An interval of late-glacial to early Holocene sedimentation, spanning the period 12.9–8.4 ka BP (14C dated by accelerator mass spectrometry), is contained within 15 cm of gyttja in a core from a small lake on southwestern Cumberland Peninsula, Baffin Island. This sediment was subsampled in continuous 2.5 mm increments for diatom analysis. Extremely low sediment accumulation rates (~1.8 cm ka−1) are characteristic of the initial phase of organic sedimentation, but they increase rapidly (to 14.2 cm ka−1) after 9 ka BP. The first 0.5 cm of gyttja contained an acidophilous diatom flora resembling that of underlying mineral sediments. Thereafter, and throughout the late glacial and earliest Holocene, diatom floras were dominated by alkaliphilous and circumneutral species of Fragilaria. Around 9 ka BP, shifts to acidophilous floras dominated by Brachysira brebissonii, and, later, Eunotia rhomboidea and Frustulia rhomboides vars. saxonica and crassinervia, suggest a period of natural lake acidification. High diatom production accompanied the lowered lake-water pH, which reflects, respectively, the paleolimnological response to an early Holocene climatic optimum, and progressive depletion of lake alkalinity sources. There is no evidence of diatom or sediment responses attributable to the Younger Dryas oscillation, implying that deglacial reorganizations of the North Atlantic Ocean did not necessarily affect paleoclimatic conditions in the southern Cumberland Sound region.