Late Cenozoic Sr isotope evolution of the Arctic Ocean: constraints on water mass exchange with the lower latitude oceans

Direct measurements of the Sr isotope composition of the different water masses that comprise the chemically and thermally stratified modern Arctic Ocean yield 87Sr86Sr ratios that are homogeneous and identical (i.e. within an error of < 1 × 10−5) to the 87Sr86Sr ratio of the lower latitude oceans. Variations in the 87Sr86Sr ratios of planktonic foraminifera from the Arctic Ocean, which define the Sr isotope composition of shallow Arctic seawater, follow the Sr isotope variations of the lower latitude oceans over the past 0.9 Ma. These new Sr isotope data support the chronostratigraphy previously established for the Pleistocene Arctic Ocean. Moreover, the results indicate that although the Arctic Ocean is nearly an enclosed basin and river-runoff constitutes a significant portion of the modern surface waters, the Arctic Ocean has been in Sr isotope equilibrium with the world ocean throughout most of the Quaternary. This conclusion is clearly supported by data for samples younger than 0.9 Ma, and is probably valid for the Sr isotope compositions of samples that are as old as 1.6 Ma. Because the Sr isotope data suggest water mass exchange between the Arctic and the world ocean during the Pleistocene, it follows that the Arctic Ocean has probably been an important factor in global climate over this time interval.