Freshwater diatom evidence for Southern Westerly Wind evolution since ~18 ka in northwestern Patagonia

Abstract

We report a fossil diatom record from small closed-basin Lago Lepu´e (43◦S) to examine past changes in freshwater ecosystems and hydrologic balance in northwestern Patagonia since ~18 ka. The record starts with abundant staurosiroids and the heavily silicified Aulacoseira granulata suggesting deep turbulent mixing during a low lake level stand between ~18- 16.4 ka. A. distans increased shortly after ~16.4 ka and achieved maximum abundance between ~15.4-13.6 ka, while A. granulata disappeared at ~15.8 ka and A. alpigena rose at ~14.9 ka to its maximum between ~13-12 ka. We infer turbulent, cold, and circumneutral to slightly acid lake conditions contemporaneous with a steady lake level rise that started at ~16.4 ka and culminated between ~13-12 ka. These trends reversed between ~11-7.8 ka with the dominance of Discostella stelligera and staurosiroids, suggesting warmer lake conditions and shallower mixing. Subsequent changes include increases of A. distans with D. stelligera between ~7.8-5.8 ka, dominance of the former between ~5.8-3.3 ka, a rapid increas e in A. perglabraat ~3.3 ka, and ensuing diversification of benthic acidophilous species. We infer a rapid lake-level decline between ~11-7.8 ka, with subsequent rising pulses at ~7.8 ka and ~5.8 ka, a multimillennial-scale lake acidificationtrend, and overall high lake levels with centennial-scale reversals between ~6-0 ka. Coherent variations in terrestrial and aquatic ecosystem changes recorded in the same core suggest negative hydrologic balance between ~18-16.4 ka and ~11–7.8 ka, positive balance between ~14.9-12 ka and ~6–0 ka, with transitional conditions in the interim, overprinted by millennial-scale changes and enhanced variability since ~6 ka. Covariation with paleoclimate records at regional, pan-Patagonian, and hemispheric scale suggests millennial to centennial-scale variability superimposed upon a multi-millennial pacing of Southern Westerly Wind evolution since ~18 ka.