Comparison of the luminescence properties of waters depositing flowstone and stalagmites at Lower Cave, Bristol

Abstract

Discharge and luminescence properties of waters feeding a flowstone in Lower Cave, Bristol, were measured at both three weekly and 30 minute sampling intervals within rainfall events over the period April 1994–April 1995 and July–December 1995, respectively. Results are compared with the discharge and luminescence of lower (<0·01 ml s−1) discharge waters supplying five stalagmites in the cave (Baker et al., 1997). When sampled at three weekly intervals, the flowstone waters exhibit a statistically significant relationship between discharge and luminescence intensity (r=0·75), with luminescence maxima in late autumn and early winter. When compared with the stalagmite waters, the flowstone waters have a higher luminescence intensity at 420 nm (28±11 vs. 12±1 luminescence units) and a higher wavelength of maximum luminescence intensity (430±2 vs. 419±1 nm). The relationship between discharge and luminescence is weaker for the flowstone than for the fastest dripping stalagmite waters (r=0·75 vs. r=0·85), suggesting that hysteresis and other non-linear effects on the luminescent fraction in the waters may be important under higher flow regimes (>0·01 ml s−1) feeding flowstones. When 30 minute sampling results are considered, dilution and hysteresis effects can be observed in the discharge–luminescence relationship, both on an intra-event basis and over the winter as a whole. Exhaustion of organic matter within the karst groundwater system over the hydrological year may also be important. A very rapid flux of luminescent organic matter in early autumn generates a non-linear, order of magnitude change in water luminescence intensity. Flowstone water luminescence properties can be interpreted as having: (1) higher concentrations of luminescent organic matter compared with stalagmites (from luminescence intensity data); (2) a greater proportion of humic acid in relation to fulvic acid (from luminescence wavelength data); (3) a more rapid response to surface rainfall events; and (4) a consequently greater proportion of non-linear responses to surface rainfall variations. These results are considered in terms of the utility of the analysis of luminescence variations in solid samples of stalagmites and flowstones in aiding the reconstruction of past rainfall variations, and it is suggested that stalagmites are more useful than flowstones. © 1998 John Wiley & Sons, Ltd.