Pleistocene-Holocene-Modern Evolution of Groundwater in a Limestone Aquifer: Geochemical and Geochronologic Links

小行星9526714 拟议的研究将调查石灰岩含水层如何随着时间的推移而演变。 巴巴多斯隆起的珊瑚礁阶地包括一个10公里尺度的大气地下水流系统，具有独特的地层、岩性和水文特性，非常适合了解地下水流和流体-岩石相互作用的路径和时间尺度。 这些独特的性质包括：（1）含水层岩石成分、矿物学和年龄、土壤成分和孔隙度-渗透率的变化，这些变化都在构造抬升的地下水集水区内和之间以可预测的模式变化;（2）从密集的威尔斯井、泉和洞穴中获取水、岩石、矿物和土壤样品;以及（iii）0至100万年的时间尺度，在此时间尺度内，可以使用新开发的同位素技术研究含水层的演变。 为了重建含水层的更新世-全新世-现代演化，将从现代流动系统中采集地下水、石灰岩和土壤样本，以及在更新世和全新世地下水流动期间在石灰岩孔隙中作为洞穴沉积物和胶结物生长的碳酸盐矿物。 最近开发的和常规的同位素技术（例如，238 U-234 U-230 Th，87 Sr/86 Sr， 13 C/12 C，18 O/16 O，15 N/14 N）将用于确定1）洞穴沉积物和胶结物中生长带的年龄，2）现代地下水和胶结物生长的地下水中溶解离子的来源，3）过去百万年中地下水溶解离子来源的时间变化。 在确定高分辨率的时间变化在一个活跃的，良好的约束地下水流系统中的溶解离子的来源，这项研究将是前所未有的。 对石笋的初步研究提供了过去5500年的时间序列记录。 在该样品的方解石生长层中记录的地下水中的Sr同位素变化反映了在全新世中期至晚期补给更新世石灰岩含水层的降雨量的变化，将为含水层的其他部分和延长的时间尺度构建类似的记录。 这些将与现代流动系统的详细研究相结合。 该含水层的岩石学，地球化学，地质年代学和水文分析将提高我们的理解（I）岩溶模式，孔隙度，含水层岩石矿物学，土壤发育和气候变化对地下水迁移和化学演化的控制，以及（ii）工业和农业污染对地下水水质的影响。 这些关于地下水演化速率的数据是朝着建立碳酸盐含水层发展预测模型迈出的重要一步。

9526714 Banner The proposed research will investigate how a limestone aquifer evolves through time. Barbados's uplifted coral-reef terraces comprise a 10-km scale meteoric groundwater flow systems with unique stratigraphic, lithologic, and hydrologic properties that are ideally suited to understanding the pathways and time scales of groundwater flow and fluid-rock interaction. These unique properties include (I) variations in aquifer-rock composition, mineralogy and age, soil composition, and porosity-permeability that all vary in predictable patterns within and between tectonically-uplifed groundwater catchments: (ii) access to water, rock, mineral, and soil samples from densely-spaced wells, springs and caves; and (iii) the 0 to 1 million year time scale within which the aquifer's evolution can be studied using newly-developed isotope techniques. To reconstruct the aquifer's Pleistocene-Holocene-modern evolution, samples will be taken of groundwater, limestones, and soils from the modern flow system, and carbonate minerals that grew as speleothems and cements in the pore spaces of the limestones during groundwater flow in the Pleistocene and Holocene epochs. Recently developed and conventional isotopic techniques (e.g., 238U-234U-230Th, 87Sr/86 Sr, 13C/12C, 18O/16O, 15N/14N) will be used in order to determine 1) the ages of growth zones in the speleothems and cements, 2) the sources of dissolved ions in the modern groundwaters and in the groundwaters from which the cements grew, and 3) temporal variations in the sources of dissolved ions for the groundwaters during the past million years. In determining high-resolution temporal variations in the sources of dissolved ion in an active, well-constrained groundwater flow systems, this study will be unprecedented. An initial study of a stalagmite provides a time series record for the past 5500 years. Sr isotope variations in groundwater recorded in the calcite growth layers of this sample reflect changes in the amount of rainfall tha t recharged the Pleistocene limestone aquifer during the mid to late Holocene, Similar records will be constructed for other parts of the aquifer and over extended temporal scales. These will be integrated with a detailed study of the modern flow system. The petrologic, geochemical, geochronologic, and hydrologic analysis of this aquifer will improve our understanding of (I) the controls of karsification patterns, porosity, aquifer rock mineralogy, soil development, and climate change on groundwater migration and chemical evolution, and (ii) the impact of industrial and agricultural pollution on groundwater quality. Such data on the rates of groundwater evolution represent an important step toward developing predictive models for the development of carbonate aquifers.