USING LOSS-ON-IGNITION TO DELINEATE ZONES OF HIGH ORGANIC CONTENT IN LAKE SEDIMENT FROM ICE-PROXIMAL LAKES, SOUTHWESTERN WISCONSIN

Vegetation dynamics at the edge of the Laurentide Ice Sheet during the Last Glacial Maximum are poorly understood. These ecosystems represent the species that would initiate recolonization of the glaciated landscape following deglaciation. On-going research in the Driftless Area of southwestern Wisconsin is analyzing lacustrine sediment from former ice-proximal lakes to better understand the local ecology during the last glacial period. Many of these lakes were formed by outwash from the Green Bay Lobe aggrading on the Wisconsin River valley; this dammed tributary valleys to form lakes that persisted for several thousand years following the peak of glaciation. Numerous locations around the Driftless Area were drilled for lake sediment to be analyzed for paleo-environmental proxies such as pollen, loss-on-ignition (LOI), and charcoal. Among these proxies, LOI provides a preliminary way to investigate the amount of organic content present in the lake sediment in order to provide guidance on intervals from the lake cores that would receive detailed charcoal and pollen analysis.

Sediment from an initial location of interest, Marsh Valley in northwest Dane County, WI, was sampled at fixed 8-cm intervals and processed for charcoal and pollen. Initial analyses indicated low counts for both charcoal and pollen. We recognized that the lake sediment consists of interbedded zones of silty sandy layers, unfavorable for proxy preservation, and more productive organic-rich layers, which required a refined processing plan. A more intensive 1-cm sampling resolution was used to constrain the organic-rich zones along the core that would result in higher pollen and charcoal counts. Subsequent pollen processing resulted in a significant increase in pollen grain counts.

The methods applied to Marsh Valley were extended to 6 other locations along the lower Wisconsin River valley between the glacial margin (to the east) and the confluence with the Mississippi River (to the west) to further refine focus areas for continuing study. Results from pollen counts and LOI analyses show a direct relationship between organic material and optimal proxy preservation. Preliminary LOI analyses provide a general indication of the amount of organic material throughout the core and determine which zones are more suitable for pollen processing.