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P. Vlag N. Thouveny D. Williamson V. Andrieu M. Icole A. J. van Velzen 《Geophysical Journal International》1997,131(3):724-740
Rock magnetic properties of the maar lake sediments of Lac St Front (Massif Central, France) reflect environmental changes during the last climatic cycle. High magnetic concentrations are measured in the sediments deposited under glacial climatic conditions, while lower concentrations correspond with more temperate climatic periods. Low- and high-temperature measurements indicate that the remanence is carried by (titanium-poor) magnetite. However, some maghemite and haematite is present in sediments deposited under temperate conditions.
Normalized intensities and coercivities of the anhysteretic remanent magnetization (ARM) are clearly higher for the sediments deposited during the temperate climatic periods of the Eemian, St Germain I, II and Mid-glacial than for glacial sediments, but other magnetic parameters hardly differ between these groups. Due to slight differences in magnetic composition and possible effects of grain interactions, it is not straightforward to relate this different ARM behaviour to magnetic grain-size variations. For the Holocene sediments, rock magnetic parameters indicate a larger grain size. This trend is also suggested by granulometric experiments with an optical laser granulometer. Dissolution of smaller grains is the most likely explanation for this larger grain size.
Changes in magnetic composition and grain size are extremely limited for the glacial sediments, but magnetic concentration varies considerably. Magnetic concentration maxima in the glacial sediments of Lac St Front correlate with those of the nearby Lac du Bouchet (Thouveny et al. 1994). Correlating the susceptibility records of these sequences with the δ18 O record of the GRIP ice cores (Thouveny et al. 1994) suggests that magnetic concentration maxima may correspond with short cold climatic episodes, associated with Heinrich events. 相似文献
Normalized intensities and coercivities of the anhysteretic remanent magnetization (ARM) are clearly higher for the sediments deposited during the temperate climatic periods of the Eemian, St Germain I, II and Mid-glacial than for glacial sediments, but other magnetic parameters hardly differ between these groups. Due to slight differences in magnetic composition and possible effects of grain interactions, it is not straightforward to relate this different ARM behaviour to magnetic grain-size variations. For the Holocene sediments, rock magnetic parameters indicate a larger grain size. This trend is also suggested by granulometric experiments with an optical laser granulometer. Dissolution of smaller grains is the most likely explanation for this larger grain size.
Changes in magnetic composition and grain size are extremely limited for the glacial sediments, but magnetic concentration varies considerably. Magnetic concentration maxima in the glacial sediments of Lac St Front correlate with those of the nearby Lac du Bouchet (Thouveny et al. 1994). Correlating the susceptibility records of these sequences with the δ
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Low-Temperature Oxidation of Magnetite in Loess-Paleosol Sequences: a Correction of Rock Magnetic Parameters 总被引：4，自引：0，他引：4
Low-temperature oxidation under atmospheric conditions affects the magnetic properties of magnetite in natural rocks: the coercivities of magnetite grains increase and other parameters change accordingly. It was recently shown that heating to 150°C largely removes the effects of low-temperature oxidation (van Velzen and Zijderveld, 1995). Heating may therefore serve as a detection tool for the presence of the effect of low-temperature oxidation.
In the present study, a collection of loess and paleosol samples from various loess regions of the world is examined for the influence of low-temperature oxidation. In all samples of the collection a decrease of coercivities was found after heating to 150°C. Generally loess samples were affected to a larger extent than paleosol samples. The original range of remanent coercivities(B
cr)of 21-58 mT changed to 20-42 mT after heating. The IRM capacity of the samples decreased from 0 up to 25%. ARM showed changes between a decrease of 10% and an increase of 15%. The grain-size indicative parameter IRM/ARM is considerably influenced by the heating and therefore by low-temperature oxidation. The changes in susceptibility are limited and will not influence the interpretation of large-scale features of the susceptibility record as a paleoclimate proxy. Small variations, however, may be obscured by the varying influence of oxidation in the outcrop, which can significantly modify the rock-magnetic record.
Rock-magnetic parameters used to determine magnetic mineral content and grain sizes should be corrected for the effect of low-temperature oxidation. To this end heating to 150°C is recommended. The occurrence of the changes is in itself already an indication for the presence of magnetite. Low-temperature oxidation will not only be due to recent weathering in the outcrop, but also to earlier oxidation processes in the source area, during transport and deposition of the loess and during pedogenesis. Truly fresh sediment samples are only influenced by this earlier oxidation. In that case heating will reveal the degree of ancient low-temperature oxidation, which may be related to climate at the time of deposition and pedogenesis. 相似文献
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Rock magnetic parameters are often used to recognize variations in the original magnetic mineralogy and for normalizing purposes in palaeointensity studies. Incipient weathering, however, is shown to have a profound but partly reversible influence on the rock magnetic properties of the marls of the Early Pliocene Trubi formation in southern Sicily (Italy). The remanence in the marls resides in single-domain (SD) magnetite grains, but the remanent coercive force (Hcr ) shows a strong variation and most values observed are anomalously high ( Hcr ) range 36–188 mT).
The enhanced coercivities are attributed to stress in the magnetite grains induced by surface oxidation at low temperature. Upon heating to 150 °C a reduction of coercivities occurs that can be explained by a stress reduction as a result of a reduction of Fe2- gradient due to a higher diffusion rate at elevated temperature. After heating to 150 °C, coercivities are quite uniform throughout the outcrop and the values are characteristic of SD magnetite (Hcr range 30–38 mT). The bulk susceptibility increases by 4–24 per cent, and the isothermal remanent magnetization (IRM) decreases by 5–11 per cent. The increase in anhysteretic remanent magnetization (ARM) is large: 20–242 per cent. The magnitude of the changes is related to the degree of weathering.
Another effect of heating the marl samples to 150 °C is a substantial reduction of the coercivities of the secondary overprint in the natural remanent magnetization. After heating. separation of the secondary and primary components by alternating-field demagnetization is more efficient. The usual difficulties of thermal demagnetization above 300 °C may thus be avoided by a combination of moderate heating to 150 °C and subsequent alternating-field demagnetization. 相似文献
The enhanced coercivities are attributed to stress in the magnetite grains induced by surface oxidation at low temperature. Upon heating to 150 °C a reduction of coercivities occurs that can be explained by a stress reduction as a result of a reduction of Fe
Another effect of heating the marl samples to 150 °C is a substantial reduction of the coercivities of the secondary overprint in the natural remanent magnetization. After heating. separation of the secondary and primary components by alternating-field demagnetization is more efficient. The usual difficulties of thermal demagnetization above 300 °C may thus be avoided by a combination of moderate heating to 150 °C and subsequent alternating-field demagnetization. 相似文献
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Data assimilation methods provide a means to handle the modeling errors and uncertainties in sophisticated ocean models. In this study, we have created an OpenDA-NEMO framework unlocking the data assimilation tools available in OpenDA for use with NEMO models. This includes data assimilation methods, automatic parallelization, and a recently implemented automatic localization algorithm that removes spurious correlations in the model based on uncertainties in the computed Kalman gain matrix. We have set up a twin experiment where we assimilate sea surface height (SSH) satellite measurements. From the experiments, we can conclude that the OpenDA-NEMO framework performs as expected and that the automatic localization significantly improves the performance of the data assimilation algorithm by successfully removing spurious correlations. Based on these results, it looks promising to extend the framework with new kinds of observations and work on improving the computational speed of the automatic localization technique such that it becomes feasible to include large number of observations. 相似文献
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