Abstract
Marine isotope substage 11c (~426–366 ka) is widely regarded as the most recent Pleistocene interglacial that is broadly analogous to the Holocene. The timing and magnitude of sea‐level changes during MIS 11 have been much debated, particularly concerning whether sea levels were significantly higher than during the Holocene. Empirical evidence derived from terrestrial (onshore) records in northern Europe that might contribute to this debate has until now been rare. Here, we present data from three contemporaneous fluvial sequences in the lower reach of the ancestral River Thames, in which intrusion of saline water is marked by the appearance of fossils (ostracods, molluscs, vertebrates and foraminiferids) indicative of brackish and nearshore environments. The maximum input of saline water to each site was estimated using trace‐element ratios (Sr/Ca and Mg/Ca) for valves of the ostracods Cyprideis torosa (Jones), Scottia browniana (Jones) and Ilyocypris spp. Correlation with the Hoxnian pollen record places the onset of brackish conditions in the Lower Thames within the late‐temperate substage (Ho III) of the interglacial. Comparison with the tidal reach of the modern Thames shows that palaeosalinity values reconstructed at Clacton, presently situated on the coast, are equivalent to areas much further upstream during MIS 11. This is consistent with global records that place the MIS 11 sea‐level maximum later in the interglacial than the time represented by the Clacton sequence. The data have important implications for palaeobiogeographical reconstructions of the southern North Sea basin, particularly with respect to the first breaching of the Weald–Artois anticline and the opening of the English Channel, and the ability of early human populations to colonise Britain during MIS 11.