Investigation of an early Pleistocene marine osmium isotope record from the eastern equatorial Pacific

Osmium isotope composition (¹⁸⁷Os/¹⁸⁸Os) and concentrations of Os, Ir and Pt are reported for an early Pleistocene section from the ODP Site 849 in the eastern equatorial Pacific. Using the data obtained in this study, the contributions from detrital and extraterrestrial particulate matter to Os concentration and ¹⁸⁷Os/¹⁸⁸Os of sediment are estimated. Our calculations show that detrital contributions to sedimentary Os are too small (<2%) to significantly shift measured bulk sediment ¹⁸⁷Os/¹⁸⁸Os away from seawater values. A moderate but significant negative correlation between ¹⁸⁷Os/¹⁸⁸Os and ³He/¹⁸⁸Os indicate that the average particulate extraterrestrial Os flux to this site is 1.21 ± 0.47 pg cm⁻² kyr⁻¹, which constitutes ?3% of total Os burial flux. The estimates of detrital and extraterrestrial Os are used to calculate the seawater ¹⁸⁷Os/¹⁸⁸Os in the early Pleistocene. The most notable features of this early Pleistocene ¹⁸⁷Os/¹⁸⁸Os record are: (1) glacial-interglacial ¹⁸⁷Os/¹⁸⁸Os differences are insignificant within errors of estimates, (2) glacial ¹⁸⁷Os/¹⁸⁸Os values are higher compared to those reported for the late Pleistocene glacials. Comparison of ¹⁸⁷Os/¹⁸⁸Os values at Site 849 to the late Pleistocene records suggests that average seawater ¹⁸⁷Os/¹⁸⁸Os change has been modest (∼5%) since the early Pleistocene. Assuming that ¹⁸⁷Os/¹⁸⁸Os difference between the glacial periods of the late and the early Pleistocene results solely from temperature dependence of weathering rates, it has been calculated that average surface temperature during the late Pleistocene glacials was 0.8 ± 0.2 °C lower than glacials in the early Pleistocene. This inference is consistent with temperature estimates based on a recent study of pCO₂ reconstruction in the Pleistocene. This observation based on limited studies of marine ¹⁸⁷Os/¹⁸⁸Os records seems to suggest that temperature played an important role in influencing chemical weathering during the Pleistocene glacials. However, more studies are needed to confirm if this temperature-weathering feedback was operational throughout the Pleistocene. A significant down core Ir-³He co-variation coupled with similar burial fluxes of Ir at Site 849 and at LL44 GPC-3 in the north Pacific point to the utility of Ir concentration as a point paleoflux tracer. However, a twofold difference in Ir burial fluxes between the eastern and the western equatorial Pacific suggests that calibration in space and time is required to use Ir concentration as a robust indicator of paleoflux through time. Significant co-variation of concentrations of Os and total alkenone during the glacials coupled with lighter δ¹³C of benthic foraminifera indicates that productivity and carbon burial played a dominant control on scavenging of Os at Site 849. In a broader context, this data set encourages future investigation of response of PGE behavior to paleoceanographic processes.