Dissected Hydrologic System at Grand Canyon: Interaction Between Upper and Lower World Waters in Modern Springs and Ancient Travertines

Crossy0310323大峡谷的壮观景观提供了一个深入解剖(1-2公里深)的含水层系统的独特景观。活动泉和第四纪导线为了解科罗拉多高原的水化学提供了关键数据，也为了解该系统如何演变并与第四纪峡谷切割相互作用提供了记录。我们的初步数据表明，现代和古代的钙华沉积泉水，以及科罗拉多河本身，记录了一个“两水的故事”：沿着断层上升的深度循环的水与高原的地表水和地下水的混合。我们建议结合测绘、水和气体分析以及钙华地球化学来检验这一模型。钙华出现在主要的泉水产层(如Muav石灰岩-光明天使页岩接触带)与拉拉米德和前寒武纪的地下室穿透断层相交的地方。根据对泉水化学和气体分析的初步研究，我们认为大量的钙华产自富CO_2的深层咸水，比高原含水层的水更高的87Sr/86Sr。两个钙华形成区的泉水记录了较高的He/Ar和He/N_2，3He/4He比值为0.15Ra，表明地幔来源的He及其载气CO2可能来自附近的同时代火山岩田。大峡谷中心泉水中的87Sr/86Sr介于0.710-0.735之间，我们将测试深循环水域贡献的放射性成因锶是否可以解释先前观测到的位于李氏渡口和米德湖之间的科罗拉多河87Sr/86Sr的增加。这项工作的更广泛的科学影响延伸到科罗拉多高原的更长期(上更新世)记录：我们提出，大洗涤海槽中的放射性成因华拉派石灰岩(~5 Ma)是由泉水形成的，与今天在钙华地区看到的类似(大峡谷石灰华的87Sr/86Sr与已报道的华拉派L.的值相当)。选定地区的快速钙华沉积也造成了第四纪阶地层序的普遍胶结和保存，这使得直接对沉积事件进行U系列测年和计算切削率成为可能。到目前为止，测年表明，钙华堆积的主要脉冲发生在过去的350ka。以前的研究人员认为，钙华生长时期与冰川事件/地区潮湿相对应。然而，现代春季流量的变化与微震活动相对应，表明地震活动/火山活动影响了深水在断层上的运动。钙华记录，结合泉水化学和地球化学分析，将提供对该地区第四纪历史和演变的水文系统的定量了解。

Crossy0310323 The spectacular landscape at Grand Canyon provides a unique view of a deeply dissected (1-2 km deep) aquifer system. Active springs and Quaternary travertines provide key data for understanding the hydrochemistry of the Colorado Plateau, as well as a record for understanding how this system has evolved and interacted with Quaternary canyon incision. Our preliminary data suggest the modern and ancient travertine-depositing springs, and the Colorado River itself, record a "tale of two waters": a mixing of deeply-circulated waters rising along faults with surface- and groundwaters of the plateau. We propose to combine mapping, water and gas analysis, and travertine geochemistry to test this model. Travertine occurs where major spring-producing horizons (such as the Muav Limestone-Bright Angel Shale contact) intersect Laramide and Precambrian basement-penetrating faults. We suggest on the basis of a pilot study of spring water chemistry and gas analysis that large volumes of travertine are produced from deep saline waters that are rich in CO2 and higher in 87Sr/86Sr than waters derived from the plateau aquifers. Spring waters in two travertine-forming localities record high He/Ar and He/N2, and 3He/4He ratios of 0.15 Ra suggesting contributions of mantle-derived He and its carrier gas CO2 potentially from nearby coeval volcanic fields. 87Sr/86Sr in central Grand Canyon springs ranges from 0.710 to 0.735 and we will test whether radiogenic Sr contributed by deeply-circulated waters can account for previously observed increases in Colorado River 87Sr/86Sr between Lee's Ferry and Lake Mead. Broader scientific impacts of the work extend to the longer-term (Plio-Pleistocene) record of the Colorado Plateau: we propose that the radiogenic Hualapai Limestone (~5 Ma) in the Grand Wash trough was formed by spring waters similar to those seen in today's travertine localities (87Sr/86Sr of Grand Canyon travertine is comparable to values reported for the Hualapai Ls.). Rapid travertine deposition in selected localities has also caused pervasive cementation and preservation of Quaternary terrace sequences and this allows direct U-series dating of aggradation events and calculation of incision rates. Dating so far suggests that major pulses of travertine accumulation occurred over the last 350 ka. Previous workers suggested that travertine growth episodes correspond with glacial events/ regional wetness. However, modern spring discharge variations correspond to microseismicity, suggesting that seismicity/volcanism influence the movements of deep water up faults. The travertine record, combined with spring water chemistry and geochemical analysis will provide a quantitative understanding of both the Quaternary history and evolving hydrologic system of the region.