PP41C-1464
A 17 kyr Record of the Southern Westerlies From Lago Cardiel, Patagonia, Argentina
The location of the Southern Westerlies (SW) stormtracks is related to the pressure gradient between the position and strength of the South Pacific High Pressure and the Circum Antarctic Low Pressure that also determines the location of the Subpolar oceanic front. Inferences on late Quaternary behavior of the SW's stormtracks in southern South America were made on the basis of precipitation-sensitive sensitive proxies such as records of past changes in vegetation, fire frequency, lake levels, river-runoff, glacier extent and marine conditions. Thus, higher levels of precipitation have been taken to imply higher westerly cyclonic activity. However, east of the Andes, higher levels of precipitation can be also related to enhanced incursions of easterly moisture during times when SW's are weak. Throughout southern South America conditions were markedly colder and drier during fullglacial times, indicating that the SW's were weaker and that westerly as well as easterly moisture was reduced given the much lower SST's. Multi-proxy evidence from Lago Cardiel (49°S, 76 m water depth, 379 km2 area, ~ 10km east of the Andean cordillera) sediments show that the lake at the end of the Last Glacial Maximum (LGM) was small, shallow and saline, according to ostracode assemblages represented by Limnocythere rionegroensis. By 13 cal kyr BP the lake had nearly dried out indicating that despite overall rapidly increasing temperatures after 17 cal kyr BP precipitation did not increase south of latitude 40°S. After 13 cal kyr BP lake level rose rapidly, culminating in a +55 m shoreline at 10,7 cal kyr BP. Presence of planktic diatoms and the highest ostracode diversity represented by Limnocythere patagonica, Kapcypridopsis megapodus, Eucypris fontana and E. virgata indicate a large and deep lake which suggests that the SW were weak allowing easterly moisture to advance towards the Andes. By 7,5 cal kyr BP lake level dropped to a level close to present. Benthic diatoms became abundant, and the modern ostracode assemblages, consisting of mainly E. fontana and L. patagonica in minor abundance, was established indicating reduction in precipitation to present-day levels. The occurrence of drift deposits after ~ 6,8 cal kyr BP reveals the presence of a wind- driven lake current, which indicates strong and persistent SW at a latitude of 49°S during the middle and late Holocene.
PP41C-1465
Abrupt climate changes in south-central Chile during the last deglaciation (18-10 kyr) revealed by the bulk organic geochemical record of Puyehue Lake sediments (40° S)
Paleoclimate data from the mid-latitudes of the Southern Hemisphere yield conflicting interpretations of interhemispheric synchrony/asynchrony of the climate system. In order to improve our understanding of the past climate changes in southern South America, we investigated the sedimentary record of Puyehue Lake, at the northern boundary of the southern westerly wind belt in south-central Chile (40° S). We analyzed the elemental (C, N) and stable isotopic (d13C, d15N) 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 by analyzing 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 (fT) preserved in the lake sediments. Our approach is validated using surface sediment samples, which show a strong relation between fT and distance to the main rivers and to the shore. We further apply 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 kyr cal. BP, which triggered a rapid increase in lake productivity, lagging 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 cover in the lake watershed (low fT). A second warming pulse at 12.8 kyr cal. BP is inferred from a substantial increase in lake productivity and a concomitant expansion of the vegetation in the lake watershed, demonstrating that the Puyehue glacier had significantly retreated from the watershed. This second warming pulse is synchronous with a 2° C increase in SST and corresponds to the beginning of the Younger Dryas cold event. These results contribute to the mounting evidence that the climate in the mid-latitudes of the southern Hemisphere was warming during the Younger Dryas, in agreement with the bipolar see-saw hypothesis.
PP41C-1466
Lacustrine Carbonate Records of Climate Variability in SW Patagonia
The strength and the latitudinal position of the Southern Hemisphere westerlies play a large role in controlling the amount and distribution of rainfall in southern South America. Understanding past changes in regional hydrology in southern Patagonia can therefore be used to provide insight into variations in atmospheric circulation in the high southern latitudes. Here we present a lacustrine paleoclimate record from a unique alkaline lake in SW Patagonia. As part of an ongoing project in the region investigating past climate and vegetation dynamics, we collected a series of sediment cores from Lago Guanaco, a small closed-basin lake located in Torres del Paine National Park (51°S, 72°W), and present a geochemical record based on the carbonate fraction that spans the last 13,000 years. Based on isotope data on the late Holocene portion of the record, the fine-fraction calcium carbonate is isotopically similar to the modern Chara growing in the lake and argues for a dominant algal origin for the carbonate. Marl deposition is largely constrained to the late glacial portion of the record where fine-fraction weight % carbonate values exceed 75% followed by an abrupt decrease to 0 % at 10,500 cal yr BP. During the early Holocene, carbonate is rare. Carbonate deposition abruptly increases in the middle Holocene at 4000 cal yr BP and continues at moderately high levels towards the present. Carbonate concentration culminates (>30%) in the very late Holocene at ~300 cal yr BP. When compared to other geochemical and palynological parameters derived from Lago Guanaco, it appears that carbonate deposition is largely associated with wet periods and may reflect an enhanced contribution of alkaline groundwater discharging into the lake. If this is the case, the carbonate stratigraphy argues for an increased groundwater influx from the middle Holocene to present, which may be related to an overall increase in precipitation and/or expanded ice associated with Neoglaciation that recharged the local aquifer. In addition to our carbonate stratigraphy, we will present modern surface and groundwater geochemical data (isotopic and elemental), and δ13C and δ18O on the fine-fraction carbonate to better understand the processes of carbonate formation in the lake and characterize the regional hydrology.
PP41C-1467
Enhanced Southern Westerly Activity During the Last Glacial Interglacial Transition in SW Patagonia
Much of the ongoing controversy regarding synchrony or bipolar asynchrony has centered on the timing and
structure of temperature changes during the Last Glacial Interglacial Transition (LGIT) in the southern mid-
latitudes, in particular the Patagonian region (40°-56°S) of South America. South America is the
only continuous continental landmass in the Southern Hemisphere that intersects the entire westerly wind belt
and lies in the critical climatic interface between mid- to high latitudes. The region between 50°-
55°S is of particular interest considering its proximity to the polar front and the southern margin of the
westerly winds, and the possibility of monitoring paleoclimate changes at or near this important oceanic-
atmospheric boundary. Present and past climate studies, including modeling simulations, have emphasized
that interactions between changes in the Antarctic Circumpolar Current, the southern westerly winds, sea ice,
and the polar front are important drivers for deep ocean circulation changes, variations in atmospheric
CO2, and equator-to-pole temperature and pressure gradients. Yet, the behavior of the westerly winds
and the adjacent Southern Ocean during the LGIT is still poorly understood despite their fundamental
influence on modern hemispheric and global climate.
In this study we compare sediment cores from small closed-basin lakes and a bog from the Ultima Esperanza
area of SW Patagonia (52°S), an area affected solely by the southern westerlies. Lake sediment cores
from three lakes show lacustrine sedimentation over the last 17,000 years. In contrast, the Dumestre bog
shows subaerial peat deposition since 15 ka interrupted by a prominent lacustrine phase dated between
14.5-11.5 ka. During this interval palynological records indicate the preponderance of cold-resistant herbs,
along with aquatic taxa, followed by a rapid expansion of Nothofagus woodlands/forests, a lake regressive
phase in Dumestre, and disappearance of laminated calcite in the lakes analyzed. These findings suggest
that cold conditions persisted during the LGIT until 11.5 ka in SW Patagonia, with important changes in
hydrological changes during the Antarctic Cold Reversal (ACR) and the Younger Dryas (YD) chrons. In
particular we identify a lake transgressive phase that led to a shallow lake between 14.5-13.5 ka, followed by
a lake level rise that persisted until 11.5 ka.
These findings indicate that SW Patagonia experienced important millennial-scale changes in the southern
westerlies during the LGIT. We suggest that the westerly wind belt intensified at 51°S during the ACR
and became stronger during YD time. An abrupt decline in precipitation at the beginning of the Holocene,
coupled with an increase in temperature, led to forest expansion, terrestrialization of the Dumestre site, and
cessation of calcite deposition in our SW Patagonian sites.
PP41C-1468
Chronicles from the End of the Word: the Holocene climate variability in Tierra del Fuego
Latest advances in the chronology and environmental importance of Antarctic paleoclimate records point towards a larger heterogeneity than previously thought. Thus, realistic inter-hemispheric correlations rely in the development of a tight array of well constrained records with a dense latitudinal coverage. Climatic records from southernmost Patagonia are hence critical corner-stones to link these Antarctic paleoclimatic archives with their South American counterparts. At 55° S on the Island of Tierra del Fuego, Lago Fagnano is located in one of the most substantially and extensively glaciated regions of southernmost South America during the Late Pleistocene. This elongated lake is the largest (~110 km long) and southernmost non-ice covered water body in the world. Existing on-shore geomorphological reconstructions combined with new lacustrine subsurface data, allowed us to better constrain the magnitude and chronology of the Fagnano glacier fluctuations since the LGM. The former Fagnano glacier flowed eastwards from the Darwin Cordillera fed by more than 50 tributary glaciers. The glacier spread over the low ranges and lowlands through three different lobes and was drained by four main outwash basins directly into the Atlantic Ocean. During the maximum ice-expansion, the ice-covered area was ca. 4000 km2 with a maximum length of ca. 132 km. A set of submerged frontal moraines covered by lacustrine infilling identified in the seismic survey suggests occasional eastward re-advances of the paleo-glacier within the overall westward deglaciation pattern. These re-advances may correspond to cold events such as the Antarctic Cold Reversal (ACR), the Huelmo- Mascardi Cold Event (HMCE) and/or the Younger Dryas Chronozone (YDC). The ongoing development of a robust age model blended with a multi-proxy dataset will potentially clarify remaining controversial issues dealing with the geographical extension and chronology of these cold episodes during the last deglaciation. A multi-proxy study of selected cores retrieved from the deepest part of the lacustrine basin allows characterizing the Holocene sedimentary record. Detailed petrophysical, sedimentological and geochemical studies of a complete laminated sequence reveal fluctuations in major and trace elements, as well as total organic matter content and palynological data suggesting an apparent cyclicity. These results provide a unique dataset that can be compared with other marine and continental archives to improve our understanding of the forcing mechanisms behind climate change and to validate the outcome of existing ocean and atmospheric climatic models for the Southern Hemisphere.
PP41C-1469
Glacial Chronologies Along the Andes (30-40°S) and the Role of the Westerlies
In arid and semi-arid mountain areas, like e.g the Central Andes, glaciers are moisture limited and thus more sensitive to precipitation changes rather than temperature changes. Glacial chronologies from such regions can therefore be out-of-phase with global temperature records, and reflect past changes in precipitation and related atmospheric circulation patterns, respectively. We have applied 10Be surface exposure dating at several locations along the Andes in order to establish glacial chronologies and to compare those with other paleoclimate proxies. At ~30°S, in the Valle Encierro and in the Cordon de Dona Rosa, pronounced moraines could be dated to between ~16 and 13 ka. Although today most precipitation at both locations occurs during austral winter and is related to the seasonal northward shift of the westerlies, we suggest that the Lateglacial advances at 30°S reflect the intensification and/or southward shift of the tropical circulation, which is well documented in numerous other records. An earlier, more extensive glacial advance at 30°S is dated to ~39 ka. This roughly coincides with the maximum glaciation that we dated in the Valle Rucachoroi at ~39°S (Argentina), and with 14C dated glacial advances in the northern Chilean Lake District (~40°S). We suggest that the westerlies were the moisture source for this "early local glacial maximum", and that the westerlies did not provide sufficient moisture north of ~39°S to allow for maximum glacial advances in-phase with the global temperature minimum during MIS 2. We will also present new exposure ages from moraines near Bariloche (Argentina, ~41°S) and from the Cerro Fredes (Chile, ~31.4°S), which provide minimum estimates of ~100 and 130 ka, respectively, for preserved older glacial deposits.
PP41C-1470
Tracking Late Quaternary Changes in the Southern Westerlies at High Resolution by Integrating Empirical Data (Marine, Fjord, Lake and Stalagmite Archives) from Southern South America Coupled With Model Results
Southernmost South America is best located to study past changes in the location/strength/extension of the SWW as it is the only land within the SWW pathway. We present here new results based on archives from marine, fjord and terrestrial environments located in the Chilean mid- to high latitudes aiming to frame the SWW variability at millennial to centennial timescales. During the last glacial period (70-20 kyr) the SWW and the subtropical front were located about 5° in latitude further north. The last deglacial warming started at ~19 kyr BP synchronously to Antarctic sea-ice retreat, and lasted about 6 kyr. As a consequence, the Patagonian Ice Sheet on the adjacent land collapsed within two main events (17.5-15.8 and 12-11.5 kyr) separated by glacier advances during the Antarctic Cold Reversal (14-12.5 kyr) which suggests a stagnation in the deglacial southward shift of the SWW. In the early Holocene (12-9 kyr BP) southern Chile was dry and warm, while southernmost Chile was wet and cold due to a southward position (and/or contraction) of the SWW. Cooler and wetter conditions during the middle and late Holocene in the mid- and high latitudes suggest a northward latitudinal shift (and/or extension) of the system. While changes in Antarctic atmospheric CO2 concentration are following the trends in the SWW over the last glacial time, the deglaciation and the early to mid-Holocene, the system is decoupled over the last 5 kyr. The Medieval Warm Period (MWP; 1.2-0.75 kyr) is characterized by wet and cold conditions in southern Chile, while the Little Ice Age (LIA; 0.65-0.1 kyr) was dry and relatively warm. On the contrary, the MWP was dry and the LIA wet in central Chile. This pattern may be explained by changes in the intensity (and/ or extension) of the SWW. Spectral analyses and new modeling results suggest that the SWW are sensitive to changes in the Southern Hemisphere insolation and solar activity.
PP41C-1471
Reconstruction of paleoproductivity and paleotemperature recorded in sediment cores of Drake Passage and Scotia Sea, Antarctica
To reconstruct paleoproductivity related to paleoclimate change of the Southern Ocean, biogeochemical
indicators, such as organic carbon and total nitrogen contents, and their stable isotopes ratios
(δ13C, δ15N), biogenic opal, and biogenic barium contents, were determined using
the sediment core GC98-06 (core length 251cm, in Drake Passage) and GC03-PW02 (core length 784cm, in
Scotia Sea). And they were analysed GDGTs using high performance liquid chromatography-mass
spectrometry (HPLC-MS) with an Agilent 1100 HPLC system connected to a Bruker Daltonics microTOF-HS
time-of-flight mass spectrometer. The GC98-06 and GC03-PW02 cores have three interglacial periods
(MIS1, MIS3, MIS5) and three glacial periods (MIS2, MIS4, MIS6).
The average of C/N ratio (9.34 and 7.65, respectively) and δ13C values (-24.4‰ and -
24.6‰, respectively) in GC03-PW02 (Scotia Sea) and GC98-06 (Drake Passage) indicated that the
organic matter in the sediment originated from marine primary production by phytoplankton. The BIT index as
an indication of fluvial terrestrial input, is 0.03 (GC03-PW02) and 0.06 (GC98-06), suggesting little
contribution of terrestrial organic matter.
TOC, TN, Biogenic Opal and Biogenic Barium contents in the glacial periods (i.e., MIS 2, MIS 4, MIS 6 stage)
are lower than in the interglacial periods. These results reflect that marine productivity increased during the
interglacial periods.
The average of TEX86-derived temperatures was lowest (0.5°C) in MIS1 periods and the highest
(3.8°C) in MIS4 period of Drake Passage, and also the lowest (2.7°C) in MIS1 periods of Scotia
sea. These TEX86-based temperature demonstrate that MIS1 period is unexpectedly the coldest in both
sites.
http://www.agu.org/yoonsh
PP41C-1472
Atmospheric modulation of western Antarctic Peninsula Holocene climate variability: Insights from the TEX86 record of Palmer Deep
A novel TEX86-derived sea surface temperature (SST) record was generated from well-dated hemipelagic sediments drilled in Palmer Deep, on the western Antarctic Peninsula continental shelf (ODP Hole 1098B; 64°51.162'S, 64°12.4795'W; 1010 m). Regional surface sediment samples with paired CTD casts are the first to demonstrate the potential utility of the TEX86 proxy in carbonate-poor western Antarctic Peninsula continental margin sediments. Average down core TEX86-derived temperatures at Site 1098 ranged between 1 and 6°C during the Holocene (0-12 ka). A long-term cooling of 3°C between 12 and 0 ka is punctuated by millennial scale temperature variability. While the absolute SST values and associated uncertainties should be regarded with caution until a more robust calibration of the TEX86 proxy at colder (0-5°C) temperatures emerges, both the long term and millennial scale variations in ODP Site 1098 TEX86 derived SSTs are consistent with temperature trends observed regionally (in both ice and marine sediment cores) and globally distributed Holocene paleoclimate records. The observed linkages suggest atmospheric (via the Southern Hemisphere westerly winds) control over the Holocene hydrography of Palmer Deep.
PP41C-1473
Increased Ice-age Influence of Antarctic Intermediate Water.
A depth transect of three ODP sites collected along the central Chile Margin constrain Antarctic Intermediate Water (AAIW) distributions and regional export production over the last 30 ka. Reduced Re and Cd, and increased Mn are proxies for higher bottom water oxygenation; 230Th-normalized burial of opal is a proxy for productivity. Mn/Al is high during the glacial interval at all three sites, suggesting high oxygenation and the retreat of the oxygen minimum zone during this period. At Site 1233, within the core of modern AAIW, Re and Cd are unchanged from detrital values throughout the last 30 ky, implying continuously oxic conditions. In contrast, at the northern sites 1234 and 1235, which reside below and above AAIW respectively, Re and Cd rise rapidly from low glacial values at ~15ka, signifying lower oxygen concentrations at the sea floor during Holocene time relative to ice-age conditions. Local productivity, recorded in Th-normalized opal burial, is highest during the glacial interval at both sites 1233 and 1234, and varies independently from the redox proxies. We conclude that local productivity does not drive bottom water oxygenation here, and that ventilation of the shallow subsurface southeast Pacific increased during the last ice age, with an expanded depth range of AAIW relative to the present.
PP41C-1474
Meltwater Induced Changes in Micronutrient Availability and Productivity in the Southern Ocean During LGM
The Southern Ocean plays an important role in the CO2 air-sea balance and have regulated atmospheric CO2 concentrations over glacial-interglacial cycles. Increased productivity resulting from higher nutrient deposition over Antarctica could have contributed to lower the atmospheric CO2 content during ice ages. Although Fe-fertilization has been observed in waters of present Southern Ocean, there is still a lack of convincing evidence for such events in the past. Nutrients supply and consumption were reconstructed by analyzing the Zn incorporated in diatom frustules and diatom-bound δ 15N from sedimentary core RC13-259 from the Antarctic sector of the South Atlantic (54°S, 5°W). These data evidence for the first time a greater availability of westerlies-driven micronutrients to phytoplankton during the LGM. These changes in concert with higher nitrate consumption are resulting from massive meltwater discharge to the Southern Ocean. The widespread presence of meltwaters and free-drifting icebergs may have stimulated nutrient drawdown contributing in turn to the sequestration of CO2 during glacial times.