Carbonate-associated sulfates (CAS) are

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ar ...

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

found in association with

carbonate minerals Carbonate minerals are those minerals containing the carbonate ion, . Carbonate divisions Anhydrous carbonates *Calcite group: trigonal **Calcite CaCO3 **Gaspéite (Ni,Mg,Fe2+)CO3 **Magnesite MgCO3 **Otavite CdCO3 **Rhodochrosite MnCO3 **Sider ...

, 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 an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...

. In the ocean, the source of this sulfate is a combination of riverine and atmospheric inputs, as well as the products of marine

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

reactions and

biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...

remineralisation In biogeochemistry, remineralisation (or remineralization) refers to the breakdown or transformation of organic matter (those molecules derived from a biological source) into its simplest inorganic forms. These transformations form a crucial link ...

. CAS is a common component of most carbonate rocks, having concentrations in the

parts per thousand In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...

within

biogenic A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...

carbonates and

parts per million In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...

within abiogenic carbonates. Through its abundance and sulfur isotope composition, it provides a valuable record of the global

sulfur cycle The sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element ( CHNOPS), being a const ...

across time and space.

Importance of sulfur (and CAS) to biogeochemistry

Sulfur compounds play a major role in global climate,

nutrient cycling A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cycli ...

, and the production and distribution of

. They can have significant effects on cloud formation and greenhouse forcing, and their distribution responds to the

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

of the atmosphere and oceans, as well as the evolution of different metabolic strategies. We can resolve the response of sulfur to biogeochemical change by measuring the abundance and isotopic composition of different sulfur species in different environments at different times. But ''how'' do abundance and isotopic composition of different sulfur reservoirs inform our understanding of biogeochemical processes? The

oxidation and reduction Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

of sulfur species often involves the breakage or formation of

chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...

s involving S atoms. Because the

thermodynamic stability In chemistry, chemical stability is the thermodynamic stability of a chemical system. Thermodynamic stability occurs when a system is in its lowest energy state, or in chemical equilibrium with its environment. This may be a dynamic equilibriu ...

of certain bonds is often greater when they involve heavier isotopes, an oxidation or reduction reaction can enrich the reactant pool (reservoir) or product pool in compounds containing the heavier isotope, relative to each other. This is known as an isotope effect. 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. The largest sulfur pool on Earth is that of marine or "seawater" sulfate. Traditionally, the isotopic composition of seawater sulfate is obtained by analysis of sulfate minerals within

evaporite An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocea ...

s, which are somewhat sparse in the geologic record, often poorly preserved, and necessarily associated with complicated and excursive events such as local

sea level change Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cryo ...

. Marine

barites Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...

are similarly limited. Carbonate-associated sulfate (CAS) provides geochemists with a more ubiquitous source of material for the direct measurement of seawater sulfate, provided the degree of secondary alteration and

diagenetic Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...

history of the carbonate and CAS can be constrained.

Sulfate and the global sulfur cycle

Earth's

is complex.

Volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

es release both reduced and oxidized sulfur species into the atmosphere, where they are further oxidized by reaction with oxygen to SO₂ and various

s. These oxidized sulfur species enter groundwater and the oceans both directly (rain/snow) or by incorporation into biomass, which decays to sulfates and

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...

s, 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

thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...

, 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 The sulfate minerals are a class of minerals that include the sulfate ion () within their structure. The sulfate minerals occur commonly in primary evaporite depositional environments, as gangue minerals in hydrothermal Vein (geology), veins and as ...

, which precipitate out in

tidal flats Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal fl ...

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

s, and

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') a ...

s as

deposits or are incorporated into the structure of

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...

and

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

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 Sulfur (16S) has 23 known isotopes with mass numbers ranging from 27 to 49, four of which are stable: 32S (95.02%), 33S (0.75%), 34S (4.21%), and 36S (0.02%). The preponderance of sulfur-32 is explained by its production from carbon-12 plus succ ...

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 In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can b ...

, 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.

Recording seawater sulfate

A Diagram and Explanation of How Sulfur Isotope Fractionation Might Act on Marine Sulfate Pool

Carbonate-associated sulfate (CAS) represents a small fraction of seawater sulfate, buried (and to some extent, preserved) with carbonate sediments. Thus, the changing δ³⁴S value of CAS through time should theoretically scale with the changing amount of reduced sulfur species being buried as metal-sulfides and the correspondingly enriched ocean. 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 Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

. The more positive the δ³⁴S of marine sulfate, the more sulfate reduction and/or burial/removal of reduced, ³⁴S-depleted sulfur species must be occurring. There are some limitations, however, to the use of carbonate-associated sulfate's isotopic composition as a proxy for the isotopic composition of marine sulfate (and thus as a

proxy Proxy may refer to: * Proxy or agent (law), a substitute authorized to act for another entity or a document which authorizes the agent so to act * Proxy (climate), a measured variable used to infer the value of a variable of interest in climate ...

for the response of the sulfur cycle to major climatological and geobiological events) through time. First, there is the question of: how representative is a particular carbonate rock's CAS of marine sulfate at the time of the rock's deposition? Various diagenetic processes (meaning: deformation by burial and

exhumation Burial, also known as interment or inhumation, is a method of final disposition whereby a dead body is placed into the ground, sometimes with objects. This is usually accomplished by excavating a pit or trench, placing the deceased and objec ...

, exposure to

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...

and meteoric fluids carrying sulfur species from more modern sources, etc.) can alter the abundance and isotopic composition of CAS. And so, carbonate mineral crystals used as a sulfur cycle proxy must be carefully selected to avoid highly altered or recrystallized material. Significant to this problem is the position that carbonate-associated sulfate occupies in the structure of carbonate minerals.

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

and reflectance spectroscopy have revealed how the replacement of the carbonate group with sulfate ion

tetrahedra In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

expands the crystal lattice. (It follows that higher Mg-content in the carbonate, which itself depends on the ocean's weathering inputs, pH, etc. and increases the distortion of the crystal lattice and rock volume, can also allow for the incorporation of ''more'' sulfate into the mineral structure.) Any processes that further distort the crystal lattice can cause sulfate to be lost from or added to the carbonate mineral, possibly overprinting the marine sulfate signal from the time of deposition. 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. If the host carbonate has been altered in this way, CAS may contain a mixture of signals that is difficult to characterize.

Measuring

Measuring abundance

In measuring the abundance and isotopic composition of CAS, it is important to know exactly ''what'' is being measured: CAS within particular shell fragments, corals,

microbialite Microbialite is a benthic sedimentary deposit made of carbonate mud (particle diameter < 5 μm) that is formed with the mediation of microbes. The constituent carbonate mud is a type of

microfossils A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

or mineral phases, using fine tweezers and drills under a microscope. The fragments, sediments, or powders should be cleaned (likely by

sonication A sonicator at the Weizmann Institute of Science during sonicationSonication is the act of applying sound energy to agitate particles in a sample, for various purposes such as the extraction of multiple compounds from plants, microalgae and seawe ...

) 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. Next, the clean samples must be measured. In one method, these samples are "digested" in an acid, likely

HCl HCL may refer to: Science and medicine * Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia * Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development * Hollow-cathode lamp, a spe ...

, which will liberate CAS from inclusions or the mineral lattice by dissolving the calcite mineral. The resulting sulfate ions are precipitated (often by mixture with

barium chloride Barium chloride is an inorganic compound with the formula Ba Cl2. It is one of the most common water-soluble salts of barium. Like most other water-soluble barium salts, it is white, highly toxic, and imparts a yellow-green coloration to a flame. ...

to produce

barium sulfate Barium sulfate (or sulphate) is the inorganic compound with the chemical formula Ba SO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium an ...

), and the solid sulfate precipitate is filtered, dried, and transferred to an

elemental analysis Elemental analysis is a process where a sample of some material (e.g., soil, waste or drinking water, bodily fluids, minerals, chemical compounds) is analyzed for its elemental and sometimes isotopic composition. Elemental analysis can be qualita ...

pipeline, which may involve the combustion of the sample and the mass balance of its various combustion products (which should include CO₂ and SO₂). Knowledge of the ratio of sulfur to oxygen and other components in the elemental analysis pipeline allows one to calculate the amount of sulfate introduced to the pipeline by the sample. This, along with the precise measurement of the original sample's mass and volume, yields a sulfate concentration for the original sample. The "combustion" and reaction to SO₂ can also bypassed by instead passing the acid-dissolved sample through an

ion chromatography Ion chromatography (or ion-exchange chromatography) separates ions and polar molecules based on their affinity to the ion exchanger. It works on almost any kind of charged molecule—including large proteins, small nucleotides, and amino acid ...

column, wherein different ions' polarity determines the strength of their interactions with polymers in the column, such that they are retained in the column for different amounts of time. The concentration of CAS may also be measured by spectroscopic methods. This could mean using the characteristic X-ray-induced fluorescence of sulfur, oxygen, carbon, and other elements in the sample to determine the abundance and ratios of each component, or the energy spectrum of an electron beam transmitted through the sample. It is also important to calibrate your measurement using standards of a known sulfate concentration, so that the strength/intensity of the signal associated with each sample can be mapped to a particular abundance.

Measuring isotopic composition

The

abundance Abundance may refer to: In science and technology * Abundance (economics), the opposite of scarcities * Abundance (ecology), the relative representation of a species in a community * Abundance (programming language), a Forth-like computer prog ...

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, it is important to have both the abundance/concentration of CAS in a sample ''and'' its isotopic composition to understand its place in the marine sulfate record. As mentioned above, different biogeochemical processes produce different isotope effects under equilibrium and disequilibrium conditions: microbial sulfur reduction and sulfur disproportionation can produce equilibrium and

kinetic isotope effect In physical organic chemistry, a kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. Formally, it is the ratio of rate constants for th ...

s of many 10s of per mille. The sulfur isotope composition of the ocean (or a lake, lagoon, or other body) is critical to understanding the extent to which those processes controlled the global sulfur cycle throughout the past. Just as the carbon and oxygen isotope composition of the carbonate host rock can illuminate temperature and local climate history, the sulfur and oxygen isotope composition of CAS can illuminate the cause and effect relationships between that history and the sulfur cycle. Isotopic composition of CAS and carbonate host rock can both be measured by "elemental analysis" wherein sulfate or carbonate is "burned" or otherwise volatilized and the ionized isotopes are accelerated along a path, the length and duration of which is a function of their masses. The ratio of different isotopes to one another is assessed by comparison to blanks and standards. However, SO₂, the analyte used in this method, presents some difficulties as the isotopic composition of the component oxygen may also vary, affecting the mass measurement. SO₂ can also "stick" to or react with other compounds in the mass spectrometer line. 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. These methods,

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

and clumped isotope mass spectrometry, are discussed in greater detail in their primary articles. The isotopic composition of CAS is often discussed in terms of δ³⁴S, which is a way of expressing the ratio of the isotope ³⁴S to ³²S in a sample, relative to a standard such as the Canyon Diablo Troilite. δ³⁴S (expressed in ‰) is equal to

\left \frac-1 \right 1000

. The isotope effect of a particular process (microbial sulfate reduction, for example) is often expressed as an ε value (also in ‰) which refers to the difference in the δ value of the reactant pool and the product pool. While studies of the sulfur isotope composition of seawater sulfate, CAS, marine barite, and evaporites typically discuss the relative ³⁴S enrichment of depletion of these pools, there are other minor but stable isotopes of sulfur that can also be measured, though to lower precision given their rarity. These include ³³S and ³⁶S. Mass-dependent and mass-independent fractionation of minor sulfur isotopes may also be an important gauge for the sulfur cycle through geologic time. ³³S and ³⁶S must, however, be measured at high-precision via fluorination to SF₆ before passing through a mass spectrometer.

Interpreting measurements

Interpreting the sulfur isotope composition of CAS can be complex. As discussed above, if seawater sulfate at a particular horizon in the geologic record gets heavier (i.e. more enriched in ³⁴S relative to seawater sulfate before it) that could mean that the ³⁴S-depleted products of sulfur-reducing reactions are being buried as sulfide minerals and removed from the oceans, possibly because of an instance of ocean anoxia or an increase in dissimilatory sulfate reduction by marine microorganisms. But it could also mean that the CAS measured at that particular horizon was derived not from seawater sulfate at the time of carbonate deposition, but from fluids moving through the sediment or porous rock from a later time, in which sulfate could have been enriched by processes in a more oxidizing world. 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

and evidence of alteration. Such samples could include

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

shell fragments (which are made of stable, low-Mg calcite that visibly resists alteration after cementation).

Some important insights from CAS studies

The CAS record can preserve evidence of major changes in

of the ocean in response to climate. For example, the Great Oxygenation Event led to the oxidation of reduced sulfur species, increasing the flux of sulfate into the oceans. This led to a corresponding depletion of ³⁴S in the marine sulfate pool — a depletion recorded in the sulfur isotope composition of marginal marine evaporite deposits and CAS in marine carbonates. Before the Great Oxygenation Event, when atmospheric and marine oxygen was low, it is expected that oxidized sulfur species like sulfate would have been much less abundant. 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). The Great Oxygenation Event lead not just to the oxygenation of Earth's oceans, but to the development of the ozone layer. Prior to this, the Archean Earth was exposed to high-energy radiation that caused mass-independent fractionation of various pools, including sulfur (which would lead to an expected negative δ³⁴S excursion in the marine sulfate pool). The marine sulfate record preserved in CAS complicates this view, as late or Neo-Archean CAS samples seem to have positive δ³⁴S. 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. 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.

Ongoing improvements to CAS studies

Much of the ongoing work in the field of carbonate-associated sulfate is dedicated to characterizing sources of variation in the CAS record, answering questions like: how are sulfate ions incorporated into the mineral structure of different Ca-carbonate and Ca-Mg-carbonate morphotypes, mechanistically speaking? And which morphotypes are most likely to contain CAS derived from primary marine sulfate? Just as for other geochemical proxies, the utility and reliability of CAS measurements will improve with the advent of more sensitive measurement techniques, and the characterization of more isotope standards.

References

{{Reflist Isotopes of sulfur Sulfate minerals