西藏西部聂尔错古湖近20 000-2 000年的微体古生物与环境及气候变化

西藏西部聂尔错古湖37/ZK7的钻孔剖面深10m，含丰富的介形类和轮藻类等微体水生生物化石。介形类数量多，但分异度低。频繁出现、含量较多的是LeucocytheremirabilisKaufmann和LimnocytheredubiosaDaday，其他是为数不多的Limnocytherellina，Eucypris和Candona。轮藻类产出层位多，但其藏卵器保存不好，多系营养体碎片及卵膜。据介形类生态环境分析，结合剖面铀系测年和沉积物特征及相关地化背景值等，聂尔错古湖在近20000-2000年间的古环境变化大致情况如下：①距今20000年前后，湖水较深，水温较低，水质微咸，气候较凉湿转暖湿。②约18700-18100aBP，湖水蒸发量较大，湖泊收缩，湖水逐渐变浅，含盐度增加，气候偏暖干。③约18100-11800aBP，气候冷湿、湖水较深，水温偏低，水质较淡，但是其间有多次气候波动，随着气候的波动，湖泊水体深度和水化学性质发生变化，显示湖区气候较冷湿与凉偏湿频繁交替，晚期凉偏干。④约11800-10000aBP，气候偏暖湿（11800-11000aBP）转冷干（11000-10000aBP），湖泊生态环境恶化，介形类数量急剧减少，应是对全球仙女木事件的响应。⑤10000aBP进入全新世后，气候偏暖湿（10000-8700aBP，7000-4800aBP）间偏冷湿（8700-7000aBP）；4800-3000aBP转为偏暖干（4800-4600aBP，4400-4300aBP，3800-3000aBP）与偏冷湿（4600-4400aBP，4300-3800aBP）交替；3000-2100aBP以冷干气候为主，湖泊趋于干涸。Micropaleontology,environmentalandclimaticchangesoftheNyêrCopaleolakeinwesternTibetbetween20000-2000aBP

Micropaleontology, environmental and climatic changes of the Nyêr Co paleolake in western Tibet between 20 000-2000 a BP

The section of the 10m deep 37/ZK7 hole in Nyêr Co paleolake, western Tibet, contains abundant aquatic microfossils such as ostracods and charophytes. Of these, the ostracods are the most abundant group but have a low diversity. The ostracods that occur frequently and have a high abundance are Leucocythere mirabilis Kaufmann and Limnocythere dubiosa Daday, while others including Limnocytherellina, Eucypris and Candona that are of much less abundance. Charophytes occur in many horizons, but their oogonia are poorly preserved and what have been preserved are mostly the fragments of vegetal bodies and the membranes of ova. According to the analyses of the eco environment of ostracods, combined with the U series dating and sedimentary data of the section, as well as the relevant geochemical background values, the paleoenvironmental changes of the Nyêr Co Paleolake at 20000-2000 a BP were largely as follows. ① Around 20000 a BP, the lake water was relatively deep, the water temperature relatively low and the water quality slightly salty. The climate was relatively cool wet and then became warm wet. ② During 18700-18100 a BP, the lake water had a high evaporation quantity, so that the lake area shrank and the lake became shallow gradually. The salinity increased and the climate was slightly warm dry. ③ During 18100-11800 a BP, the climate was cold wet. The lake water was relatively deep, the water temperature slightly low and the water quality relatively tasteless.However, there were several climatic fluctuations and the lake depth and hydrochemical properties changed with climatic oscillations in this period, suggesting that the climate in the lake district was characterized by a repeated alternation of relatively cold wet and cool slightly wet and became cool slightly dry at the late stage. ④ During 11800-10000 a BP, the climate became slightly warm wet(11800-11000 a BP) and then became cold dry(11000-10000 a BP). The eco environment of the lake deteriorated and the number of ostracods decreased abruptly, which should be the response to the global Dryas event in the study area. ⑤ At 10000 a BP in the Holocene, the climate was slightly warm wet(10000-8700 aBPand 7000-4800 aBP), with slightly a cold wet climate (8700-7000 aBP); during 4800-3000 a BP, the climate became alternately slightly warm dry(4800-4600, 4400-4300 and 3800-3000 a BP) and slightly cold wet(4600-4400 and 4300-3800 a BP); and during 3000-2100 a BP, the climate was mainly cold dry and by then the lakes had been nearly dried up.