GEOCHEMICAL SIGNATURES OF PARENT MATERIALS IN LAKE SEDIMENTS IN NORTHERN MINNESOTA, USA

Lake sediment records are often used to reconstruct changes in past climate and ecosystems at both local and regional scales. Bulk geochemistry of sediments is often used as a broad, somewhat qualitative indicator of sediment sources. To improve our understanding of geochemical records, it is necessary to acquire data concerning local sources of sediment and the geochemistry of that sediment in order to fingerprint local versus regional sources of clastic sediment. Thus by understanding the geochemical signatures of catchment-derived sediments, we can identify these signatures in lake sediments and use this knowledge to better understand past climate and environmental change. Two important features of the catchment- its geology and topography- control the nature of transported material as well as its rate.

We sampled the composition of catchment parent material and sediments to understand transport and transformation mechanisms in four lakes in northern Minnesota, U.S.A. with varying bedrock types. Bedrock types included basalts, gabbros and granites, and catchments ranged from 20 to 120 km². Sources of clastic sediments were identified as either overland flow within the catchment or aeolian. The geochemistry of these rocks and sediments were identified by abundance of major, trace and rare earth elements (REEs) measured with X-Ray Fluorescence (XRF) and Inductively coupled plasma mass spectroscopy (ICP-MS). The chemical index of variation was significantly different between lake sediment (71.9) and parent material (62.1) samples, but all were still below 75, indicating that the parent material is low maturity and the lake sediments experienced low chemical and physical weathering. The bulk geochemistry of these samples was investigated with principal components analysis, showing the associations of elements as they moved from catchment to lake bottom. REEs generally retained their correlations between parent material and lake sediments, while others (Sr, Ca) de-coupled during the weathering process.