A first evaluation of the spatial gradients in δ18O recorded by European Holocene speleothems

Oxygen isotope data for well dated Holocene speleothems from Europe have been compiled for the first time. The data were analysed at 1 ka time slices through the Holocene by taking averages of 50 year duration. After filtering the data to exclude high altitude, high latitude and sites proximal to the Mediterranean Sea, the data exhibit surprisingly tight linear correlations between speleothem O isotope values and longitude. The slope of the data on δ18O vs. longitude plots changes systematically from the early to the late Holocene, exhibiting a much steeper zonal gradient in the early Holocene. Changes in the isotope gradient through the course of the Holocene reflect both a gradual increase in δ18O in speleothems from the western margin of the transect and a simultaneous decrease in speleothem δ18O on the eastern end of the transect. These changes follow summer insolation trends through most of the Holocene, but show marked deviations from c. 4 ka to the present day. Steeper early Holocene zonal isotope gradients are attributed primarily to a combination of early Holocene warming in the west and intense convective rainfall over the European continent in summer time driven by high early Holocene summer insolation. Although the absolute δ18O values preserved in speleothems do not precisely reflect the equilibrium values with respect to the waters from which they are precipitated, the tight isotope-longitude correlations indicate that speleothems are reliable recorders of combined rainfall O isotope signals and air temperature.