沙漠边缘厚层黄土上部光释光测年的初步研究*

对处于中国西北黄土高原沙漠边缘曹岘厚层黄土剖面上部进行了光释光年代初步研究。实验结果表明,45~63μm石英颗粒的光释光信号以快组分为主,适合应用单片再生剂量法(SAR)测年。在自然和再生剂量预热温度为260℃持续10秒,检测剂量预热温度为160℃持续0秒的条件下,石英单片再生剂量法获得的光释光年龄随样品深度而增加。但是,剖面底部S₁古土壤和L₂顶部黄土样品的光释光年龄仅为69.1±5.5ka和72.0±4.6ka,表现出30 ％ ~50 ％ 的年龄低估。利用多片再生剂量法(MAR)得到的年龄与SAR方法所得年龄无显著差异,即出现了类似的年龄低估现象。在20.1~18.8ka时段内,该剖面的沉积速率超过500cm/ka。末次冰盛期的寒冷气候,与沙漠的距离以及近邻黄河的地貌特点共同造成了如此高的沉积速率。     

Chronology of Upper Pleistocene sequences at Sidi Messaoud (wadi Noun, southwestern Morocco) based on C, optical and U-series dating

The Sidi Messaoud site provides a 40 m thick sedimentary infilling of great importance for the study of environmental changes and landscape evolution during the Upper Pleistocene in the Western Sahara. In this study, chronological results obtained by the ¹⁴C, U-series and optical dating techniques are compared. Each individual age, depositional environment and post-deposition history of the various samples are evaluated, in order to explain the methodological reasons for discordances and concordances. Stratigraphic and palaeoenvironmental implications for the last 120 ka history in southwestern Morocco are also inferred: the wadi Noun gorges had been incised before MIS 5, and later, during MIS 3, the valley has been filled in with a mean sedimentation rate of 1 m/ka.     

Monsoon triggered formation of Quaternary alluvial megafans in the interior of Oman

A vast bajada consisting of coalescing low-gradient (< 0.3°) alluvial fans exceeding 100 km in length formed along the southwestern margin of the Oman Mountains. It comprises an old fan sequence of inferred Miocene to Pliocene age termed Barzaman Formation, diagenetically highly altered to dolomitic clays, and a thin veneer of weakly cemented Quaternary gravels. A combination of remote sensing, lithological analyses and luminescence dating is used to interpret the complex aggradation history of the Quaternary alluvial fans from the interior of Oman in the context of independent regional climate records. From satellite imagery and clast analysis four fans can be discerned in the study area. While two early periods of fan formation are tentatively correlated to the Miocene–Pliocene and the Early Pleistocene, luminescence dating allows the distinction of five phases of fan aggradation during the Middle–Late Pleistocene. These phases are correlated with pluvial periods from Marine Isotope Stage (MIS) 11 through 3, when southern Arabia was affected by monsoonal precipitation. It is concluded that the aggradation of the alluvial fans was triggered by the interplay of increased sediment production during arid periods and high rainfall with enhanced erosion of hillslopes and transport rates during strong monsoon phases. However, the lack of fine-grained sediments, bioturbation and organic material implies that although the Quaternary fans are sourced by monsoonal rains they formed in a semi-arid environment. Thus, it appears that, in contrast to the Oman Mountains, the interior was not directly affected by monsoonal precipitation.     

Climate controls on late Pleistocene landscape evolution of the Okhombe valley, KwaZulu-Natal, South Africa

Hillslopes in central and western parts of KwaZulu-Natal, South Africa are often mantled by colluvial sediments of the Masotcheni Formation. These sediments have accreted in response to several cycles of deposition, pedogenesis and incomplete erosion. Climatic controls on these cycles are incompletely known. Results from fieldwork, micromorphology, stable carbon isotope analysis and Optically Stimulated Luminescence dating of Masotcheni Formation sediments from Okhombe valley in the Drakensberg foothills are combined. Deposition in the area had at least 11 phases, starting before 42 ka and ending before 0.17 ka. The first six deposits (from before 42 ka to after 29 ka) resulted from the interplay between slope processes and fluvial redistribution under cold conditions. Solifluction was the most important slope process. No deposits have been found from the Last Glacial Maximum, arguably because this period was too dry. The last five deposits (from about 11 ka to before 0.17 ka) resulted from fluvial redistribution of upslope material and older deposits under increasing precipitation. Current extreme gully erosion in the Masotcheni Formation indicates a lack of available upslope material, leaving downslope deposits as the only sediment source for fluvial redistribution. This model for landscape response to climate change may be able to explain how climate controlled landscape processes in other Masotcheni Formation sites in KwaZulu-Natal. In the research area and elsewhere, this proposition may be tested with numerical landscape evolution models.     

Geoarchaeology of Pleistocene open-air sites in the Vila Nova da Barquinha-Santa Cita area (Lower Tejo River basin, central Portugal)

This paper aims to provide insight into human occupation and landscape change during the Pleistocene in a central area of the Lower Tejo basin (Portugal). Detailed geomorphological mapping, coupled with lithostratigraphy, sedimentology and luminescence dating, supports the identification of a complete terrace staircase sequence. It consists of six gravely terraces located below the culminant (Pliocene) basin unit. A chronological framework for the sedimentary sequences and associated human industries is proposed and correlated with marine oxygen isotope stages (MIS): T1 terrace, not dated; T2, not dated; T3, >300 ka; T4, ∼300-160 ka (MIS8, MIS7 and MIS6); T5, ∼136-75 ka (MIS5); T6, ∼62-30 ka (MIS3); colluvium and aeolian sands, ∼30-14 ka (MIS2); valley fill deposits, ∼14 ka to present (MIS1). The oldest artefacts were found at the base of the T4 terrace, with the local stratigraphic level dated to ≥175 ± 6 ka (Middle Pleistocene). The lithic assemblages collected from distinct stratigraphic levels (T4, T5 top, T6 terraces and colluvium) are characterized by the predominance of opportunistic technological choices, a feature that can be attributed partly to the preferential exploitation of the available raw material, dominated by local-sourced quartzites and quartz pebbles. The adaptation to local raw material (texture and volume), together with subsistence patterns and behaviours, could explain the rarity of Acheulian types (handaxes and cleavers) and picks in the T4 terraces of the Tejo tributaries; this is in contrast to the same terrace of the Tejo valley, in which these types are found. Interpretation of the environmental conditions (controlled by climate and glacio-eustatic sea-level changes) affecting the hunter-gatherer human groups is also presented.     

内蒙古狼山地区晚第四纪泥石流发育特征及其构造意义

内蒙古狼山山前断裂活动强烈,致使泥石流灾害发育,研究狼山山前泥石流的发育特征对深入了解晚第四纪以来狼山的构造抬升具有重要意义。通过对狼山山前泥石流堆积特征和地貌特征的调查,发现研究区内泥石流沉积物颗粒粗大,分选磨圆均较差,胶结及风化程度随年代的变老而变强,山前堆积台地从老到新由上到下呈阶梯状排列。利用光释光(OSL)对厘定的4期古泥石流堆积进行测年,4期泥石流分别形成于113.33±12.31ka BP、65.33±6.71ka BP、34.67±3.52ka BP、5.00±0.57ka BP,进而计算出4期构造活跃期狼山中段的平均下切速率分别为0.44m/ka、0.93m/ka、1.11m/ka、2.8m/ka。水流下切速率的增加来源于构造隆升的驱动,由此表明狼山晚第四纪以来可能处于一个加速隆升的阶段;113.33ka以来水流下切速率的空间分布揭示了狼山中段存在隆升速率由西高东低向东高西低转换的特点。本研究对重建晚第四纪阴山西段隆升过程和河套盆地古地理格局具有重要意义。     

Single grain optical dating of glacigenic deposits

Determining the age of glacigenic sediments is difficult for many geochronological methods because of the lack of suitable materials for analysis. Luminescence dating can be applied to the mineral grains making up the glacigenic sediments. However a major source of uncertainty in previous studies has been whether the mineral grains were exposed to sufficient daylight prior to deposition for the luminescence signal to be reset. Measurements of the optically stimulated luminescence signal from single sand-sized quartz grains offers the potential for explicitly identifying if a sediment contains grains that were not exposed to sufficient daylight to reset their signal. Statistical analysis of the resulting data can then reject those grains to allow the age of the sample to be determined. This study is the first to apply single grain optical dating to glacigenic sediments, and demonstrates the issues involved by analysis of samples from Chile and Scotland. Ages from 2.4±0.5 to 17.3±1.5 ka are produced. Comparison of the results with independent age control suggests that the ages are reliable. The results also show that the extent of bleaching at deposition varies considerably from one sample to another. For the most incompletely bleached sample, luminescence measurements based on the average of many hundreds or thousands of grains would have overestimated the age of the sample by 60 ka, but the single grain method proposed here was able to reliably date it.     

Radiometric dates for the Middle Palaeolithic sequence of Payre (Ardèche, France)

The archaeological site of Payre (France) yielded Middle Palaeolithic layers with Neanderthal remains, which are partly bracketed by two stalagmitic flowstones. To obtain a precise chronological framework for the human occupation and the faunal remains of this site, several dating methods were used: uranium-series (U-series by alpha spectrometry and TIMS) and electron spin resonance (ESR) on stalagmitic flowstones, combined ESR/U-series on teeth and bones, and thermoluminescence (TL) on burnt flints. Most of the ages obtained for levels H to C range from 300 to 140 ka. The age-spread obtained for the anthropic (burnt flints) and faunal remains (bones and teeth) suggests that the dated levels represent several human occupation periods, near the transition from MIS8 to MIS7 (levels G–F) and from MIS6 to MIS5 (levels D–E).     

Possible resetting of quartz OSL signals during earthquakes—Evidence from late Pleistocene injection dikes, Dead Sea basin, Israel

Clastic dikes are formed either by passive deposition of clastic material into pre-existing fissures (depositional dikes), or by fracturing and injection of clastics during earthquakes (injection dikes). We proposed to use optically stimulated luminescence (OSL) dating to distinguish between the two modes of formation and hypothesized that (1) depositional dikes filled from above show OSL ages younger than the host rock; and (2) injection dikes filled from below show the same OSL ages as that of the host rock. We studied the mechanisms of clastic-dike formation and their ages within the seismically active Dead Sea basin, where hundreds of dikes crosscut the late Pleistocene (70–15 ka) lacustrine sediments of the Lisan Formation. Field observations and analysis of magnetic tensors show unequivocally that most of these dikes were emplaced by injection, inferred to be due to seismically triggered fluidization–liquefaction during earthquakes. Twenty-eight samples were collected from the Lisan source material and dikes that, based on field observations, are unmistakably either depositional dikes or injection dikes.

Quartz single aliquot OSL ages of the source Lisan layers are between 43 and 34 ka, and are typical for the Lisan Formation. The ages of both depositional and injection dikes are between 15 and 17 ka, younger than the Lisan host rock. Depositional dikes show a highly scattered distribution of single grain ages, suggesting several episodes of infill. Single grain ages of injection dikes are of latest Pleistocene to Holocene, and do not contain recently bleached grains that infiltrated from above. These results imply that the OSL signals were reset at the time of fluidization–liquefaction and buildup of fluid pressure within the injection dikes. If this resetting mechanism has a physical ground, then OSL dating is an important tool for constraining the ages of earthquake-induced injection dikes and recovering paleoseismic data from them.     

Rapid in situ assessment of luminescence-bleaching depths for deriving burial and exposure chronologies of rock surfaces

Recent developments in luminescence dating offer new ways to date exposure and burial durations of rocks. The new rock surface dating methods ideally require high-resolution data, faster sample preparation and measurement times, and field screening methods to select samples with appropriate luminescence characteristics and bleaching histories. Presented here is a demonstration of an EMCCD (electron multiplying charge coupled device) based system capable of imaging high-resolution infrared stimulated luminescence (IRSL) and infrared-photoluminescence (IRPL) from rock samples. The IRPL can be detected at both 880 nm and 955 nm. Using this instrument, the entire luminescence-depth profile can be reconstructed by imaging a single surface cut perpendicular to the exposed rock face. We demonstrate the possibility of reconstructing luminescence-depth profiles suitable for rock surface dating from large (cm-scale) rock samples, without using a regeneration dose for normalisation of the natural luminescence signals. Based on the different bleaching characteristics of the IRSL and IRPL emissions at 880 nm and 955 nm, we show that it is possible to gain reliable estimates of bleaching depths from measurement of as few as two images of the IRPL signal (one for each emission), or from measurement of the IRSL decay curve. We thus by-pass laborious sample preparations and the need for a gamma source to estimate the bleaching depth, thereby extending the 2D luminescence-depth profile imaging technique to other laboratories that lack access to ionising radiation facilities. This study also makes a significant progression towards development of a field instrument for in situ relative exposure dating, and sample screening for rock surface burial dating.