29 Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts (Posters)

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.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Anoxia Intensified in Shallow Waters During the Steptoean Positive Carbon Isotope Excursion (SPICE) in Eastern Nevada

Paper number: 29-6
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

During the Cambrian, repeated trilobite extinction events, called biomeres, disproportionately affected shallow-water trilobite groups. One of these extinction events, the Marjumiid-Pterocephaliid biomere, is approximately concurrent with the Steptoean Positive Carbon Isotope Excursion (SPICE). The SPICE is a 4 to 6‰ δ13C  excursion identified globally at the base of the Paibian stage. Global and local redox proxies record expanded anoxic conditions during this event. It is hypothesized that the Marjumiid-Pterocephaliid biomere was triggered by this expanding anoxia. Here we test this hypothesis by using a carbonate-based redox proxy, the cerium anomaly, to describe local shallow-water conditions in Nevada. To our knowledge, this is the first stratigraphic record of cerium through the SPICE in the Great Basin region.

Our study site is located in the Pahranagat Range, Eastern Nevada. During the Cambrian, a shallow, epicontinental ocean deposited carbonate rocks in this region. We sampled rocks through the Cambrian-aged Highland Peak and Dunderberg Formations. We identified a 4‰ δ13C excursion in the Dunderberg Formation, which we identify as the SPICE. Preliminary cerium anomaly data show an increase in the intensity of anoxia during the rising limb of the SPICE. This shift occurred within a shoal/reef facies and can not be attributed to deepening. Cerium anomaly values begin to return to baseline conditions during the peak of the SPICE excursion. These data agree with faunal diversity patterns in the region, with trilobite diversity decreasing dramatically at approximately the base of the SPICE but recovering through the SPICE interval. This dataset provides direct evidence of anoxia spreading into shallow water environments and refines the timing of redox shifts during the SPICE.

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

doi: 10.1130/abs/2025AM-414749

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