GSA Connects 2021 in Portland, Oregon

Paper No. 15-3
Presentation Time: 8:40 AM


AGIC, Heda, University of California Santa BarbaraDept Earth Science, 1006 Webb Hall, Santa Barbara, CA 93106-0001; Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106, PORTER, Susannah, Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106, COHEN, Phoebe, Department of Geosciences, Williams College, Williamstown, MA 01267 and JUNIUM, Christopher, Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244

Isolating a primary carbon cycle signature in Proterozoic rocks and thus interpreting deep time δ13Corg records is a challenge. Shale δ13Corg records the isotopic composition of organic C derived from a breadth of carbon sources, some of which may be decoupled from the exogenic C cycle. In addition to bulk and compound-specific δ13Corg, primary organic C is recorded in organic-walled microfossils (OWM), comprising the remains of both prokaryotes and eukaryotes, including benthic mat-builders. We analyzed the C-isotopic composition of >250 OWM from the Tonian Chuar Group in Arizona, USA, using nano-EA-IRMS with the goal to understand heterogeneous carbon components and acquire insight into OWM palaeoecology.

The broadest range of OWM C-isotope values (up to -35.6 to -13‰ in a single sample) occurs in rocks deposited in relatively deep water that have Fe-speciation data consistent with a ferruginous environment. A narrower spread (-27 to -22‰) is found in relatively shallow environments deposited under oxic conditions. Assuming that surface waters would contain heavier δ13C values due to export of respired light-C to depth, this suggests water column habitat heterogeneity of the Chuar OWM. Moreover, the microfossils are consistently offset from the bulk δ13Corg and do not follow changes in the bulk shale δ13Corg. OWM were also picked from the strata recording positive δ13Corg excursions in Tanner and Awatubi members (up to -15 ‰). In these samples, δ13COWM ranges from -33 to -18.8‰, and the average OWM value is -25.4‰ (n=51).

C-isotope values from a potential prasinophyte Simia are heavier than both values from mat-builders and bulk δ13Corg in 11 of 13 samples. However, even though it is enriched relative to the bottom-dwelling mat-builders and thus likely planktonic, Simia’s isotopic composition does not change significantly nor does it reflect the bulk δ13Corg of samples that record positive excursions. As excursions are not recorded in short-lived OWM, there are likely other factors causing positive δ13Corg values in Tanner and Awatubi members. These results show that OWM can be used to tease apart carbon components contributing to bulk C-isotope records and offer insights into Precambrian carbon cycle.