The fabric and its formation mechanism of porous-compact laminae in the stalagmite from Shihua Cave, Beijing

The stalagmites in Shihua Cave, Beijing have been intensely studied as archives of regional palaeoclimate. They yield high resolution time series of multi-proxy records which have greatly improved the understanding of the relationship between speleothem formation and climate changes. The stalagmites studied in this cave were composed of fibrous calcite with clear visible and fluorescent laminae. Recently, porous-compact layers at the top 4 mm were found on an actively growing stalagmite (XMG). It consists of visible transparent-opaque layers. A noteworthy observation is that the laminae of calcite fabrics occurred on the top of XMG which formed after the cave open for tourism. The results from scanning electron microscopy analysis on this stalagmite indicates that the opaque microcrystalline calcite (MCC) porous sub-layers are composed of granular mosaic structures, comparing to the compact transparent fibrous columnar calcite (FCC) sub-layers with structures almost oriented perpendicular to the growth surface. Further analysis on the modern carbonate of this site was carried to investigate its formation mechanisms. Based on the size and habit of crystal, these modern carbonate crystallite formations can be categorized into three types. Type I contains larger rhombohedral grains, most of which are formed in the dry season of January–March. Most crystals with c -axis (along the growth direction) represent scales larger than 10 μm and structures of FCC. Type II includes the mixture of large rhombohedrons and small microcrystalline with a scale of less than 4 µm during the transition period of dry to rainy season, with deposition in April−May and October−December. During this period, the scale of large crystals with a diameter of more than 10 µm gradually decreases and the microcrystalline crystals begin growing. Type III consists of calcite precipitates with a spherical habit in the rainy season of June–September. Crystallites with diameters of less than 1 µm can be observed in this period, which is highly related to the new water recharge during this season. To investigate their elemental compositions, both the rhombohedral crystallites and microcrystalline were analyzed through an elemental spectrum analyzer. The results show that the elemental consistency is highly associated with the scales of crystallites. Trace elements, including silicon, aluminum, magnesium, etc. could be obtained on the microcrystalline specimen. On the contrary, only the basic formation elements of calcium carbonate as carbon, oxygen and calcium are detected on rhombohedral structures. Based on these observations, there are two hypotheses on this formation mechanism. Hypothesis I is that, FCC corresponded to the relatively decreased drip rates. During the dry season, the recharge contains high calcite saturation from a stable soil-water-rock interaction over the cave. On the other hand, new water inputs during the rain seasons of increased drip rates result in increased variables including lower index of calcite saturation, complex soil-water-rock interaction and variable amounts of prior calcite precipitation. Hereby, it presents a result of increased MCC with more impurities. Hypothesis II is that, the changed structure might be related to the cave tourism conditions. As the new structure is observed after the cave open to the public, it is expected to suggest a correlation to the tourist patterns. Moreover, both the increased CO2 level and aerosol levels inside cave systems are highly associated with increased visitors, which result in more venting as well as external air inputs. It is likely that the impurities from the external aerosol are the key factors of MCC Formation during the transition period. These anthropogenic factors might play a vital role in the thicker microcrystalline calcite layers formation.