环境地球化学国家重点实验室知识库

A 13.5-cm long stalagmite from Yelang Cave in central Guizhou provides another example of AMS C-14 dating on stalagmites which are not suitable for Th-230/U dating due to very low U content and low Th-230/Th-232 ratio. This record duplicates the published stalagmite record of 20120824-13 from the same cave (Zhao et al., 2015) not only on delta O-18 and delta C-13 records, but also on major growth hiatuses, facilitating speleothem delta O-18 and delta C-13 as climate and vegetation proxies respectively. Based on the two stalagmite records, climate and vegetation conditions can be deciphered as follow: relatively cool and dry climates with C4 dominated plants during 33.4-36 ka; very cold and dry climates with poor vegetation coverage during the Last Glacial Maximum; cold and dry climates with poor vegetation during Younger Dryas; warming and very wet climates with the best vegetation (mainly C3 plants) coverage during 9-11.5 ka BP corresponding to a maximum insolation; drying climate and decreased vegetation intensity from 9 ka to 8 ka; relatively good vegetation coverage during 5-6 ka; the summer monsoon strength decreased from 5 ka to 1.5 ka BP, but increased during the Medieval Warm Period to produce wet climates and abundant vegetation; and strongly decadal variations in moisture budget and strong deforestation due to human impact during the past 600 years. High Fe, Mn and Al contents from detritus caused dark bands near the hiatuses. Strong positive correlation of Mg and Sr with high Mg/Sr values (>23 in this study) reflects source control on the Mg and Sr variations, so that Mg/Sr is not a function of cave temperature in such a case. For pure calcite without detrital influence, Mg/Sr may register changes in cave temperature with higher Mg/Sr indicating warmer temperature. This study calls for the attention to find out climatic conditions in central Guizhou during the Bollinge-llerod warming and the middle Holocene (6-8 ka) intervals because of lacking deposition during these intervals in Yelang Cave. Using stalagmite delta O-18 alone to decipher climatic conditions on decadal-to-millennial scales needs to consider multiple factors such as amount, source, and temperature effects on the delta O-18.