Effects of glaciation on karst hydrology and sedimentology during the Last Glacial Cycle: The case of Granito cave, Central Pyrenees (Spain)

In Alpine regions, speleothem development on karst systems largely occurs during warm interglacial or interstadial phases due to their limited growth during cold stages. Still, recent attention has been given to the role of clastic sediments in caves, less dependent on temperature conditions. Yet, only a small number of caves worldwide preserve both speleothems and detrital deposits. Here we present an outstanding record of fine-grain laminated sediments and carbonate speleothems from the Granito cave (South Central Pyrenees, Spain) associated with seasonal to annual hydroclimatic pulses through the Last Glacial Cycle (LGC). Analysis of cave clastic facies together with new absolute dates on glacial deposits along the valley has provided new insights into the karst-glacial interactions in relation with long-term landscape evolution, with support of geomorphological, sedimentological, mineralogical, palynological, geochemical and geochronological data (U/Th series, OSL and IRSL).
The Granito cave was formed before 153.6±1.2 ky (Marine Isotopic Stage, MIS 6), as indicated by the oldest stalagmites dated in the cave, although speleothem formation occurred preferentially during MIS 5 and the Holocene interglacial stages. A 60 m thick clastic deposit was IRSL dated from 71.8±5.6 ky to after 26.5±2.5 ky corresponding with the maximum ice extent (MIE) during the LGC and subsequent glacial stabilization at 30.3±1.7 – 36.2±2.2 ky. The sediment infill was produced by water ponded in the cave coeval with the presence of glacial ice blocking the cave entrance. The cave clastic sequence includes channel, slackwater and backswamp facies, interpreted to result from pulses of sediment-laden water flow under vadose and phreatic flow regimes. The cave sediment’s mineralogical composition points to an exogenous source associated with glacial till sediments overlying the hillslope above the cave and along the Ara River valley. The palynological results obtained from the cave sediments show the dominance of an open landscape composed of steppe vegetation indicating cold glacial climatic conditions. The opening of the cave occurred before 14.9±1.5 ky, the stabilization age of the innermost Ara glacier moraine, giving rise to scouring and cut-and-fill sequences within the detrital infill. Speleothem growth during the Holocene indicates the return to warm climatic conditions with the development of a soil and vegetation cover above the cave. This study shows how a combined interpretation of clastic and speleothem lithofacies is critical for elucidating landscape evolution and surface-groundwater palaeohydrological changes in northern Iberia during the LGC.