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Title: Biostratigraphy and palaeolimnology of Late-glacial and Holocene lake marls at Quidenham Mere, Norfolk
Author: Horne, David Crawford
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1999
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In southern Britain there are relatively few natural lakes that have records extending back to the beginning of the Late-glacial. The exceptions are the Meres of central East Anglia that are infilled with organic sediments, providing detailed vegetational records. Of these, Quidenham Mere is unique because its sedimentary infill is composed largely of lake marls. These contain abundant molluscs and ostracods, as well as beautifully preserved pollen, covering most of the Late-glacial and Holocene. At Quidenham Mere, substantial thicknesses of sediment ( c.12 m) have accumulated relatively rapidly (at times 0.8 cm yr�'), which has enabled detailed palaeoenvironmental reconstructions. The history of the Mere has been reconstructed using a range of fossils, including molluscs, ostracods, pollen and Chara remains. A multiple profile approach utilising three cores, one from the centre and two from the lake margin, has allowed a more comprehensive reconstruction, than if a single profile had been used on its own. This approach was adopted because although Central cores are likely to yield continuous sedimentary records, they will not necessarily reflect the lake-level fluctuations that can be so obvious in � marginal sequences, where the frequency of shells is invariably higher. The use of geochemical proxies (e.g. stable isotopes and trace elements) of ostracod, mollusc and bulk carbonates have also contributed to a more meaningful understanding of the palaeolimnology of the Mere. The poor correlation (r < 0.7) between the 8180 and 813C values indicate that the Mere has been hydrologically open throughout its history. Obtaining a chronology from the lake sediments at Quidenham Mere has proved to be extremely difficult because of their calcareous nature, which renders them unsuitable for radiocarbon dating. In an attempt to provide a chronology, a variety of methods including radiocarbon, U-series (TIMS) and 210Pb dating have been utilised with varying success. U-series (TIMS) dating of the marls themselves proved problematic because of contamination by detrital thorium. Few terrestrial plant macrofossils could be recovered for radiocarbon dating, so the age model has relied largely on ages inferred from pollen stratigraphy. However, dating the Marginal cores on the basis of the pollen stratigraphy was not straightforward, because of the surprising difficulty in correlating the Central and Marginal sequences. The preservation of fossils in the lake sediments was remarkable. Remains of softtissues, including Zenker's organs and the hemipenis, survived in no fewer than 70 specimens (belonging to five species). The ramus of a furca, an important taxonomic character preserved in one specimen allowed attribution to Pseudocandona marchica. The remains of spermatozoa in a specimen of Candonopsis kingsleii, represents the first discovery of sperms in the entire fossil record. A number of proxies from the three profiles have each provided valuable information concerning the trophic status of the lake, together with the response of lake-level to climate change. The basal sediments consist of substantial thicknesses of chalk slopewash deposits that accumulated in shallow pools during the early part of the Late-glacial. The presence of a soil across the basin reflects a cessation in slope movement and a stabilisation in the climate during the Aller0d phase of the Late-glacial interstadial. This soil contains land snails, such as Punctum pygmaeum and Vertigo geyeri, whilst a fragment of wood has yielded a date of 11,385 � 75 yr B.P. The Younger Dryas has been difficult to distinguish palynologically but can be defined by the marked decline in values of 8180. The environment during this interval was characterised by a small, possibly ephemeral water-body supporting species such as Pisidium obtusa/e. In the early to mid Holocene, between I 0,000 and 5000 yr B.P., faunal evidence suggests that the water-depth fluctuated markedly, showing two main transgressions in lake-level, first in the early half of the Coryfu!r Querrus-Ulmus zone and the second shortly before the elm decline. After the elm decline (5000 yr B.P.) there was a regression in lake-level and a shift in the trophic status of the lake to a more nutrient rich state. Anthropogenically-induced disturbance (from 5000 yr B.P.), resulted in increased runoff from the catchment that may have directly controlled the carbon pool of the Mere and ultimately the 813C of the microfossils. Throughout the Anglo-Saxon and Medieval periods, Quidenham Mere was used for retting hemp, an activity that appears to have caused severe depletions in the aquatic invertebrates. The significance of the Quidenham sequence is discussed with respect to other important records that span this interval, including those from Hockham and Diss Mere, together with other sites across Europe. The lake-level variations, reconstructed from Quidenham Mere, appear synchronous with events seen in several other European lakes, suggesting a common climatic control.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral