11:30 〜 11:45
[MIS18-20] Can Mg and Li isotopes in the authigenic and detrital fractions of sediments be used to trace paleoweathering? Case study: The EOT
キーワード：Magnesium isotopes, Lithium isotopes, The Eocene-Oligocene Transition, Silicate weathering
Future warming beyond 2oC could lead to the crossing of a threshold beyond which, positive feedbacks within the Earth System could 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 global atmospheric CO2 and temperature [3].
The Eocene-Oligocene Transition (EOT; ~34 Ma) marks the appearance of large Antarctic ice sheets associated with a large surge in erosion as 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 isotope (d26Mg), Li isotope (d7Li), rare-earth element (REE), and major element measurements for the authigenic and detrital phases of marine sediments from ODP Site 738 off the coast of East Antarctica (Kerguelen Plateau).
The d26Mg and d7Li records of the authigenic and silicate phases from Site 738 display a large fractionation during the EOT, in a similar manner to previous Pb and Nd isotope records. This fractionation could be due to one of many processes: (1) increase in weathering intensity; (2) lower clay formation; (3) carbonate weathering; and (4) grain size variation. Changing the method used to extract the authigenic and silicate phase may allow us to discriminate between these processes. Overall, our new data sets suggest continental ice sheet expansion over Antarctica led to increased silicate weathering, atmospheric 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] Bluth & Erel (1995) Nature 373, 415-418. [4] Basak & Martin (2013) Nat. Geosci. 6, 121-124. [5] Scher et al. (2011) Geology 39, 383-386.
The Eocene-Oligocene Transition (EOT; ~34 Ma) marks the appearance of large Antarctic ice sheets associated with a large surge in erosion as 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 isotope (d26Mg), Li isotope (d7Li), rare-earth element (REE), and major element measurements for the authigenic and detrital phases of marine sediments from ODP Site 738 off the coast of East Antarctica (Kerguelen Plateau).
The d26Mg and d7Li records of the authigenic and silicate phases from Site 738 display a large fractionation during the EOT, in a similar manner to previous Pb and Nd isotope records. This fractionation could be due to one of many processes: (1) increase in weathering intensity; (2) lower clay formation; (3) carbonate weathering; and (4) grain size variation. Changing the method used to extract the authigenic and silicate phase may allow us to discriminate between these processes. Overall, our new data sets suggest continental ice sheet expansion over Antarctica led to increased silicate weathering, atmospheric 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] Bluth & Erel (1995) Nature 373, 415-418. [4] Basak & Martin (2013) Nat. Geosci. 6, 121-124. [5] Scher et al. (2011) Geology 39, 383-386.