Internal structures and U–Pb ages of zircons from a tuff layer in the Meishucunian formation, Yunnan Province, South China

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life, represented by prominent biological evolution from the first appearance of soft-bodied animals from the late Neoproterozoic to the sudden diversification of animals with mineralized skeletons in the Cambrian. In South China several areas contain many fossils and are well exposed, suitable for the investigation of PC/C boundary. However, geochronological relationships are still poorly known because of lack of combined detailed investigations of internal structures of zircons and in-situ U–Pb dating.We focus on the internal structure of zircons from a tuff layer within Bed 5 in the Meishucun section on which we undertook cathodoluminescence (CL) imaging and in-situ U–Pb dating with LA-ICP-MS and nano-SIMS. Over 600 zircons from the tuff layer were classified into three types based on their CL images: oscillatory rims, inherited cores and dull structures. U–Pb dating of the internal structure of the zircons by LA-ICP-MS clearly shows a distinct unimodal age population dependent on the structure: 531 ± 17 Ma for the oscillatory rims and 515 Ma for the dull structures. The clear oscillatory zonation, the prismatic morphology, and their occurrence indicate that the oscillatory rims were formed from felsic magmatism, and that the U–Pb nano-SIMS age of 536.5 ± 2.5 Ma records the depositional age of the tuff. Our results indicate that the PC/C boundary is situated below Bed 5, and therefore the bottom of Zone 1 (Marker A) is more appropriate for the PC/C boundary than is the top of Zone 1 (Marker B). The age of a positive anomaly (P2) in the early Cambrian is estimated to be ca. 536 Ma.     

Geochemical characteristics of Carboniferous greenstones in the Inner Zone of Southwest Japan

Abstract Greenstones, representing remnants of paleo-oceanic crust, occur in Permian and Jurassic accretionary complexes of the Inner Zone in the Southwestern Japan arc. The formation age of most of the greenstones is early Carboniferous, based on fossil ages for overlying limestones and Sm-Nd isotope ages of the greenstones themselves. The geochemistry of such greenstones is similar to those of present-day oceanic islands. Greenstones of the Permian accretionary complex (Akiyoshi belt) are alkalic and tholeiitic in composition. Some alkali basalts show peculiar features from an EM-1 mantle source, such as the Gough Island and Tristan da Chunha basalts in the South Atlantic. Greenstones of the Jurassic accretionary complex (Tamba belt) are also alkali and tholeiitic basalts with both basalt types in the northern part of the Tamba belt coming from strongly depleted characters similar to a mid-ocean ridge basalt source mantle. The variable geochemistry of the oceanic basalts is explained by hypothesis on existence of a Carboniferous mantle plume below the spreading ridge which divides the Farallon and Izanagi plates. The Akiyoshi belt seamounts and/or oceanic islands of the Farallon plate and Tamba belt seamounts and/or oceanic islands of the Izanagi plate formed simultaneously by the upwelling of the thermal plume. Some part of the Akiyoshi belt basalts originated locally from an EM-1 mantle source, while basalts from the northern parts of the Tamba belt have a normal-type mid-ocean ridge basalt (N-MORB) source component. Existence of an N-MORB signature is consistent with the presence of a spreading center in a Carboniferous 'Pacific Ocean' that caused separation of the Farallon and Izanagi plates. Disparity in accretion ages of the basaltic rocks in the Permian and Jurassic may have been caused by differences in the relative motion of the two plates.     

Relation of fracture resistance to fabric for granitic rocks

Double-torsion specimens of two granitic rocks were prepared in several directions with reference to microcracks fabric. Even for the same rock and at the same stress levels, the observed crack velocities in two granitic rocks were dependent on both the propagation direction and the opening direction. The maximum difference by several orders of magnitude was found for both rocks. The highest crack velocity was observed when the subcritical crack was parallel to most of the preexisting cracks. The maximum critical stress intensity factor was about twice as high as the minimum one in different directions. An analysis for a thin plate having anisotropic elasticity under torsional load showed that the observed difference in the crack velocity and the critical stress intensity factor was not an error due to conventional equations derived on the assumption of isotropic elasticity but the true material's property. As the preferred orientation of microcracks has been pointed out for many granitic rocks, we can conclude that the anisotropic nature of the fracture resistance of the two granitic rocks used in this study was not exceptional. A region of a transport-limited velocity was not found for rocks, even at the velocity of 10^–2 m/s, that is almost equal to the theoretical limit of the stress corrosion cracking.     

A study on the time-dependent microfracturing and strength of Oshima granite

Uniaxial compressive loading experiments where axial strain rate or dilatant volumetric strain rate was kept constant were carried out using a servo-controlled hydraulic testing machine. The rock studied was Oshima granite. In order to investigate the effect of the strain rate on the strength, a theory based on the assumption of redistribution of the pre-existing microcracks due to subcritical crack growth was examined. Results expected from the theory were well borne out by the observations. The strength was expressed by power-functions of both axial strain rate and dilatant strain rate. The stress corrosion index of Oshima granite appeared to be 31 ± 3, which agreed well with the values reported previously.When the axial strain rate was held constant, the acoustic emission rate increased exponentially and the b-value, the exponent in the frequency-magnitude relation of the acoustic emissions, decreased with increasing applied stress. When the dilatant volumetric strain rate was kept constant, the acoustic emission rate and the b-value remained constant and independent of the stress increase. Furthermore, when the dilatant strain rate was held constant at a higher value, the b-value was constant at a lower level. As the decrease in the b-value indicates the higher dilatant strain rate, the b-value is one of the most useful key parameters in the fracture predictions.     

Quantitative micro‐X‐ray fluorescence scanning spectroscopy of wet sediment based on the X‐ray absorption and emission theories: Its application to freshwater lake sedimentary sequences

Micro‐X‐ray fluorescence scanning spectroscopy of marine and lake sedimentary sequences can provide detailed palaeoenvironmental records through element intensity proxy data. However, problems with the effects of interstitial pore water on the micro‐X‐ray fluorescence intensities have been pointed out. This is because the X‐ray fluorescence intensities are measured directly at the surfaces of split wet sediment core samples. This study developed a new method for correcting X‐ray fluorescence data to compensate for the effects of pore water using a scanning X‐ray analytical microscope. This involved simultaneous use of micro‐X‐ray fluorescence scanning spectroscopy and an X‐ray transmission detector. To evaluate the interstitial pore water content from the X‐ray transmission intensities, a fine‐grained sediment core retrieved from Lake Baikal (VER99‐G12) was used to prepare resin‐embedded samples with smooth surfaces and uniform thickness. Simple linear regression between the linear absorption coefficients of the samples and their porosity, based on the Lambert–Beer law, enabled calculation of the interstitial pore spaces and their resin content with high reproducibility. The X‐ray fluorescence intensities of resin‐embedded samples were reduced compared with those of dry sediment samples because of: (i) the X‐ray fluorescence absorption of resin within sediment; and (ii) the sediment dilution effects by resin. An improved micro‐X‐ray fluorescence correction equation based on X‐ray fluorescence emission theory considers the instrument's sensitivity to each element, which provides a reasonable explanation of these two effects. The resin‐corrected X‐ray fluorescence intensity was then successfully converted to elemental concentrations using simple linear regression between the data from micro‐X‐ray fluorescence scanning spectroscopy and from the conventional analyzer. In particular, the calculated concentration of SiO₂ over the depth of the core, reflecting diatom/biogenic silica concentration, was significantly changed by the calibrations, from a progressively decreasing trend to an increasing trend towards the top of the core.     

A new algorithm to retrieve aerosol over the gulf of cambay in India

Ocean color analysis and aerosol retrieval in coastal regions are made difficult by water turbidity. An algorithm has been proposed which uses the data at a blue wavelength instead of those in near-infrared wavelengths for the aerosol retrieval. The quasi-homogeneous effects are assumed for the correction of water leaving radiance with soil particles at 0.412 μm. The proposed algorithm is examined using SeaWiFS data on December 24, 2000 around India. Over the coastal waters, extremely large values of optical thickness are extracted from the operational SeaWiFS algorithm, whereas our proposed algorithm produces a smooth transition in values of optical thickness from the turbid waters to the surrounding regions.     

Evaluation of global warming impacts for different levels of stabilization as a step toward determination of the long-term stabilization target

In order to estimate the benefit attributable to alleviating global warming for a kind of cost–benefit analysis of global warming mitigation, global warming impacts were quantitatively evaluated for a pathway of unmitigated CO₂ emissions and three pathways to stabilize the atmospheric CO₂ concentration at different levels, keeping unchanged the assumed conditions on population and GDP growths, although the GDP losses which will be caused due to the warming mitigation for the three stabilization pathways are taken into account. The evaluation results show that global warming will reduce the world total number of deaths caused by thermal stress owing to the large decrease in the cold-related deaths; it will increase the water stress in some regions, while it will decrease the stress in other regions; reductions in CO₂ emissions will decrease the probability of THC collapse and terrestrial biodiversity loss; and it will enhance an increase in the wheat production potential.     

Improved Kakioka automatic standard magnetometer (KASMMER)

The KASMMER (Kakioka Automatic Standard Magnetometer) has been constructed in 1972 for the standard magnetic observation at Kakioka (Yanagiharaet al., 1973). Then KASMMER system has been improved successively for the high level standard magnetic observation. The purpose of the present paper is to introduce an outline of the improved KASMMER system.The KASMMER system consists of four parts which are four optical pumping magnetometers, a fluxgate magnetometer (Supporting system to the optical pumping magnetometers), a calibration system and a data acquisition system (computer system). The calibration system consists of a proton magnetometer and a magnetic theodolite (Di-72). Details for each part are described and the ability of the improved system is shown to be higher than that of classical magnetic observatory systems.     

Decay model for biocide treatment of unballasted vessels: application for the Laurentian Great Lakes

A biocide decay model was developed to assess the potential efficacy and environmental impacts associated with using glutaraldehyde to treat unballasted overseas vessels trading on the Laurentian Great Lakes. The results of Monte Carlo simulations indicate that effective glutaraldehyde concentrations can be maintained for the duration of a vessel's oceanic transit (approximately 9-12 days): During this transit, glutaraldehyde concentrations were predicted to decrease by approximately 10% from initial treatment levels (e.g., 500 mgL(-1)). In terms of environmental impacts, mean glutaraldehyde concentrations released at Duluth-Superior Harbor, MN were predicted to be 100-fold lower than initial treatment concentrations, and ranged from 3.2 mgL(-1) (2 SD: 2.74) in April to 0.7 mgL(-1) (2 SD: 1.28) in August. Sensitivity analyses indicated that the re-ballasting dilution factor was the major variable governing final glutaraldehyde concentrations; however, lake surface temperatures became increasingly important during the warmer summer months.     

Helium Isotopes in Pacific Waters from Adjacent Region of Honshu, Japan

We have measured helium isotopic ratios of thirty-seven Pacific water samples from various depths collected in adjacent regions of Honshu, Japan. The ³He/⁴He ratios vary significantly from 0.989 R _atm to 1.208 R _atm where R _atm is the atmospheric ratio of 1.39 × 10⁻⁶. The mid-depth (750–1500 m) profile of ³He/⁴He ratios at ST-1 located Northwestern Pacific Ocean east of Japan (Off Joban; 37°00′ N, 142°40′ E) is significantly different from that at ST-2 of the Northern Philippine Sea south of Japan (Nankai Trough; 33°07′ N, 139°59′ E), suggesting that these waters were separated by a topographic barrier, the Izu-Ogasawara Ridge. Taking ³He/⁴He data of the Geosecs expeditions in the western North Pacific, an extensive plume of 15% excess ³He relative to the air may be traced at ST-1 over 12,000 kilometers to the northwest of the East Pacific Rise where the mantle helium may originate. The 20% excess found at ST-2 may be attributable to the additional source of the subduction-type mantle helium in the Okinawa Trough. A 15% excess of ³He has also been discovered at a depth of about 1000∼1500 m at ST-3 adjacent to Miyakejima Island (33°57′ N, 139°22′ E) and ST-4 of Sagami Bay (35°00′ N, 139°22′ E). It is confirmed that mid-depth all over the western North Pacific water is affected by the mantle helium with a high ³He/⁴He ratio. This revised version was published online in July 2006 with corrections to the Cover Date.