ALBERT

Beschreibung: The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250 m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~ 6 m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~ 230 mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid delta 26Mg composition becomes progressively enriched in 26Mg with depth from values similar to seawater (i.e. -0.8 per mil, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8 ± 0.2 per mil within the sediments located below 100 mbsf. Type I dolomites have a delta 26Mg of -3.5 per mil, and exhibit apparent dolomite-pore fluid fractionation factors of about -2.6 per mil consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have delta 26Mg values ranging from -2.5 to -3.0 per mil and are up to -3.6 per mil lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in 26Mg and depletion in total Mg concentration below ~ 200 mbsf is likely the result of Mg isotope fractionation during dolomite formation, The 26Mg enrichment of pore fluids in the upper ~ 200 mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.

Beschreibung: In this study we examine the behavior of stable Sr isotopes between strontianite [SrCO3] and reactive fluid during mineral dissolution, precipitation, and at chemical equilibrium. Experiments were performed in batch reactors at 25 °C in 0.01 M NaCl solutions wherein the pH was adjusted by bubbling of a water saturated gas phase of pure CO2 or atmospheric air. The equilibrium Sr isotope fractionation between strontianite and fluid after dissolution of the solid under 1 atm CO2 atmosphere was estimated as Δ88/86SrSrCO3-fluid = δ88/86Sr SrCO3 − δ88/86Srfluid = −0.05 ± 0.01‰. On the other hand, during strontianite precipitation, an enrichment of the fluid phase in 88Sr, the heavy isotopomer, was observed. The evolution of the δ88/86Srfluid during strontianite precipitation can be modeled using a Rayleigh distillation approach and the estimated, kinetically driven, fractionation factor αSrCO3-fluid between solid and fluid is calculated to be 0.99985 ± 0.00003 corresponding to Δ88/86SrSrCO3-fluid = −0.15‰. The obtained results further support that under chemical equilibrium conditions between solid and fluid a continuous exchange of isotopes occurs until the system approaches isotopic equilibrium. This isotopic exchange is not limited to the outer surface layer of the strontianite crystal, but extends to ∼7–8 unit cells below the crystal surface. The behavior of Sr isotopes in this study is in excellent agreement with the concept of dynamic equilibrium and it suggests that the time needed for achievement of chemical equilibrium is generally shorter compared to that for isotopic equilibrium. Thus it is suggested that in natural Sr-bearing carbonates an isotopic change may still occur close to thermodynamic equilibrium, despite no observable change in aqueous elemental concentrations. As such, a secondary and ongoing change of Sr isotope signals in carbonate minerals caused by isotopic re-equilibration with fluids has to be considered in order to use Sr isotopes as environmental proxies in aquatic environments.

Beschreibung: Magnesium calcites were synthesized from aqueous solutions supersaturated with respect to calcite at 25, 40, 60, and 80 °C in gas tight batch reactors for up to 35 days. Any amorphous material still present in the precipitates was removed using a partial dissolution treatment. Resulting purified Mg-calcite had Mg contents ranging from 6 to 32 mol% MgCO3. An isotopic steady-state was attained between the fluid and the precipitated solids within two weeks at 25 °C. δ18O values derived from the experiments at steady-state, depend on both temperature and the Mg content of the calcite in accord with: 1000lnαMg-calcite–H2O=18,030/T−32.42+(6×108/T3–5.47×106/T2+16,780/T−17.21)×CMg where αMg-calcite–H2O represents the calcite–water oxygen isotope fractionation factor, T refers to the temperature in °K and CMg denotes the mole percent of MgCO3 in the calcite. These results indicate that the addition of 5 mol% MgCO3 into the calcite increases 1000lnαMg-calcite–H2O by 0.88 as compared to that of pure calcite at 25 °C. This difference could lead to a 4.2 °C decrease in estimated formation temperature estimates. These results demonstrate that the accurate interpretation of oxygen isotope fractionation in magnesium calcites requires explicit provision for the effect of magnesium on oxygen isotope fractionation factors. Highlights ► The effect of Mg on calcite–aqueous fluid oxygen isotope fractionation was investigated. ► Mg incorporation is increasing the calcite–aqueous fluid oxygen isotope fractionation. ► This effect is reduced at higher temperatures. ► Obtained results are important for paleo-temperature estimation. Gadget timed out while loading

Beschreibung: The surface sediments of two mud mounds (‘‘Mound 11’’ and ‘‘Mound 12’’) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded d18Ocarbonate values ranging between 34.0 and 37.7 % Vienna standard mean ocean water (VSMOW) and d13Ccarbonate values from -52.2 to -14.2 % Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The d18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (d18Oporefluid = 0 % VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (d18Oporefluid &5 %) in Mound 11. A positive correlation between d13Ccarbonate and d18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (d13Cporefluid &0 %) and (2) bicarbonate which formed during the AOM (d13Cporefluid &-70 %). Furthermore, the d18Oporefluid composition, with values up to ?4.7 % Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boronenriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (d13CCH4 = -38 %). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).

Beschreibung: The Pagassitikos Gulf in Greece is a semi-enclosed bay with a maximum depth of 102 m. According to the present-day bathymetric configuration and the sea level during the latest Pleistocene, the gulf would have been isolated from the open sea, forming a palaeolake since ~32 cal. ka b.p. Sediment core B-4 was recovered from the deepest sector of the gulf and revealed evidence of a totally different depositional environment in the lowest part of the core: this contained light grey-coloured sediments, contrasting strongly with overlying olive grey muds. Multi-proxy analyses showed the predominance of carbonate minerals (aragonite, dolomite and calcite) and gypsum in the lowest part of the core. Carbonate mineral deposition can be attributed to autochthonous precipitation that took place in a saline palaeolake with high evaporation rates during the last glacial–early deglacial period; the lowest core sample to be AMS 14C dated provided an age of 19.53 cal. ka b.p. The palaeolake was presumably reconnected to the open sea at ~13.2 cal. ka b.p. during the last sea-level rise, marking the commencement of marine sedimentation characterised by the predominance of terrigenous aluminosilicates and fairly constant depositional conditions lasting up to the present day.

Beschreibung: Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the Phanerozoic in marine waters. Consequently, traditional and non-traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties. This paper tests the sensitivity of brachiopod shell magnesium isotope (δ26Mg) data to diagenetic alteration. We apply a dual approach by: (i) performing hydrothermal alteration experiments using meteoric, marine, and burial reactive fluids; and (ii) comparing these data to naturally altered, ancient brachiopod shells. The degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy. The absence of luminescence might indicate both well-preserved shell material, but also the secondary enrichment of quenching elements such as iron along diagenetic pathways. Complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells. Brachiopod shell magnesium isotope values respond to differential fluid temperature, chemistry, and experiment durations. The patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals. Generally, the range in bulk δ26Mg from experimentally altered (1.52‰) and that of bulk samples from ancient, diagenetically altered brachiopod valves (1.53‰) exceed the geochemical variability of δ26Mgbrachiopod bulk values of most recent specimens (1.26‰) in the lower and upper range. More 26Mg enriched (0.8‰) and more 26Mg depleted (0.7‰) values, respectively, are found in altered shells in comparison to unaltered ones. The data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives. Consequently, we propose tentative guidelines for magnesium isotope research applied to ancient carbonates.