Environmental conditions for the presence of magnetofossils in the Last Glacial Maximum inferred from magnetic parameters of sediments from the Ulleung Basin, East Sea

Abstract

Magnetic characterization of sediments from the Ulleung basin in the East Sea (Japan Sea) was carried out to decipher changes in environmental conditions over the last ~39,000years. Magnetic susceptibility (χ), intensities of natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), partial ARM (pARM), and saturation isothermal remanent magnetization (SIRM) were measured. For selected samples, IRM acquisition, hysteresis and first-order reversal curve (FORC) measurements were made. The ratios of rock-magnetic parameters, such as χARM/χ, SIRM/ARM and S-ratio, were determined to identify magnetic grain size and mineralogical variations along with electron microscope observations of magnetic mineral extracts. Down-core variations in rock-magnetic parameters for the 5.21-m-long sediment core reveal distinct intervals with different grain size, concentration and composition of magnetic minerals. The uppermost portion and the high magnetic intensity zone (HIZ) in marine isotope stage (MIS) 2 are characterized by higher concentrations of fine-grained magnetite, whereas the MIS 1 and 3 intervals contain lower concentrations of coarser-grained magnetic minerals with increasing influence of hematite and/or iron sulphide minerals. The HIZ interval is inferred to contain high occurrences of fine-grained bacterial magnetosomes. Based on geochemical proxies, East Sea conditions for the HIZ interval, coincident with the last glacial maximum (LGM), can be summarized as having low total organic carbon (TOC), enhanced eolian contributions from terrestrial organic particles and iron-bearing minerals under the influence of an intensified East Asian winter monsoon, along with iron-reducing, but not sulphidic, conditions that favoured growth and preservation of magnetotactic bacteria. The delicate balance between reductive dissolution of magnetofossils under sulphidic conditions and production and preservation of magnetotactic bacteria in overlying iron-reducing sediments for the HIZ interval is responsible for the resultant abundance variations of magnetosomes in the studied sediments.