mis 3 to mis 1 temporal and lgm spatial variability in arctic ocean sea ice cover reconstruction from biomarkers using the sea ice proxy ip25 and phytoplankton derived biomarkers brassicasterol and dinosterol arctic sea ice conditions were reconstructed for marine isotope stage mis 3 to 1 with special emphasis on the last glacial maximum lgm in sediment cores from the northern barents sea continental margin across the central arctic ocean to the southern mendeleev ridge our results suggest more extensive sea ice cover than present day during latter part of mis 3 increasing sea ice growth during mis 2 and decreased sea ice cover during the last deglacial the summer ice edge remained north of the barents sea even during extremely cold i e last glacial maximum lgm as well as warm periods i e bolling allerod during the lgm the western svalbard margin and the northern barents sea margin areas were characterized by high concentrations of both ip25 and phytoplankton biomarkers interpreted as a productive ice edge situation caused by the inflow of warm atlantic water in contrast the lgm central arctic ocean north of 84 n was covered by thick permanent sea ice throughout the year with rare break up indicated by zero or near zero biomarker concentrations the spring summer sea ice margin significantly extended southwards to the laptev sea shelf southern lomonosov ridge and southern mendeleev ridge during the lgm our proxy reconstructions are very consistent with published model results based on the north atlantic arctic ocean sea ice model naosim

distribution of clay minerals in surface sediments from the eastern barents and south western kara seas surface samples from the eastern barents and south western kara seas have been analysed for clay mineralogy transport paths the role of regional sources and local bedrock outcrops and the influence of hydrodynamic and glacigenous processes for clay distribution on the shelves are discussed in relation to central arctic ocean deep sea and sea ice sediments franz josef land and novaya zemlya show significantly different clay mineral associations although smectite concentrations are fairly high franz josef land can be excluded as a source for central arctic sea ice sediments which are relatively rich in smectite in the kara sea smectite concentrations in coastal sediments surpass even the franz josef land concentrations the large cyclonic gyre in the eastern barents sea between novaya zemlya and franz josef land which serves as a mixing zone between arctic and north atlantic water is apparently reflected within the smectite distribution pattern with the exception of franz josef land the area of investigation is typically low in kaolinite in particular coastal areas and areas north of novaya zemlya influenced by the inflow of arctic waters show the lowest kaolinite concentrations a high kaolinite occurrence within the nansen basin is most probably related to franz josef land and emphasizes the importance of long range downslope transport of sediments across the continental slope the surface water circulation pattern in close interaction with local outcrops onshore novaya zemlya and locally restricted occurrences within the eastern barents sea significantly alter the illite dispersal pattern illite concentrations are lowest around franz josef land chlorite is generally low in the area of investigation submarine outcrops and important chlorite occurrences onshore novaya zemlya bias its distribution pattern

mount st helens 1980 and mount pelee 1902 flow or surge the far reaching directed blast of mount st helens and the devastating nuees ardentes of mount pelee produced deposits that in many places consist of three main layers a basal gravelly or sandy layer 1 a massive or bedded ash layer 2 and a capping ash cloud layer 3 these thin and in part landscape mantling deposits are generally ascribed to pyroclastic surges and the dune bedding seen in parts of layer 2 has reinforced this interpretation there are several reasons however for preferring a pyroclastic flow origin 1 much of layer 2 is unequivocal pyroclastic flow 2 the tripartite subdivision and landscape mantling habit is similar to that displayed by ignimbrites of the low aspect ratio type 3 the deflation of a pyroclastic surge and the subsequent deposition of particulate material will tend not to conserve the fine ash and dust that are abundant in valley ponded parts of layer 2 and cannot explain the observed tendency for layer 2 to show stronger fines depletion on ridges than in valley bottoms and 4 the coarseness and the variance of grain size found within dune bedded bed sets in mount st helens layer 2 are like those of very weak pyroclastic surges and the dune bedding most likely resulted from local minor turbulence as could have been caused by surface roughness in a mountainous terrain littered with tree stumps and fallen trees in a thin depositing pyroclastic flow the directed blast and nuees ardentes at mount st helens and mount pelee are interpreted to have been violently emplaced pyroclastic flows producing deposits of low aspect ratio type the characteristics of which stem from an exceptionally high flow velocity

mobility of pyroclastic flows and surges at the soufriere hills volcano montserrat the soufriere hills volcano on montserrat has produced avalanche like pyroclastic flows formed by collapse of the unstable lava dome or explosive activity pyroclastic flows associated with dome collapse generate overlying dilute surges which detach from and travel beyond their parent flows the largest surges partially transform by rapid sedimentation into dense secondary pyroclastic flows that pose significant hazards to distal areas different kinds of pyroclastic density currents display contrasting mobilities indicated by ratios of total height of fall h run out distance l area inundated a and volume transported v dome collapse flow mobilities characterised by either l h or a v2 3 resemble those of terrestrial and extraterrestrial cold rockfalls dade and huppert 1998 in contrast fountain fed pumice flows and fine grained secondary pyroclastic flows travel slower but for comparable initial volumes and heights can inundate greater areas

emplacement of the 18 may 1980 lateral blast deposit ene of mount st helens washington facies variations east northeast of mount st helens preserve a record of depositional processes in the 18 may 1980 lateral blast cloud this paper reports new field grain size and component data from the ene sector of the timber blowdown zone and presents a model for blast flow and sedimentation the first erupted ejecta was rich in juvenile components and extends to the distal blowdown limit the last erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge the blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high density turbidity current the possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns depositional conditions were strongly influenced by the rate of suspended load fallout from the blast within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered settling zone able to detach gravitationally and drain into local depressions the resulting proximal facies resembles a low aspect ratio ignimbrite fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered settling zone component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers with increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions building up antidune like bedforms wavelengths of these bedforms range from 20 to 1 m and decrease away from vent there is a systematic relationship between antidune migration direction and depositional slope the transition from proximal ignimbrite like to distal surge like facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high velocity pyroclastic flows

the interaction of ash flows with ridges using both laboratory experiments and theoretical models we examine the different flow regimes that may develop when an ash flow encounters a ridge for very small ridges all the flow may pass over the ridge for intermediate size ridges the flow may be partially blocked with a fraction of the flow reflected upstream as a travelling bore in this case the remainder of the flow which does pass over the ridge is hydraulically controlled at the ridge crest finally if the ridge is sufficiently high then the flow will be totally blocked new laboratory experiments show that the sedimentation patterns associated with these flow regimes may be very different most importantly flows that involve partial blocking and the formation of upstream propagating bores display enhanced sedimentation upstream of the ridge analogous to valley ponded and caldera fill deposits in contrast under some circumstances if the flow is able to scale a ridge the deposit may be relatively unaffected by the presence of the ridge the minimum ridge height that leads to total blocking of the flow increases with mass eruption rate and has a complex variation with distance from the source in a one dimensional channel the minimum ridge height that causes blocking increases with distance downstream this is because the flow becomes less dense through sedimentation of particles and entrainment of air and so requires less energy to scale a ridge of a particular height in axisymmetric flow the minimum ridge height initially decreases with distance downstream as the flow spreads radially but subsequently increases as the flow becomes less dense through sedimentation and entrainment a new quantitative model of dilute ash flows propagating over ridges indicates that flows with mass fluxes in excess of 108 109 kg s can partially scale barriers as high as 1000 m at distances of tens of kilometres from the source whereas smaller flows are likely to be totally blocked by such an obstacle our results shed new insight on the possible long range transport mechanism of several large flows including the ata fisher and aniakchak pyroclastic flows

clay mineral signature of the nw atlantic boundary undercurrent abstract surface sediments were sampled along 2 transects across the iceland and irminger basins and in the labrador sea clay mineral assemblages deduced from x ray analyses of the carbonate free

the caldera forming eruption of volcan ceboruco mexico of magma erupted 95 of which was deposited as fall layers during most of the deposition of p1 eruptive intensity mass flux was almost constant at 4 8 107 kg s 1 producing a plinian column 25 30 km in height size grading at the top of p1 indicates however that mass flux waned dramatically and possibly that there was a brief pause in the eruption during the post p1 phase of the eruption a much smaller volume of magma erupted although mass flux varied by more than an order of magnitude we suggest that caldera collapse began at the end of the p1 phase of the eruption because along with the large differences in mass flux behavior between p1 and post p1 layers there were also dramatic changes in lithic content p1 contains 8 lithics post p1 layers contain 30 60 and magma composition p1 is 98 rhyodacite post p1 layers are 60 90 rhyodacite however the total volume of magma erupted during the jala pumice event is close to that estimated for the caldera these observations appear to conflict with models which envision that after an eruption is initiated by overpressure in the magma chamber caldera collapse begins when the reservoir becomes underpressurized as a result of the removal of magma the conflict arises because firstly the p1 layer makes up too large a proportion 75 of the total volume erupted to correspond to an overpressurized phase and secondly the caldera volume exceeds the post p1 volume of magma by at least a factor of three the mismatches between model and observations could be reconciled if collapse began near the beginning of the eruption but no record of such early collapse is evident in the tephra sequence the apparent inability to place the jala pumice eruptive sequence into existing models of caldera collapse which were constructed to explain the formation of calderas much greater in volume than that at ceboruco may indicate that differences in caldera mechanics exist that depend on size or that a more general model for caldera formation is needed

the 15 september 1991 pyroclastic flows at unzen volcano japan a flow model for associated ash cloud surges large scale collapse of a dacite dome in the late afternoon of 15 september 1991 generated a series of pyroclastic flow events at unzen volcano pyroclastic flows with a volume of 1 106 m3 as dre descended the northeastern slope of the volcano changing their courses to the southeast due to topographic control after they exited a narrow gorge an ash cloud surge rushed straight ahead detaching the main body of the flow that turned and followed the topographic lows to the east the surge swept the kita kamikoba area which had been devastated by the previous pyroclastic flow events and transported a car as far as 120 m following detachment the surge lost its force after it moved several hundred meters but maintained a high temperature the deposits consist of a bottom layer of better sorted ash unit 1 a thick layer of block and ash unit 2 and a thin top layer of fall out ash unit 3 unit 2 overlies unit 1 with an erosional contact the upper part of unit 2 grades into better sorted ash at distal block and ash flow deposits the bottom part of unit 2 also consists of better sorted ash and the contact with the unit 1 deposits becomes ambiguous video footage of cascading pyroclastic flows during the 1991 1995 eruption traveling over surfaces without any topographic barriers revealed that lobes of ash cloud protruded intermittently from the moving head and sides and that these lobes surged ahead on the ground surface this fact together with the inspection by helicopter shortly after the events suggests that the protruded lobes consisted of better sorted ash and resulted in the deposits of unit 1 the highest ash cloud plume at the oshigadani valley exit and the thickest deposition of fall out ash over kita kamikoba and ohnokoba indicate that abundant ash was also produced when the flow passed through a narrow gorge in the model presented here the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration main body an overriding and intermediate fluidization zone and an overlying dilute cloud release of pressurized gas in lava block pores due to collisions among blocks and the resulting upward current caused a zone of fluidization just above the main body the mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current where the ash was deposited as surge deposits the main body which had high internal friction and shear near its base then overran the surge deposits partially eroding them when the upward current of gas fluidization waned better sorted ash suspended in the fluidization zone was deposited on block and ash deposits in the distal places of block and ash deposits unit 2 probably was deposited in non turbulent fashion without any erosion of the underlying layer unit 1

character paleoenvironment rate of accumulation and evidence for seismic triggering of holocene turbidites canada abyssal plain arctic ocean abstract four box cores and one piston core show that holocene sedimentation on the southern canada abyssal plain for the last 8010 120 yr has consisted of a continuing rain of pelagic organic and ice rafted clastic sediment with a net accumulation rate during the late holocene of 10 mm 1000 yr and episodically emplaced turbidites 1 5 m thick deposited at intervals of 830 to 3450 yr average 2000 yr the average net accumulation rate of the mixed sequence of turbidites and thin pelagite interbeds in the cores is about 1 2 m 1000 yr physiography suggests that the turbidites originated on the mackenzie delta or its clinoform and 13c values of 27 to 25 in the turbidites are compatible with a provenance on a delta extant displaced neritic and lower slope to basin plain calcareous benthic foraminifers coexist in the turbidite units their joint occurence indicates that the turbidites originated on the modern continental shelf and entrained sediment from the slope and rise enroute to their final resting place on the canada abyssal plain the presence of middle pleistocene diatoms in the turbidites suggests in addition that the turbidites may have originated in shallow submarine slides beneath the upper slope or outer shelf small but consistent differences in organic carbon content and 13c values between the turbidite units suggest that they did not share an identical provenance which is at least compatible with an origin in slope failures the primary provenance of the ice rafted component of the pelagic beds was the glaciated terrane of northwestern canada and the provenance of the turbidite units was pleistocene and holocene sedimentary deposits on the outer continental shelf and upper slope of the mackenzie delta largely local derivation of the sediment of the canada abyssal plain indicates that sediment accumulation rates in the arctic ocean are valid only for regions with similar depositional sources and processes and that these rates cannot be extrapolated regionally the location of an elliptical zone of active seismicity over the inferred provenance of the turbidites suggests that they were triggered by large earthquakes distal turbidite sediment accumulation rates were more than two orders of magnitude greater than pelagic sediment accumulation rates on the canada abyssal plain during the last 8000 years this disparity reconciles the discrepancy between the high accumulation rates assumed by some for the arctic ocean because of the numerous major rivers and large ice sheets that discharge into this small mediterranean basin and the low pelagic sedimentation rates that have been reported from the arctic ocean

distribution of chlorite and kaolinite in eastern atlantic sediments off north africa chlorite in the atlantic sediments is predominantly derived from the atlas mountains kaolinite originates in the southern sahara and sahel zone these minerals may be used to reconstruct the tracks of two superimposed wind systems trade winds and harmattan wind a modified method for determining relative abundances of chlorite and kaolinite is presented

smectite composition as a tracer of deep circulation the case of the northern north atlantic abstract the link between smectite composition in sediments from the northern north atlantic and labrador sea and deep circulation is being further investigated through detailed studies of the x ray pattern of smectites and cation saturations this allows clear distinction of dominant terrigenous sources associated to the main components of the modern western boundary undercurrent time variations of smectite characteristics in two piston cores from the inlet and outlet of the western boundary undercurrent gyre in the labrador sea indicate 1 a more southern circulation of north east atlantic deep water during the late glacial 2 a step by step transition to the modern pattern of deep circulation during the late glacial holocene transition with intensification of north east atlantic deep water and davis strait overflow 3 an expansion of davis strait overflow and labrador sea water circulation in relation to ice surges and deposition of detrital layers 4 an intensified circulation of north east atlantic deep water during the younger dryas and 5 a very recent increased influence of denmark strait overflow water beginning between 4 4 and

influences of urban fabric on pyroclastic density currents at pompeii italy 1 flow direction and deposition 1 to assess ways in which the products of explosive eruptions interact with human settlements we performed volcanological and rock magnetic analyses on the deposits of the a d 79 eruption at the pompeii excavations italy during this eruption the roman town of pompeii was covered by 2 5 m of fallout pumice and then partially destroyed by pyroclastic density currents pdcs anisotropy of magnetic susceptibility measurements performed on the fine matrix of the deposits allowed the quantification of the variations in flow direction and emplacement mechanisms of the parental pdcs that entered the town these results integrated with volcanological field investigations revealed that the presence of buildings still protruding through the fallout deposits strongly affected the distribution and accumulation of the erupted products all of the pdcs that entered the town even the most dilute ones were density stratified currents in which interaction with the urban fabric occurred in the lower part of the current the degree of interaction varied mainly as a function of obstacle height and density stratification within the current for examples the lower part of the eu4pf current left deposits up to 3 m thick and was able to interact with 2 to 4 m high obstacles however a decrease in thickness and grain size of the deposits across the town indicates that even though the upper portion of the current was able to decouple from the lower portion enabling it to flow over the town it was not able to fully restore the sediment supply to the lower portion in order to maintain the deposition observed upon entry into the town

basaltic volcanism and extension near the intersection of the sierra madre volcanic province and the mexican volcanic belt three groups miocene pliocene and pleistocene off oceanic type basalts associated with normal faulting have been identified in an area surrounding the city of guadalajara mexico although most basalts within these groups have compositional characteristics of an asthenospheric source each group is also associated with lavas that have subduction related traits the first group the san cristobal plateau basalts has a minimum volume of 1 800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre existing extensional basin of unknown configuration 10 m y ago the relatively rapid eruption of these basalts along with their volume structural association and chemistry suggest an upwelling mantle plume beneath the guadalajara region this region may be the earliest indication of the separation of the jalisco block from north america the second basalt group the guadalajara basalts consists of small volumes of porphyritic basalts and basaltic andesites 3 3 5 0 ma in age that outcrop near the northern outskirts of guadalajara and near the town of hostotip aquillo to the northwest the youngest basalts in the guadalajara area are the 0 4 1 4 ma santa rosa suite of basalts hawaiites mugearites close to the contemporary 0 2 ma andesitic central volcano v tequila these either flowed from nearby cinder cones toward the rio santiago canyon or erupted in the canyon to dam the river the flat lying tertiary 23 27 ma ash flows of the sierra madre volcanic province that lie to the north of guadalajara are faulted north northeast into horsts and grabens that in one case can be dated as being older than 21 8 ma this fault trend has been reactivated closer to guadalajara as normal faults 200 to 300 m displacement cut the miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city also hidden by the young volcanic succession there is the north west southeast fault zone that extends to the gulf of california as the tepic zacoalco graben system faulting of this zone is well displayed in the region of the santa rosa dam in the santiago canyon northwest of guadalajara two styles of faulting are found there an older 1 ma 3 has been approximately north northeast for at least 4 m y characteristic of the mexican basin and range province

revised stratigraphy and eruption rates of ceboruco stratovolcano and surrounding monogenetic vents nayarit mexico from historical documents and new radiocarbon dates abstract more than a dozen new radiocarbon dates reconstruct the eruptive history of ceboruco volcano six of these further constrain previous results for the important plinian jala eruption which occurred near 1060 55 yr bp a calibrated radiocarbon age of ad 990 1020 was obtained as best overlap range for all samples pottery fragments found directly underneath the pumice deposit indicate that this area was inhabited by human populations that witnessed the eruption this age therefore represents an important time marker in the prehistory of this region because an area of 560 km2 was devastated and covered by a thickness of 50 cm of pumice and ash fallout after the prominent jala eruption vei 6 at least seven major lava flows and several smaller domes were issued from ceboruco s crater analysis of historical documents allows us to conclude that most of these eruptions took place well before the arrival of the spanish conquerors to this area in 1528 ceboruco s last historic eruption 1870 1872 produced a ca 7 km long viscous dacite lava flow its emplacement was accompanied by block and ash flow activity and deposition of ash fallout because a repeat of such an eruption in the future would seriously endanger the nearby population and disrupt important life lines a detailed discussion of eyewitness accounts and other documents including drawings and paintings is provided some of these materials are made available to the broader public for the first time in the present article several surrounding monogenetic vents were previously dated by others using the k ar and the ar ar methods because these dating methods are often not suitable for very young rocks we also dated several holocene monogenetic vents by the radiocarbon method these dates together with geologic mapping allowed calculating eruption rates and recurrence intervals of different types of eruptions accordingly an andesite dacite lava flow accompanied by block and ash flow activity was erupted from ceboruco s crater on average every 143 years during the last 1000 years in contrast a monogenetic eruption forming either a scoria cone or a dome has occurred in the area surrounding ceboruco on average every 1000 years during the holocene during the period between ad 1000 and ad 1500 ceboruco displayed an elevated activity experiencing a rhyodacite plinian eruption 3 4 km 3 dre followed by the emplacement of six andesitic dacitic lava flows also with a total volume of 3 4 km 3 all these figures indicate that the ceboruco volcanic region is one of the most active areas in the entire trans mexican volcanic belt consequently production of a detailed volcanic hazard map and implementation of other preventive measures aimed at reducing potential losses in case of renewed volcanic activity should be mandatory

late quaternary coccoliths at the north pole evidence of ice free conditions and rapid sedimentation in the central arctic ocean calcareous nannofossils from approximately the past 7000 yr of the holocene and from oxygen isotope stage 5 are present at 39 analyzed sites in the central arctic ocean this indicates partly ice free conditions during at least some summers the depth of holocene sediments in the nansen basin is about 20 cm or more where influenced by turbidites on the gakkel ridge calcite preservation is excellent and holocene strata are consistently documented to be at least 4 to 20 cm thick in the amundsen basin preservation of nannofossils is sometimes poor holocene specimens are commonly only found at the surface although at one site they extend down to 26 cm the holocene assemblage is consistently recorded in the upper 5 to 20 cm of sediment on the lomonosov ridge and down to 15 cm depth in the makarov basin this implies that linear sedimentation rates for the holocene in all investigated areas are centimetres per thousand years sediments from the last glacial period oxygen isotope stages 2 4 are relatively thin suggesting significantly reduced sedimentation millimetres per thousand years during this time

organic carbon carbonate and clay mineral distributions in eastern central arctic ocean surface sediments abstract results from a detailed sedimentological investigation of surface sediments from the eastern arctic ocean indicate that the distribution of different types of sediment facies is controlled by different environmental processes such as sea ice distribution terrigenous sediment supply oceanic currents and surface water productivity in comparison to other open ocean environments total organic carbon contents are high with maximum values in some deep basin areas as well as west and north of svalbard in general the organic carbon fraction is dominated by terrigenous material as indicated by low hydrogen index values and high c n ratios probably transported by currents and or sea ice from the eurasian shelf areas the amount of marine organic carbon is of secondary importance reflecting the low productivity environment described for the modern ice covered arctic ocean in the area north of svalbard some higher amounts of marine organic matter may indicate increased surface water productivity controlled by the inflow of the warm westspitsbergen current wsc into the arctic ocean and reduced sea ice cover this influence of the wsc is also supported by the high content of biogenic carbonate recorded in the yermak plateau area the clay mineral distribution gives information about different source areas and transport mechanisms illite the dominant clay mineral in the eastern central arctic ocean sediments reaches maximum values in the morris jesup rise area and around svalbard indicating north greenland and svalbard to be most probable source areas kaolinite reaches maximum values in the nansen basin east of svalbard and in the barents sea possible source areas are mesozoic sediments in the barents sea and franz josef land in contrast to the high smectite values determined in sea ice samples smectite contents are generally very low in the underlying surface sediments suggesting that the supply by sea ice is not the dominant mechanism for clay accumulation in the studied area of the modern central arctic ocean

global large deep focus earthquakes source process and cascading failure of shear instability as a unified physical mechanism abstract we apply a multiple source inversion method to systematically study the source processes of 25 large deep focus depth 400 km earthquakes with mw 7 0 from 1994 to 2012 based on waveform modeling of p pp sh and ssh wave data the earthquakes are classified into three categories based on spatial distributions and focal mechanisms of the inferred sub events 1 category one with non planar distribution and variable focal mechanisms of sub events represented by the 1994 mw 8 2 bolivia earthquake and the 2013 mw 8 3 okhotsk earthquake 2 category two with planar distribution but focal mechanisms inconsistent with the plane including eighteen earthquakes and 3 category three with planar distribution and focal mechanisms consistent with the plane including six earthquakes we discuss possible physical mechanisms for earthquakes in each category in the context of plane rupture transformational faulting and shear thermal instability we suggest that the inferred source processes of large deep focus earthquakes can be best interpreted by cascading failure of shear thermal instabilities in pre existing weak zones with the perturbation of stress generated by a shear instability triggering another and focal mechanisms of the sub events controlled by orientations of the pre existing weak zones the proposed mechanism can also explain the observed great variability of focal mechanisms the presence of large values of clvd compensated linear vector dipole and the super shear rupture of deep focus earthquakes in the previous studies in addition our studies suggest existence of relationships of seismic moment source duration 3 and moment source dimension 3 in large deep focus earthquakes

glaciomarine sedimentation in epicontinental seas exemplified by the northern barents sea abstract sediments on high arctic shelves result from modern processes and the effect of former glaciations based on data from the northern barents sea an area with input from large and numerous surging glaciers we define two principal zones with different environmental regimes and corresponding sedimentary facies 1 a glacier proximal zone influenced by grounding line processes and the immediately adjacent areas affected by glacial sediment input and 2 a glacier distal sea ice and current controlled zone which also includes a wide sediment starved region dominated by biogenic carbonate accumulation characteristic of the glacier proximal zone are glacial surges which affect sedimentation rates and leave a diagnostic pattern of sea floor morphologies extensive ice gouging causes a homogeneous sediment texture in the glacier distal zone fine grained mud supplied from sea ice and infrequent coarser material deposited from icebergs is reworked by modern oceanographic processes on shallow banks in 30 50 m of water carbonates accumulate from a prolific bottom fauna formed in response to extensive reworking and nutrient supply

sediments in arctic sea ice implications for entrainment transport and release despite the arctic sea ice cover s recognized sensitivity to environmental change the role of sediment inclusions in lowering ice albedo and affecting ice ablation is poorly understood sea ice sediment inclusions were studied in the central arctic ocean during the arctic 91 expedition and in the laptev sea east siberian arctic region expedition 1992 results from these investigations are here combined with previous studies performed in major areas of ice ablation and the southern central arctic ocean this study documents the regional distribution and composition of particle laden ice investigates and evaluates processes by which sediment is incorporated into the ice cover and identifies transport paths and probable depositional centers for the released sediment in april 1992 sea ice in the laptev sea was relatively clean the sediment occasionally observed was distributed diffusely over the entire ice column forming turbid ice observations indicate that frazil and anchor ice formation occurring in a large coastal polynya provide a main mechanism for sediment entrainment in the central arctic ocean sediments are concentrated in layers within or at the surface of ice floes due to melting and refreezing processes the surface sediment accumulation in central arctic multi year sea ice exceeds by far the amounts observed in first year ice from the laptev sea in april 1992 sea ice sediments are generally fine grained although coarse sediments and stones up to 5 cm in diameter are observed component analysis indicates that quartz and clay minerals are the main terrigenous sediment particles the biogenous components namely shells of pelecypods and benthic foraminiferal tests point to a shallow benthic marine source area apparently sediment inclusions were resuspended from shelf areas before and incorporated into the sea ice by suspension freezing clay mineralogy of ice rafted sediments provides information on potential source areas a smectite maximum in sea ice sediment samples repeatedly occurred between 81 n and 83 n along the arctic 91 transect indicating a rather stable and narrow smectite rich ice drift stream of the transpolar drift the smectite concentrations are comparable to those found in both laptev sea shelf sediments and anchor ice sediments pointing to this sea as a potential source area for sea ice sediments in the central arctic ocean sea ice clay mineralogy is significantly different from deep sea clay mineral distribution patterns the contribution of sea ice sediments to the deep sea is apparently diluted by sedimentary material provided by other transport mechanisms

reappraisal of the 1982 eruptions of el chichon volcano chiapas mexico new data from proximal deposits additional data from proximal areas enable a reconstruction of the stratigraphy and the eruptive chronology of phases iii and iv of the 1982 eruption of el chichon volcano phase iii began on 4 april at 0135 gmt with a powerful hydromagmatic explosion that generated radially fast moving 100 ms 1 pyroclastic clouds that produced a surge deposit s1 due to the sudden reduction in the confining pressure the process continued by tapping of magma from a deeper source causing a new explosion the ejected juvenile material mixed with large amounts of fragmented dome and wall rock which were dispersed laterally in several pulses as lithic rich block and ash flow f1 partial evacuation of juvenile material from the magmatic system prompted the entrance of external water to generate a series of hydromagmatic explosions that dispersed moisture rich surge clouds and small volume block and ash flows iu up to distances of 3 km from the crater the eruption continued by further decompression of the magmatic system with the ensuing emission of smaller amounts of gas rich magma which with the strong erosion of the volcanic conduit formed a lithic rich plinian column that deposited fallout layer b associated with the widening of the vent an increase in the effective density of the uprising column took place causing its collapse block and ash flows arising from the column collapse traveled along valleys as a dense laminar flow f2 in some places flow regime changes due to topographic obstacles promoted transformation into a turbulent surge s2 which attained minimum velocities of approximately 77 ms 1 near the volcano the process continued with the formation of a new column on 4 april at 1135 gmt phase iv that emplaced fall deposit c and was followed by hydromagmatic explosions which produced pyroclastic surges s3

a shift in heavy and clay mineral provenance indicates a middle miocene onset of a perennial sea ice cover in the arctic ocean during the arctic coring expedition acex a 428 m thick sequence of upper cretaceous to quaternary sediments was penetrated the mineralogical composition of the upper 300 m of this sequence is presented here for the first time heavy and clay mineral associations indicate a major and consistent shift in provenance from the barents kara western laptev sea region characterized by presence of common clinopyroxene to the eastern laptev east siberian seas in the upper part of the section characterized by common hornblende amphibole sea ice originating from the latter source region must have survived at least one summer melt cycle in order to reach the acex drill site if considering modern sea ice trajectories and velocities this shift in mineral assemblages probably represents the onset of a perennial sea ice cover in the arctic ocean which occurred at about 13 ma thus suggesting a coeval freeze in the arctic and antarctic regions

mt pelee martinique may 8 and 20 1902 pyroclastic flows and surges abstract the distribution stratigraphic relationships and fragmental components of the may 8 and 20 1902 pyroclastic flows from mt pelee martinique together with eyewitness accounts suggest the following explanation for those eruptions the eruptions were vertically directed magmatic perhaps initiated phreatically and contained abundant juvenile lithics from congealed magma of the dome and neck this resulted in a two part eruption column having 1 a dense lithic charged part which collapsed into the crater and flowed out of a pre existing notch in its side giving rise to pyrochlastic flows and 2 a magmatically derived column containing gases juvenile vitric material and crystals which largely by passed the neck and dome and escaped into the atmosphere all of the energy of the flows was apparently focused through the notch they emerged fully turbulent and flowed down riviere blanche gravity segregation of large and abundant fragments soon resulted in a dense high concentration poorly fluidized block and ash flow confined to the valley leaving above a fully turbulent high energy ash cloud surge as the ash cloud surge moved down the mountain it continued to expand outward the process of gravity segregation continued as the ash cloud surge expanded resulting in secondary block and ash underflows toward st pierre the secondary block and ash flows developed on a gently sloping upland surface 100 m or more above the valley of riviere blanche the turbulent fragment depleted surges above the secondary block and ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m including st pierre the surges refracted around obstacles and in one place moved up a small valley in a direction opposite to the main flows

a gravity current model for the may 18 1980 mount st helens plume observations of the stratospheric plume from the may 18 1980 mount st helens eruption suggest that it spread in the crosswind direction as an intrusive gravity current as it was transported downwind grain size analyses of the plinian tephra are consistent with this model suggesting that to distances of many hundreds of kilometers turbulent atmospheric diffusion played a secondary role in plume spreading and tephra dispersal

sources and dispersal patterns of clay minerals in surface sediments from the continental shelf areas off alaska the three mineral suites were recognized in the gulf of alaska the glaciomarine sediments of fjords and the eastern shelf consist typically of the primary phyllosilicates micas and chlorites derived locally from argillite graywacke and metavolcanics under conditions of mild chemical weathering more glycol expandable clay minerals gecm in the north gulf substantiate clay supply to that region primarily from the yakataga and poul creek formations and to a lesser extent from the copper river the northern gulf clay mineral suite is transported to central prince william sound and northeast kodiak shelf by the alaska coastal current in the western gulf clay minerals adjacent to the alaska peninsula are derived principally from the susitna river via cook inlet whereas those of the outer shelf presumably have their provenance in kodiak island and the katmai ash submarine deposit in the southeast bering sea a progressive northwestward decrease in gecm from 65 to 11 is caused by a gradually decreased sediment export from the alaska peninsula and the aleutians with a possible minor increase in illite from the kuskokwin river the yukon river is the predominant clay source for the north bering sea as signified by an abrupt increase in that area of gecm 20 to 35 and of kaolinite chlorite ratios clay mineralogy indicates that the central chukchi sea is the major repository of the yukon clays suggesting that clay mineral stratigraphic studies have implications for interpretation of the quaternary paleogeography of beringia in the beaufort sea nearshore influence of different fluvial inputs and their dispersals by currents are sharply defined the lack of a definite pattern to the distribution of clay minerals in the outer continental shelf may be due to haphazard transport of clays by ice rafting and or reworking and redistribution of sediments resulting from ice gouging of the sea substrate movement of sediment to the west in the alaskan portion of the inner shelf of beaufort sea is evident an apparent seaward increase in illite gecm ratio in the colville delta is tentatively ascribed to regeneration of degraded illite micas by k fixation in saline waters an isolated area of sediment rich in expandable clay minerals in the central beaufort shelf presumably is relict

lipid distribution in surface sediments from the eastern central arctic ocean abstract during the arctic 91 expedition with rv polarstern numerous short multicorer and long cores kastenlot cores including surface sediments were recovered along a transect crossing the eastern part of the arctic ocean in this paper we present data on the concentration and distribution of short c17 c19 and long chain c27 c29 c31 n alkanes as well as fatty acids 16 0 16 1 18 0 18 1 in the surface sediments from this transect these compounds among others have been used to distinguish between autochthonous marine organic matter and allochthonous terrigenous organic matter supply changes in short chain n alkane as well as fatty acid contents of the sediments allow us to distinguish between regions characterized by higher or lower marine productivity organic carbon contents of the surface sediments are generally high and vary between 0 3 and 2 in general the high organic carbon content of the sediments is derived from a high input of terrigenous organic matter marine productivity in the arctic ocean is mainly controlled by reduced sea ice cover which allows a higher production rate of phytoplankton regions with a higher marine organic matter content are the area nw of svalbard the yermak plateau a part of the lomonosov ridge and makarov basin and the morris yesup rise the pattern of distribution of long chain n alkanes in arctic ocean surface sediments is mainly controlled by sea ice and or direct transport from the adjacent shelf areas although dilution by turbidites influences the deep basins

stratified flow in pyroclastic surges stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions particle transport is assumed to be by turbulent suspension and calculations contained herein show that this is likely for many cases including the 18 may 1980 blast at mount st helens the discussion centers on the rouse number pn which represents a ratio of particle settling velocity to scale of turbulence the brunt vaisala frequency n which is the maximum possible frequency of internal waves the froude number fr representing the ratio of inertial forces to gravitational forces and the richardson number ri a ratio of buoyant restoring forces to turbulent mixing forces the velocity or flow power dependence of bed form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge this produces a decrease in dune wavelength with increasing distance from vent migration direction of bed forms is related tofr as it is defined for a continuously stratified flow proximal to distal facies variations in surge deposits reflect increasingpn andri as the flows move away from their sources this produces the progression from sandwave to massive to planar facies with increasing distance from vent where the long axis of topography is at low angles to the flow direction massive facies in topographic lows may from concurrently with sandwave facies on highs due to the higher particle concentration in the lows where long axis of topography is at high angles to flow direction denser lower parts of the surge may be dammed or blocked blocked material tends to form massive flows that may move down slope independent of the overriding surge a model incorporating turbulent transport stratified flow and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers during the initial high energy waxing phase of the eruptive event pn is sufficiently low that only coarse but poorly sorted material is deposited to form relatively coarse bottom layers as the event wanes remaining finer material is deposited through a thin bed load to produce overlying bedded and cross bedded veneer deposits throughout most of the event blocking occurs to produce relatively thick and massive deposits in valley bottoms

recent sediment accumulation rates for the western margin of the barents sea abstract we investigate patterns processes and rates of sediment accumulation within the marginal ice zone of the western barents sea the barents sea is among the most productive of the arctic marginal seas with approximately half of the sediment burial flux derived from marine rather than terrestrial sources sediment accumulation rates were quantified by 210 pb geochronology at 14 stations ranging in water depths from 173 to 503 m along a south north latitudinal gradient of 6 75 81 n the average sediment accumulation rate for all stations is 0 7 0 4 mm yr in general lower sediment accumulation rates are associated with coarser sediment fractions at shallower water depths 200 m where currents remove fine grained sediments and transport material to the deeper regions higher sediment accumulation rates are detected in deeper water stations and are largely associated with specific sedimentation events and or features of the barents seafloor although the c n ratio of surface sediments 0 2 cm from all stations indicates a predominance of sediments of marine origin c n 9 0 1 0 there is no distinct depositional pattern indicating preferential burial in areas more heavily influenced by the marginal ice zone we conclude that a combination of mixed sediment sources large shifts in the location of the marginal ice zone over time and the benthic boundary layer processes obscure the productivity signal preserved in these seafloor sediment deposits of the western barents sea

late quaternary sedimentation and glacial history of the western svalbard continental margin abstract glacigenic sediments recovered in shallow cores from the western svalbard continental slope are subdivided into five facies associations based on grain size sedimentary structure mineralogy petrography and geochemistry two diamicton facies are recognised one of which is interpreted as hemipelagic mud with variable amounts of ice rafted debris ird and the other as a product of mass movement the diamictons are associated with melting of icebergs during glacial melt events and debris flow deposition on the submarine fans during peak glaciation respectively laminated to layered mud and turbidites seem to be closely related to eustatic fall in sea level and erosion of banks located on the continental shelf resulting in accumulation of fine grained organic rich deposits and thin silt and sand layers on the continental slope middle weichselian was characterised by several phases of extensive iceberg production and input of ird the first phase 60 55 ka is correlated with a glacier advance on svalbard but the following phases 54 44 ka can not be correlated with glacier advances on land this demonstrates that there may have been more glaciations than recorded on svalbard and that ice sheets at high northern latitudes fluctuated more frequently than previously assumed the composition of the ird deposited after 54 ka suggests that the ice sheets during these advances were located in the eastern svalbard barents sea area the late weichselian growth of the svalbard barents sea ice sheet occurred probably in two steps nucleation of a continental based ice sheet between 27 and 22 5 ka followed by a rapid advance to the shelf edge when the ice margin reached the soft clay rich sediments on the continental shelf eustatic lowering of the sea level during the initial phase led to bank exposure and erosion bringing large quantities of fine grained organic rich sediments to the deep sea during peak glaciation glaciers provided a localised supply of sediments to the shelf troughs and the upper slope which were redistributed by debris flows the marine based svalbard barents sea ice sheet started to retreat around 14 5 ka through massive iceberg discharge the retreat was interrupted by a short lived advance accompanied by erosion of the shelf banks and redistribution of organic rich sediments to the continental slope the final ice recession started around 12 ka and ended close to 9 ka when the fjords of svalbard were essentially ice free large amounts of meltwater released during the removal of the svalbard barents sea ice sheet cooled down the surface waters of the norwegian greenland sea and formed a widespread sea ice cover between 12 and 8 ka input of terrigenous material was greatly reduced during the early part of holocene the present interglacial is characterised by erosion of the shelf and upper slope by bottom currents leaving a prominent lag deposit

geochemical and mineralogical characterization of surficial sediments from the northern rias implications for sediment provenance and impact of the source rocks abstract a multidisciplinary study of the elemental geochemistry and mineralogical characteristics of the marine surficial sediment in the northern rias nw iberian peninsula has been carried out the linkages between the marine sediment composition and their potential sources were examined the influence of the river borne sediments is only detected in the innermost part of the three rias regional variations of the mineral assemblages are governed by the source rock composition of the different geological complexes and the relative source rock contribution controlled by the continental hydrology mineralogical composition of the ortigueira ria and adjacent shelf surficial sediments are mainly made up of mafic and ultramafic rocks of the cape ortegal complex indicated by the high content of mg mn and chrysotile and riebeckite minerals in areas nearby ortegal complex the imprint of heavy minerals present in the surrounding rocks has also been recorded barqueiro and viveiro rias bed sediments are influenced by granitic and metamorphic rocks from the ollo de sapo complex as revealed by the high contribution of muscovite and quartz mining activities in the continental domain left strong imprints on marine surficial sediments pyrite content is high in the innermost areas of the ortigueira ria since this mineral is exploited in the mera river basin whereas high muscovite percentages characterize the viveiro ria owing to the abundance of granitic rocks and its exploitation in the landro river basin quartz content is high nearby cape estaca de bares induced by the presence of an important excavation of this material

the role of fluidization in the emplacement of pyroclastic flows 2 experimental results and their interpretation abstract although often quoted as being important in the emplacement of pyroclastic flows little relevant experimental fluidization work has been done fluidization processes are first reviewed for a simple system against which ignimbrite samples can be compared results from fluidization experiments on ignimbrite samples show that their behaviour differs radically from any simple system principally because of their extremely poor sorting at a certain gas velocity u ie whose value cannot be predicted some ignimbrite samples begin to expand whilst at a higher gas velocity u mp the samples begin to show segregation structures the minimum fluidization velocity u mf in simple systems is replaced by u mp the value of the latter in pyroclastic materials cannot be easily or reliably predicted from published u mf correlations and a new empirical method for determining u mp is presented from the experimental data during fluidization ignimbrite samples expand much less than more conventional materials due to the bypassing of gas through segregation channels the total amount of expansion is also limited e g a 100 m thick pyroclastic flow will from fairly high gas velocities deflate to form an ignimbrite which is not less than 70 85 m thick during fluidization the poor sorting causes only part of the weight of the ignimbrite samples to be supported by the gas flow the degree of support increasing with an improvement in sorting segregation structures and grain size and compositional variations within fluidized ignimbrite samples show varying behaviour depending on the sorting of the sample after subjecting a sample to high gas velocities the net result is a grain size and compositionally zoned bed which becomes richer upwards in pumice and fine material once formed these features cannot be destroyed under laboratory conditions implying that some segregation structures should be present in any ignimbrite where u mp was exceeded

deposition of organic carbon in arctic ocean sediments terrigenous supply vs marine productivity during the arctic 91 expedition with rv polarstern several kastenlot cores were recovered along a profile crossing the eastern part of the arctic ocean in this paper we present results of detailed sedimentological and organic geochemical investigations on three of these cores which have yielded a wide range of data for the interpretation of the flux and type of organic matter in general the high organic carbon content of the sediments is derived from a high input of terrigenous organic matter the variation in content and composition of the organic carbon in the sediments of core ps2174 5 reflect glacial interglacial changes interglacial stages 7 and 5e reveal a low organic carbon content 0 5 and as indicated by high hydrogen indices low c n ratios higher content of n alkanes c 17 and c 19 and a high opal content reflect higher marine productivity during the glacial stages stages 6 upper 5 and 4 organic carbon values are significantly higher 0 7 1 3 low hydrogen indices high c n ratios low short chain n alkanes and low opal contents provide evidence for a higher input of terrigenous organic matter and reduced marine productivity changes in the content and composition of the organic carbon are believed to vary with the fluctuations in sea level and sea ice coverage

late quaternary paleoceanography of the eurasian basin arctic ocean we reconstructed late quaternary deep 3000 4100 m and intermediate depth 1000 2500 m paleoceanographic history of the eurasian basin arctic ocean from ostracode assemblages in cores from the lomonosov ridge gakkel ridge yermak plateau morris jesup rise and amundsen and makarov basins obtained during the 1991 polarstern cruise modern assemblages on ridges and plateaus between 1000 and 1500 m are characterized by abundant relatively species rich benthic ostracode assemblages in part reflecting the influence of high organic productivity and inflowing atlantic water in contrast deep arctic eurasian basin assemblages have low abundance and low diversity and are dominated by krithe and cytheropteron reflecting faunal exchange with the greenland sea via the fram strait major faunal changes occurred in the arctic during the last glacial interglacial transition and the holocene low abundance low diversity assemblages from the lomonosov and gakkel ridges in the eurasian basin from the last glacial period have modern analogs in cold low salinity low nutrient greenland sea deep water glacial assemblages from the deep nansen and amundsen basins have modern analogs in the deep canada basin during termination 1 at intermediate depths diversity and abundance increased coincident with increased biogenic sediment reflecting increased organic productivity reduced sea ice and enhanced inflowing north atlantic water during deglaciation deep nansen basin assemblages were similar to those living today in the deep greenland sea perhaps reflecting deepwater exchange via the fram strait in the central arctic early holocene faunas indicate weaker north atlantic water inflow at middepths immediately following termination 1 about 8500 7000 year b p followed by a period of strong canada basin water overflow across the lomonosov ridge into the morris jesup rise area and central arctic ocean modern perennial sea ice cover evolved over the last 4000 5000 years late quaternary faunal changes reflect benthic habitat changes most likely caused by changes in the import of cold deepwater of greenland sea origin and warmer and middepth atlantic water to the eurasian basin through the fram strait and export of arctic ocean deepwater

late pleistocene changes in terrestrial biomarkers in sediments from the central arctic ocean abstract biomarkers in late pleistocene sediments collected from the integrated ocean drilling program iodp arctic coring expedition acex hole m0004c central arctic ocean were investigated the major biomarkers are long chain n alkanes n fatty acids and n alkan 1 ols indicating fresh organic matter om derived predominantly from higher plants the dominance of terrestrial biomarkers is attributed to severe om degradation caused by slow sedimentation in oxygen rich benthic water and or low primary production due to permanent sea ice coverage hopanes representing thermally altered om tend to be abundant in samples with abundant ice rafted debris ird an om rich dark grey layer deposited during marine isotope stage mis 6 contains a significant amount of branched glycerol dialkyl glycerol tetraethers branched gdgts suggesting ice erosion of organic rich continental soil followed by transportation to the central arctic by drifting ice the labile refractory index i e the abundance ratio of long chain n alkan 1 ols to the sum of long chain n alkanes and long chain n alkan 1 ols decreases above the dark grey layer indicating that the om became more refractory this change suggests that coverage of the source region by om rich soil decreased because of extensive glacial erosion during mis 6

clay mineralogy of late cenozoic sediments in the cesar cores alpha ridge central arctic ocean the clay sized 2 m fraction of the silty and arenaceous lutites constituting cesar cores 14 and 103 alpha ridge central arctic ocean is composed predominantly of mica 40 60 with subequal percentages 10 20 of kaolinite and chlorite and lesser amounts 5 of smectite quartz plagioclase and potassium feldspar calcite and dolomite also occur but only intermittently in the upper 1 2 m dissolution is probably responsible for their absence in other units the silty lutites have a constant mineralogy throughout the 4 5 m long 4 25 ma section whereas some of the arenaceous intervals in the upper 2 1 m have markedly higher amounts of kaolinite calcite dolomite and to a lesser extent smectite the silty lutites were most likely derived from the beaufort sea shelf during nonglacial periods whereas the distinctive components in the sandy layers suggest that they were transported from the canadian arctic islands and greenland by glacial ice the absence of kaolinite peaks in the lower

oceanic and atmospheric variations during the last deglaciation in the fram strait arctic ocean a coupled high resolution organic geochemical and sedimentological study abstract from a high sedimentation rate core obtained from the yermak slope arctic ocean we deduced short term climatic fluctuations of the last deglaciation lasting from 17 000 to 9000 years bp by using a coupled organic geochemical and sedimentological approach we reconstructed near bottom current conditions and assessed their significance in relation with comparable global thermohaline circulation patterns and surface water conditions highest concentrations and fluctuations of specific marine phytoplankton biomarkers were obtained in the bolling allerod when the major deglaciation of the svalbard barents sea ice sheet sbis occurred the marine biomarker record and the sedimentological record are consistent with climate reconstructions obtained from greenland ice cores and thermohaline circulation variations however beginning with the younger dryas yd episode bottom currents decreased during this time period but surface waters remained very productive the atmospheric conditions of the western fram strait reconstructed from biomarker data and thermohaline circulation signals reconstructed from sedimentological data were decoupled in the yd maximum insolation values of solar radiation and fohn like winds enabled open water conditions and increased productivity values in the northern fram strait at least in summer

shock waves dead zones and particle free regions in rapid granular free surface flows shock waves dead zones and particle free regions form when a thin surface avalanche of granular material flows around an obstacle or over a change in the bed topography understanding and modelling these flows is of considerable practical interest for industrial processes as well as for the design of defences to protect buildings structures and people from snow avalanches debris flows and rockfalls these flow phenomena also yield useful constitutive information that can be used to improve existing avalanche models in this paper a simple hydraulic theory first suggested in the russian literature is generalized to model quasi two dimensional flows around obstacles exact and numerical solutions are then compared with laboratory experiments these indicate that the theory is adequate to quantitatively describe the formation of normal shocks oblique shocks dead zones and granular vacua such features are generated by the flow around a pyramidal obstacle which is typical of some of the defensive structures in use today

ignimbrite depositional facies the anatomy of a pyroclastic flow a model is presented for the depositional regimes in a pyroclastic flow to explain the origin of facies in the taupo ignimbrite and to compare these with other published examples in this model a pyroclastic flow consists of a head a body and a tail the head which is where fluidization caused by air ingestion occurs generates layer 1 deposits on lithological grounds these are divided into layers 1 p which is fines depleted and rich in pumice and 1 h which is fines depleted and rich in lithics and crystals layer 1 p represents material thrown forwards from the flow head and is termed the jetted deposits layer 1 h represents material sedimented from within the flow head and is termed the ground layer the body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction these parts generate the layer 2 deposits which include the valley pond ignimbrite and its associated ignimbrite veneer deposit localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee side lenses behind obstacles at its outer limits all of the flow on interfluves is affected by air ingestion fluidization producing the distant facies which combines features of both layer 1 and 2 deposits one important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow

reconciling pyroclastic flow and surge the multiphase physics of pyroclastic density currents abstract two end member types of pyroclastic density current are commonly recognized pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows we provide scaling relations that unify these end members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the stokes number s t the stability factor t and the dense dilute condition d d we recognize five types of particle behaviors within a fluid eddy as a function of s t and t 1 particles sediment from the eddy 2 particles are preferentially settled out during the downward motion of the eddy but can be carried during its upward motion 3 particles concentrate on the periphery of the eddy 4 particles settling can be delayed or fast tracked as a function of the eddy spatial distribution and 5 particles remain homogeneously distributed within the eddy we extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current this stratification may favor particle interactions in the basal part of the flow and d d determines whether the flow is dense or dilute using only intrinsic characteristics of the current our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents

settling behaviour of concentrated dispersions and some volcanological applications abstract experiments have been carried out on the settling of non aggregating poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied plastic beads were added as a light component experiments were carried out at solids concentrations between 10 and 60 vol dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence above a critical concentration 45 to 60 depending on the grain size distribution particle segregation was suppressed and the dispersion settled to form a virtually ungraded bed hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes floating rafts of plastic and capping layers of fines elutriated by escaping pore fluid buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations two mechanisms of elutriation pipe growth were observed at intermediate concentrations pipes grew progressively upwards in the deposit as the latter aggraded whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows reverse grading of pumice pumice rafts elutriation pipes and layers of elutriated fines should characterise deposits from pumice rich submarine mass flows despite differences in the fluid phase the experiments also have implications for fluidization mechanisms in pyroclastic flows hindered settling of a concentrated poorly sorted dispersion is a type of self fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias

thermohaline circulation in the arctic mediterranean seas the renewal of the deep north atlantic by the various overflows of the greenland scotland ridges is only one manifestation of the convective and mixing processes which occur in the various basins and shelf areas to the north the arctic ocean and the greenland iceland and norwegian seas collectively called the arctic mediterranean the traditional site of deep ventilation for these basins is the greenland sea but a growing body of evidence also points to the arctic ocean as a major source of deep water this deep water is relatively warm and saline and it appears to be a mixture of dense brine enriched shelf water with intermediate strata in the arctic ocean the deep water exits the arctic ocean along the greenland slope to mix with the greenland sea deep water conversely very cold low salinity deep water from the greenland sea enters the arctic ocean west of spitsbergen within the arctic ocean the lomonosov ridge excludes the greenland sea deep water from the canadian basin leaving the latter warm saline and rich in silica in general the entire deep water sphere of the arctic mediterranean is constrained by the greenland scotland ridges to circulate internally therefore it is certain of the intermediate waters formed in the greenland and iceland seas which ventilate the north atlantic these waters have a very short residence time in their formation areas and are therefore able to rapidly transmit surface induced signals into the deep north atlantic

paleoceanographic development in the sw barents sea during the late weichselian early holocene transition abstract the late weichselian early holocene variability of the north atlantic current has been studied with focus on the zonal component of this meridional transport during the transition from glacial to interglacial conditions the investigated sediment core is from 409 m water depth in the sw barents sea eight accelerator mass spectrometry ams 14c dates show that the core covers the last 20 000 cal yr b p with a centennial scale resolution during late weichselian early holocene planktic foraminiferal assemblages were analyzed using the 100 m size fraction and foraminiferal planktic and benthic 13c and 18o isotopes were measured furthermore a range of physical and chemical analyses has been carried out on the bulk sediment samples four time periods have been identified which represent the varying oceanographic conditions in ingoydjupet a glacial trough located off the north coast of norway in the sw barents sea 1 the late glacial before ca 15 000 cal yr b p influenced by the nearby ice sheets with high amounts of sea ice or iceberg transported detritus 2 the late oldest dryas stadial and the bolling allerod interstadial ca 15 000 12 700 cal yr b p with cold surface water conditions influenced by the collapse of the nearby ice sheets high amounts of sea ice or iceberg transported detritus and melt water and weak subsurface inflow of atlantic water 3 the younger dryas cold stadial 12 700 11 650 cal yr b p with low primary productivity and extensive sea ice cover and 4 the preboreal and early holocene 11 650 6800 cal yr b p cal yr b p with strong influx of atlantic water into the area near absence of ice rafted debris and generally ameliorated conditions in both surface and bottom water masses as seen from a high flux of foraminifera and increased marine primary production

control of organic carbon accumulation in the late quaternary equatorial atlantic ocean drilling program sites 664 and 663 productivity versus terrigenous supply organic geochemical records of the last 940 kyr are presented for equatorial atlantic ocean drilling program odp sites 663 and 664 and discussed with regard to the development of ocean productivity and african paleoclimate proportions of marine and terrigenous organic matter om are estimated from elemental pyrolytic isotopic and petrologic data spectral analyses reveal a strong power at the eccentricity and obliquity band indicating a close response of tropical organic sedimentation to the climatic evolution at high latitudes the orbital covariance of organic carbon with biogenous opal and terrigenous records favor that glacially enhanced dust supply and surface water mixing were primary controls for deposition of organic carbon wind borne supply of terrigenous om contributes 26 to 55 and 0 to 39 to the bulk om based on microscopic and isotopic records respectively admixture of c4 plant matter was approximated to contribute up to 16 to the bulk organic fraction during peak glacial conditions

terrigenous events and climate history of the sophia basin arctic ocean periods of enhanced terrigenous input to the ocean s basins of the north atlantic have been reported for the last glacial period we present a set of new sediment cores recovered from the sophia basin north of svalbard which exhibit widespread ice rafted debris layers reflecting enhanced terrigenous input throughout the last 200 ka b p their consistent stratigraphic position sedimentological character high sedimentation rate and geochemical characteristic point to synchronously deposited layers which we name terrigenous input events ties owing to their higher densities they generate excellent reflectors for sediment penetrating acoustic devices and prominent acoustic layers in the imagery of sedimentary structures therefore ties can be used for regional acoustic stratigraphy each of the events can be linked to major glacial activity on svalbard however the early weichselian glaciation is not recorded as a tie and in agreement with other work might not have occurred on svalbard as a major glacial advance to the shelf break nonsynchronous timing of western and northern sources on svalbard points against sea level induced iceberg discharge events

reconstructing the origin and trajectory of drifting arctic sea ice recent studies have indicated that drifting arctic sea ice plays an important role in the redistribution of sediments and contaminants here we present a method to reconstruct the back ward trajectory of sea ice from its sampling location in the eurasian arctic to its possible site of ori gin on the shelf based on historical drift data from the international arctic buoy program this method is verified by showing that origins derived from the backward trajectories are generally con sistent with other indicators such as comparison of the predicted backward trajectories with known buoy drifts and matching the clay mineralogy of sediments sampled from the sea ice with that of the seafloor in the predicted shelf source regions the trajectories are then used to identify regions where sediment laden ice is exported to the transpolar drift stream from the new siberian islands and the central kara plateau calculation of forward trajectories shows that the kara sea is a major contributor of ice to the barents sea and the southern limb of the transpolar drift stream

the nature and timing of caldera collapse as indicated by accidental lithic fragments from the ad 1000 eruption of volcan ceboruco mexico one way to determine the mechanics of caldera formation is through quantitative component analysis of accidental lithic fragments in pyroclastic deposits erupted before and during caldera collapse all previous studies of this sort however are based on pyroclastic deposits from large volume 10 km3 caldera eruptions in this study we use quantitative component analysis of lithic fragments to determine the mechanics of the ad 1000 small volume 3 4 km3 dre caldera eruption of volcan ceboruco located in western mexico during this eruption caldera collapse occurred in such a way that lithic fragments of decreasing depths were preferentially erupted with time prior to caldera collapse deep origin lithics 6 km depth and vent derived lithics were erupted deposits emplaced during collapse however contain a distinctly different population of lithic fragments such as lithics from mid depth origin 1 km depth and from the vent by the end of the eruption the total amount of lithic material in the deposits had increased from 15 wt before collapse to as much as 90 wt yet this material contains essentially no lithics of deep origin this suggests that collapse resulted in the obstruction of both deep origin lithics and magma from reaching the surface based on these data we suggest that collapse occurred as a result of the fracturing and subsequent failure of the overlying magma reservoir roof that occurred along inward dipping faults the collapse forced the fragmentation depth and progressive erosion of lithic material from deep to shallow depths which subsequently sealed the conduit stopping the eruption

mapping recent sea ice conditions in the barents sea using the proxy biomarker ip25 implications for palaeo sea ice reconstructions abstract an analysis of the sea ice proxy ip 25 and the open water phytoplankton indicator brassicasterol has been carried out for 93 surface sediments collected from the barents sea and some additional data from previous studies have been included to augment the analysis further consistent with a specific and selective sea ice diatom origin for ip 25 this biomarker was identified in virtually all sediments from seasonally ice covered locations but was absent from the majority of sites beyond the maximum sea ice extent msie for the period 1983 2002 in contrast brassicasterol was detected in virtually all sediments from the region and concentrations were not at all associated with the msie instead more localised regions of brassicasterol concentrations were identified and these likely reflect different influences on production and deposition when the ip 25 and brassicasterol concentrations were combined together in the form of the previously proposed p b ip 25 index the aforementioned association between ip 25 occurrence and the msie was clarified further and in addition provided more detailed descriptions of sea ice cover in particular p b ip 25 data indicated seasonal sea ice or marginal ice zone conditions for the region of the barents sea within the msie according to previously defined p b ip 25 ranges we also amalgamated the biomarker data from the current study with those derived previously by muller et al 2011 and compared these larger datasets with satellite derived mean spring sea ice concentrations significantly although the correlation between the combined ip 25 concentration dataset with the satellite data was very similar to that reported previously by muller et al 2011 the corresponding correlation using the p b ip 25 values was poorer with the larger dataset and some possible reasons for this are discussed finally in a small number of cases ip 25 was identified in sediments somewhat distal from the msie and it is hypothesised that the occurrence of ip 25 in these sediments results from allochthonous input following sediment advection between shelf and trough environments this is the first reported example that provides evidence for the allochthonous input of ip 25 into marine sediments and it is recommended that this should be considered carefully when carrying out palaeo sea ice reconstructions based on the occurrence of this biomarker in the future

arctic ocean deep sea record of northern eurasian ice sheet history the sediment composition of deep sea cores from the central arctic ocean the fram strait and the yermak plateau was analyzed for several parameters to reconstruct the history of marine paleoenvironment and terrestrial glaciation in the last 200 000 years layers with high amounts of coarse terrigenous ice rafted debris ird and often high contents of smectite were deposited during extensive glaciations in northern eurasia when ice sheets reached the northern continental margins of the barents and kara seas and discharged icebergs into the arctic ocean intercalated layers with relatively low ird and smectite contents but abundant planktic foraminifers in the coarse fraction were deposited during periods of atlantic water inflow to the arctic ocean and seasonally open waters leads in a sea ice cover with only few icebergs in the arctic ocean high ird contents in the sediments reflect the presence of ice sheets on the kara and barents seas shelves and the hinterland during the entire oxygen isotope stage 6 ca 190 130 ka in substage 5b ca 90 80 ka at the stage boundary 5 4 around 75 ka and in late stage 4 early stage 3 ca 65 50 ka these results are in excellent correlation with those from recent field work in northern scandinavia european russia siberia and on the shelves relatively low amounts of ird in central arctic ocean sediments from the late weichselian glacial maximum ca 24 18 ka correlate well with the recent reconstruction of a very limited eastern ice sheet extension during this time oxygen and carbon isotope records of planktic foraminifers from the analyzed sediment cores show a number of prominent excursions which can be interpreted as evidence for freshwater events in the arctic ocean the synchroneity of freshwater events and ird input suggests a common source strongest events were associated with deglaciations of the barents and kara seas after the ice sheets had blocked the outflow of large rivers for several millennia the outflow of freshwater from large ice dammed lakes occurred at ca 130 80 75 and 52 ka freshwater events in the central arctic ocean during the last deglaciation ca 18 ka were relatively small compared to the previous events this indicates that during most of the late weichselian glacial maximum a river outflow from northern siberia to the arctic ocean was possible atlantic water inflow to the arctic ocean and seasonally open waters in the ice leads occurred during the interglacials ofoxygen isotope stage 1 and substage 5e during several interstadials stage 3 substages 5a and 5c and to a lesser degree within stadials and glacials stages 2 4 and 6 with the exception ofthe interglacials these periods were times ofstrong ice growth on the continents as revealed by terrestrial data the coincidence suggests that open waters in the arctic ocean and the nordic seas were an important moisture source in addition to more southerly sources which fostered the growth of ice sheets on northern eurasia

a biomarker based reconstruction of sea ice conditions for the barents sea in recent centuries variations in sea ice occurrence for the barents sea since c ad 1700 have been determined by analysis of the abundance of the sea ice biomarker ip25 in three marine sediment box cores obtained from locations in the north southeast and southwest parts of the region depth age models for each core were established using excess 210pb activity profiles comparisons between these depth age models with those reported previously for the barents sea suggest that the proxy sea ice record may alternatively extend back to c ad 1500 1600 sedimentation accumulation rates and bulk densities were combined with ip 25 concentrations resulting in temporal changes to ip25 fluxes which have been interpreted in terms of sea ice variability the ip25 sea ice proxy data are also compared with other bulk organic geochemical parameters total organic carbon and c n ratios and with sediment particle size distributions the data indicate ice free conditions for the southwest barents sea for the past c 300 yr consistent

arctic ocean evidence for late quaternary initiation of northern eurasian ice sheets a high resolution multiparameter stratigraphy allows the identification of late quaternary glacial and interglacial cycles in a central arctic ocean sediment core distinct sandy layers in the upper part of the otherwise fine grained sediment core from the lomonosov ridge lat 87 5 n correlate to four major glacials since ca 0 7 ma the composition of these ice rafted terrigenous sediments points to a glaciated northern siberia as the main source in contrast lithic carbonates derived from north america are also present in older sediments and indicate a northern north american glaciation since at least 2 8 ma we conclude that large scale northern siberian glaciation began much later than other northern hemisphere ice sheets

a two layer model for the evolution and propagation of dense and dilute regions of pyroclastic currents we present calculations of a simplified two layer pyroclastic flow model which incorporates different physics to represent the dense and dilute regions of column collapse pyroclastic density currents the two layers evolve separately but are coupled through mass exchange as suspended ash in the dilute cloud settles into the underlying dense basal layer the runout distance of the upper dilute current and the associated runout time are found to increase with increasing initial column height and decreasing particle size likewise steeper slopes increase the propagation velocity and thus the runout distance the independent runout distance of the basal flow exhibits opposite behaviour to that of the parent dilute current increasing with decreasing initial column height and increasing particle size observed runout distances can be calculated for basal flows using a coulomb friction law but deposit morphology is not well reproduced and is more realistic when an empirical slope dependent sedimentation rate is included dominant flow behaviour is controlled by the rate of mass transfer from the parent suspension current into the dense underflow tall column collapses which contain fine grained particles transfer their mass slowly to the dense basal flow and are well described by dilute cloud assumptions however for short columns containing coarse grains the particulate mass is rapidly transferred from the collapsing dilute current into the basal flow the bulk of the pyroclastic current material propagates as a concentrated suspension for the majority of its travel distance and is better described by the physics of granular avalanches

dissolved organic matter in pore water of arctic ocean sediments environmental influence on molecular composition abstract marine organic matter om sinks from surface water to the seafloor via the biological pump benthic communities which use this sedimented om as an energy and carbon source produce dissolved om dom in the process of degradation enriching the sediment pore water with fresh dom compounds in the oligotrophic deep arctic basin particle flux is low but highly seasonal we hypothesized that the molecular signal of freshly deposited primary produced om would be detectable in surface sediment pore water which should differ in dom composition from bottom water and deeper sediment pore water the study focused on i the molecular composition of the dom in sediment pore water of the deep eurasian arctic basins ii the signal of marine vs terrigenous dom represented by different compounds preserved in the pore water and iii the relationship between arctic ocean ice cover and dom composition composition based on mass spectrometric information obtained via 15 t fourier transform ion cyclotron resonance mass spectrometry was correlated with environmental parameters with partial least square analysis the fresh marine detrital om signal from surface water was limited to pore water from

organic carbon burial rates across the arctic ocean from the 1994 arctic ocean section expedition abstract estimates of organic carbon burial rates were obtained from the canada us 1994 arctic ocean section expedition using a relationship between dissolved ammonium gradients in sediment pore water and organic carbon burial rates data from 9 locations more than 200 km west of the lomonosov ridge indicate that organic carbon burial rates average 0 01 g c m 2 year 1 at 4 stations within 100 km of the lomonosov ridge carbon burial rate increases 9 fold to an average of 0 09 g c m 2 year 1 enhanced carbon burial rates and organic carbon concentrations at the lomonosov ridge and in the eurasian basin appear to be related to higher terrestrial carbon supply to the eurasian basin and to the role that the lomonosov ridge plays in the circulation and mixing of the different water masses from the two major basins on either side of the lomonosov ridge

a multiproxy approach to reconstruct the environmental changes along the eurasian continental margin over the last 150 000 years abstract sediment cores located along the eurasian continental margin arctic ocean have been studied to reconstruct the environmental changes in terms of waxing and waning of the barents kara sea ice sheets atlantic water inflow and sea ice distribution over the last 150 kyr the stratigraphy of the cores is based on stable oxygen isotopes ams 14 c and paleomagnetic data we studied variations in marine and terrigenous input by a multiproxy approach involving direct comparison of sedimentological and organo geochemical data extensive episodes of northern barents sea ice sheet growth during marine isotope stages mis 6 and 2 have been supported by at least subsurface atlantic water inflow moisture bearing storms low summer insolation and minimal calving of ice ice advance during mis 4 was probably restricted to the shallow shelf between mis 4 and mis 2 large ice sheet fluctuations correspond to contemporary laurentide surging events and indicate short term climatic changes in the arctic ocean as has been recorded in lower latitudes in contrast in low precipitation areas in eastern eurasia glacial activity was rather limited only distinct ice rafted debris ird input during termination ii and early mis 3 reflects severe glaciations on the northern severnaya semlya margin during mis 6 and mis 4 we conclude that 1 oscillations of ice sheets are less frequent along the eastern eurasian margin than in areas with continuous moisture supply like the western eurasian margins and that 2 major fluctuations of the kara sea ice sheet during the last 150 kyr apparently followed the major interglacial glacial mis 5 4 and mis 7 6 transitions rather than the precession 23 kyr and the tilt 41 kyr cyclicity of the earth s orbit as observed for the scandinavian sis and the svalbard ice sheets respectively mangerud j jansen e landvik j y 1996 late cenozoic history of the scandinavian and barents sea ice sheets in solheim a riis f elverhoi a faleide j j jensen l n cloetingh s eds impact of glaciations on basin evolution data and models from the norwegian margins and adjacent basins global and planetary chance special issue 12 pp 11 26 surface and or subsurface atlantic water masses coupled with seasonally ice free conditions penetrated continuously to at least the franz victoria trough during the last 150 kyr however sustained periods of open water were largely restricted to substages 5 5 5 1 and the holocene as indicated by distinct carbonate dissolution and higher accumulation of marine organic matter mom signals of periodic open water conditions along the northern margin of severnaya semlya are of less importance higher production of foraminifera probably due to atlantic water inflow occurred between 38 and 12 14 c kyr and corresponds to periodic atlantic water advection penetrating into the arctic ocean however marine organic proxies indicate a continuous decrease of surface water productivity from the western to the eastern eurasian continental margin due to a more extensive sea ice cover over the last 150 kyr

new aspects of organic carbon deposition and its paleoceanographic implications along the northern barents sea margin during the last 30 000 years we studied variations in terrigenous tom and marine organic matter mom input in a sediment core on the northern barents sea margin over the last 30 ka using a multiproxy approach we reconstructed processes controlling organic carbon deposition and investigated their paleoceanographic significance in the north atlantic arctic gateways variations in paleo surface water productivity are not documented in amount and composition of organic carbon the highest level of mom was deposited during 25 23 ka as a result of scavenging on fine grained reworked and tom rich material released by the retreating svalbard barents sea ice sheet during the late weichselian a second peak of mom is preserved because of sorptive protection by detrital and terrigenous organic matter higher surface water productivity due to permanent intrusion of atlantic water and high suspension load release by melting sea ice during 15 9 11 2 ka

slow unconfined spreading of a mudflow mudflows are natural highly concentrated water clay grain mixtures that flow in mountain streams after long or intense rainy periods and may cause considerable damage if they overflow on the alluvial fan the possibility of predicting the extent of these flows on the basis of material and flow parameters is examined the simplest realistic case of a yield stress mudflow moving through a narrow open channel followed by a wide long plane is considered it is demonstrated that the unconfined flow of a yield stress fluid over an inclined plane cannot be uniform even in steady state the flow width should increase continuously from the channel exit a complete treatment of the flow equation on the basis of the long wave approximation including an appropriate three dimensional expression for the constitutive equation makes it possible to establish a system of equations from which flow characteristics at any point longitudinal and lateral mean velocities and fluid depth can be deduced in particular for a herschel bulkley fluid with a flow index of 1 3 it is found that the lateral extent will increase proportionally to the distance from the channel exit to the power 9 20 and that in the sheared part the fluid depth in a cross section will have a parabolic distribution experiments have been carried out with fine mud suspensions at different solid fractions whose rheological behavior is similar to that of natural mudflows the theory is in fair agreement with experimental data concerning fluid depth distribution but systematically overestimates lateral extent by 30 this is certainly due to the fact that the assumption of lateral extent much smaller than flow length is not respected in our tests

living benthic foraminifers from the central arctic ocean faunal composition standing stock and diversity abstract fifty short sediment cores collected with a multiple corer and five box cores from the central arctic ocean were analysed to study the ecology and distribution of benthic foraminifers to work out living faunal associations standing stock and diversity separate analyses of living rose bengal stained and dead foraminifers were carried out for the sediment surface the size fractions between 63 and 125 m and 125 m were counted separately to allow comparison with former arctic studies and with studies from the adjacent norwegian greenland sea barents sea and the north atlantic ocean benthic foraminiferal associations are mainly controlled by the availability of food and competition for food while water mass characteristics bottom current activity substrate composition and water depth are of minor importance off spitsbergen in seasonally ice free areas high primary production rates are reflected by high standing stocks high diversities and foraminiferal associations 125 m that are similar to those of the norwegian greenland sea generally in seasonally ice free areas standing stock and diversity increase with increasing food supply in the central arctic ocean the oligotrophic permanently ice covered areas are dominated by epibenthic species the limited food availability is reflected by very low standing stocks and low diversities most of these foraminiferal associations do not correspond to those of the norwegian greenland sea the dominant associations include simple agglutinated species such as sorosphaerae placopsilinellae komokiacea and aschemonellae as well as small calcareous species such as stetsonia horvathi and epistominella arctica those of the foraminiferal species that usually thrive under seasonally ice free conditions in middle bathyal to lower bathyal water depth are found under permanently ice covered conditions in water depths about 1000 m shallower if present at all

the emplacement of low aspect ratio ignimbrites by turbulent parent flows 1 a quantitative model for a radially spreading suspension driven gravity current is used to interpret parent flow conditions of a volumetrically wide range of pyroclastic deposits known from the recent and ancient geologic records of explosive volcanism the analysis which requires no freely adjustable parameters yields values of the solids concentration time of duration or equivalently a minimal estimate of eruptive flux of solids and average speed for the ground hugging deposit forming gravity currents a consistent picture emerges in which the initial volumetric concentration of pyroclastic solids in the parent flows is of the order 10 2 or less and the expanded flows travel with average speeds in the range 25 250 m s 1 to reach their farthest extent in a matter of only minutes further application of the model suggests that depositional structures in a widespread and thin ignimbrite are likely to reflect the intensity of mass flux in the parent flow and thus the intensity and overall volume of the parent eruption conditions in small magnitude pyroclastic currents of recent history can result in cross stratified deposits primary sedimentary structures or their absence in some regionally widespread and massive deposits known only in the geologic record in contrast were probably emplaced under conditions of intense sheet flow transport driven by the parent gravity currents

transport and deposition in the may 18 1980 mount st helens blast flow a new statistical sampling strategy for pyroclastic deposits has yielded data that reveal the depositional characteristics of the may 18 1980 blast flow of mount st helens samples were collected from each layer along a profile parallel to flow lines as interpreted from tree blow down directions the decrease in the mass of sediment of different grain sizes with distance supports a turbulent gravitational mechanism of suspended sedimentation and transport within a dilute blast cloud having a density only slightly greater than that of the atmosphere traditional sampling strategies would have been unable to yield data suitable to test this settling mechanism hypothesis

tracking nutrient and productivity variations over the last deglaciation in the arctic ocean we have reconstructed the surface water environment of the arctic ocean over the last 50 000 years using measurements of the organic nitrogen and carbon isotope ratios carbonate and total organic carbon concentrations toc and terrestrial biomarkers lignin and long chain n alkanes in four multicores variations in nitrogen isotope ratios that are concordant with toc and carbonate concentrations representing foraminifera and excluding ice rafted debris reflect differences in relative nutrient utilization of phytoplankton in the surface waters however 15n variations also appear to be dependent on the stratification of the water column and therefore potentially track the exchange of nutrients between deep and surface waters low last glacial maximum lgm 15n values and higher holocene values are opposite to those recorded in the southern ocean the arctic ocean with higher nutrient utilization today compared to the lgm therefore acts as a counterpart to the southern ocean although the global impact on carbon dioxide variations compared to the southern ocean is probably low

stable isotope stratigraphy sedimentation rates and salinity changes in the latest pleistocene to holocene eastern central arctic ocean a high resolution study including oxygen and carbon stable isotopes as well as carbonate and total organic carbon contents has been performed on undisturbed near surface 0 40 cm sediment sequences taken in the eastern arctic ocean during the international arctic 91 expedition based on the oxygen stable isotope records measured on neogloboquadrina pachyderma sin and ams 14c dating the upper 10 to 20 cm of the sediment sequences represent isotope stage 1 and the base of termination i 15 7 ka can be identified very well stage 1 sedimentation rates vary between 0 4 and 2 0 cm kyr in general glacial stage 2 sedimentation rates are probably lower and vary between 0 4 and 0 7 cm kyr the glacial interglacial shifts in 18o values of n pachyderma sin may reach values of 1 3 to 2 5 indicating 1 that in addition to the glacial interglacial global ice volume signal changes in surface water salinity have effected the isotope records and 2 that these salinity changes have varied laterally glacial interglacial differences in salinity were small in the lomonosov ridge area 0 0 4 and relatively high in the morris jesup rise area up to 1 4 this implies that the supply of low saline waters onto the eurasian shelves and its further transport into the central arctic ocean via the transpolar drift should have continued during the last glacial and should have significantly influenced the surface water characteristics in parts of the central arctic on the morris jesup rise on the other hand the glacial low saline water signal at that time was strongly reduced in comparison to the modern situation at the glacial interglacial stage 12 boundary a strong meltwater signal is recorded in a sharp depletion in 18o as well as 13c this central arctic ocean meltwater event can be correlated from the makarov basin through the lomonosov ridge and amundsen basin to the eastern gakkel ridge the beginning of this event is ams 14c dated at 15 7 ka i e significantly older than the major decrease in the global ice volume signal which occurs between 9 and 13 5 ka large amounts of freshwater meltwater were probably supplied from the eurasian continent due to the decay of the barents sea ice sheet causing this distinct early meltwater anomaly in the central arctic ocean the extension of a well oxygenated surface near water mass in the arctic ocean and at least seasonal open ice conditions and some increased bioproductivity were probably established at the end of termination i as indicated by the increase in 13c to modern values as well as increased carbonate i e foraminifers coccoliths ostracodes and total organic carbon contents

recent distribution and accumulation of organic carbon on the continental margin west off spitsbergen the study compiles the controlling factors for organic matter sedimentation patterns from a suite of organogeochemical parameters in surface sediments off spitsbergen and direct seabed observations using a remotely operated vehicle rov in addition we assess its storage rates as well as the potential of carbon sinks on the northwestern margin of the barents sea with short sediment cores from a selected fjord environment storfjord while sedimentation in the fjords is mainly controlled by river meltwater discharge and coastal erosion by sea ice glaciers resulting in high supply of terrigenous organic matter atlantic water inflow and thus enhanced marine organic matter supply characterizes the environment on the outer shelf and slope local deviations from this pattern particularly on the shelf are due to erosion and out washing of fine grained material by bottom currents spots dominated by marine productivity close to the island have been found at the outer isfjord and west off prins karls forland as well as off the kongsfjord krossfjord area and probably reflect local upwelling of nutrient rich atlantic water derived water masses accumulation rates of marine organic carbon as well as reconstructed primary productivities decreased since the middle of the last century negative correlation of the isfjord temperature record with reconstructed productivities in the storfjord could be explained by a reduced annual duration of the marginal ice zone in the area due to global warming extremely high accumulation rates of marine organic carbon between 5 4 and 17 2 g m 2 yr 1 mark the storfjord area and probably high latitude fjord environments in general as a sink for carbon dioxide

late cenozoic sedimentary environments in the amundsen basin arctic ocean analyses of sediment cores recovered on the arctic 91 cruise with rv polarstern to the amundsen basin and lomonosov ridge identify four main sedimentary facies and associated depositional sedimentary processes these include 1 homogenous mud facies interpreted as hemipelagic deposits most abundant in the central parts of the amundsen basin 2 fining upward cycle facies interpreted as fine grained distal turbidite deposits present in all amundsen basin cores but most abundant laterally in the basin 3 massive diamicton facies found in the north pole core and 4 laminated diamicton facies abundant in the upper parts of lomonosov ridge cores both diamictons are heavily influenced by ice rafted debris and the massive diamicton may result from redeposition of laminated diamicton downslope from the lomonosov ridge the lomonosov ridge core exhibits a sedimentation rate of 29 mm ka while the sedimentation rates of turbidite rich sediments of the amundsen basin may be up to 10 times as high clay minerals show that the eastern kara sea and the laptev sea is the source area for most of the sediments while some clay mineral zones may have their source in northern greenland or the barents sea shelf

sedimentation of tephra by volcanic plumes i theory and its comparison with a study of the fogo a plinian deposit sao miguel azores sedimentation of ejecta from volcanic plumes has been studied as a function of distance from the source in the fogo a plinian deposit sao miguel azores the fogo a trachytic pumice deposit is reversely graded and can be divided into two parts on the basis of pumice colour abundance of syenite accessory lithic clasts and distribution the lower syenite poor part was dispersed to the south and was clearly influenced by wind the upper syenite rich part is coarsegrained and has a nearly symmetrical distribution around the vent elongation of isopachs to the east indicate a weak wind influence the grain size variations of lithic and crystal components in the upper coarse part were studied total accumulation and accumulation per unit area expressed in kg m2 show good fits to a gaussian function at distances greater than 7 km for grain diameters less than 2 cm these results agree with a theoretical model for a radially spreading turbulent current moving over a quiescent fluid the gaussian coefficient is shown to be a function of grain size and the flow rate of material into the umbrella region of the eruption column the coefficient is therefore also a function of column height the column height deduced from these data is 21 km which is in broad agrrement with the column height of 27 km deduced from maximum clast dispersal using the method of carey and sparks 1986 the accumulation of clasts larger than 2 cm agrees with a theory for the fallout of clasts from the margins of the ascending eruption column which treats the plume as a succession of large eddies that decrease their mass of particles as an exponential function of time calculations are also presented for the influence of the radial inflow of surrounding air into the column on the deposition of clasts these calculations constrain the wind speed during the later part of the fogo a eruption to be at most a few metres per second the study has allowed four different dynamic categories of clast behaviour to be recognised in eruption columns

a laboratory study of ash flows hot ash flows propagate from volcanic vents during some explosive volcanic eruptions as a flow entrains and heats ambient air and simultaneously deposits pyroclasts part of the flow may become less dense than the air and ascend from the flow if entrainment of air is more important than sedimentation in controlling the flowbehaviour then a large buoyant ash cloud may form above the flow conversely if sedimentation of particles dominates then a much smaller fraction of the particles are able to ascend in the buoyant cloud we have carried out a series of analog laboratory experiments to simulate the motion and buoyancy generation in dilute ash flows propagating along the ground the first series of experiments investigated buoyancy generation through entrainment using mixtures of methanol ethylene glycol and water the second series examined the role of sedimentation upon the generation of buoyancy using particle laden fresh water currents propagating in a saline ambient fluid the experiments showed that the rate of entrainment increases while the sedimentation rate decreases as the downward slope of the terrain increases suggesting that massive ash laden clouds are more likely to rise from flows traveling on steeper slopes changes in slope angle also control the motion of ash flows the experiments suggest that flow separation and hydraulic jumps resulting from sudden changes in slope angle can enhance entrainment or sedimentation and increase buoyancy observations of the the interactions of ash flows with topography at redoubt volcano in april 1990 and at mount st helens on may 18 1980 are consistent with our laboratory observations

sedimentation from gravity currents generated by turbulent plumes sedimentation from radially spreading gravity currents generated at the top of ascending sediment laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current for radial spreading this model predicts a gaussian distribution of sediment accumulation away from the source with an exponential constant b which depends on flow rate q and particle settling velocity v b nv q in the experiments described sedimentation occurs from gravity currents generated by ascent of buoyant particle laden plumes of fresh water in a tank of salty water the sediment accumulation shows close agreement with the theoretical model and the gaussian decay constant b can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line in s s0 br2 where r is the radial distance s is the sediment mass flux per unit area and s0 is the value of s at r 0 data from the dispersal of volcanic ejecta from a large c 24 km high plinian eruption column in the azores also show good agreement with the theory confirming that it is general and independent of scale and the nature of the fluid the experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity sedimentation of coarse grain sizes between the source and the corner occurs from the inclined plume margins and does not behave as predicted by the theoretical model

development of lithic breccias in the 1982 pyroclastic flow deposits of el chichon volcano mexico abstract pyroclastic flow deposit f1 volume 0 02 km 3 produced during phase iii of the 1982 eruption of el chichon volcano mexico contains basal lithic breccias the breccia layers are well exposed in el platanar gully between 2 and 4 km east of the volcano crater the lithic breccias are inversely graded as defined by lithics dense juvenile blocks and pumice supported in a coarse sandy matrix composed of the same constituents the contact between the main body of the pyroclastic flow deposit and the lithic breccias is generally sharp and planar but not erosive in some outcrops it is gradational and is only shown by an alignment of lithic clasts the origin of these beds is interpreted to be due to a hydraulic jump in the moving pyroclastic flow formed after a pronounced slope break from 11 to 3 at the site where the flow began to be funnelled into the el platanar gully we have investigated the possible modes of formation of the lithic breccias with analog laboratory experiments the experiments show that coarse localized segregations could form through a number of mechanisms the field observations interpreted with the assistance of the laboratory results suggest that pyroclastic flow 1 which produced deposit f1 moved as a kind of density stratified flow with a basal lithic rich zone transporting larger particles and an upper less dense zone transporting smaller particles in suspension at the slope break flow 1 lost competence and dumped the largest particles forming a piled up breccia downstream somewhat smaller lithic particles may have been deposited as ballistics from a low angle jetlike structure comprising a hydraulic jump this deposit thins with distance downstream once the deposit was sufficiently thick on its upstream end particles may have been re entrained into the jet to be deposited further along the flow in a low angle downstream prograding bouldery dune reverse grading in the dune beds may result from kinetic sieving we suggest that planar contacts between the lithic breccias and the associated pyroclastic flow deposit may be the result of strong density and velocity gradients between the two parts of the flow

petrology of organic matter in modern and late quaternary deposits of the equatorial atlantic climatic and oceanographic links abstract organic petrologic and geochemical analyses were performed on modern and quaternary organic carbon poor deep sea sediments from the equatorial atlantic the study area covers depositional settings from the west african margin odp site 959 through the equatorial divergence odp site 663 to the pelagic equatorial atlantic response of organic matter om deposition to quaternary climatic cycles is discussed for odp sites 959 and 663 the results are finally compared to a concept established for fossil deep sea environments littke r sachsenhofer r f 1994 organic petrology of deep sea sediments a compilation of results from the ocean drilling program and the deep sea drilling project energy and fuels 8 1498 1512 organic geochemical results obtained from equatorial atlantic deep sea deposits provide new aspects on the distribution of sedimentary om in response to continental distance atmospheric and oceanographic circulation and depositional processes controlling sedimentation under modern and past glacial interglacial conditions the inventory of macerals in deep sea deposits is limited due to mechanical breakdown of particles degree of oxidation and selective remineralization of labile mostly marine om nevertheless organic petrology has a great potential for paleoenvironmental studies especially as a proxy to assess quantitative information on the relative abundance of marine vs terrigenous om discrepancies between quantitative data obtained from microscopic and isotopic 13 c org analyses were observed depending on the stratigraphic level and depositional setting strongest offset between both records was found close to the continent and during glacial periods suggesting a coupling with wind born terrigenous om from central africa since african dust source areas are covered by c4 grass plants supply of isotopically heavy om is assumed to have caused the difference between microscopic and isotopic records

transport and deposition of a pyroclastic surge across an area of high relief the 18 may 1980 eruption of mount st helens washington the 08 32 pdt 18 may 1980 mount st helens eruption began as an explosion blast that produced a laterally flowing pyro clastic current blast surge it moved outward in a 180 arc at supersonic speeds and then slowed to subsonic speeds supersonic flow severely eroded the ground and produced a ground layer formerly layer a0 above the ground layer lies the deposit derived from the blast surge the blast surge deposits formed in three continuous and concurrent stages the first stage was movement of the transport system that is the blast surge carrying fragments most of the distance from the vent the next two stages involved the depositional system this system developed from rapidly accumulating sediment by gravity segregation at the base of the blast surge two stages of sediment gravity flows with independent flow regimes formed sequentially in rapid succession from the moving blast surge each moved downhill due to gravity independently of the generating blast surge that was driven by explosive expansion units repeated in some sections are deposits derived from divergent and crossing surge lobes caused by irregular topography locally ponded massive deposits in valleys and depressions formed from gas rich pyroclastic flows and originated in two ways 1 topographic blocking by steep volcano facing slopes blocking includes diversion or damming of the lower elevation high density part of the blast surge that could not surmount high ridges 2 also occurring to form valley ponds was drainage of just deposited or almost deposited material from steep slopes 30 35 moving downhill in all slope directions into separate valleys or depressions distances traveled by the blast surge and rates at which coarsest fragments decrease laterally are related to the topographic grain surge runout was farther where flow directions paralleled topographic trends and least where trends were at right angles to the flow obstacle interference depositional patterns thinning thickening rapid fine to coarse facies changes caused a spread in median diameter values that are as great in local areas near the volcano as they are over the width of the devastated region

experimental studies of particle laden plumes experiments are described in which buoyant plumes of fresh water and solid particles are generated in a tank of salt water dependence of the behavior of the plumes on particle concentration and grain size was studied at low concentrations behavior was similar to single component plumes and the principal effect of the particles was to reduce the buoyancy flux measurements of particle concentration across the plumes demonstrated that there were substantial amounts of particle reentrainment from an outer sedimenting veil reentrainment caused the plume margins to become denser than the ambient fluid total particle flux in the plumes increased with height and can be several times greater than the flux at the plume source plumes with high particle concentration exhibit several kinds of convective instability which generated gravity currents and rapid sedimentation gravity currents were generated at the margins of concentrated plumes these flows are attributed to reentrainment causing the flow margins to have excess density when the plume density was close to but less than the density of the ambient fluid a fountain forms high in the plumes which collapsed asymmetrically to generate a gravity current on one side of the plume when the density of the particle laden flow was greater than the ambient fluid a symmetrical fountain formed and gravity currents moved across the tank floor depositing sediment at a well defined distance from the source buoyant fluid ascended from the flow fronts to form a ring of plumes around the central fountain these plumes coalesced to form a broad buoyant cloud the plumes are generated by the sedimentation from the gravity currents which resulted in the fresh water and some entrained particles being released all the experimental plumes ascended to the top of the tank and spread out as gravity currents beneath the upper surface particles often fall from this layer in discrete parcels of particle rich fluid with shapes similar to thermals the fall velocities were much greater than the terminal velocities of individual particles and the sedimenting region is inhomogeneous these observations indicate that sedimentation is enhanced by convective instabilities when particle rich fluid is emplaced above particle free fluid the experiments indicate that there may be several different mechanisms of generating pyroclastic flows and surges from the collapse of volcanic eruptions columns the experiments also demonstrate that the sedimentation of particles from a plume has an important influence on dynamical behavior

the transport and deposition of pyroclastic material from the 1000 ad caldera forming eruption of volcan ceboruco nayarit mexico the complex eruption sequence from the 1000 a d caldera forming eruption of volcan ceboruco known as the jala pumice offers an exceptional opportunity to examine how pyroclastic material is transported and deposited from pyroclastic density currents over variable topography three main pyroclastic surge deposits s1 s2 and s3 and two pyroclastic flow deposits marquesado and north flank pfds were emplaced during this eruption pyroclastic surge deposits are massive planar or cross bedded poor to well sorted and display fluctuations in thickness median diameter sorting and lithology as a function of distance topography and flow dynamics marquesado pyroclastic flow deposits reveal lateral variations from massive poorly sorted deposits located within 5 km of ceboruco to planar bedded moderately well sorted deposits located 15 km away over the nearly horizontal topography to the south of ceboruco north flank pyroclastic flow deposits also reveal lateral variations from massive poorly sorted deposits located within 4 km of ceboruco to planar bedded moderately well sorted deposits located 8 km away atop an escarpment that steeply rises 230 m from the northern valley floor field observations granulometric analyses component analyses and crystal sedimentation calculations along flow parallel sampling transects all suggest that both surges and flows were density stratified currents where deposition occurred from a basal region of higher particle concentration that was supplied from an overlying dilute layer that transports particles in suspension this supports the idea of a transition between flow and surge end members with variations in particle concentration topography greatly affects the transport and depositional capacity of the pyroclastic density currents as a result of blocking either by topographic obstacles or by abrupt breaks at the base of volcano slopes whereas the origin of jala pumice surge deposits phreatomagmatic versus magmatic appears to have little impact on their flow dynamics