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

玻利维亚南部的Uyuni/Coipasa/Poopo盆地曾经是一个巨大的(大约有博内维尔古湖大小的)古湖沉积的数百英里长的石灰岩覆盖的海岸线和解剖的硅藻土，也许没有什么比这更明显的记录了低纬气候的变化。这些矿床恰如其分地成为了数十年来研究的重点，包括美国和法国团队的几项主要取心努力。这些湖泊记录引发了一场激烈的辩论，争论的焦点是过去区域热带日照和全球温度在控制湖泊和冰川水分平衡方面的相对作用。尽管在湖泊沉积上花费了所有的研究努力，但地质年代学仍然处于混乱状态。尤其是，乌尤尼盆地的长岩芯测年结果显示，30-10ka这一关键时期的一致性很差，一个年表支持湖平面上区域夏季日照控制的概念，而另一个年表则与此背道而驰。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表明湖泊深度。然而，这些证据可以在解剖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对其进行了调查。这些暴露是如此令人印象深刻，我们相信它们将为重建过去约120ka的湖泊深度和湖泊年代学提供关键。到目前为止，我们的结果表明，在过去的120ka期间，该湖只有两次达到高海拔，从17(？)到14ka，从108ka到94ka。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/普波盆地的湖泊水位变化。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，以及湖泊凝灰岩和软体动物的U-Th和14C测年，建立一个连贯的湖泊水位波动的地层学和年代学。我们还建议利用湖泊碳酸盐中的87Sr/86Sr和234U/238U比值作为追踪径流与地下水输入到湖泊的比例变化的工具，并从岩心推断湖泊深度。我们修订的湖泊水位年表将反过来对湖泊水位阶段和全球温度变化以及陆地和海洋低纬度季节性日照之间的潜在联系进行重要测试。

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.