Many paleoclimate studies have shown that the midcontinent of North America experienced a warmer and drier-than-present climate during the early and middle Holocene and relatively humid climate during the late Holocene interspersed with several centennial-scale megadroughts. In response to this climate variability, Holocene vegetation history in the Great Lake forests broadly comprises two shifts: the early Holocene northward movement of the ecotone between mixed forest and deciduous forest – mainly signaled by changes in pollen abundances of Quercus and Pinus, and shifts in the abundance in prairie with a general eastwards expansion between 7,000 and 5,000 years BP. Superimposed on these ecotonal shifts are the range shifts of individual taxa and episodic declines in abundance of individual tree taxa, which appear to be linked to hydroclimatic variability (e.g. Fagus decline during the droughts, Booth et al, 2012). Most recent Holocene paleoecological research has been carried out in the northern Great Lakes states (MI, MN, WI) and the Holocene climate and vegetation dynamics in the south-central Great Lakes (IN, OH) remains relatively unknown. This study focuses on climate and vegetation changes during the Holocene at Spicer Lake in northern Indiana (41°44'52" N, 86°31'19" W, 237 elevation), a kettle lake near the southeastern edge of Michigan.
Results/Conclusions
This study reports radiocarbon, loss-on-ignition, pollen, and XRF analyses to reconstruct the vegetation, climate and environment at Spicer Lake. 14 14C dates are used to build the Bacon age model, and the sedimentation rate is estimated about 0.14cm/yr during the Holocene. The main pollen types at Spicer Lake are Acer, Fagus, Quercus and Ulmus. Pinus and Picea pollen are abundant before 11,000 years B.P., then decrease during the Holocene as temperatures rise. Loss-on-ignition data suggest sharp increases in mineral sediments and decrease of organic carbon at 10.5, 7.8, 6.4, 4.5, 2.5, 1.2, and 0.2 ka BP. Fagus abundances first rise at about 7,000 years B.P., and show large variations thereafter, with peaks roughly at 6.5, 5.5, 2.5, and 1.2 ka BP. Most of the increases of Fagus pollen abundance correspond well to the peaks in mineral content after 6,700 years BP. Time-series analyses of pollen abundances for individual taxa shows that Fagus abundances have the lowest autocorrelation relative to the previous time step. Increases of Fagus and minerals suggest moist climates occur at 6.4, 2.5 and 1.2 ka BP, with high lake level and increasing shoreline erosion of clays from the surrounding upland.