Abstract The isotopic composition of vanadium (V) in hotspot lavas provides new insight into the source of deep mantle heterogeneities. We measured V isotopic compositions in Paleogene Baffin Island lavas, which erupted above the Iceland plume head, to constrain the mantle plume source. These lavas have V isotopic compositions that are lighter (−0.96 ± 0.02‰, 2 s.e., n = 25) than most other mantle‐derived lavas (δ 51 V = −0.89 ± 0.02, 2 s.e., n = 69). This difference is statistically significant with a 99.8% confidence interval. The measured 51 V/ 50 V ratio in Baffin Island is difficult to reconcile with the assimilation of continental crust, fractional crystallization, crystal accumulation, isotopic fractionation during mantle melting, or the recycling of sediments. Possibly, the Baffin Island lavas sample an ancient (>3 Ga) melt‐depleted mantle reservoir. However, this cannot explain the similarly low 51 V/ 50 V measured in younger lavas related to the Iceland plume. Instead, the V isotopic composition of Baffin Island lavas may be consistent with the incorporation of core‐affected material into the Iceland plume. The advection of bulk core into the lowermost mantle is improbable, so we hypothesize that, if the V isotopic difference is due to core‐mantle interactions, V partitioned into core‐exsolved oxides that imparted a core‐like V isotopic composition on the Baffin Island mantle source.

Abstract The isotopic composition of vanadium (V) in hotspot lavas provides new insight into the source of deep mantle heterogeneities. We measured V isotopic compositions in Paleogene Baffin Island lavas, which erupted above the Iceland plume head, to constrain the mantle plume source. These lavas have V isotopic compositions that are lighter (−0.96 ± 0.02‰, 2 s.e., n = 25) than most other mantle‐derived lavas (δ51V = −0.89 ± 0.02, 2 s.e., n = 69). This difference is statistically significant with a 99.8% confidence interval. The measured 51V/50V ratio in Baffin Island is difficult to reconcile with the assimilation of continental crust, fractional crystallization, crystal accumulation, isotopic fractionation during mantle melting, or the recycling of sediments. Possibly, the Baffin Island lavas sample an ancient (>3 Ga) melt‐depleted mantle reservoir. However, this cannot explain the similarly low 51V/50V measured in younger lavas related to the Iceland plume. Instead, the V isotopic composition of Baffin Island lavas may be consistent with the incorporation of core‐affected material into the Iceland plume. The advection of bulk core into the lowermost mantle is improbable, so we hypothesize that, if the V isotopic difference is due to core‐mantle interactions, V partitioned into core‐exsolved oxides that imparted a core‐like V isotopic composition on the Baffin Island mantle source.

AbstractOxic pelagic clays are an important component of seafloor sediment that may hold valuable information about past ocean chemistry due to their affinity for and accumulation of biogeochemically important metals. We present a new approach to calculating site‐specific sedimentation rates (SRs) by comparing authigenic sediment thorium isotope compositions (230Th/232Th) to seawater dissolved 230Th/232Th in a suite of deep (>3,000 m) pelagic core sites. We extracted the authigenic sediment fraction using an HHAc leach protocol, which major element chemistry (Al, Mn, Fe, Ti) suggested was less affected by lithogenic contamination than the HCl leach. Four different methods were tested for extracting the appropriate initial 230Th/232Th from seawater: using either the nearest water column station (methods 1 and 2) or a regionally averaged profile (methods 3 and 4) and using either the bottommost profile measurement (methods 1 and 3) or linear regression of the profile and extrapolation to the seafloor (methods 2 and 4). Method 3 outperformed the other methods in reconstructing previously published SRs from pelagic clays in the North Pacific. The new thorium‐based SRs were then combined with estimates from the total sediment thickness on ocean crust and non‐lithogenic cobalt accumulation to determine the best estimates for SRs of oxic pelagic clays. The Pacific has the lowest SR (median 0.28 cm/kyr), while the Atlantic is higher (median 0.46 cm/kyr) and the Indian Ocean is highest (median 0.75 cm/kyr). These new estimates are consistent with the expected spatial patterns of sedimentation, but they revise the absolute SR values downward from available gridded SR maps.

Mechanisms regulating material transfer from subducted slabs to arc magmas remain debated, centered on metasomatized mantle wedge interactions versus mélange mobilization at the slab-mantle interface. The South Sandiwch Islands arc offers a unique setting to distinguish between these models due to the significant barium isotope contrast between altered oceanic crust and sediments, the latter displaying unusually light barium isotope compositions compared to the global sediment range. Here we show substantial barium isotope variations coupled with invariant strontium isotope ratios in arc lavas, consistent with mélange mobilization beneath the arc. Northern arc lavas display a broader range of barium isotope values than expected from slab inputs, suggesting barium isotope fractionation during slab material transport, potentially driven by phengite-related barium retention within the mélange. Notably, sediments, rather than altered oceanic crust, emerge as the dominant source of barium in arc lavas. While a comparison of barium isotope data from four additional arcs indicates mantle wedge metasomatism remains visible in several cases, mélange mobilization is consistent with available data across all of these subduction zones.

The isotopic composition of barium (δ138Ba) has emerged as a powerful tracer of deep-ocean circulation, water mass provenance, and the oceanic Ba cycle. Although the δ138Ba of water masses is primarily controlled by the balance between pelagic barite precipitation and Ba resupply from ocean circulation, questions remain regarding the isotopic offset associated with pelagic barite formation and how the resultant Ba isotope compositions are transmitted through the water column to marine sediments. To address these questions, we conducted a time series study of dissolved, particulate, and sedimentary Ba chemistry in the Gulf of Aqaba (GOA), in the northern Red Sea, from January 2015 to April 2016. These data span significant seasonal changes in hydrography, primary productivity, and aerosol deposition, revealing three principal findings. First, the dissolved Ba chemistry of the GOA is vertically uniform across the time series, largely reflecting water mass advection from the Red Sea, with mean dissolved Ba concentrations of 47.9 ± 4.7 nmol kg−1 and mean δ138Ba = +0.55‰ ± 0.07‰ (±2 SD, n = 18). Second, despite significant variations in particulate matter composition and flux, the δ138Ba of sinking particulate Ba maintained a consistent isotope composition across different depths and over time at +0.09‰ ± 0.06‰ (n = 26). Consequently, these data imply a consistent Ba isotope offset of −0.46‰ ± 0.10‰ (±2 SD) between sinking particulates and seawater. This offset is similar to those determined in previous studies and indicates that it applies to particulates formed across diverse environmental conditions. Third, barite-containing sediment samples deposited in the GOA exhibit δ138Ba = +0.34‰ ± 0.03‰, which is offset by approximately +0.2‰ relative to sinking particles. While the specific mechanism driving this offset remains unresolved, our results highlight the importance of performing site-specific proxy validations and exercising careful site selection when applying novel paleoceanographic proxies.

Studies of magnesium (Mg) isotope ratios in subduction zone lavas have revealed small but significant offsets from the mantle value with enrichments in the heavy isotopes. However, the very high concentration of Mg in the mantle contrasts with much lower concentrations in the subducted igneous crust and oceanic sediments, making these subduction components unlikely vehicles of the Mg isotope anomalies in arc lavas. Only serpentinites, which in various proportions form part of oceanic plates, have high Mg contents comparable to fresh mantle rocks, and they have thus been regarded as a potential source of exotic Mg in the source of arc magmas.In this study we analyzed serpentinite samples from different oceanic settings for their Mg isotopic compositions. The majority of samples are indistinguishable from the depleted mantle (delta 26Mg=-0.24 parts per thousand +/- 0.04 parts per thousand) irrespective of their origin. Only a small number of seafloor-weathered serpentinites are slightly enriched in the heavy isotopes (up to delta 26Mg=-0.14 parts per thousand +/- 0.03 parts per thousand), implying that bulk serpentinites are unlikely sources of isotopically anomalous Mg in subduction zones.We also developed a partial dissolution method in which 5 % acetic acid for 180 min was shown to fully dissolve the minerals brucite and iowaite while leaving the serpentine mineral chrysotile essentially undissolved.Partial dissolution of 11 bulk serpentinite samples revealed Mg isotopic composition of brucite (+/- iowaite) that is systematically similar to 0.25 parts per thousand heavier than that of coexisting serpentine. Thus, preferential breakdown of brucite and/or iowaite in a subducted slab prior to serpentine could preferentially release isotopically heavy Mg, which could subsequently be transported into the source region of arc magmas. Such a scenario would require brucite/iowaite breakdown to occur at pressures in excess of 3 GPa and produce fluids with very high concentrations of Mg that could be transported to arc magma source regions. Whether these conditions are met in nature has yet to be experimentally investigated.

Cadmium (Cd) has a nutrient-like distribution in the ocean, similar to the macronutrient phosphate. Significant isotope fractionation induced by the biological cycling of Cd makes it a potential tracer for nutrients and productivity. However, the Cd flux and Cd isotope composition of marine sediments may also be influenced by local redox conditions and partial remineralization of organically hosted Cd. These confounding factors are underconstrained and render it challenging to use Cd as a reliable paleoproxy. To understand the relative importance of each of these processes, we examined the Cd isotope systematics of 69 modern sediments deposited across a wide range of environments. We complement these data with four profiles of particulate Cd isotope compositions from the Southern Ocean. We report three main results. First, we show that the sedimentary flux of Cd is tightly coupled to that of organic matter. Second, most Cd burial occurs in regions with some bottom-water oxygen, and the flux of CdS to anoxic regions is, globally, minor. Finally, we find that remineralization can substantially modify sedimentary Cd isotope compositions, though it is challenging to relate pelagic and sedimentary processes. For example, we find that the relationship between sedimentary Cd isotope compositions and surface seawater [Cd] is the reverse of that predicted by isotope reactor models. Likewise, sedimentary Cd isotope compositions are anti-correlated with bottom-water oxygen. While this pattern is consistent with preferential remineralization of isotopically heavy Cd, profiles of marine particulate matter reveal the reverse, whereby the Cd isotope composition of large particles, which are most likely to reach the seafloor, becomes increasingly 'heavy' with depth. These results highlight how productivity, redox, and remineralization all influence the flux and isotope composition of Cd to marine sediments. While our study suggests that there is no simple way to relate sedimentary Cd isotopes to surface nutrient utilization, our data point toward several potential controls that could form the basis of novel proxies for local redox conditions and remineralization.

The oxygenation state of the Mid-Proterozoic (1.8 – 0.8 Ga) ocean and atmosphere is heavily debated and has implications for the relationship between environmental O2 and the emergence of complex life. While recent geochemical proxy studies of Mesoproterozoic marine sedimentary rocks inform a picture of deep water redox dominated by widespread ferruginous (iron-rich and anoxic) conditions, a growing number of datasets indicate that short-lived ocean oxygenation events were recurrent features of this time interval. The spatial extent of previously reported Mesoproterozoic oxygen pulses is often unclear as they are predominantly observed via localized environmental proxies. In this study, we use the vanadium (V) and thallium (Tl) isotope paleoredox proxies to provide a global marine redox perspective during a short interval of the Mesoproterozoic. We reconstructed seawater V (δ51VSW) and Tl (ε205TlSW) isotopic compositions from shales from Unit 2 of the 1.38-1.39 Ga Xiamaling Formation (North China craton) that show an up-section shift in average δ51VSW values from 0.02‰ to 0.15‰ and a short-lived perturbation in ε205TlSW from −2.5 to −4.2. Mass-balance models for both isotopic systems are consistent with these isotopic shifts representing an expansion of oxic water column and porewater conditions from a more anoxic (ferruginous) baseline ocean state. The short-lived nature of the ε205TlSW perturbation versus the shift in δ51VSW suggests that environments sufficiently oxygenated to support widespread Mn oxide burial were only established for a fraction of this overall oxic expansion. The ocean residence time of V and Tl requires that this oxygenation event was global in extent and may reflect either a purely oceanic phenomenon such as global deepening of the oxycline, or alternatively, global ocean-atmosphere equilibration during a short-lived episode of elevated atmospheric oxygenation.

Mantle plumes with higher 3 He/ 4 He than the upper mantle imply solar-like gases captured during planetary accretion are preserved in the deep Earth, possibly in the outer core [1].We hypothesize that, if mantle plumes entrain core-derived helium, that they might also incorporate vanadium (V) from the core.Most mantle-derived peridotites, komatiites, ocean island basalts, and mid-ocean ridge basalts have invariant δ 51 V (-0.856 ± 0.020 ‰ [3], where δ 51 V = ([ 51 V/ 50 V) sample/ 51 V/ 50 V) standard ] -1) × 10 3 )

The barium isotope composition of sedimentary barite (BaSO4, barium sulfate) is emerging as a powerful tracer of the sources and cycling of Ba in modern and ancient marine environments. To reliably use Ba isotopes to interrogate the marine Ba cycle, it is important to identify and constrain processes that fractionate the isotope composition of Ba in BaSO4. Of particular interest is ion exchange: micro-scale dissolution and precipitation that occurs in mineral-fluid systems at chemical equilibrium. This process is often important in systems where minerals, such as BaSO4, and a fluid remain in contact for prolonged periods of time; however, the impact of ion exchange on Ba isotope compositions in BaSO4 is unknown. To constrain the rate and isotopic effect associated with ion exchange in BaSO4-fluid systems, we conducted a series of experiments under marine-relevant conditions and interpreted the results using a multi-phase time-dependent numerical reactor model. From a series of isotope-tracer experiments, we find that BaSO4-fluid ion exchange progresses at a rate between 5 and 53 pmol m(-2) s(-1). In a parallel set of experiments used to assess mass-dependent isotope fractionation of Ba, the combined effect of BaSO4 dissolution and precipitation while at chemical equilibrium was found to result in the continued evolution of Ba isotopes and produced a modeled offset of Delta Ba-138(barite)-dBa =-0.10 +/- 0.05 parts per thousand at isotopic equilibrium. We then constrained the magnitude of isotopic fractionation during BaSO4 dissolution by fitting our data in the numerical reactor model and using previous estimates of Ba isotope fractionation during BaSO4 precipitation (alpha(precipitation) = 0.99968 +/- 0.00002). At chemical equilibrium, we find our data are best explained by an alpha(dissolution) = 0.99978 +/- 0.00006, implying that BaSO4 dissolution releases isotopically 'light' Ba to solution. Since the magnitude of the isotope effects associated with BaSO4 precipitation and dissolution are imbal-anced, ion exchange will tend to alter the isotope composition of co-located BaSO4 and fluids until the two phases are offset by approximate to 0.10 parts per thousand. The importance of this effect on sedimentary BaSO4 likely depends on several factors and we suggest multiple site-screening criteria to maximize the utility of this emerging proxy.

Fluids mediate the transport of subducted slab material and play a crucial role in the generation of arc magmas. However, the source of subduction-derived fluids remains debated. The Kamchatka arc is an ideal subduction zone to identify the source of fluids because the arc magmas are comparably mafic, their source appears to be essentially free of subducted sediment-derived components, and subducted Hawaii-Emperor Seamount Chain (HESC) is thought to contribute a substantial fluid flux to the Kamchatka magmas. Here we show that Tl isotope ratios are unique tracers of HESC contribution to Kamchatka arc magma sources. In conjunction with trace element ratios and literature data, we trace the progressive dehydration and melting of subducted HESC across the Kamchatka arc. In succession, serpentine (<100 km depth), lawsonite (100-250 km depth) and phengite (>250 km depth) break down and produce fluids that contribute to arc magmatism at the Eastern Volcanic Front (EVF), Central Kamchatka Depression (CKD), and Sredinny Ridge (SR), respectively. However, given the Tl-poor nature of serpentine and lawsonite fluids, simultaneous melting of subducted HESC is required to explain the HESC-like Tl isotope signatures observed in EVF and CKD lavas. In the absence of eclogitic crust melting processes in this region of the Kamchatka arc, we propose that progressive dehydration and melting of a HESC-dominated mélange offers the most compelling interpretation of the combined isotope and trace element data.

Abstract 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? Here, 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 day−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.

The oxygenation state of the Mid-Proterozoic (1.8 – 0.8 Ga) ocean and atmosphere is heavily debated and has implications for the relationship between environmental O 2 and the emergence of complex life. While recent geochemical proxy studies of Mesoproterozoic marine sedimentary rocks inform a picture of ocean redox dominated by widespread ferruginous (iron-rich and anoxic) conditions [1], a growing number of datasets indicate that short-lived ocean oxygenation events were recurrent features in this time interval [2–5]. The spatial extent of previously reported Mesoproterozoic oxygen pulses is unclear as they are observed via fundamentally localized environmental proxies. In this study, we report a novel combined application of the vanadium (V) and thallium (Tl) isotope paleoredox proxies to provide a global perspective to this time interval. We reconstructed seawater V (δ 51 V SW ) and Tl (ε 205 Tl SW ) isotopic compositions for shales from Unit 2 of the 1.38-1.39 Ga Xiamaling formation (North China craton) and observed an up-section shift to higher average δ 51 V SW values and a short-lived negative perturbation in ε 205 Tl SW that are stratigraphically near-coincident. Mass balance models for both isotopic systems are consistent with these isotopic shifts representing an expansion of oxic sedimentary environments from an anoxic (ferruginous) baseline ocean state, with the delayed shift in ε 205 Tl SW

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.

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 (delta V-51=similar to-0.23 +/- 0.06 parts per thousand) during the late-Ediacaran (similar to 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. (C) 2021 Elsevier B.V. All rights reserved.

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.

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.

Radium-226 ( 226 Ra; t 1/2 = 1,602 yr) and Ba are strongly correlated to one another in the ocean. However, the extent to which this correlation reflects an active biogeochemical coupling versus a passive sharing of the same circulation is unclear. The GEOTRACES GP15 section provides an ideal dataset with which to test these hypotheses. Radium-226 activities and Ba concentrations were determined in the Pacific along 152°W from 55°N to 20°S on samples collected between September– November 2018. We isolated the circulation component of the distribution by performing an optimum multiparameter analysis of the encountered water masses and modeled the distributions of 226 Ra and Ba based solely on physical transport of these water masses. The difference between the modeled and observed 226 Ra and Ba distributions reflects three significant non-conservative processes that influence 226 Ra and Ba distributions in the Pacific. First, in the north Pacific, intermediate waters (0–1,500 m) exhibit substantial deficits in 226 Ra and Ba that are likely caused by their incorporation into barite. Second, significant positive 226 Ra and Ba anomalies are found in deep waters (below 3,000 m) along the entire GP15 section, reflecting accumulation of 226 Ra and Ba from sediment diffusion and particle dissolution, respectively. Third, there are persistent positive 226 Ra anomalies between 5°N and 20°S around 2,500 m. Importantly, there are no corresponding Ba anomalies, suggesting a solute source unique