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| 2. |
- Applegate, Patrick, et al.
(författare)
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Challenges in the Use of Cosmogenic Exposure Dating of Moraine Boulders to Trace the Geographic Extents of Abrupt Climate Changes : The Younger Dryas Example
- 2011
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Ingår i: Abrupt Climate Change. - Washington DC : American Geophysical Union (AGU). - 9780875904849 ; , s. 111-122
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Bokkapitel (refereegranskat)abstract
- Cosmogenic exposure dating has sometimes been used to identify moraines associated with short-lived climatic events, such as the Younger Dryas (12.9-11.7 ka). Here we point out two remaining challenges in using exposure dating to identify moraines produced by abrupt climate changes. Specifically, (1) a commonly applied sampling criterion likely yields incorrect exposure dates at some sites, and (2) geomorphic processes may introduce bias into presently accepted nuclide production rate estimates. We tit a geomorphic process model that treats both moraine degradation and boulder erosion to collections of exposure dates from two moraines that were deposited within a few thousand years of the Younger Dryas. Subsampling of the modeled distributions shows that choosing boulders for exposure dating based on surface freshness yields exposure dates that underestimate the true age of the moraine by up to several thousand years. This conclusion applies only where boulders do not erode while buried but do erode after exhumation. Moreover, one of our fitted data sets is part of the global nuclide production rate database. Our fit of the moraine degradation model to this data set suggests that nuclide production rates at that site are several percent higher than previously thought. Potential errors associated with sampling strategies and production rate estimates are large enough to interfere with exposure dating of moraines, especially when the moraines are associated with abrupt climate changes. We suggest sampling strategies that may help minimize these problems, including a guide for determining the minimum number of samples that must be collected to answer particular paleoclimate questions.
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| 3. |
- Applegate, Patrick J., et al.
(författare)
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An assessment of key model parametric uncertainties in projections of Greenland ice sheet behavior
- 2012
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Ingår i: The Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 6:3, s. 589-606
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Tidskriftsartikel (refereegranskat)abstract
- Lack of knowledge about the values of ice sheet model input parameters introduces substantial uncertainty into projections of Greenland Ice Sheet contributions to future sea level rise. Computer models of ice sheet behavior provide one of several means of estimating future sea level rise due to mass loss from ice sheets. Such models have many input parameters whose values are not well known. Recent studies have investigated the effects of these parameters on model output, but the range of potential future sea level increases due to model parametric uncertainty has not been characterized. Here, we demonstrate that this range is large, using a 100-member perturbed-physics ensemble with the SICOPOLIS ice sheet model. Each model run is spun up over 125 000 yr using geological forcings and subsequently driven into the future using an asymptotically increasing air temperature anomaly curve. All modeled ice sheets lose mass after 2005 AD. Parameters controlling surface melt dominate the model response to temperature change. After culling the ensemble to include only members that give reasonable ice volumes in 2005 AD, the range of projected sea level rise values in 2100 AD is similar to 40 % or more of the median. Data on past ice sheet behavior can help reduce this uncertainty, but none of our ensemble members produces a reasonable ice volume change during the mid-Holocene, relative to the present. This problem suggests that the model's exponential relation between temperature and precipitation does not hold during the Holocene, or that the central-Greenland temperature forcing curve used to drive the model is not representative of conditions around the ice margin at this time (among other possibilities). Our simulations also lack certain observed physical processes that may tend to enhance the real ice sheet's response. Regardless, this work has implications for other studies that use ice sheet models to project or hindcast the behavior of the Greenland Ice Sheet.
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| 4. |
- Applegate, Patrick
(författare)
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Modeling the statistical distributions of cosmogenic exposure dates from moraines
- 2010
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Ingår i: Geoscientific Model Development. - : European Geosciences Union. ; 3, s. 293-307
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Tidskriftsartikel (refereegranskat)abstract
- Geomorphic process modeling allows us to evalu- calculating the mean does not improve this mismatch. Theate different methods for estimating moraine ages from cos- extreme estimators (youngest date and oldest date) performmogenic exposure dates, and may provide a means to iden- well under specific circumstances, but fail in other cases. Wetify the processes responsible for the excess scatter among suggest a simple estimator that uses the skewnesses of in-exposure dates on individual moraines. Cosmogenic expo- dividual data sets to determine whether the youngest date,sure dating is an elegant method for estimating the ages of mean, or oldest date will provide the best estimate of morainemoraines, but individual exposure dates are sometimes bi- age. Although this method is perhaps the most globally ro-ased by geomorphic processes. Because exposure dates may bust of the estimators we tested, it sometimes fails spectac-be either “too young” or “too old,” there are a variety of ularly. The failure of simple methods to provide accuratemethods for estimating the ages of moraines from exposure estimates of moraine age points toward a need for more so-dates. In this paper, we present Monte Carlo-based models phisticated statistical treatments.of moraine degradation and inheritance of cosmogenic nu-clides, and we use the models to examine the effectivenessof these methods. The models estimate the statistical dis-tributions of exposure dates that we would expect to obtainfrom single moraines, given reasonable geomorphic assump-tions. The model of moraine degradation is based on priorexamples, but the inheritance model is novel. The statisticaldistributions of exposure dates from the moraine degradationmodel are skewed toward young values; in contrast, the sta-tistical distributions of exposure dates from the inheritancemodel are skewed toward old values. Sensitivity analysisshows that this difference is robust for reasonable parame-ter choices. Thus, the skewness can help indicate whether aparticular data set has problems with inheritance or morainedegradation. Given representative distributions from thesetwo models, we can determine which methods of estimatingmoraine ages are most successful in recovering the correctage for test cases where this value is known. The mean isa poor estimator of moraine age for data sets drawn fromskewed parent distributions, and excluding outliers before calculating the mean does not improve this mismatch. Theextreme estimators (youngest date and oldest date) performwell under specific circumstances, but fail in other cases. Wesuggest a simple estimator that uses the skewnesses of in-dividual data sets to determine whether the youngest date,mean, or oldest date will provide the best estimate of moraineage. Although this method is perhaps the most globally ro-bust of the estimators we tested, it sometimes fails spectac-ularly. The failure of simple methods to provide accurateestimates of moraine age points toward a need for more so-phisticated statistical treatments.
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| 5. |
- Bartlett, Sofia R., et al.
(författare)
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Sequencing of Hepatitis C Virus for Detection of Resistance to Direct-Acting Antiviral Therapy : A Systematic Review
- 2017
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Ingår i: HEPATOLOGY COMMUNICATIONS. - : JOHN WILEY & SONS LTD. - 2471-254X. ; 1:5, s. 379-390
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Forskningsöversikt (refereegranskat)abstract
- The significance of the clinical impact of direct-acting antiviral (DAA) resistance-associated substitutions (RASs) in hepatitis C virus (HCV) on treatment failure is unclear. No standardized methods or guidelines for detection of DAA RASs in HCV exist. To facilitate further evaluations of the impact of DAA RASs in HCV, we conducted a systematic review of RAS sequencing protocols, compiled a comprehensive public library of sequencing primers, and provided expert guidance on the most appropriate methods to screen and identify RASs. The development of standardized RAS sequencing protocols is complicated due to a high genetic variability and the need for genotype- and subtype-specific protocols for multiple regions. We have identified several limitations of the available methods and have highlighted areas requiring further research and development. The development, validation, and sharing of standardized methods for all genotypes and subtypes should be a priority.
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| 6. |
- Clason, Caroline C., et al.
(författare)
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Modelling Late Weichselian evolution of the Eurasian ice sheets forced by surface meltwater-enhanced basal sliding
- 2014
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Ingår i: Journal of Glaciology. - 0022-1430 .- 1727-5652. ; 60:219, s. 29-40
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Tidskriftsartikel (refereegranskat)abstract
- We simulated the Late Weichselian extent and dynamics of the Eurasian ice sheets using theshallow-ice approximation ice-sheet model SICOPOLIS. Our simulated Last Glacial Maximum ice-sheetextents closely resemble geomorphological reconstructions, and areas of modelled fast flow areconsistent with the known locations of palaeo-ice streams. Motivated by documented velocity responseto increased meltwater inputs on Greenland, we tested the sensitivity of the simulated ice sheet to thesurface meltwater effect (SME) through a simple parameterization relating basal sliding to local surfacemelt rate and ice thickness. Model runs including the SME produce significantly reduced ice volumeduring deglaciation, with maximum ice surface velocities much greater than in similar runs that neglectthe SME. We find that the simple treatment of the SME is not applicable across the whole ice sheet;however, our results highlight the importance of the SME for dynamic response to increased melting.The southwest sector of the Scandinavian ice sheet is most sensitive to the SME, with fast flow in theBaltic ice stream region shutting off by 15 kaBP when the SME is turned on, coincident with a retreat ofthe ice-margin position into the Gulf of Bothnia.
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| 7. |
- Glover, Katherine C., et al.
(författare)
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Deglaciation, basin formation and post-glacial climate change from a regional network of sediment core sites in Ohio and eastern Indiana
- 2011
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Ingår i: Quaternary Research. - : Cambridge University Press (CUP). - 0033-5894 .- 1096-0287. ; 76:3, s. 401-410
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Tidskriftsartikel (refereegranskat)abstract
- Many paleoclimate and landscape change studies in the American Midwest have focused on the Gate Glacial and early Holocene time periods (similar to 16-11 ka), but little work has addressed landscape change in this area between the Last Glacial Maximum and the Late Glacial (similar to 22-16 ka). Sediment cores were collected from 29 new lake and bog sites in Ohio and Indiana to address this gap. The basal radiocarbon dates from these cores show that initial ice retreat from the maximal last-glacial ice extent occurred by 22 ka, and numerous sites that are similar to 100 km inside this limit were exposed by 18.9 ka. Post-glacial environmental changes were identified as stratigraphic or biologic changes in select cores. The strongest signal occurs between 18.5 and 14.6 ka. These Midwestern events correspond with evidence to the northeast, suggesting that initial deglaciation of the ice sheet, and ensuing environmental changes, were episodic and rapid. Significantly, these changes predate the onset of the Belling postglacial warming (14.8 ka) as recorded by the Greenland ice cores. Thus, deglaciation and landscape change around the southern margins of the Laurentide Ice Sheet happened similar to 7 ka before postglacial changes were felt in central Greenland. (C) 2011 University of Washington.
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| 8. |
- Kelly, Meredith A., et al.
(författare)
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Late glacial fluctuations of Quelccaya Ice Cap, southeastern Peru
- 2012
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 40:11, s. 991-994
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Tidskriftsartikel (refereegranskat)abstract
- The last glacial-interglacial transition (ca. 18-11 ka) was interrupted by abrupt climate events that differed in each hemisphere. During the Antarctic Cold Reversal (ca. 14.5-12.9 ka), the Southern Hemisphere high and mid latitudes cooled, while the Northern Hemisphere warmed. The pattern of change then reversed during the Younger Dryas (ca. 12.9-11.7 ka), which was characterized by cold conditions in much of the Northern Hemisphere. Well-dated paleoclimate records serve to reveal the possible mechanisms for these events. Here we present a reconstruction of the late glacial fluctuations of Quelccaya Ice Cap, located in the southern tropics, based on 38 new radiocarbon ages. Quelccaya was retreating from its Last Glacial Maximum (ca. 21 ka) extent by ca. 17.2 ka, and was located upvalley from its late glacial moraines by 13.6-12.8 ka. Quelccaya experienced a significant readvance that culminated at 12.5-12.4 ka, and then receded several kilometers to near, or within, its late Holocene extent by ca. 11.6 ka. This record provides the most detailed evidence yet of glacier fluctuations in the southern tropics during late glacial time.
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| 9. |
- Kleman, Johan, et al.
(författare)
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Durations and propagation patterns of ice sheet instability events
- 2014
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 92, s. 32-39
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Tidskriftsartikel (refereegranskat)abstract
- Continued atmospheric and ocean warming places parts of the West Antarctic Ice Sheet at risk for collapse through accelerated ice flow and grounding line retreat over reversed bed slopes. However, understanding of the speed and duration of ice sheet instability events remains incomplete, limiting our ability to include these events in sea level rise projections. Here, we use a first-order, empirical approach, exploring past instability events in the Fennoscandian (FIS) and Laurentide (LIS) ice sheets to establish a relationship between catchment size and the duration of instability events. We also examine how instabilities propagate through ice sheet catchments, and how this propagation is controlled by topography and existing flow organisation at the onset of an event. We find that the fastest documented paleo-collapses involved streaming or surging in corridors that are wide compared to their length, and in which fast flow did not resume after the event. Distributed ice stream networks, in which narrow ice streams were intertwined with slow-flow interstream ridges, are not represented among the fastest documented events. For the FIS and LIS, there is geological evidence for instability events covering areas of similar to 100,000 km(2), with durations between 100 and 300 yr. Comparison of the spatial patterns and topographic contexts of Lateglacial collapse events in former Northern Hemisphere ice sheets and the current WAIS suggest that only a minor part of the WAIS area may be at risk for unimpeded collapse, and that negative feedbacks will likely slow or halt ice drawdown in remaining areas. The Pine Island Glacier (PIG) and Thwaites Glacier (TG) catchments in West Antarctica are likely to respond in very different ways to possible further grounding line retreat. The PIG may experience a minor collapse over its main trunk, but the bed topography favours a less dramatic retreat thereafter. The TG is probably not as close to a threshold as PIG, but once efficient drainage has progressed inwards to reach the Bentley Subglacial Basin (BSB) and Bentley Subglacial Trench (BST), a full collapse of the area may occur. The likely time perspective for a BSB BST collapse is the time required for 100-200 km of grounding line retreat in the TG system plus 100-300 years for an actual collapse event.
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