Evaluation of Duluth Complex anorthositic series (AS3) zircon as a U-Pb geochronological standard: new high-precision isotope dilution thermal ionization mass spectrometry results

U-Pb zircon geochronology is increasingly called upon to achieve the resolution of absolute time at the 0.1% to 1% level for rocks of Phanerozoic to Hadean age. Doing so requires accurate calibration of the several methods (conventional isotope dilution thermal ionization mass spectrometry [ID-TIMS], Pb evaporation, high-resolution ion microprobe [e.g. SHRIMP], and laser ablation inductively coupled plasma mass spectrometry [LA-ICPMS]) currently in use, in numerous laboratories, for the analysis of U and Pb isotopes in accessory minerals. Toward this end, the geochronological community would benefit from the establishment, distribution and widespread analysis of one or more standard reference materials. Among the candidates is natural zircon from the Duluth Complex anorthositic series of the Midcontinent Rift system of North America. These zircons, first dated by conventional ID-TIMS at 1099.1 ± 0.5 Ma, have been subsequently adopted as a geochronological standard by a number of high resolution ion microprobe facilities. A new and independent analysis of the systematics of a large set of single zircons (n = 27) from the same mineral separate yields indistinguishable ²⁰⁷Pb/²⁰⁶Pb, upper intercept, and U-Pb concordia dates for the AS3 zircons. The concordia date, based on a subset of 12 concordant and equivalent zircons, of 1099.1 ± 0.2 Ma (±1.2 Ma considering systematic uncertainties in Pb/U tracer calibration and U decay constants) is indistinguishable from previously published results. We further document the absence of inherited Pb in the AS3 zircons, and discuss strategies for avoiding certain domains within the AS3 zircons exhibiting small amounts of radiation-induced, surface and fracture-correlated, recent Pb loss. Although the AS3 zircons do not represent the ideal (and elusive) homogeneous closed U-Pb system, we conclude that these and similar zircons from the Duluth Complex anorthositic series can provide a suitable geochronological reference standard for numerous U-Pb zircon analytical methods, given appropriate preparation guided by the results of this study. Our high-precision data set also serves as a useful confirmatory test of the currently accepted U decay constants.     

Earthquake swarms of Mt Cameroon, West Africa

Historical and recent instrumental studies of the central region of the Cameroon Volcanic Line clearly indicate the occurrence of earthquake swarms of volcanic origin. Analyses of more than 3000 micro-earthquakes recorded between 1985 and 1992 show a well-defined seismic pattern characterised by single and swarm events with duration magnitudes between 2 and 3 at depths down to 20 km. On average, the earthquakes here occur at the rate of about 2 events every 3 days with occasional earthquake swarms, which greatly increase this number. The seismic swarms comprising felt earthquakes are shown to be sometimes preceded by, simultaneous with, or followed by swarms from Bimbia and Equatorial Guinea. Mapped epicentres of some of these swarms correspond to regions of volcanic gas emissions and are parallel to the fissures on Mt Cameroon and to the inferred direction of the underlying shear zone.The quiescent periods between swarms are seen to double each year since 1986. This observation was used to predict a major seismic swarm which occurred in 1993. The data coupled with historical data, are used to infer the involvement of a magma chamber in the generation of the earthquake swarms in the region. Since the installation of the network, no eruption has been observed on Mt Cameroon. This warrants more observation in order to study the seismicity that may precede, accompany or follow an eruption of the mountain.     

Seismic stratigraphy and paleogeographic evolution of Fairway Basin,Northern Zealandia,Southwest Pacific: from Cretaceous Gondwana breakup to Cenozoic Tonga–Kermadec subduction

We present the first comprehensive seismic‐stratigraphic analysis of Fairway Basin, which is situated on the rifted continent of Zealandia in the Tasman Sea, southwest Pacific, between Australia and New Caledonia. The basin is 700 km long, 150 km wide, and has water depths of 500–3000 m. We describe depositional architecture and paleogeographic evolution of this basin. Basin formation was concurrent with two tectonic events: (i) Cretaceous rifting during eastern Gondwana breakup and (ii) initiation and Cenozoic evolution of Tonga–Kermadec subduction system to the east of the basin. To interpret the basin history we compiled and interpreted 2D seismic‐reflection profiles and make correlations with DSDP boreholes and the geology of New Caledonia. Five seismic‐stratigraphic units were defined. The deepest and oldest unit, FW3, folded and faulted can be correlated with volcaniclastic sediments and magmatic rocks in New Caledonia that are associated with Mesozoic Gondwana margin subduction. Alternatively, given the basin location 200–300 km west of New Caledonia and inboard of the ancient plate boundary, the unit could have formed as Gondwana intra‐continental basin with no known correlative. The overlying unit FW2b records syn‐rift deposition, probably associated with Cretaceous Gondwana breakup. Subaerial erosion supplied terrigenous sediment into the deltas in the northern part of the basin, as suggested by the truncation surfaces on the basement highs and sigmoid reflector geometries within unit FW2b respectively. Above, unit FW2a records post‐rift sedimentation and passive subsidence as the Tasman Sea opened and the Fairway Basin drifted away from Australia. Subsidence led to the flooding of the basement highs and burial of wave‐cut surfaces. Eocene compressive deformation resulted in minor folding and tilting within the Fairway Basin and was associated with the formation of many diapiric structures. The top of unit FW2 is an extensive unconformity that is associated with erosion and truncation on surrounding ridges. Above this unconformity, unit FW1b is interpreted as a turbidite system sourced from topography created during the Eocene tectonic event, which we interpret as being related to Tonga–Kermadec subduction initiation. Pelagic carbonate sedimentation is now prevalent. Unit FW1a has progressively draped the basin during Oligocene to Pleistocene subsidence. Many small volcanic cones were erupted during this final phase of subsidence, either as a delayed consequence of subduction initiation, or related to Tasmantid and Lord Howe hotspot trails. The northern Fairway Ridge remains close to sea level and its reef system continues to supply carbonate detrital sediments into the basin, most likely during sea‐level lowstands. Fairway Basin contains a nearly continuous record of tectonic and paleoclimatic events in the southwest Pacific since Cretaceous time. Its paleogeographic history is a key piece in the puzzle for understanding patterns of regional biodiversity in the southwest Pacific.     

Queens as Mothers: the role of the traditional safety net of care and support for HIV/AIDS orphans and vulnerable children in Ghana

The paper examines the significance of the traditional safety net to provide security and help to orphans and vulnerable children (OVC). (Abbreviation OVC, as conceptualised by UNICEF (2001)). It questions the scenario of societal breakdown due to HIV/AIDS by asking how the traditional safety net operates, nature of solutions to the OVC problem it provides and its sustainability. No geographical region in Ghana has been so affected by the HIV/AIDS pandemic than the Manya Krobo District in Ghana. While analysing the role of the work of the Manya Krobo Queen Mothers’ Association (QMA) and five private caregivers, the perceptions of three OVC are presented: on how it is to be an OVC, what their dreams and hopes are for the future, and their awareness of HIV/AIDS. It is observed that even though the QMA serves as a good role model for how to address the OVC problem, it has its limitations. The traditional extended family system is under pressure, but not about to break down. As the effectiveness of its support will depend on the contribution of the wider Ghanaian society, localised solutions have to be scaled up to make an impact. Thus, the breakdown scenario needs to be nuanced and contextualised. The vulnerability of the local place under study relates to unemployment and migration, economic poverty and social–cultural marginalisation. These factors are intertwined with the HIV/AIDS epidemic.     

Estimation of sampling error uncertainties in observed surface air temperature change in China

This study examines the sampling error uncertainties in the monthly surface air temperature (SAT) change in China over recent decades, focusing on the uncertainties of gridded data, national averages, and linear trends. Results indicate that large sampling error variances appear at the station-sparse area of northern and western China with the maximum value exceeding 2.0 K² while small sampling error variances are found at the station-dense area of southern and eastern China with most grid values being less than 0.05 K². In general, the negative temperature existed in each month prior to the 1980s, and a warming in temperature began thereafter, which accelerated in the early and mid-1990s. The increasing trend in the SAT series was observed for each month of the year with the largest temperature increase and highest uncertainty of 0.51 ± 0.29 K (10 year)⁻¹ occurring in February and the weakest trend and smallest uncertainty of 0.13 ± 0.07 K (10 year)⁻¹ in August. The sampling error uncertainties in the national average annual mean SAT series are not sufficiently large to alter the conclusion of the persistent warming in China. In addition, the sampling error uncertainties in the SAT series show a clear variation compared with other uncertainty estimation methods, which is a plausible reason for the inconsistent variations between our estimate and other studies during this period.

     

The 63 μm radiation field in the earth's thermosphere and its influence on the atomic hydrogen temperature

The thermal escape of hydrogen from the Earth's atmosphere is strongly affected by its temperature at the exobase. It has been suggested recently that the hydrogen temperature might be significantly lower than the thermospheric temperature as a result of a collisional exchange of energy with atomic oxygen. The tendency is to cool the hydrogen since the energy of the excited ${}^3\text{P}{}_1$ level of oxygen can be lost from the atmosphere via magnetic dipole emission of the 63 μm line (${}^3\text{P}{}_2\text{?}{}^3\text{P}{}_1$). We present here a detailed calculation of the net cooling effect as a function of altitude throughout the thermosphere. The calculations have been performed for both day and night conditions and for periods of maximum and minimum solar activity conditions. It is found that its effect on ΔT/T varies from a very small value to a maximum of ～3%. We also provide the theoretical framework for describing deviations of the 63 μm emission from local thermodynamic equilibrium and show that these effects can cause the emission to be reduced by as much as 40% near 500 km.     

On hydraulic falls of single-layer flow

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The impact of the last European deglaciation on the deep-sea turbidite systems of the Celtic-Armorican margin (Bay of Biscay)

The compilation of results obtained on three giant piston cores from the Whittard, Shamrock and Guilcher turbidite levees reveals a high-resolution stratigraphic record for the Bay of Biscay. Due to the abundance of reworked sediments in these sedimentary environments, a specific methodological approach, based on an X-ray-assisted subsampling phase associated with sedimentological, geochemical and micropalaeontological analyses, was implemented. With an accurate chronological framework, this multi-proxy investigation provides observations on the ‘Fleuve Manche’ palaeoriver and the British-Irish Ice Sheet (BIS) histories over the last 20,000 years. The results obtained highlight the direct influence of the decay of the BIS on the Bay of Biscay deep-sea clastic sedimentation during the last European deglacial phase. During this period, the annual BIS cycle of meltwater seems enough to generate seasonal turbidity currents associated with exceptional sedimentation rates in all the Celtic and Armorican turbidite systems. With very high sedimentation rates, the turbidite levees represent the main deep-sea clastic depositional area. Long coring combined with a very careful subsampling method can provide continuous high-resolution palaeoenvironmental signals.     

Marine microgels: Optical and proteomic fingerprints

Dissolved organic matter (DOM) is a major carbon reservoir for the global carbon cycle, and its molecules play a key role in the biogeochemistry of the ocean. Colloidal DOM macromolecules assemble to form polymer hydrogels known as marine microgels. Marine microgels represent one of the most dynamic pools of organic carbon in the ocean. However, their optical characteristics and their contribution to ocean optical properties are largely unknown. In this work, we explore the optical and proteomic properties of spontaneously assembled DOM polymer microgels. Microgels from cultures and from Puget Sound seawater were sorted and counted using a dual-laser (365 nm/365 nm) high-speed cell sorter. This sorter has been adapted to interface with a scanning monochromator to measure the fluorescence emission spectrum of the microgels over the range from 300 to 850 nm. Surprisingly, the microgels show a broad fluorescence emission from 420 to 520 nm when excited with UV light. The microgels were classified according to their blue autofluorescence, and by three criteria that are used to define microgels: 1) staining with chlortetracycline 2) the ability to undergo phase transitions at low pH, and 3) dispersion following calcium chelation by EDTA.