Sea‐ice conditions in the Okhotsk Sea during the last 550 kyr deduced from environmental magnetism

Abstract

[1] Reconstructing past sea-ice conditions in the Okhotsk Sea is important because sea-ice conditions vary in response to global climate changes, which in turn may affect global ocean circulation through intermediate water mass formation. We conducted an environmental magnetic study of six cores from three stations in the central Okhotsk Sea to better understand temporal and spatial sea-ice variations. Intercore correlations and age estimations are based mainly on geomagnetic paleointensity; an oxygenisotope stratigraphy is available for one station. Magnetic susceptibility (MS) minima are accompanied by maxima in color b*, the ratio of the anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (k ARM /SIRM), and the S-ratio, which indicates a higher proportion of biogenic to terrigenous magnetic components. This reflects enhanced ocean productivity. First-order reversal curve diagrams and IRM component analyses support the dominance of biogenic magnetite at MS minima. In contrast, color b*, k ARM /SIRM, and S-ratio values are low when MS is high, which indicates an increased proportion of the terrigenous component that was probably transported as icerafted debris (IRD). For the southern two stations, IRD accumulation increased in glacial and deglacial periods, which implies mobile sea-ice conditions even in full glacials. This was succeeded by extremely enhanced ocean productivity in early interglacials, which suggests nearly ice-free conditions. For the northernmost station, on the other hand, IRD accumulation was low in glacials and increased in early interglacials, which indicates perennial sea-ice coverage with little mobility in glacials. Succeeding ocean-productivity enhancement was delayed compared to the southern stations.

Influence of sampling on magnetic susceptibility anisotropy of soft sediments: comparison between gravity and piston cores

“…Natural remanent magnetization (NRM) measurements with stepwise alternating field (AF) demagnetization were performed using a cryogenic magnetometer system with an in-line static AF demagnetizer (2G Enterprises model 760, Applied Physics Systems, Inc., Mountain View, CA, USA) in a magnetically shielded room of GSJ, AIST. Generally, the maximum angular deviation (Kirschvink 1980) values were less than 5°, which indicates that stable primary magnetization was preserved (Inoue and Yamazaki 2010;Yamazaki et al 2013). The mean paleomagnetic inclinations of the cores are close to those expected from the geocentric axial dipole model for the respective site latitudes.…”

“…All cores contain a few thin volcanic ash layers and some detrital components. The results of a paleomagnetic and rock magnetic study of these cores were reported in Inoue and Yamazaki (2010) and Yamazaki et al (2013). The ages of the bottom of the gravity cores range from approximately 120 to 160 ka.…”

Paleomagnetic inclination variations during the last 200 kyr in the Okhotsk Sea and their relation to persistent non-axial-dipole field

“…Relative paleointensity and magnetic properties of the three major sites, GC1 + PC7, GC8 + PC6, and GC9 + PC5, were already reported by Inoue and Yamazaki (2010) and Yamazaki et al (2013). The procedure of paleo-and rock magnetic measurements of other gravity-core sites, GC3, GC5, GC6, GC10, GC11, and GC12, was the same as that of Inoue and Yamazaki (2010) and Yamazaki et al (2013). Stepwise alternatingfield (AF) demagnetization showed univectorial behavior in general except for a soft secondary component that was removed at AF of 10 mT or less.…”

“…Discrete samples for paleo-and rock magnetic measurements were taken sequentially from the half-split core surfaces using plastic cubes of 7 cm 3 each. Relative paleointensity and magnetic properties of the three major sites, GC1 + PC7, GC8 + PC6, and GC9 + PC5, were already reported by Inoue and Yamazaki (2010) and Yamazaki et al (2013). The procedure of paleo-and rock magnetic measurements of other gravity-core sites, GC3, GC5, GC6, GC10, GC11, and GC12, was the same as that of Inoue and Yamazaki (2010) and Yamazaki et al (2013).…”

“…Three gravity cores out of nine were reoccupation of the three piston-core sites to make up for disturbed surface sediments. Composite core sections, GC1 + PC7, GC8 + PC6, and GC9 + PC5, were established at the three sites (Yamazaki et al 2013). Discrete samples for paleo-and rock magnetic measurements were taken sequentially from the half-split core surfaces using plastic cubes of 7 cm 3 each.…”