Concentration and isotopic composition of bromine and chlorine in Antarctic sea ice

This work presents the results from the first investigation of Br and Cl isotopic partitioning in Southern Ocean sea ice. The sea ice samples were collected during the Australian-led Sea Ice Physics and Ecosystem eXperiment-2 (SIPEX-2) marine voyage in austral spring of 2012 (26 Sept – 10 Nov), within the area bounded by 115–125°E and 62–66oS off the East Antarctic coast. Multicollector inductively coupled plasma mass spectrometry was used to provide isotopic data for bromine and chlorine. Bromine concentrations ranged from 710 ± 20 μg L−1 to 31440 ± 570 μg L−1 and chlorine concentrations ranged from 218 ± 12 mg L−1 to 7945 ± 351 mg L−1. Bromine and chlorine concentrations were positively correlated with a slope of 0.0038, consistent with the seawater Br/Cl ratio of 0.0035. Bromine isotopic compositions (denoted δ81Br with respect to SMOB isotopic reference) varied from −0.64 to 0.10‰ with a mean of −0.16‰ and standard deviation of 0.04‰. Chlorine isotopic compositions (denoted δ37Cl with respect to SMOC isotopic reference) varied from −1.10‰ to 0.43‰ with a mean of −0.26‰ and standard deviation of 0.10‰. Despite their strong concentration correlation, there was a much less consistent relation between Br and Cl isotopic compositions and great variability within individual sea ice cores. The samples showing the largest degree of fractionation in their Br and Cl isotopic compositions were found deepest in the sea ice samples. Multiple parameters may influence isotopic fractionation processes in sea ice, including ice texture, age, salt content, and algal biomass content (represented by chlorophyll-a concentrations), although no single factor was found to consistently correlate with the bromine or chlorine isotopic signature. We consider the possibility of isotopic fractionation resulting from brine rejection and desalinization processes.