Abstract— <p>Samples of Fe–Mn nodules and underlying sediments from the Gulf of Finland (the Baltic Sea) and the Kara Sea are studied. Qualitative and quantitative analysis of the molecular composition of sterols (cholesterol, brassicasterol, campesterol, stigmasterol, and β-sitosterol) is performed by gas chromatography-mass spectrometry. It is established that the nodules function as independent geochemical systems, forming a new combination of organic components with comparable proportions of autochthonous and allochthonous organic matter. Sterol concentrations in the nodules are shown to be an order of magnitude lower than in the underlying sediments. Diagenetic processes in the nodules lead to the selective preservation of resistant terrigenous sterols (C₂₉) while simultaneously accumulating a significant proportion of marine-derived sterols (C₂₇, C₂₈). Regional differences are identified: the relatively homogeneous composition of the nodules from the Baltic Sea contrasts with the wide range of terrigenous C₂₉ sterol proportions (24‒53%) in the Kara Sea, which reflects the spatial inhomogeneity of sedimentation and diagenetic conditions on the Arctic shelf.</p>