Lakewater paleothermometry from Deep Lake, Minnesota during the deglacial–Holocene transition from combined δ18O analyses of authigenic carbonate and aquatic cellulose

Abstract

Carbonate, cellulose, and bulk organic isotope data from varved sediments of Deep Lake (Minnesota) provide a record of paleohydrologic and paleoclimatic change during the deglacial–Holocene transition from 12,000 to 7500cal BP. Deep Lake became enriched in 18O in response to increased influence of maritime tropical moisture from the Gulf of Mexico between 12 000 and 9500cal BP culminating in regional expansion of Pinus and Quercus, and then again as a result of increased aridity with the establishment of more zonal circulation and prairie vegetation by 8500cal BP. Summer lake water temperatures simultaneously increased during the deglacial–Holocene transition, as suggested by decoupling of carbonate δ18O and cellulose-inferred lake water δ18O values between 11,000 and 9500cal BP, and then declined briefly around 8600cal BP, possibly in response to the cooling event associated with the final drainage of Glacial Lakes Agassiz and Ojibway into the Labrador Sea. Although the carbonate-cellulose paleothermometer generated unrealistically high values, these can readily be explained by imposing kinetic effects that occur during rapid carbonate precipitation and conservative estimates of a seasonal lag effect in the synthesis of cellulose with respect to carbonate. These new results suggest that climatic variations during the deglacial–Holocene transition in northwestern Minnesota were driven mainly by changes in large- scale atmospheric circulation and that Lake Agassiz played a comparatively minor role except during its final drainage.