Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core,and selected auxiliary records

The Greenland ice core from NorthGRIP (NGRIP) contains a proxy climate record across the Pleistocene–Holocene boundary of unprecedented clarity and resolution. Analysis of an array of physical and chemical parameters within the ice enables the base of the Holocene, as reflected in the first signs of climatic warming at the end of the Younger Dryas/Greenland Stadial 1 cold phase, to be located with a high degree of precision. This climatic event is most clearly reflected in an abrupt shift in deuterium excess values, accompanied by more gradual changes in δ¹⁸O, dust concentration, a range of chemical species, and annual layer thickness. A timescale based on multi‐parameter annual layer counting provides an age of 11 700 calendar yr b2 k (before AD 2000) for the base of the Holocene, with a maximum counting error of 99 yr. A proposal that an archived core from this unique sequence should constitute the Global Stratotype Section and Point (GSSP) for the base of the Holocene Series/Epoch (Quaternary System/Period) has been ratified by the International Union of Geological Sciences. Five auxiliary stratotypes for the Pleistocene–Holocene boundary have also been recognised. Copyright © 2008 John Wiley & Sons, Ltd.     

Size and compositional constraints of Ganymede's metallic core for driving an active dynamo

Ganymede has an intrinsic magnetic field which is generally considered to originate from a self-excited dynamo in the metallic core. Driving of the dynamo depends critically on the satellite's thermal state and internal structure. However, the inferred structure based on gravity data alone has a large uncertainty, and this makes the possibility of dynamo activity unclear; variations in core size and composition significantly change the heat capacity and alter the cooling history of the core. The main objectives of this study is to explore the structural conditions for a currently active dynamo in Ganymede using numerical simulations of the thermal history, and to evaluate under which conditions Ganymede can maintain the dynamo activity at present. We have investigated the satellite's thermal history using various core sizes and compositions satisfying the mean density and moment of inertia of Ganymede, and evaluate the temperature and heat flux at the core-mantle boundary (CMB). Based on the following two conditions, we evaluate the possibility of dynamo activity, thereby reducing the uncertainty of the previously inferred interior structure. The first condition is that the temperature at the CMB must exceed the melting point of a metallic core, and the second is that the heat flux through the CMB must exceed the adiabatic temperature gradient. The mantle temperature starts to increase because of the decay of long-lived radiogenic elements in the rocky mantle. After a few Gyr, radiogenic elements are exhausted and temperature starts to decrease. As the rocky mantle cools, the heat flux at the CMB steadily increases. If the temperature and heat flux at the CMB satisfy these conditions simultaneously, we consider the case as capable of driving a dynamo. Finally, we identify the Dynamo Regime, which is the specific range of internal structures capable of driving the dynamo, based on the results of simulations with various structures. If Ganymede's self-sustained magnetic field were maintained by thermal convection, the satellite's metallic core would be relatively large and, in comparison to other terrestrial-type planetary cores, strongly enriched in sulfur. The dynamo activity and the generation of the magnetic field of Ganymede should start from a much later stage, possibly close to the present.     

峨眉山地区大气漂尘化学组成研究

为了揭示峨眉山大气漂尘的化学特征，研究小组分别在1998，1999，2000年使用小型大气采样器进行采样，并测得样品中含有水溶性化学成分（Na^ ,K^ ,Mg^2 ,Ca^2 ,SO4^2-,NO3^-,NH4^ 和Cl^-)和酸溶性化学成分（Pb和Zn)。这一地区三年的大气成分与1990年的相似，而且峨眉山的Na^ ,Cl^-,SO4^2-,NO3^-,Ca^2 ,NH4^ ，Pb和Zn浓度分别比日本山形县鹤岗的要高出4－15，3－19，3－5，3－6，13－23，4－7，11－28和10－23倍之多。     

Small-scale mélange fabric between serpentinite block and matrix: Magnetic evidence from the Mitsuishi ultramafic rock body, Hokkaido, Japan

The Mitsuishi ultramafic rock body in Hokkaido, Japan, consists mainly of serpentinized peridotites that originated from a depleted mantle. This study aims to show new evidence of small-scale mélange fabric of serpentinite matrix in the rock body. Each serpentinite block in the serpentine matrix shows large and stable intensities of natural remanent magnetization (NRM). However, the directions of serpentinite blocks' NRM in the matrix are randomly scattered. A Curie temperature (Tc) of 580 °C corresponding to pure magnetite was also observed. Additionally, there is no evidence of heating over 580 °C after serpentinization. The blocks in the matrix must have obtained crystallization remanent magnetization (CRM) during serpentinization. The directions of the blocks' characteristic remanent magnetization (ChRM) are also scattered. It shows that serpentinite blocks were magnetized prior to uplifting. The results of the study indicate that the magnetic carrier of the serpentinite blocks in the matrix is mainly composed of magnetite, and it can keep original magnetization before uplifting. The results also imply that the scattering directions of NRM indicate the presence of small-scale mélange fabric of serpentinite matrix.     

230Th/234U and 14C dating of a lowstand coral reef beneath the insular shelf off Irabu Island, Ryukyus, southwestern Japan

Abstract   High-resolution seismic reflection profiles delineated the distribution of mound-shaped reflections, which were interpreted as reefs, beneath the insular shelf western off Irabu Island, Ryukyus, southwestern Japan. A sediment core through one of the mounded structures was recovered from the sea floor at a depth of −118.2 m by offshore drilling and was dated by radiometric methods. The lithology and coral fauna of the core indicate that the mounded structure was composed of coral–algal boundstone suggesting a small-scaled coral reef. High-precision α-spectrometric ²³⁰Th/²³⁴U dating coupled with calibrated accelerator mass spectrometric ¹⁴C ages of corals obtained reliable ages of this reef ranging from 22.18 ± 0.63 to 30.47 ± 0.98 ka. This proves that such a submerged reef was formed during the lowstand stage of marine oxygen isotope stages 3–2. The existence of low-Mg calcite in the aragonitic coral skeleton of 22.18 ± 0.63 ka provides evidence that the reef had once been exposed by lowering of the relative sealevel to at least −126 m during the last glacial maximum in the study area. There is no room for doubt that a coral reef grew during the last glacial period on the shelf off Irabu Island of Ryukyus in the subtropical region of western Pacific.     

The 10 ka multiple vent pyroclastic eruption sequence at Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand: Part 2. Petrological insights into magma storage and transport during regional extension

The six eruption episodes of the 10 ka Pahoka–Mangamate (PM) sequence (see companion paper) occurred over a ?200–400-year period from a 15-km-long zone of multiple vents within the Tongariro Volcanic Centre (TgVC), located at the southern end of the Taupo Volcanic Zone (TVZ). Most TgVC eruptives are plagioclase-dominant pyroxene andesites and dacites, with strongly porphyritic textures indicating their derivation from magmas that ascended slowly and stagnated at shallow depths. In contrast, the PM pyroclastic eruptives show petrographic features (presence of phenocrystic and groundmass hornblende, and the coexistence of olivine and augite without plagioclase during crystallisation of phenocrysts and microphenocrysts) which suggest that their crystallisation occurred at depth. Depths exceeding 8 km are indicated for the dacitic magmas, and >20 km for the andesitic and basaltic andesitic magmas. Other petrographic features (aphyric nature, lack of reaction rims around hornblende, and the common occurrence of skeletal microphenocrystic to groundmass olivine in the andesites and basaltic andesites) suggest the PM magmas ascended rapidly immediately prior to their eruption, without any significant stagnation at shallow depths in the crust. The PM eruptives show three distinct linear trends in many oxide–oxide diagrams, suggesting geochemical division of the six episodes into three chronologically-sequential groups, early, middle and late. Disequilibrium features on a variety of scales (banded pumice, heterogeneous glassy matrix and presence of reversely zoned phenocrysts) suggest that each group contains the mixing products of two end-member magmas. Both of these end-member magmas are clearly different in each of the three groups, showing that the PM magma system was completely renewed at least three times during the eruption sequence. Minor compositional diversity within the eruptives of each group also allows the PM magmas to be distinguished in terms of their source vents. Because petrography suggests that the PM magmas did not stagnate at shallow levels during their ascent, the minor diversity in magmas from different vents indicates that magmas ascended from depth through separate conduits/dikes to erupt at different vents either simultaneously or sequentially. These unique modes of magma transport and eruption support the inferred simultaneous or sequential tapping of small separate magma bodies by regional rifting in the southern Taupo Volcanic Zone during the PM eruption sequence (see companion paper).     

On build-up of magnetic energy in the solar atmosphere

The dynamic response of the solar atmosphere is examined with the use of self-consistent numerical solutions of the complete set of nonlinear, two-dimensional, hydromagnetic equations. Of particular interest are the magnetic energy build-up and the velocity field established by emerging flux at the base of an existing magnetic loop structure in a stationary atmosphere. For a plasma with a relatively low beta ( = 0.03) the magnetic energy build-up is approximately twice that of the kinetic energy, while the build-up in magnetic energy first exceeds but is eventually overtaken by the kinetic energy for a plasma with an intermediate beta ( = 3). The increased magnetic flux causes the plasma to flow upward near the loop center and downward near the loop edges for the low beta plasma. The plasma eventually flows downward throughout the lower portion of the loop carrying the magnetic field with it for the intermediate beta plasma. It is hypothesized that this latter case, and possibly the other case as well, may provide a reasonable simulation of the disappearance of prominences by flowing down into the chromosphere (a form of disparition brusque).The National Center for Atmospheric Research is sponsored by the National Science Foundation.Now at the School of Science and Engineering, The University of Alabama in Huntsville, Huntsville, Alabama 35807.     

Palynological evidence for the astronomical origin of lignite-detritus sequence in the Middle Pleistocene Marathousa Member, Megalopolis, SW Greece

The Marathousa Member, Middle Pleistocene strata in the fluvio-lacustrine Megalopolis basin, southwest Greece, displays distinct but complicated lithological cycles comprising first-order alternation of lignites and detrital muds and second-order alternation expressed by frequent intercalation of organic layers. Palynological evidence indicates that the lithological cycles are driven by the Earth’s orbital forcing. All the lignite seams yield temperate oak forest whereas the detrital beds provide semi-arid steppe mainly of Artemisia. This means that the first-order lithological cycle represents the glacial/interglacial cycle (i.e., the 100-kyr eccentricity cycle), providing a timescale of at least 350 kyr to the Marathousa Member. Pollen also detects smaller-scale climate fluctuations in many of the subordinate organic layers, with the total number of fluctuations being five in a complete lignite-detritus couplet. This means that the second-order lithological cycle reflects the 21-kyr insolation cycle. A tentative phase relation between the lithological cycles and the astronomical cycles is shown based on palynostratigraphy and electron spin resonance dating. Lacustrine environments with increased water tables are implied for the glacial periods sedimentologically, in contrast to local swamp vegetation for the interglacial periods. The subordinate organic layers were formed under intermediate environments (climate, water depth, etc.) between full glacials and interglacials.