Monohydrocalcite (MHC) has previously been known only from fresh-water and terrestrial environments. However, as documented in the present paper, MHC is a common mineral (up to 55%) in debris fragments from submarine ikaite (CaCO3·6H2O) tufa columns in the cold marine Ikka fjord, SW Greenland. The columns form where alkaline fresh water from submarine springs mixes with cold seawater and precipitates ikaite. Petrographical and geochemical data indicate that MHC represents a transitional diagenetic phase in the recrystallization process of metastable ikaite to final calcite. The stable-isotope data (δ13C and δ18O) document a closer relationship of MHC with the precursor ikaite than with calcite. Replacement of ikaite by MHC is suggested to take place by dissolution and precipitation in a semi-closed pore-water system dominated by alkaline spring water and with only limited access of seawater. In this environment, nucleation of calcite is inhibited by phosphate ions. Apart from calcite, aragonite and hydrous Mg carbonates were also observed in the tufa debris. The Mg carbonates represent late diagenetic phases, probably formed from recrystallization of MCH in normal seawater environments.