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BANDING IN MCMURDO FAST ICE

We report results from field investigation of banded features with differing optical scattering density occurring in columnar-congelation first-year landfast sea ice samples from the Eastern side of McMurdo Sound. To decipher the origin of these features, an analysis of the bands’ spacing is combined with high-resolution measurements of the chlorinity (as a proxy to salinity), total gas content and gas composition (O2, CO2, N2) in the white and dark portions of the bands at various levels in the core. It is shown that the banding corresponds to a 24 hours cycle throughout the core. It cannot therefore be attributed to surface temperature-driven growth rate fluctuations, since those are unlikely to show a daily cycle, or to sustain significant contrasts below 0.5 m depth. Total gas content and chlorinity show sympathetic fluctuations between the dark and the white layers forming the banding, therefore precluding a biological origin for the banding cycle. Boundary layer fluctuations driven by tidal currents emerge as the best candidate to explain the observed features. INTRODUCTION In many parts of the Earth’s polar regions, columnar sea ice shows the formation of bands, on the scale of up to tens of mm, with differing optical scattering density. The bands are formed during “direct” freezing (congelation) at the ice-water interface (as opposed to “frazil” or “platelet” ice crystals formation), but their exact nature and their formation processes are as yet unclear. Banding results from the regular alternation of darker and whiter ice layers, that are obvious when observing “half cores” (Figure 1, left) or thick sections in transmitted light (Figure 1, middle). In the latter case, however, the contrast is inverted. The layers which are more strongly scattering, appearing white in reflection but dark in transmission, will be described below as the “white layers”, and we will refer to a “band” as a given dark-white layer association. A common intuitive interpretation of this banding process from the sea ice research community is that it results from growth rates fluctuations and associated gas phase 1 Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles, Belgium 2 School of Chemical and Physical Sciences, Victoria University, Wellington, New Zealand 3 Industrial Research Ltd., PO Box 31310, Lower Hutt, New Zealand Figure 1: Structural features of core McM99 4C shown on two half cores (left), thick sections (middle) and thin sections viewed between crossed polarizers (right). Bottom right shows the reproducibility of features between cores McM99 4C and McM99 4D. changes at the interface, the whiter layers reflecting a higher bubble content. However, little attention has been paid until now to the detailed processes involved. It is the aim of this paper to provide a geometrical, thermodynamical and physico-chemical data set to get a better insight into those processes. SAMPLE LOCATION AND ANALYTICAL PROCEDURE Four landfast first year sea ice cores have been collected in the framework of a study on biogeochemical properties of sea ice at Arrival Heights, on the Eastern side of McMurdo Sound (Figure 2). A fuller description of the ice at that location and season is given by Trodahl et al. (2001). The cores were shipped back to Brussels in a reefer container equipped with a datalogger showing that the temperature was kept below –28 °C at all times, therefore preventing post-sampling relocation of impurities. Two of those cores have been sliced in two halves and thick and thin sections performed following the standard microtoming procedure of Langway (1958), in order to describe the structural and textural properties of the sea ice cover. To achieve the high resolution imposed by the scale of the banded features, chlorinity has been measured as a proxy to the bulk salinity. Cl measurements were performed using a Dionex DX100 ion Chromatograph, a technique that only requires 0.5 ml of sample volume. Precision of the measurements was ±5 %. Total gas volume (content in ml STP of gas per kg of ice) was measured using a melting-refreezing method coupled with a Toepler pump extraction (Blunier et al., 1993; Raynaud et al., 1988). The relative standard error was ±2–5 % (Martinerie et al., 1994). Gas composition was measured using the dry extraction technique (crushing at –55 °C in the cold room) described for glacial ice by Raynaud et al. (1982) and Barnola et al. (1983). Since the samples are crushed at very low temperature (–55 °C) and under vacuum (10 Torr), the technique measures the gas composition both from gas bubbles and gas dissolved in the brines of the sea ice. The extraction line is used in conjunction with a Varian 3300 gas chromatograph. Oxygen and nitrogen were analyzed using a molecular sieve column and a thermal conductivity detector. Carbon dioxide was analyzed using an Haysep column and flame ionization detector following catalytic transformation into methane. Different standard mixtures of gases (Praxair) were used for calibration. Given the small amounts of samples and low total gas contents, the precision of the measurements was ±5 % of the measured CO2 concentrations and ±0.8 % for O2 and N2. Because of the relatively large amounts of ice necessary for the gas measurements (about 40 g for total gas content and 80 g for gas composition), we had to use two Figure 2: Location map of the sampling site at Arrival Height in McMurdo Sound, Antarctica. Mount Erebus Hu t P oin t P en in su la MMurdo Ice helf McMurdo Sound ROSS ICE SHELF ROSS SEA