Postglacial Mud Research Articles

Seismic airgun sound propagation in shallow water of the East Siberian shelf and its prediction with the measured source signature

Seismic airgun sound was measured with an autonomous passive acoustic recorder as a function of distance from 18.6 to 164.2 km in shallow water (<70 m) at the continental shelf of the East Siberian Sea in September 2019. The least-square regression curves were derived in the zero-to-peak sound pressure level, sound exposure level, and band level in a frequency range between 10 and 300 Hz using the initial amplitude scaled from the near-field hydrophone data. In addition, propagation modeling based on the parabolic equation with the measured source spectrum was performed for range-dependent bathymetry, and the results were compared with the band level of the measurements. The sediment structure of the measurement area was a thin layer of iceberg-scoured postglacial mud overlying a fast bottom with high density based on grounding events of past ice masses. The observed precursor arrivals, modal dispersion, and rapid decrease in spectrum level at low frequencies can be explained by the condition of the high-velocity sediment. Our results can be applied to studies on the inversion of ocean boundary conditions and measurement geometry and basic data for noise impact assessment.

Geological and groundwater flow model of a submarine groundwater discharge site at Hanko (Finland), northern Baltic Sea

Three-dimensional geological and groundwater flow models of a submarine groundwater discharge (SGD) site at Hanko (Finland), in the northern Baltic Sea, have been developed to provide a geological framework and a tool for the estimation of SGD rates into the coastal sea. The dataset used consists of gravimetric, ground-penetrating radar and shallow seismic surveys, drill logs, groundwater level monitoring data, field observations, and a LiDAR digital elevation model. The geological model is constrained by the local geometry of late Pleistocene and Holocene deposits, including till, glacial coarse-grained and fine-grained sediments, post-glacial mud, and coarse-grained littoral and aeolian deposits. The coarse-grained aquifer sediments form a shallow shore platform that extends approximately 100–250 m offshore, where the unit slopes steeply seawards and becomes covered by glacial and post-glacial muds. Groundwater flow preferentially takes place in channel-fill outwash coarse-grained sediments and sand and gravel interbeds that provide conduits of higher hydraulic conductivity, and have led to the formation of pockmarks on the seafloor in areas of thin or absent mud cover. The groundwater flow model estimated the average SGD rate per square meter of the seafloor at 0.22 cm day−1 in autumn 2017. The average SGD rate increased to 0.28 cm day−1 as a response to an approximately 30% increase in recharge in spring 2020. Sensitivity analysis shows that recharge has a larger influence on SGD rate compared with aquifer hydraulic conductivity and the seafloor conductance. An increase in recharge in this region will cause more SGD into the Baltic Sea.

Oceanic Distribution, Behaviour, and a Winter Aggregation Area of Adult Atlantic Sturgeon, Acipenser oxyrinchus oxyrinchus, in the Bay of Fundy, Canada.

Seasonal distribution of adult Atlantic sturgeon was examined using pop-up satellite archival tags (PSATs) and ultrasonic transmitters deployed in the Saint John River, New Brunswick, Canada. Seven MK10 PSATs programmed for release in June 2012 and seven MiniPAT PSATs programmed for release in February and April 2013 were deployed in August 2011 and 2012, respectively. Eleven of 14 PSATs surfaced and transmitted depth and temperature data archived for the duration of their deployment (121–302 days). Among these eleven PSATs, five were recovered and 15-sec archival data was downloaded. Following exit from the Saint John River in the fall, tagged fish occupied a mean monthly depth of 76.3–81.6 m at temperatures as low as 4.9˚C throughout the winter before returning to shallower areas in the spring. The majority of ultrasonic detections occurred in the Bay of Fundy, but fish were detected as far as Riviere Saint-Jean, Quebec, approximately 1500 km from the Bay of Fundy (representing long-distance migratory rates of up to 44 km/day). All PSATs were first detected in the Bay of Fundy. Tags that released in February and April were found 5–21 km offshore of the Saint John Harbour, while tags that released in June were first detected in near shore areas throughout the Bay of Fundy. The substrate at winter tag release locations (estimated from backward numerical particle-tracking experiments) consisted primarily of moraines and postglacial mud substrate with low backscatter strength, indicative of soft or smooth seabed. Based on the proximity of winter tag release locations, the consistent depths observed between fish, and previous research, it is suspected that a winter aggregation exists in the Bay of Fundy. This study expands the understanding of the marine distribution and range of Atlantic sturgeon on the east coast of Canada.

Anthropogenic modification of a fjord: Bay of Islands, Newfoundland

The majority of Newfoundland fjords are relatively pristine. However, the Bay of Islands fjord, site of Corner Brook (Newfoundland's second-largest city with a population of 20 000), shows considerable anthropogenic modification. Submarine slides have been triggered by dock and road construction. Dredge spoil and a scuttled barge litter the seafloor. A large mound of wood chips extending into the Bay of Islands is waste from a paper mill. The Bay of Islands, western Newfoundland, is a fjord bounded at its seaward end by an arcuate submarine moraine (Fig. 1a) up to 100 m thick (Josenhans et al. 1990; Josenhans & Zevenhuizen 1993; Josenhans & Lehman 1999). The moraine formed after retreat of ice that formerly extended far out on the continental shelf (Shaw et al. 2006). The deepest areas of the fjord (295 m) are immediately landward of the shallow (30 m) moraine. The fjord has three principal arms, the largest of which (Humber Arm, Fig. 1a, c), averages 100 m deep and shallows steeply at its head where the Humber River enters. Humber Arm has many of the attributes of fjords elsewhere; it hosts thick glacimarine sediments (Fig. 1b) overlain by postglacial mud (Shaw et al. 2000 …

Landforms in a Pacific fjord system: Douglas Channel, British Columbia, Canada

Douglas Channel is part of a complex network of fjords on the Pacific north coast of British Columbia, Canada, an area in which fjords formed under the influence of westward-moving ice of the Cordilleran Ice Sheet (Barrie et al. 2014). The Douglas Channel fjord system (Fig. 1a) extends from the Pacific coast to the town of Kitimat located 150 km inland, and has a maximum depth of 690 m. The fjords are developed within the Coast Range, which has peaks extending up to 1500 m in the study area. The fjord system is located on the North American Plate, east of its boundary with the Pacific Plate. The region is subject to heavy precipitation (>2700 mm a−1 at Kitimat) and melt from snow pack in the interior. Fig. 1. Sun-illuminated multibeam bathymetry and seismic data from the Douglas Channel area, British Columbia. Multibeam acquisition system Kongsberg EM120. Frequency 12 kHz. Grid-cell size 5 m. ( a ) The entire fjord system. ( b ) Moraine and submarine slide complex at Caamano Sound, showing bank of postglacial mud (A) and residual of unfailed sediment (B). Headwall scar, dashed line. Transport direction …

Post-glacial mud depocentre in the southern Beibu Gulf: acoustic features and sedimentary environment evolution

Abstract This study documents the Southern Beibu Gulf Mud Depocentre (SBGMD) for the first time using high-resolution sub-bottom seismic reflection profiles. The SBGMD is located near the southern mouth of the Beibu Gulf in water depths of 50–80 m, and covers an area of more than 11 000 km 2 . The acoustic architecture of the SBGMD is characterized by a homogenous seismic unit surrounded by an erosive area where gullies and sand/mud waves occur. The SBGMD unconformably overlies a parallel-bedded seismic unit. Based on 14 C-dating, together with acoustic, lithological and geochemical analysis of gravity core SO31, which was sampled within the high-sedimentation-rate area of the SBGMD, the post-glacial sedimentary history may be summarized as follows: (1) from 12.8 to 11.6 cal ka BP, the SBGMD was deposited in a low-energy, brackish/freshwater and intensely terrestrial-influenced environment; (2) from 11.6 to 8.4 cal ka BP, the SBGMD underwent marine transgression; and (3) from 8.4 cal ka BP to present, a continuous shallow-marine environment has prevailed. Comparing the age and depth of the SBGMD base with the established sea-level curve, lacustrine sedimentation is predicted to have occurred in the SBGMD area before 12.8 cal ka BP.

Megaflutes in a continental shelf setting, Placentia Bay, Newfoundland

Megaflutes – erosional scours normally found in deep water on continental slopes – were identified in 1978 on sidescan sonograms and seismic reflection profiles from Placentia Bay on the south coast of Newfoundland, Canada. Data from recent extensive multibeam sonar surveys provide an opportunity to describe the morphology and distribution of the megaflutes in detail, and to consider the formative processes. They occur on the east side of the outer bay, at a depth of ~200m, in a 2–3km-wide swath that continues to the south into Halibut Channel, over a total distance of ~100km. The megaflutes have been formed by removal of a layer of postglacial mud, exposing underlying glaciomarine sediments and releasing a volume of 4.5km3. They occur in a range of forms, including single, multiple, and coalescent types, and in some areas at least their inception was related to pre-existing pockmarks. Radiocarbon dates from piston cores are used to demonstrate that megaflute formation post-dated ca. 9ka. Megaflute formation in Placentia Bay has been attributed to a ‘reverse flow’ from the tsunami generated by the 1929 Grand Banks earthquake, and this is still the accepted hypothesis. We argue that the return flow from a tsunami did not generate the megaflutes, and suggest instead that they could be formed during south-flowing density currents generated when volumes of cold saline water stored in the deep (>250m) basins at the head of Placentia Bay are intermittently displaced and spilled south in a shallow channel at the east side of the bay after intense coastal surface dense water formation events, perhaps during very cold winters.

Holocene tephras in lake cores from northern British Columbia, Canada

Sediment cores recovered from alpine and subalpine lakes up to 250 km apart in northern British Columbia contain five previously unrecognized tephras. Two black phonolitic tephras, each 5–10 mm thick, occur within 2–4 cm of each other in basal sediments from seven lakes in the Finlay River – Dease Lake area. The upper and lower Finlay tephras are slightly older than 10 220 – 10 560 cal year B.P. and likely originate from two closely spaced eruptions of one or two large volcanoes in the northern Cordilleran volcanic province. The Finlay tephras occur at the transition between deglacial sediments and organic-rich postglacial mud in the lake cores and, therefore, closely delimit the termination of the Fraser Glaciation in northern British Columbia. Sediments in Bob Quinn Lake, which lies on the east edge of the northern Coast Mountains, contain two black tephras that differ in age and composition from the Finlay tephras. The lower Bob Quinn tephra is 3–4 mm thick, basaltic in composition, and is derived from an eruption in the Iskut River volcanic field about 9400 cal years ago. The upper Bob Quinn tephra is 12 mm thick, trachytic in composition, and probably 7000–8000 cal years old. A fifth tephra occurs as a cryptotephra near the top of two cores from the Finlay River area and is correlated to the east lobe of the White River tephra (ca. 1150 cal year B.P.). Although present throughout southern Yukon, the White River tephra has not previously been documented this far south in British Columbia. The tephras are valuable new isochrons for future paleoenvironmental studies in northern British Columbia.

Temporal and spatial complexity in post-glacial sedimentation on the tectonically active, Poverty Bay continental margin of New Zealand

On the eastern Raukumara Ranges of the New Zealand East Coast, active tectonics, vigorous weather systems, and human colonisation have combined to cause widespread erosion of the mudstone- and sandstone-dominated hinterland. The Waipaoa River sedimentary dispersal system is an example that has responded to environmental change, and is now New Zealand's second largest river in terms of suspended sediment discharge. This paper presents new sediment accumulation rates for the continental shelf and slope that span century to post-glacial time scales. These data are derived from radiochemical tracer, palynological, tephrostratigraphic, and seismic methods. We hypothesise on the temporal and spatial complexity of post-glacial sedimentation across the margin and identify the broad extent of sediment dispersal from the Waipaoa system. The ∼15 km 3 Poverty Bay mid-shelf basin lies adjacent to the mouth of the Waipaoa River, reaching a maximum thickness of ∼45 m. A post-glacial mud lobe of an additional ∼3 km 3 extends through the Poverty Gap and out onto the uppermost slope, attaining 40 m thickness in a structurally controlled sub-basin. Here, an offset in the last-glacial erosion surface indicates that deposition was sympathetic with fault activity and the creation of accommodation space, implying that sedimentation was not supply limited. Contrary to classical shelf sedimentation models, the highest modern accumulation rate of 1 cm y −1 occurs on the outer-shelf sediment lobe, approximately ∼2 times the rate recorded at the mid-shelf basin depocentre, and ∼10 times faster than the excess 210Pb rates estimated from the slope. Pollen records from slope cores fingerprint Polynesian then European settlement, and broaden the spatial extent of post-settlement sedimentation initially documented from the Poverty Bay mid-shelf. Changes in sub-millennial sedimentation infer a 2–3-times increase in post-settlement accumulation on the shelf but a smaller 1–2 times increase on the slope. Over longer time scales, seismic evidence infers slower but steady sedimentation since the last transgression, and that significant cross-shelf sediment pathways pre-date the increase in sedimentation resulting from colonisation and deforestation. From a summation of coastal bedload, shelf and slope sediment mass accumulation, the total sediment budget for the Holocene is ∼1 Mt y −1. Under modern conditions a larger proportion of the Waipaoa sediment dispersal system likely extends onto the slope and beyond.

Storm-induced Redistribution of Deepwater Sediments in Lake Ontario

High-resolution seismic reflection profiles, side-scan sonar profiles, and surface sediment analyses for grain size (% and, silt & clay), total organic carbon content, and carbonate content along shore-perpendicular transects offshore of Olcott and Rochester in Lake Ontario were utilized to investigate cm-thick sands or absence of deepwater postglacial sediments in water depths of 130 to 165 m. These deepwater sands were observed as each transect approached and occupied the “sills,” identified by earlier researchers, between the three deepest basins of the lake. The results reveal thin (0 to 5-cm) postglacial sediments, lake floor lineations, and sand-rich, organic, and carbonate poor sediments at the deepwater sites (> 130 m) along both transects at depths significantly below wave base, epilimnetic currents, and internal wave activity. These sediments are anomalous compared to shallower sediments observed in this study and deeper sediments reported by earlier research, and are interpreted to indicate winnowing and resuspension of the postglacial muds. We hypothesize that the mid-lake confluence of the two-gyre surface current system set up by strong storm events extends down to the lake floor when the lake is isothermal, and resuspends and winnows lake floor sediment at these locations. Furthermore, we believe that sedimentation is more likely to be influenced by bottom currents at these at these sites than in the deeper basins because these sites are located on bathymetric highs between deeper depositional basins of the lake, and the bathymetric constriction may intensify any bottom current activity at these sites.

Late Quaternary Ice Cap Extent and Deglaciation, Húnaflóaáll, Northwest Iceland: Evidence from Marine Cores

The extent of the Last Glacial Maximum (LGM) and the timing of deglaciation around the Northwest Peninsula of Iceland are poorly understood. To provide information on these issues, we report sedimentological, foraminiferal, isotopic, and chronological data from marine piston cores B997-322PC, -323PC, -326PC1, and -326PC2 from a transect along Reykjafjardaáll/Húnaflóaáll, a large trough that extends from the north Iceland coast toward the shelf break, ca. 66.8°N and 20°W. Cores B997-322PC, -323PC, and -326PC1 recovered diamictons (stiff, pebbly muds) overlain by fine-grained, postglacial muds, with intermittent occurrences of iceberg-rafted clasts and volcanic shards. No glacial con-structional features (moraines) were noted on the seismic profiles. In the outermost cores (B997-322PC and 323PC) the diamictons contained small but persistent numbers of foraminifera which gave dates between 25 and 44 ka B.P.; foraminifera are absent in the diamicton at the base of core B997-326PC1. 14C dates immediately above the diamictons have ages of ca. 13 ka B.P., which we take to represent the date of deglaciation of Húnaflóaáll and the withdrawal of the ice margin to the adjacent Strandir coast. Foraminifera are used to construct a bio- and isotope stratigraphy of cores B997-322PC and 323PC. In the diamictons, the fauna is dominated by cold-water benthic foraminifera including Elphidium excavatum and Cassidulina reniforme; Cassidulina neoteretis is also a persistent component. In B997-322PC and -323PC the deglacial sequence is interrupted by an interval of IRD deposition, a dramatic reduction in foraminiferal numbers, and heavy δ18O values; we equate this interval with the Younger Dryas cold event. Alternative working hypotheses are advanced for the origin of the diamictons and their importance to mapping the LGM ice extent. The 12-ky hiatus between the diamictons and the overlying marine muds indicates that the sequence is not conformable. The diamictons probably represent glacial-marine sediments overrun by late Weichselian ice streaming down the trough. The absence of a thick, deglacial sequence of glacial-marine sediments indicates a dramatic retreat of the ice cap to the present coast by ∼13 ka B.P.

The Dwyka Group in the northern part of Kwazulu/Natal, South Africa: sedimentation during late Palaeozoic deglaciation

Abstract Multiple sedimentary sequences, up to and over 300 m thick, and comprising the Late Palaeozoic Dwyka Group in the southeastern part of the African subcontinent, were deposited during deglaciation of the Gondwana ice sheet. Attention is focused on the northern part of KwaZulu/Natal in South Africa where the Dwyka Group, represented by glaciogenic deposits, rests on an uneven surface of Archaean basement rocks. These deposits occur on the northwestern flank of an elongate pre-Dwyka crustal downwarp which had a significant influence on the mode of sedimentation as it progressively subsided. Stratigraphic sections from selected localities in northern KwaZulu/Natal display the thickness variations, and marked vertical and lateral lithofacies changes that characterise the Dwyka Group of this region. Diamictite consistently occurs at the base of the Dwyka sequences but is also interstratified with mainly arenaceous and conglomeratic lithotypes which predominate in the upper 60–160 m. These heterolithic sequences reflect sedimentation from a retreating marine ice sheet during a rise in relative sea-level which commenced towards the close of the Carboniferous Period in southwestern Gondwana. As the margin of the ice sheet receded towards an Archaean upland region in the northwest, this ice mass was locally grounded and stabilised on bathymetric highs of the irregular basement topography. Release of glacial debris produced diachronous sedimentary sequences as the ice sheet was temporarily pinned on successive basement highs during its episodic retreat. Matrix- and clast-supported conglomerates and associated sandstones testify to a plexus of sediment gravity flow and subaqueous outwash processes that occurred near the grounding line. Thick, homogeneous, blanket-like diamictite facies, in the southeastern and southern part of the region, suggest an unimpeded retreat of the sediment-laden marine sheet in the deeper parts of the basin. Rapid dissipation of the ice in the closing stages of deglaciation was followed by accumulation of postglacial muds which mantled the glaciogenic sediments. In the northernmost part of the study area proximal products of deglaciation, represented by conglomeratic deposits and associated sandstones, are preserved in deep partly exhumed glaciated valleys and depressions along the southern periphery of the Archaean upland. The distinctive character of the Dwyka Group in northern KwaZulu/Natal is partly attributable to the complex tectonic setting in which it occurs and reflects an interplay of various factors that controlled the styles of sedimentation during deglaciation.

Late Quaternary sedimentation in St. George's Bay, southwest Newfoundland: acoustic stratigraphy and seabed deposits

Shallow seismic reflection data collected in St. George's Bay, southwest Newfoundland, reveal a complex pattern of subsurface topography and acoustic facies. Two basins in the inner bay are underlain by glacially overdeepened valleys that extend to depths in excess of 180 m. Within the thick Quaternary sequence in the inner bay we recognize eight acoustic units. Units 1 (ice contact), 2 (subaqueous outwash), and 3 (draped glaciomarine) record the presence and retreat of a major Late Wisconsinan ice margin. Unit 4 (postglacial mud) has resulted from reworking of glaciogenic sediments in response to changes in relative sea level. Unit 5 (postglacial sand) is a shoreface deposit on the seaward front of the former moraine. Unit 6 (postglacial delta) was formed by fluvial reworking of glaciogenic sediments during the postglacial lowstand of relative sea level. Unit 7 (postglacial barrier–platform) comprises seaward-fining clinoform prisms that have prograded into the basins, and underlie gravel beach-ridge plains at Stephenville and Flat Island. Unit 8 (postglacial spillover) results from entrainment of coarse sediment on the shallow sill and subsequent progradation into the basins of the inner bay.Seabed sediment in the basins is mud where sampled. The submarine platforms associated with the barriers at Stephenville and Flat Island are largely sandy. The sill is covered by a gravel veneer, with irregular patches of sand that coalesce and thicken seawards. Extensive areas of gravel ripples testify to the continued mobility of much of the coarse sediment on the sill.A major ice front in the inner bay was present prior to 13.7 ka BP. The draped glaciomarine sediments, dated at 13.7–11.2 ka BP in nearby Port au Port Bay, were deposited after withdrawal of the ice front to the vicinity of the present coast, during a readvance, and subsequently. During the postglacial lowstand of relative sea level much of the present sill area was emergent.

A comparison of models of glacial sedimentation along the eastern Canadian margin

The distribution and thickness of glacial and postglacial sediments of the eastern Gulf of Maine, Scotian Shelf, Grand Banks of Newfoundland, Labrador Shelf and Hudson Strait have been mapped using seismic reflection systems and lithological, chronological and biostratigraphic data from cores and seabed samples. Deposits of till, represented by incoherent seismic reflections, commonly occur as sheets but also as linear ridges, lenses and tongues. Discontinuous multiple till sequences, whose thicknesses range from 1–300 m occur across the shelves and pinch out in water depths of 500–700 m. Till thickness and distribution is in part controlled by erodability and geomorphology of the underlying substrate. Glaciomarine sediments up to 300 m thick, represented by medium to high intensity, parallel, continuous coherent seismic reflections occur as conformable to slightly ponded deposits. Glaciomarine sediments can interfinger, conformably overlie or abruptly terminate against the till surfaces. On the southern shelves, from the Gulf of Maine to northeast Newfoundland Shelf and also in eastern Hudson Strait, the glaciomarine sediments are typically interbedded with the tills at former ice-sheet buoyancy zones where much of the material has been derived from subglacial meltout. On the Labrador Shelf, subglacial meltout did not occur as the till and glaciomarine sediments are of different lithology and do not interfinger. From the Gulf of Maine to the Grand Banks of Newfoundland, acoustically weakly stratified, ponded postglacial muds were eroded from the shallow bank surfaces by a marine transgression and deposited in the basins of the shelf. On the Labrador Shelf, iceberg scouring has reworked the entire bank top surface into an ice-keel turbate up to 10 m thick and released the fines which now occupy the basins. The thick (mid-late Wisconsinan) multiple till sections and associated glaciomarine sediments in the basins of the southern shelves suggest the presence of an ice shelf for 20,000 years, beneath which large volumes of sediment were deposited. In contrast, the relatively thin (mid-late Wisconsinan) glacial sequence of the Labrador Shelf and the lack of subglacial deposits within the glaciomarine sequence may indicate either a rapid ice-sheet retreat for that area or an ice shelf largely devoid of englacial debris.

The Quaternary geology of the Labrador Shelf

The regional distribution of glacial and postglacial sediments on the Labrador Shelf was mapped by medium- and high-resolution seismics and shows a succession of tills that extend to the continental shelf edge at approximately 600 m. The uppermost of these tills is less laterally extensive and does not occur on the Labrador Shelf bank tops in water depths less than 160 m. We infer that the last glacial episode was less extensive than the preceding ones. Glaciomarine stratified silts and postglacial muds are found overlying the upper till in the deep basins that separate the banks. The upper till, glaciomarine silts, and postglacial muds are interpreted to constitute one complete deglacial cycle. Radiocarbon age determination suggests this deglacial sequence began at approximately 20 000 years BP. On the northernmost Labrador Shelf and Hudson Strait, seismic evidence indicates a late glacial readvance locally deposited a till that overlies the glaciomarine sediments associated with the last glacial advance on the Labrador Shelf. The stratigraphic position of this Hudson Strait till suggests the local readvance occurred at approximately 8000 years BP. The acoustic stratigraphic framework (data) is ground truthed by a regionally extensive piston core and grab sample network. Multidisciplinary analysis of these sediment samples included X-ray, textural, geotechnical, paleontological, lithological, trace-element, and 14C analyses. The results confirm the acoustic interpretations and show that each of the stratigraphic units defined have unique and regionally consistent properties. Geotechnical analyses from the upper till indicate low shear strengths (± 25 kPa) and low to normal consolidation ratios (0.8–2.8), which we interpret to indicate deposition under low basal loading as a result of hydrostatic support on an ice shelf in a marine setting. Provenance studies on the sands and gravels from each of the stratigraphic units of the last deglacial sequence (i.e., till, glaciomarine silts, postglacial muds) indicate that the till is mostly derived from mainland Labrador and transported by an easterly ice flow. Since limestone outcrops are rare to absent on the Labrador mainland and continental shelf, high limestone concentrations (up to 80%) within the glaciomarine sediments that overlie the upper till indicate a northern and distal source for that unit. We interpret an active ice margin overlying the Paleozoic limestone outcrops in Hudson Strait and Ungava Bay as being the dominant source for the glaciomarine silts. The absence of limestone within the postglacial muds that overlie the glaciomarine silts indicates a significant change in provenance, which we attribute to the disappearance of the active ice margin over the limestone terranes. The depositional style and sedimentary structures within the glaciomarine silts indicate low bottom currents and a uniform rain out of ice-rafted (limestone) debris that extends to the southernmost part of the Labrador Shelf. The depositional style of the postglacial muds indicates a significant increase in bottom currents, with deposition restricted to the deep basins. We suggest these increased currents resulted from the disappearance of the Hudson Strait ice barrier, which allowed the Canadian current to develop and combine with the west Greenland current. This combination of currents occurred at approximately 8000 years BP and marks the inception of the Labrador current, which presently traverses the Labrador Shelf.

The geology and origin of narrow closed channels in the Sound of Jura

Synopsis Geological and geophysical investigations in the southern part of the Sound of Jura reveal numerous deep and narrow closed channels in a sequence of thick post-glacial muds. It is proposed that the channels were formed subsequent to the deposition of the sediments by the erosive action of the strong tides observed in the Sound. Several problems related to this mode of origin are considered, but more data are required to provide greater insight into the post-glacial history of the Sound.

Grain Size and Mineralogy of Sediment Cores from Western Lake Huron

Grain size and mineralogical analyses along the lengths of 12 cores recovered from the Alpena and Manitoulin Basins, when combined with ages determined by C-14 dating of surficial and basal sediments, reveal an approximate sedimentary history of this part of Lake Huron. Reddish glaciolacustrine clays, essentially all 10 ϕ, crop out on the flanks of the sublacustrine basins and underlie gray post-glacial muds in the central basin regions. The post-glacial muds become more coarse in their upper portions where they are very fine silt. Detrital carbonates which comprise approximately 30% of older sediments are reduced in abundance by an order of magnitude in more recent deposits. We interpet these phenomena as recording lowered lake levels and reduced sedimentation rates since the Nipissing high stand which ended about 4,000 years ago.

Aliphatic and olefinic hydrocarbons in recent sediments of Greifensee, Switzerland

Qualitative and quantitative determinations of aliphatic and olefinic hydrocarbons in Recent sediment cores of Greifensee, Switzerland show that n-heptadecane is most abundant in the surface sediments (8–24 μg/g dry sediment). This compound is due to the extensive phytoplankton production in the highly-eutrophic lake. The levels of n-heptadecane decrease rapidly in the uppermost 10 cm of the core (to less than 2 μg/g), probably by microbial degradation processes. An 80-week incubation experiment provided evidence of anaerobic degradation of sedimentary-heptadecane. The predominance of n-heptadecane in the older section of lacustrine chalk (back to approximately 11,000 yr B.P.) can still be considered as a qualitative historic record of primary productivity in the lake. In postglacial mud (> 11,000 yr B.P.), n-heptadecane is accompanied by equally abundant n-alkanes and an unresolved complex mixture of branched and cyclic hydrocarbons. These latter were present in sedimentary rocks outcropping in the catchment basin of the lake and were eroded, transported by glaciers and rivers, and finally redeposited in the postglacial Greifensee. This group of hydrocarbons is absent in the lacustrine chalk section (~ 11,000-100 yr B.P.) but appears again in the youngest sediments (100 yr B.P. to present). Thus changes in erosion rate in the past and acceleration of erosion through human activities are recorded in the hydrocarbon content of the sediment cores.

Surficial Sediments of Lake Erie

On the basis of extensive echosounding and grab sampling, three major units have been recognized in Lake Erie: till and bedrock, glaciolacustrine clay, and postglacial muds. These units represent the late glacial and postglacial evolution of the basin and occur in an offshore younging sequence. The main basin of the lake is subdivided by residual glacial moraines into four depositional basins: Western, Sandusky, Central, and Eastern basins. The sediment texture has been defined by moment measures (mean, standard deviation, skewness, and kurtosis), the trends of which are related to the mixing of two primary grain-size populations in the sand- and clay-size ranges. A third grain-size mode in the silt size, composed of fine quartz with some carbonate, has been recognized. This mode has a modifying effect on the symmetry of the two primary populations and may, to some extent, be sufficiently abundant to behave as a discrete population. The trends in the textural characteristics, particularly skewness and kurtosis, have been utilized to define energy regimes at the sediment–water interface which indicates three distinct sedimentary or hydraulic regions: 1) Western basin region — Fine-grained sediment accretion in shallow water related to an imbalance in sediment budget, with high input loadings of fine-grained sediment, and deficit in coarse materials, with an excess of input over sediment export to the Central basin region. This results in net sediment accretion in shallow water with texture in disequilibrium with environmental energy, which produces mixing and suspension, followed by redeposition; 2) Central basin region — West to East coarsening of sediment in textural equilibrium with hydraulic energy, as it relates to increasing fetch under westerly and southwesterly prevailing winds; 3) Eastern basin region — Deepwater basin with sediments showing decreasing size offshore with increasing water depth. The deepwater sediment is modified by the influx of substantial quantities of the silt-size material derived from shoreline erosion in the north shore of the Central basin region.The interrelationships of parameters indicate textural dependence on mineralogic composition, particularly important being the relationship of clay concentration to mean grain size. This has particular value in modelling the physical behavior of clay-associated geochemical elements such as phosphorus.

The Surficial Sediments of Lake Huron

On the basis of extensive sampling and echo sounding, three major lithological units are recognized in the main basin of Lake Huron: (1) glacial till and bedrock; (2) glaciolacustrine clay; and (3) postglacial mud. Sand is a lesser unit in the Huron surficial sediments. Owing to the wide range in bathymetric complexity, postglacial muds occur in basins of three distinct types:Type A. Regular basins in which mud forms a continuous cover.Type B. Irregular basins with undulating bottom topography. Glaciolacustrine clays outcrop in the crests and mud fill occurs in the hollows. Mud cover is greater than 50%.Type C. As for type B but with mud cover less than 50%.The sediment distribution pattern is essentially simple with a natural superposition of sediment units reflecting the glacial and postglacial history of the lake. A bedrock escarpment from Point Clark to Thunder Bay divides Lake Huron into two distinct morphological regions. To the south of the escarpment, the lake has a gentle topography. A second low amplitude escarpment, trending northeast from Ipperwash, divides the southern region of the lake into two large depositional basins. To the north of the major escarpment, the lake is much deeper and has a complex bottom topography. The northern region is separated into two large basins of discontinuous sediment deposition by a rise of glacial sediments trending north from Thunder Bay. The inshore region of Lake Huron and the two escarpments are composed of glacial till and bedrock. The till is overstepped in the deeper water by glaciolacustrine clays which are themselves overstepped by postglacial muds. Postglacial mud accumulation is continuous in the southern basins due to the gentle relief of the lake bottom. In the northern region of the lake, mud accumulation is discontinuous due to the undulating nature of the lake bottom. Mud fills the hollows leaving glaciolacustrine clay exposed at the top of the undulations in this region.The surficial sediments contain variable amounts of quartz, clay minerals, organic carbon, and carbonates. Quartz content is greatest in the coarser inshore sediments while clay minerals and organic carbon are greatest in the liner offshore sediments. Carbonate is low throughout the lake, except along the eastern edge. Blite is the dominant clay mineral with lesser amounts of chlorite and kaolinite.Amphipods, oligochaetes, and chironomids are the major benthic organisms in the Huron sediments. Amphipods are most numerous at a water depth of 70 m, oligochaetes at 140 m, and chironomids in the shallowest depths.The textural characteristics of the sediments, defined by moment measures, can be interpreted as resulting from variable mixing of a clay and a sand end member population.Both end member populations are leptokurtic and asymmetrically skewed due to the introduction of a silt size mode predominantly in the form of a carbonate. The sand end member population is positively skewed and occurs in the inshore zone comprising the coastal nearshore region and the shallow mid-lake regions. The clay end member is negatively skewed and occurs in the depositional basins. Between these two extremes there is a gradual prograding from sand to clay related to a progressive mixing of the two populations. This mixing is believed to be a direct function of declining energy with increasing water depth.Sediment composition reflects both the source materials and the textural properties. The sediments of the southern basin are derived predominantly from glacial tills whereas the composition of the sediments of the northern basin has been substantially modified by dilution with reworked glaciolacustrine clays.