Palaeosalinity change in the Taw Estuary, south-west England: response to late Holocene river discharge and relative sea-level change.

Present models of Holocene estuary evolution are driven largely by changes in relative sea-level (RSL) with little reference to long-term changes in fluvial regime and regional climate. Recent US studies of estuarine sequences have shown that decadal-centennial scale fluctuations in river discharge and freshwater inflow can be inferred by changes in estuarine palaeosalinity and that the timing of these events reflect changes in regional precipitation. It is therefore becoming apparent that estuarine sequences may hold an archive of mid-late Holocene climate change information, as well as being recorders of RSL change. The principal aim of this study is to produce a palaeosalinity-based climate record for southern England during the late Holocene, based on changes in climate-driven freshwater influx into the estuarine environment. The late Holocene palaeosalinity record of the inner Taw Estuary will be reconstructed using diatom salinity index as a proxy for salinity. Nine periods of below-average or above-average palaeosalinity have been recognised in the Taw Estuary since 300 cal.yr.BC. Four intervals of high river discharge are identified at 520-780, 850-1030, 1215-1315, and 1420-1900 cal.yr.AD. Five intervals of low river discharge are identified at 300-520, 780-850, 1030-1215, 1315-1420, and 1900-2000 cal.yr.AD. This shows that there has been significant climatic variation in southern Britain since c.300 cal.yr.AD, with climatic shifts evident in the estuarine record. In order to validate this record, the fluvial geomorphic history of the lower Taw valley was also investigated. There is a strong correspondence between the dry and wet climatic periods identified in the estuary and the geomorphic fluvial history and flood record of the lower Taw valley. Comparisons with other proxy climate records in the UK and Europe show a high degree of correspondence with the Taw Estuary palaeosalinity-based climate record. As the inner estuary environment will also be influenced by RSL change during the late Holocene, RSL change since c.6600 cal.yr.BP was successfully reconstructed in the Taw Estuary, with eleven new validated SLIPs providing evidence of former MSL. The magnitude and rates of RSL rise in north Devon are compared with other RSL records in southern Britain, suggesting that the isostatic history is similar to other areas bordering the Bristol Channel and with the central south coast of England. Fluctuations in palaeosalinity in the late Holocene are seen to be mainly controlled by centennial-scale changes in climate-driven river discharge, rather than RSL change