We review recent observations and models concerning the dynamics of Cryogenian global glaciation and their biological consequences.

The sulfur biogeochemical cycle integrates the metabolic activity of multiple microbial pathways (e.g., sulfate reduction, disproportionation, and sulfide oxidation) along with abiotic reactions and geological processes that cycle sulfur through various reservoirs. The sulfur cycle impacts the global carbon cycle and climate primarily through the remineralization of organic carbon. Over geological timescales, cycling of sulfur is closely tied to the redox state of Earth's exosphere through the burial of oxidized (sulfate) and reduced (sulfide) sulfur species in marine sediments. Biological sulfur cycling is associated with isotopic fractionations that can be used to trace the fluxes through various metabolic pathways. The resulting isotopic data provide insights into sulfur cycling in both modern and ancient environments via isotopic signatures in sedimentary sulfate and sulfide phases. Here, we review the deep-time δ 34 S record of marine sulfates and sulfides in light of recent advances in understanding how isotopic signatures are generated by microbial activity, how these signatures are encoded in marine sediments, and how they may be altered following deposition. The resulting picture shows a sulfur cycle intimately coupled to ambient carbon cycling, where sulfur isotopic records preserved in sedimentary rocks are critically dependent on sedimentological and geochemical conditions (e.g., iron availability) during deposition. 593 Annu. Rev. Earth Planet. Sci. 2015.43:593-622. Downloaded from www.annualreviews.org Access provided by 99.5.235.72 on 08/04/16. For personal use only.

New mapping in eastern Bhutan, in conjunction with U-Pb detrital zircon and δ 13 C data, defi nes Lesser Himalayan tectonostratigraphy. The Daling-Shumar Group, 2-6 km of quartzite (Shumar Formation) overlain by 3 km of schist (Daling Formation), contains ~1.8-1.9 Ga intrusive orthogneiss bodies and youngest detrital zircon peaks, indicating a Paleoproterozoic deposition age. The Jaishidanda Formation, 0.5-1.7 km of garnet-biotite schist and quartzite, stratigraphically overlies the Daling Formation beneath the Main Central thrust, and yields youngest detrital zircon peaks ranging from ~0.8-1.0 Ga to ca. 475 Ma, indicating a Neoproterozoic-Ordovician(?) deposition age range. The Baxa Group, 2-3 km of quartzite, phyllite, and dolomite, overlies the Daling-Shumar Group in the foreland, and yields ca. 0.9 Ga to ca. 520 Ma youngest detrital zircon peaks, indicating a Neoproterozoic-Cambrian(?) deposition age range. Baxa dolo mite overlying quartzite containing ca. 525 Ma detrital zircons yielded δ 13 C values between +3‰ and +6‰, suggesting deposition during an Early Cambrian positive δ 13 C excursion. Above the Baxa Group, the 2-3 km thick Diuri Formation diamictite yielded a ca. 390 Ma youngest detrital zircon peak, suggesting correlation with the late Paleo zoic Gondwana supercontinent glaciation. Finally, the Permian Gondwana succession consists of sandstone, siltstone, shale, and coal. Our deposition age data from Bhutan: (1) reinforce suggestions that Paleoproterozoic (~1.8-1.9 Ga) Lesser Himalayan deposition was continuous along the entire northern Indian margin; (2) show a likely east ward continuation of a Permian over Cambrian unconformity in the Lesser Hima-layan section identifi ed in Nepal and northwest India; and (3) indicate temporal overlap between Neoproterozoic-Paleozoic Lesser Himalayan (proximal) and Greater Himalayan-Tethyan Himalayan (distal) deposition.

Global carbon cycle perturbations throughout Earth history are frequently linked to changing paleogeography, glaciation, ocean oxygenation, and biological innovation. A pronounced carbonate carbon-isotope excursion during the Ediacaran Period (635 to 542 million years ago), accompanied by invariant or decoupled organic carbon-isotope values, has been explained with a model that relies on a large oceanic reservoir of organic carbon. We present carbonate and organic matter carbon-isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the Sturtian and Marinoan glacial events of the Cryogenian Period (approximately 720 to 635 million years ago). Growth of the organic carbon pool may be related to iron-rich and sulfate-poor deep-ocean conditions facilitated by an increase in the Fe:S ratio of the riverine flux after Sturtian glacial removal of a long-lived continental regolith.

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Early Silurian (~431 Ma) carbonate rocks record a ca. 4.5‰ positive excursion in the stable isotopic composition of carbonate carbon ( 13 C carb). Associated with this isotopic shift is a macroevolutionary turnover pulse known as the 'Ireviken Event'. The onset of this carbon isotope excursion is commonly associated with a shallowing-upward facies transition that may have been accompanied by climatic change, as indicated by a parallel positive shift (~0.6‰) in the stable isotopic composition of carbonate oxygen (δ 18 O carb). However, the relationships among carbon cycle perturbations, faunal turnover, and environmental changes remain enigmatic. Here we present a suite of new isotopic data across the Ireviken Event from multiple sections in Gotland, Sweden. These samples preserve no systematic change in δ 18 O carb but show positive excursions of equal magnitude in both carbonate ( 13 C carb) and organic ( 13 C org) carbon. In addition, the data reveal a synchronous perturbation in sulfur isotope ratios, manifest as a ca. 7‰ positive excursion in carbonate-associated sulfate ( 34 S CAS) and a ca. 30‰ positive excursion in pyrite (δ 34 S pyr). The increase in δ 34 S pyr values is accompanied by a substantial, concomitant increase in stratigraphic variability of δ 34 S pyr. The relatively constant offset between the  13 C carb and  13 C org excursions throughout the Ireviken Event could be attributed to increased organic carbon burial, or possibly a change in the isotopic composition of CO 2 sources from weathering. However, a positive correlation between carbonate abundance and  13 C carb suggests that local to regional changes in dissolved inorganic carbon (DIC) during the shallowing-upward sequence may have been at least partly responsible for the observed excursion. The positive excursion recorded in  34 S CAS suggests a perturbation of sufficient magnitude and duration to have impacted the marine sulfate reservoir. An inverse correlation between CAS abundance and  34 S CAS supports the notion of decreased sulfate concentrations, at least locally, consistent with a concomitant increase in pyrite burial. A decrease in the offset between  34 S CAS and δ 34 S pyr values during the Ireviken Event suggests a substantial reduction in the isotopic fractionations ( pyr) expressed during microbial sulfur cycling and pyrite precipitation through this interval. Decreased  pyr and the concomitant increase in stratigraphic variation in δ 34 S pyr are typical of isotope systematics observed in modern shallow-water environments, associated with increased closed-system behavior and/or oxidative sedimentary reworking during early sediment diagenesis. While the isotopic trends associated with the Ireviken Event have been observed in multiple locations around the globe, many sections display different magnitudes of isotopic change, and moreover, are typically associated with local facies changes. Due to the stratigraphic coherence of the carbon and sulfur isotopic and abundance records across the Ireviken Event, and t...

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