Buoyancy-controlled melt-out: a missing end-member of ice rafted sedimentation

A previously undescribed mechanism of subaqueous sediment deposition from sediment-laden glacial icebergs is documented, in which progressive melting of sediment-laden ice results in gradual buoyancy loss and slow sinking to the sediment–water interface. Unlike dropstone impact or iceberg grounding, descent occurs almost imperceptibly and produces little or no deformation of the underlying sediment. Continued in situ melting deposits sediment directly onto the substrate, forming localized, cone-shaped or nest-like accumulations of clasts or mixed sediment. This process was observed during laboratory experiments originally designed to simulate dropstone deposition. In some cases, partial sediment release reduced bulk density sufficiently for the remaining ice to regain buoyancy and lift off, leaving annular or nest-like sediment accumulations surrounding a central low. These deposits resemble isolated mounds and clustered coarse-grained features previously attributed to iceberg dumping or grounding but lacking diagnostic deformation structures (e.g., Fecht and Tallman, 1977; Thomas and Connell, 1985). Although demonstrated using centimeter-scale experimental analogs, the process is applicable to natural icebergs capable of transporting coarse supraglacial debris. The mechanism provides a physically simple explanation for the emplacement of coarse sediment on fine-grained substrates without impact structures and should be considered an additional end-member process in glaciolacustrine and glaciomarine environments.