FLOODPLAIN RESPONSE TO LATE HOLOCENE CLIMATE CHANGE AND HUMAN LAND-USE AT THE MANNA SITE (36PI04), UPPER DELAWARE VALLEY, PA

The Manna site (36PI04) is a nationally registered archaeological site located in the Delaware Water Gap National Recreation Area that documents fluvial and pedogenic responses to climate change and human land-use during the late Holocene. A portion of this site is embedded within a 3.5 m thick floodplain-overbank succession that has a basal age of 2040±40 cal yrs BP (Beta-257433). Particle size analysis and soil morphology indicate at least 10 distinct fining upward successions bounded by weakly developed (Entisol-Inceptisol) loamy sand and sandy loam soils. Radiocarbon ages, historic/prehistoric artifacts and geochemical markers were used to calculate an interpolated mean sediment accumulation rate of 0.15 cm yr^-1, which is rapid for this region. Within the succession, an interval of increased rate of sedimentation, coarser mean grain-size, and more weakly developed soil formation coincides with the transition from the Medieval Warm Period to the Little Ice Age (LIA). This suggests increased flood frequency and higher flood magnitudes during the LIA. The mean soil organic matter δ¹³C value for this succession is -24‰, 2‰ heavier than the -26‰ mean for C3 dominant ecosystems. There are two significant excursions, -21.13‰ and -20.34‰, occurring around 1830 cal yrs B.P. (Beta-227477) and 310 cal. yrs B.P. (Beta-258885), respectively. Phytolith data from the 310 cal. yrs B.P. excursion indicate an increase in both Panicoideae and Chloridoideae subfamilies of C4 grasses. Both excursions coincide with known dry intervals previously documented elsewhere in the Mid-Atlantic region, suggesting drought-induced moisture stress on plants stable carbon isotopic values and the distribution of C4 grasses through the late Holocene. However, archaeobotanical evidence of prehistoric maize based food production coinciding with the upper excursion interval suggests human land-use may have influenced sedimentation and the composition and distribution of floral communities and their associated isotopic values. This research demonstrates the paleoenvironmental significance of the Manna site and the potential δ¹³C isotopes from soil organic matter and phytolith analysis hold for reconstructing moisture stress in the Middle Atlantic USA.