New insight into acquisition of natural remanent magnetizations in a tidal depositional setting: A study from a Holocene tidal mud sediment in the southwestern coast of the Korean Peninsula

The high sedimentation rates in tidal flats are useful for paleomagnetic and paleoenvironmental reconstruction studies. However, the tidal flat sediments in the Korean Peninsula have received little or no attention for their sedimentary magnetism. Therefore, we aimed to investigate the acquisition and preservation of natural remanent magnetizations (NRMs) to explore the usability of tidal flat sediment as a recorder of Holocene paleomagnetic secular variation (PSV). Paleomagnetic and mineral magnetic analyses and dry density and X-ray fluorescence data from a tidal sediment core (QJS29 of 11 m in length in total) in the Dongjin River estuary revealed primary NRM acquisition and preservation depending on the depth and sample dry density in a 2–3.9 m depth interval. The major NRM carriers in the samples were mixtures of low-coercivity ferrimagnetic minerals, probably detrital magnetite and greigite of diagenetic and biogenic origins. With increasing depth, NRM acquisition efficiency and remanence stability increased while sample dry density decreased. Well-defined stable characteristic remanence (ChRM) components were observed in the tidal muds below 3.075 m depth. The correlation of the stratigraphic record of the ChRM and relative paleointensity with the available local reference curves reflected a local PSV between ca. 4 kyr before present (B.P.) and ca. 2 kyr B.P., suggesting a temporal lag in remanence acquisition (lock-in) from sedimentation. Our results suggest that a postdepositional remanence acquisition mechanism could dominate PSV-related ChRMs even though the remanence carriers are a combination of detrital, diagenetic, and biogenic sources. Thus, this study improves the usability of muddy tidal sediments as PSV recorders.