Post-glacial history of Lake of the Woods

Lake of the Woods covers ~4000 square kilometers and lies in the giant glacial Lake Agassiz basin of Ontario, Manitoba, and Minnesota, which was once the largest lake in the world. Ten sediment cores were collected by University of Manitoba geologists Jim Teller and Trevor Mellors, using a gravity coring system operated by a team from the Limnological Research Center at the University of Minnesota. These cores reveal dramatic hydrological changes in the lake during the past 12,000 years.

Sediments in the six cores studied consist mainly of laminated silty clay. Sedimentological, mineralogical, paleobiological, and stratigraphic analyses, constrained by 38 new AMS radiocarbon dates, reveal the post-glacial interplay of varying climate and differential isostatic rebound.

The most notable sediment characteristic in the cores is the presence of buried soils, which reflect periods when the level of the lake declined and sediments on its floor were exposed to weathering. Two periods of soil development are recorded, specifically (1) during an initial phase after the Lake of the Woods basin became isolated from Lake Agassiz ~10,500 to 10,000 years ago; and (2) during a 3,600-year-long negative hydrological budget and chronological hiatus related to mid-Holocene warming and drying, which is represented by <1 m of sediment.

Sediment in the older part of the sequence commonly contains ostracodes and carbonate, but these disappear by 7,600 years ago, probably because carbonate availability in glacial sediment in the watershed declined when the watershed became more stable and weathered. A distinct pink laminae in the lower part of the sequence records overflow from the Lake Superior basin 11,300 years ago that resulted from a short re-advance of glaciers that forced water to spill over into the lake.

Studies of one core by the research team of Kathleen Ruhland and John Smol at Queen's University and Andrew Paterson at the Ontario Ministry of Environment and Climate Change show that conditions in Lake of the Woods became favorable for siliceous microfossils, including diatoms, after the lake basin became isolated from glacial Lake Agassiz shortly after 10,500 years ago.

Diatoms are a good indicator of changing Holocene conditions in the lake, and distinct shifts in diatom assemblage composition and in sedimentary chlorophyll-a track late-Holocene changes in Lake of the Woods; they also show that modern environmental conditions were established after the middle Holocene drying phase ended about 3,500 years ago.