Reconstructions of late Holocene paleofloods and glacier length changes in the Upper Engadine, Switzerland (ca. 1450 BC–AD 420)

Abstract

The relationship between summer-autumn floods in Central Europe and climate warming is poorly constrained by available instrumental, historical, proxy and model data. To investigate this relationship, a complete record of paleofloods, regional glacier length changes (and associated climate phases) and regional glacier advances and retreats (and associated climate transitions) are derived from the varved sediments of Lake Silvaplana (ca. 1450 BC–AD 420; Upper Engadine, Switzerland). In combination, these records provide insight into the behavior of floods (i.e. frequency) under a wide range of climate conditions. Eighty-five paleofloods are identified from turbidites in the sediments of Lake Silvaplana. Regional glacier length changes (and associated cool and/or wet and warm and/or dry climate phases) are inferred from centennial anomalies in the square root of low-pass (LP) filtered Mass Accumulation Rates (MAR LP 1/2). Regional glacier advances and retreats (and associated cooling and/or wetting and warming and/or drying climate transitions) are inferred from centennial trends in MAR LP. This is the first continuous record of glacier length changes in the Lake Silvaplana catchment for this time period. These data agree with regional records of land-use, glacier activity and lake levels. More frequent turbidites are found during cool and/or wet phases of ca. 1450 BC to AD 420. However, no relationship to climate transitions is discerned. Consistently, June–July–August (JJA) temperatures dating ca. 570 BC–AD 120 are inversely correlated to the frequency of turbidites. The rate that turbidite frequency increases with cooler JJA temperatures is not linear. Finally, 130 analogues for a 21st century climate in the Alps between ca. 570 BC–AD 120 (i.e. 50 year windows with a warming trend and average JJA temperature exceeding AD 1950–AD 2000 values from nearby meteo station Sils Maria) are considered. These reveal that turbidites are less frequent than between ca. 1450 BC–AD 420.