fractionation of sulfur isotopes during bacterial sulfate reduction in deep ocean sediments at elevated temperatures abstract a numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid so 4 2 and 34 s data from four sites drilled during ocean drilling program odp leg 168 in the cascadia basin at 48 n where basement temperatures reach up to 62 c there is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow which promotes bacterial sulfate reduction below the sulfate minimum zone at elevated temperatures pore fluid 34 s data show the highest values 135 yet found in the marine environment the bacterial sulfur isotopic fractionation factor is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and rayleigh fractionation plots yield erroneous values for by as much as 15 in diffusive and advective pore fluid regimes model results are consistent with 1 077 0 007 with no temperature effect over the range 1 8 to 62 c and no effect of sulfate reduction rate over the range 2 to 10 pmol cm 3 d 1 the reason for this large isotopic fractionation is unknown but one difference with previous studies is the very low sulfate reduction rates recorded about two orders of magnitude lower than literature values that are in the range of mol cm 3 d 1 to tens of nmol cm 3 d 1 in general the greatest 34 s depletions are associated with the lowest sulfate reduction rates and vice versa and it is possible that such extreme fractionation is a characteristic of open systems with low sulfate reduction rates

coeval giant landslides in the canary islands implications for global regional and local triggers of giant flank collapses on oceanic volcanoes abstract giant landslides are an important part of the evolution of most intra plate volcanic islands they often proceed in catastrophic events likely to generate voluminous debris avalanches and eventually trigger destructive tsunamis although knowledge of the timing of their recurrence is a key factor regarding the hazard assessment in coastal environments only a few of them have been well dated in this contribution we focus on the la orotava event on tenerife which we date with the unspiked k ar technique between 534 and 523 ka such narrow temporal interval is compatible within uncertainties with the age of the cumbre nueva collapse on the neighboring island of la palma we thus examine here the possible common triggering mechanisms at the global regional and local scales both events occurred shortly after the climax of the oxygen isotopic stage 14 during the rapid transition towards the interglacial stage 13 reinforcing the hypothesis of a control from global paleoclimatic changes on the destabilization of oceanic islands intense volcanic pulses at the regional scale also lead to the synchronous overgrowth of several volcanic islands in the archipelago but coeval destabilization on tenerife and la palma appears significantly controlled by the intrinsic morphology of the edifices with contrasted instability thresholds for shield volcanoes and volcanic ridges respectively finally we propose that the two events may be genetically linked dynamic transfer of voluminous debris avalanches during a giant landslide episode can induce isostatic readjustments generate significant ground acceleration and finally produce a large tsunami three processes which can concur to trigger large scale flank collapse on a neighboring mature unstable volcanic island

volcanic evolution of central basse terre island revisited on the basis of new geochronology and geomorphology data twenty six new and seven previous k ar ages obtained on groundmass separates for samples from the axial chain massif guadeloupe f w i associated with geomorphological investigations allow us to propose a new model of the volcanic evolution of the central part of basse terre island the axial chain is composed of four edifices moustique mateliane capesterre and icaque mounts showing coeval activity from 681 12 to 509 10 ka which contradicts a previous hypothesis that flank collapse affected them successively our geomorphological reconstruction shows that the axial chain can be considered as a single large volcano named the southern axial chain volcano sca rather than a succession of several smaller volcanoes it raises questions regarding the formation of a large depression within the sca volcano prior to the construction of the sans toucher volcano between 451 13 and 412 8 ka given presently available evidence a slump affecting the western part of the sca volcano is the most probable scenario to reconcile the complete age dataset and the present day morphology of central basse terre finally our study shows that the sca volcano had a post activity volume of 90 km3 implying a construction rate of 0 5 km3 kyr this value strongly constrains interpretations of magma generation processes throughout the lesser antilles arc

cyclic 100 ka glacial interglacial migration of subseafloor redox zonation on the peruvian shelf the coupling of subseafloor microbial life to oceanographic and atmospheric conditions is poorly understood we examined diagenetic imprints and lipid biomarkers of past subseafloor microbial activity to evaluate its response to glacial interglacial cycles in a sedimentary section drilled on the peruvian shelf ocean drilling program leg 201 site 1229 multiple and distinct layers of diagenetic barite and dolomite i e minerals that typically form at the sulfate methane transition smt occur at much shallower burial depth than the present smt around 30 meters below seafloor these shallow layers co occur with peaks of 13c depleted archaeol a molecular fossil of anaerobic methane oxidizing archaea present day non steady state distributions of dissolved sulfate also suggest that the smt is highly sensitive to variations in organic carbon flux to the surface shelf sediments that may lead to shoaling of the smt reaction transport modeling substantiates our hypothesis that shallow smts occur in response to cyclic sediment deposition with a high organic carbon flux during interglacials and a low organic carbon flux during glacial stages long diffusion distances expectedly dampen the response of deeply buried microbial communities to changes in sediment deposition and other oceanographic drivers over relatively short geological time scales e g glacial interglacial periods however our study demonstrates how dynamically sediment biogeochemistry of the peru margin has responded to glacial interglacial change and how these changes are now preserved in the geological record such changes in subsurface biogeochemical zonation need to be taken into account to assess the role of the subseafloor biosphere in global element and redox cycling

reconstruction of primary productivity from the barium contents in surface sediments of the south atlantic ocean abstract the aim of the present study is an evaluation of the applicability of biogenic barium as a proxy for productivity for this purpose 190 surface sediment samples from the south atlantic ocean were analysed for their barium and aluminium concentrations biogenic barium is estimated by subtracting the calculated terrigenous barium obtained from the terrigenous ba al ratio and the amount of al in the sample from the total ba content in the sample based on the accumulation rates of biogenic barium export production is estimated using three different algorithms proposed by paleoceanography 7 1992 163 global biogeochem cycles 9 1995 289 geomar report 38 1995 105 primary productivity was calculated from these different export productions and compared with measurements of recent primary productivity in the overlying surface waters only the primary productions calculated on the basis of the algorithm of paleoceanography 7 1992 163 yield productivity values comparable to those existing in ocean surface waters this study further reveals that it is not sufficient to use a constant generally applicable organic carbon biogenic barium ratio as is postulated by global biogeochem cycles 9 1995 289 this ratio has to be assessed regionally for the sediments of the cape basin in the eastern south atlantic ocean a new algorithm is developed which gives plausible primary productivities for the overlying surface waters

deep sulfate methane transition and sediment diagenesis in the gulf of alaska iodp site u1417 abstract sediment samples from the gulf of alaska goa iodp expedition 341 site u1417 have been analyzed to understand present and past diagenetic processes that overprint the primary sediment composition no sulfate methane transition zone smtz was observed at the zone of sulfate depletion but a 200 m thick sulfate and methane free sediment interval occurred between the depth of sulfate depletion 200 m and the onset of methanogenesis 440 m we suggest that this apparent gap in biogeochemical processing of organic matter is caused by anaerobic oxidation of methane fueled by sulfate which is released during dissolution of barites at the upper boundary of the methane rich layer beneath the methanogenic zone at 650 m depth pore water sulfate concentrations increase again indicating sulfate supply from greater depth feeding into a deep inverse smtz a likely explanation for the availability of sulfate in the deep sub seafloor at u1417 is the existence of a deep aquifer related to plate bending fractures which actively transports sulfate rich water to and potentially along the interface between sediments and oceanic crust such inverse diagenetic zonations have been previously observed in marine sediments but have not yet been linked to subduction related plate bending with the discovery of a deep inverse smtz in an intra oceanic plate setting and the blocking of upward methane diffusion by sulfate released from authigenic barite dissolution site u1417 provides new insights into sub seafloor pore fluid and gas dynamics and their implications for global element cycling and the deep biosphere

reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores the morpho structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass wasting processes we present here a multi disciplinary study of the successive stages of development of faial azores during the last 1 myr using high resolution digital elevation model dem and new k ar tectonic and magnetic data we reconstruct the rapidly evolving topography at successive stages in response to complex interactions between volcanic construction and mass wasting including the development of a graben we show that 1 sub aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca 0 85 ma followed by its partial destruction over half a million years 2 beginning about 360 ka a new small edifice grew on the ne of the island and was subsequently cut by normal faults responsible for initiation of the graben 3 after an apparent pause of ca 250 kyr the large central volcano cv developed on the western side of the island at ca 120 ka accumulating a thick pile of lava flows in less than 20 kyr which were partly channelized within the graben 4 the period between 120 ka and 40 ka is marked by widespread deformation at the island scale including westward propagation of faulting and associated erosion of the graben walls which produced sedimentary deposits subsequent growth of the cv at 40 ka was then constrained within the graben with lava flowing onto the sediments up to the eastern shore 5 the island evolution during the holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano tectonic depression we conclude that the whole evolution of faial island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation each pulse has been separated by considerable periods of volcanic inactivity during which the faial graben gradually developed we propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement as observed historically when immediate vertical collapse of up to 2 m was observed along the western segments of the graben at the end of the capelinhos eruptive crises 1957 58

incoming plate faulting in the northern and western pacific and implications for subduction zone water budgets abstract the greatest uncertainty in the amount of water input into the earth at subduction zones results from poor constraints on the degree of mantle serpentinization in the incoming plate recent studies suggest that the depth of serpentinization within the incoming plate mantle is likely controlled by the depth of extensional faulting caused by lithospheric bending at the outer rise and trench we explore the maximum depth of extension within the incoming plate at northwestern pacific subduction zones in order to estimate the depth limit of serpentinization and to identify any significant variation between subduction zone segments we relocate trench earthquakes to identify which events occurred within the incoming plate and determine accurate depths for 63 incoming plate earthquakes occurring during 1988 2011 by inverting teleseismic broadband p and sh waveforms we observe that the top 10 15 km of the incoming plate mantle experiences extensional faulting at all of the subduction zones with a reasonable sample of earthquakes 60 of the total number of extensional earthquakes occur at crustal depths or within the top 5 km of the incoming plate mantle 80 occur above 10 km within the mantle and 95 occur above 15 km there is evidence for variation throughout the different regions of study for example extensional earthquakes occur down to 20 km below the crust in the western aleutians and izu bonin we propose that the incoming plate mantle is most strongly hydrated in the upper 5 km and that partial serpentinization exists regionally within the incoming plate mantle to 15 km making reasonable assumptions about the degree of serpentinization and incorporating previous estimates of crustal water we calculate that the total water carried into the northern and western pacific subduction zones is generally higher than previous estimates and is approximately 4 6 10 8 tg myr or 45 70 10 3 tg myr per kilometer of subduction zone

development of a sequential extraction procedure for iron implications for iron partitioning in continentally derived particulates abstract the development of a sequential extraction procedure for iron in modern and ancient sediments is presented the scheme recognizes seven operationally derived iron pools 1 carbonate associated fe fe carb including siderite and ankerite 2 easily reducible oxides fe ox1 including ferrihydrite and lepidocrocite 3 reducible oxides fe ox2 including goethite hematite and akaganeite 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u as such this is the first extraction scheme specifically developed to allow the separate identification of magnetite and the first to allow a complete evaluation of fe carbonate phases such as siderite and ankerite the scheme was developed following tests on pure mineral phases to evaluate the minerals solubilized by each technique and to determine optimum extraction times further tests on mixtures of pure minerals and on grain size separated sediments from two major us rivers and two glacial meltwaters validate the specificity of the scheme for different pools of iron minerals and demonstrate a high degree of reproducibility for each analytical stage the data obtained for the riverine and glacial sediments are additionally discussed in relation to the dominant modes of transport of different iron minerals in fine grained continental sediments

caldera or flank collapse in the fogo volcano what age consequences for risk assessment in volcanic islands abstract the target of this study is the fogo island in cape verde where the hazards caldera and or flank collapse and the timing 120 ka or younger are still controversial using high resolution dem field geological and structural analysis and high precision k ar dating we produced an age calibrated volcanic stratigraphy of fogo s summit cha das caldeiras for the last ca 220 ka from this we infer the following evolution and associated processes 1 the fogo volcano formed during seven stages of construction and partial destruction 2 three flank collapses can be recognised the biggest of which occurred between 60 and 43 ka 3 this collapse occurred retrogressively producing at least two distinct collapse blocks 4 the innermost collapse was only partial forming a flat step where a new volcano pico do fogo grew and formed the cha das caldeiras literally flat of the calderas 5 the removal of the buttressing eastern flank by the outermost collapse can be responsible for the seemingly caldera structure 6 the growing load of the young volcano can produce a new flank failure following the innermost fault 7 the young and closely spaced collapse ages here reported indicate a significant risk for the inhabitants of fogo in particular and for the whole cape verde archipelago and atlantic seaboard if a tsunami is produced by a future catastrophic flank collapse

morpho structural evolution of a volcanic island developed inside an active oceanic rift s miguel island terceira rift azores the evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass wasting processes more specifically volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation the island of s miguel azores has developed during the last 1 myr inside the active terceira rift a major tectonic structure materializing the present boundary between the eurasian and nubian lithospheric plates in this work we depict the evolution of the island based on high resolution dem data stratigraphic and structural analyses high precision k ar dating on separated mineral phases and offshore data bathymetry and seismic profiles the new results indicate that 1 the oldest volcanic complex nordeste composing the easternmost part of the island was dominantly active between ca 850 and 750 ka and was subsequently affected by a major south directed flank collapse 2 between at least 500 ka and 250 ka the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main e w collapse scar 3 since 250 kyr the western part of this succession furnas area was affected by multiple vertical collapses associated plinian eruptions produced large pyroclastic deposits here dated at ca 60 ka and less than 25 ka 4 during the same period the eastern part of the landslide scar was enlarged by retrogressive erosion producing the large povoacao valley which was gradually filled by sediments and young volcanic products 5 the fogo volcano in the middle of s miguel is here dated between ca 270 and 17 ka and was affected by at least one southwards flank collapse 6 the sete cidades volcano in the western end of the island is here dated between ca 91 and 13 ka and experienced mutliple caldera collapses a landslide to the north is also suspected from the presence of a subtle morphologic scar covered by recent lava flows erupted from alignments of basaltic strombolian cones the predominance of the n150 and n75 trends in the island suggest that the tectonics of the terceira rift controlled the location and the distribution of the volcanism and to some extent the various destruction events

multistage collapse of eight western canary island landslides in the last 1 5 ma sedimentological and geochemical evidence from subunits in submarine flow deposits volcaniclastic turbidites in the madeira abyssal plain provide a record of major landslides from the western canary islands in the last 1 5 ma these volcaniclastic turbidites are composed of multiple fining upward turbidite sands known as subunits the subunits indicate that the landslides responsible for the sediment gravity flows occurred in multiple stages the subunits cannot result from flow reflection or splitting because the compositions of volcanic glasses from each individual subunit in an event bed are subtly different this indicates that each subunit represents a discrete failure as part of a multistage landslide this has significant implications for geohazard assessments as multistage failures reduce the magnitude of the associated tsunami the multistage failure mechanism reduces individual landslide volumes from up to 350 km3 to less than 100 km3 thus although multistage failure ultimately reduce the potential landslide and tsunami threat the landslide events may still generate significant tsunamis close to source

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

large scale flank collapse events during the activity of montagne pelee martinique lesser antilles 1 a horseshoe shaped structure already identified on the southwestern flank of montagne pelee martinique lesser antilles arc was previously interpreted as resulting of a flank collapse event but no debris avalanche deposits were observed at the time new offshore high resolution bathymetry and geophysical data aguadomar cruise december 1998 to january 1999 r v l atalante lead us to identify three debris avalanche deposits on the submarine western flank of montagne pelee extending down to the grenada basin they display morphological fronts and hummocky morphology on bathymetric data speckled pattern on backscatter data and hyperbolic facies on 3 5 khz and seismic profiles new on land geological studies lead us to identify two other horseshoe shaped structures on the same flank of the volcano the three submarine deposits have been traced back to the structures identified on land which confirms the occurrence of repeated flank collapse events during the evolution of montagne pelee the ages of the last two events are estimated at 9 ka and 25 ka on the basis of 14c and 238u 230th dates every flank collapse produced debris avalanches which flowed down to the caribbean sea we propose that the repeated instabilities are due to the large asymmetry of the island with western aerial and submarine slopes steeper than the eastern slopes the asymmetry results from progressive loading by accumulation of volcanic products on the western slopes of the volcano and development of long term gravitational instabilities meteoric and hydrothermal fluid circulation on the floor of the second flank collapse structure also creates a weakened hydrothermalized area which favors the recurrence of flank collapses

radiometric dating of three large volume flank collapses in the lesser antilles arc abstract it is now recognised that flank collapses are a recurrent process in the evolution of the lesser antilles arc volcanoes large magnitude debris avalanche deposits have been identified off the coast of dominica martinique and st lucia with associated volumes up to 20 km 3 deplus c le friant a boudon g komorowski j c villemant b harford c segoufin j cheminee j l 2001 submarine evidence for large scale debris avalanches in the lesser antilles arc earth planet sci lett 192 145 157 we present new radiometric dating of three major events using the k ar cassignol gillot technique in the qualibou depression of st lucia a collapse has been constrained by dome emplacement prior to 95 2 ka in dominica where repetitive flank collapse events have occurred le friant a boudon g komorowski j c deplus c 2002 l ile de la dominique a l origine des avalanches de debris les plus volumineuses de l arc des petites antilles c r geoscience 334 235 243 the plat pays event probably occurred after 96 2 ka inside the depression caused by this event scotts head which is interpreted as a proximal megabloc from the subsequent soufriere avalanche event has been dated at 14 1 ka providing an older bound for this event on martinique three different domes within the carbets structure dated at 337 5 ka constrain the age of this high magnitude event finally these results obtained from three of the most voluminous flank collapses provide constraints to estimate the recurrence of these events which represent one of the major hazards associated with volcanoes of the lesser antilles arc

barium peaks at glacial terminations in sediments of the equatorial atlantic ocean relicts of deglacial productivity pulses in sediment cores recovered from oligotrophic settings of the western and eastern north equatorial atlantic ocean ceara rise and sierra leone rise distinct peaks of excess barium were detected at glacial interglacial transitions these ba maxima are unrelated to any other potential productivity proxy e g organic carbon carbonate or opal moreover they coincide with minima in organic carbon contents despite the lack of correlation between excess barium and organic carbon we ascribe these ba spikes to pulses of productivity that occurred during glacial interglacial transitions the discrepancy between barium and organic carbon in these transitional sediment intervals is attributed to the action of downward progressing oxidation fronts during deglacial nonsteady state depositional conditions the oxidation fronts were initiated due to the overall low sedimentation rates prevailing in the study areas and an increase in bottom water oxygen concentrations at the onset of enhanced nadw production during interglacial periods the fronts led to a very efficient oxidation of the organic carbon initially present leaving peaks in solid phase barium as relicts of these short time productivity events this assumption is supported by the depth arrangement of the solid phase peaks of ba fe and mn along the 6 5 oxygen isotope stage boundary in sediments of the ceara rise which show striking similarities to the distribution of these elements in oxidized sapropel intervals in eastern mediterranean sediments it remains unclear whether productivity was proportional to the magnitude of the baexcess maxima possibly higher dissolved ba concentrations in intermediate and deep waters during glacial interglacial transitions caused the precipitation of barite to increase out of proportion of the postulated deglacial productivity pulses

water takes a deep dive into an oceanic tectonic plate a tectonic plate descending into the mariana trench carries sea water deep into earth s interior it seems that much more water enters earth at this location than was thought with implications for the global water budget the pacific plate carries sea water deep into earth s interior

simultaneous formation of iron rich layers at different redox boundaries in sediments of the amazon deep sea fan high resolution examinations of the solid phase have been performed on a gravity core that shows the typical amazon fan stratigraphy of the last 20 000 years the sedimentary record is characterized by two pronounced enrichments of solid phase fe the first fe peak from 0 34 to 0 37 m sediment depth consists of an indurated fe oxyhydroxide crust located close to the pleistocene holocene boundary this peak represents a widespread nonsteady state diagenetic feature on the fan the second fe peak is a pronounced enrichment of authigenic fe sulfides located between 6 15 and 6 45 m in the sediment the formation of this maximum is attributed to sulfate reduction that is mainly driven by anaerobic methane oxidation based on so42 pore water profiles and calculations of the time of formation of the fe sulfide enrichment we conclude that both fe rich layers formed more or less simultaneously the condition that caused both fe enrichments is a period of nonsteady state diagenesis induced by a pronounced decrease in sedimentation and organic carbon accumulation rates between the pleistocene and the holocene during this depositional change the redox boundaries and reaction fronts were fixed at particular sediment levels for a prolonged time thus producing higher concentrations of the authigenic minerals than possible under steady state diagenetic conditions

large scale catastrophic flank collapses in a steep volcanic ridge the pico faial ridge azores triple junction abstract large scale flank collapses are common in the geological evolution of volcanic ocean islands in the atlantic to date catastrophic lateral collapses in the azores islands have been difficult to identify leading to suggestions that a lack of events may relate to the relatively small size of the islands here we show evidence for two major collapses on the northern flank of pico island pico faial volcanic ridge central azores suggesting that this island had a collapse incidence similar to that of other atlantic volcanic islands the study is based on the analysis of 1 offshore and onshore high resolution digital elevation models 2 field data focused on the n flank and 3 new k ar ages on selected lava flow samples pico sub aerial northern flank is marked by two conspicuous arcuate shaped depressions concave towards the sea here interpreted as landslide scars a main debris field is observed offshore the largest depression this deposit has 20 km of maximum length covers ca 150 km2 is composed of meter to hectometer blocks and has an exposed volume here estimated between 4 and 10 km3 though the actual volume probably exceeds 10 km3 debris flow towards the ese was apparently determined by the slope of the narrow wnw ese s jorge channel young lava flows cascade over the interpreted scars thus concealing the older volcanic sequence s affected by the landslide s new k ar ages measured on these lava flows provide a minimum age of ca 70 ka for the large scale collapse s in pico s northern flank

offshore evidence for a huge landslide of the northern flank of tahiti nui french polynesia a bathymetric analysis of the flanks of tahiti nui french polynesia reveals the existence of a major catastrophic north directed landslide during its eruptive history the related debris avalanche da deposits extend north 70 km from the island shore and have a total volume of about 800 km3 two contrasted facies of the debris are recognized 1 relatively small sized material including a few blocks with well developed flow structures elongate downslope directed ridges and transversal pressure ridges and 2 a scattering of large kilometer sized individual blocks that form in part a prominent bathymetric hummock in the proximal central part of the landslide deposits bathymetric analyses suggest that the tahitian failure evolved through two main successive shots the da debris were rapidly overlain by subsequent volcanic material producing a smooth surface down to 1500 m below sea level the on land and offshore structures indicate a listric fault geometry for the landslide scar the volume removed by the landslide is thus estimated between 400 km3 and 450 km3 discrepancy in the volume of the scar and the debris derived from it is due in part to expansion of the debris to fragmentation of the avalanche and to incorporation of sediments comparison with a recently recognized southern tahitian landslide clouard et al 2001 and with landslides recognized around the hawaiian islands and the canary islands shows that the huge northern tahitian landslide described here has a reduced mobility in its central part perhaps the older tetiaroa eruptive complex to the north acted as a buttress to inhibit movement of the da products the northern and southern tahiti nui landslides moved in opposite directions away from the main central e w directed rift zone that served as the main magma feeding structure then the huge northern landslide strongly controlled the later volcano structural evolution by capturing subsequent volcanic activity to the north despite the northwestward displacement of the pacific plate over the society hot spot

volcanic evolution of sao vicente cape verde islands the praia grande landslide abstract the island of sao vicente has undergone continuous volcanic activity from pliocene to pleistocene times the earliest evidence of activity corresponds to some 9 million years ago when a submarine edifice of alkaline basaltic affinity had already started to develop the island resulted from the growth of a single major edifice the sao vicente edifice built up in several distinguishable growth stages the early main stage occurred at 6 5 to 4 5 ma giving rise to an edifice of about 10 12 km in radius and more than 2500 m in height whose centre was located south of the present city of mindelo although the edifice in its earlier phase showed typical characteristics of a shield volcano the mindelo formation it adopted later on those of an ordinary composite volcano the madeiral monte cara formation a giant landslide event the praia grande landslide destroyed the ne sector of the edifice and left a 10 12 km large depression that was successively refilled by nephelinitic lava flows and some associated carbonatites the monte verde formation 4 5 3 1 ma the volcanic activity ceased about 3 2 ma ago when the edifice started to be deeply eroded until disappearing about 90 of its total volume only in recent times 0 3 ma very scarce and local strombolian activity has been developed aside the sao vicente edifice in the eastern sector of the island

watershed control on the carbon loading of marine sedimentary particles abstract previous investigations of the factors governing organic carbon burial on continental margins have pointed toward the important apparently protective association of carbon with mineral particles these studies have also revealed dramatic transformations of carbon particle relationships at the land sea interface riverine particles in some settings lose a large portion of their loads of sorbed terrestrial carbon upon discharge to the ocean and gradually reload to similar levels with marine carbon the eel river in northern california and the adjacent continental shelf were selected as an ideal system to investigate the rates of these processes the river is episodically subject to large floods and the shelf stratigraphy preserves a record of the resultant large pulses of sediment and carbon input to the marine environment carbon isotopic carbon to nitrogen and carbon to surface area ratios of particles in flood deposits were expected to reflect the rapid unloading of terrestrial carbon from discharged particles whereas nonflood sediments that have accumulated at slower rates on the shelf were expected to carry higher loads of marine carbon our results indicate however that particles on the eel shelf have retained their loads of terrigenous carbon and that a significant portion of the particle sorbed carbon buried on the shelf is kerogen derived from the mesozoic tertiary franciscan complex we hypothesize that rates of uplift and mass wasting in the eel watershed and rates of particle delivery to and burial on the continental shelf are so rapid that kerogen is not completely oxidized and is recycled instead the loading of carbon on clay sized particles delivered to the shelf moreover is dependent on river discharge and may reflect the relative importance of different mass wasting processes during precipitation events of varying intensity the eel river system is likely to be representative of other small mountainous rivers and indicates that processes on land may play an important role in governing the amount and character of carbon being buried on the continental margins

sources of iron for pyrite formation in marine sediments more than two hundred aerobic continental margin aerobic deep sea dysaerobic and anaerobic euxinic sediments have been examined for their variations in different operationally defined iron fractions each of which represents a different reactivity towards dissolved sulfide aerobic continental margin deep sea and dysaerobic sediments contain similar contents of highly reactive iron dithionite soluble iron plus pyrite iron poorly reactive iron iron soluble in hcl less that soluble in dithionite and unreactive iron total iron less that soluble in hcl by contrast non turbidite euxinic samples from the black sea as well as euxinic samples from the cariaco basin and framvaren are enriched in highly reactive iron these sediments contain a small lithogenous fraction and a large biogenous organic c rich fraction which decays by sulphate reduction in an iron rich water column to form pyrite rich sediment other anaerobic euxinic samples from the black sea orca basin and kau bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron degrees of pyritization dop for all the aerobic dysaerobic and anaerobic euxinix samples except those low in biogenous material are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction

recurrent large scale landsliding on the west flank of la palma canary islands a large area of debris avalanche deposits has been discovered on the western submarine flanks of the island of la palma multibeam bathymetry and its derivative backscatter data towed ocean bottom instrument tobi sidescan sonar images and 3 5 khz and airgun seismic reflection data have been used to identify at least two and possibly as many as four major landslide events the youngest of the events the cumbre nueva debris avalanche extends onshore into the valleys bounded by the caldera de taburiente and cumbre nueva ridge which mark the degraded collapse scars radiometric dating of the volcanic flows in the headwall indicate an age of between 536 and 125 ka for this landslide the debris avalanche covers an area of 780 km2 has a maximum thickness of 500 m and has an estimated volume of 95 km3 older deposits collectively referred to as the playa de la veta debris avalanche complex are probably as the name indicates an amalgamation of at least two or three events rather than the result of a single catastrophic failure the playa de la veta debris avalanche complex is associated onshore with an unconformity dated as late matuyama 1 ma to 800 ka it covers an area of 1200 km2 has a maximum thickness of 1300 m and may represent a total volume of up to 650 km3 the greater thicknesses and limited areas occupied by debris avalanches on the western flank of la palma compared to other landslides in the canary archipelago suggest that the la palma landslide masses have relatively low mobility the different debris avalanche lobes formed by each landslide event are separated by channels 2 2 5 km wide the clear relationship between channel position and the boundaries of each debris avalanche lobe indicates that debris avalanches control later channel formation and pathways the relief of the submarine flanks of the la palma volcanoes in the areas of island slope unaffected by landslides is mainly the result of constructional volcanic processes however the older submarine slopes such as in the northern taburiente volcano may also have been modified by smaller scale submarine mass wasting and sediment flows

climate controlled variability of iron deposition in the central arctic ocean southern mendeleev ridge over the last 130 000 years abstract distinct brown coloured mn rich layers are common features in quaternary sediments of the central arctic ocean but their paleoenvironmental significance is disputed glacial interglacial changes in bottom water ventilation variable riverine input and diagenetic processes have all been suggested to cause the cyclicity in optical and geochemical parameters in these deposits to better assess the nature and origin of these sediment intervals and their implications for nutrient cycles we applied bulk geochemical analyses and a sequential fe extraction scheme to a sediment record covering the last 130 ka on the southern mendeleev ridge all brown layers in this core are enriched in mn but also in fe oxyhydr oxides e g ferrihydrite lepidocrocite goethite that contribute to the characteristic sediment colour phosphorus and several trace metals as co cu mo ni are also enriched indicating that both mn and fe oxyhydr oxides adsorbed p and trace metals from the water column the metal enrichments are most probably caused by a combination of enhanced mn and fe input from arctic rivers and or the erosion of shelf deposits coupled with early diagenetic processes leading to precipitation of authigenic mn and fe oxyhydr oxides close to the sediment water interface both primary supply and diagenetic recycling of metals should have been enhanced by a more intense hydrological cycle under warmer climate conditions combined with seasonal increases in productivity and reactive organic matter export during ice free conditions in the central arctic ocean consequently the studied metal oxyhydr oxide rich layers were formed under interglacial interstadial climatic conditions in contrast sheet silicate bound fe e g chlorite a second major fe pool in these sediments 15 35 of total fe is enriched between the oxide rich layers thus the delivery of fe rich sheet silicates from the east siberian hinterland or the bering sea and subsequent deposition on the mendeleev ridge was enhanced by more intense physical weathering during glacial stadial periods these contrasts in fe speciation between the brown layers and the adjacent sediments document the direct response of biogeochemical element cycles to glacial interglacial variations in river input weathering regime and sea level in the arctic ocean

the role of farfield tectonic stress in oceanic intraplate deformation gulf of alaska 1 an integration of geophysical data from the pacific plate reveals plate bending anomalies massive intraplate shearing and deformation and a lack of oceanic crust magnetic lineaments in different regions across the gulf of alaska we argue that farfield stress from the yakutat terrane collision with north america is the major driver for these unusual features similar plate motion vectors indicate that the pacific plate and yakutat terrane are largely coupled along their boundary the transition fault with minimal translation our study shows that the pacific plate subduction angle shallows toward the yakutat terrane and supports the theory that the pacific plate and yakutat terrane maintain coupling along the subducted region of the transition fault we argue that the outboard transfer of collisional stress to the pacific plate could have resulted in significant strain in the ne corner of the pacific plate which created pathways for igneous sill formation just above the pacific plate crust in the surveyor fan a shift in pacific plate motion during the late miocene altered the yakutat collision with north america changing the stress transfer regime and potentially terminating associated strain in the ne corner of the pacific plate the collision further intensified as the thickest portion of the yakutat terrane began to subduct during the pleistocene possibly providing the impetus for the creation of the gulf of alaska shear zone a 200 km zone of intraplate strike slip faults that extend from the transition fault out into the pacific plate this study highlights the importance of farfield stress from complex tectonic regimes in consideration of large scale oceanic intraplate deformation

diagenetic alteration of magnetic signals by anaerobic oxidation of methane related to a change in sedimentation rate geochemical and rock magnetic investigations of sediments from three sites on the continental margin off argentina and uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane aom the western argentine basin represents a suitable sedimentary environment to study nonsteady state processes because it is characterized by highly dynamic depositional conditions mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals as a typical feature of these deposits distinct minima in magnetic susceptibility are observed pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate methane transition smt where hs is generated by the process of aom the released hs reacts with the abundant iron oxyhydr oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate sr which occurred during the pleistocene holocene transition we assume that the strong decrease in mean sr encountered during this glacial interglacial transition induced a fixation of the smt at a specific depth the stagnation has obviously enhanced diagenetic dissolution of iron oxyhydr oxides within a distinct sediment interval this assumption was further substantiated by numerical modeling in which the mean sr was decreased from 100 cm kyr 1 during glacial times to 5 cm kyr 1 in the holocene and the methane flux from below was fixed to a constant value to obtain the observed geochemical and magnetic patterns the smt must remain at a fixed position for 9000 yrs this calculated value closely correlates to the timing of the pleistocene holocene transition the results of the model show additionally that a constant high mean sr would cause a concave up profile of pore water sulfate under steady state conditions copyright 2005 elsevier ltd

iron oxide reduction in methane rich deep baltic sea sediments abstract methane is a powerful greenhouse gas and its emission from marine sediments to the atmosphere is largely controlled by anaerobic oxidation of methane aom traditionally sulfate is considered to be the most important electron acceptor for aom in marine sediments recent evidence suggests however that aom may also be coupled to the reduction of iron fe oxides in the baltic sea the post glacial transition from the ancylus freshwater phase to the littorina brackish marine phase a l transition around 9 7 kyr bp before present resulted in the accumulation of organic rich brackish marine sediments overlying organic poor limnic deposits rich in fe oxides methane produced in the organic rich layer diffuses into the lake sediments thus allowing for the possible coupling between fe oxide reduction and methane oxidation here we combine detailed geochemical analyses of the sediment and pore water retrieved from three sites that were drilled during the iodp baltic sea paleoenvironment expedition 347 with multicomponent diagenetic modeling to study the possible role of fe mediated aom as a mechanism for the apparent fe oxide reduction in the methane bearing lake deposits below the a l transition our results reveal a complex interplay between production oxidation and transport of methane showing that besides organoclastic fe reduction oxidation of downward migrating methane with fe oxides may also explain the elevated concentrations of dissolved ferrous fe in deep baltic sea sediments our findings imply that the transition of a lake toward a marine system could lead to reactivation of deeply buried mostly crystalline fe oxides in organic poor lake deposits through reactions with downward diffusing methane from the overlying organic rich marine sediments based on the geochemical profiles and numerical modeling we propose that a potential coupling between fe oxide reduction and methane oxidation likely affects deep fe cycling and related biogeochemical processes such as burial of phosphorus in systems subject to changes in organic matter loading or bottom water salinity

water input into the mariana subduction zone estimated from ocean bottom seismic data the water cycle at subduction zones remains poorly understood although subduction is the only mechanism for water transport deep into earth previous estimates of water flux1 3 exhibit large variations in the amount of water that is subducted deeper than 100 kilometres the main source of uncertainty in these calculations is the initial water content of the subducting uppermost mantle previous active source seismic studies suggest that the subducting slab may be pervasively hydrated in the plate bending region near the oceanic trench4 7 however these studies do not constrain the depth extent of hydration and most investigate young incoming plates leaving subduction zone water budgets for old subducting plates uncertain here we present seismic images of the crust and uppermost mantle around the central mariana trench derived from rayleigh wave analysis of broadband ocean bottom seismic data these images show that the low mantle velocities that result from mantle hydration extend roughly 24 kilometres beneath the moho discontinuity combined with estimates of subducting crustal water these results indicate that at least 4 3 times more water subducts than previously calculated for this region3 if other old cold subducting slabs contain correspondingly thick layers of hydrous mantle as suggested by the similarity of incoming plate faulting across old cold subducting slabs then estimates of the global water flux into the mantle at depths greater than 100 kilometres must be increased by a factor of about three compared to previous estimates3 because a long term net influx of water to the deep interior of earth is inconsistent with the geological record8 estimates of water expelled at volcanic arcs and backarc basins probably also need to be revised upwards9 seismic images of earth s crust and uppermost mantle around the mariana trench show widespread serpentinization suggesting that much more water is subducted than previously thought

deep sulfate reduction completely mediated by anaerobic methane oxidation in sediments of the upwelling area off namibia abstract porewater concentrations of sulfate methane and other relevant constituents were determined on four sediment cores from the high productivity upwelling area off namibia which were recovered from the continental slope at water depths of 1300 and 2000 m at all four stations a distinct sulfate methane transition zone was observed several meters below the seafloor in which both sulfate and methane are consumed nutrient porewater concentration profiles do not show gradient slope changes at the depths of the transition zones flux calculations carried out on the basis of the determined porewater profiles revealed that anaerobic methane oxidation accounts for 100 of deep sulfate reduction within the sulfate methane transition zone and consumes the total net diffusive sulfate flux a significant contribution of organic carbon oxidation to the reduction of sulfate at these depths could therefore be excluded we state that porewater profiles of sulfate with constant gradients above the transition zones are indicative for anaerobic methane oxidation controlling sulfate reduction

construction and destruction of a volcanic island developed inside an oceanic rift graciosa island terceira rift azores abstract there is a great lack of knowledge regarding the evolution of islands inside active oceanic rifts in particular the meaning of the different evolutionary steps therefore we conducted an investigation in graciosa island which lies at the northwestern end of the terceira rift in the azores triple junction with the objective of constraining the evolution of the island in terms of volcanic growth and mass wasting in particular the meaning and age of the destruction events from digital elevation model dem analysis stratigraphic and tectonic observations k ar dating on key samples and available bathymetry and gravity data we propose that graciosa comprises five main volcanic complexes separated by major unconformities related to large scale mass wasting 1 the older volcanic edifice serra das fontes complex grew until ca 700 ka and was affected by a major flank collapse towards the southwest which removed the whole sw flank the summit and a part of the ne flank 2 the baia do filipe complex developed between at least 472 ka and 433 ka in two different ways in the sw presently offshore as a main volcano and in the ne unconformably over the sub aerial remnants of the serra das fontes complex as secondary volcanic edifices 3 the baia do filipe complex was affected by a major flank collapse towards the sw again removing most of the edifice 4 the remnants of the baia do filipe complex were covered in unconformity by the serra dormida complex between ca 330 and 300 ka which in turn was unconformably covered by the younger basaltic cover complex between ca 300 ka and 214 ka these two units were affected by a third major sector collapse that removed the whole western flank the summit and part of the eastern flank of the serra dormida and basaltic cover complexes 5 despite the relatively young age of graciosa the collapse scars are not well preserved and not active anymore 6 a central type volcano has been growing since at least 60 ka at the southeastern end of the island inside the scar left by a fourth sector collapse towards the se which affected most previous complexes contemporaneously parasitic strombolian cones formed all over the island despite the location of graciosa inside the active terceira rift the new data indicate that the evolution of the island has been driven by a competition between volcano growth and repeated destruction by catastrophic sector collapses rather than by slow incremental faulting associated with the tectonics of the rift

water depth and diagenetic constraints on the use of barium as a palaeoproductivity indicator sediments underneath the high productivity areas of the coastal upwclling and equatorial divergence zones in the pacific and indian oceans are enriched in barium and on this basis the barium content of the sediments has long been suggested as a potential palaeoproductivity indicator analyses of sediments from the peru margin corroborate the pattern of enhanced barium accumulation in areas of high primary productivity in the deep ocean but also show that this pattern cannot be extended to shallow water deposits sediments from the peru shelf lack any barium enrichment whereas this element is significantly enriched in slope and basinal deposits in water columns deeper than 2000 m this depth effect is further illustrated at site 682 located in the rapidly subsiding lima basin the quaternary sequence deposited at water depths 3000 m is enriched in organic carbon opal and biogenic barium the miocene sediments although deposited under highly productive waters associated with a coastal upwclling centre do not show an enrichment in the barium record corresponding to the very high organic carbon and opal levels a pattern that is consistent with deposition in shallow basins in addition to the effect of water depth on the barium distribution overprinting of primary signals can be observed in sediments from upwclling areas undergoing strong anoxic diagenesis microcrystalline barite is partly dissolved in intervals depleted in interstitial sulphate as shown for sediments from the peru margin dissolution of barite in sulphate depleted intervals of deep water sections leads to high barium concentrations near the termination of the sulphate reducing zone where downward sulphate and upward barium diffusion foster local barite precipitation in diagenetic fronts the barium distribution in sedimentary oxic and suboxic environments at deep water depositional sites has a high potential as a palaeoproductivity indicator however barium accumulation as a proxy for ocean fertility should be used with caution in areas of varying water depths and in anoxic diagenetic environments where sulphate depletion undcrsatur ates the interstitial waters with respect to barite

causal link between quaternary paleoclimatic changes and volcanic islands evolution 1 giant landslides and resulting tsunamis represent the main geologic hazards linked to volcanic island evolution from offshore and onland studies flank failures have been identified around numerous islands in most geodynamic contexts however the triggering conditions are still poorly understood and several causes may act simultaneously to reach a critical threshold here we show that most large volume 10 km3 landslides occur at glacial stages termination and we propose that a causal relationship between flank collapse of volcanic islands and global climatic changes has existed at least since 900 kyr moreover ages reported here favour the hypothesis that major collapses occurred during the onset of glacial to interglacial transitions when sudden influx of melt water from polar ice caps causes rapid sea level rise we propose that rapid sea level rise induces enhanced coastal erosion and sudden changes of pore pressure conditions within basal layers which favour edifice failure

tracing the paleo sulfate methane transition zones and h2s seepage events in marine sediments an application of c s mo systematics 1 microbially mediated anaebic oxidation of methane aom coupled with sulfate consumption within the sulfate methane transition zone smtz in marine sediments is a widely recorded biogeochemical reaction and has profound influence on the atmospheric ch4budget marine carbon cycle and composition of sediment pore fluids recognizing the paleo smtzs in the marine sediments rock records can throw light on the variation of paleo methane fluxes and occurrences of cold seep h2s ch4 events through geologic time here we present results from carbonate carbon pyrite sulfur and molybdenum analyses for two sediment cores overlying the methane hydrate deposits in the bay of bengal the results show intimate association of isotopically depleted carbonate carbon and enriched pyrite sulfur constraining the paleo smtz within the sediment column in addition anomalous enrichments of mo concentrations indicate hydrogen sulfide seepage events here we propose a geochemical tool using c s mo sytematics to decipher the paleo smtzs in marine sediments and rocks

a past giant lateral collapse and present day flank instability of fogo cape verde islands fogo island is a large and extremely steepsided oceanic island volcano in the cape verde archipelago it has a large ca 9 km across east facing summit collapse structure the monte amarelo collapse with a probable volume of at least 150 200 km3 for most of its history the monte amarelo volcano had a small but productive central vent complex and radial rift zones fed by laterally propagating dykes shortly before the collapse the latter were replaced by north south trending arrays of en echelon vertically propagating dykes since the monte amarelo collapse the scar has partly filled with a new volcano the cha das caldeiras volcano the summit cone of this volcano the pico do fogo is a very young feature but has been abandoned in the most recent phase of activity from the 18th century onwards the same period has also seen the abandonment of earlier radial rift zones with laterally propagating dykes and their replacement with en echelon arrays of vents fed by vertically propagating dykes these form an n s trending array within the older collapse structure and are associated with seaward displacement of the eastern flank of the volcano within the old collapse structure the most recent eruptions those of 1951 and 1995 appear to be associated with episodes of flank instability manifested in n s surface fissuring and east facing normal faults these recent structural changes in the volcano parallel those which took place in the monte amarelo volcano prior to its collapse

diagenetic changes of magnetic and geochemical signals by anaerobic methane oxidation in sediments of the zambezi deep sea fan sw indian ocean abstract the zambezi deep sea fan the largest of its kind along the east african continental margin is poorly studied to date despite its potential to record marine and terrestrial climate signals in the southwest indian ocean therefore gravity core geob 9309 1 retrieved from 1219 m water depth was investigated for various geophysical magnetic susceptibility porosity colour reflectance and geochemical pore water and sediment geochemistry fe and p speciation properties onboard and onshore data documented a sulphate methane transition smt zone at 450 530 cm sediment depth where the simultaneous consumption of pore water sulphate and methane liberates hydrogen sulphide and bi carbonate into the pore space this leads to characteristic changes in the sediment and pore water chemistry as the reduction of primary fe oxyhydr oxides the precipitation of fe sulphides and the mobilization of fe oxyhydr oxide bound p these chemical processes also lead to a marked decrease in magnetic susceptibility below the smt we find a reduction of porosity possibly due to pore space cementation by authigenic minerals formation of the observed geochemical magnetic and mineralogical patterns requires a fixation of the smt at this distinct sediment depth for a considerable time which we calculated to be 10 000 years assuming steady state conditions following a period of rapid upward migration towards this interval we postulate that the worldwide sea level rise at the last glacial interglacial transition 10 000 years b p most probably caused the fixation of the smt at its present position through drastically reduced sediment delivery to the deep sea fan in addition we report an internal redistribution of p occurring around the smt closely linked to the de coupling of sedimentary fe and p and leaving a characteristic pattern in the solid p record by phosphate re adsorption onto fe oxyhydr oxides above and formation of authigenic p minerals e g vivianite below the smt deep sea fan deposits may potentially act as long term sinks for p

kerogen origin evolution and structure kerogen commonly defined as the insoluble macromolecular organic matter om dispersed in sedimentary rocks is by far the most abundant form of om on earth this fossil material is of prime importance as the source of oil and natural gas moreover kerogen can provide essential information on major topics such as past environments climates and biota this review reports the main advances in kerogen studies since the comprehensive synthesis edited by durand durand b ed kerogen insoluble organic matter from sedimentary rocks editions technip paris 1980 it is organized into eight sections the first two are concerned with the successive definitions of kerogen and the definition used here the different techniques used for kerogen isolation without loss or degradation and basic kerogen analysis the third and fourth focus on sedimentary om sources and preservation processes in relation to depositional environment including sedimentation conditions favourable for kerogen accumulation and extrapolation to past geological time great strides have been made in the latter topics over the last 25 years owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry oceanography hydrology and soil science along with the development of powerful analytical tools the next two sections deal with the different kerogen classifications by type and kerogen evolution and maturation upon burial in sediments structural modelling of coal and kerogen based on physical and or chemical structural analysis is described in the following section although only statistical the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source maturity or physicochemical properties finally the last section explores some of the advances in kerogen understanding expected for the near future the review includes a list containing about 500 references

widespread seawater circulation in 18 22 ma oceanic crust impact on heat flow and sediment geochemistry on the basis of heat flow measurements seismic mapping and sediment pore water analysis we demonstrate widespread and efficient ventilation of the 18 22 ma oceanic crust of the northeast equatorial pacific ocean recharge and discharge appear to be associated with basement outcrops including seamounts and north south trending faults along which sediment cover thins out and volcanic rocks are exposed low temperature hydrothermal circulation through the volcanic crust leads to the reduction of heat flow through overlying sediments with measured heat flow values that are well below those expected from conductive cooling curves for lithosphere of this age typically dissolved pore water oxygen decreases from the sediment surface downward reaching minimum values at mid depth and rising again in the lower part of the cores investigated clearly indicating oxygen rich seawater circulation through the oceanic crust underneath the sediments if the residence time of the circulating fluids in the upper crust is short or the fluid flux is large oxic conditions may be preserved and oxygen can diffuse upwards into the sediments this process leading to widespread oxic conditions in the near basement sediments may cause the oxidation of residual reduced material stored in the deeper sediments resulting in downward fluxes of the reaction products into the basement and from there back into the oceans considering the widespread existence of this type of off axis ventilation the net effect of the resulting return flow of reaction products on biogeochemical cycles and element fluxes e g carbon and nitrogen may be very large

catastrophic flank collapses and slumping in pico island during the last 130 kyr pico faial ridge azores triple junction abstract the pico island constitutes the easternmost sub aerial domain of a steep wnw ese volcanic ridge which has developed within the nubia eurasia diffuse plate boundary azores triple junction the island comprises three volcanic systems from older to younger the topo volcano the fissural system and the pico stratovolcano from a high resolution digital elevation model 10 m and new bathymetric stratigraphic structural and high precision k ar data we reconstruct the main successive stages of growth and partial destruction of the island over the last 200 kyr we especially concentrate on the central sector of the island which has recorded gradual movements through slumping and catastrophic flank collapses since ca 130 kyr the remmants of the topo volcano are partly exposed on pico s se flank and are here dated between 186 5 and 115 4 ka topo was significantly destroyed by n and s directed large scale flank collapses between ca 125 and 70 ka on pico s n flank collapse seems to have removed all the unstable material but in the s the collapse structure is composite including a major flank collapse and a remnant slump complex that is still active a first episode of deformation occurred between ca 125 and 115 ka along the master fault of the slump between ca 115 and 69 ka most of the unstable material was removed by a major flank collapse leaving behind a still considerable volume of unstable material that comprises the active slump this first collapse was catastrophic and generated a large debris deposit recognized on the high resolution bathymetry with a minimum run out of ca 17 km the scar was partially filled by volcanic products erupted from volcanic cones developed within the slump depression and possibly also from the early wnw ese fissural system subsequent deformation in the slump area affected in part the filling units leading to the individualization of secondary curved faults younger volcanic products have gradually masked the mass wasting scars unlike the well known hilina slump hawaii pico s slump evolution might be controlled by an active regional tectonics

catastrophic collapse of the volcanic island of hierro 15 ka ago and the history of landslides in the canary islands landslides play an important role in the evolution of many volcanic islands producing huge fields of blocky volcanic debris on their submarine slopes sidescan sonar images presented in this paper provide evidence for a large debris avalanche el golfo on the northern flank of hierro island in the canary islands angular blocks as much as 1 2 km across and 200 m high cover the debris avalanche surface el golfo avalanche is related to both the canary debris flow and a volcaniclastic turbidite found in the madeira abyssal plain 600 km west of the canaries dating of the turbidite and the failure scarp onshore indicates that the failure probably occurred between 13 and 17 ka there appears to be a general correlation between volcaniclastic turbidites in the abyssal plain sequence and landslides in the canaries during the past 750 ka tentatively this correlation suggests that seven major landslides have affected the canaries in that time

formation of collapse calderas collapse calderas are common in many volcanic regions but the mechanics of their formation is not well understood i propose that collapse calderas may form as a result of the growth of associated shallow magma chambers excess magmatic pressure in a shallow chamber gives rise to tensile stresses at the surface but the location and magnitude of the maximum tensile stress depend on the shape of and depth d to the top of the chamber this model indicates that penny shaped chambers i e sill like chambers of circular horizontal cross sections are most likely to initiate collapse calderas for such a chamber with horizontal radius a the higher the a d radius depth ratio the greater the probability of forming a collapse caldera

repeated occurrences of methanogenic zones diagenetic dolomite formation and linked silicate alteration in southern bering sea sediments bowers ridge iodp exp 323 site u1341 abstract diagenetic precipitates such as dolomite and the chemistry of residual deeply buried porewater often represent the only traces of past biogeochemical activity in marine sediments a 600 m thick sedimentary section recently drilled at integrated ocean drilling program iodp site u1341 on bowers ridge southern bering sea provides insight into such a 4 3 ma old paleo diagenetic archive hard lithified calcite dolomite layers and laminae of disseminated carbonate were recovered in diatom rich sediments over a depth range of 400 m carbon isotope values of the diagenetic carbonates between 16 6 and 14 4 vpdb and strontium isotope ratios of dolomites close to past seawater values suggest carbonate precipitation induced by the production of dissolved inorganic carbon dic during elevated rates of organic carbon mineralization primarily via sulfate reduction at shallow sediment depth below the paleo seafloor diagenetic carbonates at 280 440 m below seafloor were likely also produced by the intermittent onset of sulfate reduction coupled to the anaerobic oxidation of methane aom at sulfate methane transition zones smtz these microbially mediated processes do not occur in the sediment at this site at present but were likely connected to the presence of a methanogenic zone at 2 58 2 51 ma a minimum in sulfate concentrations in modern porewaters and low sedimentary ba al ratios resulting from former sulfate depletion are reminiscent of the presence of this large methanogenic zone the minimum in sulfate concentrations is reflected in a minimum in magnesium concentrations less radiogenic strontium and isotopically light calcium in the porewater it is proposed that magnesium was removed from the porewater during carbonate precipitation and volcanic ash alteration which occurred in the former methanogenic zone and also released strontium with a less radiogenic isotope ratio and isotopically light calcium into the porewater the isotopic composition of porewater calcium was also influenced by ammonium calcium exchange on clay minerals and carbonate recrystallization our study elucidates the response of porewater element concentrations and isotopic profiles interlinked with the formation of diagenetic carbonates to changes in the deposition of organic carbon in sediments of deeper water sites 2000 m water depth over prolonged timescales it shows that variations in biogeochemical processes in response to changes in oceanographic conditions and a dynamic subseafloor biogeochemical zonation have to also be taken into account at these deep water sites for a global assessment of organic carbon burial fluxes and remineralization

effects of turbidity flows on organic matter accumulation sulfate reduction and methane generation in deep sea sediments on the iberia abyssal plain non steady state sedimentation has affected preservation of organic matter in a pliocene pleistocene turbidite sequence sampled by the ocean drilling program at four closely spaced drill sites on the iberia abyssal plain organic carbon concentrations average ca 0 7 in sediments from sites 897 and 898 and ca 0 4 at sites 899 and 900 headspace concentrations of interstitial methane exceed 100 000 ppm in sediments from sites 897 and 898 but are essentially zero in those from sites 899 and 900 methane concentrations do not rise until interstitial sulfate concentrations are virtually depleted suggesting the presence of deep in situ methanogenic bacterial activity at sites 897 and 898 and its absence at sites 899 and 900 two factors associated with the turbidity flows that created the sedimentary sequence evidently influenced post depositional diagenesis at these sites the principal factor is that the rapidly deposited turbidite sequences at sites 897 and 898 protected organic matter from early oxic degradation and thereby permitted subsequent anoxic degradation to proceed in contrast organic matter in the more slowly deposited turbidites at sites 899 and 900 was oxidized soon after deposition and was therefore not available for later microbial utilization a lesser factor is that the turbidity flows may have obtained their entrained organic matter from different environments and consequently delivered organic matter with different characteristics both factors contributed to heterogeneity of the types and amounts of organic matter that accumulated in these deep sea sediments and this variability has influenced subsequent sulfate reduction and methanogenesis

hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank hydrothermal circulation within the sea floor through lithosphere older than one million years myr is responsible for 30 of the energy released from plate cooling and for 70 of the global heat flow anomaly the difference between observed thermal output and that predicted by conductive cooling models 1 2 hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 myr old3 4 but in view of the relatively impermeable sediments that cover most ridge flanks5 it has been difficult to explain how these fluids transport heat from the crust to the ocean here we present results of swath mapping heat flow geochemistry and seismic surveys from the young eastern flank of the juan de fuca ridge which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops we find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies

mid pleistocene climate transition drives net mass loss from rapidly uplifting st elias mountains alaska erosion sediment production and routing on a tectonically active continental margin reflect both tectonic and climatic processes partitioning the relative importance of these processes remains controversial gulf of alaska contains a preserved sedimentary record of the yakutat terrane collision with north america because tectonic convergence in the coastal st elias orogen has been roughly constant for 6 my variations in its eroded sediments preserved in the offshore surveyor fan constrain a budget of tectonic material influx erosion and sediment output seismically imaged sediment volumes calibrated with chronologies derived from integrated ocean drilling program boreholes show that erosion accelerated in response to northern hemisphere glacial intensification 2 7 ma and that the 900 km long surveyor channel inception appears to correlate with this event however tectonic influx exceeded integrated sediment efflux over the interval 2 8 1 2 ma volumetric erosion accelerated following the onset of quasi periodic 100 ky glacial cycles in the mid pleistocene climate transition 1 2 0 7 ma since then erosion and transport of material out of the orogen has outpaced tectonic influx by 50 80 such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out of sequence fault patterns the 1 2 my mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response millions of years the st elias range provides a key example of how active orogenic systems respond to transient mass fluxes and of the possible influence of climate driven erosive processes that diverge from equilibrium on the million year scale

hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single step microbial sulfate reduction coexisting dissolved sulfide and sulfate from hypersulfidic interstitial waters of a 380 m long sediment core show a large isotopic difference of up to 72 caused by in situ microbial sulfate reduction this is considerably larger than the assumed biological maximum of 46 derived from laboratory studies with pure cultures of sulfate reducing bacteria similar high fractionations inferred from sedimentary metal sulfides have been previously explained by a multistage process involving sulfide reoxidation and disproportionation of sulfur intermediates our data show that extreme isotopic differences between sulfate and the reduced sulfur species can also be generated during microbial single step fractionation this result indicates that the sulfate reducing communities and or their cellular metabolic activities in the deep biosphere may differ from those observed in near surface sediments or the water column

evidence for a persistent uplifting of la palma canary islands inferred from morphological and radiometric data abstract the morpho structural evolution of la palma canary islands is characterized by vertical movements responsible for or favoring large landslides in its northern part conglomerate terraces nowadays topographically hanged and re incised by the present day barranco de las angustias reveal a recent ongoing uplift process lava flows covered by this geological unit have been sampled and dated by the k ar cassignol technique at a mean age of 507 11 ka the minimum rate of uplift inferred of about 0 4 mm yr is similar to the one deduced from the abnormal elevation of the early submarine series staudigel and schmincke j geophys res 89 1984 11195 11215 this indicates that the northern shield may have experienced a global rise fairly constant throughout most of its geological history it probably results from the combination of regional atlas tectonic processes and the existence of a swell at the island scale this global rise together with the existence of a north south fault in the basement may have controlled triggering of the sw flank collapse of the northern shield and the subsequent concentration of magma along a southern still active topographic ridge the cumbre vieja cv morphological asymmetry of this structure may result from the destabilization of its distal western part favored by strong injections at the axis tilting processes along lateral listric faults can then explain the existence of slope break ups especially on the western side of the ridge such tectonic processes are likely responsible for local superimposed uplifting contributing to the genesis and development of huge western coastal cliffs even in sub historic and historic times we show that unlike many oceanic islands la palma island has experienced important positive vertical movements rather than subsidence

abrupt thermal transition reveals hydrothermal boundary and role of seamounts within the cocos plate 1 new thermal data from 18 24 ma lithosphere on the cocos plate delineate contrasting subsurface thermal conditions in adjacent sections of crust heat flow through seafloor created at the east pacific rise is generally suppressed by 70 relative to conductive lithospheric cooling models whereas heat flow through adjacent similarly aged lithosphere generated at the cocos nazca spreading center is consistent with these models the transition between thermal regimes is remarkably abrupt only 2 5 km wide indicating a shallow hydrothermal origin the transition is more closely associated with differences in the distribution of basement outcrops than with tectonic boundaries demonstrating the importance of the former in extracting heat from the lithosphere on a regional basis

phosphorus dynamics around the sulphate methane transition in continental margin sediments authigenic apatite and fe ii phosphates abstract the formation of authigenic phosphate minerals in marine sediments is an important process for the burial and long term storage of the bio essential nutrient phosphorus p in this context we report the composition of pore waters bulk sediments and the speciation of p in four sediment cores recovered on the continental margins off the amazon rio de la plata and zambezi rivers here pronounced sulphate methane transitions smts occur between 4 5 and 6 5 m sediment depth where sulphate is consumed by the anaerobic oxidation of methane and free hydrogen sulphide builds up in the pore waters this leads to the reductive dissolution of primary fe oxyhydr oxides feox by hydrogen sulphide and the subsequent liberation of feox adsorbed phosphate into pore waters at the smt the released phosphate builds up to significant concentrations making it available for the precipitation of authigenic minerals within and below the smt using a sequential p extraction we find consistently high contributions of carbonate fluorapatite cfa to the total p pool within the smt where it likely precipitates due to high local phosphate concentrations high alkalinity and abundant dissolved ca in the pore waters phreeqc calculations confirm these results with highest saturation states with respect to authigenic apatite calculated at all smts cfa authigenesis however is insufficient to completely consume pore water phosphate leading to diffusive loss of phosphate from the smt while the upward decrease in phosphate above the smt is relatively gentle the significantly steeper phosphate gradient into underlying sediments suggests the formation of another authigenic mineral phase sequential p extraction and phreeqc results clearly show that the dominant authigenic phase below the smt is not cfa instead geochemical conditions below the sulphidic zone at all four sites are favourable for the precipitation of fe ii phosphate minerals e g vivianite these conditions are summarized as the absence of sulphate and free hydrogen sulphide but the presence of fe 2 and phosphate in pore waters and low calcium carbonate contents in the sediment while we did not directly detect or quantify fe ii phosphate minerals fe bound p clearly increases in the sediments below the sulphidic zones at all studied sites and this is where highest pore water saturation states with respect to vivianite are calculated with phreeqc in addition geochemical inverse models performed in phreeqc show that vivianite formation is an important geochemical process controlling the observed patterns of dissolved p and fe ph and redox potential of the pore waters analysed below the smt we therefore argue that fe ii phosphate minerals are formed in these ocean margin sediments on a large scale and pore water and sediment data indicate that this fe bound p largely originates from feox adsorbed phosphate liberated within the sulphidic smt this study suggests that under specific but not unusual depositional conditions smts are hotspots for biogeochemical p cycling in marginal marine sediments worldwide with cfa precipitating within the smt and fe ii phosphates forming below it it also adds to the growing evidence that fe ii phosphates may contribute significantly to the long term burial of reactive p phases in fe rich methanic marine sediments and thus act as a previously underestimated sedimentary p sink

mineralogical geochemical and isotopic characterization of authigenic carbonates from the methane bearing sediments of the bering sea continental margin iodp expedition 323 sites u1343 u1345 abstract during expedition 323 of the integrated ocean drilling program to the bering sea july 5 september 4 2009 three sites were drilled along the bering sea northeastern continental margin u1343 down to 745 meters below sea floor mbsf u1344 745 mbsf u1345 150 mbsf diagenetic carbonates are present at all sites within the clayey diatom rich oozes of the bering sea where pore waters are also characterized by extremely high methane concentrations we here present mineralogical elemental and isotopic data obtained from the authigenic carbonate rich intercalations within the clay rich pleistocene sediments deposited along the bering sea continental margin the mineralogy of the authigenic carbonates is generally represented by composite mixtures of very small crystals of magnesian calcite dolomite and iron rich carbonates with the latter phases occurring below 260 mbsf at site u1343 below 200 mbsf at site u1344 and below 130 mbsf at site u1345 element geochemistry shows that ca mg fe ba mn sr and u are enriched in the carbonate rich intercalations relative to the background sediments due to their incorporation into the carbonates and into other authigenic phases e g barite and pyrite the oxygen and carbon isotopic compositions of the authigenic carbonate minerals show that they were sequentially precipitated from pore waters at different temperatures i e different burial depths and with different isotopic compositions of dissolved inorganic carbon dic the authigenic mg calcite precipitated early during diagenesis and shallow burial from a 13 c depleted dic pool whereas dolomite and fe rich carbonates formed during later diagenesis and deeper burial from a 13 c enriched dic pool these authigenic carbonate occurrences are interpreted as resulting from microbial sulfate reduction combined with anaerobic oxidation of methane and methanogenesis that was intimately linked to the alteration of silicates especially iron rich clay minerals

tsunami deposits in santiago island cape verde archipelago as possible evidence of a massive flank failure of fogos volcano abstract massive flank failures of volcanic edifices generate tsunami waves these low frequency but high magnitude hazards remain poorly documented because of the scarcity of observations offshore deposits are studied only by geophysical surveys and the failure rheologies are poorly constrained marine conglomerates found at unusually high elevations in hawaii and in the canary islands were previously interpreted as being the result of giant tsunami waves generated by massive flank failures this study focuses on the search for and interpretation of sedimentary evidence of a tsunami around the coastline of santiago island which is located 55 km east of the active fogo volcano the bordeira caldera in fogo island which opens to the east was formed by the monte amarelo flank collapse and the active edifice is now nested in the failure s scar the only evidence of a tsunami was found north of tarrafal bay the deposits display many similarities with the tsunami conglomerate described in hawaii and the canary islands erosive contact with the substratum rip up clasts of paleo soil marine bioclasts never found in growth or live position internal organisation into different facies with distinct characteristics of clast size up to boulders sorting from moderately to very poorly sorted and clast fabrics landward or seaward orientations this organisation suggests that the tsunami consisted of two main waves the absence of tsunami deposits elsewhere on santiago island coupled with the relatively low elevation of the conglomerate in tarrafal