Two studied sediment cores from the Korea Strait contain mud sequences (14 m and 32.62 m in thickness) that were deposited during the last 6,000 years. The sediments have uniform lithology and geochemical properties, however, marked down-core changes in magnetic properties suggest that diagenesis has significantly impacted the magnetic properties. An expanded view of early diagenetic reactions that affect magnetic mineral assemblages is evident in these rapidly deposited continental shelf sediments compared to deep-sea sediments. The studied sediments are divided into four descending intervals, based on magnetic property variations. Interval 1 is least affected by diagenesis and has the highest concentrations of detrital magnetite and hematite, and the lowest solid-phase sulfur contents. Interval 2 is characterized by the presence of paramagnetic pyrite and sharply decreasing magnetite and hematite concentrations, which suggest active reductive dissolution of detrital magnetic minerals. Interval 3 is marked by a progressive loss of hematite with depth, and coincides with the minimum magnetite concentration. Interval 4 has an increasing down-core enhancement of authigenic greigite, which apparently formed at depths of 3--30 m below the sediment-water interface due to arrested pyritization reactions. These results indicate delays of thousands of years for acquisition of magnetizations carried by greigite, which suggests that studies of geomagnetic field behavior from greigite-bearing continental shelf sediments should be conducted with care. Also, virtually complete destruction of detrital magnetic minerals at depth suggests that magnetic studies of rapidly deposited shelf sediments are unlikely to provide a meaningful signature associated with syn-depositional environmental processes.