GEOCHEMICAL RB-SR ISOCHRON METHOD FOR FINGERPRINTING THE IMPACTS OF TIMBER HARVEST ON SEDIMENT PROVENANCE

Human alterations to landscapes can greatly influence watershed-scale sedimentary processes, therefore in this study we test the viability of a novel Rb-Sr isotope and elemental geochemical method for fingerprinting the impacts of timber harvest on sediment provenance. We analyzed a ~1200-year sediment core record from Little Kennebago Lake, Maine, USA that captures negligible anthropogenic activity pre-1900, timber transport downstream during log drives (1900 – 1952), and increased road construction (1952 – 2018). The pre-logging record Rb-Sr isotope data comprise an isochron corresponding to a Devonian age of 361.0 ± 11.3 Ma (MSWD = 2.1, n = 33). None of the bedrock or glacial till sources that we analyzed fall along the pre-logging isochron, nor did any binary or ternary bulk rock mixtures. However, the linearity of all pre-logging samples and coincidence of the isochron age with Acadian metamorphism in the region suggests that sediment was derived from a single Devonian source in the undisturbed watershed, though that exact bedrock or glacial till source remains unclear.

The onset of logging coincides with a shift to a scattered errorchron with an apparent Carboniferous age of 334.4 ± 16.5 Ma (MSWD = 5, n = 10). No Carboniferous rocks exist in the region. Therefore, we interpret this age to have no geologic meaning and instead reflect the introduction of new sediment sources due to logging. Logging also coincides with subtle changes in elemental geochemistry. The Big Island Pond Pluton (BIPP) underlies most of the northern watershed and is the only catchment lithology capable of producing each geochemical change, therefore we conclude that it was the primary new source of sediment introduced by logging. We find little variation between log drive and road period sediment geochemistry, suggesting that sediment provenance was constant once altered by the initial onset of logging. As such, we conclude that tree removal was the driver of changes to sediment provenance. Our Rb-Sr isochron method clearly identified a shift in human impacts on geomorphic processes in the watershed and represents a promising tool that should be further tested in other watersheds experiencing sediment issues.