Interglacial Extreme Floods and Their Implications for Climate Instability in the Ipswichian (Eemian) Stage for the River Great Ouse, Southeast England, U.K.

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Abstract This paper outlines a sedimentological study of an extreme flood event for the Last Interglacial or the Ipswichian (Eemian) Stage. A channeled bedrock surface and associated fossiliferous diamicton deposits occur below the Ipswichian floodplain alluvium at Woolpack Farm and the adjacent area in the lower middle reach of the River Great Ouse, southeast England. Detailed geological observations suggest that these features were derived from an extreme flood event, probably connected to an exceptionally large storm surge from the North Sea. The waves accompanying the surge not only caused deep entrenchment of the ground but also entrained and transported the brackish-water gastropod Mercuria confusa to the southern margins of the Fenland basin, to an elevation about 2.5 m above the normal marine to brackish influence in this region. At the recession stage, cohesive debris flows developed, emplacing in the scoured channels diamicton deposits containing numerous tree and animal remains. The catastrophic impact exerted by the flood on the landscape has led to its preservation in the bedrock. In comparison with modern storm surges and in light of the absence of comparable geological features in the Holocene, the flood must have been far greater in magnitude, a response to intensified atmospheric circulation perturbation under unstable atmospheric or climatic conditions during the thermal maximum of the Ipswichian at circa 125 ka. Stable climatic conditions probably developed afterwards as indicated by the subsequent accretion of thick argillaceous alluvium characteristically under frequent, relatively low-amplitude flood conditions.