A Holocene calcrete from North Yorkshire, England: implications for interpreting palaeoclimates using calcretes
A Holocene calcrete from North Yorkshire, England: implications for interpreting palaeoclimates using calcretes
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来自英格兰北约克郡的全新世钙质凝结物：使用钙质凝结物解释古气候的意义
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发表时间：
1992
期刊：
影响因子：
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通讯作者：
V. Wright
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文献类型：
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作者：
G. E. Strong;J. Giles;V. Wright
英国北约克郡西坦菲尔德晚德文世丁灵顿期（距今26000 - 13000年）的冰川砾石被富碳酸盐溶液胶结，形成了一个强烈硬化的钙质结砾岩层。低镁水泥出现晶簇晶石，针纤维，肺泡间隔结构，微晶灰岩和micropinnacles，指示渗流带胶结。一些复杂的孔分配结构归因于沉淀沿着弯月形膜也发生。这些分区分隔了包气带以空气为主的微环境和以水为主的微环境。丰富的渗流组构表明该堆积体不是地下水钙质结砾岩。此外，大部分碳酸盐似乎是通过生物调解沉淀的。
碳和氧同位素数据表明，碳酸盐并不是由于冻结而形成的，就像一些“北极”土壤碳酸盐一样。该地区的花粉历史，因为德文西期表明，这种钙质结砾岩沉淀在低温下，这与广泛报道的发生在炎热的干旱或半干旱地区的土壤中的钙质结砾岩，并建议古钙质结砾岩不应该被用作绝对的古气候指标。
在凉爽和潮湿的气候下，厚的钙质结砾岩虽然是局部的，但却不寻常，这可能反映了砾石的排水良好，砾石中石灰石碎屑中富含CaCO 3，以及森林覆盖下高度的生物活动，这创造了有利于碳酸盐沉淀的当地环境。
Glacial gravels of Late Devensian Dimlington Stadial age (26 000–13 000 years BP) at West Tanfield, North Yorkshire, England, have been cemented by carbonate-rich solutions to produce a strongly indurated calcrete horizon. The low-Mg cements occur as drusy spar, needle fibres, alveolar septal structures, micrite and micropinnacles, indicative of vadose-zone cementation. Some complex pore partition structures attributed to precipitation along meniscus films also occur. These partitions separate air-dominated and water-dominated microenvironments of the vadose zone. The abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete. In addition, much of the carbonate appears to have been precipitated by biological mediation.
Carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing, as has been suggested for some ‘arctic’soil carbonates. The pollen history of the area since the Devensian suggests that this calcrete precipitated at low temperatures; this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi-arid regions, and suggests that palaeo-calcretes should not be used as absolute palaeoclimatic indicators.
The unusual occurrence, albeit localized, of a thick calcrete under a cool and wet climate probably reflects the well-drained nature of the gravels, the abundance of CaCO3 as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover, which created a local environment favouring carbonate precipitation.