Seven glacial cycles in the middle-late Pleistocene of northwest Europe: Geomorphic evidence from buried tunnel valleys

The deep-ocean marine isotope record and the Antarctic and Greenland ice caps record numerous glacial and interglacial cycles since the middle Pleistocene, and yet evidence for similar numbers of ice-sheet advances over the continent and shallow shelves of northwest Europe is absent. Here, we document seven generations of regionally correlatable subglacial tunnel valleys that record the geomorphic imprint of ice sheets traversing the North Sea basin between ca. 500 and 40 ka, consistent with that predicted by proxy records of glacial and interglacial climate change. Over 180 subglacial tunnel valleys that incise into Pleistocene sediments in the North Sea basin were mapped over similar to 60,000 km(2) of three-dimensional seismic-reflection data. Using a subset of these data, we identified seven separate episodes of subglacial erosion that can be correlated regionally in the UK sector of the central North Sea. The characteristics of the valley morphologies, orientations, and infill stratigraphy indicate that each set of tunnel valleys formed during a separate ice-sheet advance and retreat cycle. Stratigraphic data suggest that the tunnel valleys formed significantly later than the Brunhes-Matuyama reversal event at 780 ka and before the Last Glacial Maximum (marine isotope stage [MIS] 2; 21 ka). These results imply a more complicated glacial history for northwest Europe and more glaciations than the three-glaciation model traditionally interpreted from the terrestrial record for the past 500 k.y. Our data provide the most complete documentary evidence for repeated advance and retreat of the northwest European ice sheets since the middle Pleistocene, and for the first time indicate that terrestrial ice-sheet advances in the North Sea can be matched in number with the cold events recorded in oceanic and/or ice-core proxies of climate change over the past 500 k.y.