A new Glacial Isostatic Adjustment model for Antarctica

A new Glacial Isostatic Adjustment model for Antarctica

Abstracts / Quaternary International 279-280 (2012) 9–120 uplifted by the combined effects of tectonic, halokinetic, and isostatic processes are pres...

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Abstracts / Quaternary International 279-280 (2012) 9–120

uplifted by the combined effects of tectonic, halokinetic, and isostatic processes are preserved at elevations of 20-30 m above modern sea level in various tectonic settings between Peniche and São Pedro de Muel. These sections include sandy and pebbly shoreface deposits in association with ancient dunes, cliff-front colluvium, ripple-laminated mud flats, channel deposits, and peat bogs. Two prominent high stand complexes with luminescence (OSL) and radiocarbon age control date to about 35 and 42 ka, recording a jump in relative sea level of at least 5m near the time of Heinrich Event 4 at 39 ka. Uplift rates of 1-2 mm yr-1 are likely if these deposits represent eustatic sea level stands at roughly 40 m below modern during MIS 3, as suggested by recent eustatic sea level reconstructions. Evidence exists for additional high stand deposits dating to about 62 and 95 ka. The high stand deposits were locally reworked by colluvial and fluvial activity during the MIS 2 lowstand, and buried by an aeolian dune field that extends up to 15 km inland. OSL ages on aeolian, fluvial, and colluvial deposits in the study area show that episodes of geomorphic activity are closely linked to cold/arid phases of MIS 4, 3, and 2, especially Heinrich Events. These findings highlight the magnitude of geomorphic response to climatic and sea level change on the Portuguese margin during the last glacial stage and provide paleoenvironmental context for newlydiscovered archaeological sites in the region. RAINFALL-RUNOFF MODELLING AND PALAEOFLOOD HYDROLOGY APPLIED TO RECONSTRUCT CENTENNIAL SCALE RECORDS OF FLOODING AND AQUIFER RECHARGE IN UNGAUGED EPHEMERAL RIVERS Gerardo Benito. Centre for Environmental Sciences, Spain E-mail address: [email protected]

In this study we propose a multi-source data approach for quantifying long-term flooding and aquifer recharge in ungauged ephemeral rivers. The methodology is applied to the Buffels River, at 9000 km2 the largest ephemeral river in Namaqualand (NW South Africa), a region with scarce stream flow records limiting research investigating hydrological response to global change. Daily discharge and annual flood series (1965-2006) were estimated from a distributed rainfall-runoff hydrological model (TETIS) using rainfall gauge records located within the catchment. The model was calibrated and validated with data collected during a two year monitoring programme (2005-2006) at two stream flow stations, one each in the upper and lower reaches of the catchment. In addition to the modelled flow records, non-systematic flood data were reconstructed using both sedimentary and documentary evidence. The palaeoflood record identified at least 25 large floods during the last 700 years; with the largest events reaching a minimum discharge of 255 m3s-1 (450 year return period) in the upper basin, and 510 m3s-1 (100 year return period) in the lower catchment. Since AD 1925, the flood hydrology of the Buffels River has been characterised by a decrease in the magnitude and frequency of extreme events, with palaeoflood discharges five times larger than the largest modelled floods during the period 1965-2006. Large floods generated the highest hydrograph volumes, however their contribution to aquifer recharge is limited as this depends on other factors such as flood duration and storage capacity of the unsaturated zone prior to the flood. Floods reaching flows associated with 5-10 year return periods (120-140 m3s-1) and flowing for 12 days are able to fully saturate the Spektakel aquifer in the lower Buffels River basin.

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concentrated within transverse debris bands that are creating controlled moraine. These originate up ice of the subglacial overdeepening, either as subglacial foliation/ogive formation at the base of the icefall, or englacial tunnel fills presently emerging as englacial/supraglacial eskers that document the bi-passing of the overdeepening by debris-charged drainage pathways. The melt-out of these debris concentrations results in polygenetic hummocky moraine complexes that comprise three different process-sediment-landform associations: (1) chaotic hummocky moraine, with minor elements of linearity remaining from the stagnation of controlled moraine; (2) heavily channelized moraine complexes, resulting from dissection by a confined proglacial drainage network; and (3) discontinuous push moraine ridges formed during ice-marginal pushing of the actively retreating snout. The debris patterns observed at Kvíárjökull indicate that debris entrainment is predominantly not related to supercooling and therefore the landform-sediment imprint of the foreland cannot be unequivocally related to the supercooling process or used as a diagnostic signature of late Pleistocene supercooling. This is relevant for the interpretation of palaeo-moraine complexes that are suggested to be signatures of supercooling, for example the Laurentide Ice Sheet moraines. A NEW GLACIAL ISOSTATIC ADJUSTMENT MODEL FOR ANTARCTICA Michael J. Bentley. Durham University, United Kingdom E-mail address: [email protected]

We present a glacial isostatic adjustment (GIA) model which is based on a new reconstruction of ice mass changes in Antarctica during the last glacial cycle. The ice sheet reconstruction was constrained using a continent-wide dataset of glacial geological and glaciological data, and generated using a numerical ice sheet model driven by spatial and temporal variations in temperature and accumulation rate. This reconstruction of the Antarctic ice sheet was then incorporated into a global history of ice mass change in order to solve the sea-level equation and make predictions of past relative sea-level change, as well as present-day land uplift and geoid rates, for a range of earth models. The earth model component of the GIA model was then tuned using near-field observations of relative sealevel change. The sparse distribution of both relative sea-level data and glacial geological data limits the degree to which we can constrain past ice mass changes and Earth structure using the above procedure, therefore, a range of uncertainty will feed into our new GIA model parameters. We estimate this range, and attempt to differentiate between possible solutions using a newly reprocessed data set of GPS-determined vertical rates. The model has been developed primarily to provide improved estimates of present-day gravity changes due to GIA in Antarctica that can be applied to the interpretation of GRACE data; thus enabling tighter constraints to be placed on the magnitude and source of present-day ice mass change in Antarctica. But there are a number of conclusions that we can also draw about the deglaciation of Antarctica that are of interest to the wider community, including LGM ice sheet volume, partitioning between East And West Antarctic Ice Sheets, contribution to meltwater pulse-1a, and Holocene melt history. RAPID THINNING AND RETREAT OF THE MARGUERITE TROUGH ICE STREAM, WESTERN ANTARCTIC PENINSULA IN THE EARLY HOLOCENE Michael J. Bentley. Durham University, United Kingdom E-mail address: [email protected]

THE DEBRIS-CHARGED GLACIER LANDSYSTEM, A NEW LANDSYSTEM MODEL, BASED ON KVÍÁRJÖKULL, ICELAND Georgina Lucy Bennett. ETH Zürich, Switzerland E-mail address: [email protected]

A number of well-established landsystem models are based on Icelandic glaciers: the plateau ice-field, lowland surging and active temperate landsystems. We present a new addition to the archive based on the Icelandic glacier, Kvíárjökull: the debris-charged glacier landsystem, developed through a detailed assessment of glacier retreat and landform production between 1945 and 2008 using a combination of digital photogrammetry, field assessments and geomorphological mapping. The glacier snout at Kvíárjökull is charged with large volumes of debris,

Developing detailed records of past ice stream retreat is important in order to constraint rates of ice stream change, and to determine the forcing mechanisms responsible for retreat. Here, we focus on geological records of the Marguerite Trough on the west side of the Antarctic Peninsula, which was occupied by a fast-flowing ice stream during the latter part of the last glacial cycle. Previous work has determined its initial retreat history to the mid-shelf following the local last glacial maximum. Here we report new cosmogenic age data from erratics deposited on the flanks of a coastal site in Marguerite Bay that show very rapid thinning of 350 m in the early Holocene. Synthesised together with other previous terrestrial and marine geological work we demonstrate that the final demise of the ice stream was extremely rapid, and resulted in > 100 km of grounding line retreat. The retreat apparently led to simultaneous deglaciation of a large