251 Phanerozoic Earth System Shifts in the Marine Sedimentary Record

This session explores Earth System shifts documented as perturbations to the Phanerozoic geochemical, lithological, and biogeochemical marine records, leading to climate shifts, anoxia, ocean acidification, and extinction events.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 303AB

A Window of Opportunity: Waulsortian Mud Mounds as a Sedimentological Response to Geochemical Fluctuations Associated with the Tournaisian Carbon Isotope Excursion

Paper number: 251-8
Program: Technical Sessions
Time: 9:50 AM – 10:05 AM

Authors

Abstract

The Tournaisian Carbon Isotope Excursion (TICE) is one of the largest positive δ13C excursions of the Phanerozoic (~7‰) and marks a major disruption to the global carbon cycle during the early Mississippian. A global carbon cycle disruption of this magnitude would have had consequences on biological, sedimentological, and geochemical processes across the Earth system. Studies investigating this interval have noted increased marine anoxia, shifts in reef builder communities, and the emergence of deeper water microbial carbonate buildups (i.e. Waulsortian mounds). Despite broad interest across subdisciplines, researchers have yet to relate the temporal and geochemical feedback dynamics in the carbon cycle between carbonate sedimentation shifts (i.e. mound emergence) and the TICE. Our data, generated at Bat Mountain in Southern California, are the first to establish a temporal relationship between the TICE and mound emergence. These results provide the foundation for establishing global carbon cycle Earth system feedbacks between the TICE, ocean geochemistry change (e.g. redox), and sedimentological shifts (e.g. deep water carbonate mounds).

We present new δ¹³C data from the Tin Mountain Formation in the southern Funeral Mountains of California that document the full duration of the TICE for the first time at this site. We find that Waulsortian mud mound emergence is stratigraphically restricted to the rising limb of the excursion and terminates before the brief return to lower δ¹³C values that separate the double peak structure of the TICE. Mounds are absent from both older and younger beds, indicating that mound growth was confined to a narrow window of elevated δ¹³C values. In addition, we document the δ¹³C of mound facies within one of our measured sections. The δ¹³C values closely match those of temporally equivalent laterally adjacent beds, suggesting that mound development did not overprint the broader geochemical conditions that define the rising limb of the TICE. Comparisons with mound-bearing successions elsewhere in North America reveal a similar temporal restriction equivalent to the early TICE, suggesting that mound emergence required both a  ramp-style depositional setting and specific geochemical conditions associated with the TICE. Together, these findings underscore the role of microbial systems—such as Waulsortian mud mounds—as both sensitive archives and active participants in biogeochemical feedbacks during intervals of global carbon cycle instability.

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

doi: 10.1130/abs/2025AM-414164

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