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Calcium isotope record of Phanerozoic oceans: Implications for chemical evolution of seawater and its causative mechanisms |
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| Authors: | Juraj Farkaš Florian Böhm John Blenkinsop Robert van Geldern Silke Voigt |
| Institution: | a University of Ottawa and Ottawa-Carleton Geoscience Centre, Ottawa, Ont., Canada K1N 6N5 b Leibniz Institute of Marine Sciences, IFM-GEOMAR, D-24148 Kiel, Germany c Carleton University and Ottawa-Carleton Geoscience Centre, Ottawa, Ont., Canada K1S 5B6 d Leibniz Institute for Applied Geosciences, Stilleweg 2, 30655 Hannover, Germany e Palaeontological Institute of the Friedrich-Alexander University, D-91054 Erlangen, Germany |
| Abstract: | A total of 280 brachiopods of Ordovician to Cretaceous age, complemented by published data from belemnites and planktonic foraminifera, are used to reconstruct the evolution of calcium isotope composition of seawater (δ44/40CaSW) over the Phanerozoic. The compiled δ44/40CaSW record shows a general increase from ∼1.3‰ (NIST SRM 915a) at the beginning of the Ordovician to ∼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 δ44/40CaSW 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 δ44/40CaSW 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 δ44/40CaSW. As to the causes of the short-term oscillations recorded in the secular δ44/40CaSW 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  and  ratios, induced by variable proportions of silicate vs. carbonate weathering rates on the continents. |
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