The boundary between Late Permian and Early Triassic stratigraphic sequences is characterized by the onset of a marked shift in the isotopic composition of both marine carbonates and sulfates. The abrupt 3? decrease in the carbon isotope ratio of carbonates and the more gradual 5? increase in the sulfur isotope ratio of sulfates can be explained by changes in the nature of the Earth's ecosystems induced by the massive Permo-Triassic extinctions. We propose that the continental ecosystem responsible for the preservation and burial of organic matter during Permian time was disrupted and that the relatively efficient marine food web of Permian time was replaced by a less efficient system, which allowed for a greater fraction of the marine organic matter to reach the seafloor. These changes were reflected by a major shift in the depositional environment for the reduced component of material accumulating in sediments. During the Permian, organic carbon burial on the continents dominated, whereas during the Triassic, deposition of organic carbon on the seafloor dominated, allowing sulfides to become an important component of the reduced phase. ¿ 1999 American Geophysical Union