CALCIUM ISOTOPES AS NEW TOOL FOR PALEOCEANOGRAPHIC AND PALEOCLIMATIC STUDIES - VARIATION OF CA-ISOTOPES (dSUP>44/42CA) IN PHANEROZOIC AND PRECAMBRIAN MARINE CARBONATES

High-precision analytical protocol for the measurements of the Ca isotopes using MC-ICP-MS has been recently developed and employed for the determination of Ca isotope composition in modern seawater and selected well-preserved Phanerozoic-Precambrian marine carbonates (brachiopods and micrites). The standard-sample bracketing technique, using modern seawater as an internal reference material, allowed for a precise (external reproducibility ±0.15‰ 2SD) determination of calcium isotope composition in terms of a delta notation. The overall d44/42Ca-change in the analyzed set of marine carbonates exhibits a narrow range ~ 0,7‰ of variation.

This limited natural fractionation, however, exceeds the achieved analytical precision. All the measured d44/42Ca values in marine carbonates are isotopically lighter than the modern seawater; a fact that is in accord with previous studies done by TIMS. The detected temporal change in the d44/42Ca in carbonate samples has not confirmed any unidirectional Phanerozoic-Precambrian trend as an analogy to a negative- d18O evolution trend. On the contrary, ‘detrended' d18O time-series (believed to monitor relative oscillations in SST) could be, in general, negatively correlated with the d44/42Ca; this is true particularly for the Neo-Proterozoic and the Early Paleozoic.

This general negative correlation between d44/42Ca and detrended-d18O might indicate that fractionation of Ca isotopes is to certain level related to processes that drive the fractionation of O isotopes. For the Ca isotope system the higher precipitation rates during calcification due to higher SST might be the reason for negative coupling of these two isotope systems. However, this correlation is not utterly matching the Late Paleozoic and the Early Mesozoic section and suggests that the d44/42Ca signal is more complex and possibly reflects combined information on a paleo-temperature (precipitation rate) and the relative balance between Ca-input and output in the oceans. Probably the successful advent of pelagic calcifiers in the Late Paleozoic followed by the Mesozoic proliferation of coccolithophores and foraminifera significantly controlled the oceanic Ca-budget through the enhanced output (biofixation of Ca) and thus systematically changing the d44/42Ca value of past seawater.