NOAA/WDS Paleoclimatology - Furong Cave Geochemical Data from 17000 to 6000 Cal Yr BP

To understand oxygen and carbon stable isotopic characteristics of aragonite stalagmites and evaluate their applicability to paleoclimate, the isotopic compositions of active and fossil aragonite speleothems and water samples from an in situ multi-year (October 2005–July 2010) monitoring program in Furong Cave located in Chongqing of China have been examined. The observations during October 2005–June 2007 show that the meteoric water is well mixed in the overlying 300–500-m bedrock aquifer, reflected by relatively constant δ18O, ±0.11–0.14‰ (1σ), of drip waters in the cave, which represents the annual status of rainfall water. Active cave aragonite speleothems are at oxygen isotopic equilibrium with drip water and their δ18O values capture the surface-water oxygen isotopic signal. Aragonite-to-calcite transformation since the last glaciation is not noticeable in Furong stalagmites. Our multi-year field experiment approves that aragonite stalagmite δ18O records in this cave are suitable for paleoclimate reconstruction. With high U, 0.5–7.2 ppm, and low Th, 20–1270 ppt, the Furong aragonite stalagmites provide very precise chronology (as good as ±20s yrs (2σ)) of the climatic variations since the last deglaciation. The synchroneity of Chinese stalagmite δ18O records at the transition into the Bølling–Allerød (t-BA) and the Younger Dryas from Furong, Hulu and Dongge Caves supports the fidelity of the reconstructed East Asian monsoon evolution. However, the Furong record shows that the cold Older Dryas (OD) occurred at 14.0 thousand years ago, agreeing with Greenland ice core δ18O records but ∼200 yrs younger than that in the Hulu record. The OD age discrepancy between Chinese caves can be attributable to different regionally climatic/environmental conditions or chronological uncertainty of stalagmite proxy records, which is limited by changes in growth rate and subsampling intervals in absolute dating. Seasonal dissolved inorganic carbon δ13C variations of 2–3‰ in the drip water and 5–7‰ in the pool and spring waters are likely attributed to variable degrees of CO2 degassing in winter and summer. The variable δ13C values of active deposits from −11‰ to 0‰ could be caused by kinetically mediated CO2 degassing processes. The complicated nature of pre-deposition kinetic isotopic fractionation processes for carbon isotopes in speleothems at Furong Cave require further study before they can be interpreted in a paleoclimatic or paleoenvironmental context.

为明确文石（aragonite）石笋（stalagmite）的氧、碳稳定同位素特征并评估其在古气候研究中的适用性，本研究对中国重庆芙蓉洞原位长期（2005年10月—2010年7月）监测项目采集的现代活动与化石文石洞穴沉积物（speleothems）、水样的同位素组成开展了分析。2005年10月—2007年6月的观测结果显示，大气降水在上覆300~500米基岩含水层中充分混合，洞穴内滴水的δ¹⁸O值相对恒定（±0.11~0.14‰，1σ），这一特征反映了降雨的年际同位素状态。洞穴内现代活动文石洞穴沉积物与滴水处于氧同位素平衡状态，其δ¹⁸O值能够记录地表水的氧同位素信号。芙蓉洞石笋中自末次冰期以来的文石向方解石（calcite）转化现象并不显著。本长期野外实验证实，该洞穴中文石石笋的δ¹⁸O记录适用于古气候重建。芙蓉洞文石石笋铀（U）含量较高（0.5~7.2 ppm）且钍（Th）含量较低（20~1270 ppt），可为末次冰消期以来的气候演化提供极高精度的年代学框架（精度可达±20s yr（2σ））。来自芙蓉洞、葫芦洞与董哥洞的中国石笋δ¹⁸O记录在进入博令-阿勒罗德暖期（Bølling–Allerød，简称t-BA）与新仙女木事件（Younger Dryas，简称YD）的转换节点处的同步性，证实了重建的东亚季风演化序列的可靠性。但芙蓉洞记录显示，寒冷的老仙女木期（Older Dryas，简称OD）发生于14.0千年前，这与格陵兰冰芯δ¹⁸O记录一致，但比葫芦洞的记录年轻约200年。中国各洞穴间老仙女木期年代的差异，可归因于区域气候/环境条件的不同，或是石笋代用记录的年代学不确定性——这种不确定性受绝对测年中生长速率变化与亚采样间隔的限制。滴水水中溶解无机碳的δ¹³C存在2~3‰的季节变化，池水与泉水中的δ¹³C变化幅度则为5~7‰，这一现象可能与冬夏两季不同程度的CO₂脱气作用有关。现代活动沉积物的δ¹³C值变化范围为-11‰~0‰，这可能由动力学调控的CO₂脱气过程所导致。芙蓉洞洞穴沉积物中碳同位素沉积前的动力学同位素分馏过程具有复杂特性，在将其用于古气候或古环境解译前，仍需开展进一步研究。