J. P. Francois

Covariability of the Southern Westerlies and atmospheric CO2 during the Holocene

A suite of mechanisms has been proposed to account for natural variations in atmospheric CO 2 during the Holocene; all of which have achieved limited success in reproducing the timing, direction, and magnitude of change. Recent modeling studies propose that changes in the latitudinal position and strength of the Southern Hemisphere Westerly Winds (SWW) can greatly influence large-scale ocean circulation and degassing of the deep ocean via changes in wind-driven upwelling in the Southern Ocean. The extent to which the hypothesized SWW–Southern Ocean coupled system could account for changes in atmospheric CO 2 is uncertain, because of a lack of observations on the behavior of the SWW in the past, the paucity of appropriate records of productivity changes in the Southern Ocean, and our limited understanding of the sensitivity of the Southern Ocean biological and/or physical system to SWW forcing. Here we report a reconstruction of the behavior of the SWW during the past 14 k.y. based on terrestrial ecosystem proxies from western Patagonia, South America. The reconstructed variations in the intensity of zonal flow correspond to the timing and structure of atmospheric CO 2 changes, and are consistent with the modeled magnitude of CO 2 changes induced by varying strengths of the SWW. The close match between data and models supports the view that the SWW–Southern Ocean coupled system underpins multimillennial CO 2 variations during the current interglacial and, possibly, during glacial cycles over the past 800 k.y.

Renewed glacial activity during the Antarctic cold reversal and persistence of cold conditions until 11.5 ka in southwestern Patagonia

Resolving the timing, direction, and magnitude of paleoclimate changes in the southern midlatitudes is a prerequisite for determining the mechanisms underlying abrupt and widespread climate changes between the hemispheres during the Last Glacial-Interglacial transition (LGIT). This issue is still debated, with previous studies producing apparently discordant fi ndings. Here we show evidence for a glacial readvance and a cold episode between ca. 14.8 and 12.6 ka in southwestern Patagonia (50°S), contemporaneous with the Antarctic cold reversal. This was followed by ice recession under cold but relatively milder conditions until ca. 11.5 ka, when paleovegetation records indicate the onset of warm interglacial conditions. These fi ndings differ from those reported in northern Patagonia (~40°S), where deteriorating conditions before 13.5 ka were followed by the coldest part of the LGIT that lasted until ca. 11.5 ka. We interpret the apparent blend of Greenlandic and Antarctic cold phases as evidence for their co-occurrence in the southern middle latitudes in Patagonia, and hypothesize that the position of the Antarctic Polar Front modulated the strength of these cold events in regions to the north or south of it.

Climate Change in Southern South America During the Last Two Millennia

Paleoclimate records from southern South America can be used to address important questions regarding the timing and nature of late-Holocene climate variability. During the last 30 years, many areas of southern South America have experienced rapid climatic and ecological changes that are driven by global and hemispheric-scale ocean-atmosphere processes. In order to place these recent changes in a longer-term context, we first present an overview of the modern climate processes relevant for the interpretation of paleoclimate records in southern South America, and then review records that have been developed from various archives that span the last two thousand years. Multiple paleoclimate records provide evidence for an overall decrease in temperature and an increase in westerly wind intensity that culminates in the last few hundred years during the time of the European Little Ice Age. We also find evidence for aridity generally coincident with the Medieval Climate Anomaly in several paleoclimate archives. Although much work has been done in this region, high-resolution well-dated archives are still needed from sensitive locations to improve our understanding of past and present climate change. From the paleoclimate records that we have compiled, we infer that warming, retreat of glaciers, and reconfiguration of precipitation patterns during the past century is unique within the context of the last 2000 years.