Seasonal dripwater Mg/Ca and Sr/Ca variations driven by cave ventilation: Implications for and modeling of speleothem paleoclimate records
Seasonal dripwater Mg/Ca and Sr/Ca variations driven by cave ventilation: Implications for and modeling of speleothem paleoclimate records
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DOI：
10.1016/j.gca.2011.03.025
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发表时间：
2011-06
影响因子：
5
通讯作者：
Corinne I. Wong;J. Banner;M. Musgrove
中科院分区：
文献类型：
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作者：
Corinne I. Wong;J. Banner;M. Musgrove
在德克萨斯州中部的一个洞穴中进行的一项为期4年的研究量化了控制滴水成分的多种机制，以及这些机制在不同的滴水地点是如何变化的。我们每4-6周监测一次洞穴空气成分、原位方解石生长、滴水成分和滴水速率。根据滴水成分的地球化学变化，圈定了三组滴水点（1-3组）。利用宿主碳酸盐岩和洞穴环境中矿物-溶液反应的定量模型来确定可以解释滴水成分变化的机制。Mg/Ca（和Sr/Ca）和Sr同位素的共变是描述Mg/Ca和Sr/Ca变化是由水-岩相互作用（即方解石或白云石再结晶）还是先前方解石沉淀（PCP）决定的关键。第1组滴水组成反映了水岩相互作用程度的狭窄范围，随后是不同数量的方解石沉淀（PCP）。第2组水滴的组成受不同数量的水-岩相互作用的控制，PCP几乎没有影响。第3组水滴组成由水岩相互作用和PCP的不同程度决定。第1组滴注点的Mg/Ca和Sr/Ca有季节性变化，而其他滴注点没有。与之前大多数滴落水Mg/Ca / sr /Ca研究的结果相反，这些季节变化（在第1组滴落点）与水通量（即降雨量和/或滴落率）的变化无关，而是与洞穴空气中二氧化碳浓度的变化显著相关。这些结果与较低的洞穴空气CO2相一致，这与洞穴大气的凉爽季节通风，方解石降水增强以及通过PCP导致水滴地球化学演化有关。第1组滴水Mg/Ca和Sr/Ca的季节性以及PCP作为一种机制可以解释这种季节性的证据，对许多其他可能存在季节性洞穴通风的地区有两个含义：(1)洞穴微量元素记录可能提供季节性信号；(2)这些记录可能偏向于记录方解石沉积季节的气候条件。此外，我们利用我们的结果构建了一个正演模型，该模型说明了洞穴中Mg/Ca和Sr/Ca变化的类型，这些变化可能是由滴水成分的不同控制引起的。该模型为解释来自洞穴的高频率Mg/Ca和Sr/Ca变化的古滴水控制提供了基础，在这些洞穴中进行长期监测研究是不可行的。
A 4-year study in a central Texas cave quantifies multiple mechanisms that control dripwater composition and how these mechanisms vary at different drip sites. We monitored cave-air compositions, in situ calcite growth, dripwater composition and drip rate every 4–6 weeks. Three groups of drip sites are delineated (Groups 1–3) based on geochemical variations in dripwater composition. Quantitative modeling of mineral-solution reactions within the host carbonate rock and cave environments is used to identify mechanisms that can account for variations in dripwater compositions. The covariation of Mg/Ca (and Sr/Ca) and Sr isotopes is key in delineating whether Mg/Ca and Sr/Ca variations are dictated by water–rock interaction (i.e., calcite or dolomite recrystallization) or prior calcite precipitation (PCP). Group 1 dripwater compositions reflects a narrow range of the extent of water–rock interaction followed by varying amounts of prior calcite precipitation (PCP). Group 2 dripwater compositions are controlled by varying amounts of water–rock interaction with little to no PCP influence. Group 3 dripwater compositions are dictated by variable extents of both water–rock interaction and PCP. Group 1 drip sites show seasonal variations in dripwater Mg/Ca and Sr/Ca, whereas the other drip sites do not. In contrast to the findings of most previous dripwater Mg/Ca–Sr/Ca studies, these seasonal variations (at Group 1 drip sites) are independent of changes in water flux (i.e., rainfall and/or drip rate), and instead significantly correlate with changes in cave-air CO2concentrations. These results are consistent with lower cave-air CO2, related to cool season ventilation of the cave atmosphere, enhancing calcite precipitation and leading to dripwater geochemical evolution via PCP. Group 1 dripwater Mg/Ca and Sr/Ca seasonality and evidence for PCP as a mechanism that can account for that seasonality, have two implications for many other regions where seasonal ventilation of caves is likely: (1) speleothem trace-element records may provide seasonal signals, and (2) such records may be biased toward recording climate conditions during the season when calcite is depositing. Additionally, we use our results to construct a forward model that illustrates the types of speleothem Mg/Ca and Sr/Ca variations that would result from varying controls on dripwater compositions. The model provides a basis for interpreting paleo-dripwater controls from high frequency Mg/Ca and Sr/Ca variations for speleothems from caves at which long term monitoring studies are not feasible.