Late Pleistocene and Holocene hydroclimate change in the Southeastern United States: Sedimentary, pedogenic, and stable carbon isotope evidence in Tennessee River floodplain paleosols

In order to understand hydroclimate variability of future climate change, it is important to know the timing and range of natural climate change in the past. The Southeastern United States (SE) is situated along the poleward extent of projected subtropical drying, where the expression of past hydrological balances remains unclear. The lack of high-resolution paleohydroclimate records in the SE forces climate modelers to base interpretations on better-understood regions of North America. The SE likely experienced significant changes in precipitation regimes resulting from its position at the convergence of several oceanic and continental air masses. To reconstruct precipitation variability, this study examines multiple floodplains along the Tennessee River as hydroclimate proxy archives in the SE. Changes in precipitation balances are interpreted from high-resolution trends in sedimentary, pedogenic, and stable carbon datasets that span the last 14 ka BP (kiloannum before present).

Variation in overbank sedimentation, identified from grain-size trends indicate precipitation increased from 11.0-8.0 ka BP. During this pluvial interval, stable carbon isotope values of soil organic matter in floodplain paleosols show negative excursions (~1.0-1.5 permil) from average modern values. A period of aridity, evident from an overall excursion to less negative stable carbon isotope values, increased abundance of fine-grain sizes, and increased paleosol development occurs between 8.0-5.0 ka BP. Most notably, the transition out of the mid-Holocene (~5.0 ka BP) is marked by an abrupt change to more negative carbon isotope values (~1.5 permil). Wavelet analysis of composite sedimentary and stable carbon isotope data time series indicate statistically significant (p≤0.1) 200- to 500-yr and ~1000-yr periodicities. Periods of aridity in the SE are correlated with the eastward intensification of the North Atlantic Subtropical High (NASH). Pluvial conditions result from the westward intensification of the NASH. The timing of wet and dry periods recorded by floodplain paleosol is in agreement with the onset and frequency of such changes observed in regional records. The floodplain datasets presented here provide the first long-term record of the onset and duration of centennial-scale changes in precipitation that occurred during the late Pleistocene and Holocene in the SE.