The causes underlying Holocene glacier fluctuations remain elusive, despite decades of research efforts. Cosmogenic nuclide dating has allowed systematic study and thus improved knowledge of glacier-climate dynamics during this time frame, in part by filling in geographical gaps in both hemispheres. Here we present a new comprehensive Holocene moraine chronology from Mt. San Lorenzo (47°S) in central Patagonia, Southern Hemisphere. Twenty-four new 10Be ages, together with three published ages, indicate that the Río Tranquilo glacier approached its Holocene maximum position sometime, or possibly on multiple occasions, between 9,860 ± 180 and 6,730 ± 130 years. This event(s) was followed by a sequence of slightly smaller advances at 5,750 ± 220, 4,290 ± 100 (?), 3,490 ± 140, 1,440 ± 60, between 670 ± 20 and 430 ± 20, and at 390 ± 10 years ago. The Tranquilo record documents centennial to millennial-scale glacier advances throughout the Holocene, and is consistent with recent glacier chronologies from central and southern Patagonia. This pattern correlates well with that of multiple moraine-building events with slightly decreasing net extent, as is observed at other sites in the Southern Hemisphere (i.e., Patagonia, New Zealand and Antarctic Peninsula) throughout the early, middle and late Holocene. This is in stark contrast to the typical Holocene mountain glacier pattern in the Northern Hemisphere, as documented in the European Alps, Scandinavia and Canada, where small glaciers in the early-to-mid Holocene gave way to more-extensive glacier advances during the late Holocene, culminating in the Little Ice Age expansion. We posit that this past asymmetry between the Southern and Northern hemisphere glacier patterns is due to natural forcing that has been recently overwhelmed by anthropogenic greenhouse gas driven warming, which is causing interhemispherically synchronized glacier retreat unprecedented during the Holocene.
<p>Recent temperature history of the Juneau Icefield</p><p>&#160;</p><p>Mass loss from Alaskan glaciers makes a significant contribution to current sea-level rise. The Juneau Icefield (JIF) of southeast Alaska is one of the world largest, and longest-studied, ice fields, and is currently in a documented state of thinning, retreat, and negative mass balance. The climatological context of this glacier change is critical to understanding its causes, the future of the region, and perhaps that of similar mountain glaciers. Do these changes primarily reflect changes in accumulation or ablation? Are mean air temperatures in the region increasing? If so, during which season, ablation or accumulation, are the changes strongest?<br><br>Here we investigate the recent temperature history of the Juneau Icefield, using a combination of reanalysis data and in situ temperature observations from the Juneau Icefield Research Program.&#160; On the whole, we find a significant trend in annual average temperature since the 1950&#8217;s of 0.19&#176;C per decade. Interestingly, this warming is entirely a winter-season signal. We find no significant trend in summer-season temperatures, but a winter time trend of nearly 0.5&#176;C per decade, over twice that of the annual average. This pattern is consistent between the reanalysis products and the local temperature observations across the icefield. Using the in situ measurements from stations across the icefield, we find that the magnitude of the winter-season warming (and that of the annual mean warming) depends strongly on surface elevation: the higher the surface elevation the larger the trend in warming. These results have implications for the cause of recent glacier changes. While there is little evidence for a change in ablation-season temperatures, these results point toward changes in both the length of the ablation season and perhaps the phase of winter precipitation. The elevation-dependence of these trends may have further implications for the future stability of the JIF.</p>
<p>A well-resolved glacial chronology is crucial to compare sequences of glacial/climate events within and between regions, and thus, to unravel mechanisms underlying past climate changes. Important efforts have been made towards understanding the Holocene climate evolution of the Southern Andes; however, the timing, patterns and causes of glacial fluctuations during this period remain elusive. Advances in surface exposure dating techniques, together with the establishment of a Patagonian <sup>10</sup>Be production rate, have opened new possibilities for establishing high-resolution glacial chronologies at centennial/decadal scale. Here we present a new comprehensive Holocene moraine chronology from Mt. San Lorenzo (47&#176;S) in central Patagonia, Southern Hemisphere. Twenty-four new <sup>10</sup>Be ages, together with three published ages, indicate that the R&#237;o Tranquilo glacier approached its Holocene maximum position sometime, or possibly on multiple occasions, between 9860 &#177; 180 and 6730 &#177; 130 yr. This event(s) was followed by a sequence of slightly smaller advances at 5750 &#177; 220, 4290 &#177; 100 (?), 3490 &#177; 140, 1440 &#177; 60, between 670 &#177; 20 and 430 &#177; 20, and at 390 &#177; 10 yr ago. By comparing our results with other glacier chronologies from central and southern Patagonia, we explore the role of the Southern Westerly Winds as a pacemaker of the Holocene glacier fluctuation in southern South America.&#160;</p>
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