PS 2-21 - Evidence of a dry early Holocene in north central Wisconsin

Abstract: The timing and intensity of climatic changes in the early to mid-Holocene are not well understood. Slight increases in non-arboreal pollen 8000-6000 BP (Before Present= 1950 CE) suggest drier conditions, similar to but less severe than the mid-Holocene drying in Minnesota. However, optically stimulated luminescence (OSL) dating in Upper Michigan indicates that dune activity peaked 8000-10,000 cal yr BP, indicating the driest period of the Holocene.

We used a multi site and multi proxy approach to investigate how early Holocene climate changes affected vegetation fire, and water levels across a landscape in northern Wisconsin. A deep-water sediment core from Fallison Lake, WI (12.7m water depth, 6.3m sediment) was analyzed for pollen and charcoal to identify vegetation and fire regime changes. Changes in lake levels were reconstructed using loss on ignition (LOI), in a series of four shallow-water cores from the Lake. The Fallison Lake record was compared to pollen, LOI, and Sphagnum establishment ages obtained from four kettle peatlands located high and low in the watershed to provide a landscape-scale perspective on early Holocene environmental change.

The pattern of early Holocene vegetation history for our sites is generally similar to previous studies in the region. The most surprising feature of our analysis is a layer of grey inorganic material that appeared ~8500-9200 BP. This feature is present in all cores we analyzed, including peat surfaces, suggesting regional aeolian deposition, approximately synchronous with the peak of dune activation in Upper Michigan. White pine (Pinus strobus) pollen increased from < 10% before to >40% after the inorganic deposition and jack-red pine (Pinus banksiana and/or P. resinosa) declined from 80% to < 30%. This switch to the more mesic pine species would be consistent with wetter and/or less seasonally extreme climate and occurs in pollen diagrams from across the region between 8000-9000 BP. The charcoal accumulation rate (CHAR) gradually decreases late in the jack/red pine zone and remains low through the white pine zone, with lower background and fewer peaks than previously. Lake levels fluctuated within 2-3m below modern levels before the aeolian deposit, generally rising after 9000 BP. Peat establishment on lake sediment in several nearby bog ponds is also consistent with low water stands 9000-10,500 BP. Lake levels remained low 8000-5500 BP when white pine was rapidly expanding, suggesting that it responded more strongly to decreasing seasonality rather than increasing moisture.