Fluorescence intensity variations of speleothem-forming groundwaters: Implications for paleoclimate reconstruction

Groundwater discharges onto 16 speleothems and surface climate parameters have been measured in an oolitic limestone site in England. Groundwater recharge is demonstrated to lag behind periods of hydrologically effective precipitation by 30-50 days. With the exception of short periods of high rainfall intensity or quantity, summer rainfall has no effect on groundwater discharge onto the speleothems because of the development of a soil moisture deficit. The first groundwater fluorescence intensity increase was demonstrated to lag the autumn increase in groundwater discharge by 10-20 days, suggesting that the first groundwater comprises "old" stored groundwater, whereas later water derives from the overlying soil and has a high organic acid concentration. Despite the lags, the structure of autumn and winter moisture availability is reflected within the annual fluorescence intensity variations, suggesting that fluorescent organic acids, when trapped within speleothem calcite, may provide a proxy for past winter moisture variations for temperate latitudes.