WACSWAIN project: Investigation of seasonality and annual signals in major chemical components and water isotopes in a deep ice core from Skytrain ice rise (Antarctica)

Abstract:

For any kind of ice core analysis and especially for dating, the identification of annual layers and therefore the deduction of a present day accumulation rate is crucial. Seasonal signals in ice cores are most commonly detected in variations of the water isotope content (δ¹⁸O). They are also found in atmospheric chemical markers, such as H₂0₂ and Methane Sulfonic Acid (MSA). In coastal Antarctic areas, sodium is also expected to show a seasonal variation, which is caused by changes in wind intensity and thus mobilisation strength from sea ice and open ocean between summer and winter. In this study, the 651 m deep Skytrain ice core was analysed using a continuous flow system (CFA) at the British Antarctic Survey. The top 250 m were investigated for seasonal variations. The analytical setup includes a fluorescence detection system for H₂O₂ and calcium, an ICP-MS instrument for cation measurements, including sodium, magnesium and calcium, a fast ion chromatography system for anion analysis and a Picarro CRDS for water isotope analysis. Clear seasonal cycles could be observed in the water isotope data, which are supported by discrete measurements from snowpit samples from the same location. Similar seasonal variation was observed in the H₂O₂, calcium and sodium data, leading to a near surface snow accumulation rate of about 20 cm/year. The typical seasonal amplitudes and phase shifts of major ion and isotope variations were investigated. The sodium ICP-MS data, showing the most distinctive variation, was subsequently used to perform annual layer counting for initial dating of the shallow part of the core. It was found that at a depth of about 250 m, an age of at least 1500 years before present is expected. At greater depths, the annual layer counted age-depth-relationship no longer followed its expected exponential decrease. This led to the conclusion that at this depth the annual layers are thinned and can no longer be reliably counted, given the depth resolution of the ICP-MS instrument (3.8 cm).