The diagenetic history of the skeletal elements of Late Pleistocene-Holocene Ophiura sarsi from the shelf off northern Norway (Andfjorden, Malangsdjupet) is elucidated by comparison with natural and induced degradation of the skeletal elements of Recent ophiuroids (brittle stars) and asteroids (sea stars) from Danish waters. Dissolution features ("core-and-rind") in the trabeculae of fossil and Recent echinoderm stereom are initiated during death and early decay of organic tissue in the

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... The trabeculae have a polycrystal­line lamellar ultrastructure and lose their older central part during later stages of dissolution, which are dependant on undersaturation of the sea-water with regard to CaC03• The presence of undersaturated sea-water is supported by palaeoecological studies (Thomsen & Vorren 1984, 1986) implying oxygen deficient periods in the Late Pleistocene and an increased biogenic production in the Holocene. Pyrite framboids are situated in the secondary voids within the trabeculae and in the pore space of the stereom of the Late Pleistocene elements. No pyrite is observed within the polycrystalline lamellar ultrastructure of the trabeculae. The Late Pleistocene "pyritization" took place during oxygen deficient periods at the sediment-water interface or within the reduced zone of the topmost sediment.