Carbon isotope variations of high magnitude recorded in carbonate rocks from the Stenian-Tonian Lajeado Group, Southeast Brazil

The Lajeado Group is a Stenian-Tonian metasedimentary unit comprising a thick carbonate succession that was involved in the Brasiliano – Pan-African Ribeira orogenesis in Southeast Brazil. The carbonate rocks record significant carbon isotope variations, which provide further information about the Mesoproterozoic Earth system. The δ13C values obtained along a composite vertical section vary between −5 and +5‰ with two negative isotope excursions with magnitudes higher than 6‰. The coincidence between the isotopic trend of the Lajeado carbonate succession with equivalent units studied worldwide suggests that the deposition of the lower and middle parts of the unit may have occurred at 1.2–1.1 Ga, which is in agreement with recently published geochronological data. Despite the similarities, the Lajeado Group displays larger carbon isotope variations and negative isotope excursions of higher magnitudes when compared to coeval marine settings. We argue that the basin conditions may have been more favourable to build up large particulate (POC) and dissolved organic carbon (DOC) reservoirs. When oxidized they significantly impacted the dissolved inorganic carbon (DIC) pool, causing the negative excursions. If our scenario holds, the Lajeado Group may be considered an example of record of fluctuations in atmospheric pO2 levels and the oxidation state of the planet during the late Mesoproterozoic. •Carbonate rocks from the Lageado Group record major variations of δ13Ccarb values.•These variations are similar to those from other Mesoproterozoic units worldwide.•Such C isotope shifts may record fluctuations in atmospheric oxygen levels.