USING SINGLE-FOSSIL ORGANIC CARBON ISOTOPES TO ELUCIDATE THE CRYOGENIAN TAISHIR ANOMALY

The Neoproterozoic Tsagaan-Olom Group of southwestern Mongolia records one of the largest negative carbon isotopic anomalies in Earth’s history, known as the Taishir anomaly. Background isotope values in the Taishir Formation are ~ +8‰, but plummet abruptly at the beginning of the third supersequence and reach a low of −7.5‰. This −16‰ δ¹³C anomaly is puzzling for multiple reasons. First, there are ambiguous correlations between the Taishir anomaly and other anomalies in Namibia, Canada, Australia, and the UK. Second, there is a lack of correlation between carbon and oxygen isotopic values. Third, the source of the light carbon required to create this anomaly is unclear. There are multiple hypotheses as to the driver of the Taishir excursion, including changes in theδ¹³C values of global seawater, massive organic carbon oxidation, and the precipitation of enriched authigenic carbonates.

In this study we look at single red algal microfossils (OSM, organic spore-like microfossils) found in the Cryogenian Taishir and Ediacaran Ol Formations of the Tsagaan-Olom Group. The goal of this work is to better understand the survival and distribution of eukaryotic life through global glaciations. We employ a novel method of measuring δ¹³C values of single microfossils, which we then compare to bulk organic and inorganic carbon isotope values in order to describe patterns and differences between bulk organic, inorganic, and fossil data. Analyzing these fossils individually can provide a short-term view of Cryogenian environmental conditions and variability, as well as information on the global carbon cycle.

Our preliminary data was collected from 35 fossils across supersequence T3 in the Taishir Formation and two horizons in the Ol Formation. The horizon from supersequence T3 is located within the descending limb of the anomaly, and shows a range from −20.1‰ to −22.4‰ and a mean of −21.3‰. One horizon from the Ol Formation is located at the bottom of the formation and shows a range from −22.1‰ to −26.3‰ and a mean of −24.6‰. The second, from the top of the measured section, shows a range from −21.3‰ to −29.1‰ and a mean of −25.7‰.