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ABSTRACT: Stable Oxygen and Carbon Isotope Study of Recent Sediments and Cements, Lee Stocking Island, Bahamas: Organic vs. Inorganic Precipitation

W. Fred Falls, D. F. Williams, C. G. St. C. Kendall, R. F. Dill

Stable isotope analyses of modern carbonates from the tidal inlet along the north end of Lee Stocking Island indicate a strong correlation of both carbon and oxygen isotope values with grain size. Carbonate samples used in this study include the soft, recently formed, outer surface of several large columnar stromatolites, bedded muds from within the tidal channel, and marine hardgrounds from the shallow shoals adjacent to the channel. All samples are predominantly aragonite and were divided into mud-, silt-, and sand-size particles and bleached of organics prior to isotopic analysis. The range in isotopic values for all carbonate samples is 3.3 to 4.6^pmil for ^dgr13C and -0.3 to -1.4^pmil for ^dgr18O. The ^dgr18O and ^dgr13C va ues of the mud-size aragonite from all of these samples are depleted relative to the silt- and sand-size ooids and peloids and have isotope values lighter than 1.4^pmil for ^dgr13C and 0.8^pmil for ^dgr18O. The ^dgr13C values for green calcerous algae from the restricted platform behind Lee Stocking are similar to values for the stromatolite mud. ^dgr13C values for the hardground and channel-based muds are intermediate between the ^dgr13C values for the calcareous algae, and coarse-grained sediment. These data suggest that the green calcareous algae which flourish in the restricted platform, are a significant source of aragonite mud in and around the channel but are not the only source.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990