Mineral Changes to the Tufa Columns of Ikka Fjord, SW Greenland

Abstract

The submarine tufa columns of Ikka Fjord in Southwest Greenland have been studied during multiple field campaigns since 1995. The fjord contains close to thousand columns previously shown to consist of the metastable carbonate mineral ikaite (CaCO3·6H2O), which requires near-freezing conditions to remain stable over longer periods of time. During a field campaign to Ikka Fjord in the summer of 2019, seawater temperatures of 6–9 °C and visual physical changes to the columns were observed. These are the highest recorded seawater temperatures measured in Ikka Fjord in over three decades of research. In response, three selected columns at three different locations were sampled at their bases, middle, and top sections for mineralogical analysis. These samples were supplemented by a four further column samples and an extensive hydrographical campaign during fieldwork in the summer 2021. Here, we report the results of the mineralogical analyses performed by X-ray diffraction and µ-Raman Spectroscopy on these column samples. The results show that the columns analysed now consist of the less hydrated carbonate minerals, monohydrocalcite (CaCO3·H2O), aragonite, and calcite (CaCO3). One of the columns has completely altered into monohydrocalcite, whereas the other columns have crusts of ikaite and cores of monohydrocalcite ± aragonite and calcite. This change is interpreted as a dehydration reaction and mineral alteration from ikaite to monohydrocalcite continuing to aragonite ± calcite in response to being bathed in warming seawater. Hydrographic profilers and static dataloggers recorded seawater temperatures of 4–8 °C in the column-containing fjord areas during June–August 2021. The upper parts of the columns are particularly exposed to temperatures > 6 °C, considered to be the long-term stability threshold of ikaite in Ikka Fjord. The mineral dehydration reactions are irreversible. It is therefore predicted in a warming Arctic, ikaite will only appear as new growth on the columns for a short period, and that with time, the columns of Ikka Fjord will change mineralogy into mainly monohydrocalcite.