The physical properties record of the AND-1B sediment core A quantitative approach to describing past ice shelf/ice sheet dynamics


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gerhard.kuhn [ at ] awi.de

Abstract

In austral summer 2006/07 a more than 1200 m long sediment core was drilled beneath McMurdo Ice Shelf near Ross Island (Antarctica) with the purpose of contributing to a better understanding of the Late Cenozoic history of the Antarctic Ice Sheet (ANDRILL-MIS Project). Dating back to about 13 Ma this core offers the great potential to study the long-term global cooling trend at an ice-proximal location. High-resolution multi-sensor core logger measurements of whole-core physical properties serve as a numerical expression of lithologic changes in the core and therefore, represent a quantitative tool for approaching past ice dynamics. This is especially applicable for the repeating sequences of diatomites and diamtictites in the upper half of the core with a prominent cyclicity between 140-300 mbsf. Rather abrupt high-amplitude variations in wet-bulk density (WBD) and magnetic susceptibility (MS) reflect rapid changes between two main end-member facies generated by the alternation between a grounded ice sheet and open marine conditions. For the whole core, the WBD signal, ranging from 1.4 kg/cu.m in the diatomites to 2.3 kg/cu.m in diamitctites from the lower part of the core, represents the influence of three variables: (i) the degree of compaction seen as reduced porosities with depth from about 55 % in the top part to about 25 % at the bottom, (ii) the clast content with clasts being almost absent in diatomite deposits and (iii) the individual grain density (GD). GD itself ranges from about 2.15 kg/cu.m in diatomites to 2.9 kg/cu.m for volcanic sandstones and thereby reflects the variety of lithologies as well as the influence of cement (mainly pyrite and carbonate) on the matrix grain density. The calculation of residual porosities demonstrates the strong imprint of glacial loading for especially diamictites from the upper 150 m. This over-consolidation signature preserved in the sediments can be used to characterize the Pleistocene ice sheet in terms of ice thickness derived from past stresses. MS on the other hand mainly documents a marine vs. terrestrial source of sediments where the latter can be divided into younger local material from the McMurdo Volcanic Province and basement clasts from the Transantarctic Mountains. Values range over several orders of magnitude from <10 (10-5 SI) in the diatomites to 8000 (10-5 SI) in single clasts (mainly dolerite). Synchronous minima and maxima in both WBD and MS support dramatic changes in the depositional environment, driven by oscillations in ice extent in response to climate fluctuations. According to the age model, cyclicity occurs on Milankovich timescales with changes in obliquity being the dominant forcing until the Early Pleistocene. Additionally, small-amplitude variations within diatomite units propose sub-Milankovich forcing as superimposed control on system dynamics.



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Conference (Talk)
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Not peer-reviewed
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Published
Event Details
33rd International Geological Congress, August 6-14, 2008, Oslo, Norway...
Eprint ID
19451
Cite as
Magens, D. , Niessen, F. , Kuhn, G. and Helling, D. (2008): The physical properties record of the AND-1B sediment core A quantitative approach to describing past ice shelf/ice sheet dynamics , 33rd International Geological Congress, August 6-14, 2008, Oslo, Norway.. .


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