The origin of white calcite silts forming 0.5 to 3-cm-thick lenses in alluvial fan deposits ¹⁴C-dated to the Last Glacial Maximum in the Dronne Valley (Dordogne, southwest France) is investigated using microscopic imagery, chemistry, and O and C stable isotopes. The calcite silts, composed mainly of aggregates of 3–5-μm euhedral crystals, do not resemble secondary precipitations of pedological origin because of the strata-like pattern and the lack of clearly identifiable biological structures. Their association with evidence of ice formation in the soil (platy structure, involutions) suggests that they were deposited in a deep seasonal frost context. Their isotopic composition differs significantly from those of detrital carbonates and of Holocene bioprecipitation and seems to be best explained by precipitation under closed-system conditions. Calculation of the isotopic composition of calcite that would have formed in equilibrium with groundwater of regional LGM aquifers provides values that are in the range of the composition of the calcite silts for a precipitation temperature close to 0°C. Therefore, these deposits are interpreted as cryogenic calcite precipitated from waters close to saturation with respect to calcite freezing at the base of/within icings or within the ground, possibly from frost blisters. Similar calcite precipitation at the outlet of karstic springs may have been abundant in the calcareous terrains of southwest France during the LGM, although still unrecognized in the geological record.

中文翻译：

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法国西南部维勒图雷冲积扇中的末次冰期最大低温方解石矿床

白色方解石淤泥在冲积扇沉积物中形成 0.5 至 3 厘米厚的透镜体的起源¹⁴使用显微图像、化学以及 O 和 C 稳定同位素对 Dronne 山谷（法国西南部多尔多涅省）的末次盛冰期 C 进行了研究。主要由 3-5 μm 自形晶体聚集体组成的方解石淤泥，由于地层状模式和缺乏清晰可辨的生物结构，与土壤学起源的次生沉淀物不同。它们与土壤中冰形成的证据（板状结构、内卷）的关联表明它们沉积在深厚的季节性霜冻环境中。它们的同位素组成与碎屑碳酸盐和全新世生物沉淀的同位素组成显着不同，似乎最好用封闭系统条件下的沉淀来解释。计算与区域 LGM 含水层地下水平衡形成的方解石的同位素组成，提供的值在降水温度接近 0°C 的方解石淤泥组成范围内。因此，这些沉积物被解释为低温方解石从接近饱和的水域沉淀出来，相对于在冰层底部/内部或地下的方解石冻结，可能来自霜泡。在 LGM 期间，在法国西南部的钙质地形中，岩溶泉出口处可能存在类似的方解石降水，尽管在地质记录中仍未被识别。这些沉积物被解释为低温方解石，从接近饱和的水域沉淀，方解石在冰层底部/内部或地下冻结，可能来自霜泡。在 LGM 期间，在法国西南部的钙质地形中，岩溶泉出口处可能存在类似的方解石降水，尽管在地质记录中仍未被识别。这些沉积物被解释为低温方解石，从接近饱和的水域沉淀，方解石在冰层底部/内部或地下冻结，可能来自霜泡。在 LGM 期间，在法国西南部的钙质地形中，岩溶泉出口处可能存在类似的方解石降水，尽管在地质记录中仍未被识别。