EVIDENCE FOR REDUCTION IN MID-HOLOCENE DECADAL-SCALE CLIMATIC VARIABILITY IN THE NORTH-CENTRAL UNITED STATES

An early Holocene reduction in ENSO variability is documented in a growing number of studies. Less data are available regarding changes in longer, decadal-scale climate cycles through the Holocene. We present sedimentologic, geochemical, and image analysis data from annually laminated Derby Lake, Michigan (43°16’15” N lat., 85°7’30” W lon.) suggesting similar changes in decadal-scale variability occurred during the Mid-Holocene. Decadal-scale cycles are expressed in Derby Lake sediment as changes in the carbonate content, with carbonate alternating between high values (40-80%) and low values (<10%) for periods of a few years to multiple decades. Transitions between carbonate-rich and carbonate-depleted sediments are typically abrupt, occurring within 1 to 10 varve years, and form visually striking cm-scale bundles of light carbonate-rich laminations and dark carbonate-depleted laminations. These cycles are documented visually, as well as quantified in sediment grayscale intensity, carbon coulometry, and stable isotope data. Beginning in the early Holocene (8400 cal yr BP), Derby Lake varves were dominantly carbonate rich, with some carbonate depletion events occurring around 8200 cal yr BP. A period of low lake level followed, and Mid-Holocene Derby Lake sediments are dominated by elevated carbonate abundance, and a paucity of carbonate depletion events. Beginning gradually after 5300 cal yr BP, carbonate depletion events recur and continue to increase in frequency and intensity until 1500 cal yr BP, where cycles begin to taper off, with a final carbonate-depletion cycle ending in the Little Ice Age. We suggest that carbonate-depletion cycles are environmentally driven phenomena reflecting changes in decadal-scale climatic variability in Michigan through the Holocene. Links to the Pacific ocean-atmosphere system are suspected drivers in these changes, and the phenomena may also be linked to similar changes observed in the ENSO system during these times.