117-5 Interpreting Seasonal Sedimentation in Upper Midwest Varved Lakes for High-Resolution Hydroclimate Reconstructions

Abstract:

The Upper Mississippi River Region (UMRR) is experiencing substantial climate change due to natural variability with superimposed anthropogenic forcing. Long-term records are required to contextualize past millennium environmental change, yet instrumental records are short, local, and often discontinuous. Our aim is to use varved lake sediments to produce an annual record of hydroclimate variability in the UMRR that spans the past millennium. There are established methods that use tree ring records calibrated by instrumental records, and varved sediments may be used as a complimentary archive in locations where tree ring chronologies are spatiotemporally limited. This calibration can be applied to achieve a continuous reconstruction that extends well beyond the temporal limitations of instrumental data.

Freeze cores preserving an undisturbed sediment water interface and varve structure were collected from Steel Lake, LaSalle Lake, and Elk Lake, MN over the past two years. While excellent for preserving high-resolution stratigraphy, freeze cores are challenging to analyze, particularly in the case of scanning that requires core exposure to standard lab temperatures (well above freezing). To address this, Steel Lake sediments were used to develop a method for high-resolution (200µm) X-ray fluorescence (XRF) core scanning. The sediment remains frozen throughout scanning when segmented and placed in a custom-made cold box. Measurement drift was observed during core scans, which we attribute to surface water content changes and variable lab room conditions. To account for drift, we developed a bilinear piecewise mathematical correction that approximates the drift in time and depth space for individual scans using multiple quality control points as anchors.

High-resolution XRF-derived geochemical data enables analysis of rhythmic sedimentation, characterization of seasonal deposition and its variability, and offers a framework for geochemically assisted semi-automated varve counting. Preliminary results suggest that iron deposition associated with lake mixing exhibits greater interannual variability in both seasonal expression and magnitude than previously recognized. Varves containing four visually distinct laminae have been identified, suggesting greater variability in seasonal depositional patterns than was previously resolved. To further constrain depositional regimes, smear slide microscopy and X-ray diffraction analyses of individual varves and seasonally deployed sediment traps are being used to identify seasonal mineral assemblages and inform future carbonate isotope analyses. These seasonal signatures will improve interpretation of climatic variability, stratification regimes, productivity, and other paleoenvironmental processes.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-17585

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