SCALES OF INFLUENCE: MULTI-PROXY INDICTORS OF HIGH INTENSITY PRECIPITATION EVENTS IN NEW HAMPSHIRE LAKES

Limnic systems, especially lakes, accumulate deposits, which can be used to interpret climatic, geologic, biologic, and anthropogenic watershed scale histories. In New Hampshire, humans, and now climate change, have heavily impacted many of the watersheds since the Colonial Period. These impacts are often seen in lake sediment records. Although lake records of climate change exist in a wide variety of proxy indicators, watershed processes operating across many different spatial scales can smooth and lengthen the distribution of sedimentary deposits related to individual events. Even very large events, such as hurricanes, can be masked by mixing of “event sediments” with earlier deposits. We wonder will high intensity, short duration events can stand out above low intensity, long duration events and, if so, which proxy best documents that change?

This project addresses these questions to better understand the magnitude of precipitation needed to create distinct and recognizable deposits in lakes. Between the fall of 2017 and 2018, we used a 60 cm-long Uwitee gravity-corer to collect nine sediment cores, including the sediment-water interface, from four New Hampshire lakes: two cores each from Norway Pond (Hancock) and Pleasant Lake (Deerfield), one from Newfound Lake (Bristol), and four cores from Spofford Lake (Chesterfield) in south-central NH. All the cores were subsampled at either 0.5 cm or 1 cm intervals. The subsamples were analyzed for several physical and chemical properties, including Pb210 age, carbon content, particle-size, frequency-dependent magnetic susceptibility and mineral geochemistry. These data help us identify sedimentary properties that correspond to episodes of regional flooding and consequent terrestrial erosion including two particularly high magnitude events for New Hampshire: Tropical Storm Irene, in 2011, and the Hurricane of 1938. The different sampling resolutions in side-by-side cores provide insight into the impact of sampling resolution on the detection of short duration, high-magnitude sedimentary responses to particular events.

Preliminary results from Norway Pond show a dip in magnetic susceptibility around the time of the 1938 hurricane and a general upwards trend in loss on ignition. Pleasant Lake does not show any signal that might point to a response from the hurricanes in 1938 or 2011. But the Pleasant Lake accumulation rates, calculated from Pb210 results, indicate exceptionally high sedimentation rates over the last century which might obscure storm-related deposition.