Multiple phases of North African humidity recorded in lacustrine sediments from the Fazzan Basin, Libyan Sahara

The Fazzan Basin of south-west Libya is at present arid with less than 20 mm of rainfall per annum. However, regionally extensive limestones, lacustrine sands and coquina (fossiliferous carbonate rock) deposits show that the Fazzan Basin previously contained a large palaeolake, indicating that the climate in the past was more humid. Optically stimulated luminescence (OSL) dating techniques have been applied to key lacustrine deposits within the basin in an attempt to provide an internally consistent chronology for this humidity record. Results indicate that palaeolake sediments within the Fazzan Basin record a very long history of palaeohydrological change, ranging from present day arid conditions to humidity capable of sustaining a lake with an approximate area of 76,250 km². The existence of humid periods in mid oxygen isotope stage 5 and the early Holocene is confirmed. An older lacustrine event, tentatively correlated to oxygen isotope stage 11, is also recognized. In addition, evidence is presented for at least two humid phases beyond the age range over which the conventional OSL dating technique is applicable. This study demonstrates that OSL dating of palaeolake sediments within the Fazzan Basin offers the potential to provide a detailed record of North African humidity spanning several glacial–interglacial cycles.     

古湖相碳酸盐沉积和化石贝壳的87Sr/86Sr及其意义

位于极端干旱区的额济纳盆地，是我国内陆地区代表性的盆地之一。这里不仅具有典型的大陆性气候、脆弱的生态系统，而且在平缓的戈壁滩下面，沉积有厚达200-400余米的古湖相地层。盆地东南与最高达近500m沙丘的巴丹吉林沙漠比邻，构成了特征性的盆地-沙漠系统，是世界少有的自然景观，也是目前研究热点之一。 考察研究工作揭示，巴丹吉林沙漠在蹑今30000a左右存在一湿润时期（Wunnemann等，1998；杨小平，2002；杨小平等，2003），并与临区腾格里沙漠相应时期发育的大湖期可以对比（Zhang等，2002，2004）。 新近的考察工作发现，在盆地东南部古日乃与巴丹吉林沙漠过渡区，存在有多级古沙丘被水夷平的、高度不等的平台，在这些平台之上，保存有水体快速蒸发环境条件下沙粒物质被钙质胶结的坚硬、类似砂岩的顶盖，其高度在大范围内可以对比，同时伴有湖相碳酸盐沉积、根管等。这些地貌、沉积与盆地北部沙砾石层中大量出现的贝壳化石和湖相地层，构成了古湖泊存在的地质证据。 14C测年结果表明，分布于第二级平台上的碳酸盐结核其形成年代为距今25000a前（25630±270a、25530±230a、25640±220a，半衰期5568a，下同），掰鳃类贝壳化石的年代为距今28000a(28530±430a、28630±320a、28780±340a、28560±280a)。 对这两组样品进行87Sr/86Sr分析结果表明（表1），碳酸盐沉积的87Sr/86Sr比值变化较为显著而贝壳化石的87Sr/86Sr比值变化相对较小，且贝壳化石的87Sr/86Sr比值的平均值小于碳酸盐沉积的87Sr/86Sr比值。 由于贝壳化石和碳酸盐沉积的87Sr/86Sr比值在一定意义上代表了古湖水的锶同位素比值，而87Sr/86Sr比值又与湖泊水体盐度相关（刘秀明等，2000），因此可以推测，贝壳化石所代表的古湖泊发育时期的水体较碳酸盐结核形成时期的盐度要低，也就是说古湖泊发育早期的为淡水，后期水体盐度有所增高。这是与贝壳化石本身代表的淡水种属相一致。由于后期湖泊水拉的快速下降和水体的快速蒸发，使得碳酸盐结核沉积的87Sr/86Sr比值增高，而这种古湖泊水位的快速下降和结核的形成是直接与气候变化相关联的。反映了该时期额济纳盆地古湖泊高湖面的存在和气候快速变化的历史。     

Late Pleistocene palaeolake in the interior of Oman: a potential key area for the dispersal of anatomically modern humans out‐of‐Africa?

Relict fluvial and lacustrine deposits in the interior of Oman near Saiwan consist of waterlain breccias with pebble imbrications and current ripples, covered by algal laminites containing calcified reeds and charcoal pieces. Geomorphological evidence suggests that the palaeolake covered a maximum surface of 1400 km² and had a maximum depth of about 25 m. Optically stimulated luminescence (OSL) and thermally transferred OSL dating indicate that this palaeolake existed sometime between 132 and 104 ka. The rich archaeological evidence in the area suggests substantial human occupation, possibly at the time when Palaeolake Saiwan existed. Interestingly, the lithic traditions of the archaeological material show no clear relation to coeval findings from neighbouring areas, putting a question mark on the origin of the culture found at Saiwan. This finding raises questions concerning the origin of the population inhabiting Saiwan, as the site lies along one possible route for the dispersal of anatomically modern humans out‐of‐Africa. Copyright © 2011 John Wiley & Sons, Ltd.     

Disentangling late quaternary fluvial and climatic drivers of palaeohydrological change in the Najaf Sea basin,Western Iraq

The water resource provided by lake basins in the western desert of Iraq is important for human occupation of areas outside the Tigris-Euphrates floodplain, both in the past and into the future. This paper presents the first geomorphological and geochronological study of the date of formation of the Najaf Sea and the only such study of any lake basin to the west of Mesopotamia. Geomorphological shoreline features and a palaeochannel linking to the Euphrates were studied and dated using optically stimulated luminescence (OSL) and radiocarbon dating. Provenance was determined using heavy mineral analysis. Past environments in the Najaf Sea were reconstructed by molluscan analysis. The earliest OSL ages date from c. 30 000 and 22 000 years ago and seem to predate lake formation. Younger OSL ages date the highest lake level at c. 19 m asl to between 1620–1760 AD (base) to 1906–1974 AD (near surface). The radiocarbon ages are affected by a freshwater reservoir effect, but the maximum ages recorded for either of the c. 15 m and c. 17 m asl shorelines are c. 800 cal. BC. This predates the first archaeological sites in the Najaf basin and is similar to maximum ages of c. 850 and c. 1100 cal. BC from the associated palaeochannel. This timing does not seem to be linked to a humid climate event. We therefore conclude that the establishment of the Najaf Sea in the Najaf basin occurred as a result of an avulsion event within the Euphrates system that diverted flow to the basin. The trigger for this avulsion event likely related to rapid sediment accumulation and may have been either autogenic or driven by human activity. This study therefore suggests that Najaf Sea formation facilitated human expansion beyond the Tigris- Euphrates floodplain and occurred due to avulsion of the Euphrates.     

Late Quaternary lake-level fluctuations in the Mababe Depression: Middle Kalahari palaeolakes and the role of Zambezi inflows

A systematic drilling and optical dating programme on Middle Kalahari beach ridge (relict shoreline) sediments has enabled the identification of multiple episodes of lake high stands of an extensive palaeolake system at the terminus of the Okavango Delta, northern Botswana. This paper presents 23 ages from the Mababe Depression and establishes four shoreline construction phases in the late Quaternary coeval with other sub-basin lake high stands (Lake Ngami). These synchronous lake phases result from a coalescence of the sub-basins into a unified palaeolake, Lake Thamalakane, covering an area of  32,000 km². Six additional ages are also presented from the Chobe enclave to the north of the basin where shoreline ridges were emplaced at the same time as Lake Thamalakane phases. This suggests that increased flow in the Chobe and Zambezi system significantly contributed to the Middle Kalahari lake phases in both the post-glacial and Holocene periods. The integration of these new data and their compatibility with other regional and tropical palaeo-archives is discussed in the light of understanding Quaternary climate drivers within the Kalahari.     

Evolution of a Late Pleistocene palaeolake in Dali Nor area of southeastern Inner Mongolia Plateau,China

Lithological,geochronological,granulometric,and scanning electron microscope(SEM)analyses of eight lacustrine outcrops indicate the existence of a large palaeolake in Dali Nor area during 149-24.6 ka(MIS5-3)and a small one in upstream of the Xilamulun River during 12.96-11.69 ka.The large palaeolake recorded its highest water level above 1300 m during the MIS3 and covered the whole Dali Nor basin and upstream of the Xilamulun River within an area of about 3000 km~2.However,the small palaeolake with a highest water level above 1200 m,covered an area of about 600 km~2.Besides the reconstruction of palaeolake areas,this study also looked into the evolutionary process of the palaeolake during the Late Pleistocene.Our data provide insights on the timing of appearance of the large palaeolakes and a better understanding of the mechanism responsible for variations in the environment conditions of dry lands in middle latitudes.