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Vanadium, a potentially toxic metal, is enriched in the environment from anthropogenic releases, particularly during fossil fuel production and use and steel manufacturing. Metal stable isotopes are sophisticated tools to trace pollution; however, only recent analytical advances have allowed for the accurate and precise measurement of vanadium isotope ratios (δ51V). To examine its potential as a tracer in terrestrial and aquatic ecosystems, δ51V was measured in soil, plant, lichen, marten, and lake sediment from sites near vanadium emissions at oil sands mines (Alberta, Canada) and in the sediment and biota (algae, zooplankton, fish) from a remote subarctic lake (Northwest Territories, Canada). Samples from Alberta had distinct δ51V values with marten liver the lowest (-1.7 ± 0.3‰), followed by lichen (-0.9 ± 0.1‰), soil (-0.7 ± 0.1‰), sediment (-0.5 ± 0.2‰), and plant root (-0.3 ± 0.2‰). Average values were lower than Alberta bitumen and petroleum coke (-0.1 ± 0.1‰). Plant roots had systematically higher δ51V than the soil from which they grew (Δ51Vplant-soil = 0.4 ± 0.1‰), while δ51V of lichen and aquatic biota were lower (0.1-0.3‰) than likely crustal sources. These δ51V measurements in terrestrial and aquatic biota demonstrate promise for tracer applications, although further study of its biological fractionation is needed.

Subduction zones are the focal points of mass transfer between the surface and deep Earth. Despite their significance, there remains substantial debate regarding the specific mechanisms of material transport from the slab to the overlying magmatic arc. Broadly, models accounting for slab material transport focus on the relative sequence of events promoting arc volcanism and, in particular, whether mobilization of the down-going slab leads or lags mixing with the mantle wedge. To address these uncertainties, we outline the utility of barium (Ba) isotope mass balance in subduction zones as a means to test different slab material transport models. Barium is a highly fluid-mobile element that is significantly enriched in arc magmas and is thus thought to be a sensitive tracer of slab material transport in arcs. We also present qualitative Ba isotopic mass balances for two well-characterized subduction zones—the Aleutian and Ryukyu magmatic arcs—by analyzing the Ba isotope systematics of their respective subduction inputs and outputs. Despite the narrow (and similar) Ba-isotope range of slab inputs to both systems, we find that erupted magmas exhibit systematic variations indicative of a small negative isotope fractionation during Ba mobilization (≈20–40 ppm AMU−1). We suggest that AOC (altered oceanic crust) is not the principal source of these negative isotope values using other geochemical parameters (e.g., Rb/Ba, Pb isotopes), and infer that the Ba isotope composition of AOC—though contributing a minor amount of Ba in these systems—is isotopically heavier than the overlying sediment package and the depleted mantle. Altogether, these findings are significant as they indicate that the magnitude of isotope fractionation associated with Ba mobilization is small relative to the likely isotopic contrast between subduction inputs in other subduction zones, such as beneath areas of strong ocean upwelling (e.g., South Sandwich, Kamchatka). Thus, we propose that the Ba isotope composition of erupted arc magmas holds great promise for constraining the importance of different slab components, which could help address uncertainties regarding the mechanism of slab material transport in subduction zones.

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.

Thallium (Tl) isotope compositions of ocean island basalts (OIBs) have been proposed as a novel tracer of subducted oceanic crust and sediments in ocean island basalt sources, which could act as direct confirmation that deep mantle recycling eventually resurfaces through mantle upwelling to form ocean island basalt magmatism. However, it is unknown if oceanic crust that went through an active subduction zone would retain the Tl isotope compositions recorded in hydrothermally altered oceanic crust and authigenic marine sediments. In this study we present Tl isotope and concentration data for samples of subducted oceanic crust from five different locations: Zambezi Belt, Zambia; Cabo Ortegal complex, Iberian Massif, Spain; Raspas Complex, southwest Ecuador; Syros island, Cyclades, Greece; Tian Shan, northwest China. Thallium concentrations in most samples follow strong linear relationships with K, Rb, Cs and Ba, which strongly suggest that the mineral phengite is the primary control of Tl abundances in subducted oceanic crust. This conclusion is consistent with recent Tl data sets for arc lavas that imply residual phengite in the arc lava source regions as a strong control of Tl recycling. We find that Tl isotope compositions vary widely and systematically in each location depending on the protolith, metamorphic and metasomatic history of the samples. Samples from Cabo Ortegal and Raspas Complex reveal Tl isotope compositions similar to their protoliths, which were comprised of low-temperature altered oceanic crust. Tian Shan metamorphic rocks and Zambian eclogites reveal invariant Tl isotope values indistinguishable from average mantle, which is best explained by overprinting by metamorphic fluids that contained high concentrations of Tl and other alkali metals. Samples from Syros reveal a range of Tl isotope compositions from normal mantle towards values for pelagic clay sediments. The sediment-like values in Syros likely arose from fluids released from the surrounding melange matrix that consists of serpentinite, metagabbros and metasediments. Each of the three Tl isotope ranges observed for subducted oceanic crust samples here are mirrored by individual OIB locations. Cabo Ortegal and Raspas Complex display Tl isotope compositions identical to St. Helena, suggesting that the HIMU component likely comprises subduction modified low-temperature altered oceanic crust. Thallium isotope ratios in Zambia and Tian Shan eclogites and blueschists are identical to lavas from Iceland, whereas Syros metamorphic rocks overlap almost exactly with lavas from Hawaii. Our data, therefore, show that Tl isotope compositions of oceanic crust and sediments can be traced through the subduction process and eventually is expressed largely unmodified in ocean island basalts.

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 22(5), (2021): e2020GC009608, https://doi.org/10.1029/2020GC009608.

Isotopic measurements of lunar and terrestrial rocks have revealed that, unlike any other body in the solar system, the Moon is indistinguishable from the Earth for nearly every isotopic system. This observation, however, contradicts predictions by the standard model for the origin of the Moon, the canonical giant impact. Here we show that the vanadium isotopic composition of the Moon is offset from that of the bulk silicate Earth by 0.18 ± 0.04 parts per thousand towards the chondritic value. This offset most likely results from isotope fractionation on proto-Earth during the main stage of terrestrial core formation (pre-giant impact), followed by a canonical giant impact where ~80% of the Moon originates from the impactor of chondritic composition. Our data refute the possibility of post-giant impact equilibration between the Earth and Moon, and implies that the impactor and proto-Earth mainly accreted from a common isotopic reservoir in the inner solar system., Here, the authors show that Earth and Moon are characterized by different vanadium isotope compositions, which is most likely resulting from vanadium isotope fractionation of the bulk silicate proto-Earth during the main stage of terrestrial core formation—followed by a canonical giant impact scenario, where 80% of the Moon originates from an impactor of chondritic composition.

Description, CAIs, the oldest solids in the Solar System, probably formed further away from the Sun than previously thought., Calcium-aluminum–rich inclusions (CAIs) in meteorites carry crucial information about the environmental conditions of the nascent Solar System prior to planet formation. Based on models of 50V–10Be co-production by in-situ irradiation, CAIs are considered to have formed within ~0.1 AU from the proto-Sun. Here, we present vanadium (V) and strontium (Sr) isotopic co-variations in fine- and coarse-grained CAIs and demonstrate that kinetic isotope effects during partial condensation and evaporation best explain V isotope anomalies previously attributed to solar particle irradiation. We also report initial excesses of 10Be and argue that CV CAIs possess essentially a homogeneous level of 10Be, inherited during their formation. Based on numerical modeling of 50V–10Be co-production by irradiation, we show that CAI formation during protoplanetary disk build-up likely occurred at greater heliocentric distances than previously considered, up to planet-forming regions (~1AU), where solar particle fluxes were sufficiently low to avoid substantial in-situ irradiation of CAIs.

For reasons that remain unclear, the initial appearance of large, morphologically complex life on Earth seems to have taken place in deep-marine environments. We provide new perspective on this topic by applying for the first time the vanadium (V) isotope paleoredox-proxy. We use shales in two different sections that preserve Doushantuo Member IV (South China) to reconstruct a global seawater V isotope composition ( δ 51 V = ∼ − 0.23 ± 0.06 ‰ ) during the late-Ediacaran (∼567 to ≥ 560 million years ago) that is much lighter than today. A mass-balance model informed by this composition is only reconciled by a global ocean in which hydrogen sulfide-rich (‘euxinic’) conditions were commonly present on continental shelves. Higher surface temperatures are a known driver of widespread euxinia in Earth's past, and if this was also the case during the late-Ediacaran, then relegation of large complex life to deep-marine settings at this time was probably driven to some extent by the persistently cooler and sulfide-poor conditions offered by this refuge.

The role that dissolved iron (dFe) rich Circumpolar Deep Water (CDW) might play in sustaining the consistently observed discrete patches of high chlorophyll biomass over Pennell Bank (PB) and Mawson Bank (MB) in the Ross Sea, was investigated during January/February 2011. Over a 26-day period, hydrographic and trace metal clean water sampling was carried out adjacent to both of these banks, in some cases repeatedly. Particulate sampling was also accomplished at selected stations by in situ pumping. The results indicate that the dFe content of the CDW is in fact reduced by on-shelf mixing with Antarctic Surface Water as it transitions into modified CDW (MCDW). Our stations above PB, where the maximum bloom is encountered, show no evidence of MCDW presence. In contrast, above MB, where there is a smaller persistent bloom, MCDW was observed. Although both of these stations displayed the imprint of sedimentary Fe input connected to the strong tidal cycles above the banks, the stronger near-bottom density gradient that MCDW produces appears to contribute to reduced vertical mixing of the sedimentary source. Thus, ironically, the presence of MCDW may be hindering the Fe supply to the surface waters, rather than being the source, as originally hypothesized.

Vanadium (V) isotopes have been hypothesized to record irradiation processes in the early solar system through production of the minor 50V isotope. However, because V only possesses two stable isotopes it is difficult to distinguish irradiation from other processes such as stable isotope fractionation and nucleosynthetic heterogeneity that could also cause V isotope variation. Here we perform the first detailed investigation of V isotopes in ordinary and carbonaceous chondrites to investigate the origin of any variation. We also perform a three-laboratory inter-calibration for chondrites, which confirms that the different chemical separation protocols do not induce V isotope analytical artifacts as long as samples are measured using medium resolution multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). Vanadium isotope compositions (51V/50V) of carbonaceous chondrites correlate with previously reported nucleosynthetically derived excesses in 54Cr. Both 51V and 54Cr are the most neutron-rich of their respective elements, which may suggest that pre-solar grains rich in r-process isotopes is the primary cause of the V–Cr isotope correlation. Vanadium isotope ratios of ordinary chondrite groups and Earth form a weaker correlation with 54Cr that has a different slope than observed for carbonaceous chondrites. The offset between carbonaceous and non-carbonaceous meteorites in V–Cr isotope space is similar to differences also reported for chromium, titanium, oxygen, molybdenum and ruthenium isotopes, which has been inferred to reflect the presence in the early solar system of two physically separated reservoirs. The V isotope composition of Earth is heavier than any meteorite measured to date. Therefore, V isotopes support models of Earth accretion in which a significant portion of Earth was formed from material that is not present in our meteorite collections.

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: Conway GEOTRACES - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González., National Science Foundation (U.S.) (Grant OCE-0608600), National Science Foundation (U.S.) (Grant OCE0938349), National Science Foundation (U.S.) (Grant OCE-1243377), National Science Foundation (U.S.) (Grant OCE-1546580)

Isotopes of the actinide elements protactinium (Pa), thorium (Th), and uranium (U), and the lanthanide element neodymium (Nd) are often used as complementary tracers of modern and past oceanic processes. The extraction of such elements from low abundance matrices, such as seawater and carbonate, is however labor-intensive and requires significant amounts of sample material. We here present a combined method for the extraction of Pa, Th, and Nd from 5 to 10 L seawater samples, and of U, Th, and Nd from

Vertical distributions of iron (Fe) concentrations and isotopes were determined in the total dissolvable and dissolved pools in the water column at three coastal stations located along the Peruvian margin, in the core of the Oxygen Minimum Zone (OMZ). The shallowest station 121 (161 m total water depth) was characterized by lithogenic input from the continental plateau, yielding concentrations as high as 456 nM in the total dissolvable pool. At the 2 other stations (stations 122 and 123), Fe concentrations of dissolved and total dissolvable pools exhibited maxima in both surface and deep layers. Fe isotopic composition (δ56Fe) showed a fractionation toward lighter values for both physical pools throughout the water column for all stations with minimum values observed for the surface layer (between -0.64 and -0.97 ‰ at 10 - 20 m depth) and deep layer (between -0.03 to -1.25 ‰ at 160 - 300 m depth). An Fe isotope budget was established to determine the isotopic composition of the particulate pool. We observed a range of δ56Fe values for particulate Fe from + 0.02 to -0.87 ‰, with lightest values obtained at water depth above 50 m. Such light values in the both particulate and dissolved pools suggest sources other than atmospheric dust deposition in the surface ocean, including lateral transport of isotopically light Fe. Samples collected at station 122 closest to the sediment show the lightest isotope composition in the dissolved and the particulate pools (-1.25 and -0.53 ‰ respectively) and high Fe(II) concentrations (14.2 ± 2.1 nM) consistent with a major reductive benthic Fe sources that is transferred to the ocean water column. A simple isotopic model is proposed to link the extent of Fe(II) oxidation and the Fe isotope composition of both particulate and dissolved Fe pools. This study demonstrates that Fe isotopic composition in OMZ regions is not only affected by the relative contribution of reductive and non-reductive shelf sediment input but also by seawater-column processes during the transport and oxidation of Fe from the source region to open seawater.

Sequestration of carbon by the marine biological pump depends on the processes that alter, remineralize, and preserve particulate organic carbon (POC) during transit to the deep ocean. Here, we present data collected from the Great Calcite Belt, a calcite-rich band across the Southern Ocean surface, to compare the transformation of POC in the euphotic and mesopelagic zones of the water column. The 234Th-derived export fluxes and size-fractionated concentrations of POC, particulate inorganic carbon (PIC), and biogenic silica (BSi) were measured from the upper 1000 m of 27 stations across the Atlantic and Indian sectors of the Great Calcite Belt. POC export out of the euphotic zone was correlated with BSi export. PIC export was not, but did correlate positively with POC flux transfer efficiency. Moreover, regions of high BSi concentrations, which corresponded to regions with proportionally larger particles, exhibited higher attenuation of > 51 μm POC concentrations in the mesopelagic zone. The interplay among POC size partitioning, mineral composition, and POC attenuation suggests a more fundamental driver of POC transfer through both depth regimes in the Great Calcite Belt. In particular, we argue that diatom-rich communities produce large and labile POC aggregates, which not only generate high export fluxes but also drive more remineralization in the mesopelagic zone. We observe the opposite in communities with smaller calcifying phytoplankton, such as coccolithophores. We hypothesize that these differences are influenced by inherent differences in the lability of POC exported by different phytoplankton communities.

The concentration and the major phase composition (particulate organic matter, CaCO3, opal, lithogenic matter, and iron and manganese oxyhydroxides) of marine particles is thought to determine the scavenging removal of particle-reactive TEIs. Particles are also the vector for transferring carbon from the atmosphere to the deep ocean via the biological carbon pump, and their composition may determine the efficiency and strength of this transfer. Here, we present the first full ocean depth section of size-fractionated (1–51 µm, >51 µm) suspended particulate matter (SPM) concentration and major phase composition from the US GEOTRACES North Atlantic Zonal Transect between Woods Hole, MA and Lisbon, Portugal conducted in 2010 and 2011. Several major particle features are notable in the section: intense benthic nepheloid layers were observed in the western North American margin with concentrations of SPM of up to 1648 µg/L, two to three orders of magnitude higher than surrounding waters, that were dominated by lithogenic material. A more moderate benthic nepheloid layer was also observed in the eastern Mauritanian margin (44 µg/L) that had a lower lithogenic content and, notably, significant concentrations of iron and manganese oxyhydroxides (2.5% each). An intermediate nepheloid layer reaching 102 µg/L, an order of magnitude above surrounding waters, was observed associated with the Mediterranean Outflow. Finally, there was a factor of two enhancement in SPM at the TAG hydrothermal plume due almost entirely to the addition of iron oxyhydroxides from the hydrothermal vent. We observe correlations between POC and CaCO3 in large (>51 µm) particles in the upper 2000 m, but not deeper than 2000 m, and no correlations between POC and CaCO3 at any depth in small (

Sediment is actively being subducted in every convergent margin worldwide. Yet, geochemical data for arc lavas from several subduction zones, such as Northern Tonga and Costa Rica have revealed either attenuated or limited evidence for sediment in their mantle source regions. Here we use thallium (Tl) isotopes to trace slab components in lavas from the Tonga-Kermadec and Central American arcs. In general, both arcs display Tl isotope data that are most compatible with addition of sediment to the sub-arc mantle from the subducting slab. This evidence is particular strong in the Tonga-Kermadec arc where pelagic clays dominate the Tl budget along the entire arc. Contributions from altered oceanic crust as well as the Louisville Seamount chain that subducts underneath Northern Tonga are not visible in Tl isotopes, which is likely due to the very high Tl concentrations found in pelagic sediments outboard of the Tonga-Kermadec arc. Lavas from Central America reveal variable and systematic Tl isotope compositions along-strike. In particular, lavas from Nicaragua are dominated by contributions from sediments, whereas Costa Rican samples reveal a significant altered oceanic crust component with little influence from sediments on thallium isotope composition. The absence of a sediment signature in Costa Rica corresponds with the Cocos Ridge and the seamount province subduction, which results in a thinner sediment cover. Furthermore, the subducted sediment is dominated by carbonates with very low Tl concentrations and, therefore, small amounts of carbonate sediment added to the mantle wedge do not contribute significantly to the overall Tl budget. A review of Tl isotope and concentration data from the Aleutians, Marianas, Tonga-Kermadec and Central American arcs demonstrate that pelagic sediments are detectable in most arcs, whereas altered oceanic crust components only become appreciable when sediment Tl concentrations are very low (e.g. carbonate) or if sediments are no longer a significant component of the subducting slab (e.g. slab melting in Western Aleutians). As such, Tl isotopes are a promising tool to trace sediment subduction although this requires at least some pelagic sediment is present in the subducted sediment package. We suggest that thallium partitioning between the slab and mantle wedge is most likely controlled by retention in phengite or by partitioning into fluids. Residual phengite likely produces high Cs/Tl ratios because Tl should be more compatible in phengite than is Cs, however, this conclusion needs experimental verification. The stability of phengite is lower at higher fluid contents, which results in hyperbolic relationships between Cs/Tl and possible indicators of fluids such as Sr/Nd and Ba/Th. Thus, combined Tl isotopic and elemental systematics not only provide detailed information about the specific slab components that contribute to arc lavas, but also potentially shed light on the mineralogy and physical conditions of subducting slabs.

The US GEOTRACES program will generate marine particulate trace metal data over spatial scales and depth resolutions never before sampled. In preparation for these analyses, we conducted a four laboratory intercomparison exercise to determine our degree of intercalibration and to examine how several total digestion procedures perform on marine particles collected on polyethersulfone (PES, Pall Supor) filters. In addition, we present a new chemical method for complete dissolution of PES filters using a combination of sulfuric acid and hydrogen peroxide called Piranha reagent that can be conducted using minimal specialized equipment. Intralaboratory subsampling variability across 142 mm particulate matter filters, for subsamples representing approximately 10 L filtered seawater, was measured at an element-dependent 1% to 9% (RSD: 1δ/x-, %), whereas interlaboratory variability accounted for an additional 5% to 42% variability. Lab- and element-specific trends in recoveries are discussed, though all digestion methods tested appear to completely solubilize particulate material. We recommend rigorous determination of digest-acid and/or filter process blanks, as some particulate elements (namely Pb and Zn) have natural abundances that approach blank values.

Scleractinian corals have a global distribution ranging from shallow tropical seas to the depths of the Southern Ocean. Although this distribution is indicative of the corals having a tolerance to a wide spectrum of environmental conditions, individual species seem to be restricted to a much narrower range of ecosystem variables. One way to ascertain the tolerances of corals, with particular focus on the potential impacts of changing climate, is to reconstruct their growth history across a range of environmental regimes. This study examines the spatial and temporal distribution of the solitary scleractinian corals Desmophyllum dianthus, Gardineria antarctica, Balanophyllia malouinensis, Caryophyllia spp. and Flabellum spp. from five sites in the Drake Passage which cross the major frontal zones. A rapid reconnaissance radiocarbon method was used to date more than 850 individual corals. Coupled with U-Th dating, an age range of present day back to more than 100 thousand years was established for corals in the region. Within this age range there are distinct changes in the temporal and spatial distributions of these corals, both with depth and latitude, and on millennial timescales. Two major patterns that emerge are: (1) D. dianthus populations show clear variability in their occurrence through time depending on the latitudinal position within the Drake Passage. North of the Subantarctic Front, D. dianthus first appears in the late deglaciation (~17,000 years ago) and persists to today. South of the Polar Front, in contrast, early deglacial periods, with a few modern occurrences. A seamount site between the two fronts exhibits characteristics similar to both the northern and southern sites. This shift across the frontal zones within one species cannot yet be fully explained, but it is likely to be linked to changes in surface productivity, subsurface oxygen concentrations, and carbonate saturation state. (2) at locations where multiple genera were dated, differences in age and depth distribution of the populations provide clear evidence that each genus has unique environmental requirements to sustain its population.

The toxigenic diatom Pseudo-nitzschia cuspidata, isolated from the U.S. Pacific Northwest, was examined in unialgal batch cultures to evaluate domoic acid (DA) toxicity and growth as a function of light, N substrate, and growth phase. Experiments conducted at saturating (120 μmol photons · m(-2) · s(-1) ) and subsaturating (40 μmol photons · m(-2) · s(-1) ) photosynthetic photon flux density (PPFD), demonstrate that P. cuspidata grows significantly faster at the higher PPFD on all three N substrates tested [nitrate (NO3 (-) ), ammonium (NH4 (+) ), and urea], but neither cellular toxicity nor exponential growth rates were strongly associated with one N source over the other at high PPFD. However, at the lower PPFD, the exponential growth rates were approximately halved, and the cells were significantly more toxic regardless of N substrate. Urea supported significantly faster growth rates, and cellular toxicity varied as a function of N substrate with NO3 (-) -supported cells being significantly more toxic than both NH4 (+) - and urea-supported cells at the low PPFD. Kinetic uptake parameters were determined for another member of the P. pseudodelicatissima complex, P. fryxelliana. After growth of these cells on NO3 (-) they exhibited maximum specific uptake rates (Vmax ) of 22.7, 29.9, 8.98 × 10(-3) · h(-1) , half-saturation constants (Ks ) of 1.34, 2.14, 0.28 μg-at N · L(-1) , and affinity values (α) of 17.0, 14.7, 32.5 × 10(-3) · h(-1) /(μg-at N · L(-1) ) for NO3 (-) , NH4 (+) and urea, respectively. These labo-ratory results demonstrate the capability of P. cuspidata to grow and produce DA on both oxidized and reduced N substrates during both exponential and stationary growth phases, and the uptake kinetic results for the pseudo-cryptic species, P. fryxelliana suggest that reduced N sources from coastal runoff could be important for maintenance of these small pennate diatoms in U.S. west coast blooms, especially during times of low ambient N concentrations.

© The Author(s), 2016. This is the authors version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geochimica et Cosmochimica Acta 181 (2016): 217-237, doi:10.1016/j.gca.2016.03.010. Sediment transport from the subducted slab to the mantle wedge is an important process in understanding the chemical and physical conditions of arc magma generation. The Aleutian arc offers an excellent opportunity to study sediment transport processes because the subducted sediment flux varies systematically along strike (Kelemen et al., 2003) and many lavas exhibit unambiguous signatures of sediment addition to the sub-arc mantle (Morris et al., 1990). However, the exact sediment contribution to Aleutian lavas and how these sediments are transported from the slab to the surface are still debated. Thallium (Tl) isotope ratios have great potential to distinguish sediment fluxes in subduction zones because pelagic sediments and low temperature altered oceanic crust are highly enriched in Tl and display heavy and light Tl isotope compositions, respectively, compared with the upper mantle and continental crust. Here, we investigate the Tl isotope composition of lavas covering almost the entire Aleutian arc a well as sediments outboard of both the eastern (DSDP Sites 178 and 183) and central (ODP Hole 886C) portions of the arc. Sediment Tl isotope compositions change systematically from lighter in the Eastern to heavier in the Central Aleutians reflecting a larger proportion of pelagic sediments when distal from the North American continent. Lavas in the Eastern and Central Aleutians mirror this systematic change to heavier Tl isotope compositions to the west, which shows that the subducted sediment composition is directly translated to the arc east of Kanaga Island. Moreover, quantitative mixing models of Tl and Pb, Sr and Nd isotopes reveal that bulk sediment transfer of ~0.6-1.0% by weight in the Eastern Aleutians and ~0.2-0.6% by weight in the Central Aleutians can account for all four isotope systems. Bulk mixing models, however, require that fractionation of trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd in the Central and Eastern Aleutians occurs after the sediment component was mixed with the mantle wedge. Models of Sr and Nd isotopes that involve sediment melting require either high degrees of sediment melting (>50%), in which case trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd of Aleutian lavas need to be produced after mixing with the mantle, or significant fluid additions from the underlying oceanic crust with Sr and Nd isotope compositions indistinguishable from the mantle wedge as well as high Sr/Nd ratios similar to that of low (

The GEOTRACES program requires the analysis of large numbers of seawater samples for ^(232)Th, ^(230)Th, and ^(231)Pa. During the GEOTRACES international intercalibration exercise, we encountered unexpected difficulties with recovery and contamination of these isotopes, ^(232)Th in particular. Experiments were carried out to identify the source of these issues, leading to a more streamlined and efficient procedure. The two particular problems that we identified and corrected were (1) frits in columns supplied by Bio-Rad Laboratories caused loss of Th during column chemistry and (2) new batches of AG1-X8 resin supplied by Bio-Rad Laboratories released more than 100 pg of ^(232)Th during elution of sample. To improve yields and blanks, we implemented a series of changes including switching to Eichrom anion exchange resin (100-200 μm mesh) and Environmental Express columns. All Th and Pa samples were analyzed on a Neptune multi-collector inductively-coupled-plasma mass spectrometer (MC-ICP-MS) using peak hopping of ^(230)Th and ^(229)Th on the central SEM, with either ^(232)Th, ^(236)U (or both) used to monitor for beam intensity. We used in-house laboratory standards to check for machine reproducibility, and the GEOTRACES intercalibration standard to check for accuracy. Over a 1-y period, the 2 s.d. reproducibility on the GEOTRACES SW STD 2010-1 was 2.5% for ^(230)Th, 1.8% for ^(232)Th, and 4% for ^(231)Pa. The lessons learned during this intercalibration process will be of value to those analyzing U-Th-Pa and rare earth elements as part of the GEOTRACES program as well as those using U-series elements in other applications that require high yields and low blanks, such as geochronology.

Deep-sea scleractinian corals precipitate aragonite skeletons that provide valuable archives of past ocean conditions. During calcification biological mediation causes variability in trace metal incorporation and isotopic ratios of the aragonite such that signals caused by environmental controls can be overwhelmed. This complicates the interpretation of geochemical proxies used for paleo-reconstructions. In this study we examine the environmental controls on the Mg/Li ratio of 34 individuals from seven genera of deep-sea scleractinian corals: Desmophyllum, Balanophyllia, Caryophyllia, Enallopsammia, Flabellum, Trochocyanthus, and Lophelia. In addition we examine the microscale distributions of Mg and Li in Desmophyllum and Balanophyllia using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Both Mg/Ca and Li/Ca ratios increased by more than a factor of two in the center of calcification regions compared to the outer, fibrous regions of the coral skeleton. As a result, replicate ~ 10 mg subsamples of coral show less variability in the Mg/Li ratio than Mg/Ca. Microscale Mg and Li results are consistent with Rayleigh-type incorporation of trace metals with additional processes dominating the composition within centers of calcification. Comparison of Mg/Li to seawater properties near the site of collection shows that the ratio is not controlled by either carbonate ion or salinity. It appears that temperature is the major control on the Mg/Li ratio. For all 34 samples the temperature correlation (R2 = 0.62) is significantly better than for Mg/Ca (R2 = 0.06). For corals of the family Caryophyllidae the R2 value increases to 0.82 with the exclusion of one sample that was observed to have an altered, chalky texture. Despite this excellent correlation the scatter in the data suggests that the Mg/Li ratio of deep-sea corals cannot be used to reconstruct temperature to better than approximately ± 1.6 °C without better temperature control and additional calibration points on modern coral samples.

International audience; In this paper, we applied a reliable technique for measuring Fe isotope variations in coastal seawater at nanomolar levels. Iron was directly pre-concentrated from acidified seawater samples onto a nitrilotriacetic acid chelating resin and further purified using anion-exchange resin. Sample recovery, determined using a standard addition method, was essentially quantitative. Iron was then determined using a high-resolution multicollector ICP-MS (Neptune) coupled to an ApexQ desolvation introduction system. The external precision for δ56Fe values was 0.11‰ (2s) when using total a Fe quantity between 25 and 100 ng. We initially applied this technique to measure the Fe isotope composition of dissolved Fe from several coastal environments in the north-eastern United States and we observed a range of δ56Fe values between -0.9‰ and 0.1‰ relative to the IRMM-14 reference material. Iron isotope compositions of several reference water materials for inter-laboratory comparisons were also reported. Our results suggest that iron in coastal seawater, derived from benthic diagenesis and/or groundwater has negative Fe isotopic signatures that are distinct from other iron sources such as atmospheric deposition and rivers. Dans cet article, nous appliquons une technique fiable pour mesurer les variations isotopiques du Fer dans l'eau de mer côtière au niveau nanomolaire. Le Fer a été directement préconcentréà partir d'échantillons d'eau de mer acidifiés par chélation sur résine à l'aide d'acide nitrilotriacétique et puis ensuite purifiéà l'aide de résine échangeuse d'anions. La récupération de l'échantillon, déterminé selon une méthode d'addition de standard, est essentiellement quantitative. Le Fer a été ensuite déterminé en utilisant un multicollecteur ICP-MS à haute résolution (Neptune) coupléà un système d'introduction par désolvatation de type ApexQ. La précision externe pour δ56Fe était de 0.11‰ (2s) lors de l'utilisation d'une quantité totale de Fer comprise entre 25 et 100 ng. Nous avons d'abord appliqué cette technique pour mesurer la composition isotopique du Fer, pour du Fer dissous provenant de plusieurs milieux côtiers du Nord Est des Etats-Unis. Nous avons observé des valeurs de δ56Fe comprises entre -0.9‰à 0.1‰ par rapport au matériau de référence IRMM-14. Les compositions isotopiques en fer de plusieurs eaux de référence utilisées pour des comparaisons inter-laboratoires sont également présentées. Nos résultats suggèrent que les eaux marines côtières, provenant de la diagenèse benthique, et les eaux souterraines ont des signatures isotopiques du Fer négatives qui sont distinctes de celles des autres sources de fer tel que les dépôts atmosphériques et les rivières.