Surface mass balance and stable oxygen isotope ratios from shallow firn cores on Fimbulisen, East Antarctica E. SCHLOSSER, 1 H. ANSCHU ¨ TZ, 2 E. ISAKSSON, 3 T. MARTMA, 4 D. DIVINE, 5 O.-A. NØST 3 1 Institute of Meteorology and Geophysics, University of Innsbruck, nnsbruck, Austria E-mail: elisabeth.schlosser@uibk.ac.at 2 NGI Oslo, Oslo, Norway 3 Norwegian Polar Institute, Tromsø, Norway 4 Institute of Geology, Tallinn University of Technology, Tallinn, Estonia 5 Department of Mathematics and Statistics, University of Tromsø, Tromsø, Norway ABSTRACT. The mass balance of Antarctica is one of the crucial factors for determining sea-level change in a warming climate. The marginal zones of the continent, namely the ice shelves, are most sensitive to climate change. During the 2009/10 austral summer an extensive glaciological field campaign was carried out on Fimbulisen, an ice shelf in East Antarctica, to investigate its recent surface mass balance. Shallow (10–18 m) firn cores were drilled and accumulation rates and stable-isotope ratios determined. For firn-core dating, two different methods were compared: (1) seasonal variations of stable oxygen isotope ratios (d 18 O), and (2) dielectric profiling, including using the volcanic eruptions of Pinatubo, Philippines (1991), and El Chicho ´n, Mexico (1982), as time markers. The mean annual accumulation for the period 1992–2009 ranges from 298 to 349 mm w.e. a –1 . The interannual variability at the drilling sites is >30%. Accumulation rates show a weak decreasing trend during the past 20–30 years, which is statistically significant only for one of the cores. Stable-isotope ratios were compared to the snowfall temperature of Neumayer station. Neither the temperatures nor the d 18 O values show any trend for the investigated time period. INTRODUCTION The mass balance of Antarctica is one of the crucial factors for determining sea-level change in a warming climate. Higher precipitation due to higher saturation vapour pres- sure of warmer air could mitigate sea-level rise (e.g. Oerlemans, 1982). However, recent studies find no or only weak evidence for increase in temperature and/or surface mass balance (SMB). Combining modelling with field data, Monaghan and others (2006) investigated possible trends in Antarctic precipitation since the International Geophysical Year (IGY, 1957/58) and found no increase in the past 50 years. Accumulation studies with shallow firn cores find both positive and negative trends depending on the region and time period investigated (e.g. Isaksson and Melvold, 2002). Contradictory results have been found for the behaviour of temperature. Apart from the Antarctic Penin- sula, which clearly shows a strong warming trend during the past century, only weak and often statistically insignificant near-surface temperature trends are found, the sign of the trend being dependent on the investigation period (e.g. Turner and others, 2005; Monaghan and others, 2008). Generally, the spatial interpolation between the few avail- able stations remains a problem, in spite of the use of high- resolution numerical models. The marginal zones of the continent, namely the ice shelves, are the regions that are most sensitive to climate change. They are located in a temperature regime where positive temperatures and melting of snow already occur in the present climate (Kaczmarska and others, 2006). Thus, small changes in temperature and/or precipitation can have a large effect on the mass balance. Since ice shelves are in hydrostatic equilibrium with the ocean, the mass-balance change itself is not important quantitatively but as an indicator of climate change. Even more relevant is the strong influence the ice shelves have on ice-sheet dynamics and glacier outflow, especially in West Antarctica (Jenkins and others, 2010). Thus it is important to monitor the present conditions and possible recent changes on the ice shelves as exactly as possible. In this study, we present results from four shallow firn cores drilled on Fimbulisen, Dronning Maud Land (DML), in the 2009/10 austral summer, which cover approximately the past three decades. This study is part of a larger project, ‘Fimbulisen top-to-bottom’, with the overall aim of determining the total mass balance of the ice shelf combining glaciological and oceanographic methods (http:// fimbul.npolar.no). FIELD AREA AND PREVIOUS WORK Fimbulisen is one of the ice shelves that bound DML and is centred approximately at the Greenwich Meridian, with a north–south extent from 69.58 S to 71.58 S (see Fig. 1b). It is the largest ice shelf in the King Haakon VII Sea, the part of the Southern Ocean that borders DML in the north. The central part of the ice shelf is fed by Jutulstraumen, the largest outlet glacier in DML. Jutulstraumen drains an area of 124 000 km 2 , and the ice velocity is 1 km a –1 at the grounding line. Jutulstraumen causes the build-up of a fast-moving tongue within the ice shelf, Trolltunga (Fig. 1), surrounded by slower- moving ice to the west and east. Trolltunga sticks out into the sea at its northern end, which might lead to an accumulation regime slightly different from the rest of the ice shelf, due to higher wind speeds and possibly slightly higher temperatures because of the surrounding ocean water. Annals of Glaciology 53(60) 2012 doi: 10.3189/2012AoG60A102 70