Testing seawater Sr isotopic variability on a glacial-interglacial timescale: An application of latest high-precision thermal ionization mass spectrometry

Abstract

Stability in seawater87Sr/86Sr ratios over a glacial-interglacial timescale has gained the consensus, yet inter-site inconsistency exists in the previous data employed for validation of this issue. This study tests if the use of state-of-the-art Thermo Finnigan ™ TRITON multi-collector thermal ionization mass spectrometer (TIMS) can place more rigorous constraints on the seawater87Sr/ 86Sr evolution at such a narrow timescale, by utilizing modern seawater, benthic foraminifera (150 ka-present) and corals (30 ka-present) (all sampled from the western Pacific realm). Application of a high-intensity setting (mass88Sr beam at 20 V) with internal and external precisions of ±0.000005 (2 SE) and ±0.000006 (2 SD), respectively, generates remarkably consistent 150 ka record of seawater Sr isotopes, such that 94.4% of foraminiferal87Sr/86Sr data fall within a ±0.000006 envelope relative to a regression function. Undoubtedly the 150 ka 87Sr/86Sr trend exhibits no short-term, orbitally-paced variation. Meanwhile, unlike previous inference, our87Sr/ 86Sr record cannot be attributed to a simple linear function, implying that higher-order, minor (amplitude ≤0.000010)87Sr/ 86Sr oscillation might have been present on the timescale greater than glacial-interglacial cycle. Our observations are demonstrative data-based verification and improvement upon previous knowledge of the glacial-interglacial seawater87Sr/86Sr ratios. We emphasize that high-precision87Sr/86Sr analysis with the TRITON TIMS certainly enables us to discriminate minute Sr isotopic details of (paleo-)oceanographic significance. Copyright © 2010 by The Geochemical Society of Japan.