Origin of the sedimentary magnetic record at Ocean Drilling Program Sites on the Owen Ridge, western Arabian Sea

Abstract

The 3.2‐m.y. whole core magnetic susceptibility record obtained during Ocean Drilling Program leg 117 from the Owen Ridge in the western Arabian Sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information. The salient features of this record are that (1) it correlates strongly with variations in the concentration and flux of eolian dust; (2) it is driven strongly at Earth orbital periodicities; and (3) there is a significant change in the spectral character at 2.4 m.y., which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas. In view of its potential paleoclimatic significance, we have examined the origin of the rock magnetic signal from the Owen Ridge in more detail. We find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content, there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis. We ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process. The first occurs at about 1.5 m depth and is characterized by a loss of fine‐grained ferrimagnetic material. The second stepshift in magnetic properties occurs at a depth of 6–8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity (magnetite) and high coercivity (hematite and/or goethite) magnetic mineral components. Scanning electron microscope and energy dispersive Xray analyses show that the bulk of the coarse‐grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the Fe:Ti ratio of these grains across the lower diagenetic front. This suggests that titanomagnetite grains with higher Ti contents may be more resistant to the process of reductive diagenesis, and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front.