Geochemical cycles of sulfur and carbon on the early Earth

Abstract

The current popular dogma postulates that the geochemical cycles of S and C during the Archean were entirely different from the modern ones: (1) the global S and C cycles were not linked with each other; (2) oxidative recycling of OM and FeS2 was not important, resulting in very low-SO4 oceans (i.e., <1/200 of today); and (3) UV photolysis of volcanic SO2 to generate native S and SO4 was the principal reaction for the formations of sulfide- and sulfate minerals in Archean sedimentary rocks. However, this model fails to explain many important geological and geochemical features in Archean rocks.We have recently discovered the abiotically formed organic matter (OM) in 3.46 Ga-old cherts that were deposited in a deep submarine environments by hydrothermal fluids. A large quantity of this type of OM must have been produced in the Archean oceans because of the high contents of CO2 and O2 in the atmosphere and of the higher submarine hydrothermal activity. Compared to biogenic OM, abiotic organic matter would be more reactive to SO4 during bacterial sulfate reduction and thermochemical sulfate reduction. This explains the sulfur isotopic characteristics of the Archean sedimentary rocks (i.e., small d34S fractionations and MIF-S).