332-10 From SPICE to MDICE: Thirty Million Years of Cambrian-Ordovician Marine Carbon Cycle Evolution Recorded in Western Maryland, USA

Abstract:

The late Cambrian to Middle Ordovician (494–458 Ma) was a pivotal interval in Earth’s history in terms of both the evolution of the biosphere and the carbon cycle. Frequent biotic turnover events in the late Cambrian and earliest Ordovician were often accompanied by excursions in the marine carbon isotope record, such as the end-Marjuman extinction and the SPICE (Steptoean Positive Isotopic Carbon Excursion). The concurrence of these events suggests a link between extinctions and the carbon cycle disruptions represented by isotope excursions. However, while the Early to Middle Ordovician saw continued carbon cycle instability, it also saw an end to these extinction events and a marked increase in biodiversity referred to as the Great Ordovician Biodiversification Event (GOBE). Fundamentally, our ability to understand the complex relationships between the biosphere and the carbon cycle hinges upon identifying which aspects of the marine carbon isotope record reflect global or local changes to the carbon cycle and which are diagenetic artifacts.

Here, we present a new high-resolution Cambrian-Ordovician carbonate carbon isotope (δ¹³C) record from the Chesapeake and Ohio Canal National Historical Park near Clear Spring, Maryland, USA. We measured, logged, and sampled over 2000 meters of strata consisting of shallow carbonate facies at 1- to 3-meter resolution to produce ~1100 δ¹³C and δ¹⁸O measurements. Using previously collected biostratigraphic data for temporal context, we identified four globally recognized carbon isotope excursions present in our section. The late Cambrian SPICE is preserved ~100 meters above the base of the section as a +5.5‰ δ¹³C shift over ~70 meters. Next, the Ordovician TSICE (Top Skullrockian Isotopic Carbon Excursion) appears as a sharp, asymmetrical +3‰ excursion spanning ~50 meters. In the Floian, our section preserves a +4‰ excursion spanning nearly 500 meters that appears in a few other localities globally but is yet unnamed. The MDICE (Middle Darriwilian Isotopic Carbon Excursion), a +6‰ excursion, is preserved near the top of our studied section. Ultimately, the preservation of these four global excursions, plus local δ¹³C features, in a location with relatively continuous deposition and consistent shallow carbonate facies provides an exciting opportunity to investigate carbon cycle dynamics leading into and during the GOBE. 

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.