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At least 433 records · Page 24Linked to original sources

USGS1991

Shoreline deposits and diagenesis resulting from two Late Pleistocene highstands near +5 and +6 metres, Durban, South Africa

In exposures of Pleistocene rocks on the east coast of South Africa, eight sedimentary facies were distinguished on the basis of petrology, grain size, internal structures and field relationships. These are interpreted as deposits of surf zone, breaker zone, swash zone, backbeach, boulder beach and dune environments. Three phases of deposition and diagenesis are recognized. As a result of the stabilising effect of pre-existing coastal facies, the deposits from successive sea level stands are stacked vertically in a narrow coast-normal strip. Early cementation prevented erosion of the deposits during subsequent transgressions. Deposition of subsequent facies took place on an existing coastal dune (Facies 1). A terrace was cut into this dune at a sea level 4.5 to 5 m above present. At this sea level, clastic shoreline sediments were deposited which make up the main sedimentary sequence exposed (Facies 2–7). The steep swash zone, coarse grain size, and comparison with modern conditions in the study area indicate clastic deposition on a high-energy, wave-dominated, microtidal coastline. Vertical stacking of progressively shallower water facies indicates progradation associated with slightly regressive conditions, prior to stranding of the succession above sea level. During a subsequent transgression to 5.5 or 6 m above present sea level, a second terrace was cut across the existing facies, which by then were partly lithified. A boulder beach (Facies 8) deposited on this terrace is indicative of high wave energy and a rocky coastline, formed by existing cemented coastal facies. Comparison with dated deposits from other parts of the South African coast suggest a Late Pleistocene age for Facies 2–8. Deposition was terminated by subsequent regression and continuing low sea levels during the remainder of the Pleistocene. Cementation of the facies took place almost entirely by carbonate precipitation. The presence of isopachous fibrous cements suggests early cementation of Facies 1, 2, 3 and 4 under marine conditions, initially as aragonite which has since inverted to calcite. Facies 5, 6 and 7 are cemented only by equant calcite spar, evidence of cementation in the meteoric phreatic and vadose zones. Lowering of the water table during regression caused the remaining pore space in Facies 1, 2, 3 and 4 to be filled with equant calcite spar. Decementation in a 130 cm wide zone is attributed to water table shifts associated with the later transgression which deposited Facies 8. The vertical stacking of the two depositional sequences may be attributed to rapid cementation of Facies 2, 3, 4, 5, 6 and 7 under humid, subtropical conditions. This lithified sequence then acted as a focus for deposition of coarse-grained shoreline facies (Facies 8) during the subsequent transgression.

Marine Geology↗

USGS1991

A giant submarine slope failure on the northern insular slope of Puerto Rico

A large amphitheater-shaped scarp, approximately 55 km across, was imaged on the northern insular slope of Puerto Rico using long-range sidescan sonar and bathymetric data. This scarp results from the removal of more than 1500 km 3 of Tertiary strata. A review of seismic-reflection profiles, stratigraphic data, and subsidence models of the northern insular margin of Puerto Rico were used to infer that large-scale slope failure was induced by the tectonic oversteepening of the insular slope and was responsible for the formation of the scarp. The oversteepening probably was caused by the most recent episode of convergence of the Caribbean and North American plates, which began between approximately 4 and 2.5 m.y. ago. The Tertiary strata have been tilted approximately 4.5° to the north in the last 4 m.y.

Marine Geology↗

USGS1991

Modes of cross-shore sediment transport on the shoreface of the Middle Atlantic Bight

The mechanisms responsible for onshore and offshore sediment fluxes across the shoreface zone seaward of the surf zone were examined in a 3-year field study. The study was conducted in the southern part of the Middle Atlantic Bight in the depth region 7–17 m using instrumented tripods supporting electromagnetic current meters, pressure sensors, suspended sediment concentration sensors, and sonar altimeters. The observations embraced fairweather, moderate energy, swell-dominated, and storm conditions. Cross-shore mean flows ranged from near zero during fairweather to > 20 cm s −1 during the storm; oscillatory flows were on the order of 10 cm s −1 during fairweather and 100 cm s −1 during the storm. Suspended sediment concentrations at about 10 cm above the bed were < 0.1 kg m −3 under fairweather conditions, 1–2 kg m −3 under moderate swell conditions, and > 5 kg m −3 during the storm. Three methods were applied to evaluate the relative importance of incident waves, long-period oscillations, mean flows and gravity in effecting shoreward or seaward sediment flux: (1) an energetics transport model was applied to instantaneous near-bottom velocity data, (2) higher moments of near-bottom flows were estimated and compared, and (3) suspended sediment fluxes were estimated directly from the instantaneous products of cross-shore velocity and suspended sediment concentration. The results show that measurable contributions were made by all four of the processes. Most significantly, mean flows were seen to dominate and cause offshore fluxes during the storm and to contribute significantly to onshore and offshore flux during fairweather and moderate energy. Incident waves were, in all cases, the major source of bed shear stress but also caused shoreward as well as seaward net sediment advection. Low-frequency effects involving wave groups and long-period waves made secondary contributions to cross-shore sediment flux. Contrary to expectations, low-frequency fluxes were just as often shoreward as seaward. Whereas cross-correlations between suspended sediment concentration and the instantaneous near-bottom current speed were high and in phase under storm conditions, they were weak and out of phase during fairweather conditions. This suggests that simple energetics models are probably inadequate for predicting fairweather transport of suspended sediment.

Middle Atlantic Bight↗

USGS1992

Holocene depocenter migration and sediment accumulation in Delaware Bay: A submerging marginal marine sedimentary basin

The Holocene transgression of the Delaware Bay estuary and adjacent Atlantic coast results from the combined effect of regional crustal subsidence and eustasy. Together, the estuary and ocean coast constitute a small sedimentary basin whose principal depocenter has migrated with the transgression. A millenial time series of isopach and paleogeographic reconstructions for the migrating depocenter outlines the basin-wide pattern of sediment distribution and accumulation. Upland sediments entering the basin through the estuarine turbidity maximum accumulate in tidal wetland or open water sedimentary environments. Wind-wave activity at the edge of the tidal wetlands erodes the aggraded Holocene section and builds migrating washover barriers. Along the Atlantic and estuary coasts of Delaware, the area of the upland environment decreases from 2.0 billion m2 to 730 million m2 during the transgression. The area of the tidal wetland environment increases from 140 million to 270 million m2, and due to the widening of the estuary the area of open water increases from 190 million to 1.21 billion m2. Gross uncorrected rates of sediment accumulation for the tidal wetlands decrease from 0.64 mm/yr at 6 ka to 0.48 mm/yr at 1 ka. In the open water environments uncorrected rates decrease from 0.50 mm/yr to 0.04 mm/yr over the same period. We also present data on total sediment volumes within the tidal wetland and open water environments at specific intervals during the Holocene.

Delaware Bay↗

USGS1992

Mineralogy of the silt fraction in surficial sediments from the United States mid-Atlantic shelf, slope and rise

An analysis of the abundances and distributions of silt-sized heavy minerals from the U.S. mid-Atlantic outer continental shelf, slope, and rise shows that heavy minerals constitute a substantially greater weight percent of the silt fraction than that of the sand fraction regardless of environment and sediment texture. Concentrations of silt-sized heavy minerals progressively decrease from the shelf where they average 6.94%, to the slope and rise where they average 4.45% and 3.45%, respectively. A mixed amphibole-garnet+staurolite-epidote-pyroxene association dominates the silt-sized heavy mineral assemblage on the slope and rise; an ilmenite-amphibole-epidote association predominates on the shelf. Downslope trends in detrital nonmicaceous silt-sized heavy mineral abundances are related to hydraulic sorting rather than to chemical weathering. Elevated concentrations of the authigenic pyrite, siderite, dolomite+ankerite, and, possibly, phosphorite in the surficial slope sediment suggests that formation of silt-sized heavy minerals by diagenetic processes is relatively more important there than on the continental shelf or rise.

Marine Geology↗

USGS1992

Late Cretaceous to Miocene phosphatic sediments in the Georges Bank Basin, U.S. North Atlantic outer continental shelf

Phosphorite and phosphatic sediments are present in the Georges Bank Basin in marine, Late Cretaceous to Miocene strata equivalent to the Dawson Canyon Formation and Banquereau Formation of offshore Nova Scotia. The Late Cretaceous to Paleocene phosphorite occurs predominantely as sand- and gravel-sized pellets and as cement in conglomeratic aggregates. The Eocene and Miocene phosphate occurs mainly as fine-very fine sand-size spheroidal-avoidal pellets in unconsolidated clayey silts. The older phosphorites form intraformational conglomerates that are the result of a winnowed finer-grained matrix, leaving lag deposits of phosphorite. We present evidence that most of the Eocene and Miocene phosphate is primary and formed during marine trangressions. Our observations extend the geographic and temporal limits of the major phosphogenic system of the Western North Atlantic northward and through time. However, compared to the well-known phosphorite deposits along the southeastern margin of the U.S.A., these northern deposits are not of commercial scale due to a high terrigenous input and the lack of a mechanism capable of driving persistant upwelling.

Marine Geology↗

USGS1992

Effect of sea-level variation on upper-slope depositional processes offshore of Tiber delta, Tyrrhenian Sea, Italy

The upper slope and outer shelf in front of the Tiber River mouth (east-central Tyrrhenian Sea) exhibits a series of about 15 gullies that are not clearly related to present-day erosional or depositional processes. An extensive, high-resolution, seismic-reflection profiling survey of a 450 km 2 area in front of the Tiber delta shows several generations of similar features within the older depositional sequences underlying the outer continental shelf. The gully relief appears dominantly depositional, probably developed during lowstand periods when the Tiber River mouth was relatively near the shelf break. The position of the gullied intervals, including those on the modern slope relict from the last lowstand, shows a successive northward shift with time. This northward shift indicates continued tilting of this part of the eastern Tyrrhenian continental margin, probably resulting from a continued subsidence to the north.

Marine Geology↗

USGS1992

Collapse and flow of lowstand shelf-margin deposits: An example from the eastern Tyrrhenian Sea, Italy

The upper slope of the eastern Tyrrhenian Sea margin has a complex morphology shaped by Quaternary tectonism and by sedimentation phases controlled by changing sea levels. Sediment slides of widely varying size and shape are common in Quaternary deposits of the upper slope, particularly where gradients are steep. Our study of a large sediment failure in lowstand prograded coastal deposits west of Cape Licosa indicates that the nature of shelf-margin deposition is an additional important control on failure. The failure zone has a mobilization surface showing in-situ deformation in the sediment above it; an upper failure surface; a head scarp; and a zone of ponded sediment debris downslope from the exposed surface of failure. The basal mobilization surface is roughly parallel to the seafloor and coincident with a major downlap surface. The failed section is less that 20 m thick and local in extent, but deformation on the basal mobilization surface extends outside the immediate area of the failure. Directly downslope of the slide scarp are internally stratified mounds that show no evidence of deformation or movement. Most of the prograded deposit experienced in-situ deformation that evolved into the collapse of part of the sediment pile above the mobilization surface. A portion of the mobilized sediment flowed a few kilometers basinward and accumulated at the base of a slope-parallel ridge. Sediment failure occurred on the Licosa shelf margin following a major pulse of coastal sedimentation along the lowstand shoreline. On many continental margins, sea-level lowering is thought to be an important cause of failure unconsolidated sediment deposited during previous high-stand conditions. The Licosa slide demonstrates that sea-level fall has another, equally important but indirect, role in sediment failure. As sea level falls and reaches its lowstand position, streams are at their peak efficiency and a coarsening-upward clastic coastal wedge is rapidly emplaced at relatively steep gradients on the shelf margin, and these are conditions that inherently favor sediment instability and failure.

Marine Geology↗

USGS1993

Mesozoic stratigraphy and paleoenvironments of the Georges Bank Basin: A correlation of exploratory and cost wells

The Exxon 975-1, Conoco 145-1, and Mobil 312-1 hydrocarbon exploratory wells and the Continental Offshore Stratigraphic Test (COST) G-1 and G-2 wells were drilled in the southeastern part of the Georges Bank Basin. We used drill cuttings and logs from these wells to describe and correlate the dominant lithostratigraphic units and to document lateral changes in the depositional environments. The strata penetrated by the Exxon 975-1 and COST G-1 wells are much more terrestrial than at the seaward (downdip) COST G-2, Conoco 145-1, and Mobil 312-1 wellsites. Oldest rocks penetrated by the exploratory wells represent a Middle Jurassic carbonate-evaporite sequence that correlates to the Iroquois Formation. The Iroquois records nonmarine to marginal-marine sabkha, tidal-flat, and restricted lagoonal paleoenvironments in the Exxon 975-1 and COST G-1 wells, but progressively changes to inner neritic, littoral, and lagoonal paleoenvironments at the Mobil 312-1 wellsite. The nonmarine deltaic siliciclastics of the overlying Mohican Formation, Misaine Shale, and Mic Mac-Mohawk Formations are thicker and the marine carbonates of the Scatarie and Bacarro Limestones are usually thinner in the Jurassic strata of the Exxon 975-1 and COST G-1 wells than at the other wellsites. Similarly, lower delta-plain and delta-front facies of the Early Cretaceous Missisuaga and Logan Canyon Formations at the Exxon 975-1 and COST G-1 wellsites reflect a greater terrestrial influence than the laterally-equivalent shallow marine to delta-front facies present at the downdip wellsites. ?? 1993.

Georges Bank Basin↗

USGS1993

Neogene folding and faulting in southern Monterey Bay, Central California, USA

The goal of this study was to determine the Neogene structural history of southern Monterey Bay by mapping and correlating the shallow tectonic structures with previously identified deeper occurring structures. Side scan sonographs and Uniboom seismic reflection profiles collected in the region suggest that deformation associated with both compressional and transcurrent movement is occurring. Strike-slip movement between the North American and Pacific plates started as subduction ceased 21 Ma, creating the San Andreas fault system. Clockwise rotation of the Pacific plate occurred between 3.4 and 3.9 Ma causing orthogonal convergence between the two plates. This plate rotation is responsible for compressional Neogene structures along the central California coast. Structures exhibit transpressional tectonic characteristics such as thrust faulting, reverse faulting and asymmetrical folding. Folding and faulting are confined to middle Miocene and younger strata. Shallow Mesozoic granitic basement rocks either crop out or lie near the surface in most of the region and form a possible décollement along which the Miocene Monterey Formation has decoupled and been folded. Over 50% of the shallow faults strike normal (NE-SW) to the previously identified faults. Wrench fault tectonics complicated by compression, gradual uplift of the basement rocks, and a change in plate convergence direction are responsible for the observed structures in southern Monterey Bay.

Marine Geology↗

USGS1993

Assessing the paradigm of mutually exclusive erosion and deposition of mud, with examples from upper Chesapeake Bay

A paradigm of cohesive sediment transport research is that erosion and deposition are mutually exclusive. Many laboratory studies have shown that there is a velocity/stress threshold below which erosion does not occur and a lower threshold above which deposition does not occur. In contrast, a deposition threshold is not included in standard noncohesive sediment transport models, allowing erosion and deposition to occur simultaneously. Several researchers have also modeled erosion and deposition of mud without a deposition threshold. This distinction can have important implications for suspended sediment transport predictions and for data interpretation. Model-data comparisons based on observations of in situ erosion and deposition of upper Chesapeake Bay mud indicate poor agreement when the sediments are modeled as a single resuspended particle class and mutually exclusive erosion and deposition is assumed. The total resuspended sediment load increases in conjunction with increasing bottom shear stress as anticipated, but deposition is initiated soon after the shear stress begins to decrease and long before the stress falls below the value at which erosion had previously begun. Models assuming no critical stress for deposition, with continuous deposition proportional to the near bottom resuspended sediment concentration, describe the data better. Empirical parameter values estimated from these model fits are similar to other published values for estuarine cohesive sediments, indicating significantly greater erodability for higher water content surface sediments and settling velocities appropriate for large estuarine flocs. The apparent failure of the cohesive paradigm when applied to in situ data does not mean that the concept of a critical stress for deposition is wrong. Two possibilities for explaining the observed discrepancies are that certain aspects of in situ conditions have not been replicated in the laboratory experiments underlying the cohesive paradigm, and that in situ sediment behavior is better described as a sequence of particle classes than as the single particle class modeled here. However, the in situ measurements needed to resolve these questions are very difficult and data generally are not available. For practical modeling purposes, allowing continuous deposition of a single resuspended particle class may often give quite satisfactory results.

Marine Geology↗

USGS1993

Hydrothermal palygorskite and ferromanganese mineralization at a central California margin fracture zone

Ferromanganese oxyhydroxide crusts and nodules associated with palygorskite were recovered from the Santa Lucia Escarpment where the Morro Fracture Zone intersects the central California continental margin. Palygorskite was found in pure, high-Mg, low-Al, boxwork-textured veins, and disseminated in poorly consolidated palygorskite-rich mudstone. The purity of the palygorskite boxwork blades and the boxwork structure suggest formation by direct precipitation rather than by diagenetic or detrital processes. Interaction of hydrothermal fluids with oceanic basalt and/or deeper ultramafic rocks produced a Mg-Si enriched fluid supersaturated with respect to palygorskite that precipitated directly from the fluid at or near the seafloor. The close association of Fe-Mn crusts and nodules with both the palygorskite-rich mudstone and boxwork-vein palygorskite suggests a genetic link between the three types of mineralization. Mixed origin hydrothermal-hydrogenetic Fe-Mn crusts, with up to 50% hydrothermal input, formed contemporaneously with and subsequent to palygorskite formation. Fe-Mn nodules collected in the same dredge are of combined hydrogenetic and diagenetic origin and appear to be unrelated to hydrothermal mineralization that produced the crusts and palygorskite. The thickness of the Fe-Mn crusts and rare diatom fragments within the mudstone suggest an age of formation between 13 and 5 Ma.

Marine Geology↗

USGS1993

Biological interference of optical backscatterance sensors in Tampa Bay, Florida

Optical backscatterance (OBS, D&A Instruments, Inc. 1 ) sensors for measuring suspended-solids concentrations have been deployed in Tampa Bay to monitor resuspension of bottom sediments. This paper describes biological factors that affected the OBS sensors deployed in Tampa Bay and discusses deployment strategies that minimize biological interference. Phytoplankton may interfere with the OBS sensors when the suspended-solids concentration is near or below the sensor response threshold. Fish swimming in front of the OBS sensors caused spikes in the OBS sensor output, so the median average was more appropriate than the mean average. An algal slime on the OBS sensors caused excessive backscatterance that dominated the backscatterance from suspended material. Because of the fouling problem, deployments were limited to less than a week, and OBS sensors were cleaned daily, if possible. Calibration of OBS sensors with water samples collected from Tampa Bay was satisfactory when biological interference was not significant. When properly deployed, the OBS sensors can successfully monitor sediment resuspension in Tampa Bay and similar subtropical estuaries.

Marine Geology↗

USGS1993

Enzymatic iron and uranium reduction by sulfate-reducing bacteria

The potential for sulfate-reducing bacteria (SRB) to enzymatically reduce Fe(III) and U(VI) was investigated. Five species of Desulfovibrio as well as Desulfobacterium autotrophicum and Desulfobulbus propionicus reduced Fe(III) chelated with nitrilotriacetic acid as well as insoluble Fe(III) oxide. Fe(III) oxide reduction resulted in the accumulation of magnetite and siderite. Desulfobacter postgatei reduced the chelated Fe(III) but not Fe(III) oxide. Desulfobacter curvatus, Desulfomonile tiedjei , and Desulfotomaculum acetoxidans did not reduce Fe(III). Only Desulfovibrio species reduced U(VI). U(VI) reduction resulted in the precipitation of uraninite. None of the SRB that reduced Fe(III) or U(VI) appeared to conserve enough energy to support growth from this reaction. However, Desulfovibrio desulfuricans metabolized H 2 down to lower concentrations with Fe(III) or U(VI) as the electron acceptor than with sulfate, suggesting that these metals may be preferred electron acceptors at the low H 2 concentrations present in most marine sediments. Molybdate did not inhibit Fe(III) reduction by D. desulfuricans . This indicates that the inability of molybdate to inhibit Fe(III) reduction in marine sediments does not rule out the possibility that SRB are important catalysts for Fe(III) reduction. The results demonstrate that although SRB were previously considered to reduce Fe(III) and U(VI) indirectly through the production of sulfide, they may also directly reduce Fe(III) and U(VI) through enzymatic mechanisms. These findings, as well as our recent discovery that the S o -reducing microorganism Desulfuromonas acetoxidans can reduce Fe(III), demonstrate that there are close links between the microbial sulfur, iron, and uranium cycles in anaerobic marine sediments.

Marine Geology↗

USGS1994

New uranium-series ages of the Waimanalo Limestone, Oahu, Hawaii: implications for sea level during the last interglacial period

The Waimanalo Formation (limestone) of Oahu has been correlated with the last interglacial period based on U-series dating of corals by T.-L. Ku and colleagues. The limestone consists of growth-position corals and overlying coral conglomerate. An apparent bimodal distribution of ages for the growth-position corals (mean age = 133 ka) and the overlying coral conglomerate (mean age = 119 ka) has been interpreted to represent two distinct high stands of sea that occurred within the last interglacial period. Both growth-position corals and overlying, conglomerate coral occur in an outcrop east of Kaena Point and consist mainly of Pocillopora and Porites. U-seriesages of growth-position corals that show closed-system conditions are 120 ± 3 ka and 127 ± 4 ka; overlying conglomerate corals have U-seriesages that range from 120 ± 3 ka to 138 ± 4 ka. At Kahe Point, conglomerate corals have ages of 120 ± 3 ka and 134 ± 4 ka. These data show that the growth position corals are not systematically older than the conglomerate corals; thus, there is no evidence for two distinct high stands of sea. Waimanalo deposits at Kahe Point and Mokapu Point (new U-seriesages of 134 ± 4 ka and 127 ± 3 ka) have beach deposits as high as 12.5 m and, at Mokapu Point, growth-position corals as high as 8.5 m. A last-interglacial sea-level stand of +8.5 to +12.5 m conflicts with estimates of +6 m from a number of tectonically stable coastlines and islands in the western Atlantic Ocean. We infer, therefore, that Oahu may be undergoing uplift at a low rate. This uplift may be due to compensatory lithospheric flexure, because the island of Hawaii has been subsiding throughout much of the Quaternary from volcanic loading. Because of this possible uplift, Oahu and islands like it elsewhere in the Pacific cannot be used as reference points for sealevel during the last interglacial period.

Hawai'i↗

USGS1994

Build your own low-cost seismic/bathymetric recorder annotator

An inexpensive programmable annotator, completely compatible with at least three models of widely used graphic recorders (Raytheon LSR-1811, Raytheon LSR-1807 M, and EDO 550) has been developed to automatically write event marks and print up to sixteen numbers on the paper record. Event mark and character printout intervals, character height and character position are all selectable with front panel switches. Operation is completely compatible with recorders running in either continuous or start-stop mode.

Marine Geology↗

USGS1994

Sediments in Arctic sea ice: Implications for entrainment, transport and release

Despite the Arctic sea ice cover's recognized sensitivity to environmental change, the role of sediment inclusions in lowering ice albedo and affecting ice ablation is poorly understood. Sea ice sediment inclusions were studied in the central Arctic Ocean during the Arctic 91 expedition and in the Laptev Sea (East Siberian Arctic Region Expedition 1992). Results from these investigations are here combined with previous studies performed in major areas of ice ablation and the southern central Arctic Ocean. This study documents the regional distribution and composition of particle-laden ice, investigates and evaluates processes by which sediment is incorporated into the ice cover, and identifies transport paths and probable depositional centers for the released sediment. In April 1992, sea ice in the Laptev Sea was relatively clean. The sediment occasionally observed was distributed diffusely over the entire ice column, forming turbid ice. Observations indicate that frazil and anchor ice formation occurring in a large coastal polynya provide a main mechanism for sediment entrainment. In the central Arctic Ocean sediments are concentrated in layers within or at the surface of ice floes due to melting and refreezing processes. The surface sediment accumulation in central Arctic multi-year sea ice exceeds by far the amounts observed in first-year ice from the Laptev Sea in April 1992. Sea ice sediments are generally fine grained, although coarse sediments and stones up to 5 cm in diameter are observed. Component analysis indicates that quartz and clay minerals are the main terrigenous sediment particles. The biogenous components, namely shells of pelecypods and benthic foraminiferal tests, point to a shallow, benthic, marine source area. Apparently, sediment inclusions were resuspended from shelf areas before and incorporated into the sea ice by suspension freezing. Clay mineralogy of ice-rafted sediments provides information on potential source areas. A smectite maximum in sea ice sediment samples repeatedly occurred between 81°N and 83°N along the Arctic 91 transect, indicating a rather stable and narrow smectite rich ice drift stream of the Transpolar Drift. The smectite concentrations are comparable to those found in both Laptev Sea shelf sediments and anchor ice sediments, pointing to this sea as a potential source area for sea ice sediments. In the central Arctic Ocean sea ice clay mineralogy is significantly different from deep-sea clay mineral distribution patterns. The contribution of sea ice sediments to the deep sea is apparently diluted by sedimentary material provided by other transport mechanisms.

Marine Geology↗

USGS1994

Contrasts in Arctic shelf sea-ice regimes and some implications: Beaufort Sea versus Laptev Sea

The winter ice-regime of the <80 km wide Alaskan Beaufort Sea shelf is characterized by compression and shearing, resulting in the formation of major grounded pressure ridge systems stabilizing the fast ice on the mid-shelf, and essentially no open-water areas. In contrast, the winter ice-regime of the 500-km wide Laptev Sea shelf is controlled by winds blowing from land to sea, and is therefore dilational. A perennial polynya borders the hundreds of kilometers-wide and very smooth fast ice offshore. In this body of open water, rapidly forming ice is continuously advected offshore by the mean wind field, making the Laptev Sea the single major ice factory for the Arctic Ocean and Transpolar Drift. Conversely, with summer warming this dark polynya turns into an area of high heat gain, which results in the retreat of the ice edge to a much higher latitude and greater distance (>500 km) from the mainland than in the Beaufort Sea. As a result, the annual freeze-up does not incorporate old, deep-draft ice, and with a lack of compression, such deep-draft ice is not generated in situ, as on the Beaufort Sea shelf. The Laptev Sea has as much as 1000 km of fetch at the end of summer, when freezing storms move in and large (6 m) waves can form. Also, for the first three winter months, the polynya lies inshore at a water depth of only 10 m. Turbulence and freezing are excellent conditions for sediment entrainment by frazil and anchor ice, when compared to conditions in the short-fetched Beaufort Sea. We expect entrainment to occur yearly. Different from the intensely ice-gouged Beaufort Sea shelf, hydraulic bedforms probably dominate in the Laptev Sea. Corresponding with the large volume of ice produced, more dense water is generated in the Laptev Sea, possibly accompanied by downslope sediment transport. Thermohaline convection at the midshelf polynya, together with the reduced rate of bottom disruption by ice keels, may enhance benthic productivity and permit establishment of open-shelf benthic communities which in the Beaufort Sea can thrive only in the protection of barrier islands. Indirect evidence for high benthic productivity is found in the presence of walrus, who also require year-round open water. By contrast, lack of a suitable environment restricts walrus from the Beaufort Sea, although over 700 km farther to the south. We could speculate on other consequences of the different ice regimes in the Beaufort and Laptev Seas, but these few examples serve to point out the dangers of exptrapolating from knowledge gained in the North American Arctic to other shallow Arctic shelf settings.

Marine Geology↗