T81 Deep-Time Carbon Cycles, Redox Changes, and the Evolution of the Biosphere I

Tuesday, 27 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

INVESTIGATING CONTROLS ON THE EXPRESSION OF THE STEPTOEAN POSITIVE CARBON ISOTOPE EXCURSION THROUGH META-ANALYSIS

Paper number: T81-6
Program: Technical Sessions
Time: 2:35 PM – 2:50 PM

Authors

Abstract

The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent chemostratigraphic feature in the Lower Paleozoic. It has been used to correlate Upper Cambrian carbonate strata globally, and is cited as intimately linked to the Crepicephalus-Aphelaspis trilobite extinction event and the Sauk II-Sauk III megasequence transition. Despite the global nature of the SPICE event, regional/local conditions serve as a control on its expression in the rock record. In light of this, and to better understand how reliable the SPICE event is as a chemostratigraphic tool for correlation, we have created the “SPICEraq,” a database comprising 78 SPICE-bearing sections containing 6669 individual δ13C analyses. In this study, we quantitatively evaluate the variability in SPICE records, and document that, while the excursion is a global signature, its stratigraphic expression is influenced by such conditions as paleolatitude, paleocontinent, water depth, and facies. While the magnitude of the SPICE excursion is generally consistent (an ~4‰ V-PDB increase), peak δ13C values are quite variable (+0.35 to +5.87‰). Sections located between 30 and 60°S paleolatitude ca. 500 Ma record δ13C values ~1 to 2‰ lower than those from lower paleolatitudes. Sections deposited in shallow water facies record lower δ13C values than those from deeper-waters. The stratigraphic thickness of the excursion varies widely (<3 to ~884 m) and is significantly impacted by all categorical variables investigated. The rising limb of the SPICE is immediately preceded by a small negative δ13C excursion in 75% of sections with pre-SPICE data. While 32% of sections record a δ13C plateau during peak SPICE, its presence in the δ13C record is not influenced by any conditions investigated herein. Altogether, these analyses indicate that regional/local conditions impact the stratigraphic expression of δ13C records, and thus care should be taken to use robust, quantitative measures to compare and correlate excursions.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-354196

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