page_title: Carbonate-associated sulfate
section_title: Sulfate and the global sulfur cycle
hierarchical_section_title: Carbonate-associated sulfate / Sulfate and the global sulfur cycle
caption_attribution_description: English: A cartoon/schematic showing inputs and outputs of the sulfur cycle, with an inset cartoon explaining how seawater sulfate can be incorporated into biogenic and abiogenic carbonate minerals and preserved.
context_page_description: Carbonate-associated sulfates are sulfate species found in association with carbonate minerals, either as inclusions, adsorbed phases, or in distorted sites within the carbonate mineral lattice. It is derived primarily from dissolved sulfate in the solution from which the carbonate precipitates. In the ocean, the source of this sulfate is a combination of riverine and atmospheric inputs, as well as the products of marine hydrothermal reactions and biomass remineralisation. CAS is a common component of most carbonate rocks, having concentrations in the parts per thousand within biogenic carbonates and parts per million within abiogenic carbonates. Through its abundance and sulfur isotope composition, it provides a valuable record of the global sulfur cycle across time and space.
context_section_description: Earth's sulfur cycle is complex. Volcanoes release both reduced and oxidized sulfur species into the atmosphere, where they are further oxidized by reaction with oxygen to SO₂ and various sulfates. These oxidized sulfur species enter groundwater and the oceans both directly (rain/snow) or by incorporation into biomass, which decays to sulfates and sulfides, again by a combination of biological and abiological processes. Some of this sulfate is reduced through microbial metabolism (microbial sulfate reduction or MSR) or by hydrothermal processes, yielding sulfides, thiosulfate, and elemental sulfur. Some reduced sulfur species are buried as metal-sulfide compounds, some are cyclically reduced and oxidized in the oceans and sediments indefinitely, and some are oxidized back into sulfate minerals, which precipitate out in tidal flats, lakes, and lagoons as evaporite deposits or are incorporated into the structure of carbonate and phosphate minerals in the ocean (i.e. as CAS).
  Because oxidation-reduction reactions with sulfur species are often accompanied by mass-dependent fractionation, the sulfur isotope composition of the various pools of reduced and oxidized sulfur species in the water column, sediment, and rock record is a clue to how sulfur moves between those pools or has moved in the past. For example, sulfur at the time of Earth's formation should have (barring some accretion-related fractionation process for which there is little evidence) had a δ³⁴S value of about 0‰, while sulfate in the modern oceans (the dominant marine sulfur species) has a δ³⁴S of about +21‰. This implies that, over geologic time, a reservoir of correspondingly depleted (i.e. ³⁴S-poor) sulfur was buried in the crust and possibly subducted into the deep mantle. This is because sulfate's reduction to sulfide is typically accompanied by a negative isotope effect, which (depending on the sulfate-reducing microorganism's enzymatic machinery, temperature, and other factors) can be tens of per mille. This effect can be compounded through sulfur disproportionation, a process by which some microbes reduce sulfate to sulfides and thiosulfate, both of which can be ³⁴S-depleted by tens of per mille relative to the starting sulfate pool. Depleted sulfides and thiosulfate can then be repeatedly oxidized and reduced again, until the final, total sulfide pool that is measured has δ³⁴S values of -70 or -80‰. The formation of a "lighter" S-isotope pool leaves behind an enriched pool, and so the enrichment of seawater sulfate is taken as evidence that some large amount of reduced sulfur (in the form, perhaps, of metal-sulfide minerals) was buried and incorporated into the crust.
caption: A cartoon/schematic of the sulfur cycle, describing inputs and outputs for seawater sulfate.
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