PP33B-1271
Signatures of Glacial Erosion and Retreat in the Landscape: Cosmogenic and Numerical Modeling Constraints
We use cosmogenic radionuclide (CRN) exposure ages to constrain numerical simulations of deglaciation histories in the Middle Boulder Creek drainage, Colorado Front Range, and the Animas River valley, San Juan Mountains, Colorado. We present 18 new 10Be exposure ages from glacially polished bedrock sampled in the Middle Boulder Creek valley. All of these ages are younger than the ~19-22 ka terminal moraine age based on 26Al and 36Cl measurements by Schildgen (2000) and Benson et al. (2005). Exposure ages decrease with distance upvalley from the moraine, and the youngest ages in the uppermost valley are uniformly ~13 ka. We include 4 10Be ages in a cross section across the mid-valley, which show a pattern of Last Glacial Maximum (LGM) ages (12-14 ka) within the glacial footprint, and older exposure ages (~40 ka) near the trim lines. A similar age trend is seen in the Animas River valley in southwestern Colorado, which was occupied by a lobe of the LGM ice sheet that capped the San Juan mountains. Deglaciation began here ca. 19.4 ka, based on a 10Be depth profile in a proglacial terrace. A longitudinal transect of exposure ages from glacially polished samples indicates that terminus retreat proceeded at ~15 m/yr until complete deglaciation ca. 12.3 ka. Neither valley has obvious recessional deposits within the LGM glacial footprint. The first-order trend in each valley is a monotonic glacial retreat, but there are other possible retreat scenarios. For instance, we would like to test whether the same trend in 10Be concentrations could be generated by episodic retreat punctuated by periods of readvance. To investigate these scenarios, we modified the GC2D numerical glacier simulation (see Kessler et al., 2006) to incorporate a CRN accumulation layer. This layer can contain any starting value of CRN concentration. Production over each timestep is scaled to DEM latitude and altitude. Production is taken to be zero in areas covered by more than 10 m of ice. The CRN inventory can also decline due to glacial erosion. We incorporate a selectable erosion rule based on basal sliding or total ice velocity, ice discharge, ice power, or basal shear stress, and calculate the reduction in CRN inventory by the depth stripped in each timestep. We then simulate a glacier responding to equilibrium line altitude (ELA) changes imposed stepwise, gradually, or including short periods of lowering during an overall rise. Each scenario generates a pattern of ages in the CRN layer that can be compared with the map pattern of measured 10Be concentrations. Initial results show that a step-function ELA rise to its present value causes a retreat that is too rapid to explain the range of ages observed in both valleys. A steady ELA rise can replicate the age-distance trend of an individual valley (within error). An episodic retreat with readvances results in a distinctive pattern of discordant ages between tributaries of the same glacial valley. Our CRN dataset includes several samples chosen to discriminate between this pattern and one typical of monotonic retreat.
PP33B-1272
Moraine pebbles and boulders yield indistinguishable 10Be ages: A case study from Colorado, USA
Cosmogenic exposure dating of moraines over the last two decades has vastly improved knowledge on the timing of glaciation worldwide. Due to a variety of geologic complications, such as moraine degradation, snow cover, bedrock erosion and isotopic inheritance, samples from multiple large boulders (>1-2 m) often lead to the most accurate moraine age assignments. However, in many cases, large boulders are not available on moraines of interest. Here, I test the suitability of collections of pebbles as a sample type for exposure dating. Twenty-one 10Be ages from two Pleistocene lateral moraine crests in Pine Creek valley in the upper Arkansas River basin, Colorado, were calculated from both pebble and boulder samples. Eleven 10Be ages from a single-crested Pinedale lateral moraine have an unexpected bimodal age distribution; one mode is 22.0±1.4 ka (3 boulders, 2 pebble collections), the other is 15.0±0.9 ka (1 boulder, 5 pebble collections). Ten 10Be ages from a single-crested Bull Lake lateral moraine are much more scattered, ranging between 3 and 72 ka, with no statistical difference between pebble (n=5) and boulder (n=5) ages. The preliminary interpretation of the two age modes from the Pinedale moraine is that two glacier maxima of similar extent were attained during the late Pleistocene. Unlike the Pinedale moraine, there is no cluster of 10Be ages from the Bull Lake moraine, suggesting that moraine degradation has led to anomalously young exposure ages. Regardless of interpretations of moraine age, that 10Be ages from pebble collections and boulder tops are indistinguishable on moraines of two different ages, and in two different modes of the Pinedale moraine, suggests that pebble collections from moraine crests may serve as a suitable sample type in some settings.
PP33B-1273
Constraining Glacial Chronologies Using Beryllium-10 Depth Profiles
Establishing a chronology of glacial advances and retreat in the Polar Regions is critical to understanding hemispheric and inter-hemispheric climate change. To date, this has proved to be a difficult task as radiocarbon dateable material is either absent or rare in ice-contact and/or ice-distal features. Progress is being made via the application of surface exposure dating to boulders resting on moraines. However, a major problem with the boulder dating method is the issue of varying amounts of inheritance, resulting in scatter in boulder ages from a single moraine. We propose an alternative method to constrain glacial advances and retreat by sampling surface and sub-surface samples from deltas and other ice-contact features. Beryllium-10 (10Be) concentrations were measured in an amalgamation of surface cobbles and four subsurface sand/gravel samples down to a depth of ~~1.5 m from each profile. A non-linear least squares regression of the 10Be concentrations and shielding depth yield model exposure ages and quantify the inherited component of the 10Be accumulated during pre-exposure and/or transport. Five depth profiles were sampled in east Greenland, three from an ice-contact glaciomarine delta in southern Kjove Land and one each from a raised marine beach and glaciolacustrine delta in southern Milne Land. Initial results indicate that profile exposure ages from the ice- contact delta are in agreement within uncertainties with radiocarbon age and boulder exposure ages, while the other features are slightly younger than radiocarbon ages. In the case presented, all ages are considered minimum ages, as no corrections have been made for erosion and/or glacio-isostatic rebound. In cases where boulder exposure ages show a high degree of scatter and/or inheritance, profile ages may provide a more accurate deglacial age since the sediments are well mixed with respect to pre-exposure, and inheritance can be quantified and accounted for in the model exposure age.
PP33B-1274
Cosmogenic Surface-Exposure Age Limits for Latest-Pleistocene Glaciation and Paleoclimatic Inferences in the American Fork Canyon, Wasatch Mountains, Utah, U.S.A.
The Wasatch Mountains of north-central Utah bordered the eastern shore of Lake Bonneville and were occupied by numerous valley glaciers during the latest Pleistocene. Stratigraphic and morphostratigraphic observations near the mouths of Little Cottonwood and Bells Canyons reveal that glaciers in these two valleys began constructing terminal moraines before Lake Bonneville reached its maximum shoreline elevation at about 19-17 cal. ka. Although the chronology of the lake highstand is well constrained by numerous radiocarbon dates, the timing of deglaciation in the Wasatch Mountains is relatively unclear. Moreover, there is considerable disagreement over the climatic conditions (cold/dry vs. cool/wet) that led to the expansion of glaciers and the lake. To address these issues, we explore the glacial record in the American Fork canyon by combining field mapping with cosmogenic 10Be surface-exposure dating and numerical modeling of glacier mass balance and ice flow to limit the extent, timing and climate of the last glaciation in the Wasatch Mountains. Six of ten cosmogenic surface-exposure ages from a terminal moraine in the canyon are tightly clustered (ranging from 14.4 ± 1.3 to 15.2 ± 1.1 ka; 2σ analytical error) and yield an error-weighted mean age of 14.8 ± 0.4 ka (2σ, MSWD = 0.26; individual age calculations based on a high latitude/sea level production rate of 4.98 ± 0.34 atoms g SiO2-1 yr-1 scaled for elevation and latitude). This age is consistent with previously reported cosmogenic-exposure dates from elsewhere in the range, and suggests that ice retreat in the Wasatch Mountains began as much as 4 kyr later than in other Rocky Mountain ranges and was in phase with the hydrologic fall of Lake Bonneville from the Provo shoreline. Numerical glacier modeling experiments (based on methods of Plummer and Phillips, 2003) simulate maximum ice extent in the American Fork, Little Cottonwood and Dry Creek canyons under a broad range of potential temperature and precipitation changes. The combined results of these experiments suggest that if glaciers in the Wasatch Mountains were being fed by substantial precipitation derived from Lake Bonneville (as suggested by previous studies of this region), temperature depression was likely equal to or less than 7-9° C. Latest-Pleistocene temperature depressions greater than 9° C, which are also suggested by previous studies of this region, would have been accompanied by less-than-modern precipitation.
PP33B-1275
Be-10 Ages From Northern Alaska Range Moraines Help Constrain the Timing of the Penultimate Glaciation in Eastern Beringia
Alaska and the adjoining Yukon Territory is currently the site of uncertainty concerning the timing of the penultimate glaciation. While the timing of the Last Glacial Maximum across this region is generally synchronous (~18-25 ka), several ages for the penultimate glaciation have been proposed, including 50-60 ka, 140 ka, and 250 ka, correlating with marine isotope stage (MIS) 4/early MIS 3, MIS 6, and MIS 8, respectively. Furthermore, an out-of-phase, late MIS 5 age has also been suggested in western Beringia. Here, 10 new 10Be ages are presented from a key reference locality in the Delta River valley, Alaska, situated in the north- central Alaska Range. Because boulders are absent at this moraine sequence, quartzose pebble collections from moraine crest surfaces are used. Five pebble samples taken from the Delta (penultimate glaciation) moraine range between 24.7 and 68.0 ka and average 53.5 ± 11.0 ka after excluding a young outlier (24.7 ± 0.6 ka). Because previous research has shown that exposure ages typically date moraine stabilization, emphasis should be placed on older ages within a suite of exposure ages where inheritance is of minimal concern. Five pebble samples taken from the up-valley, Donnelly moraine (late Wisconsin glaciation) range between 11.5 and 65.3 ka, and average 13.3 ± 2.0 ka after removing one sample due to obvious inheritance (65.3 ± 2.1 ka). A nearby site in the north-central Alaska Range also shows promise for exposure dating late Quaternary moraines. The Fish Lake valley, located approximately 70 km southeast of the Delta River valley, is host to an extensive sequence of moraines that spans between the Little Ice Age and the penultimate glaciation; 17 boulder samples from the penultimate and late Wisconsin moraines collected in the summer of 2006 will also be presented. 10Be ages from these boulders will serve as an important comparison to the pebble-based ages from the Delta River valley. Although 10Be dating of pebble collections is untested, the average ages are in broad agreement with recent exposure dating studies elsewhere in Alaska and the Yukon supporting a MIS 4/early MIS 3 age for the penultimate glaciation in eastern Beringia.
PP33B-1276
10Be Surface-Exposure Chronology of Moraines Deposited During the Last Glacial Maximum in the New Zealand Southern Alps
Timing, amplitude and driving mechanisms of the Last Glacial Maximum (LGM) remain still controversial due to scarcity of well-dated and reliable records from extra-polar areas in general, and from the Southern Hemisphere in particular. We present a new LGM glacier reconstruction based on more than 100 10Be surface-exposure ages of moraine boulders from the New Zealand Southern Alps. High precision and internal consistency characterize data sets from two key basins, which indicate that maximum ice extent was achieved more than 10 ky prior the termination of the LGM. Glacial conditions persisted throughout the LGM in New Zealand's Southern Alps, despite rapidly rising local insolation at that time. The palaeosnowline estimates of Porter (1975), taken together with data from this study, suggest that snowlines were between ~875 and ~750 m lower during the LGM. This glacial record exhibits a tight correspondence in timing and duration to LGM cooling as shown by South Island pollen stratigraphy, temperature and carbon dioxide signatures from Antarctic ice cores, global sea-level reconstructions, and chronologies of mountain glacier activity from both polar hemispheres. The striking agreement among these records suggests that a global mechanism drove glacial activity in the Southern Alps during the LGM, and that insolation cannot be the main driving force.
PP33B-1277
A Chronology of Late-Glacial and Holocene Advances of Quelccaya Ice Cap, Peru, Based on 10Be and Radiocarbon Dating
The Quelccaya Ice Cap region in the southeastern Peruvian Andes (~13-14°S latitude) is a key location for the development of late-glacial and Holocene terrestrial paleoclimate records in the tropics. We present a chronology of past extents of Quelccaya Ice Cap based on ~thirty internally consistent 10Be dates of boulders on moraines and bedrock as well as twenty radiocarbon dates of organic material associated with moraines. Based on results from both dating methods, we suggest that significant advances of Quelccaya Ice Cap occurred during late-glacial time, at ~12,700-11,400 yr BP, and during Late Holocene time ~400-300 yr BP. Radiocarbon dating of organic material associated with moraines provides maximum and minimum ages for ice advances and recessions, respectively, thus providing an independent check on 10Be dates of boulders on moraines. The opportunity to use both 10Be and radiocarbon dating makes the Quelccaya Ice Cap region a potentially important low-latitude calibration site for production rates of cosmogenic nuclides. Our radiocarbon chronology provides a tighter constraint on maximum ages of late-glacial and Late Holocene ice advances. Upcoming field research will obtain organic material for radiocarbon dating to improve minimum age constrains for late-glacial and Late Holocene ice recessions.
PP33B-1278
Evidence for Multiple Late Quaternary Glaciations in the Southernmost Cordillera Blanca, Peru
Surface-exposure dating with in-situ-produced cosmogenic isotopes has provided the basis for a growing framework of glacial chronologies in the tropical Andes. In the Peruvian Andes, long chronologies (>400 ka) with relatively small local last glacial maximum (LLGM) advances have been reported for the central Cordillera Blanca (ca. 9°30'S) and Junin Plain (11°00'S), whereas preliminary data suggest a shorter record (<40 ka) in the intervening Cordillera Huayhuash (10°15'S). These seemingly contradictory findings raise several questions: Was the LLGM a relatively minor event in the Peruvian Andes, far exceeded by bigger, older advances? Which combination of geographic and geomorphic factors increases the likelihood that evidence of older advances will be preserved? With these questions in mind, we sought a site with both high peaks and a high-altitude plateau. The glaciated Nevado Jeulla Rajo massif (10°00'S, 77°16'W, peaks ca. 5600 masl) marks the southern end of the Cordillera Blanca and the Callejon de Huaylas valley in the central Peruvian Andes. The Conococha Plain (ca. 4050 masl) borders the western side of the massif. Large lateral moraines extend onto the Conococha Plain from the west-facing valleys and multiple moraine loops lie upvalley, closer to active ice margins. Surface-exposure dating (10Be) indicates that the largest lateral moraines from Jeullesh Valley are compound features deposited during the LLGM (ca. 30 ka) and a late-glacial readvance (ca. 16 ka). The LLGM/late-glacial moraines cross-cut an older pair of lateral moraines (ca. 70 ka) that may provide evidence for a smaller advance during marine isotope stage 4. Although the LLGM/late-glacial moraines are impressively large (ca. 150 m high), they do not represent the maximum ice extent in the region. Fluvial outwash deposits beyond the termini of the moraines on the Conococha Plain are underlain by lodgement till that is up to 20 m thick and extends ca. 6 km across the width of the Plain to the Rió Santa. The wide distribution of the till suggests that at least one older glaciation was far more extensive than any of the late Quaternary advances that we have dated by 10Be. The combination of high peaks, a high-altitude plateau, and an active fault may be ideal for enhancing preservation of older moraines and till deposits.
PP33B-1279
Cosmogenic 10Be Dating of Early and Latest Holocene Moraines on Nevado Salcantay in the Southern Peruvian Andes
A two-fold sequence of nested lateral and end moraines was mapped in a glacial trough emanating from the southwest flank of Nevado Salcantay (6271 m; ~13°S latitude), the highest peak in the Cordillera Vilcabamba of southern Peru. The field area is situated 25 km due south of the archaeological site of Machu Picchu. Outer and inner moraines in the sequence were deposited by valley glaciers that terminated ~5 km and ~3 km, respectively, from their headwall on the Salcantay summit massif. Cosmogenic 10Be surface exposure dating of granitic boulders sampled on the Salcantay moraines is underway and has provided the first numerical ages for these deposits. Initial results indicate ages of 8.1 ± 0.1 10Be ka for the outer moraine and 200 ± 20 10Be years for the sharp-crested inner moraine. These ages are derived using the CRONUS-Earth 10Be exposure age calculator (version 2.0) and expressed with respect to the Lal- Stone production rate scaling scheme using the standard atmosphere. The outer and inner moraine ages correspond to glacial events during the early and latest Holocene, respectively. Further 10Be dating of the mapped moraines and similar deposits observed in adjacent drainages on Nevado Salcantay is anticipated to yield a high-resolution chronology of valley glaciation in this segment of the southern Peruvian Andes. The new results bridge an important gap between existing Andean glacier records to the north and south, and complement available ice core and lacustrine paleoclimate records in the vicinity, thereby expanding spatial and temporal coverage for identifying patterns of Holocene climate change in the tropical Andes. Notably, the inner moraine age correlates with the timing of the Little Ice Age as defined in northern mid- and high latitude glacier records, and suggests considerable expansion of valley glaciers in the southern Peruvian Andes during this climatic minimum. Apart from their paleoclimatic significance, the initial results also demonstrate the utility of 10Be exposure dating for historical surface deposits.
PP33B-1280
10-Be Constraints on the Timing of the Last Glacial Maximum and Deglaciation in the Northern Peruvian Andes
Eighteen 10Be ages were determined on quartzite boulders from two latest Pleistocene moraines in the northern Peruvian Andes at 7°S. Pleistocene moraines in this area are only a few hundred meters below the highest summits and represent small glaciers sensitive to climate change. A moraine corresponding to the local Last Glacial Maximum (LGM) yields a mean age of 19.2 +/- 1.1 10Be ka using the scaling of Lal (1991) and the production rate of Stone (2000). This age agrees fairly well with the onset of deglaciation inferred from other records in the tropical Andes including 10Be dating of moraines in the Cordillera Blanca, glaciogenic sediment input into Lakes Junin and Titicaca, and Huascaran d18O, as well as the initiation of warming seen in many marine records throughout the tropics at ~19 ka. These data do not seem to support an early local LGM in the tropical Andes, although ongoing cosmogenic work at our field site seeks to better clarify this issue. A deglacial moraine in an adjacent valley has a mean age of 15.8 +/- 1.4 10Be ka and best represents the timing of ice withdrawal from this region. Numerous other moraines throughout Peru and northern Bolivia have also been dated to ~15 10Be ka (Farber et al, 2005; Smith et al, 2005). Other records from the southern tropics indicate drying at this time, perhaps in response to a northward shift of the intertropical convergence zone associated with a resumption of thermohaline circulation, which may explain this deglacial event. While Schaefer et al. (2006) found a near-synchronous termination of the LGM in the mid-latitudes of both the Northern and Southern Hemispheres at ~17 10Be ka, the ~15 10Be ka age of moraines from the tropical Andes may indicate an asynchronous onset of the last deglaciation between the low and mid-latitudes.
PP33B-1281
In situ 10Be ages constraining the glacial, periglacial and sea-level history of Andøya, northern Norway
Northwards transport of Atlantic water by the Norwegian Current warms the Norwegian Arctic coast 5-10°C above the annual mean for latitude 69°N. A reduction in heat transport would cause a shift from anomalously warm and maritime open-ocean climate to a cold and continental regime with continuous/seasonal sea-ice cover. Palaeoclimatic archives from this region thus facilitate testing of key hypotheses on the role of ocean, sea-ice, atmospheric processes, and feedbacks in driving Arctic climate variability during the shift from glacial to peak interglacial climates. Its proximity to the shelf break and diversity of geomorphological features has made the island Andøya (69°N, 15°E) a key area for reconstructing ice-age history and palaeoclimatic investigations for more than a century. Even if lacustrine sediments span the last 20 ka, however, evidence has not yet been conclusive with regard to the ice-sheet extent during the Late Weichselian glacial maximum (~LGM) and the Late Glacial phase. We have measured in-situ cosmogenic 10Be in surface rock samples from selected marginal moraine ridges formed by continental ice sheets and local glaciers, glacial erratics, ice-moulded bedrock, fossil rock glaciers and raised shorelines in order to resolve the Late Weichselian glacial, periglacial and sea level history of northern Andøya. The results so far show that the most comprehensive glacial erosion phase occurred prior to 40 ka with glacial erosion up to at least 100 m asl on both the western and eastern side of the northern part of the island. Small talus-derived rock glaciers on the eastern side were active around 30-25 ka, implying that any ice sheet present at that time must have been thinner than 200 m. Remnants of marginal moraines formed by a local glacier isolated from the continental ice sheet on the western side give >25 10Be ka. Beach ridges on the western side of the island developed at c. 20 m asl suggest a limited ice extent around 20 ka. Younger marginal moraine formed by low-elevation cirque- and valley glaciers show a maximum extent around 18-16 ka. Talus- derived rock glaciers on the western side of the island overrun the c. 20 ka shoreline and became inactive c. 15 ka, constraining the timing of suitable conditions for rock glacier formation close to the present sea level.
PP33B-1282
Slow Vertical Downwasting of Cold-Based Ice Sheets in the Rondane Area, Central Scandinavia
The rate of ice-sheet downwasting following the Late Weichselian glacial maximum (LGM) c. 20 ka is crucial for estimating the vertical extent prior to and during the last deglaciation in central Scandinavia. The Scandinavian Ice Sheet was cold-based in this region, and air temperature is suggested to be the critical factor influencing the rate of downwasting until the final deglaciation took place close to 10 ka. Depending on when the deglaciation started, the rate of downwasting necessary to explain recent ‘maximum' and ‘minimum' estimates for ice-sheet thickness may differ by 1-2 orders of magnitude. Here we present figures for the rate of downwasting based on OSL-dated waterlain sediments exposed to daylight prior to deposition and 10Be exposure dates on erratic boulders and lateral meltwater channels cut into bedrock in different altitudes. The results indicate a very slow vertical downwasting until c. 15 ka, and a somewhat increase prior to c. 13 ka. After a stillstand related to the Younger Dryas (12.8-11.5 ka) the downwasting accelerated until all ice was gone close to 10 ka. The ice-sheet surface was locally lowered c. 120 m from the LGM to 13.8 ka, or about 2.4 cm/year on an average. The results imply that a much thinner continental ice sheet that hitherto suggested by ice-sheet modelling is necessary to explain field observations related to the last deglaciation in the Rondane area.
PP33B-1283
Cosmogenic 3He exposure dating of the Quaternary lavas at Fogo, Cape Verdes: Dating flank collapse and magmatic reorganisation
Construction and destruction of young ocean island volcanoes is often episodic and governed by periods of intense volcanism and flank collapse. Volcano flank collapses can potentially trigger large tsunamis, which pose significant natural hazard to coastal communities when reaching the main land. However factors leading up to flank instability followed by collapse are not well understood. Both endogenetic (increased volcanism, reorganisation of magmatic system, shift in stress direction, etc) as well as exogenetic (sea level rise, climate change, etc) processes have been proposed (Keating and McGuire, 2000). The lack of quantitative age constraints on the duration and timing of volcanic processes leading to flank instability hamper interpretation to the underlying causes. Here we present cosmogenic 3He exposure age results from a study to date Quaternary lavas of Fogo, Cape Verdes. The growth of Fogo occurred in two main stages, separated by a large flank collapse removing the summit and eastern flank of the pre–collapse Monte Amarelo volcano. The collapse produced a 9 km wide escarpment with a headwall cliff that is still 1 km high despite the infilling with over 1.5 km of post–collapse lava flows from the Cha das Caldeiras volcano. Dyke swarms in the Monte Amarelo volcano indicate that the collapse was preceded by a structural reconfiguration of an initially radial dyke swarm to a single, dominantly north–south swarm across the summit region. This coincided with an eastward shift in magmatic activity. The recent history of the Cha das Caldeiras volcano has seen similar structural reconfiguration of dyke swarms and eastward shift of volcanism resulting in the extinction of post–collapse volcanic rift zones in the west of the island. Dating these events is vital for understanding the timing of (future) volcano flank collapses of Fogo. Lava flows erupted immediately prior to, and after the collapse bracket its age and exposure ages from two pre- collapse ankaramite flows yield 24 and 110 ka, while 8 post-collapse flows yield 55 – 48 ka (2 flows), 25 – 22 ka (2 flows) to 14 – 8 ka (4 flows). We infer two possible scenarios; (i) the collapse occurred between 22 and 24 ka, (ii) the young exposure age of the pre-collapse flow is an artefact of ash cover/erosion and collapse occurred between 55 and 110 ka. This has profound implications for the interpretation of future flank collapses on Fogo. The post-collapse flows are among the youngest flows on the western side and the early Holocene ages imply that the magmatic reconfiguration and possible onset of renewed instability of the eastern flank has been developing for several thousand years. If the collapse occurred between 22 and 24 ka, flank instability may be in its early stages. If the collapse occurred between 55 and 110 ka, the duration of the pre-Monte Amarelo collapse reorganization may be comparable to the duration of the present one, with the prospect of a near-future collapse likely.
PP33B-1284
A Cosmogenic 10Be Chronology of the Last Deglaciation of Western Ireland, and Implications for Sensitivity of the Irish Ice Sheet to Climate Change
AMS 14C dates of fossiliferous marine mud identify two readvances of the Irish ice sheet from the north and central lowlands of Ireland into the northern Irish Sea Basin during the Killard Point Stadial at approximately 16.5 cal ka, with subsequent deglaciation occurring by ~15.0 – 15.5 cal ka. Killard Point Stadial moraines have been mapped elsewhere in Ireland, but have previously remained undated. Here we report sixteen 10Be surface exposure dates that constrain retreat of the Killard Point Stadial ice margin from western Ireland. Eight 10Be dates from the Ox Mountains (13.9 – 18.1 10Be ka) indicate that final deposition of the moraine occurred 15.6 + 0.5 10Be ka (mean age). Eight 10Be dates from Furnace Lough (14.1 – 17.3 10Be ka, mean age of 15.6 + 0.4 10Be ka) are statistically indistinguishable from the Ox Mountain samples, suggesting that the moraines were deposited during the same glacial event. Given the agreement between the two age groups, and their common association with a regionally significant moraine system, we combine them to derive a mean age of 15.6 + 0.3 10Be ka. This age is in excellent agreement with the timing of deglaciation from the Irish Sea Basin (~15.3 + 0.2 cal ka), and suggests the onset of near-contemporaneous retreat of the Irish Ice Sheet from its maximum Killard Point Stadial limit. A reconstruction of the ice surface indicates that the Irish Ice Sheet reached a maximum surface elevation of ~500 m over the central Irish Lowlands during the Killard Point Stadial, suggesting a high sensitivity of the ice sheet to small changes in climate.
PP33B-1285
Improving the accuracy of basin-averaged skyline shielding factors by concidering surface morphometrics
The determination of basin-averaged denudation rates from cosmogenic nuclide concentrations in stream sediments depends on the surface production rate, scaling methods of cosmic ray intensities, and the correction algorithms for skyline, snow and vegetation shielding. While much work has been devoted to the calculation of skyline shielding factors (Dunne et al. 1999, Codilean 2006), the pitfalls of and potential solutions to the derivation of skyline shielding factors for large areas has never been addressed. Specifically, the resolution of common topographic datasets, 30 to 90 m, are coarse enough that significant underestimations, up to nearly 20 percent, of the shielding factor can occur. This effect is greatest in mountainous regions with high relief, i.e. exactly those landscapes which are most often studied with cosmogenic methods. By combining measurements of surface roughness from high resolution topographic data with cosmogenic ray shielding laws, we determined an empirical model for the calculation of accurate skyline shielding factors. Codilean A. 2006. Calculation of the cosmogenic nuclide production topographic shielding scaling factor for large areas using DEMs. Earth Surface Process and Landforms 31: pp. 785-794. Dunne J, Elmore D & Muzikar P. 1999. Scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces. Geomorphology 27: pp. 3-11.
PP33B-1286
New Production Rate Estimates for In Situ Cosmogenic 14C From Lake Bonneville, Utah and Northwest Scotland
Accurate and precise quantification of the relationship between past climatic changes and surficial processes is necessary to yield a better understanding of environmental responses to current climatic change. Terrestrial in situ cosmogenic nuclide (TCN) analytical techniques have enabled broad advances in Quaternary geologic and surficial process research over the past two decades, yet the lack of a commonly accepted framework for TCN measurements and spatial and temporal production rate scaling has limited the ability to draw robust comparisons on a global scale. The CRONUS-Earth and CRONUS-EU projects seek to remedy this situation through a multidisciplinary investigation of TCN production and measurement systematics. Under the CRONUS-Earth project, we aim to improve the calibration of in situ cosmogenic 14C (in situ 14C) production rates. In situ 14C is a particularly useful nuclide for late Quaternary surficial process studies by virtue of its short half-life (5.73 ka) - unique among commonly measured TCNs. Lifton et al. (2001, GCA 65, p. 1953) estimated the in situ 14C production rate in quartz based on measurements of wave-cut quartzite bedrock benches associated with the highstand of Pleistocene Lake Bonneville, Utah (17.4 ± 0.3 cal ka). To allow direct comparison of production rates for commonly measured TCNs in the same samples, CRONUS-Earth resampled the Lake Bonneville site in 2005, and sampled Younger-Dryas-age (11.6 cal ka) glacial and landslide deposits in northwest Scotland in 2006, with additional sites pending. Recent stepwise etching experiments have suggested the need for a more robust quartz pretreatment protocol than was employed by Lifton et al. (2001). When combined with improved extraction procedures (e.g., Miller et al., 2006, Quat. Geochron. 1, p. 74), a downward revision of the Lifton et al. (2001) time-integrated site production rate estimate of up to 15.3% may be required, from 52.9 ± 1.7 to 44.8 ± 2.9 14C at g-1 yr-1. Initial data from a new Lake Bonneville sample yields a time-integrated site production rate of 46.7 ± 0.8 14C at g-1 yr-1 - consistent with the revised estimate. We plan to present and compare additional measurements from the Lake Bonneville and Scotland samples as well.
PP33B-1287
In-Situ Cosmogenic 36Cl Production Rate Calibration from Basaltic Flows of Mount Etna (Sicily, 38° N)
One of the CRONUS-EU goals is to provide high quality calibration sites from independently dated surfaces. Several previous studies have been conducted on 36Cl production rate calibration (e.g. Stone et al. 1996, Phillips et al. 2001), which, however, used different protocols and yielded 36Cl production rates with up to 40% discrepancies. The objectives of this study are 1- to understand the source of these discrepancies and 2- to calibrate 36Cl production rates from its target elements Ca and K. As a first step we focused on testing the chemical protocol by performing a sequential 36Cl extraction experiment on whole rock grains and Ca-rich plagioclase from the same sample. The sample was collected at Mt. Etna on a pahoehoe flow, which has a K-Ar fossil exposure time of (10±3) kyr. Cosmogenic 3He was also precisely measured within cogenetic olivine phenocrysts of this sample (Blard et al. 2005) and yields an exposure time of (10.4±1.5) kyr. Both, total Cl and 36Cl concentrations from the first dissolution steps are high, 5800 ppm (whole rock) and 450 ppm (plagioclase) Cl, and 107 - 106 atoms 36Cl/g of rock dissolved. After about 20% dissolution of the plagioclase sample, Cl is almost completely removed (1-3ppm) and 36Cl concentrations reach a plateau value of 2*105 atoms/g of rock. Using the Stone et al. (1996) and Evans et al. (1997) 36Cl production rates for the target elements Ca and K, respectively, this plateau concentration yields an exposure age which is in excellent agreement with K-Ar dating and cosmogenic 3He ages. On the contrary, in the whole rock sample total Cl concentrations remain high (>330ppm) resulting in a considerable 36Cl production from capture of low-energy neutrons by 35Cl, an additional and still not well-constrained 36Cl production mechanism. The resulting exposure ages from the whole rock are 35-45% higher than the independent 3He ages. For 36Cl production rate calibration from Ca, we will use separated Ca-rich plagioclase from various Mt. Etna lava flows of different elevation and independently determined ages between 400 yr and 41 kyr. To better constrain the 36Cl production rate from K, separated sanidine (K-rich feldspar) from a 15 kyr old lava flow of volcano Payun-Matru (Argentina, 36° S) will be used. Stone J.O., et al. (1996), Geochim. Cosmochim. Acta 60 679-692; Phillips F.M., et al. (2001), Chem. Geol. 175 689-701; Blard P.H., et al. (2005), EPSL 236 613-631; Evans J.M. et al. (1997), Nucl. Instr. and Meth. in Phys. Res. B 123 334-340
PP33B-1288
Constraints on Chlorine-36 Production Rates Using Beryllium-10 at Lake Bonneville, UT
The CRONUS-Earth geological calibration at the Lake Bonneville shoreline has provided a unique opportunity to constrain the chlorine-36 production rates based on the well-known beryllium-10 production rate by comparing the inventories of both nuclides in the same samples. Lake Bonneville, an ancient glacial lake, occupied a large area surrounding the current Great Salt Lake in Utah during the last glacial period. The highest stage, called the Bonneville shoreline, was very brief and lasted from 18.9-17.2 cal ka before catastrophic flooding lowered the lake level to the Provo shoreline. The geological calibration at Lake Bonneville shoreline consists of two different sites: the Tabernacle Hill basalt and the Promontory Point quartzite. The Tabernacle Hill basalt is a radiocarbon age-constrained flow that represents the Provo shoreline. The basalt erupted after the catastrophic flood into the Provo lake based on the dating of the bounding flood and tufa deposits. The Promontory Point site is a quartzite wave-cut bench at the Bonneville shoreline level. The cutting action of the waves removed significant amounts of rock and exposed a fresh surface at the base of the current cliffs. The sampling was done close to the cliffs in order to minimize the possible effects of muon production prior to exposure by the cutting action and flood. The first chlorine-36 results from Tabernacle Hill have shown that most samples fall within the time frame indicated by the carbon-14 dating and the average (16.7 +/- 0.8 ka) is within one sigma of the lower age limit. Each sample was sent to several labs and analyzed for multiple cosmogenic nuclides. The comparison of beryllium-10 concentrations from the Promontory Point quartzite with chlorine-36 inventories in rocks of varying chemistry enables closer constraints to be placed on chlorine-36 production rates. This new information will allow tests to be performed on the current production schemes and provide insight into the chlorine-36 production rate discrepancy.
PP33B-1289
Radiocarbon Analysis of Single-Year Tree Rings at the Last Glacial-Deglacial Transition
Variations in solar insolation originate mainly from the changes in solar radiation and Earth's orbit shift. The number of sunspots is correlated with the amount of solar radiation, which has been responsible for climate changes. The sunspot scarcity in the Maunder Minimum stage, resulting from extraordinary weakening of solar magnetic activity, is considered to have played an important role in a serious cold spell from the 14th to the 19th century (Little Ice Age; Miyahara et al, 2004). Since historical observations of the sunspots are available only for the last 400 years (Versteegh, 2005), climate changes in pre-historic age cannot be reconstructed by the historical data. Variations of the solar activity can be identified using cosmogenic nuclides that are generated in the Earth's atmosphere under the action of cosmic ray fluxes. The amount of cosmogenic nuclides from samples such as tree rings, ice and sediment cores are a good indicator of the variation of the solar activity for the longer time interval. To clarify the solar activity at the end of the last glaciation, we tried to measure the radiocarbon concentration for single-year interval tree rings in fossil trunks (ca. 15.5 cal kyr BP; Horiuchi et al., 2007) of conifer buried in the Towada-Hachinohe tephra from Towada Volcano, NE Japan. As a primary step of investigating the solar cycle at the last deglaciation, we checked up pretreatment methods and attempted a high-precision 14C measurement. The results of the 14C measurement for 33 single-year interval tree rings, we detected about 7-year cycle and 14- year cycle. In recent solar variations, 22-year solar cycle, double length cycle of 11-year solar cycle, appears accompanied by 11-year solar cycle. Therefore, the length of the solar cycle at the transition from glacial to deglacial stage is slightly shorter than that of present days. Our data show a slightly shorter periodicity in the 33-year interval of the last deglaciation, suggesting the increase in sunspots at this transition. Radiocarbon analysis of tree rings from the buried forest may be a means towards the elucidation of correlation between solar activity and climate changes.