Geological Society of America

68 D21. Laying the Foundation for the Modern Earth System at the Neoproterozoic-Paleozoic Transition

Monday, 10 October 2022
8:00 AM - 12:00 PM
Mile High Ballroom 4C

EDIACARAN SHURAM EXCURSION INTERPRETED, REINTERPRETED, AND MISINTERPRETED

Paper number: 68-2
Program: Technical Sessions
Time: 8:05 AM – 8:20 AM

Authors

Huan Cui Presenting Author
Mississippi State University, Mississippi State, MS
John Valley
University of Wisconsin-Madison, Madison, WI
Alan Kaufman
University of Maryland, College Park, MD
Shuhai Xiao
Virginia Tech, Blacksburg, VA
Chuanming Zhou
Nanjing Institute of Geology, Palaeontology, and Center for Excellence in Life and Palaeoenvironment Chinese Academy of Sciences, Nanjing
Jean-Marc Baele
University of Mons, Mons
Kouki Kitajima
University of Wisconsin-Madison, Madison, WI
Ian J. Orland
University of Wisconsin, Madison, WI
Adam Denny
University of Wisconsin-Madison, Madison, WI
Adam Denny
Pacific Northwest National Laboratory, Richland, WA
John H. Fournelle
University of Wisconsin, Madison, WI

Abstract

Since the earliest report of the Ediacaran Shuram excursion (SE) almost 30 years ago, the origin of this negative carbon isotope (δ¹³C) anomaly has been hotly debated. Numerous hypotheses have been proposed, ranging from primary to diagenetic, from local to open ocean signals. As yet, no consensus has been reached. Recently, we applied secondary ion mass spectrometry (SIMS) to study SE carbonates at intra-shelf (Jiulongwan) and outer-shelf (Zhongling) sections of an Ediacaran rimmed basin in South China.

Our investigation shows that diagenesis of the SE at Jiulongwan is mainly limited to neomorphism and dolomitization. Coupled SIMS analysis consistently shows δ¹³C of ∼–8‰ in both the micritic matrix and secondary dolomite crystals. The mean values of SIMS δ¹³C data measured from the matrix and secondary dolomite are statistically indistinguishable within each sample, suggesting sediment-buffered diagenesis of δ¹³C. Importantly, no positive δ¹³C or any extensive dissolution-cementation texture has been found. These results suggest a depositional or fabric-retentive early authigenic origin for the SE, and argue against previously published hypotheses that the SE was caused by meteoric water diagenesis or late burial diagenesis.

We also revisited the Zhongling section and discovered remarkableμm-scale heterogeneity of δ¹³C in authigenic calcite cements, with extremely negative values down to –37.5‰. We interpret these cements as methane-derived authigenic calcite resulting from microbial sulfate reduction and anaerobic oxidation of methane during deposition.

We propose that variation of the SE — a notable phenomenon that has been reported in many basins — was modulated by methane oxidation under variable local redox and water depth conditions. The SE likely reflects local carbon cycle anomalies coupled with different degrees of methane oxidation in individual Ediacaran basins and globally triggered by enhanced seawater sulfate and marine transgression during an atmospheric oxygenation event. Our study demonstrates that previously published hypotheses that argue for a post-depositional origin for the SE can be tested by fabric-specific in situ analysis in a detailed petrographic context.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-377531

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