CENTENNIAL- AND MILLENNIAL-SCALE CHANGES OF THE SOUTHERN WESTERLY WINDS SINCE 15 KA IN SOUTHWESTERN PATAGONIA

PATRICIO MORENO ; ISABEL VILANOVA; RODRIGO VILLA-MARTINEZ; DE POL-HOLZ, R.

Ushuaia

Congreso; VI Congreso Argentino de Cuaternario y Geomorfología; 2015

CADIC-CONICET

We present a paleovegetation and paleofire record from Lago Cipreses (51°S), southwestern Patagonia, to explore millennial and centennial-scale hydroclimate changes over the last 15,000years. Local and regional precipitation exhibits a positive correlation with the southern westerly wind (SWW) intensity (Morenoet al., 2014) supporting rainforests and icefields along thePatagonian Andes. Recent studies along western Patagonia detected an association between tree-ring-dated paleofires and negative anomalies in precipitation/positive anomalies in temperature associated with positive phasesof the Southern Annular Mode (SAM) (Holz andVeblen, 2012). Hence, hydrologic inferences based on paleovegetation, past lake-level changes and paleofire-regime shifts from south western Patagonia constitute a valuable source of information for reconstructing past variations of the SWW and SAM. Lago Cipreses records a virtually treeless, open landscape dominated by cold-resistant herbs and shrubs commonly found in high Andean environments andhumid sectors of the Pacific slopes, suggesting cold and hyperhumid conditions during the Antarctic Cold Reversal (ACR, ~12.8-15 ka, ka= 1000 calendar yearsbefore present). Nothofagus then increased rapidly leading to the establishment of Magellanic forests inresponse to warming under hyperhumid conditions during the Younger Dryas chron(YDC ~11-12.8 ka). Closed-canopy forests dominated between 9.3-11 and 6-7.5 ka,indicating cool/wet phases, alternating with open forest intervals between 10.1-10.5and 7.5-9.3 ka during the warm/dry Early Holocene. This was followed by a highly variable phase since 6 ka, within which we detect centennial-scale variations in forest cover, paleofires, and lake-level change which we interpret as secular changes in temperature and precipitation akin to the modern functioning of SAM. Because precipitation in the study area is positively correlated to zonal wind speeds and this relationship extends over a broad swath of the southern mid-latitudes, we can infer the past behavior of the SWW at a zonal scale based on precipitation-sensitive stratigraphic records from SW Patagonia.     Our results and interpretations are consistent with a previous study from the Última Esperanza sector (51°S), which detected a lake transgressive phase in Pantano Dumestre between ~11.5-14.7 ka, i.e. the ACR and YDC, implying strong SWW influence during those intervals (Morenoet al., 2012). Glacier lobes originating from an expanded South Patagonian Icefield achieved their maxima in the nearby Torres del Paine (Garciaet al., 2012; Moreno et al., 2009) and Lago Argentino areas (Strelinet al., 2011) during the ACR and underwent recession during theYDC, contemporaneous with a regressive glaciolacustrine phase recorded in the Lago Eberhard site in the Última Esperanza sector (Morenoet al., 2012; Sagredo et al., 2011).     We detect a conspicuous warm/dry multi-millennial phase between 9.3-7.5 ka, implying reduced SWW influence at 51°S. This phase was heralded by a precursor event between 10.1-10.5 ka which caused a brief, rapid decline in arboreal vegetation.    We observe stronger SWW influence starting at 7.5 ka and the onset of SAM-like variability at centennial timescales at 6 ka.     Radiocarbon- and cosmogenic nuclide-dated moraines from the Lago Argentino area (Strelinet al., 2014) constrain the timing and extent of neoglacialadvances in the SW Patagonian region. Comparison of this chronology with the timing of our hydroclimate anomalies since 10 ka suggests that neoglacial events were initiated by negative SAM-like states, and their termination driven by positive SAM-like states. We note that the onset of SAM-like variability in SW Patagonia was nearly contemporaneous with the onset neoglaciations in Patagonia, the commencement of ENSO-like variability in the tropics and subtropics (Moy etal., 2002), and a rising trend in the atmospheric CO2concentrations, suggesting that stronger (Fletcherand Moreno, 2011; Moreno et al., 2010) and more variable SWW might drive enhanced degassing of the Southern Ocean and influence the intensity of Hadley and Walker circulation over the Pacific Ocean.