Weak precipitation δ²H response to large Holocene hydroclimate changes in eastern North America

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 (δ²H_MAP). However, competing influences on δ²H_MAP values in the mid-latitudes, such as changes in moisture sources and in the seasonal distribution of precipitation, can generate confounding effects. To test δ²H_MAP 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 δ²H_MAP values during the Holocene. The pollen-inferred precipitation increased by 100–200 mm during both cold and warm seasons, but modeled δ²H_MAP 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 (δ²H_C29) records spanning the Holocene from Vermont, Pennsylvania, and Massachusetts closely follow modeled δ²H_MAP trends and confirm only a small decline in δ²H_MAP 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 δ²H_MAP history despite the effects of deglaciation on atmospheric circulation. Soil evaporation also did not significantly alter δ²H_C29 values from the values predicted using the pollen-derived reconstructions. The results affirm that δ²H_C29 values faithfully detected anticipated isotopic changes in δ²H_MAP values, but that important paleoclimate events may not always yield strong changes in δ²H_MAP values.