HR: 09:45h
AN: PP51F-08 INVITED     [Abstracts]
TI: Recent sedimentation beneath the McMurdo Ice Shelf, Antarctica
AU: * Riesselman, C R
EM: criessel@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford Unviersity Braun Hall, Bldg. 320, Stanford, CA 94305-2115 United States
AU: Barrett, P J
EM: Peter.Barrett@vuw.ac.nz
AF: Antarctic Research Center, School of Earth Sciences, Victoria University of Wellington, Wellington, 6015 New Zealand
AU: Dunbar, R B
EM: dunbar@stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford Unviersity Braun Hall, Bldg. 320, Stanford, CA 94305-2115 United States
AU: Dunbar, G B
EM: gavin.dunbar@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University Building 61 Mills Road, Canberra, ACT 0200 Australia
AB: Accurate reconstructions of past ice shelf behavior are critical to understanding Antarctic's dynamics. While coastal and sub-ice shelf sedimentary records are best suited to address the question of stability, our recovery of such records has historically been limited by technological challenges. Here we report on a unique set of cores collected from the seabed beneath the McMurdo Ice Shelf (MIS), part of the Ross Ice Shelf, the world's largest such feature. In January and February 2003, 60-cm gravity cores were recovered from two sites in the Windless Bight region of the MIS in support of future ANDRILL drilling. Site 1 (77° 53.3'S, 167° 05.1'E) is 5 km east of the ice shelf edge, with an ice thickness of 70m and Site 2 (77° 50.1'S, 167° 20.2'E) is 12 km from the ice edge, with an ice thickness of 143 m; both were located along the axis of a 900 m deep depression surrounding Ross Island. Three AMS radiocarbon dates provide a chronology for each core. Though the absence of organic matter precluded the determination of a basal age for the Site 1 core, an age of 24,550 ¹⁴C yrs BP at 34 cm places this core within the LGM. The Site 2 core spans the late Pleistocene/Holocene transition, with a basal date of 12,800 ¹⁴C yrs BP. Analysis of bulk organic C and N isotopes and wt. % C, N, CaCO₃ and SiO₃ reveal significant differences between of the two sites. Surface sediments from Site 1 exhibit low carbonate concentrations. Wt. % CaCO₃ increases downcore by an order of magnitude to a maximum value of 3.4% at 10 cm, accompanied by increased numbers of calcareous foraminifera. Apart from the top of the core, CaCO₃, SiO₃, and bulk organic carbon concentrations covary at Site 1. CaCO₃ from Site 2 is consistently lower than 0.4%, however these sediments are rich in biogenic silica; some intervals exceed 25% SiO₃ with abundant diatom valves. Although CaCO₃ concentration is very low, there is an inverse relationship between CaCO₃ and SiO₃ contents downcore. The abundance of biosiliceous material at Site 2 and relative absence at Site 1 suggests that sedimentation of current-driven biodetritus under the ice shelf is extremely variable, and that heterogeneity of the seabed across short distances may be a common feature of sub-ice shelf deposits. The sedimentological and chemical differences from these two sites will be discussed in conjunction with water column observations, and reinforce the importance of drawing large-scale conclusions about Antarctic glacial processes from multiple sites.
DE: 0728 Ice shelves
DE: 4207 Arctic and Antarctic oceanography (9310, 9315)
DE: 4924 Geochemical tracers
DE: 9310 Antarctica (4207)
DE: 9605 Neogene
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005