Intra- and Inter-annual Fluorescence Intensity Variations in Drip Water, Heshang Cave, Central China: Implications for Speleothem Palaeoclimatology

Abstract:

Cave drip water acts as a signal carrier for the soil-rock-air system leading to the capture of climatic and environmental information in stalagmites. This paper seeks to develop an understanding of the environmental and climatic factors which control fluorescence variations in dripwater from in Heshang Cave, Central China. This information is essential to unravelling the significance of organic fluorescence in stalagmites and its utility in quantitative paleoclimate reconstructions. On the seasonal time scale, drip water fluorescence is largely controlled by the decomposition and translocation of dissolved organic matter in the soil, related to climate factors like temperature and precipitation. On the inter-annual time scale, longer duration monitoring data in scarce, yet this is needed to fully comprehend the influence of climate in stalagmite fluorescence time series. This study presents nine consecutive years of monthly drip water fluorescence intensity and drip rate data from two perennial drip sites in Heshang Cave. Drip water fluorescence was generally characterized by intensities in spring/summer and low intensities in autumn/winter. In dry hydrologic years, little seasonality in fluorescence signals was observed, but the opposite was observed in wet years. On the inter-annual time scale, the annual mean intensities of drip water fluorescence positively correlated with local annual rainfall with a 1-year lag (R²_HS4=0.94; R²_HS6=0.74). This indicates that rainfall is the main control on total drip water fluorescence (integrating across a hydrologic year), despite significant degrees of intra-annual fluorescence variation being observed between wet and dry years. These findings are of direct relevance for paleoclimate reconstruction using fluorescence intensities in stalagmites from the Asian monsoon region.

Key words: fluorescence; dissolved organic matter; drip water rates; seasonality; precipitation