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S U M M A R YA mixed-polarity zone, representing alternations between remagnetized and non-remagnetized strata, has been documented within the lower few metres of the CRP-1 core (Ross Sea, Antarctica). Detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single-domain particles of a ferrimagnetic iron sulphide mineral, while the reversed-polarity magnetization of non-remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions. Scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite (Fe 3 S 4 ). Based on microtextural relationships, it is not possible to determine the relative timing of formation for much of the greigite. However, a significant proportion of the greigite has grown on the surface of authigenic siderite (FeCO 3 ) grains that occur as microconcretions and as cement surrounding detrital matrix grains. In such cases, microtextural relationships indicate that siderite post-dates early diagenetic pyrite and that greigite post-dates the siderite. Siderite usually forms in environments with abundant dissolved iron and carbonate, but without dissolved pore water H 2 S. This set of geochemical conditions occurs in methanic settings below the sulphate reduction zone (in which early diagenetic pyrite forms). We interpret the observed remagnetization of the lower part of the CRP-1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide, thereby causing precipitation of greigite. The distribution of siderite (and associated greigite) in the lower part of the CRP-1 core is patchy, which accounts for the apparent alternation of polarities. This study is part of a growing catalogue of remagnetizations involving greigite, which suggests that occurrences of greigite should be treated with caution in palaeomagnetic and environmental magnetic studies.The reliability of a magnetic polarity stratigraphy depends on the validity of the assumption that the magnetic polarity recorded by a sedimentary sequence was acquired at or near the time of deposition. This assumption does not always hold, and it must be tested by carefully investigating the nature, origin and age of the magnetic minerals carrying the observed characteristic remanent magnetization (ChRM). There have recently been an increasing number of cases where the ferrimagnetic iron sulphide minerals greigite (Fe 3 S 4 ) and pyrrhotite (Fe 7 S 8 ) carry ChRMs with magnetic polarities opposite to those carried by coexisting magnetite or where the ChRM is contradictory to the expected polarity for the depositional age of the sedimentary sequence. These observations suggest a late, and sometimes complex, diagenetic origin for the iron sulphides (e.g.