GC53A-01 INVITED 13:40h
Late Quaternary Paleoenvironmental History of the Peru-Chile Current System and Adjacent Continental Chile
A combined analysis of terrigenous and biogenic compounds in marine sediments from the Chilean continental margin allows detailed reconstructions of the paleoclimatic and paleoceanographic history of this region during the last ca. 120,000 years. Based on several sediment cores recovered during two German cruises and ODP Leg 202 (Site 1233), we found evidence for changes both in continental rainfall, most likely induced by latitudinal shifts of the Southern Westerlies, and marine productivity as well as sea surface temperature and salinity changes within the Peru-Chile Current system on time scales ranging from Milankovitch to centennial-scale. On Milankovitch time-scales, we found strong evidence for precession-controlled shifts of the Southern Westerlies implying for example generally more humid conditions during the LGM and a trend towards more arid climates during the deglaciation culminating in the early Holocene. These shifts are paralleled by paleoceanographic changes indicating generally higher productivity during the LGM mainly caused by increased advection of nutrients from the south through an enhanced Peru-Chile current. North of 33°S, these general productivity patterns are complicated by additional impacts from the tropics resulting in maximum paleoproductivity during the deglaciation and prior to the LGM. On shorter time-scales, extremely high resolution sediment cores from the southern Chilean margin provide evidence of significant short-term Holocene climate variability with bands of variability centred at ca. 900 and 1500 years, periodicities also well known from Northern Hemisphere records. Recently drilled ODP Site 1233 allowed to prolong these records into the last glacial. The available data show millennial-scale SST changes that closely follow the temperature pattern known from Antarctic ice-cores. Including other records from the Southern Hemisphere mid-latitudes, our data suggest a quasi-hemisphere-wide response that is consistent with the bipolar see-saw mechanism but may also imply a more prominent role of the Southern Hemisphere in the origin and transfer of millennial-scale climate variations during the last glacial. In addtion, Site 1233 offers the excellent opportunity to compare continental and marine paleoenvironmental signals within the same well-dated archive suggesting a lagged response of the terrestrial signal most likely related to climate inertia of the Patagonian ice-sheet.
GC53A-02 INVITED 13:50h
The Last Transition From Extreme Glacial to Extreme Interglacial Climate in NW Patagonia: Regional and Global Implications
The study of interhemispheric climate linkages during and since the last ice age has benefited from the recent development of high-resolution ice core and marine records from the mid- and high latitudes of the Southern Hemisphere. Few paleoclimate records from terrestrial environments in these regions, however, have the temporal continuity, time resolution, and adequate chronologic control to allow a detailed examination of the timing, rates, direction, and phasing of climate change at millennial timescales. Stratigraphic, palynologic, and charcoal records from small, high-sediment accumulating lakes in the Chilean Lake District (41°S) afford useful data for examining the interval between the LGM to the early Holocene (25-8 ka, ka=cal kyr BP). Millennial-scale changes in glacier extent and vegetation patterns within this interval match key events both in the Northern Hemisphere and Antarctic records, in particular the EPICA Dome C data, highlighting the role of an atmosphere-based conduit for the global propagation of abrupt climate changes. The onset of the last termination in NW Patagonia is marked by glacial collapse and the expansion of rainforest trees at 17.7 ka. An apparent antiphase relationship among the polar hemispheres between 17.7-14.7 ka, might reflect the hemispheric-scale effects of a quasi-total shut down of Atlantic Meridional Overturning circulation driven by Heinrich event 1. Extreme glacial and interglacial modes in the position/strength of the westerlies at multi-millennial timescales in the southern westerlies accompany important shifts in the abundance and composition of rainforest communities during the last termination and the early Holocene. At millennial timescales, this interval is characterised by successive warming pulses interrupted by a generalized reversal in trend with cooling events starting at 14.7 and 13.4 ka. Fires between 12.9-11.5 ka, i.e. Younger Dryas Chron (YDC), led to the expansion of opportunistic rainforest species and decline in all other trees, vines, and epiphytes. No species indicative of warming increase during this interval, suggesting that rainfall variability at sub-millennial timescales might have set the stage for fire occurrence in a cool-temperate environment dominated by North Patagonian species and cold-resistant conifers. Nowadays, El Nino events are teleconnected with negative anomalies in summer precipitation in the Chilean Lake District. Thus, it is possible that an ENSO-like signal during the YDC may have triggered forest fires in NW Patagonia. Acknowledgements: Fcyt 1030766, ICM P02-051-F
GC53A-03 INVITED 14:00h
High Resolution Fossil Chironomid Records From Lake Mascardi (41°S) During The Late Glacial: Additional Evidence Of The Huelmo/Mascardi Cold Reversal Event.
Despite the increasing number of palaeoclimatic studies from southern South America (Hajdas et al.,2004) there are still unresolved questions concerning the synchrony of climatic events between the northern and southern hemisphere and the extent of climate variability in the south. In particular, there is conflicting evidence about whether or not the Younger Dryas (YD) cold period, which is well-known from the North Atlantic region, was also evident in southern South America. So far, most of the evidence for climate change for these mid-latitude South American regions has been derived from glacial, pollen and beetle records but in an attempt to resolve these questions I have used chironomid midges as an independent proxy in one of the few studies of its kind in Patagonia. This study shows a high resolution chironomid stratigraphy, covering the Late Glacial cooling and early Holocene (between 15,000 and 11,000 cal yr BP) in Lake Mascardi, Argentina. An intensive radiocarbon chronology allowed to pinpoint the onset and timing of cold and warm episodes and to establish comparisons between previous multiproxy studies developed in the lake. The dynamics of the chironomid fauna of Lake Mascardi shows a cyclical pattern which appears to be linked to cold/warm climatic changes. Changes in chironomid assemblages indicated that a climatic cooling did occur during the YD and agreed with the Late Glacial scenario proposed by Hajdas et al. (2004). Based in a refined chronology, their results show the Huelmo/Mascardi Cold Reversal (HMCR) that encompasses the YD and a transition period, between 13,500 and 13,200 cal yr BP, preceding the onset of the YD event. The outstanding correlation between Hajdas' chronology and the chironomid results allowed to determine the exact phasing of cold/warm events confirming the HMCR between 13,300 and 11,500 cal yr BP (11,400-10,200 14C yr BP) as well as other short term climate oscillations occurred at Lake Mascardi. Hajdas, I., Bonani, G., Moreno, P.I., Ariztegui, D., 2003. Precise radiocarbon dating of Late-Glacial cooling in mid-latitude South America. Quaternary Research 59, 70- 78.
GC53A-04 14:10h
High-resolution Stable Isotope Record From Lago Cardiel ($49\deg$S), Argentina - Implications for Past Hydroclimatic Conditions in Patagonia Since the Late Pleistocene
Lacustrine authigenic carbonates are excellent archives of the prevailing water conditions at the time of precipitation. Their stable carbon and oxygen isotope compositions provide powerful proxies to reconstruct past environmental conditions in a high-resolution mode. Our study site, Lago Cardiel, is a large closed basin located at the southern tip of Patagonia (Argentina) on the eastern edge of the modern rain shadow caused by the combination of persistent westerly winds and the topographic barrier of the Andes. This geographical setting combined with the fact that the lake catchment area was not glaciated during the Last Glaciation makes it very sensitive to past atmospheric changes in the hydrological conditions. Previous studies on shorelines and seismic sequence stratigraphy calibrated with sedimentary core data revealed former large lake level fluctuations, which can now be assessed and constrained using stable isotope data from authigenic precipitated carbonates. During the last ca. 16,000 years Lago Cardiel underwent substantial changes in volume. The oxygen isotope record is to a large extend controlled by the precipitation/evaporation ratio leading to more negative oxygen isotopic values in times of wetter hydroclimatic conditions and vice versa. A clear stepwise structure characterized the Late Glacial-Holocene transition (LG-H) with the lowest lake level between 13,160 and 12,900 cal yr BP revealing relative positive oxygen isotope values. Conversely, a rapid lake level rise between 12,600 and 10,300 cal yr BP is characterized by a decrease in the oxygen isotope value of more than 3 \permil. The oxygen isotope record reveals a mid-Holocene lake level highstand and indicates highly variable environmental conditions in the late Holocene as a result of an intensification of the El Ni\~{n}o/Southern Oscillation (ENSO). The observed changes in the hydroclimatic condition of Patagonia are of more than just regional significance since they have substantial intra-hemispheric linkages controlling partly the dust content of Antarctic ice cores. The comparison of these results with those from other Patagonian sites shows a significant latitudinal gradient at critical time windows such as the LG-H transition. The latter point towards the complexity of the climate system and the need for more refined climate models.
GC53A-05 14:20h
Climate Development in Southern Santa Cruz (Argentina) During the last 1600 Years Gained from Proxies of the Salsa-Project
The DEKLIM project SALSA: "South Argentinean Lake Sediment Archives and Modelling" investigates lacustrine sediments with an integrated research strategy to understand quaternary ecosystem changes in space and time. SALSA studies are carried out in the Pali Aike Volcanic Field (PAVF; Santa Cruz province, southern Patagonia, Argentina). A few maar-like structures in the PAVF contain permanent lakes varying from 500 to 3700 m in diameter and exhibiting up to 100 m water depth. We will present results from short gravity cores of two different maars: Laguna Potrok Aike and Laguna Azul covering the last 1600-1300 years, respectively. An interpretation integrating the prominent sediment, isotopes and microfossil data from both lakes suggests the following climatic changes during the last 1.6 ka AD: Between the 5th and the 11th century relatively frequent moist/dry fluctuations were noticed. Between the 11th and the 20th century conditions were relatively moist except for a dry/warm period between the 13th and 15th century. The change from dry to moister conditions was dated to the 15th century. Since the middle of the 20th century climate is characterized by increasing drought.
http://www.salsa.uni-bremen.de
GC53A-06 14:30h
The Varve Record of Puyehue Lake (Meridional Chile), AD 1412-2002 .
Puyehue Lake is located in the Southern Volcanic Zone of Chile (Southern Lake District, 40°S). This monomictic lake is under the influence of the oceanic winter polar front (WPF) and constitutes a powerful sedimentary archive for paleoenvironmental reconstruction. The underflow (PUI) and interflow (PUII) coring sites were selected by a high resolution seismic survey (RUG). The sedimentation is driven by the annual blooms of diatoms in association with thermal lake cycles. For each site, one short core (50-60 cm) is analyzed by continuous cover of thin sections for varve counting and chronology for the AD 1412-2002 time window. The varve chronology is constrained by 210Pb,137Cs sedimentation rate estimates, and by the AD 1960 Valdivia'seismite. The sediment laminations are especially emphasized by the continuous grey-scale measurements from thin sections. Detrital clayey material characterizes the dark levels, and biogenic diatom mud defines the light levels. Varve counting is derived from the occurrence of light layers, i.e. formed at the end of winter after nutrient turn-over. In both cores, the grey-scale is influenced by instantaneous tephra layers related to the regional volcanic activity (Puyehue and Cordon Caulle volcanoes). For instance, in PUII, we identify major historical eruptions (AD 1921, 1837, 1790, 1564, and 1544) in accordance with the varve age model. In addition, we measure the total varve thickness. Lower than 500 m until AD 1780, the varve thickness gradually increases by a factor of four in PUII. The first period suggests a decrease of lake productivity, in agreement with a cooling. The age fits with the end of Little Ice Age in Southern America. Between AD 1780-2002, the thicker varves could be driven by a reinforcement of the WPF since the 19e century. Our local observations require further investigations to be extrapolated over regional or global scale.
http://www.ulg.ac.be/urap/
GC53A-07 14:40h
A Holocene Record of Hydrological Fluctuations in the Northern Chilean Altiplano (Lago Chungar\'{a})
Holocene records of moisture availability in the Central Andes and the Altiplano show contrasting and even opposite signals and time-transgressive millennial-scale climatic changes across the region, particularly between the Titicaca Basin and the Atacama Altiplano. A multiproxy study of a 13 kyr record of Lago Chungar\'{a} (18° 15' S, 69° 10' W, 4520 m a.s.l.) provides new data to solve some of the paleoclimate controversies as regional moisture availability patterns during the early and mid Holocene and the onset of modern ENSO conditions. Lago Chungara originated after the emplacement of the Parinacota volcano debris avalanche that blocked the Chungar\'{a} River prior to 13 cal. kyrs ago. A seismic survey and fifteen Kullenberg cores allowed a detailed 3-D reconstruction of the 8 m long sedimentary sequence. The chronological model is based on 5 AMS $^{14}$C on bulk organic matter and 3 U/Th dates on authigenic carbonates and shells. To assess the typically large (and variable) reservoir effect, we dated modern sediments and waters, and constrained the model with time markers based on a $^{210}$Pb age model for the last 150 yrs, volcanic ashes of known age, and the U/Th dates. We performed high-resolution analyses by an X-ray fluorescence core scanner and magnetic, sedimentological, mineralogical, isotopic, and biological (pollen, diatoms and ostracodes) analyses on selected cores. Statistical analyses helped to separate the volcanism from climate as the key driving forces in the hydrological and sedimentological evolution of the lake. Three main lacustrine units are identified on top of the pre Parinacota avalanche substrate. The basal unit (13 - 7.2 cal. kyrs BP) is a finely laminated diatomite. The middle unit (7.2 - 4.5 cal. kyrs BP) is banded and it is composed of diatomites and carbonate-rich layers. The diatomaceous upper unit (after 4.5 cal. kyrs BP) is banded to massive, and it contains abundant volcanic layers (lapilli, ash layers). Increased volcanic activity after 6 cal kyrs BP and climate-driven hydrological changes seem to have been conducive to increase carbonate precipitation during the middle unit. Seismic features of the middle unit in littoral areas evidence an alluvial progradation compatible with lowstand intervals and a subsequent lake level rise episode during the upper unit. The reconstructed hydrological variability in the Lake Chungar\'{a} shows fluctuating lake levels during the Pleistocene-Holocene transition and the early Holocene (13 to 7.2 cal kyrs BP), and the mid Holocene (7.2 - 4.5 cal kyrs BP), and the highest lake levels during the late Holocene. Preliminary frequency analyses in laminated basal unit suggest that ENSO-like variability was also present during the early Holocene in the Altiplano. The presence of mid - Holocene arid intervals supports the regional extent of this period of reduced moisture through the region.
GC53A-08 14:50h
A High-Resolution Biogenic Silica Record From Lake Titicaca, Peru-Bolivia: South American Millennial-Scale Climate Variability From 18-60 Kya
Sediments recovered from a deep basin in Lake Titicaca, Peru-Boliva, were analyzed for biogenic silica (BSi) content by extraction of freeze dried sediments in 1% sodium carbonate. Sediments were dated using an age model developed from multiple $^{14}$C dates on bulk sediments. The BSi record shows distinct fluctuations in concentration and accumulation rate from 18 to 60 kya. Multi-taper method spectral analysis reveals a significant millennial-scale component to these fluctuations centered at 1370 years. High BSi accumulation rates correlate with enhanced benthic diatom preservation, suggesting that the BSi record is related to variations in lake water level. Modern-day Lake Titicaca lake level and precipitation are strongly related to northern equatorial Atlantic sea surface temperatures, with cooler SSTs related to wetter conditions. Subsequently, the spectral behavior of the GRIP ice core $\delta ^{18}$O record was investigated in order to estimate coherency and linkages between North Atlantic and tropical South American climate. GRIP data exhibit a significant 1370-year spectral peak which comprises approximately 26% of the total variability in the record. Despite a high degree of coherency between millennial-scale periodicities in Lake Titicaca BSi and GRIP $\delta ^{18}$O records, the Lake Titicaca silica record does not show longer term cooling cycles characteristic of D-O cycles found in the GRIP record. Rather, the Lake Titicaca record is highly periodic and more similar in nature to several Antarctic climate proxy records. These results suggest that while South American tropical climate varies in phase with North Atlantic climate, additional forcing mechanisms are manifest in the region which may include tropical Pacific and Southern Ocean variability.
GC53A-09 15:00h
Paleoenvironmental and Paleoclimate Changes Since 21,000 Cal Years BP in the Northeastern part of Brazil Inferred From Sediment Records in Lagoa do Caco (Maranhao State, Brazil)
Two cores from Caco Lake, Maranhao State (North Brazil) record different histories of sediment accumulation on the margin and center of the lake that reflect changes in lake level. Seismic profiles, mineralogy and organic geochemical studies, backed by radiocarbon dating, reveal variable climatic and environmental conditions over the last 21 Cal Kyr BP. During the Last Glacial Maximum, regional climate was predominantly dry but was interrupted by short humid phases as reflected by a succession of very thin layers of sand and organic matter. The late glacial climate was relatively wet and included two rapid lake-level increases accompanied by forest expansion. The two wet phases were separated by a phase where the lake level remained stable and the forest changes were marked by the development of cool "Podocarpus" forest. These humid climate periods differed significantly from present warm tropical conditions.. The Holocene period is characterized by progressive increase of lake level, which reaches his maximum at around 7,000 Cal years BP. The period between 4,000 Cal years BP and the present shows high variability in lake level. Comparing with other South American and African records, we conclude that Late Glacial humid conditions were controlled by intensification of the ITCZ or shifts of its position, resulting in southeasterly trade wind variations and in interconnection between northern South America and the Atlantic tropical ocean-atmosphere system. The climatic variability during the Holocene is probably the result of sub-Milankovitch solar cycles and regional responses to these global forcings that are related to Atlantic and Pacific variability and their interconnections.
GC53A-10 15:10h
Solar and Volcanic Modulation of Little Ice Age Climate in the Tropical Andes, Venezuela
The underlying causes of late-Holocene climate variability in the tropics are incompletely understood. Here, we report a 1500-year reconstruction of climate history in the Venezuelan Andes using lake sediment records from four sites. This reconstruction is based upon accelerator mass spectrometry (AMS) radiocarbon and Pb-210 dating, sedimentology, magnetic susceptibility, geochemistry, pollen and stable isotope (C, N) measurements. In the Laguna Mucubaji watershed four distinct glacial advances occurred between 1250 and 1810 A.D. The earliest advance began during an extended period of higher global volcanic activity. The subsequent three advances were coincident with minima in solar activity (reconstructed from Be-10 and C-14 records). The Mucubaji glacial activity in the Venezuelan Andes coincides with other records of Little Ice Age (LIA) glacial advances in S. America. Comparison of modern glacier equilibrium line altitudes (ELAs) in Venezuela with the Mucubaji LIA glacier ELA indicates an ELA depression of at least 300 m. Both a decline in temperature and increase in precipitation are required to explain the ELA depression. The precipitation increase is supported by increased catchment erosion recorded in L. Blanca sediments. Pollen records from two sites in the Venezuelan Andes also indicate wetter and colder conditions during the LIA.
GC53A-11 15:20h
A 7000 C-14 Year Record of Environmental Change From North Central Cuba: Implications for Regional Sea Level and Climate Change
Paleoenvironmental records from the Caribbean are rare. To fill in this gap, a study of Laguna de la Leche, a 67 square kilometer coastal lake located in north central Cuba, was undertaken. Sediment cores were extracted from this lake and its vicinity in order to study to the Holocene sea level and climate history of the area. The cores were studied using pollen, benthic foraminifera, and stable isotopes of oxygen and carbon. A chronology was established by radiocarbon dating. The results show that from approximately 7000 C-14 years to 5000 C-14 years B.P., Laguna de la Leche was a marsh dominated by cattail (Typha domingensis). From 5000 C-14 years to the present, water level rose, turning the marsh in to a shallow, saline lake. Beginning around 1700 C-14 years B.P., mangroves expanded across the area, and are presently prograding into Laguna de la Leche. It is anticipated that over the next several hundred years, Laguna de la Leche may be completely infilled by mangroves. The cause of the rise in lake level appears to have been driven mostly by relative sea level rise. Pollen records from Florida and southern Alabama show an abrupt transition from oak-grassland to pine dominated systems at roughly 5000 C-14 years B.P. This shift from dryer to wetter conditions has been interpreted as representing an increase in the water table due to regional sea level rise (Watts and Hansen, 1994), and is coincident with the marsh-lake transition at Laguna de la Leche. The possibility that an increase in precipitation occurred at 5000 C-14 years B.P. is still being studied, but an oxygen isotope record from Haiti (Hodell et al., 1991) provides no evidence for such a climatic shift.
GC53A-12 15:30h
A comparison of speleothem and lake sediment core \delta$^{18}$O records from the north-central Yucatan Peninsula, Mexico
Paleoclimate studies in the Maya lowlands of Mesoamerica using lake sediment cores revealed relationships between times of drought and Maya cultural change. Here we test lake-based climatic inferences using oxygen isotopes of calcite from a stalagmite collected in a cave near Hobonil, Mexico, on the northern-central Yucatan Peninsula. The 30-cm stalagmite (Hobo-2) yielded a basal U/Th age of 1746\pm123 years ($\sim$250 AD.). This suggests that it grew throughout the Early and Late Classic Periods (250 - 800 AD.) when the Maya reached their cultural apex, and during the terminal Classic Period ($\sim$800-1000 AD.) when the civilization declined. Samples were drilled every 0.5 mm along the speleothem growth axis, yielding a mean temporal sampling resolution of 2.5 years, assuming a constant growth rate of $\sim$0.2 mm per year. The mean \delta$^{18}$O value over the stalagmite's length is -3.3\permil with a range of $\sim$2\permil (from -2.3 to -4.3\permil). Oxygen isotope variation in Hobo-2 is interpreted to reflect the \delta$^{18}$O of weighted mean annual precipitation, which is correlated with rainfall amount in the region. Evaporation effects on \delta$^{18}$O are also possible because the stalagmite was collected relatively near the cave entrance. We define dry conditions as periods when \delta$^{18}$O values exceed 1-standard deviation of the mean. The longest duration of dry climate for the last 1750 yrs occurred during the Terminal Classic period between ~800 and 1000 AD, confirming previous paleolimnological evidence for a protracted drought at that time. Dry events of shorter duration are centered at $\sim$620, 1410, and 1620 AD. The speleothem chronology is preliminary and based on the assumption of constant growth rate. Nonetheless, the oxygen isotope record of Hobo-2 is very similar to the \delta$^{18}$O records from shell carbonate in nearby Lakes Chichancanab and Punta Laguna, providing independent evidence of the climate history of the north-central part of the Yucatan Peninsula. Additional U/Th dates on speleothems will provide more accurate ages for climate events than has been possible with radiocarbon dating of lake sediment cores. Even AMS $^{14}$C dates on definitively terrestrial organic matter have calibrated age errors of +/-100 years. Because the karsted Yucatan Peninsula has abundant caves, speleothems hold much promise for reconstructing past climate and assessing its role in Maya cultural transformation.