Robust Patterns and Process Insight from Multi-Site, Multi-Proxy, Multi-Region Holocene Paleohydrologic Reconstructions

Holocene moisture trends provide a useful context for historic hydrologic variations and potential future changes, and serve as key benchmarks for assessing paleoclimate dynamics. Many different approaches exist for paleohydrologic reconstruction, and comparisons offer an opportunity to validate key patterns and gain information based on differences. We consider multiple datasets from two mid-latitude regions of North America to evaluate centennial-to-millennial hydrologic variability during the Holocene. Comparisons of proxies at individual sites (i.e., pollen-inferred precipitation, reconstructed lake levels, hydrogen isotope ratios of biomarkers) and comparisons of proxies across sites in different hydrologic settings (i.e., closed vrs open lakes) provide insight into regional changes, while differences between the semi-arid (Wyoming) and humid (New England) regions reveal shifts in continental-scale moisture gradients.

Pollen-inferred precipitation records were developed using the modern analog technique, which matches fossil pollen data to the climates of equivalent modern pollen assemblages. Lake moisture budgets were quantified in terms of mm/yr of precipitation minus evapotranspiration based on water-level histories systematically reconstructed from transects of sediment cores and geophysical data. Comparisons of the budgets for open and closed lakes, which have modern lake water isotopic values consistent with no or substantial evaporative losses respectively, were used to differentiate precipitation and evapotranspiration trends. Hydrogen isotopic values of aquatic versus terrestrial biomarkers, as well as comparisons with oxygen isotopic values of carbonate sediment, provide additional constrains on the magnitude of past evaporation trends that enrich the heavy isotope composition of lake water and thus aquatic substrates.

In New England, pollen-inferred precipitation trends closely agree (Adj. R2 = 0.80) with replicated lake-level reconstructions, which show >300 mm/yr increase in effective moisture over the Holocene as well as repeated multi-century fluctuations. The trends indicate significant changes in the east-west moisture gradient before ca. 8000 BP and at ca. 5000 BP, in part due to changes in warming-induced evaporation in Wyoming.