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  • 1
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    GSD-1G and MPI-DING Reference Glasses for In Situ and Bulk Isotopic Determination (2011)
    Wiley-Blackwell
    In:  Geostandards and Geoanalytical Research (35). pp. 193-226.
    Details
    Publication Date: 2017-07-21
    Description: This paper contains the results of an extensive isotopic study of United States Geological Survey GSD-1G and MPI-DING reference glasses. Thirteen different laboratories were involved using high-precision bulk (TIMS, MC-ICP-MS) and microanalytical (LA-MC-ICP-MS, LA-ICP-MS) techniques. Detailed studies were performed to demonstrate the large-scale and small-scale homogeneity of the reference glasses. Together with previously published isotopic data from ten other laboratories, preliminary reference and information values as well as their uncertainties at the 95% confidence level were determined for H, O, Li, B, Si, Ca, Sr, Nd, Hf, Pb, Th and U isotopes using the recommendations of the International Association of Geoanalysts for certification of reference materials. Our results indicate that GSD-1G and the MPI-DING glasses are suitable reference materials for microanalytical and bulk analytical purposes.
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  • 2
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    Arctic Ocean evidence for late Quaternary initiation of northern Eurasian ice sheets (1997)
    GSA, Geological Society of America
    In:  Geology, 25 (9). pp. 783-786.
    Details
    Publication Date: 2017-04-10
    Description: A high-resolution multiparameter stratigraphy allows the identification of late Quaternary glacial and interglacial cycles in a central Arctic Ocean sediment core. Distinct sandy layers in the upper part of the otherwise fine-grained sediment core from the Lomonosov Ridge (lat 87.5°N) correlate to four major glacials since ca. 0.7 Ma. The composition of these ice-rafted terrigenous sediments points to a glaciated northern Siberia as the main source. In contrast, lithic carbonates derived from North America are also present in older sediments and indicate a northern North American glaciation since at least 2.8 Ma. We conclude that large-scale northern Siberian glaciation began much later than other Northern Hemisphere ice sheets.
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  • 3
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    A reflection on Mg, Cd, Ca, Li and Si isotopic measurements and related reference materials (2004)
    Wiley-Blackwell
    In:  Geostandards and Geoanalytical Research, 28 (1). pp. 139-148.
    Details
    Publication Date: 2019-01-22
    Description: This contribution aims to report the reflections we had with the scientific community during two international workshops on reference materials for stable isotopes in Davos (2002) and Nice (2003). After evaluating the isotopic homogeneity of some existing reference materials, based on either certificates, literature data or specific inter-laboratory rounds, we confirm these as primary reference materials or propose new ones relative to which stable isotope compositions should be reported. We propose DSM-3 for Mg, NIST SRM 915a for Ca, L-SVEC for Li and NBS28 for Si. Cadmium does not yet have a well identified delta zero material, although three commercial mono-elemental Cd solutions have yielded the same isotopic composition relative to one another. In order to scale the linearity of any mass spectrometer, some secondary reference materials are also proposed: Cambridge-1 solution for Mg, the “Münster-Cd” and JEPPIM Cd solutions for Cd and the “Big Batch” silicate for Si. The team from Nancy propose to prepare a mixed spike solution for Li isotopes. Well-characterised natural samples such as ocean or continental waters, diatoms, sponges, rocks and minerals are needed to validate the entire analytical procedure, particularly to take into account the effect of sample mineralisation and of chemical manipulations for elemental separation prior to analysis.
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  • 4
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    Discovery of Miocene to early Pleistocene deposits on Mayaguana, Bahamas: Evidence for recent active tectonism on the North American margin (2011)
    GSA, Geological Society of America
    In:  Geology, 39 (6). pp. 523-526.
    Details
    Publication Date: 2017-05-18
    Description: We report here the discovery of Miocene, Pliocene, and early Pleistocene shallow-marine carbonates on Mayaguana Island (southeastern Bahamas) that have so far not been observed on any other Bahamian island. Spanning more than 17 m.y., but 〈12 m thick, this stratigraphic succession only occurs along the northern coast of the island, indicating that the Mayaguana Bank underwent minor subsidence throughout the late Cenozoic and was tilted toward the south during the Quaternary. In addition to considerably extending the stratigraphic record of the Bahamas Islands, our findings demonstrate that these carbonate banks were at different elevations and subsided at different rates during the Neogene. The young age of the tilting event detected on Mayaguana further shows that parts of the southeastern margin of North America have recently undergone tectonic activity a long way from its actual boundary with the Caribbean plate.
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  • 5
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    Calcium isotopic composition of various standards and seawater (2003)
    Wiley-Blackwell
    In:  Geostandards Newsletter, 27 (1). pp. 13-19.
    Details
    Publication Date: 2019-01-22
    Description: A compilation of δ44/40Ca (δ44/40Ca) data sets of different calcium reference materials is presented, based on measurements in three different laboratories (Institute of Geological Sciences, Bern; Centre de Géochimie de la Surface, Strasbourg; GEOMAR, Kiel) to support the establishment of a calcium isotope reference standard. Samples include a series of international and internal Ca reference materials, including NIST SRM 915a, seawater, two calcium carbonates and a CaF2 reference sample. The deviations in δ44/40Ca for selected pairs of reference samples have been defined and are consistent within statistical uncertainties in all three laboratories. Emphasis has been placed on characterising both NIST SRM 915a as an internationally available high purity Ca reference sample and seawater as representative of an important and widely available geological reservoir. The difference between δ44/40Ca of NIST SRM 915a and seawater is defined as -1.88 O.O4%o (δ44/42CaNISTSRM915a/Sw= -0.94 0.07%o). The conversion of values referenced to NIST SRM 915a to seawater can be described by the simplified equation δ44/40CaSa/Sw=δ44/40CaSa/NIST SRM 915a - 1.88 (δ44/42CaSa/Sw=δ44/42CaSa/NIST SRM 915a - 0.94). We propose the use of NIST SRM 915a as general Ca isotope reference standard, with seawater being defined as the major reservoir with respect to oceanographic studies.
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  • 6
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    Cellular calcium pathways and isotope fractionation in Emiliania huxleyi (2006)
    GSA, Geological Society of America
    In:  Geology, 34 (8). pp. 625-629.
    Details
    Publication Date: 2019-09-23
    Description: The marine calcifying algae Emiliania huxleyi (coccolithophores) was grown in laboratory cultures under varying conditions with respect to the environmental parameters of temperature and carbonate ion concentration [CO32-] concentration. The Ca isotope composition of E. huxleyi's coccoliths reveals new insights into fractionation processes during biomineralization. The temperature-dependent Ca isotope fractionation resembles previous calibrations of inorganic and biogenic calcite and aragonite. Unlike inorganically precipitated calcite, the [CO32-] concentration of the medium has no significant effect on the Ca isotope composition of the coccoliths. These results indicate a decoupling of the chemical properties of the bulk medium and the calcifying vesicle. Cellular Ca pathways of E. huxleyi indicate that fractionation cannot occur at the crystal surface, as occurs during inorganic precipitation. The dominant processes leading to the observed Ca isotope fractionation pattern in E. huxleyi are most likely the dehydration of the Ca aquocomplex at the plasma membrane and the attachment of dissolved Ca to proteins of Ca channels. The independence of Ca isotope fractionation from [CO32-] and the small temperature dependence of E. huxleyi are also important for defining the isotopic signature of the oceanic Ca sink. Since coccolithophores contribute to about half the global CaCO3 production, a relatively uniform isotopic composition of the oceanic Ca sink is further supported.
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  • 7
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    Clathrites: Archives of near-seafloor pore-fluid evolution (δ44/40Ca, δ13C, δ18O) in gas hydrate environments (2005)
    GSA, Geological Society of America
    In:  Geology, 33 (3). pp. 213-216.
    Details
    Publication Date: 2017-08-04
    Description: Aragonitic clathrites are methane-derived precipitates that are found at sites of massive near-seafloor gas hydrate (clathrate) accumulations at the summit of southern Hydrate Ridge, Cascadia margin. These platy carbonate precipitates form inside or in proximity to gas hydrate, which in our study site currently coexists with a fluid that is highly enriched in dissolved ions as salts are excluded during gas hydrate formation. The clathrites record the preferential incorporation of 18O into the hydrate structure and hence the enrichment of 16O in the surrounding brine. We measured δ18O values as high as 2.27‰ relative to Peedee belemnite that correspond to a fluid composition of −1.18‰ relative to standard mean ocean water. The same trend can be observed in Ca isotopes. Ongoing clathrite precipitation causes enrichment of the 44Ca in the fluid and hence in the carbonates. Carbon isotopes confirm a methane source for the carbonates. Our triple stable isotope approach that uses the three main components of carbonates (Ca, C, O) provides insight into multiple parameters influencing the isotopic composition of the pore water and hence the isotopic composition of the clathrites. This approach provides a tool to monitor the geochemical processes during clathrate and clathrite formation, thus recording the evolution of the geochemical environment of gas hydrate systems.
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  • 8
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    Seawater calcium isotope ratios across the Eocene-Oligocene transition (2011)
    GSA, Geological Society of America
    In:  Geology, 39 (7). pp. 683-686.
    Details
    Publication Date: 2019-09-23
    Description: During the Eocene-Oligocene transition (EOT, ca. 34 Ma), Earth's climate cooled significantly from a greenhouse to an icehouse climate, while the calcite (CaCO3) compensation depth (CCD) in the Pacific Ocean increased rapidly. Fluctuations in the CCD could result from various processes that create an imbalance between calcium (Ca) sources to, and sinks from, the ocean (e.g., weathering and CaCO3 deposition), with different effects on the isotopic composition of dissolved Ca in the oceans due to differences in the Ca isotopic composition of various inputs and outputs. We used Ca isotope ratios (δ44/40Ca) of coeval pelagic marine barite and bulk carbonate to evaluate changes in the marine Ca cycle across the EOT. We show that the permanent deepening of the CCD was not accompanied by a pronounced change in seawater δ44/40Ca, whereas time intervals in the Neogene with smaller carbonate depositional changes are characterized by seawater δ44/40Ca shifts. This suggests that the response of seawater δ44/40Ca to changes in weathering fluxes and to imbalances in the oceanic alkalinity budget depends on the chemical composition of seawater. A minor and transient fluctuation in the Ca isotope ratio of bulk carbonate may reflect a change in isotopic fractionation associated with CaCO3 precipitation from seawater due to a combination of factors, including changes in temperature and/or in the assemblages of calcifying organisms.
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