Carbonate-associated sulfate
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.
Sulfur compounds play a major role in global climate, nutrient cycling, and the production and distribution of biomass.
They can have significant effects on cloud formation and greenhouse forcing, and their distribution responds to the oxidation state of the atmosphere and oceans, as well as the evolution of different metabolic strategies.
The extent to which such a mass-dependent reaction operates in the world's oceans or atmosphere determines how much heavier or lighter various reservoirs of sulfur species will become.
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 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).
[40][41][6] 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.
The enrichment of marine sulfate in 34S should in turn scale with things like: the level of oxygen in the oceans and atmosphere, the initial appearance and proliferation of sulfur-reducing metabolisms among the world's microbial communities, and perhaps local-scale climate events and tectonism.
Various diagenetic processes (meaning: deformation by burial and exhumation, exposure to groundwater and meteoric fluids carrying sulfur species from more modern sources, etc.)
[53] And so, carbonate mineral crystals used as a sulfur cycle proxy must be carefully selected to avoid highly altered or recrystallized material.
X-ray diffraction and reflectance spectroscopy have revealed how the replacement of the carbonate group with sulfate ion tetrahedra expands the crystal lattice.
[54][55][56][57][58] On balance, CAS preserves and records the isotopic composition of seawater sulfate at the time of its deposition, provided the host carbonate has not been completely recrystallized or undergone replacement via sulfur-bearing fluids after burial.
The fragments, sediments, or powders should be cleaned (likely by sonication) and exposed only to deionized and filtered water, so that no contaminant sulfur species are introduced, and the original CAS is not further reduced, oxidized, or otherwise altered.
The abundance of CAS in a particular sample depends as much on the circumstances of a particular carbonate rock's formation and diagenetic history as it does on the processes acting on the marine sulfate pool that generated it.
Thus, if high precision is needed, sulfate samples are reduced to sulfides, which are then fluorinated to produce the inert and stable-isotopologue-free compound SF6, which can be passed through a specialized mass spectrometer.
It could mean that there is a hitherto uncharacterized kinetic isotope effect associated with the incorporation of sulfate into a particular carbonate texture (shrubs vs. nodules vs. acicular cements vs. other conformations).
Distinguishing between the effects of true changes in ancient ocean dynamics/chemistry and the effects of early- and late-stage diagenesis on CAS isotope composition is possible only through careful analyses that: compare the CAS record to the seawater sulfate record preserved in evaporites and marine barite, and carefully screen samples for their thermodynamic stability and evidence of alteration.
[3][64][4] Such samples could include brachiopod shell fragments (which are made of stable, low-Mg calcite that visibly resists alteration after cementation).
For example, the Great Oxygenation Event led to the oxidation of reduced sulfur species, increasing the flux of sulfate into the oceans.
Exactly how much less may be estimated from the δ34S value of sediments in modern analog environments like anoxic lakes, and their comparison to preserved Archean-age seawater sulfate (as found in CAS).
[59] The CAS record may (or may not) preserve evidence of the rise of microbial sulfate reduction, in the form of a negative δ34S excursion between 2.7 and 2.5 Ga.[69][70] The variation in sulfur isotope composition of sulfate associated with the different components of a carbonate or phosphate rock may also provide insights into the diagenetic history of a sample and the degree of preservation of the original texture and chemistry in different types of grains.
