[MTT51-06] Intensification of Antarctic chemical silicate weathering during the EOT revealed by Mg and Li isotopes
キーワード：Magnesium, Lithium, Eocene-Oligocene Transition, Silicate Weathering, Antarctica
Future warming beyond 2oC 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 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 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 [6, 7] and the δ26Mg and δ7Li records of the authigenic and silicate phases from Site 738 reveal a dramatic intensification of silicate weathering across the EOT, displaying similar variation to previous Pb and Nd isotope records [4, 5]. 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.
The Eocene-Oligocene Transition (EOT; ca. 34 Ma) marks the sudden appearance of 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 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 [6, 7] and the δ26Mg and δ7Li records of the authigenic and silicate phases from Site 738 reveal a dramatic intensification of silicate weathering across the EOT, displaying similar variation to previous Pb and Nd isotope records [4, 5]. 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.