Abstract

A high-resolution C-isotope record based on δ13Corg from organic-rich shales and δ13Ccarb 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 δ13Corg 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 δ13Cdol 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 δ13C values and contains sufficient primary carbonate δ13C data to use for calculating Δδ13C values and for comparison with other mid-Neoproterozoic successions.

The Chuar δ13C 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 δ13C anomalies may not require significant continental (and) or low-latitude glaciation as a mechanism for generating large-magnitude C-isotope shifts.

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