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Cosmogenic exposure dating of glacial boulders is commonly used to estimate the timing of past glaciations because the method enables direct dating of the duration a boulder has been exposed to cosmic rays. For successful dating, the boulders must have been fully shielded from cosmic rays prior to deposition and continuously exposed to cosmic rays ever since. A common assumption is that boulder height (the distance between the top of the boulder and the surrounding surface) is important, and that tall boulders are more likely to have been continuously exposed to cosmic rays than short boulders and therefore yield more accurate exposure ages. Here we test this assumption 'based on exposure age clustering for groups of glacial boulders (and single cobbles) Be-10 exposure ages that have recorded boulder heights (3741 boulders; 579 boulder groups with >= 3 boulders). Of the full set of boulder groups with >= 3 boulders, 21% fulfill a reduced chi square criterion (chi(2)(R) < 2) for well-clustered exposure ages. For boulder groups containing only tall boulders, the fraction of well-clustered exposure age groups is consistently larger. Moreover, this fraction of well-clustered exposure age groups increases with the minimum boulder height in each group. This result confirms the common assumption that tall boulders are generally better targets for cosmogenic exposure dating compared to short boulders. Whereas the tall boulder groups have a significantly larger fraction of well-clustered exposure age groups, there is nonetheless a dominant fraction (>50%) of the boulder groups with scattered exposure ages, highlighting the problem with prior and incomplete exposure for cosmogenic dating of glacial boulders.

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., AuthorCount:5

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., AuthorCount:6