U13B-1143
A 60,000-yr Record of Climate From Lake Qinghai: A Comparison With a Proxy Record From the South China Sea
Lake Qinghai is a closed-basin lake and the largest waterbody in China, located at the outer margin of the Asian summer monsoon today. Past fluctuations of the lake level were subject to the changes of the monsoon circulation. The reconstruction of lake level and climate over the past 60 ka is based on data from a multi-proxy investigation on two piston cores and a 26 m drill core recovered from the central subbasins of the lake, and on subbottom sediment structures as revealed by high-resolution seismic profiles of more than 300 km. The results indicate that both lake size and temperature during the Marine Isotope Stage 3 did not exceed those of the Holocene, and that the paleo-lake during the LGM separated into smaller lakes and windblown loess-like sediments deposited, indicating an extremely cold and arid climate. Seasonal catchment inflow increased at 11.6-10.7 ka though the lake was less than 6 m deep. The arid Younger Dryas equivalent at 10.7-10 ka was followed by an abrupt onset of the warm early-Holocene climate but the effective moisture at 10-8 ka was much lower than that of today. A permanent expansion of Lake Qinghai occurred at 10 ka and the lake began to increase towards the present-day dimension from about 8 ka. This reflects a stepwise enhancement of summer rainfall on the NE Tibet-Qinghai Plateau. The G. rubber δ18O record from a northern SCS core suggested that the main pattern of climate change since the MIS 3 is consistent with that documented in the Greenland GISP2 δ18O record. The SCS during the MIS 3 was overall under the conditions of a strong winter monsoon and weak summer monsoon precipitation, as also indicated by low fluvial clay content and high modal grain size. The proxy record clearly indicates deteriorated conditions during the LGM. The correlation of the SCS record with Lake Qinghai suggests that the paleoenvironmental conditions of the western Pacific marginal seas and the WPWP had a substantial impact of moisture availability on the outer margin of the Asian summer monsoon, which determined that the strength of the summer monsoon during the MIS 3 was weaker than the Holocene. During the LGM, a combined impact of southward-shifted polar front and an about 100 m decrease of the WP marginal sea levels resulted in a further increase of aridity on the NE Plateau.
U13B-1144
A Late Quaternary Climate Reconstruction Based On Combined Heat Flow And Borehole Temperature Data
Reconstructions of ground surface temperature history (GSTH) from geothermal data have contributed to an understanding of the contemporary global warming in the context of long-term climate variability. Among the reconstructions based on geothermal data are a twenty-millennium GSTH published in Geophysical Research Letters (1997, 31: 267-280) and a five-century GSTH published in Nature (2000, 403: 756-758) by the Huang- Pollack-Shen (HPS) team. But there have been questions regarding the different amplitudes and temporal coverage of these two reconstructions. Here we explain their differences and present a reconstruction based on the combination of the climate information carried independently by the data sources of these two previous studies. The HPS(2000) study is based on a global database of borehole temperatures compiled for the explicit purpose of climate reconstruction. Signals of past temperature changes at the ground surface are recorded in the rocks as transient perturbations to the steady-state subsurface temperature field. However, there are several constraints on the amount of climate information one can retrieve from a borehole temperature profile. In particular, the temporal length of a reconstruction is basically limited by the depth of the borehole temperature profile and the noise level of the borehole data. Given that most of the temperature profiles are shallower than 400 m, the HPS(2000) focused on reconstructions representing the past five centuries. The HPS(1997) reconstruction is not a "standard" borehole reconstruction in which a temperature versus depth profile is inverted to yield a GSTH. This reconstruction is based on a heat flow versus depth profile assembled from over 6,000 data entries residing in the heat flow database compiled for the original purpose of mapping the heat loss from the interior of Earth. Compared to the data used in HPS(2000), the data set of HPS(1997), while more abundant, is much noisier. We converted the heat flow versus depth profile into a 2000 m temperature versus depth profile prior to inversion. Data shallower than 100 m were excluded from the analysis due to the concern over high noise level. The inversion of this converted profile yielded a highly smoothed late Quaternary climate history. To integrate the climate information preserved in the borehole temperature database and the heat flow database, we use the century-long trends from HPS(2000) supplemented by the SAT instrumental record to generate a globally representative temperature-depth profile from the surface down to 300 m depth at 10 m depth intervals. We then extend this synthetic temperature-depth profile down to 2000 m with the converted heat flow data of the HPS(1997) study. The inversion of this composite temperature data set shows an average late Pleistocene temperature some 4K below the reference level (1961-1990 mean), and a long mid-Holocene warm episode averaging about 1K above the reference level. The temperature in the Medieval Warm Interval some 600-1,000 years ago was at or slightly above the reference level. The Little Ice Age is apparent around 200-500 years ago, averaging almost 1K below the reference level. http://www.geo.lsa.umich.edu/~ shaopeng/
U13B-1145
Millennial-scale Hydrological Fluctuations in Western Mediterranean During the Last 20 ka: The Sedimentary Record of Lake Estanya (NE Spain)
The impact of North Atlantic changes in Western Mediterranean Sea has been widely documented by marine records (Cacho et al., 2001; Martrat et al., 2004 and 2007; Moreno et al., 2005). Although this teleconnection has also been recorded in several mountain records of northern Spain (Allen et al., 1996; Leira and Santos, 2002; González-Samperiz et al., 2006), the environmental effects and the hydrological amplitude of these fluctuations in continental, low-land areas of Iberian Peninsula are still poorly understood. The multi-proxy study of sediment cores from karstic Lake Estanya (Pre-Pyrenees, NE Spain) provides the first continuous record of the hydrological evolution in this area during the deglaciation. The chronological model, based on 14 AMS radiocarbon dates shows an age of ca. 20,000 calendar years BP for the base of the sequence. Seismic stratigraphy revealed a sedimentary filling up to 15 m thick consisting on three main units identifiable through the lake basin and deposited during the Lateglacial, Early-to-Mid Holocene and Late Holocene, respectively. Sediment analyses have been carried out in cores from the deepest area of the lake. Sedimentary facies have been defined after the integration of sedimentological and mineralogical proxies (X-ray diffraction and SEM), high-resolution analyses of light elements (Al, Si, P, S, K, Ca, Ti, Mn and Fe) by XRF core-scanner and stable isotope analyses of ostracods, carbonate phases and organic matter. Increased run-off and higher lake levels are interpreted from massive, clastic facies while more concentrated waters and shallower conditions were more frequent during deposition of laminated and organogenic gypsum-rich facies. Lateglacial sequence shows fluctuating but shallow lake conditions followed by a large increase in water availability during the Early Holocene and lowered lake levels and arid conditions for the Mid-to-Late Holocene (4.2 – 0.8 ka). Fluctuating but higher lake levels occurred during the last 800 years. Millennial-scale hydrological phases recorded in Lake Estanya are coherent with western Mediterranean reconstructions (Frigola et al., 2007) and central Europe lacustrine records (Magny, 2004), but show similarities with northern African records (Lamb et al., 1995) suggesting a complex interplay of mid-latitude and subtropical dynamics in the climatic evolution of the Iberian Peninsula. http://www.ipe.csic.es/limnogeologia/principal.htm
U13B-1146
High-Resolution Speleothem Records of Climate Variability from the Northwestern Iberian Peninsula During the Last 30,000 Years
The late Quaternary climatic evolution of the northern Iberian Peninsula has not yet been well documented despite the rich human history of occupation preserved in cave sites and the significance of this location in the modern North Atlantic Oscillation. Existing geomorphologic records suggest that in the northern Iberian Peninsula the maximum glacial extent was reached as much as 10 ka before the last global glacial maximum (LGM), but do not resolve rapid climate changes. Here, we present stable isotope and elemental climate records from two well-dated stalagmites from the northern coast of Spain, which span from 28,000 to 4800 years B.P., with average deposition rates of 20 and 50 mm/ka. The climate variability described by these new speleothem records will be placed in a regional context by comparison with available lacustrine records from nearby mountains. We aim to (1) describe the sequence of climate changes that occurred since the LGM, (2) identify the timing and describe the structure of abrupt climate oscillations and (3) test hypotheses concerning the forcing mechanisms of hydrological variations for the last 30 ka. Oxygen isotopic values, with a total variability of 3 ‰, are highest from 22.8 to 18.2 ka, coinciding with the global LGM. After that, a hiatus was detected ending at 15.4 ka, likely pointing to a cold and dry period without speleothem formation, which may be related to Heinrich Event 1. The postglacial decrease in δ18O features higher frequency variability including a 0.7 ‰ excursion to lower values during the Younger Dryas (12.8 to 11.8 ka). The Holocene values are lower and ranging 2 ‰, pointing to significant hydrological variability during the last 10 ka. Carbon isotopic values, ranging from -11 to -2 ‰, are very similar to the oxygen profile in both records and along the entire time interval recorded. After testing that calcite was deposited under equilibrium conditions, we suggest a probable common forcing behind the hydrological and soil-vegetation patterns of variation. Our high resolution (48 hour) analysis of oxygen isotopes in modern precipitation at the site reveals a large (6 ‰) range which is highly correlated with the daily CPC NAO index and only weakly correlated with air temperature or precipitation amount. Oxygen isotopes in rain and dripwater might thus be most strongly regulated by the dynamics of precipitation (moisture source and condensation height and temperature), superimposed on the trend in δ18O of seawater from ice volume effects. Thus, the negative isotopic excursion during the Younger Dryas and those events during the Holocene could reflect precipitation generated under conditions more similar to those characterizing the modern negative NAO phase. Mg/Ca ratios rise sharply after 8000 years BP likely reflecting sea level rise and greater proximity to the coast which increases sea salt aerosol Mg deposition.
U13B-1147
A New Noble Gas Paleoclimate Record from Texas: Perils and Promise Explained
Typically, it is assumed that because of dispersion, signals from rapid climate change are lost in groundwater systems. This plus the uncertainties in 14C groundwater dating have meant that significant negative shifts in noble gas temperatures (NGTs) have usually been associated with large scale events such as the Last Glacial Maximum (LGM). New noble gas measurements on 49 samples from the Carrizo aquifer in South Texas have been used to construct a detailed paleoclimate record over the past ~10ka (Castro et al., 2007). The build up of radiogenic 4He was used to constrain a 3D hydrogeological model of unprecedented detail for the Carrizo and confirms earlier work (Castro and Goblet, 2003) suggesting that the large NGT drop in this area occurred ~6ka and not at the LGM. A study of the effect of dispersion using aquifer characteristics derived from the 3D model demonstrate that rapid shifts in climate can be recorded well into the Pleistocene. The NGT record using standard models matches mean air temperature (T) in the recharge area, but shows a ~ 5 °C bias to low T relative to present ground T, with a sudden further drop in T at about 1ka and a minimum ~ 8 °C below present day values at about 6ka. This is significantly younger than the LGM and well below the expected temperature at this time. Hall et al. (2005) suggested an NGT model that assumes that O2 depletion at the water table enhances noble gas partial pressures and this model successfully corrects for the 5 °C temperature bias seen in recharge area samples while simultaneously improving NGT model fits. The model also shows a marked change in the amount of excess air seen in samples, which drops suddenly at ~ 1ka. We interpret the temperature drop and the reduction of excess air at this time as indicating a change mostly in recharge conditions rather than atmospheric T, with the water table approaching the surface. It is not an indicator of the LGM as had previously been assumed (Stute et al., 1992). Castro and Goblet, 2003, Geophys. Res. Lett., 30 (24), 2251, doi:10.1029/2003GL018875. Castro et al., 2007, Earth Planet. Sci. Lett., 257, 170-187, doi:10.1016/j.epsl.2007.02.030. Hall et al., 2005, Geophys. Res. Lett. 32(18) L18404, doi: 10.1029/2005GL023582. Stute et al., 1992, Science, 256, 1000-1001.
U13B-1148
Quantitative Estimates of Precipitation and Temperature Change and Associated Climate- Vegetation Feedbacks During Heinrich Events in Florida, USA
Several paleoecological reconstructions from Lake Tulane in subtropical central Florida (USA) have revealed a series of sharp vegetation switches from open scrub oak and prairie communities towards pine dominated forests during the past 60 ky. A recently improved age model shows that the pine phases are coeval with North Atlantic stadials that are terminated by Heinrich events. Based on these vegetation reconstructions, the pine phases are now considered to represent warmer and wetter conditions in Florida, which points to an antiphase relationship between climate in Florida and the North Atlantic region. We have developed a number of pollen-climate inference models using surface sediment samples from the southeastern USA to quantitatively estimate the past climate changes associated with the vegetation shifts in Florida of the past 60 ky. The models are derived from partial least squares (PLS) and weighted-averaging PLS (WA-PLS) of summer and winter precipitation and growing-season temperature with bootstrap cross-validated prediction errors. Results show that summer rainfall increased by 80-90 mm/yr during Heinrich pine phases, while temperature increased between 0.5-1.0 degrees C. These reconstructions support the previous qualitative interpretations and permit testing with climate models. A regional vegetation-atmosphere model forced by the reconstructed climate shifts is used to investigate runoff and evaporation changes during Heinrich events and assess vegetation-climate feedback processes.
U13B-1149
Holocene paleoclimates of Canada north of 50°: Comparison with high-resolution multi-proxy records
High-latitude regions are naturally more sensitive to climate perturbations due in part to strong surface albedo feedback. In this study, we use a large network of modern and fossil pollen data to investigate climate variability above 50°N in North America during the past 12,000 years. Climate variables such as July and January, and annual precipitations are reconstructed at 100-year intervals to evaluate climate variability at several temporal and spatial scales. Results show both low and high frequency climate variability occurring throughout the Holocene. Climatologically reasonable space-time patterns are presented. Estimates of the range of climate variability on time scales of 104 and 103 compare well with high-resolution continental to hemispheric scale temperature reconstructions using multiple proxy records. High-latitude reconstructions show slightly higher sensitivity compared to lower mid-latitude reconstructions. Recent observed regional warming is unprecedented in terms of millennial-scale temperature variability found during the Holocene.
U13B-1150
Preliminary Chronostratigraphy of Pingualuit Impact Crater Lake Sediments (Nunavik, Canada): a Possible Arctic Terrestrial Record of Several Glacial/Interglacial Cycles
The Pingualuit Crater Lake is a 1.4 Ma perfectly circular depression. It is 3.4 km in diameter and hosts a lake with a maximum depth of 267 m with no surface connection to other surrounding water bodies. It is located in the northernmost part of the Ungava Peninsula in Nunavik, Canada (61°N, 73°W) and, due to its unique morphometry (shape, depth), the lake bottom may have escaped glacial erosion, possibly preserving a continuous Arctic terrestrial record of several glacial/interglacial cycles. Previous attempts to core the lake have resulted in the collection of only 14 cm of sediments that spanned about the last ~5000 years. Here, in order to test the possible accumulation of several glacial/interglacial cycles, we will present initial high resolution physical (CAT-scan, Multi Sensor Core Logger, color reflectance, high resolution digital images), magnetic (point source magnetic susceptibility) and sedimentological (detailed visual description, grain size) properties of about 10 m of sediments recently cored using a UWITEC piston percussion corer under harsh climatic conditions. The initial results revealed the presence of at least two decimetre-thick intervals composed of laminated, dark grey clayey silts characterized by a relatively low density and magnetic susceptibility that contrast sharply with the thicker and more abundant over- and underlying light grey, denser, sandy sediments. The sediment characteristics in the darker laminated intervals are also similar to the ones observed in the small surface gravity core sampled at the same site. Moreover, smear slides performed in these two intervals revealed the presence of diatoms and chrysophytes, suggesting that these two intervals represent ice-free conditions and thus possible interglacials, whereas the more abundant light grey and more sandy sediments likely reflect glacial intervals. This interpretation will be tested by ongoing paleomagnetic measurements (i.e., magnetostratigraphy), as well as radiocarbon and thermoluminescence dating.
U13B-1151
Regional Arctic and Hemispheric Teleconnections expressed in the paleoenvironmental record of El'gygytgyn Lake, NE Russia
The common goal of recovering long high-resolution records is in testing relevant questions of Earth system dynamics, as well as documenting the drivers of regional and global scale change. Lake El'gygytgyn, located 100 km north of the Arctic Circle in NE Russia is a target for deep drilling a continuous record back to ~3.6 My in Spring 2009. Pilot cores dating to 250ka to 300 ka provide the impetus for evaluating the sensitivity of the Arctic to regional and global climate events on millennial timescales. A clear record of the Younger Dryas, rapid change within MIS 3, and events including interstadials 19, 20, events within Stage 5, and at the end of stage 6 seen in Greenland and marine records suggest that oceanographic and atmospheric changes over the North Atlantic are reflected in hydrologic and seasonal temperature proxies. Rapid events are recorded despite demonstrated precessional influences and the suggested upwind influence of the Eurasian Ice sheet and dramatic changes in continentality due to changes in sea level across the Bering/Chukchi shelves and the extent and seasonal persistence of sea ice in the Arctic Ocean and deeper Bering Sea. Regionally, lake cores throughout Beringia reflect patterns of precipitation and temperature that point to persistent zonal differences in the response of the landscape to environmental change.
U13B-1152
Last glacial- Interglacial Cycle Palaeoclimatology and Palaeoecology Reconstruction: Ostracod Stable Isotope Records From Ohrid Lake and Palaeolake "Les Echets"
delta18O and delta13C signals are inferred from lacustrine ostracod shells to document the terrestrial climate changes from the penultimate glacial maximum (Rissian) to the last interglacial (Eemian). These proxies are recorded in two sediment cores taken in Ohrid Lake (Southern Balkans) and palaeolake Les Echets (SE France), sites chosen to reflect the climate dynamics under north Atlantic and Mediterranean influences. The d18O signal shows high fluctuations probably linked to the water balance effects (evaporative enrichment) associated with the very complex hydrological settings of both lakes. The d13C seems to respond more accurately to climate changes as evidenced by others proxies (Pollen, magnetic susceptibility). The ostracod d13C is directly linked to the d13C of the Dissolved Inorganic Carbon (DIC), which in our case is controlled first by the isotopic composition of DIC in the incoming water and, secondly, by a number of lake-internal processes (production, carbon deposition, and CO2 exchange with the atmosphere). At the beginning of the Eemian, the negative excursions of d13C are likely due to an increase of the biogenic CO2 production in the soils (mainly related to the plant respiration and microbial activity). Conversely, the high values observed at the Rissian late glacial are a consequence of the sparse vegetation and weak soil activities. The agreement between the d13C variations in both lakes with other well constrained proxy shows the high potential of our records in monitoring climate- induced vegetation changes. Overall our results are consistent with the main climate trends documented in other records (ice and marine sequences) but reveal complex leads and lags. One of the most striking features of both records is the interruption of the warming trend (a Dansgaard-Oeshger like event) during transition from IS6 to IS5 already mentioned by others studies conducted in the northern Atlantic Ocean. Such rapid climate shifts are very well known in Greenland ice cores but only from early IS4 and younger. To our knowledge, so far no continental data show this rapid event before and during transition from IS6 to IS5 in the northern hemisphere; therefore comparing continental and marine data as far as penultimate glacial stage 6 will help to better constrain the forcing mechanisms responsible for such rapid climate changes.
U13B-1153
A Younger Dryas glacierization in the Venezuelan Andes
The Younger Dryas (YD) was an abrupt climate change event that has been well recorded in marine archives, but is still uncertain in terrestrial records from the tropics. For instance, the YD is recorded in the Cariaco Basin as an event of increased upwelling intensity and decreased terrigenous input; however continental records of a coincident climate change event across the region are inconclusive. Moreover, pollen data from the Colombian Andes suggest that there was a local cold stadial that coincides with the YD, but paleoenvironmental records from the Venezuelan Andes during the same time interval are not clear. Here we present a high-resolution late- glacial sequence from the Pico Espejo region of Venezuela. Our record is based on radiocarbon dated lake sediments that were analyzed using magnetic susceptibility, scanning X-ray fluorescence and loss-on-ignition methods. The sequence contains a period of increased glacial-sedimentary flux between ca. 13,000 and 12,000 cal yr BP, which coincides with the timing of the YD. Glacio-lacustrine sediments also increased between ca. 11,000 to 10,000 cal yr BP. Combined, these data suggest the northern tropical Andes experienced cooling events during the YD, and from ca. 11,000 to 10,000 cal yr BP.
U13B-1154
Climate Variability of Southern South America During the Late Holocene: The Multi-proxy Sedimentary Record of Chilean Fjord Sediments (46°S)
High-resolution paleoclimate records from the Southern Hemisphere are essential to improve the understanding of the interhemispheric pattern of paleoclimate changes. Due to their intermediate location between the terrestrial and marine realms of southern South America, the sediments deposited in the fjords of Southern Chile contain a high resolution record of paleoclimate changes that occurred in the area since the last deglaciation. In this project, a 2m long gravity core from the Quitralco fjord (PC29A, 46°S) has been studied by a multi-proxy sedimentological and geochemical approach. To better understand the spatial variability of the sediment composition and to characterize the marine and terrestrial end-members, we also analyzed soil, river and surface sediment samples from the same area (44-47°S). The multi-proxy analysis includes high resolution XRF core scanning (ITRAX core scanner) with calibration by ICP-AES, biogenic silica, TIC, bulk and clay mineralogy, elemental and isotopic composition of the bulk organic matter (C/N, d13C, d15N), grain-size, magnetic susceptibility and loss-on-ignition. The analysis of surface, river and soil samples demonstrates that the terrestrial end-member of the sediment is rich in iron, titanium and olivine, and is characterized by low d13C and high C/N values. When applied to sediment core PC29A, these proxies allow us to estimate variations in the input of terrestrially-derived particles during the last 1400 years and therefore to reconstruct the intensity of the terrestrial runoff, which is directly linked to the intensity of precipitation in the Andes. Between 750 and 1400 AD, the sediments of site PC29A show low Ti concentrations, high Rb/Sr ratios and a high content in biogenic silica, which indicate dry climate conditions and high biogenic productivity during the Medieval Warm Period (MWP). The high TOC values and low C/N ratios between 1100 AD and 1400 AD also suggest high marine productivity at the beginning of the second millennium AD. Around 1500 AD, the Fe and Ti concentrations increase significantly and the TOC values decrease, indicating higher terrestrial supply and therefore wetter climate conditions until the beginning of the 20th century. This wet period is believed to represent the local signature of the Little Ice Age (LIA), which seems to be characterized in southern Chile by an intensification of the Westerlies along a wide latitudinal range. Our data demonstrate that the MWP and LIA are global climate events, not only restricted to the Northern Hemisphere.
U13B-1155
Evidence for Mid-latitude Lake-level Response to Millennial and Milankovitch Forcing: The Past 65 kyr at Mono Lake, CA
Mono Lake, California (38°N, 119°W) lies in the western Great Basin in the rain shadow of the Sierra Nevada, and preserves high-elevation strandlines indicating that lake level was much higher in the past. Comparison of stratigraphy-based relative lake level with geochemical records from three sites around the lake (overlapping between 40-32 ka), reveals high carbonate fluxes at times of high lake level on precessional (20-kyr) time-scales. Although absolute values are different in different locations representing slightly different paleoenvironmental settings, the timing and relative amplitude of changes are similar on orbital as well as millennial time-scales. Within the established regional 100-kyr pattern of wet glacials-dry interglacials, we find that during the last glaciation (Marine Isotope Stages (MIS) 4-3-2) times of high spring insolation were generally wetter, represented by lacustrine silts rather than sand facies and greater carbonate flux to the deep-lake sediments. This interval is also characterized by high-amplitude variability. Age model errors of ~1 kyr or less during MIS 3 allow correlation of sharp increases in carbonate deposition with Dansgaard-Oeschger warmings in Greenland. If the high carbonate-high lake relationship holds at millennial time-scales, this would indicate increases in lake level coeval with warming, opposite to the 100-kyr regional pattern. Alternatively, carbonate precipitation may be triggered by lake level peaking and beginning to drop, suggesting that lake level began to fall about the time of Greenland warming. Regardless, the correspondence in phasing between Mono Lake and Greenland, also demonstrated in the nearby Santa Barbara Basin, suggests that Mono Lake responded rapidly to the atmosphere-ocean changes invoked to explain the global nature of the D-O events. The suppression of this variability during times of low spring insolation may be due to a reduced sensitivity of the recorder, and we are looking for additional proxies to explore these intervals more carefully.
U13B-1156
Combined U-decay Series and Oxygen Isotope Dating of a Mid-Pleistocene Lacustrine Sequence: The Amora Formation, Dead Sea Basin, Israel
Dating of mid-Pleistocene carbonate sediments is challenged by the lack of suitable absolute radiometric methods. Here, we present a combined approach that utilized the U and Th isotopes with floating δ18O stratigraphy and paleomagnetic constraints, and established a high-resolution chronology of the mid to upper Pleistocene Lake Amora in the Dead Sea basin. The application of the δ18O record as a floating chronometer is based on the correlation found between δ18O values of synchronously deposited upper-Pleistocene and Holocene lake sediments, East Mediterranean foraminifers and Judean Mountains speleothems (Kolodny et al., 2005). The lacustrine Amora Formation consists of laminated aragonite and detritus, Ca-sulfate minerals, halite and clastic units. The sediments were depsoited in the lacustrine environment of the paleo-Dead Sea basin and were later uplifted and tilted by the rising Sedom diapir, exposing ~330 m of the formation on the eastern flanks of Mt. Sedom. δ18O values range between 6.0 and –1.0‰, shifting periodically between glacial and interglacial sequences throughout the sedimentary section, marking corresponding shifts in the global marine records. Paleomagnetic data indicate the entire section was deposited after the 780 ka Matuyama-Brunhes magnetic transition. Data compilation renders the age of the base of the exposed Amora Fm. to be ~750 to 700 ka BP (MIS 18 to 17), and the age of its capping sediments to be between ~200 and 130 ka BP (MIS 6 and the transition to MIS 5). Climatic-limnologic shifts throughout the sedimentation period are recorded by the lithological, chemical and isotopical properties of the sediments, and are correlated to global and regional events. A prominent ~6 meter thick salt layer precipitated during MIS 11 (~400 ka BP) and marks a significant lake level decline. The only other time a similar salt unit precipitated from the Dead Sea basin lakes during last ~750 ka, was before ~10 ka (Pleistocene-Holocene transition). The striking similarity between the MIS11 and Holocene sedimentary records supports the assumption that the lacustrine deposits reflect trends in in global climatic changes. Reference Kolodny Y., Stein M., and Machlus M. (2005) Geochim. Cosmochim. Acta 69, 4045-4060.
U13B-1157
A Speleothem Record of Tropical Moisture Transport
It has been suggested that the tropics may modulate changes in climate normally associated with high latitude forcing. One suggested mechanism is moisture transport from the Atlantic to the Pacific across Central America. We present a long-term, high- resolution speleothem record from western Costa Rica that documents change in local precipitation in the tropics during the Late Pleistocene (96.9 to 44.4 ka) and has important implications for moisture transport. Oxygen isotope signatures preserved in tropical speleothem calcite, precisely dated using uranium-series, provide a direct record of local moisture variability. The δ18O values in our speleothem range from -10.0‰ to -5.2‰ VPDB. The δ18O values and amount of precipitation are inversely correlated in the tropics due to the amount effect. Essentially, progressive rainout of a given air mass drives the δ18O value of the water to increasingly negative values due to localized convection. High precipitation intervals at the study site are due to either 1) an increase in Atlantic-sourced moisture transported across the isthmus, or 2) increased evaporation and convection of local Pacific waters. Progressive rainout from the Atlantic to the Pacific is documented in modern surface waters and speleothems, so excursions toward lower δ18O values may correspond to enhanced moisture transport from the Atlantic. Paleosalinity data are consistent with this inference. However, the stable isotope data from this study also correspond with sea surface temperature (SST) anomalies in the tropical Atlantic, as well as changes in the SST gradient of the two ocean basins. These temperature patterns and the corresponding precipitation record from the study area are analogous to the modern response to ENSO. Although our paleoprecipitation data do not distinguish between the above two scenarios as of yet, with additional analysis they may provide important constraints for models that invoke tropical moisture transport as a climate modulator.
U13B-1158
The Malili Lakes, Indonesia: Potential Archives of Long Records of Terrestrial Paleoclimate from the Equatorial Western Pacific
The tropical western Pacific Ocean exerts a dominant influence on the Earth's atmospheric heat and moisture budgets. Continental drilling of lake deposits through DOSECC and ICDP has begun to provide critical new information on the Plio-Pleistocene history of tropical continental climate, yet no potential drilling sites have been identified in the tropical western Pacific. Here we report results from the first seismic reflection studies ever conducted on large lakes in Indonesia that highlight the potential of Lake Towuti, South Sulawesi, for obtaining long terrestrial records of western Pacific climate. We have collected nearly 500 km of high-resolution seismic reflection (CHIRP) data from Lakes Matano (164 km2, 590 m deep) and Towuti (560 km2, 203 m deep), two of the Malili Lakes in central Sulawesi. These tectonic lakes formed about 2 Myr BP, and thus have the potential to provide long records of Indonesian climate. Seismic stratigraphic analyses of sediments from Lake Matano document numerous slump and slide deposits, the proximity of which to Matano's deepest structural troughs as well as gravity core data suggests that sediments accumulating in Matano's deep basins represents the distal end of catastrophic sedimentary processes. Although seismic data from shallow sites indicate the presence of unconformities related to climate-driven water level changes in Matano, attesting to Lake Matano's climate sensitivity, sedimentation in Matano appears to be too complex to warrant scientific drilling. On the other hand, well-stratified sediments are present across much of Lake Towuti. The deepest basins in Lake Towuti are far removed from slump deposits and fluvio-deltaic inputs, and contain highly stratified sediments ideal for paleoenvironmental analysis. A series of high-amplitude reflectors in the upper 110 meters of sediment (our maximum penetration with the CHIRP) can be traced across much of the Towuti basin. Acoustic properties of these reflectors, together with channel features and other sequences, suggest substantial changes in Towuti's water level in response to Pleistocene environmental changes. Lithologic, geophysical, and geochemical analyses of sediment cores from Towuti show that the sediments contain a wealth of climate indicators that may be used to reconstruct continental climate in the western Pacific. In short, we suggest that Lake Towuti has the potential to provide long, powerful new records of Pleistocene climate from the equatorial western Pacific.
U13B-1159
Precessional(?) Variability in East African Aridity During the Past Few Hundred Thousand Years
We used scanning X-ray fluorescence to investigate fluctuations in calcium abundance in the upper 200 m of cores MAL05-1B and 1C recovered by the Lake Malawi Drilling Project from a central lake site. Conspicuous variations in calcium concentrations reflect the abundance of endogenic calcite, which accumulates during periods of relatively dry conditions in the Malawi basin. Major calcium peaks occur at fairly regular intervals down core. The power spectrum of calcium on a depth scale reveals a large concentration of variance in the 20-meter- band. More detailed analyses were undertaken on MAL05-1C which has a well-constrained age-depth model built on radiocarbon, paleointensity, inclination, 10Be, and OSL dating [Scholz et al., in press]. We attribute the cycles of calcium fluctuation in MAL05-1C to monsoonal climate driven by variations in local insolation dominated by eccentricity-modulated precession. Scholz, C.A., T.C. Johnson, A.S. Cohen, J.W. King, J.A. Peck, J.T. Overpeck, M.K. Talbot, E.T. Brown, L. Kalindekafe, P.Y.O. Amoako, R.P. Lyons, T.M. Shanahan, I.S. Castaneda, C.W. Heil, S.L. Forman, L.R. McHargue, K. Beuning, J. Gomez, and J. Pierson, East African megadroughts between 135-75 kyr ago and implications for early human history, Proceedings of the National Academy of Sciences, in press.
U13B-1160
A 75,000-year TEX86-based Temperature Record from Lake Malawi, East Africa
We present a record of tropical paleotemperature from one of the few continental sites available to date, Lake Malawi. Using the TEX86 molecular paleotemperature proxy, we have extended the previous Lake Malawi temperature record from 23,000 to 75,000 years. The record, taken from hole 2a Site MAL05-2A drilled during the Lake Malawi Drilling Project in 2005, ends at around 75 ky bp at a near shore sand unconformity. TEX86 temperatures were generally stable between 26 and 27°C from 73 to 70 ky bp. This is followed by ~10 kyr of continuous cooling, during which temperatures dropped 5 - 6°C. This cold period reached its maximum at 60 ky bp, with temperatures as low as 20°C, 1-1.5°C colder than the LGM. Crenarchaeotal lipid abundance during this cold period is five times higher than anywhere else in record, but TOC and Si/Ti ratios do not suggest this to be a time of upwelling. The cooling observed correlates well with a decrease in December and January insolation over Lake Malawi. Following this cold period, we observe a short period of sudden warming with temperatures increasing by 3-4°C in only 300 years. After this, temperature variability in Lake Malawi does not seem to be driven by changes in insolation. Between 58 and 48 ky bp temperatures varied between 23.5 and 25°C. Stable temperatures, varying between 23 and 24°C, are observed from 48 to 21 ky bp. Sediment cores from the Indian Ocean show a similar temperature stability for this time period while temperature records from Antarctica and the equatorial Atlantic reveal a gradual decrease in temperature during this time. No temperature variations correlating with DO events were evident from our record.
U13B-1161
Vegetation Response to Climatic Variations in the southern African tropics during the Late- Pleistocene and Holocene
Pollen records from Lake Malawi, Africa spanning the last 135 kyr show substantial and abrupt vegetation response to multiple episodes of extreme aridity during the mega-drought period (130-90 ka). In contrast, vegetation composition and relative abundance remained fairly constant throughout the last 75 ka with no significant change during the Last Glacial Maxima (LGM) (35-15 ka). During the extremely arid mega-drought time period, fluctuations in pollen production define three distinct zones. The first zone, from 123-117 ka, is characterized by increasing amounts of grass, and decreasing amounts of both Podocarpus and evergreen forest taxa (i.e. Celtis, Ixora, Myrica, Macaranga), which, when matched with charcoal data, suggests a short period of extreme aridity. The disappearance of Brachystegia in this interval in conjunction with a peak in Amaranthaceae suggests conversion of the surrounding miombo woodland to an open grassland community probably caused by increased seasonality with a more prolonged and arid dry season. Peak amounts of Podocarpus (30-40%) along with diminishing levels of grass distinguish zone two (117-105 ka). This assemblage defines zone 2 as a period marked by a cool, dry climate resulting in expansion of montane forest taxa to lower elevations. Marine palynological records from the Angola Margin and Congo Fan show similar peak Podocarpus percentages at this time (oxygen isotope stage 5d) indicating similar latitudinal climates across the African continent. Zone three (105-75 ka) shows the highest and most consistent levels of Poaceae. This evidence, along with markedly low levels of most other taxa, indicates that this period contained the most sustained long-lasting dry spells during the past 135 ka. This episode in African history was severe enough as to cause the disappearance of forest taxa such as Uapaca and Brachystegia as well as montane taxa ( Podocarpus, Olea spp. and Ericaceae) within the pollen source area of Lake Malawi. The resultant semi-desert vegetation would have been inhospitable for early humans living within or traveling through the Lake Malawi region.
U13B-1162
Episodes of Midwestern USA Stalagmite Deposition Correlated With Major Stage Boundaries of the Marine Oxygen Isotope Record
Using precise U/Th dating techniques with a multi-collector inductively coupled plasma mass spectrometer, we have dated the episodic onset and cessation of calcite deposition in a long-lived stalagmite from Crevice Cave, Missouri USA (38o N, 90o W). Stalagmite CC-99-3-E was recovered as 4 broken pieces and totals approximately 87 cm in length. Top and basal dates show that calcite deposition episodically spanned the time window from ~ 400-120 ka. Four prominent hiatuses can be visually recognized as distinct, opaque white or white/red colored layers in the polished sections which stand in contrast to the yellowish translucent calcite. The timing of non- deposition represented by each hiatus was constrained by milling 100-200 mg sub-samples for dating directly above and below each hiatus. Our preliminary results show that each hiatus is centered on a significant glacial or stadial stage of the marine oxygen isotope record, and that the onset of deposition following each hiatus is centered on a stage boundary. Specifically, the four hiatuses represent the following glacial or stadial periods, following the Marine Isotope Stage (MIS) numbering scheme of Bassinot et al. (1994): MIS 6.6-6.2 with the onset of deposition following at ~ 130 ka, MIS 8.4-8.2 with the onset of deposition following at ~ 245 ka, MIS 8.6, with the onset of deposition following at ~ 290 ka, and MIS 10.4-10.2, with the onset of deposition following at ~ 334 ka. In sum, the pattern of calcite deposition and non-deposition at our inland site follows the overall global pattern of climate change (ice volume linked to insolation forcing) with remarkable fidelity and provides further support for the growing utilization of speleothems for understanding past climate change at continental sites around the world. Furthermore, details of the well-dated calcite deposition record hold promise for continued refinements of our understanding of the specific climatic conditions associated with each MIS. For example, with the exception of MIS 8.6, the periods of non-deposition during MIS's 6.6-6.2, 8.4-8.2, and 10.4-10.2 are all periods of higher global ice volume than MIS 4.2. It is interesting then that MIS 8.6 was a time of non-deposition, and that MIS 4.2, based on several other speleothems in our collection, was a time of prolific calcite deposition. Bassinot, F.C. et al., 1994. The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic reversal. Earth and Planetary Science Letters 126, 91-108.
U13B-1163
Oaks and ice: Linking Continental North American and North Atlantic Climate Proxies Between 9.5 and 14 k yr BP
Climate and plant growth data from agriculturally important continental regions is often limited in time series length and resolution. Recent work on the dendrochronology of buried sub-fossil oak (Quercus macrocarpa, Quercus bicolor) collected from streams in Missouri and Iowa, USA has yielded climate-sensitive tree-ring data within the last 14 k yr BP. We present the development of the American Long Oak Chronology (ALOC) and report on variations in tree growth and North Atlantic climate between 9.5 and 14 k yr BP. Oaks dated using 14C and density dating methods show decreased growth and depleted δ 13C coincident with North Atlantic climate proxies. The growth (basal area increment) of oak trees was significantly (p = 0.01) less (6.5 cm2 yr-1) during the Younger Dryas compared to growth (11.2 cm2 yr-1) before and after. Mean maximum annual basal area increment (17.0 cm2 yr-1) was reduced by 54 percent during the Younger Dryas. Between-tree growth variance is about 75 percent less during the Younger Dryas than it is during the post glacial period. This variance reduction within a tree population likely reflects limits on growth switching from highly variable endogenous factors (forest competition) to regional exogenous factors (climate). Three point moving averages of the mean ring width of the oak trees and temporally paired δ 18O values from Greenland ice cores (GRIP, GISP2) are correlated (r = 0.57) during the post glacial climate period. Values of δ 13C in oak wood are found to be positively and significantly correlated (r = 0.78) with temporally paired δ 18O values. The termination of the Younger Dryas in mid-continental North America oak was marked by an expected and abrupt increase in the climate sensitive stable isotope δ 13C at about 11,550 yr BP. These results indicate that climate changes during the glacial-Holocene transition in mid- continental North America were approximately synchronous with those in the North Atlantic (~ 4,500 km northeast of the study site) and suggests that post glacial climate variations had marked effects on the growth rates of trees. http://www.missouri.edu/~guyetter
U13B-1164
Large floods and rapid deglaciation of the Lake Michigan Lobe and environs, ca. 19 to 18 ka
Our collective research indicates that features attributed to large floods are associated with events separated in time by about 900 years. The oldest event is interpreted from a long and narrow morainal gap floored by coarse sand and gravel outwash that likely formed by a plungepool and migration of this nickpoint via overflow of Glacial Lake Wauponsee across the Marseilles Moraine near Oswego, Illinois. The weighted mean of four AMS C-14 ages of tundra plant stems and leaves identified from the base of the overlying lake fill is 18,900 ± 35 cal yr BP (15,710 ± 35 yr BP). The age of another overflow channel across the Marseilles Moraine is not yet known, but similar channel floor elevations relative to the Oswego channel suggest that they were contemporaneous. Landforms attributed to a younger large flood are located downstream of where the Fox River breaches the Woodstock Moraine in Algonquin, Illinois. Radiocarbon ages of tundra plant fossils preserved in sediment of ice-walled and slackwater lakes associated with the Woodstock and West Chicago Moraines indicate the younger floods likely predate 18,080 cal yr BP (14,860 ± 40 yr BP; UCIAMS-26265). A statistically similar age of 18,000 cal yr BP (14,830 ± 50 yr BP; B-207031) was obtained from organics preserved in a slackwater lake deposit in the Illinois River valley near Havana. The slackwater lake basin is floored by coarse gravelly sand attributed to the latest stage of flooding. At this time, the similar C-14 ages are the only evidence that link the proglacial and distal sites (more than 150 km apart) to the same flood. Based on continuity of morainic ridges, we believe that the large floods that eroded scarps into the southern margin of the Valparaiso Moraine north and parallel to the Kankakee River in NW Indiana and NE Illinois are contemporaneous with the younger large floods discussed above. The scarp was eroded by meltwater from interlobate areas of the Lake Michigan and Huron-Erie Lobes. The rate of deglaciation of the Lake Michigan Lobe (LML) is estimated from the youngest ages obtained from ice- walled lake plains and the age of the southern maximum Wisconsin Episode LML (23,960 cal yr BP (20,000 yr BP)). Overall, the rate of retreat from the Shelbyville Moraine to the Lake Border Moraines is about 60 m/yr, but the initial meltback/meltdown rate (from the Shelbyville to the Marseilles Moraine) was about double that, 135 m/yr. The initial faster meltback rate is attributed to more effective drainage away from the glacier margin.
U13B-1165
Gains in efficiency and scientific potential of continental climate reconstruction provided by the LRC LacCore Facility, University of Minnesota
Lacustrine sediment cores comprise an integral archive for the determination of continental paleoclimate, for their potentially high temporal resolution and for their ability to resolve spatial variability in climate across vast sections of the globe. Researchers studying these archives now have a large, nationally-funded, public facility dedicated to the support of their efforts. The LRC LacCore Facility, funded by NSF and the University of Minnesota, provides free or low-cost assistance to any portion of research projects, depending on the specific needs of the project. A large collection of field equipment (site survey equipment, coring devices, boats/platforms, water sampling devices) for nearly any lacustrine setting is available for rental, and Livingstone-type corers and drive rods may be purchased. LacCore staff can accompany field expeditions to operate these devices and curate samples, or provide training prior to device rental. The Facility maintains strong connections to experienced shipping agents and customs brokers, which vastly improves transport and importation of samples. In the lab, high-end instrumentation (e.g., multisensor loggers, high-resolution digital linescan cameras) provides a baseline of fundamental analyses before any sample material is consumed. LacCore staff provide support and training in lithological description, including smear-slide, XRD, and SEM analyses. The LRC botanical macrofossil reference collection is a valuable resource for both core description and detailed macrofossil analysis. Dedicated equipment and space for various subsample analyses streamlines these endeavors; subsamples for several analyses may be submitted for preparation or analysis by Facility technicians for a fee (e.g., carbon and sulfur coulometry, grain size, pollen sample preparation and analysis, charcoal, biogenic silica, LOI, freeze drying). The National Lacustrine Core Repository now curates ~9km of sediment cores from expeditions around the world, and stores metadata and analytical data for all cores processed at the facility. Any researcher may submit sample requests for material in archived cores. Supplies for field (e.g., polycarbonate pipe, endcaps), lab (e.g., sample containers, pollen sample spike), and curation (e.g., D-tubes) are sold at cost. In collaboration with facility users, staff continually develop new equipment, supplies, and procedures as needed in order to provide the best and most comprehensive set of services to the research community. http://www.laccore.org