Abstract:
Magnetic properties are increasingly used for paleoclimatic and paleoceanographic studies. Utilizing recently developed proxies, an environmental magnetic study was conducted on the uppermost 12 m sediments of Integrated Ocean Drilling Program Site U1337 in the eastern equatorial Pacific Ocean. This interval is above the Fe-redox boundary, and covers the past approximately 800 k.y. The site is located near the present southern boundary of the Intertropical Convergence Zone (ITCZ), and is thus expected to be sensitive to variations of the ITCZ position. The ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (k (sub ARM) /SIRM), first-order reversal curve diagrams, and IRM acquisition curves indicate that the magnetic mineral assemblage consists of a dominant biogenic component and a minor terrigenous component. Two groups, the biogenic soft (BS) and hard (BH), are identified for the biogenic component, and probably correspond to different magnetofossil morphology. The BH component, probably carried by elongated magnetofossils, increases in sediments of glacial periods, which are probably in less oxic conditions due to increased ocean productivity. This demonstrates that magnetofossil morphology, which can be discriminated by the rock magnetic technique, is a sensitive indicator of slight oxic-suboxic environmental fluctuations in sediments. Temporal variations of the terrigenous component, most likely transported as eolian dust, were estimated from the k (sub ARM) /SIRM ratio and S ratio (ratio of a moderate field IRM to SIRM, representing relative contribution of lower- and higher-coercivity magnetic minerals); significant glacial-interglacial variations occurred at marine isotope stage (MIS) 10 and before, but not after. In addition, coeval upcore increases in sedimentation rates and the BH component were observed, suggesting increased productivity. These observations may indicate that the position of the ITCZ was more southward than today ca. 250 ka and before.