A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)
A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)
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DOI：
10.1016/s0016-7037(01)00804-3
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发表时间：
2002-03-01
影响因子：
5
通讯作者：
Schlosser, P
中科院分区：
文献类型：
--
作者：
Aeschbach-Hertig, W;Stute, M;Schlosser, P
在马里兰州东南部Aquia含水层的封闭部分，地下水中低碳-14的活动表明，这些水的大部分至少在3万年前渗透。然而，溶解无机碳的放射性碳含量似乎受到同位素交换的影响，可能与地层中次生方解石沉积有关，导致高估了C-14年龄。鉴于Aquia含水层的地球化学使广泛使用的C-14测年方法的应用复杂化，放射性成因He的积累似乎为建立年代学提供了一个可行的替代方法。在Aquia含水层中观测到的He浓度随流动距离的准线性增加，完全可以用原位产生的放射性成因He的积累来解释。为了确定氦-4的积累速率，对Aquia砂中的U和Th浓度进行了测定，并有足够的置信度来建立He时间尺度。溶解的大气惰性气体浓度被用来计算入渗时的年平均地面温度。这些稀有气体温度（NGTs）清楚地表明，在比现在冷得多的条件下，水渗透了。NGTs与氯化物浓度相关，证实了该含水层中氯化物变化构成气候信号的假设。而δ (18)O和δ tad的稳定同位素比值不能提供Aquia含水层过去气候变化的清晰记录，NGTs与稳定同位素比值的相关性较弱。NGT记录表明，末次盛冰期（LGM）中纬度沿海地区的年平均气温比全新世低（9.0 +/- 0.6）℃。这一差异略低于从该地区花粉数据得出的估计，但比北美更南部地区惰性气体研究表明的大约5摄氏度的相当均匀的冷却要大得多。较大的降温归因于劳伦泰德冰盖的影响，该冰盖的最大延伸距离接近我们的研究地点250公里。版权所有2002爱思唯尔科学有限公司
Low C-14 activities in groundwater of the confined part of the Aquia aquifer in southeastern Maryland suggest that most of this water infiltrated at least 30,000 years ago. However, radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange, possibly with secondary calcite deposits in the formation, leading to overestimated C-14 ages. Whereas the geochemistry of the Aquia aquifer complicates the application of the widely used C-14 dating method, the accumulation of radiogenic He seems to provide a viable alternative for establishing a chronology. The quasi-linear increase of He concentrations with flow distance observed in the Aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic He. U and Th concentrations in Aquia sand were measured in order to determine the accumulation rate of He-4 with sufficient confidence to establish a He time scale.Concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration. These noble gas temperatures (NGTs) clearly show the presence of water that infiltrated under much cooler conditions than at present. NGTs are correlated with chloride concentrations, corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal. In contrast, the stable isotope ratios delta(18)O and deltaD do not provide a clear record of past climatic changes in the Aquia aquifer and the correlation between NGTs and stable isotope ratios is weak. The NGT record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum (LGM) were (9.0 +/- 0.6) degreesC colder than during the Holocene. This difference is slightly lower than estimates derived from pollen data for this region, but considerably larger than the rather uniform cooling of about 5degreesC indicated by noble gas studies in more southern locations of North America. The larger cooling is ascribed to the influence of the Laurentide ice sheet, which at its maximum extension came as close as 250 km to our study site. Copyright (C) 2002 Elsevier Science Ltd.