Extraterrestrial and climatic forcing of the marine osmium isotope record

Variations in the late Eocene marine Os isotope record (Chapters 1 and 2) show that an abrupt decrease in the 187Os/188Os of pelagic marine sediments occurs globally coincident with a major impact event. The magnitude of this excursion and the duration of the subsequent recovery are used to place constraints on the size of the projectile, and the residence time of Os in the Eocene ocean, respectively. High-resolution data from multiple sites reveal evidence that the 187Os/ 188 of the global ocean is unlikely to have been homogeneous, and that an episode of increased delivery of cosmic dust fluxes approximately 100 ka prior to the impact event also influenced the marine Os isotope record.;Chapter 3 critically evaluates the recently proposed Younger Dryas impact hypothesis from a geochemical perspective, based on the results from Chapters 1 and 2. This work failed to reproduce the previously reported high Ir concentrations in selected terrestrial sections. Instead Ir concentrations are close to that of average upper continental crust (Ir ∼ 22 pg/g). Measured 187 Os/188 isotopic ratios in a putative impact horizon are also similar to average crustal values (∼1.40), and therefore unsupportive of a significant meteoritic Os contribution to these sediments. Additionally, two high temporal resolution marine Os records (14--9 ka) do not show an excursion to lower ratios in the seawater 187Os/ 188 record. In short, the results on this study provide no evidence supporting the hypothesis that a chondritic impact occurred at the onset of the Younger Dryas. Chapter 4 presents new Os isotope data that are used to evaluate previous claims that seawater 187Os/188 shifts to lower values during glacial maxima, and gradually increases toward more radiogenic, present-day values as climate warms. Our results do not record a systematic shift of seawater 187Os/188 covarying with global climate. Rather, results of this research show that all sites investigated have slightly different Os isotopic compositions, reflecting a heterogeneous ocean with respect to osmium at the millennial time scale.