A High-Resolution Record of Rainfall Variability During the Past Three Millennia from a Stalagmite at DeSoto Caverns (Alabama, USA)

Hydroclimate changes in the Southeast USA are poorly documented due to the paucity of high-resolution paleo-records. We have investigated a cored stalagmite from the DeSoto Caverns using petrographic techniques and stable oxygen and carbon isotopes in an effort to provide a highly resolved (bi-annual) time series of rainfall variability in the Southeast for the past three millennia. DeSoto Caverns have recently been the focus of intense monitories studies under the current climate state. Oxygen and carbon isotope records are constrained by 15 precise 230Th/234U age determinations from the upper 32 cm of the stalagmite. The stalagmite is subdivided into two main units. The lower unit, (from 31.9 cm to 11.5 cm, corresponding to the time interval between ~3200 and 2035 years BP) contains continuous layered sequences of highly regular, tightly packed, aragonite botryoids (0.3 ± 0.34 mm2, n=30) showing excellent preservation. δ18O and δ13C time series show a regular periodicity of 30 ±3 yrs. In contrast, the upper unit (from 32 cm to 0 cm) exhibits at least 10 unconformities that are limited to the past 2035 years and whose durations vary between 62 and 265 years. These unconformities of variable thickness exhibit three distinct features: (i) an erosional surface at the bottom, (ii) a debris infill associated with a calcite layer at the middle, thought to be washed in during periods of high water flow and (iii) large aragonite botryoids (1.39± 0.38 mm2, n=30) with dissolution marks at the top. δ18O values range from -5% to -2.2% (VPDB) and show δ18O-depletions at the start of the unconformities. δ13C values range from -11 % to -2% (VPDB) and show a significant correlation with δ18O. Large aragonite botryoids with dissolution marks, lower δ18O and δ13C values and the presence of a calcite layer with debris infill suggest that the unconformities were caused by unusually high rainfall rates alternating with drier intervals. The stalagmite record offers compelling proxy evidence of a lower but regular rainfall pattern during the period between ~3200 and 2035 years BP followed by a phase change to high rainfall variability accompanied by high rainfall events alternating with droughts during the last two millennia.