In this study, we measure the silicon-isotopic composition d 29Si,‰, relative to NBS28 quartz standard) of dissolved silicon and biogenic silica collected by sequential melting from spring Antarctic pack ice collected near 117°E-64.5°S. This work aims to investigate the use of Si isotopes to quantify the activity of sea-ice diatoms in the different brine structures and the influence of sea-ice diatoms on the spring ice edge blooms. From three cores with contrasted physico-chemical characteristics, we report significant isotopic fractionations linked to diatom activity with distinct silicon biogeochemical dynamics between different brine structures. The diatoms in snow ice and in brine pockets of frazil or congelation ice have the heavier silicon-isotopic composition (+0.53 to +0.86‰), indicating that they grow in a closed system and use a significant fraction of the small dissolved silicon pool available. In the brine channels and skeletal layer, diatoms display a relatively lower silicon-isotopic composition (+0.41 to +0.70‰), although it is still relatively positive compared to expected equilibrium fractionation. This suggests that the diatoms have grown in a semi-closed system where the dissolved silicon pool (i.e., brine) is partially replenished. The silicon-isotopic composition (+0.63‰) of the sea-ice diatoms is much heavier than the one of biogenic silica in the seasonal ice zone mixed layer (+0.09‰). Our results suggest that sea-ice diatoms either contribute to an insignificant part of the whole diatom biomass in the upper water layer, or that they are directly exported below the mixed layer.
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