High resolution, U/Th dated (32,000 to 11,000 years), oxygen and carbon isotope proxy climate records from a stalagmite in Desoto Caverns, Alabama, USA.

摘要

This study addresses the question whether speleothems from DeSoto Caverns (Childersburg, AL) can serve as archives of paleoclimate conditions for the Southeast, USA. The focus of the study involves determining present-day controls of cave water delta18O and delta13C followed by interpretation of stalagmite delta18O and delta 13C variability in comparison to climate events of the past.;The monitoring program involved a 3-year study of cave waters and local rainfall (Tuscaloosa, AL) during years characterized by a significant trend from wet to dry conditions. Decreasing recharge of the cave aquifer was expressed as an interannual trend of declining drip flow rates, which was punctuated by seasonal oscillations due to varying rates of evapotranspiration. Amount-weighted mean monthly rainwater delta18O range from -1.5 to -8.3‰, show a mean seasonal amplitude of ∼4‰, and exhibit an interannual trend toward 18O-enrichment that I interpret as being governed by global atmospheric circulation patterns. The cave's aquifer attenuates seasonal delta18O variability, records 20% of rainfall's interannual 18O-enrichment, and is biased toward winter rainfall delta 18O.;Cave waters display strong seasonal variability in dissolved inorganic carbon (DIC) and delta13C, which range from 0.2 to 6.0 mM and 2.7 to -12.9‰ (VPDB), respectively. The data suggest the strongest seasonal controls are cave air ventilation/stagnation and varying CO 2 fluxes through the soil horizon and epikarst. delta13C of active speleothems imply the precipitating aragonite captures the seasonality observed in source dripwaters and time-series delta13C records of stalagmites carry the imprints of drip annual means entailing climate-driven delta 13C seasonal biases.;A fossil stalagmite provided a high-resolution proxy record of rainfall variability between 31.9 and 11.3 ka. I propose a more southerly polar jet stream (PJS) promoted increased winter rainfall amounts during cold phase events while warm phases result in a higher PJS position and decreased winter rainfall. The Younger Dryas was characterized by a dramatic change in the PJS path as warm air from the Gulf of Mexico infiltrated deep into the continent's interior and substantially decreased winter rainfall. Establishment of near modern climate conditions greatly enhanced deposition rates before changes in flow paths through the epikarst prevented stalagmite deposition since 11.3 ka.