Environmental rock‐magnetism of pelagic clay: Implications for Asian eolian input to the North Pacific since the Pliocene

Abstract

We conducted a rock‐magnetic study of pelagic sediments in order to document variations of Asian eolian input to the North Pacific since the Pliocene. The materials studied consist of five pelagic‐clay (red‐clay) cores of several meters long taken from the central North Pacific and surface sediments of box cores obtained at 40 sites along two lines, a N‐S line along 175°E and an E‐W line along 20°N. The magnetic susceptibility (concentration of magnetic minerals) and S ratio (relative abundance of low‐coercivity magnetic minerals) of the five pelagic‐clay cores have decreased synchronously since about 2.5 Ma. This age coincides with the onset of the northern hemisphere glaciation and Chinese loess deposition. Geographical distribution of S ratios measured on the surface sediments revealed that they are remarkably low in the central North Pacific. These facts suggest that S ratio can be used as a proxy for Asian eolian dust. Mössbauer spectroscopy showed that hematite dominates the high‐coercivity minerals. Observation with a transmission electron microscope suggests two sources of magnetic minerals in the pelagic clay: biogenic magnetite produced in situ and detrital magnetite and hematite. The variations of S ratio and magnetic susceptibility with time and region can be explained by a model that Asian eolian dust has higher (lower) hematite (magnetite) concentration than other sources of magnetic minerals and that its input to the central North Pacific has increased since 2.5 Ma. We recognized subtle S‐ratio fluctuations of about 400 kyr in recurring intervals superimposed on the decreasing trend since 2.5 Ma. Rock‐magnetic parameters diagnostic of magnetic grain size, on the other hand, showed small or no variations with age, although eolian grain size is generally thought to have increased significantly since the Pliocene based on sedimentological studies.