SULFUR ISOTOPIC VARIABILITY OF CAS ALONG AN EARLY TRIASSIC ENVIRONMENTAL TRANSECT

2005 Salt Lake City Annual Meeting (October 16–19, 2005)
Paper No. 76-5

Presentation Time: 9:10 AM-9:25 AM

SULFUR ISOTOPIC VARIABILITY OF CAS ALONG AN EARLY TRIASSIC ENVIRONMENTAL TRANSECT
MARENCO, Pedro J., Earth Sciences, University of Southern California, ZHS 117, Los Angeles, CA 90089-0740, marenco@usc.edu, CORSETTI, Frank A., Earth Sciences, Univ of Southern California, Los Angeles, CA 90089, KAUFMAN, Alan J., Geology, Univ of Maryland, College Park, MD 20742, and BOTTJER, David J., Department of Earth Sciences, Univ. of Southern California, Los Angeles, CA 90089-0740 The S isotopic composition of carbonate associated sulfate (CAS) has been studied in marine limestones and dolostones of all ages and has been used to interpret global redox conditions in ancient marine environments. However, while CAS in recent sediments appears to record the S isotopic composition of seawater sulfate, environmental and diagenetic gradients in the S isotopic composition of CAS in ancient sediments is largely unexplored. Here we present the results of a petrographic, stable isotopic and trace element geochemical analysis of limestone, dolostone and gypsum formed along an Early Triassic environmental transect in the western USA. Within-section δ³⁴S varied up to 7‰ in limestone samples, with lower δ³⁴S values corresponding to higher Mn/Sr. Notably, δ³⁴S values of dolomitic samples were up to 5‰ lower than limestone samples from the same section. The largest variation between sections was ~13‰, with higher δ³⁴S values in the most offshore samples exhibiting the lowest Mn/Sr, and lower δ³⁴S values in the most nearshore samples with the highest Mn/Sr. δ³⁴S values varied logarithmically with Ca/Mg, suggesting diagenetic effects associated with dolomitization. The δ³⁴S values of gypsum exhibiting Spathian ⁸⁷Sr/⁸⁶Sr was within 1‰ of closely associated limestones and dolostones with low Mn/Sr. Gypsum samples with more radiogenic ⁸⁷Sr/⁸⁶Sr values showed similar δ³⁴S values to dolostones with high Mn/Sr. The results suggest significant variation in the S isotopic composition of CAS with environment and diagenetic grade. The decrease in δ³⁴S with increased Mn/Sr may have resulted from proximity to terrestrial meteoric sources, perhaps with isotopically depleted S from the oxidative weathering of shales and organic-rich soils. Dolomitizing fluids were also characterized by lower δ³⁴S values that were incorporated as CAS at the time of dolomitization. Such trends would not be apparent without a detailed, basin scale analysis; thus studies based on isolated sections without detailed environmental analysis may be problematic. Although CAS chemostratigraphy is a useful tool in understanding ancient environments, CAS samples should undergo trace element and petrographic analysis to understand potential environmental and diagenetic variations.
2005 Salt Lake City Annual Meeting (October 16–19, 2005) General Information for this Meeting

Session No. 76 Weathering, Sedimentation, and Diagenesis in Major Element Cycles I: In Memory of Raymond Siever Salt Palace Convention Center: 151 DEF 8:00 AM-12:00 PM, Monday, 17 October 2005 Geological Society of America Abstracts with Programs, Vol. 37, No. 7, p. 184
© Copyright 2005 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.