PP32A-01
Clear signature of Last Glacial Maximum advances in Southern Tibet
The extent and timing of glacial fluctuations during the Late Quaternary provide essential information on past climatic changes, which in turn enhance our ability to predict future climate change. Controversy exists with regard to the glacial history of the Himalayas and Tibet, particularly on the geologic signature, timing and magnitude of the Last Glacial Maximum (LGM) relative to previous advances. Indeed, Quaternary glaciations in Tibet and the Himalayas, and in other parts of the Northern Hemisphere may have been asynchronous. Here, based on cosmogenic radionuclide (CRN) dating of 7 glacial features (68 samples), we demonstrate a clear signature of an LGM glacial advance in Southern and Western Tibet, but also that the largest glaciation (in terms of preserved deposits) took place ~40 ka ago (Marine Isotope Stage MIS-3). Further, it appears that most of the global and local temperature minima in the various climate proxy curves (e.g. Marine oxygen Isotope Stages MIS, Specmap) were coeval with the prominent glacial advances found in Tibet. These data indicate that glacial advances in Tibet and elsewhere in the Northern Hemisphere are synchronous and modulated by global climate variability, but that their extent may vary due to local conditions.
PP32A-02
Glacial Chronologies Spanning the Past >100 Ky Along the Eastern Sierra Nevada From 10Be Surface Exposure Dating
A deeper understanding of the timing and extent of global paleoclimate variations includes knowledge of the age, duration, and extent of alpine glaciations around the world. Despite numerous past studies, the 10 ky-200 ky time span has been poorly explored due either to the paucity of dating techniques with sufficient resolution in this interval or to the absence of datable materials. Terrestrial cosmogenic nuclide (TCN) surface exposure dating allows more accurate, high-resolution chronologies in the 102-105 yr temporal window to be developed for geomorphic features that serve as records for past alpine glaciations. Answers to fundamental global climate questions have remained elusive at time scales where TCN surface exposure dating methods have proven an effective tool. Our study focuses on field sites where glacial landforms are well preserved for the past >100 ky. Along the eastern escarpment of the Sierra Nevada, the use of glacial deposits of different ages with high-resolution TCN dates permit a robust discrimination of the climate history in the region. Previous paleoclimate studies using TCN surface exposure dating methods along the eastern Sierra Nevada were generally constructed for glacial moraines using 36Cl. We construct a record of moraines and outwash terrace ages using 10Be data from quartz, which can be directly compared to existing 36Cl chronologies and other 10Be data. Our 10Be TCN ages on glacial landforms between Sonora Pass and Mono Basin (including deposits in the West Walker River, Buckeye Creek, Robinson Creek, Green Creek, Virginia Creek, and Mill Creek catchments) illustrate the viability of our approach. We produced >100 10Be ages for boulder samples and depth profiles from 8 glacial moraines and 4 outwash terraces spanning MIS 2 through 6. Our data for the Last Glacial Maximum (MIS 2) are remarkably tightly clustered and support a central age with a small error (~3-6%; e.g. 17.2 ± 0.5 ka and 19.5 ± 1.2 ka). A similar clustering even holds for an outwash surface associated with the penultimate glaciation (MIS 6) for which the error is ~7% (136 ± 10 ka). We believe that with judicious selection of surface boulders and depth profiles to sample, it is possible to consistently generate geomorphic ages with <10% errors: this sets an important lower limit on our ability to delineate differences in the ages of glacial deposits (e.g. moraines of different ages and associated outwash surfaces). In short, our results generate within the study area a paleoclimate record spanning the past >100 ky using 10Be that allows comparison to LGM and older glacial chronologies globally.
PP32A-03 INVITED
Allerod Collapse of the Newfoundland Ice Cap, northwestern Atlantic Ocean
Control on the timing of deglaciation of the island of Newfoundland was mostly restricted to offshore records although a few lakes have yielded latest Pleistocene paleovegetation proxies of paleoclimate. Twenty-three cosmogenic 10Be ages on boulders throughout Newfoundland have a range of 11.6 to 14.1 ka and mean of 13.1 +/- 0.1 ka (standard error). The samples indicate near-synchronous deglaciation (collapse) in all the regions sampled over a >100,000 km2 area: St. John's Highlands (on the Great Northern Peninsula once believed to be a nunatak); head of White Bay (northcentral NF); Gaff Topsails Region central NF; Anniopsquotch Mountains and Burgeo region (southwestern NF); Granite Lake (southcentral NF); head of Baie d'Espoir (southern NF); and northern Burin Peninsula (southeastern NF). The large (>2 m) boulders with flat horizontal surfaces are unlikely to have been influenced by snow cover, exhumation through till, boulder erosion, or inheritance. Lake bottom (most bulk sediment) conventional and AMS radiocarbon dates support this exposure chronology, and UMISM simulations of ice sheet decay (driven by a scaled GISP temperature record) is consistent with a rapid melting of the ice cap after retreating from (modern) continental shelf regions. Coincidence of the collapse of the Newfoundland Ice Sheet with the timing of Allerod warming provides an example of the sensitivity of large peripheral ice caps to rapid regional climate change.
PP32A-04
A Holocene Chronology of Alpine Glaciation for the Western United States
Our research will develop a Holocene glacial chronology based on cosmogenic dating of boulders from moraine crests at several sites across the western United States. The chronology will address spatial and temporal glacier variability in response to postulated Holocene climate forcings. A number of studies have interpreted several Holocene glacial advances in the western U.S. (e.g., LIA, Neoglacial, Early-Holocene) (see, Burke and Birkeland, 1983; Davis, 1988 and Osborn and Bevis, 2001) but age control is based largely on relative dating techniques. Surface exposure dating using cosmogenic nuclides now provides a robust method to reevaluate and re-date several of these poorly defined glacial chronologies and develop a high-precision glacial record across the western U.S. for the Holocene epoch. Development of this chronology will provide new constrains on the extent of major Holocene climate forcings and their effects on the mass balance of western North America alpine glaciers as well as providing a better framework for understanding climate forcing during deglaciation. We will present initial data from three cirque moraines in the Medicine Bow Mountains, Wyoming as well as ongoing and future work in the western United States.
PP32A-05
Be-10 dating of historical glaciations in the Southern Alps of New Zealand
Understanding the natural variability of climate change during the Holocene period is key to correctly evaluate the ongoing climate change. Glaciers are among the most sensitive recorders of climate changes, and deposited spectacular Holocene moraine sequence in many areas on Earth. The moraine record of New Zealand's Southern Alps offers the unique opportunity to reconstruct in detail Holocene glaciations in southern mid-latitudes, an area where paleo records are particularly sparse. At least five Holocene glaciations have been mapped in several mountain glacier systems. However, until now a uniform and reliable dating tool for Holocene moraines has been missing. Due to very recent methodological progress, glacial advances throughout the Holocene period including the historical time-scale can now be accurately dated by Be-10 surface exposure dating. We present more than 50 Be-10 boulder ages from Holocene moraines deposited by the Hooker, Muller, and Tasman glacier in the Lake Pukaki area spanning the last 6,000 years, including three glacial events during the last millenium. The cosmogenic chronology is of high internal consistency and shows that glaciations during the latest part of the Holocene period in the Southern mid-latitudes differ from the pattern known from northern latitudes.
PP32A-06 INVITED
Structure of the Last Glacial-Interglacial Transition in New Zealand's Southern Alps
There is a debate concerning the timing and magnitude of paleoclimate change during the last glacial to interglacial transition in the middle to high latitudes of the Southern Hemisphere. To address the controversy, we obtained an extensive 10Be chronology consisting of 41 boulder ages from well-preserved moraines in the central part of the South Island of New Zealand, including the Ben Ohau Range, ~44°S. This moraine chronology documents glacier/advance retreat events reflecting paleoclimate changes in the southern mid- latitudes. The most extensive glacial events during this period occurred between 14-13 and at ~12 ka, which were followed by recessional moraine building events at ~11 and ~10 ka. We conclude that 1) the paleoclimate of New Zealand from ~15 to 10 ka was multifaceted and it did not include a scenario whereby only one major glacial event occurred, for example, coeval with the Antarctic Cold Reversal (ACR) or Younger Dryas (YD) chron 2) despite net overall recession, ice repeatedly deposited moraines until the beginning of the Holocene. We speculate that the New Zealand moraine record contains a signature of both regional and global climate dynamics at the end of the last ice age.
PP32A-07
The late glacial paleoclimate of the central Altiplano constrained by cosmogenic 3He dating and 'clumped-isotope' paleothermometry
One of the ongoing debates in paleoclimatology is whether climate change in the tropics is simply a response to global climate change, or actually forces it. The Altiplano is a highland area (>3600 m) located in the tropical Andes that is particularly well-suited to address this question since both paleo-glacier footprints and paleolake shorelines are very well preserved in this region. Because glacier extents and lake levels have differential sensitivity to change in temperature and in precipitation, they can be used to place tight constraints on the amplitude of past changes in these atmospheric variables. However, because the timing of the deglaciation in this region is still uncertain and poorly documented, this task requires new chronological constraints on paleoglacier fluctuations. New cosmogenic 3He dates from several glacial moraines and striated rocks of the Cerro Tunupa (Bolivia, 20 ° S) show that, in the central part of the Altiplano, glaciers persisted at their maximum extent between 18 and 15 ka, synchronously with the highest level of paleolake Tauca at 17-15 ka (Placzek et al., 2006) (Clayton and Clapperton, 1997). Abrupt glacial retreat occurred approximately 15 ka and was followed by a small amplitude readvance during the Younger Dryas. This result is different from previous moraine dating (Smith et al., 2005) that indicates an earlier (~34 ka) local glacial maximum in the northern part of the Altiplano. This discrepancy most probably reflects spatial variations in past precipitation. Consequently, climatic conditions inferred from the past extent of mountain glaciers must take into account local atmospheric variations before being interpreted in a global context. To this end, we have coupled a numerical modeling approach with an independent estimate of past air temperature based on "clumped isotope" thermometry, a technique based on the temperature dependence of the abundances of 13C-18O bonds in carbonate minerals (Ghosh et al., 2006). This technique was applied to shallow-water biogenic carbonates from the Tauca paleolake. To reproduce the synchronous paleoglacier and paleolake maxima, climate modeling shows that air temperatures dropped between 5 and 7 ° C, and that local precipitation increased four-fold between 17 and 15 ka in the central Altiplano. This temperature change is consistent with the drop in diurnal temperature of about 5 ° C estimated from clumped isotopes measurements of algal bioherms. This quantitative reconstruction suggests that the Tauca paleolake episode (from 17 to 15 ka) is the result of wet and cold (rather than wet and warm) conditions. References Clayton J.D. and Clapperton C.M. (1997) J. of Quat. Science 12, 169-182. Ghosh P. et al (2006) GCA 70, 1439-1456. Placzek C. et al., 2006. Geol. Soc. of Am. Bull.118, 515-532. Smith J. A., et al (2005) Science 308, 678-681.
PP32A-08
Constraining the last glacial maximum and deglaciation of Mauna Kea with cosmogenic surface exposure dating
We have sampled glacially deposited boulders on several moraines associated with Pleistocene glacial advances On Mauna Kea, Hawaii. We retrieved ultramafic xenoliths on these boulders and have measured cosmogenic 3He concentrations in olivine and clinopyroxine mineral grains to obtain surface exposure ages. Our two oldest-dated moraines, stratigraphically most distal to the ice cap, yield ages of 21.6 ±1.6 ka BP and 23.8 ±1.7 ka BP. We interpret these ages to indicate the onset of ice cap retreat and the end of the LGM local to Mauna Kea. Samples from a moraine that cross-cuts our older-dated moraine yield a boulder mean age of 14.9 ±1.9 ka BP. This age, combined with a single boulder age near the summit of 16.2 ±1.2 ka BP imply a rapid and complete deglaciation before 13.7 ka BP. Surface exposure ages from striated and well-polished bedrock samples upslope from the Makanaka terminal moraines yield a mean age of 19.6 ± 1.4 ka BP. Assuming that there was no inherited cosmogenic 3He in these, one interpretation of these ages suggests a minimum retreat from the LGM extent of ~1.5 km after 21.6 ka BP followed by a 2 kyr readvance culminating in complete deglaciation by 14.9 ka BP. Our ages are the first to show that deglaciation of Mauna Kea, Hawaii began no later than 20.0 ka BP and that the duration of readvance culminating in final deglaciation could have been as long as 2 kyr.