An apparently complete carbonate-rich Cretaceous–Tertiary boundary interval in ODP section 119-738C-20R-5 from the southern Kerguelen Plateau provides a unique insight into processes of magnetization acquisition in marine carbonates. The boundary interval is characterized by a 1-m-thick clay-rich zone. Distinct depositional lamina are preserved within the basal 15 cm of this zone; the upper part is bioturbated. Previous studies have demonstrated that the bulk of the detrital fraction in the laminated and bioturbated carbonates has the same local source, and hence, the two intervals likely had similar initial detrital assemblages. Magnetic properties of these rocks, however, differ significantly. The laminated sediments have a higher content of non-silicate-bound iron, yet approximately an order of magnitude lower intensity of the natural remanent magnetization compared to the bioturbated rocks.Our detailed rock magnetic study indicates that in the bioturbated interval the dominant iron-bearing phase is single-domain magnetite, likely of biogenic origin. In the laminated interval, apart from a small ferromagnetic fraction with multi-domain-like behavior, non-silicate-bound iron is mainly sequestered in paramagnetic phases, likely poorly-crystalline oxyhydroxides. It appears that a shut-down of biological productivity after the K–T event allowed preservation of the initial detrital/early authigenic iron phases that are dominated by reactive iron oxyhydroxides. With recovery of the normal biological activity as evidenced by resumption of bioturbation, the oxyhydroxides had been replaced with biogenic magnetite. Thus produced biochemical magnetization led to a several-fold increase in the remanence. Our results suggest that in areas where bioavailable iron constitutes a significant part of the detrital input, such as in pelagic marine environments distant from clastic sources, the biochemical remanent magnetization may be the dominant process of magnetization acquisition.