Variations of the southern margin of the Southern westerlies during the last 14,000 years
Few land areas in the southern mid-latitudes have the potential for tracking the glacial-to-interglacial evolution of the westerly wind belt. Among those, SW Patagonia is key considering that: (i) local precipitation is positively correlated to the hemispheric-scale westerly flow, (ii) the regional vegetation shows a distinct zonation driven by E-W, N-S, and altitudinal gradients in moisture/temperature, (iii) there is potential for developing precisely dated paleoclimate records on high sediment-accumulating, closed-basin lakes located along these climate gradients. Here we present high-resolution pollen, charcoal and stable isotopic records of climate change over the last 14,000 years from Lago Guanaco (51°S, 71°W). The pollen record shows an open landscape dominated by Poaceae, Ephedra, and other shrubs between 14-11.5 ka (ka=1000 cal yr BP), suggesting cold and relatively humid conditions. Nothofagus expanded at 11.5 ka, suggesting a temperature rise at the Pleistocene-Holocene transition. This was followed by an expansion in Compositae and decline of Nothofagus between 9-7 ka, indicative of lower moisture levels. During this interval we observe high bulk organic d13C values, which we attribute to increased lake productivity, driven by higher temperatures. Stepwise increases in Nothofagus, along with corresponding declines in Poaceae, are evident at ~7, 4.5, 2.9 and 0.6 ka. In each case the arboreal taxa attained progressive percentage maxima that persisted until the next pulse. We interpret these pulses as periods of enhanced precipitation brought by stronger westerlies. Furthermore, the increase in Nothofagus at 4.5 ka coincides with an abrupt decrease in bulk organic d13C, suggesting cooler conditions. An emerging pattern is evident when comparing our results with paleoclimate records from NW and Central Patagonia. The pattern indicates an in-phase relationship of moisture changes between NW-SW Patagonia (dry between 9-7 ka, and onset of wetter conditions at ~7 ka), and antiphasing with the Andean region of Central Patagonia. Possible explanations for this contrasting pattern involve: (i) focusing of the westerlies in central Patagonia and subsequent northward and southward shifts of their boundaries after ~7 ka, or (ii) a southward shift of the westerlies to latitudes south of 51°S, along with enhanced advection of Atlantic moisture to the eastern flank of Central Andean Patagonia, or (iii) generalized weakening of the westerlies between 9-7 ka. The Lago Guanaco d13C record shows a good correspondence with temperature changes reconstructed from marine and ice core records from the Southern Ocean and Antarctica, indicating an in-phase relationship with the high southern latitudes during the Holocene.