文摘
Detailed mineral magnetic measurements, integrated with grain-size distribution and X-ray diffraction (XRD) analyses, were made on the marine sediments of Core MD98-2172, retrieved from the Eastern Timor Sea. Values of magnetic susceptibility in this core drop sharply down-core from .85 m deep below sediment/water interface and are very low at .35 m. However, both XRD and grain-size distribution results show no sudden change in terrigenous input during sedimentation. Mineral magnetic results indicate that the depth of .85 m may be an oxic/anoxic boundary. Therefore, the sediments below .85 m have been subjected to intense reductive diagenesis, whereas the sediments above .85 m are seldom affected. The magnetic properties of the sediments shallower than 3.85 m are dominated by pseudo-single domain (PSD) magnetite, with little down-core variation in its content and grain size. Below .85 m, the magnetic mineral assemblages that have survived in the sediments may record different stages of the reductive diagenesis: (1) the sediments from the 3.85-.35 m interval are at the stage of iron oxide reduction; PSD magnetite is the major magnetic contributor, but it becomes less abundant and coarser down-core; (2) the sediments below .35 m are at the stage of sulphate reduction; ferrimagnetic minerals almost vanish and paramagnetic minerals contribute to down-core susceptibility variations, including pyrite as evidenced by high-temperature magnetic susceptibility measurements. However, the susceptibility variations below .35 m of Core MD98-2172 show obvious periodicity, despite the intense effect of reductive diagenesis. Furthermore, the down-core susceptibility variations are coincident with fluctuations in the quantity of fine detrital particles (<8 μm), which may come mainly from the advection of the Indonesia Throughflow (ITF) and/or river input from Timor. Therefore, for Core MD98-2172, susceptibility variation below .35 m, which potentially correspond to fluctuations in the quantity of fine particles, may record the histories of the development of the ITF and precipitation on Timor.