Onshore-offshore Transect Research Articles

Coastal water quality in the Kihei and Lahaina districts of the island of Maui, Hawaiian Islands. Impacts from physical habitat and groundwater seepage: implications for water quality standards

Water quality in terms of nutrient concentrations, turbidity, suspended solids, and chlorophyll a, was studied during 2000–2001 at a total of 27 beaches in the Kihei and West Maui–Lahaina districts of Maui, Hawaiian Islands. Nitrate concentrations varied widely and averaged as much as 15–20 µM at some beaches. Most turbidities fell in the range 1–10 nephelometric turbidity units. Resuspension of bottom sediments on shallow reef flats appeared to be a cause of some of the highest turbidity. Onshore-offshore transect studies showed that turbidity and concentrations of nitrate and chlorophyll a declined dramatically with distance from the shoreline. The dramatic gradients in water quality within the first 100 m from the shoreline and the fact that some violations of water quality criteria within that zone appear to be the result of natural phenomena, underscore the difficulty of assessing the quality of coastal waters based on traditional parameters.

Geophysical characteristics of the continental crust along the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT): a review

The Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT) of the Lithoprobe program included 1200 km of normal-incidence seismic profiles and seven wide-angle seismic profiles across Archean and Proterozoic rocks of Labrador, northern Quebec, and the surrounding marine areas. Archean crust is 33–44 km thick. P-wave velocity increases downwards from 6.0 to 6.9 km/s. There is moderate crustal reflectivity, but the reflection Moho is unclear. Archean crust that stabilized in the Proterozoic is similar except for greater reflectivity and a well-defined Moho. Proterozoic crust has similar or greater thickness, variable lower crustal velocities, and strong crustal reflectivity. Geodynamic processes of Paleoproterozoic growth of the Canadian Shield are similar to those observed in modern collisional orogens. The suturing of the Archean Core Zone and Superior provinces involved whole-crustal shearing (top to west) in the Core Zone, linked to thin-skinned deformation in the New Quebec Orogen. The Torngat Orogen sutures the Nain Province to the Core Zone and reveals a crustal root, in which Moho descends to 55 km. It formed by transpression and survived because of the lack of postorogenic heating. Accretion of the Makkovik Province to the Nain Province involves delamination at the Moho and distributed strain in the juvenile arcs. Delamination within the lower crust characterizes the accretion of Labradorian crust in the southeastern Grenville Province. Thinning of the crust northwards across the Grenville Front is accentuated by Mesozoic extension that reactivates Proterozoic shear zones. The intrusion of the Mesoproterozoic Nain Plutonic Suite is attributed to a mantle plume ponding at the base of the crust.

Proterozoic evolution of the northeastern Canadian Shield: Lithoprobe Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT), introduction and summary

The Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT) has focused on studies of the processes by which the northeastern Canadian (Laurentian) Shield evolved, firstly by the 1.9–1.8 Ga coalescence of Archean cratons and then by the addition of juvenile crust from the south in the period 1.9–0.9 Ga. The transect area is a microcosm of continental accretion and also offers a unique opportunity to link the geological evolution of the Canadian Shield with once-adjacent Greenland.

The High Pressure belt in the Grenville Province: architecture, timing, and exhumation

We propose that the Grenvillian allochthonous terranes may be grouped into High Pressure (HP) and Low Pressure (LP) belts and examine the HP belt in detail in the western and central Grenville Province. The HP belt is developed in Paleo- and Mesoproterozoic rocks of the pre-Grenvillian Laurentian margin and characterized by Grenvillian eclogite and co-facial HP granulite in mafic rocks. Pressure–temperature (P–T) estimates for eclogite-facies conditions in well-preserved assemblages are about 1800 MPa and 850°C. In the central Grenville Province, HP rocks formed at ~1060–1040 Ma and underwent a single stage of unroofing with transport into the upper crust by ~1020 Ma, whereas farther west they underwent two stages of unroofing separated by penetrative mid-crustal recrystallization before transport to the upper crust at ~1020 Ma. Unroofing processes were comparable in the two areas, involving both thrusting and extensional faulting in an orogen propagating into its foreland by understacking. In detail, thrusting episodes preceded extension in the western Grenville Province, whereas in the central Grenville Province, they were coeval, resulting in unroofing by tectonic extrusion. In the central Grenville Province, the footwall ramp is well preserved, but any former ramp in the western Grenville Province was obliterated by later lower crustal extensional flow. Continuation of the HP belt into the eastern Grenville Province is not established, but likely on geological grounds. However, the pattern of deep crustal seismic reflection in the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT) line contrasts with that father west, suggesting that, if present, the HP rocks were exhumed by a different mechanism.

Review of crustal architecture and evolution in the Ungava Peninsula — Baffin Island area: connection to the Lithoprobe ECSOOT transect

Within the Trans-Hudson Orogen in northern Quebec and southern Baffin Island, parautochthonous Archean basement and Paleoproterozoic cover, as well as allochthonous (accreted) Paleoproterozoic units, are exposed in a series of antiformal culminations and complementary synclinoria. The parautochthonous rocks of the Superior Province margin and two assemblages of accreted Paleoproterozoic units (Ungava and Meta Incognita crustal terranes) define a basic tripartite crustal architecture which is characterized by polyphase deformation and metamorphic histories. Early structures and mineral assemblages are recognized in all three crustal components and shown to be temporally distinct. The earlier structures and assemblages are overprinted by younger elements and mineral assemblages that are related to at least three regional contraction episodes and one thermal event that are common to all crustal components of the orogen. Close correlation of regional aeromagnetic data in northern Quebec and southern Baffin Island with the three crustal components offers a first-order approximation of the distribution of basement lithological units across Hudson Strait. Continuation of the aeromagnetic domains beneath Ungava Bay to northeastern Quebec and northern Labrador allows for the southward extrapolation of the tripartite Trans-Hudson Orogen crustal architecture into the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect area.

Early Oligocene dinoflagellate cysts from the Upper Rhine Graben (SW Germany): paleoenvironmental and paleoclimatic implications

Using principal component analysis and cyst diversity and equity trends, we can recognize four distinct dinoflagellate cyst (dinocyst) assemblages from four Rupelian (Early Oligocene) cores in the Mainz Embayment of the northern Rhine Graben (SW Germany). These assemblages are the Spiniferites ramosus (PC1), Thalassiphora pelagica (PC2), Homotryblium tenuispinosum (PC3), and Vozzhennikovia spinula (PC4) assemblages. The four cores provide an onshore–offshore transect in the Mainz Embayment. The H. tenuispinosum assemblage shows high factor loadings in proximal to intermediate cores, which is interpreted to reflect temporary high-salinity conditions. Mean dinocyst diversity and equity increase with distance from the Mid-Rupelian shoreline, indicating increasingly stable paleoenvironmental conditions towards the center of the Mainz Embayment. Within individual cores, changes in dinocyst assemblages through time are related to paleoenvironmental and paleoclimatological changes. The three proximal to intermediate cores show dominance of the H. tenuispinosum assemblage repeatedly alternating with high factor loadings of the T. pelagica assemblage. In both cases, dinocyst diversity and equity tend to be reduced. Highest factor loadings of the S. ramosus assemblage occur in intervals where neither of the above assemblages is dominant and tend to coincide with dinocyst diversity and equity maxima. We interpret this distribution pattern to denote different paleoceanographic conditions, reflecting drier and more humid phases in the Early Oligocene of Central Europe. During relatively dry periods, increased salinity conditions prevailed in proximal to intermediate settings of the Mainz Embayment, as reflected by the dominance of the H. tenuispinosum assemblage. During more humid periods, increased runoff led to higher nutrient availability and the formation of a pycnocline separating slightly less saline surface waters from higher saline deeper waters, thus impeding vertical circulation. These environmental conditions are documented in high loadings of the T. pelagica assemblage which is indicative of increased eutrophication and/or oxygen-depleted bottom waters. Transitions between drier and more humid periods, i.e. episodes of normal marine conditions, are characterized by high loadings predominantly of the S. ramosus assemblage as well as increased dinocyst diversity and equity values. We propose that the alternations between drier and more humid phases may be related to variations in the ocean–atmosphere moisture flux from the North Atlantic into Central Europe bearing a high-latitude climate signal.

Taphonomic Trends Along a Forereef Slope: Lee Stocking Island, Bahamas. II. Time

Abstract The Shelf and Slope Experimental Taphonomy Initiative (SSETI) Program was established to measure taphonomic rates in a range of continental shelf and slope environments over a long period of time. For this report, mollusk shells were deployed for one and two years at seven different environments of deposition (EODs) along two onshore-offshore transects off Lee Stocking Island in the Bahamas. The experimental sites were located: in sand channels on the platform top (15 m) and the platform edge (33 m); on ledges down the wall (70–88 m); on the upper (183 m) and lower (210–226 m) talus slope below the wall; and on the crest (256–264 m) and in the trough (259–267 m) of large sand dunes. Shell condition was assessed using a range of taphonomic attributes including dissolution, abrasion, edge alteration, discoloration, and changes in shell weight. After two years, taphonomic alteration was not particularly intense in any EOD. No species was particularly susceptible or resistant to taphonomic alteration...

Crustal structure of the Grenville Province in southeastern Labrador from refraction seismic data: evidence for a high-velocity lower crustal wedge

The crustal structure of the eastern Grenville and Makkovik provinces was determined using two onshore–offshore refraction seismic lines of the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT). A gravity high in the Hawke River terrane correlates with increased P-wave velocities in the upper 30 km of the crust (6.2–6.7 km/s in the upper and middle crust and 6.9–7.1 km/s below) which we interpret as structure inherited from the Labradorian orogen. Velocities in the adjacent Groswater Bay terrane are 6.0–6.55 km/s in the upper and middle crust and 6.6–6.95 km/s in the lower crust. The entire Grenville crust is underlain by a 15–20 km thick high-velocity lower crustal (HVLC) wedge consisting of an upper layer (7.1–7.4 km/s) and a lower layer (7.6–7.8 km/s). The HVLC wedge is interpreted as an underplated layer formed during Iapetan rifting. This interpretation is based on the correlation with the 615 Ma Long Range dykes onshore and the eastward termination of the wedge at the Cartwright Arch. Similar HVLC layers are found offshore western Newfoundland, suggesting that the underplating may be a continuous feature along the passive Grenvillian margin. The Cartwright Arch is characterized by velocities of 6.4 km/s and 4 km thick sediment sequences (4.3–5.7 km/s) in the surrounding basin, interpreted as an extensional basin with basaltic magmatism within the arch. The Grenville front is clearly marked by a decrease of velocities in the Makkovik Province (5.8–6.4 km/s in the upper and middle crust, 6.65–6.85 km/s in the lower crust) and a gradual thickening of the crust (not including the HVLC layer) from 30 km in the Grenville Province to 35 km in the Makkovik Province.

Palynofacies prediction of distance from sediment source: A case study from the Upper Cretaceous of the Pyrenees

Quantitative variations in the size, shape, and preservation state of microscopic plant debris assemblages correlate with the actual distance of the sample site from the coastal siliciclastic sediment source. A case study from a shelf to basin transect in the Upper Cretaceous of the Spanish Pyrenees reveals that (1) the diameter of equant, black wood phytoclasts, (2) the ratio of equant- to lath-shaped, black wood phytoclasts, and (3) several other parameters show predictable changes over a 55 km onshore-offshore transect. These parameters primarily reflect sedimentological sorting based on particle size and density, rather than paleoecological gradients. When only samples from the transgressive systems tract are considered, multiple regression models predict the distance from the source with a standard error of ±5 km (r 2 = 0.84–0.93). These parameters also allow the maximum flooding surface to be more precisely located within mudstone-dominated sections. By applying the lateral regression relationship to stratigraphic data, a quantitative estimate can also be made for the scale of lateral facies shifts through depositional sequences (here ∼10 km).

Regional and seasonal differences in light absorption by yellow substance in the Southern Bight of the North Sea

Absorption spectra (250–800 nm) of yellow substance were measured in 1993 and 1994 along five onshore–offshore transects in the Southern Bight of the North Sea in different seasons. All spectra had a common shape, so the amount of yellow substance could be established using one reference wavelength as a proxy variable. The exponential slope parameter at 380 nm ( S) of the absorption spectra ranged from −0.016 to −0.023 per nm when regression was based upon the 250–650 nm wavelength range; values were 25% lower when fitting was performed over the more restricted waveband 360–540 nm, partly due to a minor shoulder at 260–275 nm in all spectra. The concentration of yellow substance, expressed as the absorption coefficient at 380 nm ( a 380; m −1), varied over an order of magnitude, from low values down to 0.17 m −1 in the English Channel, up to 1.00–1.25 m −1 near the Rhine and Scheldt outflow, to maxima of up to 1.75 m −1 in the neritic–estuarine waters at the entrance to the shallow Wadden Sea. This regional distribution pattern was in agreement with the known water-mass circulation and with the location of sources of yellow substance: highest concentrations close to the shore under the influence of freshwater inflow, lower values with increasing distance from the coast, where Atlantic Ocean water is the major water-mass component. No significant seasonal variation in yellow-substance concentration was found anywhere when a correction was applied for salinity. Extrapolation to zero salinity yielded an absorption coefficient at 380 nm of 5.38 m −1 for the freshwater input at both the Texel and Walcheren transects, but of 3.29 m −1 at the Noordwijk transect, reflecting differences in yellow-substance concentration between the Rhine, the Meuse, and Lake IJssel, as previously noted in fluorescence measurements. Yellow-substance concentration was not only correlated with salinity but also, albeit much less, with chlorophyll concentrations; the contribution of phytoplankton to the yellow-substance pool was marginal but significant. A relation appeared to exist between yellow-substance absorbance at 380 nm and yellow-substance fluorescence intensity; fluorescence measurements in the southern North Sea can be translated to absorption (more appropriate for ocean colour detection by remote sensing) by equating 1 mFl unit to an absorption coefficient at 380 nm of 0.056 m −1, and using the exponential relation a ys( λ)= a ys( λ ref)exp(− S( λ− λ ref)) for extrapolation to UV–Vis spectral absorption.

A reassessment of the sources and importance of land-derived organic matter in surface sediments from the Gulf of Mexico

Organic matter in surface sediments from two onshore-offshore transects in the northwestern Gulf of Mexico was characterized by a variety of techniques, including elemental, stable carbon, radiocarbon, and molecular-level analyses. In spite of the importance of the Mississippi River as a sediment source, there is little evidence for a significant terrigenous input based on the low carbon:nitrogen ratios (8–5) and the enriched δ 13C values of bulk sedimentary organic carbon (−19.7‰ to −21.7‰). Radiocarbon analyses, on the other hand, yield depleted Δ 14C values (−277‰ to −572‰) which indicate that a significant fraction of the sedimentary organic carbon (OC) in all these surface sediments must be relatively old and most likely of allochthonous origin. CuO oxidations yield relatively low quantities of lignin products (0.4–1.4 mg/100 mg OC) along with compounds derived from proteins, polysaccharides, and lipids. Syringyl:vanillyl and cinnamyl:vanillyl ratios (averaging 1.6 and 0.5, respectively) and acid:aldehyde ratios for both vanillyl and syringyl phenols (averaging 0.8 and 1.2, respectively) indicate that the lignin present in sediments originates from nonwoody angiosperm sources and is highly degraded. The δ 13C values of lignin phenols in shelf sediments are relatively depleted in 13C (averaging −26.3‰) but are increasingly enriched in 13C at the slope sites (averaging −17.5‰ for the two deepest stations). We interpret these molecular and isotopic compositions to indicate that a significant fraction (≥50%) of the lignin and, by inference, the land-derived organic carbon in northwestern Gulf of Mexico sediments ultimately originated from C 4 plants. The source of this material is likely to be soil organic matter eroded from the extensive grasslands of the Mississippi River drainage basin. Notably, the mixed C 4 and C 3 source and the highly degraded state of this material hampers its recognition and quantification in shelf and slope sediments. Our data are consistent with higher than previously estimated inputs of land-derived organic carbon to regions of the ocean, such as the Gulf of Mexico, with significant sources of terrigenous C 4-derived organic matter.

Crustal structures of Grenville, Makkovik, and southern Nain provinces along the Lithoprobe ECSOOT Transect: regional seismic refraction and gravity models and their tectonic implications

Crustal structures of the eastern Grenville, Makkovik, and southern Nain provinces are determined using seismic reflection-refraction and gravity data along the Lithoprobe Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT). Within the Grenville Province, the velocity model contains a 5 km thick upper crust and a variable-thickness middle to lower crust. The total crustal thickness varies from 25 to 43 km, with the thickest crust in the south and thinnest crust in the north. A high-velocity, lower crustal wedge is coincident with a strong band of northward-dipping reflectors. The two-dimensional velocity structure is compatible with modelling of a 60 mGal gravity high over the Hawke River terrane. In the Makkovik Province, the thickness of upper crustal velocities increases to 17 km. The velocity decrease in the upper to middle crust from the Grenville Province to the Makkovik Province is similar to that of refraction models across the Grenville Front in Ontario and Quebec. It is possibly related to a decrease in metamorphic grade from south to north and (or) a larger volume of unmetamorphosed plutons in the Makkovik Province. A lower crustal layer is coincident with a region of increased reflectivity in the lower crust. There are no major crustal discontinuities associated with terrane boundaries within the Makkovik Province. The base of the crust is consistent with a change from north- to south-dipping reflectors beneath the Cape Harrison domain. Alternatively, it may consist of a thick zone of complex velocity variations, consistent with a zone of diffusive reflectivity observed to the north of the Allik domain.

Small scale spatial structure in zooplankton from high-frequency acoustical measurements

Volume backscattering was measured at multiple discrete depths at frequencies of 0.165 and 1.1 MHz from a mooring on the shelf-slope break at the northern edge of the San Pedro Basin near Los Angeles, CA. Attempts to understand temporal variability and vertical structure in the data from the mooring led us to make additional measurements along an onshore–offshore transect at this location at frequencies of 0.265, 1.1, 1.8, and 3.0 MHz. These measurements, when processed with an inverse algorithm to determine plankton size and abundance, revealed both an onshore–offshore trend and small scale structure in this environment. Results of sampling the same environment with a multiple opening–closing net sensor system (MOCNESS) are consistent with the acoustical measurements, but at the same time highlight the difficulty of sampling a heterogeneous spatial distribution with a net system. [Work supported by ONR, NSF, and Tracor Applied Sciences.]

Proterozoic orogens of the northeastern Canadian Shield: new information from the Lithoprobe ECSOOT crustal reflection seismic survey

As part of the Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT), Lithoprobe acquired 1250 km of deep seismic reflection data along the coast of Labrador and across Ungava Bay, to image evidence of Proterozoic crustal accretion to the Archean nuclei of the Nain and Superior provinces of the Canadian Shield. The relatively pristine Archean crust of the Nain Province has low reflectivity and generally lacks systematic reflector orientations. Reworking of Archean crust on the margins of the Makkovik Province has little effect on this weak signature. In contrast, the Archean crust in the Eastern Churchill (Rae) Province appears to have been overprinted by a strongly developed, whole-crustal, easterly dipping reflection fabric, interpreted to result from Proterozoic collision of the Nain and Superior provinces in the paired New Quebec and Torngat orogens. Juvenile Proterozoic crust in the Makkovik and Grenville provinces also shows strong whole-crustal dipping reflection fabrics, interpretable as outwardly verging structures associated with collisional mobile belts. Crustal thickness varies from 35 to 45 km in Proterozoic provinces, except where thinner in areas probably affected by Mesozoic extension associated with rifting of the Labrador Sea. Dipping reflectors in the mantle are commonly associated with strong lower-crustal dipping reflections in a manner similar to that observed in some modern orogens. The ECSOOT data show that Proterozoic crust in this area has structural forms comparable with those of modern orogens and, inferentially, its tectonic development was controlled by very similar collisional processes.

Comparative Spatial Variability in Faunal Composition along Two Middle Devonian Paleoenvironmental Gradients

Previous studies of spatial patterns among fossil assemblages have demonstrated the utility of gradient analysis as an investigative tool in paleobiology. However, because lateral samples in these studies have routinely been widely spaced (commonly tens of kilometers apart), there is little sense of the degree to which faunas vary locally, relative to variation observed at larger scales along hypothesized transects. To address this concern, the present study assesses preserved macrofaunal spatial variability in composition among samples collected at both outcrop (small) and basinal (large) scales, along two onshore-off shore transects in the Middle Devonian Hamilton Group within the Appalachian Basin

Climatic Cyclicity and Terrigenous Sediment Influx to the Early Turonian Greenhorn Sea, Southern Utah

ABSTRACT Rhythmically alternating beds of limestone and shale in pelagic deposits of the Cenomanian-Turonian North American Western Interior Seaway have been widely attributed to climatic cyclicity in the Milankovitch band. Whether these rhythmic beds record cycles of terrigenous dilution or of productivity remains controversial. We present results of a study of early Turonian strata of the Tropic Shale and Tununk Member of the Mancos Shale in southern Utah, aimed at testing the dilution model in a prodeltaic environment within 130 km of the paleoshoreline. The study transect consisted of four localities spaced along a 110 km onshore-offshore transect. In each of the four sections, we studied a stratigraphic interval characterized by beds of limestone or marlstone that alternate with beds of marly shale or calcareous shale. Along the transect, content of quartz sand and coarse silt in each of the beds generally decreases and content of calcium carbonate increases. In each section, quartz sand and coarse silt content varies antithetically with calcium-carbonate content. These observations suggest that the limestone/marlstone beds accumulated during times when rates of terrigenous sediment transport to offshore sites were relatively slow. Conversely, the shaly beds were deposited during times of enhanced terrigenous sediment flux. These ob ervations support the hypothesis that limestone-shale bedding rhythms record wet and dry phases of climate cycles. In Utah, the limestone/marlstone beds contain smaller proportions of mixed-layer illite/smectite clays than do the shaly beds. Climatic cyclicity apparently led to cyclic variations in the proportions of clay minerals supplied to the prodeltaic environments, or to variation in the loci of accumulation of different clay minerals. Analysis of intensity of bioturbation and preservation of fecal pellets also indicates that the limestone/marlstone beds accumulated under more oxic benthic conditions than did the shaly beds. Detailed examination of the alternating beds of limestone/marlstone and shale in southern Utah indicates that the deposits are characterized by a distinctive succession of facies. These deposits are better interpreted as bedding cycles, composed of three gradational lithologies, than as bedding rhythms, composed of two alternating lithologies. Data on grain size and calcium-carbonate content suggest that the bedding cycles correspond to shoaling-upward parasequences that developed in response to small-scale changes in relative sea level. Our study suggests that climatically modulated changes in fresh-water discharge and terrigenous sediment supply were mainly responsible for these sea-level changes and for cyclic variations in benthic oxygenation in prodeltaic environments of the earl Turonian Western Interior Seaway.

Analysis of Cretaceous Bedding Cycles in an Onshore-Offshore Transect of the Western Interior Basin: ABSTRACT

Analysis of Cretaceous Bedding Cycles in an Onshore-Offshore Transect of the Western Interior Basin: ABSTRACT

Distributions and fluorochrome-staining properties of submicrometer particles and bacteria in the North Atlantic

Sub-micrometer particles have recently been shown to exist in marine water at concentrations exceeding 10 7 particles ml −1. Their presence has important implications for ocean optics, global biogeochemical models and trophic relationships in the microbial food web. Small particles that were stainable by Acridine Orange (AO) and 4′,6-diamidino-2-phenylindole (DAPI) were enumerated and sized using a quantitative fluorescence microscopy imaging system along an onshore-offshore transect from the mouth of Chesapeake Bay to the Sargasso Sea. The particles were characterized by staining with DAPI, a stain specific for double-stranded DNA and generally indicative of a living cell or viral particle, and AO, a more general bio-polymer stain indicative of organic matter. Two distinct particle populations were measured in the 0.2–1.0 μm size range: (1) typical bacteria; and (2) abundant small, dimly fluorescing (SD) particles. Surface concentrations of organic (AO-staining), SD particles ranged from 3×10 7 ml −1 near the mouth of Chesapeake Bay to 4×10 6 ml −1 in the Sargasso Sea. A variable proportion of the SD particles were DAPI-positive, probably very small bacteria and viruses. The DAPI-positive SD particles constituted 9–29% of the total organic SD particles at coastal and shelf stations, and 25–61% in a vertical profile in oligotrophic waters. The vertical distribution of SD particles in oligotrophic waters showed higher numbers in the surface layer and lower numbers below the sub-surface chlorophyll maximum, suggesting an association of the particles with biological productivity. Our carbon estimates, based on measured particle size spectra and abundances, and reasonable values for particle carbon density, agree with recent measurements of bulk elemental particulate carbon in the 0.2–0.7 μm size fraction in the Sargasso Sea. The particle volume ml −1 of the total SD particles ranged from equal to twice the bacterial biovolume ml −1, indicating a significant carbon pool.

Trace metal concentrations in sediments of the Eastern Mississippi Bight

Sediments collected from 3 onshore-offshore transects (12 stations total) in the Eastern Mississippi Bight at 5 different times during 1987–1988 have been analyzed for total Ag, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, Sn, Th and Zn. The data show considerable variation both spatially and temporally, largely as a result of natural variability in grain size and mineralogy. Clay-rich samples from deep water were always more metal-rich than sandy samples from shallow areas. Many samples from near the Mississippi River Delta appeared to be enriched in Ba by about a factor of two, probably as a result of contamination from oil well drilling mud, but there were few other indications of pollution influences. Many samples from throughout the area gave Mn/Fe and Cd/Fe ratios a factor of two or more lower than their Mississippi River source material. This shows that biochemical activity in the sediment is capable of solubilizing reducible and adsorbed metals.

Engineering geology of nearshore areas off Richards Island, N.W.T.: a comparison of stable and actively eroding coastlines

Nearshore areas off northern Richards Island can be expected to show considerable variability in lithology, strengths, and geothermal setting both in a temporal and a spatial sense. Drilling and laboratory studies carried out along onshore–offshore transects at a stable coastal site and an actively eroding coastal site have identified six major stratigraphic units of Holocene and pre-Holocene (Wisconsinan) age. The main factors controlling the geotechnical properties of these sediments and their distribution are the occurrence of shallow permafrost beneath areas seasonally covered by landfast sea ice, rapid degradation of permafrost in areas farther offshore, ice content of thawing pre-Holocene sediments, and variability in coastal processes. Key words: nearshore deposits, engineering geology, permafrost, physical properties, acoustic properties.