Speleothem annual layers revealed by seasonal SIMS δ ¹⁸O measurements

Abstract

In-situ seasonal δ ¹⁸O measurements of section 236.3–235.6 cm of speleothem HS4, from Qingjiang Valley of the Middle reaches of Yangtze River, China, were performed by an Secondary Ionization Mass Spectrometry (SIMS) with Oka (Chinese primary calcite standard GBW04481) and UWC-3 (international calcite standard from University of Wisconsin). The potential of using SIMS δ ¹⁸O measurements to establish speleothem time series has been explored and the differences between conventional and SIMS δ ¹⁸O values have been discussed. During a 3-day period, UWC-3 δ ¹⁸O has been measured on Cameca IMS 1280 Ion Microprobe Mass Spectrometer against “Oka” external standard. The measured mean value of UWC-3 (δ ¹⁸O_VPDB= −17.85‰±0.22‰, 1SD) matches well with its recommended value (δ ¹⁸O_VPDB=−17.83‰±0.08‰), suggesting that the instrument was stable. The same method applied on HS4 produced δ ¹⁸O measurements at seasonal resolution with distinct annual cycles and the total cycle number in agreement with that from Mg/Ca cycles and lamination layer counting of the same section, so it offers an alternative for accessing speleothem time series. However, compared with conventional δ ¹⁸O values of HS4, SIMS δ ¹⁸O values are more negative by 0.90‰ with larger seasonal variation. The main reasons might come from the micro-cracks, micro-pores or liquid inclusions existing in HS4, and organic materials in the speleothem might be another factor affecting the SIMS δ ¹⁸O values, indicating that to obtain reliable speleothem SIMS δ ¹⁸O values, both compaction and purity of samples are crucial.