Improved magnetic anomalies of the Antarctic lithosphere from satellite and near-surface data

The Antarctic magnetic anomaly map compiled marine and airborne surveys collected south of 60°S through 1999 and used Magsat data to help fill in the regional gaps between the surveys. Ørsted and CHAMP satellite magnetic observations with greatly improved measurement accuracies and temporal and spatial coverage of the Antarctic, have now supplanted the Magsat data. We combined the new satellite observations with the near-surface survey data for an improved magnetic anomaly map of the Antarctic lithosphere. Specifically, we separated the crustal from the core and external field components in the satellite data using crustal thickness variations estimated from the terrain and the satellite-derived free-air gravity observations. Regional gaps in the near-surface surveys were then filled with predictions from crustal magnetization models that jointly satisfied the near-surface and satellite crustal anomalies. Comparisons in some of the regional gaps that also considered newly acquired aeromagnetic data demonstrated the enhanced anomaly estimation capabilities of the predictions over those from conventional minimum curvature and spherical harmonic geomagnetic field models. We also noted that the growing number of regional and world magnetic survey compilations involve coverage gaps where these procedures can contribute effective near-surface crustal anomaly estimates.     

On the construction of geomagnetic timescales from non-prejudicial treatment of magnetic anomaly data from multiple ridges

When marine magnetic-anomaly data are used to construct geomagnetic polarity timescales, the usual assumption of a smooth spreading-rate function at one seafloor spreading ridge forces much more erratic rate functions at other ridges. To eliminate this problem, we propose a formalism for the timescale problem that penalizes non-smooth spreading behaviour equally for all ridges. Specifically, we establish a non-linear Lagrange multiplier optimization problem for finding the timescale that (1) agrees with known chron ages and with anomaly-interval distance data from multiple ridges and (2) allows the rate functions for each ridge to be as nearly constant as possible, according to a cumulative penalty function. The method is applied to a synthetic data set reconstructed from the timescale and rate functions for seven ridges, derived by Cande & Kent (1992) under the assumption of smooth spreading in the South Atlantic. We find that only modest changes in the timescale (less than 5 per cent for each reversal) are needed if no one ridge is singled out for the preferential assumption of smoothness. Future implementation of this non-prejudicial treatment of spreading-rate data from multiple ridges to large anomaly-distance data sets should lead to the next incremental improvement to the pre-Quaternary geomagnetic polarity timescale, as well as allow a more accurate assessment of global and local changes in seafloor spreading rates over time.     

Periodicity of magnetic intensities in magnetic anomaly profiles: the Cenozoic of the South Atlantic

Spectral analyses of several published magnetic anomaly profiles from Candé & Kent (1992a) were undertaken prior to analysing, in the same way, raw magnetic anomaly data from similar parts of the South Atlantic. It was found that similar and distinct medium and short wavelengths were present in both the published and raw data. When these are converted into the time domain using the average rate of spreading for each profile, these periodicities appear similar, possibly identical, to those expected from the long-term eccentricity orbital parameters (Fischer, DeBoer & Premoli Silva 1990). While such correlations are not necessarily causative, they suggest that magnetohydro-dynamical processes near the core-mantle boundary may be affected by gravitational changes due to planetary orbital perturbations.     

On the roughness of Mesozoic oceanic crust in the western North Atlantic

Seismic reflection profiles from Mesozoic oceanic crust around the Blake Spur Fracture Zone (BSFZ) in the western North Atlantic have been widely used in constraining tectonic models of slow-spreading mid-ocean ridges. These profiles have anomalously low basement relief compared to crust formed more recently at the Mid-Atlantic Ridge at the same spreading rate. Profiles from other regions of Mesozoic oceanic crust also have greater relief. The anomalous basement relief and slightly increased crustal thickness in the BSFZ survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation. If so, the intracrustal structures observed may be representative of an atypical tectonic regime.     

Periodicity of magnetic intensities in magnetic anomaly profiles: the Cretaceous of the Central Pacific

Spectral analyses of long magnetic anomaly profiles of Cretaceous age in the Phoenix and Hawaiian lineations within the Pacific Basin show two to five non-harmonic wavelength peaks, with probabilities >90 per cent, within the range of several tens to hundreds of kilometres for each profile. For each profile with four peak wavelengths, their ratios correlate strongly ( r > 0.98) with the ratios for the Earth's orbital eccentricity periodicities for this time, and hence the profile peaks necessarily correlate with each other. These observations suggest that the magnetic anomaly signal is influenced by the Earth's changing orbital parameters, probably through their influence on the internal geomagnetic field. This implies that assumptions usually made to convert the magnetic signal to the magnetization of the oceanic crust may be suspect. The observations, if confirmed, indicate that orbital periodicities can assist in improving the age calibration of the global polarity timescale and that the predicted orbital behaviour of the Earth is confirmed for the last 150 Ma.     

A climatological study of the effect of sea-surface temperature on North Atlantic hurricane intensification

The climatic influence of sea-surface temperature (SST) on intensification is examined for North Atlantic hurricanes by averaging hourly intensity increases from best-track data over the period 1986–2013 in 4° by 4° latitude–longitude grid cells. Independent monthly SST data over the same period are averaged in the same cells. After removing cells with cold water or fast moving hurricanes, the SST effect on intensification, at the climate scale, is quantified by regressing intensification onto SST while controlling for average intensity. The regression is performed using a generalized linear model from a gamma family and a logarithmic link function. The model shows a statistically significant relationship, with higher intensification values associated with higher SST values. On average, mean intensification increases by 16% [(9,?20)% uncertainty interval] for every 1 °C increase in mean SST. A clustered region where the model underpredicts intensification is noted over the southeastern Caribbean Sea, perhaps related to the fresh water plume from the Orinoco River.     

A Maastrichtian palaeomagnetic pole for the Pacific plate from a skewness analysis of marine magnetic anomaly 32

The asymmetry (skewness) of marine magnetic anomaly 32 (72.1–73.3  Ma) on the Pacific plate has been analysed in order to estimate a new palaeomagnetic pole. Apparent effective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire Pacific plate and spanning a great-circle distance of ~12  000  km. The data were inverted to obtain a palaeomagnetic pole at 72.1°N, 26.8°E with a 95 per cent confidence ellipse having a 4.0° major semi-axis oriented 98° clockwise of north and a 1.8° minor semi-axis; the anomalous skewness is 14.2° ± 3.7°. The possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible effect on our palaeopole analysis over the range of relevant spreading half-rates, ~25 to ~90  mm  yr⁻¹ . The new pole is consistent with the northward motion for the Pacific plate indicated by coeval palaeocolatitude and palaeoequatorial data, but differs significantly from, and lies to the northeast of, coeval seamount poles. We attribute the difference to unmodelled errors in the seamount poles, mainly in the declinations. Comparison with the northward motion inferred from dated volcanoes along the Hawaiian–Emperor seamount chain indicates 13° of southward motion of the Hawaiian hotspot since 73  Ma. When the pole is reconstructed with the Pacific plate relative to the Pacific hotspots, it differs by 14°–18° from the position of the pole relative to the Indo–Atlantic hotspots. This has several possible explanations including bias in one or more of the palaeomagnetic poles, motion between the Pacific and Indo–Atlantic hotspots, and errors in plate reconstructions relative to the hotspots.     

北疆西部近50 a来气候、水文变化趋势及其与北大西洋/北极涛动的关系

 根据北疆西部11个气象站点1957—2007气象资料,主要代表性河流1960—2006年流量数据以及1957—2007年NAO和AO时间序列数据,采用趋势分析,Mann-Kendall检验以及相关分析等方法,研究了北疆西部地区近50 a来气候、水文的变化趋势,分析了气候变化对径流的影响,初步探讨了NAO和AO对区域气候变化的影响。研究结果表明,近50 a来,北疆西部地区增温、增湿显著,但二者变化幅度存在季节差异。受气候变化的影响,奎屯河年径流量表现出显著增加趋势。进一步分析发现,奎屯河、四棵树河年径流量的变化主要受温度因子控制,卡琅古尔河、精河和博尔塔拉河的年径流量则主要受降水因子控制。研究表明,艾比湖水域面积同样受气候变化的影响,尤其是与年降水量增加紧密相关,二者相关系数为0.584。相关分析证实NAO和AO确实对北疆西部地区的气候变化具有显著影响。具体体现在,NAO和AO显著的影响研究区冬季温度和全年的温度变化;NAO对整个区域年降水具有显著影响。温度比降水对NAO和AO的响应更敏感。