Books by Jean Pierre Francois
This paper discusses cultural (material) and environmental evidence for the Pleistocene-Holocene ... more This paper discusses cultural (material) and environmental evidence for the Pleistocene-Holocene transition in the Andean Central Patagonia. Fossil pollen analysis in Laguna Shaman, geomorphologic observations and archaeological surface surveys in upper Cisnes river basin and stratigraphic excavations at one site -El Chueco- are integrated, summing up the information recently gathered by our research team. The exposition of this data has a twofold aim. On one hand, it is intended to shape an image of the scenery the first occupants faced while initially exploring the region. On the other, we try to set the environmental and palaeoenvironmental constraints and possibilities for the survival of the archaeological record of Pleistocene-Holocene age. Finally, the evidence is discussed in the frame of the chronological data for the wider Central Patagonian archaeological record.

Paleoclimate records from southern South America can be used to address important questions regar... more 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.

El bosque relicto de Olivillo de Santa Inés (32°10´S; 71°30´O) ha sido escasamente estudiado y ge... more El bosque relicto de Olivillo de Santa Inés (32°10´S; 71°30´O) ha sido escasamente estudiado y generalmente se le compara con el ubicado en el Parque Nacional Bosque Fray Jorge. Sin embargo, mediante diversos estudios de la vegetación al interior del bosque, muestran la existencia de 3 formaciones vegetacionales que se desarrollan bajo condiciones de heterogeneidad ambiental específicas. Las formaciones: 1) Aextoxicon punctatum-Lomatia dentata y 2) Aextoxicon punctatum, son características para Santa Inés, mientras que, 3) Aextoxicon punctatum- Myrceugenia correifolia es común entre Santa Inés y Fray Jorge. Los cambios en la heterogeneidad ambiental biótica (tipos de dosel) y abiótica (pendientes, exposiciones y altitud) se ven expresados en aumentos y disminuciones del número de especies y cambios en la dominancia de los elementos florísticos presentes en las distintas formaciones. Así mismo, los cambios en la heterogeneidad ambiental (biótica y abiótica) al interior del bosque promueve variados ambientes de regeneración presentándose un mayor éxito en la regeneración de A. punctatum y M. correifolia cuando ocurren pequeñas aberturas en el dosel (< 64 m2) y en las quebradas, mientras que claros mayores a > 64 m2 propician la llegada de especies pioneras como Aristotelia chilensis y Chusquea sp. que impiden la regeneración de las especies relictas. Finalmente en las quebradas se aprecia un amplio ecotono con especies relictas y esclerófilas coexistiendo.
Papers by Jean Pierre Francois

Here we report results from a high-resolution palynological record and stratigraphic/geochronolog... more Here we report results from a high-resolution palynological record and stratigraphic/geochronologic data related to a Neoglacial event in Torres del Paine National Park, southern Chile (51 S, 71 W), to investigate climatic variations in Southwest Patagonia during the last 5000 years. The record reveals a stepwise expansion of Nothofagus-dominated woodlands and forests with discrete pulses at 4400, 2900, 1300, and 570 cal yr BP. Superimposed upon this trend we identify a relative opening of the woodlands between 4100-2900 and 2300-1300 cal yr BP. Closed-canopy forests dominated the landscape between 570-60 cal yr BP, followed by a rapid decline at the end of the 19th century that coincided with intense fire activity and the appearance of Rumex cf. acetocella, an exotic species introduced by European settlers. We interpret these changes as variations in the intensity and/or position of the southern margin of the westerly winds, which culminated with a net eastward shift of the forest-steppe ecotone during the Little Ice Age. We propose that millennial-scale changes in either the latitudinal position and/or the overall strength of the southern westerlies may be responsible for vegetation changes, fire occurrence, and the dynamic behavior of Patagonian glaciers during the last 5000 years. Because the modern maximum in near-surface wind velocities and precipitation is located between 48 and 50 S, we suggest that the core of the southern westerlies may have achieved this modern position w570 years ago.

Quaternary Science …, 2012
A 19 cal ka BP pollen and charcoal record from Lake Shaman (44 S; 71 W, Chile) was analyzed to es... more A 19 cal ka BP pollen and charcoal record from Lake Shaman (44 S; 71 W, Chile) was analyzed to establish vegetation, fire and climate dynamics of the forest-steppe ecotone in Central Chilean Patagonia. Lake Shaman record indicates that the upper Río Cisnes valley was free of ice at around 19 cal ka BP. From this date and until 14.8 cal ka BP, a grass steppe with high proportions of shrubs associated to colder and drier conditions than present developed in this area. A continuous increase of Nothofagus accompanied by a decline in the steppe shrubs and sudden dominance of paludal over aquatic plants from 11 cal ka BP was associated to effective moisture increase but still under modern values. The replacement of the colddry grass-shrub steppe by a similar-than-present forest-steppe ecotone suggests an increase in temperature indicating the onset of the Holocene. At the same time, moderate fire activity suggested by the charcoal record could be related to major fuel availability as consequence of Nothofagus forest expansion. Between 8 and 3 cal ka BP, the record indicates the easternmost position of the forest-steppe ecotone suggesting the highest effective moisture with the establishment of seasonality between 5 and 3 cal ka BP. From 3 cal ka BP, the record indicates a retraction of the forest-steppe ecotone accompanied by a high pollen record variability and an increased fire activity. These late changes suggest decreased effective moisture associated with a high climatic variability. At regional and extra-regional scale, climatic changes at Lake Shaman's record are mostly associated to changes (latitudinal shifts and/or strengthening/weakening) of past Southern Westerlies that were previously recorded along Patagonia from the Lateglacial to the mid-Holocene. During the Late Holocene, a regional pattern characterized by high record variability emerges throughout Central Chilean Patagonia. This variability would be related to (1) low magnitude Southern Westerlies changes probably associated to ENSO and/or SAM or (2) the complex relationships between vegetation, fire and human occupations during the last 3 cal ka.

Subduction, isostatic rebound, and changes in global sea levels, combined with the last glaciatio... more Subduction, isostatic rebound, and changes in global sea levels, combined with the last glaciation, have shaped the geography of the channels of Western Patagonia. Current archaeological research in this area includes some ten sites that allow us to characterize the occupation of this territory by marine hunter-gatherers. The studied archaeological sites also inform about the various geomorphological changes that the coastline has undergone. Archives dating back six thousand years ago and archaeological contexts yield new insights about the location, distribution, and position of the shoreline and its changes over time. We present a set of data, including new sites and AMS radiocarbon determinations, which supports the hypothesis that landforms have risen or subsided, and provide the bases for a working model in which archaeological ages can inform the chronology of changes in the region's coastal morphology. This paper suggest that human occupations between 6200 and 4400 cal BP recorded on high terraces of the Guiatecas Archipelago indicate higher local sea-levels, while the sites immediately on the waterfront are 2000 years younger. On the other hand, sites younger than 3300 cal BP on the modern coastline of the Chonos archipelago undergo permanent shaping, mainly due to local tectonics affecting vertical movement. Considering previously published and new data provided in this paper, we suggest preliminary uplift rates between 0.57 and 5.42 m/ka for the Guaitecas Archipelago, 0.31e1.48 m/ka for the northern sector of the Chonos Archipelago, and 0.85 m/ka in the central sector.
LGM Romania Pollen XRF Magnetic susceptibility Biomass burning Grass steppe Boreal and temperate ... more LGM Romania Pollen XRF Magnetic susceptibility Biomass burning Grass steppe Boreal and temperate tree refugia a b s t r a c t
An Isotopic Record of Holocene Variations in the Southern Hemisphere Westerly Wind Field From SW Patagonia
The southern westerly winds are an important feature of atmospheric circulation in the mid- to hi... more The southern westerly winds are an important feature of atmospheric circulation in the mid- to high-latitudes of the Southern Hemisphere. Recent observations and model-based studies have shown that both the latitudinal position and the overall strength of the wind field can directly influence Southern Ocean circulation and have important implications for the global carbon cycle. Here we present a lacustrine

Late Holocene Lacustrine Records of Climate and Vegetation Change From Southernmost South America
ABSTRACT The westerly wind field is one of the most prominent atmospheric circulation features in... more ABSTRACT The westerly wind field is one of the most prominent atmospheric circulation features in the Southern Hemisphere and has a major impact on the climate of southern South America as well as Southern Ocean hydrography. Southernmost South America is well-located to investigate past changes in the westerly winds because regional precipitation variability is controlled by the location and intensity of the wind field and it is the only landmass to extend within the core of the westerlies. Here we present late Holocene lacustrine records of climate change related to the westerlies from southern Patagonia, Chile. We focus on Lago Guanaco, a small hydrologically closed-basin lake in Southern Patagonia, and use stable isotope and pollen data from this site and three additional lakes in order to reconstruct changes in moisture balance related to the westerlies. Lago Guanaco (51°S, 72°W) is located close to the Nothofagus forest-Patagonian Steppe transition in the eastern region of Parque Nacional Torres del Paine. The location and composition of this important biological discontinuity is highly sensitive to the W-E precipitation gradient throughout Patagonia. The 4.75 m sediment core we obtained from the center of the lake has high concentrations of organic mater in addition to ostracodes and bivalves, which are relatively rare in Chilean Patagonia. Eleven AMS radiocarbon dates on organic and carbonate fractions indicate that the record spans the last ~14,400 cal yr BP and modern dates from core tops suggest little influence by old carbon sources. Changes in moisture balance and forest density/proximity are reflected in downcore variations in delta18Obivalve and delta18Oostracode, the Nothofagus/Poaceae paleovegetation index, and the C/N ratio of bulk decalcified organic matter. Combined, these variables document changes in the isotopic composition of the lake water, which largely reflect the isotopic composition of precipitation and the influence of evaporation, as well as shifts in the forest/steppe ecotone during the last 1800 years. More negative isotopic values at ~1350 cal yr BP and at the onset of the Little Ice Age (LIA) at ~450 cal yr BP correspond to cooler and/or wetter conditions. Increases in C/N and paleovegetation index values culminate between 100 and 400 cal yrs BP and are indicative of forest expansion and increased terrestrial matter input to the lake. Coincidently, enrichment of delta18Obivalve and delta18Oostracode are indicative of increased evaporation during spring/summer months. Taken together, the data indicate that during peak LIA conditions, summer precipitation was reduced while annual moisture balance increased to allow for forest expansion. An enhanced summertime poleward displacement of the westerlies can account for the observed change in the precipitation/evaporation regime.

Lacustrine Records of Holocene Climate Change From Southernmost South America
ABSTRACT The westerly wind field is one of the most prominent atmospheric circulation features in... more ABSTRACT The westerly wind field is one of the most prominent atmospheric circulation features in the Southern Hemisphere, which has a major impact on the climate of southern South America and hydrographic conditions in the Southern Ocean. Recent studies have argued that latitudinal shifts in the westerly boundaries during the LGM played a role in the modulation of atmospheric CO2 through positive feedback mechanisms within the Southern Ocean. However, significant discrepancies exist among paleoclimate records and GCM simulations. Here we present lacustrine records of late Holocene climate variability from two sites in southern South America. The first, a 4.75 meter sediment core from Laguna Guanacos (51°S, 72°W) in Southern Patagonia, provides a continuous record of climatic change related to the westerly wind regime during the last ~11,000 14C yrs. The sediment cores obtained from this small, shallow closed-basin lake reveal high concentrations of organic mater and biogenic carbonate. Eleven AMS radiocarbon dates on organic and carbonate fractions indicate that the record spans the last ~13,000 calendar years and modern dates from core tops suggest little influence by old or dead carbon sources. Combined pollen, stable isotope and elemental data indicate that significant shifts in the forest-steppe ecotone have occurred during the late Holocene. Increases in C/N and Nothofagus/Graminae index values culminating between 100 and 550 14C yrs BP are indicative of forest expansion related to cooler and moister conditions during the Little Ice Age. A large &gt;20/00 decrease in bulk organic delta13C and a concomitant decrease in Nothofagus and rise in Rumex during the last 100 years, marks extensive burning of the region and the replacement of a forested landscape by an anthropogenic-set steppe. A positive linear relationship between delta18O and delta13C on bi-valves and ostracodes indicates that the lake behaves as a closed system and will be sensitive to changes in moisture balance. Sediments from the second site, Lago Fagnano in Tierra del Fuego (54°S, 68°W), exhibit abundant mm- to cm-scale interbedded dark and light laminae. AMS radiocarbon dates on the pollen fraction indicate that the 2.4 meter long core spans the last 5300 years 14C yrs. Isotopic data from these two sites will be compared and discussed in the context of late Holocene westerly variability.
Postglacial vegetation, fire and climate dynamics at Central Patagonia (Lake Shaman, 44°S), Chile. Please contact me if you request a pdf version
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The Carpathian Mountains were one of the main mountain reserves of the boreal and cool temperate ... more The Carpathian Mountains were one of the main mountain reserves of the boreal and cool temperate flora during the Last Glacial Maximum (LGM) in East-Central Europe. Previous studies demonstrated Lateglacial vegetation dynamics in this area; however, our knowledge on the LGM vegetation composition is very limited due to the scarcity of suitable sedimentary archives. Here we present a new record of vegetation, fire and lacustrine sedimentation from the youngest volcanic crater of the Carpathians (Lake St Anne, Lacul Sfânta Ana, Szent-Anna-tó) to examine environmental change in this region during the LGM and the subsequent deglaciation. Our record indicates the persistence of boreal forest steppe vegetation (with Pinus, Betula, Salix, Populus and Picea) in the foreland and low mountain zone of the East Carpathians and Juniperus shrubland at higher elevation. We demonstrate attenuated response of the regional vegetation to maximum global cooling. Between ∼22,870 and 19,150 cal yr BP we find increased regional biomass burning that is antagonistic with the global trend. Increased regional fire activity suggests extreme continentality likely with relatively warm and dry summers. We also demonstrate xerophytic steppe expansion directly after the LGM, from ∼19,150 cal yr BP, and regional increase in boreal woodland cover with Pinus and Betula from 16,300 cal yr BP. Plant macrofossils indicate local (950 m a.s.l.) establishment of Betula nana and Betula pubescens at 15,150 cal yr BP, Pinus sylvestris at 14,700 cal yr BP and Larix decidua at 12,870 cal yr BP. Pollen data furthermore support population genetic inferences regarding the regional presence of some temperate deciduous trees during the LGM (Fagus sylvatica, Corylus avellana, Fraxinus excelsior). Our sedimentological data also demonstrate intensified aeolian dust accumulation between 26,000 and 20,000 cal yr BP.

Chronological and geoarchaeological investigations on an anthropogenic shell accumulation layer in the Longotoma dune field (Central Chile)
Along the Chilean–Peruvian coast, numerous archaeological sites associated with shell accumulatio... more Along the Chilean–Peruvian coast, numerous archaeological sites associated with shell accumulations provide evidence for the role of coastal environments as a natural corridor for the human occupation of South America. In particular, the semi-arid coastline of northern Central Chile (∼33–27° S) is a key area for investigating the relationship between human occupation history and past climate changes, as palaeoenvironmental studies record important climatic fluctuations in the area throughout the Holocene. In this study, we present a consistent chronology of an anthropogenic shell accumulation layer and an associated palaeosol, found in the stratigraphy of palaeodunes in the Longotoma dune system, northern Central Chile (32°24′ S, 71°23′ W). Infrared Stimulated Luminescence (IRSL) dating of feldspars is used, for the first time in this context, to establish the chronological framework together with radiocarbon dating. The consistency between IRSL- and radiocarbon ages demonstrates the potential of IRSL techniques for studies in comparable settings and at similar time scales, where Optically Stimulated Luminescence (OSL) dating of quartz is problematic. The chronostratigraphical results (i) allow to infer different phases of dune activity as well as an intermittent period of interdunal soil formation at the end of the 6th millennium BP, when the cultural layer and the palaeosol formed; (ii) allocate an age range of ∼4500–3500 a to alluvial deposits below the modern dune corridor; and thereby (iii) give further evidence for the presence of Archaic coastal hunter-gatherer populations in the La Ligua Bay and in northern Central Chile.

Reconstruction of palaeoprecipitation based on pollen transfer functions e the record of the last 16 ka from Laguna Potrok Aike, southern Patagonia
Based on modern pollen samples from different vegetation units in southern Patagonia, showing a c... more Based on modern pollen samples from different vegetation units in southern Patagonia, showing a close relation to yearly amounts of precipitation and mean annual temperatures, different polleneclimate transfer functions are developed and tested. Comparing the performance of MAT (Modern Analogue Techniques), WA (Weighted Average), as well as WAPLS (Weighted Average Partial Least Square) statistical techniques, it is possible to determine the statistically most robust model (WAPLS for precipitation). This transfer function is then used to estimate palaeoprecipitation amounts based on Laguna Potrok Aike pollen results for the last 16,000 years. Generally, the results of the precipitation model indicate less precipitation during the Lateglacial and alternating wet and dry periods during the Holocene. The Holocene started with higher amounts of precipitation until about 8 ka cal. BP, followed by a period with lower amounts between 8 and 2.5 ka cal. BP, while the Late Holocene shows a general increase in precipitation. Comparisons with former shoreline reconstructions and carbonate concentrations in the sediments of Laguna Potrok Aike not always show similarities due to the complex environmental factors recorded by these proxies. Moreover, changes in the moisture availability due to the interplay of precipitation and temperature, cannot be reconstructed directly. Nevertheless, the general long-term trend of palaeoprecipitation is in accordance with the absolute moisture content in the air, which is determined mainly by temperature: during cold periods with less absolute moisture, the model shows less precipitation. Moreover, the model also points to a relation with the position and strength of the Southern Hemisphere Westerlies.

A 19 cal ka BP pollen and charcoal record from Lake Shaman (44 S; 71 W, Chile) was analyzed to es... more A 19 cal ka BP pollen and charcoal record from Lake Shaman (44 S; 71 W, Chile) was analyzed to establish vegetation, fire and climate dynamics of the forest-steppe ecotone in Central Chilean Patagonia. Lake Shaman record indicates that the upper Río Cisnes valley was free of ice at around 19 cal ka BP. From this date and until 14.8 cal ka BP, a grass steppe with high proportions of shrubs associated to colder and drier conditions than present developed in this area. A continuous increase of Nothofagus accompanied by a decline in the steppe shrubs and sudden dominance of paludal over aquatic plants from 11 cal ka BP was associated to effective moisture increase but still under modern values. The replacement of the colddry grass-shrub steppe by a similar-than-present forest-steppe ecotone suggests an increase in temperature indicating the onset of the Holocene. At the same time, moderate fire activity suggested by the charcoal record could be related to major fuel availability as consequence of Nothofagus forest expansion. Between 8 and 3 cal ka BP, the record indicates the easternmost position of the forest-steppe ecotone suggesting the highest effective moisture with the establishment of seasonality between 5 and 3 cal ka BP. From 3 cal ka BP, the record indicates a retraction of the forest-steppe ecotone accompanied by a high pollen record variability and an increased fire activity. These late changes suggest decreased effective moisture associated with a high climatic variability. At regional and extra-regional scale, climatic changes at Lake Shaman’s record are mostly associated to changes (latitudinal shifts and/or strengthening/weakening) of past Southern Westerlies that were previously recorded along Patagonia from the Lateglacial to the mid-Holocene. During the Late Holocene, a regional pattern characterized by high record variability emerges throughout Central Chilean Patagonia. This variability would be related to (1) low magnitude Southern Westerlies changes probably associated to ENSO and/or SAM or (2) the complex relationships between vegetation, fire and human occupations during the last 3 cal ka.

The position and intensity of the southern westerly wind belt varies seasonally as a consequence ... more The position and intensity of the southern westerly wind belt varies seasonally as a consequence of changes in sea surface temperature. During the austral winter, the belt expands northward and the wind intensity in the core decreases. Conversely, during the summer, the belt contracts, and the intensity within the core is strengthened. Reconstructions of the westerly winds since the last glacial maximum, however, have suggested that changes at a single site reflected shifts throughout the entire southern wind belt1, 2, 3, 4. Here we use sedimentological and pollen records to reconstruct precipitation patterns over the past 12,500 yr from sites along the windward side of the Andes. Precipitation at the sites, located in the present core and northern margin of the westerlies, is driven almost entirely by the wind belt5, and can be used to reconstruct its intensity. Rather than varying coherently throughout the Holocene epoch, we find a distinct anti-phasing of wind strength between the core and northern margin over multi-millennial timescales. During the early Holocene, the core westerlies were strong whereas the northern margin westerlies were weak. We observe the opposite pattern in the late Holocene. As this variation resembles modern seasonal variability, we suggest that our observed changes in westerly wind strength can best be explained by variations in sea surface temperature in the eastern South Pacific Ocean.

A suite of mechanisms has been proposed to account for natural variations in atmospheric CO2 duri... more A suite of mechanisms has been proposed to account for natural variations in atmospheric CO2 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 infl uence 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 CO2 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 fl ow correspond to the timing and structure of atmospheric CO2 changes, and are consistent with the modeled magnitude of CO2 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 CO2 variations during the current interglacial and, possibly, during glacial cycles over the past 800 k.y.

Resolving the timing, direction, and magnitude of paleoclimate changes in the Southern midlatitud... more 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 discordante findings. 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.

Here we report results from a high-resolution palynological record and stratigraphic/geochronolog... more Here we report results from a high-resolution palynological record and stratigraphic/geochronologic data related to a Neoglacial event in Torres del Paine National Park, southern Chile (51_S, 71_W), to investigate climatic variations in Southwest Patagonia during the last 5000 years. The record reveals a stepwise expansion of Nothofagus-dominated woodlands and forests with discrete pulses at 4400, 2900, 1300, and 570 cal yr BP. Superimposed upon this trend we identify a relative opening of the woodlands between 4100–2900 and 2300–1300 cal yr BP. Closed-canopy forests dominated the landscape between 570–60 cal yr BP, followed by a rapid decline at the end of the 19th century that coincided with intense fire activity and the appearance of Rumex cf. acetocella, an exotic species introduced by European settlers. We interpret these changes as variations in the intensity and/or position of the southern margin of the westerly winds, which culminated with a net eastward shift of the forest–steppe ecotone during the Little Ice Age. We propose that millennial-scale changes in either the latitudinal position and/or the overall strength of the southern westerlies may be responsible for vegetation changes, fire occurrence, and the dynamic behavior of Patagonian glaciers during the last 5000 years. Because the modern maximum in near-surface wind velocities and precipitation is located between 48_ and 50_S, we suggest that the core of the southern westerlies may have achieved this modern position w570 years ago.

The Southern Hemisphere westerly winds influence the spatial distribution of precipitation in Sou... more The Southern Hemisphere westerly winds influence the spatial distribution of precipitation in Southern South America and play a significant role in the global carbon cycle, yet little is known about how this important atmospheric circulation feature has varied in the past. Here, we present a sediment core record of late Holocene variability from Lago Guanaco, a small closed-basin lake located in Torres del Paine National Park, Chilean Patagonia. The park is located in the core of the modern wind field and variations in the intensity of the atmospheric circulation directly influence the hydrology of this region. We combine stable isotopic measurements of biogenic carbonate and bulk organic matter to identify two periods of increased evaporation between 900–550 and 400–50 calendar years before present (cal yr BP). The first interval is coincident with the Medieval Climate Anomaly (MCA) while the more recent period is broadly coincident with the timing of the Little Ice Age (LIA). During the LIA interval, we observe simultaneous monotonic increases in the d18O of biogenic carbonate and Nothofagus dombeyitype pollen, which we interpret as indicative of significant changes in the intensity of the Southern westerlies during the last millennium. The isotopic and palynological variations in the Guanaco record are coincident with geochemical variations found in an Antarctic ice core record from Siple Dome, suggesting that the signal preserved in Lago Guanaco is regional rather than local, and that the LIA intensification was accompanied by a poleward shift in the southern margin of the westerlies. In addition, we interpret four periods of increased lake productivity centered on 900, 650, 500, and 200 cal yr BP from simultaneous increases in the d13C of bulk organic material and biogenic carbonate. These increases in lake productivity are most likely tied to increases in summer temperatures.
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Books by Jean Pierre Francois
Papers by Jean Pierre Francois