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Author(s): Joerg M. Schaefer (corresponding author) [1, 2]; Robert C. Finkel [1, 3]; Greg Balco [4]; Richard B. Alley [5]; Marc W. Caffee [6]; Jason P. Briner [7]; Nicolas E. [...]

Millennial-scale cold reversals in the high latitudes of both hemispheres interrupted the last transition from full glacial to interglacial climate conditions. The presence of the Younger Dryas stadial (~12.9 to ~11.7kyr ago) is established throughout much of the Northern Hemisphere, but the global timing, nature and extent of the event are not well established. Evidence in mid to low latitudes of the Southern Hemisphere, in particular, has remained perplexing [1-6]. The debate has in part focused on the behaviour of mountain glaciers in New Zealand, where previous research has found equivocal evidence for the precise timing of increased or reduced ice extent [1-3]. The interhemispheric behaviour of the climate system during the Younger Dryas thus remains an open question, fundamentally limiting our ability to formulate realistic models of global climate dynamics for this time period. Here we show that New Zealand's glaciers retreated after ~13 kyr bp, at the onset of the Younger Dryas, and in general over the subsequent ~1.5-kyr period. Our evidence is based on detailed landform mapping, a high-precision [sup.10]Be chronology [7] and reconstruction of former ice extents and snow lines from well-preserved cirque moraines. Our late-glacial glacier chronology matches climatic trends in Antarctica, Southern Ocean behaviour and variations in atmospheric C[O.sub.2]. The evidence points to a distinct warming of the southern mid-latitude atmosphere during the Younger Dryas and a close coupling between New Zealand's cryosphere and southern high-latitude climate. These findings support the hypothesis that extensive winter sea ice and curtailed meridional ocean overturning in the North Atlantic led to a strong interhemispheric thermal gradient [8] during late-glacial times, in turn leading to increased upwelling and C[O.sub.2] release from the Southern Ocean9, thereby triggering Southern Hemisphere warming during the northern Younger Dryas., The transition of Earth's climate from the last ice age to the warm conditions of the Holocene epoch (the past ~11.5 kyr) is not yet fully explained. Polar ice-core records [...]

The glacial geology of two massifs, Muztag Ata and Kongur Shan, in western Tibet was examined to help define the timing and style of glaciation in the semiarid regions of western Tibet. Remote sensing, geomorphic mapping, and [sup.10]Be terrestrial cosmogenic nuclide (TCN) surface-exposure dating of boulders on the moraines and sediment in depth profiles show that glaciers advanced at least 12 times during at least the last two glacial cycles. Over this time, the style of glaciation changed progressively from one that produced ice caps to one that produced less extensive and more deeply entrenched valley glaciers. The timing of the two earliest glaciations is poorly defined, but they likely occurred prior to the penultimate glacial cycle (the Karasu glacial stage) and the early part of the last glacial cycle or during the penultimate glacial cycle (the Subaxh glacial stage). In contrast, the timing of later glacial advances (the Olimde glacial stage) is relatively well defined showing quasiperiodical oscillations on millennial time scales (17.1 [+ or -] 0.3 ka, 13.7 [+ or -] 0.5 ka, 11.2 [+ or -] 0.1 ka, 10.2 [+ or -] 0.3 ka, 8.4 [+ or -] 0.4 ka, 6.7 [+ or -] 0.2 ka, 4.2 [+ or -] 0.3 ka, 3.3 [+ or -] 0.6 ka, 1.4 [+ or -] 0.1 ka, and a few hundred years before the present). These data suggest that since the global Last Glacial Maximum (LGM), the glaciers in western Tibet likely responded to Northern Hemisphere climate oscillations (rapid climate changes), with minor influences from the south Asian monsoon. This study provides the first well-defined glacial geologic evidence to suggest that glaciers in western Tibet respond to rapid climate changes on millennial time scales throughout the Late Glacial and Holocene. Keywords: glacial geology, Tibet, Muztag Ata, Kongur Sham geochronology, terrestrial cosmogenic nuclide surface-exposure dating, midlatitude westerlies, Late Glacial, Holocene, rapid climate changes.

New geologic mapping, tectonic geomorphologic, [sup.10]Be terrestrial cosmogenic nuclide, and (U-Th)/He zircon and apatite thermochronometric data provide the first numerical constraints on late Cretaceous to late Quaternary exhumation of the Inyo Mountains and vertical slip and horizontal extension rates across the eastern Inyo fault zone, California. The east-dipping eastern Inyo fault zone bounds the eastern flank of the Inyo Mountains, a prominent geomorphic feature within the western Basin and Range Province and eastern California shear zone. (U-Th)/He zircon and apatite thermochronometry yield age patterns across the range that are interpreted as indicating: (1) two episodes of moderate to rapid exhumation associated with Laramide deformation during the late Cretaceous/ early Tertiary; (2) development of a slowly eroding surface during a prolonged period from early Eocene to middle Miocene; (3) rapid cooling, exhumation, and initiation of normal slip along the eastern Inyo fault zone, accommodated by westward tilting of the Inyo Mountains block, at 15.6 Ma; and (4) rapid cooling, exhumation, and renewed normal slip along the eastern Inyo fault zone at 2.8 Ma. Fault slip continues today as indicated by fault scarps that cut late Pleistocene alluvial fan surfaces. The second episode of normal slip at 2.8 Ma also signals onset of dextral slip along the Hunter Mountain fault, yielding a Pliocene dextral slip rate of 3.3 [+ or -] 1.0 mm/a, where a is years. Summing this dextral slip rate with estimated dextral slip rates along the Owens Valley, Death Valley, and Stateline faults yields a net geologic dextral slip rate across the eastern California shear zone of 9.3 + 2.2/-1.4 to 9.8 + 1.4/-1.0 mm/a.

The history of alluvial fan and terrace formation within a stretch of the Ouarzazate basin along the southern margin of the Central High Atlas is reconstructed using geomorphological and [sup.10]Be terrestrial cosmogenic nuclide (TCN) methods. Alluvial fan and terrace incision was controlled partially by a drop in base level during the Pliocene or early Pleistocene as the outlet channel, the Draa river, progressively cut through the Anti-Atlas to the south of the Ouarzazate foreland basin, the drainage of which started to become external after a long period of internal drained conditions. The alluvial fans and terrace surfaces have abandonment ages that date to at least the past four glacial cycles. Their formation was strongly modulated by climate on glacial--interglacial time scales as base level dropped. This demonstrates a strong climatic control on sediment transfer and landscape denudation during the Quaternary and provides a model for understanding sediment transfer in other intracontinental mountain belts. Furthermore, these data show that mean rates of fluvial incision in this region range between 0.3 and 1.0 mm [a.sup.-1] for the latter part of the Quaternary. This study provides the first comprehensive TCN chronology for the Atlas Mountains, and it illustrates the applicability and limitations of TCN methods.

The interpretation of differentially weathered mountainous areas along the fringes of Pleistocene ice sheets is fundamental for determining ice-sheet behavior and thickness during the last glaciation. Two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice, but that it was differentially erosive as a function of its spatially variable basal thermal regime. Cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fiord landscapes on northeastern Baffin Island shed light on Laurentide Ice Sheet (LIS) dynamics and thickness. Tots on weathered upland surfaces have minimum [sup.10]Be ages between ca. 50 and ca. 170 ka (n = 12), whereas the majority of erratics perched in the uplands range from ca. Ill to ca. 13 ka 0t = 14), indicating that the whole landscape was glaciated during the Last Glacial Maximum (LGM). Glacially sculpted bedrock (n = 7) near sea level reflects the age of deglaciation, and increasing amounts of isotopic inheritance in high-elevation sculpted bedrock (n = 3), higher elevation intermediately weathered bedrock (n = 11), and highest elevation intensely weathered bedrock reflect the weakening of erosive power as the ice sheet transitioned from fiords to interfiord plateaus. Cold-based glaciation of uplands was contemporaneous with warm-based glaciation in fiords, suggesting the presence of ice streams. [sup.26]Al and [sup.10]Be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure, burial, and limited glacial erosion. The paired isotope data reveal a minimum duration of burial of 80-500 k.y., presumably by cold-based ice, and indicate that the tors have an age of at least 150-580 ka. These data, together with other reconstructions of ice streams in the eastern Canadian Arctic, suggest that ice streams were a large influence on northeastern LIS dynamics throughout the late Quaternary. Thus, the northeastern LIS was sensitively tied with North Atlantic thermohaline circulation and abrupt climate change. Keywords: fjord, cosmogenic radionuclide, Laurentide Ice Sheet, weathering zone, glacier erosion, basal thermal regime.

Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya. The glacial stages that have been identified are: the Indus Valley glacial stage, dated at older than 430 ka; the Leh glacial stage occurring in the penultimate glacial cycle or older; the Kar glacial stage, occurring during the early part of the last glacial cycle; the Bazgo glacial stage, at its maximum during the middle of the last glacial cycle; and the early Holocene Khalling glacial stage. The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation. A possible explanation is that uplift of Himalayan ranges to the south and/or of the Karakoram Mountains to the west of the region may have effectively blocked moisture supply by the south Asian summer monsoon and mid-latitude westerlies, respectively. Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene. Keywords: terrestrial cosmogenic nuclide, surface exposure dating, Transhimalaya, glacial geology, geochronology, Ladakh.

Large fans and terraces are frequent in the Khumbu Himal within the high Himalayan valleys south of Mt. Everest. These features are composed of massive matrix- and clast-supported diamicts that were formed from both hyperconcentrated flows and coarse-grained debris flows. Cosmogenic radionuclide (CRN) exposure ages for boulders on fans and terraces indicate that periods of fan and terrace formation occurred at c. 16, c. 12, c. 8, c. 4 and c. 1.5 ka, and are broadly coincident with the timing of glaciation in the region. The dating precision is insufficient to resolve whether the surfaces formed before, during or after the correlated glacial advance. However, the sedimentology, and morphostratigraphic and geomorphological relationships suggest that fan and terrace sedimentation in this part of the Himalaya primarily occurs during glacier retreat and is thus paraglacial in origin. Furthermore, modern glacial-lake outburst floods and their associated deposits are common in the Khumbu Himal as the result of glacial retreat during historical times. We therefore suggest that Late Quaternary and Holocene fan and terrace formation and sediment transfer are probably linked to temporal changes in discharge and sediment load caused by glacier oscillations responding to climate change. The timing of major sedimentation events in this region can be correlated with fans and terraces in other parts of the Himalaya, suggesting that major sedimentation throughout the Himalaya is synchronous and tied to regional climatic oscillations. Bedrock incision rates calculated from strath terrace ages average c. 3.9 mm [a.sup.-1], suggesting that the overall rate of incision is set by regional uplift.

Seventy-three new ~Be/26Al ages from 57 moraine boulders and 2 tors, together with 43 previously published cosmogenic exposure ages from 41 moraine boulders, allow us to critique the use of cosmogenic exposure (CE) dating of moraine boulders in Alaska. Boulder exhumation during moraine degradation likely gives rise to the largest uncertainty in constraining the timing of initial moraine stabilization following ice retreat. Isotopic inheritance appears to be most important for moraines deposited close to their cirque headwalls. Boulder-surface (bedrock) erosion rate can be roughly constrained and leads to a range in moraine stabilization ages. Snow-cover history is difficult to constrain, but its effect is thought to be minor for the tall boulders sampled. Despite these complications, the CE ages provide important new information regarding the timing of the last and penultimate glaciations in Alaska. Three penultimate moraines yielded CE ages that overlap with marine isotope stage (MIS) 4/early MIS 3 (45-65 ka) rather than MIS 6 (ca. 140 ka). Based on a combination of our new CE chronologies and existing 14C ages from six study areas, glaciers retreated from their local late Wisconsin maxima: ca. 24-27 ka, Kokrines Hills (west-interior Alaska); ca. 24-26 ka, northeastern Brooks Range (NE Alaska); ca. 21-23 ka, Yukon Tanana Upland (east-interior Alaska); ca. 22 ka, Ahklun Mountains (SW Alaska); ca. 20 ka, western Alaska Range (central Alaska); ca. 16-18 ka, Chuilnuk Mountains (SW Alaska). Overall, glacier retreat was concurrent with the peak of the last global glacial maximum, probably in response to limited moisture availability. Keywords: cosmogenic exposure dating, moraine, Alaska, last glacial maximum, penultimate glaciation.

Several giant ice avalanches were initiated by slope failure from ice caps due to strong ground motion during the 14 November 2001 [M.sub.w] = 7.9 Kokoxili earthquake on the Kunlun fault. Four ice avalanches were identified on the north slope of the Burhan Budai Shan several kilometers east of the Kunlun Pass, and two were identified on the south slope of the eastern Yuxi Feng, which is ~50 km west of the Kunlun Pass. These ice avalanches originated from steep-sided ice caps and progressed over and past the termini of outlet valley glaciers. In the Burhan Budai Shan, the ice avalanches comprised ice and snow that reached 2-3 km down valley beyond the snouts of the contemporary glaciers. Detailed study of the largest ice avalanche (B2) shows that the initial movement over the contemporary glacier was turbulent in nature, having a velocity >35 [ms.sup.-1]. Beyond the contemporary glacier, the ice avalanche was confined within steep valley walls and entrenched paraglacial fans. Before coming to rest, this ice avalanche moved as a Bingham plastic flow at a velocity of [less than or equal to] 21 [ms.sup.-1]. These ice avalanches transported little rock debris, and it is thus unlikely that they are important in contributing to the landscape evolution of this region. Yet, given the appropriate geologic and climatic conditions, ice avalanching may be an important process in the landscape evolution of high mountainous terrains. The frequency of such events is unknown, but such phenomena may become more common in the future as a consequence of increased glacier and slope instability caused by human-induced climate change. Ice avalanches, therefore, likely constitute a significant geologic hazard in the near future. Keywords: ice avalanches, earthquakes, glaciers, Kunlun Mountains, Tibet, remote sensing.

We report [sup.26]Al/[sup.10]Be based ages of Sierra Nevada caves that constrain detailed late Pliocene and Quaternary river incision histories for five river canyons. Rapid incision of ~0.2 mm/yr from 2.7 to ca. 1.5 Ma slowed markedly to ~0.03 mm/yr thereafter, likely reflecting the combined effects of a transient erosional response to Pliocene rock uplift and periodic mantling of riverbeds with glacially derived sediment in the late Quaternary. While ~400 m of incision has occurred in the past 2.7 m.y., outpacing interfluve erosion and thereby increasing the local relief, canyons as deep as 1.6 km existed prior to that time. These new erosion rates strengthen the case for tectonically driven late Cenozoic uplift. Keywords: caves, cosmogenic dating, bedrock incision, landscape evolution, Sierra Nevada.

Glacial successions in the Anyemaqen and Nianbaoyeze Mountains of northeastern Tibet are reassessed and new glacial chronologies are presented for these regions. Cosmogenic radionuclide and optically stimulated luminescence dating indicates that two glacial advances occurred in marine isotope stage (MIS)-3 and MIS-2. In the Anyemaqen Mountains, a third advance occurred in the Early Holocene. We suggest that glaciation was synchronous in the Anyemaqen and Nianbaoyeze Mountains, as well as in other glaciated areas of Tibet and the Himalaya that are influenced by the Asian monsoon. The maximum extent of glaciation occurred early in the last glacial cycle (MIS-3) during a time of increased insolation when the monsoon intensified and supplied abundant precipitation, as snow at high altitude, to feed high-altitude glaciers. This suggests that precipitation, as snow, is fundamental in controlling glaciation in these regions. However, the occurrence of glacial advances during the insolation minimum of MIS-2 suggests that, despite reduced precipitation at this time, the annual temperatures were cold enough to maintain positive glacier mass balances. The numerically defined chronologies for the Anyemaqen and Nianbaoyeze Mountains presented here provide a framework for comparing glacial advances in other parts of high Asia. Keywords: Tibet, glaciation, monsoon, cosmogenic radionuclide dating, optically stimulated luminescence dating.

Well-preserved latero-frontal moraines in the eastern San Bernardino Mountains of southern California provide evidence for several glacial advances during the late Quaternary and mark the southwesternmost limit of glaciation in the Western Cordillera. Using geomorphology and [sup.10]Be cosmogenic radionuclide dating, a succession of moraines from three glaciated valleys is dated to 18-20 ka (Last Glacial Maximum), 15-16 ka (Heinrich Event 1), 12-13 ka (Younger Dryas Stade), and 5-9 ka (early-middle Holocene). These ages substantiate the view that glaciation throughout the American Cordilleras was synchronous during the late Quaternary. Furthermore, these data show that glacial advances in southern California occur when a significant decrease in summer temperature is coupled with an increase in moisture flux producing high winter snowfall. This allows for perennial snow accumulation that may, under appropriate conditions, persist to form glacial ice. Keywords: cosmogenic radionuclide dating, glaciation, San Bernardino Mountains, Last Glacial Maximum, Younger Dryas.

Moraine successions in glaciated valleys south of Mount Everest provide evidence for at least eight glacial advances during the late Quaternary. Cosmogenic radionuclide (CRN) surface exposure dating of moraine boulders defines the timing of each glacial advance and refines the previous glacial chronologies. The CRN data show that glaciation was most extensive during the early part of the last glacial (marine oxygen isotype stage [MIS] 3 and earlier), but limited during MIS 2 (the global Last Glacial Maximum) and the Holocene. A previously assumed Neoglacial advance is dated to 3.6 [+ or -] 0.3 ka and the CRN dates confirm a glacial advance ca. 1 ka. These results show that glaciations on the south side of Everest were not synchronous with the advance of Northern Hemisphere ice sheets, yet glaciations within the Himalaya, the world's highest mountain belt, were synchronous during the late Quaternary. The existence of glacial advances during times of increased insolation suggests that enhanced moisture delivered by an active south Asian summer monsoon is largely responsible for glacial advances in this part of the Himalaya. These data allow us to quantify the importance of global climate change and monsoon influence on glaciation in the Himalaya. Keywords: cosmogenic radionuclides, Himalaya, glacial geology, geochronology, monsoon, Everest.

An expansion of alpine glaciers during the latest Pleistocene produced an extraordinarily well defined end moraine system in the Ahklun Mountains, southwestern Alaska. These moraines, deposited during the Mount Waskey advance, are several kilometers beyond modern glacier termini, and ~80 km upvalley of the late Wisconsin Ahklun Mountains ice cap terminal moraine. Eleven cosmogenic [sup.10]Be and [sup.26]Al exposure ages on moraine boulders, combined with radiocarbon ages from a lake core upvalley of a moraine deposited during the Mount Waskey advance, suggest that the advance culminated between 12.4 and 11.0 ka, sometime during, or shortly following, the Younger Dryas event (ca. 12.9-11.6 ka). We believe that the Mount Waskey advance was a consequence of cooling during the Younger Dryas. These data further strengthen emerging evidence for Younger Dryas-age cooling of the North Pacific region. Keywords: Alaska, cosmogenic exposure dating, glaciers, Younger Dryas, late glacial.

Moraines and associated landforms in the upper Hunza Valley, Karakoram Mountains, northern Pakistan, provide an excellent record of multiple glaciations. During the late Quaternary, glaciers advanced at least eight times. By using [sup.10]Be and [sup.26]Al surface-exposure dating on moraine boulders and scoured bedrock, we determined the timing of glaciation for four of these glacial advances: ca. 54.7-43.2 ka (Borit Jheel glacial stage), ca. 25.7-21.8 ka (Ghulkin I glacial stage), ca. 18.4-15.3 ka (Ghulkin II glacial stage), and ca. 10.8-9.0 ka (Batura glacial stage). For two of the older advances, the Yunz and Shanoz glacial stages, our data set a limit of >60 ka. Although, at present, the uncertainties in dating make it impossible to describe unequivocally the climate processes controlling glaciations in the Hunza Valley, the results suggest that precipitation changes related to oscillations in the southwest Asian monsoonal system combine with cooling that is broadly associated with Heinrich events to cause glacial advances in this region. Keywords: cosmogenic radionuclide dating, glaciation, Hunza Valley, Karakoram Mountains, southwest Asian summer monsoon.