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11.
Layers with high magnetic susceptibility contain heavy minerals that are the most likely to preserve signatures of impact in metamorphosed Precambrian rocks. The following impact indicators are resistant to alteration and metamorphism and are likely to be found in heavy mineral layers: impact diamond, SiC, shocked zircon, high-pressure TiO2 phases, reidite, and high-pressure chromite. Coesite and stishovite could also be present but are not likely to be preserved in rocks metamorphosed above the greenschist facies. Looking for layers with high magnetic susceptibility will also identify layers that might contain concentrations of zircon for dating. 相似文献
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Olivier Henry Michael Ablain Benoit Meyssignac Anny Cazenave Dallas Masters Steve Nerem Gilles Garric 《Journal of Geodesy》2014,88(4):351-361
Determining how the global mean sea level (GMSL) evolves with time is of primary importance to understand one of the main consequences of global warming and its potential impact on populations living near coasts or in low-lying islands. Five groups are routinely providing satellite altimetry-based estimates of the GMSL over the altimetry era (since late 1992). Because each group developed its own approach to compute the GMSL time series, this leads to some differences in the GMSL interannual variability and linear trend. While over the whole high-precision altimetry time span (1993–2012), good agreement is noticed for the computed GMSL linear trend (of $3.1\pm 0.4$ mm/year), on shorter time spans (e.g., ${<}10~\hbox {years}$ ), trend differences are significantly larger than the 0.4 mm/year uncertainty. Here we investigate the sources of the trend differences, focusing on the averaging methods used to generate the GMSL. For that purpose, we consider outputs from two different groups: the Colorado University (CU) and Archiving, Validation and Interpretation of Satellite Oceanographic Data (AVISO) because associated processing of each group is largely representative of all other groups. For this investigation, we use the high-resolution MERCATOR ocean circulation model with data assimilation (version Glorys2-v1) and compute synthetic sea surface height (SSH) data by interpolating the model grids at the time and location of “true” along-track satellite altimetry measurements, focusing on the Jason-1 operating period (i.e., 2002–2009). These synthetic SSH data are then treated as “real” altimetry measurements, allowing us to test the different averaging methods used by the two processing groups for computing the GMSL: (1) averaging along-track altimetry data (as done by CU) or (2) gridding the along-track data into $2^{\circ }\times 2^{\circ }$ meshes and then geographical averaging of the gridded data (as done by AVISO). We also investigate the effect of considering or not SSH data at shallow depths $({<}120~\hbox {m})$ as well as the editing procedure. We find that the main difference comes from the averaging method with significant differences depending on latitude. In the tropics, the $2^{\circ }\times 2^{\circ }$ gridding method used by AVISO overestimates by 11 % the GMSL trend. At high latitudes (above $60^{\circ }\hbox {N}/\hbox {S}$ ), both methods underestimate the GMSL trend. Our calculation shows that the CU method (along-track averaging) and AVISO gridding process underestimate the trend in high latitudes of the northern hemisphere by 0.9 and 1.2 mm/year, respectively. While we were able to attribute the AVISO trend overestimation in the tropics to grid cells with too few data, the cause of underestimation at high latitudes remains unclear and needs further investigation. 相似文献
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Observational estimate of climate sensitivity from changes in the rate of ocean heat uptake and comparison to CMIP5 models 总被引:1,自引:0,他引:1
Troy Masters 《Climate Dynamics》2014,42(7-8):2173-2181
Climate sensitivity is estimated based on 0–2,000 m ocean heat content and surface temperature observations from the second half of the 20th century and first decade of the 21st century, using a simple energy balance model and the change in the rate of ocean heat uptake to determine the radiative restoration strength over this time period. The relationship between this 30–50 year radiative restoration strength and longer term effective sensitivity is investigated using an ensemble of 32 model configurations from the Coupled Model Intercomparison Project phase 5 (CMIP5), suggesting a strong correlation between the two. The mean radiative restoration strength over this period for the CMIP5 members examined is 1.16 Wm?2K?1, compared to 2.05 Wm?2K?1 from the observations. This suggests that temperature in these CMIP5 models may be too sensitive to perturbations in radiative forcing, although this depends on the actual magnitude of the anthropogenic aerosol forcing in the modern period. The potential change in the radiative restoration strength over longer timescales is also considered, resulting in a likely (67 %) range of 1.5–2.9 K for equilibrium climate sensitivity, and a 90 % confidence interval of 1.2–5.1 K. 相似文献
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This paper is the continuation of a recent paper (written by the first author), in which an axiomatic zatrikean (i.e., chess-like) pre-geometry together with a rotationally induced zatrikean pre-geometry were developed. In the present paper, the number of alyssoids joining two geobits is determined for bothQ andR pre-metrics. Furthermore, the functional forms ofQ andR pre-geometric dilation factors are determined and calculated. It has been verified that these functions contain discontinuities and, therefore, a method for smoothing them out is proposed and applied. 相似文献
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Guy Masters 《Geophysical Journal International》1979,57(2):507-534
Summary. The observations of the periods of free oscillation of the Earth provide direct constraints on the density distribution in the Earth. These in turn allow constraints to be placed on the size of departures from a state of adiabaticity and chemical homogeneity. These departures are quantified in terms of a stratification parameter '8' first introduced as an index of chemical homogeneity. The resolving power theory of Backus & Gilbert is used to determine the ability of the observed free oscillations to constrain η in the lower mantle and outer core. The results suggest that the outer core is not strongly chemically stratified although a significantly thermally stable core cannot be excluded. The free oscillations also apparently require a compositional difference between the inner and outer cores. 相似文献
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