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Methane seeps as ammonite habitats in the U.S. Western Interior Seaway revealed by isotopic analyses of well-preserved shell material

Neil H. Landman, J. Kirk Cochran, Neal L. Larson, Jamie Brezina, Matthew P. Garb and Peter J. Harries
Methane seeps as ammonite habitats in the U.S. Western Interior Seaway revealed by isotopic analyses of well-preserved shell material
Geology (Boulder) (April 2012) 40 (6): 507-510

Abstract

Methane seep deposits are common in the Upper Cretaceous Pierre Shale of the U.S. Western Interior. They contain a rich fauna including ammonites, bivalves, gastropods, sponges, corals, echinoids, crinoids, and fish. In an effort to understand the role of ammonites in these ecosystems, we examined a seep from the upper Campanian Didymoceras cheyennense Zone in Custer County, South Dakota, that contains molluscs with well-preserved shell material permitting isotopic analyses. Values of delta (super 13) C of the micritic limestone at the seep range from -46.94 per mil to -11.49 per mil, confirming the influence of anaerobic oxidation of methane on the isotopic composition of the dissolved inorganic carbon reservoir. The ammonites also consistently display light values of delta (super 13) C ranging from -13.71 per mil to 0.68 per mil. These values are generally lighter than those in nonseep specimens from age-equivalent rocks elsewhere in the basin (-1.75 per mil to 3.42 per mil). In a single specimen of Baculites corrugatus from the seep, light delta (super 13) C values occur throughout ontogeny. These data suggest that ammonites incorporated isotopically light methane-derived carbon in their shells and lived near the vent fluids and methane-oxidizing bacteria. Both juvenile and adult specimens are present, implying that these ammonites spent their entire lives at the seep and formed an integral part of an interwoven community. The values of (super 87) Sr/ (super 86) Sr in the limestone and well-preserved fossils at the seep (0.707690-0.707728) are higher than that of the open ocean at this time (0.707659). These elevated values suggest that the seep fluids were imprinted with a radiogenic Sr signature, perhaps derived from equilibration with granitic deposits at depth during the initial uplift of the Black Hills.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 40
Serial Issue: 6
Title: Methane seeps as ammonite habitats in the U.S. Western Interior Seaway revealed by isotopic analyses of well-preserved shell material
Affiliation: American Museum of Natural History, Division of Paleontology (Invertebrates), New York, NY, United States
Pages: 507-510
Published: 20120410
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 20
Accession Number: 2012-050095
Categories: StratigraphyInvertebrate paleontologyIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2012148; accessed on April 30, 2012
Illustration Description: illus. incl. sect., sketch map
N43°28'00" - N43°52'00", W104°04'00" - W102°40'00"
Secondary Affiliation: State University of New York at Stony Brook, USA, United StatesBlack Hills Museum of Natural History, USA, United StatesSouth Dakota School of Mines and Technology, USA, United StatesBrooklyn College, USA, United StatesUniversity of South Florida, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201226
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