Isotopic Composition Of Seawater Research Articles

Calibration of the carbon isotope composition (δ13C) of benthic foraminifera

AbstractThe carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air‐sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one‐to‐one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one‐to‐one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.

Eastern equatorial Pacific benthic foraminiferal distribution and deep water temperature changes during the early to middle Miocene

Abstract We investigate changes in benthic foraminiferal assemblages and estimate bottom water temperatures (BWT), based on Mg/Ca ratios of Oridorsalis umbonatus , over three selected intervals of major climate change during the late early to middle Miocene at Integrated Ocean Drilling Program (IODP) Sites U1337 (4463 m water depth) and U1338 (4200 m water depth), eastern equatorial Pacific Ocean. The targeted intervals are: (1) the onset of the Miocene Climatic Optimum (MCO; at ~ 16.9 Ma) and associated carbon cycle perturbation (Site U1337); (2) the episode of peak warmth within the MCO centered at 15.6 Ma (Site U1338); (3) the major cooling step at ~ 13.8 Ma related to east Antarctic ice-sheet expansion (Site U1338). Assemblages from these three intervals are mainly composed of cosmopolitan, long ranging (e.g., Paleogene to recent), lower bathyal to abyssal species, including calcareous elongated forms that became extinct during the middle Pleistocene. The onset of the MCO and interval of peak warmth had little impact on benthic foraminiferal accumulation rates (BFAR), abundances and assemblage composition. A transient BWT warming of 2.6 °C at the onset of the interval of peak warmth concurred with a benthic stable oxygen isotope (δ 18 O) decrease of 1‰, suggesting that the δ 18 O decrease was largely controlled by BWT changes. A substantial increase in BFAR and in the abundances of Epistominella exigua and epifaunal taxa after ~ 13.83 Ma coincides with increased opal accumulation rates, reflecting enhanced and seasonally pulsed organic carbon (C org ) flux to the seafloor during the global cooling step. During this interval, BWT decrease by 1.7 °C concurrent with a 1.0‰ increase in the stable oxygen isotope composition of seawater (δ 18 O sw ). Comparable trends at Ocean Drilling Program (ODP) Site 1146 in the South China Sea, with a 2.2 °C cooling and 0.9‰ increase in δ 18 O sw at ~ 13.8 Ma, further support that the benthic δ 18 O increase during the major cooling step was largely controlled by ice volume changes.

Marine redox conditions during deposition of Late Ordovician and Early Silurian organic-rich mudrocks in the Siljan ring district, central Sweden

The Late Ordovician Period witnessed the second largest mass extinction in the Phanerozoic Eon and the Hirnantian glaciation. To infer ocean redox conditions across the Ordovician-Silurian transition, we measured the U (as δ238U relative to standard CRM145=0‰) and Mo (as δ98Mo relative to standard NIST SRM 3134=+0.25‰) isotope compositions of 26 organic-rich mudrock samples from the Late Ordovician (Katian) Fjäcka Shale and the Early Silurian (Aeronian-Telychian) Kallholn Formation (Siljan ring district, Sweden). The magnitude of Re, Mo, and U enrichments, ReEF/MoEF and UEF/MoEF ratios, and sedimentary Fe speciation point to locally euxinic bottom water conditions during deposition of the Fjäcka Shale. The same proxies suggest that black shales of the Kallholn Formation were deposited under transiently euxinic conditions with the chemocline situated near the sediment-water interface, whereas gray shales stratigraphically equivalent to the upper Kallholn Formation were deposited from oxygenated bottom waters. These observations are consistent with higher δ98Mo and δ238U in the Fjäcka Shale compared with the Kallholn Formation.Because the Fjäcka Shale was deposited from persistently euxinic bottom waters, the Mo and U isotope compositions from these rocks can be used to estimate the extent of global ocean euxinia and ocean anoxia (euxinic plus ferruginous conditions), respectively. Elevated MoEF and Mo/TOC ratios in the euxinic Fjäcka Shale suggest no more than moderate basin restriction from the open ocean as well as non-quantitative removal of Mo from the euxinic bottom waters, thus pointing to Mo isotope fractionation between seawater and the euxinic sediments. Hence, we infer that even the highest δ98Mo (+1.28‰) preserved in the Fjäcka Shale is only a minimum estimate for the Mo isotope composition of coeval global seawater. Correcting for seawater-sediment Mo isotope fractionation, the δ98Mo of late Katian seawater may have been +1.4–2.1‰, which corresponds to ~10–70% Mo removal into the euxinic sink. The average authigenic δ238U of the Fjäcka Shale is −0.05‰ to +0.02‰ after correcting for a range of possible detrital δ238U values, thus yielding an overall average of ~0‰. Taking into account isotope fractionation during U removal to euxinic sediments, we infer that late Katian seawater δ238U was about −0.85‰ to −0.60‰. A steady-state U isotope mass balance model reveals that 46–63% of riverine U input was removed in anoxic settings. Based on the Mo and U isotope data, we infer that euxinic and anoxic waters may have covered <1% and at least 5% (potentially tens of percent) of the total seafloor area during the late Katian, respectively, based on previously published models that relate the magnitude of Mo and U burial fluxes to the areal extent of euxinic and anoxic seafloor. By comparison, only 0.21–0.35% and <1% of the total seafloor area was covered by anoxic waters today and during the Cenozoic, respectively. The difference between the estimated extent of ocean anoxia (euxinic plus ferruginous) and ocean euxinia points to an appreciable extent of ferruginous water masses during the late Katian. Integration of our data with previous studies thus supports the hypothesis that ocean oxygenation intensified during the subsequent Hirnantian glaciation (when seawater δ98Mo temporarily reached values similar to today). Hence, environmental stresses related to glaciation, not an expansion of ocean anoxia, may have triggered the first phase of the Hirnantian mass extinction.

A record of Neogene seawater &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B reconstructed from paired &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B analyses on benthic and planktic foraminifera

Abstract. The boron isotope composition (δ11B) of foraminiferal calcite reflects the pH and the boron isotope composition of the seawater the foraminifer grew in. For pH reconstructions, the δ11B of seawater must therefore be known, but information on this parameter is limited. Here we reconstruct Neogene seawater δ11B based on the δ11B difference between paired measurements of planktic and benthic foraminifera and an estimate of the coeval water column pH gradient from their δ13C values. Carbon cycle model simulations underscore that the ΔpH–Δδ13C relationship is relatively insensitive to ocean and carbon cycle changes, validating our approach. Our reconstructions suggest that δ11Bsw was ∼ 37.5 ‰ during the early and middle Miocene (roughly 23–12 Ma) and rapidly increased during the late Miocene (between 12 and 5 Ma) towards the modern value of 39.61 ‰. Strikingly, this pattern is similar to the evolution of the seawater isotope composition of Mg, Li and Ca, suggesting a common forcing mechanism. Based on the observed direction of change, we hypothesize that an increase in secondary mineral formation during continental weathering affected the isotope composition of riverine input to the ocean since 14 Ma.

Modeling the paleo-seawater radiogenic strontium isotope record: A case study of the Late Jurassic-Early Cretaceous

While uniformitarianism underlies many aspects of the Earth sciences, modeling geochemical cycles throughout Earth history requires critically assessing the reliability and constancy of projecting modern fluxes and their isotope ratios into deep geologic time. In this study, we develop an iterative technique for modeling the radiogenic strontium isotope composition (R87=87Sr/86Sr) of seawater (sw) during the Phanerozoic utilizing as a constraint the composite record of McArthur et al. (2012) (McArthur, J. M., Howarth, R. J., & Shields, G. A. (2012). Strontium isotope stratigraphy. In F. M. Gradstein, J. G. Ogg, M. Schmitz, & G. Ogg (Eds.), The Geologic Time Scale 2012 (pp. 127–144): Elsevier). The model yields estimates for the Sr isotope composition of the riverine, submarine groundwater discharge, and diagenetic input fluxes. We use the Late Jurassic-Early Cretaceous segment of the marine Sr isotope record as a case study to demonstrate the efficacy of the new method. To begin, 87Sr/86Sr ratios are calculated for a combined riverine and submarine groundwater discharge flux (tw=total weathering) using a prescribed time-variable ddtR87,sw determined from the LOWESS FIT 5 of McArthur et al. (2012). The calculated R87,tw values are checked against the range of R87,sw for the time interval of interest, as well as older carbonates available for weathering, to determine the reasonableness of the estimates. From these estimates, different prescribed R87,tw values are tested to identify the best match with target seawater 87Sr/86Sr ratios and the range of ddtR87,sw. For model runs focusing on long time intervals (≳10Myr), a single prescribed R87,tw value does not produce a reasonable match to the Sr isotope record. Testing the frequency of variation in R87,tw indicates that during the Early Cretaceous, R87,tw varied at least every 5Myr, if not more frequently. The minimum frequency at which R87,tw changes likely varied throughout the Phanerozoic, as it depends on the variability of ddtR87,sw and residence time of Sr. Model results also demonstrate the significance of estimating variation in the total hydrothermal (high and low temperature) input flux of Sr along with seawater Sr concentrations in the geologic past, which are key for properly estimating R87,tw.

An evaporite-based high-resolution sulfur isotope record of Late Permian and Triassic seawater sulfate

Variations in the sulfur isotope composition of dissolved marine sulfate through time reflect changes in the global sulfur cycle and are intimately related to changes in the carbon and oxygen cycles. A large shift in the sulfur isotope composition of sulfate at the Permian/Triassic boundary has been recognized for long time and a number of studies were carried out to understand the causes and significance of this shift. However, data for the Middle and Late Triassic are very sparse and the stratigraphic evolution of the sulfur isotope composition of seawater is poorly constrained due to the small number of samples analyzed and/or due to the limited stratigraphic intervals studied. Moreover, in the last few years the Triassic timescale has significantly changed due to a wealth of new radiometric and stratigraphic data. In this study we show that for the Late Permian and the Triassic it is possible to obtain a precise reconstruction of the evolution of the sulfur cycle, for parts of it at sub-million year resolution, by analyzing exclusively gypsum and anhydrite deposits. We base our reconstruction on new data from the Middle and Late Triassic evaporites of Northern Switzerland and literature data from evaporites from Germany, Austria, Italy and the Middle East. We propose a revised correlation between the well-dated marine Tethyan sections in northern Italy and the evaporites from Northern Switzerland and from the Germanic Basin calibrated to the newest radiometric absolute age scale. This new correlation allows for a precise dating of the evaporites and constructing a composite sulfur isotope evolution of seawater sulfate from the latest Permian (Lopingian Epoch) to the Norian. We show that a rapid positive shift of approximately 24‰ at the Permian-Triassic boundary can be used to constrain seawater sulfate concentrations in the range of 2–6mM, thus higher than previous estimates but with less rapid changes. Finally, we discuss two possible evolution scenarios for the Middle Triassic negative shift to values of 15‰ that subsequently remain constant during the Late Triassic.

Thorium distributions in high‐ and low‐dust regions and the significance for iron supply

AbstractThorium and uranium isotopes (232Th, 230Th, 238U, and 234U) were investigated to refine their use for estimating mineral dust deposition and Fe delivery to the ocean. U concentrations and isotope ratios were consistent with conservative behavior and can safely be described using published U‐salinity relationships and global average seawater isotopic composition. Near Barbados, waters affected by the Amazon outflow contained elevated 232Th. This signals one region where the thorium‐dust method is inaccurate because of a confounding continental input. Dissolved 232Th fluxes in this region suggest that Amazonian Fe supply to the adjacent open ocean is much larger than local atmospheric deposition. The colloidal content of dissolved Th south of Bermuda was found to be quite small (2–6%), similar to that found north of Hawaii, despite the order of magnitude higher dust deposition in the Atlantic. This finding supports the assumption that dissolved 232Th and 230Th are scavenged at the same rate despite their different sources and also sheds light on the increase of dissolved 232Th fluxes with integrated depth. Outside the region influenced by Amazon River waters, dissolved 232Th fluxes are compared with Bermudan aerosol Fe deposition to estimate that fractional Th solubility is around 20% in this region. Finally, new dissolved and soluble Fe, Mn, and Cr data from the subtropical North Pacific support the idea that Fe concentrations in the remote ocean are highly buffered, whereas 232Th has a larger dynamic range between high‐ and low‐dust regions.

Neodymium isotopes and concentrations in aragonitic scleractinian cold-water coral skeletons - Modern calibration and evaluation of palaeo-applications

Cold-water corals (CWCs) are unique archives of mid-depth ocean chemistry and have been used successfully to reconstruct the neodymium (Nd) isotopic composition of seawater from a number of species. High and variable Nd concentrations in fossil corals however pose the question as to how Nd is incorporated into their skeletons.We here present new results on modern specimens of Desmophyllum dianthus, Balanophyllia malouinensis, and Flabellum curvatum, collected from the Drake Passage, and Madrepora oculata, collected from the North Atlantic. All modern individuals were either collected alive or uranium-series dated to be <500years old for comparison with local surface sediments and seawater profiles. Modern coral Nd isotopic compositions generally agree with ambient seawater values, which in turn are consistent with previously published seawater analyses, supporting small vertical and lateral Nd isotope gradients in modern Drake Passage waters. Two Balanophyllia malouinensis specimens collected live however deviate by up to 0.6 epsilon units from ambient seawater. We therefore recommend that this species should be treated with caution for the reconstruction of past seawater Nd isotopic compositions.Seventy fossil Drake Passage CWCs were furthermore analysed for their Nd concentrations, revealing a large range from 7.3 to 964.5ng/g. Samples of the species D. dianthus and Caryophyllia spp. show minor covariation of Nd with 232Th content, utilised to monitor contaminant phases in cleaned coral aragonite. Strong covariations between Nd and Th concentrations are however observed in the species B. malouinensis and G. antarctica. In order to better constrain the source and nature of Nd in the cleaned aragonitic skeletons, a subset of sixteen corals was investigated for its rare earth element (REE) content, as well as major and trace element geochemistry. Our new data provide supporting evidence that the applied cleaning protocol efficiently removes contaminant lithogenic and ferromanganese oxyhydroxide phases. Mass balance calculations and seawater-like REE patterns rule out lithogenic and ferromanganese oxyhydroxide phases as a major contributor to elevated Nd concentrations in coral aragonite. Based on mass balance considerations, geochemical evidence, and previously published independent work by solid-state nuclear magnetic resonance (NMR) spectroscopy, we suggest authigenic phosphate phases as a significant carrier of skeletal Nd. Such a carrier phase could explain sporadic appearance of high Nd concentrations in corals and would be coupled with seawater-derived Nd isotopic compositions, lending further confidence to the application of Nd isotopes as a water mass proxy in CWCs.

The role of low-temperature (off-axis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite

Changes in the global Li-cycle, as recorded in the Li concentration and/or isotopic composition of seawater, have the potential to provide important insight into the controls on the long-term C-cycle. Understanding the magnitude and isotopic composition of the fluxes of Li into and out-of the ocean, and the controls on any variability in these, is necessary if we are to correctly interpret the paleo-record of the Li-cycle. Here the low-temperature (off-axis) hydrothermal sink is investigated using the volcanic section of the exceptionally preserved Troodos ophiolite. Using glass to define the protolith Li content, the uptake flux of Li is determined using bulk-rock analyses from four hydrologically distinct sections through the lava pile of the ophiolite. Differences in paleo-hydrological conditions in the crust appear to have played a significant role in controlling the uptake flux of Li with an ‘average’ uptake flux of equivalent to 14–21×109molyr−1 – this is considerably larger than generally assumed. Bulk-rock samples that contain a large seawater Li component have δ7Li of ∼10±2‰. Celadonite separates have a δ7Li of ∼6±1‰, considerably lighter than bulk-rock samples with the same Li content. Because celadonite is a significant repository for Li within the Troodos upper crust this means that another phase(s) must have markedly heavier δ7Li than the average bulk-rock; i.e. changes in the average mineralogy of altered crust will lead to changes in the bulk isotopic fractionation between the Li added to the upper oceanic crust and seawater (ΔSW-lava). The shallowest samples in three of the four studied sections are isotopically lighter than deeper samples (but do not contain significant celadonite), again indicating that variations in alteration conditions and/or mineralogy can lead to variations in ΔSW-lava. Comparison with other studies of altered upper oceanic crust suggests that changes in alteration conditions lead to significant changes in ΔSW-lava. These changes likely reflect both a temperature dependence of the isotopic fractionation factor and a change in the fractionation factor due to changing mineral assemblage and/or mineral compositions and abundances. A significant portion of the increase in δ7Li of seawater over the past 50Myr may be due to an increase in the bulk fractionation factor between seawater and Li added to the upper oceanic crust due to cooling bottom water.

Neodymium in the oceans: a global database, a regional comparison and implications for palaeoceanographic research.

The neodymium (Nd) isotopic composition of seawater has been used extensively to reconstruct ocean circulation on a variety of time scales. However, dissolved neodymium concentrations and isotopes do not always behave conservatively, and quantitative deconvolution of this non-conservative component can be used to detect trace metal inputs and isotopic exchange at ocean–sediment interfaces. In order to facilitate such comparisons for historical datasets, we here provide an extended global database for Nd isotopes and concentrations in the context of hydrography and nutrients. Since 2010, combined datasets for a large range of trace elements and isotopes are collected on international GEOTRACES section cruises, alongside classical nutrient and hydrography measurements. Here, we take a first step towards exploiting these datasets by comparing high-resolution Nd sections for the western and eastern North Atlantic in the context of hydrography, nutrients and aluminium (Al) concentrations. Evaluating those data in tracer–tracer space reveals that North Atlantic seawater Nd isotopes and concentrations generally follow the patterns of advection, as do Al concentrations. Deviations from water mass mixing are observed locally, associated with the addition or removal of trace metals in benthic nepheloid layers, exchange with ocean margins (i.e. boundary exchange) and/or exchange with particulate phases (i.e. reversible scavenging). We emphasize that the complexity of some of the new datasets cautions against a quantitative interpretation of individual palaeo Nd isotope records, and indicates the importance of spatial reconstructions for a more balanced approach to deciphering past ocean changes.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Redox fluctuations in the Early Ordovician oceans: An insight from chromium stable isotopes

Chromium (Cr) stable isotopes are a useful tracer of changes in redox conditions because changes in its oxidation state are accompanied by an isotopic fractionation. Recent co-precipitation experiments have shown that Cr(VI) is incorporated into the calcite lattice, suggesting that carbonates are a potential sink for dissolved Cr in the ocean and could effectively capture the Cr isotope composition of ambient seawater. Thus, the Cr isotopic composition (δ53Cr) of carbonates has the potential to record changes in the oxygenation state of ancient oceans. To test the potential of this system, we measured δ53Cr values of limestones and dolostones from an Early Ordovician marine carbonate platform (Cerro La Silla section of the Precordillera, San Juan, Argentina). An increasing number of studies support the hypothesis that the Early Ordovician oceans were stratified with respect to oxygen and that the global ocean experienced multiple episodes of anoxic expansion and contraction, making this time period ideal for testing the usefulness of the Cr isotope system. The most prominent feature of the Cerro La Silla Cr isotope profile is a positive excursion (of ~+0.5‰) during the Late Tremadocian, accompanied by exceptionally low Cr concentrations (<0.05mg/kg) at the peak of the excursion. Initial 87Sr/86Sr ratios remain relatively stable (0.70897±0.00001) over this time period, indicating that this excursion is not due to major changes in terrestrial runoff or hydrothermal input into the global ocean. It is likely, then, that the excursion reflects changes in redox conditions, with the most tenable cause being reductive drawdown of dissolved Cr(VI). The lack of major perturbations in carbon and sulphur isotope trends over the Early Ordovician, however, indicates that the Tremadocian Cr isotope positive excursion was not in response to changes in the redox state of the global ocean, but instead is a perturbation in response to changes in local redox conditions.

Chromium isotopic composition of core-top planktonic foraminifera.

The chromium isotope system (53 Cr/52 Cr expressed as δ53 Cr relative to NIST SRM 979) is potentially a powerful proxy for the redox state of the ocean-atmosphere system, but a lack of temporally continuous, well-calibrated archives has limited its application to date. Marine carbonates could potentially serve as a common and continuous Cr isotope archive. Here, we present the first evaluation of planktonic foraminiferal calcite as an archive of seawater δ53 Cr. We show that single foraminiferal species from globally distributed core tops yielded variable δ53 Cr, ranging from 0.1‰ to 2.5‰. These values do not match with the existing measurements of seawater δ53 Cr. Further, within a single core-top, species with similar water column distributions (i.e., depth habitats) yielded variable δ53 Cr values. In addition, mixed layer and thermocline species do not consistently exhibit decreasing trends in δ53 Cr as expected based on current understanding of Cr cycling in the ocean. These observations suggest that either seawater δ53 Cr is more heterogeneous than previously thought or that there is significant and species-dependent Cr isotope fractionation during foraminiferal calcification. Given that the δ53 Cr variability is comparable to that observed in geological samples throughout Earth's history, interpreting planktonic foraminiferal δ53 Cr without calibrating modern foraminifera further, and without additional seawater measurements, would lead to erroneous conclusions. Our core-top survey clearly indicates that planktonic foraminifera are not a straightforward δ53 Cr archive and should not be used to study marine redox evolution without additional study. It likewise cautions against the use of δ53 Cr in bulk carbonate or other biogenic archives pending further work on vital effects and the geographic heterogeneity of the Cr isotope composition of seawater.

Comparative geochemistry of four ferromanganese crusts from the Pacific Ocean and significance for the use of Ni isotopes as paleoceanographic tracers

Ferromanganese (Fe-Mn) crusts are potential archive of the Ni isotope composition of seawater through time. In this study we aim at (1) understanding Ni isotope fractionation mechanisms and metal enrichment processes in Fe-Mn deposits, (2) addressing global vs. local control of Ni isotope composition of these deposits. Two Fe-Mn crusts from the North Pacific Ocean (Apuupuu Seamount, Hawaii) and two Fe-Mn crusts from the South Pacific Ocean (near Rurutu Island, Austral archipelago of French Polynesia) were characterized for their elemental geochemistry and Ni isotope composition. Geochemical analyses were performed at millimeter intervals in order to provide time-resolved record of Ni isotopes. Chronology and growth rates were determined using cosmogenic 10Be isotope abundances. The results show that, despite different growth rates, textures and geochemical patterns, Fe-Mn crusts from both North and South Pacific Oceans have fairly homogenous Ni isotope compositions over the last ∼17Ma, yielding average δ60/58Ni values of 1.79±0.21‰ (2sd, n=31) and 1.73±0.21‰ (2sd, n=21) respectively. In one crust sample, however, layers directly in contact with the altered substrate show anomalously light δ60/58Ni values down to 0.25±0.05‰ (2se) together with rejuvenated 10Be/9Be ratios correlating with elevated Ni/Mn ratios. Such patterns are best explained by protracted fluid–rock interactions leading to alteration of Mn-phases after crust formation. Isotopically light Ni would be the result of Ni isotope fractionation during adsorption rather than the contribution of external Ni sources (e.g. hydrothermal sources) having light Ni isotope compositions. The combination of our results with previously published data on Fe-Mn crusts indicates that the average Ni isotope composition in deep waters has not changed through the Cenozoic (∼70Ma). We propose that Ni isotope variations in Fe-Mn crusts may not only record variations of Ni sources to the oceans, but also post-depositional processes depending on the growth history and geological settings of Fe-Mn crusts.

Sex-associated variations in coral skeletal oxygen and carbon isotopic composition of &amp;lt;i&amp;gt;Porites panamensis&amp;lt;/i&amp;gt; in the southern Gulf of California

Abstract. Coral δ18O variations are used as a proxy for changes in sea surface temperature (SST) and seawater isotope composition. Skeletal δ13C of coral is frequently used as a proxy for solar radiation because most of its variability is controlled by an interrelationship between three processes: photosynthesis, respiration, and feeding. Coral growth rate is known to influence the δ18O and δ13C isotope record to a lesser extent than environmental variables. Recent published data show differences in growth parameters between female and male coral in the gonochoric brooding coral Porites panamensis; thus, skeletal δ18O and δ13C are hypothesized to be different in each sex. To test this, this study describes changes in the skeletal δ18O and δ13C record of four female and six male Porites panamensis coral collected in Bahía de La Paz, Mexico, whose growth bands spanned 12 years. The isotopic data were compared to SST, precipitation, photosynthetically active radiation (PAR), chlorophyll a, and skeletal growth parameters. Porites panamensis is a known gonochoric brooder whose growth parameters are different in females and males. Splitting the data by sexes explained 81 and 93 % of the differences of δ18O, and of δ13C, respectively, in the isotope record between colonies. Both isotope records were different between sexes. δ18O was higher in female colonies than in male colonies, with a 0.31 ‰ difference; δ13C was lower in female colonies, with a 0.28 ‰ difference. A difference in the skeletal δ18O could introduce an error in SST estimates of ≈ 1.0 to ≈ 2.6 °C. The δ18O records showed a seasonal pattern that corresponded to SST, with low correlation coefficients (−0.45, −0.32), and gentle slopes (0.09, 0.10 ‰ °C−1) of the δ18O–SST relation. Seasonal variation in coral δ18O represents only 52.37 and 35.66 % of the SST cycle; 29.72 and 38.53 % can be attributed to δ18O variability in seawater. δ13C data did not correlate with any of the environmental variables; therefore, variations in skeletal δ13C appear to be driven mainly by metabolic effects. Our results support the hypothesis of a sex-associated difference in skeletal δ18O and δ13C signal, and suggest that environmental conditions and coral growth parameters affect skeletal isotopic signals differently in each sex. Although these findings relate to one gonochoric brooding species, they may have some implications for the more commonly used gonochoric spawning species such as Porites lutea and Porites lobata.

Lead Sources to the Amundsen Sea, West Antarctica

The global prevalence of industrial lead (Pb) contamination was exemplified decades ago by the predominance of anthropogenic Pb in samples of Antarctic surface ice and in Southern Ocean surface waters. Decreases in environmental Pb contamination corresponding with the near-global phase-out of leaded automobile gasoline beginning in the 1970s have since been observed. Measurements of Pb concentration in snow and ice core samples from Antarctica show that recent fluxes of industrial Pb to Antarctica have similarly declined. Here, we present measurements of Pb concentrations and isotopic compositions in seawater and surface sediments from the Amundsen Sea continental shelf including the Amundsen Sea Polynya. Both sets of measurements show that most (∼60-95%) of the Pb at our sites, at the time of sampling, is natural in source: that is, derived from the weathering of Antarctic continental rocks. These fluxes of natural Pb then become entrained into polynya waters either from sediment resuspension or from the transport of sediment-laden glacial melt waters to the polynya.

Surface water mass composition changes captured by cores of Arctic land-fast sea ice

In the Arctic, land-fast sea ice growth can be influenced by fresher water from rivers and residual summer melt. This paper examines a method to reconstruct changes in water masses using oxygen isotope measurements of sea ice cores. To determine changes in sea water isotope composition over the course of the ice growth period, the output of a sea ice thermodynamic model (driven with reanalysis data, observations of snow depth, and freeze-up dates) is used along with sea ice oxygen isotope measurements and an isotopic fractionation model. Direct measurements of sea ice growth rates are used to validate the output of the sea ice growth model. It is shown that for sea ice formed during the 2011/2012 ice growth season at Barrow, Alaska, large changes in isotopic composition of the ocean waters were captured by the sea ice isotopic composition. Salinity anomalies in the ocean were also tracked by moored instruments. These data indicate episodic advection of meteoric water, having both lower salinity and lower oxygen isotopic composition, during the winter sea ice growth season. Such advection of meteoric water during winter is surprising, as no surface meltwater and no local river discharge should be occurring at this time of year in that area. How accurately changes in water masses as indicated by oxygen isotope composition can be reconstructed using oxygen isotope analysis of sea ice cores is addressed, along with methods/strategies that could be used to further optimize the results. The method described will be useful for winter detection of meteoric water presence in Arctic fast ice regions, which is important for climate studies in a rapidly changing Arctic. Land-fast sea ice effective fractionation coefficients were derived, with a range of +1.82‰ to +2.52‰. Those derived effective fractionation coefficients will be useful for future water mass component proportion calculations. In particular, the equations given can be used to inform choices made when engaging in end member determination for working out the component proportions of water masses.

The imprint of anthropogenic CO2 emissions on Atlantic bluefin tuna otoliths

Otoliths of Atlantic bluefin tuna (Thunnus thynnus) collected from the Mediterranean Sea and North Atlantic Ocean were analyzed to evaluate changes in the seawater isotopic composition over time. We report an annual otolith δ13C record that documents the magnitude of the δ13C depletion in the Mediterranean Sea between 1989 and 2010. Atlantic bluefin tuna in our sample (n=632) ranged from 1 to 22years, and otolith material corresponding to the first year of life (back-calculated birth year) was used to reconstruct seawater isotopic composition. Otolith δ18O remained relatively stable between 1989 and 2010, whereas a statistically significant decrease in δ13C was detected across the time interval investigated, with a rate of decline of 0.05‰yr−1 (−0.94‰ depletion throughout the recorded period). The depletion in otolith δ13C over time was associated with the oceanic uptake of anthropogenically derived CO2.

Chromium fluxes and speciation in ultramafic catchments and global rivers

The role of ultramafic rock weathering in global chromium (Cr) budgets and how these may be linked to changes in Earth's climate over geological time was investigated by compiling dissolved Cr speciation and fluxes in (i) 73 of the worlds large rivers, representing ~35% of global river discharge to oceans, and in (ii) an ultramafic catchment (Putah Creek watershed) in the California Coast Range Mountains, USA. Weathering of ultramafic rocks creates ideal conditions for Cr release and redox cycling. Alkaline river water draining ultramafic catchments is naturally enriched in Cr (up to 582nmol/L). Chromium concentrations increase with the extent of rock–water interaction (as indicated by correlations with Mg and HCO3− concentrations and pH), whereas Cr cycling depends on the availability of electron donors and acceptors (e.g. Fe(II), organics, Mn(III/IV)-oxides). Thus, dissolved Cr is exported from ultramafic watersheds as both the soluble hexavalent Cr(VI) species (MgCrO4, CrO42−), and also as trivalent Cr(III) species (CrOH(CO3)22−), Cr(OH)3). The latter Cr(III) species were previously thought to have low solubility. Ultramafic catchments have higher area normalized Cr and major ion fluxes for a given runoff when compared to global rivers and may thus have a disproportional impact on global Cr-budgets, long-term carbon cycling and moderation of the Earth's climate over million-year timescales. Riverine export fluxes of Cr are linearly correlated with fluxes of Ca+Mg and alkalinity in both ultramafic catchments and global rivers. This suggests that silicate weathering is a key control on Cr fluxes and that major element weathering fluxes may be a useful proxy for estimating Cr and other trace element fluxes. Globally, present-day riverine dissolved Cr fluxes to oceans are spatially variable and estimated to be 1.7×109mol/yr, three times higher than previously reported. However, when geochemical reactions in estuaries are considered riverine Cr fluxes may be lower. Throughout Earth history, Cr weathering fluxes also may have varied with changes in the global distribution of ultramafic rocks, Cr concentrations in continental rocks, or climactic conditions (e.g. atmospheric CO2 levels or runoff). Such variation should be considered when interpreting both modern and past Cr seawater isotopic compositions and residence times.

Neodymium isotopic composition and concentration in the western North Atlantic Ocean: Results from the GEOTRACES GA02 section

The neodymium (Nd) isotopic composition of seawater is commonly used as a proxy to study past changes in the thermohaline circulation. The modern database for such reconstructions is however poor and the understanding of the underlying processes is incomplete. Here we present new observational data for Nd isotopes and concentrations from twelve seawater depth profiles, which follow the flow path of North Atlantic Deep Water (NADW) from its formation region in the North Atlantic to the northern equatorial Atlantic. Samples were collected during two cruises constituting the northern part of the Dutch GEOTRACES transect GA02 in 2010. The results show that the different water masses in the subpolar North Atlantic Ocean, which ultimately constitute NADW, have the following Nd isotope characteristics: Upper Labrador Sea Water (ULSW), εNd=−14.2±0.3; Labrador Sea Water (LSW), εNd=−13.7±0.9; Northeast Atlantic Deep Water (NEADW), εNd=−12.5±0.6; Northwest Atlantic Bottom Water (NWABW), εNd=−11.8±1.4. In the subtropics, where these source water masses have mixed to form NADW, which is exported to the global ocean, upper-NADW is characterised by εNd values of −13.2±1.0 (2sd) and lower-NADW exhibits values of εNd=−12.4±0.4 (2sd). While both signatures overlap within error, the signature for lower-NADW is significantly more radiogenic than the traditionally used value for NADW (εNd=−13.5) due to the dominance of source waters from the Nordic Seas (NWABW and NEADW). Comparison between the concentration profiles and the corresponding Nd isotope profiles with other water mass properties such as salinity, silicate concentrations, neutral densities and chlorofluorocarbon (CFC) concentration provides novel insights into the geochemical cycle of Nd and reveals that different processes are necessary to account for the observed Nd characteristics in the subpolar and subtropical gyres and throughout the vertical water column. While our data set provides additional insights into the contribution of boundary exchange in areas of sediment resuspension, the results for open ocean seawater demonstrate, at an unprecedented level, the suitability of Nd isotopes to trace modern water masses in the strongly advecting western Atlantic Ocean.

New insights into hydrological exchange between the South China Sea and the Western Pacific Ocean based on the Nd isotopic composition of seawater

Rare Earth Element (REE) concentrations and Nd isotopic compositions (εNd) of 16 seawater profiles collected in the northern South China Sea (SCS) and the Philippine Sea were investigated (1) to establish the distribution of the Nd isotopic composition of water masses along the tropical Western Pacific and the SCS until now underexplored, (2) to constrain hydrological exchanges between the SCS and the Philippine Sea through the Luzon Strait, and (3) to test, in the context of the semi-closed marginal sea (SCS), the possible impact of the process of exchange of Nd between SCS water masses and unradiogenic sediments from its north-western margin.εNd values for mid- and deep-water masses of the Philippine Sea and the SCS range from −2.3 to −4.4 and generally increase slightly as water depth increases. In the Philippine Sea, εNd values for the North Pacific Intermediate Water (NPIW) reach −2.7±0.4 at mid-depths (500 to 1400m). Below ~1800m, the Pacific Deep Water (PDW) is characterised by less radiogenic Nd (−4.1±0.5) indicating the intrusion of southern-sourced water. For most of the stations in the Northern SCS, water masses below 1500m (PDW) display homogenous εNd values (~−4.1) identical to those of the PDW in the Philippine Sea. εNd values for the South China Sea Intermediate Water (SCSIW, 500–1500m) vary from −3.0 to −3.9 as a result of the vertical mixing of the NPIW with the PDW in the SCS. Seawater εNd values for the SCS display local modification (~−5.3 to −7.0) in areas where water passes above sediment drift deposit systems. This implies locally confined “boundary exchange’’ with unradiogenic sediments (around −11) insufficient to alter the Nd isotopic composition of the PDW in the Northern SCS. In addition, εNd values analysed for the first time in nepheloid layers collected along the north-western margin of the SCS do not exhibit any significant modification of the seawater εNd which remains into a narrow range between −3.7 and −4.3, similar to that of the PDW. A compilation of εNd from the sea surface water of the SCS indicates lower εNd values in the western and the central SCS (−3.3 to −9.5), induced by sediments riverine input which are characterised by unradiogenic εNd (around −11). Finally, as significant variations are observed in seawater εNd at mid-depth between the SCS and the Philippine Sea, εNd values could be used in the future to track intermediate water mass exchange in the Luzon Strait, a process which, at present, remains poorly understood.