An important role in the cycling of marine trace elements is scavenging, their adsorption and removal from the water column by sinking particles. Boundary scavenging occurs when areas of strong particle flux drive preferential removal of the trace metals at locations of enhanced scavenging. Due to its uniform production and quick burial via scavenging, 230 Th is used to assess sedimentary mass fluxes; however, these calculations are potentially biased near regions where net lateral transport of dissolved 230 Th violates the assumption that the flux of particulate 230 Th to the seabed equals its rate of production in the water column. Here, we present a water column transect of dissolved 230 Th along 152° W between Alaska and Tahiti (GEOTRACES GP15), where we examine 230 Th profiles across multiple biogeochemical provinces and, novelly, the lateral transport of 230 Th to distal East Pacific Rise hydrothermal plumes. We observed a strong relationship between the slope of dissolved 230 Th concentration‐depth profiles and suspended particle matter inventory in the upper‐mid water column, reinforcing the view that biogenic particle mass flux sets the background 230 Th distribution in open ocean settings. We find that, instead of the region of enhanced particle flux around the equator, hydrothermal plumes act as a regional boundary sink of 230 Th. At 152° W, we found that the flux‐to‐production ratio, and thereby error in 230 Th‐normalized sediment flux, is between 0.80 and 1.50 for hydrothermal water, but the error is likely larger approaching the East Pacific Rise.

Unsupported 231Pa/230Th ratios have been used widely as a paleoproxy for ocean circulation and as a paleoproductivity proxy; however, some of the inherent assumptions for these proxies have not been thoroughly tested, which would impact how the ratio is interpreted in different regions. Both applications of the ratio influence interpretations of past climate changes, so it is important to determine the extent to which each process impacts the sedimentary 231Pa/230Th values. Here, we compare 231Pa/230Th ratios between the Atlantic, Indian, and Pacific Oceans, and within the Atlantic Ocean, as a test of whether or not this ratio serves as a reliable proxy for water mass ventilation age, which is closely related to ocean circulation.In this study, we present new 231Pa and 230Th measurements from Indian and Atlantic Ocean sediments and from Indian Ocean seawater samples, alongside previously published Atlantic Ocean water column and sedimentary 231Pa/230Th ratios. The observed water column dissolved 231Pa/230Th profiles do not show the expected increase in ratio values with water mass age, which disagrees with the conceptual model for the use of 231Pa/230Th as a paleocirculation proxy. Dissolved 231Pa/230Th varies between 0.35 and 0.75 within the depth range of NADW, and there is no apparent correlation with water mass age. The observed 231Pa/230Th values and profiles are strikingly similar between the western North Atlantic, Western Indian, and North Pacific Oceans, even though these basins have significantly different water mass histories and deep water mass ages.The water column 231Pa/230Th values then determine the ratio value of the underlying sediments. So, we then compare 231Pa/230Th values from Holocene sediments between the ocean basins to determine if the observations agree with our understanding of modern ocean circulation. Atlantic and Indian Ocean sediments are indistinguishable from each other with respect to their 231Pa/230Th ratios. Taken together, these results indicate that factors other than ventilation age must significantly impact 231Pa/230Th ratios, and the evidence suggests that fractionation and scavenging intensity may strongly impact particulate and sedimentary 231Pa/230Th.

Carbon mobilization at subduction zones can be constrained by focusing on Sr as a proxy, as Sr readily substitutes for Ca-a major cation in the crystal structures of carbonates. Unmelted high/ultrahigh pressure (HP/UHP) pelitic metasediments exhumed from less than 100 km depth along the slab-mantle interface show loss of Sr and retention of most other incompatible trace elements, when compared with their potential protoliths, for example, average shales/average upper continental crust/average subducting sediments. Sr and CaO/SiO2 in subducting sediments and unmelted HP/UHP metasediments are correlated, suggesting that Ca-carbonate is one of the major hosts for Sr in subducting sediments, and that Sr and carbonates are lost during subsolidus devolatilization of subducting HP/UHP metasediments beneath forearcs. In turn, the elevated Sr in arc magmas suggests that carbonated Sr-rich wedge peridotites may be dragged down into the source of arc magmas via wedge convection, where they contribute to C and Sr during partial melting. Decoupling of Sr from most of the other trace elements in subduction zone metasediments suggests that previous mass balance attempts using Sr, Nd, and Pb concentrations and isotope ratios to constrain the relative contributions of mantle peridotites, subducting sediments and subducting oceanic crust to arc melts need to be reexamined. Metasediments could have contributed more to arc magmas than previously estimated.

A common technique for determining the mass accumulation rates of marine sediments is through measurements of constant flux proxies. These proxies, like unsupported 230Th and extraterrestrial 3He, are buried at a known rate at the seafloor, and thus their concentration in sediments is inversely proportional to the vertical mass accumulation rate of ambient sediments. These two proxies have varied assumptions regarding their behavior that have been difficult to test, particularly in regions of very low sedimentation. We present new measurements of helium and thorium isotopes in coretop sediments from the South Pacific Gyre. Our main finding is that 230Th- and 3He-derived mass accumulation rates of these sediments differ by a factor of 2-10, with 230Th-based mass accumulation rates systematically higher than those derived from 3He. While the final cause of the discrepancy is difficult to elucidate, we suggest that up to a factor of 2 worth of the sedimentary 230Th deficit can be explained by lateral transport of 230Th, while the remainder must be explained by other processes, such as ancient coretops undercorrected for post-depositional 230Th decay. Regardless of the mechanism, our findings have critical implications for the application of constant flux proxies in the South Pacific Gyre, with one example being the calibration of dust deposition models in this region. Ultimately, additional testing is required to determine the accuracy of the 3He and 230Th techniques for estimating sediment accumulation rates in regions of the open ocean with extremely low sedimentation rates.

Five sediment cores were collected along a cruise tract from Hawaii to Alaska in August 2017 (C-Disk-IV cruise) with the objective of characterizing the behavior of Ac-227, Ra-228, and Ra-226 and their fluxes into the overlying water column, information that is essential to the interpretation of the distribution of these tracers in the ocean, for example, as measured on GEOTRACES cruises. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that incorporated molecular diffusion, bioturbation, sedimentation, distribution coefficients (kd), and the fraction of each isotope released to pore water by parent decay (called F). Fits to these profiles used kd values determined in lab experiments for C-Disk-IV sediments. Ra kd values (1000-3000 mL g(-1)) agreed with previous estimates for deep-sea sediments, and Ac kd values (3500-22,000 mL g(-1)) correlated with those for Ra but were about 7 times greater. Two independent approaches were used to quantify the benthic fluxes of Ac-227 and 228Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agreed within uncertainty, and the average Ac-227 and Ra-228 sediment fluxes for the Northeast Pacific are 90 & PLUSMN; 20 and 600 &200 dpm m-2-yr(-1), respectively. The 226Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific, averaging 1300 & PLUSMN; 200 dpm m-2-yr(-1). This is over 2x higher than the water column inventory of Ra-226 in this region (600 dpm m-2-yr(-1)), and indicates the importance of lateral 226Ra export from the N. Pacific. The largest 227Ac and Ra isotope fluxes in the study area are near the center of the Northeast Pacific (37 degrees N). Smaller Ac-227, Ra-228 and Ra-226 fluxes occur north of 40 degrees N, primarily due to dilution of their Pa and Th ancestors by higher sediment accumulation rates.

The long-lived radioisotopes of Th and Pa are unique tracers for quantifying rates of biogeochemical processes in the ocean. However, their generally low concentrations (sub-fg/kg for 230Th and 231Pa and pg/kg for 232Th) in seawater make them difficult to measure. Here, we present a new approach to determine 232Th and 230Th using Nobias PA-1 chelating resin following a bulk-extraction technique, and report for the first time the use of this resin to measure 231Pa concentrations. This method has high extraction efficiency (>80%) at pH of 4.4 ± 0.2 and the lowest procedural blanks reported in the literature: 1.0 ± 0.2 pg, 0.10 ± 0.03 fg, and 0.02 ± 0.01 fg for 232Th, 230Th, and 231Pa, respectively, representing 3%, 0.02%, and 0.01% of the total dissolved 232Th, 230Th, and 231Pa found in 5 L of a typical low-concentration surface seawater sample from the subtropical Pacific Ocean. The procedure yields data with high precision for all three isotopes (0.76% for 232Th, 0.89% for 230Th, and 0.96% for 231Pa, 2σ), allowing us to reliably measure Th and Pa in the oceans even at concentrations as low as those found in surface waters of the South Pacific Ocean. The accuracy of this method was confirmed by the analysis of well-characterized standard solutions (SW STD 2010-1 and SW STD 2015-1) and seawater samples collected aboard the FS Sonne (cruise SO245) during the UltraPac cruise in the South Pacific Ocean. Simultaneous and rapid extraction of 232Th, 230Th and 231Pa from seawater, as well as the high precision and accuracy of this method makes it ideal for both spatially and temporally high-resolution studies.

One of the primary sources of micronutrients to the sea surface in remote ocean regions is the deposition of atmospheric dust. Geographic patterns in biogeochemical processes such as primary production and nitrogen fixation that require micronutrients like iron (Fe) are modulated in part by the spatial distribution of dust supply. Global models of dust deposition rates are poorly calibrated in the open ocean, owing to the difficulty of determining dust fluxes in sparsely sampled regions. We present new estimates of dust and Fe input rates from measurements of dissolved and particulate thorium isotopes(230)Th and(232)Th on theFS SonneSO245 section (GEOTRACES process study GPpr09) in the South Pacific. We first discuss high-resolution upper water column profiles of Th isotopes and the implications for the systematics of dust flux reconstructions from seawater Th measurements. We find dust fluxes in the center of the highly oligotrophic South Pacific Gyre that are the lowest of any mean annual dust input rates measured in the global oceans, but that are 1-2 orders of magnitude higher than those estimated by global dust models. We also determine dust-borne Fe fluxes and reassess the importance of individual Fe sources to the surface South Pacific Gyre, finding that dust dissolution, not vertical or lateral diffusion, is the primary Fe source. Finally, we combine our estimates of Fe flux in dust with previously published cellular and enzymatic quotas to determine theoretical upper limits on annual average nitrogen fixation rates for a given Fe deposition rate.

The Southern Ocean hosts complex connections between ocean physics, chemistry, and biology. Changes in these connections are hypothesized to be responsible for significant alterations of ocean biogeochemistry and carbon storage both on glacial‐interglacial timescales and in the future due to anthropogenic forcing. Isotopes of thorium (230Th and 232Th) and protactinium (231Pa) have been widely applied as tools to study paleoceanographic conditions in the Southern Ocean. However, understanding of the chemical behavior of these isotopes in the modern Southern Ocean has been limited by a paucity of high‐resolution observations. In this study, we present measurements of dissolved 230Th, 231Pa, and 232Th on a meridional transect along 170°W from 67°S to 54°S in the Pacific sector of the Southern Ocean, with high vertical and meridional sampling resolution. We find Th/Pa fractionation factors below 1, highlighting the preferential removal of Pa relative to Th in a region with low lithogenic inputs where the particle flux is dominated by biogenic opal. We also find steep gradients in all three of these isotopes along neutral density surfaces from north to south, demonstrating the importance of isopycnal mixing in transporting these nuclides to the Southern Ocean. Our results suggest that 231Pa and 230Th in the Southern Ocean are highly sensitive tracers of physical transport that may find use in studies of Southern Ocean biogeochemical‐physical connections in the past, present, and future.

Significance Plankton in the sunlit surface ocean photosynthesize, fixing dissolved CO 2 into particulate organic carbon (POC). This POC sinks and is respired, releasing CO 2 into subsurface waters that are sequestered from the atmosphere. The depth scale over which this regeneration happens strongly affects atmospheric CO 2 , but estimates to date have been sparse and challenging to interpret. We use a new geochemical method to determine POC regeneration depth scales at unprecedented resolution in the South Pacific Ocean, finding shallow regeneration in both oxygen-deficient zone and oligotrophic gyre settings. Our results imply decreased future ocean carbon storage due to gyre expansion and two opposing feedbacks to expanding oxygen-deficient zones, the net effects of which on ocean carbon storage require future research.

•Kinetics of thorium (Th) scavenging are quantified in a SE Pacific hydrothermal plume.•Calculated adsorption and desorption rates of Th in hydrothermal plume to determine that scavenging may be irreversible.•Greater hydrothermal depletion of total 230Th relative to 228Th and 234Th.•Removal signature of 230Th is likely integrated across many scavenging events as waters flow northward above the EPR ridge axis.

Enhanced ocean carbon storage during the Pleistocene ice ages lowered atmospheric CO2 concentrations by 80 to 100 ppm relative to interglacial levels. Leading hypotheses to explain this phenomenon invoke a greater efficiency of the ocean's biological pump, in which case carbon storage in the deep sea would have been accompanied by a corresponding reduction in dissolved oxygen. We exploit the sensitivity of organic matter preservation in marine sediments to bottom water oxygen concentration to constrain the level of dissolved oxygen in the deep central equatorial Pacific Ocean during the last glacial period (18,000–28,000 years BP) to have been within the range of 20–50 μmol/kg, much less than the modern value of ~168 μmol/kg. We further demonstrate that reduced oxygen levels characterized the water column below a depth of ~1,000 m. Converting the ice age oxygen level to an equivalent concentration of respiratory CO2, and extrapolating globally, we estimate that deep‐sea CO2 storage during the last ice age exceeded modern values by as much as 850 Pg C, sufficient to balance the loss of carbon from the atmosphere (~200 Pg C) and from the terrestrial biosphere (~300–600 Pg C). In addition, recognizing the enhanced preservation of organic matter in ice age sediments of the deep Pacific Ocean helps reconcile previously unexplained inconsistencies among different geochemical and micropaleontological proxy records used to assess past changes in biological productivity of the ocean.

The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the previous IDP2014. For the first time, the IDP2017 contains data for a large suite of biogeochemical parameters as well as aerosol and rain data characterising atmospheric trace element and isotope (TEI) sources. The TEI data in the IDP2017 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at crossover stations. The IDP2017 consists of two parts: (1) a compilation of digital data for more than 450 TEIs as well as standard hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing an on-line atlas that includes more than 590 section plots and 130 animated 3D scenes. The digital data are provided in several formats, including ASCII, Excel spreadsheet, netCDF, and Ocean Data View collection. Users can download the full data packages or make their own custom selections with a new on-line data extraction service. In addition to the actual data values, the IDP2017 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering and for statistical analysis. Metadata about data originators, analytical methods and original publications related to the data are linked in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2017 as section plots and rotating 3D scenes. The basin-wide 3D scenes combine data from many cruises and provide quick overviews of large-scale tracer distributions. These 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of tracer plumes near ocean margins or along ridges. The IDP2017 is the result of a truly international effort involving 326 researchers from 25 countries. This publication provides the critical reference for unpublished data, as well as for studies that make use of a large cross-section of data from the IDP2017.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.

The measurable supply of Th-232 to the ocean can be used to derive the supply of other elements, which is more difficult to quantify directly. The measured inventory of an element divided by the derived supply yields a replacement time estimate, which in special circumstances is related to a residence time. As a proof of concept, Th-based supply rates imply a range in the replacement times of the rare earth elements in the North Atlantic that is consistent with the chemical reactivity of rare earth elements related to their ionic charge density. Similar estimates of replacement times for the bioactive trace elements (Fe, Mn, Zn, Cd, Cu, and Co), ranging from <5 years to >50,000 years, demonstrate the broad range of elemental reactivity in the ocean. Here we discuss how variations in source composition, fractional solubility ratios, or noncontinental sources, such as hydrothermal vents, lead to uncertainties in Th-based replacement time estimates. We show that the constraints on oceanic replacement time provided by the Th-based calculations are broadly applicable in predicting how elements are distributed in the ocean and for some elements, such as Fe, may inform us on how the carbon cycle may be impacted by trace element supply and removal.

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 (230Th, 231Pa) and primordial (232Th) 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, 230Th and 231Pa provide unique insights into the rates of scavenging. We find strong deficits in 230Th and 231Pa 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 230Th and 231Pa, expressed as distribution coefficients between solid and solution. A comparison of 230Th and 231Pa inventories with mantle-derived 3He as well as a mass balance of 230Th and 231Pa suggests continuous scavenging removal over the course of the entire 4000km transect. Unlike the two radiogenic isotopes, 232Th is enriched above what would be expected from assuming identical scavenging behavior to 230Th, indicating a hydrothermal source of colloidal, unreactive 232Th.

The equatorial Pacific Ocean is one of the major high-nutrient, low-chlorophyll regions in the global ocean. In such regions, the consumption of the available macro-nutrients such as nitrate and phosphate is thought to be limited in part by the low abundance of the critical micro-nutrient iron. Greater atmospheric dust deposition could have fertilized the equatorial Pacific with iron during the last ice age--the Last Glacial Period (LGP)--but the effect of increased ice-age dust fluxes on primary productivity in the equatorial Pacific remains uncertain. Here we present meridional transects of dust (derived from the (232)Th proxy), phytoplankton productivity (using opal, (231)Pa/(230)Th and excess Ba), and the degree of nitrate consumption (using foraminifera-bound δ(15)N) from six cores in the central equatorial Pacific for the Holocene (0-10,000 years ago) and the LGP (17,000-27,000 years ago). We find that, although dust deposition in the central equatorial Pacific was two to three times greater in the LGP than in the Holocene, productivity was the same or lower, and the degree of nitrate consumption was the same. These biogeochemical findings suggest that the relatively greater ice-age dust fluxes were not large enough to provide substantial iron fertilization to the central equatorial Pacific. This may have been because the absolute rate of dust deposition in the LGP (although greater than the Holocene rate) was very low. The lower productivity coupled with unchanged nitrate consumption suggests that the subsurface major nutrient concentrations were lower in the central equatorial Pacific during the LGP. As these nutrients are today dominantly sourced from the Subantarctic Zone of the Southern Ocean, we propose that the central equatorial Pacific data are consistent with more nutrient consumption in the Subantarctic Zone, possibly owing to iron fertilization as a result of higher absolute dust fluxes in this region. Thus, ice-age iron fertilization in the Subantarctic Zone would have ultimately worked to lower, not raise, equatorial Pacific productivity.