Description: The distribution of dissolved silicon isotopes (δ30Si) was examined along the US GEOTRACES East Pacific Zonal Transect (GP16) extending from Peru to Tahiti (10°S and 15°S latitude). Surface waters in the subtropical gyre displayed high δ30Si due to strong utilization of silicic acid (DSi). In contrast, surface waters close to the Peruvian coast where upwelling prevailed were less depleted and only moderately fractionated. δ30Si of water masses along the transect were compared with the results of an Optimum Multiparameter Analysis that quantified the fractional contributions of endmember water masses in each sample. Strong admixture of intermediate waters obscured the expected heavy isotopic signatures of Subantarctic Mode Water and Antarctic Intermediate Water. Isotope values were nearly homogenous below 2000 m (Average: +1.3 ± 0.1 ‰, 1 s.d.) despite the 25 μmol kg‐1 range in the DSi content among water masses. This homogeneity confirms prior observations and model results that predict nearly constant δ30Si values of +1.0 to +1.2 ‰ for Pacific deep waters with [DSi] 〉 100 μmol kg‐1. Waters above the East Pacific Rise (EPR) influenced by hydrothermal activity showed a small increase in [DSi] together with dissolved iron, but overall stations close to the EPR were slightly depleted in [DSi] (3 to 6 μmol kg‐1) with no significant shift in δ30Si compared to adjacent waters. Hydrothermal [DSi] appears to precipitate within the conduit of the EPR or upon contact with cold seawater resulting in a negligible influence of hydrothermal fluids on δ30Si in deep water. Key Points Surface waters have a large range in dissolved silicon isotopes covering nutrient‐rich coastal upwelling to oligotrophic waters Deep water masses with DSi concentrations 〉 100 μmol kg‐1 show homogenous silicon isotope signatures despite up to 25 μmol kg‐1 differences in [DSi] Hydrothermal fluids have a negligible effect on Si isotope distributions in the deep Pacific

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: In this study, we used stable isotopes of oxygen (δ18O), deuterium (δD), and dissolved inorganic carbon (δ13CDIC) in combination with temperature, salinity, oxygen and nutrient concentrations to characterize the coastal (71-78 °W) and an oceanic (82-98 °W) water masses (SAAW-Subantarctic Surface Water; STW-Subtropical Water; ESSW-Equatorial Subsurface water; AAIW-Antarctic Intermediate Water; PDW-Pacific Deep Water) of the Southeast Pacific (SEP). The results show that δ18O and δD can be used to differentiate between SAAW-STW, SAAW-ESSW and ESSW-AAIW. δ13CDIC signatures can be used to differentiate between STW-ESSW (oceanic section), SAAW-ESSW, ESSW-AAIW and AAIW-PDW. Compared with the oceanic section, our new coastal section highlights differences in both the chemistry and geometry of water masses above 1000 m. Previous paleoceanographic studies using marine sediments from the SEP continental margin used the present-day hydrological oceanic transect to compare against, as the coastal section was not sufficiently characterized. We suggest that our new results of the coastal section should be used for past characterizations of the SEP water masses that are usually based on continental margin sediment samples.

Description: An extensive data set of biogenic silica (BSi) fluxes is presented for the Peruvian oxygen minimum zone (OMZ) at 11ºS and 12ºS. Each transect extends from the shelf to the upper slope (∼1000 m) and dissects the permanently anoxic waters between ∼200 – 500m water depth. BSi burial (2100 mmol m‐2 yr‐1) and recycling fluxes (3300 mmol m‐2 yr‐1) were highest on the shelf with mean preservation efficiencies (34±15%) that exceed the global mean of 10 – 20%. BSi preservation was highest on the inner shelf (up to 56%), decreasing to 7% and 12% under anoxic waters and below the OMZ, respectively. The data suggest that the main control on BSi preservation is the rate at which reactive BSi is transported away from undersaturated surface sediments by burial and bioturbation to the underlying saturated sediment layers where BSi dissolution is thermodynamically and/or kinetically inhibited. BSi burial across the entire Peruvian margin between 3ºS to 15ºS and down to 1000m water depth is estimated to be 0.1 – 0.2 Tmol yr‐1; equivalent to 2 – 7% of total burial on continental margins. Existing global data permit a simple relationship between BSi rain rate to the seafloor and the accumulation of unaltered BSi, giving the possibility to reconstruct rain rates and primary production from the sediment archive in addition to benthic Si turnover in global models.

Description: Realistic prediction of the near-future response of Arctic Ocean primary productivity to ongoing warming and sea ice loss requires a mechanistic understanding of the processes controlling nutrient bioavailability. To evaluate continental nutrient inputs, biological utilization and the influence of mixing and winter processes in the Laptev Sea, the major source region of the Transpolar Drift, we compare observed with preformed concentrations of dissolved inorganic nitrogen (DIN), phosphorus (DIP), silicic acid (DSi) and silicon isotope compositions of DSi (δ30SiDSi) obtained for two summers (2013, 2014) and one winter (2012). In summer, preformed nutrient concentrations persisted in the surface layer of the southeastern Laptev Sea, while diatom-dominated utilization caused intense northward drawdown and a pronounced shift in δ30SiDSi from +0.91 to +3.82 ‰. The modeled Si isotope fractionation suggests that DSi in the northern Laptev Sea originated from the Lena River during the spring freshet, while in the southeastern Laptev Sea it was continuously supplied by it during the summer. Primary productivity fueled by river-borne nutrients was enhanced by admixture of DIN- and DIP-rich Atlantic-sourced waters to the surface, either by convective mixing during the previous winter or by occasional storm-induced stratification breakdowns in late summer. Substantial enrichments of DSi (+240 %) and DIP (+90 %) beneath the Lena River plume were caused by sea ice-driven redistribution and remineralization. Predicted weaker stratification on the outer Laptev shelf will enhance DSi utilization and removal through greater vertical DIN supply, which will limit DSi export and reduce diatom-dominated primary productivity in the Transpolar Drift. Key Points - Surface DIN, DIP, DSi and Si isotope dynamics are controlled by marine and riverine inputs and uptake by phytoplankton - Strong DIP and DSi enrichments beneath the Lena River plume are due to sea ice-driven nutrient redistribution and remineralization - Enhanced DSi utilization in the Laptev Sea will lead to a reduced diatom-dominated primary productivity in the Transpolar Drift