Thematic Articles

The sulfur cycle is arguably the most important geochemical cycle on Mars because the transfer of sulfur places limits on Mars’s differentiation processes, sedimentary, geomorphic and aqueous processes, past climate, and current and past habitability. The presence of sulfur-rich compositions on Mars is suggested by meteorite data, in situ bulk chemical and mineralogical analyses, remote sensing data from dust and surfaces, and geochemical models. The inferred sulfur-rich nature of Mars may have resulted in an Fe–(Ni–)S core that has been liquid throughout Mars’s history. On the surface, Mg- and Ca-sulfates are widespread and Fe3+-sulfates are found locally. It is likely that these minerals occur in a variety of hydration states and host much of the mineral-bound hydrogen in the Martian subsurface.

Life, as recorded in almost every corner of the oceans and continents, has evolved to take advantage of chemical gradients. Organisms, both big and small, utilize reduction–oxidation (redox) reactions to gain the energy required to live and grow. Although aerobic respiration (using O2) is the most popular form, other modes of respiration use oxygen alternatives and drive additional element cycles (for example, nitrogen, sulfur, and metals such as iron and manganese). These alternative metabolisms, and especially those cycling sulfur, helped shape Earth’s long history and much of the world we see today. Sulfur is a fundamental constituent in macroscopic and microscopic worlds alike and is a key oxidant in the anaerobic biosphere. By reconstructing the distribution of sulfur metabolisms throughout the marine realm, we can better understand the role that sulfur plays in marine biogeochemical cycling and Earth-surface processes.

The amount of sulfate in the early ocean was tied directly to oxygen levels in the atmosphere and the deep ocean. These concentrations and other environmentally diagnostic biogeochemical pathways of the sulfur cycle can be expressed through isotope fractionation between sulfate and pyrite. The balance between rising oxygen and sulfate concentrations and varying hydrothermal iron inputs led to a pattern of iron, oxygen, and sulfide domination that varied in time and space in the early deep ocean and was more complex than previously recognized. Through all this change, no element played a bigger role than sulfur as a recorder of early oxygenation of the biosphere and the coevolution of life.

While other volcanic gas species are more abundant than sulfur, it is the measurement of sulfur dioxide emissions that has played the key geochemical role in volcano monitoring for decades. Recently, this sphere of volcano surveillance has undergone a revolution: the instruments suitable for the task have become cheaper, more compact, less power hungry, and more capable than their predecessors. It is now possible to measure multiple gas species simultaneously, at high time resolution, and even to image volcanic clouds remotely. This technological explosion is leading to the installation of a global network of volcanic-emission sensors. This network will underpin the geochemical surveillance and hazard assessment at volcano observatories worldwide and will yield new insights into the degassing and eruptive style of volcanoes and the impact of volcanic clouds on the atmosphere.

Sulfur is a ubiquitous element whose variable valence states (S2-, S0, S4+, S6+) allow it to participate in a wide variety of geochemical and biogeochemical processes. Depending on its redox state and controlling species, sulfur dissolved in magma may be fractionated into a water-rich phase and sulfur-bearing minerals. Retrieving information on the original sulfur abundance and isotopic signature of a magma is challenging and requires deciphering the different processes that may have operated during its evolution en route to the surface. Advances made in thermodynamic modeling, experimentation on sulfur solubility and diffusion in silicate melts, and microanalytical techniques for probing sulfur’s speciation and isotopic signature at the micrometer scale are providing an outstanding picture of sulfur evolution in magmas.

Sulfur is a widely distributed element on Earth and in the solar system. Its multiple valence states (S2- to S6+) allow it to participate in numerous geochemical and biochemical processes. It may be one of the light elements in the Earth’s core and may have been crucial in core formation. Sulfur is an essential component in all life on Earth and likely supported earliest life. Sulfur geochemistry is used to understand the early evolution of Earth’s atmosphere and hydrosphere, and serves as a monitor of volcanic SO2 and H2S and as a tracer of anthropogenic sources of sulfur. Recent advances in the use of multiple sulfur isotopes (32S, 33S, 34S, and 36S) and in situ isotopic measurements will help to develop sulfur stable isotopes as a vital tracer in the Earth and planetary sciences and will provide applications for understanding inorganic and biogenic processes.