Ioana Cristina Stefanescu , Bryan N Shuman, Laurie D Grigg, Adriana Bailey, Vania Stefanova, W. Wyatt Oswald

In eastern North America, annual precipitation increased by >40% over the Holocene, largely in response to melting of the Laurentide Ice Sheet. The change substantially raised lake levels and transformed conifer-dominated ecosystems into mesic deciduous forests. δ2H values of terrestrially derived leaf-wax n-alkanes can facilitate diagnoses of the climate dynamics involved by reconstructing δ2H values of mean annual precipitation (δ2HMAP). However, competing influences on δ2HMAP values in the mid-latitudes, such as changes in moisture sources and in the seasonal distribution of precipitation, can generate confounding effects. To test δ2HMAP sensitivity to potential changes associated with the final Holocene phases of deglaciation in eastern North America, we used 14 fossil-pollen records to reconstruct monthly precipitation changes and to model δ2HMAP values during the Holocene. The pollen-inferred precipitation increased by 100-200 mm during both cold and warm seasons, but modelled δ2HMAP values changed by only ~10‰, because isotopically-heavy summer precipitation increased by nearly as much as the cold-season isotopically-light winter precipitation. Three new leaf wax n-C29-alkane (δ2HC29) records spanning the Holocene from Vermont, Pennsylvania, and Massachusetts closely follow modeled δ2HMAP trends and confirm only a small decline in δ2HMAP values over the Holocene. Because the shifts in precipitation seasonality accurately predict the n-alkane records, changes in moisture sources or pathways appear to play only a minor role in the regional δ2HMAP history despite the effects of deglaciation on atmospheric circulation. Soil evaporation also did not significantly alter δ2HC29 values from the values predicted using the pollen-derived reconstructions. The results affirm that δ2HC29 values faithfully detected anticipated isotopic changes in δ2HMAP values, but that important paleoclimate events may not always yield strong changes in δ2HMAP values.