[M-IS16] 古気候・古海洋変動

[MIS16-19] Authigenic and detrital Mg and Li isotopes as a tracer of Antarctic chemical silicate weathering across the EOT

*Adam David Sproson^1,2,3、Toshihiro Yoshimura¹、Yusuke Yokoyama^1,3、Takahiro AZE³、Tsuyoshi Ishikawa⁴、Shigeyuki Wakaki⁴、Naohiko Ohkouchi¹ (1.Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan、2.Submarine Resources Research Center, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan、3.Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan、4.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology 200 Monobe Otsu, Nankoku City, Kochi 783-8502, Japan)

キーワード：Chemical weathering, Eocene-Oligocene Transition, Mg & Li isotopes

Future warming beyond 2C could lead to the crossing of a threshold beyond which positive feedbacks within the Earth System, such as permafrost thawing, increased bacterial respiration and forest dieback, would create a “Hothouse Earth” [1]. Understanding the interplay of feedback mechanisms is therefore of utmost importance to predicting future climate change. On million-year timescales, the hydrolysis of silicate minerals and subsequent precipitation of carbonate minerals in the ocean acts as a negative feedback within the carbon-cycle [2]. On shorter, more relevant, timescales, the production of fine-grained material and associated subglacial chemical weathering during expansion and contraction of glaciers may influence atm. CO₂ and temperature [3].

The Eocene-Oligocene Transition (EOT; ca. 34 Ma) marks the sudden appearance of large ice sheets on Antarctica which caused a surge in erosion recorded by Nd isotopes, Pb isotopes and clay mineralogy [4, 5]. However, the response of chemical silicate weathering is not well understood. Here, we present Mg and Li isotope and rare-earth element (REE) data for the authigenic and detrital phases of marine sediments from ODP Site 738 on the Kerguelen Plateau. Magnesium and Li isotopes fractionate significantly during chemical weathering, with the retention of the heavier ²⁶Mg and ⁷Li in weathering residues [6, 7]. During secondary mineral formation, the heavier (lighter) isotopes of Mg (Li) become preferentially incorporated, driving the δ²⁶Mg (δ⁷Li) to higher (lower) values.

The δ²⁶Mg and δ⁷Li records of the authigenic and silicate phases from Site 738 display similar variation to previous Pb and Nd isotope records, revealing a dramatic intensification of silicate weathering across the EOT. This intensification is well correlated to oxygen isotopes suggesting continental ice sheet expansion over Antarctica led to increased silicate weathering, atm. CO₂ drawdown and further cooling. Such feedbacks may help reverse future warming as ice sheets begin to retreat.

[1] Steffen et al. (2018) PNAS 115, 8252-8259. [2] Berner (2006) GCA 70, 5653-5664. [3] Kump & Alley (1994) 46-60. [4] Basak & Martin (2013) Nature Geoscience 6, 121-124. [5] Scher et al. (2011) Geology 39, 383-386. [6] Wimpenny et al. (2011) EPSL 304, 260-269. [7] Huh et al. (1998) GCA 62, 2039-2051.