text: Pyrite (FeS) is a mineral formed by the reaction of hydrogen sulfide (HS) and bioreactive iron (Fe). In oxic bottom waters pyrite can only form in sediments where HS is present. However, in iron-rich euxinic environments, pyrite formation can occur at higher rates in both the water column and in sediments due to higher concentrations of HS. Therefore the presence of euxinic conditions can be inferred by the ratio of pyrite-bound iron to the total iron in sediments. High ratios of pyrite-bound iron can be used as an indicator of past euxinic conditions. Similarly, if >45% of the bioreactive iron in sediments is pyrite-bound, then anoxic or euxinic conditions can be inferred. While useful, these methods do not provide definitive proof of euxinia because not all euxinic waters have the same concentrations of bioreactive iron available. These relationships have been found to be present in the modern euxinic Black Sea.