Ridge Sediments Research Articles

Multiscale spatio-temporal variability of sedimentary deposits in the Var turbidite system (North-Western Mediterranean Sea)

The Var turbidite system is a small sandy system located in the Ligurian Basin (Mediterranean Sea). It is active during present sea-level highstand and shows four types of sediment transfer processes: (1) low-density turbidity surges generated by small-scale failures (2) low-magnitude, high-frequency (yearly) hyperpycnal turbid plumes, (3) high-magnitude, less-frequent, hyperpycnal currents and (4) high-magnitude flows generated by large slope failures. These processes have different imprints on the morphology of the system. Inversely, the topography plays a role on the behaviour of these flows. The depositional and erosional architecture of the system has been investigated in detail on the basis of SAR imagery and a set of cores using an interface corer collected repetitively from a dense group of sites. The inner terraces located in the upper part of the turbidite system are generally depositional and thus provide a detailed record of recent sediment transfer processes. But the lower the elevation, the more terraces are affected by turbulent flow erosion. Downward, the channel-floor is a complex area where flows mainly by-pass but locally erode or deposit. The levee is dominantly depositional, but only records high-magnitude events, able to spill over. Low-magnitude, high-frequency events, such as yearly hyperpycnal currents are confined in the upper part of the system and thus have little control on system architecture. Moreover, they provide only thin deposits that can be misinterpreted in terms of involved process, and will probably be erased from the geological record. High-magnitude events have strong control on the system architecture as they erode the channel-floor and participate in the construction of the Var Sedimentary Ridge.

Changes in North Atlantic Deep Water strength and bottom water masses during Marine Isotope Stage 3 (45–35 ka BP)

The strength of the North Atlantic Meridional Overturning Circulation during climatically highly variable Marine Isotope Stage (MIS) 3 has attracted much attention in recent years. Here we present high-resolution Nd isotope compositions of past seawater derived from authigenic Fe–Mn oxyhydroxides recovered from drift sediments on the Blake Ridge in the deep western North Atlantic (ODP Leg 172, Site 1060, 3481 m water depth). The data cover the period from 45 to 35 ka BP, tracing circulation changes during major Heinrich iceberg discharge event 4 (H4, ∼40–39 ka BP). The Nd isotope record suggests that there was no northern-source water (NSW) mass like modern NADW at the deeper part of Blake Ridge at any time between 45 and 35 ka. This is fundamentally different from the hydrographic situation during the Holocene where NADW extends below 4500 m at this location. The ɛ Nd of past deep water recorded in the Blake Ridge sediments was least radiogenic during Dansgaard/Oeschger (D/O) Interstadial (IS) 8 ( ɛ Nd = −11.3) and most radiogenic immediately preceding IS 9 ( ɛ Nd = −9.8). More radiogenic compositions were also recorded during H4 (−10.2 ≤ ɛ Nd ≤ −9.9). The Nd isotope variability in MIS 3 matches that of a physical bottom current strength reconstruction from the same location. Neither record follows the pattern of Northern Hemisphere D/O climatic cycles. In our record, reduced mixing with northern source waters started in stadial 12 and lasted until after H4 in stadial 9, followed by a rapid increase in NSW contribution thereafter. This major change in the Nd isotope record predates the iceberg discharge event Heinrich 4 by more than 3 ka indicating a shallowing of the water mass boundary between Glacial North Atlantic Intermediate Water and Southern Source Water beneath. This early change in bottom water properties at the deep Blake Ridge suggests that North Atlantic deep water advection may already have decreased several thousand years before the actual iceberg discharge event and associated freshening of the surface waters in the North Atlantic. The change can thus not be attributed to climatic events in the North Atlantic but may be related to changes in flux of deep water from the South.

Seabed morphology and gas venting features in the continental slope region of Krishna–Godavari basin, Bay of Bengal: Implications in gas-hydrate exploration

Increased oil and gas exploration activity has led to a detailed investigation of the continental shelf and adjacent slope regions of Mahanadi, Krishna–Godavari (KG) and Cauvery basins, which are promising petroliferous basins along the eastern continental margin of India. In this paper, we analyze the high resolution sparker, subbottom profiler and multibeam data in KG offshore basin to understand the shallow structures and shallow deposits for gas hydrate exploration. We identified and mapped prominent positive topographic features in the bathymetry data. These mounds show fluid/gas migration features such as acoustic voids, acoustic chimneys, and acoustic turbid layers. It is interesting to note that drilling/coring onboard JOIDES in the vicinity of the mounds show the presence of thick accumulation of subsurface gas hydrate. Further, geological and geochemical study of long sediment cores collected onboard Marion Dufresne in the vicinity of the mounds and sedimentary ridges shows the imprints of paleo-expulsion of methane and sulfidic fluid from the seafloor. To understand the origin of the mounds and their relationship with gas hydrate/cold seep formation, we analyzed the multi-channel seismic reflection (MCS) data close to the mounds. The MCS data show that the subsurface layers beneath the mounds are folded. Below the folded overburden, we observe zones of no coherent reflections probably originating from Miocene sequence. Since the study area is located in shale tectonics regime where Miocene sequences are known to be overpressured, we interpret the zones of no coherent reflections as Miocene shale diapirs. The upward movement of shale diapirs has folded the overburden layer and resulted in the formation of numerous faults/fractures. These faults act as permeable pathways for fluid/gas movement facilitating the formation of gas hydrate and cold seeps close to shale diapiric mound.

“Glacial curvilineations”: New glacial landforms produced by longitudinal vortices in subglacial meltwater flows

The glacial landscape of the Dobrzyń Plateau in central Poland contains a complex suite of enigmatic bedforms consisting of elongate and sinuous sediment ridges occurring within an anabranched network of tunnel channels terminating at a former ice margin marked by extensive glaciofluvial fans. The term “glacial curvilineations” is proposed to describe these enigmatic bedforms that occur as fields of parallel, sinuous ridges separated by troughs. The regional pattern of glacial curvilineations replicates the morphology and pattern of tunnel channel margins and suggests a common genesis. Tunnel channels and glacial curvilineations are eroded in the interbedded diamictons and glaciofluvial sediments of the plateau. We propose that tunnel channels and glacial curvilineations are the products of subglacial meltwater erosion. Curvilineations are erosional remnants produced by longitudinal vortices within the tunnel channel-forming flows. Parallelism of curvilineations reflects spacing of turbulent structures within the subglacial flow. Tunnel channel margins also control the sinuosity of curvilineations by dictating both the path and curvature of longitudinal vortices. Arguments for meltwater erosion are strengthened by the presence of extensive glaciofluvial fans at tunnel channel termini: they are the depositional counterparts to the extensive subglacial fluvial dissection of the plateau. Understanding curvilineation genesis may shed light on glacier hydrologic processes. Tunnel channels develop abruptly in the plateau surface suggesting localized point sources of meltwater, such as drainage of subglacial and/or supraglacial lakes. The extent of tunnel channels and lateral continuity of glacial curvilineations suggests broad sheet-like flows as wide as a few kilometres.

Submarine end moraines on the continental shelf off NE Greenland – Implications for Lateglacial dynamics

Favourable sea-ice conditions gave way to an acoustic survey offshore NE Greenland during RV Polarstern ARK-XXIV/3 leg in 2009. The acquired data set clearly depicts sediment ridges in an area of app. 18 × 9 km. The ridges are found in water depths between 270 and 350 m. The sediment ridges are 2.5–9 km long, 50–250 m wide and 5–25 m high. In profile, most of these ridges are characterized by steep slopes towards Northwest and gentle slopes towards Southeast. Their internal structure, imaged by parametric echo-sounding data, shows that they are positive sedimentation features rather than erosive remnant structures. Arcuate shape, joint orientation and position on a basal till are indicative for end moraines. Because they are positioned within the Westwind Trough on a basal till that extends further east, we consider these ridges end moraines of the Westwind ice stream reported by Evans et al. (2009), Marine geophysical evidence for former expansion and flow of the Greenland Ice Sheet across the north-east Greenland continental shelf. Journal of Quaternary Science (2008), doi: 10.1002/jqs.1231.). Based on our hydro-acoustic data, we interpret these end moraines to be formed by short-lived re-advances during an overall recession of the ice margin. However, they could also be deposited during halts of the grounding line (comparable to De Geer moraines) though their morphological characteristics are slightly different from most published De Geer moraines. The ages for the moraine deposition can be inferred from a thin sedimentary drape indicating timing between Lateglacial and early Holocene. This set of end moraines is direct evidence for a dynamic behaviour of the marine-based ice stream during the last deglaciation on the NE Greenland shelf.

Litho‐ and chronostratigraphy of the Late Weichselian in Vendsyssel, northern Denmark, with special emphasis on tunnel‐valley infill in relation to a receding ice margin

Lithostratigraphy and chronostratigraphy of samples from 18 deep boreholes in Vendsyssel have resulted in new insight into the Late Weichselian glaciation history of northern Denmark. Prior to the Late Weichselian Main advance c. 23–21 kyr BP, Vendsyssel was part of an ice‐dammed lake where the Ribjerg Formation was deposited c. 27–23 kyr BP. The timing of the Late Weichselian deglaciation is well constrained by the Main advance and the Lateglacial marine inundation c. 18 kyr BP, and thus spans only a few millennia. Rapid deposition of more than 200 m of sediments took place mainly in a highly dynamic proglacial and ice‐marginal environment during the overall ice recession. Mean retreat rates have been estimated as 45–50 m/yr in Vendsyssel with significantly higher retreat rates between periods of standstill and re‐advance. The deglaciation commenced in Vendsyssel c. 20 kyr BP, and the Troldbjerg Formation was deposited c. 20–19 kyr BP in a large ice‐dammed lake in front of the receding ice sheet, partly as glaciolacustrine sediments and partly as rapid and focused sedimentation in prominent ice‐contact fans, which make up the Jyske Ås and Hammer Bakker moraines. In the northern part of central Vendsyssel, at least four generations of north–south orientated tunnel valleys are identified, each generation related to a recessional ice margin. This initial deglaciation was interrupted by a major re‐advance from the east c. 19 kyr BP, which covered most of Vendsyssel. An ice‐dammed lake formed in front of the ice sheet as it retreated towards the east; the Morild Formation was deposited here c. 19–18 kyr BP. Related to this stage of deglaciation, eight ice‐marginal positions have been identified based on the distribution of large tunnel‐valley systems and pronounced recessional moraines. The Morild Formation consists of glaciolacustrine sediments, including the sediment infill of more than 190 m deep tunnel valleys, as well as the sediments in recessional moraines, which were formed as ice‐contact sedimentary ridges, possibly in combination with glaciotectonic deformation. The character of the tunnel‐valley infill sediments was determined by proximity to the ice margin. During episodes of rapid retreat of the ice margin, tunnel valleys were quickly abandoned and filled with fine‐grained sediments in a distal setting. During slow retreat of the ice margin, tunnel valleys were filled in an ice‐proximal environment, and the infill consists of alternating layers of fine‐ to coarse‐grained sediments. At c. 18 kyr BP, Vendsyssel was inundated by the sea, when the Norwegian Channel Ice Stream broke up, and a succession of marine sediments (Vendsyssel Formation) was deposited during a forced regression.

Morphosedimentological record and human settlements as indicators of West-African Late Holocene climate variations in the littoral zone of the Iwik peninsula (Banc d’Arguin – Mauritania)

Abstract The geomorphological, sedimentological and archaeological studies of coastal features in the Iwik-Aouatil area show two categories of coastal sedimentary units: very extensive sand flats and linear relief features covered by a thin veneer of anthropogenic Anadara senilis shell-middens. These units provide an opportunity to discuss the return to arid conditions in western Sahara during the Late Holocene, when climate changes induced the alternating occurrence of more or less dry episodes, from around 5 ka until today. These constructions reflect the global climate change within the general framework known for the region. Two different processes occurred: (1) constant deposition of wide sand-flats and (2) construction of sedimentary ridges at specific moments. These processes demonstrate the existence of (1) constant sand discharge during the whole period as sand availability is not a limiting factor and (2) a substantially increased sand influx during the 4.2-3.2 ka episode. Because of sea-level stability in the Banc d’Arguin area during the Late Holocene, the variability of morphogenetic agents is considered to be the likely cause of fluctuations, influencing sand input rate and implying that either sand was delivered in greater abundance (aridity crisis and stronger winds) or littoral drift was more powerful (stronger waves and more frequent storms in the northern part of the Atlantic ocean), or both simultaneously. The action of these two driving forces in the depositional processes observed in the Iwik-Aouatil coastal plain is evaluated with respect to the occurrence of episodes of rapid climate changes (RCCs) and North Atlantic Oscillation (NAO) changes. A climate pattern combining episodes of RCC and NAO positive (NAO+) index during the 4.5-3.2 ka period is convenient for both sand supply, which accelerates the southwestward migration of barchan’s lines, and sand transportation, which improves the efficiency of the littoral drift.

Os‐isotope insights into major environmental changes of the Arctic Ocean during the Cenozoic

Recent studies of Lomonosov Ridge sediments provided evidence for an Eocene lacustrine stage in the Arctic Basin and concluded at its merging into global oceanic circulation some 17.5 Ma ago, with a ∼26 Ma gap in‐between. Here, Re‐Os data are used to document these events. High Os‐concentrations in the lacustrine unit and overlying transitional layer point to low sedimentary O2‐conditions. Highly radiogenic Os‐isotope values mark this layer suggesting massive input of old continental material. Above, Os concentrations decrease to typical oxic‐sediment values, whereas 187Os/188Os‐ratios follow the trend observed in other oceans. However, Re‐Os isochrons from layers assigned to the upper and lower limit of the 26 Ma‐gap yield identical ages, suggesting a short transitional event marking the opening of Fram Strait and a ca. 38 Ma for the inception of marine conditions. If confirmed, this new chronology would modify views about the tectonic evolution of Lomonosov Ridge.

The role of currents and sea ice in both slowly deposited central Arctic and rapidly deposited Chukchi–Alaskan margin sediments

A study of three long cores from the outer shelf and continental slope north of Alaska in the Arctic Ocean indicate that localized drift deposits occur here with sedimentation rates of more than 1.5 m/kyr during the Holocene. Currents in this area average about 5–20 cm/s but can reach 100 cm/s and these velocities transport the sediment found in these cores primarily as intermittent suspended load. These high accumulation sediments form levee-like deposits associated with margins of canyons cutting across the shelf and slope. Unlike most textural investigations of Arctic sediment that focus on the coarser ice-rafted detritus (IRD), this paper focuses on the > 95% of the sediment, which is finer than 45 μm. The mean size of this fraction varies between 6 and 15 μm in Holocene sediments from the Chukchi–Alaskan shelf and slope with the higher values closer to shore. Analysis of detailed size distributions of these Holocene deposits are compared to 34 sediment samples collected from sea ice across the Arctic Ocean and to Holocene sediment from central Arctic Ocean cores and indicate that similar textural parameters occur in all of these sediments. Principal components of these size distributions indicate that sea ice is an important link between the shelves and the central Arctic. Factor scores indicate nearly identical components in the clay and fine silt size fractions but very different components in the coarse silt for sea ice sediment and central Arctic ridge sediments compared to shelf and continental slope deposits. Sea ice must contribute to sedimentation in both of these Arctic regions, but bottom currents dominate in the slope region, forming drift deposits.

Formation of mud ridge and runnels in the intertidal zone of the Severn Estuary, UK

Flow-parallel linear bedforms, consisting of ridges and runnels have been reported widely in soft and consolidated cohesive sediments within intertidal and subtidal coastal waters. Less frequent are reports of these systems incised into soft bedrock. Despite their common occurrence there are few data concerning the hydrodynamic controls on the initiation and maintenance of ridge–runnel systems. Rather, several hypotheses have been advanced to account for them. In tidal environments a theory based upon the evolution of ‘streamwise vorticity’ transverse to the main flow direction is favoured, and has been supported recently by detailed hydrodynamic data obtained from above a ridge–runnel system on an intertidal zone of the Severn Estuary, UK. In the present paper, a detailed description of the sedimentology of the same ridge–runnel system is provided and interpreted within the framework of the ‘streamwise vorticity’ hypothesis. This objective is accomplished by measurements of the morphology and sampling the bulk characteristics of the sedimentary ridges and runnels but also with reference to ridges and runnels incised into soft bedrock beneath the sediment body. Grain-size composition, sediment compaction, radionuclide distributions and elemental distributions and X-radiography are used to describe the sediment structures within ridges and their inferred histories of development. These descriptions are interpreted with respect to the known hydrodynamic climate and the deposition and erosion behaviour of the sediments in laboratory experiments using an annular flume. Runnels contain liquid mud at low tide but contain very little fine gravel and shell debris. In contrast, fine suspended sediment is present at high concentrations in the flow. Thus fluid evorsion, aided by a dense suspension of fine sediment that impacts the mud bed, must be the primary tool to initiate and maintain the runnels, rather than scour by a coarse bedload. The ridges are composed primarily of fine silt or sand laminae some of which are approximately horizontal but most of which are slightly domed upwards such that they drape the ridge topography. This relationship between lamination and form demonstrates that the ridges are distinct accretion units and not residual masses of un-eroded planar-laminated mud incised by runnels. Laminae thicken, or thin and pinch-out, or are truncated, laterally at the sides of the runnels. Truncation occurs probably by damped spanwise turbulence structures which were recorded at this location. A notable characteristic is that the mud in the upper portion of the ridges is of low density and low compaction whilst the sediment in the lower part is more dense and well-compacted; often the two units are distinct, being separated by major reactivation surfaces; the latter indicative of occasional stripping of the surface sediments from above a residual ‘core’ at the base. The presence of coarse-grained laminae and lenses and minor reactivation surfaces throughout the sediment body indicates that high energy conditions, possibly other than the Neap–Spring cycle, are also important in developing the ridges. In this respect, consideration of the erosion potential of deposited sediment, using field and flume data, shows that the relatively low density, low-compaction mud constituting the crests of the ridges is not readily eroded by the present-day Neap–Spring tidal flow regime. Further, for the same flow regime, when the shear stresses on the bed fall below critical for suspension, potential deposition rates are very low, which supports the interpretation of the coarse-grained layers and reactivation surfaces as indicators of the significant role played by additional high-stress events, such as storm surges, in ridge-erosion and rebuilding.

Ice marginal dynamics during surge activity, Kuannersuit Glacier, Disko Island, West Greenland

The Kuannersuit Glacier surged 11 km between 1995 and 1998. The surge resulted in the formation of an ice cored thrust moraine complex constructed by subglacial and proglacial glaciotectonic processes. Four main thrust zones are evident in the glacier snout area with phases of compressional folding and thrusting followed by hydrofracture in response to the build-up of compressional stresses and the aquicludal nature of submarginal permafrost and naled. Various types of stratified debris-rich ice facies occur within the marginal zone: The first (Facies I) comprises laterally continuous strata of ice with sorted sediment accumulations, and is reworked and thrust naled ice. The second is laterally discontinuous stratified debris-rich ice with distinct tectonic structures, and is derived through subglacial extensional deformation and localised regelation (Facies II), whilst the third type is characterised by reworked and brecciated ice associated with the reworking and entrainment of meteoric ice (Facies III). Hydrofracture dykes and sills (Facies IV) cross-cut the marginal ice cored thrust moraines, with their sub-vertically frozen internal contact boundaries and sedimentary structures, suggesting supercooling operated as high-pressure evacuation of water occurred during thrusting, but this is not related to the formation of basal stratified debris-rich ice. Linear distributions of sorted fines transverse to ice flow, and small stratified sediment ridges that vertically cross-cut the ice surface up-ice of the thrust zone relate to sediment migration along crevasse traces and fluvial infilling of crevasses. From a palaeoglaciological viewpoint, marginal glacier tectonics, ice sediment content and sediment delivery mechanisms combine to control the development of this polythermal surge valley landsystem. The bulldozing of proglacial sediments and the folding and thrusting of naled leads to the initial development of the outer zone of the moraine complex. This becomes buried in bulldozed outwash sediment and well-sorted fines through surface ablation of naled. Up-ice of this, the heavily thrust margin becomes buried in sediment melted out from basal debris-rich ice and subglacial diamicts routed along thrusts. These mechanisms combine to deliver sediment to supraglacial localities, and promote the initial preservation of structurally controlled moraines through insulation, and the later development of kettled dead ice terrain.

A 35 kyr record of organic matter composition and δ13C of n-alkanes in bog sediments close to Lake Baikal: Implications for paleoenvironmental studies

We characterized the compositions of organic compounds in a Cheremushka bog sediment core (deposited over the last 35kyr), located at the eastern coast of Lake Baikal, to obtain basic information about the terrestrial organic matter (OM) which contributed to Lake Baikal sediments. The bog sediment was analyzed for the molecular composition of n-alkanes, lignin phenols and n-C24 to C30 alkanoic acids, as well as the carbon isotopic composition of plant wax derived n-C27 to C33 alkanes.Concentrations of lignin phenols [vanillyl (V) plus syringyl (S) phenols] normalized to total organic carbon (TOC) in the Holocene are twice those for the last glacial maximum (LGM), while concentrations of TOC-normalized n-C24 to C30 alkanoic acids do not change markedly in this period. Thus, the ratio of lignin phenols to n-C24 to C30 alkanoic acids increases from the LGM to the Holocene. This result is essentially consistent with pollen analysis indicating an expansion of woody plants in the Holocene and a prevailing herb-abundant environment for the LGM. The δ13C values of n-C27 to C33 alkanes (e.g. −29‰ to −33‰ for C31) indicate the presence of C3-dominant plants throughout the core.The contribution of terrestrial OM to Lake Baikal sediments was estimated using the biomarkers, on the assumption that the OM in the bog sediments is a representative of the terrestrial OM around the lake. Hence, the estimation using lignin phenol or n-C24 to C30 alkanoic acid parameters indicates that 11–24% of the TOC in the Academician Ridge sediments is land-derived for both the Holocene and the LGM, which is similar to the estimates from C/N values of bulk OM. However, the estimates for terrestrial OM using the n-C27 to C33 alkane parameter are generally higher than those using lignin phenol or n-C24 to C30 alkanoic acid parameters. The difference is thought to be associated with the difference in source and behavior of these biomarkers.

Multidisciplinary investigations exploring indicators of gas hydrate occurrence in the Krishna–Godavari Basin offshore, east coast of India

We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002–2003 in the Krishna–Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW–ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO4 2−) concentrations (28.7 to <4 mM), increasing downcore methane (CH4) concentrations (0–20 nM) and relatively high total organic carbon contents (1–2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gas-escape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna–Godavari Basin offshore.

Bed Ribbing Instability Explanation: Testing a numerical model of ribbed moraine formation arising from coupled flow of ice and subglacial sediment

Ribbed moraines are large (up to 16 km long) ridges of sediment produced transverse to ice flow direction that formed widely beneath palaeo‐ice sheets. Since ice sheet stability is sensitive to conditions operating at the bed, an understanding of ribbed moraine genesis will provide critical information on ice sheet dynamics. Currently, there is no consensus on ribbed moraine formation and various competing hypotheses have been presented to account for their genesis. Only one of these theories has been developed into a physically based numerical model that quantitatively describes ribbed moraine formation. This theory, known as the Bed Ribbing Instability Explanation (BRIE), argues that ribbed moraines are produced by a naturally arising instability in the coupled flow of ice and till. BRIE demonstrates that transverse subglacial ridges (i.e., ribbed moraine) spontaneously grow under certain parameter combinations, and it predicts their wavelength (spacing between ridges). The model represents a significant advance because it is the first time a theory of subglacial bedform generation has been developed to make quantitative predictions which can be formally tested. This paper discusses the types of tests that are currently possible and reports the results from the first testing of BRIE. This analysis centers on the ability of BRIE to predict the primary characteristics of ribbed moraine, which are patterning and wavelength. Results show that BRIE successfully predicts the correct ribbed moraine pattern and appropriate wavelengths. The tests fail to falsify the model, and it is concluded that BRIE remains a viable explanation of ribbed moraine formation.

A model for spiral flows in basal ice and the formation of subglacial flutes based on a Reiner‐Rivlin rheology for glacial ice

Flutes are elongated sediment ridges formed at the base of glaciers and ice sheets. In this paper, we show that flutes can be the product of a corkscrew‐like spiral flow in basal ice that removes sediment from troughs between flutes and deposits it at their crests, as first suggested by Shaw and Freschauf. In order to generate the type of basal ice flow required for this mechanism, the viscous rheology of ice must allow for the generation of deviatoric normal stresses transverse to the main flow direction. This type of behavior, which is commonly observed in real nonlinearly viscous and viscoelastic fluids, can be described by a Reiner‐Rivlin rheology. Here, we develop a mathematical model that describes the role of these transverse stresses in generating spiral flows in basal ice and investigate how these flows lead to the amplification of initially small basal topography and the eventual formation of assemblies of evenly spaced subglacial flutes.

Late Quaternary lake-level fluctuations in the Mababe Depression: Middle Kalahari palaeolakes and the role of Zambezi inflows

A systematic drilling and optical dating programme on Middle Kalahari beach ridge (relict shoreline) sediments has enabled the identification of multiple episodes of lake high stands of an extensive palaeolake system at the terminus of the Okavango Delta, northern Botswana. This paper presents 23 ages from the Mababe Depression and establishes four shoreline construction phases in the late Quaternary coeval with other sub-basin lake high stands (Lake Ngami). These synchronous lake phases result from a coalescence of the sub-basins into a unified palaeolake, Lake Thamalakane, covering an area of ∼ 32,000 km 2. Six additional ages are also presented from the Chobe enclave to the north of the basin where shoreline ridges were emplaced at the same time as Lake Thamalakane phases. This suggests that increased flow in the Chobe and Zambezi system significantly contributed to the Middle Kalahari lake phases in both the post-glacial and Holocene periods. The integration of these new data and their compatibility with other regional and tropical palaeo-archives is discussed in the light of understanding Quaternary climate drivers within the Kalahari.

Field investigation of ridge–runnel dynamics on an intertidal mudflat

Flow-parallel linear ridge–runnel ( R– R) bedforms composed of mainly cohesive sediments have been investigated at an intertidal site located at Hills Flats in the Severn Estuary, UK. It is argued that the sequences of sand–mud laminae in the ridge sediments indicate strongly that R– R are depositional bedforms. Faint R– R topography scoured in the underlying soft bedrock and parallel rows of coarser particle now found at the base of ridges are evidence of secondary vortices that may have provided a catalyst for mud ridge growth. Bed shear stress remains below the erosion threshold for all but the most recent and weak surficial mud deposits which are removed by evorsion [mechanical erosion by turbulent flows that may also carry sand and/or gravel] during Spring tidal flow ca. 60 cm/s. Differences in flow characteristics between ridges and runnel are minor. Corrasion [to erode or be eroded by abrasion] by sparse coarse sediment largely prevents net deposition in the runnels. Over time a slight imbalance between mud deposition and erosion rates allows slow growth of the ridges suggesting that these mudflats may be especially sensitive to slight changes in the local hydrodynamic regime and/or sediment supply.

Estimates of biogenic methane production rates in deep marine sediments at Hydrate Ridge, Cascadia margin.

Methane hydrate found in marine sediments is thought to contain gigaton quantities of methane and is considered an important potential fuel source and climate-forcing agent. Much of the methane in hydrates is biogenic, so models that predict the presence and distribution of hydrates require accurate rates of in situ methanogenesis. We estimated the in situ methanogenesis rates in Hydrate Ridge (HR) sediments by coupling experimentally derived minimal rates of methanogenesis to methanogen biomass determinations for discrete locations in the sediment column. When starved in a biomass recycle reactor, Methanoculleus submarinus produced ca. 0.017 fmol methane/cell/day. Quantitative PCR (QPCR) directed at the methyl coenzyme M reductase subunit A gene (mcrA) indicated that 75% of the HR sediments analyzed contained <1,000 methanogens/g. The highest numbers of methanogens were found mostly from sediments <10 m below seafloor. By considering methanogenesis rates for starved methanogens (adjusted to account for in situ temperatures) and the numbers of methanogens at selected depths, we derived an upper estimate of <4.25 fmol methane produced/g sediment/day for the samples with fewer methanogens than the QPCR method could detect. The actual rates could vary depending on the real number of methanogens and various seafloor parameters that influence microbial activity. However, our calculated rate is lower than rates previously reported for such sediments and close to the rate derived using geochemical modeling of the sediments. These data will help to improve models that predict microbial gas generation in marine sediments and determine the potential influence of this source of methane on the global carbon cycle.

Devensian Late-glacial environmental change in the Gordano Valley, North Somerset, England: a rare archive for southwest Britain

The Late-glacial and Holocene environmental history of the Gordano Valley, North Somerset, UK has been reconstructed using pollen, sediment particle size and mineralogical analyses and radiocarbon dating. A Devensian sediment ridge across the valley confined the waters of a small lake, within which the initial sedimentation was minerogenic. Radiocarbon dating of overlying organic-rich deposits suggests that this began late in the Dimlington Stadial c. 18,000–15,000 Cal. BP. Petrographic analyses indicate the minerogenic sediments were partly wind-blown in origin. Climatic amelioration during the Windermere Interstadial c. 15,000 Cal. BP encouraged a shift from minerogenic to biogenic sedimentation. A brief return to minerogenic sedimentation between c. 10,400 and c. 9,520 Cal. BP was followed by uninterrupted fen peat accumulation throughout the Holocene. The later minerogenic horizon appears to represent the Loch Lomond Stadial. Few stratigraphic sequences preserving the complete Devensian Late-glacial–Holocene transition exist in southwest Britain, making the sedimentary archive of the Gordano Valley valuable regionally for reconstructing Late-glacial climate change.

Ice Scours in the Sediments of Glacial Lake Iroquois, Prince Edward County, Eastern Ontario

Straight or slightly curved ice scours are found in thin glacilacustrine sediment of eastern Lake Iroquois, especially near the crest of an escarpment in Prince Edward County. They are large (to 3.57 km long and 174 m wide), shallow (about 1 m deep) and oriented in a nearly westerly direction. Irregular ridges of sediment have been pushed up along the sides and at the western end of some scours. Bedrock is near the ground surface, but had little influence on the formation of the scours. Based on their shape, location and pattern, we conclude that the scours were most likely formed in shallow water of the short-lived Sydney phase of Lake Iroquois by lake ice driven by prevailing northeasterly winds from the retreating Laurentide Ice Sheet.