在洞穴中形成的无定形碳酸钙及其对洞穴堆积物研究的影响
摘要
浅褐色沉积物是古气候研究中最有价值的大陆构造之一，因为它们可以使用绝对测年法进行评估，并且还提供有价值的气候替代指标。然而，沉积后的矿物学转化等改变过程可能会对其地球化学数据的古气候应用产生显著的影响。本文介绍了采用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技术的创新采样和测量方案，证明了在-10°C的洞穴中的滴水中的碳酸盐沉淀含有无定形碳酸钙（ACC），后来会转变为纳米晶方解石。还通过分馏法测定了方解石，ACC和水中稳定的氧同位素，证明ACC相对于方解石为 18O-耗尽的（> 2.4±0.8‰）。这反过来又对于基于流体的流体包裹体研究具有严重的后果，因为ACC向方解石的闭合系统转化可能会导致流体夹杂水中的负氧同位素偏移，导致原始组成的变差。 ACC形成增加了浅褐色沉积物对变化的敏感性，因为它与外部解决方案的交互可能导致原始代理信号的部分丢失。因此本文建议在研究的浅褐色沉积物位置对新鲜沉淀碳酸盐进行矿物学分析，以确定ACC形成的潜在影响。
Formation of amorphous calcium carbonate in caves and its implications for speleothem research
Abstract
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 18O-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.
