Comprehensive petrological and isotope-geochemical study of marble was carried out in the upper part of the Sortavala Group of the northeastern lense at the Ruskeala deposit in the Northern Ladoga area. The petrological study showed that the carbonate rocks of the Sortavala Group underwent medium-temperature low-pressure amphibolite-facies metamorphism. The mineral assemblages of Ruskeala marbles were formed at temperature of 550–600°C and pressure of ~3–5 kbar in equilibrium with a mixed water-carbon dioxide fluid with $${{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}$$ ~ 0.5–0.8. Dolomite marbles contain admixture (up to 2%) of finely disseminated carbonaceous matter and about 8–15% of calcite. Dolomites contain small amounts of Mn (70–110 ppm) and Fe (1600–3600 ppm) and high content of Sr (122–256 ppm). The initial ⁸⁷Sr/⁸⁶Sr ratio in the dolomites is within 0.70465–0.70522, δ¹³C values vary between +0.6…+1.9 ‰, and δ¹⁸О within –13.2 … –10.2‰ (V-PDB). Calcite marbles are free of carbonaceous matter, have very low contents of Mg (0.2–0.8%), Mn (10–90 ppm) and Fe (160–640 ppm) and very high Sr concentration (850–2750 ppm). The initial ⁸⁷Sr/⁸⁶Sr ratios in the calcite marbles range from 0.70482 to 0.70489, δ¹³С from 1.5 to 2.1‰, and δ¹⁸О from –10.9 to ‒8.1 ‰ (V-PDB). Tremolite-bearing marbles have higher ⁸⁷Sr/⁸⁶Sr ratio (up to 0.70522), while their δ¹³С and δ¹⁸О values decrease to 0.1‰ and –12.2‰, respectively. The metamorphism of Ruskeala carbonate even under medium-temperature amphibolite-facies conditions was essentially isochemical process, which retained the unique Sr and C chemostratigraphic potential of calcite and dolomite marbles for the reconstruction of ⁸⁷Sr/⁸⁶Sr and δ¹³C in the Paleoproterozoic seawater. The ⁸⁷Sr/⁸⁶Sr ratio in the 1.9–2.0 Ga Svecofennian ocean was 0.70463–0.70492, and δ¹³С value +1.5 ± 1‰. New Sr-isotope data record an increase in radiogenic Sr input in the ocean about 2 billion years ago, which was probably related to the growth of continental crust and more intense weathering. The δ¹³С in Ludicovian carbonates mark the beginning of C-isotope stasis in ocean after the Lomagundi-Jatulian anomaly of ¹³C_carb.