Deciphering geochemical signals across a Cambro-Ordovician carbonate platform

Abstract

Sulfur isotopes (δ34S) preserve a geological record of seawater sulfate that permit insights into biogeochemical cycling, surface redox conditions and microbial activity over geological time. However, isotopic records from carbonate-associated sulfate, sulfate bound within the calcium carbonate mineral matrix (CAS and δ34SCAS), frequently exhibit poorly understood stratigraphic and lateral variation that cannot be explained by evolution of the marine sulfate reservoir alone. As marine carbonate rocks are composed, in varying abundance, of cement, micrite, abiotic grains and fossils, it is important to identify both those components that can provide information about ancient marine conditions and the controls on their chemical composition. Existing bulk-rock δ34SCAS records from the Lower to Middle Ordovician San Juan Formation of the Argentine Precordillera are up to 20‰ lighter than coeval records. These observed differences with coeval carbonate records and observed rapid secular variability over short stratigraphic distances are too large a magnitude to be explained by changes to the marine sulfate reservoir alone, without implying a significantly reduced concentration in the marine sulfate reservoir and inferring deep water euxinia. This study investigates one of the possible alternative causes of variation in the δ34SCAS record. Syn-depositional processes, including microbial fractionation and physical reworking, have been observed in modern depositional environments to result in sulfur isotope fractionation in shallow porewaters. Characteristics such as sedimentation rate, organic carbon loading, grain size and shape, and mineralogy potentially all influence the distribution, size and microecology of microbial redox zones and porewater exchange with the water column. We consider the scale of δ34SCAS variation in a number of different depositional facies at the Cerro La Silla stratigraphic section of the San Juan Formation by linking outcrop scale observations (lithofacies) with thin section petrology (microfacies) and geochemical data to identify the cause of the observed variability. At a micron scale, XAS micron-scale imaging reveals clear differences in the distribution and abundance of sulfur- bearing phases in thin sections. This analysis is supplemented by mapping of trace element proxies of water-column and porewater chemical and redox conditions by LA-ICP-MS imaging with the aim of increasing understanding of how the conditions in which carbonate components form may contribute to the observed variable distribution of carbonate- associated sulfate (CAS). A newly presented bulk-rock δ34SCAS dataset exhibits a typical pattern of high-frequency secular variation over short stratigraphic distances and exhibits higher mean values than previous δ34SCAS records from the same section. The data is supplemented by sedimentological evidence of reworking, trace element data on porewater and water column chemistry and redox conditions, and micron-scale elemental and species mapping. These data indicate that low δ34SCAS values at the base of the section are caused by sulfide oxidation in reworked shallow sedimentary porewaters, with minimal detrital input and very low Fe and Mn concentrations, to form isotopically light sulfate which is incorporated into precipitating phreatic carbonate cements. This research supports a need for caution when interpreting δ34SCAS datasets from shallow, reworked facies and suggest that further research is required to clarify the relationship between shallow sediment processes and variability in both δ34SCAS and rare earth element records.