Browsing by Author "Hood, D"

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    Constraints on ice volume changes of the WAIS and Ross Ice Shelf since the LGM based on cosmogenic exposure ages in the Darwin-Hatherton glacial system of the Transantarctic Mountains
    (Copernicus Publications, 2010-05-02) Fink, D; Storey, BC; Hood, D; Joy, K; Shulmeister, J
    Quantitative assessment of the spatial and temporal scale of ice volume change of the West Antarctic ice sheet (WAIS) and Ross Ice Shelf since the last glacial maximum (LGM) ~20 ka is essential to accurately predict ice sheet response to current and future climate change. Although global sea level rose by approximately 120 metres since the LGM, the contribution of polar ice sheets is uncertain and the timing of any such contribution is controversial. Mackintosh et al (2007) suggest that sectors of the EAIS, similar to those studied at Framnes Mountains where the ice sheet slowly calves at coastal margins, have made marginal contributions to global sea-level rise between 13 and 7 ka. In contrast, Stone et al (2003) document continuing WAIS decay during the mid-late Holocene, raising the question of what was the response of the WAIS since LGM and into the Holocene. Terrestrial evidence is restricted to sparse coastal oasis and ice free mountains which archive limits of former ice advances. Mountain ranges flanking the Darwin-Hatherton glaciers exhibit well-defined moraines, weathering signatures, boulder rich plateaus and glacial tills, which preserve the evidence of advance and retreat of the ice sheet during previous glacial cycles. Previous studies suggest a WAIS at the LGM in this location to be at least 1,000 meters thicker than today. As part of the New Zealand Latitudinal Gradient Project along the Transantarctic, we collected samples for cosmogenic exposure dating at a) Lake Wellman area bordering the Hatherton Glacier, (b) Roadend Nunatak at the confluence of the Darwin and Hatherton glaciers and (c) Diamond Hill which is positioned at the intersection of the Ross Ice Shelf and Darwin Glacier outlet. While the technique of exposure dating is very successful in mid-latitude alpine glacier systems, it is more challenging in polar ice-sheet regions due to the prevalence of cold-based ice over-riding events and absence of outwash processes which removes glacially transported debris. Our glacial geomorphic survey from ice sheet contact edge (~850 masl) to mountain peak at 1600 masl together with a suite of 10Be and 26Al exposure ages, documents a pre-LGM ice volume at least 800 meters thicker than current ice levels which was established at least 2 million years ago. However a complex history of exposure and re-exposure of the ice free regions in this area is seen in accordance with advance and retreat of the ice sheets that feeds into the Darwin –Hatherton system. A cluster of mid-altitude boulders, located below a prominent moraine feature mapped previously as demarcating the LGM ice advance limits, have exposure ages ranging from 30 to 40 ka. Exposure ages for boulders just above the ice contact range from 1to 19 ka and allow an estimate of inheritance. Hence, we conclude that LGM ice volume was not as large as previously estimated and actually little different from what is observed today. These results raise rather serious questions about the implications of a reduced WAIS at the LGM, its effect on the development of the Ross Ice Shelf, and how the Antarctic ice sheets respond to global warming.
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    Cosmogenic nuclide exposure age constraints on the glacial history of the Lake Wellman area, Darwin Mountains, Antarctica
    (Cambridge University Press, 2010-12) Storey, BC; Fink, D; Hood, D; Joy, K; Shulmeister, J; Riger-Kusk, M; Stevens, MI
    We present direct terrestrial evidence of ice volume change of the Darwin and Hatherton glaciers which channel ice from the Transantarctic Mountains into the Ross Ice Shelf. Combining glacial geomorphology with cosmogenic exposure ages from 25 erratics indicates a pre-LGM ice volume at least 600 m thicker than current Hatherton ice elevation was established at least 2.2 million years ago. In particular, five erratics spread across a drift deposit at intermediate elevations located below a prominent moraine feature mapped previously as demarcating the LGM ice advance limits, give a well-constrained single population with mean 10Be age of 37.0 ± 5.5 ka (1σ). At lower elevations of 50–100 m above the surface of Lake Wellman, a further five samples from within a younger drift deposit range in exposure age from 1 to 19 ka. Our preferred age model interpretation, which is partly dependent on the selection of a minimum or maximum age-elevation model, suggests that LGM ice volume was not as large as previously estimated and constrains LGM ice elevation to be within ± 50 m of the modern Hatherton Glacier ice surface, effectively little different from what is observed today. © 2010, Cambridge University Press
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    Size of the West Antarctic ice sheet at the last glacial maximum: new constraints from the Darwin-Hatherton glacial system in the Transantarctic Mountains
    (University of Auckland, 2009-07-01) Storey, BC; Hood, D; Fink, D; Shulmeister, J; Riger-Kusk, M
    An understanding of how the Antarctic ice sheet has reacted to natural global warming since the last glacial maximum (LGM) 18 to 22 thousand years ago (kya) is essential to accurately predict the response of the ice sheets to current and future climate change. Although global sea level rose by approximately 120 metres since the LGM, the contribution from and rate of change of the Antarctic ice sheets is by no means certain. Mackintosh et al (2007) have suggested that the East Antarctic Ice Sheet (EAIS) made an insignificant contribution to global sea-level rise between 13 and 7 kya raising interesting questions about the initial extent and response of the West Antarctic Ice Sheet (WAIS) during that time frame. Terrestrial evidence of these changes is restricted to a few ice-free areas where glacial landforms, such as moraines, show the extent of former ice advances. One such area is the Darwin-Hatherton glacial system where spectacular moraines preserve the advance and retreat of the glacial system during previous glacial cycles. Previous researchers have suggested that the WAIS was more than 1000 metres thicker than it is today at this location at the LGM. As part of the Latitudinal Gradient Project, we mapped the moraines of the Lake Wellman area bordering the Hatherton Glacier and collected samples for cosmogenic nuclide dating, a technique that is widely used to calculate the exposure history of the glacial landscape and the amount of time that the glacial debris has been exposed to cosmic rays and not covered by ice or other glacial debris. While the technique is very successful in mid latitudes, it is more challenging in Polar Regions. Our mapping has shown that ice in the past was at least 800 metres thicker than current ice levels in this area. Our cosmogenic data suggest that this was at least 2 million years ago but for the most part our data record, as expected, a complex history of exposure and re exposure of the ice free regions in this area in accordance with advance and retreat of the ice sheets. However, a cluster of ages of 35 to 40 thousand years record a single exposure event and indicate that the ice in this area was not as thick as previous estimates for the extent of ice at the LGM. These ages are recorded from moraine boulders that are located below a prominent moraine feature mapped as representing the LGM. These results raise further questions about the size of the Antarctic ice sheets at the LGM, their contribution to global sea level change and how the Antarctic ice sheets respond to global warming.