[M-IS15] Paleoclimatology and Paleoceanography

[MIS15-02] Factors for the long-term climatic trends in the early Eocene hothouse world: Implications from the seawater osmium isotope record of the Indian Ocean sediments

*Yusuke Kuwahara¹, Kazutaka Yasukawa^1,2, Junichiro Ohta^2,3, Moei Yano^3,2, Kazuhide Mimura^3,2, Erika Tanaka^4,3, Koichiro Fujinaga^3,2, Kentaro Nakamura^1,2, Yasuhiro Kato^1,2 (1.Department of Systems Innovation, School of Engineering, The University of Tokyo, 2.Frontier Research Center for Energy and Resources, School of Engineering, The University of Tokyo, 3.Ocean Resources Research Center for Next Generation, Chiba Institute of Technology, 4.Center for Advanced Marine Core Research, Kochi University)

Keywords:Early Eocene Climate Optimum, Osmium isotope, Carbon cycle, Chemical weathering, Hothouse Earth

It is widely accepted that the early Paleogene (late Paleocene–middle Eocene) is characterized as a “hothouse” environment in the long-term (~10⁶ yr) climatic trend during the Cenozoic era [1]. The long-term warming initiated at the late Paleocene and culminated during the early Eocene, as the Early Eocene Climatic Optimum (EECO). After the EECO, the global climate turned to a long-term cooling. Although many hypotheses have been proposed, the fundamental factors accounting for the long-term warming and cooling trends during the Paleogene are still in debate.
Here we employed the Os isotopic ratio (¹⁸⁷Os/¹⁸⁸Os) of seawater as a proxy for the solid Earth activity (e.g., silicate chemical weathering, magmatism), and discuss its relationship with the early Paleogene climate trend. The marine Os isotopic ratio reflects the relative strength of two dominant influxes to the ocean: radiogenic Os flux through the weathering of continental crust (¹⁸⁷Os/¹⁸⁸Os > 1.0) and unradiogenic Os flux through the inputs of hydrothermal, mafic materials, and extraterrestrial inputs (¹⁸⁷Os/¹⁸⁸Os = 0.12–0.13) [2]. Owing to the contrasting ¹⁸⁷Os/¹⁸⁸Os values between these two influxes, the variability in ¹⁸⁷Os/¹⁸⁸Os of seawater constitutes a sensitive tracer for the continental, mantle, and extraterrestrial inputs into the marine environment.
We report the long-term early Paleogene marine Os isotopic record from the pelagic carbonate sediment in the Exmouth Plateau (ODP Site 762C) and Kerguelen Plateau (ODP Site 738C) in the Indian Ocean. In addition, we will quantitatively discuss the relative contribution between continental and mantle-like materials into the ocean by using a simple 1-box isotopic mass-balance modeling based on the marine ¹⁸⁷Os/¹⁸⁸Os record. In the presentation, we will provide new constraints for the long-term climatic trend during the Early Eocene.

[1] Westerhold et al. (2020) Science 369, 1383–1387. [2] Peucker-Ehrenbrink & Ravizza (2000) Terra Nova 12, 205-219.