EXAMINING THE EVIDENCE FOR A RECENT ACCELERATION IN THE RATE OF SEA-LEVEL RISE USING COMBINED INSTRUMENTAL AND PROXY DATA FROM THE BAY OF BISCAY

Concerns regarding the potential sea-level rise associated with anthropogenic warming of the atmosphere and oceans have resulted in increased interest in former relative sea levels (RSL). Late Holocene rates of RSL rise 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 in the last 150 years.

Recent research from the western margins of the North Atlantic has provided the first indication that modern rates of RSL rise (last 150 yrs) in this region may be more rapid than the long-term rate of rise (last 800 – 1000 yrs), and that the timing of this acceleration may indicate a link with human-induced climate change. No such data exists for areas on the eastern margins of the North Atlantic. We seek to address this imbalance by combining tide gauge and high-precision geological reconstructions of RSL from the northern (Brittany) and southern (Basque Country) marshes of the Bay of Biscay.

Considerable advances in high-resolution RSL reconstruction have been made in the last few years. These have centered on the development of microfossil transfer functions that quantify the vertical relationships between indicator species and tide level. We have developed transfer functions using a unimodal-based technique known as WA-PLS (weighted averaging partial least squares) based on 59 samples and 23 species obtained from four Basque marshes. Using component three, the observed versus foraminifera-predicted elevation relationship is robust (r2jack = 0.93). These results indicate that precise reconstructions of former sea levels are possible (RMSEP jack = 0.20 m). This relationship was employed to reconstruct past tide levels from sub-fossil assemblages contained within sedimentary sequences. We placed the foraminiferal-based reconstructions into a temporal framework to produce a RSL curve, where the chronology is established from the 137Cs and 210Pb-derived sediment accumulation rates. The resulting RSL curve represents a time span of ~200 years, with a sea-level rise rate of 1.4 mm yr-1 for the period 1810-1997, although the sea-level rise rate accelerates since 1930s (3 mm yr-1). Tide gauge data from the southern Bay of Biscay provides an estimate sea level rise of ~ 2.5 mm yr-1 since 1943.