GEOTRACES Eastern Pacific Zonal Transect Research Articles

Controls on Dissolved Silicon Isotopes Along the U.S. GEOTRACES Eastern Pacific Zonal Transect (GP16)

AbstractThe distribution of dissolved silicon isotopes (δ30Si) was examined along the U.S. 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 was compared with the results of an Optimum Multiparameter Analysis that quantified the fractional contributions of end‐member 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 2,000 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.

Distribution of 236U in the U.S. GEOTRACES Eastern Pacific Zonal Transect and its use as a water mass tracer

We report dissolved concentrations of the long-lived radioisotope 236U measured in the water column along the 2013 US GEOTRACES Eastern Pacific Zonal Transect (GP16). This transect followed a 10–15°S line from Manta, Ecuador, to Papeete, Haiti, French Polynesia, crossing the southern East Pacific Rise, intercepting one of the largest hydrothermal plumes as well as a productivity gradient, which includes the upwelling zone and associated low oxygen waters offshore from Peru and the oligotrophic sub-tropical gyre. Accelerator Mass Spectrometry was used to measure dissolved seawater 236U concentrations as low as 1 × 103 atom kg−1, which are among the lowest levels reported to date. Differences in upper water column 236U distributions from east to west are a result of variable contributions from different surface and intermediate waters encountered along the transect.The distribution of 236U, both in depth and geographically, provides complementary information to that obtained from Δ14C and helium isotopes, demonstrating that 236U concentrations are diagnostic in the identification of and contributions from the different deep and bottom water masses encountered along the EPZT (Jenkins et al., 2017). For example, we observe minimum 236U concentrations along the EPZT between 2000 and 3000 m that are consistent with contributions attributed to Pacific Deep Water. We also observe increases in 236U below 3000 m at the eastern and western termini of the EPZT. This is consistent with contributions associated with Antarctic Bottom Water and Lower Circumpolar Deep Water. Our results indicate that 236U may be used in conjunction with Δ14C and 3He isotopes as an additional tool with which to identify and resolve contributions from different water masses in the Pacific Ocean.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Seawater Carbonate Chemistry Distributions Across the Eastern South Pacific Ocean Sampled as Part of the GEOTRACES Project and Changes in Marine Carbonate Chemistry Over the Past 20 Years

The US GEOTRACES Eastern Pacific Zonal Transect in 2013 provided an opportunity to complement studies of the sources, sinks, and internal cycling of trace elements and isotopes (TEIs) with observations of seawater carbon dioxide (CO2)–carbonate chemistry. Across the Peru-Tahiti section, very large horizontal gradients in surface/mixed layer dissolved inorganic carbon (DIC) and total alkalinity (TA) from the nutrient-rich, low-oxygen coastal upwelling region adjacent to Peru to the oligotrophic central Pacific. Near the coast of Peru, upwelling of CO2 rich waters from the oxygen deficient zone (ODZ) impinged at the surface with very high partial pressures of CO2 (pCO2; >800-1,200 µatm), and low pH (7.55 to 7.8). These waters were also undersaturated with respect to aragonite, a common calcium carbonate (CaCO3) mineral. These chemical conditions are not conducive to pelagic and shelf calcification, with shelf calcareous sediments vulnerable to CaCO3 dissolution, and to the future impacts of ocean acidification. A comparison to earlier data collected from 1991 to 1994 suggests that surface dissolved inorganic carbon (DIC) and pCO2 have increased by as much as 3% and 20%, respectively, while pH and saturation state for aragonite (Waragonite) have decreased by as much as 0.063 and 0.54, respectively. In intermediate waters (~200-500 m), dissolved oxygen has decreased (loss of up to -43 µmoles kg-1) and nitrate increased (gain of up to 5 µmoles kg-1) over the past twenty years and this likely reflects the westward expansion of the ODZ across the central Eastern South Pacific Ocean. Over the same period, DIC and pCO2 increased by as much as +45 µmoles kg-1 and +145 µatm, respectively, while pH and Waragonite decreased by -0.091 and -0.45, respectively. Such rapid change in pH and CO2–carbonate chemistry over the past twenty years has implications for changing the thermodynamics and solubility of intermediate water TEIs, but also for the marine ecosystem of the upper waters, especially for the vertically migrating community present in the eastern South Pacific.

Near-field iron and carbon chemistry of non-buoyant hydrothermal plume particles, Southern East Pacific Rise 15°S

Abstract 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.

Organic complexation of iron in the eastern tropical South Pacific: Results from US GEOTRACES Eastern Pacific Zonal Transect (GEOTRACES cruise GP16)

Abstract Dissolved iron, organic iron-binding ligands, and organic carbon were determined in full water column depth profiles across the US GEOTRACES Eastern Pacific Zonal Transect (GEOTRACES cruise GP16) in late 2013. Dissolved iron concentrations exhibited subsurface maxima associated with the remineralization of organic matter at the Peru shelf and with hydrothermal inputs from the East Pacific Rise. Iron-binding organic ligands are described as ligand classes based on defined ranges in conditional stability constants. The stronger L1-type ligands were measured in large excesses in surface and intermediate waters, and these excesses were negatively correlated with Si*, a biogeochemical proxy for iron limited diatom growth. These data suggest sources of strong iron-binding ligands from iron limitation of diatom communities, both locally and in waters originating from the Southern Ocean. Benthic sources of strong ligands were associated with new iron inputs from hydrothermal activity at the East Pacific Rise and from bottom sediments. In contrast to most studies in the Atlantic basin but consistent with previous datasets from the Pacific, stronger L1 ligands in this dataset were generally restricted to the upper water column and did not show large excesses through the water column. At depth, iron-binding ligands on GP16 were instead best described as L2 and L3 ligands. Concomitant decreases in excess L1, excess total ligands and dissolved organic carbon suggest similar degradation pathways of these pools below the surface.

Water mass analysis of the 2013 US GEOTRACES eastern Pacific zonal transect (GP16)

Abstract The 2013 US GEOTRACES Eastern Pacific Zonal Transect (GP16) extended from the Peruvian coast to Tahiti, along a line that fell between 10 and 15°S. This transect sampled the Peruvian oxygen deficient zone (ODZ) and the hydrothermal plume extending from the East Pacific Rise (EPR) for a variety of trace elements and isotopes (TEIs). Here we report nutrient and hydrographic measurements collected on this cruise, as well as results from an Optimum Multiparameter Analysis (OMPA) to quantify the fractional contributions of endmember water masses in each sample. The primary goals of this study were to better understand the distribution of water masses in the eastern tropical Pacific, and to help interpret TEI measurements collected on this cruise, as well as related studies carried out in the region. In the thermocline, Equatorial Subsurface Water (ESSW) dominated the low oxygen waters of the eastern tropical South Pacific, blending into Eastern South Pacific Intermediate Water (ESPIW) and South Pacific Central Water (SPCW) further west. Below the thermocline, distributions of Antarctic Intermediate Water (AAIW) and Equatorial Pacific Intermediate Water (EqPIW) were relatively homogenous along the section between 800 and 1200 m depth. Deeper in the water column, distinct water mass signatures were found on opposite sides of the EPR: southward flowing Pacific Deep Water (PDW) dominated the basin east of the EPR, while the northward flowing Antarctic Bottom Water (AABW) and Lower Circumpolar Deep Water (LCDW) had the strongest contributions on the western side of the EPR. These findings support previous studies that indicate the Peruvian ODZ is largely contained within ESSW and that the EPR plays an important role in steering water mass distributions in the deep waters of the tropical Pacific. Overall, these results agree well with previous water mass analyses in this region and are consistent with the general circulation patterns in the eastern tropical Pacific.

Nitrogen and oxygen isotope measurements of nitrate along the US GEOTRACES Eastern Pacific Zonal Transect (GP16) yield insights into nitrate supply, remineralization, and water mass transport

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 (< 4 μM) and high δ15NNO3 and δ18ONO3 values (up to 28‰ and 25‰, respectively) were generated from consumption of NO3− by phytoplankton. Analysis of δ15NNO3 and δ18ONO3 patterns in surface waters relative to δ15NSPN provided insight into supply of nutrients to surface waters along the transect and remineralization at depth. In the subsurface oxygen deficient zone (ODZ), dissimilatory NO3− reduction to nitrite (NO2−) generated distinct elevations in δ15NNO3 and δ18ONO3 (up to ~ 37‰ and 31‰, respectively). Below the oxygen deficient zone, δ15NNO3 values (6 to 10‰) in the intermediate waters (σ0 = 27.0 to 27.5 kg m− 3; 500 m to 1500 m) were elevated relative to deep waters across the entire length of GP16, while intermediate δ18ONO3 values (2 to 3‰) were more similar to deep water values. These observations suggested that both NO3− regeneration and the transport of NO3− from the ODZ influence the intermediate waters of the central and eastern south Pacific. In order to explore this further, results from the GP16 water mass analysis and a NO3− isotope mixing model were employed to quantify the importance of those processes in the thermocline and intermediate waters across GP16. The contribution of NO3− from the ODZ contributed up to 10 μM across the thermocline, but generally < 5 μM in the intermediate waters. In contrast, it was estimated that remineralization contributed 5 to 15 μM of NO3− across the thermocline and intermediate waters. Overall, these results from GP16 confirm that NO3− reduction in the ODZ and organic matter remineralization play important roles in maintaining the widespread elevation in δ15NNO3 observed across the thermocline and intermediate waters of the eastern South Pacific.

Size distribution of particulate trace elements in the U.S. GEOTRACES Eastern Pacific Zonal Transect (GP16)

Abstract 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.

Size-fractionated distributions of suspended particle concentration and major phase composition from the U.S. GEOTRACES Eastern Pacific Zonal Transect (GP16)

Abstract 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.

Biogeochemical cycling of Fe and Fe stable isotopes in the Eastern Tropical 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.

Evolution of the south Pacific helium plume over the past three decades

AbstractThe recent GEOTRACES Eastern Pacific Zonal Transect in 2013 crossed the East Pacific Rise at 15°S following the same track as the 1987 Helios Expedition along the core of the mid‐depth helium plume that spreads westward from the East Pacific Rise (EPR) axis. The fact that several stations were co‐located with the earlier Helios stations has allowed a detailed comparison of the changes in the helium plume over the intervening 26 years. While the plume in many areas is unchanged, there is a marked decrease in plume intensity at longitude 120°W in the 2013 data which was not present in 1987. Recent radioisotope measurements along the plume track suggest that this decrease is due to the intrusion of a different water mass into the plume, rather than a modulation of hydrothermal input on the EPR axis. Analysis of GEOTRACES hydrographic data shows excess heat present in the plume up to 0.04°C, corresponding to a 3He/heat ratio of ∼2.5 × 10−18 mol J−1, similar to that found in mature hydrothermal vents. RAFOS floats deployed in 1987 indicate an average westward transport of ∼0.3 cm s−1 at 2500 m depth in the off‐axis plume, in agreement with recent estimates of ∼0.4 cm s−1 based on “aging” of the plume from 227Ac/3He ratios.

Upwelling and primary production during the U.S. GEOTRACES East Pacific Zonal Transect

AbstractThe 2013 U.S. GEOTRACES Eastern Pacific Zonal Transect (EPZT) traversed the highly productive Peruvian coastal upwelling (PCU) region. In this work, the flux of nitrate into the euphotic zone is derived for stations within the PCU using a previously developed method whereby dilution of the water column 7Be inventory by upwelled 7Be‐free water provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, 7Be profiles are used to constrain vertical diffusivity within the upper thermocline. These transport terms are applied to nitrate profiles to estimate net community production between 79°W and 104°W along the EPZT, which includes the zone of active upwelling to the edge of the oligotrophic gyre. With a simple, one‐dimensional model, the calculated upwelling rates were inversely related to mixed layer temperature and ranged from 0 to 3.0 m/d. Results using a depth‐dependent upwelling rate with a component of horizontal advection are also described. Vertical diffusivities near the base of the euphotic zone were in the range 1.7–4.5 × 10−4 m2/s. These values are compared to those generated by analysis of temperature profiles. Net community production averaged 15 mmol C/m2/d for stations between 84°W and 104°W and was 134 mmol C/m2/d for the furthest inshore station at 79°W which displayed the lowest SST and greatest rate of upwelling.

Structural Characterization of Natural Nickel and Copper Binding Ligands along the US GEOTRACES Eastern Pacific Zonal Transect

Organic ligands form strong complexes with many trace elements in seawater. Various metals can compete for the same ligand chelation sites, and the final speciation of bound metals is determined by relative binding affinities, concentrations of binding sites, uncomplexed metal concentrations, and association/dissociation kinetics. Different ligands have a wide range of metal affinities and specificities. However, the chemical composition of these ligands in the marine environment remains poorly constrained, which has hindered progress in modeling marine metal speciation. In this study, we detected and characterized natural ligands that bind copper (Cu) and nickel (Ni) in the eastern South Pacific Ocean with liquid chromatography tandem inductively coupled plasma mass spectrometry (LC-ICPMS), and high resolution electrospray ionization mass spectrometry (ESIMS). Dissolved Cu, Ni, and ligand concentrations were highest near the coast. Chromatographically unresolved polar compounds dominated ligands isolated near the coast by solid phase extraction. Offshore, metal and ligand concentrations decreased, but several new ligands appeared. One major ligand was detected that bound both Cu2+ and Ni2+. Based on accurate mass and fragmentation measurements, this compound has a molecular formula of [C20H21N4O8S2 + M]+ (M = metal isotope) and contains several azole-like metal binding groups. Additional lipophilic Ni complexes were also present only in oligotrophic waters, with masses of 649, 698, and 712 m/z (corresponding to the 58Ni metal complex). Molecular formulae of [C32H54N3O6S2Ni]+ and [C33H56N3O6S2Ni]+ were determined for two of these compounds. Addition of Cu and Ni to the samples also revealed the presence of additional compounds that can bind both Ni and Cu. Although these specific compounds represent a small fraction of the total dissolved Cu and Ni pool, they highlight the compositional diversity and spatial heterogeneity of marine Ni and Cu ligands, as well as variability in the extent to which different metals in the same environment compete for ligand binding.

Elevated trace metal content of prokaryotic communities associated with marine oxygen deficient zones

AbstractLittle is known about the trace metal content of marine prokaryotes, in part due to their co‐occurrence with more abundant particulate phases in the upper ocean, such as phytoplankton and biogenic detritus, lithogenic minerals, and authigenic Mn and Fe oxyhydroxides. We attempt to isolate these biomass signals in particulate data from the US GEOTRACES Eastern Pacific Zonal Transect (cruise GP16) in the Eastern Tropical South Pacific (ETSP), which exhibited consistent maxima in P and other bioactive trace metals, and minima in particulate Mn, in the oxygen deficient zones (ODZs) of 13 stations. Nitrite maxima and nitrate deficits indicated the presence of denitrifying prokaryotic biomass within ETSP ODZs, and deep secondary fluorescence maxima at the upper ODZ boundaries of 10 stations also suggested the presence of low‐light, autotrophic communities. ODZs were observed as far west as 99°W, more than 2300 km from the South American coast, where eolian lithogenic and lateral/resuspended sedimentary inputs were negligible, presenting a unique opportunity to examine prokaryotic metal stoichiometries. ODZ particulate P maxima can rival gyre mixed layer biomass concentrations, are highly sensitive to oxygen, and are in excess of amounts scavengable by local Fe oxyhydroxides and acid–volatile sulfides. Even after correction for lithogenic and ferruginous–scavenged metals, ODZ P‐maxima are often enriched in Cd, Co, Cu, Ni, V, and Zn, exhibiting particulate trace metal ratios to P that exceed mixed layer biomass ratios by factors of 2–9. ODZ prokaryotic communities may be largely hidden, TM–rich pools involved in the marine cycles of these bioactive trace metals.

Basin-scale transport of hydrothermal dissolved metals across the South Pacific Ocean.

Hydrothermal venting along mid-ocean ridges exerts an important control on the chemical composition of sea water by serving as a major source or sink for a number of trace elements in the ocean. Of these, iron has received considerable attention because of its role as an essential and often limiting nutrient for primary production in regions of the ocean that are of critical importance for the global carbon cycle. It has been thought that most of the dissolved iron discharged by hydrothermal vents is lost from solution close to ridge-axis sources and is thus of limited importance for ocean biogeochemistry. This long-standing view is challenged by recent studies which suggest that stabilization of hydrothermal dissolved iron may facilitate its long-range oceanic transport. Such transport has been subsequently inferred from spatially limited oceanographic observations. Here we report data from the US GEOTRACES Eastern Pacific Zonal Transect (EPZT) that demonstrate lateral transport of hydrothermal dissolved iron, manganese, and aluminium from the southern East Pacific Rise (SEPR) several thousand kilometres westward across the South Pacific Ocean. Dissolved iron exhibits nearly conservative (that is, no loss from solution during transport and mixing) behaviour in this hydrothermal plume, implying a greater longevity in the deep ocean than previously assumed. Based on our observations, we estimate a global hydrothermal dissolved iron input of three to four gigamoles per year to the ocean interior, which is more than fourfold higher than previous estimates. Complementary simulations with a global-scale ocean biogeochemical model suggest that the observed transport of hydrothermal dissolved iron requires some means of physicochemical stabilization and indicate that hydrothermally derived iron sustains a large fraction of Southern Ocean export production.