99 Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part I

This session is dedicated to studies that illuminate the biotic, geobiological, sedimentological, and geochemical contexts of the evolution of early animal life in the oceans, with a particular focus on the Cambrian Period.

Monday, 20 October 2025
8:00 AM - 12:00 PM
HBGCC, 304B

Regional correlation of a diachronous record of carbonate carbon isotope fluctuations across the Ediacaran-Cambrian Boundary in southwestern Mongolia

Paper number: 99-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

The Zuun-Arts Formation of the Zavkhan terrane in southwestern Mongolia preserves the Ediacaran-Cambrian transition in one of the thickest, most continuous, and most carbonate dominated sections globally. However, the integration of this boundary record into a global age model is complicated by extreme lateral and stratigraphic variability in the Zuun-Arts carbon isotope (δ13C) record. The goal of this study is to assess whether or not the observed lateral δ13C chemostratigraphic heterogeneity of this unit is the result of factors like local and/or diagenetic alteration or can instead be explained by diachronous deposition of this unit and thus primary δ13C values.

To address this, we (1) develop a depositional model for the Zuun-Arts Formation, (2) characterize the spatial scale at which the carbon isotope record of the Zuun-Arts Formation changes, and (3) investigate diagenetic histories of the formation. We find that, although petrographic and geochemical (δ44/40Ca, Sr/Ca, and Mg/Ca) analyses show evidence of recrystallization and neomorphism during early limestone diagenesis and dolomitization, diagenesis did not cause the observed variability of extreme (<-6‰) carbon isotope values recorded in the top of the Zuun-Arts Formation. Instead, we suggest the deposition of the Zuun-Arts Formation was highly diachronous, resulting in a fragmented record of basin-wide carbonate carbon isotope fluctuations. Such an explanation is consistent with the proposed depositional model, as well as the observation that carbon isotope trends are largely reproducible over scales <10 km, or within individual fault blocks. Together, these results imply that the Zuun-Arts Formation records multiple shifts in the composition of dissolved inorganic carbon across the Ediacaran-Cambrian transition.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412310

© Copyright 2025 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.