ANALYSIS OF LATE HOLOCENE PALEOLIMNOLOGIC RECORDS FROM THE DEAD RIVER WATERSHED, FRANKLIN COUNTY, MAINE

Lakes are sensitive to regional and watershed-specific environmental changes, and sedimentary proxies help unravel past forcings on a given lake. Jim Pond (JP) and Tea Pond (TP) are within the Dead River watershed (Franklin County, ME) but are isolated from each other. Sediment cores from each lake were compared to differentiate between regional climate effects and local watershed alteration over the past three millennia.

The TP core had been previously analyzed and dated. The 256 cm JP core was analyzed for physical and geochemical proxies including: lithologic description, bulk density, volume magnetic susceptibility (MS), and CNS stable isotopic and interparametric ratios as measured by continuous flow EA/IRMS.A JP age model was constructed using five calibrated bulk AMS ¹⁴C dates.

In the JP core, percentage of organic carbon (%OC) remained consistent between 15 and 18% and the lithology is dark brown massive gyttja. The %OC in the top 250 cm of TP ranges from 16 and 20% which is a similar to the range in JP. From 250 to 230 cm in TP, there is a decrease in %OC from 20% to 16%, corresponding to increased MS. At the same depths in JP, %OC averages ~16%, but MS is slightly higher. This trend is similar throughout both cores at depths of 200 to 180 cm. The most distinct shift in either core is observed in the upper 8cm of JP, where a drop in %OC from 21.1% to 14.5% is observed. Corresponding to this decrease in %OC, there is a rapid increase in MS values from 2.1 to 5.2 SI, and the lithology of the sediment changes to a green-gray fine-grained mud.These proxy changes are interpreted as recording an influx of inorganic sediment to the lake associated with soil destabilization of the watershed due to logging practices, which is consistent with the known history of the region. This rapid change in sediment type is recorded subtly in TP; MS values increase from close to 0 to 2 SI with a simultaneous dip in %OC. From 0 to 40 cm in both cores, there is a stark shift to more depleted δ¹³C values, likely associated with a change in carbon source associated with vegetation changes in response to climatic forcings.

The dated sediment proxy records of TP and JP suggest that environmental conditions have been largely driven by regional climate variability over the late Holocene and the local anthropogenic effects have had a greater influence in JP as compared to TP.