New evidence of warm early-Holocene summers in subarctic Finland based on an enhanced regional chironomid-based temperature calibration model

Abstract

Paleoclimate reconstructions based on biological proxies present methodological challenges, especially during non-analog conditions, such as the early Holocene. Here, two chironomid-based training sets from Finland were amalgamated to create a more accurate transfer function of summer air temperature. The aim was to reconstruct Holocene paleoclimate in northernmost Lapland, in an area that has been either too warm or too cold for reliable reconstructions using the original calibration models. The results showed that the combined calibration model had improved performance statistics. The temperature trends inferred from the downcore chironomid record using the original and combined models were very similar. However, there were major changes in their absolute values with the combined model showing greatly improved accuracy. The chironomid-based temperature reconstruction showed significant correlation with the previous pollen-based reconstructions from northwestern Finnish Lapland. However, differences were observed in the temperature trends of the early Holocene, when the chironomid-inferred temperatures rapidly increased, but the pollen-based reconstructions lagged behind suggesting that a cool climate continued for much longer. However, similar to the chironomid record, new plant macrofossil evidence from northwestern Finland also showed warmer-than-present early Holocene temperatures. Therefore, we conclude that the early Holocene was probably warm in northern Lapland.

New Aspects of High-Mountain Palaeobiogeography: A Synthesis of Data from Forefields of Receding Glaciers and Ice Patches in the Tärna and Kebnekaise Mountains, Swedish Lapland

“…Despite the reservations and uncertainties outlined above, there is little doubt that the Holocene thermal optimum for the Scandes occurred in the earliest part of the Holocene, which concurs with multi-proxy inferences from other parts of Europe and Greenland (e.g., Paus, 2013;Luoto et al, 2014). Other studies suggest a substantially later thermal optimum (e.g., Berglund et al, 1996;Seppä and Birks, 2001).…”

Characterization of the Medieval Climate Anomaly, Little Ice Age and recent warming in northern Lapland

“…The chironomid‐based mean July air temperature (T Jul ) reconstruction used the regional expanded Fennoscandian calibration model (weighted‐averaging partial least squares) combining several data sets (Nyman et al , ; Luoto, ; Luoto et al , , ). The model has been downcore validated against meteorological data (instrumental measurements) using varved lake sediments by Luoto and Ojala () and applied in its current form by Rantala et al ().…”

A chironomid-based mean July temperature inference model from the south-east margin of the Tibetan Plateau, China

“…Merged regional chironomid training sets and combined inference models have been developed in Europe (Lotter et al, 1999;Holmes et al, 2011;Heiri et al, 2011;Luoto et al, 2014). These large data sets and models provide much more robust reconstructions than smaller local temperature inference models (Heiri et al, 2011;Luoto et al, 2014).…”

“…These large data sets and models provide much more robust reconstructions than smaller local temperature inference models (Heiri et al, 2011;Luoto et al, 2014). However, the distribution of large regional inference models is limited to Europe and northern North America (e.g.…”