Abstract

Ikaite is considered a metastable mineral forming and stable only at low temperatures and therefore an indicator of low-temperature carbonate precipitation often associated with cold marine seeps. It is found world-wide but most spectacularly in Ikka Fjord in southwest Greenland as submarine carbonate tufa columns. Here, ikaite is formed as a result of submarine spring water mixing with cold seawater. As ikaite disintegrates at temperatures above 6–7°C, it has been speculated that global warming could endanger this unique habitat as well as other sites. In Ikka Fjord in situ water chemistry in and around an ikaite column measured continuously over two years showed that the column water is alkaline (pH > 9–10) throughout the year with temperatures of −1.3–6.0 °C and conductivities of 5.7–7.9 mS cm⁻¹, favoring year-round growth of columns at 4–5 cm per month. Short-term in situ measurements with needle micro sensors from both older dehydrated and calcified parts and more recently formed solid parts of an ikaite tufa column showed similar pH and temperature values, including a temperature variation over the tidal cycle. In the uppermost, recently deposited ikaite matrix, spring water escaping at the top causes passive drag of seawater into the porous ikaite matrix, leading to a mixing layer several centimeters thick that has pH values intermediate to the spring water in the column and the surrounding seawater. We conclude that the main part of the columns, consisting of fossilized ikaite (inverted to calcite) partly sealed by calcifying coralline algae and with year-round flow of alkaline freshwater through distinct channels, are resistant to warming. In the more diffuse top part of the columns, the formation of ikaite, and thus column growth, will be limited in the future due to increased fjord water temperature during the < 3 summer months a year.