In contrast, farther east at SL where climate was cooler and wetter, C(4) abundance showed no correlation with delta(18)O-inferred aridity. As climate ameliorated C(4) abundance increased (from approximately 33 to 66%) at the expense of weedy species, enhancing fuel availability and fire occurrence. At WOL C(4) abundance was negatively correlated with aragonite/calcite, suggesting that severe moisture deficits suppressed C(4) plants in favor of weedy C(3) plants (e.g., Ambrosia). From charcoal delta(13)C data we estimated that the MH abundance of C(4) plants averaged 50% at WOL and 43% at SL. The ratio of aragonite/calcite at WOL and delta(18)O at SL suggest that pronounced droughts occurred during the MH but that drought severity decreased with time. Mineral composition and carbonate delta(18)O were used to determine climatic variations, whereas pollen assemblages, charcoal delta(13)C, and charcoal accumulation rates were used to reconstruct vegetation composition, C(3) and C(4) plant abundance, and fire. We analyzed MH sediments from West Olaf Lake (WOL) and Steel Lake (SL) in Minnesota to examine the effects of warm/dry climatic conditions on prairie-woodland ecosystems. Paleorecords of the middle Holocene (MH) from the North American mid-continent can offer insights into ecological responses to pervasive drought that may accompany future climatic warming.
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