24-13 Using 15N as a Marker of the Anthropocene in the Northeast US

Session: Lake Sedimentary Records of Past Climate and Environment (Posters)

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Abstract:

Nitrogen is an essential nutrient in the biosphere, and anthropogenic activity has drastically altered N cycle processes and isotopic signals, especially through increased industrial activity, fossil fuel burning, and land use change. ¹⁵N within sedimentary records is influenced by inputs such as organic matter, vegetation, and fractionation during N cycle processes, and can be used to reconstruct environmental changes over various time scales. The northeast US provides an ideal location to investigate records of ¹⁵N as a signal of anthropogenic activity, as the region experienced widespread deforestation in the 18th–19^th centuries primarily for agriculture, followed by reforestation in the late-19 through 20^th centuries and then increased 20–21^st century urbanization, while isolated areas continued to be used for agriculture. Here we present a regional compilation of δ¹⁵N in sediment cores in the northeast US across a variety of depositional environments (kettle ponds, wetlands, lakes, and reservoirs) that represent watersheds that underwent different types, durations, and intensities of historic land use to investigate how δ¹⁵N can be used as a marker of the Anthropocene. Background δ¹⁵N values are set by the environment (wetlands <0‰, lakes and kettle ponds 0–3‰), but regardless of background values, anthropogenic sediment shifts positive across all depositional environments and land use types. However, these changes are influenced by the type and duration of land use — sustained agriculture for >50–150 years leads to shifts by >2‰, while isolated deforestation for timber harvesting leads to shifts by <2‰. Increased usage of organic fertilizer or erosion of soils due to historic land use practices likely accounts for increased anthropogenic δ¹⁵N. Individual records suggest that discrepancies in the timing of the increase in δ¹⁵N may be related to geographic differences in timing of land use, however, better age constraints are needed to fully investigate these differences.

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