Top PDF The Short-Timescale Behavior of Glacial Ice

The Short-Timescale Behavior of Glacial Ice

The Short-Timescale Behavior of Glacial Ice

During the 2006 survey when Helheim Glacier had a floating ice tongue, de Juan Verger (2011) reports that there is a tidal signal in the along-glacier, cross-glacier, and vertical directions. In all cases, the signal decays exponentially with distance away from the glacier’s edge, with the cross-glacier and vertical components decaying over an e- folding length of about 1.0 kilometers, while the along-glacier length-scale is about 2.3 kilometers. These distances translate to an order of magnitude drop in stress over a length of 3.7 kilometers and 8.5 kilometers, respectively. For reference, the thickness of Helheim Glacier was approximately 750 meters during these surveys (de Juan Verger, 2011). The de-trended response of Helheim Glacier to the semidiurnal ocean tides is out of phase, such that at high tide the de-trended position of Helheim Glacier is farther inland than at low tide. However, there is additional lag between this response and the semidiurnal ocean tides, such that the peak glacier motion is delayed relative to the peak tidal amplitudes. The best fit phase lag between the response of the along-glacier displacement and the tide gauge ranges between about 1 hour and 2 hours (30-60 degrees), though a large error on some data points allows for a range that may extend between 0 and 4 hours (0–120 degrees). The best fit values suggest an increase in phase lag with distance inland, but such a trend is dubious at best as the magnitude of the distance-variation falls below the errors of the fits.
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Simulating ice core 10Be on the glacial–interglacial timescale

Simulating ice core 10Be on the glacial–interglacial timescale

In recent years, several studies improved the knowledge on processes which influence atmospheric 10 Be in polar regions. Atmospheric production rates have been calculated within elaborative Monte Carlo simulations of cosmic ray particle cascades (Masarik and Beer, 2009; Kovaltsov and Usoskin, 2010). Long-term records of radionuclide air concentrations (e.g., Dibb, 2007; Aldahan et al., 2008; Elsässer et al., 2011) as well as high-resolution measurements of 10 Be in snow and firn (Pedro et al., 2006, 2011) have been evaluated for the un- derstanding of radionuclide transport processes while global circulation modeling studies investigated the climate influ- ence on the 10 Be deposition (Field et al., 2006; Heikkilä et al., 2008a, 2009, 2013). However, despite these efforts, sev- eral findings remain inconsistent, making the interpretation of 10 Be ice core records a matter of ongoing debate. Further research needs concern transport and deposition processes, linking the 10 Be ice concentration with the cosmogenic pro- duction in the atmosphere. Measurements of 10 Be (and short- lived 7 Be) in polar air show that its boundary layer concentra- tion is very sensitive to seasonal changes in atmospheric cir- culation processes such as the stratosphere–troposphere ex- change or vertical tropospheric mixing (Elsässer et al., 2011). So far it is up for debate as to how these processes are sub- ject to longer-term climate changes and modulate the 10 Be ice concentration. In addition to direct effects of atmospheric transport on 10 Be, atmospheric mixing has a major influence on the production signal recorded in ice core 10 Be: while ge- omagnetic changes primarily affect atmospheric 10 Be pro- duction at lower latitudes, solar-activity-based production variations are more decisive in polar areas. Detailed knowl- edge of the 10 Be atmospheric footprint is thus a crucial re- quirement for interpreting observed 10 Be time series. More- over, in addition to atmospheric transport, aerosol deposition is a crucial process influencing the 10 Be ice concentration. While during the Holocene period 10 Be variations are dom- inated by production changes related to solar and geomag- netic activity (e.g., Steinhilber et al., 2012), 10 Be ice concen- tration is certainly subject to climate modulation on longer timescales (e.g., Finkel and Nishiizumi, 1997). Here, strong
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The Antarctic Ice Sheet response to glacial millennial-scale  variability

The Antarctic Ice Sheet response to glacial millennial-scale variability

We have investigated the response of the AIS to millennial- scale climate variability and, in particular, its response to dif- ferent oceanic sensitivities using a hybrid, three-dimensional, thermomechanical ice-sheet model. The model is forced us- ing a method that has already been tested (Banderas et al., 2018) and is provided by an improved subglacial melting routine. Because SO temperature reconstructions are not available we assumed that oceanic temperatures covary with atmospheric temperature variations at millennial timescales based on Stocker and Johnsen (2003). Our simulations sug- gest that, contrary to the idea that the AIS is a slow reac- tive ice sheet, it could be more reactive to millennial-scale climate variabilities than previously thought. We found that whereas atmospheric millennial-scale variability had no ap- preciable impact on the AIS, SO warming could produce episodes of ice discharge, leading to substantial sea-level rise and grounding line migration. Although this timescale may seem short for such a large ice sheet, our simulations show, in the range of realistic values for oceanic sensitivities, that considerable grounding line retreat in the Ronne, Ross, and Wilkes Land embayment, as well as sea-level discharge of around 6 m SLE at millennial timescales, can occur. Our re- sults highlight the possibility that, via the bipolar seesaw, a slowdown of the AMOC could have accumulated more heat in the Southern Ocean, resulting in significant sea-level rise produced by the AIS on millennial timescales.
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Simulating ice core 10 Be on the glacial–interglacial timescale

Simulating ice core 10 Be on the glacial–interglacial timescale

In recent years, several studies improved the knowledge on processes which influence atmospheric 10 Be in polar regions. Atmospheric production rates have been calculated within elaborative Monte Carlo simulations of cosmic ray particle cascades (Masarik and Beer, 2009; Kovaltsov and Usoskin, 2010). Long-term records of radionuclide air concentrations (e.g., Dibb, 2007; Aldahan et al., 2008; Elsässer et al., 2011) as well as high-resolution measurements of 10 Be in snow and firn (Pedro et al., 2006, 2011) have been evaluated for the un- derstanding of radionuclide transport processes while global circulation modeling studies investigated the climate influ- ence on the 10 Be deposition (Field et al., 2006; Heikkilä et al., 2008a, 2009, 2013). However, despite these efforts, sev- eral findings remain inconsistent, making the interpretation of 10 Be ice core records a matter of ongoing debate. Further research needs concern transport and deposition processes, linking the 10 Be ice concentration with the cosmogenic pro- duction in the atmosphere. Measurements of 10 Be (and short- lived 7 Be) in polar air show that its boundary layer concentra- tion is very sensitive to seasonal changes in atmospheric cir- culation processes such as the stratosphere–troposphere ex- change or vertical tropospheric mixing (Elsässer et al., 2011). So far it is up for debate as to how these processes are sub- ject to longer-term climate changes and modulate the 10 Be ice concentration. In addition to direct effects of atmospheric transport on 10 Be, atmospheric mixing has a major influence on the production signal recorded in ice core 10 Be: while ge- omagnetic changes primarily affect atmospheric 10 Be pro- duction at lower latitudes, solar-activity-based production variations are more decisive in polar areas. Detailed knowl- edge of the 10 Be atmospheric footprint is thus a crucial re- quirement for interpreting observed 10 Be time series. More- over, in addition to atmospheric transport, aerosol deposition is a crucial process influencing the 10 Be ice concentration. While during the Holocene period 10 Be variations are dom- inated by production changes related to solar and geomag- netic activity (e.g., Steinhilber et al., 2012), 10 Be ice concen- tration is certainly subject to climate modulation on longer timescales (e.g., Finkel and Nishiizumi, 1997). Here, strong
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Ice-stream flow switching by up-ice propagation of instabilities along glacial marginal troughs

Ice-stream flow switching by up-ice propagation of instabilities along glacial marginal troughs

Here, we propose a mechanism to explain the switch- ing behavior of an ice stream that incorporates some of the switching mechanisms invoked in previous studies but in which the flow switch of Sam Ford Ice Stream is due to long- term erosion by up-ice-propagating ice streams (i.e., Scott Ice Stream). Ice streams propagating upstream and eroding Hecla & Griper Trough have eventually eroded the marginal trough to a point at which it extended to reach the mouth of Sam Ford Fiord. The changes in the depth to bedrock result- ing from the upstream erosion by Scott Ice Stream in Sam Ford system led to the (1) reorganization of the ice drainage system through the switching of Sam Ford Ice Stream from Sam Ford Trough to Scott Trough and, ultimately, (2) the shutdown of Sam Ford Ice Stream in Sam Ford Trough (Al- ley et al., 1994; Anandakrishnan and Alley, 1997; Graham et al., 2010). Such ice piracy through the switching of ice streaming most probably occurred early during Pleistocene glaciations: it is probable that ice streaming in Sam Ford Trough only occurred during a short period at the beginning of early glaciations, before its ice discharge got captured by the Scott Ice Stream. As this process repeated itself through- out glacial cycles, it accentuated the depth of Hecla & Griper Trough, which in turn facilitated the capture of Sam Ford Ice Stream by Scott Ice Stream during subsequent glaciations. The changes in the depth to bedrock associated with the up- stream propagation of Scott Ice Steam in Hecla & Griper also led to a point in time when the ice discharge of Sam Ford Fiord switched to be topographically diverted towards Hecla & Griper (i.e., the 1.5 km wide depression). From that point on, ice-streaming conditions were unlikely to occur in Sam Ford Trough and could explain (1) why Scott Trough is more than 2 times deeper than Sam Ford Trough and (2) the absence of a trough–mouth fan at the seaward end of Sam Ford Trough. Taken together, these observations suggest that
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A 2700-year annual timescale and accumulation history for an ice core from Roosevelt Island, West Antarctica

A 2700-year annual timescale and accumulation history for an ice core from Roosevelt Island, West Antarctica

M. Winstrup et al.: Timescale and accumulation history for an ice core from Roosevelt Island 763 Table 2. Marker horizons used for development and validation of the RICE17 chronology. Strata in bold were used for constraining the timescale. The statistical significance of volcanic peaks in RICE (column 3) is given in terms of the average peak size in smoothed and standardized versions of the four volcanic records (ECM, H + , nss cond, nss S; down to 249 m) computed relative to a running mean and standard deviation. Volcanic matching to WAIS Divide allows for comparison between RICE17 ages (with 95 % confidence interval indicated) and the corresponding WD2014 ages with associated uncertainties (Sigl et al., 2015, 2016). Indicated depths and ages correspond to peaks in the volcanic proxies. Below 42.3 m, decimal ages have been calculated assuming BC to peak on 1 January. Historical eruption ages (column 4) indicate the starting date of the eruption. Column 4 also indicates whether the eruption has previously been observed to cause a bipolar signal based on the compilation in Sigl et al. (2013), here updated to the WD2014 timescale. Since this compilation only identifies bipolar volcanoes back to 80 CE, volcanoes prior to this are not classified. Three exceptionally large volcanic signals observed in the RICE core are indicated in italics.
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Glacial geomorphology of Marguerite Bay Palaeo-Ice stream, western Antarctic Peninsula

Glacial geomorphology of Marguerite Bay Palaeo-Ice stream, western Antarctic Peninsula

The outer shelf is dominated by highly elongate MSGLs formed in till (e.g. O ´ Cofaigh et al., 2007), which probably relate to the final imprint of palaeo-ice stream activity. Although there is considerable variation in MSGL elongation, it generally increases downstream and towards the centre of the trough. Unlike the bedrock-floored inner- and mid-shelf there is no evidence of melt- water channels or related fluvial sediments on the sedimentary substrate of the outer-shelf (e.g. Kilfeather et al., 2011; O ´ Cofaigh et al., 2007). Bedrock channels on the inner shelf are interpreted to have formed over many glacial cycles (e.g. Anderson & Oakes-Fretwell, 2008) and thus reflect an entirely separate temporal signature of subglacial water production and flow. It is therefore not surprising that similar features are not documented on the sediment-floored outer shelf. However, it is unknown how subglacial meltwater drained through this sector of the palaeo ice-stream. Poss- ible mechanisms include: (i) water flow through meltwater channels or canals below the resol- ution of the instruments; (ii), flow as part of a dynamic network, where channels were constantly evolving due to the ‘competition’ between water flux and sediment creep; or (iii), via Darcian flow through the sediment (see Noormets et al., 2009).
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Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum

Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum

Recession of individual ice streams across the western AP shelf was therefore asynchronous between troughs in terms of the timing of initial grounding line recession. However, subse- quent retreat rates also seem to exhibit marked spatial varia- tions, if we assume that the deglaciation ages are close to the true time of grounding-line retreat, i.e. that the fact that the dates are minimum ages has not signi fi cantly altered the timing of palaeo-ice stream retreat. For example, according to the core chronologies from Anvers Trough grounding-line retreat was initially slow across the outer-mid shelf (mean retreat rates of 2 e 15 m yr 1 ) but then accelerated (47 m yr 1 ) (Heroy and Anderson, 2007; Livingstone et al., 2012). By contrast, in Marguerite Trough, retreat across the outer shelf was rapid with mean retreat rates of ~80 m yr 1 , although these rates could have been considerably greater given the overlap in the error of dates constraining retreat along a 140 km long stretch of the outer to mid-shelf part of the trough (Kilfeather et al., 2011). Variations in retreat style and rate between individual troughs are also implied from the mapped glacial geomorphology. The distribution of GZWs, which often overprint or disrupt MSGLs on the shelf, indicates the occurrence of temporary stillstands or slow-downs in the rate of grounding line retreat in the bathy- metric troughs, and show that post-LGM grounding-line retreat ranged from continuous to episodic (Larter and Vanneste, 1995; Heroy and Anderson, 2007; O Cofaigh et al., 2008; Livingstone et al., 2012, 2013). Retreat in some troughs was interrupted by a series of re-advances and ice-shelf breakup and reformation episodes (e.g., Prince Gustav Channel; Pudsey and Evans, 2001). The presence of GZWs indicating episodic retreat contrasts with areas characterised by uninterrupted MSGLs and where GZWs are absent, implying continuous, possibly rapid retreat ( O Cofaigh et al., 2008; Dowdeswell et al., 2008). Although GZWs can also form as the grounding line advances across the shelf, such GZWs are likely to be successively eroded or substantially modi fi ed when they are overridden by the advancing ice. Retreat from the inner shelf was strongly diachronous, with dates ranging from 13 to 7 cal ka BP (Heroy and Anderson, 2007). As suggested by Heroy and Anderson (2007), this vari- ability may re fl ect the in fl uence of local controls on retreat rate; notably the rugged, bedrock dominated, inner shelf which would have facilitated pinning on bedrock highs and topo- graphic constrictions.
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The Southern Hemisphere at glacial terminations: insights from the Dome C ice core

The Southern Hemisphere at glacial terminations: insights from the Dome C ice core

The ice core was drilled from 1996 to 2004 and has been analysed for stable water isotopes (δD, (Jouzel et al., 2007)) at 55 cm resolution. The analysis of the soluble impuri- ties e.g. sodium (Na + ) and calcium (Ca 2+ ) has been done by seven European laboratories with different methods, and low-resolution data along most of the core, using a previ- ous age-scale, have already been published (Wolff et al., 2006). In this study, we used the data obtained by contin- uous flow analysis (CFA), (R¨othlisberger et al., 2000), which resulted in a high-resolution record (of the order of 1 cm, corresponding to less than a year in the Holocene, approx- imately 3 years at 410 ka BP during marine isotope stage (MIS) 11, and 20 years at 800 ka BP during MIS 20). In the top part (0 to 450 ka BP), these data were downsampled to 20 years resolution by using the median of the data in each 20-a interval in order to reduce the computing time. Below that, computing time was within reasonable limits for the high-resolution data, so that 1 cm data were used for further analysis. Fluxes, being representative of atmo- spheric concentrations at sites where dry deposition is as- sumed to dominate, were calculated using the accumulation rates derived from the EDC3 timescale (Parrenin et al., 2007) (Fig. 1). Compared to nssCa 2+ and ssNa + flux, the ac- cumulation rate changed independently during terminations,
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Multi Timescale Long Short Term Memory Neural Network for Modelling Sentences and Documents

Multi Timescale Long Short Term Memory Neural Network for Modelling Sentences and Documents

In this paper, we propose a multi-timescale long short-term memory (MT-LSTM) to capture the valuable information with different timescales. In- spired by the works of (El Hihi and Bengio, 1995) and (Koutnik et al., 2014), we partition the hidden states of the standard LSTM into several groups. Each group is activated and updated at different time periods. The fast-speed groups keep the short-term memories, while the slow-speed groups keep the long-term memories. We evaluate our model on four benchmark datasets of text classifi- cation. Experimental results show that our model can not only handle short texts, but can model long texts.
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Ice-shelf damming in the glacial Arctic Ocean: dynamical regimes of a basin-covering kilometre-thick ice shelf

Ice-shelf damming in the glacial Arctic Ocean: dynamical regimes of a basin-covering kilometre-thick ice shelf

The present theoretical considerations and review of recent ice-dynamical studies suggest that the dynamically most con- sistent scenario is a fully developed glacial Arctic Ocean ice shelf of approximately uniform thickness that covers the en- tire basin. This is in line with numerical-modelling results of Colleoni et al. (2016a) and the reconstruction based on mapped ice grounding on submarine ridges and bathymet- ric highs of Jakobsson et al. (2016) shown in Fig. 1, which in turn is similar to the earlier ice-shelf scenario proposed by Mercer (1970), Hughes et al. (1977), and Grosswald and Hughes (1999). The mapped mega-scale lineation features on the Arlis Plateau and southern Lomonosov Ridge off the New Siberian Islands point to an ice shelf drafting deeper than 1000 m below present sea level, which is incompatible with the notion of a freely spreading ice shelf that only covers fractions of the Arctic Ocean: without back stresses from ice- shelf embayment towards the Fram Strait, the ice mass flux from the continental ice sheets would have been unreason- ably large (Schoof, 2007b). It is important to note that for the same physical reason, a kilometre-thick Amerasian Basin ice shelf, which grounds on the Lomonosov Ridge and thereafter spreads freely in the Eurasian Basin, is dynamically unfeasi- ble: the mass flux from the downstream grounding line on the Lomonosov Ridge would be very large, 6 causing rapid thinning and un-grounding of the ice shelf. Further, our dis- cussions have concerned an ice shelf that terminates near the Fram Strait. It is an open question whether glacial Arctic Ocean ice shelves have continued southward to the North At- lantic covering also the Nordic Seas as proposed for instance
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BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British–Irish Ice Sheet

BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British–Irish Ice Sheet

We have undertaken a survey of the glacial geomorpho- logical evidence of onshore and offshore Britain and Ireland to produce a new BRITICE map and GIS database (version 2). To our knowledge, all published geomorpho- logical evidence in the themes that we targeted and relating to the extent and behaviour of the last ice sheet that covered much of the British Isles is now included (up to the census date of 31st December 2015). The fully revised GIS database contains over 170 000 geospatially referenced and attributed elements – an eightfold increase in informa- tion from the previous version (V.1; Clark et al. 2004). The spatial coverage has been increased (Ireland and continen- tal shelf) and some new thematic layers added (cirques, crag-and-tails and glacially streamlined bedrock). The geolocation of many features has been improved. Data are stored in thematic layers including: drumlins, ribbed moraine, crag-and-tails, mega-scale glacial lineations, gla- cially streamlined bedrock (grooves, roches mouton ees and whalebacks); glacial erratics; eskers; meltwater channels (subglacial, lateral, proglacial and tunnel valleys); moraines; trimlines; cirques; trough-mouth fans and evidence defining ice-dammed lakes. Not included are themes such as raised shorelines, glacial striae, hummocky moraine, kame ter- races, periglacial and aeolian landforms, alluvial and delta-fans, which if compiled in a future version, could help constrain aspects of ice cover, retreat and dynamics. Features thought to relate to previous glaciations (e.g. Anglian Stage) are also not included and could form the basis of future work. Features relating to the later and smaller Loch Lomond Stadial ice masses are excluded, but have already been compiled in Bickerdike et al. (2016). Comments or help to correct mistakes or add
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Glacial Stratigraphy of the Ridge River Area, Northern Ontario: Refining Wisconsinan Glacial History and Evidence for Laurentide Ice Streaming

Glacial Stratigraphy of the Ridge River Area, Northern Ontario: Refining Wisconsinan Glacial History and Evidence for Laurentide Ice Streaming

Locally derived carbonates are the dominant pebble type with an average of 66% in the matrix whereas Superior and Belcher group rocks average 25% and 9% respectively. There are only minor variations in pebble composition within each till unit and between the upper and lower tills in the RRA. These results suggest that local flow did not deviate drastically from the accepted regional flow. Southwestward flowing ice emanating from northern Quebec would have transported mostly carbonate pebbles from the expansive Hudson Platform and lesser amounts of Superior Group and Belcher Group erratics hundreds of kilometres down-ice to the RRA. The abundance of omars, which comprised nearly one tenth of all pebbles examined, is a strong indicator of deposition by southwest flowing ice. Omars are the colloquial name for distinctive greywackes from the Omarolluk Formation in the Belcher Islands that contain calcareous concretions and have been used to trace ice-flow paths across much of central Canada (Prest et al., 2000). These conclusions are discussed further in Chapter 4.2.
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The importance of snow albedo for ice sheet evolution over the last glacial cycle

The importance of snow albedo for ice sheet evolution over the last glacial cycle

Abstract. The surface energy and mass balance of ice sheets strongly depends on the amount of solar radiation absorbed at the surface, which is mainly controlled by the albedo of snow and ice. Here, using an Earth system model of inter- mediate complexity, we explore the role played by surface albedo for the simulation of glacial cycles. We show that the evolution of the Northern Hemisphere ice sheets over the last glacial cycle is very sensitive to the representation of snow albedo in the model. It is well known that the albedo of snow depends strongly on snow grain size and the content of light- absorbing impurities. Excluding either the snow aging effect or the dust darkening effect on snow albedo leads to an ex- cessive ice build-up during glacial times and consequently to a failure in simulating deglaciation. While the effect of snow grain growth on snow albedo is well constrained, the albedo reduction due to the presence of dust in snow is much more uncertain because the light-absorbing properties of dust vary widely as a function of dust mineral composition. We also show that assuming slightly different optical properties of dust leads to very different ice sheet and climate evolutions in the model. Conversely, ice sheet evolution is less sensi- tive to the choice of ice albedo in the model. We conclude that a proper representation of snow albedo is a fundamental prerequisite for a successful simulation of glacial cycles.
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Modelling large-scale ice-sheet–climate interactions following glacial inception

Modelling large-scale ice-sheet–climate interactions following glacial inception

Abstract. We have coupled the FAMOUS global AOGCM (atmosphere-ocean general circulation model) to the Glim- mer thermomechanical ice-sheet model in order to study the development of ice-sheets in north-east America (Laurentia) and north-west Europe (Fennoscandia) following glacial in- ception. This first use of a coupled AOGCM–ice-sheet model for a study of change on long palæoclimate timescales is made possible by the low computational cost of FAMOUS, despite its inclusion of physical parameterisations similar in complexity to higher-resolution AOGCMs. With the orbital forcing of 115 ka BP, FAMOUS–Glimmer produces ice caps on the Canadian Arctic islands, on the north-west coast of Hudson Bay and in southern Scandinavia, which grow to oc- cupy the Keewatin region of the Canadian mainland and all of Fennoscandia over 50 ka. Their growth is eventually halted by increasing coastal ice discharge. The expansion of the ice- sheets influences the regional climate, which becomes cooler, reducing the ablation, and ice accumulates in places that ini- tially do not have positive surface mass balance. The results suggest the possibility that the glaciation of north-east Amer- ica could have begun on the Canadian Arctic islands, produc- ing a regional climate change that caused or enhanced the growth of ice on the mainland. The increase in albedo (due to snow and ice cover) is the dominant feedback on the area of the ice-sheets and acts rapidly, whereas the feedback of topography on SMB does not become significant for several centuries, but eventually has a large effect on the thicken- ing of the ice-sheets. These two positive feedbacks are mu- tually reinforcing. In addition, the change in topography per- turbs the tropospheric circulation, producing some reduction
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Interglacial and glacial variability from the last 800 ka in marine, ice and terrestrial archives

Interglacial and glacial variability from the last 800 ka in marine, ice and terrestrial archives

The kind of compilation we have made here is simplified by the fact that so few records meet our criteria. However, one consequence of that is that it is very hard to make statements about the global pattern of change and of glacial and inter- glacial strength based on such sparse geographic coverage. The deep ocean is reasonably well-represented by isotopic records, and the different benthic records show relatively lit- tle variability between them in terms of amplitude, even if there are phase differences between them. The surface ocean, represented by 10 suitable SST records, shows greater vari- ability than in the isotopic records; regional differences be- tween tropical and higher latitudes can be tentatively identi- fied. Specific proxy measurements of bottom water temper- ature are limited; some extend so far only to around 500 ka (Elderfield et al., 2010), and we are completely lacking data from the African, American and Australian continents. Thus while our datasets do represent very significant components of the Earth system including CO 2 (representing both a range
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NON-DESTRUCTIVELY MAPPING THE IN-SITU HYDROLOGIC PROPERTIES OF SNOW, FIRN, AND GLACIAL ICE WITH GEORADAR

NON-DESTRUCTIVELY MAPPING THE IN-SITU HYDROLOGIC PROPERTIES OF SNOW, FIRN, AND GLACIAL ICE WITH GEORADAR

with a power head and ~9 cm inside diameter. Core depths are measured from the surface, which was loose snow at both locations, thus depths are not exact (± 5 cm). We logged the cores in the field recording density, grain size, firn type, and estimated percent ice content. Density measurements were made approximately every 0.15 m to 0.4 m. Firn type, grain size, and estimated ice percent were recorded layer-by-layer. Herein we distinguish between seven metamorphic firn types with varying diagenesis: 1) dry snow – layer above most recent melt surface with no noticeable amount of liquid water content, 2) wet snow – layer infiltrated by current season’s melt with a noticeable amount of liquid water content, 3) faceted crystals – buried layer of dry faceted ice crystals, 4) wetted facets – faceted crystals with signs of previous wetting (i.e., slight rounding of facets, partially necked), 5) wetted firn – either firn with evidence of previous wetting (i.e., rounded grains, heavily necked) or frozen slush (same characteristics), 6) unwetted firn – firn with no evidence of previous wetting (i.e. angular ice grains, open pore space), and 7) ice layer or ice pipe – any layer that is pure ice. For layers that had inclusions of ice lenses or ice pipes, we visually estimated the percent pure ice for that layer.
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Simulation of the last glacial cycle with a coupled climate ice-sheet model of intermediate complexity

Simulation of the last glacial cycle with a coupled climate ice-sheet model of intermediate complexity

Simulation of even only one glacial cycle with a state-of- the-art coupled GCM remains computationally extremely de- manding. Thereby the possibility of an acceleration of the model runs would be desirable. Since the time scales of the ice sheets are comparable or even longer than periodicity of the orbital forcing, it is not possible to apply any acceleration technique to the ice-sheet model. However, even a relatively high resolution (ca. 50 km), modern, three-dimensional ther- momechanical ice-sheet model is computationally inexpen- sive compared to the climate models (about 1% of total CPU time). At the same time, the typical time scales of the atmosphere-ocean system is much shorter compared to or- bital time scales and, therefore, it would be justified to accel- erate the climate component by artificially stretching the time scales of the external forcing (orbital and GHGs in our case), as proposed by Lorenz and Lohmann (2004). This technique, when applied to the climate ice-sheet model, implies that the ice sheet is simulated in real time, while for the climate com- ponent orbital forcing and GHGs concentrations changes N times faster than in reality, where N is the acceleration factor. As a result, the exchange of information between the climate and the ice sheet model components occurs every year for the climate and each Nth year for the ice sheet component. Calov et al. (2009) showed that the climate component can be considerably accelerated without significant loss of accu- racy in simulation of the glacial inception. Here, we extend this analysis to the whole glacial cycle. Figure 12a shows the simulated global ice volume in three runs with acceler- ation factors of 5, 10 and 20 in comparison with the (fully synchronous) BE. With respect to simulated ice volume, the agreement between accelerated and BE remains reasonably good up to an acceleration factor of 20. The same is true for the globally averaged surface air temperature, although, pronounced millennial scale variability is already consider- ably suppressed for a two-fold acceleration (not shown). Fi- nally, the simulated deep water temperature is considerably affected by acceleration technique (Fig. 12c). Therefore, our experiments indicate that some aspects of glacial variability on orbital time scales can be successfully reproduced, even when using a large acceleration factor but considerable delay is introduced in the deep ocean evolution. The latter prob- lem can be at least partly mitigated by using an additional acceleration scheme for the deep ocean, as proposed by Liu et al. (2004).
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Modelling the northern hemisphere and Antarctic ice sheet changes through the last glacial cycle

Modelling the northern hemisphere and Antarctic ice sheet changes through the last glacial cycle

As part of the global approach to the niodelling of the last glacial cycle, the Antarctic Ice Sheet model was driven by the same sea level forcing as was applied to the northern hemisphe[r]

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Numerical reconstructions of the Northern Hemisphere ice sheets through the last glacial-interglacial cycle

Numerical reconstructions of the Northern Hemisphere ice sheets through the last glacial-interglacial cycle

CCSR1 ice also covers the Keewatin region, while small ice caps are produced in the Labrador sector with LMD5 and CCSR1, and in the Hudson Bay lowland with CCSR1 and UGAMP. Observational data (Andrews and Barry, 1978) in- dicate that the regions of ice-sheet inception in North Amer- ica were those bordering the Eastern coast, such as the Baf- fin Island and the Quebec-Labrador region, as well as the uplands of Northeastern Keewatin. This is concordant with our reconstructions, except for the Labrador sector where small ice caps are only produced with two models. More- over, the advance of ice in the Middle West region is highly discordant with the geological data. The excess of ice in this area, simulated by using UGAMP outputs as climate forc- ing, seems to be directly related to a high precipitation ratio added to a small anomaly of temperature. Paleoenvironmen- tal records indicate that, at the early beginning of the glacia- tion, climate in the Rocky Mountains regions was as warm as, or warmer than present (Clark et al., 1993). Hence, the Cordilleran ice sheet does not appear to have developed be- fore the late isotopic stage 5 or 4 (i.e. ∼ 75 kyr BP). At that time, the ice advanced over the Southern British Columbia and into the Northern Puget lowland, whereas northern ar- eas were later covered by ice, which is in contradiction with our modeling results. This discordance can be explained by the shortcomings of our approach. Actually, according to a study carried out by Clark and Bartlein (1995), the Cordilleran ice sheet started to grow when the Laurentide ice sheet was high enough to induce a displacement of the jet stream causing precipitation to fall over the Rocky Moun- tains (Roe and Lindzen, 2001). Such a glaciation sequence cannot be represented with our methodology because it does not account for the feedbacks of the ice sheets on the atmo- spheric circulation. Moreover, the use of LGM climate snap- shots in the climate forcing induces an artifact albedo effect (see Sect. 2.2) in regions covered by snow at the LGM, and hence favours the glaciation process at any time of the last glacial-interglacial cycle.
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