[M-IS18] Paleoclimatology and paleoceanography

[MIS18-22] The Paleogene seawater osmium isotope record reconstructed from carbonate sediments in the Indian Ocean: Implications for the chemical weathering feedback against the Eocene hyperthermals

★Invited Papers

*Yusuke Kuwahara¹, Kazutaka Yasukawa^3,1, Junichiro Ohta^3,2, Moei Yano^2,3, Kazuhide Mimura², Erika Tanaka⁴, Koichiro Fujinaga^2,3, Kentaro Nakamura^1,2, Yasuhiro Kato^3,1 (1.Department of Systems Innovation, School of Engineering, The University of Tokyo, 2.Ocean Resources Research Center for Next Generation, Chiba Institute of Technology, 3.Frontier Research Center for Energy and Resources, School of Engineering, The University of Tokyo, 4.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology)

Keywords:Eocene hyperthermals, Paleocene-Eocene Thermal Maximum, Osmium isotopic ratio, Carbon cycle, Chemical weathering

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 of the Earth’s history. In addition, multiple global warming events called “hyperthermals” on shorter (10⁴–10⁵ yr) timescales occurred in the hothouse Earth [1]. Among them, the Paleocene-Eocene Thermal Maximum (PETM), which was the most drastic global warming event in the Early Paleogene, has been gathering much attention as an analog of current global warming [2].
During the recovery from the PETM, enhanced silicate weathering on land due to the high temperature and accelerated hydrological cycle could have worked as a negative feedback mechanism through the drawdown of atmospheric CO₂ [3]. However, whether this system repeatedly works against other hyperthermal events (e.g. Eocene Thermal Maximum (ETM) 2 and ETM3) is still unclear. Here we employed the Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os) of seawater as a proxy of silicate weathering flux. The marine Os isotopic ratio reflects the relative strength of two dominant influxes to the ocean: radiogenic Os flux (¹⁸⁷Os/¹⁸⁸Os = 1.0−1.4) through the weathering of continental crust, and unradiogenic Os flux (¹⁸⁷Os/¹⁸⁸Os = ~0.13) through the mantle-origin input (the hydrothermal activities, and the weathering of juvenile basalt) and extraterrestrial inputs. Owing to the contrasting ¹⁸⁷Os/¹⁸⁸Os values between these influxes, and a relatively short residence time of Os in the ocean (~10⁴ yr), the variability in ¹⁸⁷Os/¹⁸⁸Os of seawater constitutes a sensitive tracer for the continental, mantle, and extraterrestrial input into the marine environment [4].
We report the 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 present the possible range of riverine Os flux calculated by simple 1-box isotopic mass-balance modeling based on the marine ¹⁸⁷Os/¹⁸⁸Os record. In the presentation, we will discuss the effectiveness of chemical weathering feedback for the early Eocene hyperthermals.

[1] Zachos et al. (2008) Nature 451, 279-283. [2] Zeebe & Zachos (2016) Nature Geoscience 9, 325-329. [3] Ravizza et al. (2001) Paleoceanography 16, 155-163. [4] Peucker-Ehrenbrink & Ravizza (2000) Terra Nova 12, 205-219.