Small ice cap instability in the presence of fluctuations

Phenomena associated with small ice cap instability (SICI) are investigated using a general circulation model (GCM: NCAR Community Climate Model version 0) and a noise-forced nonlinear energy balance model (EBM). Both make use of idealized boundary conditions consisting of an all-land planet without topography and mean annual insolation. Ice is prescribed to exist on surface areas for which the instantaneous temperature lies below freezing. The adjustable parameters of the EBM were chosen to match the GCM solutions. For the regions in parameter space where SICI might occur we do not find the corresponding icefree steady state solution with the GCM. Our simulations with the EBM show that SICI phenomena in the presence of fluctuations are strongly dependent on the amplitude of the noise forcing. When the strength of noise forcing is adjusted to match the fluctuations in the GCM, we do not find a SICI in the EBM. With weaker levels of forcing the SICI reappears. In all cases steady stable ice caps smaller than a critical size are not found to exist.     

Integration of sedimentology and ground‐penetrating radar for high‐resolution imaging of a carbonate platform

Ground‐penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south‐east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground‐penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground‐penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground‐penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea‐level fluctuations controlling the Upper Jurassic carbonate production.     

Weathering and polymerization of tektites: An x-ray photoelectron spectroscopy (XPS) investigation

Abstract— Tektites are natural glasses formed from terrestrial material that was melted and displaced by the impact of an extraterrestrial body. The surface and near-surface compositions of tektite glass results from fractionation during impact and ejection, and/or postsolidification weathering. The first goal of this study was to characterise the surface and near-surface (in the order of tens of angstroms) chemical composition of two tektites by x-ray photoelectron spectroscopy (XPS), and to estimate the importance of weathering vs. fractionation during flying. In order to separate the chemical modification due to weathering from that due to fractionation during ballistic flight, we studied two samples from the Australasian tektite strewn field. One of them was collected in a hot desert area (Nullarbor Plain, Australia) and the other, in a humid climate (Thailand). Our study reveals the presence of well-developed leached layers in both tektites. In the Australian tektite, Si is depleted in the topmost layers (a few tens of angstroms). A more complex chemical zoning is defined in the tektite from Thailand. These leached layers are comparable to those observed in weathered glasses, and therefore we conclude that weathering is responsible for the chemical composition of the surface and near-surface compositions. The second goal was to investigate the chemical environment of O, N and C in the glass. The O peak was resolved into two bridging O components (Si-O-Si and Al-O-Si) that are comparable to O environments in artificial glasses. The binding energy of the C_1s electron is typical for C-C and C-H bonds in hydrocarbons; minor organic acid components are also present. Nitrogen is only observed on the surface of the Thailand tektite. The binding energy of N_1s is comparable to that of ammonia, and the surface enrichment in N is interpreted as due to sorption related to interactions between glass and fluid buffered by the organic material in the soil.     

Sea‐level reconstructions from the Peri‐Vocontian Zone (South‐east France) point to Valanginian glacio‐eustasy

The Valanginian is a period of global environmental change as illustrated by sedimentary, palaeontological, geochemical and climatic perturbations. A production crisis in most of the carbonate platforms suggests important changes in palaeoenvironmental conditions. During the same time interval, a major positive excursion in δ¹³C, the Weissert Event, suggests perturbations of the carbon cycle from the latest Early Valanginian to the Early Hauterivian. In order to better understand the link between these changes, sea‐level fluctuations have been reconstructed in detail from the Middle Berriasian to the earliest Hauterivian. Sections from the Peri‐Vocontian Zone (South‐east France) have been investigated because of the good quality of outcrops on the carbonate platforms, their margins and in the Vocontian Basin. Sections ranging from the most proximal zone (Swiss Jura) to the basin were interpreted in terms of sequence stratigraphy and cyclostratigraphy, and correlated at high resolutions. Using the identified small, medium and large‐scale sequences as well as depositional geometries, sea‐level fluctuations were reconstructed. Two main trends are evidenced during the studied interval: (i) the peak amplitude (magnitude) of the sea‐level fluctuations increased gradually from the Middle Berriasian to the Early Valanginian, and reached a maximum (more than 50 m) from the middle Early Valanginian to the Valanginian/Hauterivian boundary; and (ii) sea‐level variations were quite symmetrical during the Late Berriasian, slightly asymmetrical during the Early Valanginian and strongly asymmetrical (fast sea‐level rise, slow fall) from the latest Early Valanginian to the earliest Hauterivian. Moreover, three orders of sea‐level fluctuations were recognized in the sedimentary rocks of the Peri‐Vocontian Zone. Platform‐basin correlations and cyclostratigraphic interpretations of the basinal sections evidence an astronomical control on the sea‐level variations, mainly by the two eccentricity cycles of 100 and 400 kyr. The increase in the amplitude of the sea‐level fluctuations and their change from symmetrical to asymmetrical can be related to the onset of a major cooling event in the Early Valanginian. Fast transgressions followed by slower regressions would correspond to waxing and waning of high‐latitudinal ice during most of the Valanginian, especially from the latest Early Valanginian to the latest Late Valanginian. Glacio‐eustatic sea‐level fluctuations in tune with the 100 and 400 kyr eccentricity cycles are in agreement with glaciations during the Valanginian.     

准噶尔盆地南缘新生代构造特征及其与砂岩型铀矿成矿作用初析

准噶尔盆地南缘盆山结合部中新生界沉积巨厚,新生代变形强烈,是研究新盆山耦合的理想场所,也是我国砂岩型铀矿找矿的远景区段。本文在综合前人资料和野外观测分析基础上,根据新生代构造活动特征,将准噶尔盆地南缘划分为博格达山前和西部断褶带两个构造分区,博格达山前以强烈的逆冲推覆为特征,发育多条活动的逆冲推覆断裂; 乌鲁木齐以西至乌苏南的西部断褶带则发育三-四排的褶皱-逆冲断裂构造带。对采自博格达山前逆冲推覆断裂带内的方解石和断层泥,利用电子自旋共振测年手段,推测博格达山前的富康-吉木萨尔断裂带和北三台断裂带分别在0.7~1Ma和0.25Ma期间,经历了一期重要的逆冲推覆作用。结合盆地南缘砂岩型铀矿的展布规律及其成矿条件的分析,探讨了新生代构造运动对砂岩型铀矿成矿的控制作用,认为西部断褶带的第一排构造带具有较好的成矿前景,而博格达山前由于新生代构造活动强烈而相对成矿不利,为此提出了准噶尔盆地南缘砂岩型铀矿成矿“构造优先权”的构造控矿模式,进而指出了区域找矿的优选区段。     

一种新型人工“流体包裹体”：融合二氧化硅毛细管技术

利用融合二氧化硅毛细管技术制作了纯H2O体系、纯CO2体系、H2O-NaCl体系和H2O-CO2体系的人工包裹体样品,并对样品进行了显微测温和激光拉曼光谱测试工作。实验结果显示毛细管样品中的流体成分具有代表性,而且常规的流体包裹体显微测温和显微激光拉曼光谱分析技术完全适于毛细管样品的测试。对样品的显微测温和拉曼光谱研究...     

THE KNUD RASMUSSEN MEMORIAL EXPEDITION Introduction

Jacobsen, N. Kingo, et.al. Report of activities at Tugtuligssuaq, Melville Bugt, 1978–79. Geografisk Tidsskrift 80: 29–44. Copenhagen, June 1980.

A joint cooperative Greenland/Danish expedition to the Melville Bugt area, Tugtuligssuaq/Kap Seddon to throw light on the Eskimos' immigration routes to Greenland seen in relation to climatic fluctuations. Studies of geography, botany, zoology, archaeology, and ethnohistory have been carried out.     

Geochemical and Isotopic Evidence for Crustal Assimilation Beneath Krafla, Iceland

The Krafla volcanic system consists of a central volcano andassociated fissure swarm in the NE axial rift zone of Iceland.Lavas spanning the whole of Krafla's exposed volcanic history(estimated to be 0-> 300 ka) have been analysed and rangein composition from olivine tholeiite to rhyolite. Major-elementcompositions suggest that fractional crystallization exertsthe main control over the differentiation process. However,K₂O and the very incompatible trace elements, Rb, Th, and U,are all enriched beyond the extent expected by closed-systemfractional crystallization. Fractionation coupled with periodicreplenishment and tapping of the reservoir is unlikely to beresponsible for this enrichment, despite the geophysical evidencesuggesting a large number of inflations and deflations of ashallow magma reservoir (Tryggvason, 1986). Th- and O-isotope results confirm the work of previous authorsthat crustal assimilation is operating on a local scale beneathKrafla. A model is suggested, fitting both the Th- and O-isotopicdata, which involves the partial melting and incorporation ofa hydrothermally altered wall-rock contaminant during fractionalcrystallization (i. e., AFC processes). This process of partialmelting is likely to enhance the most highly incompatible elementconcentrations (e. g., increasing Rb/Zr) more than expectedby closed-system fractional crystallization.     

Multiple slump scars in the Tortonian type area (Piedmont Basin, northwestern Italy)

Multiple intraformational discordances, recognized in the Tortonian (Upper Miocene) type area, occur at the top of a shelf sequence, directly overlain by slope sediments. The discordances display smooth, concave upward lower surfaces and are covered by sediments identical to the surrounding strata. Inferred lateral extent of single discordances is 500–700 m. Thickness of sediments overlying the discordances ranges from 5 to 25 m. The intraformational discordances are interpreted as scars formed by downslope sliding of whole packets of strata. The slumps may have been triggered by tectonic oversteepening of shelf sediments due to tilting of the sea floor. The slump scars appear comparable in position and size to some small scale modern examples recorded on margins near the shelf-break.     

A catchment scale evaluation of the SIBERIA and CAESAR landscape evolution models

Landscape evolution models provide a way to determine erosion rates and landscape stability over times scales from tens to thousands of years. The SIBERIA and CAESAR landscape evolution models both have the capability to simulate catchment–wide erosion and deposition over these time scales. They are both cellular, operate over a digital elevation model of the landscape, and represent fluvial and slope processes. However, they were initially developed to solve research questions at different time and space scales and subsequently the perspective, detail and process representation vary considerably between the models. Notably, CAESAR simulates individual events with a greater emphasis on fluvial processes whereas SIBERIA averages erosion rates across annual time scales. This paper describes how both models are applied to Tin Camp Creek, Northern Territory, Australia, where soil erosion rates have been closely monitored over the last 10 years. Results simulating 10 000 years of erosion are similar, yet also pick up subtle differences that indicate the relative strengths and weaknesses of the two models. The results from both the SIBERIA and CAESAR models compare well with independent field data determined for the site over different time scales. Representative hillslope cross‐sections are very similar between the models. Geomorphologically there was little difference between the modelled catchments after 1000 years but significant differences were revealed at longer simulation times. Importantly, both models show that they are sensitive to input parameters and that hydrology and erosion parameter derivation has long‐term implications for sediment transport prediction. Therefore selection of input parameters is critical. This study also provides a good example of how different models may be better suited to different applications or research questions. Copyright © 2010 John Wiley & Sons, Ltd and Commonwealth of Australia