High-resolution δ13C stratigraphy of the Chuar Group (ca. 770–742 Ma), Grand Canyon: Implications for mid-Neoproterozoic climate change

Abstract

A high-resolution C-isotope record based on δ 13 C org from organic-rich shales and δ 13 C carb from dolomites in the ca. 770–742 Ma Chuar Group provides important new data for evaluating the signifi cance 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 signifi cant long-term (10 6 –10 7 m.y.) sea-level change nor glaciomarine deposits. Intrabasinal correlation of δ 13 C 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 δ 13 C 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 refl ect predominantly primary organic carbon δ 13 C values and contains suf