Sulfur Isotopic Insights into the Modern and Ancient Marine Sulfur Cycles

The marine sulfur cycle plays a key role in regulating Earth's surface oxygen (O₂) levels through its interactions with the carbon and iron cycles. Our understanding of the sulfur cycle has traditionally come from measurements of the sulfur isotopic compositions of marine sulfate (SO₄²⁻) and sulfur-bearing materials in marine sediments. Because the residence time of SO₄²⁻ in seawater is long (Myr) compared to the mixing time of Earth's oceans (kyr), the concentration and sulfur isotopic composition of marine SO₄²⁻ are homogeneous in modern seawater and are assumed to have been homogeneous throughout most of the Phanerozoic Eon (541 Ma to the present). This assumption of homogeneity, when combined with sulfur isotopic composition measurements, has enabled box model reconstructions of the relative fluxes of oxidized versus reduced sulfur leaving the oceans at times in Earth's past. Such reconstructions have informed our understanding of the interactions between Earth's tectonics, climate, and elemental cycles. This thesis tests some of the key assumptions made in sulfur cycle box models and attempts to better understand sulfur isotopic variability in geologic archives using a combination of measurements and modeling. Measurements of the sulfur isotopic composition (i.e., δ³⁴S) of SO₄²⁻ in Permo-Carboniferous brachiopod shells demonstrate that more precise records of SO₄²⁻ δ³⁴S may be generated via careful sampling that avoids diagenetically altered phases (Chapter II). Furthermore, measurements of heterogeneous carbonate associated sulfate (CAS) δ³⁴S within carbonates deposited across the End-Permian mass extinction (EPME) in South China show that a lack of careful sampling can substantially alter our understanding of the marine sulfur cycle at times in Earth's past (Chapter III). Simple models constructed in each of these studies indicate that changes in the δ³⁴S of the sulfur input to the ocean, the δ³⁴S offset (i.e., Δδ³⁴S) between the oxidized and reduced sulfur output fluxes, and the amount of SO₄²⁻ incorporated during diagenetic alteration - all assumed to be negligible in many studies of the marine sulfur cycle - may viably explain these data. Development of a sediment diagenesis model that includes sulfur isotopic species demonstrates that variations in organic matter rain rate, ferric iron input, sedimentation rate, bottom water O₂ concentration, and bottom water SO₄²⁻ concentration may all affect Δδ³⁴S in a given sedimentary environment (Chapter IV). Application of this model to pore water SO₄²⁻ and hydrogen sulfide H₂S δ³⁴S data from International Ocean Discovery Program (IODP) Expedition 361, IODP Expedition 363, and R.V. Knorr cruise KN223 sites shows that Δδ³⁴S is ubiquitously large in these deep ocean sedimentary environments (Chapter V). Cluster analysis of pore water [SO₄²⁻] profiles collected during previous deep ocean cruises successfully extracts and groups profiles that are similar to those observed on these three cruises (Chapter VI). Comparison of cluster data to a compilation of recent marine pyrite (FeS₂) δ³⁴S data confirms that pyrite burial in shelf sediments constitutes the majority of pyrite burial occurring globally in the modern day. However, changes in sea level or in other variables that affect sediment deposition may plausibly force an increase in deep ocean pyrite burial and a corresponding change in the global Δδ³⁴S. Future studies of the modern and ancient marine sulfur cycles must carefully consider the geologic and geochemical context of sulfur isotopic measurements - including sea level changes, sedimentation rate changes, and measured or presumed concentrations of other redox-active species - if interpretations of such data are to be robust.