Geological record of the recent anthropogenic impact on salt marshes from the eastern cantabrian coastagricultural occupation, environmental regeneration and response to climate change

Resumen

Analysis of salt marshes in the Urdaibai, Plentzia and Santoña estuaries in the eastern Cantabrian coast, provided key information to define criteria to distinguish between areas of natural and anthropogenically-influenced evolution in the sedimentary record. A multi-proxy approach based on foraminifera, sand, organic matter and heavy metal content, together with aerial photography, was developed to characterize the two possible evolution paths. Radiocarbon and short-lived radionuclides ascribed an age to the sediments.Reconstructions of recent sea-level changes from natural salt marsh geological records provided a relative sea-level rise rate of 2.0 mm yr-1 for the 20th century, starting in the mid-l920s. This figure is in general agreement with the nearest instrumental record from Santander. The Brest tide-gauge record further north in the Bay of Biscay showed a similar trend (with a similar inflexion point), while the rates differed. When comparing the calculated relative sea-level rate for the last century to the calculated late Holocene background rate since 1300 CE (0.5 mm yr-1), the modern rate is 4 times higher and can be related to the anthropogenic impact of global climate change.Regenerated salt marshes in the same coastal area have shown increasing amounts of sand and foraminifera and very high sedimentation rates ( 14-18 mm yr-1) during the regeneration process, while the already regenerated environments show much lower sedimentation rates (0.9-6 mm yr-1), abundant agglutinated foraminiferal assemblages and enrichment of heavy metals due to recent industrialization or mining activities . The regeneration process in the Urdaibai estuary took place between the 1950s and 1960s, in the Plentzia estuary slightly later, since 1973-1984 CE, and in the Santoña estuary during the late 1950s-early 1970s but in particular areas much sooner, before the 1940s. In all locations the regeneration process was very rapid (less than 1O years), due to high sedimentation rates related to the abundant sediment available in estuaries, which permitted salt marshes to keep pace with the observed sea-level rise during the 20th century.These results are relevant to future predictions of coastal evolution and environmental management of modem coastal areas taking into account future sea-level rise, particularly in the eastern Cantabrian coast where extensive reclaimed land is still present, as these environments accrete sediment very fast to reach equilibrium with the tidal frame. The restoration of currently reclaimed tidal wetlands can serve as a cost-effective adaptation measure against climate change consequences on the coastal zone.