Holocene floodplain vegetation dynamics and sea-level change in the lower Aire valley, Yorkshire
Kirby, Jason Robert
PublisherUniversity of Hull
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Perimarine river valleys are a rich resource for studying palaeoecological change which may provide information concerning catchment and sea-level change. This thesis reconstructs the environmental history of the lower Aire valley during the Holocene, and investigates the factors influencing floodplain and vegetational development.Reconstruction of environmental change in the lower Aire valley in the mid- to late-Holocene is based on lithological and palaeoecological records from three sites in the upper, middle, and lower parts of the study reach. Techniques used include pollen, diatom, wood macrofossil, loss on ignition, and radiocarbon analysis.Paludification of the valley floor was time transgressive, apparently responding to gradually rising sea level, beginning at c. 7000 BP (c. 8000 to c. 7600 cal. yrs BP) at the lowermost site, whereas conditions were not wet enough for preservation of organic sediment in the upper reach until c. 4200 BP (c. 5000 to c. 8500 cal. yrs BP). Accumulation of floodplain peat was interrupted by the deposition of finely laminated humic clays some time after c. 7000 BP (c. 8000 to c. 7600 cal. yrs BP) in the lower tract of the Aire valley, near Goole, suggesting a change to lagoonal conditions. This was apparently caused by the ponding of freshwater against the rising estuary. It is also possible that drainage was impeded, associated with widespread deposition of organic sediment in the lower valley areas, which may have contributed to the creation of a lagoonal environment. The lagoon had silted up by c. 6000 BP (c. 7200 to c. 6600 cal. yrs BP), probably due to an increase in tidal asymmetry, and range, which resulted in a net surplus of sediment into the floodbasins and enabled the re-invasion of fen carr onto the site.The main period of organic sedimentation lasted for several millennia at each of the study sites, during which time Alnus glutinosa fen carr communities dominated the wet floodplain backswamp areas. During mid-Holocene times, the vegetation of the surrounding dryland, was colonised by a mixed woodland, with Tilia, Ulmus, Quercus, and probably, Corylus avellana and Fraxinus excelsior. Pinus sylvestris was also prevalent in the region.Alnus carr was progressively replaced by fen meadow communities, and then saltmarsh or freshwater reedswamp communities, due to a phase of positive sea-level tendency, which was recorded throughout the lower Aire valley between c. 4600 and c. 2700 BP (c. 5500 to c. 2700 cal. yrs BP). Remnants of a possible upper peat unit and diatom evidence from the upper clastic sediment is tentatively interpreted as indicating the contraction of estuarine conditions and a phase of negative sea-level tendency some time during the late-Iron Age.