MID-HOLOCENE CLIMATIC VARIATIONS IN THE MIDCONTINENT OF NORTH AMERICA: MULTI-PROXY ANALYSES OF SEDIMENTS FROM TWO MINNESOTA LAKES

We analyzed sediment cores from Steel Lake (SL, 46º 58’ N, 94º 41’ W) and Lake Olaf (LO, 46º 37’ N, 96º 11’ W), located in the prairie-forest ecotone of Minnesota, for d¹⁸O and d¹³C of bulk carbonate, d¹³C of charcoal, mineral composition, charcoal abundance, and pollen assemblages. These data provide new information on Holocene climatic changes in the mid-continent of North America. The SL chronology is based on 26 AMS ¹⁴C dates of terrestrial plant macrofossils, and the LO chronology is established with 8 AMS ¹⁴C dates of charcoal flakes. An abrupt transition to warm/dry climatic conditions of the middle Holocene occurred at 7.8 cal ka, as indicated by increases in d¹⁸O (-9.8‰ to -7.3‰, SL), quartz content (10% to 43%, SL), aragonite/calcite ratio (0.7 to 1.4, LO), and non-arboreal pollen abundance (34% to 56%, SL). This transition probably resulted from a major reorganization in global atmospheric circulation, but it appeared to postdate the 8.2 ka event. Within the middle Holocene, our isotopic, mineralogical, and biological data exhibit two distinct climatic periods with warmer/drier conditions between 7.8 and 5.8 cal ka than between 5.8 and 3.5 cal ka. Superimposed on this pattern are millennial-scale shifts in drought intensity and prairie vegetation, as evidenced by oscillations in aragonite/calcite ratio, % Poaceae pollen, charcoal abundance, and C₄-plant abundance. For example, the aragonite/calcite ratio of the LO sediments reaches peak values at 7.5 cal ka, 6.2 cal ka, and 5.0 cal ka, suggesting driest climatic conditions at these times, and minimal values at 6.8 cal ka and 5.5 cal ka, suggesting wetter phases of the middle Holocene. These oscillations have a ~1300-year pacing but do not appear to coincide with the ice-rafted-debris events in the North Atlantic.