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Late Glacial and Deglacial Fluctuations of Mono Lake, California

机译：加利福尼亚莫诺湖的晚期冰川和冰川消融波动

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摘要

Anthropogenic climate change risks significant changes in the global distribution of precipitation. Across the western United States, modelling studies show significant reductions in wetness that imply weighty societal and ecological impacts. But the validity of the model projections need to be ground-truthed. Paleo-hydroclimate data are useful reference points to assess a model's ability to hindcast past hydroclimate. If the hindcast matches the paleodata, it brings confidence to a model's ability to predict future hydroclimatic change.;The foremost metric of hydroclimate in the geologic record is the surface area of lakes in hydrologically closed basins. In such basins, a lake's surface area is determined by the balance between precipitation and evaporation. The lake will expand when the balance is positive, and it will contract when the balance is negative.;In this dissertation, I develop a 25-9 ka record of lake fluctuation from the Mono Basin, a hydrologically closed basin in east-central California. I deduced the fluctuations using three pieces of evidence: stratigraphy; geomorphology; and geochronology. These pieces of evidence were determined from a study of the Mono Basin's Late Pleistocene lithostratigraphic unit: the Wilson Creek Formation.;There are 19 tephra intercalated in the Wilson Creek tephra. They are named by their reverse depositional order (Ash 19 is the oldest and Ash 1 is the youngest). Uncertainty on their ages cause confusion as to the paleo-hydroclimate record of the Mono Basin. The age of Ash 19, for example, is important because its deposition marks the onset of relatively high lake levels that occurred during the last glaciation. There are two principal interpretations of Ash 19's age: 40 ka, which is based on lacustrine macrofossil 14C data; and 66 ka, which is underpinned by paleomagnetic intensity data. In chapter 2, I tested these end-member interpretations. I used the U/Th method to date carbonate deposits that underlie and cut across Ash 19. The U/Th data show that Ash 19 must have been deposited between these two dates: 66.8 +/- 2.8 ka; and 65.4 +/- 0.3 ka. These dates are, therefore, more consistent with the 66 ka interpretation of Ash 19's age. Thus the onset of relatively high lake levels in the Mono Basin corresponds with the rapid drawdown of atmospheric CO2 during Marine Isotope Stage 4. The coincidence between the drop in atmospheric CO2 and lake level rise is suggestive of a causal link.;In chapter 3, I determined Mono Lake's fluctuations 25-9 ka. This time encompasses three climatic intervals: the coolest time of the last glaciation, termed the Last Glacial Maximum (LGM); the period corresponding to the rapid termination of the last glaciation, termed the deglaciation; and the early Holocene, a period of inordinate warmth that immediately followed the last glaciation's termination. In this study, I used stratigraphic and geomorphic evidence in conjunction with 14C and U/Th dates. I measured the 14C dates on bird bones and charcoal. And I measured the U/Th dates on carbonates. Together the data showed that the lake's rises and falls concurred with North Atlantic climate. Periods of aberrant warmth in the North Atlantic concurred with low stands of Mono Lake. On the other hand, extreme cooling in the North Atlantic correlated with Mono Lake high stands. The timing of these lake fluctuations also corresponds with variations in other tropical and mid-latitude hydroclimatic records. The global harmony in the hydroclimatic records suggests a unifying conductor. I hypothesize that the conductor is tropical atmospheric circulation.;In chapter 4, I present evidence on the peculiar case of an extreme low stand of Mono Lake. The low stand is dubbed the "Big Low". The principal evidence underpinning the Big Low derives from a sedimentary sequence exposed along the canyon walls of Mill Creek. The strata show that the lake fell below 1,982 m between the deposition of Ashes 5 and 4---making this low stand the lowest recognized level of Mono Lake during the Wilson Creek Formation. Observations from dispersed sequences corroborate this interpretation. And three data constrain the age of the Big Low to be between ~24.4-20.5 ka: a carbonate U/Th date on a littoral conglomerate associated with the Big Low; a carbonate U/Th date that underlies Ash 4; and a carbonate U/Th date that cuts across Ash 5. Thus the interval that the Big Low must occur within encompasses the LGM. The timing of this low stand, therefore, corresponds with summer temperature minima, suggesting that the fall was due not to an increase in evaporation but due to a decrease in precipitation. This finding is counter to conventional wisdom: that the LGM was a relatively wet interval. (Abstract shortened by ProQuest.).
机译：人为气候变化有可能使全球降水分布发生重大变化。在整个美国西部，模型研究表明，湿度显着降低，这意味着沉重的社会和生态影响。但是，模型预测的有效性需要得到证实。古水文气候数据是评估模型后预报过去水文气候能力的有用参考点。如果后生与古数据相匹配，则将为模型预测未来水文气候变化的能力增添信心。地质记录中最重要的水文气候指标是水文封闭盆地中的湖泊表面积。在这样的盆地中，湖泊的表面积取决于降水和蒸发之间的平衡。当天平为正时，湖泊将扩张，当天平为负时，湖泊将收缩。;本文，我从加利福尼亚中东部的水文封闭盆地莫诺盆地开发了一个25-9 ka的湖泊波动记录。 。我使用三项证据推论了这种波动：地层学；地貌和年代学。这些证据是根据对莫诺盆地晚更新世岩相地层学单元威尔逊溪地层的研究确定的；威尔逊溪地底中插有19根地弗拉。它们以相反的沉积顺序命名（灰烬19最老，灰烬1最年轻）。他们年龄的不确定性引起了莫诺盆地古水文气候记录的混乱。例如，灰烬19的年龄很重要，因为它的沉积标志着最后一次冰期期间相对较高的湖泊水位的开始。 Ash 19的年龄有两种主要解释：40 ka，这是基于湖相大化石14C数据得出的； 66 ka，这是由古磁强度数据支持的。在第二章中，我测试了这些最终成员的解释。我使用U / Th方法对位于灰烬19下方并穿过灰烬19的碳酸盐沉积物进行了测年。U/ Th数据表明，灰烬19必须已经沉积在这两个日期之间：66.8 +/- 2.8 ka；以及和65.4 +/- 0.3 ka。因此，这些日期与Ash 19年龄的66 ka解释更加一致。因此，莫诺河流域相对较高的湖泊水位的发生与海洋同位素第4阶段大气CO2的快速下降相对应。大气CO2的下降与湖泊水位上升之间的吻合表明存在因果关系。我确定了Mono Lake的波动幅度为25-9 ka。这段时间包含三个气候间隔：最后一次冰川降温的最冷时间，称为最后一次冰川最大值（LGM）；与最后一次冰期迅速终止相对应的时期，称为冰消期；和早期的全新世，这是最后一次冰河终止后的一个非常温暖的时期。在这项研究中，我结合14C和U / Th日期使用了地层和地貌证据。我测量了鸟骨头和木炭上的14 C日期。我用碳酸盐测量了U / Th日期。数据一起显示该湖的起伏与北大西洋的气候一致。北大西洋的异常温暖时期与莫诺湖的低潮同时发生。另一方面，北大西洋的极端降温与莫诺湖的高海拔相关。这些湖泊波动的时间也与其他热带和中纬度水文气候记录的变化相对应。水文气候记录中的全球和谐表明是统一的导体。我假设导体是热带大气环流。在第四章​​中，我提供了关于莫诺湖极低水位的特殊情况的证据。低位被称为“大低位”。大低谷的主要证据来自沿着米尔克里克峡谷壁暴露的沉积层序。地层表明，该湖在灰烬5和灰烬4的沉积之间落在1,982 m以下-使得该低水位是威尔逊溪组期间Mono湖公认的最低水平。来自分散序列的观察证实了这种解释。三个数据将大低谷的年龄限制在〜24.4-20.5 ka之间：与大低谷相关的沿岸砾岩上的碳酸盐U / Th日期；构成灰烬4的碳酸盐U / Th日期;碳酸盐U / Th日期跨越灰烬5。因此，大低点必须发生的时间间隔包括LGM。因此，这种低潮的时机与夏季最低温度相对应，表明下降的原因不是蒸发增加，而是降水减少。这一发现与传统观点相反：LGM是一个相对潮湿的区间。 （摘要由ProQuest缩短。）。

著录项

  • 作者

    Ali, Guleed Ahmed Hussein.;

  • 作者单位

    Columbia University.;

  • 授予单位 Columbia University.;
  • 学科 Paleoclimate science.;Geology.;Geochemistry.
  • 学位 Ph.D.
  • 年度 2018
  • 页码 255 p.
  • 总页数 255
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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