By using bulk samples, rock magnetic measurements were performed to discriminate between pyrrhotite‐ and greigite‐bearing shallow marine sediments that are now uplifted above sea level in southwestern Taiwan. Thermal demagnetization of a composite isothermal remanent magnetization (IRM) was found to be effective in differentiating between the two types of sediments. To check the thermal instability and estimate the true unblocking temperature (TB) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature step during demagnetization. While pyrrhotite‐bearing samples showed unambiguous TB temperature spectra, greigite‐bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instability of greigite is a practical and important clue for its identification. Zero‐field warming of IRM from 5 to 300 K sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation.
Summary We conducted a mineral magnetic study on sand samples from the Taklimakan desert, Tarim Basin in western China. Modern sands were collected at 19 sites in the central and western parts of the Taklimakan desert. We carried out low‐field susceptibility measurements, ARM and IRM acquisitions, high‐ and low‐temperature measurements and hysteresis measurements on these samples. Nearly stoichiometric magnetite is the dominant magnetic mineral of these samples. The presence of titanomagnetite is unlikely, and contributions from a high‐coercivity mineral(s) are minor. Magnetic grain size is estimated to be that of larger pseudo‐single domain or even greater. We compared the hysteresis data against loess samples from both pristine and weathered loess sections from the Chinese Loess Plateau. We found an apparent decreasing grain size trend from the Taklimakan to the central Loess Plateau through the western Loess Plateau. These lines of evidence favour the idea that the Taklimakan desert is one of the potential dust source areas, even though it is located more than 2000 km west of the central Loess Plateau.
Abstract. We reconstructed paleoenvironmental changes from a sediment archive of a lake in the floodplain of the Ilan Plain of NE Taiwan on multi-decadal resolution for the last ca. 1900 years. On the basis of pollen and diatom records, we evaluated past floods, typhoons, and agricultural activities in this area which are sensitive to the hydrological conditions in the western Pacific. Considering the high sedimentation rates with low microfossil preservations in our sedimentary record, multiple flood events were. identified during the period AD 100-1400. During the Little Ice Age phase 1 (LIA 1 -AD 1400-1620), the abundant occurrences of wetland plant (Cyperaceae) and diatom frustules imply less flood events under stable climate conditions in this period. Between AD 500 and 700 and the Little Ice Age phase 2 (LIA 2 -AD 1630-1850), the frequent typhoons were inferred by coarse sediments and planktonic diatoms, which represented more dynamical climate conditions than in the LIA 1. By comparing our results with the reconstructed changes in tropical hydrological conditions, we suggested that the local hydrology in NE Taiwan is strongly influenced by typhoontriggered heavy rainfalls, which could be influenced by the variation of global temperature, the expansion of the Pacific warm pool, and the intensification of El Niño-Southern Oscillation (ENSO) events.
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