Applications of organic geochemistry to paleolimnological reconstructions: a summary of examples from the Laurentian Great Lakes

The organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions, evaluate histories of climate change, and assess impacts of humans on local ecosystems. The elemental, isotopic, and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area, and they serve as proxies of organic matter delivery and accumulation. Sedimentary records from the North American Great Lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions. The records of these lakes date from retreat of the Laurentide ice sheet around 12 ka, include the mid-Holocene Hypsithermal, and show consequences of recent human changes. Low Corg/Ntotal ratios indicate that most of the sediment organic matter in the Great Lakes is from algal production, yet changes in biomarker molecule compositions also show that varying amounts of land-plant organic matter have been delivered to the lakes. Elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative δ13C values in the organic matter of sediments that were deposited in the 1960s and 1970s. Increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the Great Lakes. Accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the Great Lakes. Emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising, yet some potentially important measurements remain underutilized.