Fine-grained Marine Sediments Research Articles

Geotechnical and geophysical properties of deep marine fine-grained sediments recovered during the second Ulleung Basin Gas Hydrate expedition, East Sea, Korea

This study investigates the geotechnical and geophysical properties of minimally disturbed fine-grained marine sediments recovered during the second Ulleung Basin Gas Hydrate expedition (UBGH2), East Sea, Korea. Conventional laboratory geotechnical engineering test methods were used to characterize the sediments from a geotechnical engineering perspective. The sediments were mostly uncompacted and classified as highly plastic silty soils according to the Unified Soil Classification System (USCS); they exhibited high compressibility when subjected to an increase in effective stress (or a decrease in pore water pressure), low hydraulic conductivities, and low coefficient of consolidation (or pressure diffusion). It is expected that the application of a depressurization method for gas hydrate production at the investigated sites would cause a significant amount of settlement and compaction around the production hole. Because the shear wave velocity of gas hydrate-bearing sediments is much less sensitive to the stress state and is much higher than that of sediments without gas hydrates, it is possible to estimate the degree of hydrate saturation from shear wave velocity data measured in gas hydrate-bearing sediments. In addition, the prior presence of gas hydrates in the sediments was verified through a comparison of water content, compressibility, and electrical resistivity of the sediments.

Selenium as paleo-oceanographic proxy: A first assessment

Selenium (Se) is an essential trace element, which, with multiple oxidation states and six stable isotopes, has been suggested as a potentially powerful paleoenvironmental proxy. In this study, bulk Se concentrations and isotopic compositions were analyzed in a suite of about 120 samples of fine-grained marine sedimentary rocks and sediments spanning the entire Phanerozoic. While the Se concentrations vary greatly (0.22–72ppm), the δ82/76Se values fall in a fairly narrow range from −1 to +1‰ (relative to NIST SRM3149), with the exception of laminated black shales from the New Albany Shale formation (Late Devonian), which have δ82/76Se values of up to +2.20‰. Black Sea sediments (Holocene) and sedimentary rocks from the Alum Shale formation (Late Cambrian) have Se to total organic carbon ratios (Se/TOC) and δ82/76Se values close to those found in modern marine plankton (1.72±0.15×10−6mol/mol and 0.42±0.22‰). For the other sedimentary sequences and sediments, the Se/TOC ratios show Se enrichment relative to modern marine plankton. Additional input of isotopically light terrigenous Se may explain the Se/TOC and δ82/76Se data measured in recent Arabian Sea sediments (Pleistocene). The very high Se concentrations in sedimentary sequences that include the Cenomanian–Turonian Oceanic Anoxic Event (OAE) 2 may reflect an enhanced input of volcanogenic Se to the oceans. As the latter has an isotopic composition not greatly different from marine plankton, the volcanogenic source does not impart a distinct signature to the sedimentary Se isotope record. The lowest average δ82/76Se values are observed in the OAE2 samples from Demerara Rise and Cape Verde Basin cores (δ82/76Se=−0.14±0.45‰) and could reflect fractionation associated with microbial or chemical reduction of Se oxyanions in the euxinic water column. In contrast, a limiting availability of seawater Se during periods of increased organic matter production and burial may be responsible for the elevated δ82/76Se values and low Se/TOC ratios in the black shales of the New Albany Shale formation. Overall, our results indicate that to unlock the full proxy potential of marine sedimentary Se records, we need to gain a much more detailed understanding of the sources, chemical speciation, isotopic fractionations and cycling of Se in the marine environment.

Characteristics of the Grain-Size Distribution of Fine-Grained Marine Sediments, off Pacific Side of Northeast Japan

Characteristics of the Grain-Size Distribution of Fine-Grained Marine Sediments, off Pacific Side of Northeast Japan

Geochemical modeling of sedimentary rocks in the central Hokkaido, Japan: Episodic deformation and subsequent confined basin-formation along the eastern Eurasian margin since the Cretaceous

Late Cretaceous to Paleogene tectonic episodes around the eastern Eurasian margin are described utilizing one-dimensional (1D) basin modeling technique on the basis of organic maturation; vitrinite reflectance and T max parameter values of the Rock-Eval pyrolysis. Fine-grained marine sediment of the Yezo Supergroup in central Hokkaido is one of the most extensive Cretaceous constituents along the plate margin. To evaluate organic maturation within the Yezo Supergroup, a surface section (Hakkin-zawa) and a deep borehole (MITI Yubari) were selected, from which quite different maturation trends were obtained. Notably low and constant maturation was suggested throughout the thick Hakkin-zawa section, and it was confirmed on the basis of various biomarker analyses. It requires anomalously rapid burial, probably related to thrust-stacking or large-scale slumping, followed by prompt tilting/exhumation event. In sharp contrast, Late Cretaceous strata in the MITI Yubari do not indicate coeval tectonic disturbances in a short period. A wider view of the ancient convergent margin suggests that deformation of the forearc propagated westward during the Cretaceous. High maturation levels in the uppermost thrust sheet around the MITI Yubari are optimized in 1D geochemical modeling on the assumption that a thick missing Paleogene unit has been eroded out as a result of thrust-forming contraction scheme emerged during the Neogene. A similar Cenozoic burial pattern is adopted for the once tilted and exhumed Hakkin-zawa section in order to match the present maturation levels. Although such an active subsidence of a ‘forearc’ is generally interpreted as an effect of the subduction erosion, heavy mineral composition of the shallow marine to fluvial Paleogene implies uplift and exhumation of ultramafic rocks to the east (trench-side). Thus the confined basin-formation in central Hokkaido is to be understood in a different tectonic framework that may reflect transcurrent motions on the plate margin.

Correlations for Zero Effective Stress Void Ratio of Fine-Grained Marine and Riverine Sediments

The void ratio at zero effective stress is the origin for measuring strains in fine-grained marine and riverine sediments and similar materials deposited in land impoundments, and is thus an important parameter in consolidation calculations. However, it is difficult to determine accurately by experiment, and existing correlations are unsatisfactory. New empirical correlations are presented that relate the zero effective stress void ratios of fine-grained sediments to the initial void ratios or moisture contents of the slurries from which they form. The new correlations are statistically very strong and enable zero effective stress void ratios to be estimated with relatively small errors. They are applicable to a wide range of materials, but must be applied with caution to materials with very high plasticity or high organic contents.

Clay slides in the Målselv valley, northern Norway: Characteristics, occurrence, and triggering mechanisms

The distribution of a large number of clay slides in the Målselv valley, northern Norway, is analysed and put into context with the stratigraphic organization of the valley-fill sediments. About 32% of the landslides larger than 10 4 m 3 occur close to the valley margins, where mud is either exposed or at shallow depth. About 57% of the landslides occur mid-valley, where relatively thin (< 15 m) coarse-grained deltaic sediments overlay fine-grained marine and glaciomarine sediments, and about 11% of the landslides occur in front of ice-contact deposits. The slide-prone areas are all characterized by the occurrence of heterogeneous sediments (interbedded clay, silt and sand), in addition to the presence of steep slopes eroded by rivers. The heterogeneous nature of the sediments probably enhanced groundwater drainage and leaching of salts from the clay, increasing sensitivity. Thus, the distribution and organization of the valley-fill sediments and groundwater drainage probably controlled the position of the slide scars and sliding planes. Since deglaciation of the valley (11,500 BP–present), isostatic rebound has enhanced fluvial incision and the creation of steep slopes due to a fall in relative sea level of up to 80 m. Arcuate-shaped, ‘bottleneck’ landslide scars ranging from c. 10 4 to 10 7 m 3 in size is the dominant morphological signature of the slides, typical for quick clay slides or earth flows involving fluid mud. Their most common triggering mechanism is probably erosion at the toe slopes by the river Målselv or its tributaries. River erosion close to the valley margin, where glaciomarine and marine sediments are present, seems to give the most severe slides. The overall valley-fill organization controls the distribution of clay slides, which may apply to other fjord valleys having similar sediment distribution.

Mapping of permafrost surface using ground-penetrating radar at Kangerlussuaq Airport, western Greenland

Abstract Kangerlussuaq Airport is located at 67°N and 51°W in the zone of continuous permafrost in western Greenland. Its proximity to the Greenlandic ice sheet results in a dry sub-arctic climate with a mean annual temperature of − 5.7 °C. The airport is built on a river terrace mostly consisting of fluvial deposits overlying fine-grained marine melt-water sediments and bedrock. A ground-penetrating radar (GPR) survey was performed to study the frozen surface beneath the airfield. The measurements were carried out in late July 2005 on the southern parking area in Kangerlussuaq Airport. Five years earlier, in autumn 2000, three test areas were painted white in order to reduce further development of depressions in the asphalt pavement. GPR profiles crossing the white areas show a distinct difference in depth to the permafrost surface under the painted areas compared to the natural black asphalt surface. GPR data also show a correlation between structures in the river terrace and depressions in the paved surface. The combination of large dark areas of asphalt pavement and perhaps a change in snow removal routines on the southern parking area have caused an increase in solar heating, a lowering of the permafrost surface and the formation of several depressions in the pavement of the southern parking area. The depressions can be clearly seen after rainfall. To calibrate the GPR survey, sediment samples from a borehole were analyzed with respect to water content, grain size and content of organic material. To find the exact depth of the permafrost a trench was dug down to the frozen surface.

Fate and effects of picric acid and 2,6-DNT in marine environments: Toxicity of degradation products

The toxicity of transformation products of 2,6-dinitrotoluene (2,6-DNT) and 2,4,6-trinitrophenol (picric acid) were assessed in spiked sandy and fine-grained marine sediments and in seawater. Toxicity of pore water from sediments spiked with 2,6-DNT decreased for the macro-alga, Ulva fasciata, zoospores as biotransformation proceeded, but increased for the copepod, Schizopera knabeni, nauplii. The primary biotransformation product of 2,6-DNT, 2-amino-6-nitrotoluene, was also more toxic than the parent compound to copepod nauplii, but not to alga zoospores, in spiked seawater tests. Two biotransformation products of picric acid, picramic acid and 2,4-DNP, were more toxic than their parent compound. Porewater toxicity from picric acid-spiked sediments decreased significantly at the end of six-months incubation. Fine-grained sediment spiked with either ordnance compound had lower toxicity than its sandy counterpart after six months, suggesting faster microbial transformation in the former and production of less toxic products. Photo-transformation of 2,6-DNT in seawater resulted in a reduction in toxicity.

Multiple mechanisms of remagnetization involving sedimentary greigite (Fe 3S 4)

Sedimentary greigite (Fe 3S 4) is being increasingly implicated as the carrier of late diagenetic remagnetizations in fine-grained marine and terrestrial sediments. We have conducted detailed scanning electron microscope observations on polished sections from such sediments, coupled with elemental microanalysis, to identify greigite and to characterize the mode of occurrence and the relationships between the observed greigite and other authigenic and detrital mineral phases. This detailed observational work, in conjunction with recently published work, has enabled identification of several mechanisms for remagnetization involving greigite: (1) neoformation of greigite on the surfaces of early diagenetic framboidal and nodular pyrite; (2) greigite growth within cleavages of detrital sheet silicate grains; (3) neoformation of greigite on the surfaces of authigenic clays (smectite, illite); (4) greigite nucleation on the surfaces of siderite; (5) greigite growth on the surface of gypsum that resulted from earlier oxidation of nodular pyrite. Many other mechanisms for greigite neoformation are conceivable. The key variables are appropriate redox conditions for generation of sulphide and availability of iron from the dissolution of a wide range of possible reactive detrital and authigenic iron-bearing minerals. Documentation of a wide range of mechanisms for neoformation of greigite provides compelling evidence that sediments containing greigite should be routinely suspected of remagnetization, which will complicate or compromise studies of environmental magnetism and geomagnetic field behaviour.

Tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from New Zealand

Research Article| July 01, 2005 Tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from New Zealand Christopher J. Rowan; Christopher J. Rowan 1Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK Search for other works by this author on: GSW Google Scholar Andrew P. Roberts Andrew P. Roberts 1Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK Search for other works by this author on: GSW Google Scholar Geology (2005) 33 (7): 553–556. https://doi.org/10.1130/G21382.1 Article history received: 28 Nov 2004 rev-recd: 17 Jan 2005 accepted: 22 Jan 2005 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Christopher J. Rowan, Andrew P. Roberts; Tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from New Zealand. Geology 2005;; 33 (7): 553–556. doi: https://doi.org/10.1130/G21382.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Detailed scanning electron microscope observations, coupled with elemental microanalysis, confirm the presence of a chemical remanent magnetization carried by authigenic greigite (Fe3S4) in uplifted Neogene marine sediments from the Hikurangi Margin of New Zealand. Normal polarity samples from the studied section have declinations that are deflected ∼60° clockwise of reversed polarity samples, indicating the presence of two distinct magnetizations separated by several million years of tectonic rotation about a vertical axis. However, although multiple generations of iron sulfide growth are observed petrographically, we see no clear differences in the relative timing of greigite formation between samples carrying these two magnetizations. Not only can the diagenetic growth of greigite in fine-grained marine sediments occur long after deposition, obscuring tectonic and magnetostratigraphic information, but such remagnetizations are also difficult to distinguish from a more primary signal in the absence of constraints from field tests. Our observations emphasize that considerable care is necessary when interpreting paleomagnetic data from greigite-bearing sediments. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic stratigraphy of the Plio-Pleistocene Ross Island flexural moat-fill: a prognosis for ANDRILL Program drilling beneath McMurdo-Ross Ice Shelf

Ross Island volcanic complex began forming with the emplacement of the basaltic shield volcanoes of Mt. Bird and Mt. Terror between ca. 4.6 and 1.3 Ma, though it has developed most significantly over the last 1 Ma during an eruptive phase resulting in the 3794-m-high composite vent of Mt. Erebus. Throughout this time, loading of the lithosphere at the southern end of the Terror Rift by the Ross Island volcanic pile has progressively depressed the crust, resulting in a subcircular flexural moat around the periphery of the island. Multichannel seismic reflection data collected from the McMurdo-Ross Ice Shelf (MRIS) reveal the stratigraphic architecture of the moat-fill on the southeastern side of Ross Island. The moat region has accommodated a well-stratified, regionally extensive sedimentary succession of at least 1.2 km below the seafloor in the deepest part of the depression. Three seismic stratigraphic units are identified that generally thicken and dip towards Ross Island and are bounded by angular (onlap) unconformities. We infer that the three units were deposited in accommodation space created during discrete phases of volcanic load-induced subsidence: 1. Unit III. Moderate to low-amplitude discontinuous reflectors are dislocated and tilted by normal faulting and interpreted to represent coarse-grained glacigenic and fine-grained marine sediments with likely intercalated volcanic ash. These strata may have started to accumulate during loading of the crust by Mt. Bird between ca. 4.6 and 3.8 Ma. 2. Unit II. Moderate to high-amplitude continuous reflectors that onlap Unit III and are interpreted to represent coarse-grained glacigenic and fine-grained marine sediments with likely intercalated volcanic ash. These strata are inferred to have accumulated in the crustal depression resulting from loading by Mt. Terror between ca. 1.8 and 1.3 Ma. 3. Unit I. Relatively continuous low-amplitude to seismically-opaque reflectors (Unit IB), onlap Unit II and grade upwards into moderate to high amplitude reflectors below the seafloor (Unit IA). Unit IB is interpreted to represent fine-grained pelagic marine sediments and volcanic ashes which may have accumulated during a phase of rapid subsidence at the initiation of Mt. Erebus loading between ca. 1.0 and 0.8 Ma. Unit IA is interpreted to represent progressive infilling of the flexural moat with more coarse-grained, probably glacigenic sediments during continuation of Mt. Erebus loading between ca. 0.8 and 0.2 Ma. These phases of load-induced subsidence may have periodically overdeepened the moat, making it likely that the sedimentary fill is relatively continuous and has largely escaped erosion by ice grounding during past glacial expansions of the West Antarctic Ice Sheet (WAIS) and the MRIS. The age relationships provided by radiometric dating of the various volcanic loads imply that the sediments here have the potential to provide a relatively continuous and high-resolution glacimarine record of the past behavior of the WAIS–MRIS and the climate history of the western Ross Sea region back to ∼5 Ma. This stratigraphic record is scheduled to be drilled by the ANDRILL Program in the austral summer of 2006. From the interpretation of seismic facies, we present a stratigraphic prognosis for the proposed drill site.

The Jurassic of the Netherlands

A recent revision of the lithostratigraphy of the Netherlands has triggered an extensive re-evaluation of existing ideas on the Jurassic structural and depositional history. Significant advances can be attributed to the incorporation of sequence stratigraphic concepts. In the course of the Triassic and Jurassic, structural complexity increased progressively. The Jurassic sedimentary succession can be subdivided into three depositional megasequences. Megasequence I (Rhaetian– Aalenian) reflects the period between the so-called early and mid-Cimmerian tectonic phases. Megasequence II (Aalenian – Middle Callovian) covers the period of activity of the mid-Cimmerian phase. Megasequence III (Middle Callovian – Ryazanian) corresponds with the period between the mid-Cimmerian and late Cimmerian phases (particularly after pulse II). In this latter megasequence, six stages (IIIa–f) are recognised. Sediments deposited during the Rhaetian and Ryazanian bear a stronger affinity with the Jurassic succession than with Triassic and Cretaceous sediments respectively. These stages are thus treated here as an integral part of the Jurassic succession. During the Rhaetian–Bajocian the area subsided relatively uniformly. A sheet of predominantly fine-grained marine sediments of great lateral uniformity was deposited. During the Toarcian, in particular, basin circulation was largely restricted. The cooling that followed the thermal Central North Sea dome uplift triggered an important extensional phase during the Aalenian–Callovian. The rift phase resulted in the formation of several smaller basins, each with its own characteristic depositional succession. The basins fall into three structural provinces: the eastern province (Lower Saxony Basin, E–W-striking); the northern province (Central Graben, N–S-striking); and the southern–central system (Roer Valley Graben – Broad Fourteens, with a strong NW–SE strike). The mid-Cimmerian event started to affect the Dutch basins during the Bajocian. Sedimentation ceased in the Dutch Central Graben while it persisted in a predominantly coarse-grained, shallow marine facies in the southern basins (Roer Valley Graben, West Netherlands Basin). Extensional tectonics in the Central Graben were initiated during the Middle Callovian, with the deposition of continental sediments. During the Oxfordian–Kimmeridgian, marine incursions gradually became more frequent. Marine deposition in the other basins in the south persisted into the Oxfordian, at which time deposition became predominantly continental. Marine conditions gradually returned in the south during the Ryazanian–Barremian, with a series of advancing partial transgressions from the north. The present- day distribution of Jurassic strata in the Netherlands was determined largely by erosion associated with Late Cretaceous – Paleocene uplift.

Facies development and sequence architecture of a late Quaternary fluvial-marine transition, Canterbury Plains and shelf, New Zealand: implications for forced regressive deposits

The Canterbury Plains, South Island, New Zealand, comprise a c. 7500 km 2 coarse-grained, braidplain that accumulated during Quaternary glacio-eustatic, sea-level fluctuations. The adjacent Canterbury Bight shelf covering c.13,000 km 2, comprises coeval shelf–slope deposits, that are punctuated by advances of the braidplain onto the shelf during periods of sea-level fall. This study examines the sedimentological and stratal characteristics of outcropping last glacial braidplain deposits, and then traces oscillations in the position of the fluvial-marine transition over several late Quaternary sea-level cycles using high-resolution seismic reflection profiles of the Canterbury shelf and slope. Outcropping last glacial Burnham Formation sediments display numerous, aggradationally stacked massive and cross-stratified gravel deposits with minor intercalated sand and mud. The gravels accumulated as longitudinal bars and channel fills within an extensive braidplain succession, with some evidence of frozen ground conditions during deposition based on sedimentological features. High-frequency (3.5 kHz) seismic reflection data of the subsurface Canterbury shelf identify up to seven unconformity-bound, Milankovitch-duration depositional sequences. These sequences are inferred to correlate with successive 100-ka, sea-level cycles spanning Oxygen Isotope Stages 16 to 1 (last c. 700 ka). Each sequence displays a distinctive stratigraphic motif comprising four recurring seismic units: 1. Basinward of the glacial maximum shoreline, wedge-shaped units displaying steeply dipping clinoforms that onlap the continental slope are interpreted as “perched lowstand deltas” belonging to the lowstand prograding wedge systems tract (LST). 2. Irregular hummocky units up to 10 m thick, containing high-amplitude discontinuous reflectors, are interpreted as representing stranded coastal deposits of the transgressive systems tract (TST). 3. Low-amplitude seismic units which offlap and downlap onto the TST, infilling local paleotopography, and interpreted as comprising fine-grained marine sediments of the highstand systems tract (HST). 4. Basinward thickening units (up to 40 m thick), containing a strongly progradational series of offlapping, inclined (0.5–1.0°), high-amplitude reflectors, that downstep towards the basin are interpreted as coarse-grained, fluvio-deltaic sediments, similar to the last glacial Burnham Formation, deposited during glacio-eustatic sea-level fall, or forced regression. We assign this unit to the regressive systems tract (RST), which displays a gradational lower boundary overlain by a sharp planar regionally extensive sequence boundary or ravinement surface. 2-D forward stratigraphic modelling, constrained by outcrop and seismic data, indicates that rivers of the Canterbury region did not incise during eustatic sea-level fall. This may be the case elsewhere, too, where a coastal plain is flanked by a lower gradient shelf. On the Canterbury shelf, fluvial incision did not occur during Quaternary forced regressions, but instead, subaerial accommodation was created and filled in by thick, fluvio-deltaic deposits, as contemporary rivers graded to the glacial maximum shoreline. Incision was restricted to three zones: (1) The lowstand shelf break, where canyons of limited extent formed by nickpoint retreat; (2) the transgressive coastline, where rivers incised due to coastal erosion; and (3) the inner braidplain adjacent to the Southern Alps, where degradation was caused by tectonic uplift.

Enhanced erodibility of fine-grained marine sediments by Hydrobia ulvae

The common mud snail Hydrobia ulvae is a widespread and dominant deposit feeder on fine-grained substrata along the European Atlantic coastline. Previous studies have shown that mud snail activities such as grazing, faecal pellet production and mucous production may influence physical properties of the surface sediment layer and thus depositional and erosional processes. To quantify the influence of Hydrobia density on key parameters such as erosion threshold and erosion rate, a short-term laboratory experiment was conducted. Snails were placed on fine-grained sediment at densities of 10 000 and 50 000 ind m −2 and erosion experiments were carried out one, three and five days after establishment of the sediment beds. Controls without H. ulvae were treated the same way. The presence of H. ulvae significantly increased the erosion rate and decreased the erosion threshold compared to snail-free control plots. The erosion rate was increased by a factor of 2 to 4 when H. ulvae were present and showed stronger influence by the snail than the erosion threshold. Snail density did not affect the erosion threshold, but the erosion rate doubled with an increase in density from 10 000 to 50 000 ind m −2. The erosion rate was only marginally different after one day but the difference increased over time and the erosion rate was significantly different for all treatments after five days. No significant time dependence was observed for the erosion threshold. The results generally confirm results obtained in situ and differences can be related to different hydrodynamic conditions under field and laboratory conditions.

Anisotropic permeability and bimodal pore-size distributions of fine-grained marine sediments

We present preliminary results illustrating that overconsolidated fine-grained sediments without a compactional fabric but favourably aligned microfractures can show significant anisotropic permeability. At much reduced effective stress, substantial flow can occur parallel to the fabric such that anisotropy indices — calculated by dividing horizontal permeability by vertical permeability — increase by factors above 10. SEM observations show no intense particle alignments in the materials, but the presence of parallel oriented microfractures; mercury-intrusion porosimetry data indicate that this enhanced flow is due to microfractures opening when favourably oriented. Such an effect is displayed as a distinctly bimodal pore-size distribution when intruded by mercury parallel to the fabric; identical samples with the fabric oriented perpendicular to intrusion direction show only unimodal pore-size distributions. While the results, at present, are only directly applicable to the shallow subsurface, it is proposed that directional bimodal pore-size distribution needs further investigation for deeper, more consolidated sediments in light of the potential ramifications highlighted by the results. For example, in some instances shales may not be as efficient hydrocarbon seals as traditionally thought.

WATER STRIDERS FROM THE PALAEOGENE OF DENMARK WITH A REVIEW OF THE FOSSIL RECORD and EVOLUTION OF SEMIAQUATIC BUGS (HEMIPTERA: GERROMORPHA).. By Nils Moller Andersen Biologiske Skrifter 50, The Royal Danish Academy of Sciences &amp; Letters, Copenhagen. 1998. pp. 157. Softcover. DKK200.00. ISBN 87-7304-295-1.

Fine-grained marine sediments from the Palaeocene/Eocene boundary in northern Jutland have recently yielded abundant compression fossils of non-marine organisms. The pondskaters and their relatives from this assemblage, together with a single specimen from strata of a similar age in northern Germany and three inclusion fossils in Baltic amber of Eocene/Oligocene age, are monographed in the opening section of this scholarly treatize. Apart from a monobasic genus of Hydrometridae previously described from these strata by Andersen, and a gerrid nymph in amber that was described in 1856, all are new to science. The new gerrid genus Palaeogerris is based on no fewer than thirty specimens assigned to three species, including the stately P. grandis which, at 3 cm is length, is exceeded in size only by the extant Oriental Gigantometra, to which the new genus is closely related. A new monobasic genus of Hydrometridae is described and a second species is added to that previously recognized in this material, all preserved as compression fossils. Another new genus is placed in Macroveliidae for lack of apomorphies. The amber fossils yielded two rather uninteresting gerrids and a new genus of Veliidae represented by a single specimen. The middle third of the work reviews and reinterprets all previously described Cenozoic and Mesozoic fossil Gerromorpha, providing descriptive notes or re-descriptions and examining their assignment to genera and higher taxa, reassigning or querying placements as necessary and describing a new Limnoporus species from the Middle Eocene of western Canada. Mesozoic fossils of Gerromorpha are unfortunately and unaccountably rare. Andersen accepts a Lower Cretaceous species from Brazil as a genuine hydrometrid and two more of similar age from Australia as possibly a veliid and a mesoveliid. He doubts the assignment of Engynabis to Gerromorpha, is silent as to the affinities of Karanabis (described in Nabidae but transferred to Gerridae by Popov in 1968), ignores Velia cornuta Weyenbergh (said by Handlirsch in 1907 to be merely an artefact), dismisses Chresmodidae as being non-hemipteran (with good reason) and overlooks Brodie's mention (in 1845), without descriptions, of ‘Hydrometra’ and ‘Velia(?)’ from the Jurassic of Britain. If genuine, Brodie's fossils would be both the earliest recognition of fossil Gerromorpha in print and the oldest known representatives of the group. In the concluding section of the work, Andersen discusses the implications of the fossil record in the areas of phylogeny, phylogenetic classification, palaeoecology, palaeobiology and historical biogeography, taking care always to define his terminology, explain his methodology and weigh his evidence. Thus, when he uses morphological evidence to make deductions about the relationships among the various gerromorphan taxa, the minimum ages of the various lineages, past climatic influences or the era when Hemiptera first ventured onto the water surface or initiated the practices of post-copulatory mate guarding and the maintenance of territories, the reader is impelled to share his conclusions and reservations alike. This is palaeontology as it should be done: the critical study of once-living organisms.

Impact of Isostatic Uplift and Ditching of Sulfidic Sediments on the Hydrochemistry of Major and Trace Elements and Sulfur Isotope Ratios in Streams, Western Finland

Acid soil drainage is of major economic concern for the arable crop farmers and fishermen in midwestern Finland. Almost all the farmland in this region consist of acid sulfate soils developed on sulfide-bearing fine-grained marine and lacustrine sediments. During rain storms in autumn and snowmelt in spring these soils are flushed, resulting in low pH and high metal concentrations in affected watercourses. A study of 40 first- and second-order streams during the 1997 autumn high water flow event demonstrated that (1) the pH in peat/till dominated catchments is controlled by organic acids (pH > 4.5), while in streams draining acid sulfate soil pH is controlled by sulfuric acid (4.5 > pH > 3.0); (2) the δ34S(sulfate) values with a range of −16 to +7 ‰ VCDT show two populations with medians of −6.2 and +3.2 ‰, the first is dominated by original sedimentary sulfide and the second is most likely related to secondary sulfide formation in the acid sulfate soils; (3) the concentrations of Al, Ca, Cd, Cl, Co, Cu, Mn, Ni, Na, Zn, and SO42- are considerably higher in streamwaters in the investigated area than in headwater streams from the whole country as a result of extensive leaching of the acid sulfate soils; and (4) As, Ba, Cr, Fe, Pb, Sb, and V concentrations are controlled by soil and water pH, dissolved organic matter, and suspended particles and are not like the other trace elements, significantly enriched in the streamwaters.

In situ conditions and interactions between microbes and minerals in fine-grained marine sediments; a TEM microfabric perspective

Microbes, their exocellular secretions, and their impact on the mineralogy and microfabric of fine-grained continental margin sediments were investigated by transmission electron microscopy. Techniques were used that retained the in situ spatial relations of both bio-organic and mineralogical constituents. Photomicrographs were taken of characteristic mineral-microbe associations in the first meter of burial at conditions ranging from aerobic to anaerobic. Single-celled prokaryotes, prokaryotic colonies, and eukaryotic organisms were observed as were motile, sessile, and predatory species. Bacterial cells dominate the assemblage. The most commonly observed mineral-biological interaction was the surrounding, or close association, of isolated heterotrophic bacterial cells by clay minerals. Almost without exception, the external surfaces of the bacteria were covered with secreted exocellular slimes composed of cross-linked polysaccharide fibrils. These fibrils act to bind sediment grains into relatively robust microaggregates, roughly {le} 25 {micro}m in diameter. These exocellular polymers can significantly impact the interaction between microbes and minerals, as well as the chemical and physical transport of fluids and dissolved aqueous species through the sediment. Although pore water chemical profiles from the field sites studied have dissolved Fe and Mn, no close association was found between the microbes imaged and precipitated metal oxyhydroxides or other authigenic minerals, such asmore » is commonly reported from laboratory cultures.« less

A plug flow-through reactor for studying biogeochemical reactions in undisturbed aquatic sediments

A slow flow, plug-through reactor was developed for measuring equilibrium and kinetic parameters of biogeochemical reactions on intact sections of sediment cores. The experimental approach was designed to preserve the structural, geochemical and microbiological integrity of the sediment sections and, hence, retrieve reaction parameters that apply to in-situ conditions. Inert tracer breakthrough experiments were performed on a variety of unconsolidated surface sediments from lacustrine, estuarine and marine depositional environments. The sediments studied cover wide ranges of composition, porosity (46–83%) and mean grain size (10 −4−10 −2 cm). Longitudinal dispersion coefficients were determined from the breakthrough curves of Br −. The curves were also used to check for early breakthrough or trailing, that is, features indicative of non-ideal flow conditions. Sediment plugs that exhibited these features were eliminated from further experiments. Dimensionless equilibrium adsorption coefficients ( K) of NH 4 +, were calculated from measured retardation times between the breakthrough of NH 4 + and Br −. The values of K at 5°C vary between 0.3 and 2.3, with the highest value obtained in a fine-grained marine sediment, the lowest in a coarse-grained lake sediment. The values for the marine and estuarine sediments agree with values reported in the literature. The dependencies of K on ionic strength (range 0.2-0.7 m) and temperature (range 5–25°C) in an estuarine sediment confirm that the main sorption mechanism for NH 4 + is ion exchange. The reactor was used in recirculation mode to measure steady-state rates of dissimilatory SO 4 2− reduction in a salt-marsh sediment. Recirculation homogenizes solute concentrations within the reactor, hence facilitating the derivation of reaction rate expressions that depend on solution composition. The rate of microbial S0 4 − reduction was found to be nearly independent of the dissolved SO 4 2− concentration in the range of 2.2−1 mM. Fitting of the experimental rates to a Monod relationship resulted in a maximum estimate of the half-saturation concentration, K s, of 240 μM. This value is comparable to those reported for a pure culture of SO 4 2−-reducing bacteria, but is significantly smaller than the only other K s value reported in the literature for SO 4 2− utilization in a natural marine sediment.

The problem of defining geochemical baselines. A case study of selected elements and geological materials in Finland

Although the term ‘geochemical baseline’ appears in the international geochemical mapping programmes IGCP 259 and 360, it has never been well defined. Several considerations relevant to such a definition are discussed. A geochemical baseline for an element refers to its natural variations in concentration in the surficial environment. Geochemical baselines were studied in Finland by comparing results from regional geochemical mapping programmes based on samples of till, clay and organic stream sediment. The geochemical background changes regionally with the basic geology and locally with the type and genesis of the overburden. Baseline concentrations depend on sample material collected, grain size and extraction method. In Finland, concentrations of potentially harmful elements tend to be higher in fine-grained marine and lacustrine sediments than in glacial till. Concentrations are also systematically higher in the < 0.06 mm fraction than in the < 2 mm size fraction of till samples. Only small proportions of the total heavy metal concentrations in Finnish marine clays are bioavailable. Geochemical baselines are needed for environmental legislation and political decision-making, especially in the assessment of contaminated soil. In many areas of Finland, natural concentrations of several heavy metals exceed the guide or limit values designated for contaminated soils. Thus baselines must always be verified in any assessment of sites for contamination.