Assessment of quantitative Holocene temperature reconstructions based on multiple proxies from the sediment record of Lake Loitsana, Sokli, NE Finland

Abstract:

Four biotic proxies (pollen, plant macrofossils chironomids and diatoms) are employed to quantitatively reconstruct variations in mean July air temperatures (T_jul) at Lake Loitsana, NE Finland, during the Holocene. The aim: assessing the reliability of these temperature reconstructions and the timing of highest T_jul. The reconstructed T_jul values are evaluated in relation to local-scale/site-specific processes associated to the Holocene lake development at Loitsana, as these factors have been shown to significantly influence the fossil assemblages found in the Lake Loitsana sediments. Our study shows that the reconstructions are influenced, at least to some extent, by local factors, and highlights the benefit of using multi-proxy data in Holocene climate reconstructions. While pollen-based temperatures follow the classical trend of gradually increasing early Holocene T_jul with a mid-Holocene maximum July warming, the aquatic/wetland assemblages reconstruct higher than present T_jul already during the early Holocene, i.e. at the peak of summer insolation. We conclude that the relatively low early Holocene July temperatures recorded by the terrestrial pollen are the result of site-specific factors possibly combined with a delayed response of the terrestrial ecosystem compared to the aquatic ecosystem.