Lars B. Clemmensen

Aeolian sediment in raised bog deposits, Halland, SW Sweden: a new proxy record of Holocene winter storminess variation in southern Scandinavia?

Cores of peat taken from two raised bogs in the near-coastal part of Halland, SW Sweden, were examined for their content of wind-transported clastic material. The Boarps Mosse core has a length of 262 cm and covers the last 2850 years (age model based on three radiocarbon dates), while the Hyltemossen core has a length of 347 cm and records the last 7000 years (age model based on eight radiocarbon dates). The peat contains aeolian quartz silt and sand (from trace amount to more than 10 weight /% of the dry peat sample), and the grain-size composition of the minerogenic sediment indicates that the material is composed of both far-travelled dust and local wind-eroded silt and sand. Grains larger than 0.2mm are present in many intervals and probably record niveo-aeolian transport in connection with severe snowstorms. A systematic count of quartz grains between 0.2-0.35mm and larger than 0.35mm was carried out to determine variations in aeolian sand influx (ASI) and winter wind climate with time. The Boarps Mosse site is situated in a hilly terrain and is primarily open to southerly and northerly winds, while the Hyltemossen site is situated in a more flat landscape open to winds from most directions. Furthermore, at Boarps Mosse dense forest stands seem to have protected the site from significant aeolian sand influx until about AD 1000, while aeolian sand influx apparently was less influenced by local forest development at Hyltemossen. However, using periods with low ASI values as marker horizons it is possible to identify a number of periods of high ASI values during the last 2500 years. These periods, which probably record high winter storminess, are dated to 130, 300, 400, 475, (700), 900, (1000), (1100), (1150), (1350), (1450), 1675, (1800), 1875, 1950-2000, (2050), 2150, 2225 and (2300) cal. yr BP. In addition, the Hyltemossen site documents peak storminess at 2450, 2725, 2850-3050, 3275, 3675, 4100-4200, 4550, 4900, 5100-5200 and 5450-5700 cal. yr BP. Most of the aeolian activity phases in Halland correlate with well-known cold periods in northwest Europe, suggesting a link between winter storminess in southern Scandinavia and expansion of the polar vortex.

Tracks and trackways of Myotragus balearicus Bate (Artiodactyla, Caprinae) in Pleistocene aeolianites from Mallorca (Balearic Islands, Western Mediterranean)

Tracks and trackways of the ruminant goat, Myotragus balearicus Bate, 1909 are abundant in the Late Pleistocene aeolianites of the south coast of Mallorca, Spain. The carbonate aeolianites form impressive dunes and related sand ramps that were initiated as echo dunes in front of a 20–40 m high paleocliff of Miocene reefal limestone. Where the dunes built up to the cliff top, they covered it as an aeolian ramp. The dunes and sand ramps were active during the dry summer months but were passive during the wet winter period. The small, goat-like animals were abundant on the island, and for unknown reasons crossed the aeolian accumulations in large numbers. This study deals with the tracks formed in the dune deposits. All preserved trackways indicate impression into moist sand. Special features of the tracks include the structure produced by the withdrawal of the foot, here called the ‘axis’, and a disturbance zone of plastic deformation around this. On dune crests, the disturbance zone surrounds the axis more or less symmetrically. However, in addition, a ‘pressure pad’ of dislocated, slightly rotated sediment bound by curved microfaults is commonly produced posterior to the axis by propulsive pressure of the foot. On steep stoss and lee slopes, the pressure pad becomes oriented in a down-slope position as a result of gravitational slip of the walking animal. Combination of disturbance of the sediment in this way by manus followed by overprinting of similar disturbance by pes produces highly complicated track structure. This structure may be characteristic of artiodactylous mammals in soft sand, particularly aeolian deposits. The structure is designated as Bifidipes aeolis isp. nov.

The late Holocene coastal dunefield at Vejers, Denmark: characteristics, sand budget and depositional dynamics

Abstract The coastal dunefield at Vejers (west coast of Jutland), which is now stabilized by vegetation, covers an area of approximately 120 km 2 and occurs in a very high-energy wind regime. Along the coastline a narrow belt of 5–15 m high dune ridges occur. The dune ridges are gradually replaced inland by 10–20 m high parabolic or irregular dune forms. In the central part of the dunefield a large aeolian sand plain (ca. 50 km 2 ) occurs at the windward side of a large and up to 20 m high parabolic dune. The 3D structure of the dunefield deposits have been studied by geomorphological analysis, sedimentological facies analysis of borings, trenches and natural exposures, and most importantly by georadar mapping. The aeolian deposits which overlie a middle to late Holocene barrier spit depositional system can be divided into a Lower unit that drapes the underlying barrier system topography, and an Upper unit that includes the present dunes. The boundary between the two aeolian units is a well-developed Phragmites peat. The Lower aeolian unit is composed of two aeolian subunits separated by an organic-rich horizon. Also the Upper unit is composite and composed of a basal aeolian sand cover and overlying dune or sand plain deposits. The dune deposits locally are composed of up to four depositional subunits separated by immature soils. The base of the Lower aeolian unit formed around 300 A.D., whereas the Phragmites peat at the base of the Upper aeolian unit has been dated to ca. 1000 A.D. From historical sources we know that the present dunefield primarily formed between 1550 and 1850 A.D. The sand content in the Upper unit is estimated to 550 × 10 6 m 3 , which yields sand transport rates between 25 m 3 (m width) −1 yr −1 (accumulation in 1000 years), and 83 m 3 (m width) −1 yr −1 (accumulation in 300 years). The sand in the dunefield originated from beach deposits. A large but pulsating supply of sand was supplied to the beaches by southwards running coastal currents. The composition of the Vejers dunefield deposits indicate that periods of dunefield growth alternated with periods of dunefield stabilization during the last ca. 1700 years. The final and most important phase of dunefield growth took place during ‘the Little Ice Age’. This period was characterized by an overall cold and stormy climate and a relative low sea level, and resulted in an increased availability of sand in the shorezone and a high influx of sand into the dunefield. Short periods of decreased storminess are recorded by the immature soils. Dunefield growth prior to 1000 A.D. was also linked to cold and stormy climatic intervals.

Evolution of a dune from crescentic to parabolic form in response to short-term climatic changes: Råbjerg Mile, Skagen Odde, Denmark

Abstract Rabjerg Mile, a complex parabolic dune, reaches a height of 20 m above the deflation plain. The width of the dune is 900 m and the length of the whole dune is 1500 m. The dune is covered by 0.5–1.5 m high transverse dune ridges. The dune has migrated approximately 1 km since 1887 A.D., accompanied by a change in the migration direction from ESE (100°) to ENE (68°). The morphology of Rabjerg Mile through the last 100 years is illustrated by a geographical information system (Arc/Info) applied to a series of high-quality topographical maps. Based on the digitized contour lines, the evolution of Rabjerg Mile can be followed from an initial crescentic form in 1887 A.D. to a complex parabolic dune in this century. Dune dynamics and sites of erosion and deposition are evaluated from the digital terrain models. Changes in vegetation cover, documented from the maps and air photos, indicate that vegetation in the study area steadily increased with time. Calculations of resultant drift directions and gross bedform-normal transport are based on climatic data from the Skagen changed from an almost perfect transverse bedform (1887–1924) to an oblique imperfectly transverse bedform (1966 to present). The bedform alignment followed the gross bedform-normal transport rule in most years. The evolution of a parabolic dune after 1924 is related to a period of reduced wind energy and increased vegetation cover. This parabolic dune was maintained after 1960 when wind energy increased and the wind regime was more complex. At the same time the dune became markedly asymmetric because of the frequent occurrence of strong southerly and easterly winds.

Erg-margin deposits in the Lower Jurassic Moenave Formation and Wingate Sandstone, southern Utah

The Moenave Formation (Dinosaur Canyon Member) and the closely associated Wingate Sandstone in southern Utah represent the southwestern erg margin of the Lower Jurassic Wingate erg. The erg-margin deposits, which extend across a 50- to 100-km-wide region, are composed of dune, eolian sand-sheet, mud-flanked eolian, sabkha, ephemeral-stream and lacustrine facies. The succession is divided into a basal erg-margin association (8-27 m) dominated by eolian facies and an overlying erg-margin association (as much as 90 m thick) composed mainly of fluvial and lacustrine facies. The sediments of the basal association are composed of three drying-upward or locally drying-wetting-upward sequences that can be traced from the erg margin 100 km slightly upwind (that is, toward the west-northwest) into the outer erg-margin zone. Sequences of the inner erg-margin zone contain dune facies in their upper parts, sequences in the intermediate erg-margin zone contain sand-sheet facies, and those in the outer erg-margin zone contain sabkha facies. The basal association thins markedly toward the west-northwest. Sediments of the overlying association contain up to six drying-upward or drying-wetting-upward sequences but are apparently only fully preserved in the outer erg-margin zone. Individual sequences show much lateral variation but characteristically contain lower, muddy, ephemeral-stream deposits and upper, sandy, ephemeral-stream deposits; sand-sheet deposits; or incipientdune deposits. The general time trend is interpreted as a gradual change toward a more humid climate. The stacked drying-upward or drying-wetting-upward sequences suggest that this long-term climatic change was characterized by a number of fluctuations between relatively arid and humid conditions. These climatic fluctuations apparently controlled both sediment yield to the basin and the depositional conditions in the erg-margin region. During arid intervals, ephemeral braided streams transported sandy bed-load into the erg-margin area, where eolian reworking took place. During humid intervals, sheet-floods and high-sinuosity rivers transported a more mud-rich sediment load into the basin, where eolian activity was minimal.

A Late Triassic lake system in East Greenland: facies, depositional cycles and palaeoclimate

Abstract The Upper Triassic Fleming Fjord Formation of the Jameson Land Basin in East Greenland contains a well-exposed succession, 200–300 m thick, of lake deposits. The Malmros Klint Member, 100–130 m thick, is composed of cyclically bedded intraformational conglomerates, red siltstones and fine-grained sandstones and disrupted dolomitic sediments (paleosols). The cyclicity is composite with cycles having mean thicknesses of (25), 5.9 and 1.6 m. The overlying Carlsberg Fjord beds of the Orsted Dal Member, 80–115 m thick, are composed of structureless red mudstones rhythmically broken by thin greyish siltstones. This unit also has a composite cyclicity with cycles having mean thicknesses of 5.0 and 1.0 m. The uppermost Tait Bjerg Beds of the Orsted Dal Member, 50–65 m thick, can be divided into two units. A lower unit is composed of cyclically bedded intraformational conglomerates or thin sandstones, red mudstones, greenish mudstones and yellowish marlstones. An upper unit is composed of relatively simple cycles of grey mudstones and yellowish marlstones. Recognized cycles have mean thicknesses of 5.6 and 1.6 m. The lake deposits contain evidence of seasonal, orbital and long-term climatic change. Seasonal change is documented by numerous desiccation surfaces especially in the Malmros Klint Member and Carlsberg Fjord beds, orbital change is suggested by the composite cyclicity, and long-term climatic change is indicated by the systematic upwards change in sedimentary characteristics of the lake deposits. The sedimentary features of the Malmros Klint Member suggest lacustrine deposition in a dry climate that fluctuated between desert and steppe conditions, the Carlsberg Fjord beds probably record lacustrine lake deposition in a rather constant dry (steppe) climate, while the Tait Bjerg Beds record lake sedimentation in a climate that fluctuated between dry (steppe) and warm moist temperate. In the Tait Bjerg Beds the upward change in cycle characteristics indicates a shift towards more humid conditions. Climatic deductions from sedimentary facies are in good agreement with climate maps of Laurasia, as simulated by numerical climate models. Palaeomagnetic data indicate a northward drift of East Greenland of about 10° from ca. 25°N to ca. 35°N in the Middle to Late Triassic. The Fleming Fjord Formation which represents ca. 5 m.y. of the Late Triassic interval was deposited during latitudinal drift of 1–2°. It is possible that the observed long-term upward shift in climatic indicators within the formation can be ascribed to plate drift, but southward shift of climatic belts could also have been of importance.

Vertical sections through dinosaur tracks (Late Triassic lake deposits, East Greenland) – undertracks and other subsurface deformation structures revealed

Tracks and trackways of theropod dinosaurs (Grallator footprints) are abundant in the Late Triassic lake sediments of East Greenland. For this study we selected a rather diffuse theropod track preserved on the upper surface of a red heterolithic mudrock, and a better preserved track seen on the upper surface of a greyish mudrock. In order to examine undertracks and other subsurface deformation structures, both slabs were sectioned vertically at closely-spaced intervals, perpendicular to the length of the axis of the impression of digit III. Each section was subsequently polished and internal structures revealed. The digit impressions of both tracks were associated with well-defined undertracks which were cut by deep and narrow claw imprints at the distal end of the digit impressions. Marginal ridges at the tracking surfaces were typically associated with subsurface marginal folds. The marginal ridges were asymmetrically developed suggesting an outward movement of the proximal part of the foot, probably during the kick-off; this is in contrast to what is observed in tracks from Lower Jurassic theropods. The study shows that cross-sections through dinosaur tracks display large structural variation and it is suggested that some disturbed layers in continental deposits could be the result of trampling by vertebrates.

Aeolian sand-sheet deposits in the Lower Cambrian Neksø sandstone formation, bornholm, Denmark: sedimentary architecture and genesis

Abstract The Lower Cambrian Nekso Sandstone Formation, which hitherto has been thought to be of pure fluvial origin, locally contains a significant amount of aeolian deposits. These aeolian sediments form relatively thin, tabular sand sheet deposits separated by straight to somewhat irregular erosional bounding surfaces and associated pond deposits. These bounding surfaces can be classified after rank, of after genesis. The aeolian bounding surfaces in the Nekso Sandstone Formation are probably multugenetic and signify initial ground-water-controlled deflation of the sand-sheets followed by a ground-water rise or low-energy flooding and pond sedimentation. The sand-sheets and intervening pond deposits form a multistorey succession (7 m thick) that records systematic fluctuations in available sand supply in the source area, air flow characteristics, ground-water level and aeolian accumulation rate. Apparently, the recorded sedimentary pattern reflects episodic growth of a sand-sheet deposit related to systematic climatic fluctuations.

Lacustrine Facies and Stromatolites From the Middle Triassic of East Greenland

ABSTRACT The Middle Triassic Edderfugledal Member of East Greenland is interpreted as lacustrine because of: (A) nonmarine fossils such as conchostracans and small bivalves, (B) non-marine trace fossils such as Isopodichnus, Cruziana, Pelecypodichnus and Fuersichnus, (C) stromatolites with algal moulds, (D) sedimentary structures, which indicate shallow water, weak to moderately strong wave action and frequent subaerial exposure, (E) absence of tidal current generated structures and (F) great lateral continuity of single beds, which indicate a low topographic gradient in the basin. Six major facies are distinguished within this ancient lake content: (1) green mudstone (open lacustrine), (2) yellow dolostone (carbonate mudflat), (3) flat pebble conglomerate (beach la or stromatolite breccia), (4) stromatolitic limestone (nearshore lacustrine), (5) greyish wave rippled sandstone (shoreline sandflat) and (6) reddish sandstone and mudstone (alluvial mudflat). The stromatolitic columns, commonly between 5 and 15 cm high, are subcylindrical to hemispheroidal, discrete and laterally linked. They consist of mm-thin couplets of organic-rich micrite laminae with both coccoid and filamentous algal moulds and dolomicrite laminae. The member displays well developed cyclic sedimentation both in the basal open lacustrine unit (the Sporfjeld Beds) and in the overlying shallow lacustrine unit (the Pingel Dal Beds). In the Sporfjeld Beds open lacustrine facies dominate and marginal lacustrine facies are restricted to carbonate mudflats. In the Pingel Dal Beds open l custrine facies become relatively rare and shoreline sandflats and alluvial mudflats are added to the marginal lake environment. This change in sedimentation is explained mainly in terms of intermittent tectonic uplift. These tectonic events were probably accompanied by a gradual change towards more humid conditions during the Middle Triassic.

Large-scale aeolian sand movement on the west coast of Jutland, Denmark in late Subboreal to early Subatlantic time—a record of climate change or cultural impact?

Abstract Holocene dunefield construction on the west coast of Jutland was episodic. One of the most intense phases of inland sand movement and dunefield construction took place in late Subboreal to early Subatlantic time. Evidence from optically stimulated luminescence dating supplemented by radiocarbon dating and/or archaeological data from four different dunefields places the onset of this important phase of aeolian activity to about 700 BC and indicates that sand movement continued for up to 700 years before the dunefields were stabilized. The onset of this phase of dunefield construction may be related to an abrupt climatic change in the North Atlantic region at about 800 BC and a likely increase in storminess. The coastal landscapes of western Jutland were nearly treeless at the end of Subboreal time promoting large-scale aeolian sand movement.