Palaeoclimatic interpretation of high-resolution oxygen isotope profiles derived from annually laminated speleothems from Southern Oman

Abstract

High-resolution stable isotope profiles of three contemporaneously deposited stalagmites from a shallow cave in Southern Oman provide an annually resolved record of Indian Ocean monsoon rainfall variability for the past 780 years. Uranium-series age dating and counts of annual growth bands enable an excellent age calibration. Although modern speleothems do not grow in perfect isotopic equilibrium, oxygen isotope ratios (δ 18O) are a proxy for the amount of monsoon rainfall. This is supported by the statistically significant correlation between δ 18O and the thickness of annual bands, whereas δ 18O is inversely correlated with annual band thickness. Additionally, overlapping δ 18O profiles are very similar in pattern and range, indicating that sample specific noise did not blur the climatic signal. The longest oxygen isotope profile, derived from stalagmite S3, clearly shows the transition at ∼1320 AD from a generally wetter Medieval Warm Period to a drier Little Ice Age that lasted from approximately AD 1320–1660 in Southern Oman. The decrease in monsoon rainfall since the 1960s is also obvious in meteorological records from Northern Africa and India, indicating that our speleothem-based rainfall records do not only reflect local monsoon rainfall variability.