Relative sea-level changes in the Basque coast (northern Spain, Bay of Biscay) during the Holocene and Anthropocene: The Urdaibai estuary case

In order to reconstruct the environmental evolution process of the Urdaibai estuary in response to sea-level changes in the Basque coast (northern Spain, Bay of Biscay) during the last 8500 years, 10 boreholes were drilled in different estuarine areas using a rotary drill until the pre-Quaternary basement was reached. One manual short core (50 cm) was obtained from a salt marsh environment located in the middle part of the estuary. Micropalaeontological (benthic foraminifera), sedimentological (grain size) and geochemical (14C, 210Pb and 137Cs radioisotopes and total Pb and Zn) analyses were performed on these materials aiming to quantify the sea-level changes both of natural origin (Holocene) or derived from recent human activities (Anthropocene). Based on the obtained results, and by comparison with previously published information for this coastal area, the Holocene environmental evolution of the Urdaibai estuary has been interpreted as the result of relative sea-level variation that exhibits a rapid rise until 7000 cal BP followed by a moderate rise since then, and a stabilization during the last 3000 years until modern rates of sea-level rise (20th century: 1.7 ± 0.2 mm y−1) were reached.