The Nickel isotope composition of the authigenic sink and the diagenetic flux in modern oceans

We investigated Ni isotope composition in a stratigraphic sequence of pelagic clays collected during ODP core leg 185 on site 1149 in Western Pacific Ocean near the Izu-Bonin subduction trench in order to determine the Ni isotope composition of the authigenic Mn-oxides sink and evaluate the Ni isotope composition of the diagenetic input flux. This predominant oxic sink likely controls the Ni isotope budget in modern oceans. The sequence presented here is a 170 m-thick sequence of pelagic sediments deposited on some of the oldest oceanic crust of the seafloor, and the base was dated at 104 Ma. Nickel isotope values (δ60/58Ni relative to NIST SRM 986) vary in the range of 0.04 ± 0.04 to 1.03 ± 0.03‰. There is a trend between depth in the stratigraphic sequence and increasing Mn/Fe, Mn/Al, higher Ni concentration and heavier Ni isotope values. This trend is accounted for by authigenic Mn-oxyhdroxides precipitation in the sediment and scavenging of dissolved metals in the sediment porewaters. This indicates that authigenic oxide minerals in deep-sea pelagic clays are a relevant sink for Ni in modern oceans. Results enable us to determine the authigenic oxic output flux of 3.58 × 108 mol/yr of Ni with a Ni isotope composition of 1.2‰, this oxic output is ~0.4‰ lighter than the oxic output flux in FeMn crusts and nodules. This study shows that Ni isotope variations along the stratigraphic sequence could be the result of mixing between a pure pelagic clays end-member at ~0.1‰ and a pure Mn-oxyhydroxides end-member at ~1.2‰. We suggest that to keep the system at steady-state conditions the oxic output flux is compensated for by a diagenetic input flux of 3.7 × 108 mol/yr which is characterized by a highly fractionated Ni isotope composition of ~2.9‰

Nickel isotopes and rare earth elements systematics in marine hydrogenetic and hydrothermal ferromanganese deposits

Attention is now being given to Ni isotope systematics in hydrogenetic marine ferromanganese (Fe-Mn) crusts as paleoceanographic proxies. Previous work focused on identifying both mineralogy (post-depositional) and source effects (Gall et al. 2013; Gueguen et al. 2016), in particular regarding hydrothermal inputs in the oceans and the response of Ni isotope biogeochemical cycling through time. The most important sink for Ni in the oceans is the Fe-Mn oxides sink, but estimation of its Ni isotope composition is only based on hydrogenetic Fe-Mn crusts. In this study, we investigated a range of Fe-Mn deposits including Fe-Mn deposits variably affected by hydrothermal inputs, including hydrothermal deposits from the Lau back-arc basin (South West Pacific) and Lo'ihi seamount (Hawaii), hydrogenetic crust and nodules from the Bauer Basin (Pacific Ocean). Nickel isotope ratios were measured by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using a double-spike (61Ni and 62Ni) correction method. The combination of Ni isotopes and rare earth element (REE) geochemistry show that Ni isotope fractionation in Fe-Mn deposits is essentially controlled by formation processes of the deposits (such as the rate of formation, the initial Mn-phase and sorption processes) which are also related to the depositional environment. Consistent with previous studies, pure hydrogenetic crusts are characterized by isotopically heavy Ni isotope signatures (δ60/58Ni values range from ‰ 0.9 and 2.5‰) and well-developed positive Ce anomalies. In contrast, mixed hydrothermal‑hydrogenetic crust and nodules from the Bauer Basin (East Pacific) display negative Ce anomaly and lighter δ60/58Ni values (0.3‰ to 0.4‰), which are interpreted as the result of far-field hydrothermal inputs of Fe-Mn precipitates from the East Pacific Rise. Nickel in hydrothermal deposits from the Lau Basin (0.5 and 1.1‰) and Lo'ihi seamount (−0.8 to −1.5‰) is isotopically lighter than in hydrogenetic Fe-Mn crusts. Light δ60/58Ni values in Lo'ihi deposits is due to the removal of Ni during Ni adsorption from seawater and from the hydrothermal fluid (between 0 and 1.4‰) on Fe-oxides followed by isotope fractionation between the fluid and the mineral phase. Results suggest that Ni isotopes in hydrothermal Fe-rich deposits are strongly fractionated relative to the seawater/fluid source due to partial removal of Ni on Fe-phases. Hydrothermal Mn-oxides deposits from the Lau Basin acquired their Ni isotope signature through Ni adsorption and continuous exchange of Ni with seawater. We propose that the systematic difference in Ni isotope signatures between hydrogeneous and hydrothermal Fe-Mn deposits is related to the mechanisms of Ni uptake into oxide minerals (e.g., birnessite vs. todorokite; Fe-oxides vs. Mn-oxides) which depend on the rate of formation and the source of Mn and Fe to marine ferromanganese deposits (i.e., depositional environment) rather than Ni sources

Light Zn and Cu isotope compositions recorded in ferromanganese crusts during the Cenozoic as evidence for hydrothermal inputs in South Pacific deep seawater

International audienceThis study presents a high-resolution record of Cu and Zn isotopes in four Fe-Mn crusts from the North and South Pacific oceans. North Pacific crusts were collected on the Apuupuu seamount south of the Hawaiian archipelago and South Pacific crusts were recovered near Rurutu Island in the Tahiti archipelago. Major and trace element compositions suggest that Cu and Zn in these crusts is of hydrogenous origin, i.e., precipitated from seawater, and they may therefore mirror deep seawater metal isotope. We show that Cu and Zn display different isotopic patterns between the North and the South Pacific Oceans but show similar temporal evolution within each geographical area. Copper and Zn isotope composition of both North Pacific crusts vary between 0.57 ‰ to 0.73 ‰ for δ65/63CuNIST976 and 0.97 ‰ to 1.25 ‰ for δ66/64ZnJMC-Lyon. In contrast, South Pacific crusts show resolvable temporal variations, with Cu and Zn isotopic ratios increasing sharply over the last ∼ 6 Ma from 0.16 ‰ to 0.51 ‰ and 0.67 ‰ to 1.09 ‰ respectively. Notably, we observed a positive correlation between δ65/63CuNIST976 and δ66/64ZnJMC-Lyon values in Fe-Mn crusts from the South Pacific. The correlation suggests mixing between two components in Fe-Mn crusts, a hydrothermal component with δ65/63CuNIST976 ∼ 0.2 ‰ and δ66/64ZnJMC-Lyon ∼ 0.7 ‰, and a Pacific deep seawater component with δ65/63CuNIST976 ∼ 0.7 ‰ and δ66/64ZnJMC-Lyon ∼ 1.2 ‰. These values are fractionated from modern dissolved Cu and Zn by a factor of -0.3 ‰ and 0.5 ‰ respectively. We suggest that the deep Southern Pacific Ocean received sustained hydrothermal input during the last 6 Ma, which was recorded in the Cu and Zn isotope composition of Fe-Mn crusts precipitated thousands of kilometers away. Our study highlights that hydrothermal venting may be a significant source of Cu and Zn in the deep oceans despite their extensive precipitation within hydrothermal vent fields. We show that this source could be persistent through time, and thus, it could have a significant impact on the biogeochemical cycling of Cu and Zn in seawater which would ultimately be recorded by Fe-Mn crusts

Comparing orthomagmatic and hydrothermal mineralization models for komatiite-hosted nickel deposits in Zimbabwe using multiple-sulfur, iron, and nickel isotope data

Trojan and Shangani mines are low-grade (<0.8 % Ni), komatiite-hosted nickel sulfide deposits associated with ca. 2.7 Ga volcano-sedimentary sequences of the Zimbabwe craton. At both mines, nickel sulfide mineralization is present in strongly deformed serpentinite bodies that are enveloped by a complex network of highly sheared, silicified, and sulfide-bearing metasedimentary rocks. Strong, polyphase structural–metamorphic–metasomatic overprints in both the Trojan and Shangani deposits make it difficult to ascertain if sulfide mineralization was derived from orthomagmatic or hydrothermal processes, or by a combination of both. Multiple S, Fe, and Ni isotope analyses were applied to test these competing models. Massive ores at Shangani Mine show mass-dependent fractionation of sulfur isotopes consistent with a mantle sulfur source, whereas S-isotope systematics of net-textured ore and disseminated ore in talcose serpentinite indicates mixing of magmatic and sedimentary sulfur sources, potentially via post-magmatic hydrothermal processes. A restricted range of strongly mass-independent Δ33S values in ore samples from Trojan Mine likely reflects high-temperature assimilation of sulfur from supracrustal rocks and later superimposed low-temperature hydrothermal remobilization. Iron isotope values for most Ni-bearing sulfides show a narrow range suggesting that, in contrast to sulfur, nearly all of iron was derived from an igneous source. Negative Ni isotope values also agree with derivation of Ni from ultramafic melt and a significant high-temperature fractionation of Ni isotopes. Fe isotope values of some samples from Shangani Mine are more fractionated than expected to occur in high-temperature magmatic systems, further suggesting that hydrothermal processes were involved in either low-grade ore formation (liberation of Ni from olivine by sulfur-bearing hydrothermal fluids) or remobilization of existing sulfides potentially inducing secondary Ni-sulfide mineralization

Geochemistry of the coccoliths: proxy of surface water conditions or of resilience of coccolithophores facing climate change ?

International audienceCoccolithophores, the pelagic calcite ocean producers, experience ocean acidification and warming of Anthropocene. Concomitantly, they can recorde changes in pH and temperature (SST) in surface waters, through the incorporation of elements (B, Sr, Li, Mg) and isotopes (d 11 B) in their biominerals (coccoliths) during biocalcification. Yet, geochemistry of coccoliths is still relatively unexplored so far, stressing the need for calibration of such proxies in coccoliths. In this work, we studied the geochemistry of coccoliths deposited on top-core sediments of worlwide ocean. Elemental and isotopic compositions of these archives were then plotted against in situ data of surface-pH and SST to investigate potential relationships. A positive relationship was found between the B/Ca ratio of large sedimentary coccoliths (5-12 µm, mainly C. leptoporus) and surface-pH as predicted by thermodynamics, indicating these biominerals could be used as an archive of surface acidification. Conversely, the relation was negative for those of small size (3-5 µm, essentially consisting of E. huxleyi and G. oceanica), suggesting an increase of pH in the internal vesicle where coccoliths are formed when pH is decreasing in surface waters. Intra-vesicular pH regulation is favourable for the precipitation of calcite. We also showed that Li/Mg in coccoliths-rich sediments could be used to estimate SST. Our work constitutes an important step towards the calibration of proxies in coccoliths either to reconstruct surface water conditions, or to explore biocalcification mechanisms that will help to better anticipate and reconstruct the evolution of the pelagic calcification

Zinc isotopic variations in ureilites

The Ureilite Parent Body (UPB) was a C-rich planetary embryo disrupted by impact. Ureilites are fragments of the UPB mantle and among the most numerous achondrites. Zinc isotopic data are presented for 26 unbrecciated ureilites and a trachyandesite (ALM-A) from the same parent body. The δ66Zn values of ureilites range from 0.40 to 2.71 ‰ including literature results. Zinc isotopic compositions do not correlate with the compositions of olivine cores, with C and O isotopic compositions, with Zn abundances, nor with shock grades. The wide range of δ66Zn displayed by the ureilites is chiefly explained by evaporation processes that took place during the catastrophic breakup of the UPB. During breakup, the high temperatures of the UPB mantle allowed Zn to evaporate, regardless of the intensity the shock suffered by the ureilitic debris. For the most shocked of them, post-shock heating permitted greater evaporation, and heavier Zn isotopic compositions. The surface of the UPB was certainly much colder than the mantle before the breakup. Therefore, crustal rocks were probably less prone to Zn evaporation. ALM-A, the sole crustal rock analyzed at present, has a δ66Zn value (0.67 ‰) significantly higher than those of regular chondrites. This result indicates that its mantle source displayed already non-chondritic Zn isotopic compositions before the breakup of the UPB

Variable Ni isotope fractionation between Fe-oxyhydroxides and implications for the use of Ni isotopes as geochemical tracers

Nickel (Ni) isotopes have recently emerged as a new biogeochemical tracer in marine environments, but our understanding of the mechanisms of Ni isotope fractionation in natural systems with regards to its fractionation by mineral surfaces is incomplete. This study aims to provide experimental constraints on Ni isotope fractionation during adsorption to goethite and 2-line ferrihydrite, two Fe minerals that vary in terms of distinct crystalline properties. We conducted two types of adsorption experiments: one with variable pH (5.0 to 8.0) and constant initial Ni concentration, one at a constant pH of 7.7 and variable initial Ni concentrations. Isotopic measurements were made on both the solid phase and the supernatant solutions in order to determine the Ni isotope fractionation factors (Δ60/58Nimin-aq = δ60/58Nimin − δ60/58Niaq) between the mineral and aqueous phases. Our results show preferential adsorption of lighter Ni isotopes during adsorption of Ni to Fe oxyhydroxides presumably under conditions of near equilibrium conditions. Adsorption to goethite generates the greatest fractionation, with Δ60/58Nimin-aq = −0.77 ± 0.23‰ (n = 14, 2sd), whereas adsorption to 2-line ferrihydrite samples yield Δ60/58Nimin-aq = −0.35 ± 0.08‰ (n = 16, 2sd). Using Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, we found that Ni forms an inner-sphere complex and that its coordination environment does not vary significantly with pH nor with surface loading. In addition, we found no evidence of Ni incorporation into the mineral. We suggest that the more than two-fold increase in Ni isotope fractionation in goethite relative to 2-line ferrihydrite is due to the lower Ni-Fe coordination number in the second shell, which results in the formation of a weaker surface complex and thus favors the adsorption of lighter Ni isotopes. These results show that Ni isotope fractionation during sorption by Fe-oxyhydroxides is dependent on mineralogy, which has important implications for the use of Ni isotopes as environmental tracers and the interpretation of their record in sedimentary rocks

Large nickel isotope fractionation caused by surface complexation reactions with hexagonal birnessite

Manganese oxides are an important sink for Ni in the ocean. To explore the potential of Ni stable isotopes as a geochemical tracer, we conducted two types of sorption reactions between Ni and hexagonal birnessite in 0.05 M NaNO3 media: one where we varied pH from 5 to 8 (constant initial Ni concentration = 170 μmol/L), and a second where we varied the initial dissolved Ni concentration from 17 to 426 μmol/L (constant pH = 7.7). Isotopic measurements were made on both the solid phase and the supernatant solutions to determine the Ni isotope fractionation factors (∆60/58Nimin-aq = δ60/58Nimin − δ60/58Niaq) between the mineral and aqueous phases. Nickel extended X-ray absorption fine structure (EXAFS) spectroscopy showed Ni in two distinct bonding environments: one where Ni atoms incorporate into the MnO2 sheet and a second where Ni atoms associate with the mineral surface sharing oxygens with 3 Mn tetrahedra (TCS, triple corner sharing). As pH and net negative surface charge increase, the coordination of Ni shifts to higher proportions of incorporation. The number of structural vacancies in birnessite, which are locations for TCS coordination of Ni, are controlled by pH and increase with decreasing pH. These vacancies are preferentially occupied by lighter Ni isotopes leading to fractionation factors, ∆60/58Nimin-aq, ranging from −2.76‰ (lowest TCS) to −3.35‰ (maximum TCS). These Ni isotopic fractionation factors are among the largest observed in natural geological and biological materials to date. Our findings reveal a relationship between Ni coordination environment and pH that may ultimately be used as an isotopic geochemical tracer of past ocean conditions. However, the results are inconsistent with current isotopic fractionation factors for marine ferromanganese deposits relative to seawater and point to unaddressed processes that modify Ni isotopic fractionation for ferromanganese deposits. Further research is needed to develop Ni as an isotopic tracer

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 ∼17 Ma, 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 is best explained by 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 (∼70 Ma). 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

Comparison of Sodium Selenite and Selenium-Enriched Spirulina Supplementation Effects After Selenium Deficiency on Growth, Tissue Selenium Concentrations, Antioxidant Activities, and Selenoprotein Expression in Rats

Selenium contributes to physiological functions through its incorporation into selenoproteins. It is involved in oxidative stress defense. A selenium deficiency results in the onset or aggravation of pathologies. Following a deficiency, the repletion of selenium leads to a selenoprotein expression hierarchy misunderstood. Moreover, spirulina, a microalga, exhibits antioxidant properties and can be enriched in selenium.. Our objective was to determine the effects of a sodium selenite or selenium-enriched spirulina supplementation. Thirty-two female Wistar rats were fed for 12 weeks with a selenium-deficient diet. After 8 weeks, rats were divided into 4 groups and were fed with water, sodium selenite (20 μg Se/kg body weight), spirulina (3 g/kg bw), or selenium-enriched spirulina (20 μg Se/kg bw + 3 g spirulina/kg bw). Another group of 8 rats was fed with normal diet during 12 weeks. Selenium concentration and antioxidant enzyme activities were measured in plasma, urine, liver, brain, kidney, heart, and soleus. Expression of GPx (1, 3), Sel (P, S, T, W), SEPHS2, TrxR1, ApoER2, and megalin were quantified in liver, kidney, brain, and heart. We showed that a selenium deficiency leads to a growth delay, reversed by selenium supplementation despite a minor loss of weight in week 12 for SS rats. All tissues displayed a decrease in selenium concentration following deficiency. The brain seemed protected. We demonstrated a hierarchy in selenium distribution and selenoprotein expression. A supplementation of sodium selenite improved GPx activities and selenoprotein expression while a selenium-enriched spirulina was more effective to restore selenium concentration especially in the liver, kidney, and soleus