Abstract

Ikka Fjord in southwest Greenland is the type locality for ikaite, a metastable hexahydrate of calcium carbonate. Here, ikaite forms submarine tufa columns rising up to 18 m above the fjord bottom. The columns are spectacular examples of abiogenic carbonate precipitation in a cold seawater environment and so far represent the only known formation of ikaite in seawater. We have analyzed the 2H, 13C, 14C, 18O and 87Sr contents and major-element and minor-element compositions of ikaite and of the different water types in and around Ikka Fjord in order to understand the formation of the Ikka columns. Water from inside the columns is fresh but alkaline, rich in ions of sodium, bicarbonate, and carbonate, and has a stable-isotope composition identical to precipitation falling on the nearby Grønnedal-Ika igneous complex, and we conclude that the tufa columns form over submarine springs issuing from the bottom of Ikka Fjord. Dissolution of unidentified sodium carbonate minerals in carbonatites in the igneous complex probably accounts for the peculiar chemistry of the springs. Ikaite supersaturation and precipitation is controlled by mixing of spring water and seawater at the cold (< 6°C) fjord bottom, and precipitation is accelerated by the formation of hydrated CaCO3o (aq) ion pairs, while phosphate ions in the spring water act as inhibitor for precipitation of anhydrous carbonates. We propose that the spring water seeps in Ikka Fjord literally create their own conduits in the form of the vertical, chimney-like columns with ikaite forming continuously at the tops of the columns. The only limits to upward growth are winter ice and the fresh-water layer capping the fjord during summer.

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