Holocene relative sea-level changes in, Donegal, north-west Ireland: an empirical test for glacial isostatic adjustment models.
Dr Jason Kirby -- Subject Head for Geography and Environmental Sciences, Liverpool John Moores University
Abstract: This paper presents a new mid-late Holocene relative sea-level record compiled from saltmarsh deposits in north-western Ireland. The record is derived from 10 radiocarbon and 12 210Pb dated sea-level index points, collected from a 2 metre sediment sequence at Bracky Bridge, Donegal. The relationship between these samples and a former tidal height (i.e. indicative meaning) has been established using the known elevation of modern foraminferal and diatom assemblages in relation to Irish datum. GIA models have been presented that indicate a mid to late-Holocene RSL high-stand of between +0.5 and +2 m above present in Donegal, with RSL gradually falling to the present day. Despite numerous attempts to alter the input parameters of these models, the resulting RSL predictions still do not fit with existing geological sea level data, particularly for western Donegal. Furthermore, good quality validated sea-level data are lacking which hampers attempts to resolve this discrepancy.
Our reconstruction shows that there has been 1.8 metres of relative sea-level rise in the past 2500 years and we do not find evidence in support of a RSL high-stand above present sea level. Whilst data for the mid-Holocene are limited, we present a dated basal peat sequence which appears to constrain a possible RSL high stand which reached a minimum altitude of +0.48 m Irish Grid c.2900 BC. The stratigraphic data suggest that RSL then fell before gradually rising to the present day from (or before) c. 700 BC. These findings have implications for the inputs to geophysical models used to estimate the pattern of postglacial RSL in Ireland (resulting in the over-prediction of the amount of isostatic rebound) and the geographical positioning of the Holocene 0 m isobase.