SCIENCE CHINA Earth Sciences • RESEARCH PAPER •
February 2012 Vol.55 No.2: 323–331 doi: 10.1007/s11430-011-4291-1
Magnetic characterization and paleoclimatic significances of late Pliocene-early Pleistocene sediments at site 882A, northwestern Pacific Ocean JIANG ZhaoXia1,2 & LIU QingSong1* 1
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing100029, China; 2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China Received May 31, 2010; accepted February 21, 2011; published online January 2, 2012
Aeolian dust, a primary terrigenous component of ocean sediments, has been widely used to reconstruct the paleoclimatic evolution because its transported distance, grain size and concentration are sensitive to climate changes. To further characterize the aeolian dust, the deposits at site Ocean Drilling Program (ODP) 882A in northwestern Pacific Ocean are divided into four grain-size fractions (64 m) using the gravitative differentiation method. Detailed rock magnetism results show that magnetite and hematite are dominant magnetic minerals for the dust components. In addition, the aeolian dust (64 m) contributes little to the magnetic enhancement of the sediments at 2.73 Ma. These new results greatly improve our understanding of paleoenvironmental evolution during late Pliocene-early Pleistocene in this area. eolian dust sediment, rock magnetism, magnetite, hematite, ODP site 882A, the major glaciation in the Northern Hemisphere Citation:
Jiang Z X, Liu Q S. Magnetic characterization and paleoclimatic significances of late Pliocene-early Pleistocene sediments at site 882A, northwestern Pacific Ocean. Sci China Earth Sci, 2012, 55: 323–331, doi: 10.1007/s11430-011-4291-1
Marine sediment, a faithful recorder for the physical, chemical and biological processes of the earth evolution, is significant for the study of geologic history and climatic evolution [1]. In recent years, a number of proxies, e.g., 18O, organic carbon, carbonate, and bioprotein, magnetic parameters, have been used on the sediments of many sea areas to study the geochronology, geochemistry, ancient life and paleomagnetism [2–14]. Previous studies revealed that the terrigenous deposits (aeolian dust and ice-rafted detritus, IRD) were abundant in pelagic sediments. In general, the particle size of IRD is >64 m, whereas the particle size for
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© Science China Press and Springer-Verlag Berlin Heidelberg 2012
aeolian dust in pelagic sediments is 64 m component is too
Figure 2 SEM images for samples from different fractions.
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low to get reliable hysteresis parameters. Day plot indicates that magnetic mineral is dominated by pseudo-singledomain (PSD) magnetic grains (Figure 3). In addition, the coarser components are closer to the multi-domain (MD) region, further demonstrating the systematic changes of both the magnetic grain sizes and particle size of the bulk samples. To further analyze magnetic grain size of sediments, the correlation diagrams of ARM-SIRM and -SIRM are constructed. ARM/SIRM and /SIRM are widely employed as grain size indicators. In general, finer particles have higher ARM/SIRM but lower /SIRM ratios [45, 46]. Figure 4 shows that ARM/SIRM and /SIRM follow linear trends, which suggests that magnetic grain size for a single fraction is approximately uniform, and the magnetic enhancement is controlled dominantly by the increase of magnetic mineral concentration. In addition, the slope of ARM-SIRM decreases from 64 m component, which indicates that the relation of magnetic grain size for different fractions is coeval with that of sediment particle size. 2.2
Magnetic mineralogy analysis
The hysteresis loops and IRM acquisition curves are shown in Figure 5. After a paramagnetic correction, samples of
