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Processes controlling dissolved barium (dBa) were investigated along the GEOTRACES GA03 North Atlantic and GP16 Eastern Tropical Pacific transects, which traversed similar physical and biogeochemical provinces. Dissolved Ba concentrations are lowest in surface waters (∼35–50 nmol kg⁻¹) and increase to 70–80 and 140–150 nmol kg⁻¹ in deep waters of the Atlantic and Pacific transects, respectively. Using water mass mixing models, we estimate conservative mixing that accounts for most of dBa variability in both transects. To examine nonconservative processes, particulate excess Ba (pBa_(xs)) formation and dissolution rates were tracked by normalizing particulate excess ²³⁰Th activities. Th-normalized pBa_(xs) fluxes, with barite as the likely phase, have subsurface maxima in the top 1,000 m (∼100–200 μmol m⁻² year⁻¹ average) in both basins. Barite precipitation depletes dBa within oxygen minimum zones from concentrations predicted by water mass mixing, whereas inputs from continental margins, particle dissolution in the water column, and benthic diffusive flux raise dBa above predications. Average pBa_(xs) burial efficiencies along GA03 and GP16 are ∼37% and 17%–100%, respectively, and do not seem to be predicated on barite saturation indices in the overlying water column. Using published values, we reevaluate the global freshwater dBa river input as 6.6 ± 3.9 Gmol year⁻¹. Estuarine mixing processes may add another 3–13 Gmol year⁻¹. Dissolved Ba inputs from broad shallow continental margins, previously unaccounted for in global marine summaries, are substantial (∼17 Gmol year⁻¹), exceeding terrestrial freshwater inputs. Revising river and shelf dBa inputs may help bring the marine Ba isotope budget more into balance.

Dissolved Mn(II) in the hydrothermal plume is known to be microbially oxidized to form Mn(III/IV) oxides, and the Mn oxides scavenge other trace elements in seawater. In the GEOTRACES GP16 cruise, dissolved Mn (dMn) and particulate Mn (pMn) were found to be transported over 4000 km westwards from the Southern Eastern Pacific Rise. Previous studies in this plume showed different removal rates of dMn and pMn as well as pMn size distribution between the near-field (80 km) plumes. In order to understand Mn cycling in these plumes, spatial distribution, oxidation states, and mineral structures of Mn in small size fraction (SSF; 0.8–51 μm) and large size fraction (LSF; >51 μm) particles from the near-field and far-field plumes were examined using micro X-ray fluorescence spectrometry (μ-XRF), X-ray absorption near-edge structure spectroscopy (XANES), chemical species mapping, and extended X-ray absorption fine-structure spectroscopy (EXAFS). In the near-field plume, pMn in the SSF is dominated by oxidized Mn with Mn(III) fractions of ∼30%. They are a mixture of δ-MnO₂ and triclinic birnessites that is known to be formed as a result of autocatalytic Mn(II) oxidation at the surface of freshly-formed δ-MnO₂, suggesting that both microbial and autocatalytic Mn oxidation occur in the near-field plume. The LSF pMn in the near-field plume is also oxidized and often found in large aggregates several hundreds of μm in size. These aggregates settle out in the near-field and during transport, and are not found in the far-field plume. In the far-field plume where Mn oxides are not newly formed, pMn in the SSF is oxidized, but their Mn(III) fractions are smaller than in the near-field pMn. Unlike the SSF, the far-field plume LSF pMn is dominated by reduced Mn, implying very slow aggregation of pMn in the far-field plume. The different characteristics of pMn between the near-field and far-field plumes affect its scavenging of other trace elements. In the near-field plume, Co, Mo, ²³¹Pa are associated with pMn, but not in the far-field plume. ²³¹Pa is adsorbed to pFe rather than pMn in the far-field plume, and Pb is adsorbed to pFe in the entire plume. The result shows that freshly-formed Mn oxides in the near-field plume have higher scavenging capacity than the far-field plume pMn. Our findings suggest that the mineralogical age of Mn oxides may be an important parameter that controls the scavenging of many other trace elements and isotopes.

Early studies revealed relationships between barium (Ba), particulate organic carbon and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. But, what controls the distribution of barium (Ba) in the oceans? Herein, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba ratio (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ∼50% of the dBa inventory (upper 500 m of the Arctic water column) was supplied by nonconservative inputs. Calculated areal dBa fluxes are up to 10 µmol m-2 d-1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. The Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin Bay-derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a sedimentary remineralization and terrigenous sources (e.g., submarine groundwater discharge or fluvial particles) are contributors. Plain Language Summary We investigated the barium cycle in the Arctic Ocean. The oceanic barium cycle is supported by the interplay of seawater mixing, river inputs, sediment inputs, and particle formation and export from the water column. We determined that the distribution of dissolved barium in the upper 500 m of the Arctic Ocean is largely set by a shelf sediment source; this is newly described, as previous literature assumed rivers and seawater mixing were the predominant contributors to the distribution. This discovery fits in with recent findings that the shelf sediments are a major source of radium and other trace metals to the surface Arctic Ocean. This is important to consider as the warming climate continues to erode Arctic ice cover (sea ice or glacial). Monitoring the relative sources of Ba to the water column can help define how warming impacts Arctic Ocean biogeochemistry.

Despite decades of research, barite formation in the ocean water column has been widely discussed since most of the world´s ocean mesopelagic zone, in which barite forms, is generally undersaturated with respect to this mineral. Recent evidence from experimental work and also from observations in microenvironments of intense organic matter mineralization in the ocean support that barite forms via transient amorphous precursor phases that evolve to barite crystals. This crystallization pathway is further supported by the close association of barite particles with extracellular polymeric substances (EPS) at depths of higher bacterial production. Barite particles association with exopolymers demonstrates that microbial processes and exopolymer production play a major role in promoting locally high concentrations of Ba and barite precipitation. Scanning and high-resolution transmission electron microscopy analyses from particulate samples collected using multiple-unit large volume in-situ filtration systems have shown how these amorphous precursor phase nucleate, demonstrating that phosphate groups in EPS and bacterial cells are the sites for binding Ba. EDX maps have shown the nature of these P-rich nanometer-sized amorphous particles that evolve to poorly crystallized barite and to micrometer-sized barite crystals. The strong link between organo-mineralization and microbial processes further supports the role that such processes play in biomineralization in the ocean. The distribution of particulate Ba and Ba isotopes in the water column is also consistent with such precipitation mechanisms. Hence, processes involved in barite precipitation including primary production, export production, organic matter degradation, bacterial respiration, EPS formation, aggregation, and sinking, need to be taken into account when interpreting temporal and spatial variability in the Ba:Corg ratios and barite accumulation in marine sediments.

Deep-sea hydrothermal vents are a source of Fe to the ocean with potential impact on surface ocean primary productivity. Long-range horizontal transport of hydrothermally derived Fe in suspended pa...

The sinking of organic matter to the deep ocean leaves extremely low concentrations of major and trace nutrients for photosynthetic organisms at the sunlit surface. As a result, marine phytoplankton make use of alternative sources of essential elements and have evolved to substitute some elements by others in various biochemical processes. A particularly intriguing example is that of Zn, which is used in many biochemical functions but is often depleted down to picomolar concentrations in surface seawater. Laboratory data show that many phytoplankton species are able to achieve high growth rates by replacing Zn with Cd or Co in cultures. One documented biochemical replacement occurs in some carbonic anhydrases that are used in the acquisition of inorganic carbon for photosynthesis. Field data show the existence of such enzymes in surface seawater and indicate a replacement of Zn by Cd and Co in the surface waters of the eastern tropical South Pacific. Those results point at interesting opportunities for future research. ▪ The dearth of essential elements in surface seawater has caused marine phytoplankton to substitute some trace metals by others in various biochemical processes. ▪ Many species can substitute Cd and/or Co for Zn as a metal center in carbonic anhydrase enzymes that are used in the acquisition of inorganic carbon for photosynthesis. ▪ Field data show the presence of such enzymes in the sea and indicate a replacement of Zn by Cd and Co in the surface upwelling waters of the eastern tropical South Pacific. ▪ New analytical and molecular tools provide opportunities to elucidate the unusual biochemistry of marine phytoplankton.

Particle cycling rates in marine systems are difficult to measure directly, but of great interest in understanding how carbon and other elements are distributed throughout the ocean. Here, rates of particle production, aggregation, disaggregation, sinking, remineralization, and transport mediated by zooplankton diel vertical migration were estimated from size-fractionated measurements of particulate organic carbon (POC) concentration collected during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise at Station P in summer 2018. POC data were combined with a particle cycling model using an inverse method. Our estimates of the total POC settling flux throughout the water column are consistent with those derived from thorium-234 disequilibrium and sediment traps. A budget for POC in two size fractions, small (1–51 µm) and large (> 51 µm), was produced for both the euphotic zone (0–100 m) and the upper mesopelagic zone (100–500 m). We estimated that POC export at the base of the euphotic zone was 2.2 ± 0.8 mmol m−2 d−1, and that both small and large particles contributed considerably to the total export flux along the water column. The model results indicated that throughout the upper 500 m, remineralization leads to a larger loss of small POC than does aggregation, whereas disaggregation results in a larger loss of large POC than does remineralization. Of the processes explicitly represented in the model, zooplankton diel vertical migration is a larger source of large POC to the upper mesopelagic zone than the convergence of large POC due to particle sinking. Positive model residuals reveal an even larger unidentified source of large POC in the upper mesopelagic zone. Overall, our posterior estimates of particle cycling rate constants do not deviate much from values reported in the literature, i.e., size-fractionated POC concentration data collected at Station P are largely consistent with prior estimates given their uncertainties. Our budget estimates should provide a useful framework for the interpretation of process-specific observations obtained by various research groups in EXPORTS. Applying our inverse method to other systems could provide insight into how different biogeochemical processes affect the cycling of POC in the upper water column.

Knowledge of the chemical speciation of particulate manganese (pMn) is important for understanding the biogeochemical cycling of Mn and other particle-reactive elements. Here, we present the synchr...

Over the past decade, the GEOTRACES and wider trace metal geochemical community have made substantial contributions towards constraining the marine cobalt (Co) cycle and its major biogeochemical processes. However, few Co speciation studies have been conducted in the North and equatorial Pacific Ocean, a vast portion of the world’s oceans by volume and an important endmember of deep thermohaline circulation. Dissolved Co (dCo) samples, including total dissolved and labile Co, were measured at-sea during the GEOTRACES Pacific meridional transect (GP15) along the 152° W longitudinal from 56° N to 20° S. Along this transect, upper ocean dCo was linearly correlated to dissolved phosphate (slope = 82 ± 2 µM:M) due to phytoplankton uptake and remineralization. As depth increased, dCo concentrations became increasingly decoupled from phosphate concentrations due to co-scavenging with manganese oxide particles in the mesopelagic. The transect revealed an organically-bound coastal source of dCo to the Alaskan Stream associated with low salinity waters. An intermediate-depth hydrothermal flux of dCo was observed off the Hawaiian coast at the Loihi Seamount, and the elevated dCo was correlated to estimated xs3He at and above the vent site; however, the Loihi Seamount likely did not represent a major source of Co to the Pacific basin. Elevated concentrations of dCo within oxygen minimum zones (OMZs) in the equatorial North and South Pacific were consistent with the suppression of oxidative scavenging, and we estimate that future deoxygenation could increase the OMZ dCo inventory by 13–28 % over the next century. In North Pacific Deep Water (NPDW), a fraction of elevated ligand-bound dCo appeared protected from scavenging by the high biogenic particle flux in the North Pacific basin. This finding is counter to previous expectations of low dCo concentrations in the deep Pacific due to scavenging over thermohaline circulation. Compared to a Co global biogeochemical model, the observed transect displayed more extreme inventories and fluxes of dCo than predicted by the model, suggesting a highly dynamic Pacific Co cycle.

In order to better understand the sources, sinks and hydrodynamic/biogeochemical influences on particulate matter distribution and variability in Arctic basins, we combined data from two 2015 fall ...

International audience; The goal of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign is to develop a predictive understanding of the export, fate, and carbon cycle impacts of global ocean net primary production. To accomplish this goal, observations of export flux pathways, plankton community composition, food web processes, and optical, physical, and biogeochemical (BGC) properties are needed over a range of ecosystem states. Here we introduce the first EXPORTS field deployment to Ocean Station Papa in the Northeast Pacific Ocean during summer of 2018, providing context for other papers in this special collection. The experiment was conducted with two ships: a Process Ship, focused on ecological rates, BGC fluxes, temporal changes in food web, and BGC and optical properties, that followed an instrumented Lagrangian float; and a Survey Ship that sampled BGC and optical properties in spatial patterns around the Process Ship. An array of autonomous underwater assets provided measurements over a range of spatial and temporal scales, and partnering programs and remote sensing observations provided additional observational context. The oceanographic setting was typical of late-summer conditions at Ocean Station Papa: a shallow mixed layer, strong vertical and weak horizontal gradients in hydrographic properties, sluggish sub-inertial currents, elevated macronutrient concentrations and low phytoplankton abundances. Although nutrient concentrations were consistent with previous observations, mixed layer chlorophyll was lower than typically observed, resulting in a deeper euphotic zone. Analyses of surface layer temperature and salinity found three distinct surface water types, allowing for diagnosis of whether observed changes were spatial or temporal. The 2018 EXPORTS field deployment is among the most comprehensive biological pump studies ever conducted. A second deployment to the North Atlantic Ocean occurred in spring 2021, which will be followed by focused work on data synthesis and modeling using the entire EXPORTS data set.

Particulate organic carbon (POC) produced in the surface ocean sinks through the water column and is respired at depth, acting as a primary vector sequestering carbon in the abyssal ocean. Atmospheric carbon dioxide levels are sensitive to the length (depth) scale over which respiration converts POC back to inorganic carbon, because shallower waters exchange with the atmosphere more rapidly than deeper ones. However, estimates of this carbon regeneration length scale and its spatiotemporal variability are limited, hindering the ability to characterize its sensitivity to environmental conditions. Here, we present a zonal section of POC fluxes at high vertical and spatial resolution from the GEOTRACES GP16 transect in the eastern tropical South Pacific, based on normalization to the radiogenic thorium isotope (230)Th. We find shallower carbon regeneration length scales than previous estimates for the oligotrophic South Pacific gyre, indicating less efficient carbon transfer to the deep ocean. Carbon regeneration is strongly inhibited within suboxic waters near the Peru coast. Canonical Martin curve power laws inadequately capture POC flux profiles at suboxic stations. We instead fit these profiles using an exponential function with flux preserved at depth, finding shallow regeneration but high POC sequestration below 1,000 m. Both regeneration length scales and POC flux at depth closely track the depths at which oxygen concentrations approach zero. Our findings imply that climate warming will result in reduced ocean carbon storage due to expanding oligotrophic gyres, but opposing effects on ocean carbon storage from expanding suboxic waters will require modeling and future work to disentangle.

The downward flux of sinking particles is a prominent Hg removal and redistribution process in the ocean; however, it is not well-constrained. Using data from three U.S. GEOTRACES cruises including the Pacific, Atlantic, and Arctic Oceans, we examined the mercury partitioning coefficient, Kd, in the water column. The data suggest that the Kd varies widely over three ocean basins. We also investigated the effect of particle concentration and composition on Kd by comparing the concentration of small-sized (1–51 μm) suspended particulate mass (SPM) as well as its compositional fractions in six different phases to the partitioning coefficient. We observed an inverse relationship between Kd and suspended particulate mass, as has been observed for other metals and known as the “particle concentration effect,” that explains much of the variation in Kd. Particulate organic matter (POM) and calcium carbonate (CaCO3) dominated the Hg partitioning in all three ocean basins while Fe and Mn could make a difference in some places where their concentrations are elevated, such as in hydrothermal plumes. Finally, our estimated Hg residence time has a strong negative correlation with average log bulk Kd, indicating that Kd has significant effect on Hg residence time.

Particle composition is an important parameter that influences sinking velocity of marine particles. Most current studies, however, are limited by either a lack of routine measurements of particle ...

Mechanisms underlying barite precipitation in seawater and the precise depths of barite precipitation in the water column have been debated for decades. Here we present a detailed study of water column barite distribution in the mesopelagic zone at diverse stations in the open ocean by analyzing samples collected using multiple unit large volume in-situ filtration systems in the Pacific, Atlantic and Indian oceans. Our results demonstrate that barite is an organo-mineral particularly abundant at intermediate depths throughout the world’s ocean regardless of saturation state with respect to barite. This is confirming the notion of precipitation at depths of intense organic matter mineralization. Our observations further support the link between barite formation and microbial activity, demonstrated by the association of barite particles with organic matter aggregates and with extracellular polymeric substances. Evidence for microbial mediation is consistent with previous experimental work showing that in bacterial biofilms Ba binds to phosphate groups on cell surfaces and within extracellular polymeric substances. This organoaccumulation promotes high concentrations of Ba leading to saturated microenvironments and nucleation sites favoring precipitation. The distribution of Ba isotopes in the water column and in particulate matter is also consistent with the proposed precipitation mechanism., European Union (EU), Agencia Estatal de Investigacion, Ministerio de Ciencia e Innovacion, Spain CGL2017-92600-EXP PID2019-104624RB-I00, Junta de Andalucia RNM-179 BIO 103 P18-RT-3804 P18-RT-4074, Unidad Cientifica de Excelencia (University of Granada) UCE-PP2016-05 OCE-1443577

Although iron availability has been shown to limit ocean productivity and influence marine carbon cycling, the rates of processes driving iron's removal and retention in the upper ocean are poorly constrained. Using 234Th‐ and sediment‐trap data, most of which were collected through international GEOTRACES efforts, we perform an unprecedented observation‐based assessment of iron export from and residence time in the upper ocean. The majority of these new residence time estimates for total iron in the surface ocean (0‐250 m) fall between 10 and 100 days. The upper ocean residence time of dissolved iron, on the other hand, varies and cycles on sub‐annual to annual timescales. Collectively, these residence times are shorter than previously thought, and the rates and timescales presented here will contribute to ongoing efforts to integrate iron into global biogeochemical models predicting climate and carbon dioxide sequestration in the ocean in the 21st century and beyond. Plain Language Summary Iron is a key micronutrient for organisms living in the upper ocean and thus, its availability is one of the key factors controlling the removal of carbon dioxide via phytoplankton growth in much of the global ocean. Until very recently, measurements of internal iron cycling were scarce. This includes estimates of how much iron leaves the surface ocean via sinking particles. Due to the lack of observations, models struggle to reproduce observed patterns in global surface iron distributions. For the first time, we constrain the rate of iron loss from the upper ocean along three basin‐wide transects and bring together all preexisting estimates to determine the timescales on which different forms of iron are retained in the upper ocean. Overall, our findings suggest that iron cycles more rapidly between the surface and the subsurface ocean than previously estimated and we encourage the modeling community to utilize the wealth of data presented here to explore the global consequences of these findings.

The processes of particle aggregation and disaggregation are of paramount importance for ocean biogeochemical cycles. Particle aggregation leads to a transfer of particulate material and of their chemical constituents into large size fractions that can settle rapidly through the water column, thereby contributing to the ocean biological pump. In contrast, particle disaggregation redistributes material into smaller size classes and places limits on the size of the largest aggregates. In spite of their preeminent importance for ocean biogeochemistry, rates of particle (dis)aggregation in the ocean cannot be measured directly and are notoriously difficult to constrain. Indeed, current estimates obtained in a variety of oceanographic environments range over several orders of magnitude and suffer from appreciable uncertainties.The goal of the Export Processes in the Ocean from Remote Sensing (EXPORTS) program is to develop a predictive understanding of the export and fate of global ocean net primary production for present and future climates. As part of this program, an extensive oceanographic campaign took place in summer 2018 in the Gulf of Alaska, during which various measuring and sampling platforms including a large-volume filtration (LVF) system have been deployed at different depths in the euphotic and mesopelagic zones at stations centered around a drifting Lagrangian float. Here we present the status of our ongoing effort to estimate the rates of particle (dis)aggregation in the mesopelagic zone of EXPORTS stations based on concomitant measurements of the concentration of particulate organic carbon (POC), lithogenic elements (Al and Ti), and thorium-234 (a naturally-occurring particle-reactive radionuclide), in different size fractions sampled from LVF and bottles. The rates of particle (dis)aggregation, as well as remineralization and sinking, are estimated from the quantitative combination of this diverse dataset with a simplified model of the cycling of POC, Al, Ti, and Th-234 in the upper water column using a least-squares procedure that accounts for both data and model errors. Rate estimates and their errors obtained at different stations and at different depths in the upper 500 m are presented and discussed in the context of independent measurements bearing on the mesopelagic ecosystem of the eastern North Pacific.

Data from GEOTRACES cruises GN01 (HLY1502) and GN04 (PS94) have been archived at the Biological & Chemical Oceanography Data Management Office (BCO-DMO; https://www.bco-dmo.org/deployment/638807) and PANGAEA (https://www.pangaea.de/?q=PS94&f.campaign%5B%5D=PS94) websites, respectively. The inorganic carbon data are available at the NOAA Ocean Carbon Data System (OCADS ; doi :10.3334/CDIAC/OTG.CLIVAR_ARC01_33HQ20150809). In addition to these national data archives, the data used in this paper is available as a supplementary downloadable excel file; International audience; A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river‐influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high‐resolution pan‐Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and ~25‐50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle‐reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 ± 0.4 Sv (106 m3 s‐1). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.

Goldschmidt 2020 Online, 21-26 june 2020, [EN] The reliability of pelagic barite as a proxy for productivity and past oceanic conditions has been broadly demonstrated. However, despite decades of research, processes leading to barite precipitation in the undersaturated marine water column have not been clearly identified. Recent experimental work has shown that extracellular polymeric substances are capable of bioaccumulating Ba providing adequate conditions for barite precipitation. These experiments demonstrated that Ba is initially bound to phosphate groups in bacterial cells and exopolymers. Diverse compositions were observed in barite particles formed in laboratory experiments from the initial P-rich precipitates to intermediate phases and the almost complete substitution of phosphate by sulfate, resulting in the formation of barite crystals. Subsequent mineralogical and crystallographic analyses of barite from size-fractionated particulate material from the ocean water column in the Atlantic have also supported this hypothesis by demonstrating that marine barite in the ocean also forms from an initial amorphous P-rich phase that eventually evolves into barite. New studies at other ocean sectors have further supported this mechanism as responsible for pelagic barite formation. Barite particles are commonly associated with exopolymers evidencing that microbial processes and exopolymer production play an essential role in promoting locally high concentrations of Ba and barite precipitation through an amorphous P-rich precurso

We observed large water column anomalies in helium isotopes and trace metal concentrations above the Loihi Seamount. The 3He/4He of the added helium was 27.3 times the atmospheric ratio, clearly marking its origin to a primitive mantle plume. The dissolved iron to 3He ratio (dFe:3He) exported to surrounding waters was 9.3 ± 0.3 × 10 6 . We observed the Loihi 3He and dFe “signal” at a depth of 1100 m at several stations within ∼100 – 1000 km of Loihi, which exhibited a distal dFe:3He ratio of ∼ 4 × 10 6 , about half the proximal ratio. These ratios were remarkably similar to those observed over and near the Southern East Pacific Rise (SEPR) despite greatly contrasting geochemical and volcanic-tectonic origins. In contrast, the proximal and distal dMn:3He ratios were both ∼ 1 × 10 6 , less than half of that observed at the SEPR. Dissolved methane was minimally enriched in waters above Loihi Seamount and was distally absent. Using an idealized regional-scale model we replicated the historically observed regional 3He distribution, requiring a hydrothermal 3He source from Loihi of 10.4 ± 4.2 mol a−1, ∼2% of the global abyssal hydrothermal 3He flux. From this we compute a corresponding dFe flux of ∼40 Mmol a−1. Global circulation model simulations suggest that the Loihi-influenced waters eventually upwell along the west coast of North America, also extending into the shallow northwest Pacific, making it a possibly important determinant of marine primary production in the subpolar North Pacific.

The dependence of thorium scavenging by particles on particle composition is examined at selected stations of the U.S. GEOTRACES North Atlantic Section (GA03). Scavenging is here described by the apparent, first-order rate constant of Th adsorption onto particles ( k 1 ), as estimated from an inversion of Th radioisotope and radioactive parent data. Our k 1 estimates are regressed against particle phase data using two different models. Model I considers biogenic particles (POC + PIC + bSi), lithogenic particles, Mn (oxyhydr)oxides, and Fe (oxyhydr)oxides as regressors, and k 1 as the regressand. Model II considers ln(POC + PIC + bSi), ln(lithogenic particles), ln(Mn (oxyhydr)oxides), and ln(Fe (oxyhydr)oxides) as regressors, and ln( k 1 ) as the regressand, where ln() denotes the natural logarithm. Thus, models I and II posit that the effects of particle phases on k 1 are, respectively, additive and multiplicative. These models are applied to three groups of stations: (i) all selected stations, (ii) stations west of the Mauritanian upwelling region (“western stations”), and (iii) stations within that region (“eastern stations”). We find that model II appears to better describe the effect of particle composition on k 1 than model I. Particle composition explains a larger fraction of the variance of k 1 for the eastern stations ( R 2 = 0.60 for model I and 0.67 for model II) than for the western stations ( R 2 = 0.26 for model I and 0.39 for model II). When considering all stations, the variance of k 1 explained by particle composition is intermediate ( R 2 = 0.50 for model I and 0.51 for model II). According to model II, the variance of k 1 explained by particle composition is predominantly due to biogenic particles at the eastern stations and to Mn (oxyhydr)oxides at the western stations. Additionally, we find that particle composition does not explain a significantly different proportion of variance of k 1 than particle concentration. It is thus concluded that, at our selected stations, (i) biogenic particles and Mn (oxyhydr)oxides more strongly influence Th scavenging than any other phases considered and (ii) particle composition and particle concentration have comparable effects on this process.

Author(s): Bourne, HL; Bishop, JKB; Lam, PJ; Ohnemus, DC | Abstract: Concentrations of Cd and P were determined in particle samples collected using the multiple unit large volume in situ filtration system (MULVFS) from 50 profiles at 34 different locations throughout the Atlantic, Pacific, and Southern Oceans since 1991. Consistent methodology has been used. This data set of Cd:P in size fractionated particles gives insight into the processes that lead to differences in regional Cd:P particle values as well as how the formation and remineralization of these particles lead to dissolved deep water ratios that increase from the North Atlantic to the North Pacific. With large spatial and temporal variation, this data set allows us to study the effects of an El Nino, upwelling, large-scale in situ Fe fertilization, low-oxygen conditions, and seasonal variation on the Cd:P in particles. Overall, Cd:P tends to be higher (~1–2nmmol/mol) in particles gathered in biologically dynamic waters and is much lower (typically ~0.1nmmol/mol) in oligotrophic regions. Using multiple linear regression analysis, we investigate how euphotic zone parameters important to photosynthesis including nitrate, phosphate, silicate, temperature, and euphotic zone depth affect the Cd:P ratio in particles. Using the results of the analysis, we create global seasonal maps of predicted particulate Cd:P distributions. We find that three factors—local dissolved nitrate, silicate concentrations, and euphotic zone depth—can predict 59% of the variation in particulate Cd:P. We verified our projections using in situ filtration samples collected during GEOTRACES expeditions GA03 (North Atlantic) and GP16 (South Pacific).

The basin-scale distributions of iron (Fe) and Fe isotopes provide important insights into the biogeochemical cycling of this growth-limiting micronutrient in the ocean. Here we present new observations of dissolved Fe concentrations and stable isotope ratios (δ56Fe) from the US GEOTRACES Eastern Pacific Zonal Transect GP16. The western portion of the transect is characterized by low dissolved Fe concentrations with a heavy δ56Fe signature of + 0.4 to + 0.6‰, similar to the dust-influenced North Atlantic deep waters. This is punctuated by Fe inputs from hydrothermal vents along the East Pacific Rise, with a δ56Fe of − 0.3‰. One striking feature of the transect is a large plume of high dissolved Fe and low δ56Fe (0 to − 0.5‰) in the east, near the Peru margin. Here, maximum dissolved Fe occurs between 1000 and 3000 m and the elevated concentrations persist over 1000 km from the margin. The region of markedly lower δ56Fe extends even further, to roughly 4000 km offshore. The mid-slope depth at which this plume occurs (1000–3000 m) is at odds with current conceptual and numerical models of Fe inputs along continental margins, which predict a shallower and more restricted dissolved Fe maximum (upper-slope; ~ 100–1000 m). Here, we explore four possible explanations for the mid-slope Fe plume: (1) Fe fluxes are actually higher from mid-slope sediments; (2) the mid-slope plume is transported from a remote region (3) mid-slope Fe originates from resuspended sediments in a very persistent form, which remains in the dissolved phase for longer than Fe released from the upper-slope; (4) Fe is supplied from upper-slope sediments, and then transferred to greater depth by reversible scavenging onto sinking particles. Simple modeling is used to show that both input of persistent Fe from mid-slope sediments and reversible scavenging could explain the data. Flux of a more persistent chemical form of Fe from the mid-slope would be consistent with other tracers such as particle composition and 228Ra, which suggest that lithogenic sediments are preferentially resuspended at this depth, but may be at odds with the low δ56Fe signature. Reversible scavenging is consistent with both Fe concentrations and δ56Fe. Whatever its provenance, the plume observed near the Peru margin impacts Fe concentrations and δ56Fe throughout the entire eastern South Pacific region, suggesting that the roles of persistent Fe input and reversible scavenging should be appraised in fully coupled iron-carbon cycle models in order to better understand the global cycling of Fe and δ56Fe.

Particles play an important role in the cycling of many trace elements in the ocean by transporting them to the deep sea. As a particle's settling speed and rates of interaction with dissolved species and other particles depend partly on its size, it is essential to understand the distribution of trace elements in small (suspended) vs. large (sinking) particles and its controlling factors. Here we present the size-distribution of ten particulate trace elements (pTEs; Al, Cd, Co, Cu, Fe, Mn, Ni, P, Ti, and V) from the Eastern Tropical South Pacific collected during the U.S. GEOTRACES GP16 cruise. The surface waters at all stations (surface), subsurface waters west of 100°W (subsurface), oxygen minimum zone (OMZ) off Peru, hydrothermal plume from the East Pacific Rise (HT), and benthic nepheloid layer (BNL) show distinct pTE concentrations. In these regions, certain elements are found more in the large size fraction (LSF, > 51 μm): V within the surface, Al, Ti, and Fe in the subsurface, and most elements in the OMZ, HT, and BNL, whereas pTEs are > 80% partitioned to the small size fraction (SSF, 0.8–51 μm) in the rest of the section. The higher partitioning of pTEs to the LSF in these regions is likely a result of higher V quota of large-size particles (e.g., diatom) (surface) and enhanced particle aggregation relative to disaggregation in these regions caused by the cycling of organic particles (subsurface), increased particle density (OMZ, BNL, HT) or lack of zooplankton-mediated disaggregation (OMZ). Several groups of elements appear to have similar concentration distribution and size partitioning depending on the particle phase (lithogenic, biogenic, Fe oxyhydroxides, and Mn oxides) that each element is mainly associated with. This result implies that the elements occurring in the same particle phase not only have similar sources and sinks, but also are affected by similar particle cycling processes within the ocean.

In this study, stable isotope measurements of nitrate (δ15NNO3 and δ18ONO3) and suspended particulate nitrogen (δ15NSPN, > 51 μm) were made on the 2013 US GEOTRACES Eastern Pacific Zonal Transect (GP16). This cruise extended from the nutrient-rich coastal waters of the Peru upwelling system to the oligotrophic waters of the south Pacific subtropical gyre, where low surface nitrate (NO3−) concentrations (

Particles play an important role in the marine biogeochemical cycling of iron, an essential micronutrient. Particles of biogenic, lithogenic and authigenic origin can act as sources or sinks of dissolved iron and therefore influence its distribution and availability to phytoplankton. However, not all particulate Fe (pFe) is readily accessible to phytoplankton and various treatments are used to estimate the bioavailable, or labile fraction of pFe. Here we report concentration and iron stable isotope data for the “ligand-leachable” fraction of pFe, following the application of a pH 8 oxalate-EDTA leach to 0.8–51 μm-sized particles collected during the US GEOTRACES East Pacific Zonal Transect (GP16). Across the GP16 section, we identified two regions of high labile pFe concentrations and differing δ56Felabile. Within a hydrothermal plume originating from the East Pacific Rise, nearly all of the pFe was labile, with concentrations as high as 42 nM. Particulate δ56Felabile within the hydrothermal plume averaged − 0.26 ± 0.14‰ (n = 67), which is similar to both dissolved hydrothermal plume δ56Fe and the δ56Fe of primary vent fluids from previous studies, suggesting that vent fluid Fe is quantitatively precipitated as labile Fe oxyhydroxides and that the chemical form of hydrothermal dissolved and particulate Fe are the same, though they are found in different size classes. Elevated pFe concentrations were also observed over the Peruvian continental shelf and extending westwards, coincident with the secondary nitrite maximum at the top of the oxygen minimum zone (OMZ). Again, most of the pFe was within the labile fraction, but the δ56Felabile associated with the OMZ was lighter than in the hydrothermal plume (− 0.68 ± 0.32‰, n = 41), reflecting its origin within reducing continental margin sediments. The open ocean away from these features was characterized by labile and total pFe concentrations of

In the ocean, dissolved cobalt is affected by both nutrient cycling and scavenging onto manganese oxides. The latter process concentrates Co in pelagic sediments, resulting in a small deep water inventory. While the flux of scavenged cobalt to sediments appears steady on timescales > 100,000 years, its residence time in the water column is short, approximately 130 years. Using results from recent GEOTRACES expeditions, we show net removal of dissolved Co from the deep ocean on the order of 0.043 pM year− 1, which corresponds to a turnover time of 980 years. Scavenging in deep ocean water masses is too slow to match cobalt accumulation rates in marine sediments, requiring most of the scavenging flux to derive from the mesopelagic ocean (< 1500 m depth) where nutrient cycling is active. Based on differences between the Co:P stoichiometry in particles sinking from the euphotic zone and dissolved Co:P remineralization ratios, we calculate areal scavenging rates in the North Atlantic and South Pacific basins on the order of 1.5 and 0.7 μmol m− 2 year− 1, respectively, which agree with long-term accumulation rates in Atlantic and Pacific sediments. In both basins, over 50% of the scavenged flux of cobalt occurs in the upper 500 m, resulting in decadal turnover times in the mesopelagic. An assessment of sources suggests that the marine cobalt cycle is approximately in balance, but that this inventory may be sensitive to long term trends in the intensity of oxygen minimum zones, which account for ~ 25% of the annual cobalt source to the modern oceans.

Hydrothermal circulation and subsequent eruption of seawater at mid-ocean ridges and back-arc basins has great potential to modulate deep ocean biogeochemistry, acting as both a source and a sink for many trace elements and their isotopes. The influence of hydrothermal vents as a source of iron and manganese has been demonstrated in all ocean basins. However, the long-range impact of scavenging by hydrothermal particles has yet to be documented in detail. We use dissolved and particulate measurements of long-lived radiogenic ( 230 Th, 231 Pa) and primordial ( 232 Th) radionuclides to investigate the nature and geographic scale of scavenging processes occurring within a hydrothermal plume in the Southeast Pacific Ocean sampled during the GEOTRACES GP16 section. Due to their radioactive disequilibrium with respect to production by their parent uranium isotopes, 230 Th and 231 Pa provide unique insights into the rates of scavenging. We find strong deficits in 230 Th and 231 Pa coincident with elevated particulate Mn and Fe(OH) 3 , indicating that trace metal scavenging is widespread and likely the result of the strong affinity of trace metals for nanoparticulate metal surface sites. The chemical composition of the particulate material is closely linked to the scavenging intensity of 230 Th and 231 Pa, expressed as distribution coefficients between solid and solution. A comparison of 230 Th and 231 Pa inventories with mantle-derived 3 He as well as a mass balance of 230 Th and 231 Pa suggests continuous scavenging removal over the course of the entire 4000 km transect. Unlike the two radiogenic isotopes, 232 Th is enriched above what would be expected from assuming identical scavenging behavior to 230 Th, indicating a hydrothermal source of colloidal, unreactive 232 Th.

Marine particles play key roles in the cycling of most elements in the ocean. Here we present full water column sections of size-fractionated (1–51 μm; > 51 μm) concentrations of suspended particulate matter (SPM) and major particle phase composition, including particulate organic matter (POM), calcium carbonate (CaCO 3 ), opal , lithogenic particles, and iron and manganese (oxyhydr)oxides, as well as the Redfield (C:N:P) stoichiometry of particles, from the U.S. GEOTRACES GP16 Eastern Pacific Zonal Transect (EPZT). The GP16 cruise sampled the oxygen deficient waters of the productive eastern upwelling Peru margin westward to Tahiti through the East Pacific Rise (EPR) hydrothermal plume around 12°S in October–December 2013. In this region of relatively low mineral dust deposition, the sum of POM and CaCO 3 concentrations accounted for > 80% of small size fraction (SSF) SPM for most of the section. Some exceptions to this include the southern EPR hydrothermal plume, where iron oxyhydroxides accounted for almost 60% of SSF SPM, and the coastal upwelling zone, where opal and lithogenic particles accounted for ~ 50% of SSF SPM. We discuss possible mechanisms to explain a large sediment resuspension feature on the deep Peru continental slope . Large size fraction (LSF) particles generally had relatively higher contributions of opal and lower contributions of POM compared to SSF particles, reflecting an unusually high size partitioning of opal to the LSF. Distributions of CaCO 3 and other phases were more strongly controlled by particle dynamics than by dissolution. Our particle phase data are consistent with a conceptual model where particle production, aggregation, and disaggregation processes dominate in the euphotic, 100–300 m, and 300–500 m depth zones, respectively. Our direct measurements of particle composition showed 1) euphotic zone C vs P relationships (SSF: 48 ± 13; LSF: 65 ± 14) that were significantly lower than canonical Redfield values (~ 106), but that were consistent with previously published large scale inverse and box model estimates of exported C:P for this region, and 2) PIC:POC ratios (LSF: 0.088) that were remarkably similar to estimated rain ratios from a biogeochemical-transport box model of alkalinity and nitrate in the low latitude Pacific.

Iron (Fe)-poor surface waters limit phytoplankton growth and their ability to remove carbon (C) from the atmosphere and surface ocean. Over the past few decades, research has focused on constraining the global Fe cycle and its impacts on the global C cycle. Hydrothermal vents have become a highly debated potential source of Fe to the surface ocean. Two main mechanisms for transport of Fe over long distances have been proposed: Fe-bearing nanoparticles and organic C complexation with Fe in the dissolved (dFe) and particulate (pFe) pools. However, the ubiquity and importance of these processes is unknown at present, and very few vents have been investigated for Fe-Corg interactions or the transport of such materials away from the vent. Here we describe the near-field contributions (first ~100 km from ridge) of pFe and Corg to the Southern East Pacific Rise (SEPR) plume, one of the largest known hydrothermal plume features in the global ocean. Plume particles (>0.2 μm) were collected as part of the U.S. GEOTRACES Eastern Pacific Zonal Transect cruise (GP16) by in-situ filtration. Sediment cores were also collected to investigate the properties of settling particles. In this study, X-ray absorption near edge structure (XANES) spectroscopy was used in two complementary X-ray synchrotron approaches, scanning transmission X-ray microscopy (STXM) and X-ray microprobe, to investigate the Fe and C speciation of particles within the near-field non-buoyant SEPR plume. When used in concert, STXM and X-ray microprobe provide fine-scale and representative information on particle morphology, elemental co-location, and chemical speciation. Bulk chemistry depth profiles for particulate Corg (POC), particulate manganese (pMn), and pFe indicated that the source of these materials to the non-buoyant plume is hydrothermal in origin. The plume particles at stations within the first ~100 km down-stream of the ridge were composites of mineral (oxidized Fe) and biological materials (organic C, Corg). Iron chemistry in the plume and in the core-top sediment fluff layer were both dominated by Fe(III) phases, such as Fe(III) oxyhydroxides and Fe(III) phyllosilicates. Particulate sulfur (pS) was a rare component of our plume and sediment samples. When pS was detected, it was in the form of an Fe sulfide mineral phase, composing ≤0.4% of the Fe on a per atom basis. The sediment fluff layer contained a mixture of inorganic (coccolith fragments) and Corg bearing (lipid-rich biofilm-like) materials. The particle morphology and co-location of C and Fe in the sediment was different from that in plume particles. This indicates that if the Fe-Corg composite particles settle rapidly to the sediments, then they experience strong alteration during settling and/or within the sediments. Overall, our observations indicate that the particles within the first ~100 km of the laterally advected plume are S-depleted, Fe(III)-Corg composites indicative of a chemically oxidizing plume with strong biological modification. These findings confirm that the Fe-Corg relationships observed for non-buoyant plume particles within ~100 m of vent sites are representative of particles within the first ~100 km of the advecting non-buoyant plume, and demonstrate that the export of hydrothermal pFe is facilitated through physical-chemical association with Corg.

Oxygen minimum zones (OMZs) are thought to be regions of decreased carbon attenuation in the upper ocean with a biological pump that could deliver a greater percentage of exported carbon to the mesopelagic relative to surrounding waters. However, much is still unknown about carbon cycling through these zones and the areas of extreme oxygen minima (nM O 2 ) or oxygen deficient zones (ODZs) within the OMZs. Paired sampling for 234 Th (t½ ~ 24.1 days) and particulate organic carbon (POC) was performed along a zonal transect between 77 and 152°W during the U.S. GEOTRACES Southeastern Tropical Pacific campaign in 2013 in order to constrain the magnitude of carbon export and remineralization through the Peruvian OMZ. POC export varied by an order of magnitude from the coast to 152°W, reflecting a decrease in POC: 234 Th ratios (> 51 μm) with distance offshore and the influence of upwelling at the coast. Modeling indicated that 234 Th fluxes could be underestimated at coastal stations by up to 4-fold without adjustment for the impact of upwelling, which in turn would produce much lower carbon export estimates. Low carbon Export:NPP ratios ( 7500 km that resulted in, on average, 3% of the POC exported from the euphotic zone reaching 100 m below the Ez. Although the highest percentages (> 10%) of total exported POC at 100 m below the Ez were observed in the coastal ODZ region, the observed remineralization was also most pronounced these stations. While an average of 75% of the carbon export from the euphotic zone remained at Ez + 100 m in the gyre, a range of 10 to 50% was observed at ODZ stations, reflecting increased attenuation. Local subsurface minima in light transmission and maxima in fluorescence were observed in the regions of greatest remineralization at the upper ODZ boundary, suggesting that complex bacterial community dynamics play a role in increased attenuation through these zones. With ODZs and OMZs predicted to grow worldwide with climate change, these areas require further large-scale and seasonal studies to assess the permanency of these attenuation features and the impact of high Gyre and lower ODZ transfer of POC on the overall efficiency of carbon export in the Pacific.

Transmissometers record high-frequency, high-sensitivity measurements of beam attenuation due to particles (beam c P ), enabling determination of POC and SPM concentrations (via regression). Modern instruments have unique sensitivities to particle loads of various composition, but can exhibit unresolved thermal effects: even well-calibrated and monitored instruments exhibit asymmetry in upcast and downcast traces. On the US GEOTRACES EPZT transect, three independently deployed 650 nm, 25-cm pathlength (CStar) transmissometers and a broad-angle turbidity meter (Seapoint) were deployed at up to 36 stations. We report a procedure for improving agreement in upcast and downcast transmissometry via simple thermal modeling of instrument temperature using CTD temperature traces. Observed particle features from thermally corrected beam c P and turbidity include surface-derived biogenic phases, hydrothermal plumes, benthic nepheloid layers, and low-oxygen particle maxima. Optical responses to particles from a wide range of oceanographic regimes are examined against measured particle composition. In contrast to the transmissometers, the turbidity meter expresses a noted sensitivity to Fe(OH) 3 . The derived turbidity/ c P optical ratio thus shows promise in discriminating regions of elevated Fe(OH) 3 content at high vertical spatial resolution.