Carbonate-Associated Sulfate in Modern Sediments of South Florida: Diagenetic Relationships

University Of Missouri-Columbia, Columbia MO

Investigators

Abstract

Carbonate-Associated Sulfate in Modern Sediments of South Florida: Diagenetic Relationships Biogeochemical cycling of sulfur at and near the earth.s surface is intimately linked.through a complex set of feedbacks.to ambient redox conditions, including the availability of atmospheric oxygen. Our ability to delineate and quantify these cycles and the associated biospheric impacts hinges on the strong preferential utilization of 32S relative to 34S during bacterial sulfate reduction. Historically, temporal trends in the relative magnitudes of critical redox pathways have been estimated by monitoring the 34S of the ocean as manifested in a limited sedimentary record of gypsum. Consequently, the models are only as good as the data, which fail to provide even minimal constraints for most of the Precambrian. During the earth.s early history, gypsum deposition was less abundant, and preservation is poor due to its vulnerability during weathering. In light of these complications, carbonate-associated sulfate or CAS, which is complementary to but generally not overlapping with a promising new barite approach, has emerged as a viable alternative. CAS, which often occurs at concentrations of hundreds to thousands of ppm, is a component of most limestones and dolostones and shows great promise for continuous, high-resolution records of ancient seawater sulfate. Interest in the method is peaking, yet applications greatly outnumber studies tracking even the basic systematics of CAS and the potential loss of seawater signals through diagenesis. Despite encouraging initial results for CAS in ancient sediments, no systematic attempt has been made to test the method in a modern setting where the controlling factors are more easily and independently constrained. Furthermore, any attempts at modern calibration have neglected the precursor of the most abundant carbonate component of the geologic record.carbonate mud. and the effects of diagenesis. The primary objective of the proposed study is to understand how CAS is incorporated, distributed and preserved in modern carbonate sediments in south Florida. Specifically, we will observe isotopic and concentration trends across a carefully chosen set of well-characterized depositional settings in Florida Bay and at lagoonal sites seaward of the Keys. The resulting diversity translates into spatial and temporal gradients in the parameters that control sulfur cycling, such as the availability of labile organic compounds, salinity and the physical and biological properties of the sediment. What is the overall fidelity of the seawater isotopic signal recorded in bulk mud samples that are undergoing carbonate dissolution, precipitation and mineral transformations on early diagenetic time scales within the confines of an evolving pore-water sulfate reservoir? Although we recognize the profound differences between modern and ancient settings, it is these bulk mud samples that will illuminate the conditions under which primary signals may or may not be preserved. Various coarser sketetal components will also be tracked from life through burial to depths up to several meters. These goals are facilitated by Lyons. experience working in modern settings, including south Florida, although many of the questions asked will stem from the ultimate goal of testing the robustness of a paleoenvironmental proxy. In particular, Lyons. past work with sediment diagenesis, carbonate sedimentology/geochemistry and sulfur geochemistry will be most helpful. The tight focus of this study.as requested by the panel following review of an earlier version of this proposalwill allow us to track CAS within a rigorous organic and inorganic geochemical context. The multi-component approach, including rate measurements and concentration and isotopic determinations for a wide range of relevant species, is designed to provide the biogeochemical sulfur mass balance necessary for understanding ppm levels of sulfate within reactive carbonate grains. In the end, CAS may help fill the many gaps in the .Claypool curve. through use of continuous sections of shallow-water carbonates from even the oldest rocks. But as with all carbonate proxies for ancient seawater, our interpretations are only as good as our understanding of the multiple diagenetic pathways.