Climatic interpretation of the recently extended Vostok ice records

A new ice core drilled at the Russian station of Vostok in Antarctica reached 2755 m depth in September 1993. At this depth, the glaciological time scale provides an age of 260 ky BP (±25). We refine this estimate using records of dust and deuterium in the ice and of ¹⁸O of O₂ in the entrapped air. ¹⁸O of O₂ is highly correlated with insolation over the last two climatic cycles if one assumes that the EGT chronology overestimates the increase of age with depth by 12% for ages older than 112 ky BP. This modified age-depth scale gives an age of 244 ky BP at 2755 m depth and agrees well with the age-depth scale of Walbroeck et al. (in press) derived by orbital tuning of the Vostok D record. We discuss the temperature interpretation of this latter record accounting for the influence of the origin of the ice and using information derived from deuterium-excess data. We conclude that the warmest period of stage 7 was likely as warm as today in Antarctica. A remarkable feature of the Vostok record is the high level of similarity of proxy temperature records for the last two climatic cycles (stages 6 and 7 versus stages 1–5). This similarity has no equivalent in other paleorecords.     

Response to the eruption of Mount Pinatubo in relation to climate sensitivity in the CMIP3 models

The radiative flux perturbations and subsequent temperature responses in relation to the eruption of Mount Pinatubo in 1991 are studied in the ten general circulation models incorporated in the Coupled Model Intercomparison Project, phase 3 (CMIP3), that include a parameterization of volcanic aerosol. Models and observations show decreases in global mean temperature of up to 0.5 K, in response to radiative perturbations of up to 10 W m⁻², averaged over the tropics. The time scale representing the delay between radiative perturbation and temperature response is determined by the slow ocean response, and is estimated to be centered around 4 months in the models. Although the magniude of the temperature response to a volcanic eruption has previously been used as an indicator of equilibrium climate sensitivity in models, we find these two quantities to be only weakly correlated. This may partly be due to the fact that the size of the volcano-induced radiative perturbation varies among the models. It is found that the magnitude of the modelled radiative perturbation increases with decreasing climate sensitivity, with the exception of one outlying model. Therefore, we scale the temperature perturbation by the radiative perturbation in each model, and use the ratio between the integrated temperature perturbation and the integrated radiative perturbation as a measure of sensitivity to volcanic forcing. This ratio is found to be well correlated with the model climate sensitivity, more sensitive models having a larger ratio. Further, if this correspondence between “volcanic sensitivity” and sensitivity to CO₂ forcing is a feature not only among the models, but also of the real climate system, the alleged linear relation can be used to estimate the real climate sensitivity. The observational value of the ratio signifying volcanic sensitivity is hereby estimated to correspond to an equilibrium climate sensitivity, i.e. equilibrium temperature increase due to a doubling of the CO₂ concentration, between 1.7 and 4.1 K. Several sources of uncertainty reside in the method applied, and it is pointed out that additional model output, related to ocean heat storage and radiative forcing, could refine the analysis, as could reduced uncertainty in the observational record, of temperature as well as forcing.     

Does use of tropical beaches by tourists and island residents result in damage to fringing coral reefs? A case study in Moorea French Polynesia

Although coral reefs worldwide are subject to increasing global threats, humans also impact coral reefs directly through localized activities such as snorkeling, kayaking and fishing. We investigated five sites on the northern shore of Moorea, French Polynesia, and quantified the number of visitors on the beach and in shallow water. In field surveys, we measured total coral cover and colony sizes of two common genera, Porites and Acropora, a massive and branching morphology, respectively. One site, which hosted over an order of magnitude more people than the other four, had significantly less total coral cover and supported very little branching Acropora. In addition, size frequency distributions of both the branching and massive genera were skewed toward smaller colony sizes at the high use site. Our results demonstrated that the use of tropical beaches may result in less coral cover, with branching colonies rare and small.     

Precambrian basement around Wadi Halfa,Sudan: a new perspective on the evolution of the East Saharan Craton

This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. There is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African ( 500–900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and whole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northernmost Sudan. These results indicate the presence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisses. Several episodes of crust formation, inferred from zircon ages, are preserved in the gneisses : 2.6, 2.4, 2.0, 1.7, 1.2 and 0.72 Ga. Nd model ages for the same units are invariably older than the zircon ages, yet still record a predominantly late Archaean and Palaeoproterozoic history, with depleted mantle model ages between 1.3 and 2.8 Ga. The earliest recorded Pan-African magmatic event is about 720 Ma and dates the beginning of collisional deformation. A younger Pan-African volcanic sequence ( 650 Ma) has isotopic compositions of Sr and Nd compatible with derivation from late Prote rozoic asthenospheric mantle. A 530 Ma anorogenic A-type granite also has isotopic compositions suggesting derivation from a primitive source. The inferred tectonic evolution began with rifting to form an oceanic re-entrant. This was followed by subduction leading to collision at about 700 Ma, accompanied by post-orogenic rifting at about 650 Ma.     

阿尔金索尔库里地区元古代流纹岩锆石SHRIMP U-Pb定年及其地质意义

阿尔金索尔库里北山彩虹沟塔什达坂群卓阿布拉克组流纹岩锆石SHRIMP U-Pb 年龄为920±20Ma (MSWD=1.50),代表流纹岩的喷出时间。岩石的SiO₂介于68.33%~70.60%,K₂O>Na₂O,K₂O/Na₂O平均值6.42,属弱过铝质(ACNK平均为1.25)钙碱性系列(σ平均为2.76),为一套典型的壳源流纹岩岩石系列。稀土元素含量高(平均为211.3×10^-6),轻稀土富集(L/H平均为6.57),Eu亏损中等(δEu平均为0.30)。微量元素以富集Rb、Th、K、Zr,而相对亏损Ba、Sr、Eu、Ti、Nb、Ta为特征。岩石微量及稀土元素具有典型的板内火山岩特征,而岩石显著的低Sr特征(25.0×10^-6~65.3×10^-6,平均值为36.8×10^-6))表明其并非源自加厚的地壳,而是起源于斜长石稳定的正常地壳。阿尔金流纹岩是由进入上地壳高位岩浆房的拉斑玄武岩浆的底侵作用直接使上地壳部分熔融,形成酸性火山岩的原始岩浆。该岩浆体系沿区域断裂构造体系上升,并经历了较强的结晶分异和演化,最终形成阿尔金索尔库里北山流纹岩系列。     

Modelling of solar quiet magnetic field variations near a conductivity anomaly

Summary. A simplified model of the solar quiet-time ionospheric current system is used to calculate the induced currents in a model earth. The conductivity is assumed to be constant below a depth of about 400 km and zero above that depth. The current induced in the north—south conductivity anomaly under the Rocky Mountains is then estimated from the time-varying potential difference between points at 30 and 45° latitude at the surface of the conducting sphere. The purpose of these calculations is to investigate whether variations in the latitude of the northern hemisphere current system vortex will substantially alter the relationship between the observed magnetic field components at the Earth's surface and the local magnetic field gradient caused by the conductivity anomaly. We find that a 10° shift in the latitude of the ionospheric current focus causes a change of 6 per cent or less in the transfer function from the field components to the gradient in the total field. Thus such latitude shifts cannot explain much of the magnetic field gradient variation at periods near 24 hr that has been observed near Boulder, Colorado.     

Provenance and pathways of late Quaternary turbidites in the deep-water Agadir Basin,northwest African margin

A series of individual turbidites, correlated over distances >100 km, are present in the recent fill of the Agadir Basin, offshore northwest Africa. The aim here is to unravel multiple turbidite source areas and flow pathways, and show how turbidite provenance studies contribute to interpretation of flow processes. Agadir Basin turbidites are sourced from four main areas, with the majority originating from the siliciclastic Morocco Shelf; their sand-mud distribution is strongly controlled by flow sediment volume, with relatively low-volume flows dying out within the Agadir Basin and large-volume flows bypassing significant sediment volumes to basins further downslope. Two large-volume volcaniclastic turbidites are attributed to a Canary Islands landslide source, while several small mud-dominated turbidites are interpreted to be locally sourced from hemipelagic-draped seamounts (e.g. Turbidite AB10). Finally, Turbidite AB1 (∼1 ka) is only present in the western Agadir Basin, and is linked to recent “re-activation” of the Sahara Slide headwall. The muddy suspension clouds of three large-volume flows, all linked to large-scale landslides, have covered huge areas of seafloor and flowed along or even slightly upslope for long distances. It is proposed that northeastwards-flowing bottom currents have aided transport of these dilute flow fractions into and across the Agadir Basin.