266-6 Filtered geochemical constraints on the environmental drivers of early animal extinction

Session: The Neoproterozoic Earth and Life Co-evolution (Posters)

Presenting Author:

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 δ³⁴S_CAS proxy has been widely applied to reconstruct global ocean redox changes throughout Earth’s history, compiled δ³⁴S_CAS 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 δ³⁴S_CAS 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 δ³⁴S_CAS as a primary record of seawater sulfate δ³⁴S values. To mitigate this, X-ray Fluorescence Microscopy (XFM) and calcium isotopic ratios (δ^44/40Ca) 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 δ^44/40Ca 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 δ^44/40Ca cutoff value between −0.5 and −0.9‰ relative to coeval seawater (0.4‰)). Highly variable δ^44/40Ca values within this cutoff range may indicate transient diagenetic conditions and thus non-primary δ³⁴S values.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9539

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.