Early afforestation on islands of proglacial Lake Ojibway as evidence of post-glacial migration outposts

摘要

At the end of the last glacial period in the northern hemisphere, meltwater from receding ice sheets accumulated into large proglacial lakes, potentially limiting postglacial afforestation. We explored whether former islands of proglacial Lake Ojibway (Canada) (hilltops in the current landscape) could have acted as migration outposts and thus accelerated the postglacial migration. We extracted sediments from two small lakes located on “paleo-islands” and used XRF to detect changes in soil erosion and vegetation biomass. We also used plant macro-remains and wood charcoal to determine if (and which) tree species colonized the sites and to detect local fire events. Organic sediment accumulation started around 9657 and 9947?cal. yr BP at Lakes Perché and Despériers, respectively, before the level of Lake Ojibway started to decrease and liberate parts of the studied landscape ca 9400?cal. yr BP. Lithogenic elements (Ti, K, Sr, Fe, Zr, and Rb) decreased between the beginning of organic sediment accumulation and 8800–8700?cal. yr BP, indicating reduced soil erosion, possibly due to soil stabilization by vegetation. Then, the S/Ti ratio, a proxy of organic matter increased around 8800 and 8400?cal. yr BP. The earliest tree macro-remains ( Larix laricina and Pinus spp.) were found between 9850 and 9500?cal. yr BP. Local fires were detected around 9820 and 8362?cal. yr BP. Early afforestation occurred on the islands of Lake Ojibway, 200 and 450?years before its level started to decrease, confirming that some islands acted as migratory outposts accelerating postglacial migration.