The end of the Paleoproterozoic carbon isotope excursion: New time constraints

The isotopic composition of carbon in carbonate sediments underwent a major positive excursion at 2.2 -2.1 Ga . In order to apply the carbon isotope record for chemostratigraphic correlation of sedimentary sequences, it is important to have good knowledge of the secular variations of marine carbon isotope ratios. Unfortunately, very few reliable isotopic ages constrain the timing of the Paleoproterozoic carbon isotope excursion. The Peräpohja belt in northern Finland provides several stratigraphic sections of sedimentary carbonates covering the end of the carbon isotope excursion at ca. 2.1 Ga. Carbonate rocks are present on several stratigraphic levels, and they mostly represent shallow water deposition on a tidal flat environment. They are light colored and laminated and have distinct features suggesting inter- and supratidal origin, such as intraformational conglomerates, stromatolites, microbial laminites, teepees and stone rosettes. The supra and intertidal carbonates are pervasively dolomitized. Locally the dolomite units contain layered calcite-dolomite rock that was deposited in a deeper water environment, below the fair weather wave base.
Based on earlier studies from the Fennoscandian Shield, the Paleoproterozoic carbon isotope excursion terminated between 2115±6 and 2062±2 Ma. New data from the Peräpohja belt are in agreement with these rough time limits and, in addition, provide a more accurate constraint for the shift from high to low δ¹³C values. The pyroclastic Hirsimaa Formation is a continuous marker bed, underlain by dolomite units of the Poikkimaa Formation and overlain by dolomite beds of the Rantamaa Formation. U-Pb zircon isotope analyses from the Hirsimaa Formation, measured using the SHRIMP II ion-microprobe at VSEGEI; St Petersburg, yielded a preliminary U-Pb date of 2106±8 Ma for the deposition of the Hirsimaa Formation. The δ¹³C values of dolomite in the underlying and overlying sedimentary units show systematic stratigraphic variations superimposed on a generally decreasing trend from about 10 to 1 ‰. It appears that the age of the Hirsimaa Formation closely approximates the end of the carbon isotope excursion at a time, when the marine δ¹³C values were at 4 - 6 ‰. The systematic carbon isotope fluctuations superimposed on the general trend may serve as a correlation tool allowing different sections to be correlated across the basin, and possibly globally.