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Graphical abstract Highlights • Ferro-manganese nodules from the Kara Sea mainly were formed by diagenetic process. • Studied nodules are characterized by the alternation of Mn- and Fe-P-rich laminae. • Buserite, birnessite and X-ray amorphous phase are main Mn-bearing phases. • Amorphous Fe-P phase, goethite and framboidal pyrite are main Fe-bearing phases. • Ferro-manganese nodules from the Kara Sea have negative Ce and Y anomalies. • Diagenetic nodules differ significantly from the other types of Fe-Mn concretions. Abstract Sixteen ferromanganese nodules collected from a large area in the Kara Sea were studied by optical and scanning electron microscopy, X-ray powder diffraction, X-ray fluorescence analysis, inductively coupled plasma mass spectrometry, and electron microprobe analysis. The geological setting, chemical and mineralogical features suggest that Fe-Mn concretions of the Kara Sea mainly were formed by diagenetic process, while hydrogenetic process is secondary if present. The studied nodules are characterized by the alternation of Mn- and Fe-P-enriched laminae. The Fe-P-rich laminae are almost completely composed of an amorphous phase under X-ray. We consider that the phosphatization process did not occur during formation of the Fe-Mn crusts and nodules from the Kara Sea. Several Mn-bearing phases are present in the studied samples (buserite-1, birnessite and X-ray amorphous phase). All studied nodules have similar contents of the main chemical elements. In all cases, the samples are depleted in light REE, enriched in heavy REE, and have negative Ce and Y anomalies. Our data indicate that diagenetic crusts and nodules differ significantly in terms of chemical composition from other types of Fe-Mn concretions. Based on their Co, Ni, Cu, Ce, Zr and Mo contents, we could differentiate diagenetic concretions from hydrogenetic and hydrothermal (both high and low-temperature) concretions. [ABSTRACT FROM AUTHOR]