Antarctic Paleotopography Estimates at the Eocene-Oligocene Climate Transition: Implications for Climate-Ice Models
Abstract
Paleoclimate models for the rapid growth of Antarctic ice in the latest Eocene have advanced substantially in recent years. However, one generally recognized limitation of these models is that they are based on present topography, corrected only for removal of modern ice. For West Antarctica this results in large areas below sea level that would not host ice. In the Ross Embayment, part of the recently active West Antarctic rift system, there are reasons to suspect that other factors may have contributed to significant vertical motions since the Eocene. The clearest example is the sampling of ~26 Ma Oligocene shallow-marine sediments slightly above Paleozoic basement in DSDP Site 270 at a depth of about 1 km below present sea level. We present a model for 34-Ma paleotopography that, in addition to accounting for the load of the modern ice, also accounts for thermal contraction that results from tectonic extension and accounts for sediment redistribution and loading since 34 Ma. With support from plate-motion data, geologic observations, and limited crustal-thickness data, we assume that the West Antarctic Rift was an orogenic highland in the Early Cretaceous, with a crustal thickness of about 50 km. Three phases of extension transformed this highland to the present lowlands and basins. The first, at 100-80 Ma, affected the western and southern margins of Marie Byrd Land and the adjacent Eastern Basin. The second occurred about 70-55 Ma, as suggested Cande and Stock [2004], affecting the central Ross Embayment, including Iselin Bank, Central High, and possibly reaching as far as Siple Dome. The third, synchronous with spreading at Adare Trough about 45-25 Ma, primarily affected basins adjacent to the Transantarctic Mountains. Stretching factors are everywhere at least 2.0, and higher in sedimentary basins. Sediment thickness is fairly well mapped in the Ross Sea, but must be estimated from extremely limited data under ice. Vertical responses to changes in ice thickness, sediment thickness, and resulting changes in water load are calculated using a flexure model with effective elastic thickness tentatively set at 50 km, using 10-km grid cells. Predicted subsidence for thinly sedimented structural highs is reasonably well constrained in the range of 400-1200 m, which restores many marine areas to 100-300 m above sea level. Predicted subsidence in basins is larger but sensitive to details of the model. Structural highs in the western Ross Sea and potentially more than half of the area under the Ross Ice Shelf and adjacent areas with the modern bed below sea level restore to subaerial lowlands in late Eocene. This study shows that paleotopography models with more regions above sea level should be considered in coupled climate-ice modeling. These models could result in more rapid or earlier accumulation of ice on West Antarctica.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V11F..08W
- Keywords:
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- 0726 Ice sheets;
- 8109 Continental tectonics: extensional (0905);
- 9310 Antarctica (4207)