The study of tidewater glacier systems from the Last Glacial Maximum to the Present in southern Patagonia provides a relatively continuous high-resolution sedimentary record of climate change. This record enables us to establish the style and rate of sedimentation in the basin, as well as to determine the glacial history of the fjord with respect to the surrounding regions and hemispheric climate forcing. A comprehensive marine geologic study was conducted in Marinelli Fjord (54°S), southern Chile. A major transition is sedimentation at 12.5 ka marks the retreat of Marinelli Glacier with no evidence for glacial re-advance. This is consistent with the climatic trends for southern South America where the final deglaciation of the region ended at ∼12 ka, indicating a Southern Hemisphere climatic forcing. Marinelli Glacier has undergone a modern rapid retreat that is unprecedented in the time interval we recorded and is not related to hemispheric-scale forcing mechanisms.