A high-resolution record of mid-Holocene climate change from Diss Mere, UK
Given that recent anomalous positive values in the North Atlantic Oscillation (NAO) may be related to anthropogenic forcing, the need exists to document more fully NAO evolution in palaeoclimate records. To this end, this thesis investigates whether NAO-type variability can be identified in biochemical varves of mid- to late-Holocene age from a lake sediment sequence in Diss Mere, UK. The varved record was examined using a number of cutting-edge technologies, with an emphasis placed on the time-series analysis approach to determine spectral properties of high-resolution palaeo-environmental time-series constructed from laminae-thickness measurements (a palaeoproductivity indicator) and XRF core-scanning for inter-annual records, principally, of elemental Ca-abundance, a proxy for summer temperature. For comparison, a low-resolution record of organic carbon burial was taken to record change in winter temperature. The in-situ chemical, biological and clastic constituents of individual varves were also examined using SEM-BSEI to investigate the annual cycle of sedimentation and variety of varve-fabric types. The results indicate that varve deposition is characterised by statistically significant interannual and bidecadal cycles that correspond to periodicities found in instrumental and other proxy records of the NAO. A shift from interannual to bidecadal cycles after 4000 Cal. BP is coincident with a rapid transition at Diss towards decreased seasonality during the late-Holocene that is also reflected in a distinct change in varve character and phytoplankton dynamics. This appears comparable, if not analogous, to the evolution of the modern NAO where a dominance of decadal variability since the 1950s is coincident with a tendency towards an NAO positive. It is hypothesised that change at 4000 Cal. BP and seasonality cycles thereafter have implications for the degree of longer-term predictability that may exist in the mean state of the NAO due to forcing by solar activity and orbital precession.