<p>Rare earth elements (REEs) were studied in ferromanganese nodules and bivalve shell carbonates from Lake Krasnoe (Karelian Isthmus, West Russia). This lake is a model example of modern lacustrine manganese ore genesis. The results revealed important features of the formation of modern manganese ores in the diagenesis zone of lake sediments: (1) carbonates of bivalve shells and the Mn-oxide crusts on them show very strong positive Eu anomalies but insignificant negative Ce anomalies; (2) manganese oxide nodules exhibit strong negative Eu anomalies; therefore, the negative cerium anomalies are insufficient; (3) REEs in the studied Fe–Mn-ore nodules are associated with organic matter (C_org), phosphates and iron hydroxides; (4) a positive correlation between carbon (<i>δ</i>¹³C, ‰, V-PDB) and oxygen isotopes (<i>δ</i>¹⁸O, ‰; SMOW) versus Ce/Ce* is observed. The authors believe that the obtained regularities in the distribution of REE content, Eu and Ce anomalies, and C- and O-isotope composition (<i>δ</i>¹³C, <i>δ</i>¹⁸O) result from changes in the physicochemical conditions in the ore body lake sediments during early diagenesis, which oxidize organic matter (C_org), and active participation of microorganisms in manganese ore formation.</p>