Seafloor mapping for geohazard assessment: state of the art

During the last two decades, increasing use of full-coverage sonic mapping of the seafloor has made us more aware of the large and different number of seafloor processes and events bearing significant geohazard potential. This awareness combines with the increasing use of the seafloor for infrastructure and with the high density of population and settlement on the coast. Seafloor mapping is the first step in making a census of the geohazard-bearing features present in a given offshore area. It often provides the only tool for a comprehensive, although non-specific, seafloor geohazard assessment over large areas that are scarcely groundtruthed by acoustic prospection and seafloor sampling. However, the characterization of geohazard features on a morphological basis alone is limited, and more detailed investigations are needed to define the character and state of activity of potentially hazardous features. Such investigations include the use of deep-tow or autonomous platforms designed to acquire high-resolution data at depth as well as in situ measurements, both being very expensive activities not applicable over large areas. Thus seafloor mapping is often not only the first and the main but also the only tool for a comprehensive seafloor geohazard assessment. This special issue represents an example of the diversity of approaches to seafloor geohazard assessment and summarizes the present state of this discipline. Both the diverse technologies applied and the specific aims of offshore geohazard assessment brought different communities to deal with the study of seafloor processes and events from remarkably distinct viewpoints. We identified three end members in offshore geohazard assessment: (1) geohazard assessment ??sensu stricto??, (2) ??engineering?? geohazard assessment, (3) ??non-specific?? geohazard assessment. These are being conducted by industry, academia and public agencies in charge of civil protection and land-use planning and management. Understanding the needs and geohazard perception of the different groups is a necessary step for a profitable collaboration in such an interesting and rapidly developing field of marine geology.     

Product formation from the gas-phase reactions of OH radicals and O3 with a series of monoterpenes

The formation yields of nine carbonyl products are reported from the gas-phase OH radical-initiated reactions (in the presence of NO _x ) and the O₃ reactions with seven monoterpenes. The products were identified using GC/MS and GC-FTIR and quantified by GC-FID analyses of samples collected on Tenax solid adsorbent cartridges. The identities of products from camphene, limonene and -pinene were confirmed by comparison with authentic standards. Sufficient quantities of products from the 3-carene, limonene, -pinene, sabinene and terpinolene reactions were isolated to allow structural confirmation by proton NMR spectroscopy. The measured total carbonyl formation yields ranged from non-detectable for the OH radical reaction with camphene and the O₃ reactions with 3-carene and limonene to 0.5 for the OH radical reaction with limonene and the O₃ reaction with sabinene.     

The mid-IR radio correlation at high angular resolution: NGC253

We present high angular resolution (1.2), narrow band (/=0.1) images of the nucleus of NGC253 at three wavelengths in the mid-IR (8.5, 10.0, and 12.5 m). We find that most of the mid-IR flux in the nucleus of NGC253 derives from a very small region 120 pc in diameter. Within this small region there are three spatially and spectrally distinct IR components: two bright compact sources, and a surrounding envelope of low level, diffuse emission. The mid-IR and 6cm radio are loosely correlated in position but not in brightness. The implication is that the mid-IR - radio correlation may begin to break down on small spatial scales relevant to individual star forming regions and large individual sources. These variations in the mid-IR spectra and in the radio-IR correlation imply that the nuclear emission is excited by more than one mechanism and source type.     

Alkyl nitrate formation from the atmospheric photoxidation of alkanes; a revised estimation method

The available experimental data concerning the yields of alkyl nitrates in the reactions of alkyl peroxy radicals with NO have been used to derive a revised expression for the estimation of alkyl nitrate yields in the atmospheric photooxidation of alkanes as a function of temperature and pressure. This revised expression gives more reasonable predictions of alkyl nitrate yields under high altitude tropospheric conditions than that which has been previously published.     

Sediment accumulation and its effects on a Mississippi River oxbow lake

Recent sediment accumulation rates were measured in Moon Lake, a large (10.1 km²) Mississippi River oxbow lake in northwestern Mississippi. Moon Lake, which receives channeled inflow from an intensively cultivated soybean, rice, and cotton watershed (166 km²) and limited overland flow from surrounding lands, exhibited depositional patterns that were associated with (1) points of inflow, (2) flow patterns, and (3) lake morphology. From 1954 to 1965, 70 percent of the lake bottom experienced accumulation rates greater than 2 cm/yr. Accumulation rates exceeded 4 cm/yr in areas of delta formation. Changes in cropping systems during the 1960s, from cotton to soybeans and rice which require less cultivation, resulted in significantly (a = 0.01) less sediment accumulation during the period 1965- 1982 when 86 percent of the lake averaged less than 2 cm/yr sediment deposition. If current sediment accumulation rates continue, open water habitat in the lake will be reduced by only 3 to 7 percent during the next 50 years.Contribution of the Sedimentation Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Oxford, MS 38655     

Petrogenesis and syntectonic emplacement in the early Proterozoic of south-central Finland: a reversely zoned diorite-granodiorite and a granite

The Hernemäki and Sydänmaa granitoid plutons in southern Finland lie within the Svecokarelian metagreywackes, with an approximate age of 1.9 Ga. The Hernemäki porphyritic granite was emplaced and deformed early in the regional D₂ deformation, and acquired a rectilinear foliation. The Sydänmaa pluton has a mainly concentric foliation, and its intrusion spanned D₂. It was probably emplaced as a ballooning pluton. The magmas of the two plutons were possibly derived from a similar source, but evolved independently. Both plutons show plagioclasebiotite-hornblende fractionation. The Sydänmaa pluton is reversely zoned, with a granodiorite rim and a diorite core.Crystal-residual liquid segregation in a deep magma chamber produced a normal geochemical stratification. This fractionated magma was drained from the top down, and injected into a higher, final chamber. Hence, the granodiorite was emplaced before the diorite. There is a component of normal zonation, which suggests that early hornblende crystals were incorporated in the most fractionated granodiorite.

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Zusammenfassung Die Hernemäki- und Sydänmaa-Granite in Südfinnland liegen innerhalb der svecokarelischen Metagrauwacken und haben ein ungefähres Alter von 1,9 Ga. Der porphyrische Hernemäki-Granit intrudierte frühzeitig während der regionalen D, Deformation. Er wurde zur gleichen Zeit deformiert, woraus eine geradlinige Schieferung resultierte. Der Sydänmaa-Granit, dessen Intrusoin die D,-Phase überdauerte, zeigt dagegen eine überwiegend konzentrische Schieferung. Sehr wahrscheinlich stieg er als ballonförmiger Granit auf.Die Magmen der zwei Plutone entstammen eventuell derselben Quelle, entwickelten sich aber unabhängig voneinander. Beide zeigen Plagioklas-Biotit-Hornblende Fraktionierung. Der Sydänmaa-Granit ist umgekehrt zoniert, er hat einen Granodiorit-Rand und einen Diorit-Kern. Residuale Flüssigkeitsdifferenzierung in einer tiefgelegenen Magmenkammer erzeugte eine normale geochemische Abfolge. Dieses fraktionierte Magma floß vom Top des Plutons nach unten ab und wurde in eine höher gelegene Kammer injiziert. Also intrudierte der Granodiorit vor dem Diorit. Es existiert eine Komponente normaler Zonierung, was für eine Aufnahme früh gebildeter Hornblendekristalle in den am stärksten fraktionierten Granodiorit spricht.

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Résumé Les plutons granitoïdes de Hernemäki et Sydänmaa (Finlande méridionale) sont inclus dans les métagrauwackes svecocaréliennes, avec un âge approximatif de 1,9 Ga. Lors du début de la déformation régionale D₂, le granite prophyrique de Hernemäki s'est mis en place et a été déformé, acquérant ainsi une structure linéaire. Le pluton de Sydänmaa, dont l'intrusion a dépassé la phase D₂, possède une schistosité essentiellement concentrique. Il a été probablement mis en place par »ballooning«. Il est possible que les magmas des deux plutons dérivent d'une source similaire, mais ils ont évolué de manière indépendante. Les deux plutons montrent un fractionnement par cristallisation du plagioclase, de la biotite et de la hornblende. Celui de Sydänmaa présente une zonation inverse: bordure granodioritique et coeur dioritique. La séparation des cristaux et du liquide résiduel dans une chambre magmatique profonde a engendré une stratification géochimique normale. La partie supérieure de cet ensemble fractionné a ensuite été injectée dans une chambre définitive plus élevée, suivie des parties plus profondes: de la sorte, la granodiorite s'est mise en place avant la diorite. Il existe une composante de zonation normale, qui laisse supposer que de la hornblende, cristallisée lors des premiers stades, a été incorporée à la fraction granodioritique ultime.

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Fixed-ammonium in clays associated with crude oils

The association of ammonium (NH₄⁺) silicates with organic-rich sedimentary environments has stimulated interest in the chemical cycle of N, and its possible application as an indicator of in situ organic maturation reactions or crude oil migration. Fixed-NH₄ in clay minerals was determined from three hydrocarbon occurrences of similar depositional environment but different ages, depth and thermal maturity, to determine whether anomalously high NH₄-substitution occurs near mature hydrocarbons. Results show higher fixed-NH₄ concentrations in marginally mature mudstones than in immature sediments. The highest fixed-NH₄ concentrations were found in clays from sandstone reservoirs containing migrated crude oil.Fixed-NH₄ in clays from Holocene oil seep sediments in the Gulf of Mexico continental slope, offshore Louisiana, averages 0.08 wt % and increases with depth in shallow cores (420 cm), reflecting an early diagenetic trend that is apparently not influenced by migrating crude oil. Programmed pyrolysis shows that the sediments are thermally immature (av.T_max = 419°C). High Hydrogen Index values (av.= 359mg/g) are the result of biodegraded crude oil, and a high Oxygen Index (av.= 182mg/g) reflects the presence of authigenic carbonate.Fixed-NH₄ averages 0.16 wt % in Wilcox Group (Eocene) mudstones enclosing two sandstone reservoirs at Fordoche Field, onshore Louisiana. In comparison to these mudstones, anomalously high NH₄-fixation appears to occur in reservoir clay minerals. Pyrolysis shows that the sediments are marginally mature for crude oil generation (av.T_max = 432°C). Average Hydrogen Index (187 mg/g) and Oxygen Index values (75 mg/g) are consistent with oil-prone Type II and Type III kerogen. Increased pyrolysis Production Index values and solvent extraction shows the presence of migrated crude oil. This suggests that a reaction which releases NH₃ during crude oil generation or migration is recorded byNH₄⁺ substitution in clays.Fixed-NH₄ and total organic carbon (TOC) at Fordoche Field show no statistically significant correlation, suggesting that NH₄⁺ substitution in clay minerals is not simply related to the amount of organic matter in the section, but is also influenced by the presence of crude oil. Once NH₄⁺ has been fixed in clays, it is a more stable hydrocarbon proximity indicator than pore fluid tracers, because it is less influenced by later chemical or geological changes.     

Influence of salt front position on the occurrence of uncommon marine fishes in the Hudson River estuary

Six species of marine fishes, the Atlantic cutlassfish Trichiurus lepturus; planehead filefish, Monacanthus hispidus; guaguanche, Sphyraena guachancho; pigfish, Orthopristic chrysoptera; freckled blenny, Hypsoblenius ionthas; and short bigeye, Pristigenys alta, were observed for the first time in the Hudson River estuary in 1985. Their occurrence was associated with low freshwater runoff and the resulting upstream penetration of the salt front to historic levels. These conditions may have facilitated the dispersal of marine fishes from coastal areas into the lower Hudson River estuary.     

Large-eddy simulation of turbulent flow above and within a forest

A large-eddy simulation has been performed of an atmospheric surface layer in which the lower third of the domain is occupied by a drag layer and heat sources to represent a forest. Subgridscale processes are treated using second-order closure techniques. Lateral boundaries are periodic, while the upper boundary is a frictionless fixed lid. Mean vertical profiles of wind velocity derived from the output are realistic in their shape and response to forest density. Similarly, vertical profiles of Reynolds stress, turbulent kinetic energy and velocity skewness match observations, at least in a qualitative sense. The limited vertical extent of the domain and the artificial upper boundary, however, cause some departures from measured turbulence profiles in real forests. Instantaneous turbulent velocity and scalar fields are presented which show some of the features obtained by tower instrumentation in the field and in wind tunnels, such as the vertical coherence of vertical velocity and the slope of structures revealed by temperature patterns.