bluuebunny/crossref_metadata_2025 huggingface dataset:
Relating erosion shear stress to tidal flat surface colour
Authors: Riethmüller R. , Hakvoort J. H. M. , Heineke M. , Heymann K. , Kühl H. , Witte G.
Published: April 2008

The relationships between erosion shear stress and bio-geochemical parameters, and between the optical reflectance spectra and bio-geochemical parameters of different tidal flats were investigated in the North Frisian part of the Wadden Sea. Erosion shear stress shows clear dependencies both on fine-grain fraction <63 µm and on the benthic diatom chlorophyll
a
concentration present in the uppermost 1 mm sediment layer. The strongest dependence of erosion shear stress on the benthic diatom chlorophyll
a
surface concentration was found for muddy areas. This dependence vanishes for sandy surface sediments. The reflectance spectra show two main classes. The first class contains information on the fine-grain fraction <63 µm of areas not covered by phytobenthos. The second class corresponds to the phytobenthos which can be subdivided into benthic diatoms and other macrophytes. A significant correlation was found between reflectance spectra and the amount of the fine-grain fraction <63 µm present and also between reflectance spectra and the benthic diatom chlorophyll
a
concentration. Thus, erosion shear stress can be related to tidal flat surface colour yielding a basis to map the erodibility of tidal flats using optical remote sensing.
Biological control of avalanching and slope stability in the intertidal zone
Authors: Shaikh Masroor A. , Meadows Azra , Meadows Peter S.
Published: April 2008

Sediment structure and stability are important features of soils and sedimentary environments, and are of great importance for the maintenance of estuarine, and intertidal banks and channels. Avalanching and slope stability are critical factors controlling the shape of intertidal dunes. The role of biological activity in these processes, either in the form of vegetation and roots, animal burrows, algal mats or microbial glues, is now a forefront area of research. The objective of our research has been to quantify the effects of biological activity on the avalanching of intertidal sediments. Natural microbial and meiofaunal communities and particularly burrowing infauna such as
Corophium volutator
and
Nereis diversicolor
dramatically increase angles of avalanche and factors of safety. They do not significantly alter angles of repose. Factors of safety, duration of avalanches, and percentage increase in sediment volume, all increase with increasing angles of avalanche. Video analysis of the avalanching process shows that different slope failure mechanisms are associated with the different types of biological activity, and probably reflect specific chemical and mechanical stabilizing effects produced by the different types of biological activity. These latter include tubes, extracellular polymeric materials, and a small element of compaction. These mechanisms include rotational failure, block and wedge failure and translational failure. The significance of our results for the stability of slopes under field conditions is discussed, and it is concluded that field seeding experiments in the intertidal zone with the species that we have used would be very productive.
An estimation of the intrinsic permeability of argillaceous rocks and the effects on long-term fluid migration
Authors: Tokunaga T. , Hosoya S. , Tosaka H. , Kojima K. , Hosoya S.
Published: April 2008

The effect of intrinsic permeability of argillaceous rocks on long-term petroleum migration is discussed from results of both laboratory experiments and numerical sensitivity studies. Results of our experimental studies on one-dimensional mechanical compaction of both muddy slurries and mudstones show that porosity(
φ
)-permeability(
K
) relationships are linear on double logarithmic scales, i.e.
φ
and
K
satisfy the relationship:
K
=
K
0
(
φ/φ
0
)

a

, where
K
0
is initial permeability,
φ
0
is initial porosity, and ‘
a
’ is a parameter depending on the samples used. Comparison with published
φ
-
K
relationships of mudstones indicates that the above mentioned relationship could be extrapolated to natural condition when
φ
>0.3. On the other hand, the relationship becomes deviated from measured data in the low porosity range.

Considering the importance of
φ
-
K
relationships in
φ
<0.3 on the timing of expulsion of petroleum from source rocks, this paper discusses the way to find out an appropriate
φ
-
K
relationship through trial and error matching of results of basin simulation using different
φ
-
K
relationships with observed pore pressure and porosity data. We also present one case study using an imaginary siliciclastic sedimentary basin and discuss the potential of a numerical study to establish one of the appropriate relationships, showing how calculated results become varied by choosing different relationships.
A comparison between 1-D, 2-D and 3-D basin simulations of compaction, water flow and temperature evolution
Authors: Throndsen T. , Wangen M.
Published: April 2008

Is 3-D basin modelling really necessary? The answer depends on the type of problem under consideration. The Institute for Energy Technology (IFE) 3-D Basin Simulator offers the possibility to investigate certain aspects of this issue. It is a prototype 1-D, 2-D or 3-D basin simulator for reconstruction of the time-dependent pressure and temperature evolution of compacting sedimentary basins. Comparison of results, from 1-D, 2-D and 3-D simulations confirms that there may be significant differences due to water flow and pressure behaviour. The advantage of 2-D simulations as compared to 1-D is seen in overpressured configurations where 2-D allows more realistic calculations, while 1-D is less predictable. When going from 2-D to 3-D advantages are evident in configurations with pronounced 3-D basin geometries in combination with rapid processes and overpressure. The differences in water flow velocities and consequently the pressure calculations, may be quite significant. The rates in 3-D may be several orders of magnitude higher than in 2-D due to lateral focusing effects, while the overpressure still remains high. One consequence is, for example, reduced compaction in 3-D as compared to 2-D, with notable effects on basin geometry and temperature evolution of individual layers. Temperature anomalies due to increased convection in 3-D seem to be of less importance, unless processes of lateral focusing of water flow reach extreme levels.
Comparative studies of pre- and post-drilling modelled thermal conductivity and maturity data with post-drilling results: implications for basin modelling and hydrocarbon exploration
Authors: Ho T. T. Y. , Jensen R. P. , Sahai S. K. , Leadholm R. H. , Senneseth O.
Published: April 2008

The primary objective of this paper is to demonstrate the successful application of maturation modelling using velocity-based thermal conductivity in the whole cycle of hydrocarbon exploration (frontier, developing and mature) on the Halten Terrace, Offshore Norway. This seismic geochemical method, used as a critical technique for selecting a favourable block in the early stages of exploration in the early 80s, enabled Conoco and its partners to make the first oil discovery and the subsequent discovery of the giant Heidrun Field in the area north of the 62nd parallel.

Additional data (modelled and measured) on the thermal conductivity and Ro values are now available, and provide an excellent opportunity to compare the original (pre-drilling) basin models and related modelled results of the early 80s with 1990s (post-drilling) state-of-the-art data and models. The results of the comparative study indicate that the velocity-based conductivity compares very well with measured data, particularly in an overpressured area where conventional porosity (modelled) based thermal conductivities were inadequate. The pre-drilling predicted values of heat flow, geothermal gradient and vitrinite reflectance (based on Arrhenius equation) and oil window limits also compared favourably with post-drilling measured results. Thus, the method of calculating thermal conductivity from seismic data provides a useful tool to integrate geochemistry and geophysical (seismic) data, to calibrate maturation models, and to enhance the value of geochemistry and basin modelling in hydrocarbon exploration.
Dating of crustal fluid flow by the Rb-Sr isotopic analysis of sphalerite: a review
Authors: Walshaw R. D. , Menuge J. F.
Published: April 2008

The sphalerite Rb-Sr isochron technique is a relatively new and powerful geochronological tool allowing the direct dating of zinc sulphide mineralization. Recently, insights have been gained into the Rb-Sr isotope systematics of sphalerite, in particular the crystallographic residence sites of Rb and Sr and the mechanisms by which the Rb/Sr ratio of sphalerite is fractionated over that of its parent hydrothermal fluid. This, along with independent testing against three other dating techniques, has resulted in the vindication of the sphalerite Rb-Sr isochron technique. The resultant isochron ages have allowed very precise chronological constraints to be placed on the genesis of American and Australian Mississippi Valley-type deposits, greatly reducing the controversy which has surrounded this class of base metal deposit.
Reconstructing fluid history: an integrated approach to timing fluid expulsion and migration on the Carboniferous Derbyshire Platform, England
Authors: Hollis Cathy
Published: April 2008

Galena-sphalerite-baryte-fluorite mineralization and non-economic reserves of liquid hydrocarbon and bitumen are hosted on the Derbyshire Platform within Lower Carboniferous limestone which was deposited during Variscan back-arc extension. The limestone also hosts a sequence of burial calcite cements, precipitated dominantly in crosscutting and refractured extensional vein systems, which typically follow Caledonian-Variscan trends. The calcite cements are often intergrown with galena, sphalerite, baryte and fluorite as well as bitumen, whilst epifluorescence reveals hydrocarbon inclusions. Paragenetic relationships, in conjunction with geochemical results, permit definition of a precise timing for mineralization and hydrocarbon emplacement, and modelling of the source, composition and migration pathways of the mineralizing fluids.
Geochemical constraints on the origin and timing of palaeofluid flow in the Presqu’ile barrier reef, Western Canada Sedimentary Basin
Authors: Qing Hairuo
Published: April 2008

The Mississippi Valley-type ore deposits at Pine Point are spatially and genetically associated with saddle dolomite cements. The origin and timing of fluid flow that produced these ore deposits are, therefore, constrained by paragenesis and geochemistry of the saddle dolomite cements. Because saddle dolomites occur continuously across the sub-Watt Mountain unconformity, dolomitization and associated mineralization must have occurred after the sub-Watt Mountain exposure during burial. The lateral continuity of saddle dolomite along the barrier for 400 km suggests that dolomitization and mineralization were probably associated with the lateral fluid migration along the barrier reef.

From northeastern British Columbia to Pine Point, over a lateral distance of 400 km, saddle dolomites display remarkable trends of decreasing
87
Sr/
86
Sr ratios (0.7106 to 0.7081) and homogenization temperatures of fluid inclusions (178°C to 92°C) with a corresponding increase in δ
18
O values (−16‰ to −7‰ PDB). These regional trends suggest that hot, radiogenic basinal fluids moved eastward up-dip along the Presqu’ile barrier reef. The movements of basinal fluids were probably related to tectonic compression and sedimentary loading on the western margin of the Western Canada Sedimentary Basin during either the late Devonian-early Carboniferous (Antler Orogeny) or the Jurassic-early Tertiary (Columbia-Laramide Orogenies).

The δD values of aqueous fluid inclusions from Pine Point dolomite are very low (−80‰ SMOW to −100‰ SMOW) compared with that of Devonian seawater (−10‰ SMOW) suggesting an input of some Columbia-Laramide meteoric waters. If the low δD values are caused by a mixture of fluids from primary inclusions containing Devonian seawater with secondary inclusions that formed later during the Columbia-Laramide Orogenies, the fluid migration and associated dolomitization and mineralization could be interpreted as late-Devonian events. However, if the measured fluids were mostly from the primary inclusions, the low δD values indicate an entrapment of some Columbia-Laramide meteoric waters with Devonian formation waters at the time of dolomitization. This would suggest a Jurassic to early Tertiary age for dolomitization, which is supported by similar low δD values of present-day Devonian formation waters that consist of a mixture of Laramide meteoric waters and original connate brines. The light δD values of Pine Point dolomite inclusions could also occur as a result of reaction of fluids with organic matter associated with generation of oil and gas, which occurred at the maximum burial during the Laramide Orogeny.
On the vertical and lateral distribution of hydrocarbon migration velocities during secondary migration
Authors: Sylta Øyvind , Pedersen J. I. , Hamborg M.
Published: April 2008

Recent improvements in the techniques for modelling the flow of oil and gas through carrier systems allow us to compute the column heights of migrating oil and gas stringers. The calculations are performed throughout the history of the basin and suggest that secondary migration occurs at low saturation, and very often with velocities in excess of 100 km/Ma. The migrating hydrocarbon stringer columns are modelled to be very thin, i.e. in the centimetre range, over large areas. This paper discusses the lateral and vertical distribution of the above properties in order to elucidate and quantify the extreme focusing that hydrocarbons can experience during secondary migration. A result of these very efficient migration mechanisms is that we can successfully explore for hydrocarbons, even in traps that require lateral migration distances in excess of 1000 km from source to trap.
ESR isochron dating of the Nojima Fault gouge, southwest Japan, using ICP-MS: an approach to fluid flow events in the fault zone
Authors: Fukuchi Tatsuro , Imai Noboru
Published: April 2008

The absolute age of the earliest fault movement during the Quaternary period of the Nojima Fault, which moved in the 1995 Southern Hyogo Prefecture Earthquake (SHPE) in Japan, has been determined from clay grains in the fault gouge by election spin resonance (ESR) isochron dating using inductively coupled plasma mass spectrometry (ICP-MS). According to ICP-MS analysis, the concentrations of minor elements tend to increase with decreasing clay grain size, and they may adhere to the surface of clay grains, especially montmorillonite, in the fault gouge. The adherence of minor elements is possibly attributed to electrical charges caused on the surface of montmorillonite by substituting Al for some of the Si in the tetrahedral sites of the Si2O5sheets.238U,232Th and208Pb also increase with decreasing grain size, whereas the238U/232Th ratio is almost unchanged within the measurement error. This indicates that a closed system in the238U- and232Th- series had pertained in the fault gouge collected since the earliest fault movement. On the other hand, the208Pb/232Th ratio decreases with the grain size, and the208Pb/232Th ratio from a grain of <1 µm is about 14% of that from <500 µm.232Th decays to radioactively stable208Pb and produces220Rn in the decay process, but220Rn which is an inactive and non-ionized gas cannot be absorbed electrically on the surface of montmorillonite. Therefore, most of the220Rn gases produced on the surface of clay grains may be released from the fault fracture zone. A radon anomaly in ground water observed before the 1995 SHPE may be attributed to radon loss by huge clay mineralization in the Earth’s crust. The ESR isochron age of the fault clay gouge of the Nojima Fault is estimated as 0.84 ± 0.12 Ma to 1.23 ± 0.1 Ma from each clay grain by considering the calibration of radon loss, water content and defect creation efficiency by α-rays. This age is consistent with a date of about 0.53 −3 Ma that was estimated as the beginning of the Nojima Fault from recent drilling core data.
Experimental investigation of molecular transport and fluid flow in unfaulted and faulted pelitic rocks
Authors: Krooss B. M. , Schloemer S. , Ehrlich R.
Published: April 2008

Laboratory experiments have been performed to investigate the fluid transport properties of fine-grained (pelitic) sedimentary rocks with respect to their hydrocarbon sealing efficiency. The experiments comprised the molecular transport (diffusion) of gases and the pressure-driven volume flow (Darcy flow). Diffusion coefficients of methane and nitrogen in water-saturated rock samples were measured at 90°C from an equimolar mixture of the two gases (10 MPa gas pressure). Methane had lower effective diffusion coefficients but higher steady-state molecular transport rates than nitrogen. Permeability coefficients measured with water on selected unfaulted natural samples, under controlled effective stress up to 47 MPa, ranged between 10
−22
and 10
−19
m
2
. The permeability reduction with increasing stress mostly followed an exponential relationship. Permeabilities of faulted mudstones from compressive fault zones ranged from 2 up to 20 × 10
−21
m
2
. No systematic relationship between permeability and microfault frequency within the samples or distance of samples from microfault planes could be identified. Permeability measurements were also performed on macroscopically homogeneous Carboniferous shales from a compressive tectonic stress field. These samples, which showed distinct sonic velocity anisotropies, had permeability coefficients in the nanodarcy (nDarcy; 10
−21
m
2
) range. Only in one case was a significant permeability anisotropy associated with the sonic velocity anisotropy.
Simulating polyphase faulting with a tensorial 3D model of fault growth
Authors: Maillot B. , Cowie P. , Lague D.
Published: April 2008

Fault growth in brittle media has previously been extensively studied via a numerical approach using scalar representation of the stress and strain fields. More realistic simulations and further investigations of fault array evolution demand a fully tensorial three dimensional (3D) representation of these fields. In this paper we present a tensorial model of the spontaneous birth and growth of faults in a 3D medium. The medium is elastic and attenuating up to a stress threshold, determined by the Mohr-Coulomb criterion, where brittle failure is modelled by a partial shear stress drop. Elastic radiations generated by the rupture are explicitly extrapolated in time by a finite-difference scheme of the equation of dynamics until the static state is reached. We do not consider the dynamic process explicitly; further ruptures can only be triggered by the resulting static stress field, or by the imposed straining of the medium. Preliminary simulations of 3D straining of a 2D plate show how a pre-existing fault set (appearing as a perturbed stress field)can influence the development of a second fault set. We believe that our model provides a valuable tool for the study of fault development and in particular for the assessment of the effects of anisotropic stresses around faults on strain accumulation and the spatial organization of crustal deformation.
The influence of fault compaction on fault zone evolution
Authors: Henderson J. R.
Published: April 2008

A numerical simulation of coupled fluid flow and failure in a fault zone is presented, that shows the impact of the dynamics of fault growth on the relationship between seismic and sub-seismic fault scaling. A tabular fault zone is modelled which undergoes non-uniform compaction, leading to development of pressure gradients in the fault zone. Increased pore fluid pressures result in a reduction of the fault-normal stresses, and hence frictional failure. The model suggests a possible physical mechanism for the development of non-power-law scaling of fault displacements, implying that observations of such behaviour are real, rather than the result of poor sampling. The fault displacement scaling law appears to result from the dynamics of the fault evolution, which depend upon the relative rates of compaction of fault material and fluid flow within the fault zone. When fault compaction is slow, event sizes show a power-law scaling relationship. When compaction is fast, pressure gradients are unable to dissipate and the distribution of these event sizes is non-power-law. It is suggested that the dynamics of fault evolution may be inferred from geological evidence, with the relics of repeated small events (e.g. aligned calcite fibres) implying a relatively high compaction rate, whereas cataclasites would suggest relatively low compaction rates.
Relating microscale rock-fluid interaction to macroscale fluid flow structure
Authors: Leary P. C.
Published: April 2008

Borehole logs of rock-property spatial fluctuations in the metre to kilometre scale range have power-law Fourier power-spectra that scale inversely with spatial frequency to a power near unity,
S
(
k
) ∞ 1/
k
α
. The spectral scaling is ‘universal’ in the sense that a narrow range of scaling exponents, α
v
≈ 1.1 ± 0.1 for vertical wells and α
h
≈ 1.34 ± 0.1 for horizontal wells, describes rock density, elastic modulus, porosity and lithology fluctuation spectra of a sample of 50 well logs; for 35 vertical logs the bounds on α
v
hold for both sedimentary and crystalline rock types. The power-law nature of rock-property fluctuation spectra are modelled as long range spatial correlations arising from short range (grain scale) uncorrelated random fluctuations. Such long range random spatial correlations occur in thermodynamic order-disorder phase transitions. Applying to rock the statistical physics of thermodynamic order-disorder transitions, rock heterogeneity observed in borehole logs formally emerges from grain-scale elastic interactions and long range spatial organization of finite strain induced grain-scale defects associated with fluid percolation. If fluid flow paths in rock are significantly influenced by long range correlated random structures, reservoir management cannot be accurately conducted from flow models constrained by small-scale sampling of the reservoir rock; macro scale measurements of site-specific long range random correlation structures are needed.
Volcano-tectonic variability along segments of the Mid-Atlantic Ridge between Azores platform and Hayes fracture zone: evidence from submersible and high-resolution sidescan sonar data
Authors: Gràcia Eulàlia , Parson Lindsay M. , Bideau Daniel , Hekinian Roger
Published: April 2008

Three contrasting segments of the Mid-Atlantic Ridge (MAR) between the Azores platform and Hayes fracture zones have been studied to determine variations in the magmato-tectonic processes between segment centre and segment ends. Two of the segments (OH1 and OH3) were surveyed with the submersible
Nautile
and the third (Lucky Strike or PO1), with high resolution deep-tow side-scan sonar (TOBI). Segment OH1 is a long and robust segment with a wide and shallow axial valley floor. Submersible observations support the inference that this segment is characterized by intense volcanic activity concentrated principally at the segment centre. Segment OH3, with a narrow and deep rift valley, is dominated by tectonic features at the segment centre, whereas the most recent volcanic constructions are found to outcrop in its southern part. Lucky Strike contains a shallow centrally located volcanic platform but tectonic features predominate all along the segment. The only evidence of fresh volcanism are a lava lake and fresh sheet-flows located at the middle and northern part of the platform, respectively. Each of the three segments show ‘bull’s eye’ gravity lows suggesting a focused mantle diapiric source.
Fault-controlled magma transport through the mantle lithosphere at slow-spreading ridges
Authors: Allerton S. , Macleod C. J.
Published: April 2008

At the ends of spreading segments at slow-spreading ocean ridges the axial valleys are usually asymmetric and bounded by large valley-wall faults, in contrast to segment centres, which are usually symmetric, with relatively small faults. These morphological variations are believed to reflect differences in the thermal structure of the lithosphere, caused by focused upwelling of partially molten mantle beneath segment centres. This gives rise to a thinner crust but thicker, and consequently stronger, lithosphere at segment ends, which is likely to provide a barrier to melt migration in comparison with segment centres. High-resolution sidescan sonar images of parts of the Mid-Atlantic Ridge reveal that flattopped seamounts occur preferentially at segment ends. Many of these seamounts are situated asymmetrically within the axial valley, and show a spatial association with the large segment-end faults, raising the possibility that melt may be channelled through the lithospheric mantle by such faults. If this is the case, then one might expect to encounter gabbros within mantle shear zones. We here document one such case in the Lizard Ophiolite, Cornwall, Southwest England: the Carrick Luz shear zone, a 100m wide dykelike body of gabbro and gabbro mylonite occurring within mantle ultramafic rocks. The shear zone shows a progression from penetrative mylonite fabrics, discrete ultramylonitic shear planes, through to cataclasites and fault gouges. The penetrative mylonite fabrics themselves deform, and are in turn cut by, mafic dykes. Shear direction and sense is the same for all of the fault rock types, ductile and brittle, and after correction for the regional tilt of the ophiolite section, are consistent with normal faulting. Gabbro mylonitic shear zones such as this within the lithospheric mantle provide a mechanism for weakening the lithospheric mantle. They might be expected to have a high acoustic impedance contrast with the surrounding ultramafic rocks, and are strong candidates for dipping seismic reflectors observed in old oceanic crust.
Tracing the evolution of hydrothermal fluids in the upper oceanic crust: Sr-isotopic constraints from DSDP/ODP Holes 504B and 896A
Authors: Teagle D. A. H. , Alt J. C. , Halliday A. N.
Published: April 2008

Whole rock and secondary mineral
87
Sr/
86
Sr determinations from ODP Holes 504B and 896A have been compiled into a profile for the upper 2 km of
in situ
ocean crust to constrain the composition and evolution of recharge and discharge fluids. There is a gradual trend from elevated
87
Sr/
86
Sr whole rock ratios at the top of the crust towards primary MORB compositions at the base of the sheeted dykes. Rocks from the mineralized lava-dyke transition zone have
87
Sr/
86
Sr ratios clustering in the range of black smoker fluids. Igneous and alteration phases are not in Sr-isotopic equilibrium, therefore whole rock strontium isotopic ratios reflect the proportion of secondary minerals and the extent of plagioclase recrystallization. The
87
Sr/
86
Sr of anhydrite directly records the chemical evolution of seawater during recharge. There is a slight decrease in anhydrite
87
Sr/
86
Sr with depth in the volcanic rocks but anhydrite from the lowermost dykes yields diabasic
87
Sr/
86
Sr ratios. Epidotes in cross-cutting veins have higher
87
Sr/
86
Sr than the background alteration recorded by amphibole in the lower dykes. This elevated signature reflects the
87
Sr/
86
Sr of 504B black smoker-type fluids and requires less fluid evolution than the background alteration, suggesting channelling of recharge fluids locally.
Petrological investigations of low temperature hydrothermal alteration of the upper crust, Juan de Fuca Ridge, ODP Leg 168
Authors: Hunter A. G. ,   DP Leg 168 Scientific Party
Published: April 2008

Ten sites were drilled across the eastern flank of the Juan de Fuca Ridge during ODP Leg 168 to investigate the nature of ridge-flank hydrothermal circulation and crustal evolution in a region of varying sediment-basement topography. The sites were divided into: (i) the Hydrothermal Transition (HT) transect, a zone of gradually thickening sedimentary cover, extending from the ridge-flank boundary for a distance of 20 km to the east; (ii) the Buried Basement (BB) transect, which consists of uniformly, flat lying basement, capped by a regionally continuous thick sedimentary sequence; and (iii) the Rough Basement (RB) transect, represented by a region of basement highs and troughs, located ∼100 km east of the ridge axis, and capped by a variable thickness of sediment.

Across the flank, a progressive sequence of alteration was recognized, produced as the hydrothermal system evolved from open oxidative conditions involving a slightly modified seawater derived fluid, to a more restricted/closed, non-oxidative environment in which more strongly modified seawater derived fluids interacted with the basement. Alteration commenced with the formation of chlorite at relatively high temperatures in non-oxidative to oxidative conditions, followed by the first wide spread stage of low temperature oxidative alteration with the formation of iron oxyhydroxides and celadonite. Progressive burial, along with continued alteration of the basement, restricted fluid circulation resulting in the formation of saponite ± pyrite over a range of temperatures under non-oxidative conditions. The final stage of alteration is represented by carbonate, which formed under non-oxidative conditions by interaction of strongly modified seawater and the crust.
Drilling of sediment-hosted massive sulphide deposits at the Middle Valley and Escanaba Trough spreading centres: ODP Leg 169
Authors: James R. H. , Duckworth R. C. , Palmer M. R. ,   The ODP Leg 169 Shipboard Scientific Party
Published: April 2008

Massive sulphide deposits actively forming from hydrothermal systems within sedimented environments have been drilled during Ocean Drilling Program Leg 169 at two locations along the Juan de Fuca/Gorda spreading centres. The Bent Hill Massive Sulphide and Ore Drilling Program deposits, Middle Valley, include iron- and zinc-rich massive and semi-massive sulphides underlain by a well-developed feeder zone characterized by sulphide impregnations and cross-cutting copper-rich veins. Ridge-parallel normal faulting is probably involved in providing high-permeability pathways for focused discharge at the seafloor, and this is a key element in creating these large ore deposits. In strong contrast, massive sulphide recovered from the Central Hill hydrothermal site, Escanaba Trough, suggests mineralization forms only a thin (5–15 m) veneer over the sediment sequence. Interstitial waters recovered from this area have chlorinities both significantly higher and lower than seawater. The only way to explain this variation is that the fluids contain a hydrothermal component which has undergone supercritical phase separation at depth. Diffuse discharge of hydrothermal fluids through the sediments evidently precludes the formation of a large ore deposit in this area.
Holocene uplift and subsidence of the Helike Delta, Gulf of Corinth, Greece
Authors: Soter Steven
Published: April 2008

The southwestern coast of the Gulf of Corinth, known as Aigialeia, lies in a region of rapid tectonic uplift and extension. Using age and elevation data from raised relic shorelines, and Lambeck’s model for local isostatic sea-level rise, I re-examine the uplift of the coastal footwall block in Aigialeia. The average Holocene uplift rate is 2.4±0.8m ka
−1
, significantly higher than the Quaternary uplift rates associated with the raised terraces near Corinth on the southeastern coast. The footwall movement in Aigialeia consists of coseismic uplift events separating periods of relatively aseismic uplift. A footwall uplift of about 2 m apparently accompanied the earthquake that destroyed and submerged ancient Helike 373
bc
. The city was built on a Gilbert-type fan delta adjacent to the area of raised relic shorelines. Using dated samples from bore holes drilled in the delta, I estimate that the delta itself subsided by at least 3 m during the earthquake. The opposition between gradual uplift and coseismic subsidence events apparently resulted in a relatively small absolute net displacement of the delta during Holocene time.
Late Cenozoic emergence of the islands of the northern Lau-Colville Ridge, southwest Pacific
Authors: Nunn P. D.
Published: April 2008

The Lau-Colville Ridge is part of an island arc abandoned during Pliocene time as a result of development of the back-arc Lau Basin. Throughout much of Plio-Pleistocene time, the ridge subsided, and its volcanic peaks were submerged and cloaked with (reef) limestone. Uplift, mostly during Pleistocene time, caused the northern part of the ridge to emerge by at least 315m. Islands along the northern Lau-Colville Ridge exhibit ten well-defined terraces above
c.
10m whose ages are constrained by those of anomalously young lavas and avian phosphates. Low-level emerged shorelines may be all of Holocene age and exhibit a recurrence interval of coseismic uplift events of 1500 years.
Holocene coastal tectonics in NE Brazil
Authors: Bezerra Francisco H. R. , Lima-Filho Francisco P. , Amaral Ricardo F. , Caldas Luciano H. O. , Costa-Neto Leão X.
Published: April 2008

We have identified two beachrock facies in NE Brazil which can be used as reliable sea-level indicators. Facies (a) represents the lower foreshore and the upper shoreface, being mainly characterized by coarse unsorted sandstones with trough cross-stratification, whereas facies (b) corresponds to sediments deposited on the middle to lower foreshore and is usually characterized by medium to coarse, better sorted sandstones which form seaward-dipping, swash cross beds. The age of the beachrock bodies ranges from
c.
7000 to 1150 cal. a
bp
. Although the relative sea-level record is complicated by oscillations which are probably due to minor climatic changes, it was possible to identify a general rise at
c.
7000 cal. a
bp
which reached its maximum of +2 m
c.
5000 cal. a
bp
and then started to fall to its present level some 300 years later. Our sea-level data are consistent with glacio-hydroisostatic models for the region but dates of 4080 cal. a
bp
on shells in growth position at +5 m above sea level and 2780 cal. a
bp
on a beachrock at sea level east of the Carnaubais fault point to rapid, possibly coseismic, late Holocene emergence.
Carbonate-siliciclastic sedimentation on a high-energy, ocean-facing, tropical ramp, NE Brazil
Authors: Testa Viviane , Bosence Dan W. J.
Published: April 2008

The inner portion of the continental shelf of Rio Grande do Norte State, Brazil has been investigated by Landsat-TM imagery and by scuba diving. The area experiences high-energy, shoreline and shelf-margin parallel currents driven by a combination of oceanic, tidal and wave processes. The high meso-tidal range (<4 m) further distributes the high-energy waters over the entire inner shelf region (0–25 m). This shelf is a high-energy, distally steepened ramp in which the distribution of carbonate and siliciclastic facies belts is controlled by the energy of the environment, the sources of siliciclastic sediment and the Quaternary history of the area. Carbonate sediments are generated throughout the sublittoral zone and are dominated by calcareous red and green algae. Small, low-diversity coral patch reefs occur in shallow areas. Quartz-rich sands form beaches and subaerial longitudinal dunes, and an offshore, submarine, sandstone outcrop interpreted as a former Quaternary shoreline. This sandstone has been eroded during the Holocene transgression to generate quartz sands deposited in a train of transverse dunes migrating parallel to the coast. Longitudinal sand ribbons comprising bioclastic gravelly sands attest to high-energy, shore-parallel currents in the inner shelf. Branching coralline algae (maërl) form stabilized sea-floor areas. Preliminary
14
C dating indicates a Pleistocene highstand period when coarse algal sands accumulated. Holocene lowstand conditions spread quartz-rich sands over the inner ramp area, which were reworked during the Holocene transgression. Present-day erosion, transport and
in situ
carbonate production results in mixing of these earlier carbonate and siliciclastic units. The Brazilian ramp has similar, but narrower morphology, when compared with the distally steepened ramps of the Yucatan and west Florida. All three ramps are swept by coastline-parallel currents. In Brazil, the higher-energy, windward-facing, inner ramp has calcareous algal sands and patch reefs compared with the inner ramps of the leeward-facing Gulf of Mexico examples that accumulate molluscan sands. Outer ramp facies in each example are planktonic foraminiferal oozes.
Cool-water carbonate deposition on the West Shetland Shelf: a modern distally steepened ramp
Authors: Light Janice M. , Wilson John B.
Published: April 2008

The West Shetland Shelf (WSS) is part of a carbonate province ranging through 11° of latitude on the western seaboard of the British Isles. Bioclastic gravels and sands and quartzitic shelly sediments, ranging in thickness from a few centimetres to 2 m have been accumulating since the Flandrian transgression (13 ka
bp
). A storm wave-base is recognized at the shelf margin (200 m) below which deposits are winnowed and reworked by an upper slope northerly-flowing contour current. Samples from a 143 km transect of 15 stations from the south of Shetland to 260 m on the upper continental slope have been analysed. Bivalve molluscs, barnacles and attached serpulids are the major carbonate contributors with bryozoans, gastropods and echinoderms of secondary importance. The free-living serpulid
Ditrupa arietina
and erect branching bryozoans are important sediment contributors on the outer shelf. The deep-water coral,
Lophelia pertusa
, has been recorded from depths of 200–500 m.

The WSS is a distally steepened carbonate ramp shelving at an angle of <1° to the open ocean with continual sweeping of long-period waves, frequent storms and deep-water contour currents.
A review of sediment distributions and processes in Oligo-Miocene ramps of southern Italy and Malta (Mediterranean divide)
Authors: Pedley Martyn
Published: April 2008

Carbonate ramps lying on the divide between the Eastern and Western Mediterranean basins are considered in the context of the facies that dominate their inner to outer ramp profiles. They developed during the final stages of Tethyan ocean closure. Consequently, the ramp successions accumulated within an increasingly microtidal regime. The ramp facies associations are typical of mid- to late Tertiary developments found throughout the old Tethyan belt. A detailed consideration is given to carbonate productivity and location of the carbonate factory, substrate stability, resedimentation mechanisms, and synsedimentary tectonics. Gravity sliding in mid-ramp situations is examined and its importance in maintaining ramp profile is considered. The relative importance of all processes and their bearing on sustainable ramp development in both current-facing and leeward situations is discussed.
Facies architecture and geometry of a prograding carbonate ramp during the early stages of foreland basin evolution: Lower Eocene sequences, Sierra del Cadí, SE Pyrenees, Spain
Authors: Gilham Rodney F. , Bristow Charlie S.
Published: April 2008

This paper describes the Early Eocene sedimentological evolution of the Sierra del Cadí carbonate platform during the early development of the southeastern Pyrenean foreland basin, Spain. The platform developed as a south-facing ramp system located either across the southern margin of a growth anticline within the newly formed basin or along the axial margin of the basin. Sequences I and II both comprise basal transgressive systems tracts (TSTs) dominated by a thick succession of outer ramp lime mudstones, shallowing upwards into a highstand systems tract (HST) of mid- and inner ramp bioclastic shoals, sand banks and lagoonal deposits. In Sequence III the TST comprises a thick, wedge-shaped unit of east to west backstepping, inner and mid-ramp
Nummulites
shoals and lagoonal deposits, progressively overlain by a thick succession of outer ramp lime mudstones and marls of the HST. No lowstand systems tracts are recognized. Sequence boundaries correspond to flooding surfaces which grade basinwards into correlative conformities, interpreted to represent the onset of high tectonic activity in the orogenic wedge and resulting high flexural subsidence across the foreland and drowning of inner ramp regions. TSTs represent continued high flexural subsidence creating accommodation space infilled by high rates of outer ramp sediment accumulation. In Sequences II and III, TSTs show a pronounced west to east thickening, interpreted to represent the effect of variable subsidence across the margins of an uplifting growth anticline within the otherwise flexurally subsiding foreland. Progradational successions that characterize HSTs developed during periods of reduced tectonic subsidence, both within the foreland and in the orogenic belt, allowing the growth and expansion of carbonate ramp facies belts.
Controls on development of Mid-Triassic ramps: examples from southern Hungary
Authors: Török Ákos
Published: April 2008

During Mid-Triassic time the palaeogeographical-tectonic setting of the northern Tethyan passive margin and epicontinental basins favoured the ramp morphology, as broad flat areas were available for flooding and establishment of carbonate sedimentation. Throughout this time, in what is now southern Hungary, homoclinal ramps developed, which also exhibit some evidence of partial distal steepening. The nearly 1 km thick Anisian-Ladinian Muschelkalk carbonates are grouped into six depositional units. In each ramp unit facies types were identified representing coastal sabkhas, inner ramp peritidal facies, backshoal lagoons, oolite shoals, storm to fair-weather influenced mid-ramps, proximal to distal shell beds and low-energy outer ramps. Mid- and inner ramp carbonates make up the majority of the successions. The deepest outer ramp facies correspond to the peak transgression of a second-order Triassic sea-level rise. In the Hungarian Muschelkalk three major transgressive-regressive sequences were identified. In Germany, Poland and Spain much thinner and slightly coarser Muschelkalk carbonate successions were formed in the same Mid-Triassic interval. The major sediment redistribution processes were related to storms that generated coquinas, and mud flow deposits. Slow, early diagenesis and the predominance of unstable muddy substrate led to the formation of small-scale slumps, slides and plastic deformation. The fauna reflects the prevalence of carbonate mud and temporarily high-stress conditions. A non ‘cool water’ origin of this ramp is inferred from the presence of evaporites, early dolomization of inner ramp carbonates, the biota and the palaeogeographical setting. The major controlling factors of Mid-Triassic ramps were the gentle antecedent morphology, the subtropical climate, the high production of carbonate mud and the lack of reef-building organisms, as well as the passive margin setting, which resulted in a moderate and relatively uniform subsidence rate over a wide area.
Middle Devonian reef and mud mounds on a carbonate ramp: Mader Basin (eastern Anti-Atlas, Morocco)
Authors: Kaufmann Bernd
Published: April 2008

The Mader Basin is a palaeogeographical unit of the eastern Anti-Atlas, once situated on the northwestern margin of Gondwana, which was a mid-latitudinal (30–45°S) carbonate province during Devonian time. In this basin, isolated reef and mud mounds are intercalated within a 200–300 m thick Middle Devonian succession. The mounds were established on a 40 km wide, tectonically controlled homoclinal ramp, which developed between an area of uplift (Mader Platform) and another area of strong subsidence (depocentre of the Mader Basin). The bathymetric gradient of this ramp is reflected by a facies pattern varying from shallow ramp to basinal environments and by different faunal associations of the carbonate mounds. The lithology of the Mader Basin carbonate mounds consists of massive, stromatactis-bearing, bioclastic wackestones, with the bulk of the mound volume consisting of microspar. High accumulation rates (0.2–0.8 m/ka) and the purity of mound carbonates (>95% CaCO
3
) compared with the marly inter- and off-mound facies strongly favour autochthonous carbonate production. In addition, several indications (calcimicrobes in the immediate neighbourhood of stromatactis, dark crusts surrounding stromatactis and alignment of stromatactis parallel to the accretionary mound surfaces) suggest a close relationship between microbial carbonate production and stromatactis formation. Microbial communities probably flourished on the mound surfaces, precipitating fine-grained carbonates and consolidating the steep mound flanks by their mucilagenic secretions. Once embedded, these communities decayed and left behind stromatactis cavities, which were subsequently filled by calcite cements. Drowning of the Mader Basin carbonate ramp was caused by progressive deepening related to basinal subsidence, which was superimposed by rapid eustatic sea-level rises. Onlapping of poorly oxygenated basinal waters was the direct cause for the termination of mound growth.
Coevolution of the Earth’s environment and life: Goldilocks, Gaia and the anthropic principle
Authors: Watson Andrew J.
Published: April 2008

Observation suggests that the Earth’s surface environment is maintained by processes in which non-living and living causes are linked inextricably. Once established on Earth, life rapidly became a dominant influence on the evolution of the planetary environment. But life was also shaped by that evolution, constrained and directed by the physical and chemical processes that moulded the planet’s surface. Life and the planetary environment form a closely coupled entity, a view of the Earth as a complex system which is prefigured in the writings of James Hutton.

Hutton compared the workings of the Earth’s surface to the body of an animal, being both wasted and repaired continually. In modern times, James Lovelock has argued that it is a property of this system that it acts to maintain the planet in a habitable condition. Alternatively, perhaps it is pure chance that the planet has always remained hospitable for life — it could just as easily have followed an infinity of different evolutionary paths, many and perhaps most of which would lead rapidly to global extinction. Consideration of the fates of our near-neighbour planets, Mars and Venus, and the dangerous nature of the inner solar system, leads to the conclusion that there is indeed a substantial element of luck involved in the Earth’s biosphere having survived as long as it has. The fact that our own existence is dependent on it having survived makes it nearly impossible to accurately assess
a priori
the probability of survival.

Abstract models such as ‘Daisyworld’ can capture some of the complex behaviour of the Earth-life system. This may include periods of stasis and sudden changes to new states, the stasis being an example of regulatory behaviour where the system is dominated by negative feedback, and the sudden changes being essentially the opposite — brief but traumatic periods where the dominant feedbacks are positive. GEOCARB, a biogeochemical model for the Phanerozoic which links changes in the long-term carbon cycle to planetary temperature, shows examples of both regulatory and destabilizing behaviour in a less abstract system, and suggests that such responses have indeed characterized Earth history.

I conclude that the properties of the Earth-life system are complex and not easily predictable. The longevity of Hutton’s animal (nearly 4 Ga) is no guide to its future life expectancy, and even if the system as a whole lasts many aeons into the future, any given species (such as humans) is most unlikely to.