[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-12] Intense continental weathering after Marinoan snowball Earth

*Yusuke Sawaki¹, Takeshi Ohno², Tsuyoshi Komiya¹ (1.The University of Tokyo, 2.Gakushuin University)

Keywords:Snowball Earth, radiogenic Sr isotope, limestone

The Ediacaran period records one of the most dramatic episodes of biological evolution in Earth’s history. To track the environmental changes that occurred during the Ediacaran, multi-geochemical proxies have been reported from Ediacaran strata by many researchers. Based on the Snowball Earth hypothesis proposed by Hoffman et al. (1998), intense continental weathering was expected after the Marinoan snowball Earth for the deposition of cap carbonate. Strong terrestrial meltwater influx just after Marinoan snowball Earth is recently suggested by stable isotope geochemistry of lithium, magnesium, calcium, and strontium. However, these stable isotopic values of carbonate rocks depend on a number of parameters, thus interpretation of these geochemical proxies is not so straightforward. In addition, the existence of meltwater is a different story from continental weathering rate. In that sense, there is no robust evidence to support intense continental weathering after snowball Earth. The flux of continental weathering can be estimated from the ⁸⁷Sr/⁸⁶Sr ratio of carbonate rock, because this value simply relies solely on continental weathering influx relative to hydrothermal influx. Because existing ⁸⁷Sr/⁸⁶Sr values are sparse for the interval after the Marinoan snowball Earth, we report new stratigraphic profiles of ⁸⁷Sr/⁸⁶Sr ratios by using drill core samples collected at the Tianping section.
Rock powders prepared from carbonate rocks were dissolved in 2 M acetic acid. After removing coexisting matrix elements, Sr isotope ratios were measured with a MC-ICP-MS. We removed diagenetically altered rocks on the basis of Sr content (< 200 μg/g) and Mn/Sr ratio (> 1.0), and such rocks were characterized by high modal abundances of quartz and K-feldspar. Reconstructed ⁸⁷Sr/⁸⁶Sr of least-altered carbonate rocks in the lower part of the Doushantuo Fm decreased from ca. 0.709 to 0.708. These values are higher compared with the ⁸⁷Sr/⁸⁶Sr ratios reported from Early Ediacaran strata in the other areas, demonstrating that continental weathering influx was eleveated after Marinoan snowball Earth. This result is consistent with the Snowball Earth hypothrsis rather than the slushball scenario.