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

A high-resolution C-isotope record based on delta(13)C(org) from organic-rich shales and delta(13)C(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 (8-15parts per thousand) 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 (10(6)-10(7) m.y.) sea-level change nor glaciomarine deposits. Intrabasinal correlation of delta(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 delta(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 reflect predominantly primary organic carbon delta(13)C values and contains sufficient primary carbonate delta(13)C data to use for calculating Deltadelta(13)C values and for comparison with other mid-Neoproterozoic successions. The Chuar delta(13)C shifts are in phase with dolomite-poor/dolomite-rich lithostratigraphic 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 delta(13)C anomalies may not require significant continental (and) or low-latitude glaciation as a mechanism for generating large-magnitude C-isotope shifts.