Turbidite and bottom-current evolution revealed by anisotropy of magnetic susceptibility of redox sediments in the Ulleung Basin, Sea of Japan

Abstract

Anisotropy of magnetic susceptibility (AMS) from above and below 7.31 m in a core from the southern Ulleung Basin shows clear differences on an equal area projection of the lower hemisphere. Rather concentrated steep inclination of K 3 and horizontal inclination of K 1 and K 2 (K 1⩾K 2⩾K 3) are located within the upper part, and the latter two axes lie perpendicular to each other near the bedding plane. In contrast, random distribution of the three axes and extremely high value of shape parameters (Q=(K 1−K 2)/[(K 1+K 2)/2−K 3]) are evident in the lower part, indicating complete destruction of the original sedimentary structure. This result is consistent with data from X-radiographs, which show numerous conglomerates, distortions and cleavages in the lower part. According to age models by Liu et al. (2010) and Zou et al. (2010), the bottom age at 7.31 m is 48 cal ka BP, and the time domain is discussed below. The degree of AMS (P) is low, 1–1.08, and linearly related to the foliation (F) (R 2=0.95, N=176). The relationship between F and linearity (L) implies oblate aligning patterns that are typically sedimentary in origin. At least five redox couplets were found with the aid of S ratios and other rock magnetic parameters, and in most cases, the original signals of climate survived early diagenesis. The paleomagnetically reoriented AMS show corresponding changes with millennial events in the last 48 cal ka. Clearly tilted K 3 directions and reduction of P and F occurred within DO1-BA warm events, when melt water pulse 1A nd the YD cold event took place. The turbulent conditions therein, synchronous with coarsening of sediments, provide evidence of strong bottom currents and possible directional changes, as evidenced by different K 3 tilting directions. A NE current direction in the last 4.5 cal ka is consistent with in situ measurements of bottom currents. Responses of AMS, mainly to climatic modulation, show on the one hand, the limited influence of diagenesis on rock magnetic signals and, on the other hand, the dramatic change of hydrodynamic conditions and terrigenous inputs during rapid sea level rise during the last deglaciation.