Summer temperature gradients in northwest Europe during the Lateglacial to early Holocene transition (15-8 ka BP) inferred from chironomid assemblages

摘要

We present a series of summer air temperature isotherm maps based on chironomid-inferred temperatures from northwest Europe, covering the Lateglacial and early Holocene (15-8 ka BP). These maps are the first of their kind, and use data derived from 22 Lateglacial sites and 34 early Holocene sites. The isotherms are generated by weighted spatial interpolation (kriging). The major patterns of chironomid-inferred summer temperatures are spatially well-resolved in both the Lateglacial and early Holocene. The isotherm maps indicate that there was a strong west to east gradient during the Lateglacial Interstadial (GI-1) due to the influence of thermohaline circulation in the regions bordering the north Atlantic, which diminishes eastwards. A strong north to south temperature gradient is also apparent, particularly in eastern regions, influenced by the extent of the Scandinavian ice-cap. Peak temperatures are achieved early in the Interstadial in the south of the region but occur towards the end of the Interstadial in the north. Holocene warming varies spatially and temporally and is earliest in the south and east, but later in the north and west. During the period covered in our study maximum warmth is reached ca. 10 ka BP. The chironomid-based Lateglacial isotherm maps are compared with previously published isotherm maps from the same region based on beetle-inferred temperatures. While the trends shown in the two datasets are similar, beetle-inferred temperatures are often warmer than chironomid-inferred temperatures. This is especially marked in GI-1e and may be due to microdimatic effects causing the chiron-omids to underestimate air temperatures and/or the beetles to over-estimate air temperatures. The spatial coherence between sites in both the Lateglacial and early Holocene suggest that the chironomid-based temperature estimates are largely reliable, although data testing suggests that estimates from southern Scandinavia may be less reliable perhaps due to high topographical relief influencing local climate. More data points are required, particularly from northwest Scotland, southwest England and Wales, northeast France, Denmark, Finland and the Baltic States, to confirm trends and provide even coverage and a denser network of sites.