Decompacting a Late Holocene sea-level record from Loch Laxford, northwest Scotland.

Abstract

This thesis assesses the degree to which sediment compression affects a Late Holocene sea-level reconstruction from Loch Laxford, northwest Scotland. The reconstruction is based on analysis of a short sediment core (~0.7 m) that records a progressive increase in organic content up-core with loss on ignition rising from 5% at the base to ~60% at the surface. The core is decompacted using site-specific geotechnical data and a modified version of a previously published compaction model. Compression properties at the site, measured using geotechnical laboratory tests upon contemporary surface samples have significant correlations with key physical properties and environmental parameters. These data are combined to reconstruct compression properties downcore. Model predictions of bulk density, void ratio and effective stress downcore closely match those observed in situ (r2 = 0.80, r2 = 0.78 and r2 = 0.98 respectively), providing confidence in model accuracy. Modelled decompaction of the core produces estimates of post-depositional lowering (PDL, m), providing quantification of the degree to which sea level index points developed from the sample core have been lowered since their formation. A maximum PDL value of 0.013±0.005 m (2σ error) is identified between 0.35 and 0.45 m below ground level. Compaction therefore has minimal effect on reconstructed sea level at Loch Laxford and can be ruled out as a key control on the relative sea-level history at Loch Laxford.
The research also assesses the potential for geotechnical methods, particularly downcore void ratio, to identify erosive breaks in sediment successions that are not evident in routine litho- or bio-stratigraphic analysis. This allows a first-order assessment of the suitability of cores for high resolution sea-level studies without the need for the development of expensive age-depth models and time-consuming laboratory analyses.