Bertrand Sebastien

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Bertrand
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Sebastien
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  • Preprint
    Late Holocene sea-surface temperature and precipitation variability in northern Patagonia, Chile (Jacaf Fjord, 44°S)
    ( 2009-06-12) Sepulveda, Julio ; Pantoja, Silvio ; Hughen, Konrad A. ; Bertrand, Sebastien ; Figueroa, Dante ; Leon, Tania ; Drenzek, Nicholas J. ; Lange, Carina B.
    A high-resolution multi-proxy study including the elemental and isotopic composition of bulk organic matter, land plant-derived biomarkers, and alkenone-based sea surface temperature (SST) from a marine sedimentary record obtained from the Jacaf Fjord in northern Chilean Patagonia (~ 44°20'S) provided a detailed reconstruction of continental runoff, precipitation, and summer SST spanning the last 1750 years. We observed two different regimes of climate variability in our record: a relatively dry/warm period before 900 cal yr BP (lower runoff and average SST 1°C warmer than present-day) and a wet/cold period after 750 cal yr BP (higher runoff and average SST 1°C colder than present-day). Relatively colder SSTs were found during 750-600 and 450-250 cal yr BP, where the latter period roughly corresponds to the interval defined for the Little Ice Age (LIA). Similar climatic swings have been observed previously in continental and marine archives of the last two millennia from central and southern Chile, suggesting a strong latitudinal sensitivity to changes in the Southern Westerly Winds, the main source of precipitation in southern Chile, and validating the regional nature of the LIA. Our results reveal the importance of the Chilean fjord system for recording climate changes of regional and global significance.
  • Article
    Precipitation as the main driver of Neoglacial fluctuations of Gualas glacier, Northern Patagonian Icefield
    (Copernicus Publications on behalf of the European Geosciences Union, 2012-03-15) Bertrand, Sebastien ; Hughen, Konrad A. ; Lamy, Frank ; Stuut, Jan-Berend W. ; Torrejon, Fernando ; Lange, Carina B.
    Glaciers are frequently used as indicators of climate change. However, the link between past glacier fluctuations and climate variability is still highly debated. Here, we investigate the mid- to late-Holocene fluctuations of Gualas Glacier, one of the northernmost outlet glaciers of the Northern Patagonian Icefield, using a multi-proxy sedimentological and geochemical analysis of a 15 m long fjord sediment core from Golfo Elefantes, Chile, and historical documents from early Spanish explorers. Our results show that the core can be sub-divided into three main lithological units that were deposited under very different hydrodynamic conditions. Between 5400 and 4180 cal yr BP and after 750 cal yr BP, sedimentation in Golfo Elefantes was characterized by the rapid deposition of fine silt, most likely transported by fluvio-glacial processes. By contrast, the sediment deposited between 4130 and 850 cal yr BP is composed of poorly sorted sand that is free of shells. This interval is particularly marked by high magnetic susceptibility values and Zr concentrations, and likely reflects a major advance of Gualas glacier towards Golfo Elefantes during the Neoglaciation. Several thin silt layers observed in the upper part of the core are interpreted as secondary fluctuations of Gualas glacier during the Little Ice Age, in agreement with historical and dendrochronological data. Our interpretation of the Golfo Elefantes glaciomarine sediment record in terms of fluctuations of Gualas glacier is in excellent agreement with the glacier chronology proposed for the Southern Patagonian Icefield, which is based on terrestrial (moraine) deposits. By comparing our results with independent proxy records of precipitation and sea surface temperature, we suggest that the fluctuations of Gualas glacier during the last 5400 yr were mainly driven by changes in precipitation in the North Patagonian Andes.
  • Preprint
    Changes in sub-fossil chironomid assemblages in two Northern Patagonian lake systems associated with the occurrence of historical fires
    ( 2012-09-06) Araneda, Alberto ; Jana, Patricia ; Ortega, Carolina ; Torrejon, Fernando ; Bertrand, Sebastien ; Vargas, Patricia ; Fagel, Nathalie ; Alvarez, Denisse ; Stehr, Alejandra ; Urrutia, Roberto
    Patagonia is commonly seen as an exceptionally pristine area because of its wildlife and practically unpolluted waters. However, during the twentieth century the burning of natural forests was one of the most important human activities in Northern Chilean Patagonia. Some estimations indicate that three million hectares were burned during the first three decades of the century being its impacts rarely assessed. Hence the objective of this study was to evaluate the impacts of the historical fires in Lake Burgos (45º42’S) and Lake Thompson (45º38’S) in Chilean Patagonia. The impact was measured by evaluating chironomid assemblage since they are sensitive enough to be used as an indicator of aquatic ecosystem health. Fires have a direct and drastic effect on a lake watershed but also indirectly affect a lake ecosystem, changing sedimentation patterns or increasing nutrient inputs. In the studied lakes the periods with higher prevalence of fires were identified by charcoal analysis, while organic matter and magnetic susceptibility allowed to confirm the pre-fire and post-fire periods. The chironomid composition was evaluated through a PCA and an analysis of similarity (ANOSIM) to test the significance among periods while a DCA was applied to the chironomid assemblage downcore to assess compositional structure and taxa turnover. In Lake Burgos the ANOSIM test indicated significant differences between the pre-fire and fire periods (p<0.05), while in Lake Thompson differences were not significant. However, in Lake Thompson the PCA clearly separated the pre-fire from the fire period but not the fire from the post-fire periods. In both lakes chironomid composition changed in relation to the period of higher prevalence of fires, which in turn implies catchment changes, pollution, and other anthropogenic impacts. Particularly a marked change in mesotrophic/eutrophic taxa was detected, reflecting an increase in nutrient input due to deforestation. Our findings point out that the lacustrine ecosystems are still affected by the impact of fires and the subsequent increase in nutrient supply that occurred almost fifty years ago. No sign of reverting to pre-disturbance conditions was observed, which makes these lakes highly sensitive to current human-induced impacts.
  • Preprint
    Reconstruction of the Holocene seismotectonic activity of the Southern Andes from seismites recorded in Lago Icalma, Chile, 39°S
    ( 2006-09-14) Bertrand, Sebastien ; Charlet, Francois ; Chapron, Emmanuel ; Fagel, Nathalie ; De Batist, Marc
    South-central Chile is one of the most geodynamically active areas in the world, characterised by frequent volcanic eruptions and numerous earthquakes, which are both recorded in lake sediments. In Lago Icalma (39°S), long piston and short gravity coring, as well as 3.5 kHz high-resolution seismic profiling, has been carried out in order to study the Holocene sedimentary infill of the lake, with a special focus on earthquake-triggered deposits. Macroscopic description of sediment cores and detailed grain-size analyses allow us to identify four types of seismically-induced deposits, or “seismites”: slump deposits, chaotic deposits, turbidites s.s. and homogenites. Homogenites are characterized by the occurrence of three distinct units on grain-size profiles (coarse base, thick homogenous unit topped by a thin layer of very fine sediment) and by the typical distribution of the grain-size parameters in a skewness-sorting diagram, while turbidites s.s. are characterized by a continuous fining upward trend. Radiocarbon, 210Pb dating, and tephrochronology allow us to demonstrate that the regional seismotectonic activity was probably very high between 2200 and 3000 cal. yr. BP as well as between 7000 and 8000 cal. yr. BP and that none of the historically documented earthquakes have triggered any seismite in Lago Icalma. The most recent seismite recognized in the sediments of Lago Icalma is a slump deposit dated at 1100 ± 100 AD, i.e. older than the period covered by historical records. The remarkable record of seismites between 2200 and 3000 cal. yr. BP is probably influenced by a major eruption of Sollipulli volcano at 3000 cal. yr. BP, which has rejuvenated the stock of terrigenous particles available for erosion, by depositing a thick layer of pumices all over the watershed of Lago Icalma and by clearing the vegetation covering the volcanic ash soils. This paper demonstrates that the record of seismically-triggered deposits in lake sediments is not only controlled by the intensity of the triggering earthquake and the occurrence of unstable sediment along the lake slopes but also by the presence of particles available for erosion/remobilisation in the watershed.
  • Preprint
    Postglacial fluctuations of Cordillera Darwin glaciers (southernmost Patagonia) reconstructed from Almirantazgo fjord sediments
    ( 2017-10) Bertrand, Sebastien ; Lange, Carina B. ; Pantoja, Silvio ; Hughen, Konrad A. ; Van Tornhout, Evi ; Wellner, Julia S.
    Most outlet glaciers of the Cordillera Darwin Icefield (CDI; Patagonia, 54⁰S) are currently transitioning from calving to land-based conditions. Whether this situation is unique to the modern climate or also occurred during the Holocene is entirely unknown. Here, we investigate the Holocene fluctuations of outlet glaciers from the northern flank of the CDI using a multi-proxy sedimentological and geochemical analysis of a 13.5 m long sediment core from Almirantazgo fjord. Our results demonstrate that sedimentation in Almirantazgo fjord started prior to 14,300 cal yr BP, with glacier-proximal deposits occurring until 13,500 cal yr BP. After 12,300 cal yr BP, most glaciers had retreated to land-locked locations and by 9800 cal yr BP, Almirantazgo fjord was a predominantly marine fjord environment with oceanographic conditions resembling the present-day setting. Our sediment record shows that during the first part of the Holocene, CDI glaciers were almost entirely land-based, with a possible re-advance at 7300–5700 cal yr BP. This is in clear contrast with the Neoglaciation, during which CDI glaciers rapidly re-advanced and shrank back several times, mostly in phase with the outlet glaciers of the Southern Patagonian Icefield (SPI). Two significant meltwater events, indicative of rapid glacier retreat, were identified at 3250–2700 and 2000–1200 cal yr BP, based on an increase in grain-size mode and related inorganic geochemical parameters. This interpretation is additionally supported by concomitant decreases in organic carbon of marine origin and in Cl counts (salinity), reflecting higher terrestrial input to the fjord and freshening of the fjord waters. Overall, our record suggests that CDI outlet glaciers advanced in phase with SPI glaciers during the Neoglaciation, and retreated far enough into their valleys twice to form large outwash plains. Our results also highlight the potential of fjord sediments to reconstruct glacier variability at high resolution on multi-millennial timescales.
  • Preprint
    Nature, origin, transport and deposition of andosol parent material in south-central Chile (36-42°S)
    ( 2007-07) Bertrand, Sebastien ; Fagel, Nathalie
    The andosols of south-central Chile (36-42°S) are developed on yellow-brown loams that cover the region with a thickness of several meters. In the literature, several hypotheses concerning the nature, origin, mode of transport and deposition of the andosol parent material have been advanced but no general agreement has been found. In this paper, we test these hypotheses by analyzing new representative outcrops located around Icalma (38°50’S) and Puyehue (40°40’S) lakes by a plurimethodological approach. Our data demonstrate that the andosol parent material has the typical mineralogical and geochemical signature of the regional volcanism and that these deposits are postglacial in age. The grain size of the deposits and the morphology of the coarse grains evidence that most of these particles haven’t been re-transported by wind but are direct volcanic ash falls deposited throughout the Late Glacial and Holocene. Because of the prevailing westerly winds, most of them have been transported to the East. Following the deposition of the volcanic particles, weathering and pedogenetic processes have transformed part of the volcanic glasses and plagioclases into allophane and have wiped out the original layering. This work demonstrates that most of the andosols that occur in the Andes and in the eastern part of the Intermediate Depression of south-central Chile are developed on volcanic ashes directly deposited by successive volcanic eruptions throughout the Late Glacial and Holocene.
  • Preprint
    Climate variability of southern Chile since the Last Glacial Maximum : a continuous sedimentological record from Lago Puyehue (40°S)
    ( 2007-04-29) Bertrand, Sebastien ; Charlet, Francois ; Charlier, Bernard ; Renson, Virginie ; Fagel, Nathalie
    This paper presents a multi-proxy climate record of an 11 m long core collected in Lago Puyehue (southern Chile, 40°S) and extending back to 18,000 cal yr BP. The multi-proxy analyses include sedimentology, mineralogy, grain size, geochemistry, loss-on-ignition, magnetic susceptibility and radiocarbon datings. Results demonstrate that sediment grain size is positively correlated with the biogenic sediment content and can be used as a proxy for lake paleoproductivity. On the other hand, the magnetic susceptibility signal is correlated with the aluminium and titanium concentrations and can be used as a proxy for the terrigenous supply. Temporal variations of sediment composition evidence that, since the last glacial maximum, the Chilean Lake District was characterized by 3 abrupt climate changes superimposed on a long-term climate evolution. These rapid climate changes are: (1) an abrupt warming at the end of the last glacial maximum at 17,300 cal yr BP; (2) a 13,100-12,300 cal yr BP cold event, ending rapidly and interpreted as the local counter part of the Younger Dryas cold period, and (3) a 3400-2900 cal yr BP climatic instability synchronous with a period of low solar activity. The timing of the 13,100-12,300 cold event is compared with similar records in both hemispheres and demonstrates that this southern hemisphere climate change lags behind the northern hemisphere Younger Dryas cold period by 500 to 1000 years.
  • Preprint
    Geochemistry of surface sediments from the fjords of Northern Chilean Patagonia (44–47°S) : spatial variability and implications for paleoclimate reconstructions
    ( 2011-09-22) Bertrand, Sebastien ; Hughen, Konrad A. ; Sepulveda, Julio ; Pantoja, Silvio
    The Patagonian fjords have a clear potential to provide high-resolution sedimentary and geochemical records of past climate and environmental change in the Southern Andes. To improve our ability to interpret these proxy records, we investigated the processes that control fjord sediment inorganic geochemistry through a geochemical, mineralogical and sedimentological analysis of surface sediment samples from the fjords of Northern Chilean Patagonia. A simple terrestrial index based on measurements of salinity and fraction of terrestrial carbon was used to estimate the terrestrial input/river discharge at each site. Our results demonstrate that, under the cold climate conditions of Patagonia, chemical weathering is weak and the inorganic geochemical composition of the fjord sediments is primarily controlled by hydrodynamic mineralogical sorting, i.e., the intensity of river discharge. Our results suggest that the distribution of Fe, Ti and Zr in surface sediments is controlled by their association with heavy and/or coarse minerals, whereas Al is independent of hydrodynamic processes. The elemental ratios Fe/Al, Ti/Al and Zr/Al are therefore well suited for estimating changes in the energy of terrestrial sediment supply into the fjords through time. Zr/Al is particularly sensitive in proximal environments, while Fe/Al is most useful in the outer fjords and on the continental margin. In the most proximal environments, however, Fe/Al is inversely related to hydrodynamic conditions. Caution should therefore be exercised when interpreting Fe/Al ratios in terms of past river discharge. The application of these proxies to long sediment cores from Quitralco fjord and Golfo Elefantes validates our interpretations. Our results also emphasize the need to measure Al-based elemental ratios at high precision, which can be achieved using simultaneous acquisition ICP-AES technology. This study therefore constitutes a strong basis for the interpretation of sedimentary records from the Chilean Fjords.
  • Preprint
    Geochemical evidence (C, N and Pb isotopes) of recent anthropogenic impact in south-central Chile from two environmentally distinct lake sediment records
    ( 2009-10-29) Fagel, Nathalie ; Bertrand, Sebastien ; Mattielli, Nadine ; Gilson, Delphine ; Chirinos, Luis ; Lepoint, Gilles ; Urrutia, Roberto
    In this paper, we compare the elemental and isotopic (C, N, Pb) geochemistry of lake sediments from two contrasted environments in South-Central Chile. The first lake, Laguna Chica de San Pedro (LCSP), is situated in the urbanized area of the Biobio Region (36°S). The second lake, Lago Puyehue (40°S), is located 400 km to the southeast of LCSP and belongs to an Andean national park. Our aim is to identify environmental impacts associated with increasing industrial activities and land-degradation during the last 150 years. In LCSP, shifts in C/N atomic ratios, δ13C and δ15N from 1915–1937 to the late 80’s are attributed to successive land-degradation episodes in the lake watershed. Based on a Pb isotopic mixing model, we estimate that up to 20% of lead in LCSP sediments is supplied from urban atmospheric pollution. By contrast, human impact in the watershed of Lago Puyehue is very limited. We observe no change in organic geochemistry during the last 150 years and lead contamination remains lower than 5%, even during the last decades. Although contamination levels are much higher in LCSP than in Lago Puyehue, a peak in anthropogenic Pb is recorded during the same period (1974–1976) at both sites. This maximum contamination level is consistent with increased industrial activity in the vicinity of Concepción.
  • Preprint
    Using the N/C ratio to correct bulk radiocarbon ages from lake sediments : insights from Chilean Patagonia
    ( 2012-06-12) Bertrand, Sebastien ; Araneda, Alberto ; Vargas, Patricia ; Jana, Patricia ; Fagel, Nathalie ; Urrutia, Roberto
    The offset between AMS radiocarbon ages obtained on bulk lake sediments and the true age of deposition was evaluated at four sites in Northern Chilean Patagonia. Our results show that the bulk radiocarbon ages are systematically older by 300 to 1100 years. In this region free of carbonate and carbonaceous rocks, we argue that this difference results from variable inputs of terrestrial organic carbon from the Holocene soils that cover the lake watersheds. For the four studied lakes, the age offset is clearly related to the fraction of terrestrial carbon preserved in the lake sediments, which was estimated using the N/C ratio of the bulk organic matter. We propose that N/C measurements can be used to significantly improve chronologies based on radiocarbon dating of bulk lake sediments.
  • Preprint
    Late Holocene covariability of the southern westerlies and sea surface temperature in northern Chilean Patagonia
    ( 2014-09) Bertrand, Sebastien ; Hughen, Konrad A. ; Sepulveda, Julio ; Pantoja, Silvio
    The climate of Chilean Patagonia is strongly influenced by the southern westerlies, which control the amount and latitudinal distribution of precipitation in the southern Andes. In austral summer, the Southern Westerly Wind Belt (SWWB) is restricted to the high latitudes. It expands northward in winter, which results in a strong precipitation seasonality between ~35 and 45°S. Here, we present a new precipitation seasonality proxy record from Quitralco fjord (46°S), where relatively small latitudinal shifts in the SWWB result in large changes in precipitation seasonality. Our 1400 yr record is based on sedimentological and geochemical data obtained on a sediment core collected in front of a small river that drains the Patagonian Andes, which makes this site particularly sensitive to changes in river discharge. Our results indicate Fe/Al and Ti/Al values that are low between 600 and 1200 CE, increasing at 1200–1500 CE, and high between 1500 and 1950 CE. Increasing Fe/Al and Ti/Al values reflect a decrease in mean sediment grain-size from 30 to 20 μm, which is interpreted as a decrease in seasonal floods resulting from an equatorward shift of the SWWB. Our results suggest that, compared to present-day conditions, the SWWB was located in a more poleward position before 1200 CE. It gradually shifted towards the equator in 1200–1500 CE, where it remained in a sustained position until 1950 CE. This pattern is consistent with most precipitation records from central and southern Chile. The comparison of our record with published regional sea surface temperature (SST) reconstructions for the late Holocene shows that equatorward shifts in the SWWB are systematically coeval with decreasing SSTs and vice versa, which resembles fluctuations over glacial-interglacial timescales. We argue that the synchronicity between SST and SWWB changes during the last 1400 years represents the response of the SWWB to temperature changes in the Southern Hemisphere.
  • Preprint
    Tephrostratigraphy of the late glacial and Holocene sediments of Puyehue Lake (Southern Volcanic Zone, Chile, 40°S)
    ( 2008-05-29) Bertrand, Sebastien ; Castiaux, Julie ; Juvigne, Etienne
    We document the mineralogical and geochemical composition of tephra layers identified in the late Quaternary sediments of Puyehue Lake (Southern Volcanic Zone of the Andes, Chile, 40°S) to identify the source volcanoes and to present the first tephrostratigraphic model for the region. For the last millennium, we propose a multi-criteria correlation model based on five tephra layers identified at seven coring sites. The two upper tephras are thin fine-grained green layers composed of more than 80% rhyodacitic glass shards, and associated to the AD 1960 and AD 1921-22 eruptions of the Puyehue-Cordon de Caulle volcanic complex. The third tephra is a sandy layer dominated by orthopyroxene, and related to the AD 1907 eruption of Rininahue maar. An olivine-rich tephra was deposited at the end of the 16th century, and a tephra characterized by a two pyroxene association marks the second half of the first millennium AD. In addition, we detail the tephra succession of an 11.22 m long sediment core covering the last 18,000 years. The results demonstrate that the central province of the Southern Volcanic Zone has been active throughout the last deglaciation and the Holocene, with no increase in volcanic activity during glacial unloading.
  • Preprint
    Bulk organic geochemistry of sediments from Puyehue Lake and its watershed (Chile, 40°S) : implications for paleoenvironmental reconstructions
    ( 2009-03-04) Bertrand, Sebastien ; Sterken, Mieke ; Vargas-Ramirez, Lourdes ; De Batist, Marc ; Vyverman, Wim ; Lepoint, Gilles ; Fagel, Nathalie
    Since the last deglaciation, the mid-latitudes of the southern Hemisphere have undergone considerable environmental changes. In order to better understand the response of continental ecosystems to paleoclimate changes in southern South America, we investigated the sedimentary record of Puyehue Lake, located in the western piedmont of the Andes in south-central Chile (40°S). We analyzed the elemental (C, N) and stable isotopic (δ13C, δ15N) composition of the sedimentary organic matter preserved in the lake and its watershed to estimate the relative changes in the sources of sedimentary organic carbon through space and time. The geochemical signature of the aquatic and terrestrial end-members was determined on samples of lake particulate organic matter (N/C: 0.130) and Holocene paleosols (N/C: 0.069), respectively. A simple mixing equation based on the N/C ratio of these end-members was then used to estimate the fraction of terrestrial carbon (ƒT) preserved in the lake sediments. Our approach was validated using surface sediment samples, which show a strong relation between ƒT and distance to the main rivers and to the shore. We further applied this equation to an 11.22 m long sediment core to reconstruct paleoenvironmental changes in Puyehue Lake and its watershed during the last 17.9 kyr. Our data provide evidence for a first warming pulse at 17.3 cal kyr BP, which triggered a rapid increase in lake diatom productivity, lagging the start of a similar increase in sea surface temperature (SST) off Chile by 1500 years. This delay is best explained by the presence of a large glacier in the lake watershed, which delayed the response time of the terrestrial proxies and limited the concomitant expansion of the vegetation in the lake watershed (low ƒT). A second warming pulse at 12.8 cal kyr BP is inferred from an increase in lake productivity and a major expansion of the vegetation in the lake watershed, demonstrating that the Puyehue glacier had considerably retreated from the watershed. This second warming pulse is synchronous with a 2°C increase in SST off the coast of Chile, and its timing corresponds to the beginning of the Younger Dryas Chronozone. These results contribute to the mounting evidence that the climate in the mid-latitudes of the southern Hemisphere was warming during the Younger Dryas Chronozone, in agreement with the bipolar see-saw hypothesis.