Large-scale karst subsidence and accelerated fluvial aggradation during MIS6 in NE Spain: climatic and paleohydrological implications

Fluvial systems crossing evaporitic bedrock are commonly affected by karstic subsidence, which may result in river adjustments recorded by anomalous alluvial thickening. A remarkable study case is the Gállego River, at the Central Ebro Basin (NE Spain), where dissolution of evaporite bedrock gave rise to a subsidence induced depositional basin 30 km-long by 8 km-wide, infilled with alluvial sediments in excess of 110 m in thickness. A detailed morpho-sedimentary analysis of alluvial deposits supported with optically stimulated luminescence dating has revealed that paleokarst subsidence and accelerated fluvial aggradation occurred during discrete time periods, the most recent one being at 140-155 kyr (end of MIS 6). Subsequent alluvial terrace formation occurred at ∼110 kyr, 63-48 kyr and ∼17 kyr, linked to fluvio-glacial activity at the Pyrenean headwaters. Large-scale bedrock dissolution and associated alluviation appear to be sensitive to 1) long lasting periods of glacial outwash discharges; 2) high effective runoff from the extensive valley slopes, and 3) increased surface-groundwater exchange and groundwater circulation. The last large-scale subsidence episode (140-155 kyr) correlates with hydrological proxies reflecting a prolonged (141-149 kyr) salinity drop in the western Mediterranean, previously interpreted as linked to a Heinrich event, but which may instead represent an anomalous period of enhanced river discharge fed by extensively glaciated mountains.