No abstract

No abstract

Red clay widely occupies the seafloor of pelagic environments in middle latitudes, and potentially preserves long paleoceanographic records. We conducted a rock-magnetic study of Pacific Ocean red clay to elucidate paleoenvironmental changes. Three piston cores from the western North Pacific Ocean and IODP Hole U1365A cores in the South Pacific Ocean were studied here. Principal component analyses applied to first-order reversal curve diagrams (FORC-PCA) reveals three magnetic components (endmembers EM1 through EM3) in a core of the western North Pacific. EM1, which represents the features of interacting single-domain (SD) and vortex states, is interpreted to be of terrigenous origin. EM2 and EM3 are carried by non-interacting SD grains with different coercivity distributions, which are interpreted to be of biogenic origin. The EM1 contribution suddenly increases upcore at a depth of ~ 2.7 m, which indicates increased eolian dust input. The age of this event is estimated to be around the Eocene–Oligocene (E/O) boundary. Transmission electron microscopy reveals that EM2 is dominated by magnetofossils with equant octahedral morphology, while EM3 has a higher proportion of bullet-shaped magnetofossils. An increased EM3 contribution from ~ 6.7 to 8.2 m suggests that the sediments were in the oxic–anoxic transition zone (OATZ), although the core is oxidized in its entire depth now. The chemical conditions of OATZ may have been caused by higher biogenic productivity near the equator. FORC-PCA of Hole U1365A cores identified two EMs, terrigenous (EM1) and biogenic (EM2). The coercivity distribution of the biogenic component at Hole U1365A is similar to that of the lower coercivity biogenic component in the western North Pacific. A sudden upcore terrigenous-component increase is also evident at Hole U1365A with an estimated age around the E/O boundary. The increased terrigenous component may have been caused by the gradual tectonic drift of the sites on the lee of arid continental regions in Asia and Australia, respectively. Alternatively, the eolian increase may have been coeval in the both hemispheres and associated with the global cooling at the E/O boundary.

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.

[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.

Magnetic mineral inclusions in silicates are widespread in sediments as well as in igneous rocks. Because they are isolated from surrounding environment, they have potential to preserve original magnetic signature even in chemically altered sediments. Such inclusions may provide proxies to help differentiating the source of the host silicate. We measure magnetism of quartz and feldspars separated by chemical digestion of pelagic red clay. The samples are from the upper 15 m of sediments recovered at Integrated Ocean Drilling Program Site U1366 in the South Pacific Gyre. The quartz and feldspars account for 2.3-22.7 wt% of the samples. X-ray diffraction analyses detect both plagioclase feldspar and potassium feldspar. Plagioclase is albite-rich and abundant in the top ~ 7.4 m of the core. Potassium feldspar mainly occurs below ~ 10.4 m. The dominance of albite-rich plagioclase differs from a previous investigation of coarser fraction of sediments from the South Pacific. Saturation isothermal remanence (SIRM) intensities of the quartz and feldspars are 7.45 × 10 −4 to 1.98 × 10 −3 Am 2 /kg, accounting for less than 1.02% of the SIRM of the untreated bulk samples. The depth variations of the silicate mineralogy and the previously reported geochemical end-member contributions indicate that quartz and/or plagioclase above 8.26 m is likely to be Australian dust. In contrast, the relative abundance and the magnetic properties of quartz and feldspars vary below 10.42 m, without clear correlation with geochemical end-member contributions. We consider that these changes trace a subdivision of the volcanic component. Our results demonstrate that magnetism of inclusions can reveal additional information of mineral provenance, and chemical separation is an essential approach to reveal the environmental magnetic information carried by magnetic inclusions. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. remanence; IODP: Integrated Ocean Drilling Program. Authors' contributionsYU designed the study, conducted chemical separation, XRD analysis, and magnetic measurements of feldspars and quartz. TS conducted magnetic measurements of untreated samples. YU, TS, and TY interpreted the data. YU wrote the paper with input from TS and TY. All authors read and approved the final manuscript.

Anisotropy of magnetic susceptibility (AMS) has been used extensively for determining mineral orientation fabrics. One of the advantages of the AMS method for studying sediment and rock fabrics is that it is sensitive to very weak deformation. Because of its high sensitivity, however, AMS of marine sediments may easily be influenced by artificial deformation during coring and sampling. We compare magnetic susceptibility and its anisotropy between three pairs of gravity and piston cores taken at the same sites in the Okhotsk Sea. We report for the first time that artificial AMS caused by deformation is dependent on the sampling methods used. The sedimentary fabric is preserved, but declinations of the maximum axis of susceptibility (K max ) are inconsistent between piston and gravity cores after orientation with remanent magnetization directions. In terms of the sample coordinate, K max declinations in the three gravity cores are oriented along the core-splitting surface, whereas K max declinations in the three piston cores are perpendicular to the splitting surface. We attribute the artificial AMS to the stress created by the deformation of core liners when being split. When interpreting AMS data from sediment cores, it is necessary to investigate the influence of sampling using the sample coordinates. In this paper, we also report over-sampling and under-sampling of piston cores from a comparison of down-core magnetic susceptibility variations between piston and gravity cores. It is noteworthy that under-sampling as well as over-sampling can occur in the uppermost few meters of piston cores.

No abstract