A record of the upper Cochiti polarity transition from the Southern Hemisphere (FIJI)

We sampled the upper Cochiti polarity transition recorded in the Suva Marl in Fiji (18° S, 178° E). The Suva Marl accumulated at an average rate of 83.2 m/m.y. and provides a unique opportunity to address the question of whether present-day overprints bias the transition records. Fiji has undergone a 30° counterclockwise rotation since the deposition of the Suva Marl and hence a present day overprint onto the transitions would bias the VGPs toward the east of the site. Replicate transition records yield VGPs over both Asia and the Americas. Progressive demagnetization reveals a normal polarity overprint which was acquired prior to the rotation of Fiji, and therefore does not produce an easterly bias to the transition data even after correction for the rotation. We collected an oriented block sample across a portion of the transitional interval in the upper Cochiti reversal. Subsampling of this block into 1.0 cm thick wafers cut parallel to bedding provides considerably greater detail during the transition. These detailed data suggest that the oscillatory movement of the VGPs in these sections may be a result of the averaging caused by standard, detailed minicoring and not by the sediments or the remanence acquisition process.     

A critical appraisal of the Cayconi Formation, Crucero Basin, southeastern Peru

The Cayconi Formation of the Crucero Basin, Puno Department, southeastern Peru, has been described as a 800–1000 m sequence of Oligocene and Miocene fanglomerate and lacustrine sedimentary rocks, interlayered with mafic and silicic volcanic rocks, and unconformably overlying Paleozoic and Cretaceous successions. On the basis of new field and petrological investigations, key aspects of the stratigraphic relationships of the rocks comprising this formation, and hence the viability of this lithostratigraphic name, are questioned. Thus, several sedimentary units previously assigned to the Cayconi Formation are reinterpreted as Cretaceous or older. We further argue that the formational terminology fails to accomodate the great variety of volcanic rocks, which are, moreover, disposed in isolated eruptive fields. We therefore propose establishment of the Crucero Supergroup as a broad, yet flexible framework for lithostratigraphic subdivision of the Tertiary sequences of the Cordillera Oriental of southeastern Peru. This new nomenclature accomodates the voluminous two-mica ash flow tuffs (Macusani Volcanics) and associated rocks of the Quenamari Meseta, a succession excluded from the existing lithostratigraphic classification scheme. It also permits distinction between the petrogenetically-contrasted upper Oligocene — Lower Miocene and Middle Miocene — Upper Miocene volcanic suites, which dominate, respectively, the Picotani and Quenamari Groups comprising the proposed Crucero Supergroup. Finally, the economically important granitic/rhyolitic intrusive centers cogenetic with the volcanic rocks are readily assignable to intrusive lithodemes in each group.     

藏南深反射测线附近地表地质观察研究成果

本文简要介绍了ＩＮＤＥＰＴＨ项目地质组１９９４年和１９９５年间沿ＩＮＤＥＰＴＨ—Ｉ路线进行的野外填图工作的初步研究成果。它们是：①在帕里西北确定了高喜马拉雅与特提斯喜马拉雅之间拆离带的存在；③在帕里北发现了一套特提斯沉积，从而提出帕里西北的拆离带有可能穿过南北方向的亚东－谷露裂谷，而没有大规模错开；③在特提斯喜马拉雅带中确定了两条具一定规模的逆冲断裂；④发现康马穹窿存在两期构造变形，早期为上盘向北运动，晚期为横弯造穹作用。最后，结合深反射资料进一步讨论了康马穹窿与藏南拆离带之间的关系。     

Turbulent kinetic energy budgets from a large-eddy simulation of airflow above and within a forest canopy

The output of a large-eddy simulation was used to study the terms ofthe turbulent kinetic energy (TKE) budget for the air layers above andwithin a forest. The computation created a three-dimensional,time-dependent simulation of the airflow, in which the lowest third ofthe domain was occupied by drag elements and heat sources to representthe forest. Shear production was a principal source of TKE in theupper canopy, diminishing gradually above tree-top height and moresharply with depth in the canopy. The transfer of energy to subgridscales (dissipation) was the main sink in the upper part of the domainbut diminished rapidly with depth in the canopy. Removal ofresolved-scale TKE due to canopy drag was extremely important,occurring primarily in the upper half of the forest where the foliagedensity was large. Turbulent transport showed a loss at the canopytop and a gain within the canopy. These general features have beenfound elsewhere but uncertainty remains concerning the effects ofpressure transport. In the present work, pressure was calculateddirectly, allowing us to compute the pressure diffusion term. Wellabove the canopy, pressure transport was smaller than, and opposite insign to, the turbulent transport term. Near the canopy top andbelow, pressure transport acted in concert with turbulent transport toexport TKE from the region immediately above and within the uppercrown, and to provide turbulent energy for the lower parts of theforest. In combination, the transport terms accounted for over half ofthe TKE loss near the canopy top, and in the lowest two-thirds of thecanopy the transport terms were the dominant source terms in thebudget. Moreover, the pressure transport was the largest source ofturbulent kinetic energy in the lowest levels of the canopy, beingparticularly strong under convective conditions. These resultsindicate that pressure transport is important in the plant canopyturbulent kinetic energy budget, especially in the lowest portion ofthe stand, where it acts as the major driving force for turbulentmotions.     

Marine geochemistry—2: scavenging redux

Inorganic surfaces sorb dissolved and particulate phases from seawater onto organic films from a variety of sources. Heavy metals such as Au, Pt, Mn and Cd come to the coatings primarily from particulate phases. This observation confirms a previous finding of this laboratory for the uptake of U, Pu and Po. With depth in the open ocean there are greater contributions from the dissolved phases. Microbial and photochemical processes can enrich surfaces in some metals through reduction reactions. Copper and Mn enrichments in the films are attributed to the photo-reductions of Cu(II) to Cu(I) and manganese oxides to Mn(II), respectively, while the uptake of Mo is a consequence of the reduction, mediated by organisms, of molybdate to Mo(V) or Mo(IV). Cadmium, on the other hand, appears to be bioaccumulated in the films.     

Evaluation of a Carbon Adsorption Method for Sampling Gasoline Vapors in the Subsurface

Monitoring of the vapor phase has emerged as a very convenient method for detecting volatile organic contaminants in the subsurface. It can provide a reliable way of placing ground water monitoring and recovery wells. The most common method uses a driveable ground probe (DGP) to extract a vapor-phase sample followed by direct injection of the vapor into a portable gas chromatograph (GC). However, many regional offices of regulatory agencies and consultants do not have ready access to such equipment. This research explores an alternative–the carbon adsorption method—in which the vapor is withdrawn by the DGP but concentrated on a small activated carbon trap (150mg). The carbon traps can be returned to a central laboratory for solvent extraction and GC analysis. This provides the advantages of increased sensitivity, reduction in field equipment and convenience of in-lab analyses (multiple GC injections are possible). A simple DGP and carbon trap system was constructed and tested at a field site. Vapor-phase concentrations of target compounds present in gasoline were mapped quite conveniently, ranging from 10,000μg/liter (vapor phase) to less than 10μg/L. These concentrations were also shown to decrease in the direction of the ground surface, as expected. Measurements of target compounds in soil showed that the vapor phase contributed a large fraction of the total contaminant burden where a non-aqueous-phase layer (NAPL) had been identified; as important, however, is the rather uniform contamination of the soil outside the NAPL region. Finally, the concentrations of target compounds in the vapor phase and ground water could be related in a manner roughly described by a simple equilibrium model, although exceptions were noted.     

Experimental determination of the solubility of carbon dioxide in molten basalt at low pressure

We report the first measurements of CO₂ solubility in molten basalt at pressures comparable to those at which submarine basalts erupt. A basalt from the Juan de Fuca ridge was equilibrated with CO₂-rich vapor at 1200°C, 100–1500 bar for up to four hours. After quenching, the glass was analyzed for dissolved carbonate ions by infrared spectroscopy. No forms of dissolved CO₂ other than carbonate were detected. CO₂ solubility is roughly a linear function of pressure at these low pressures. The experimentally determined solubility differs from previous estimates based on CO₂ concentrations of submarine glasses, on CO₂ solubilities in basaltic liquids at significantly higher pressures, and on CO₂ concentrations of glasses equilibrated with H₂OCO₂ vapor. Our results are compatible with those obtained previously at higher pressures on a molten Kilauea tholeiite only if there is a significant positive dependence of carbonate solubility on temperature.CO₂ contents of mid-ocean ridge glasses measured by infrared spectroscopy are generally higher than would be expected based on solubilities at the hydrostatic pressures for the water depths from which the glasses were recovered, but the lowest dissolved CO₂ contents agree with the experimentally determined solubilities. We propose that submarine glasses with low CO₂ contents were quenched from magmas that were able to degas because they rose slowly from depth. The common occurrence of glasses with dissolved CO₂ contents in excess of the experimentally determined solubility suggests they were quenched from magmas that ascended too rapidly to degas fully. In conjunction with our solubility data, the highest CO₂ contents allow minimum estimates of depths to magma chambers. Depths of 2.3 km beneath the ridge are indicated for the East Pacific Rise at 21°N, in agreement with geophysical constraints.     

A history of glacial stratigraphy in China

Glacial stratigraphy had a late start in China, and it fell to Li Szeguang, an outstanding geologist with a little experience of the European Alps, to inject a systematic approach into the study of Chinese Pleistocene glacial stratigraphy starting in the early 1920s. Several diamictons in low latitude mountains of E. China were attributed to glaciation. A formal stratigraphy had to await his detailed and long term studies of the Lushan (29°30'N) on which he based a three-fold sequence of Poyang (Gunz correlate), Da Gu (= Mindel) and Lushan (Riss). A Dali (Würm) glaciation was added on the basis of evidence from Yunnan Province. Based on much morphological and erosional evidence as well as an assumption of glacial provenance for the widespread bouldery clays, and despite early critical reviews, this work became the dominant hypothesis in Chinese glacial stratigraphy for over 50 years. Echoes of it still remain in the literature, despite mounting sedimentological evidence that the diamictons are weathered debris flow and alluvial accumulations (with some thin, high-level solifluction earths).Increasingly, glacial stratigraphy is now being based on the glaciated west of China from Yunnan to western Xinjiang and it is here that the definitive glacial stratigraphy will be established. At least four glaciations arealready authenticated in many localities in this vast region, although the resolution of this stratigraphy may never match that of the classic loess stratigraphy of Shaanxi and the yet-to-be studied stratigraphy of the thick lacustrine successions in the high desert basins of the west.