HR: 1340h
AN: PP13A-1025    [Abstracts]
TI: Calcium isotope record of Phanerozoic oceans: Implications for chemical evolution of seawater and its causative mechanisms
AU: * Farkas, J
AF: Harvard University, Department of Earth and Planetary Sciences, 20 Oxford Street, Cambridge, MA 02138, United States
AU: * Farkas, J
AF: University of Ottawa, Department of Earth Sciences, Ottawa, ON K1N6N5, Canada
AU: Boehm, F
AF: IFM-GEOMAR, Leibniz Institute of Marine Sciences, Kiel, D-24148, Germany
AU: Wallmann, K
AF: IFM-GEOMAR, Leibniz Institute of Marine Sciences, Kiel, D-24148, Germany
AU: Blenkinsop, J
AF: Carleton University, Department of Earth Sciences, Ottawa, ON K1S5B6, Canada
AU: Eisenhauer, A
AF: IFM-GEOMAR, Leibniz Institute of Marine Sciences, Kiel, D-24148, Germany
AU: van Geldern, R
AF: Leibniz Institute for Applied Geosciences, Stilleweg 2, Hannover, 30655, Germany
AU: Munnecke, A
AF: Palaeontological Institute, Friedrich-Alexander University, Erlangen, D-91053, Germany
AU: Voigt, S
AF: IFM-GEOMAR, Leibniz Institute of Marine Sciences, Kiel, D-24148, Germany
AU: Veizer, J
AF: University of Ottawa, Department of Earth Sciences, Ottawa, ON K1N6N5, Canada
AB: Calcium isotope data (δ^44/40Ca) of 280 brachiopods of Ordovician to Cretaceous age, complemented by published data from belemnites and planktonic foraminifera, are used to reconstruct the evolution of Ca isotope composition of seawater (δ^44/40Ca_SW) over the Phanerozoic. The compiled δ^44/40Ca_SW record shows a general increase from about 1.3‰ (NIST SRM 915a) at the beginning of the Ordovician to about 2‰ at present. Superimposed on this trend is a major long-term positive excursion from the Early Carboniferous to Early Permian as well as several short-term, mostly negative, oscillations. A numerical model of the global cycles of calcium, carbon, magnesium and strontium was used to estimate whether the recorded δ^44/40Ca_SW variations can be explained by varying magnitudes of input and output fluxes of calcium to the oceans. The model uses the record of marine ⁸⁷Sr/⁸⁶Sr ratios as proxy for seafloor spreading rates, a record of oceanic Mg/Ca ratios to estimate rates of dolomite formation, and reconstructed atmospheric CO₂, discharge and erosion rates to estimate continental weathering fluxes. The model results indicate that varying magnitudes of the calcium input and output fluxes cannot explain the observed δ^44/40Ca_SW trends, suggesting that the isotope signatures of these fluxes must also have changed. As a possible mechanism we suggest variable isotope fractionation in the sedimentary output flux controlled by the dominant mineralogy in marine carbonate deposits, i.e. the oscillating 'calcite-aragonite seas'. The ultimate control of the calcium isotope budget of the Phanerozoic oceans appears to have been tectonic processes, specifically variable rates of oceanic crust production that modulated the hydrothermal calcium flux and the oceanic Mg/Ca ratio, which in turn controlled the dominant mineralogy of marine carbonates, hence the δ^44/40Ca_SW. As to the causes of the short-term oscillations recorded in the secular δ^44/40Ca_SW trend, we tentatively propose that these are related to variable rates of dolomite formation and/or to changing chemical composition of the riverine flux, in particular Ca/HCO₃ and Ca/SO₄ ratios, induced by variable proportions of silicate vs. carbonate weathering rates on the continents.
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL (0460)
DE: 4870 Stable isotopes (0454, 1041)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting