CAN SEDIMENT CARBON ISOTOPES DETECT LONG-TERM CHANGES IN FOREST-GRASSLAND DYNAMICS OF THE SHENANDOAH VALLEY, CENTRAL VIRGINIA?

The Shenandoah Valley of central Virginia has undergone significant vegetation shifts over the last millennia due to climate variability and human activities. Because C3 and C4 plants have distinct carbon isotope values (δ13C; average of -27‰ for C3 plants and –13‰ for C4 plants), they have been used to detect vegetation changes in various ecosystems, but their application in understanding long-term history of forest-grassland dynamics in the Shenandoah Valley remains unexplored. Our study focused on reconstructing historical vegetation dynamics to reveal the extent and timing of C3 forest dominance in Virginia through analysis of sediment samples from a sinkhole pond.

We collected 150cm sediment profile from Maple Pond in the Shenandoah Valley Sinkhole Complex, for analysis of carbon isotopes and other proxy data. Maple Pond (~480 m elevation) is situated in dry, sandy soils and oak-pine forest at the base of alluvial fans from down west slope of the Blue Ridge. Two radiocarbon dates on charcoal fragments in the lower core showed the pond age to be >15,000 years (95%CI). Sediment core δ13C values range from -29.1 to -26.0‰, with an average value of -28.1‰, suggesting that Maple Pond landscape was consistently forested throughout the studied period. Nonetheless, several samples in the middle depths showed modest δ13C excursions ranging from -26.8‰ to -27.19‰, indicating brief periods of vegetation change. These results indicate relatively stable forest cover across time with periodic grassland expansions, possibly related to fires, natural or human ignited. We postulate that pond sediments here at the base of Blue Ridge better represent forests on the slopes than the lower Valley, which may have had larger proportions of grassland. Ongoing analyses of charcoal fragments in the core, and studies of additional sites, may lend new insights to our interpretation. The findings from this research will contribute to the documentation and understanding of the long-term ecological history of the Shenandoah Valley.