Jia-Fu Zhang

Dating fluvial sediments by optically stimulated luminescence: selection of equivalent doses for age calculation

Fluvial sediments are not an ideal material for optical dating because of insufficient or/and uneven exposure to daylight prior to burial. The equivalent doses for a sample obtained using the single-aliquot regenerative-dose method usually show a large scatter which may be attributed to many factors. Here we developed a technique to evaluate the contributions from each factor. Using this technique, relatively well-bleached aliquots were identified, and the average of their equivalent doses was taken for age calculation.

Preliminary results of long-term slip rates of 1999 earthquake fault by luminescence and radiocarbon dating

For the purpose of deriving the long-term slip rates of different segments along the surface rupture of 1999 Chi-chi earthquake, radiocarbon and luminescence methods (TL and OSL) were applied to the tectonically influenced geomorphic features and deposits. We obtained six age estimates on pottery, driftwood and fluvial sediments. We found that the OSL signals of the samples collected from the coarse fluvial deposits had not been totally bleached due to rapid deposition. Based on the cumulative vertical offsets, the vertical slip rates were computed for each site. Our results reveal that the slip rate of the northern segment is relatively higher than that of the southern segment, which is consistent with the coseismic offsets that occurred in the 1999 earthquake.

Optically stimulated luminescence dating of cave deposits at the Xiaogushan prehistoric site, northeastern China

The Xiaogushan cave site is one of the most important prehistoric sites in North China. The stone and bone artifacts found in the cave are similar to European contemporaneous artifacts. Cave deposits consist of five layers that have been dated from 46,353 ± 1179 to 4229 ± 135 cal. yr BP, using radiocarbon dating techniques on charcoal and bone samples collected from Layers 2-5. In this paper, optically stimulated luminescence (OSL) techniques were applied to date six samples taken from Layers 1-3. The luminescence properties of the fine-grained and coarse-grained quartz extracts indicate that the materials are suitable for OSL dating using a single-aliquot regeneration-dose (SAR) protocol. The OSL ages obtained are broadly consistent with the stratigraphy and the associated calibrated radiocarbon ages. The dating results show that the cave was first occupied by humans about 70 ka. The human occupation of the cave may be related to climate change. An occupation hiatus is inferred to between ∼ 17 to ∼ 10 ka. The stone and bone artifacts found in Layers 2 and 3 may indicate the Middle to Upper Paleolithic transitions in the region.

Assessment of bleaching of K-feldspar grains

Based on the difference in the bleaching rate between IRSL and TL signals, a new method of assessing the bleaching of sedimentary feldspar is proposed. All measurements for De and the bleaching factor (normalized ratio of IRSL to TL) are carried out on the same grain. K-feldspar grains from two sediments, a marine and an alluvium, from Hong Kong were tested. Their bleaching factors indicate that bleaching of the marine sediment was not uniform, whereas the latter was relatively homogeneous. Furthermore, the relatively well-bleached grains in each sample were identified.

Improvement of the equivalent dose determination using aliquots of potassium feldspar

A procedure of preheat/measurement, dose/preheat/measurement and preheat/measurement is proposed, in which all the measurements for the equivalent dose determination are made on a single aliquot of K-feldspar sample with the additive-dose method. Correction for the loss of IRSL signal due to preheat/measurement cycles is made using correction factors determined both at the beginning and the end of the measurement cycle. It is suggested that the optimal preheat condition for the sample can be determined by comparing the correction factors obtained from the two stages of analysis, as a function of preheat temperature. The most appropriate preheat for our K-feldspar sample is at 235°C for 10 min.

The world’s earliest Aral-Sea type disaster: the decline of the Loulan Kingdom in the Tarim Basin

Remnants of cities and farmlands in China’s hyperarid Tarim Basin indicate that environmental conditions were significantly wetter two millennia ago in a region which is barren desert today. Historical documents and age data of organic remains show that the Loulan Kingdom flourished during the Han Dynasty (206 BCE–220 CE) but was abandoned between its end and 645 CE. Previous archaeological, geomorphological and geological studies suggest that deteriorating climate conditions led to the abandonment of the ancient desert cities. Based on analyses of lake sediments from Lop Nur in the eastern Tarim Basin and a review of published records, we show that the Loulan Kingdom decline resulted from a man-made environmental disaster comparable to the recent Aral Sea crisis rather than from changing climate. Lop Nur and other lakes within the Han Dynasty realm experienced rapidly declining water levels or even desiccation whilst lakes in adjacent regions recorded rising levels and relatively wet conditions during the time of the Loulan Kingdom decline. Water withdrawal for irrigation farming in the middle reaches of rivers likely caused water shortage downstream and eventually the widespread deterioration of desert oases a long time before man initiated the Aral Sea disaster in the 1960s.