MULTI-PROXY RECORD OF ANTHROPOGENIC CHANGE, CONROY LAKE, MONTICELLO, ME
Previous pollen analyses done on Conroy Lake (Gajewski, 1987) show increases in the Ambrosia and Rumex horizons at ~1830, which can be attributed to human settlement and town incorporation of Monticello in 1846. Historical accounts show that the early settlers utilized the land for farming and logging. A surface piston core taken in 2009 from Conroy Lake exhibits increasing trends in magnetic susceptibility (~1.5 SI to ~4 SI) and δ15N (~15‰ to ~30‰) at ~40cm core depth. These changes are hypothesized to be due to land use change associated with the pollen shifts previously identified.
Petrographic thin sections were made from the uppermost 77 cm of the 2009 piston core in order to quantitatively study the varves in light of the anthropogenic change in proxy data observed at ~40cm. Preliminary varve counts have been taken from five thin section depths. Mean varve thicknesses below 40 cm downcore are thinner, with Section M (z = 67-67.4 cm) exhibiting a mean thickness of 0.04 cm, and both Section L Upper (z = 62.5-63.4 cm) and Section L Lower (z = 64.4-65.1 cm) exhibiting a mean thickness of 0.05 cm. Above 40 cm, Section F (z = 27.9-30.9 cm) shows a mean thickness of 0.13 cm; Section E (z = 22.7-27.0 cm) a mean thickness of 0.18 cm; and Section D (z = 18.8-21.5 cm) a mean thickness of 0.17 cm. The change in varves show a statistically significant increase in thickness between pre-human appearance (mean = 0.045 cm) and post appearance (mean = 0.162 cm) (t-test t=-11.4; p<<0.0001).
Results to date suggest a connection between local environmental change and varve thickness at Conroy Lake. Ongoing analysis of 3 freeze cores, which have preserved the upper flocculent sediments in situ, will help to increase varve resolution and the ability to measure their physical properties.