THE FACTORS AFFECTING DISTRIBUTION AND PRESERVATION OF MICROTEPHRA PARTICLES IN LATEGLACIAL SEDIMENTS

Tephrochronology offers great potential for the precise correlation of late Quaternary sedimentary sequences. It is especially useful during the last glacial-interglacial transition (LGIT) (14-9 14C ka BP) where radiocarbon dating is hampered by a number of radiocarbon ‘plateaux’. Recent years have seen a number of new discoveries in the extent of LGIT tephra distributions throughout Europe. Many of these advances are due to new techniques in the extraction of far-travelled microscopic tephra (microtephra) particles of <100 micrometres in size. The work presented here stems from these discoveries where it was noticed how widely one particular and highly visible tephra varied in its within-basin distribution.

A number of field sites of varying catchment sizes have been examined in Scotland: two sites on the Isle of Skye (Loch Ashik and Loch Druim) and a site on Mull (Loch an t’Suidhe). The principle hypotheses being examined in this work are that tephra distributions and concentrations are controlled by catchment size and proximity to basin inputs. Loch Ashik and Loch Druim represent two extremes in lake area to catchment ratio: Loch Ashik has an extensive catchment with two main input areas and a single output, while Loch Druim is an isolated basin possessing virtually no catchment and no inputs/outputs.

A three-dimensional depositional model based on data gathered from these sites is in development that can be used to predict coring positions for optimal tephra shard recovery in other basins. The Loch an t’Suidhe site will make a good test of this model as it has a catchment size between that of Loch Ashik and Loch Druim with shard concentrations predicted to reflect this.

A number of consistent peaks in tephra concentration have been found throughout the cores examined thusfar, though positive identification by electron probe microanalysis (EPMA) has yet to be performed. From their stratigraphic context, however, it is reasonable to assume they are the Borrobol Tephra (12.26 14C ka yr BP), the Vedde Ash (10.3 14C ka yr BP or 11.98 ka GRIP ice core yrs BP) and Saksunarvatn Ash (9.0 14C ka yr BP or 10.24 ka GRIP ice core yrs BP. Additionally, there is one further as yet unidentified Holocene tephra.