Climatic and cave setting influences on drip water fluorescent organic matter over an annual cycle in Northwestern Spain and implications for fluorescent laminations in stalagmites

Abstract

Speleothem fluorescence may elucidate past vegetation dynamics, while microscale fluorescent laminations can provide annually resolved chronology. However, the origin of speleothem fluorescence and the mechanism responsible for the formation of micrometer scale fluorescent lamination in stalagmites, are not well constrained by monitoring studies. Here, we present results from a year-long monthly drip water monitoring from seven locations in La Vallina cave (Northwestern Spain). Fluorescence was quantified by excitation-emission matrices (EEM) spectrofluorometry. Five distinct components were resolved by parallel factor analysis (PARAFAC) modeling, including previously described humic-like, and protein-like components as well as an additional component suggesting a contribution of indigenous fluorescence sourced from the bedrock. Variations in the overlying vegetation and in the water reservoir age contribute to differences in the fluorescent components among different drip sites. While some active stalagmites feature annual to sub-annual fluorescent laminae, the drip water does not support higher abundance of humic-like fluorescence during the rainy season as a primary cause for layer formation. The constancy in humic-like fluorescence, likely to arise due to abiotic interactions with the bedrock, highlights possible other mechanisms on fluorescent layer formation, such as growth rate control over the dilation of the fluorescence signal in the stalagmite.