Stable isotope ratios can be used to infer the environmental conditions during the formation of sedimentary rock. Using stoichiometry and knowledge of redox pathways, paleogeologists can use isotopes ratios of elements to determine the chemical composition of the water and sediments when burial occurred.
Sulfur isotopes are frequently used to look for evidence of ancient euxinia. Low δS in black shales and sedimentary rocks provides positive evidence for euxinic formation conditions. The pyrite (FeS) in euxinic basins typically has higher concentrations of light sulfur isotopes than pyrite in the modern ocean. The reduction of sulfate to sulfide favors the lighter sulfur isotopes (S) and becomes depleted in the heavier isotopes (S). This lighter sulfide then bonds with Fe to form FeS which is then partially preserved in the sediments. In most modern systems, sulfate eventually becomes limiting, and the isotopic weights of sulfur in both sulfate and sulfide (preserved as FeS) become equal.
Molybdenum (Mo), the most common transition metal ion in modern seawater, is also used to look for evidence for euxinia. Weathering of rocks provides an input of MoO into oceans. Under oxic conditions, MoO is very unreactive, but in modern euxinic environments such as the Black Sea, molybdenum precipitates out as oxythiomolybdate (MoOS ). The isotope ratio for Molybdenum (δ Mo) in euxinic sediments appears to be higher than in oxic conditions. Additionally, the concentration of molybdenum is frequently correlated with the concentration of organic matter in euxinic sediments. The use of Mo to indicate euxinia is still under debate.