Controls on the Sulfur Isotopic Composition of Carbonate-Associated Sulfate

Sulfate in the modern ocean has a homogenous concentration and sulfur isotopic composition. It is well-mixed because rivers and mantle degassing deliver small amounts relative to its mass in the ocean. A similar small amount of sulfate is removed as biologic, sedimentary, and hydrothermal processes oxidize and reduce sulfur, carbon, and iron. These sulfur fluxes may have changed along with the carbon and oxygen cycles during ancient evolutionary, extinction, climatic, and tectonic transitions. The changing budget of marine sulfate is therefore key to understanding biogeochemical processes that control Earth’s surface environment. The sulfur isotopic composition of marine sulfate reflects the proportion of sulfur partitioned into reduced minerals, especially pyrite, in marine sediments and weathering rocks. In this thesis, I examine how the sulfur isotopic compositions of ancient oceans is recorded in the sedimentary rock record and examine local and global effects on the sulfur isotopic composition of Paleozoic and the Mesoproterozoic sedimentary rocks. Carbonate minerals form in many depositional environments throughout Earth history and their chemical compositions relate to that of the fluid in which they formed. Much of my thesis focuses on the sulfur isotopic composition of minor amounts of sulfate incorporated into calcite, dolomite, and aragonite called carbonate-associated sulfate. Unpacking the local biogeochemical processes and global budgets affecting the sulfur isotopic composition of ancient carbonates enriches and clarifies the paleoenvironmental information preserved in the sedimentary record. Chapter 1 is a compilation and critical comparison of proxy records of the sulfur isotopic composition of Phanerozoic seawater sulfate. I compared data from marine evaporites, barite, and carbonate-associated sulfate and showed where each record is prone to biases and which processes create variance. Only carbonate-associated sulfate data fills critical periods of biogeochemical change, but it is the most susceptible to sources of variance other than passively recording the composition of ancient oceans. However, this additional variance reflects changes in the biogeochemical processes during early diagenesis in penecontemporaneous sediments, which are the locus of the pyrite burial and sulfide reoxidation fluxes pulling on the global sulfur budget. Chapter 2 utilizes a recently-developed analytical technique to compare the carbonate-associated sulfate of diagenetic carbonates and primary marine biogenic carbonates from latest Ordovician and earliest Silurian strata on Anticosti Island, Quebec. These samples span the duration of the Hirnantian Stage glaciation of Gondwana, which coincided with and possibly caused the Late Ordovician Mass Extinction. Much of the variance observed in bulk carbonate-associated sulfate is imparted during early diagenesis and burial diagenesis, and the best-preserved calcite from ancient brachiopods faithfully reflects seawater’s sulfur isotopic composition. Seawater sulfate’s isotopic composition did not change during the glaciation and extinction, supporting prior constraints on the mass of the marine sulfate reservoir and the magnitude of sulfur flux changes. In Chapter 3, we extended the record of seawater sulfate’s sulfur isotopic composition from well-preserved brachiopod calcite from the Cincinnati Arch, Indiana-Ohio-Kentucky and Gotland, Sweden. We demonstrated that marine sulfate likely remained globally well-mixed with a constant isotopic composition for at least 30 Myr, from the earliest Late Ordovician through the late Silurian. The ocean’s sulfur isotope composition likely changed little during multiple biotic crises, periods of basin restriction, oceanographic circulation changes, and sea level and climate changes. However, the first replicate carbonate-associated sulfate measurements of individual brachiopods indicate that even the best-preserved calcite is prone to diagenetic alteration that may obscure small changes in the ocean sulfate budget. Exquisitely-preserved biogenic calcite is rare in the rock record and absent in Precambrian strata, but bulk limestones and dolomites may record changes in the composition of ancient oceans. Chapter 4 compares the sulfur isotopic composition of carbonate-associated sulfate from limestones and dolostone deposited in peritidal to basinal environments on the Capitan Reef carbonate platform in the Guadalupe Mountains, west Texas. Rocks formed in different environments at the same time have carbonate-associated sulfate with different sulfur isotope compositions. Carbonate-associated sulfate is incorporated into bulk limestone and dolostone during early marine diagenesis, and its sulfur isotopic composition reflects the diagenetic and depositional environment. Carbonates recrystallizing in low-energy environments may incorporate marine pore fluids whose sulfur isotopic compositions evolved by the action of microbial sulfate reducing organisms. The sulfur isotopic composition of rocks deposited in high-energy environments, however, reflects that of seawater sulfate because the diagenetic fluid is open to the ocean and has the same sulfur isotopic composition of seawater. Later meteoric and burial diagenetic processes to which other geochemical tracers, such as carbon and oxygen isotopes, are sensitive do not greatly affect carbonate-associated sulfate. Thus, a record of the evolution of the sulfur, carbon, and oxygen isotopic composition of ancient oceans cannot come from the same sedimentary archives. Chapter 5 considers the range of hydrothermal and sedimentary reactions that fractionate sulfur isotopes to understand the origin of unusual millimeter-scale pyrite tubes associated with a Mesoproterozoic massive sulfide deposit in the Newland Formation, Belt Supergroup, Meagher County, Montana. The petrography and sedimentology of the tubes indicates that they formed on the seafloor or in the uppermost unlithified sediments from the effluence of metalliferous fluids into euxinic seawater. The texture-specific sulfur isotopic compositions of diagenetic barite, carbonate-associated sulfate, and diagenetic and hydrothermal pyrite indicates that there was an active microbial sulfate reducing community in the sediments and possibly colonizing the vents. A dynamic set of oxidation and reduction interactions between hydrothermal fluids and seawater were controlled by this community, leading to the novel morphology and texture of vent structures. This work indicates that combining sedimentological and petrographic observations with sulfur isotope data can constrain a wide range of biogeochemical processes. It guides future sulfur geochemical examination of parts of the rock record, especially the Precambrian, with few traditional archives of ancient seawater sulfate’s chemistry. Information on both local and global controls on the sulfur isotopic composition of carbonate-associated sulfate, barite, and pyrite helps to resolve paleoenvironmental change.