Print this pageA δ34s-Based Approach for Correlation and Paleoenvironmental Reconstruction during Carbonate-Evaporite Deposition: From Example from the Ara Group, Sultanate of Oman
Sulfur isotopes, particularly those of sedimentary sulfates, provide a useful tool for correlation in the geologic record. Traditional analyses have relied on a sparsely distributed record of marine sulfate evaporites. Recent analyses of carbonate-associated sulfate (CAS) demonstrate its potential to generate a high-resolution record of seawater sulfate that improves upon existing evaporite-based records. High-resolution records of sulfate δ34S from interbedded carbonates (as CAS) and evaporites (as gypsum or anhydrite) can refine correlations in sedimentologically complex carbonate-evaporite deposits and provide information about ongoing microbial activity during evaporite deposition. Here we present high-resolution δ34S measurements from the Ara Group, Sultanate of Oman, a series of six carbonate-evaporite sequences deposited ca. 547-540 Ma. The isotopic compositions of the sulfate in Ara carbonates (δ34SCAS) and the floor and roof anhydrites (δ34SAN) that bound them were analyzed. Both δ34SCAS and δ34SAN showed secular trends that can be used to refine existing stratigraphic correlations for the Ara Group units. On top of these trends, δ34SCAS showed little scatter, while repeated enrichments (up to 4‰) were observed in δ34SAN relative to δ34SCAS. These enrichments cannot be explained from secular variation or from isotopic fractionation during evaporite deposition and require the existence of an additional 34S-depleted sink. We suggest that ongoing sulfate reduction and pyrite sequestration provided such a sink during evaporite deposition with the magnitude of the resulting δ34SAN enrichment (relative to CAS) a function of local pyrite burial (fpyr). Our data indicate fpyr ~ 0.1 during deposition of the Ara evaporites and suggest that paired δ34SCAS-δ34SAN data can be used to quantify the importance of microbial activity in a setting where direct evidence (e.g., TOC or organic biomarkers) may be scarce and physical processes are thought to dominate. Further, by linking relative offsets between evaporite δ34S and δ34SCAS to basin restriction, high-resolution evaporite δ34S profiles can be used to reconstruct pulses of freshening or evaporation within the first-order evaporite cycle.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009