The trace element composition (Al, Si, P, Ca, and Fe) of suspended particulate matter is presented for the upper 1000 m of the water column along U.S. CLIVAR/CO2 Repeat Hydrography zonal section I05 (2009) in the South Indian subtropical gyre and meridional section I09N/I08S (2007) in the eastern Indian Ocean from the Bay of Bengal to the subtropical front. Along I09N, high particulate Al and Fe concentrations are found throughout the Bay of Bengal and the northern Indian Ocean indicating significant lithogenic particle loads from river-borne sediment transport and smaller contributions from continental dust flux than current model estimates. Particles have elevated Fe:Al ratios compared to the composition of local shelf sediments, indicating that suspended particulate matter becomes enriched from scavenging of dissolved Fe that accumulates in the oxygen minimum zone in the Bay of Bengal. Along I05, retroflection of the Agulhas Return Current transports African shelf-derived lithogenic particles into the western South Indian basin, resulting in the elevated subsurface concentrations of particulate Al and Fe nearly 3000 km into the interior of the subtropical gyre. Another region of elevated dissolved Fe concentrations and Fe-enriched particulate matter was also observed along I05 in intermediate waters (>600 m) in the subtropical gyre at 32(degrees)S between 70 and 74(degrees)E, suggesting inputs from a shallow sedimentary or possibly volcanic Fe source to these stations. Comparison of data from the crossover between the I05 and I09N/ I08S sections and from the 2007 and 2016 occupations of I09N allows investigation of the temporal variability of basin-scale distributions of particle-associated trace metals with relatively short residence times in the upper water column. The most pronounced differences in trace metal distributions are associated with interannual variability in productive near-coastal and upwelling regions where mixed-layer pFe concentrations vary by a factor of 2-7, whereas Fe and Al in deeper waters and dust-impacted surface waters in oligotrophic regions show significantly less variability.

Abstract In addition to high‐temperature vents, lower‐temperature flow (LTF) (<300°C) is abundant along mid‐ocean ridges and contributes globally‐important fluxes of heat and water along with largely‐unconstrained geochemical influences on the ocean. We examined the impact of on‐axis LTF on the chemical composition of the overlying water column (<40 m above seafloor) along the 16.5°–18.0°S sector of the ultrafast‐spreading southern East Pacific Rise using autonomous underwater vehicle Sentry surveys and conductivity‐temperature‐depth rosette casts. LTF sites were typically spatially isolated from high‐temperature systems and imparted unique chemical signatures to the overlying ocean. Water column samples impacted by LTF exhibited low particulate iron:sulfur ratios and high methane: total‐dissolvable manganese ratios, whereas samples influenced by high‐temperature venting exhibited opposite trends. We confirmed that LTF imparts a distinct and measurable chemical signature to the water column, independent from high‐temperature vents. Isolated, on‐axis LTF will be important to consider when assessing hydrothermal circulation impacts upon ocean biogeochemistry.

Transport processes along the river-ocean continuum influence delivery of nutrients, carbon and trace metals from terrestrial systems to the marine environment, impacting coastal primary productivity and water quality. Although trace metal transformations have been studied extensively in the Mississippi River Delta region of the Northern Gulf of Mexico, investigations of manganese (Mn) and the presence of ligand-stabilized, dissolved manganese (Mn(III)-L) and its role in the transformation of trace elements and organic matter during riverine transport and estuarine mixing have not been considered. This study examined the chemical speciation of dissolved and particulate Mn in the water column and sediment porewaters in the Mississippi River and Northern Gulf of Mexico in March of 2021 to explore transformations in Mn speciation along the river-ocean continuum and the impact of different processes on the distribution of Mn. Total dissolved Mn concentrations were highest in the Mississippi River and decreased offshore, while Mn(III)-L contributed most to the dissolved Mn pool in near-shore waters. Porewater profiles indicated that ligand stabilization prevented dissolved Mn(III) reduction below the depth of oxygen penetration and in the presence of equimolar dissolved iron(II). Dissolved Mn(III)-L was enriched in bottom waters at all Northern Gulf of Mexico stations, and diffusive flux modelling of porewater dissolved Mn suggested that reducing sediments were a source of dissolved Mn to the overlying water column in the form of both reduced Mn(II) and Mn(III)-L. A simple box model of the Mn cycle in the Northern Gulf of Mexico indicates that Mn(III)-L is required to balance the Mn budget in this region and is an integral, and previously unconsidered, piece of the Mn cycle in the Northern Gulf of Mexico. The presence of Mn(III)-L in this system likely has an outsized impact on trace element scavenging rates, oxidative capacity, and the carbon cycle that have not been previously appreciated.

Seafloor hydrothermal venting may be an important source of marine Cu and affect the biogeochemical cycling of Cu in the oceans. The distribution of Cu and its isotope compositions (delta Cu-65) can provide insight into seafloor hydrothermal processes and their role in the mass balance of global Cu. To date, there are no published Cu isotope data for hydrothermal plumes and very few reports on Cu concentration distributions. This study presents both Cu concentrations and dissolved Cu isotope (delta Cu-65) in hydrothermal plumes from back-arc volcanoes in the Northeast Lau Basin. The dissolved Cu (dCu) concentrations range from 2.14 to 5.66 nM most of which are higher than the deep seawater concentrations. However, the concentrations for some plume samples were lower than the deep seawater dCu concentrations due to adsorption onto particles in the plumes. The delta Cu-65 of hydrothermal plumes vary from 0.25 to 1.05 parts per thousand. As plumes dispersed, the Cu isotopes from high-temperature Mata Fitu and Mata Ua vents shifted towards light values. In contrast, the delta Cu-65 in plumes from low-temperature East Mata and West Mata vents show increasingly higher values with plume dispersal. Rayleigh distillation models are built based on the adsorption of dCu onto Fe particles and complexation with organic ligands to describe the dCu isotope evolution in hydrothermal plumes. The results suggest that the adsorption and organic complexation may be the likely explanations for the observed dCu isotope compositions in plumes from high- and low-temperature venting, separately, besides the mixing with background seawater. Our measured delta Cu-65 in three fluid samples (0.08, 0.09 and 0.20 parts per thousand) are lower than that of the characterized sinks (similar to 0.3 parts per thousand) in the oceans, indicating that hydrothermal fluids might be a source of light Cu isotopes. However, the delta Cu-65 (0.53 parts per thousand on average) in plume samples are higher than 0.3 parts per thousand and these seafloor hydrothermal plumes do not seem to be a source of light Cu isotope of dCu, at least near the venting sites. Our study first reveals the Cu isotope compositions and evolution in hydrothermal plumes and provides new hints about the impact of seafloor hydrothermal venting on the modern oceanic Cu isotope budget.

Hydrothermal vents have emerged as an important source of iron to seawater, yet only a subset of iron is soluble and persists long enough to be available for surface biological uptake. The longevity and solubility of iron in seawater is governed by strong organic ligands, like siderophores, that are produced by marine microorganisms and are a part of the ocean’s dissolved iron-binding ligand pool. These ligands have been hypothesized to aid in the persistence of dissolved iron in hydrothermal environments. To explore this hypothesis, we measured iron, iron-binding ligands, and siderophores from 11 geochemically distinct sites along a 1,700 km section of the Mid-Atlantic Ridge. Siderophores were found in hydrothermal plumes at all sites, with proximity to the vent playing an important role in dictating siderophore types and diversity. The notable presence of amphiphilic siderophores may point to microbial utilization of siderophores to access particulate hydrothermal iron, and the exchange of dissolved and particulate iron. The tight coupling between strong ligands and dissolved iron within neutrally buoyant plumes across six distinct hydrothermal environments, and the presence of dissolved siderophores with siderophore-producing microbial genera, suggests that biological production of siderophores exerts a key control on hydrothermal dissolved iron concentrations.

The supply of aluminum (Al) and manganese (Mn) to the Gulf of Alaska from coastal sources is poorly constrained. Here, we investigate the seasonality of sources to better constrain Al and Mn cycling in the coastal Gulf of Alaska region and add to our understanding of seasonal and interannual inputs. We examine Mn and Al behavior over the shelf to distinguish between redox-induced release of dissolved trace metals (i.e., diffusion from sediments), sediment resuspension, and meltwater release. Data suggest that, prior to the onset of stratification in the spring, shelf sediment resuspension from deep mixing is an important mechanism for trace metal delivery to surface waters. As spring and summer ensue, increased meltwater discharge coupled with increased surface water temperatures result in stratification of the water column within the coastal Gulf of Alaska, and meltwater becomes a more important source of Al and Mn to the surface waters. The limited data available suggest that a redox-driven flux of Mn from shelf sediments is not as important as the meltwater flux during the summer. In addition, dissolved trace metal concentrations in meltwater-influenced plumes over the shelf exhibit conservative mixing, while particulate trace metal concentrations do not behave conservatively. This indicates that there are different physical controls (particle settling vs. mixing) on the spatial distributions of dissolved and particulate Al and Mn in coastal waters, the manifestation of which are likely highly variable and dependent on the trace metal composition of the river and the hydrodynamics governing this interaction at any given time.

Supply of iron (Fe) to the surface ocean supports primary productivity, and while hydrothermal input of Fe to the deep ocean is known to be extensive it remains poorly constrained. Global estimates of hydrothermal Fe supply rely on using dissolved Fe (dFe) to excess He (xs3He) ratios to upscale fluxes, but observational constraints on dFe/xs3He may be sensitive to assumptions linked to sampling and interpolation. We examined the variability in dFe/xs3He using two methods of estimation, for four vent sites with different geochemistry along the Mid-Atlantic Ridge. At both Rainbow and TAG, the plume was sampled repeatedly and the range of dFe/xs3He was 4 to 63 and 4 to 87 nmol:fmol, respectively, primarily due to differences in plume age. To account for background xs3He and shifting plume position, we calibrated He values using contemporaneous dissolved Mn (dMn). Applying this approach more widely, we found dFe/xs3He ratios of 12, 4–8, 4–44, and 4–86 nmol fmol−1 for the Menez Gwen, Lucky Strike, Rainbow, and TAG hydrothermal vent sites, respectively. Differences in plume dFe/xs3He across sites were not simply related to the vent endmember Fe and He fluxes. Within 40 km of the vents, the dFe/xs3He ratios decreased to 3–38 nmol fmol−1, due to the precipitation and subsequent settling of particulates. The ratio of colloidal Fe to dFe was consistently higher (0.67–0.97) than the deep N. Atlantic (0.5) throughout both the TAG and Rainbow plumes, indicative of Fe exchange between dissolved and particulate phases. Our comparison of TAG and Rainbow shows there is a limit to the amount of hydrothermal Fe released from vents that can form colloids in the rising plume. Higher particle loading will enhance the longevity of the Rainbow hydrothermal plume within the deep ocean assuming particles undergo continual dissolution/disaggregation. Future studies examining the length of plume pathways required to escape the ridge valley will be important in determining Fe supply from slow spreading mid-ocean ridges to the deep ocean, along with the frequency of ultramafic sites such as Rainbow. Resolving the ridge valley bathymetry and accounting for variability in vent sources in global biogeochemical models will be key to further constraining the hydrothermal Fe flux.

As previously summarized by Hammond et al. (2015), from 1983 to 2013, the NOAA Vents program conducted systematic and multidisciplinary exploration, discovery, and research related to hydrothermal vents, submarine volcanic eruptions, and associated ocean physical, chemical, and biological processes. In 2014, Vents divided into two programs, Earth-Ocean Interactions (EOI) and Acoustics, and considered a broader range of questions about how seafloor and subseafloor processes contribute to ocean health, biogeochemical cycles, ecosystem diversity, and climate change. Here, we highlight major accomplishments since 2014, including deep-sea technologies that EOI, Vents, and Pacific Marine Environmental Laboratory (PMEL) Engineering have developed to advance marine science. EOI research is driven by a need for better observational data on issues of global importance, including the role of continental margin seeps in the global methane/carbon cycle, benthic ecology, and fisheries habitat; the role of hydrothermal systems in global biogeochemical cycles, including carbon dioxide removal; the potential impact of deep-sea mining of metal sulfides on ecosystem services provided by hydrothermal vents; and how hydrothermal iron functions as an essential nutrient. NOAA Ocean Exploration, the Schmidt Ocean Institute, the Ocean Exploration Trust, and the National Science Foundation have supported and collaborated in this work. Global exploration of the deep sea with the purpose of understanding global ocean processes remains a cornerstone of EOI science.

Hydrothermal venting impacts the global-scale biogeochemical cycles of many trace metals and their isotopes. Processes in hydrothermal plumes regulate the dispersal of vent-derived metals and may vary in response to differences in the geologic setting of vent fields and/or the geochemistry of the overlying ocean water. Here we present results of analyses of dissolved Fe and Cr concentrations, and dissolved Fe isotope (656Fe) and Cr isotope (653Cr) distributions, in seawater samples collected from above TAG and Rainbow vent sites on the Mid-Atlantic Ridge during the GEOTRACES GA13 cruise. We show that profiles of dissolved Fe and Cr isotopes through the near-field hydrothermal plumes are the mirror image of each other. Oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides account for the low 656Fe values of dissolved Fe, as low as-1.83 %o at TAG and-6.94 %o at Rainbow. Plume samples with low 656Fe values are associated with elevated 653Cr values of dissolved Cr compared to background seawater (by up to +0.14 %o and +0.69 %o at TAG and Rainbow, respectively), while particulate Cr is characterised by relatively low 653Cr values (-1.02 to-1.22 %o). This striking result suggests that seawater Cr(VI) is reduced to Cr(III) and precipitates on the surface of Fe(III) particles within the hydro -thermal plume. Reduction of Cr(VI) and scavenging of Cr(III) by plume Fe-(oxyhydr)oxide particles mean that high-temperature hydrothermal systems are likely a net sink for seawater Cr at Rainbow (and also at TAG). As the removal flux of Cr is related to the flux of hydrothermal Fe(II) and the rate of Fe(II) oxidation in the hy-drothermal plume, it may (i) vary across vent sites at a global scale and (ii) change over glacial-interglacial cycles.

The dispersal of dissolved iron (DFe) from hydrothermal vents is poorly constrained. Combining field observations and a modeling hierarchy, we find the dispersal of DFe from the Trans‐Atlantic‐Geotraverse vent site occurs predominantly in the colloidal phase and is controlled by multiple physical processes. Enhanced mixing near the seafloor and transport through fracture zones at fine‐scales interacts with the wider ocean circulation to drive predominant westward DFe dispersal away from the Mid‐Atlantic ridge at the 100 km scale. In contrast, diapycnal mixing predominantly drives northward DFe transport within the ridge axial valley. The observed DFe dispersal is not reproduced by the coarse resolution ocean models typically used to assess ocean iron cycling due to their omission of local topography and mixing. Unless biogeochemical models account for fine‐scale physics and colloidal Fe, they will inaccurately represent DFe dispersal from axial valley ridge systems, which make up half of the global ocean ridge crest.

Abstract Water column physico‐chemical studies were conducted over the southern Central Indian Ridge between 24°44’S and 25°52’S to identify and chemically characterize seafloor hydrothermal activity. High turbidity values were observed between 2300 and 2700 m with two distinct layers, between water depths of 2320–2500 m and 2510–2650 m, at two closely spaced CTD stations at 24°48.62’S (CTD‐17‐P5) and 24°48.68’S (CTD‐17‐P8). Elevated concentrations of dissolved Mn (DMn: 19–112 nM), dissolved Fe (DFe: 33–88 nM), methane (CH4: 32–246 nM), elevated δ3He values (28%–88%), and stable carbon isotope ratios of CH4 confirm the hydrothermal origin. In plume layer‐1, the maximum concentrations were observed at 2375m at P8 and in plume layer‐2, the maximum concentrations were observed at 2570 m at P5. The stable isotope ratios of methane (δ13C‐CH4) show that heavier isotopes are enriched (−13.2‰ to −14.7‰) in the plume waters and are similar to vent fluids on the global mid‐oceanic ridges. Further, morphological and mineralogical studies of plume particles, collected from the plume layer‐2 maxima, clearly show the presence of barite, pyrite, chalcopyrite, and indicate possible venting of high‐temperature fluids in the vicinity of P5. Enrichment in methane relative to the other tracers and the general geochemical characteristics of these two plume layers, CH4/Mn (1.8–2.2); CH4/Δ3He (85–97 × 106), Mn/Δ3He (44–46 × 106), Fe/Δ3He (52–54 × 106), indicate that these plumes are formed from fluids released at the seafloor that circulated through ultramafic/gabbroic rocks. The high concentrations of dissolved gases and metals combined with the presence of sulfide particles in the water column provide evidence for a new ultramafic/gabbroic‐hosted hydrothermal vent field, at 24°49’S on the southern Central Indian Ridge.

AbstractOver the Ross Sea shelf, annual primary production is limited by dissolved iron (DFe) supply. Here, a major source of DFe to surface waters is thought to be vertical resupply from the benthos, which is assumed most prevalent during winter months when katabatic winds drive sea ice formation and convective overturn in coastal polynyas, although the impact of these processes on water‐column DFe distributions has not been previously documented. We collected hydrographic data and water‐column samples for trace metals analysis in the Terra Nova Bay and Ross Ice Shelf polynyas during April–May 2017 (late austral fall). In the Terra Nova Bay polynya, we observed intense katabatic wind events, and surface mixed layer depths varied from ∼250 to ∼600 m over lateral distances <10 km; there vertical mixing was just starting to excavate the dense, iron‐rich Shelf Waters, and there was also evidence of DFe inputs at shallower depths in the water column. In the Ross Ice Shelf polynya, wind speeds were lower, mixed layers were <300 m deep, and DFe distributions were similar to previous, late‐summer observations, with concentrations elevated near the seafloor. Corresponding measurements of dissolved manganese and zinc, and particulate iron, manganese, and aluminum, suggest that deep DFe maxima and some mid‐depth DFe maxima primarily reflect sedimentary inputs, rather than remineralization. Our data and model simulations imply that vertical resupply of DFe in the Ross Sea occurs mainly during mid‐late winter, and may be particularly sensitive to changes in the timing and extent of sea ice production.

Deep-sea hydrothermal venting is an important source of dissolved iron (dFe) to the oceans. Fe isotopes can be used as a potential tool to trace the dispersal of hydrothermal plumes. However, Fe isotope fractionation and its relation with Fe speciation as hydrothermal plumes disperse is still poorly constrained. In this study, we determined the Fe speciation and total and dissolved Fe isotope composition (delta 56tFe, delta 56dFe) for several hydrothermal plumes from backarc volcanoes in the Northeast Lau Basin. This combined approach provides important insights into the evolution of Fe isotopes in hydrothermal plumes. The results suggest delta(56)tFe variation in plumes is related to the loss of particulate Fe-sulfides or Fe-oxyhydroxides (FeOOH), both of which are dependant on the H2S concentrations and Fe/H2S in the source hydrothermal fluids. delta(56)dFe compositions in the hydrothermal plumes increase during plume dispersal/dilution and can be as high as 0.85 parts per thousand, demonstrating that hydrothermal plumes can export dissolved Fe with a significantly heavier delta(56)dFe than hydrothermal fluids. The reasons may be ascribed to the organic Fe complexes (FeL) and colloidal FeOOH in the dissolved phase. Another interpretation might be associated with the low pH in volcanic arc hydrothermal systems rich in magmatic CO2 and SO2, which decreases the Fe(II) oxidation rate. Further, we demonstrate for the first time that the delta(56)dFe is positively correlated with the conditional stability constants of FeL (logK'(FeL)). A Rayleigh distillation model is presented based on the mass balance of the determined FeL, and colloidal FeOOH in hydrothermal plumes, which can explain the observed Fe isotope compositions in hydrothermal plumes. Our data show how Fe isotopes are transformed within a hydrothermal plume above arc volcanoes and how these may differ from that of the original vent fluids. It adds to our understanding of the processes that have an impact on the Fe speciation and isotope composition in deep-sea hydrothermal plumes.(c) 2022 Elsevier Ltd. All rights reserved.

Hydrothermal iron supply contributes to the Southern Ocean carbon cycle via the regulation of regional export production. However, as hydrothermal iron input estimates are coupled to helium, which are uncertain depending on whether helium inputs are based on ridge spreading rates or inverse modelling, questions remain regarding the magnitude of the export production impacts. A particular challenge is the limited observations of dissolved iron (dFe) supply from the abyssal Southern Ocean ridge system to directly assess different hydrothermal iron supply scenarios. We combine ocean biogeochemical modelling with new observations of dFe from the abyssal Southern Ocean to assess the impact of hydrothermal iron supply estimated from either ridge spreading rate or inverse helium modelling on Southern Ocean export production. The hydrothermal contribution to dFe in the upper 250 m reduces 4–5 fold when supply is based on inverse modelling, relative to those based on spreading rate, translating into a 36–73% reduction in the impact of hydrothermal iron on export production. However, only the spreading rate input scheme reproduces observed dFe anomalies >1 nM around the circum-Antarctic ridge. The model correlation with observations drops 3 fold under the inverse modelling input scheme. The best dFe scenario has a residence time for hydrothermal iron that is between 21 and 34 years, highlighting the importance of rapid physical mixing to surface waters. Overall, because of its short residence time, hydrothermal Fe supplied locally by circum-Antarctic ridges is most important to the Southern Ocean carbon cycle and our results highlight decoupling between hydrothermal iron and helium supply.

Abstract. Supply of iron (Fe) to the surface ocean supports primary productivity and while hydrothermal input of Fe to the deep ocean is known to be extensive, it remains poorly constrained. Global estimates of hydrothermal Fe supply rely on using the dissolved Fe (dFe) to excess He (xs3He) ratios to upscale fluxes, but observational constraints on dFe / xs3He may be sensitive to assumptions linked to sampling and interpolation. We examined the variability in dFe / xs3He using two methods of estimation, for four vent sites with different geochemistry along the Mid-Atlantic Ridge. At both Rainbow and TAG, the plume was sampled repeatedly and the range of dFe / xs3He was 4 to 63 and 4 to 87 nmol/fmol, respectively, primarily due to differences in plume age. To account for background xs3He and shifting plume position, we calibrated He values using contemporaneous dissolved Mn (dMn). Applying this approach more widely, we found dFe / xs3He ratios of 12, 4–8, 4–44, 4–86 nmol/fmol for the Menez Gwen, Lucky Strike, Rainbow and TAG hydrothermal vent sites, respectively. Differences in plume dFe / xs3He across sites were not simply related to the vent end member Fe and He fluxes. Within 40 km of the vents, the dFe / xs3He ratios decreased to 3-38 nmol/fmol, due to the precipitation and subsequent settling of particulates. The ratio of colloidal Fe to dFe was consistently higher (0.67–0.97) than the deep N. Atlantic (0.5) throughout both the TAG and Rainbow plumes, indicative of Fe exchange between dissolved and particulate phases. Our comparison of TAG and Rainbow shows there is a limit to the amount of hydrothermal Fe released from vents that can form colloids in the rising plume. Higher particle loading will enhance the longevity of the Rainbow hydrothermal plume within the deep ocean assuming particles undergo continual dissolution/disaggregation. Future studies examining the length of plume pathways required to escape the ridge valley will be important in determining Fe supply from slow spreading mid-ocean ridges to the deep ocean, along with the frequency of ultramafic sites such as Rainbow. Resolving the ridge valley bathymetry and accounting for variability in vent sources in global biogeochemical models will be key to further constraining the hydrothermal Fe flux.

The concentrations of Rare Earth Elements (REEs) in the ocean reflect riverine, aeolian and hydrothermal inputs as well as internal cycling, removals, and transport. The chemical behaviour of this set of elements is generally similar but with subtle, predictable variations, which allow us to separate entangled physicochemical processes. Hydrothermal fluids are enriched in REEs with respect to seawater, particularly in Eu, but due to efficient scavenging by plume particles, hydrothermal vents are considered a net sink for REEs in the ocean, and an unlikely influence on wider seawater REE patterns [1]. Elderfield et al., however, hypothesised that the Eu/Sm ratio might record hydrothermal input to seawater [2]; Europium anomaly (Eu/Eu*) could be used to similar effect [3]. Our ability to assess the impact of hydrothermalism on Atlantic REE distributions has long been hindered by a paucity of dissolved REE measurements near hydrothermal vent sites. Here we present dissolved REE concentrations in deep water samples, collected from the GA13 transect at a number of vent sites along the Mid-Atlantic Ridge (MAR), and measured by Elemental Scientific seaFAST ICPMS. We reveal strong excursions in Eu/Eu* at depths in the water column that are coincident with excursions in the plume tracers dissolved Mn and 3 He at TAG vent site. Plume Eu/Eu* excursions are also present 30 km from the main vent site, providing evidence that the strong Eu anomaly present in TAG hydrothermal fluids is not wholly removed from the dissolved fraction by plume scavenging processes, and may impact REE