"When do black shales tell molybdenum isotope tales?" Geology vol 37 no 6, p. 535-538, doi: 10.1130/G25186A.1 appears in June 2009. Molybdenum isotopes in ancient black shales have been used to tell us about the amount of oxygen in the world’s oceans in the past. Lighter isotopes preferentially go into oxic sediments, while sediments laid down in water columns with high sulfide reflect the isotopic composition of the water. By looking at two similar sequences of Devonian rocks (~380 million years old), Gordon et al. constrain what types of organic-rich sediments record the signal from ancient seas. The evidence is consistent with larger areas of the seafloor with low oxygen in the middle Devonian.