Quantification of paleo-aquifer changes using clumped isotopes in subaqueous carbonate speleothems
Quantification of paleo-aquifer changes using clumped isotopes in subaqueous carbonate speleothems
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DOI：
10.1016/j.chemgeo.2018.05.046
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发表时间：
2018-08-20
期刊：
影响因子：
3.9
通讯作者：
Hodell, David A.
中科院分区：
文献类型：
--
作者：
Gazquez, Fernando;Columbu, Andrea;Hodell, David A.
在这里，我们通过确定各种水下碳酸盐岩化石（例如方解石晶石、狗齿状方解石晶体）的年龄（U/Th测年）和稳定同位素组成（δ O-18、δ C-13和δ（47））来跟踪圣乔瓦尼山含水层（意大利撒丁岛西南部伊格莱森特-苏尔西斯矿区）在过去600 ka的地下水位位置和温度和方解石涂层）。聚集同位素 (Delta(47)) 提供了碳酸盐形成温度（以及水温）的定量估计，该温度与水的氧同位素组成 (Delta O-18(w)) 无关。然后，根据聚集同位素温度 (T Delta(47)) 和碳酸盐的 δ O-18 (δ O-18(c)) 重建了古水的 δ O-18(w)。我们发现，一些高温方解石晶石在 600 ka 之前形成，温度高于约 120 ℃。低温晶石（约 70 ℃）在约 400 ka 时沉淀，冷水水下洞穴石（约 10-20 ℃）在 410 ka 至 110 ka 之间不同级别的岩溶系统的栖息池中形成。 ka，而地面流石的同期降水发生在较浅洞穴的上层，直到82 ka。我们推断，地下水位从大约400ka下降到大约250ka，下降了大约120m，下降速度相对较快，大约为0.8mm/年。考虑到第四纪撒丁岛的构造稳定性，这种高速率源自气候驱动的地表地貌过程，而不是构造隆起。古含水层水的 d18O 值范围从 MIS 5c 期间的 -6.0 +/- 0.7%（与现代洞穴水值 (-5.1%) 相似）到较冷的 MIS 8 期间的 -7.7 +/- 0.4%。这些值表明地下水反映了大气水的 Delta O-18 信号，而交代水和变质水没有显着贡献。观测到的 Delta O-18(w) 变异性可以通过影响降雨量 Delta O-18（进而影响地下水 Delta O-18）的冰期/间冰期古气候变化来解释。我们得出的结论是，水下碳酸盐洞穴的团簇同位素测温是追踪古含水层温度和 Delta O-18(w) 重建的有用工具。
Here we track the water-table position and temperature of the Mount San Giovanni aquifer (Iglesiente-Sulcis mining district, SW Sardinia, Italy) during the past 600 ka by determining the ages (U/Th dating) and stable isotope compositions (delta O-18, delta C-13 and Delta(47)) of a variety of subaqueous carbonate speleothems (e.g. calcite spars, dogtooth calcite crystals and calcite coatings). Clumped isotopes (Delta(47)) provide quantitative estimates of carbonate formation temperatures (and thus water temperatures) that are independent of the oxygen isotope composition of water (delta O-18(w)). Then, the delta O-18(w) of the paleo-water has been reconstructed from the clumped isotope temperature (T Delta(47)) and the delta O-18 of the carbonate (delta O-18(c)). We find that some high-temperature calcite spars formed prior to 600 ka at temperatures above similar to 120 degrees C. Lower-temperature spars (similar to 70 degrees C) precipitated at similar to 400 ka, and cold-water subaqueous speleothems (similar to 10-20 degrees C) formed in perched ponds at different levels of the karst systems between 410 ka and 110 ka, while coeval precipitation of subaerial flowstones occurred in the upper level of the shallower caves until 82 ka. We infer that the groundwater level dropped by similar to 120m from similar to 400 to similar to 250 ka, with a relatively rapid rate of similar to 0.8 mm/yr. Considering the tectonic stability of Sardinia during the Quaternary, this high rate derives from climate driven geomorphological processes at the surface rather than tectonic uplift. The d18O values of the paleo-aquifer water range from -6.0 +/- 0.7% during MIS 5c, and similar to modern cave water values (-5.1%), to -7.7 +/- 0.4% during the colder MIS 8. These values indicate that the groundwater reflected the delta O-18 signal of meteoric water, with no significant contributions from metasomatic and metamorphic waters. The observed delta O-18(w) variability can be explained by glacial/interglacial paleoclimate changes affecting rainfall delta O-18 (and thus groundwater delta O-18). We conclude that clumped isotope thermometry on subaqueous carbonate speleothems is a useful tool for tracking paleo-aquifer temperatures and delta O-18(w) reconstructions.