PALEOCLIMATE VARIATIONS FROM 0 TO 12.3 KA BP INFERRED FROM THREE COEVAL CALCITE AND ARAGONITE SPELEOTHEMS FROM MARENGO CAVE, INDIANA, USA

Three carbonate speleothems, including one aragonite stalagmite, were collected from Marengo Cave, southern Indiana for stable isotope analysis and regional paleoclimate reconstruction. Based on 28 U-series dates and 1188 δ¹³C and δ¹⁸O values along growth axes, the stalagmites span a growth period from 12.3 ka BP to the present day, and experienced large variations in both growth rates and stable isotopic compositions. We observed a consistent isotopic fractionation between coeval aragonite and calcite stalagmites: the aragonite speleothem was 5 to 7 ‰ enriched in ¹³C and 0.7 to 1.3 ‰ enriched in ¹⁸O with respect to the calcite stalagmites, suggesting a distinct difference in drip water chemistry. However, there is strong covariance of both isotopes between the aragonite and the calcite speleothems, indicating that both speleothems were controlled by the same external palaeoenvironmental factors (i.e. changes in temperature, precipitation, vegetation). Four distinct climatic intervals can thus be distinguished: the Younger Dryas cooling from 12.3 to 11.1 ka BP; a quick warm up from 11.1 to ~10 ka ago; an early Holocene optimum between 10 and 7.7 ka BP; and a very slow gradual cooling with millennial-scale oscillations since then. The post-glacial thermal optimum may have been a wet period because δ¹³C records display more depleted values in comparison with the previous episode, indicating an increase in biomass and/or bioactivity. The early Holocene climatic optimum was interrupted by a brief deterioration in bioactivity between 8.5 and 8.1 ka ago, which might relate to the well-known 8.2 ka cooling event but displayed no response in δ¹⁸O record. The study area experienced a very dry climate between 6.4 to 3 ka BP, which eventually led the aragonite stalagmite to stop growing. This dry period was interrupted by short-lived relatively wet periods, which caused intermittent re-growth of the aragonite speleothem. In general, the speleothem records match well with the regional pollen records and historic records. Several historic warm and cool periods known in Europe were recorded in both δ¹⁸O and δ¹³C records of the Indiana speleothems, such as the Mid-Holocene Optimum and Minimum, the Roman Climate Optimum, the Medieval warm period, and the Little Ice Age.