Lateglacial environmental and climatic changes at the Maloja Pass, Central Swiss Alps, as recorded by chironomids and pollen

Chironomids and pollen were studied in a radiocarbon-dated sediment sequence obtained from a former lake near the Maloja Pass in the Central Swiss Alps (1865 m a.s.l.) to reconstruct the Lateglacial environment. Pollen assemblages imply a vegetation development around the Maloja Pass from shrub tundra at the beginning of the Allerod to coniferous forest during the early Holocene with a lowering of the timberline during the Younger Dryas. Chironomid assemblages are characterized by several abrupt shifts in dominant taxa through the Lateglacial. The occurrence of taxa able to survive hypoxia in the second part of the Allerod and during the Preboreal, and their disappearance at the onset of the Younger Dryas cold phase suggest summer thermal stratification and unfavourable hypolimnetic oxygen conditions in the palaeo-lake during the warmer periods of the Lateglacial interstadial and early Holocene. Mean July air temperatures were reconstructed using a chironomid-temperature transfer function from the Alpine region. The pattern of reconstructed temperature changes agrees well with the Greenland delta O-18 record and other Lateglacial temperature inferences from Central Europe. The inferred July temperatures of ca 10.0 degrees C during most of the Allerod were slightly lower than modern values (10.8 degrees C) and increased up to ca 11.7 degrees C (i.e., above present-day values) at the end of the Allerod. The first part of the Younger Dryas was colder (ca 8.8 degrees C) than the second part (ca 9.8 degrees C). During most of the Preboreal, the temperatures persisted within the limits of 13.5-14.5 degrees C (i.e., ca 3 degrees C above present-day values). The amplitudes of temperature changes at the Allerod-Younger Dryas-Preboreal transitions were ca 3.5-4.0 degrees C. The temperature reconstruction also shows three short-lived cooling events of ca 1.5-2.0 degrees C, which may be attributed to the centennial-scale Greenland Interstadial events GI-1d and GI-1b, and the Preboreal Oscillation.