Other
Scientific paper
Nov 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.5117f&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 71, Issue 21, p. 5117-5134.
Other
18
Scientific paper
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 Ca/HCO3- and Ca/SO42- ratios, induced by variable proportions of silicate vs. carbonate weathering rates on the continents.
Blenkinsop John
Böhm Florian
Eisenhauer Anton
Farkaš Juraj
Munnecke Axel
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