The Late Eocene 187Os/188Os excursion: Chemostratigraphy, cosmic dust flux and the Early Oligocene glaciation
Additional Document Info
High resolution records (ca. 100 kyr) of Os isotope composition (Os-187/Os-188) in bulksediments from two tropical Pacific sites (ODP Sites 1218 and 1219) capture the complete Late Eocene Os-187/ Os-188 excursion and confirm that the Late Eocene (OS)-O-117/Os-188 Minimum, earlier reported by Ravizza and Peucker-Ehrenbrink [Earth Planet. Sci. Lett. 210 (2003) 151-165], is a global feature. Using the astronomically tuned age models available for these sites, it is suggested that the Late Eocene Os-187/Os-188 minimum can be placed at 34.5 +/- 0.1 Ma in the marine records. In addition, two other distinct features of the Os-187/Os-188 excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca. +/- 0.1 Myr. We propose a speculative hypothesis that higher cosmic dust flux in the Late Eocene may have contributed to global cooling and Early Oligocene glaciation (Oi-1) by supplying bio-essential trace elements to the oceans and thereby resulting in higher ocean productivity, enhanced burial of organic carbon and draw down of atmospheric CO. To determine if the hypothesis that enhanced cosmic dust flux ill the Late Eocene was a cause for the Os-187/Os-188 excursion can be tested by using the paired bulk sediment and leachate Os isotope composition; Os-187/Os-188 were also measured in sediment leachates. Results of analyses of leachates are inconsistent between the south Atlantic and the Pacific sites.. and therefore do not yield a robust test of this hypothesis. Comparison Of Os-187/Os-188 records with high resolution benthic foraminiferal delta O-18 records across the Eocene-Oligocene transition suggests that Os-187 flux to the oceans decreased during cooling and ice growth leading to the Oi-1 glaciation, whereas subsequent decayof ice-sheets and deglacial weathering drove seawater Os-187/Os-188 to higher values. Although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine 187oS / I HoS records are obscured by recovery from the Late Eocene Os-187/Os-188 excursion, evidence of the global influence of glaciation on supply of Os to the ocean is robust as it has now been documented in both Pacific and Atlantic records. (c) 2005 Elsevier B.V. All rights reserved.