Editing Archean Eon (section)

=== Archean Sulfur Cycle ===
The Archean sulfur cycle was marked by the absence of extensive '''oxidative weathering''' and the limited availability of free oxygen, which significantly influenced the forms of '''sulfur''' that cycled through the environment. In the absence of oxygen, sulfur predominantly existed in '''reduced''' forms, such as hydrogen sulfide (H₂S) and various '''sulfides'''. Volcanic outgassing was a primary source of these reduced sulfur species. Microbial sulfate reduction was a key biological process, where sulfate-reducing bacteria utilized sulfate (SO₄²⁻) as an '''electron acceptor''', producing H₂S. This process would have been limited by the availability of sulfate, which was much lower than today's levels due to the lack of oxidative weathering.

The interaction between reduced sulfur compounds and iron in the oceans may have contributed to the deposition of banded iron formations, which are sedimentary rocks characterized by alternating layers of iron-rich minerals and silica (or chert). Variations in sulfur '''isotope ratios''' in Archean rocks provide evidence of microbial sulfate reduction and other biologically influenced sulfur processes, offering insights into early life and the '''redox state''' of the oceans.