266 The Neoproterozoic Earth and Life Co-evolution (Posters)

This session aims to enhance discussions and foster interdisciplinary collaborations to advance understanding of the Neoproterozoic Earth and life co-evolution. Contributions from paleontologists, geochemists, sedimentologists, Earth system modelers, and related fields are highly encouraged.

Wednesday, 22 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Filtered geochemical constraints on the environmental drivers of early animal extinction

Paper number: 266-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The latest Ediacaran period represents a critical geobiological transition with the emergence of the Ediacaran fauna, and their abrupt disappearance before the Cambrian “explosion.” A major open question is: what caused this transition? Intriguingly, geologic and geochemical evidence suggests that this biotic shift followed or closely coincided with major environmental changes which have been interpreted as evidence for an environmental driver of end-Ediacaran extinction. However, paleoenvironmental interpretations of geochemical data vary greatly, particularly with respect to ocean and atmospheric redox state. To evaluate potential causal relationships between environmental and biotic change, this research will analyze the sulfur isotopic composition (δ34S) of carbonate-associated sulfate (CAS) and radiogenic strontium isotopic ratios (87Sr/86Sr) from carbonates across the Ediacaran-Cambrian (E-C) boundary, in Nevada and Mongolia. 

While the δ34SCAS proxy has been widely applied to reconstruct global ocean redox changes throughout Earth’s history, compiled δ34SCAS data across the E-C boundary are noisy, which makes assessing the trajectory of seawater sulfate δ34S challenging. A growing body of work suggests that δ34SCAS can be altered during early and late diagenesis (e.g., by sulfate reduction, or oxidation of reduced sulfur phases), compromising the fidelity of bulk rock δ34SCAS as a primary record of seawater sulfate δ34S values. To mitigate this, X-ray Fluorescence Microscopy (XFM) and calcium isotopic ratios (δ44/40Ca) will be used to screen data prior to analysis of δ34S values. 

Preliminary data from X-ray Absorption Near-Edge Structure (XANES) analyses show that multiple phases of sulfur (e.g., CAS, organic sulfur, pyrite) are disseminated throughout Ediacaran-Cambrian carbonate samples. Micro-XRF maps indicate that some, but not all, CAS is co-located with reduced sulfur phases, which may have implications for reconstructing primary S-isotope signals. Additionally, CAS appears to be concentrated in the matrix of carbonate samples suggesting a diagenetic influence. Between 82 to 20% of δ44/40Ca values in Nevada, and 87 to 65% in Mongolia, meet the threshold of being the likeliest to have preserved primary δ34S (i.e., more positive than a δ44/40Ca cutoff value between −0.5 and −0.9‰ relative to coeval seawater (0.4‰)). Highly variable δ44/40Ca values within this cutoff range may indicate transient diagenetic conditions and thus non-primary δ34S values.

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

doi: 10.1130/abs/2025AM-413850

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