湖北清江和尚洞石笋色度对温度的响应

The paleo-temperature significance of color of annual laminae stalagmite from Heshang cave, Central China

石笋在古气候重建中得到广泛应用，但石笋现有的各种替代指标的气候解释仍存在较大的不确定性，将它们定量解译到温度和降水仍存在不少的困难。本研究中，首先对湖北清江和尚洞最近100多年沉积的年层分辨石笋样品（顶部，6.1 cm长）进行切割和抛光，然后利用惠普SCANJET 8270扫描仪高分辨率扫描，获取高清晰度的电子图像。其次，使用ENVI 5.1数字化图像软件处理，通过逐层辨认和数据提取，获取了高分辨的RGB色度值序列，建立了一条1886~1993 A.D.连续的、具有季节分辨的色度记录，研究石笋年层色度R/B比值与温度的定量关系。与传统的石笋灰度相比，R/B比值能表示石笋碳酸盐对短波和长波辐射吸收的差异，因此可以有效地指示石笋中有色物质含量变化，降低了指标对气候指示意义的不确定性。结果表明，过去100多年来，HS4石笋年平均R/B值不但与石笋中的空洞分布有较好的对应，而且与宜昌气象站的器测温度呈良好的正相关关系（r=0.6586，n=71，p〈0.01），是一个理想的古温度指标。当温度升高时，岩石化学风化和土壤中微生物作用加强，土壤铁锰氧化物和有色物质输送的通量增加，滴水中有色矿物和有机质浓度增加，石笋碳酸盐中的有色物质随之增多。与此同时，方解石晶体生长快速，石笋的空洞增多，捕获滴水中有色物质能力提高，石笋颜色加深。温度对有色物质生产、输送和保存过程的协同控制，导致石笋色度R/B灵敏地响应研究区气温的变化。

Stalagmites have been widely used in paleoclimate reconstruction, although climatic interpretations remain uncertain, partly due to the many different ways in which proxy records can be understood. Colour within stalagmites has variably been interpreted to represent organic matter content related to either biological processes, climate characteristics and vegetation composition, but little attention has been paid to in-cave effects during speleothem growth. Here we present a dataset of laminae colour and porosity derived at annual resolution from stalagmite （HS4）, Heshang cave （30°27＇N, 110°25＇E; 294 m a.s.l.）, Qing River valley, Hubei Province, Central China. Records of temperature and precipitation amount collected at local meteorological observation station （Yichang）are used to understand the correlation between climate, organic matter production and the colour intensity of individual laminae.To extract information on speleothem colour intensity, the stalagmite profile was imaged using an HP SCANJET 8270 scanner in high-resolution mode. The colour value was extracted by ENVI 5.1 digital image software at seasonal resolution. We propose a colour proxy using R/B rather than the grey-scale value commonly used in speleothem studies, as this provides a more quantitative record of colour intensity, thus reducing the uncertainty of climatic interpretation. The porosity of the speleothem calcite was determined using two methods. The voids in each layer of the stalagmite surface were counted directly under a typical conventional microscope; and the porosity of the stalagmite was determined using a Toshiba digital X-ray camera system （DR）.The increase of chromogenic substances in stalagmites increased the absorption of short-wave radiation, significantly reducing the B value and increasing the colour R/B. Statistical analysis of the precipitation and temperature records show that R/B colour intensity has no correlation with precipitation amount （r=-0.0641, n=99）, but a significant correlation with atmospheric temperature （r=0.6586, n=71, p〈0.01）. A similar pattern of variation between porosity and R/B colour exists, suggesting crystal growth mechanisms may also play a role in determining colour characteristics of speleothem calcite.Two processes are suggested to drive the temperature dependency of colour intensity. Firstly, increased temperatures will enhance chemical weathering and microbiological activity in the soil. This will provide abundant iron/manganese oxides and organic matter （such as Humic matter）in the drip waters, subsequently incorporated into the speleothem calcite. Secondly, temperature is known to affect the growth kinetics of speleothem calcite, with higher temperatures increasing the abundance of crystal defect sites and porosity. Both crystal defects and porosity enhance the entrapment of chromogenic substances. We therefore argue the sensitive response of stalagmite R/B values to temperature is due to both the combined effects of soil production above the cave and incorporation into calcite at the point of speleothem growth.