Lake-Level Chronology and Climate Change in the Southern Bolivian Altiplano

在低纬度气候变化的记录中，也许没有比这更明显的了：数百英里的石灰华覆盖的海岸线和被切割的石灰岩，它们是由一个巨大的（大约是古邦纳维尔湖的大小）古湖泊所形成的，这个古湖泊曾经占据了玻利维亚南部的乌尤尼/科伊帕萨/普波盆地。这些矿床是几十年来研究的重点，包括美国和法国团队的几次主要的取芯工作。这些湖泊记录引发了一场激烈的争论，即区域热带日照与全球温度在过去控制湖泊和冰川水分平衡方面的相对作用。尽管对湖泊沉积物进行了大量研究，但地质年代学仍然处于混乱状态。特别是，来自乌尤尼盆地的长岩芯测年结果显示，在30-10 ka的关键时期内，两者的一致性很差，一个年代学支持区域夏季日照控制湖面的观点，而另一个年代学则完全反对这一观点。将湖芯记录与区域日照和全球温度等参数联系起来的一个关键挑战是，没有直接的物理证据来证明湖的深度。然而，这样的证据可以在被切割的湖泊沉积物的极好的自然暴露中找到，我们在两个野外季节前第一次进行了调查。曝光是如此令人印象深刻，我们有信心，他们将提供重建湖泊深度和湖泊年表在过去的~120 ka的关键。到目前为止，我们的研究结果表明，湖泊达到高海拔只有两次在过去的120 ka，从17（？）14 ka和108 ~ 94 ka。我们请求提供资金，以继续重建玻利维亚南部乌尤尼/科伊帕萨/普波盆地的湖面变化。我们的目的是利用湖泊沉积物和海岸线凝灰岩的自然暴露，并通过湖相凝灰岩和软体动物的U-Th和14 C测年，建立一个连贯的地层学和湖平面波动年代学。我们还建议采用87 Sr/86 Sr和234 U/238 U的比例在湖泊碳酸盐作为工具，用于跟踪径流的地下水输入到湖泊的比例变化，并推断湖泊深度的核心。我们的修订后的湖泊级年表将反过来构成一个重要的测试，湖泊级阶段和全球温度变化和低纬度季节性日照陆地和海洋之间的潜在联系。

ABSTRACTThere is perhaps no more visible a record of low-latitude shifts in climate than the hundreds of miles of tufa-encrusted shorelines and dissected diatomites laid down by an enormous (about the size of paleo-Lake Bonneville) paleolake that once occupiedUyuni/Coipasa/Poopo basin in southern Bolivia. Appropriately these deposits have been the focus of decades of research, including several major coring efforts by both American and French teams. These lake records are fuel for a heated debate over the relative role of regional tropical insolation versus global temperature in controlling moisture balance in lakes and glaciers in the past.For all the research effort expended on the lake deposits, the geochronology is still in disarray. In particular, dating of long cores from the Uyuni basin show poor agreement for the critical period 30-10 ka, with one chronology supporting the notion of regional summer insolation control on lake level, and the other flatly contradicting it.A key challenge in relating the lake-core records to such parameters as regionalinsolation and global temperature is that there is no direct physical evidence for lake depth. Such evidence, however, is available in the superb natural exposures of dissected lake deposits, which we surveyed for the first time two field seasons ago. The exposures are so impressive that we are confident that they will provide the key to reconstructing lake depths and lake chronology over the last ~120 ka. Our results so far show that the lake reached high elevations only twice in the last 120 ka, from 17 (?) to 14 ka, and from 108 to 94 ka. We request funds to continue our reconstruction of lake-level variations in the Uyuni/Coipasa/Poopo basin in southern Bolivia. Our intent is to establish a coherentstratigraphy and chronology of lake-level fluctuations using the natural exposures of lake sediments and shoreline tufas, and by both U-Th and 14 C dating of lacustrine tufas and mollusks. We also propose to employ 87Sr/86Sr and 234U/238U ratios in lake carbonates as tools for tracking changes in the proportions of runoff to ground-water inputs to the lake, and to infer lake depths from cores. Our revised lake-level chronology will in turn constitute an important test of the potential linkages between lake-level stages and variations in global temperature and low-latitude seasonal insolation over land and sea.