CARBON ISOTOPE MEASUREMENTS OF SINGLE ORGANIC-WALLED MICROFOSSILS FROM THE TONIAN CHUAR GROUP, USA REVEAL TAXON-SPECIFIC WATER COLUMN HABITATS

Early eukaryotic cells are represented in the rock record as organic-walled microfossils (OWM). Although OWM constitute the bulk of the Precambrian fossil record, little is known about their affinities, habitat, or metabolism. It is unclear if the early eukaryotes were restricted to oxygenated surface waters or also inhabited the rest of the water column (predominantly anoxic ferruginous throughout Proterozoic), yet this information is critical to understand how the environment, e.g. stepwise ocean oxygenation, impacted eukaryotic evolution.

Organic carbon isotope analyses of single microfossils can grant insight into short-term environmental variability as well as paleoecology of early eukaryotes. We analyzed C-isotopic composition of individual OWM from shales of the Tonian Chuar Group, Arizona, USA using nano-EA-IRMS method for analysis of single microfossils. OWM (>100) from 10 samples include smooth, ornamented, and envelope-bearing acritarchs, plus cell aggregates and filaments that were likely components of a benthic microbial mat. Fossil specimens show a wide spread of δ¹³C values in a sample (up to 22‰, average 12.5‰). Within well-defined species (e.g. Squamosphaera), spread is more limited (up to 5‰). Depleted values of mat-building prokaryotes Rugosoopsis, Polytrichoides, and Symplassosphaeridium (-33 to -26‰), lighter from bulkrock δ¹³C_org within samples by 5–16‰ are consistent with utilization of ¹³C-depleted C-sources from diffusing underlying pore waters, or DIC derived from respired planktonically-produced organic carbon. The most enriched δ¹³C values are observed in envelope-bearing Simia and ornamented Germinosphaera (c. -15‰), offset from the bulk δ¹³C_org by 3–11‰. Simia is consistently heavier than the mat builders in the same sample, up to 15.8‰, which suggests it probably incorporated ¹³C-enriched in surface waters, or utilized an alternative C-metabolism such as bicarbonate pumping. Polyphyletic taxon Leiosphaeridia shows a wide spread of 17‰ within a sample, likely reflecting different organisms living in different parts of the water column. Highest range in δ¹³C_OWM is observed in anoxic samples (FeHR/FeT>0.38), supporting the presence of a biological pump.