The main problem of lunar theory solved by the method of Brown

Brown's method for solving the main problem of lunar theory has been adapted for the computation by machine with the help of an algebraic processor. Brown's results are first recovered and refined. The solution is then expanded to include most terms of order nine. The terms in the series for the longitude and latitude are listed with an accuracy of 0.000 01 and of 0.000 001 for the parallax.This research was supported in parts by the National Science Foundation grant MCS 78-01425.     

Calcium-isotope fractionation in selected modern and ancient marine carbonates

The calcium-isotope composition (δ^44/42Ca) was analyzed in modern, Cretaceous and Carboniferous marine skeletal carbonates as well as in bioclasts, non-skeletal components, and diagenetic cements of Cretaceous and Carboniferous limestones. In order to gain insight in Ca²⁺_aq-CaCO₃-isotope fractionation mechanisms in marine carbonates, splits of samples were analyzed for Sr, Mg, Fe, and Mn concentrations and for their oxygen and carbon isotopic composition. Biological carbonates generally have lower δ^44/42Ca values than inorganic marine cements, and there appears to be no fractionation between seawater and marine inorganic calcite. A kinetic isotope effect related to precipitation rate is considered to control the overall discrimination against ⁴⁴Ca in biological carbonates when compared to inorganic precipitates. This is supported by a well-defined correlation of the δ^44/42Ca values with Sr concentrations in Cretaceous limestones that contain biological carbonates at various stages of marine diagenetic alteration. No significant temperature dependence of Ca-isotope fractionation was found in shells of Cretaceous rudist bivalves that have recorded large seasonal temperature variations as derived from δ¹⁸O values and Mg concentrations. The reconstruction of secular variations in the δ^44/42Ca value of seawater from well preserved skeletal calcite is compromised by a broad range of variation found in both modern and Cretaceous biological carbonates, independent of chemical composition or mineralogy. Despite these variations that may be due to still unidentified biological fractionation mechanisms, the δ^44/42Ca values of Cretaceous skeletal calcite suggest that the δ^44/42Ca value of Cretaceous seawater was 0.3-0.4‰ lower than that of the modern ocean.     

Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate

Major and trace element and Sr-Nd-Pb-O-C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sáo Tiago, Maio and Sáo Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. ²⁰⁶Pb/²⁰⁴Pb=19.3-20.4) than reported for silicate volcanic rocks from these islands (18.7-19.9; Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of ~1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated ‘¹³C and ‘¹⁸O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative Ɨ/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e.g. limestone) recycled through shallow, subcontinental-lithospheric-mantle and deep, lower-mantle sources.     

Identification of geomorphic process units in Kärkevagge, northern Sweden, by remote sensing and digital terrain analysis

Sediment transport processes in the Kärkevagge are investigated concerning their spatial and temporal characteristics due to long–term monitoring. Within this study remote sensing techniques and GIS modelling in connection with geomorphic mapping are applied for identification and characterization of geomorphic process units. Relationships between geomorphometric parameters and slope processes like solifluction, talus creep and rockfall have been analysed. Multitemporal Landsat–TM5 scenes are used as source for landcover characteristics (Normalized Difference Vegetation Index) after preprocessing involving orthorectification and topographic normalization in order to remove possible terrain–induced effects. Additionally, a digital elevation model with a resolution of 20 m for the Kärkevagge catchment is developed and parameters like slope gradient, slope aspect and profile curvature are extracted as input for the analysis of the sediment transport system. The combination of landcover information, geomorphometrical and topological features allows the definition of areas for single process activities. They show specific sediment displacement characteristics depending on material conditions, topological and geometrical features. Geomorphic process units, which show a homogenous composition, are extracted from these available layers.     

Hypolimnetic density currents traced by sulphur hexafluoride (SF6)

The artificial tracer sulphur hexafluoride (SF₆) has been used to study the density-driven deep water exchange between two sill-separated basins of Lake Lucerne, Gersauersee and Urnersee. The sources of the density gradients between the two basins are (1) salinity differences between the major inlets due to the different geology of their drainage areas, and (2) temperature differences due to spatial variation of wind forcing. Wind speeds are generally larger in Urnersee, especially in spring during the so-called Föhn events, when winds blow from the south. In contrast, Gersauersee is protected form these winds. In spring 1989, a total of 630 g of SF₆ was released at 80 to 120 m depth in the small Treib Basin located between Urnersee and Gersauersee. During about 100 days the distribution of SF₆ in the lake was determined by gaschromatography. Two models are used to quantify the exchange flow, (1) a one-box mass balance model for SF₆ in the deep part of Treib Basin, and (2) a one-dimensional diffusion/advection model describing the temporal and vertical temperature variation in Urnersee. According to the first model, the flow into the deep hypolimnion of Urnersee, decreases from 21·10⁶ m³·d^?1 at the end of March to about 8·10⁶ m³·d^?1 in late April. The second model yields similar flow rates. The decrease of the flow rate during spring, confirmed by both approaches, is consistent (1) with the decreasing strength of the density gradient above the sill during spring and early summer, and (2) with hydrographic information collected in Lake Lucerne during other years.     

Exploring the impact of diagenesis on (isotope) geochemical and microstructural alteration features in biogenic aragonite

For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre‐Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different ¹⁸O‐depleted fluids for durations between two and twenty weeks. Alteration was documented using X‐ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ₄₇ which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ¹⁸O signature supports transformation via dissolution and reprecipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple‐blue luminescence with orange bands. The δ¹⁸O and Δ₄₇ signatures reveal exchange with the aqueous fluid, whereas δ¹³C remains unaltered in all experiments, indicating a carbonate‐buffered system. Clumped isotope temperatures in high‐temperature experiments show compositions in broad agreement with the measured temperature. Experimentally induced alteration patterns are comparable with individual features present in Pleistocene shells. This study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow‐up experiments.     

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

南海张裂过程及其对晚中生代以来东南亚构造的启示——IODP建议书735-Full介绍

南海的形成揭示了大陆边缘张裂和盆地形成的复杂模式,尽管已经进行了广泛研究,但是关于基底岩石和深海盆沉积层的精确年代数据还很缺乏,这使得对南海张裂年代的估计存在很大的误差,对张裂机制和历史的各种假设没有得到验证.同时只有对南海的张裂过程有了精确地分析与刻画,才能更好地理解西太平洋边缘海盆地的形成以及它们在印支块体受印度-欧亚板块碰撞而向东南挤出、青藏高原隆升中可能起到的作用.2009年正式提交的国际综合大洋钻探计划(IODP)建议书735-Full建议在南海深海盆内的4个站位上实施钻探.这4个站位分布在南海盆地4个不同的次级构造单元上(南海东北部、西北次海盆、东部次海盆和西南次海盆),这样的站位设计会确保完成本建议书的整体研究目标,即揭示南海的张裂历史和它对晚中生代以来东南亚构造的启示.位于南海盆地最东北部的站位有助于确定该区域地壳的属性和验证古南海是否存在,位于西北次海盆的站住可能会提供南海的最早张裂年代,另外2个分别位于东部次海盆和西南次海盆的站位将重点确定2个次海盆的绝对年龄、基底矿物成分与磁化率以及2个次海盆的相对张裂次序.这些站位的水深大约在2 910～4 400 m,钻探深度预计到海底以下大约700～2 200 m,总的钻透深度为5 959 m,其中5 359 m穿透沉积层,另外600 m或400 m钻入基底.所有这些站位的位置是由已有的地球物理观测数据所确定,目前计划收集更多的地质与地球物理数据以满足IODP对井位调查数据的要求.     

Mauer – the type site of Homo heidelbergensis: palaeoenvironment and age

The mandible of Homo heidelbergensis was found 1907 in the sand pit Grafenrain at Mauer in coarse fluvial sands 24 m below the surface, deposited in a former course of the Neckar River. These ‘Mauer sands’ are overlain by a series of glacial-climate loess deposits with intercalated interglacial palaeosols, which can be correlated with Quaternary climate history, thus indicating an early Middle Pleistocene age for H. heidelbergensis. The ‘Mauer sands’ are famous for their rather rich mammal fauna, which clearly indicates interglacial climate conditions. The faunal evidence – in particular the micromammals – place the ‘Mauer sands’ into MIS 15 or MIS 13 although most stratigraphic arguments favour correlation to MIS 15 and therefore to an age of ca 600 ka.     

Gondwana breakup: no evidence for a Davie Fracture Zone offshore northern Mozambique,Tanzania and Kenya

Plate tectonic reconstructions assume a major inactive transform fault, the Davie Fracture Zone, in the West Somali Basin, along which Madagascar is thought to have migrated southwards following Gondwana breakup in the Mesozoic. Based on the interpretation of reflection seismic data, we show that the Walu Ridge offshore Kenya and the Kerimbas Basin offshore northern Mozambique are tectonically unrelated to the southward motion of Madagascar and correlate with Late Cretaceous volcanism and inversion in Kenya and the evolution of the East African Rift System respectively. Offshore Tanzania, geophysical data do not show basement structures indicating the presence of a major transform fault. These results challenge the commonly supported transform margin concept and imply a more southerly pre‐breakup position of Madagascar within Gondwana. Opening of the West Somali Basin by SW‐propagating oblique rifting and seafloor spreading is proposed.