Quantum phase transitions in disordered dimerized quantum spin models and the Harris criterion

We use quantum Monte Carlo simulations to study effects of disorder on the quantum phase transition occurring versus the ratio g=J/J' in square-lattice dimerized S=1/2 Heisenberg antiferromagnets with intra- and inter-dimer couplings J and J'. The dimers are either randomly distributed (as in the classical dimer model), or come in parallel pairs with horizontal or vertical orientation. In both cases the transition violates the Harris criterion, according to which the correlation-length exponent should satisfy nu >= 1. We do not detect any deviations from the three-dimensional O(3) universality class obtaining in the absence of disorder (where nu = 0.71). We discuss special circumstances which allow nu<1 for the type of disorder considered here.Comment: 4+ pages, 3 figure

Sedimentological Observations from the Tiskilwa Till, Illinois, and Sky Pilot Till, Manitoba

We present sedimentological observations from the Tiskilwa Till in northern Illinois, and the Sky Pilot Till in northern Manitoba, that indicate deposition of these tills by subglacial deformation. These generally homogenous tills grade downward into more heterogeneous tills that incorporate underlying sediment into their matrix, indicating entrainment of older sediments by sediment deformation. Deformed sand inclusions within these tills imply deformation of the tills and inclusions prior to deposition. The Tiskilwa Till has relatively high fabric strength throughout its thickness, whereas fabric strength in the Sky Pilot Till generally increases up-section in 2 to 3 m thick increments. Fabric orientations in both tills rotate up-section, possibly due to changes in ice-flow direction associated with the thickening and thinning of ice, and changes in ice-flow divide location. In both the Tiskilwa and Sky Pilot Tills, the change in fabric orientation occurs over intervals of ~1 m, suggesting that the maximum depth of deformation was ≤1 m insofar as any greater depth of deformation would have reoriented till fabric during maximum ice extent and retreat. In the case of the Sky Pilot Till, the up-section increase in macrofabric strength indicates that strain increased up-section. These data suggest that these tills were deposited in a time transgressive manner as strain migrated upwards with the delivery of new till either released from the ice base or advected from up-ice.Les observations sédimentologiques des tills de Tiskilwa, Illinois, et de Sky Pilot, Manitoba, indiquent que ces tills sont issus d’une déformation sous-glaciaire. Ces tills, généralement homogènes, deviennent hétérogènes vers leur base et ils incorporent du matériel sous-jacent dans leur matrice, ce qui indique un déplacement des sédiments plus âgés par déformation. La présence d’inclusions de sable dans ces tills impliquent leur déformation avant leur dépôt. Le till de Tiskilwa présente une matrice très cohérente sur toute son épaisseur tandis que celle du till de Sky Pilot augmente vers le haut tous les 2 ou 3 mètres. La rotation de l’orientation des matrices de ces deux tills est probablement associée aux changements de l’écoulement glaciaire liés à l’épaisseur de la glace et à la migration de la ligne de partage des marges glaciaires. Pour ces tills, le changement d’orientation du matériel se produit sur des intervalles d’environ 1 m, où la profondeur maximale de déformation devrait réorienter le matériel du till durant le maximum glaciaire et le retrait des glaces. Dans le cas du till de Sky Pilot, la section supérieure montre une augmentation dans la force de cohésion du matériel. Ces données indiquent que ces tills se sont déposés de manière diachronique, où la force de tension a migré vers le haut, entraînant le dépôt de matériel basal frais à partir de la base de la glace ou par advection depuis la glace

Instability of the Northeast Greenland Ice Stream over the last 45,000 years

The outlet glaciers that comprise the Northeast Greenland Ice Stream (NEGIS) have experienced accelerated retreat in recent years, yet their longterm stability remains unclear. Here, via cosmogenic surface exposure and radiocarbon ages, the authors investigate the stability of the NEGIS for the past 45 kyr

Linking the 8.2 ka Event and its Freshwater Forcing in the Labrador Sea

The 8.2 ka event was the last deglacial abrupt climate event. A reduction in the Atlantic meridional overturning circulation (AMOC) attributed to the drainage of glacial Lake Agassiz may have caused the event, but the freshwater signature of Lake Agassiz discharge has yet to be identified in (delta)18O of foraminiferal calcite records from the Labrador Sea, calling into question the connection between freshwater discharge to the North Atlantic and AMOC strength. Using Mg/Ca-paleothermometry, we demonstrate that approx. 3 C of near-surface ocean cooling masked an 1.0 % decrease in western Labrador Sea (delta)18O of seawater concurrent with Lake Agassiz drainage. Comparison with North Atlantic (delta)18O of seawater records shows that the freshwater discharge was transported to regions of deep-water formation where it could perturb AMOC and force the 8.2 ka event

Rapid early Holocene deglaciation of the Laurentide ice sheet

Author Posting. © Nature Publishing Group, 2008. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 1 (2008): 620-624, doi:10.1038/ngeo285.The early Holocene deglaciation of the Laurentide Ice Sheet (LIS) is the most recent and best constrained disappearance of a large Northern Hemisphere ice sheet. Its demise is a natural experiment for assessing rates of ice sheet decay and attendant contributions to sea level rise. Here we demonstrate with terrestrial and marine records that the final LIS demise occurred in two stages of rapid melting from ~9.0- 8.5 and 7.6-6.8 kyr BP with the LIS contributing ~1.3 and 0.7 cm yr-1 to sea level rise, respectively. Simulations using a fully coupled atmosphere-ocean general circulation model suggest that increased ablation from enhanced early Holocene boreal summer insolation may have been the predominant cause of the LIS contributions to sea level rise. Although the boreal summer surface radiative forcing of early Holocene LIS retreat is twice that of projections for 2100 C.E. greenhouse gas radiative forcing, the associated summer surface air temperature increase is the same. The geologic evidence for rapid LIS retreat under a comparable forcing provides a prehistoric precedent for a possible large negative mass balance response of the Greenland Ice Sheet by the end of the coming century.This research was funded by National Science Foundation grants ATM-05-01351 & ATM-05-01241 to D.W.O. & G.A.S., start-up funds from the University of Wisconsin-Madison and a Woods Hole Oceanographic Institution Postdoctoral Scholarship to A.E.C., and the Woods Hole Oceanographic Institution's Ocean and Climate Change Institute (D.W.O. & R.E.C.)

Rapid Holocene Deglaciation of the Labrador Sector of the Laurentide Ice Sheet

Retreat of the Laurentide Ice Sheet (LIS) following the Last Glacial Maximum 21 000 yr BP affected regional to global climate and accounted for the largest proportion of sea level rise. Although the late Pleistocene LIS retreat chronology is relatively well constrained, its Holocene chronology remains poorly dated, limiting our understanding of its role in Holocene climate change and sea level rise. Here new ¹⁰Be cosmogenic exposure ages on glacially deposited boulders are used to date the final disappearance of the Labrador sector of the LIS (LS-LIS). These data suggest that following the deglaciation of the southeastern Hudson Bay coastline at 8.0 ± 0.2 cal ka BP, the southwestern margin of the LS-LIS rapidly retreated ~600 km in 140 yr and most likely in ~600 yr at a rate of ~900 m yr⁻¹, with final deglaciation by 6.8 ± 0.2 ¹⁰Be ka. The disappearance of the LS-LIS ~6.8 ¹⁰Be ka and attendant reduction in freshwater runoff may have induced the formation of Labrador Deep Seawater, while the loss of the high albedo surface may have initiated the Holocene Thermal Maximum in eastern Canada and southern Greenland. Moreover, the rapid melting just prior to ~6.8 ¹⁰Be ka indicates that the remnant LIS may be the primary source of a postulated rapid rise in global sea level of ~5 m that occurred sometime between 7.6 and 6.5 cal ka BP.Keywords: Ice sheets, Glaciers, Climate change, Sea level, AlbedoKeywords: Ice sheets, Glaciers, Climate change, Sea level, Albed

Lost in translation: A disconnect between the science and Medicare coverage criteria for continuous subcutaneous insulin infusion

Numerous studies have demonstrated the clinical value and safety of insulin pump therapy in type 1 diabetes and type 2 diabetes populations. However, the eligibility criteria for insulin pump coverage required by the Centers for Medicare & Medicaid Services (CMS) discount conclusive evidence that supports insulin pump use in diabetes populations that are currently deemed ineligible. This article discusses the limitations and inconsistencies of the insulin pump eligibility criteria relative to current scientific evidence and proposes workable solutions to address this issue and improve the safety and care of all individuals with diabetes

Routing of western Canadian Plains runoff during the 8.2 ka cold event

The collapse of the Laurentide Ice Sheet over Hudson Bay ∼8.47 ka allowed the rapid drainage of glacial Lake Agassiz into the Labrador Sea, an event identified as causing a reduction in Atlantic meridional overturning circulation (AMOC) and the 8.2 ka cold event. Atmosphere‐ocean models simulations based on this forcing, however, fail to reproduce several characteristics of this event, particularly its duration. Here we use planktonic foraminifera U/Ca records to document the routing of western Canadian Plains runoff that accompanied ice‐sheet collapse. Geochemical modeling of the ∼7 nmol/mol increase in U/Ca at the opening of Hudson Bay indicates an increase in freshwater discharge of 0.13 ± 0.03 Sverdrups (106 m3 s−1) from routing, a sufficient magnitude to cause an AMOC reduction. We suggest that this routing event suppressed AMOC strength for several centuries after the drainage of Lake Agassiz, explaining multi‐centennial climate anomalies associated with the 8.2 ka cold event

European climate optimum and enhanced Greenland melt during the Last Interglacial

The Last Interglacial climatic optimum, ca. 128 ka, is the most recent climate interval signifi cantly warmer than present, providing an analogue (albeit imperfect) for ongoing global warming and the effects of Greenland Ice Sheet (GIS) melting on climate over the coming millennium. While some climate models predict an Atlantic meridional overturning circulation (AMOC) strengthening in response to GIS melting, others simulate weakening, leading to cooling in Europe. Here, we present evidence from new proxy-based paleoclimate and ocean circulation reconstructions that show that the strongest warming in western Europe coincided with maximum GIS meltwater runoff and a weaker AMOC early in the Last Interglacial. By performing a series of climate model sensitivity experiments, including enhanced GIS melting, we were able to simulate this confi guration of the Last Interglacial climate system and infer information on AMOC slowdown and related climate effects. These experiments suggest that GIS melt inhibited deep convection off the southern coast of Greenland, cooling local climate and reducing AMOC by ~24% of its present strength. However, GIS melt did not perturb overturning in the Nordic Seas, leaving heat transport to, and thereby temperatures in, Europe unaffected. © 2012 Geological Society of America

Palaeo-sea-level and palaeo-ice-sheet databases: Problems, strategies, and perspectives

Sea-level and ice-sheet databases have driven numerous advances in understanding the Earth system. We describe the challenges and offer best strategies that can be adopted to build self-consistent and standardised databases of geological and geochemical information used to archive palaeo-sea-levels and palaeo-ice-sheets. There are three phases in the development of a database: (i) measurement, (ii) interpretation, and (iii) database creation. Measurement should include the objective description of the position and age of a sample, description of associated geological features, and quantification of uncertainties. Interpretation of the sample may have a subjective component, but it should always include uncertainties and alternative or contrasting interpretations, with any exclusion of existing interpretations requiring a full justification. During the creation of a database, an approach based on accessibility, transparency, trust, availability, continuity, completeness, and communication of content (ATTAC3) must be adopted. It is essential to consider the community that creates and benefits from a database. We conclude that funding agencies should not only consider the creation of original data in specific research-question-oriented projects, but also include the possibility of using part of the funding for IT-related and database creation tasks, which are essential to guarantee accessibility and maintenance of the collected data