Influence of hydrological and climatic parameters on spatial-temporal variability of fluorescence intensity and DOC of karst percolation waters in the Santana Cave System, Southeastern Brazil

Abstract

Fluorescence intensity (FI) and organic carbon concentration of groundwater percolating through soil and rock into the Santana Cave were monitored at eight different cave sites between 2000 and 2002 to investigate their relationships to climatic parameters, stalactite discharge and thickness of rock overlying the cave. FI values, compared among sampling sites, are inversely proportional to depth and directly proportional to discharge; in contrast, dissolved organic matter (DOC) shows no significant spatial variability. Time-series analysis demonstrated similarities in DOC trends of different waters, but no correlation was observed with FI trends. Combined evaluation of DOC of infiltration waters, rainfall data and chemical parameters of Fe, O 2, pH, Eh in soil solution indicate that peaks in DOC content coincide with more reduced conditions in the soil and have a lag time of 2–3 months after heavy showers. Variation of FI throughout the year occurs at all sampling sites but only higher drip discharge and rimstone pool waters were correlatable to rainfall events. FI of lower discharge sampling sites shows similar trends, but no relationship between drip discharge and rainfall variation was observed. Ranges and means of FI for all drip waters were significantly higher in the 2001–2002 period than in the preceding 2000–2001 period, which correlates with a 5.5 °C increase in mean austral winter temperatures in 2001. Hence, FI variations of karst waters that form carbonate speleothems under a humid subtropical climate may provide a useful proxy in paleoenvironmental reconstruction.