SEA-LEVEL RISE OVER THE LAST 1300 YEARS IN THE SOUTHERN BAY OF BISCAY

A detailed analysis of the 20th century global sea-level curve indicates a significant decadal variability and it has been pointed out a possible link between the strength of the atmospheric and oceanic gyre-scale circulation of the North Atlantic and sea levels. The geological record also indicates that this variability have existed at other temporal scales.

In this paper, we have produced a high-resolution (1 cm resolution) sea-level curve derived from a 38 cm long core of laminated mud (clay and silt) that encompassed the last 1300 years. The core was recovered from the Urdaibai estuary (southern Bay of Biscay), which is formed by the tidal part of the Oka river, covers an area of 765 ha, and occupies the flat bottom of the 11.6 km long, 1 km wide alluvial valley. Aerial and historical photography have been used to understand the most recent environmental evolution of this site since agricultural layers have been identified in many other salt marshes in the region. The environmental reconstruction has been done based on micropaleontological (foraminifera) and geochemical proxies (δ¹³C_org and δ¹⁵N). These geochemical proxies have been used in this region for the first time as sea-level indicators. This multidisciplinary approach aims to reduce the reconstructions errors. The chronology has been developed combining multiple radionuclides (¹⁴C, ²¹⁰Pb, ¹³⁷Cs) the history of atmospheric deposition of major elements (Pb, Zn, etc.) and polycyclic aromatic hydrocarbons (PAHs).

The benthic foraminiferal content in the core was very high and dominated by typical marsh species, this is supported by the isotopic signature. The ²¹⁰Pb_xs activity showed a typical exponential decline with depth, and the depth profile did not suggest any major mixing or disruption in the sedimentation. ¹³⁷Cs shows a clear subsurface maximum in activity and supports ²¹⁰Pb derived age estimates. PAHs and concentrations of total Pb and Zn profiles seem to reflect historic atmospheric emissions, supporting ²¹⁰Pb derived ages and extending the chronology back to the early 1700s. Two 14C dates complete the chronology for the last 1300 years.

The results suggest ca. 40 cm sea-level rise over the last 1300 years with half of this amount occurring during the 20th Century.