Dissolved silicon isotope dynamics in large river estuaries

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

How Human Activities Are Disrupting the Silicon Cycle

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

Dissolved and particulate barium concentrations and isotopes in the Rainbow non-buoyant hydrothermal plume during METEOR cruise M176/2

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) 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

The Silicon Cycle in the Ocean

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

Benthic-pelagic coupling and isotopic fractionation of barium in Kiel Bight, SW Baltic Sea

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

Impact of human disturbance on the biogeochemical silicon cycle in a coastal sea revealed by silicon isotopes

Biogeochemical silicon (Si) cycling in coastal systems is highly influenced by anthropogenic perturbations in recent decades. Here, we present a systematic study on the distribution of stable Si isotopes of dissolved silicate (δ30SiDSi) in a highly eutrophic coastal system, the Baltic Sea. Besides the well-known processes, diatom production and dissolution regulating δ30SiDSi values in the water column, we combined field data with a box model to examine the role of human disturbances on Si cycling in the Baltic Sea. Results reveal that (1) damming led to increased δ30SiDSi values in water but had little impacts on their vertical distribution; (2) decrease in saltwater inflow due to enhanced thermal stratification had negligible impacts on the δ30SiDSi distribution. An atypical vertical distribution of δ30SiDSi with higher values in deep water (1.57–1.95‰) relative to those in surface water (1.24–1.68‰) was observed in the central basin. Model results suggest the role of enhanced biogenic silica (BSi) deposition and subsequently regenerated dissolved silicate (DSi) flux from sediments. Specifically, eutrophication enhances diatom production, resulting in elevated exports of highly fractionated BSi to deep water and sediments. In situ sedimentary geochemical processes, such as authigenic clay formation, further fractionate Si isotopes and increase pore-water δ30SiDSi values, which then leads to pore-water DSi flux carrying higher δ30SiDSi compositions into deep water. Our findings provide new quantitative information on how the isotope-based Si cycle responds to human perturbations in coastal seas and shed lights on shifts of Si export to open ocean

Effect of Annealing on the Properties of Antimony Telluride Thin Films and Their Applications in CdTe Solar Cells

Antimony telluride alloy thin films were deposited at room temperature by using the vacuum coevaporation method. The films were annealed at different temperatures in N2 ambient, and then the compositional, structural, and electrical properties of antimony telluride thin films were characterized by X-ray fluorescence, X-ray diffraction, differential thermal analysis, and Hall measurements. The results indicate that single phase antimony telluride existed when the annealing temperature was higher than 488 K. All thin films exhibited p-type conductivity with high carrier concentrations. Cell performance was greatly improved when the antimony telluride thin films were used as the back contact layer for CdTe thin film solar cells. The dark current voltage and capacitance voltage measurements were performed to investigate the formation of the back contacts for the cells with or without Sb2Te3 buffer layers. CdTe solar cells with the buffer layers can reduce the series resistance and eliminate the reverse junction between CdTe and metal electrodes

The Structure and Stability of Molybdenum Ditelluride Thin Films

Molybdenum-tellurium alloy thin films were fabricated by electron beam evaporation and the films were annealed in different conditions in N2 ambient. The hexagonal molybdenum ditelluride thin films with well crystallization annealed at 470°C or higher were obtained by solid state reactions. Thermal stability measurements indicate the formation of MoTe2 took place at about 350°C, and a subtle weight-loss was in the range between 30°C and 500°C. The evolution of the chemistry for Mo-Te thin films was performed to investigate the growth of the MoTe2 thin films free of any secondary phase. And the effect of other postdeposition treatments on the film characteristics was also investigated