Evidence for Lewisian limestones for isotopically heavy carbon in two-thousand-million-year-old sea water

The carbon isotope signatures of 2000-million-year-old amphibolite facies marbles from the Scottish Lewisian were measured, and the occurrence of a major excursion in carbon isotope composition of sea water at that time is confirmed by the heavy carbon isotope signatures of these rocks, which is attributed here to an origin contemporaneous with sedimentation. The excursion coincides with a major change in the redox state of the oceans, which is likely to have induced considerable changes in the carbon cycle. It is suggested that the excursion was due to increased rates of organic carbon deposition resulting from an increase in organic productivity.