ocean margins, we collected filtered seawater samples along the Congo River plume at the coast of Congo and Gabon and along an off-shelf section at 3°S, complemented by three freshwater samples collected directly from the Congo River. The water samples from the Congo margin were collected during GEOTRACES cruise GA08 with German RV Meteor (M121) in November-December 2015 with a stainless steel CTD rosette and a towed fish for surface waters. Congo River freshwater samples were taken upstream by boat in May, July and October 2017 near the center of the river at ~6°S and ~12.5°E (stations 12.1-12.3). We determine for the first time Ba and Si stable isotopes at the Congo River-dominated Southeast Atlantic margin. Based on these data we evaluate the contribution of Congo River plume to Ba and Si budgets in the South East Atlantic Ocean, and the association between Ba and Si at the Congo margin.

Description: Estuarine systems are of key importance for the riverine input of silicon (Si) to the ocean, which is a limiting factor of diatom productivity in coastal areas. This study presents a field dataset of surface dissolved Si isotopic compositions (30SiSi(OH)4) obtained in the estuaries of three of the world’s largest rivers, the Amazon (ARE), Yangtze (YRE), and Pearl (PRE), which cover different climate zones. While 30SiSi(OH)4 behaved conservatively in the YRE and PRE supporting a dominant control by water mass mixing, significantly increased 30SiSi(OH)4 signatures due to diatom utilization of Si(OH)4 were observed in the ARE and reflected a Si isotopic enrichment factor 30 of −1.0±0.4‰ (Rayleigh model) or −1.6±0.4‰ (steady state model). In addition, seasonal variability of Si isotope behavior in the YRE was observed by comparison to previous work and most likely resulted from changes in water residence time, temperature, and light level. Based on the 30 value obtained for the ARE, we estimate that the global average 30SiSi(OH)4 entering the ocean is 0.2-0.3‰ higher than that of the rivers due to Si retention in estuaries. This systematic modification of riverine Si isotopic compositions during estuarine mixing, as well as the seasonality of Si isotope dynamics in single estuaries, needs to be taken into account for better constraining the role of large river estuaries in the oceanic Si cycle.

Description: The element silicon is everywhere! In fact, silicon is the second most abundant element in Earth’s crust. Silicon in rocks and minerals breaks down and is transported from rivers and streams into the world’s oceans. Many marine organisms need silicon as it is a crucial nutrient to build their skeletons. Silicon eventually reaches the seafloor, but its journey into the abyss is not straightforward due to biological, physical, and chemical processes. All these processes transport and transform silicon, creating a cycle that we call the marine silicon cycle. The silicon cycle is directly connected to the carbon cycle, making silicon a key player in the regulation of Earth’s climate. In this article, we discuss why we need to understand the marine silicon cycle, explain the steps that happen in the ocean, and demonstrate how the marine silicon cycle affects humans.

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: Dissolved silicon is an essential nutrient for the growth of various ocean organisms that need it to build their skeletons. Most of the dissolved silicon that sustains these organisms comes from the breakdown of silicon-containing rocks on land. In recent decades, human activities have greatly disturbed the transport of silicon from land to ocean. For example, dams built to generate electricity can interrupt the transport of dissolved silicon and starve downstream areas. Fertilizers and other human pollution add large amounts of non-silicon nutrients to rivers, lakes, and reservoirs, which can stimulate organisms to grow and use up silicon before it reaches the ocean. In addition, consequences of climate change can also impact the silicon cycle. In this article, we explain how human activities have disturbed the silicon cycle and discuss how climate change may affect it in the future.

Description: Barium (Ba) isotopes are a promising new tracer for riverine freshwater input to the ocean and marine biogeochemical cycling. However, many processes that affect Ba cycling at continental margins have not yet been investigated with respect to Ba isotope fractionation. Here, we present a comprehensive data set of Ba concentration and isotope data for water column, pore water and sediment samples from Kiel Bight, a seasonally stratified and hypoxic fjord in the southwestern Baltic Sea. The surface water Ba concentration and Ba isotope inventory of the water column can generally be explained by mixing of riverine freshwater and Atlantic seawater. However, the deep-water below the seasonal pycnocline (10 - 15 m water depth) is characterized by a pronounced positive Ba concentration anomaly (up to 915 nM) that is accompanied by a δ138Ba of ~+0.25 ‰, which is lighter than expected from the seawater-freshwater mixing line (Ba: 77 nM, δ138Ba: +0.32 ‰ at a salinity of 18). Pore water profiles indicate a Ba flux across the sediment-water interface, which contributes to the enrichment in isotopically light Ba in the deep-water. Pore waters of surface sediments and deep-waters are oversaturated with respect to barite. Therefore, barite dissolution is unlikely to account for the benthic Ba flux. Water column Ba concentrations closely correlate with those of the nutrients phosphate and silica, which are removed from surface waters by biological processes and recycled from the sediment by diffusion across the sediment-water interface. As nutrient-to-Ba ratios differ among sites and from those observed in open-marine systems, we propose that Ba is removed from surface waters by adsorption onto biogenic particles (rather than assimilation) and regenerated within surface sediments upon organic matter degradation. Pore water data for subsurface sediments in Kiel Bight indicate preferential transfer of isotopically heavy Ba into an authigenic phase during early diagenesis. Quantifying the burial flux associated with this authigenic Ba phase along continental margins could potentially help to settle the isotopic imbalance between known Ba source and sink fluxes in 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

Description: Highlights • Negligible Ba removal observed in the Rainbow hydrothermal system. • Insignificant modification of Ba isotope composition of the vent fluid endmember. • Rainbow vent introduces isotopically light Ba (−0.17) to the deep Atlantic Ocean. • Hydrothermal inputs contribute 4.6 ± 2.2 Gmol/yr Ba to the ocean. Abstract The marine barium (Ba) cycle is closely connected to the short-timescale carbon cycle, and Ba serves as a valuable paleo proxy for export production, ocean alkalinity, and terrestrial inputs. However, the marine Ba budget is poorly constrained, particularly regarding the fluxes of hydrothermally sourced Ba, which hinders our understanding of the Ba cycle and use of Ba-based proxies. Recent studies have suggested a modern source-sink imbalance of Ba isotopes in the global ocean, with sources being overall isotopically heavier than the sinks, and the hydrothermal Ba inputs were considered isotopically heavy sources. In this study, we present the first investigation of Ba and its isotopes in a non-buoyant hydrothermal plume based on dissolved and particulate samples collected from the Rainbow hydrothermal vent field on the Mid-Atlantic Ridge. Our data reveal strong hydrothermal signals at near-field stations, as evidenced by helium isotopes, accompanied by elevated concentrations of dissolved and particulate Ba. Dissolved Ba isotope compositions (δ138Ba) in hydrothermally influenced deep waters (∼0.3 ) are lighter than at similar depths of far-field stations (∼0.45 ) in the Atlantic Ocean. The concentrations and isotopic compositions of dissolved and labile particulate Ba in the non-buoyant hydrothermal plume can be explained by conservative mixing between a Ba-enriched hydrothermal component and North Atlantic Deep Water. By extrapolating the correlations to the vent fluid endmember, our results suggest that there is negligible removal of Ba, and insignificant modification of Ba isotopic signatures, from the vent fluid endmember to the non-buoyant hydrothermal plume. This indicates that the Rainbow hydrothermal system introduces isotopically light Ba (−0.17 ± 0.05 ) to the deep Atlantic Ocean. We estimate that global hydrothermal inputs of Ba are 4.6 ± 2.2 Gmol/yr. These observations highlight the potential of hydrothermal Ba to be an isotopically light source component of the marine Ba isotope budget.