4.5 Article

Archean-Proterozoic unconformity on the Fennoscandian Shield: Geochemistry and Sr, C and O isotope composition of Paleoproterozoic carbonate-rich regolith from Segozero Lake (Russian Karelia)

Journal

PRECAMBRIAN RESEARCH
Volume 368, Issue -, Pages -

Publisher

ELSEVIER
DOI: 10.1016/j.precamres.2021.106459

Keywords

Paleoproterozoic; Paleosols; Iron-rich dolomite; Carbon isotope composition; Strontium isotope composition; Fennoscandia

Funding

  1. Institute of Precambrian Geology and Geochronology RAS [AAA-A18-1180115901-0, 0153-2019-0001]
  2. RSF [18-17-00247]
  3. NSERC Discovery and Acceler-ator

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This study presents the results of a paleoweathering profile in the Precambrian on Chapanshary Island in Central Karelia, which provides insights into the environmental changes and stratigraphic boundaries during that time. The profile developed in a terrestrial setting around 2.2-2.1 billion years ago, with arid to semi-arid climate and high CO2 content in the atmosphere. The findings contribute to our understanding of Precambrian environmental changes and the identification of stratigraphic boundaries.
Precambrian paleoweathering profiles provide insight into environmental changes and mark stratigraphic boundaries. However, the well-preserved and exposed stratigraphic boundaries, not affected by tectonics, are rare for the Precambrian. We report sedimentological, geochemical and Sr, C and O isotopic data for the Paleoproterozoic weathering profile in Chapanshary Island on Lake Segozero, Central Karelia, the eastern part of the Fennoscandian Shield. The Chapanshary Island weathering crust contains a unique carbonate caliche, rarely developed on the Archean granite-gneiss basement. The crust laterally grades into carbonate-cemented boulder conglomerate, which is overlain by sedimentary carbonate. The chemical index of alteration (CIA; 60-67) and plagioclase index of alteration (PIA; 53-82) values for the weathering crust correspond to weathered felsic/mafic substrate and to illite and K-rich composition of the regolith. The delta O-18 values of carbonate cement and sedimentary carbonate are highly negative (-22.3 to -17.6 parts per thousand V-PDB). The Sr-87/Sr-86 values of Fe- and Mn-rich carbonate of the regolith, boulder conglomerate cement and sedimentary carbonate are 0.7133 to 0.7242 and are exceptionally high for marine Paleoproterozoic carbonate. The high delta C-13 values (+3.9 to + 5.2 parts per thousand V-PDB) of carbonate cement and sedimentary carbonate indicate deposition during the ca. 2.22-2.06 Ga Lomagundi-Jatuli carbon isotope excursion of the early Paleoproterozoic. The Chapanshary Island weathering profile developed in a terrestrial setting with arid to semi-arid climate ca. 2.2-2.1 Ga ago under atmosphere with high CO2 content. The overlying sedimentary carbonates were deposited in a shallow-water setting (ephemeral ponds and lakes) evolved in the inner part of Fennoscandia.

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