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High-resolution ¹³C stratigraphy of the Chuar Group (ca. 770–742 Ma), Grand Canyon: Implications for mid-Neoproterozoic climate change

C.M. Dehler^,1, M. Elrick², J.D. Bloch², L.J. Crossey², K.E. Karlstrom² and D.J. Des Marais³

¹ Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA
² Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116, USA
³ Mail Stop 239-4, Ames Research Center, Moffett Field, California 94035-1000, USA

A high-resolution C-isotope record based on ¹³C_org from organic-rich shales and ¹³C_carb from dolomites in the ca. 770–742 Ma Chuar Group provides important new data for evaluating the significance of large-magnitude C-isotope anomalies in Neoproterozoic climate change. Three successive, large-magnitude isotopic excursions (8–15) are interpreted to represent primary seawater values based on a series of diagenetic tests, and they are not associated with evidence of significant long-term (10⁶–10⁷ m.y.) sea-level change nor glaciomarine deposits. Intrabasinal correlation of ¹³C_org values suggests that most Chuar shales record primary values and is consistent with previously reported H/C ratios of >0.49 indicating that Chuar shales experienced minimal thermal alteration. Although some Chuar dolomites reveal early diagenetic alteration, their ¹³C_dol values typically fall near those of coeval "least-altered" dolomites or organic-rich shales (relative to dolomite values). The Chuar carbon record is interpreted to reflect predominantly primary organic carbon ¹³C values and contains sufficient primary carbonate ¹³C data to use for calculating ¹³C values and for comparison with other mid-Neoproterozoic successions.

The Chuar ¹³C shifts are in phase with dolomite-poor/dolomite-rich litho stratigraphic sequences and with shale petrologic and mineralogical trends. These data sets collectively indicate long-term (m.y.-scale) wetter-to-drier climate change and concomitant low-amplitude sea-level change. The Chuar basin may be a proxy for mid-Neoproterozoic low-latitude basins that accommodated significant organic carbon burial during this time. Models for other Neoproterozoic long-term ¹³C anomalies may not require significant continental (and) or low-latitude glaciation as a mechanism for generating large-magnitude C-isotope shifts.

Key Words: Neoproterozoic • Grand Canyon • C-isotope record • Chuar Group • lithostratigraphy • shale geochemistry • climate change

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