Two chironomid records provide new evidence from the Apennines (Italy) to assess the Holocene climate evolution of southern Europe

Background/Question/Methods

A wealth of quantitative climate reconstructions from northern and central Europe documents a warm early to mid-Holocene (Holocene Thermal Maximum; HTM) and a late Holocene cooling. Whether southern Europe underwent a cool or warm early to mid-Holocene is in the center of recent controversial debates, since pollen-based reconstructions suggest a potential early to mid-Holocene cooling with temperatures below the pre-industrial mean. However, mid and late-Holocene vegetation changes in the Mediterranean region where strongly influenced by past changes in the precipitation regime and by early human impact. At present quantitative, vegetation-independent Holocene temperature reconstructions from Southern Europe are missing to thoroughly explore this issue. In this study we used fossil chironomid head capsules to reconstruct past changes in the chironomid assemblages of lakes and, indirectly, to infer past environmental conditions. Detailed knowledge of chironomid distribution, together with the development of transfer functions, has allowed chironomid-based palaeotemperature records to be produced. With these transfer functions, estimates of past summer temperatures, with a prediction error typically in the range 1-1.5°C, can be produced from fossil chironomid assemblages. This study presents two newly developed chironomid-based palaeotemperature records, the first ones from the Appenines in Italy, to addresses the question whether southern Europe underwent a similar Holocene temperature evolution as elsewhere in Europe. 

Results/Conclusions

The most prominent feature of the Holocene temperature reconstructions from both lakes is the pronounced thermal maximum between ca. 9300-5000 cal yr BP, with a maximum warmth recorded at ca. 7400 cal yr BP with temperatures up to ca. 1.7°C warmer than at present. A distinct cooling trend that lasted until present is evident in both records after ca. 7000-6000 cal yr BP. Until ca. 7000 cal yr BP summer insolation, the disintegrating Laurentide ice sheet (LIS) and associated ocean current changes were probably the major driving forces of the Holocene millennial scale summer temperature evolution in northern Italy. Cooling after ca. 7000-6000 cal yr BP was apparently mainly controlled by declining summer insolation. A comparison of our results with other palaeoclimate records suggests that northern, central and southern Europe underwent a similar Holocene climate history with maximum summer warmth between ca. 8000-5000 cal yr BP followed by a late Holocene cooling. Our results are in agreement with dynamic climate modeling studies, furthermore a spatial climate analysis on historical data suggests that our data might be representative for the entire central Mediterranean realm, from the Alps down to the African coast.