The Marine Os Isotopic Record in the Eocene-Oligocene Sections From the Equatorial Pacific

The marine Osmium isotope record has great potential as a tool for chemostratigraphic correlation, and as a proxy for tracking changes in inputs to seawater. In order to evaluate the utility of this record for correlation, 45 analyses have been performed on sediment samples from the Eocene-Oligocene (E-O) section of the equatorial Pacific (Leg 199, Sites 1219 and 1218) spanning ca. 2.6 Myr. The results capture the pronounced Late Eocene ¹⁸⁷Os/¹⁸⁸Os minimum and the subsequent rapid rise of ¹⁸⁷Os/¹⁸⁸Os across the E-O transition, and are in general agreement with those reported earlier in E-O sections from the Pacific, the Atlantic and the Tethys (Ravizza and Peucker-Ehrenbrink, EPSL v. 210, 151-165). In detail there are important new features in the Leg 199 data. The amplitude of the Late Eocene ¹⁸⁷Os/¹⁸⁸Os minimum is similar to that measured in the pelagic clay sequence of the Pacific core LL44-GPC3 but is smaller than recorded in the other pelagic carbonate records. In addition, ¹⁸⁷Os/¹⁸⁸Os of samples from the E-O transition at the site 1219A are systematically higher by ca. 5-10 % compared to those of the age equivalent samples from 1218A. Such comparisons suggest that marine Os isotopic records allow general stratigraphic correlation among various sections; however, precise age correlation using ¹⁸⁷Os/¹⁸⁸Os may be compromised by such small shifts in Os isotopic composition. These shifts in ¹⁸⁷Os/¹⁸⁸Os among sections may be a result of hiatus in sedimentation and/or lithological variation among cores. Further work is underway that would allow comparison of variations in ¹⁸⁷Os/¹⁸⁸Os in Late Eocene sections of 1219A and 1218A that are devoid of carbonates, where detailed logging data allow precise correlation between the two holes.