Carbon dioxide, ground air and carbon cycling in Gibraltar karst
Carbon dioxide, ground air and carbon cycling in Gibraltar karst
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DOI：
10.1016/j.gca.2016.01.041
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发表时间：
2016-07-01
影响因子：
5
通讯作者：
Ainsworth, M.
中科院分区：
文献类型：
--
作者：
Mattey, D. P.;Atkinson, T. C.;Ainsworth, M.
我们提出了一个一般的概念模型的CO2行为在岩溶含水层的包气带，空气流动通过多孔介质和洞穴的物理原理的基础上，结合洞穴监测数据的地球化学解释。这个“直布罗陀模型”将土壤中的水、空气和碳的流动与渗流带的孔隙度、地面空气的循环和洞穴的通风联系起来。直布罗陀拥有许多天然洞穴，其位置跨越岩石的整个长度和垂直范围。我们报告了一项为期8年的土壤有机质和基岩碳酸盐中的碳、渗流沃茨中的溶解无机碳以及土壤、洞穴和地面空气中的气态CO2的监测研究结果。结果表明，该制度的洞穴空气CO2的结果，从相互作用的洞穴通风与水库的CO2富集的地面空气中举行的较小的空隙的基岩。地面空气和与之密切接触的包气沃茨的pCO（2）由多个因素决定，包括补给模式、植被生产力和根系呼吸，以及土壤、表层岩溶和整个包气带中有机质向CO2的转化。数学模型和实地观测表明，地面空气受密度驱动的环流的影响，随着地表和地下温度之间的差异在符号上逆转，这种环流会季节性逆转。直布罗陀模型表明，洞穴空气pCO（2）与土壤或表岩溶带中产生的CO2没有直接关系，这是通常的假设。由有机物（OM）衰变形成的地面空气CO2被冲刷到较深的非饱和带中，是pCO（2）的重要补充来源。在直布罗陀，OM产生的CO2的增加是对深层渗流带内地面空气pCO（2）和沃茨Ca硬度的主要控制。洞穴空气中的CO2的季节性状态取决于洞穴相对于密度驱动的地面空气循环模式的位置，该模式本身由地形确定，以及由洞穴本身提供的高渗透性空气流动管道。在直布罗陀陡峭的地形中，岩石下部的洞穴在夏季充当地面空气下降的流出管道，因此在该季节具有较高的pCO（2）。岩石上部的洞穴在冬季具有较高的pCO（2），当它们作为富含CO2的地面空气上升流的流出管道时。了解季节性流动的地下空气中的包气带，以及洞穴空气中的CO2的起源和季节性制度的基础上强大的解释洞穴积土为基础的气候代理记录。(C)2016由Elsevier Ltd.出版
We put forward a general conceptual model of CO2 behaviour in the vadose zone of karst aquifers, based on physical principles of air flow through porous media and caves, combined with a geochemical interpretation of cave monitoring data. This 'Gibraltar model' links fluxes of water, air and carbon through the soil with the porosity of the vadose zone, the circulation of ground air and the ventilation of caves. Gibraltar hosts many natural caves whose locations span the full length and vertical range of the Rock. We report results of an 8-year monitoring study of carbon in soil organic matter and bedrock carbonate, dissolved inorganic carbon in vadose waters, and gaseous CO2 in soil, cave and ground air. Results show that the regime of cave air CO2 results from the interaction of cave ventilation with a reservoir of CO2-enriched ground air held within the smaller voids of the bedrock. The pCO(2) of ground air, and of vadose waters that have been in close contact with it, are determined by multiple factors that include recharge patterns, vegetation productivity and root respiration, and conversion of organic matter to CO2 within the soil, the epikarst and the whole vadose zone. Mathematical modelling and field observations show that ground air is subject to a density-driven circulation that reverses seasonally, as the difference between surface and underground temperatures reverses in sign. The Gibraltar model suggests that cave air pCO(2) is not directly related to CO2 generated in the soil or the epikarstic zone, as is often assumed. Ground air CO2 formed by the decay of organic matter (OM) washed down into the deeper unsaturated zone is an important additional source of pCO(2). In Gibraltar the addition of OM-derived CO2 is the dominant control on the pCO(2) of ground air and the Ca-hardness of waters within the deep vadose zone. The seasonal regime of CO2 in cave air depends on the position of a cave in relation to the density-driven ground air circulation pattern which is itself determined by the topography, as well as by the high-permeability conduits for air movement provided by caves themselves. In the steep topography of Gibraltar, caves in the lower part of the Rock act as outflow conduits for descending ground air in summer, and so have higher pCO(2) in that season. Caves in the upper Rock have high pCO(2) in winter, when they act as outflow conduits for rising currents of CO2-enriched ground air. Understanding seasonal flows of ground air in the vadose zone, together with the origins and seasonal regimes of CO2 in cave air underpins robust interpretation of speleothem-based climate proxy records. (C) 2016 Published by Elsevier Ltd.