MAXIMUM EXTENT OF GLACIERS AND CHRONOLOGY OF GLACIAL OUTWASH EVENTS IN THE SPANISH PYRENEES FROM REGIONALLY CORRELATED FLUVIAL TERRACES

Records of glacial activity in the Pyrenees are preserved in the adjacent Ebro basin (Spain) as fluvial outwash terraces along the Cinca and Gallego Rivers. Ages for terraces derived from OSL dates on sand lenses overlying terrace straths agree with dates related to the maximum extent of late Pleistocene moraines. This correlation suggests terrace aggradation is related to deglaciation, supporting a link to glacial activity. Time-related trends in soil development confirm radiometric ages of alluvial terraces and glacial deposits obtained by radiocarbon, luminescence, and magnetostratigraphic methods. Terrace soils show strong trends related to pedogenic accumulation of calcium carbonate and Fe oxides, carbonate morphology, and total profile thickness.

We report the first chronology of a sequence of glacial outwash events in the central Pyrenees and adjacent Ebro Basin. We document glacial retreat at ~10 ka [5-16 ka (Younger Dryas)], ~36 ka [34-46 ka; marine isotope stage 3 (MIS 3)], ~53 ka [45-58 ka (MIS 3)], ~60 ka [53-84 ka (MIS 4), and ~85 ka [75-113 ka (MIS 5b)] with the maximum extent of glaciers in the Pyrenees occurring at ~60 ka in the Cinca River valley and 85 ka in the Gállego River valley. Our terrace chronology is consistent with millennial-scale vegetation changes in the Mediterranean region, although uncertainties on our luminescence dates prevent exact matching of climatic events. The absence of fluvial terraces of ~14 kyr age, and the previously documented absence of large volumes of ice in the Pyrenees during the Last Glacial Maximum (LGM; 18-20 ka) suggest low precipitation in the Pyrenees at that time. Glacial advances and retreats in the Pyrenees Range appear linked to the position of the polar front in the North Atlantic Ocean. The position of the polar front during the YD and MIS 3, 4, and 5b was near its mean position, allowing warm subtropical water to circulate to the northwest of Iberia, resulting in transfer of heat and moisture to the atmosphere in the path of the prevailing westerlies that bring storm tracks across Iberia. The zonal position of the polar front at ~43ºN during the LGM resulted in an extreme southerly position for the jet stream such that polar conditions prevailed in Iberia while Tropical Depressions delivered enough precipitation to the Sahara to allow expansion of lacustrine deposition.