Publications
131 results found
Glasser NF, Jansson KN, Goodfellow BW, et al., 2011, Cosmogenic nuclide exposure ages for moraines in the Lago San Martin Valley, Argentina, Quaternary Research, Vol: 75, Pages: 636-646, ISSN: 0033-5894
At several times during the Quaternary, a major eastward-flowing outlet glacier of the former Patagonian Ice Sheet occupied the Lago San Martin Valley in Argentina (49°S, 72°W). We present a glacial chronology for the valley based on geomorphological mapping and cosmogenic nuclide (10Be) exposure ages (n=10) of boulders on moraines and lake shorelines. There are five prominent moraine belts in the Lago San Martin Valley, associated with extensive sandar (glaciofluvial outwash plains) and former lake shorelines. Cosmogenic nuclide exposure ages for boulders on these moraines indicate that they formed at 14.3±1.7ka, 22.4±2.3ka, 34.4±3.4ka to 37.6±3.4ka (and possibly 60±3.5ka), and 99±11ka (1ρ). These dated glacier advances differ from published chronologies from the Lago San Martin Valley based on 14C age determinations from organic sediments and molluscs in meltwater channels directly in front of moraines or in kettleholes within end moraine ridges. The moraine boulder ages also point to possible pre-LGM glacial advances during the last glacial cycle and a key observation from our data is that the LGM glaciers were probably less extensive in the Lago San Martin Valley than previously thought. © 2010 University of Washington.
Rood DH, Burbank DW, Finkel RC, 2011, Chronology of glaciations in the Sierra Nevada, California, from <sup>10</sup>Be surface exposure dating, Quaternary Science Reviews, Vol: 30, Pages: 646-661, ISSN: 0277-3791
We use 10Be surface exposure dating to construct a high-resolution chronology of glacial fluctuations in the Sierra Nevada, California. Most previous studies focused on individual glaciated valleys, whereas our study compares chronologies developed throughout the range to identify regional patterns in the timing of glacier response to major climate changes. Sites throughout the range indicate Last Glacial Maximum retreat at 18.8 ± 1.9 ka (2σ) that suggests rather consistent changes in atmospheric variables, e.g., temperature and precipitation, throughout the range. The penultimate glacial retreat occurred at ca 145 ka. Our data suggest that the Sierra Nevada landscape is dominated by glacial features deposited during marine isotope stage (MIS) 2 and MIS 6. Deposits of previously recognized glaciations between circa 25 and 140 ka, e.g., MIS 4, Tenaya, early Tahoe, cannot be unequivocally identified. The timing of Sierra Nevada glacial retreat correlates well with other regional paleoclimate proxies in the Sierra Nevada, but differs significantly from paleoclimate proxies in other regions. Our dating results indicate that the onset of LGM retreat occurred several thousand years earlier in the Sierra Nevada than some glacial records in the western US. © 2010 Elsevier Ltd.
Young NE, Briner JP, Stewart HAM, et al., 2011, Response of Jakobshavn Isbræ, Greenland, to Holocene climate change, Geology, Vol: 39, Pages: 131-134, ISSN: 0091-7613
Rapid fluctuations in the velocity of Greenland Ice Sheet (GIS) outlet glaciers over the past decade have made it difficult to extrapolate ice-sheet change into the future. This significant short-term variability highlights the need for geologic records of preinstrumental GIS margin fluctuations in order to better predict future GIS response to climate change. Using 10Be surface exposure ages and radiocarbon-dated lake sediments, we constructed a detailed chronology of ice-margin fluctuations over the past 10 k.y. for Jakobshavn Isbræ, Greenland's largest outlet glacier. In addition, we present new estimates of corresponding local temperature changes using a continuous record of insect (Chironomidae) remains preserved in lake sediments. We find that following an early Holocene advance just prior to 8 ka, Jakobshavn Isbræ retreated rapidly at a rate of ~100 m yr-1, likely in response to increasing regional and local temperatures. Ice remained behind its present margin for ~7 k.y. during a warm period in the middle Holocene with sustained temperatures ~2 °C warmer than today, then the land-based margin advanced at least 2-4 km between A.D. 1500-1640 and A.D. 1850. The ice margin near Jakobshavn thus underwent large and rapid adjustments in response to relatively modest centennial-scale Holocene temperature changes, which may foreshadow GIS response to future warming. © 2011 Geological Society of America.
Rood DH, Burbank DW, Finkel RC, 2011, Spatiotemporal patterns of fault slip rates across the Central Sierra Nevada frontal fault zone, Earth and Planetary Science Letters, Vol: 301, Pages: 457-468, ISSN: 0012-821X
Patterns in fault slip rates through time and space are examined across the transition from the Sierra Nevada to the Eastern California Shear Zone-Walker Lane belt. At each of four sites along the eastern Sierra Nevada frontal fault zone between 38 and 39° N latitude, geomorphic markers, such as glacial moraines and outwash terraces, are displaced by a suite of range-front normal faults. Using geomorphic mapping, surveying, and 10Be surface exposure dating, mean fault slip rates are defined, and by utilizing markers of different ages (generally, ~20ka and ~150ka), rates through time and interactions among multiple faults are examined over 104-105year timescales. At each site for which data are available for the last ~150ky, mean slip rates across the Sierra Nevada frontal fault zone have probably not varied by more than a factor of two over time spans equal to half of the total time interval (~20ky and ~150ky timescales): 0.3±0.1mm year-1 (mode and 95% CI) at both Buckeye Creek in the Bridgeport basin and Sonora Junction; and 0.4 +0.3/-0.1mm year-1 along the West Fork of the Carson River at Woodfords. Data permit rates that are relatively constant over the time scales examined. In contrast, slip rates are highly variable in space over the last ~20ky. Slip rates decrease by a factor of 3-5 northward over a distance of ~20km between the northern Mono Basin (1.3 +0.6/-0.3mm year-1 at Lundy Canyon site) to the Bridgeport Basin (0.3±0.1mm year-1). The 3-fold decrease in the slip rate on the Sierra Nevada frontal fault zone northward from Mono Basin is indicative of a change in the character of faulting north of the Mina Deflection as extension is transferred eastward onto normal faults between the Sierra Nevada and Walker Lane belt. A compilation of regional deformation rates reveals that the spatial pattern of extension rates changes along strike of the Eastern California Shear Zone-Walker Lane belt. South of the Mina Deflection, extension is accommoda
Reusser L, Graly J, Bierman P, et al., 2010, Calibrating a long‐term meteoric <sup>10</sup>Be accumulation rate in soil, Geophysical Research Letters, Vol: 37, ISSN: 0094-8276
<jats:p>Using 13 samples collected from a 4.1 meter profile in a well‐dated and stable New Zealand fluvial terrace, we present the first long‐term accumulation rate for meteoric <jats:sup>10</jats:sup>Be in soil (1.68 to 1.72 × 10<jats:sup>6</jats:sup> at/(cm<jats:sup>2</jats:sup>·yr)) integrated over the past ∼18 ka. Site‐specific accumulation data, such as these, are prerequisite to the application of meteoric <jats:sup>10</jats:sup>Be in surface process studies. Our data begin the process of calibrating long‐term meteoric <jats:sup>10</jats:sup>Be delivery rates across latitude and precipitation gradients. Our integrated rate is lower than contemporary meteoric <jats:sup>10</jats:sup>Be fluxes measured in New Zealand rainfall, suggesting that long‐term average precipitation, dust flux, or both, at this site were less than modern values. With accurately calibrated long‐term delivery rates, such as this, meteoric <jats:sup>10</jats:sup>Be will be a powerful tool for studying rates of landscape change in environments where other cosmogenic nuclides, such as in situ <jats:sup>10</jats:sup>Be, cannot be used.</jats:p>
Behr WM, Rood DH, Fletcher KE, et al., 2010, Uncertainties in slip-rate estimates for the Mission Creek strand of the southern San Andreas fault at Biskra Palms Oasis, southern California, Bulletin of the Geological Society of America, Vol: 122, Pages: 1360-1377, ISSN: 0016-7606
This study focuses on uncertainties in estimates of the geologic slip rate along the Mission Creek strand of the southern San Andreas fault where it offsets an alluvial fan (T2) at Biskra Palms Oasis in southern California. We provide new estimates of the amount of fault offset of the T2 fan based on trench excavations and new cosmogenic 10Be age determinations from the tops of 12 boulders on the fan surface. We present three alternative fan offset models: a minimum, a maximum, and a preferred offset of 660 m, 980 m, and 770 m, respectively. We assign an age of between 45 and 54 ka to the T2 fan from the 10Be data, which is significantly older than previously reported but is consistent with both the degree of soil development associated with this surface, and with ages from U-series geochronology on pedogenic carbonate from T2, described in a companion paper by Fletcher et al. (this volume). These new constraints suggest a range of slip rates between ~12 and 22 mm/yr with a preferred estimate of ~14-17 mm/yr for the Mission Creek strand of the southern San Andreas fault. Previous studies suggested that the geologic and geodetic slip-rate estimates at Biskra Palms differed. We find, however, that considerable uncertainty affects both the geologic and geodetic slip-rate estimates, such that if a real discrepancy between these rates exists for the southern San Andreas fault at Biskra Palms, it cannot be demonstrated with available data. © 2010 Geological Society of America.
Scherler D, Bookhagen B, Strecker MR, et al., 2010, Timing and extent of late Quaternary glaciation in the western Himalaya constrained by <sup>10</sup>Be moraine dating in Garhwal, India, Quaternary Science Reviews, Vol: 29, Pages: 815-831, ISSN: 0277-3791
Glacial chronologies from the Himalayan region indicate various degrees of asynchronous glacial behavior. Part of this has been related to different sensitivities of glaciers situated in contrasting climatic compartments of the orogen, but so far field data in support for this hypothesis is lacking. Here, we present a new 10Be-derived glacial chronology for the upper Tons valley in western Garhwal, India, and initial results for the Pin and Thangi valleys in eastern Himachal Pradesh. These areas cover a steep gradient in orographic precipitation and allow testing for different climatic sensitivities. Our data provide a record of five glacial episodes at ∼16 ka, ∼11-12 ka, ∼8-9 ka, ∼5 ka, and <1 ka. In the Thangi valley, our results indicate a glacial episode at ∼19 ka, but no data are available for younger glacial deposits in this valley. At their largest mapped extent (∼16 ka), the two main glaciers in the upper Tons valley joined and descended down to ∼2500 m asl, which represents a drop of ∼1400 m compared to the present-day glacial extent. During the Holocene the two largest glaciers produced distinct glacial landforms that allowed us to reconstruct changes in the Equilibrium Line Altitude (ELA) over ∼20 km north-south distance that is presently associated with a steep gradient in rainfall. We observe that ELA-changes have been consistently ∼2 times higher for the glacier located in a presently wetter climate, pointing at different climate sensitivities, related to the amount of precipitation that they receive. At regional scale, our data is in reasonable agreement with other published glacial chronologies from the western Himalaya and suggest that glacial advances during the Holocene have been largely synchronous in this region. Comparison of glacial chronologies from the western Himalaya with other palaeoclimatic proxy data suggests that long-term changes in glaci
Rood DH, Hall S, Guilderson TP, et al., 2010, Challenges and opportunities in high-precision Be-10 measurements at CAMS, 11th International Conference on Accelerator Mass Spectrometry, Publisher: ELSEVIER SCIENCE BV, Pages: 730-732, ISSN: 0168-583X
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Amos CB, Kelson KI, Rood DH, et al., 2010, Late quaternary slip rate on the Kern Canyon fault at Soda Spring, Tulare County, California, Lithosphere, Vol: 2, Pages: 411-417, ISSN: 1941-8264
The Kern Canyon fault represents a major tectonic and physiographic boundary in the southern Sierra Nevada of east-central California. Previous investigations of the Kern Canyon fault underscore its importance as a Late Cretaceous and Neogene shear zone in the tectonic development of the southern Sierra Nevada. Study of the late Quaternary history of activity, however, has been confounded by the remote nature of the Kern Canyon fault and deep along-strike exhumation within the northern Kern River drainage, driven by focused fl uvial and glacial erosion. Recent acquisition of airborne lidar (light detection and ranging) topography along the ~140 km length of the Kern Canyon fault provides a comprehensive view of the active surface trace. High-resolution, lidar-derived digital elevation models (DEMs) for the northern Kern Canyon fault enable identifi cation of previously unrecognized offsets of late Quaternary moraines near Soda Spring (36.345°N, 118.408°W). Predominately north-striking fault scarps developed on the Soda Spring moraines display west-side-up displacement and lack a signifi cant sense of strike-slip separation, consistent with detailed mapping and trenching along the entire Kern Canyon fault. Scarp-normal topographic profi ling derived from the lidar DEMs suggests normal displacement of at least 2.8 +0.6/-0.5 m of the Tioga terminal moraine crest. Cosmogenic 10 Be exposure dating of Tioga moraine boulders yields a tight age cluster centered around 18.1 ± 0.5 ka (n = 6), indicating a minimum normal-sense fault slip rate of ~0.1-0.2 mm/yr over this period. Taken together, these results provide the fi rst clear documentation of late Quaternary activity on the Kern Canyon fault and highlight its role in accommodating internal deformation of the southern Sierra Nevada. © 2010 Geological Society of America.
Amidon WH, Rood DH, Farley KA, 2009, Cosmogenic <sup>3</sup>He and <sup>21</sup>Ne production rates calibrated against <sup>10</sup>Be in minerals from the Coso volcanic field, Earth and Planetary Science Letters, Vol: 280, Pages: 194-204, ISSN: 0012-821X
This study calibrates the production rate of cosmogenic 3He in pyroxene, olivine, garnet, zircon and apatite as well as 21Ne in quartz and pyroxene against the known production rate of 10Be in quartz. The Devil's Kitchen rhyolite from the Coso volcanic field in southeastern California (elev. ~ 1300 m) was chosen for this study due to its young age (~ 610 ka) and diverse mineral assemblage. Based on 10Be, our two rhyolite samples have apparent exposure ages of ~ 49 and 93 ka, indicating substantial erosion after eruption. Combining data from the two samples, we estimate sea level high latitude 3He spallation production rates of 145 ± 11, 141 ± 16, and 144 ± 30 at g- 1 a- 1 (2σ) for pyroxene, olivine and spessartine garnet respectively. For zircon and apatite, we estimate apparent 3He spallation production rates of 114 ± 8 and 149 ± 28 at g- 1 a- 1 (2σ) respectively. The rates for zircon and apatite are reported as apparent production rates because we do not explicitly address the redistribution of spallation produced 3He from adjacent minerals. These estimates quantitatively account for production of 3He from both cosmogenic and radiogenic neutron reactions on 6Li within the analyzed phases and also implanted from nuclear reactions in neighboring minerals; the high U, Th and Li content of this rhyolite provides a particularly rigorous test of this correction. We estimate 21Ne production rates of 17.7 ± 1.6 and 34.1 ± 3.2 at g- 1 a- 1 (2σ) in quartz and pyroxene (Fe/Mg = 0.7 by mass) respectively. Although high U and Th contents create the potential for significant production of nucleogenic 21Ne, this component is small due to the young eruption age of the rhyolite. © 2009 Elsevier B.V. All rights reserved.
Busby CJ, Hagan JC, Putirka K, et al., 2008, The ancestral cascades arc: Cenozoic evolution of the central Sierra Nevada (California) and the birth of the new plate boundary, Special Paper of the Geological Society of America, Vol: 438, Pages: 331-378, ISSN: 0072-1077
We integrate new stratigraphic, structural, geochemical, geochronological, and magnetostratigraphic data on Cenozoic volcanic rocks in the central Sierra Nevada to arrive at closely inter-related new models for: (1) the paleogeography of the ancestral Cascades arc, (2) the stratigraphic record of uplift events in the Sierra Nevada, (3) the tectonic controls on volcanic styles and compositions in the arc, and (4) the birth of a new plate margin. Previous workers have assumed that the ancestral Cascades arc consisted of stratovolcanoes, similar to the modern Cascades arc, but we suggest that the arc was composed largely of numerous, very small centers, where magmas frequently leaked up strands of the Sierran frontal fault zone. These small centers erupted to produce andesite lava domes that collapsed to produce block-and-ash flows, which were reworked into paleocanyons as volcanic debris flows and streamflow deposits. Where intrusions rose up through water-saturated paleocanyon fill, they formed peperite complexes that were commonly destabilized to form debris flows. Paleocanyons that were cut into Cretaceous bedrock and filled with Oligocene to late Miocene strata not only provide a stratigraphic record of the ancestral Cascades arc volcanism, but also deep unconformities within them record tectonic events. Preliminary correlation of newly mapped unconformities and new geochronological, magnetostratigraphic, and structural data allow us to propose three episodes of Cenozoic uplift that may correspond to (1) early Miocene onset of arc magmatism (ca. 15 Ma), (2) middle Miocene onset of Basin and Range faulting (ca. 10 Ma), and (3) late Miocene arrival of the triple junction (ca. 6 Ma), perhaps coinciding with a second episode of rapid extension on the range front. Oligocene ignimbrites, which erupted from calderas in central Nevada and filled Sierran paleocanyons, were deeply eroded during the early Miocene uplift event. The middle Miocene event is recorded by growth f
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