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This dataset includes measurements of dissolved barium (Ba) concentrations in the South Pacific and Southern Ocean from the US GEOTRACES GP17 section (GP17-OCE, Papeete, Tahiti to Punta Arenas, Chile) on R/V Roger Revelle from December 2022 to January 2023. 

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⁻¹and 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. 

Mantle-derived, low-degree melts, such as kimberlites, carbonate-rich olivine lamproites (CROLS), and cratonic olivine lamproites, are the main carriers of diamonds. They are rare ultramafic, volatile-rich volcanic magmas, generally restricted to stable cratons, and are the deepest-sourced magmas erupted onto Earth’s surface. As hybrid magmas, their formation mechanism and mantle sources remain enigmatic and highly debated, especially the nature of the processes leading to their “enriched” isotopic signatures. The often extreme isotopic compositions of Sr, Nd, Pb, and Hf suggest that the mantle sources of these magmas vary between an ancient and geochemically depleted component and various enriched components. The enriched components could include crustal material recycled into the convective mantle or metasomatized lithospheric mantle. For the latter, discriminating between assimilation by sub-lithospheric magmas during the ascent or melting of element-enriched material from within the lithospheric mantle is paramount concerning petrogenesis. As the stable isotope composition of K, and Ba vary between surface and mantle reservoirs, they are well-suited tools for addressing the cause of different radiogenic isotopic signatures and to better constrain the mantle sources of these important magmas. Here, we use collision cell multi-collector inductively-coupled-plasma mass-spectrometry (MC-ICP-MS) and traditional MC-ICP-MS to conduct the first comprehensive whole-rock K and Ba stable isotope study on a wide range of low-degree mantle-derived melts. All the deep-seated, low-degree melts analyzed here show no correlation between melting/differentiation indices and δ41K and δ138Ba compositions, implying that any isotopic fractionation during melting or eruption was limited and that the different mantle and crustal reservoirs affecting these melts dominate their isotopic variability. Overall, kimberlites show limited δ41K and δ138Ba variability, with a median δ41K of -0.40 ± 0.06‰ (2SE) and δ138Ba of 0.00 ± 0.07‰ (2SE), within error relative to an estimated bulk silicate Earth [(BSE: δ41K= -0.42±0.07‰ (2SD) and δ138Ba=0.03±0.04‰ (2SD)], suggesting significant sublithospheric input. While the sample size is small (N=4), Canadian kimberlites from Lake De Gras display a bi-modal distribution with δ41K values slightly higher and lower relative to BSE, ascribed to crustal and lithospheric contamination. Like kimberlites, South African CROLS show limited K isotope variability with a median δ41K of -0.48 ± 0.02‰ (2SE). Their compositions are non-resolvable from two Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) xenoliths. The δ138Ba of the CROLS also shows limited variation with a median δ138Ba of 0.00 ± 0.07‰ (2SE), plotting within BSE estimations. Compared to the other low-degree mantle-derived melts, cratonic olivine/leucite-bearing lamproites from West Australia show a wide range in δ41K (-0.97‰ to +0.34‰) and δ138Ba (-0.30‰ to +0.27) values. The observed large K isotopic variation in cratonic lamproites is similar to that observed in post-collisional lamproites and is ascribed to sediment recycling. Argyle lamproites define robust correlations between potassium and barium elemental abundances, and their stable isotopes call for significant hydrothermal fluid-assisted leaching and isotopic fractionation. 

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 under-constrained 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. 

Abstract 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 Oxic 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 dissolved²³⁰Th/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 initial²³⁰Th/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. 

Abstract Groundwater-derived solute fluxes to the ocean have long been assumed static and subordinate to riverine fluxes, if not neglected entirely, in marine isotope budgets. Here we present concentration and isotope data for Li, Mg, Ca, Sr, and Ba in coastal groundwaters to constrain the importance of groundwater discharge in mediating the magnitude and isotopic composition of terrestrially derived solute fluxes to the ocean. Data were extrapolated globally using three independent volumetric estimates of groundwater discharge to coastal waters, from which we estimate that groundwater-derived solute fluxes represent, at a minimum, 5% of riverine fluxes for Li, Mg, Ca, Sr, and Ba. The isotopic compositions of the groundwater-derived Mg, Ca, and Sr fluxes are distinct from global riverine averages, while Li and Ba fluxes are isotopically indistinguishable from rivers. These differences reflect a strong dependence on coastal lithology that should be considered a priority for parameterization in Earth-system models.