IMPROVING TIDE GAUGE SEA-LEVEL RECONSTRUCTIONS USING MODELLING AND FIELD OBSERVATIONS OF HOLOCENE SEA-LEVEL CHANGES ALONG THE ATLANTIC COAST OF SW EUROPE

During the Quaternary, the main control on the sea-level changes has been the exchange of mass between ice sheets and oceans with ice sheet growth inducing sea-level lowstands. However, local and regional changes become more important as the temporal scale resolution increases. Therefore, high quality relative sea-level data reveal spatial and temporal variations in crustal movements, which are used for many applications, ranging from calibrating models of earth rheology and ice sheet reconstructions to the development of coastal lowlands and human occupation, where local sea-level curves are required to predict future sea-level change. Rates of sea-level rise obtained represent the fundamental basis for comparison with the historical and present day changes. They provide a benchmark against which one must measure the additional sea-level rise that has occurred over the last 100-150 years.

In this paper, we compare published and new sea-level field observations for the Holocene between Brittany and Algarve and with glacial isostatic adjustment (GIA) models. The results obtained indicate an offset between model predictions and field observations in the south of Portugal when model predictions fit with data from the Bay of Biscay and Brittany. Although model predictions are sensitive to several parameters, this offset could be partially explained by long term tectonic uplift identified in the South of Portugal (less than 2 mm yr^-1). This finding is very important since, until recently, tide gauge data from this region used to assess global sea-level rise where only corrected for isostatic adjustment. A more robust approach involves the use of GPS-derived land motion which will encompass all processes affecting height changes of the tide gauges. However, GPS-derived vertical velocities are based on short time data and height determination using this technique is a delicate task because of several reasons, suggesting that an independent estimate of land motion should be required, therefore, more Holocene sea-level index points are required to obtain more accurate rates of vertical land motion and to test and validate GIA models.