Sediment transport during recent cave flooding events and characterization of speleothem archives of past flooding

Extreme precipitation events have a disproportionate impact on landscape evolution and on human constructions, and the frequency and intensity of extreme events may vary with changes in mean climate in the past and in the future. We document the potential for cave sediment archives to record past changes in the frequency of flooding events by characterizing the modern processes responsible for sediment deposition and mobilization during floods and linking these to their incorporation in speleothem deposits. We focus on the Cueva Fria cave system in NW Spain, which like other caves in the region features abundant sandy sediments sourced from dominant sandstone lithologies in the catchment area, in this case delineated by our dye trace study. In this system, the extreme flood conditions in June 2010 caused overflow of the cave stream and deposition of nearly 1 m of sand in sectors near the cave stream and resulted in burial of some stalagmites in > 30 cm of sand in more distal sectors. All sectioned stalagmites growing < 1 m above the overflow channels in these three caves feature sand and silt layers deposited during flood events. Although the high detrital content of these recent materials precludes U/Th dating, radiocarbon dating (especially of actively growing stalagmites) may provide suitable chronologies for reconstruction of past flood chronologies. The basal dates of stalagmites formed in the main overflow channels in Cueva Fria and two other caves in the region cluster around 4 ka. We infer that stalagmite growth in channels was favored by a reduced incidence of flooding in the late Holocene, concomitant with regional drying recorded in numerous terrestrial and marine archives.