Some Geochronological Applications of 210Pb in the Coastal Marine and Freshwater Environments

Swan, David Samuel (1978) Some Geochronological Applications of 210Pb in the Coastal Marine and Freshwater Environments. PhD thesis, University of Glasgow.

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Radioanalytical techniques for 210Pb dating of sediments have been established, 210Pb is measured indirectly via a-spectrometric determination of its a-emitting grand-daughter 210Po, 226Ra is quantified indirectly via separation and gas-phase a-scintillation counting of its radioactive rare- gas daughter, 222Rn, Sediment cores recovered from the Cilicia Basin in the Eastern Mediterranean, Loch Lomond, a freshwater loch in west central Scotland, and Loch Goil and Gareloch, two fjord-like sea-lochs of the Clyde Sea Area, have been examined for their 210Pb and 226Ra distributions. Where appropriate 210Pb chronologies covering the last 100 to 150y have been developed. Complementary studies include radiocaesium and stable lead analyses, In interpreting 210Pb chronologies, the importance of correcting data for a number of sediment properties has been emphasised. Thus, (1) 226Ra concentrations should be measured throughout the length of cores before making correction for supported 210Pb contributions, (2) it is essential to correct 210Pb specific activities for variable water contents produced in surficial deposits by compaction processes, and (3) in the case of marine sediments, variable salt-residues from interstitial waters must be quantified and allowed for in 210Pb activity calculation Based on the exponential decrease of excess 210Pb with depth, a sedimentation rate of around 16 mg cm-2y-1(0.34mm.y-1) is estimated for the Cilicia Basin; this may, however, best be regarded as a lower limit estimate of the accumulation rate since there is considerable evidence to suggest that the sediment surface was not recovered during sampling. For Loch Lomond, a sediment accumulation rate of 22 mg. cm-2 y-1 (1.0 mm. y-1) is estimated. The presence of 137Cs in this core, largely confined to the upper 2 to 3 cms. of the deposit, is consistent both with the 210Pb-derived time-scale and with recovery of surface material, In the principal area of study, the Clyde Sea Area, 210Pb is shown to be in sufficient excess over that required for secular equilibrium with Ra to permit its use as a geo-chronological indicator in the region. In Gareloch and Loch Goil, considerable differences are apparent in the patterns and rates of recent sediment accumulation. For example, in Gareloch, a 210Pb-based sedimentation rate of 540 rag. cm-2y-1 (1.5 cm.y-1 ) is derived, while values of 200 mg. cm-2y-1 (6 mm.y-1) and 123 mg. cm-2y-1 (3.5 mm.y-1 ) are similarly estimated for two stations in Loch Goil. Sedimentation rates estimated from radiocaesium (134Cs and 137Cs) distributions are in generally good agreement to those based on 210Pb. The major source of 210Pb to the deposits is shown to involve river and stream particulate inputs. Modern fluxes of stable lead to the deposits are considerably greater than those of pre-industrial times. Both radionuclide evidence and X-radiographs of frozen cores indicate Loch Goil sediments to be homogeneously reworked by bioturbation to depths of 4 to 6 cms. Of particular interest, Ra concentrations are considerably greater in the upper few centimetres of Loch Goil sediments than those at depth. This 'excess' 226Ra is attributed to its enrichment in siliceous marine organisms, the remains of which are subsequently incorporated into bottom deposits. Sediment cores analysed in this study have been collected by a variety of coring devices, viz, conventional small- diameter gravity corers, mini-Mackereth and Craib corers, the latter two samplers being specifically designed to collect undisturbed samples of surface sediment. One of the major features of this study is the comparison of results from both Craib and gravity cores of Clyde Sea Area sediments. Highly significant differences in the vertical distribution of tracer species are evident in cores collected with these different samplers at identical locations. As a result, the applicability of conventional small-diameter gravity coring for collection of sediments is questioned, particularly in studies, such as 210Pb dating, to which recovery of vertically undisturbed material is critical.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Chemical oceanography, Geochemistry
Date of Award: 1978
Depositing User: Enlighten Team
Unique ID: glathesis:1978-78793
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:54
Last Modified: 30 Jan 2020 14:54

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