291-7 Towards reconstructing Cretaceous paleoecologies: paired oxygen and carbon isotope analyses of Western Interior Seaway fossiliferous concretions

Session: Cephalopods Through Time: Insights into Evolution, Ecology, and Environmental Reconstruction

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Authors:

Abstract:

The Cretaceous Western Interior Seaway contains fossils of abundant molluscan fauna, but interpreting the ecology and biology of these extinct organisms can be difficult, particularly those like ammonites with no modern analogues. Fossiliferous carbonate concretions from the Western Interior Seaway provide exceptional preservation of diverse molluscan assemblages and probably represent rapid formation events (<10 years), potentially offering near-instantaneous ecosystem snapshots. The preservation of shell material within concretions allows for analysis of oxygen isotopes, with implications for paleotemperature, and carbon isotopes, a less frequently interpreted but significant tool for examining intrinsic biology.

We analyzed paired δ¹³C and δ¹⁸O from 101 specimens across eight concretions, including ammonites (Baculites, Hoploscaphites), nautiloids, and benthic mollusks (bivalves, inoceramids, and gastropods). Specimens were filtered using Preservation Index criteria (PI ≥3) and analyzed using standard mass spectrometry techniques. Results demonstrate that specimens of the same species within concretions are generally isotopically comparable to each other. δ¹⁸O ranges are consistently smaller within concretions than between concretions, implying that concretions record discrete environmental conditions from others. Systematic inter-morphotype relationships persist across concretions, particularly within δ¹³C values: bivalves consistently exhibit more positive δ¹³C than cephalopods (∆δ¹³C ~2-5‰), probably indicative of lower metabolic rates. δ¹⁸O relationships are less consistent than carbon, but scaphites show consistently more negative δ¹⁸O than baculites (~1‰, equivalent to ~4°C warmer temperatures), suggesting distinct water column habitats. These findings suggest concretions may be reliable targets for paleoenvironmental reconstruction and reveal persistent ecological patterns in Cretaceous marine ecosystems. The consistent isotopic offsets between morphotypes likely reflect differences in metabolic rates and habitat depths, providing new constraints on ammonite paleoecology and Western Interior Seaway water column structure.

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

doi: 10.1130/abs/2025AM-6491

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