Carbon isotope excursions and crinoid dissolution at exposure surfaces in carbonates, West Texas, U.S.A

Dickson, J. A. D. and Saller, A. H. (2006) Carbon isotope excursions and crinoid dissolution at exposure surfaces in carbonates, West Texas, U.S.A. Journal of Sedimentary Research, 76 (3). pp. 404-410. DOI

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Carbonates from the Horseshoe Atoll, Reinecke Field, West Texas, U.S.A., show four subaerial exposure events by the presence of rhizoliths, alveolar septal fabric, and thin calcretes that subdivide the Pennsylvanian succession into five sequences. Vertical profiles of bulk rock {delta}13C analyses show values that are generally ~ +1{per thousand} (PDB), but negative shifts of up to 2{per thousand} occur beneath exposure surfaces. The bulk-rock {delta}13C values ~ +1{per thousand} could be explained as a mixture of 63% marine carbonate ({delta}13C = +4{per thousand}) and 37% pedogenic carbonate ({delta}13C = –4{per thousand}) while a negative bulk-rock excursion of {delta}13C = ~ –1{per thousand} can be explained as a mixture of 37% marine and 63% pedogenic calcite. Distinctive negative shifts in the {delta}13C profiles do not occur under some exposure surfaces that contain soil-generated features, and one negative shift occurs that apparently is unrelated to an exposure surface. Bulk-rock {delta}13C values are difficult to interpret because the volume of their various components is unknown. Furthermore, {delta}13C profiles may miss subaerial exposure surfaces. Certain styles of crinoid alteration may indicate paleo-subaerial exposure surfaces. Fossil crinoids can be preserved in a variety of ways, but crinoids with dissolution holes that subsequently either become filled with calcite cement or were crushed during burial are restricted to 2 m below exposure surfaces. Dissolution of Mg calcite crinoids is due to their relatively high Mg composition, 12 mole % MgCO3, combined with their location beneath soil-capped surfaces from which meteoric waters greatly undersaturated with respect to calcite emerged. Late-stage dissolution removed finely crystalline calcite, but crinoids by this time had stabilized to coarsely crystalline calcite and dolomite and thus were unaffected. Evaluation of other stratigraphic systems is needed to determine whether the use of "holey" crinoids to identify subaerial exposure surfaces has broad application.

Item Type: Article
Uncontrolled Keywords: 2006 AREP 2006 P IA51
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Journal or Publication Title: Journal of Sedimentary Research
Volume: 76
Page Range: pp. 404-410
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 16 Feb 2009 13:02
Last Modified: 23 Jul 2013 10:06

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