Collaborative Research: Magnetic Properties of Greenland and Antarctic Ice Cores

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The Principal Investigators have successfully measured the concentration of ultra-fine magnetic particles in ice samples from different climatic intervals from the NorthGRIP core, Greenland, using low temperature (77K) isothermal remnant magnetization (IRM) analysis and compared it with the mass concentration of aerosol dust that varies by a factor of ten. The mean IRM intensity of the ice varies by a factor of three from glacial to interglacial intervals, being lower during interglacials. The magnetic measurements are reproducible and well above the IRM intensity of ultra-pure water ice made under clean lab conditions. The IRM acquisition curves of the ice samples are compatible with a mixture of magnetite and subsidiary hematite contributions, magnetic properties that are typical of pristine loess from the Chinese loess plateau, which is considered to have the same source in the eastern Asian deserts as dust in Greenland ice. Comparison of the IRM intensity and total dust mass of the ice shows a remarkably good correlation, but also reveals a large uncorrelated magnetization, which is essentially constant over the different climatic stages and is the dominant component during intervals with very low measured dust concentrations. The uncorrelated magnetization may be due to either a constant flux of highly magnetic interplanetary dust particles (IDPs) or the more general presence of ultrafine grain aerosol dust mass that were largely undetected in the Coulter Counter measurements. The latter would imply that the actual dust concentration contrast between glacial and interglacial intervals is much smaller than estimated. These alternative hypotheses can be evaluated by obtaining comparative data from ice cores from Antarctica, where the aerosol dust flux is radically different (concentration, source) than in Greenland whereas the flux of IDPs should be about the same. The Principal Investigators will measure the magnetization of a suite of samples representing the full range of glacial and interglacial dust concentrations from the Vostok ice core. IRM acquisition curves will be generated for about 10 samples from each of 12 distinct climatic intervals using a new pulse magnetizer capable of generating fields to 2.5 T on large (~50 cc) samples to achieve saturation and high signal level. Dust concentrations will be measured on the same samples using a Coulter Counter. Additional sets of samples from selected intervals with intermediate dust concentrations will be analyzed from North GRIP to solidify the magnetization vs. dust concentration curve from that region. SEM-EDXRF analyses will be done to characterize the dust. A comparison of the magnetization vs. dust concentration relationships between NorthGRIP and Vostok will allow them to make quantitative estimates of the contributions to the uncorrelated magnetization from the flux of extraterrestrial particles and possibly, the undetected aerosol dust mass. The outcome of these novel experiments is expected to have broad significance to our understanding of the role of dust in long-term climate change.