Age determination of recent cave deposits using excess 210Pb - A new technique

Cave deposits have been widely used as proxy recorders in deciphering palaeoclimate during the last glacial/interglacial maxima (~ 120 ka) [Harmon et al., 1975; Atkinson et al., 1978; Goede and Harmon, 1983; Ayliffe and Veeh, 1989]. Palaeoclimatic studies of cave deposits for the past 1-1000 yr time scale require a precise dating technique, that until now has been lacking. Due to the multiple sources of carbon in speleothems, 14C dates obtained for recently deposited calcite are highly variable and thus, 14C dating techniques are not suitable to obtain speleothem ages for the past 1-1000 years. Here, we show for the first time that speleothems contain high concentrations of excess 210Pb and that this 210Pbexcess can be successfully employed to obtain growth rates of speleothems deposited during the last 100 years. Of two specimens analyzed, a tubular "soda straw" stalactite yielded a longitudinal growth rate of 1.1 mm/yr, while a normal icicleshaped stalactite had a lateral growth rate of 0.028 mm/yr. The mass growth rates of these two speleothems (149 and 78 mg/yr respectively) are comparable within a factor of two. Studies of fine-scale variations in the isotopic omposition of recent speleothems will help to corroborate the validity of palaeoclimate r cords obtained using longer lived isotopes and extending back into Pleistocene.