Biogeochemical and stable carbon isotopic analysis of black-share sequences
deposited during an Albian oceanic anoxic event (similar to 112 million ye
ars ago) indicate that up to 80 weight percent of sedimentary organic carbo
n is derived from marine, nonthermophilic archaea, The carbon-13 content of
archaeal molecular fossils indicates that these archaea were Living chemoa
utotrophically. Their massive expansion may have been a response to the str
ong stratification of the ocean during this anoxic event. Indeed, the sedim
entary record of archaeal membrane lipids suggests that this anoxic event m
arks a time in Earth history at which certain hyperthermophilic archaea ada
pted to low-temperature environments.