"Understanding the Climate Behaviour of the McMurdo Dry Valleys, Antarctica, From Coastal Ice Cores"

Abstract

The aim of the study is to investigate the climate signal contained in coastal ice cores adjacent to the McMurdo Dry Valleys (MDV) in the Ross Sea region, Antarctica. The MDV are one of the most climate sensitive regions in Antarctica, and ice cores from low elevation piedmont glaciers on their coastal margin contain information on climate variability during the late Holocene that is not resolved in ice cores from the Antarctic interior. Five snow profiles and three firn cores were collected along a coast-to-plateau transect across Victoria Dry Valley. The elevation of the sites range from 50m to 2400m and distance from the coast from 8km to 93km. The snow profiles are between 0.8m and 4.5m deep and provide a climate record for the last 4 to 42 years. The firn cores are between 22 and 33m long and present a climate record for the last 400 to 800 years. Chemistry concentrations (major ions, methylsulfonate, and trace elements), isotopic composition (δ^18 O and δD), density and dust content of the snow and firn samples were analysed. Borehole temperature, mass balance and ground penetrating radar measurements provided site-specific background information. Annual chemistry variations and radioactivity fallout from the nuclear testing have been used to date the snow profiles. The firn records were dated using a simple firn densification model. While dating uncertainty in the snow profiles is thought to be <±2years, dating errors associated with the modelled age of the firn cores are potentially large. The results show that the seasonality of the aerosol input differs from other ice core records in Antarctica. During summer aerosol input is 3-5 orders of magnitude higher than during winter and dominated by seasalt species from a proximal source, the open Ross Sea. During winter the aerosol input is dominated by long-travelled seasalt species derived via the East Antarctic Ice Sheet and Ross Ice Shelf. Dust input is episodic and likely caused by katabatic storms sweeping through the MDV. MDV climate appears to be sensitive to changes in solar variability. Decadal to centennial fluctuations in aerosol input are examined and explained by strong albedo differences between the MDV and their ice-covered surroundings. These albedo contrasts amplify the small change in solar energy output caused by solar activity. A conceptual model is used to explain the forcing mechanism. During the mid/late 19th century the MDV experienced a rapid warming which coincides with the termination of the last Little Ice Age cooling. The inferred warming during this time is +4.8K/deeade and +5.8K/desade at VLG I and BVG, respectively. However, the isotopic shift is partially caused through enhanced influence of Ross Sea airmass precipitation, which is in comparison to EAIS airmasses isotopically less depleted. On a shorter timescale it is suggested that MDV temperature variability is linked to the El Niño Southern Oscillation. During El Niño mode, the position of the Amundsen Sea Low shifts further to West Antarctica, intensifying katabatic flow across the Ross Ice Shelf to the western Ross Sea. The enhanced influence of colder airmasses during such events seems to be responsible for the observed cooling in the MDV since 1985.