INDIVIDUAL ORGANIC-WALLED MICROFOSSIL δ13C DECOUPLED FROM POSITIVE δ13CORG EXCURSIONS IN THE TONIAN CHUAR GROUP (Invited Presentation)

Isolating a primary carbon cycle signature in Proterozoic rocks and thus interpreting deep time δ¹³C_org records is a challenge. Shale δ¹³C_org 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 δ¹³C_org, 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 δ¹³C_org and do not follow changes in the bulk shale δ¹³C_org. OWM were also picked from the strata recording positive δ¹³C_org excursions in Tanner and Awatubi members (up to -15 ‰). In these samples, δ¹³C_OWM 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 δ¹³C_org 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 δ¹³C_org of samples that record positive excursions. As excursions are not recorded in short-lived OWM, there are likely other factors causing positive δ¹³C_org 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.