1,571 research outputs found

    Spit extension and barrier rollover at Blakeney Point and Salthouse: Historic maps and field observations

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    In the late 1990s, Brian Funnell used historic maps (1586 and 1797) and aerial photos (1952-1989) of the Blakeney area to calculate the mean extension rate of Blakeney Spit. He also charted landward rates of barrier movement (rollover) in the Salthouse area using a 1649 map. The mean centennial rate of extension of Blakeney Spit was ~3.5 m a-1 implying a likely age of the spit of 3000-4000 years. Change in barrier beach position between 1649 and the present day recorded a mean landward rollover rate of ~0.85 m a-1. These rates concur with earlier estimates and are supported by offshore data published since 2000. This hitherto unpublished work is augmented by more recent field observations from storm surges in 2007 and 2013 that record the episodic nature of barrier rollover and breaching. The most important observations are that shingle-entraining washover events at Salthouse are probably in part conditioned by local topography, favouring topographic lows. In 2013, shingle washover was accompanied by two breach channels. These temporary breach channels were cut principally by seaward drainage of floodwater trapped in the back barrier area following storm overtopping of the barrier. Moreover, the channels were sited on former creek and channel locations visible in the 1649 map. The inherited hydrological topography of the back barrier area thus continues to influence recent coastal geomorphic change

    Significance of Swaley cross-stratification in the Carstone Formation, Hunstanton, Norfolk

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    Cross-bedding in the Lower Cretaceous Carstone Formation at Hunstanton cliffs in Norfolk was first reported almost a hundred years ago, but details on the nature and significance of the cross-bedding have been largely ignored. To redress this we made detailed field observations and measurements. Fourteen cross-bedded sets were suitable for accurate measurements of set thicknesses, visible set length and foreset orientation. Eleven cross-bedded sets had NE trending foreset dip directions, while three sets had S trending foreset dip directions. Circular statistics on the directional data gave a significant NE mean dip direction of 55°. Cross-bedding geometry is broadly of ‘trough-type’ but specifically ‘swaley’, characterised by concave-upward shallow scours between 0.5-2.0 m wide and a few centimetres deep. Swaley cross-stratification is thought to form below fair-weather wave base, but above storm wave base; our calculations using published physical equations suggest the Carstone bedforms were generated on a storm dominated shoreface in 30 to 40 m water depth. Northward palaeocurrent directions are strikingly different to the predominantly southward palaeocurrents recorded in older Lower Greensand deposits of southern England. A storm surge relaxation current from the land deflected right by Coriolis forces could have resulted in an alongshore NE trending flow forming the Carstone cross-bedding. This interpretation allows for predominantly southward ocean currents interrupted by episodic storms surges and resulting relaxation currents

    Lacustrine stromatolites as multi-scale recorders of climate change: Insights from the Miocene Ebro Basin

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    Sedimentological, δ13C and δ18O data from stromatolites in a lower and middle Miocene sequence from the Ebro Basin (N Spain) are used to assess the potential of ancient lacustrine stromatolite lamination as an archive of palaeoenvironmental and palaeoclimatic change. The isotopic evolution through the studied sequence supports a general trend toward less saline conditions with time. Stromatolites and muddy-grainy laminated limestones developed in lake water that underwent little renewal, compared with other carbonate facies. The palaeoclimatic value of the stable-isotope changes and concurrent textural variations in calcite stromatolite lamination is studied at different orders of cyclicity. Stromatolite lamination consists of simple laminae (dark dense, light dense and light porous) grouped into alternating composite light and dark laminae. δ13C and δ18O analyses in consecutive composite laminae (bulk sampling) yielded a cyclic pattern that mimics textural variations. Light laminae, with lower δ13C and δ18O values, reflect higher precipitation/evaporation ratio (P/E) and more influence of biogenic 12C. Dark laminae, with higher δ13C and δ18O values, reflect drier conditions, more complete atmospheric CO2 exchange with water and photosynthetic 12CO2 uptake. Textural features of laminae support these results: the dark laminae are related to higher calcite saturation in lake water during drier periods. Isotopic values from high-resolution sampling through a 2.1-cm thick stromatolite reveal palaeoclimate variations at different temporal scales. Isotopic variation in 3rd order cyclicity of alternating light/dark simple laminae is recording seasonal P/E variations. Light and dark composite laminae (2nd order cyclicity) correspond to pluriannual dominantly-humid or -dry conditions, respectively. A gradual succession from light to dark composite laminae forms the 1st order cycles driven by decreasing P/E through longer pluriannual periods, resulting in lake level lowering. The stromatolites are thus recording lake level changes of centennial to millennial scale

    Stable isotope evidence for near-surface, low-temperature formation of Mg-(hydro)carbonates in highly altered Greek Mesozoic serpentinites

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    Authigenic magnesite, hydromagnesite and huntite associated with intensely altered and serpentinized ophiolitic rocks in Attica (mainland Greece) occur predominantly as veinlets and nodules within a totally weathered former-serpentinite groundmass. Carbonate δ18O values are consistent with post-geothermal fluid temperatures between 25-70 ˚C, but mostly between 25-30 ˚C, from a dominantly meteoric-sourced groundwater, indicating near-surface, low-temperature conditions. Despite the proximity of a volcanic centre with strong CO2 flux, 75% of the carbon isotope data imply little or no incorporation of this CO2 into the authigenic Mg-(hydro)carbonates. Indeed, many δ13C values are more negative than soil-zone calcrete values, and in this setting Mg-(hydro)carbonate δ13C below -6‰ VPDB probably indicate disequilibrium effects in alkaline groundwaters. Geothermal fluids and groundwaters were mainly routed through structural conduits. Some of the low temperature hydromagnesite subsequently dehydrated to magnesite under near-surface conditions, while huntite is likely a diagenetic transformation of hydromagnesite, forming close to the volcanic centre where fluid Mg/Ca ratios were low. The isotopic signatures are distinct from previously published Balkan-East Mediterranean magnesite data arrays but are consistent with many other ultramafic-associated magnesium carbonates worldwide; their association with likely fluid compositions provide important context for Mg-(hydro)carbonate formation as geothermal conditions cool to near surface temperatures

    Local-scale environmental gradients in ‘snail-shell’ stable isotopes from Holocene Jordanian archaeological sites

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    Reconstructing environments around archaeological sites is complicated by past land management practices and regional-scale climate proxies that can be contradictory and are often located at a distance from the sites themselves. Here we explore environmental information from fossil snail shells which, even when few in number on an archaeological site, may prove invaluable in constructing site-specific data. The palaeoecology of fossil snails and the stable isotopic composition of their shell carbonate can provide context-specific information on vegetation, water availability, and relative humidity during the occupation of a site. We studied terrestrial and aquatic snails from two later Neolithic archaeological sites in the Jordanian badia, Wadi al-Qattafi and Wisad Pools. At specific archaeological site-scale our study highlights the importance of aquatic snails in the reconstruction of semi-arid environments. At Wisad pools rare aquatic snails in contexts dating between ~8.0 and ~7.6 ka demonstrate episodes of wetness; moreover, their shell isotopic compositions indicate that local watercourses were well established, corroborating previous findings that during this period the immediate environs of Wisad Pools were host to C3 plant species more typical of the Mediterranean zone. Moreover, the δ18O signal in these snail shells allow tentative reconstruction of rainwater isotopic compositions and identify the effects of evaporation. Such fine-grained environmental information is much less evident from the terrestrial snail shell data alone, showing that an ensemble of snail shell data can be highly sensitive to environmental differentials across an archaeological site. Finally, at a regional palaeoclimate-scale our Wisad Pools snail shell stable isotope data are consistent with a sustained, Rapid Climate Change (RCC)-driven wetness between 8.6 and 7.6 ka concurrent with cold and wet conditions in the wider Levant

    Deep‐water sediment transport patterns and basin floor topography in early rift basins: Plio‐Pleistocene syn‐rift of the Corinth Rift, Greece

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    Our current understanding on sedimentary deep‐water environments is mainly built of information obtained from tectonic settings such as passive margins and foreland basins. More observations from extensional settings are particularly needed in order to better constrain the role of active tectonics in controlling sediment pathways, depositional style and stratigraphic stacking patterns. This study focuses on the evolution of a Plio‐Pleistocene deep‐water sedimentary system (Rethi‐Dendro Formation) and its relation to structural activity in the Amphithea fault block in the Corinth Rift, Greece. The Corinth Rift is an active extensional basin in the early stages of rift evolution, providing perfect opportunities for the study of early deep‐water syn‐rift deposits that are usually eroded from the rift shoulders due to erosion in mature basins like the Red Sea, North Sea and the Atlantic rifted margin. The depocentre is located at the exit of a structurally controlled sediment fairway, approximately 15 km from its main sediment source and 12 km basinwards from the basin margin coastline. Fieldwork, augmented by digital outcrop techniques (LiDAR and photogrammetry) and clast‐count compositional analysis allowed identification of 16 stratigraphic units that are grouped into six types of depositional elements: A—mudstone‐dominated sheets, B—conglomerate‐dominated lobes, C—conglomerate channel belts and sandstone sheets, D—sandstone channel belts, E—sandstone‐dominated broad shallow lobes, F—sandstone‐dominated sheets with broad shallow channels. The formation represents an axial system sourced by a hinterland‐fed Mavro delta, with minor contributions from a transverse system of conglomerate‐dominated lobes sourced from intrabasinal highs. The results of clast compositional analysis enable precise attribution for the different sediment sources to the deep‐water system and their link to other stratigraphic units in the area. Structures in the Amphithea fault block played a major role in controlling the location and orientation of sedimentary systems by modifying basin‐floor gradients due to a combination of hangingwall tilt, displacement of faults internal to the depocentre and folding on top of blind growing faults. Fault activity also promoted large‐scale subaqueous landslides and eventual uplift of the whole fault block

    Does Large-Scale Ocean Circulation Structure Life History Connectivity in Antarctic Toothfish (Dissostichus mawsoni)?

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    A multidisciplinary approach incorporating otolith chemistry, age data, and numerical Lagrangian particle simulations indicated a single, self-recruiting population of Antarctic toothfish (Dissostichus mawsoni) in the Southeast Pacific Basin (SPB) and Ross Sea, with a life history structured by the large-scale circulation. Chemistry deposited prior to capture along otolith edges demonstrated strong environmental heterogeneity, yet the chemistry in otolith nuclei, deposited during early life, showed no differences. Age data showed only adult fish in catches on the Pacific-Antarctic Ridge in the SPB and structuring of life stages consistent with transport pathways from the northern Ross Sea. Lagrangian particle simulations predicted that early life stages following the flow in the SPB would be transported to areas in the Ross Sea where juveniles are caught, whereas the circulation would facilitate adult movement along the shelf slope and back into the SPB where spawning adults are caught. These results suggest that successfully spawning fish spend only a part of their adult life history in the Ross Sea, areas in the eastern Ross Sea contribute disproportionately to the spawning population, and areas in the southwestern Ross Sea may supply fisheries in the southern Indian Ocean
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