A HIGH RESOLUTION C-ISOTOPE RECORD FROM THE MID-NEOPROTEROZOIC (~800 MA-742 MA) CHUAR GROUP, GRAND CANYON, ARIZONA

C-isotope analysis on a high-resolution sample set (<10 m stratigraphic intervals) of organic-rich mudrocks and dolomites from the marine Chuar Group (1600 m thick) reveals large-scale variability in d¹³C_org and d¹³C_dol. H/C ratios of >0.49 indicate Chuar mudrocks have not been thermally altered and intrabasinal d¹³C_org correlation suggests mudrocks record near-primary values. Intrabasinal d¹³C_dol correlation and average d¹⁸O values of -4‰ (PDB) indicate many Chuar dolomites preserve near-primary d¹³C_dol values.

Large-magnitude C-isotope shifts in the Chuar Group resemble those from other mid-Neoproterozoic C-curves. The negative d¹³C_org shift of ~11‰ in the upper Chuar Group coincides with Melanocyrillium (vase-shaped microfossil) and a U-Pb date of 742 Ma (ash), which allows regional correlation with the upper Pahrump and Uinta Mountain groups of the western United States, and perhaps with the large negative shift in the upper part (pre-Sturtian) of the global curve of Hill and Walter (2000). C-isotope variability lower in the Chuar Group may correlate with lower parts of regional and global C-curves.

There are intriguing dissimilarities between the Chuar and global carbon curves. Typical Chuar d¹³C values (relative to d¹³C_carb) range from 0 to +12‰, whereas the global C-curve ranges from -5 to +8‰. The Chuar Group also shows a pattern of thick stratigraphic intervals with near-zero d¹³C values punctuated by relatively thinner intervals with strongly positive d¹³C values, whereas the global curve shows an opposite pattern. These dissimilarities may be explained by regional/local influences on Chuar seawater chemistry, unconformities within the Chuar Group or within successions comprising the global curve, variable sediment accumulation rates, and/or comparison of C-curves with differing sampling and temporal resolutions.

The Chuar Group offers an important mid-Neoproterozoic data set that includes high-resolution C-isotope data in the context of sedimentologic, physical stratigraphic, biostratigraphic, and geochronologic data. Hypotheses that explain the unique Neoproterozoic Earth system require testing by high-resolution interbasinal correlations to demonstrate the synchroneity (or not) of stratigraphic events.