Search

Radiocarbon and dendrochronological dating of glacially overridden stumps and detrital wood indicates that two outlet glaciers of the Juneau Icefield advanced shortly before the 'Little Ice Age'. Tulsequah Glacier advanced to within 2.4 km of its all-time Holocene limit between ad 865 and ad 940. Llewellyn Glacier, one of the largest glaciers in British Columbia, advanced sometime between ad 300 and ad 500, and reached to within 400 m of its Holocene limit between ad 1035 and ad 1210, well before the climactic, 'classical' 'Little Ice Age' advances of the past several centuries. Our data show that some glaciers in western North America were extensive and expanding at times when alpine glaciers have, in the past, been assumed to be restricted. The evidence raises questions about how to define the time of the beginning of the 'Little Ice Age' and, perhaps more importantly, about the utility of the term. [ABSTRACT FROM AUTHOR]

Jökulhlaups from lakes dammed by Tulsequah Glacier in northwestern British Columbia have occurred regularly since the early twentieth century. The floods commenced after decades of downwasting and retreat of the glacier from its Holocene maximum position, which it achieved in the nineteenth century. Hydrometric data and other records are used to reconstruct the times and peak discharges of floods from two glacier-dammed lakes. Tulsequah Lake formed in the early 1900s when a tributary glacier separated from Tulsequah Glacier, creating an ice-free embayment between the two. The lake grew rapidly in surface area and volume in the first half of the twentieth century, but later, with continued glacier retreat, it decreased in size. The first jökulhlaups from Tulsequah Lake were the largest. Peak and total discharges decreased as the lake became smaller. Today, Tulsequah Lake is small, and it will disappear completely if Tulsequah Glacier retreats any further. A second lake (Lake No Lake), however, has formed and grown in size as Tulsequah Lake has diminished. Lake No Lake developed from a subglacial water body in a tributary valley, 7 km upglacier from Tulsequah Lake. Like Tulsequah Lake, Lake No Lake rapidly grew in area and volume during its youth, and in the 1970s it began to generate its own jökulhlaups. Lake No Lake appears to be following the same evolutionary path as Tulsequah Lake – its volume is now decreasing due to downwasting of Tulsequah Glacier, and its jökulhlaups are beginning to diminish. As Tulsequah Glacier continues to shrink in response to climatic warming, additional glacier-dammed lakes may form, renewing the cycle of outburst flood activity. Such behaviour can be expected in other high mountains because most alpine glaciers are retreating in response to global warming. [ABSTRACT FROM AUTHOR]

The frequency of large supraglacial landslides (rock avalanches) occurring in glacial environments is thought to be increasing due to feedbacks with climate warming and permafrost degradation. However, it is difficult to (i) test this; (ii) establish cause–effect relationships; and (iii) determine associated lag-times, due to both temporal and spatial biases in detection rates. Here we applied the Google Earth Engine supraglacial debris input detector (GERALDINE) to Glacier Bay National Park & Preserve (GLBA), Alaska. We find that the number of rock avalanches (RAs) has previously been underestimated by 53 %, with a bias in past detections towards large area RAs. In total, GLBA experienced 69 RAs during 1984–2020, with the highest frequency in the last three years. Of these, 58 % were deposited into the accumulation zone and then sequestered into the ice within two years. RA sources clustered spatially at high elevations and around certain peaks and ridges, predominantly at the boundary of modelled permafrost likelihood. They also clustered temporally, occurring mainly between May and September when air temperatures were high enough to initiate rock-permafrost degradation mechanisms. There was a chronic background debris supply from RAs, with at least one RA occurring in all but nine years; however, a debris rich period during 2012–2016 was driven by three large RAs delivering 44 % of all (1984–2020) debris (by area). Comparable investigation of slope-failures in other remote currently glaciated regions is lacking. If RA rates are similar elsewhere, especially the bias towards emplacement onto/into accumulation zones, their contribution to glacial sediment budgets has been globally underestimated. [ABSTRACT FROM AUTHOR]