189-4 Paleoecology of Late Devonian Hydrocarbon Seeps Dominated by Brachiopods (Moroccan Meseta)

Session: Paleontology, Paleoecology/Taphonomy (Posters)

Presenting Author:

Authors:

Abstract:

Benthic invertebrates that dominate marine chemosynthetic communities (e.g. hydrothermal vents and hydrocarbon seeps) are specialized to cultivate symbiotic bacteria that help them exploit their habitats’ abundant chemical energy. Strangely, among the most common fossils within ancient examples of these communities are brachiopods of the extinct clade Dimerelloidea, who dominated Paleozoic and Mesozoic seeps, presumably (1) without the ability to harbor symbionts and (2) without being fully restricted to chemosynthetic habitats. To better define the ecological tolerances of this paradoxical group, we located three Moroccan deposits containing the oldest dimerelloid brachiopod, Dzieduszyckia, and analyzed them using stratigraphy, petrography, and stable isotope geochemistry. Our primary focus is a rare in situ, well-bedded deposit ostensibly unrelated to chemosynthesis. Interrupting a deep-water carbonate sequence are anoxic Kellwasser limestones of the Frasnian-Famennian biotic crisis, immediately followed by the appearance of fibrous carbonate cements associated with mass accumulations of Dzieduszyckia spp. Early cement phases had δ¹³C_carb values as low as -11.5‰ VPDB, comparable to a nearby deposit previously classified as a seep. These moderately depleted values indicate hydrocarbon seepage with significant admixture from another carbon source, which we speculate to be background sediments from the pelagic carbonate setting. Though paleoenvironmental context was lacking for the other two ex situ deposits, we infer they were also seeps based on facies overlap with the in situ site. The reclassification of these deposits suggests that even the earliest dimerelloid brachiopods were considerably specialized to chemosynthetic habitats, warranting reappraisal of the remaining exceptions.

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

doi: 10.1130/abs/2025AM-5833

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