Sr Isotopes at the Onset of the Ice Ages

The radiogenic 87Sr/86Sr isotope ratios are used to trace sources of Sr to the oceans. Temporal variations of the marine 87Sr/86Sr ratios were reconstructed by analyzing marine macro- and microfossils (e.g. brachiopods and foraminifera). The recent research on stable 88Sr/86Sr isotope ratio (expressed by δ88/86Sr notation) opened new ways to constrain the Sr budget of the oceans. So far δ88/86Sr data are available for limited intervals of the oceans history. Here, we set to examine the behavior of radiogenic and stable isotopes of Sr during the time interval of the Pliocene to early-Middle Pleistocene – upon the onset of Ice Ages in the northern hemisphere. We collected marine fossil mollusks (gastropods, pectinids) and brachiopods from outcrops of Stirone River in the northern Apennines, Italy. The outcrops expose the sedimentary section that was deposited between ~5 to 0.9Ma within the PaleoAdriatic Sea. We determined the fractionation factors for the gastropods and pectinids to be Δ88/86Sr=-0.15±0.02‰ and used Δ88/86Srbrachiopod=-0.21‰ for brachiopods (Vollstaedt et al. 2014). The fractionation factors were used to calculate δ88/86Sr in seawater using the δ88/86Sr values of Pliocene to mid-Pleistocene fossils. While there is a distinct rise in 87Sr/86Sr ratio, the δ88/86Sr ratio in seawater remained virtually constant (0.38±0.02 ‰) during the studied time interval