Climatic and cave settings influence on drip water fluorescent organic matter with implications for fluorescent laminations in stalagmites

Abstract

Speleothem fluorescence can provide insights into past vegetation dynamics and stalagmite chronology. However, its origin and especially the formation of fluorescent laminations in stalagmites are poorly understood. We conducted a year-long monthly monitoring of drip water fluorescence in La Vallina Cave (northern Iberian Peninsula) and compared the results to drip water chemistry and active speleothems from the same sites. Drip waters were analyzed using fluorescence spectroscopy and parallel factor analysis (PARAFAC). The resulting five-component model indicates contributions from vegetation, microbial activity, and bedrock. Intra-site fluorescence variability is mainly influenced by changes in overlying vegetation, water reservoir time, and respiration rates. Contrary to prevailing views, we find no systematic increase in drip water fluorescence during rainy conditions across drip sites and seasonal variations in drip water fluorescence are absent at a location where present-day speleothem layers form. Our findings challenge the notion of a higher abundance of humic-like fluorescence during the rainy season as the primary cause for layer formation and suggest additional controls on drip water fluorescence, such as bedrock interaction and microbial reprocessing. We also propose that growth rate may control the dilation of the fluorescence signal in stalagmites, indicating other potential mechanisms for fluorescent layer formation.

The influence of fluid inclusions, organics, and calcite fabric on trace element distributions in stalagmites

“…In this study, CLSM was limited to detecting organic components excited at 488 nm and fluorescing in the window of 490-555 nm, a small fraction of the total fluorescent organic constituents delivered to speleothems in this cave (Endres et al, 2023), and potentially emphasizing components such as lignin (Sliwinski and Stoll, 2021). From FTIR, we focused on the 2,983 cm −1 band of the asymmetric CH 2 component which is expected in fats, waxes, and lipids.…”

“…The fluorescence from the applied CLSM method is commonly interpreted as a first order indication of the concentration of OM in a region of the stalagmite (Sliwinski and Stoll, 2021), although the single wavelength excitation (488 nm) and a narrow detection window of 490-555 nm are sensitive to only a narrow range of compounds and therefore may not capture all OM in stalagmites (Sliwinski and Stoll, 2021;Endres et al, 2023).…”

Characteristics and Influencing Factors of Dissolved Organic Matter in Cave Drip Water—A Case Study from Furong Cave, Southwest China

“…These organic proxies include organic carbon isotopes [4], GDGTs [5,6], branched fatty acids, and hydroxy fatty acids [7,8]. Research on fluorescent organic matter in stalagmites has primarily focused on utilizing organic luminescence laminae as a research tool; the application of stalagmite organic matter fluorescence as a proxy has been employed to reconstruct paleoenvironmental and paleoclimatic changes [9][10][11]. Despite the significant achievements of organic matter in stalagmites in the field of paleoclimate reconstruction, the wide applicability and uncertainties of the proxy indicators in stalagmite-based paleoclimate reconstructions still require validation.…”