Similar Depth Distributions Research Articles

Ultisols with Unique Soil Properties and their Implications in Carbon Enhancement Strategies for Indian Tropical Soils

Million years old Ultisols, developed as an end stage of advance soil weathering on base-poor rock systems under forest and plantation crops of the Indian sub-continent, are enriched with organic carbon (OC). Despite their remarkable acid chemical reaction, they show an upward increase in extractable bases (EB) and base saturation (BS) in the pedon, and on some occasion BS > 35% in surface horizons. Recent research suggests that the litter falls from forest species provide bases on soil surface, which causes similar depth distribution of EB and BS in Andisols of Nilgiri Hills of humid tropical southern India, developed on pre-weathered lateritic materials consisting primarily of Fe and Al oxides/ hydroxides. Under similar humid tropical (HT) climate in protected rain forest, million years old OC rich soils of Andaman and Nicobar Islands, developed on base-poor rock systems, have similar chemical and clay mineralogical properties of Ultisols except however their EB and BS, which are more than 10% and much above 50%, respectively and therefore, they are acidic Alfisols. Formation of Alfisols on the Island is a unique example on the formidable role of vegetation in soil formation, suggesting that if inland Ultisols are kept under natural protected forest ecosystem, they may phase towards Alfisols thereby facilitating OC sequestration and perform other ecosystem functions. The preservation of natural or semi-natural ecosystems is thus essential to raise the OC stocks of soils, which would help to gain various ecological benefits.

Distribution and bioavailability of dissolved and particulate organic matter in different water masses of the Southern Yellow Sea and East China Sea

As the largest marginal seas, the Southern Yellow Sea (SYS) and the East China Sea (ECS) are important areas to study the biogeochemical cycles of organic matter from different sources. Seawater samples were collected from the SYS and ECS to determine the composition of organic carbon and major biochemicals (amino acids and carbohydrates) making up the dissolved (DOM) and particulate organic matter (POM). Concentrations of dissolved organic carbon (DOC) in the surface waters exhibited a latitudinal gradient, with high values appearing in the Yellow Sea Coastal Current Water (YSCCW). As the labile fraction of DOM, the concentrations of total hydrolysable amino acids (THAA) and total dissolved carbohydrates (TDCHO) were variable and tended to decrease from inshore to offshore sites. The DOC, THAA, and TDCHO concentrations were negatively correlated with salinity, suggesting that physical mixing and lateral transport played an important role in the distribution of DOM. In contrast, phytoplankton production directly affected the particulate amino acid (PAA) concentrations. High concentrations of PAA in the bottom water along a transect were probably due to resuspension by strong currents. DOC-normalized yields of THAA and TDCHO (THAA-C% and TDCHO-C%) were used to characterize the bioavailability of DOM in different water masses. Elevated THAA-C% values occurred in regions with high phytoplankton biomass, while high TDCHO-C% values were widespread in the Yellow Sea Warm Current Water (YSWCW) and the YSCCW. THAA-C% and TDCHO-C% had similar depth distributions, higher in the surface and middle layers than in the bottom layer, suggesting that organic matter undergoes diagenetic alterations as it sinks. The degradation index (DI) of DOM was lower than that of POM, which could be explained by their different sources and degradation states. The sporadic occurrence of hot spots likely represented important sites of biological activity. • Concentration, chemical composition, and bioavailability of OM were studied. • Physical mixing and lateral transport affect the distribution of DOM. • Amino acid and carbohydrate yields decreased with water depth. • Hot spots of bioavailable DOM occur sporadically in the surface water.

A microscopic TEM study of the defect layers in cast-mono crystalline silicon wafers induced by diamond-wire sawing

Slicing silicon ingots into wafers by diamond-wire sawing (DWS) is an important step in the material production chain in the semiconductor industry. It will induce defect layers that are highly related to the stress release process and further influence wafers’ mechanical properties. This work aims to investigate the stress release in brittle crystalline silicon via the behaviors of defect layers comprising a surface phase transformed layer and a subsurface crystalline defect layer in DWS silicon wafers from the microscopic perspective. The micro-characteristics of defect layers were mainly obtained by transmission electron microscopy (TEM). The grooves’ surface contains amorphous silicon (a-Si) and diamond-cubic silicon (dc-Si), whereas indentations also contain additional Si-III and Si-XII phases, which were confirmed by both characteristic Raman peaks and the diffraction spots in the TEM image. The subsurface crystalline defect layers were characterized with a similar depth distribution of ∼0.6 µm and possess high-density planar-like defects, which were confirmed as novel defects with a large number (typically 200–400) of compactly strung-together nano-stacking faults inside and believed to be more favorable for stress release. Results show that the stress in crystal silicon can be well relaxed via plastic ways. These findings provide in-depth insights for revealing the essential characteristics of the defect layers in DWS wafers and will be beneficial for the understanding of the plastically deformed mechanisms of brittle silicon crystals.

Diamond's depth distribution systematics

The thermobarometric analysis of inclusions in lithospheric diamonds has shown that these diamonds may originate from a wide range of depths, with a global mode at ~175 ± 15 km. Studies based on diamond depth distribution at global scale, however, cannot clarify if this mode reflects a real concentration of diamonds, preferential sampling of materials from this level by ascending kimberlites, or simply a statistical distribution within the hard limits imposed by diamond stability, lithosphere thickness and mantle adiabat under typical cratonic thermal regimes. We addressed this problem by comparing depth distributions for peridotitic diamonds from the three localities that have been the most prolific for diamond geobarometry (Cullinan, Kimberley and Voorspoed, South Africa) with those of mantle xenocrysts from the same kimberlite sources. The revised P–T estimates indicate that the diamonds were formed at T higher, equal or lower than the ambient geotherm recorded by the xenocrysts. These conditions may represent old mantle thermal regimes or local thermal perturbations related to infiltration of parental fluids or melts. Nonetheless, the studied diamonds show similar depth distributions for the different localities, with a distinct mode at ⁓180 ± 10 km. The similarity of these distributions with that calculated for peridotitic diamonds worldwide, as well as the lack of systematic correlation with kimberlite sampling efficiency as recorded by mantle xenocrysts, suggests that this mode has genetic significance. Based on observed depth distributions and thermodynamic modeling of COH fluids, diamond-forming processes are predicted to become less efficient with decreasing depth from at least ⁓165 km. In addition, diamond endowment near the base of the lithosphere may be negatively affected by infiltration of carbon-undersaturated melts or fluids after diamond formation. Considering the poor correlation between diamond and xenocryst depth distributions in single kimberlites or kimberlite clusters, even limited xenocryst records from diamond favorable depths (especially from the 160–190 km interval) may correspond to significant diamond potential.

High He-ion irradiation resistance of CrMnFeCoNi high-entropy alloy revealed by comparison study with Ni and 304SS

High entropy alloys (HEAs) have presented potential applications in nuclear power plants owing to their novel atomic structure based high irradiation resistance. However, understanding of He-ion irradiation of HEAs is still lacking. In this work, we reveal He-ion irradiation resistance of HEA CrMnFeCoNi by comparison study with a pure Ni and a 304 stainless steel (304SS). It is found that the damage structure in the three materials can be characterized with He bubbles and stacking faults/stacking fault tetrahedrons ((SFs/SFTs), which show a similar depth distribution after He-ion irradiation at both RT and 450 °C. Although the He bubbles have a similar size about 2 nm after irradiation at RT, the He bubble sizes of the HEA, 304SS, and Ni increase to 4.0 ± 0.9, 5.3 ± 1.0 and 6.7 ± 1.0 nm after irradiation at 450 °C, respectively. Moreover, the density of SFs/SFTs displays in an order of Ni < 304SS < HEA at both RT and 450 °C. The He-ion irradiation at RT causes significant hardness enhancement for the three materials, however, compared to RT, after irradiation at 450 °C, the Ni presents softening, while the 304SS, especially the HEA, shows further hardening. Thus, the HEA CrMnFeCoNi possesses the smallest He bubble size, the densest SFs/SFTs, and the highest hardening, indicating the best structural stability, as well as the best He-ion irradiation resistance, which can be attributed to its low mobility of He atoms and point defects.

Plasticity in the diel vertical movement of two pelagic predators (Prionace glauca and Alopias vulpinus) in the southeastern Indian Ocean

AbstractManagement and conservation of marine predator species relies on a fundamental knowledge of their movements and behaviour. Pop‐up satellite archival tags were used to investigate the vertical movement patterns of five blue sharks (Prionace glauca) and one thresher shark (Alopias vulpinus) within the southeastern Indian Ocean. Sections of similar depth distribution, identified using a split moving window analysis, were investigated in relation to the thermal structure of the water column and activity rates. Minimum horizontal displacement of between 66 and 5,187 km for blue sharks and 16 km for the thresher shark were recorded over 863 tracking days. Maximum depths ranged from 540 to 807 m for blue sharks and 144 m for the thresher shark. All sharks displayed plasticity in their depth distribution, with diel vertical movements and surface‐oriented movements the two most common patterns. Diel movement of prey is the most likely explanation for diel vertical movements of thresher and blue sharks. This study has improved our understanding of the vertical movement patterns of these pelagic predators and the relationship between their depth distribution, temperature, and activity.

Superior NBTI in High- $k$ SiGe Transistors–Part I: Experimental

SiGe quantum-well pMOSFETs have recently been introduced for enhanced performance of transistors. Quite surprisingly, a significant reduction in negative bias temperature instability (NBTI) was also found in these devices. Furthermore, a stronger oxide field acceleration of the degradation in SiGe devices compared with Si devices was reported. These observations were speculated to be a consequence of the energetical realignment of the SiGe channel with respect to the dielectric stack. As these observations were made on large-area devices, only the average contribution of many defects to NBTI could be studied. In order to reveal the microscopic reasons responsible for the improved reliability, a detailed study of single defects is performed in nanoscale devices. To provide a detailed picture of single charge trapping, the step-height distributions for different device variants are measured and found to follow a unimodal and bimodal distribution. This finding suggests two conducting channels, one in the SiGe and one in the thin Si cap layer. We, furthermore, demonstrate that similar trap depth distributions are present among the device variants supported by a similar stress bias dependence of the capture times of the identified single defects. We conclude that NBTI is primarily determined by the dielectric stack and not by the device technology.

Intraspecific variation in vertical habitat use by tiger sharks (Galeocerdo cuvier) in the western North Atlantic.

Tiger sharks (Galeocerdo cuvier) are a wide ranging, potentially keystone predator species that display a variety of horizontal movement patterns, making use of coastal and pelagic waters. Far less, however, is known about their vertical movements and use of the water column. We used pop-up satellite archival tags with two data sampling rates (high rate and standard rate tags) to investigate the vertical habitat use and diving behavior of tiger sharks tagged on the Puerto Rico–Virgin Islands platform and off Bermuda between 2008 and 2009. Useable data were received from nine of 14 sharks tagged, tracked over a total of 529 days. Sharks spent the majority of their time making yo-yo dives within the upper 50 m of the water column and considerable time within the upper 5 m of the water column. As a result, sharks typically occupied a narrow daily temperature range (∼2°C). Dives to greater than 200 m were common, and all sharks made dives to at least 250 m, with one shark reaching a depth of 828 m. Despite some similarities among individuals, a great deal of intraspecific variability in vertical habit use was observed. Four distinct depth distributions that were not related to tagging location, horizontal movements, sex, or size were detected. In addition, similar depth distributions did not necessitate similar dive patterns among sharks. Recognition of intraspecific variability in habitat use of top predators can be crucial for effective management of these species and for understanding their influence on ecosystem dynamics.

Use of 137Cs and 210Pbex measurements on deposits in a karst depression to study the erosional response of a small karst catchment in Southwest China to land‐use change

AbstractKarst depressions represent important sinks for sediment, and such sediment can provide a valuable record of the impact of environmental change on soil erosion rates. However, the sediment dynamics of karst depressions are not well understood. This contribution reports a study of the small catchment of a karst depression in Southwest China, with a drainage area of 0.054 km2, aimed at using the sediment deposits in the depression to reconstruct the erosional response of the catchment to land‐use change and, more particularly, the deforestation which took place in 1979. 137Cs and 210Pbex are used as both chronometers and as tracers. Five cores, collected from the bottom of a depression, with an area of 2652 m2, showed similar 137Cs depth distributions, with a single 137Cs peak, which was attributed to the 1979 deforestation. The 137Cs activity associated with the peaks varied between 5.68 ± 0.64 and 9.19 ± 0.99 Bq kg–1. The average depth of sediment deposition between 1979 and 2008 deduced from the depths of the 1979 137Cs peak was 74.1 cm. The existence of relatively high 210Pbex activity of 66.33 ± 8.44 Bq kg–1 in the upper section (0–16 cm) of the core analyzed for 210Pbex suggests that recent sedimentation has been very limited. Net erosion rates on the hillslopes contributing runoff and sediment t to the depression were estimated to be 5258 t km–2 year–1 from 1979 to 1990 and 256 t km–2 year–1 from 1991 to 2008, respectively. The high sediment yield in the first period was associated with the severe soil erosion triggered by the 1979 deforestation, which resulted from the changes in land ownership immediately after the Cultural Revolution. Soil erosion has been very limited since 1990 because the thin soils had been totally removed from many parts of the karst slopes and the soils remaining on the other parts of the slopes have been protected by terracing or vegetation rehabilitation. Copyright © 2012 John Wiley &amp; Sons, Ltd.

The diffusion of silicon atoms in stack structures of La2O3 and Al2O3

The interfacial reactions and electrical characteristics of stack structures of La2O3 and Al2O3 were investigated as a function of the annealing temperature. In the case of Al2O3/La2O3/Si (ALO structure), the La2O3 in contact with the Si substrate was readily transformed into La-silicate by the diffusion of Si atoms, while in the case of La2O3/Al2O3/Si (LAO structure), interfacial reactions between the Al2O3 layer and the Si substrate were suppressed. After an annealing treatment at 700 °C, the Al2O3 in the ALO structure can play an effective role in blocking the hydration of La2O3, resulting in an unchanged interfacial layer. However, the Al2O3 layer in the LAO structure was unable to suppress the diffusion of Si atoms into the La2O3 film. When the annealing temperature reached 900 °C, both structures showed a similar depth distribution with a high content of Si atoms diffused into the films. The change in the elemental distributions via the diffusion and reaction of Si atoms affected the electrical characteristics at the interface between ALO/LAO structure and Si substrate, specifically the trap charge density (Dit) and band gap (Eg) values.

Crustal structure and seismotectonics of central Sicily (southern Italy): new constraints from instrumental seismicity

SUMMARY In this paper, we propose a new model of the crustal structure and seismotectonics for central Sicily (southern Italy) through the analysis of the depth distribution and kinematics of the instrumental seismicity, occurring during the period from 1983 to 2010, and its comparison with individual geological structures that may be active in the area. The analysed data set consists of 392 earthquakes with local magnitudes ranging from 1.0 to 4.7. We defined a new, detailed 1-D velocity model to relocate the earthquakes that occurred in central Sicily, and we calculated a Moho depth of 37 km and a mean VP/VS ratio of 1.73. The relocated seismic events are clustered mainly in the area north of Caltanissetta (e.g. Mainland Sicily) and in the northeastern sector (Madonie Mountains) of the study area; only minor and greatly dispersed seismicity is located in the western sector, near Belice, and along the southern coast, between Gela and Sciacca. The relocated hypocentral distribution depicts a bimodal pattern: 50 per cent of the events occur within the upper crust at depths less than ∼16 km, 40 per cent of the events occur within the middle and depth crust, at depths between 16 and 32 km, and the remaining 10 per cent occur at subcrustal depths. The energy release pattern shows a similar depth distribution. On the basis of the kinematic analysis of 38 newly computed focal plane solutions, two major geographically distinct seismotectonic domains are distinguished: the Madonie Mountain domain, with prevalent extensional and extensional-oblique kinematics associated with upper crust Late Pliocene–Quaternary faulting, and the Mainland Sicily domain, with prevalent compressional and compressional-oblique kinematics associated with thrust faulting, at mid to deep crust depth, along the north-dipping Sicilian Basal Thrust (SBT). The stress inversion of the Mainland Sicily focal solutions integrated with neighbouring mechanisms available in the literature highlights a regional homogeneous compressional tensor, with a subhorizontal NNW–SSE-striking σ1 axis. In addition, on the basis of geodetic data, the Mainland Sicily domain may be attributed to the SSE-ward thrusting of the Mainland Sicily block along the SBT plane. Seismogenic shearing along the SBT at mid-crustal depths was responsible for the unexpected Belice 1968 earthquake (Mw 6.1), with evident implications in terms of hazard assessment.

Modelling the fate and transport of negatively buoyant storm–river water in small multi-basin lakes

The dynamics of negatively buoyant river plumes in a small multi-basin kettle lake with steep bathymetry (Toolik Lake, AK) are simulated using a Cartesian hydrodynamic model based on the solution of the three-dimensional shallow water equations. To validate the model, model predictions are compared with results from previous analytical and laboratory studies and with field observations. The grid resolution adopted for the Toolik Lake model is 0.5 m (= Δ z) in the vertical and 20 m (= Δ x) in the horizontal, so that the ratio of the bottom slope S 0 to Δ z/Δ x is lower than 4 in 99% of the computational domain. With that resolution, the model represents correctly the rate of mixing between lake and river water and the speed of propagation of downslope gravity currents. The model provides accurate predictions of the temperature structure (RMSE = 0.25 °C) and of eddy diffusivities at the depths of the intrusions of incoming water. Measurements and modelling show similar fractions and depth distribution of river water on a cross-basin transect, which suggests that the mixing dynamics of the plume as it transits between basins are well resolved. Thus, the stage is set to quantify the ecological consequences of storm events in small lakes with several interconnected basins using coupled biological measurements and 3D modelling.

Western Arabian Sea SST during the penultimate interglacial: A comparison of U37K′ and Mg/Ca paleothermometry

Millennial‐scale records of planktonic foraminiferal Mg/Ca, bulk sediment U37K′, and planktonic foraminiferal δ18O are presented across the last two deglaciations in sediment core NIOP929 from the Arabian Sea. Mg/Ca‐derived temperature variability during the penultimate and last deglacial periods falls within the range of modern day Arabian Sea temperatures, which are influenced by monsoon‐driven upwelling. The U37K′‐derived temperatures in MIS 5e are similar to modern intermonsoon values and are on average 3.5°C higher than the Mg/Ca temperatures in the same period. MIS 5e U37K′ and Mg/Ca temperatures are 1.5°C warmer than during the Holocene, while the U37K′‐Mg/Ca temperature difference was about twice as large during MIS 5e. This is surprising as, nowadays, both proxy carriers have a very similar seasonal and depth distribution. Partial explanations for the MIS 5e U37K′‐Mg/Ca temperature offset include carbonate dissolution, the change in dominant alkenone‐producing species, and possibly lateral advection of alkenone‐bearing material and a change in seasonal or depth distribution of proxy carriers. Our findings suggest that (1) Mg/Ca of G. ruber documents seawater temperature in the same way during both studied deglaciations as in the present, with respect to, e.g., season and depth, and (2) U37K′‐based temperatures from MIS 5 (or older) represent neither upwelling SST nor annual average SST (as it does in the present and the Holocene) but a higher temperature, despite alkenone production mainly occurring in the upwelling season. Further we report that at the onset of the deglacial warming, the Mg/Ca record leads the U37K′ record by 4 ka, of which a maximum of 2 ka may be explained by postdepositional processes. Deglacial warming in both temperature records leads the deglacial decrease in the δ18O profile, and Mg/Ca‐based temperature returns to lower values before δ18O has reached minimum interglacial values. This indicates a substantial lead in Arabian Sea warming relative to global ice melting.

BENTHIC FORAMINIFERAL EFFECT ON NITROGEN AND CARBON CYCLING

Abundances of fossil planktonic and benthic foraminifera are widely used to reconstruct marine paleoenvironments. Although poorly quantified, occurrences of benthic species are partly determined by in-sediment redox conditions. Understanding the relationship between redox zonation and species-specific microhabitat selection will improve the applicability of foraminifera in reconstructing benthic habitats. By conducting protozoan-free microcosm-scale experiments under controlled conditions, we quantified changes in redox zonation after killing the meiofauna and after artificially increasing foraminiferal abundances. During these experiments, we also quantified the effect of foraminiferal abundance on organic matter mineralization rates in soft sediments typical of nearshore environments. Results show that presence or absence of a meiofauna did not affect carbon mineralization rates. However, oxygen penetration depths were considerably shallower in microcosms where the meiofauna was killed, most likely due to the absence of meiofaunal bio-irrigation. Additionally, the distinctly larger nitrate fluxes observed in these microcosms demonstrate a rapid initial release of nitrate (15 days), followed by increased nitrification and denitrification activity, indicating that meiofaunal tissue can serve as a source of labile organic matter for bacterial mineralization. Similar foraminiferal abundances and depth distributions in the microcosms with natural and increased bacterial numbers (i.e., addition of bacterial food for the meiofauna) indicate that foraminiferal grazing rates on bacteria are low and do not affect redox chemistry considerably.

Acid volatile sulfide and simultaneously extracted metals in tidal flat sediments of Jiaozhou Bay, China

It is well known that acid-volatile sulfide (AVS) plays an important role in influencing the toxicity of divalent cationic metals within anoxic sediments. In studying sediment core samples collected from tidal flats within the Jiaozhou Bay, China, we found that the AVS concentration gradually increases with depth and decreases from high tidal flat to low tidal flat areas. We evaluated the chemical activity and bioavailability of heavy metals in the tidal flat based on the molar ratio of simultaneously extracted metals (SEM) and AVS. The value of SEM/AVS is generally less than 1 in this area except for the surface layer, which suggests that the heavy metals only have chemical activity in the surface layer. SEM is most highly concentrated at the boundary of the redox layer. SEM have similar depth distributions throughout the tidal flat. The aeration of low tidal flat sediment indicates that SEM gradually move to deeper sites via interstitial water.

Seasonal dynamics and depth distribution of Planktothrix spp. in Lake Steinsfjorden (Norway) related to environmental factors

To investigate factors and mechanisms regulating toxin-producing populations of Planktothrix, we conducted a field study (2001-04) in the mesotrophic Lake Steinsfjorden, South-eastern Norway. The occurring species, Planktothrix rubescens and P agardhii, had similar depth distributions and seasonal dynamics, both forming metalimnetic blooms in 10-14m depth. By comparing the resource availability and temperature in Lake Steinsfjorden with demands determined in laboratory studies, temperature and light were identified as the most important factors controlling growth and depth distribution of Planktothrix spp. In addition, macronutrients, especially nitrogen, may have limited growth in periods. A lowering of nutrient supplies over time could in addition to the prevailing suboptimal temperature and light conditions prevent the population of Planktothrix spp. from forming blooms. On two occasions, a major decrease in Planktothrix spp. abundance in the open water could be linked to a transport towards the banks of Lake Steinsfjorden with subsequent decomposition in the littoral zone. Our results show that the depth distribution and seasonal dynamics of Planktothrix spp. in Lake Steinsfjorden is controlled by environmental factors in a similar way as in other Nordic, Central European and North American lakes.

Spatial and Temporal Variability of Zooplankton in a Great Plains Reservoir

Seasonal and daily patterns of zooplankton populations are often predictable in natural lakes. Distinct zonation and geomorphic differences in reservoirs, however, make ecological extrapolations from lakes to reservoirs uncertain. We describe the spatial and temporal distribution of zooplankton, algae, and water clarity across reservoir zones and along a depth gradient in Glen Elder Reservoir, Kansas. Daphnia species were most abundant in the lacustrine zone, with D. pulicaria numerically dominant in early spring and D. mendotae dominant later in 1999 and 2000. Rotifers (Keratella quadrata, Asplanchna spp.) were dominant in the riverine zone in 1999, 2000, and 2001. Algal biomass was not significantly different between zones through most of the sampling periods, except late April in 1999 and mid-April in 2000. Chlorophyll a exceeded 81 l gl )1 in the lacustrine zone in mid- to late-April in 1999 and 2000, and exceeded 90 l gl )1 in the riverine zone in mid-April. Water clarity was significantly lower in the riverine zone in 1999 and 2000. Most zooplankton taxa had similar depth distributions at night and day, indicating a lack of diel vertical migration behavior on a large scale. However, in small scale (30 cm vertical enclosures) laboratory experiments, both D. pulicaria and D. mendotae occupied significantly deeper depths (>25 cm) under lighted conditions in the presence of fish chemical cues compared to shallow water (7–17 cm) under dark conditions and in the absence of fish cues. These contrasting results suggest that, as in other studies, Daphnia sense cues from predators and alter their depth in the water column on small scales without natural constraints on movement and choices. However, other factors in the reservoir such as wind-generated water movements and cues from other predators may prevent depth choices similar to those seen under controlled conditions. These results illustrate biotic and abiotic differences between riverine and lacustrine zones in a large reservoir, and contrast with Daphnia depth segregation and migration patterns in natural lakes.

Expansion of coral communities within the Northern Gulf of Mexico via offshore oil and gas platforms

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 280:129-143 (2004) - doi:10.3354/meps280129 Expansion of coral communities within the Northern Gulf of Mexico via offshore oil and gas platforms Paul W. Sammarco1,2,*, Amy D. Atchison1,2, Gregory S. Boland3 1Louisiana Universities Marine Consortium (LUMCON), 8124 Highway 56, Chauvin, Louisiana 70344, USA 2Department of Oceanography and Coastal Sciences, Louisiana State University A & M College, Baton Rouge, Louisiana 70803, USA 3US Department of the Interior, Minerals Management Service, 1201 Elmwood Park Boulevard, New Orleans, Louisiana 70123-2394, USA *Present address: LUMCON. Email: psammarco@lumcon.edu ABSTRACT: Approximately 4000 oil and gas platforms exist in the northern Gulf of Mexico. These platforms provide hard substratum that extends throughout the euphotic zone, in a region where such has been rare in recent geological time. Major exceptions to this are coral reefs of the Flower Garden Banks (FGB), ~200 km S-SE of Galveston, Texas, USA. We determined whether extensive scleractinian coral populations had colonized these platforms, quantified them, and determined their population and community characteristics. We also examined the relationship between these variables and distance from the FGB, platform age and depth. Surveys were performed on 13 oil or gas production platforms down to 36 m depth, encompassing an ellipse around the FGB. Corals occurred in abundance on most of the platforms. We found 11 species, comprising 8 hermatypic scleractinians, 2 ahermatypic scleractinians and 1 hydrozoan coral. The most abundant corals were Madracis decactis, Diploria strigosa and Tubastraea coccinea. Distance from the FGB was not related to the coral community variables measured, including total coral abundances of the dominant 3 species in shallow water (<14 m), deep water (14 to 36 m), or both depths combined. Total coral abundance increased significantly with increasing platform age, and the community was best developed on platforms >12 to 15 yr old. Abundance of M. decactis increased significantly with increasing platform age in deeper water and both depths combined, as did coral species diversity (S). This was not the case with D. strigosa, indicating that it may not be associated with any particular successional sere. Neither was T. coccinea abundance associated with platform age, except in shallow water, where it decreased in abundance with increasing age, indicating that this species may be an opportunistic pioneer species. All corals combined exhibited a significantly non-uniform depth distribution, with total coral abundance peaking at ~20 and 28 m depths. M. decactis exhibited a similar bimodal depth distribution, with a disproportionately high peak at depths >27 m. D. strigosa exhibited peaks at ~10 and 23 m depth and was not found at depths >27 m. T. coccinea exhibited a near-normal distribution, with a mode at ~17 m depth. Platforms have facilitated the expansion of coral populations in the GOM. Such platforms possess an intrinsic environmental value through the presence of coral populations, and this may influence future decisions regarding their removal. KEY WORDS: Coral · Platform · Oil · Diversity · Reef · Flower Garden Banks · Artifical Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 280. Online publication date: October 14, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Strike-slip earthquakes in the oceanic lithosphere: observations of exceptionally high apparent stress

SUMMARY The radiated energies, E S, and seismic moments, M0, for 942 globally distributed earthquakes that occurred between 1987 to 1998 are examined to find the earthquakes with the highest apparent stresses (τa = µE S/M0, where µ is the modulus of rigidity). The globally averaged τa for shallow earthquakes in all tectonic environments and seismic regions is 0.3 MPa. However, the subset of 49 earthquakes with the highest apparent stresses (τa greater than about 5.0 MPa) is dominated almost exclusively by strike-slip earthquakes that occur in oceanic environments. These earthquakes are all located in the depth range 7‐29 km in the upper mantle of the young oceanic lithosphere. Many of these events occur near plate-boundary triple junctions where there appear to be high rates of intraplate deformation. Indeed, the small rapidly deforming Gorda Plate accounts for 10 of the 49 high-τa events. The depth distribution of τa, which shows peak values somewhat greater than 25 MPa in the depth range 20‐25 km, suggests that upper bounds on this parameter are a result of the strength of the oceanic lithosphere. A recently proposed envelope for apparent stress, derived by taking 6 per cent of the strength inferred from laboratory experiments for young (less than 30 Ma) deforming oceanic lithosphere, agrees well with the upper-bound envelope of apparent stresses over the depth range 5‐30 km. The corresponding depth-dependent shear strength for young oceanic lithosphere attains a peak value of about 575 MPa at a depth of 21 km and then diminishes rapidly as the depth increases. In addition to their high apparent stresses, which suggest that the strength of the young oceanic lithosphere is highest in the depth range 10‐30 km, our set of high-τa earthquakes show other features that constrain the nature of the forces that cause interplate motion. First, our set of events is divided roughly equally between intraplate and transform faulting with similar depth distributions of τa for the two types. Secondly, many of the intraplate events have focal mechanisms with the T-axes that are normal to the nearest ridge crest or subduction zone and P-axes that are normal to the proximate transform fault. These observations suggest that forces associated with the reorganization of plate boundaries play an important role in causing high-τa earthquakes inside oceanic plates. Extant transform boundaries may be misaligned with current plate motion. To accommodate current plate motion, the pre-existing plate boundaries would have to be subjected to large horizontal transform push forces. A notable example of this is the triple junction near which the second large aftershock of the 1992 April Cape Mendocino, California, sequence occurred. Alternatively, subduction zone resistance may be enhanced by the collision of a buoyant lithosphere, a process that also markedly increases the horizontal stress. A notable example of this is the Aleutian Trench near which large events occurred in the Gulf of Alaska in late 1987 and the 1998 March Balleny Sea M = 8.2 earthquake within the Antarctic Plate.

Burial, exportation and degradation of acyclic petroleum hydrocarbons following a simulated oil spill in bioturbated Mediterranean coastal sediments.

A field study was conducted in a French Mediterranean littoral (Gulf of Fos) in order to determine the role of bioturbation processes during the bioremediation of oil-contaminated sediments. Inert particulate tracers (luminophores) and Arabian light crude oil were deposited at the surface of sediment cores incubated in situ for 2, 6 and 12 months. After incubation, luminophores and hydrocarbons presented roughly similar depth distributions in the sediment, showing a continuous burial of material until 55 mm depth. Short-chain (< or = n-C25) n-alkanes were totally removed from the sedimentary column after 6 months, whereas approximately 20% of heavier n-alkanes (e.g. n-C30) and of isoprenoid hydrocarbons (pristane (Pr) and phytane (Ph)) remained at the end of the experiment. The determination of the degradation constant and the turn-over rate of individual hydrocarbon indicated that C17-25 n-alkanes were degraded two to three times faster than longer homologues and than pristane and phytane. Using the 17alpha,21beta-C30-hopane as an internal inert reference, we could demonstrate that, after 12 months of in situ incubation, 55% of the losses of the n-alkanes < or = C25 and 35% of the losses of the heavier n-alkanes and of Pr and Ph were due to biodegradation processes. These results demonstrate that the activity of benthic organisms can have a significant influence on the qualitative and quantitative fate of acyclic hydrocarbons following a petroleum contamination in marine coastal sediments.