Anoxic development of sapropel S1 in the Nile Fan inferred from redox sensitive proxies, Fe speciation, Fe and Mo isotopes

Redox conditions and the mechanisms of redox development are a critical aspect of Eastern Mediterranean sapropels, whose formation in oxygen-depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima. Sapropels in the Nile Fan formed at relatively shallow water depths under the influence of the monsoon-driven freshwater output from the River Nile. This work evaluates the redox evolution of Holocene sapropel S1 in VALPAMED cruise core MD9509, recovered at 880 mbsl in the NE Nile Fan, using a combination of geochemical element proxies, Fe speciation, Fe and Mo isotopes studies. The productivity and redox proxies (Ba/Al, Mo/Al, U/Al, V/Al, Sb/Al) show well-defined enrichments in the sapropel, but with a marked minimum at ca 8.2 ka indicative of reventilation corresponding to a well known global cooling event. Peak productivity and reducing signals occur close to the initiation of sapropel formation. The proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel S1, recovered at the same depth, but 380 km to the north (MD9501), supporting the notion (suggested in previous studies) of more reduced conditions in the Nile Fan. The MoEF vs. UEF enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non-sapropel sediments to anoxic non-sulphidic water column conditions in the sapropel. Correspondingly, the highly reactive Fe pool (FeHR) measured in Fe speciation studies is dominated by Fe(oxyhydr) oxide minerals in the background sediments, whereas pyrite (Fepy) becomes the dominant component of the FeHR pool in the sapropel. Maximum Fepy values in the sapropel coincide with peak productivity and reducing conditions, implying a clear link between trace element uptake, diagenetic bacterial sulphate reduction in anoxic porewater and Fe mobilization in the sapropel. Iron isotope compositions (δ56Fe) in the sapropel do not show any departure from primary (marine and detrital) source sediment values, and the absence of an Fe/Al vs. δ56Fe trend strongly argues against an Fe shuttle. Molybdenum isotopes, however, show marked non-conservative fractionation patterns. Background sediment δ98/95Mo values (0.2 to 0.7‰) are compatible with fractionation upon absorptive uptake by Fe (oxyhydr)oxides and pyrite. In contrast, minimum δ98/95Mo values exhibited at peak sapropel (reducing and pyrite producing) conditions are most closely modeled by Mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species. The conservative Fe isotope behavior/Mo isotope fractionation minima in the sapropel may be a characteristic of organic-rich sediment diagenesis below an anoxic non-sulphidic water body, without the operation of a benthic Fe shuttle

Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core-mantle differentiation of terrestrial planets [rapid communication]

International audienceSeven bulk chondrites, with ?57Fe/54Fe values between ?0.1? and 0? relative to IRMM-14, tend to be slightly lighter than 11 bulk iron meteorites, which have ?57Fe/54Fe values ranging from 0.04? to 0.2?. At the mineral scale, taenite from two iron meteorites, Cranbourne and Toluca, shows ?57Fe/54Fe values heavier by up to 0.3? than their kamacite counterpart, thus calling into question the significance of bulk iron meteorite data. On three pallasites (Esquel, Marjalahti and Springwater) we measured a heavier iron isotope composition for the metal fractions compared to the coexisting olivines as previously observed on two other pallasites (Eagle Station and Imilac), but the range of ?57Fe/54Fe differences (from 0.32? to 0.07?) is larger than that originally found. Troilite from two pallasites appears to be even heavier than the metal fraction, whereas schreibersite is lighter than its olivine counterpart. There is thus a general tendency for minerals within a given rock to show a heavier Fe isotope composition as the coordination number of Fe increases, although troilite is an exception to this rule. Iron meteorites are classically considered as remnants of asteroid cores and pallasites as core mantle interfaces. The simultaneous finding that the metal fractions of pallasites have a higher ?57Fe/54Fe signature than the coexisting olivines, and that the iron meteorites are slightly heavier than chondrites could be taken as an indication that planetary core mantle differentiation is accompanied by sizeable iron isotope fractionation. In this hypothesis, mass balance constraints imply that resultant planetary mantles should be isotopically lighter than the chondritic starting material. That is not observed, however, since all planetary mantles analyzed so far have ?57Fe/54Fe values equivalent to or heavier than those of chondrites. It thus appears that the moderate temperature and pressure metal silicate fractionation that occurred in pallasite and iron parent bodies is not readily transposable to planets as far as Fe isotopes are concerned. Instead, these mantle signatures could reflect equilibrium fractionation at a higher temperature, or the lack of a global core mantle equilibration at the planetary scale. Overall, these new results show that the mass-dependent isotopic scatter observed among inner solar system bodies from the bulk-rock to the planetary scale (?0.3? ?57Fe/54Fe) is more restricted than previously thought. This likely confirms a homogenization process that occurred in the protoplanetary accretion disk, between refractory inclusion condensation and chondrule formation

Influence of Intrinsic Colloid Formation on Migration of Cerium through Fractured Carbonate Rock

Migration of colloids may facilitate the transport of radionuclides leaked from near surface waste sites and geological repositories. Intrinsic colloids are favorably formed by precipitation with carbonates in bicarbonate-rich environments, and their migration may be enhanced through fractured bedrock. The mobility of Ce­(III) as an intrinsic colloid was studied in an artificial rainwater solution through a natural discrete chalk fracture. The results indicate that at variable injection concentrations (between 1 and 30 mg/L), nearly all of the recovered Ce takes the form of an intrinsic colloid of >0.45 μm diameter, including in those experiments in which the inlet solution was first filtered via 0.45 μm. In all experiments, these intrinsic colloids reached their maximum relative concentrations prior to that of the Br conservative tracer. Total Ce recovery from experiments using 0.45 μm filtered inlet solutions was only about 0.1%, and colloids of >0.45 μm constituted the majority of recovered Ce. About 1% of Ce was recovered when colloids of >0.45 μm were injected, indicating the enhanced mobility and recovery of Ce in the presence of bicarbonate

Retention of strontium in high- & low-pH cementitious matrices – OPC vs. model systems

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Iron isotope fractionation and the oxygen fugacity of the mantle

The oxygen fugacity of the mantle exerts a fundamental influence on mantle melting, volatile speciation, and the development of the atmosphere. However, its evolution through time is poorly understood. Changes in mantle oxidation state should be reflected in the Fe3+/Fe2+ of mantle minerals, and hence in stable iron isotope fractionation. Here it is shown that there are substantial (1.7 per mil) systematic variations in the iron isotope compositions (δ57/54Fe) of mantle spinels. Spinel δ57/54Fe values correlate with relative oxygen fugacity, Fe3+/ΣFe, and chromium number, and provide a proxy of changes in mantle oxidation state, melting, and volatile recycling

Iron isotope fractionation during pedogenesis in redoximorphic soils

Stable Fe isotopes provide a potential new tool for tracing the biogeochemical cycle of Fe in soils. Iron isotope ratios in two redoximorphic soils were measured by multicollector inductively coupled plasma mass spectrometry to study the relationships between pedogenic Fe transformation and redistribution processes, and mass-dependent fractionation of Fe isotopes. Redoximorphic Fe depletion and enrichment zones were sampled in addition to the bulk soil samples. A three-step sequential extraction procedure was used to separate different Fe pools, which were examined in addition to total soil digests. Significant enrichments of heavy Fe isotopes of about 0.3¿ in d57Fe were found in total soil digests of Fe-depleted zones compared with bulk soil samples and were explained by the preferential removal of light isotopes, presumably during microbially mediated Fe oxide dissolution under anoxic conditions. Accordingly, pedogenic Fe enrichment zones were found to be slightly enriched in light Fe isotopes. Distinct Fe isotope variations of >1¿ in d57Fe were found between different Fe pools within soil samples, specifically enrichments of light isotopes in pedogenic oxides contrasting with heavy isotope signatures of residual silicate-bound Fe. Our data demonstrate that pedogenic Fe transformations in redoximorphic soils are linked to isotope fractionation, revealing greater mobility of lighter Fe isotopes compared with heavier isotopes during pedogenesis. No simple quantitative relationship between Fe depletion and isotope fractionation could be inferred, however. Our findings provide new insights into the behavior of Fe isotopes in soils and contribute to the development of Fe isotopes as a tracer for the biogeochemical Fe cycle