Geochemical signatures of parent materials and lake sediments in northern Minnesota

Master of ArtsDepartment of GeographyKendra K. McLauchlanThe importance of local parent material has been recognized as a fundamental control on the geochemistry of lake sediments, but there have been relatively few broad-scale surveys of catchment sources of terrigenous lake sediments. In this paper, I present a geochemical study of catchment parent materials and lake sediments from four lakes in Northern Minnesota. Similar climate and vegetation conditions are present at all four lakes, which vary mainly in catchment parent material and lake morphometry. Geochemical data including major, trace and rare earth elements (REEs) from catchment parent material samples was compared with lake sediment geochemical data using PCA, linear regression, geological indices and elemental ratios. In homogenous till-dominated catchments, patterns of elemental variation in the catchment till could be extended to predict elemental concentrations in the lake sediments. Simple ratios, which are commonly used to analyze lake sediment geochemical data, were not good predictors of lake sediment composition, however. Catchments with mixed bedrock and till were compositionally heterogeneous, and comparison with lake sediments was difficult. Lack of grain size control and biogenic silica measurements further confounded analysis. However, ΣREE/Y ratio was found to be diagnostic of the catchment parent materials and present within the lake sediments. This study makes a contribution to an improved understanding of lacustrine sedimentary archives by analyzing the spatial linkages among catchment, water and sedimentary geochemistry

Influences of forested and grassland vegetation on late Quaternary ecosystem development as recorded in lacustrine sediments

Geosphere-biosphere interactions are ubiquitous features of the Earth surface, yet the development of interactions between newly exposed lithologic surfaces and colonizing plants during primary succession after glaciation are lacking temporal detail. To assess the nature, rate, and magnitude of vegetation influence on parent material and sediment delivery, we analyzed ecosystem and geochemical proxies from lacustrine sediment cores at a grassland site and a forested site in the northern United States. Over time, terrigenous inputs declined at both sites, with increasing amounts of organic inputs toward present. The similarities between sites were striking given that the grassland sequence began in the Early Holocene, and the forested sequence began after the last glacial maximum. Multiple mechanisms of chemical weathering, hydrologic transport, and changes in source material potentially contribute to this pattern. Although there were strong links between vegetation composition and nitrogen cycling at each site, it appears that changes in forest type, or from oak woodland to grassland, did not exert a large influence on elemental (K, Ti, Si, Ca, Fe, Mn, and S) abundance in the sedimentary sequences. Rather, other factors in the catchment-lake system determined the temporal sequence of elemental abundance