Giant flute-like scour and other erosional features formed by the 1929 Grand Banks turbidity current

Erosional features on the floor of Eastern Valley of the Laurentian Fan, in 2800 m water depth, have been mapped with SeaMARC I side-scan sonar images and Seabeam multi-beam echo-soundings, and were directly observed during a dive with the deep submersible Alvin. The most spectacular feature is a 100-m-deep flute-shaped scour, more than 1 km long. The surrounding valley is floored by an unconsolidated coarse conglomerate, which was moulded into transverse bedforms by the turbidity current that was triggered by the 1929 Grand Banks earthquake. Direct observations and seismic-reflection profiles show that the flute-shaped scour cuts through this conglomerate and into Plio-Pleistocene valley-floor sediments, thereby exposing a section through the 1929 deposit. Application of the Allen defect theory suggests that the flute is unusually deep because general channel-floor erosion was inhibited by the conglomerate veneer. Valley-floor channels typically 1 km wide and 10m deep contain series of closed depressions that occasionally deepen to 30 m. These are also interpreted as erosional scours, analogous to pools cut on the beds of bedrock rivers. The large flute was probably formed by detached flow enlarging an initial scour depression. Such scours probably play an important role in channel-floor erosion, increasing the volume of sediment transported by large turbidity currents.     

Fluid record of rock exhumation across the brittle–ductile transition during formation of a Metamorphic Core Complex (Naxos Island,Cyclades, Greece)

Fluid inclusions trapped in quartz veins hosted by a leucogneiss from the southern part of the Naxos Metamorphic Core Complex (Attic‐Cycladic‐Massif, Greece) were studied to determine the evolution of the fluid record of metamorphic rocks during their exhumation across the ductile/brittle transition. Three sets of quartz veins (V‐M2, V‐BD & V‐B) are distinguished. The V‐M2 and V‐BD are totally or, respectively, partially transposed into the foliation of the leucogneiss. They formed by hydrofracturing alternating with ductile deformation accommodated by crystal‐plastic deformation. The V‐B is discordant to the foliation and formed by fracturing during exhumation without subsequent ductile transposition. Fluids trapped during crystal–plastic deformation comprise two very distinct fluid types, namely a CO₂‐rich fluid and a high‐salinity brine, that are interpreted to represent immiscible fluids generated from metamorphic reactions and the crystallization of magmas respectively. They were initially trapped at ～625 °C and 400 MPa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end‐members with later trapping conditions of ～350 °C and 140 MPa. In contrast, brittle microcracks contain aqueous fluids trapped at 250 °C and 80 MPa. All veins display a similar δ¹³C pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history. In contrast, the δD signature is marked by a drastic difference between (i) V‐M2 and V‐BD veins that are dominated by carbonic, aqueous‐carbonic and high‐salinity fluids of metamorphic and magmatic origin characterized by δD between ?56‰ and ?66‰, and (ii) V‐B veins that are dominated by aqueous fluids of meteoric origin characterized by δD between ?40‰ and ?46‰. The retrograde P–T pathway implies that the brittle/ductile transition separates two structurally, chemically and thermally distinct fluid reservoirs, namely (i) the ductile crust into which fluids originating from crystallizing magmas and fluids in equilibrium with metamorphic rocks circulate through a geothermal gradient of 30 °C km^?1 at lithostatic pressure, and (ii) the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 °C km^?1 at hydrostatic pressure.     

A 12.5-kyr history of vegetation dynamics and mire development with evidence of Younger Dryas larch presence in the Verkhoyansk Mountains, East Siberia, Russia

Werner, K., Tarasov, P. E., Andreev, A. A., Müller, S., Kienast, F., Zech, M., Zech, W. & Diekmann, B. 2009: A 12.5‐kyr history of vegetation dynamics and mire development with evidence of Younger Dryas larch presence in the Verkhoyansk Mountains, East Siberia, Russia. Boreas, 10.1111/j.1502‐3885.2009.00116.x. ISSN 0300‐9483. A 415 cm thick permafrost peat section from the Verkhoyansk Mountains was radiocarbon‐dated and studied using palaeobotanical and sedimentological approaches. Accumulation of organic‐rich sediment commenced in a former oxbow lake, detached from a Dyanushka River meander during the Younger Dryas stadial, at ～12.5 kyr BP. Pollen data indicate that larch trees, shrub alder and dwarf birch were abundant in the vegetation at that time. Local presence of larch during the Younger Dryas is documented by well‐preserved and radiocarbon‐dated needles and cones. The early Holocene pollen assemblages reveal high percentages of Artemisia pollen, suggesting the presence of steppe‐like communities around the site, possibly in response to a relatively warm and dry climate ～11.4–11.2 kyr BP. Both pollen and plant macrofossil data demonstrate that larch woods were common in the river valley. Remains of charcoal and pollen of Epilobium indicate fire events and mark a hiatus ～11.0–8.7 kyr BP. Changes in peat properties, C31/C27 alkane ratios and radiocarbon dates suggest that two other hiatuses occurred ～8.2–6.9 and ～6.7–0.6 kyr BP. Prior to 0.6 kyr BP, a major fire destroyed the mire surface. The upper 60 cm of the studied section is composed of aeolian sands modified in the uppermost part by the modern soil formation. For the first time, local growth of larch during the Younger Dryas has been verified in the western foreland of the Verkhoyansk Mountains (～170 km south of the Arctic Circle), thus increasing our understanding of the quick reforestation of northern Eurasia by the early Holocene.     

POST-DRAINAGE ICE DAM RESPONSE AT LAKE MERZBACHER, INYLCHEK GLACIER, KYRGYZSTAN

The Inylchek glacier system in Central Tian Shan, Kyrgyzstan, comprises a large glacier‐dammed lake which usually drains once a year through a subglacial drainage system. Detailed GPS measurements on the ice dam and the analysis of Aster scenes from several subsequent years provide insight into the post‐drainage dam response and the changed ice dynamic conditions. We demonstrate that during high water levels in the lake a large part of the ice dam is afloat, lifting the ice surface up to almost 20 m in the central dam region. During this phase of extensive flotation strong calving is facilitated, which is supported by the high density of ice debris in the lake. In general, surface ice velocities are about 1.5‐2 times higher during summer than winter. Closer to the lake, however, ice velocities increase considerably after the drainage event, showing values more than three times the annual mean. The increased mass flux during the phase of high lake level needs to be compensated by replenishment of the lost ice from the dam. Therefore the ice velocities show compressive flow during the remaining part of the year. These results show that Southern Inylchek glacier is strongly influenced by the existence of the lake.     

THE EFFECTIVE STRESS PRINCIPLE: INCREMENTAL OR FINITE FORM?

Different expressions of the effective stress principle can be found in the literature, in particular some are written in finite form and others in incremental form. For the purpose of the paper we take for granted that stress–strain relationships exist or can be obtained for the effective stress coming from both formulations. We investigate the consequences of the choice of particular finite or differential forms when they are introduced in a weak form of the linear momentum balance equation of two- of three-phase porous media for its numerical solution. For partially saturated geomaterials the importance of the capillary pressure–saturation relationship is pointed out.     

Sediment transport to the Laptev Sea (Siberian Arctic) during the Holocene — evidence from the heavy mineral composition of fluvial and marine sediments

Heavy mineral studies of East Siberian river sediments, Laptev Sea surface sediments, and a sediment core of the western Laptev Sea were carried out in order to reconstruct the pathways of modern and ancient sediment transport from the Siberian hinterland to the Laptev Sea. The modern heavy mineral distribution of Laptev Sea surface sediments reflects mainly the riverine input. While the eastern and central part of the Laptev Sea is dominated by amphibole, which is supplied by the Lena River, the western part is dominated by pyroxene imported from the Siberian Trap basalts by the Khatanga River. The distribution of garnet and opaque minerals is additionally influenced by hydrodynamic processes. As a consequence of their high density, these minerals are predominantly deposited close to the river mouths. Heavy mineral and sedimentological studies of a sediment core of the western Laptev Sea were applied to reconstruct the postglacial history of the shelf area during the last 11 ka. In the lowermost interval of the core (> c. 10 ka), high accumulation rates and a heavy mineral composition similar to that of the modern Khatanga river indicate fluvial conditions. Additionally, the high mica content in this interval may indicate meltwater inflow from the Byrranga mountains. Strong variations in accumulation rates, grain-size distribution, and heavy mineral composition are observed in the time interval between c. 10 and 6 ka, which represents the main transgression of the Laptev Sea shelf. During the uppermost interval (<6 ka), rather stable conditions similar to the modem situation prevailed.