Environmental rock‐magnetism of Cenozoic red clay in the South Pacific Gyre

Abstract

Nonfossiliferous red clay can be used for elucidating long‐range environmental changes, although such studies were limited so far because of the difficulty in precise age estimation and extremely low sedimentation rates. We conducted an environmental rock‐magnetic study of Cenozoic red clay at the Integrated Ocean Drilling Program Site U1365 in the South Pacific Gyre. Magnetostratigraphy could be established only above ∼6 m below the seafloor (mbsf) (∼5 Ma). Below ∼6 mbsf, the ages of the cores were transferred from the published ages of nearby Deep Sea Drilling Project Site 596, which is based mainly on a constant Cobalt flux model, by intercore correlation using magnetic susceptibility and rare earth element content variation patterns. Rock‐magnetic analyses including first‐order reversal curve diagrams, the ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (SIRM), and IRM component analyses revealed that magnetic minerals consist mainly of biogenic magnetite and terrigenous maghemite, and that the proportion of the terrigenous component increased since ∼23 Ma. We consider that the increase reflects a growth of eolian dust flux associated with a northward shift of Australia and the site to an arid region of the middle latitudes. The increase of the terrigenous component accelerated after ∼5 Ma, which may be associated with a further growth of the Antarctic glaciation at that time. This is coeval with the onset of the preservation of magnetostratigraphy, suggesting that the primary remanent magnetization is carried by the terrigenous component.

A Geomagnetic Paleointensity Record of 0.6 to 3.2 Ma From Sediments in the Western Equatorial Pacific and Remanent Magnetization Lock‐In Depth

“…This is consistent with the TEM images showing that the dominant magnetofossil morphology is equant octahedra. The L and M components with mean coercivities and DPs similar to those observed in core MR1402‐PC1 are commonly observed in pelagic sediments in the Pacific and Indian Oceans, and the M component is considered to be carried by maghemite of probably terrigenous origin (Roberts et al, ; Shimono & Yamazaki, ; Yamazaki, ; Yamazaki, ; Yamazaki & Ikehara, ). The contribution of the M component is a little smaller in the deeper part of the core (Figure ), which is consistent with the increased k ARM /SIRM ratios suggesting a decreased terrigenous input.…”

Precessional control on ocean productivity in the Western Pacific Warm Pool for the last 400 kyr: Insight from biogenic magnetite

“…The FORC distributions of the broad component are interpreted to be carried by a mixture of interacting SD, PSD, and MD grains [Roberts et al, 2000;Pike et al, 2001;Muxworthy and Dunlop, 2002;Roberts et al, 2014]. The magnetic grains that constitute the component(s) with broad vertical spread on the FORC diagrams are inferred to have terrigenous origin [Yamazaki, 2008;Yamazaki and Ikehara, 2012;Shimono and Yamazaki, 2016]. It is hence considered that the magnetic mineral assemblages of the studied sediments are a mixture of biogenic and terrigenous components.…”

“…It is hence considered that the magnetic mineral assemblages of the studied sediments are a mixture of biogenic and terrigenous components. The broad component in the studied sediments is larger than that from other deep‐sea sediments in the Pacific and Indian Oceans [ Yamazaki , ; Yamazaki and Ikehara , ; Yamazaki and Shimono , ; Shimono and Yamazaki , ]. The larger terrigenous component, which is consistent with the relatively low k ARM /SIRM ratios of the sediments, probably indicates the proximity of the studied cores to terrigenous sediment sources.…”

“…The first-order reversal curve (FORC) diagram is a powerful tool for characterizing a magnetic particle assemblage [Roberts et al, 2000. It provides information on the distribution of coercivities (H c ) and local interaction fields (H u ) for a magnetic mineral assemblage [Pike et al, 1999], and is used to detect biogenic magnetite in sediments from a characteristic magnetic signal for a chain structure of magnetofossils, which has a sharp ridge along the H c axis with little vertical spread, called a ''central ridge'' [Chen et al, 2007;Yamazaki, 2008;Egli et al, 2010;Roberts et al, 2011Roberts et al, , 2012Roberts et al, , 2013Li et al, 2012;Yamazaki, 2012;Yamazaki and Ikehara, 2012;Yamazaki and Shimono, 2013;Chang et al, 2014;Shimono and Yamazaki, 2016]. FORC measurements were performed on selected samples using an alternating gradient magnetometer (AGM) (MicroMag 2900, Princeton Measurements Corporation) at GSJ.…”

Rock magnetism of quartz and feldspars chemically separated from pelagic red clay: a new approach to provenance study

“…We focus on samples from the top 15 m of Hole U1366C that covers Subunit Ia, Ib, and Ic. At Site U1365 which is ~ 900 km west from Site U1366, Shimono and Yamazaki (2016) reported that magnetism of bulk sediments is largely controlled by magnetofossils, especially in sediments older than ~ 23 Ma.…”