17:15 〜 18:30
[BCG04-P07] High 187Os/188Os values of Neoproterozoic seawater reconstructed from the Lesser Himalayan Black shales
キーワード：Neoproterozoic、Osmium、Lesser Himalaya
An increase in oceanic sulfate concentrations during the Neoproterozoic (0.8-0.55 Ga) was inferred from widespread sulfate evaporite deposition and the increased range of marine sulfide δ34S values (e.g., Prince et al., 2019 Geology; Fakhraee et al., 2019 Nat. Geo.). This shift has been linked to a rise of atmospheric O2 levels during this period (the so-called Neoproterozoic Oxidation Event). However, recent findings of highly fractionated Cr isotopes in Mesoproterozoic shale and lack of clear Ce anomalies in Neoproterozoic carbonates do not necessarily support significant oxidation of the Earth's surface during the Neoproterozoic (e.g., Wallace et al., 2017 EPSL; Canfield et al., 2018 Nat. Commun.).
To better understand the sulfur cycle during the Neoproterozoic, we analyzed Re-Os isotopic compositions of black shale samples collected from three outcrops in the Infra-Krol Formation of the Lesser Himalaya, India. U-Pb ages of detrital zircons from the underlying Blaini Formation provide a maximum depositional age of 656 ± 21 Ma (Dey et al., 2020 GSA Bulletin). The analyzed samples exhibited a limited spread in the 187Re/188Os ratio and did not provide a precise depositional age of the black shale unit. However, the Re-Os data from one of the outcrops yield an isochron age of 839 ± 220 Ma that is broadly consistent with the possible depositional age of the black shale unit (< 656 ± 21 Ma; Dey et al., 2020 GSA Bulletin). This indicates that Re and Os in the samples are dominant of hydrogenous origin and have not been disturbed significantly by later alteration events.
The initial 187Os/188Os ratios of all of the analyzed samples suggest a highly radiogenic 187Os/188Os value (> 1.07 ± 0.13) of ambient seawater at the time of deposition. Similarly, high values were also reported from black shale samples deposited after the Sturtian (~717–659 Ma) and Marinoan (~649–636 Ma) glaciations (e.g., Rooney et al., 2014 PNAS). In contrast, no such values have been reported from pre-Sturtian black shales. Hence, the initial 187Os/188Os ratios of the analyzed samples likely reflect global processes affecting Neoproterozoic (post-Sturtian and/or post-Marinoan) seawater chemistry. Currently, organic-rich shale is the dominant source of radiogenic Os to seawater (e.g., Dubin and Peucker-Ehrenbrink, 2015 Chem. Geol.). Hence, the high 187Os/188Os ratios of post-Sturtian seawater can be interpreted as evidence of enhanced weathering of organic-rich shales. Because sulfide minerals are common in organic-rich shales (e.g., Berner and Raiswell, 1983 GCA), this process could have contributed to the rise of marine sulfate concentrations during the Neoproterozoic. This scenario does not necessarily support the conventional view of the marine sulfur cycle during the Neoproterozoic; however, it seems to be consistent with the recent geological findings mentioned above (e.g., Wallece et al., 2017; Canfield et al., 2018).
To better understand the sulfur cycle during the Neoproterozoic, we analyzed Re-Os isotopic compositions of black shale samples collected from three outcrops in the Infra-Krol Formation of the Lesser Himalaya, India. U-Pb ages of detrital zircons from the underlying Blaini Formation provide a maximum depositional age of 656 ± 21 Ma (Dey et al., 2020 GSA Bulletin). The analyzed samples exhibited a limited spread in the 187Re/188Os ratio and did not provide a precise depositional age of the black shale unit. However, the Re-Os data from one of the outcrops yield an isochron age of 839 ± 220 Ma that is broadly consistent with the possible depositional age of the black shale unit (< 656 ± 21 Ma; Dey et al., 2020 GSA Bulletin). This indicates that Re and Os in the samples are dominant of hydrogenous origin and have not been disturbed significantly by later alteration events.
The initial 187Os/188Os ratios of all of the analyzed samples suggest a highly radiogenic 187Os/188Os value (> 1.07 ± 0.13) of ambient seawater at the time of deposition. Similarly, high values were also reported from black shale samples deposited after the Sturtian (~717–659 Ma) and Marinoan (~649–636 Ma) glaciations (e.g., Rooney et al., 2014 PNAS). In contrast, no such values have been reported from pre-Sturtian black shales. Hence, the initial 187Os/188Os ratios of the analyzed samples likely reflect global processes affecting Neoproterozoic (post-Sturtian and/or post-Marinoan) seawater chemistry. Currently, organic-rich shale is the dominant source of radiogenic Os to seawater (e.g., Dubin and Peucker-Ehrenbrink, 2015 Chem. Geol.). Hence, the high 187Os/188Os ratios of post-Sturtian seawater can be interpreted as evidence of enhanced weathering of organic-rich shales. Because sulfide minerals are common in organic-rich shales (e.g., Berner and Raiswell, 1983 GCA), this process could have contributed to the rise of marine sulfate concentrations during the Neoproterozoic. This scenario does not necessarily support the conventional view of the marine sulfur cycle during the Neoproterozoic; however, it seems to be consistent with the recent geological findings mentioned above (e.g., Wallece et al., 2017; Canfield et al., 2018).