THE BACE ACROSS A BASIN: INVESTIGATING INTRABASINAL VARIATION IN THE EDIACARAN-CAMBRIAN CARBONATE CARBON ISOTOPE RECORD OF THE ZUUN-ARTS FORMATION IN SOUTHWESTERN MONGOLIA

Around the world, carbonates from the latest Ediacaran and early Cambrian (~540-520 Ma) record high magnitude (>4‰) carbon isotope (δ¹³C) excursions (CIEs). Although the causes and environmental effects of these CIEs remain unknown, they are often used to correlate sections globally and construct age models. The oldest of these perturbations within this time window (the BAsal Cambrian negative CIE, or BACE) unofficially marks the Ediacaran-Cambrian boundary (ECB). To use secular changes in the δ¹³C record for correlation and to better understand carbon cycling dynamics, we must untangle how depositional and post-depositional histories have affected geochemical proxy records. In Mongolia, the BACE appears in the Zuun-Arts Formation, an ECB carbonate succession that records diachronous deposition within a foreland basin. As such, the Zuun-Arts Formation allows for investigation of interplays between factors that might affect expression of the BACE, including changes in ocean chemistry, sedimentology, and diagenesis.

We pair δ¹³C and δ¹⁸O records with proxies to assess marine diagenesis (δ^44/40Ca and Sr/Ca data) and integrate these data into a basin-wide depositional model. We present sedimentological and geochemical data from measured sections across a transect. Between sites spaced kilometers apart, we find the Zuun-Arts δ¹³C signal varies significantly laterally, despite being reproducible over ~500 m. We explain this variation in two ways: diachronous deposition of strata and local differences in depositional and post-depositional conditions that affect sediments’ isotopic compositions. The latter point is diagnosed via proxy records, which indicate whether sediment isotopic compositions are influenced by the compositions of a fluid (fluid-buffered) or of a precursor sediment (sediment buffered). We find that sites closer to paleoshoreline record evidence of sediment buffered conditions, while those farther record evidence of fluid-buffered conditions. At all sites, we find the nadirs of CIEs occur where proxy records indicate the most fluid-buffered conditions. Collectively, these data show that diagenesis and depositional setting can influence expressions of the BACE in Mongolia and demonstrate the necessity of similar regional studies of the BACE and other CIEs elsewhere.