C13A-01 INVITED
The Greenland ice Sheet Since LGM, an Overview.
During LGM (last glacial maximum) the Greenland ice sheet covered an area of c. 3 mio km2, 40% more than now. This appears from a compilation of geological and geophysical studies on land and on the shelf. Ice sheet break up began at c. 15 ka with the clearance of the shelves, c. 0.8 mio km2. This may have been triggered by sea level rise, and was followed by melting and calving of ice on land in the Early Holocene. However, only c. 0.4 mio km2 of land and fjords were cleared of ice during this period of optimal warmth - an indication of the ice sheet's remarkable stability. In view of ice sheet response to future greenhouse warming, this phase of deglaciation is especially interesting. Recent field work in different parts of Greenland and a compilation of curves for isostatic emergence have shown that timing, amount, and rate of change varied from area to area. This is due not only to differences in local climate, but also to local topography, capacity of drainage ways (fjords and shelf-troughs), and presence or absence of ice streams. Before 7.5 ka the present state of glaciation had been attained in most parts. After this, the margins withdrew behind their present location, possibly as much as 40 km in some areas. Pollen and other climate proxies show that cooling and Neoglacial readvance of the ice margin began at c. 5.5 ka. This culminated around 1920 when glaciers in most parts began rapid retreat. Even though this retreat in most areas amounted to a few kilometres at most, it is more significant than the retreat in "greenhouse times".
C13A-02
Cosmogenic Exposure Ages From Northeast Greenland Constrain the Extent of the Greenland Ice Sheet During the Last Glacial Maximum
Traditional reconstructions have suggested two distinctly different ice sheet regimes along the East Greenland continental margin during the Last Glacial Maximum (LGM); ice to the shelf break south of Scoresby Sund and ice extending no further than to the inner shelf at and north of Scoresby Sund. We report on new 10Be-ages from erratic boulders perched at 250 m a.s.l. on the Kap Brewster peninsula at the mouth of Scoresby Sund. The average ages, calculated with and without an assumed maximum erosion rate of 1 cm/ka indicate c.17 ka and c.15 ka, respectively. These ages overlap a period of increased sediment input to the Scoresby Sund fan between 19 and 15 ka ago. Such results suggest that glacier ice reached at least 250 m asl at the mouth of Scoresby Sund and advanced onto the outer shelf during the LGM. It further implies that sliding ice in the fjord-trough was buttressed by cold-based ice on Jameson Land, which has hitherto been regarded as ice-free during the LGM. The results presented above provide additional evidence to the presumed hypothesis that indicates LGM ice had extended onto the outer shelf in northeast Greenland. Accordingly 10Be and 26Al ages from Jameson Land and areas north thereof, relevant in this context, are pending.
C13A-03 INVITED
Late Holocene Sea-levels, Climate and Ice Sheet Dynamics in West Greenland
At the end of the Holocene altithermal, c. 5 ka cal. yr BP, the Greenland Ice Sheet (GIS) had retreated 10-30 km inland of its present position. During the subsequent neoglacial, from c. 4 ka cal yr BP onwards, the GIS re-grew, advancing to reach a maximum extent during the "Little Ice Age". This re-growth saw an increase in load on the earth's surface that, throughout West Greenland, was accompanied by a switch from early and mid Holocene relative sea-level (RSL) fall to late Holocene RSL rise. Evidence for this rise includes drowned archaeological sites, submerged freshwater peats and flooded lake basins. Recently collected sediment cores from flooded lake basins in Disko Bugt and Kangerlussuaq (and from elsewhere in West Greenland) define millennial scale trends in mid and late Holocene RSL that reflect increased ice load during the neoglacial. They provide powerful constraints on geophysical models of Holocene ice sheet history. However, such records lack the resolution required to explore the short-term dynamic interactions between the ice sheet and RSL during the last millennia. In this paper we present a high resolution RSL record from Kangerlussuaq from this period. Our approach uses thin, radiocarbon-dated salt marsh deposits that overlie bedrock. We reconstruct RSL change between c. AD 1400 and the present using seventeen radiocarbon dates on terrestrial plant macrofossils, together with an extensive database of fossil and modern diatom assemblages. Our data indicate a long-term rate of RSL rise over this period of c. 1.3 mm yr-1, close to that suggested by recent GPS observations (when corrected for a twentieth century "eustatic" sea-level rise of c. 1.5-2 mm yr-1). We also identify higher frequency variability in our record that may reflect changes in ice load associated with the "Little Ice Age" in West Greenland. High resolution RSL records, such as this, provide a new target for geophysical models of ice sheet mass balance change and a long term context for short term geodetic records of crustal motions and ice sheet dynamics.
C13A-04
Organic Remains from the Istorvet Ice Cap, Liverpool Land, East Greenland: A Record of Late Holocene Climate Change
Radiocarbon dates of emergent organic remains along the western margin of Istorvet ice cap (70.8°N, 22.2°W) indicate a time when the ice cap was smaller than at present. This ice cap, similar to others in east Greenland, exhibits "historic" moraines ~1-2 km in front of the presently retreating ice margins. At Istorvet, ice margin retreat has exposed a thin (~8 cm) organic horizon and in situ plant remains in bedrock cracks lie less than 10 m away from the present ice margin (453 m asl in 2006). Clusters of multi-species vegetation also were found on two nuntaks (to 719 m asl) located ~3 km from the historic drift limit. All organic remains were located in protected bedrock lees. On the west side of the ice-cap, vegetation is sparse but present at elevations near the ice margin. Both the ice cap geometry and the presence of overrun organic remains indicate past temperatures at least as warm as those at present. At Istorvet plant remains yielded 12 number of radiocarbon dates. These ages, when converted to calendar years, range from A.D. 400 to 1014, with the largest concentration from A.D. 800 to 1014. This work hones the conclusion of Funder (1978) who reported general climate deterioration since 800 BC. Moreover, it indicates warm conditons at this latitude at the time of Norse colonization of Greenland.
C13A-05 INVITED
Greenland outlet glacier sensitivity to surface melting and other key factors
Greenland outlet glaciers drain the vast inland ice sheet and thus deliver amounts of ice that influence global sea level and ocean thermohaline circulation. Outlet glaciers velocity has been observed to fluctuate in ways that implicate a response to warming. Observed outlet glacier velocities from publications spanning several decades are thus compared with surface air temperatures and modeled melt rates to quantify inter-annual glacier velocity sensitivity to melting. Correlation statistics indicate a seasonal dimension of flow speed sensitivity to melt. Melt alone, of course, is not the sole factor that explains a glacier's behavior. The history of a glaciers' terminus position is thus also invoked to tells a story that makes clear that the multiple factors of glacier thickness, buttressing forces, driving stress, and melt rates must all be considered simultaneously to understand glacier sensitivity to climate change.
C13A-06 INVITED
New States of Behavior: Current Status of Outlet Glaciers in Southeast Greenland and the Potential for Similar Changes Elsewhere
The Greenland Ice Sheet's contribution to sea level has doubled in the last 5 years, largely as a consequence of increased ice discharge through outlet glaciers in southeast Greenland. An analysis of satellite remote sensing data shows that just two glaciers, Helheim and Kangerdlugssuaq, might account for ~10% of the observed rise in global sea level since 2001. These two glaciers progressively thinned before entering phases of terminus retreat, rapid acceleration, and faster thinning. After periods of reaching peak speeds 2-3 times faster than historical ('normal') values, both glaciers have decelerated slightly, but are still sustaining speeds far in excess of normal conditions. We speculate that these faster flow speeds represent a new long-term state of behavior which, while not as dramatic as the short-lived periods of peak speeds, have important implications for the rate of sea level rise. Other outlet glaciers in Greenland might be close to similar switches in flow regime. We use existing data to examine which glaciers are the next most likely to change, and describe some of the warning signs we might use to predict imminent changes.
C13A-07
Changes in Greenland Outlet Glacier Systems Synchronous With Increasing Summer Sea Surface Temperatures Over the Past Decade
Analysis of satellite-derived (AVHRR and MODIS) sea surface temperature (SST) time series around Greenland show large-scale patterns of increase that are synchronous with changes in glacier termini and flow speed. In particular, the warming of summer SST in the last half of the 1990s is observed in the south of Greenland and in Disko Bay, coincident with the patterns of glacier acceleration (Rignot and Kanagaratnam. Science 2006) and, in Disko Bay, with the initial changes observed in the configuration and flow of Jakobshavns Isbrae (Joughin et al., Nature, 2004, and Luckman and Murray, GRL, 2005). The warming is observed to spread to the north along both coasts of Greenland, and be particularly pronounced on the NW coast during the first half of the present decade, also matching the patterns of glacier acceleration reported by Rignot and Kanagaratnam and others. Local reports from Disko Bay, and sea ice extent maps from satellite imagery, indicate that winter freezing of the coastal ocean has shown a dramatic decrease over the interval of change, and open water is much more common. Local changes in SST appear to coincide with changes in outlet glacier systems on a nearly annual basis, suggesting that the connection between ocean and inland ice is quite responsive to changes, whether warming or cooling. In a number of areas, maximum summer sea surface temperatures, as measured by satellite thermal IR data, have increased by four degrees C over the last 15 years, representing a significant change in the melting potential of near surface waters.
C13A-08
Short-time-scale variations in flow speed and behavior, Helheim Glacier, East Greenland
We have obtained a suite of detailed geophysical observations, spanning two summer seasons, at Helheim Glacier, East Greenland. This interdisciplinary dataset includes geodetic, seismic, radar, and lidar observations, in addition to tidal, weather, and satellite remote-sensing data. Continuous high-rate GPS observations from a period of 50 days in July--August, 2007, extend a 60-day summer-season time series obtained in 2006. Reoccupation of many glacier sites allows for a direct interannual comparison. Continuous data for periods as long as four weeks were also obtained for the first time on the lowermost part of the glacier, within a few km of the calving front. Analysis of both geodetic and seismic data show significant changes in glacier behavior between 2006 and 2007. The 2006 summer season saw a substantial readvance of the calving front compared with the minimum position recorded in 2005; a retreat of the front was observed in 2007 with respect to 2006. The 2006 summer season was seismically quiescent, with 2007 marking a return to glacial-earthquake activity. While geodetically determined glacier velocities from 2006 can be explained well by simple first- or second-order polynomial models, this is not the case for 2007, where more complex behavior occurring on shorter timescales is observed. We will present a joint analysis of seismic and geodetic data focused on elucidating the nature of short-time-scale variations in glacier flow, including glacial earthquakes.