Climate and sea-level perturbations during the initial eocene thermal maximum:: evidence from siliciclastic units in the Basque Basin (Ermua, Zumaia and Trabakua Pass), northern Spain

Based on carbon isotope stratigraphy and lithology, a revised high-resolution correlation is proposed between sections spanning the Initial Eocene Thermal Maximum (IETM) in the Basque region, northern Spain. In three studied sections, the IETM event is represented by a prominent non- to low-calcareous siliciclastic unit (SU), 4-20.5 m thick. During formation of the SU, sedimentation rates for siliciclastic material increased by a factor of two to five, whereas detrital grain size decreased significantly. Kaolinite and chlorite became more abundant in the clay fraction and calcite dissolution in the water column intensified. The SU reflects a significant change in climate and hydrological regime towards a warmer and seasonally drier climate. The drying caused perennial vegetation cover to collapse and led to increased continental erosion but also to reduced hydrodynamic energy of freshwater debouching into the oceans. A coeval relative sea-level fall, probably related to tectonic uplift or doming affecting major parts of western Europe, increased the land-sea slope and further enhanced erosion rates. Palynological studies support periodic vegetation during seasonal rain instead of permanent conifer forests. Prominent caliche soils in coeval continental deposits in northern Spain also support warm semiarid conditions. During the TETM event, net evaporation increased under the subtropical high-pressure cells, including the Basque region, whereas more northerly latitudes became more wet, The SU has a similar origin as detrital units previously assigned to the early Palaeogene so-called 'Siderolithic' discharge in other parts of western Europe. (C) 2001 Elsevier Science B.V. All rights reserved.