Institute for Geological and Geochemical Research, RCAES, Hungarian Academy of Sciences, Budaörsi út 45, Budapest, H-1112, Hungary.
Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar tudósok körútja 2., Budapest, H- 1117, Hungary.
Sci Rep. 2016 Dec 22;6:39602. doi: 10.1038/srep39602.
Speleothem deposits are among the most valuable continental formations in paleoclimate research, as they can be dated using absolute dating methods, and they also provide valuable climate proxies. However, alteration processes such as post-depositional mineralogical transformations can significantly influence the paleoclimatic application of their geochemical data. An innovative sampling and measurement protocol combined with scanning and transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy is presented, demonstrating that carbonate precipitating from drip water in caves at ~10 °C contains amorphous calcium carbonate (ACC) that later transforms to nanocrystalline calcite. Stable oxygen isotope fractionations among calcite, ACC and water were also determined, proving that ACC is O-depleted (by >2.4 ± 0.8‰) relative to calcite. This, in turn, has serious consequences for speleothem-based fluid inclusion research as closed system transformation of ACC to calcite may induce a negative oxygen isotope shift in fluid inclusion water, resulting in deterioration of the original compositions. ACC formation increases the speleothems' sensitivity to alteration as its interaction with external solutions may result in the partial loss of original proxy signals. Mineralogical analysis of freshly precipitating carbonate at the studied speleothem site is suggested in order to determine the potential influence of ACC formation.
石笋沉积物是古气候研究中最有价值的大陆地层之一，因为它们可以使用绝对测年方法进行定年，并且还提供了有价值的气候代用指标。然而，后沉积的矿物转化等变化过程会显著影响其地球化学数据在古气候研究中的应用。本文提出了一种创新的采样和测量方案，结合扫描和透射电子显微镜、X 射线衍射和傅里叶变换红外光谱，证明了从洞穴滴水中沉淀的碳酸盐中含有无定形碳酸钙 (ACC)，随后转化为纳米晶方解石。还确定了方解石、ACC 和水之间的稳定氧同位素分馏，证明了 ACC 相对于方解石是 O 亏损的（>2.4±0.8‰）。这反过来又对基于石笋的包裹体研究产生了严重影响，因为 ACC 向方解石的封闭体系转化可能导致包裹体水的氧同位素负移，从而恶化原始成分。ACC 的形成增加了石笋对变化的敏感性，因为其与外部溶液的相互作用可能导致原始代用信号的部分损失。建议对研究地点的新沉淀碳酸盐进行矿物分析，以确定 ACC 形成的潜在影响。
