bluuebunny/crossref_metadata_2025_split huggingface dataset:
Organic geochemistry as a tool to study upwelling systems: recent results from the Peru and Namibian shelves
Authors: Repeta Daniel J. , McCaffrey Mark A. , Farrington John W. , McCaffrey Mark A. 
Published: April 2008

Abstract
Organic geochemists are currently developing new, molecular-based tools for studying the historical record of coastal upwelling. Several hundred biomarkers, organic compounds with known biological sources, have been identified in sediments underlying the world’s major upwelling systems. Studies completed over the last two decades show that the distribution of many biomarkers in sediments is heavily impacted by the oceanographic and ecological conditions that prevail at the time of sediment deposition. Therefore, these biomakers have the potential to serve as useful palaeoenvironmental indicators. In order to decipher the sedimentary record on the molecular level, a better understanding of early diagenesis, and the coupling between water column biology, chemistry and biomarker distribution in sediments, is needed. Upwelling environments provide excellent sites to conduct such studies. High sediment accumulation rates and good preservation of organic matter in sediments relative to other depositional environments permit high-resolution time-series studies. We summarize here some recent research on early diagenesis, the effect of dysaerobic conditions on biomarker preservation, and the impact of El Nino Southern Oscillation (ENSO) events on the organic geochemical record. We further suggest some specific areas of future research that could substantially enhance our understanding of upwelling environments.
Evidence for aridity-driven dust flux to the northwest Arabian Sea and for decoupling of the dust and upwelling systems
Authors: Krissek L. A. , Clemens S. C. 
Published: April 2008

Abstract
Late Quaternary records of dust grain size, dust flux (mass accumulation rate; MAR) and dust mineralogy from ODP Site 722 on the Owen Ridge, northwest Arabian Sea, have been used to: (1) infer environmental conditions in the continental source areas; (2) determine the spectral response (frequency and phase relationships) of dust MAR and mineralogy to environmental changes associated with global ice volume variations and orbital insolation patterns and (3) evaluate potential associations between the continental (dust) and marine (upwelling) systems over the past 800 ky. The terrigenous dust component is composed of smectite, illite, palygorskite, kaolinite, chlorite, quartz, plagioclase feldspar and dolomite. This is consistent with the composition of source areas currently supplying sediment to the Arabian Sea. An R-mode factor analysis identifies four mineral assemblages: three reflect arid continental source conditions and one reflects more humid conditions. Total dust MAR, two of the three ‘arid’ factor assemblages, and six of the seven minerals associated with the ‘arid’ factors are coherent with the marine isotopic record of global ice volume over at least one primary orbital frequency, with input maxima during glacial (arid) hemicycles. In contrast, wind strength and upwelling intensity indicators exhibit maxima during the interglacial (humid) hemicycles, indicating a decoupling of the continental (dust MAR) and monsoon-driven upwelling systems.
Variation of South Somalian upwelling during the last 160 ka: radiolarian and foraminifera records in core MD 85674
Authors: Caulet Jean Pierre , Vénec-Peyré Marie-Thérèse , Vergnaud-Grazzini Colette , Nigrini Catherine 
Published: April 2008

Abstract

Indicators of upwelling activity and surface-water productivity for the last 160 ka have been studied in the ‘Marion Dufresne’ core MD 85674 taken off Somalia (3°11, 2 N-50°26, 3 E; 4875 m depth). Quantitative changes in the abundances of radiolarian species which are restricted to upwelling areas (
A. murrayana, C. irregularis, D. infabricatus, L. nigriniae, P. caryoforma, P. crustula
, and
P. minythorax
) were used to monitor the variation of vertical advection of deep water. These changes are compared with those recorded by the stable carbon isotopes of a foraminiferal thermocline dweller,
N. dutertrei
, and with quantitative variations of some planktonic foraminifers (
N. dutertrei, G. bulloides, G. menardii, G. sacculifer
and
G. glutinata
). Taken together, our data indicate that, under the south Somalian gyre, upwelling activity was maximal during transition between isotope stages 6 and 5, isotope stage 3, and transition between isotope stages 2 and 1 (respectively at about 130 ka, 65 to 25 ka, and 15 to 10 ka). These data also suggest that, at least during the last 60 ka, periods of increased activity in the Somalian, Arabian and Peruvian upwelling systems were synchronous.
Peruvian coastal primary palaeo-productivity during the last 200 000 years
Authors: Schrader Hans 
Published: April 2008

Abstract

Two times series of primary palaeo-productivity off the Peruvian coast over the last 200 000 years are presented for Ocean Drilling Leg 112 Sites: Site 681, Hole 681A at 11° and Site 686A, Hole 686A at 13° southern latitude. Estimates of primary palaeoproductivity are based on the transfer function PDU-3 that uses marine planktonic diatom assemblage distribution from surface sediment samples and a ten-year average primary productivity map over the area with a range of 37 to 365 gC/m
2
/year. Primary palaeoproductivity does not follow a simple glacial-interglacial mode; the record is more complex and above average palaeo-productivity occurred in Sites 681 and 686 over the following time intervals: 12 to 28, 60 to 68, and 148 to 160 ka; major increases in productivity were found during peak glaciations, and significant decreases occurred at 8, 24, 72 to 80, 112 to 128, and 160 to 168 ka. The averaged palaeo-productivity curve of 681A and 686A over the last 160 000 years is in inverse phase with the Vostok ice core atmospheric CO
2
indicating that part of the atmospheric CO
2
variations is linked to changes in the biological pump activities of the eastern Pacific boundary current system.
Transfer functions to reconstruct ocean palaeoproductivity: a comparison
Authors: Sarnthein M. , Pflaumann U. , Ross R. , Tiedemann R. , Winn K. 
Published: April 2008

Abstract

Oceanic plankton (export) productivity contributes to the control of glacial-to-interglacial changes in atmospheric CO
2
concentration. The extent of this contribution may be deciphered from global reconstructions of palaeoproductivity. We quantitatively estimate palaeoproductivity over the last 350 000 years in the eastern equatorial Atlantic, using equations based on foraminiferal assemblages and marine organic carbon accumulation rates; and make qualitative estimates using diatom and radiolarian accumulation rates. These proxydata are calibrated to data on modern primary production. When applied to the same set of marine sediment samples, the various reconstruction techniques produce productivity estimates with similar temporal productivity oscillations and a long-term similar absolute productivity level, suggesting that each provides a good signal of productivity changes.
The history of coastal upwelling off Peru (11°S, ODP Leg 112, Site 680B) over the past 650 000 years
Authors: Heinze Peter-Matthias , Wefer Gerold 
Published: April 2008

Abstract

The coarse sediment fraction (>63 μm) and benthic foraminiferal composition of Quaternary sediment cores from ODP LEG 112 Hole 680B (252.5 m water depth) were sampled from the upwelling area off Peru. High-resolution oxygen isotope stratigraphy based on analyses of the benthic foraminifera
Bolivina seminuda
allows a detailed reconstruction of the upwelling history since oxygen isotope Stage 16. Variations in the relative abundances of the benthic foraminiferal species
Nonionella auris, Bolivina costata
, and
Bolivina seminuda
document changes in fertility, which were found to be enhanced during interglacial Stages 1, 3, 5, 7, 9 and 11. Generally, bioturbated sediments are dominated by the genera
Cassidulina, Epistominella, Gyroidina, Trifarina
and
Valvulineria
. These benthic foraminifera are members of the ‘Undercurrent Fauna’, which is commonly associated with sediments that have a high terrigenous detrital component and a low organic carbon content (<3 wt%). These sedimentary deposits are indicative of high near-bottom current velocities of the Peruvian Undercurrent and well oxygenated bottom waters (0.5 to >6 ml/l O
2
). The entire core interval investigated in Hole 680B appears to have been affected by the Peruvian Undercurrent. Generally, undercurrent-dominated sections were found to correspond with glacial oxygen isotope stages.
Evolution of the Izu-Bonin intraoceanic forearc basin, western Pacific, from cores and FMS images
Authors: Pezard Philippe A. , Hiscott Richard N. , Lovell Michael A. , Collela Albina , Malinverno Alberto 
Published: April 2008

Abstract
One of the objectives of Ocean Drilling Program (ODP) Leg 126 was to investigate the origina nd evolution of the Izu-Bonin arc and forearc, both products of the subduction of Pacific lithosphere under the Philippine Sea Plate. Within the forearc basin, a full set of downhole measurements was recorded in two deep holes (792E and 793B). In addition, borehole electrical images were obtained (for the first time in the context of ODP) with the Formation MicroScanner (FMS*).
The main result of the drilling is that the forearc basin formed between 31.0 and 24.0 Ma by separation of a formerly contiguous frontal and outer arc high. The cored material shows a characteristic pattern of volcanogenic input, from turbidites and debris flows produced by volcanism and erosion of surrounding highs. The short rifting period is characterized by high sedimentation rate (300 m/Ma). In this context, the high resolution of FMS images was used to analyze the sedimentary processes associated with the deposition of deep-water volcaniclastics. The images reveal fine details of turbidite sequences that dip at low angles due to recent tectonics.
An FMS-based sedimentary log was calibrated from cores and prepared for each of the two holes, providing continuous bed-by-bed sections and permitting the investigation of trends in bed thicknesses. Palaeocurrent data were obtained from the analysis of ripple marks. During early basin history (30.2 to 29.5 Ma), the main sediment source was located to the east, in the vicinity of the modern outer arc high, with a secondary transport-mode oriented northward, along the basin axis. In the shallower section, emplaced at a lower rate from 28.9 to 27.3 Ma, axial transport from the north dominates a small component of flow from the western margin of the basin.
Initiated by rifting of the arc during the Oligocene time, basin development was followed by periods characterized also by extensional tectonics. Postdepositional extensional deformations such as normal microfaults, conjugate high-angle fractures, and dewatering veinlets were identified in the core and on FMS images. The orientation of the stress field within the arc and forearc was obtained from the analysis of borehole ellipticity. The results confirm models of stress distribution in forearc-arc-back arc regions. In particular, a rotation of the maximum horizontal stress trajectory in the overlying plate was observed, in a direction orthogonal to the plate boundary. In spite of a 90° clockwise rotation of the Philippine Sea plate since Oligocene time, the orientation of the stress field seems to have remained stable with respect to the trench axis over this period.
Automated prediction of sedimentary facies from wireline logs
Authors: Bølviken Erik , Storvik Geir , Nilsen Dag Erik , Siring Erling , Van Der Wel Dirk 
Published: April 2008

Abstract
The problem addressed is whether a computer can be programmed to identify depositional facies from a set of wireline logs. The basic approach is to let the computer learn by itself the patterns to search for by feeding it log signatures that have already been assigned facies labels. Having gone through this training phase, it can make sedimentary predictions from new data. The underlying model is a mathematical formalization of the idea that sedimentary processes have deposited lithological sequences which influence the observed log traces. Stochastic descriptions are used for these relationships. Markov chains link the lithology to the underlying sedimentary facies. The upward transition probabilities of the Markov chain are the main features which discriminate sedimentary facies.
An efficient reconstruction algorithm permits probabilistic restoration of both lithology and sedimentology. This allows the uncertainty of the conclusions to be quantified, and more than one interpretation can be put forward where appropriate. Results of the tests are promising.
Analysis of dipmeter data for sedimentary orientation
Authors: Cameron Gavin I. F. 
Published: April 2008

Abstract
Dipmeter data are logged as a vertical traverse through formations of interest. By analogy to concepts of dip analysis at surface outcrops, not all of the dip data collected by a logging run are of direct relevance to specific interpretation aims. A methodology is required whereby only relevant dip magnitude and azimuth data can be extracted from the mass of logged results. One approach involves the deductive use of log zonation and dip filtering techniques to isolate the dip results that most closely match the geological requirements. Statistical analysis for preferred orientation in the selected dataset gives a result to which clear geological significance can be attached. The principles described can be applied to a variety of sedimentary dipmeter interpretation problems, such as the determination of palaeocurrent directions, considerations of reservoir geometry and preferred bedding orientation.
Stress trajectory determinations in southwestern Ontario from borehole logs
Authors: Yassir Najwa A. , Dusseault Maurice B. 
Published: April 2008

Abstract
This paper reviews the benefits and limitations of the use of four-arm dipmeter logs to detect breakouts, and to estimate horizontal stress trajectories. An example is given from southwestern Ontario, a low seismic risk area, yet one with unusually high horizontal stresses, causing contemporaneous deformations such as pop-ups and tunnel closures. The preliminary results show that the relatively few breakouts detected follow two main trends, approximately N80°E and S80°E. The breakouts often have different orientations within the same well, but do not seem to ‘prefer’ a particular lithology. The data, when compared to other stress measurements, do show a general NE trend for compressive stress, as popularly believed. However, they are as yet too few to make statistically reliable assumptions on the stress state in the area.
Attempts to detect open fractures and non-sealing faults with dipmeter logs
Authors: Bell J. S. , Caillet G. , Adams J. 
Published: April 2008

Abstract

If enough stress-generated breakouts have been identified on four-arm dipmeter logs to map regional horizontal stress trajectories, it may be possible to use anomalous orientations to identify open fractures and non-sealing faults. These act as free surfaces and, if they were oriented obliquely to regional stresses, deflect them locally. This type of analysis is simple to apply and appears to offer a novel way to obtain key information required for optimizing the production of oil and gas. In offshore eastern Canada, the regional stress regime is well established and local deflections appear to be caused by near-by open fractures and non-sealing faults. In the Aquitaine Basin in France the regional stress signature is not homogeneous. Though some anomalous orientations appear to be caused by open fractures and faults, it is clear that many are not. Hence the method may not be applicable everywhere. Possible reasons include differences in the σ
H
:σ
h
ratios of the two areas and the geotectonic settings.
Lithological and fracture response of common logs in crystalline rocks
Authors: Bremer Martin H. , Kulenkampff Johannes , Schopper Jürgen R. 
Published: April 2008

Abstract
Log responses in crystalline rocks are a function of the lithology (rock matrix), macro- and microfractures. Basically, the porespace-geometry dependent petrophysical parameters (porosity, internal surface area, permeability, formation factor) are similarly interrelated as in sediments, but mineralogy and fractures exert greater control than in sediments.
A qualitative log interpretation method was developed and tested. Fractured and otherwise deteriorated zones with anomalous properties are identified, using a suitable combination of five standard logs. Before a quantitative well-log analysis of petrophysical parameters is possible, the logs are corrected for lithology effects, using a quick-look lithology identification program. For lithology identification seven logs are considered sufficient. Their mean lithology-dependent response in the German KTB borehole is reported.
Application of dipmeter data in structural interpretation, Niger Delta
Authors: Adams J. T. , Ayodele J. K. , Bedford J. , Kaars-Sijpesteijn C. H. , Watts N. L. 
Published: April 2008

Abstract
Modern 3D seismic techniques provide both high resolution and dense coverage for interpretation of subsurface structures, and are a great improvement on older 2D methods. Features at the lower limit of seismic resolution, however, are often not resolved and interpretation can be ambiguous. Structures at this scale, in particular small-scale faults, are critical for optimum placement of production wells and enhanced field development. Integration of structural analysis techniques from borehole dipmeter data with 3D seismic data allows re-examination of an interpretation and enhancement of such small-scale features.
Dipmeter analysis of a faulted structure in the Niger Delta has enhanced the structural model derived from 3D seismic. The faulted intervals were first identified using vertical dip trend analysis, and the faults were orientated by applying stereographic techniques to bedding dips in zones of fault-related deformation. Near-wellbore structure was then modelled by constructing dipmeter-based cross-sections using an interactive workstation, and minimum fault throw was estimated from these models. Accurate positioning of the faults on the seismic data was achieved by time-indexing the dipmeter log, and the structure was integrated with the present-day stress regime using borehole breakout analysis.
Application of these methods has allowed faults that were below seismic resolution to be identified and oriented, and for these to be explained in terms of the regional structural regime.
Enhanced resolution resistivity logging for fracture studies
Authors: Jackson P. D. , Shedlock S. , Willis-Richards J. , Green A. S. P. 
Published: April 2008

Abstract
Electrical resistivity techniques have been developed which have a higher resolution than standard logging methods. The paper describes an enhanced focusing technique which is shown to increase the resolution to an extent where individual fractures can be identified within what appears to be a single fracture from conventional focused logs. The measurements were made in the Carnmenellis Granite and the problems of making resistivity measurements in this resistive environment are discussed in terms of electric current flow in the borehole which is controlled by the proximity of connected fractures and the resistivity contrast between the fluid and the formation.
Laboratory measurements of the seismic properties of sedimentary rocks
Authors: McCann C. , Sothcott J. 
Published: April 2008

Abstract
A laboratory technique for the measurement of compressional and shear wave velocities and attenuations (quality factors) of sedimentary rocks at pressures up to 70 MPa and at frequencies up to 1 MHz was published by Winkler and Plona in 1982. In the method the rock sample is enclosed between buffer rods. The arrival times and amplitudes of the signals reflected from its top and base are compared. A similar technique has been used for many years in the non-destructive testing of metals and plastics. This paper describes the use of a single-frequency pulsed sine wave for the velocity and attenuation measurements, in addition to the transient pulse used in the Schlumberger method. Measurements were made on samples with accurately known compressional and shear wave properties and on replicated rock samples. The results demonstrate that the determinations of velocities are accurate to ± 0.3%, and that the determinations of attenuation are accurate to ±0.1 dB/cm at 0.85 MHz.
A new approach to the interpretation of nuclear borehole logs
Authors: Ølgaard P. L. 
Published: April 2008

Abstract

When interpreting nuclear borehole logs it is usually assumed for simplicity that the tool signal and a formation property, e.g. its density, are uniquely related. However, while the tool signal will usually depend primarily on one formation property, it will also be affected by other formation properties. To take this multi-parameter dependence of all the nuclear tools into account, a new approach to nuclear log interpretation has been developed. In this approach four nuclear tools are considered: the gamma density tool, the neutron porosity tool, the pulsed neutron tool and the natural gamma tool. The responses of these four tools depend on formation density (ρ), porosity (φ), neutron absorption (σ
a
), and geological type (including shale content). For each of the four tools a physical model has been developed. The signals measured with the four tools and the four models represent together four equations with four unknowns: ρ, φ, σ
a
and geological type. These four equations may be solved by iteration and a consistent set of formation properties obtained. The interpretation approach has been implemented on a personal computer. Examples of results of such interpretations are presented.
Sandstone diagenesis: framework of a forward modelling approach by integrating wireline and other geological data
Authors: Cheshire M. J. , Sellwood B. W. 
Published: April 2008

Abstract
The integration of wireline log information with other forms of geological data allows the recognition of certain diagenetic effects in sedimentary rocks. Diagenetic modification of a sandstone causes changes in the rock properties that will produce subtle shifts in the wireline log characteristics. Such shifts are often difficult to interpret. However, by reference to a now extensive diagenetic information base it is possible to construct models for processes controlling some types of diagenetic change. So it is possible both to model and predict the spatial distribution of diagenetic effects. To interpret diagenetic effects on wireline logs it is necessary to integrate existing geological databases with information derived from wireline logs in a form that can be handled by computer. Interactive modelling involves the iterative refinement of modelled features. This is done by comparing synthetic logs with actual logs over intervals from which core-derived information is available (Rotliegend and Sherwood sandstones provide ideal test-beds for this approach). Predictive models may then be generated by an interactive process of confirming or denying modelled features. This approach allows refinement of the interpretations made. Eventually we expect that a means of directly interpreting certain important diagenetic effects will be possible from wireline log data.