meteoric like fabrics forming in marine waters implications for the use of petrography to identify diagenetic environments petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments however on the basis of oxygen isotopic data we document similar fabrics forming in marine pore fluids in the shallow subsurface of great bahama bank therefore petrographic fabrics alone are not reliable indicators of diagenetic environments even for shallow water sediments in our study skeletal grainstones show two distinct diagenetic assemblages either dissolution of aragonitic grains and minimal cementation high permeability intervals or abundant blocky spar cement and neomorphism of aragonitic skeletal grains low permeability intervals these marine burial fabrics are present as shallow as 110 m below sea level well above the aragonite compensation depth a feature that must be considered for models of diagenesis in ancient carbonate sediments marine burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed

a volcanologist s review of atmospheric hazards of volcanic activity fuego and mount st helens abstract the large amount of scientific data collected on the mount st helens eruption has resulted in significant changes in thinking about the atmospheric hazards caused by explosive volcanic activity the hazard posed by fine silicate ash with long residence time in the atmosphere is probably much less serious than previously thought the mount st helens eruption released much fine ash in the upper atmosphere these silicates were removed very rapidly due to a process of particle aggregation sorem 1982 carey and sigurdsson 1982 rose and hoffman 1982 there is some evidence to suggest that particle aggregation is particularly successful in removing glass shards with high surface areas mass ratios the primary atmospheric hazard of explosive eruptions is volcanic sulfur which is converted to sulfuric acid and sulfate crystals although the mount st helens dacite magma had a very low sulfur content before eruption the eruptions did make a significant contribution to the stratospheric sulfate layer newell 1982 evidence based on measurements of s and cl in erupted rocks glass inclusions gas samples and atmospheric samples collected for both mount st helens and fuego volcanoes suggests that both volcanoes released substantial contributions of s from intrusive non eruptive magma the amount of sulfur contributed to the atmosphere by an explosive eruption thus depends not only on the volume of magma erupted and its sulfur content but also on the degree of near surface non eruptive magma the data collected to assess atmospheric hazard and to evaluate the processes and mechanisms of explosive volcanic eruptions have helped illuminate our understanding of 1 the dispersion and atmospheric fractionation of volcanic ash and 2 the determination of the size and degassing energetics of shallow magma bodies beneath volcanoes

age constraints on the origin and growth history of a deep water coral mound in the northeast atlantic drilled during integrated ocean drilling program expedition 307 sr isotope stratigraphy provides a new age model for the first complete section drilled through a deep water coral mound the 155 m long section from challenger mound in the porcupine sea bight southwest of ireland is on miocene siliciclastics and consists entirely of sediments bearing well preserved cold water coral lophelia pertusa the 87sr 86sr values of 28 coral specimens from the mound show an upward increasing trend correspond to ages from 2 6 to 0 5 ma and identify a significant hiatus from ca 1 7 to 1 0 ma at 23 6 m below seafloor the age of the basal mound sediments coincides with the intensification of northern hemisphere glaciations that set up the modern stratification of the northeast atlantic and enabled coral growth mound growth persisted throughout glacial interglacial fluctuations reached a maximum rate 24 cm k y ca 2 0 ma and ceased at 1 7 ma unlike other buried mounds in porcupine seabight challenger mound was only partly covered during its growth interruption and growth restarted ca 1 0 ma

variations in column height and magma discharge during the may 18 1980 eruption of mount st helens abstract peak eruption column heights for the b1 b2 b3 and b4 units of the may 18 1980 fall deposit from mount st helens have been determined from pumice and lithic clast sizes and models of tephra dispersal column heights determined from the fall deposit agree well with those determined by radar measurements b1 and b2 units were derived from plinian activity between 0900 and about 1215 hrs b3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs a brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height 19 km during deposition of b4 variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass balance calculations based on the volume of pyroclastic flows and coignimbrite ash peak magma discharge occurred during the period 1215 1630 hrs when pyroclastic flows were generated by collapse of low fountains through the crater breach pyroclastic flow deposits and the widely dispersed co ignimbrite ash account for 77 of the total erupted mass with only 23 derived from plinian discharge a shift in eruptive style at noon on may 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw up and tapping of deeper levels in the magma reservoir as predicted by theoretical models of magma withdrawal return to plinian activity late in the afternoon when magma discharge decreased is consistent with theoretical predictions of eruption column behavior the dominant generation of pyroclastic flows during the may 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable convective eruption column to a collapsing one

late miocene molluscs from the southwestern atlantic ocean argentina and uruguay a palaeobiogeographic analysis abstract the analysis of the structure and evolutionary history of the late miocene marine molluscs along the southwestern atlantic littoral indicates that these faunas did not give rise to the present assemblages of the argentinean province of the same region the latter being characterised by a typically transitional association in the sense of scarabino 1977 com soc malac urug 4 177 285 that developed after miocene times once the cold malvinas falkland current was completely operating the late miocene valdesian and paranaian provinces mainly composed of paratropical warm water faunas are defined in this paper the former includes assemblages recovered from the puerto madryn formation its southern limit is placed at 42 s while in the north its boundary with the paranaian province is situated approximately between 37 and 39 s the paranaian province includes assemblages belonging to the parana and camacho formations it extends northwards along the uruguayan and southern brazilian littoral but its northern boundary still remains unknown an incipient thermal gradient is recognised as the temperature values for the paranaian province are slightly higher than those for the valdesian province taking into account that in the valdesian province water temperature was in fact warm it is concluded that the cold malvinas falkland current still had a very low activity in the area during the late miocene the presence of upwelling phenomena is also suggested by the presence of widely distributed turritellid beds in the northern patagonian region

sedimentation at manop site h eastern equatorial pacific over the past 400 000 years climatically induced redox variations and their effects on transition metal cycling gravity cores recovered from manganese nodule project site h 6 33 n 92 49 w show marked downcore variations in the abundance of calcium carbonate organic carbon opal manganese and other components deposited over the past 400 000 years variations in the downcore abundance of organic carbon which ranges from 0 2 to 1 0 can be used to hindcast redox conditions in the surface sediments over this time the results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles downcore variations in solid phase mn ni and cu can be produced by such changes in redox conditions a model which predicts that solid phase mn can be trapped and buried when the mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and mn records and supports the reconstructed redox variations the history of redox variations at site h can be explained by changes with time in surface water productivity major productivity variations at the site occur over 100 kyr cycles with relatively higher productivity occurring during glacial stages thus quaternary climate changes influence surface water productivity redox conditions in sediments and the cycling of transition metals

reefs of the deep the biology and geology of cold water coral ecosystems coral reefs are generally associated with shallow tropical seas however recent deep ocean exploration using advanced acoustics and submersibles has revealed unexpectedly widespread and diverse coral ecosystems in deep waters on continental shelves slopes seamounts and ridge systems around the world advances reviewed here include the use of corals as paleoclimatic archives and their biogeological functioning biodiversity and biogeography threats to these fragile long lived and rich ecosystems are mounting the impacts of deep water trawling are already widespread and effects of ocean acidification are potentially devastating

giant carbonate mud mounds in the southern rockall trough seismic profiles sidescan sonar data bottom photography and sampling proved for the first time that there are giant carbonate mounds bioherms in the rockall trough the pelagia mounds on the northern porcupine bank are relatively isolated whereas most of the logachev mounds on the southeast rockall bank form a field of closely spaced contiguous mounds they are all found on the upper slope at depths between about 500 and 1200 m and as a result of this study about 500 have been mapped they have a variety of shapes and the larger ones are very steep sided up to 350 m high and 2 km wide at the base sediment samples show that the mounds consist of pale coloured muds mainly aragonite usually with live and or dead cold water corals at the seafloor and with buried dead corals coral thickets appear to be detected as a brush like signature on high resolution profiles and as a speckled pattern on high resolution sidescan sonar records shelly sands are found on the seabed between the mounds currents in the mound areas are strong enough to transport sands along slope as medium sized sand waves and to prevent pelagic deposition thus providing surfaces for initial coral settlement by the winnowing of fines to leave glacial dropstones and exposed rocks these observations show that the poleward directed upper slope current of the eastern north atlantic extends further south than hitherto known and that there is a southwesterly directed current on rockall bank there are waves on the logachev mounds with a wavelength of about 20 30 m that are thought to be moulded in carbonate muds by across slope directed internal tidal currents and or cascading currents mounds develop above erosional surfaces seen on seismic data the huge amount of carbonate mud seems to be produced by the rapid growth and breakdown of cold water corals living in a very favourable environment intermediate nepheloid layers formed upstream by the strong currents may provide the food source for the deep water suspension feeders conditions for giant mound growth require an ideal balance between current speed and sedimentation rate among other factors no evidence for the suggested link to methane seepage has been found so far

missing molluscs as evidence of large scale early skeletal aragonite dissolution in a silurian sea in the silurian carbonate platform succession of gotland sweden interreef limestone and marl sequences have shallow marine shelly assemblages dominated by brachiopods typical of many paleozoic benthic faunas locally from similar facies where early skeletal silicification occurred associated with bentonites there are rich and diverse assemblages of originally aragonitic molluscs quantitative faunal analysis suggests that massive depletion of skeletal aragonite chiefly molluscs took place and the entire molluscan fauna may generally be underrepresented by at least 100 fold previous workers on the gotland succession have already shown that abundant diagenetic limestones were sourced by early aragonitic remobilization our findings indicate molluscs as the probable source of this diagenetic calcite not aragonite mud as was previously thought the taphonomic window represented by the silicified faunas illustrates the importance of aragonitic shells among the original silurian shelf benthos and raises the possibility that the paucity of such shells among many fossil faunas may be largely an early taphonomic effect

growth and erosion of a cold water coral covered carbonate mound in the northeast atlantic during the late pleistocene and holocene the first detailed stratigraphic record from a deep water carbonate mound in the northeast atlantic based on absolute datings u th and ams 14c and stable oxygen isotope records reveals that its top sediment sequences are condensed by numerous hiatuses according to stable isotope data mainly sediments with an intermediate signal are preserved on the mound while almost all fully glacial and interglacial sediments have either not been deposited or have been eroded later the resulting hiatuses reduce the late pleistocene sediment accumulation at propeller mound to amounts smaller than the background sedimentation the hiatuses most likely result due to the sweeping of the mound in turn with the re establishment of vigour interglacial circulation patterns after sluggish current regimes during glacials thus within the discussion if internal fluid driven or external environmentally driven processes control the evolution of such carbonate mounds our findings for propeller mound clearly point to environmental forcing as the dominant mechanism shaping deep water carbonate mounds in the ne atlantic during the late pleistocene and holocene

questioning carbonate diagenetic paradigms evidence from the neogene of the bahamas carbonate diagenetic models have been heavily influenced by numerous studies of exposed quaternary limestones as a result meteoric diagenesis is often assumed to be the principle means of altering aragonite rich sediments into calcitic limestones however these models are limited by the scarcity of examples of aragonite rich sediments buried in seawater that have never been influenced by meteoric fluids the bahamas transect cores recovered originally aragonite rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms the bahamas transect consists of seven cores drilled in the prograding western margin of great bahama bank the two proximal cores clino and unda were drilled on the platform top and recovered shallow water platform to reef facies overlying deeper margin and proximal slope facies the five distal cores were drilled by odp leg 166 in up to 660 m of water and recovered carbonate slope facies all studied sections are neogene to pleistocene in age diagenetic environments were identified based on petrographic and scanning electron microscopy sem observations xrd mineralogy carbon and oxygen stable isotopic data and trace elements the upper 100 150 m of the two proximal cores were altered in meteoric to mixing zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor marineburial and deep burial diagenesis several of the findings of this study question current carbonate diagenetic paradigms these include 1 large scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the 3120 m elevation of the latest pleistocene lowstand rather phreatic meteoric diagenesis appears restricted to within w10 m of the land surface 2 mixing zone diagenesis includes aragonite dissolution and minor lmc cementation but does not show the cavernous porosity or dolomitization predicted by mixing zone diagenetic models current models are based on coastal mixing zones which do not appear to be applicable to these more inland and perhaps more typical locations 3 marine burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity aragonite neomorphism blocky calcite spar and calcite microspar however oxygen stable isotopic data average n 18 o 1x indicate alteration in marine pore fluids only the character of marine burial diagenesis is partially controlled by the nature of the sediment being altered we have identified two end member styles an open system style characterized by dissolution of aragonite without significant cementation and a more closed system style with aragonite dissolution

computer simulation of deep sulfate reduction in sediments of the amazon fan pore water concentration profiles of sediments at a site on the amazon fan were investigated and simulated with the numerical model cotrem column transport and reaction model to reveal the biogeochemical processes involved the pore water profiles for gravity core geob 4417 7 showed a distinct sulfate methane transition zone in which deep sulfate reduction occurs only a small sulfide peak could be observed at the reaction zone due to high amounts of iron minerals the produced sulfide is instantaneously precipitated in form of iron sulfides we present a simulation which starts from a steady state system with respect to pore water profiles for methane and sulfate furthermore sulfide iron ph pe calcium and total inorganic carbon tic were included in the simulation the program calculated mineral equilibria to mackinawite iron sulfides more stable than mackinawite iron hydroxides and calcite via saturation indices si by a module incorporating the program phreeqc parkhurst 1995 the measured sulfide and iron profiles are obtained in the simulation output by using a constant si 0 for mackinawite and calcite while a depth dependent si distribution is applied for the phreeqc phases pyrite and fe oh 3 a representing a composition and the kinetics of different iron sulfides and iron hydroxides these si distributions control the results of sulfide and iron pore water profiles especially conserving the sulfide profile at the reaction zone during the simulation the results suggest that phases of iron hydroxides are dissolved mackinawite is precipitated within and other iron sulfides are precipitated below the reaction zone the chemical reactivity of iron hydroxides corresponds to the rate of sulfide production the system h2o co2 caco3 is generally successfully maintained during the simulation deviations to the measured ph profile suggest that further processes are active which are not included in the simulation yet

late glacial and holocene palaeocurrents around rockall bank ne atlantic ocean core samples and hydrographic data from 50 to 60 n and 15 to 25 w in the northeast atlantic ocean are used to study changes in current activity from the last glacial to present we establish a new method to distinguish between the effects of changing bottom current speed and varying size input by drawing upon sediment flux data and detailed grain size analysis changes in current speed are recorded by the mean size of the sortable coarse noncarbonate silt component which increases with current vigor by winnowing away of fines our method involves the definition of an ideal sortable silt input function recorded at a site accumulating unmodified pelagic flux only the ideal profile is then compared with records from other sites to determine the history of current speed at a range of water depths the upper surface of southern source water ssw probably shoaled during the glacial as indicated by the covariation of sortable silt records from sites presently covered by ssw and lower north atlantic deep water lnadw the data suggest that production of lnadw was suppressed during the glacial increased following the last glacial maximum and declined at the start of termination 1a a second pulse of production occurred immediately before the younger dryas event intermediate waters were generally faster flowing during the glacial and may have expanded their depth domain such that a single glacial intermediate water mass covered depths from 1100 to more than 2000 m

the holocene occurrence of cold water corals in the ne atlantic implications for coral carbonate mound evolution abstract u series dating of constructional cold water corals is a powerful tool to reconstruct the evolution of corals on carbonate mounds here we have investigated the time framework of corals such as lophelia pertusa and madrepora oculata on five different mound settings of the eastern north atlantic on rockall bank and in porcupine seabight sampled at variable depth and location 610 880 m water depth we have found that the past 11 ka reflect a period generally favourable for coral development we further determined local mound growth rates and identified mound surface erosion framework collapse during times of active coral framework construction local vertical mound growth rates vary between less than 5 cm ka 1 and up to 220 cm ka 1 we interpret rates exceeding 15 cm ka 1 as representative of densely populated coral reefs during times of reduced or absent coral development mound evolution rates are by far smaller 0 to 1 the time resolution achieved here furthermore provides first evidence for reduced coral ecosystem activity at 1 8 2 0 ka 4 2 4 8 ka and between 6 and 8 2 ka within the holocene that may be related to climate driven changes of the coral growth environments during glacial periods coral growth in those areas seems apparently extremely reduced or is even absent on mounds

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

northeastern atlantic cold water coral reefs and climate u series age patterns obtained on reef framework forming cold water corals collected over a nearly 6000 km long continental margin sector extending from off mauritania 17 degrees n northwest africa to the southwestern barents sea 70 degrees n northeastern europe reveal strong climate influences on the geographical distribution and sustained development of these ecosystems over the past three glacial interglacial cycles framework forming cold water corals lophelia pertusa and madrepora oculata seem to have predominantly populated reefs canyons and patches in the temperate east atlantic and the mediterranean sea above 50 degrees n corals colonize reefs in the northern east atlantic primarily during warm climate periods with the biogeographic limit advancing from similar to 50 degrees n to similar to 70 degrees n we propose that north south oscillations of the biogeographic limit of reef developments are paced by ice ages and may occur synchronously with north south displacement of cold nutrient rich intermediate waters and surface productivity related to changes of the polar front

late miocene ground sloth footprints and their paleoenvironment megatherichnum oportoi revisited abstract fossil footprints of ground sloths are mostly restricted to the late miocene to pleistocene of south america this study is focused on the oldest known ground sloth trackways megatherichnum oportoi casamiquela 1974 from the late miocene rio negro formation of northern patagonia the section logged at the study site near carmen de patagones buenos aires province includes the uppermost part of the middle member marine and the upper member aeolian of the rio negro formation identified sedimentary facies can be grouped into four facies associations tidal flat aeolian dune dry damp interdune and wet interdune shallow lacustrine m oportoi trackways are preserved in an upper intertidal mixed flat associated trace fossils belong to a wet aeolian system developed close to the sea coast and were mostly preserved in interdune facies these include poaceae root and stem traces and nagtuichnus meuleni chlamyphorinae burrow in dry interdune facies indeterminate tetrapod footprints in damp interdune facies lockeia siliquaria and chevron like trace fossils bivalve resting and horizontal locomotion traces in wet interdune facies shallow lakes and palaeophycus tubularis in aeolian dune facies the most likely trackmaker of m oportoi is pyramiodontherium sp megatheriinae recorded from the same stratigraphic unit at chubut province with a body mass ranging from 2 5 to 3 6 t m oportoi is interpreted as a quadrupedal trackway of a late miocene ground sloth without overlap of the pes on the manus the previously proposed bipedal interpretation for the trackway requires angles of gait in excess of those permitted for such a large and heavy animal pleistocene ground sloth trackways are distinctive because of the much smaller size of the fore footprint for the latter examples a quadrupedal locomotion with overlap of the pes on the manus may be applied

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

aragonite cycles diagenesis caught in the act organic biomarker and nannofossil assemblages are used in combination with sedimentary petrology to identify the role of diagenesis for the formation of a rhythmic calcareous succession a record from the early pliocene of the maldives ocean drilling program odp leg 115 is presented containing precession and eccentricity cycles expressed as variable aragonite content additional sub milankovitch cycles are caused by rhythmic precipitation of calcite cement in the lower part of the interval comparison with palaeo productivity indicators nannofossils chlorin total organic carbon suggests that cementation occurs preferentially in intervals characterized by increasing or decreasing productivity the coupled variability in productivity and carbonate diagenesis is attributed to the effect of organic matter degradation in the sediment the observed combination of primary and diagenetic factors hampers the interpretation of the cyclicity on milankovitch and sub milankovitch scales diagenetically stable proxies for palaeo productivity such as nannofossil assemblage data were used to distinguish between palaeoclimate and diagenetic influences

chemical model for the origin of minor limestone shale cycles by anaerobic methane oxidation chemical and isotopic studies of jurassic carbonate concretions indicate an origin by anaerobic methane oxidation concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late rejuvenated phase of sulfate reduction similar chemical and isotopic characteristics are shown by diagenetic carbonate in several limestone shale sequences and an origin by anaerobic methane oxidation is proposed for these sequences also the evolution of isolated concretions into diagenetic limestones depends on the extent of and depth variations in carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive sup 34 s values than the adjacent shales

rock magnetic characterization of early redoxomorphic diagenesis in turbiditic sediments from the madeira abyssal plain rock magnetic measurements of two sediment cores from the madeira abyssal plain map north atlantic are used to investigate post depositional changes in the concentration grain size and composition of magnetic minerals in the sediments that have occurred within organic rich turbidite horizons the changes are associated with an initial stage of suboxic reductive diagenesis following depletion of porewater o2 and a later stage of oxidative diagenesis associated with the slow descent of an oxidation front through the sediment as a result of diffusion of o2 from the overlying sea water the turbidites are of late quaternary age 18o stages 1 3 and derive both from different sites on the nw african continental margin and from the flanks of the canary islands thus the turbidites are variable compositionally especially in terms of carbonate detrital magnetic mineral and organic carbon content diagenetic changes in these sediments have been identified using solid phase geochemical data u mn corg and caco3 reported previously in more than one study rock magnetic parameters of the sediments when expressed on a carbonate free basis reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains has occurred within organic rich turbidites during redoxomorphic diagenesis normalized quotients of magnetic parameters also show that reductive diagenesis is a ferrimagnetic grain size selective process but it has a minimal effect on the canted antiferromagnetic fe3 oxides in the sediment such components if present therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are dissolved progressively and bulk magnetic concentration is thus depleted there is clear evidence in both cores for the existence of ultrafine ferrimagnetic grains at depth within the suboxic zone of the organic rich turbidites beneath both active and fossil oxidation fronts these grains are most probably associated with populations of live magnetotactic bacteria which commonly inhabit such organic rich horizons and play a part in the chain of bacterially mediated reactions normally associated with suboxic diagenesis these results show that simple and rapid rock magnetic techniques can be used to characterize early diagenetic processes involving iron phases in deep sea sediments at least as effectively as more laborious time consuming and sample destructive geochemical measurements

a 3d seismic study of the morphology and spatial distribution of buried coral banks in the porcupine basin sw of ireland an industrial 3d seismic data volume supplemented by high resolution 2d seismics was used to study part of a province of buried mound structures in the porcupine basin southwest of ireland these magellan mounds and their associated moat structures interpreted as scour marks were mapped semi automatically from time structure and isopach maps image analysis techniques such as a tophat transformation mathematical morphology were applied for feature extraction size measures of both mounds and moats were derived from the resulting maps and summarised by means of some descriptive statistics spatial variability in mound occurrence and characteristics was investigated comparison with other mound structures in the area allowed the magellan mounds to be identified as coral banks associated with the growth of cold water deep sea coral species such as lophelia pertusa l and madrepora oculata l mound growth clearly started in a single event confined in time and space bottom currents and oceanographic characteristics of the surrounding water masses influenced this sudden process and the further mound development however the analysis of the 3d seismic data set did not allow us to identify unambiguously the actual cause for the sudden mound start up the mounds appear to have formed a dense cluster of structures of moderate size which are significantly elongated in a n s direction they are associated with even more elongated moats implying a periodically reversing n s directed current influence a spatial density of one mound per km2 was measured which remains more or less constant over the area investigated mound width and cross sectional area and moat shape gradually change across the mound province due to spatially changing environmental conditions at the initial growth stages of the mounds and during their further development interplay between current regime and sedimentation

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

carbonate budget of a cold water coral mound challenger mound iodp exp 307 abstract cold water coral ecosystems represent a worldwide distributed carbonate factory which is so far not considered in global carbonate budget estimations the herein presented study evaluates the carbonate budget of a complete cold water coral mound sequence from challenger mound belgica mound province porcupine seabight sw offshore ireland drilled during iodp expedition 307 reaching back to the late pliocene versus background sediment derived calcite dominated carbonate cores from adjacent drift deposits were examined for comparison studies challenger mound which developed in a fast growth phase unit m1 2 7 to 1 6 ma and a slower growth phase or mound decline unit m2 0 8 ma until today separated by a so called mound crisis displays increased carbonate accumulation rates for both phases 17 3 g cm 2 ka for unit m1 and 5 7 g cm 2 ka for unit m2 compared to adjacent drift deposits but only about 33 to 40 wt of the carbonate is derived from corals bulk sedimentation rates of on and off mound deposits suggest a decline of challenger mound since the onset of drift deposition in the area about 1 2 ma ago yet total carbonate accumulation rates as well as hemipelagic background sediment derived carbonate accumulation rates from challenger mound unit m2 still exceed the carbonate accumulation rates of the adjacent drift deposits the selective enrichment of background sediment derived carbonate by a factor of 2 0 in a cwc mound is herein observed for the first time this might be caused by the different hydrodynamic behaviour and slower settlement of marine fluff rich in calcareous detritus compared to siliciclastic sediments and its effective baffling by the coral framework and or the active catch of the calcareous plankton by the corals the carbonate accumulation rates of challenger mound are about 4 to 12 of the carbonate accumulation rates of tropical shallow water reefs but exceeds the carbonate accumulation rates of continental slopes by a factor of 3 9 to 11 8 the carbonate production for the entire belgica mound province is estimated with 3 2 10 3 t a for unit m1 and 1 0 10 3 t a for unit m2

the initial giant umbrella cloud of the may 18th 1980 explosive eruption of mount st helens abstract the initial eruption column of may 18th 1980 reached nearly 30 km altitude and released 10 17 joules of thermal energy into the atmosphere in only a few minutes ascent of the cloud resulted in forced intrusion of a giant umbrella shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m s the mushroom cloud expanded 15 km upwind forming a stagnation point where the radial expansion velocity and wind velocity were equal the cloud was initiated when the pyroclastic blast flow became buoyant the flow reduced its density as it moved away from the volcano by decompression by sedimentation and by mixing with and heating the surrounding air observations indicate that much of the flow covering an area of 600 km 2 became buoyant within 1 5 minutes and abruptly ascended to form the giant cloud calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant assuming an initial temperature of 450 c and a magmatic origin for the explosion these calculations indicate that the flow became buoyant when its temperature was approximately 150 c and the flow consisted of a mixture of 3 25 10 11 kg of pyroclasts and 5 0 10 11 kg of air if sedimentation is considered these figures reduce to 1 1 10 11 kg of pyroclasts and 1 0 10 11 kg of air

aggradation and carbonate accumulation of holocene norwegian cold water coral reefs cold water coral ecosystems present common carbonate factories along the atlantic continental margins where they can form large reef structures there is increasing knowledge on their ecology molecular genetics environmental controls and threats available however information on their carbo nate production and accumulation is still very limited even though this information is essential for their evaluation as carbonate sinks the aim of this study is to provide high resolution reef aggradation and carbonate accumulation rates for norwegian cold water coral reefs from various settings sunds inner shelf and shelf margin furthermore it introduces a new approach for the evaluation of the cold water coral preservation within cold water coral deposits by computed tomography analysis this approach allows the differentiation of various kinds of cold water coral deposits by their macrofossil clast size and orientation signature the obtained results suggest that preservation of cold water coral frameworks in living position is favoured by high reef aggradation rates while preservation of coral rubble prevails by moderate aggradation rates a high degree of macrofossil fragmentation indicates condensed intervals or unconformities the observed aggradation rates with up to 1500 cm kyr 1 exhibit the highest rates from cold water coral reefs so far reef aggradation within the studied cores was restricted to the early and late holocene available datings of norwegian cold water corals support this age pattern for other fjords while on the shelf cold water coral ages are reported additionally from the early middle holocene the obtained mean carbonate accumulation rates of up to 103 g cm 2 kyr 1 exceed previous estimates of cold water coral reefs by a factor of two to three and by almost one order of magnitude to adjacent sedimentary environments shelf slope and deep sea only fjord basins locally exhibit carbonate accumulation rates in the range of the cold water coral reefs furthermore cold water coral reef carbonate accumulation rates are in the range of tropical reef carbonate accumulation rates these results clearly suggest the importance of cold water coral reefs as local maybe regional to global carbonate sinks

low temperature magnetic properties of pelagic sediments ocean drilling program site 805c tracers of maghemitization and magnetic mineral reduction magnetic properties of pelagic sediments from the western equatorial pacific ocean drilling program odp site 805c have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth a saturation isothermal remanent magnetization mrs was given to the samples by applying a strong magnetic field 2 5 t at 20 k after cooling both in the absence zero field cooling zfc and presence of a 2 5 t magnetic field field cooling fc the thermal demagnetization of both mrszfc and mrsfc was measured from 20 to 300 k no pronounced verwey transition was observed in the data from samples shallower than the iron redox boundary below the boundary the verwey transition is visible for all samples studied we interpret this behavior as reflecting low temperature oxidation maghemitization of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary the low temperature oxidation may occur both at the water sediment interface and deeper in the suboxic zone of sediments as a result of bacteria mediated processes including the reduction of nitrate compounds mrszfc and mrsfc thermal demagnetization curves of all samples diverge for the entire temperature range measured the divergence may be caused by the low temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite maghemite system together maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low temperature properties of the sediment studied as a result low temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments the changes in effective oxidation state below the iron redox boundary detected by low temperature methods may be useful in the study of some paleoenvironmental processes

trajectories of the mount st helens eruption plume the plume of the major eruption of mount st helens on 18 may 1980 penetrated 10 to 11 kilometers into the stratosphere attaining heights of 22 to 23 kilometers wind shears rapidly converted the plume from an expanding vertical cone to a thin slightly inclined lamina the lamina was extruded zonally in the stratosphere as the lower part moved eastward at jet stream velocities while the upper part slowly moved westward in the region of nonsteady transition from the westerlies to the summer stratospheric easterlies trajectories computed to position the nasa u 2 aircraft for sampling in the plume are described plume volume after 8 hours of strong volcanic emission is estimated at 2 x 106cubic kilometers only about 1 percent of this volume is attributed to the volcano the rest was entrained from the environment

the avaatech xrf core scanner technical description and applications to ne atlantic sediments abstract x ray fluorescence xrf core scanning provides rapid high resolution down to 1 mm records of chemical composition on split sediment cores the measurements are non destructive and require very limited sample preparation the new avaatech xrf core scanner operational since 2002 covers the atomic mass range from al to u instrument parameters especially tube voltage can be adjusted to provide optimum settings for selected elements or sets thereof owing to the nature of the surface of split sediment cores particularly effects resulting from sample inhomogeneity and surface roughness results are semiquantitative yet provide reliable records of the relative variability in elemental composition downcore selected case studies from diverse sedimentary settings in the ne atlantic ocean illustrate a range of applications of xrf logging data these include preliminary stratigraphic interpretations glacial interglacial cycles provenance studies of the terrigenous sediment fraction lithological characterization early diagenetic processes and distinction between carbonate phases aragonite v calcite

paleo redox fronts and their formation in carbonate mound sediments from the rockall trough abstract piston cores from the summits of coral topped carbonate mounds at the south west rockall trough margin reveal that the sediments have undergone significant post depositional modifications affecting the original geochemical signature and mineralogical composition of the sediments this diagenetic imprint provides information about the geochemical processes within cold water coral mounds the most prominent result of diagenetic alteration of the primary sediment composition is the absence or poor preservation of aragonitic coral skeletons in certain depth intervals associated with lithification this study focuses on the enrichment of redox sensitive elements and the dissolution of primary magnetic ferric iron minerals in the depth interval below lithification levels by combining the magnetic susceptibility with the xrf signal of fe and ti specifically intervals with susceptibility variations related to the conversion of strongly magnetic into weakly magnetic iron species can be defined in all three studied cores a succession is recognised with a lithified interval with aragonite dissolution and low mg calcite precipitates that is underlain by an interval of magnetite dissolution and of iron and manganese enrichment for the most recent lithified interval it is demonstrated that initial lithification occurred before an erosional regime was in place most likely near the end of interglacial or at the start of glacial periods

large deep water coral banks in the porcupine basin southwest of ireland the porcupine basin southwest of ireland was one of the earliest sites from where the deep water corals lophelia sp and madrepora sp were recovered these deep water corals have since been found all along the atlantic margins of europe in water depths ranging from 50 to more than 2000 m recent geophysical studies have demonstrated the mound building potential of deep water corals available data indicate that three major provinces of coral bank occurrences can be identified in the porcupine basin 1 high relief surface mounds which have a dimension of 1 by 5 km and a height up to 200 m hovland mounds flanked to the north by 2 a swarm of buried mounds somewhat smaller up to 90 m and with more irregular shapes than those recognised in area 1 magellan mounds and 3 outcropping or buried conical mounds single or in elongated clusters up to 150 m high occurring on the southeastern slope of the basin belgica mounds as far as can be inferred from shallow cores the surface lithology predominantly consists of an upper layer rich in foraminiferal sand and terrigenous silty clay with intercalations of biogenic rubble the banks host a remarkable number of colonies of living and dead lophelia pertusa and madrepora oculata the living and dead assemblages are underlain by a significant layer of coral debris in a muddy matrix deep water coral debris together with a living association of the same species covers the surface of the belgica and hovland mounds which may suggest that these corals have played a significant role in the development of the mound structures the capacity for mound formation by scleractinian corals in the aphotic zone has been known for some time examples are found at different locations along the shelves and the continental margins of the north atlantic the role of the corals in these deep water build ups is still a point of debate though the genesis and initial control of mound settings in this basin might be related to hydrocarbon seeps it appears that the major development of the porcupine coral banks in recent geological times has most likely been controlled by oceanic circulation and dynamics in water masses and nutrient supply

mobility and immobility of redox sensitive elements in deep sea turbidites during shallow burial during the first few tens of thousands of years after an organic rich turbidite is emplaced on an abyssal plain surficial oxidation of the unit occurs by downwards diffusion of bottom water o2 from work on piston cores it is known that a suite of redox sensitive elements form characteristic relocation peaks in such units around the deepest reaches upper few dm achieved by oxidation fossil oxic post oxic boundaries four individual turbidites now buried at depths of 130 230 m below seafloor mbsf obtained by odp leg 157 drilling on the madeira abyssal plain map have been investigated to determine the effects of burial diagenesis on these geochemical signals over 3 7 my porewater data indicate that post oxic conditions persist to around 130 mbsf on the map below which sulphate reduction predominates all four units have retained high total organic carbon corg contents 1 4 in their lower reaches despite the fact that bottom waters have previously oxidised this same organic matter in the turbidite tops to 0 2 the apparent inertness of the turbidite corg contrasts with the reactivity of organic matter observed in hemipelagic sediment from other odp investigations in all four cases low initial caco3 contents have been quantitatively removed along with corg oxidation with additional dissolution of carbonate by undersaturated bottom waters diagenetic fractions of the elements se cd sb tl and v are present in highly localised peaks and show no evidence of migration in different combinations the elements cu co ni and zn appear to have migrated over short 5 cm distances in thin mm dark coloured sulphide bands probably sourced by reduction of a mn oxyhydroxide host phase during early 70 ky burial two examples of local diagenetic pyritisation up to 20 wt fes2 are observed in the bodies of the two turbidites buried to 130 mbsf confirming that sulphate reduction is active at this level this diagenetic pyrite is associated with high concentrations of as and lesser enrichments of ni zn co se sb and possibly mo

carbonate budget of a cold water coral carbonate mound propeller mound porcupine seabight high resolution studies from the propeller mound a cold water coral carbonate mound in the ne atlantic show that this mound consists of 50 carbonate justifying the name carbonate mound through the last 300 000 years approximately one third of the carbonate has been contributed by cold water corals namely lophelia pertusa and madrepora oculata this coral bound contribution to the carbonate budget of propeller mound is probably accompanied by an unknown portion of sediments buffered from suspension by the corals however extended hiatuses in propeller mound sequences only allow the calculation of a net carbonate accumulation thus net carbonate accumulation for the last 175 kyr accounts for only 0 3 g cm2 kyr which is even less than for the off mound sediments these data imply that propeller mound faces burial by hemipelagic sediments as has happened to numerous buried carbonate mounds found slightly to the north of the investigated area

carbonate mound development at the sw rockall trough margin based on high resolution tobi and seismic recording abstract in 2002 high resolution sidescan sonar images of a mound area at the sw rockall trough margin were recorded with the tobi deep towed sidescan sonar processed tobi images with a pixel resolution of 6 m provide a unique overview of the carbonate mound distribution and related sedimentary features around the mounds three morphologically distinct areas can be recognised on the tobi mosaic in area i between 500 and 600 m water depth upslope of the mounds giant sediment waves are found with wavelengths up to 500 m with wave crests of more than 10 km long and with an amplitude of up to 30 m reflected with a high backscatter on the tobi image area ii between 600 and 1000 m water depth along the upper margin flank is characterised by clustered and isolated carbonate mounds forming elongated ridges with an orientation perpendicular to the slope sedimentary structures such as flow ridges sediment waves local scouring at the foot of some mounds and draping of sediment around mounds indicate the influence of strong near bed currents oriented in two main current directions parallel to the margin as well as in a similar direction as the mound clusters the mound clusters are several kilometres long can be over 380 m high and are dissected by downslope directed channels on the tobi image the mounds appear as regions of high backscatter caused by the presence of cold water coral colonies on the sediment and by distinct shadows all mounds have their tops at a specific depth level 500 600 m water depth area iii between 900 and 1200 m water depth below the mounds is characterised by disturbed sediments with distinct slump scars and flow ridges 2d high resolution seismic profiling perpendicular and parallel to the slope across the mounds does in general not reveal strong internal reflectors within the mounds in contrast three strong reflectors can be observed in the sedimentary sequence underneath the mounds the upper reflector c10 of early pliocene age is an erosional unconformity above which most of the mounds and sediment waves seem to have developed the second erosional unconformity c20 of middle miocene age is formed by an apparent irregular surface that locally is dissected by some small faults reflectors c10 and c20 have been found in two different mound clusters in the mound area indicating that mound development possibly started after formation of reflector c20 the third reflector forms the acoustic basement which is locally dissected by some small faults that are located below the mounds at least two stages of mound formation are recognised on the seismic profiles

intermediate and deep water paleoceanography of the northern north atlantic over the past 21 000 years benthic foraminiferal stable isotope records from four high resolution sediment cores forming a depth transect between 1237 m and 2303 m on the south iceland rise have been used to reconstruct intermediate and deep water paleoceanographic changes in the northern north atlantic during the last 21 ka spanning termination i and the holocene typically a sampling resolution of 100 years is attained deglacial core chronologies are accurately tied to north greenland ice core project ngrip ice core records through the correlation of tephra layers and changes in the percent abundance of neogloboquadrina pachyderma sinistral with transitions in ngrip the evolution from the glacial mode of circulation to the present regime is punctuated by two periods with low benthic 13c and 18o values which do not lie on glacial or holocene water mass mixing lines these periods correlate with the late younger dryas early holocene 11 5 12 2 ka and heinrich stadial 1 14 7 16 8 ka during which time freshwater input and sea ice formation led to brine rejection both locally and as an overflow exported from the nordic seas into the northern north atlantic as earlier reported by meland et al 2008 the export of brine with low 13c values from the nordic seas complicates traditional interpretations of low 13c values during the deglaciation as incursions of southern sourced water although the spatial extent of this brine is uncertain the records also reveal that the onset of the younger dryas was accompanied by an abrupt and transient 200 300 year duration decrease in the ventilation of the northern north atlantic during the holocene iceland scotland overflow water only reached its modern flow strength and or depth over the south iceland rise by 7 8 ka in parallel with surface ocean reorganizations and a cessation in deglacial meltwater input to the north atlantic

sediment accumulation on a cold water carbonate mound at the southwest rockall trough margin abstract cold water corals build isolated and clustered carbonate mound ridges several kilometres long and wide and up to 380 m high at the southwest rockall trough margin photo and video observations as well as coring have shown that mainly the upper mound flanks and summits are covered with a thriving living coral cover and associated fauna box cores prove the presence of living coral colonies on top of a thick layer of coral debris which is to a large extent abraded and bio eroded before being buried the coral framework is filled with sediment from settling pelagic material and parts of the fauna living at the mounds resulting in a 120 cm thick layer of sediments since 10 800 yr component analysis of a piston core from the top of a carbonate mound shows a complete record of cold water coral mound facies comprising many different species living at the mounds an alternation of skeletal and cement dominated intervals was distinguished in the piston core stable isotope analysis of planktonic and benthic foraminifera indicates a continuous sedimentation pattern since the younger dryas the deeper and older part of the core showing mainly intermediate isotope values is dominated by the presence of large hiatuses spanning a total of up to 200 000 yr hiatuses in the core may reflect periods of non deposition or erosion which may be linked to glacial interglacial variability environmental conditions probably change at glacial interglacial timescales influencing the local hydrodynamic regime food supply and sedimentation rate around the carbonate mounds affecting coral growth and therefore carbonate mound development

deep sea corals evidence periodic reduced ventilation in the north atlantic during the lgm holocene transition abstract nine new intermediate water ages estimated with coupled uranium series and radiocarbon measurements on deep sea corals from the northeastern atlantic are presented together with six intermediate and deep water ages from the western north atlantic adkins et al science 280 1998 725 728 and one from the equatorial atlantic mangini et al nature 392 1998 347 348 they span the time period from 15 400 to 560 yr ago and show abrupt variations of intermediate and deep water ages from 270 up to 2320 yr both major pulses of meltwater discharge mwp 1a following heinrich event 1 and mwp 1b after the younger dryas cold event are followed by enhanced supply of southern source water at intermediate depths in the north atlantic together with reduced formation of well ventilated glacial north atlantic intermediate water 1000 2000 m this result gives strong support to numerical models predicting fast and sharp increase of bottom water age in the atlantic triggered by pulses of freshwater discharge into the north atlantic rahmstorf nature 372 1994 82 85 stocker and wright radiocarbon 40 1998 359 366

microspar development during early marine burial diagenesis a comparison of pliocene carbonates from the bahamas with silurian limestones from gotland sweden comparison of ultrastructures in pliocene periplatform carbonates from the bahamas with silurian limestones from gotland sweden reveals that despite the differences in primary sediment composition and age they reflect a similar mechanism of lithification in both sequences calcite microspar was formed as a primary cement at an early stage of marine burial diagenesis neither significant compression nor meteoric influence are necessary for the formation of calcite microspar a model is proposed for the process of microsparitic cementation of fine grained aragonite needle muds comprising four stages 1 unconsolidated aragonite dominated carbonate mud 2 precipitation of microspar that engulfs aragonite needles 3 dissolution of aragonite resulting in pitted surfaces of the microspar crystals and 4 slight recrystallization our results contradict the widespread opinion that microspar necessarily is a product of secondary recrystallization of a previously lithified micrite

late miocene continental biota in northeastern patagonia peninsula valdes chubut argentina abstract a new vertebrate faunal assemblage was recently discovered from the uppermost part of the late miocene puerto madryn formation these deposits crop out along the southwestern coast of the peninsula valdes area near punta delgada chubut province argentina the exhumed vertebrate fauna includes a range of fish bird and mammal taxa of which the latter are most varied and abundant the new findings represent the first record of continental fossil vertebrates in the puerto madryn formation and this is the first assemblage of late miocene continental vertebrates recorded to the south of rio negro province it also includes the southernmost record of hydrochoeridae rodents dendrocygninae birds and loricariidae fishes the mammals suggest that the fossil bearing sediments are huayquerian in age the climate during the accumulation of late miocene deposits in this region is inferred to have been warmer and seasonally drier than that of today

chronology and pyroclastic stratigraphy of the may 18 1980 eruption of mount st helens washington many timed observations make it possible to subdivide the 9 hour plinian eruption of mount st helens on may 18 1980 into six phases defined by eruption style the phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits the suite of pyroclastic deposits indicates that the eruption became more pumice rich and compositionally diverse with time perhaps owing to concurrent eruption of less evolved gas poor parts of the magma body with the more evolved gas rich parts the paroxysmal phase i 0832 0900 consisted of landslides lithic pyroclastic flows of a lateral blast and other explosions and a weak pre plinian column phase i pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer the early plinian phase ii 0900 1215 consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks 1010 1035 a brief period of lithic ash ejection 1035 1100 and a pumice rich pulse that accompanied growth in height of the eruption column 1100 1215 deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer the lower lithic ash layer and the middle pumice lapilli layer all of which consist of evolved white dacitic pumice 63 64 sio2 during the early ash flow phase iii 1215 1500 the height of the eruption column decreased vertical ejection of tephra ceased and pyroclastic flows were fed from intermittent fountains phase iii deposits consist of a poorly exposed sequence 12 m of ash flow tuff that consist of many thin flow units 2 m each containing pumiceous white dacite 63 64 sio2 and denser gray silicic andesite 61 62 sio2 and fine grained ash cloud deposits interbedded with a nongraded middle pumice ash layer the climactic phase iv 1500 1715 developed in two stages fountain fed pyroclastic flows followed by a short pulse 1625 1715 of vigorous vertical ejection of tephra these stages were accompanied by the peak seismic energy release and peak eruption column height respectively climactic deposits consist of a thick 35 m sequence of thick lapilli rich ash flow sheets 4 12 m each with white and gray pumice and streaky scoria bands 60 sio2 in pumice breccia clasts and the reversely graded upper pumice lapilli layer that is interbedded with fine grained ash cloud deposits during the late ash flow phase v 1715 1815 eruption intensity waned but included a brief episode of small pyroclastic flows 1745 1815 phase v deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice and minor fine ash deposits phase vi activity 1815 to may 19 1980 consisted of a low energy ash plume with transient increases in intensity while seismicity continued at depth sustained vertical discharge of phase ii prodeced evolved dacitewith high s cl ratios ash flow activity of phase iii is attributed to decreases in gas content indicated by reduced s cl ratios and increased clast density of the less evolved gray pumice climactic events are attributed to vent clearing and exhaustion of the evolved dacite

structure and development of giant carbonate mounds at the sw and se rockall trough margins ne atlantic ocean abstract high resolution seismic reflection profiling carried out in 1997 1999 showed that giant carbonate mounds occur between 500 and 1200 m water depth along both the se and sw margins of rockall trough the mounds rise 5 300 m above the surrounding seafloor and have diameters at their bases of up to 5 km buried mounds at relatively shallow depths below the seafloor are also found both individual and complex clusters of mounds can be recognized smaller and individual sometimes buried mounds are found at the upper slope on the sw rockall trough margin higher steeper and individual mounds are found deeper downslope 900 1100 m at the middle slope the mounds merge into a complex structure and form complex clusters with a very irregular upper surface and an apparent lack of internal reflectors 600 1000 m depth initial results of high resolution 3d seismic profiling at 30 m line spacing in august 2000 in two boxes of 750 m 14 5 km covering a small central part of the sw rockall trough mound area indicate two stages of mound development here the latest probably of pliocene holocene age this age based on the presence of a regionally recognizable unconformity below the mounds is also assumed for the mounds of the se rockall trough porcupine bank margin

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

aragonite loss in a cold water coral mound mechanisms and implications selective dissolution of aragonitic grains is emerging as a volumetrically significant process that affects a broad range of modern carbonate settings this study explores mechanisms and implications of aragonite loss in challenger mound a giant cold water coral lophelia pertusa mound of pleistocene age which lies on the continental slope off south west ireland a comprehensive sampling scheme allowed the integration of petrographic data with geochemical analyses of sediment and pore water the mound remains virtually unlithified and consists of stacked fining upward cycles of silty coral floatstone rudstone and bafflestone grading into wackestone whereas calcitic grains appear unaltered aragonitic grains are corroded and fragmented aragonite dissolution is attributed to organic matter oxidation at near the sediment water interface and at greater depths to the initial stages of bacterially mediated sulphate reduction when alkalinity production is outpaced by the generation of h pore water profiles indicate that undersaturated waters are diffusing towards the mound interior from two centres of sulphate reduction one located in the upper 10 m of the sediment column and a second that lies below an erosional unconformity which marks the base of the mound continued aragonite dissolution is expected to gradually lower the diagenetic potential of the challenger mound and delay lithification until deep burial when solution compaction processes come into play despite a fundamental role in predestining the final taphonomic and textural characteristics of challenger mound the processes described here are expected to leave little trace in the geological record due to a lack of cementation and calcitization assuming that similar processes have been active throughout the phanerozoic results imply that the understanding of diagenetic processes in carbonate systems may be incomplete

environmental changes and growth history of a cold water carbonate mound propeller mound porcupine seabight on and off mound sediment cores from propeller mound hovland mound province porcupine seabight were analysed to understand better the evolution of a carbonate mound the evaluation of benthic foraminiferal assemblages from the off mound position helps to determine the changes of the environmental controls on propeller mound in glacial and interglacial times two different assemblages describe the holocene and marine isotope stage mis 2 and late mis 3 31 kyr bp the different assemblages are related to changes in oceanographic conditions surface productivity and the waxing and waning of the british irish ice sheet biis during the last glacial stages the interglacial assemblage is related to a higher supply of organic material and stronger current intensities in water depth of recent coral growth during the last glaciation the benthic faunas showed high abundances of cassidulinid species implying cold bottom waters and a reduced availability of organic matter high sedimentation rates and the domination of elphidium excavatum point to shelf erosion related to sea level lowering 50 m and the progradation of the biis onto the shelf a different assemblage described for the on mound core is dominated by discanomalina coronata gavelinopsis translucens planulina ariminensis cibicides lobatulus and to a lower degree by hyrrokkin sarcophaga these species are only found or show significantly higher relative abundances in on mound samples and their maximum contribution in the lower part of the record indicates a higher coral growth density on propeller mound in an earlier period they are less abundant during the holocene however this dataset portrays the boundary conditions of the habitable range for the cold water coral lophelia pertusa which dominates the deep water reefal ecosystem on the upper flanks of propeller mound the growth of this ecosystem occurs during interglacial and interstadial periods whereas a retreat of corals is documented in the absence of glacial sediments on mound glacial conditions with cold intermediate waters a weak current regime and high sedimentation rates provide an unfavourable environmental setting for lophelia corals to grow a late pleistocene decrease is observed in the mound growth for propeller mound which might face its complete burial in the future as it already happened to the buried mounds of the magellan mound province further north

diagenetic formation of gypsum and dolomite in a cold water coral mound in the porcupine seabight off ireland authigenic gypsum was found in a gravity core retrieved from the top of mound perseverance a giant cold water coral mound in the porcupine basin off ireland the occurrence of gypsum in such an environment is intriguing because gypsum a classic evaporitic mineral is undersaturated with respect to sea water sedimentological petrographic and isotopic evidence point to diagenetic formation of the gypsum tied to oxidation of sedimentary sulphide minerals i e pyrite this oxidation is attributed to a phase of increased bottom currents which caused erosion and enhanced inflow of oxidizing fluids into the mound sediments the oxidation of pyrite produced acidity causing carbonate dissolution and subsequently leading to pore water oversaturation with respect to gypsum and dolomite calculations based on the isotopic compositions of gypsum and pyrite reveal that between 21 6 and 28 6 of the sulphate incorporated into the gypsum derived from pyrite oxidation the dissolution of carbonate increased the porosity in the affected sediment layer but promoted lithification of the sediments at the sediment water interface thus authigenic gypsum can serve as a signature for diagenetic oxidation events in carbonate rich sediments these observations demonstrate that fluid flow steered by environmental factors has an important effect on the diagenesis of coral mounds

growth history of a cold water coral covered carbonate mound galway mound porcupine seabight ne atlantic abstract during the past decade knowledge about the ecology and the environment of giant carbonate mounds has been growing continuously however still little is known about their growth dynamics three gravity cores from galway mound belgica mound province in the porcupine seabight off ireland were investigated for their sedimentological geophysical and geochemical properties to get insight into the long term development of this cold water coral covered carbonate mound these data were supplemented by radiometric age determinations on planktonic foraminifera and coral skeletons the records from three different settings on galway mound reveal a coherent growth history that in general is similar to what is known from other carbonate mounds at the irish margin however whereas other cores are often disturbed by numerous and not correlateable hiatuses galway mound in contrast appears to be characterised by only one major hiatus representing a time gap of 250 kyr several mechanisms are discussed in this study as possible causes for the observed stratigraphic record at galway mound the most likely explanation is that the hiatus has its origin in a major mass wasting event on an instable possibly glacial unit that could have acted as a slip plane the overall late quaternary growth history of galway mound fits well into existing cyclical mound development models pointing to galway mound being an actively growing mound coral bank stage at present

cortex a shipboard xrf scanner for element analyses in split sediment cores abstract the cortex corescanner texel is a non destructive xrf system for the logging of split sediment cores it is a seaworthy instrument that remains active during heavy weather conditions the scanner produces reliable counts for k to sr highly correlated with concentrations measured with atomic absorption spectrometry aas the option to scan al si and elements heavier than sr is in preparation a core section of 1 m can be scanned at 2 cm intervals within an hour a maximum resolution of 1 mm can be achieved at present each region and sedimentary environment requires separate calibration in the nearby future the generator settings will be standardized which may result in the development of a standard calibration procedure because of the high resolution of its semi quantitative records the cortex is a useful tool for detailed time series analysis inter core correlation and shipboard analysis

pink fairy armadillo meniscate burrows and ichnofabrics from miocene and holocene interdune deposits of argentina palaeoenvironmental and palaeoecological significance abstract the possible producer and environmental setting of large meniscate burrows nagtuichnus meuleni igen and isp nov occurring in late miocene and holocene aeolian deposits from argentina are discussed these are unbranched burrows with an uniform meniscate filling that range in width from 46 to 78 mm n meuleni distinguishing features are a filling composed of a discontinuous outer massive layer between a central meniscate core and the excavation boundary sets of parallel ridges in the external wall and a particular outline plus the presence of paired pits in the concave surface of menisci or hollows in the convex surface the late miocene lower member of the rio negro formation at la loberia rio negro province is interpreted as aeolian dune damp interdune and dry interdune deposits the holocene sediments from gran salitral southwestern la pampa province represent aeolian dune wet interdune and dry interdune deposits n meuleni occurs only in damp and dry interdune deposits the composite ichnofabric produced by n meuleni in the holocene of the gran salitral is composed of two suites of trace fossils a former skolithos suite includes skolithos linearis taenidium barretti polykladichnus aragonensis and root traces developed in lacustrine calcareous mudstone deposits wet interdune this suite is overprinted by n meuleni with subordinate participation of s linearis and root traces composing the nagtuichnus suite the later suite is developed in brown silt and sand of dry interdune areas the nagtuichnus ichnofabric is considered indicative of damp to dry interdune facies of late miocene to holocene age the burrow systems of the likely producers of the holocene n meuleni structures were studied in the field and in the laboratory the candidates were selected from the extant local fauna of the gran salitral including southern cavies microcavia australis rodentia tuco tucos ctenomys azarae rodentia and pink fairy armadillos or pichiciegos chlamyphorus truncatus xenarthra the most likely producer of n meuleni are pichiciegos because they have fully subterranean habits and insectivorous diet involving the production of long unbranched horizontal burrows excavated in sand which are mostly backfilled in contrast to this tuco tucos and southern cavies which are herbivores that forage aboveground use their more permanent burrows for dwelling and reproduction as observed in a live captive kept c truncatus the meniscate structure of the fill can be explained by the animal packing the excavated sand with its rump plate this mechanism also results in a massive layer of sediment in the burrow fill other significant bioglyphs that are a sort of fingerprint of pichiciegos are the outline of menisci in cross section mimicking their rump plate and pairs of rounded pits in the concave surface of menisci probably tail traces

first radioisotope potassium argon age of marine neogene rionegro beds in northeastern patagonia argentina an average age of 9 41 million years was obtained from radioisotope potassium argon age determinations of three glass concentrates of a tuff from the upper part of the marine rionegrense at punta cracker in golfo nuevo argentina this age correlates with the tortonian marine stage of europe and the chasicoan land mammal age of south america

diagenetic pyritisation under eastern mediterranean sapropels caused by downward sulphide diffusion recurrent organic rich layers sapropels in eastern mediterranean sediments are enriched in corg fe and s sulphur and fe are enriched in a zone immediately below the sapropels whereas corg is not 34s values of bulk sediments and simple mass balance calculations indicate that so42 reduction has taken place in an open system with all hs formed at or close to the sediment surface formation of pyrite in the sapropel was fe limited and consequently excess hs was able to migrate downwards downward sulphidisation this resulted in the formation of pyrite below the sapropel by reaction of this hs with solid phase ferric iron and fe2 diffusing upwards from underlying sediments the fe2 source probably includes fe hydr oxide layers formed at former oxidation fronts above previously deposited and buried sapropels this downward sulphidisation mechanism allows accumulation of twice as much s in alternating organic rich anoxic organic poor suboxic sediments compared to what is preserved in organic rich anoxic sediments

diagenetic processes in carbonate mound sediments at the south west rockall trough margin cold water coral covered carbonate mounds at the south west margin of the rockall trough form ridges several kilometres long and up to 380 m high piston cores obtained at three mound crests reveal the complex internal structure of the mound build up with alternating unlithified coral dominated intervals and lithified intervals the most recent lithified interval is covered by corals embedded in a fine grained matrix comprising ca 11 000 years of continuous mound evolution before this time 230th u dating shows the presence of several hiatuses in mound build up aragonitic coral material is absent or only present as mouldic porosity in the lithified intervals and coccoliths display widespread overgrowth downcore x ray fluorescence scanning computer tomography scan images and petrographic observations indicate different degrees of diagenetic alteration the upper boundary of the most recent lithified interval shows some erosional features but petrographic observations indicate that initial lithification of the sediments is not related to this erosive event or to long term non sedimentation but to earlier sub surface diagenesis organic matter oxidation and the subsequent lowering of the saturation state of the carbonate system drives dissolution of the unstable aragonitic coral skeletons depending on the openness of the system this can lead to precipitation of a more stable low magnesium carbonate a model is presented describing the sedimentary and diagenetic processes leading to the formation of lithified intervals

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

carbonate mound development in contrasting settings on the irish margin cold water coral carbonate mounds formed by framework building cold water corals are found in several mound provinces on the irish margin differences in cold water coral mound development rates and sediment composition between mounds at the southwest rockall trough margin and the galway mound in the porcupine seabight are investigated variations in sediment composition in the two mound provinces are related to the local environmental conditions and sediment sources mound accumulation rates are possibly higher at the galway mound probably due to a higher influx of hemipelagic fine grained non carbonate sediments in both cold water coral mound areas mound growth has been continuous for the last ca 11 000 years before this period several hiatuses and unconformities exist in the mound record the most recent unconformity can be correlated across multiple mounds and mound provinces at the irish margin on the basis of apparent age on the southwest rockall trough margin these hiatuses unconformities are associated with post depositional aragonite dissolution in and lithification of certain intervals while at galway mound no lithification occurs this study revealed that the influx and types of material transported to cold water coral mounds may have a direct impact on the carbonate mound accumulation rate and on post depositional processes significantly the logachev mounds on the sw rockall trough margin accumulate slower but because they contain lithified layers are less susceptible to erosion this net effect may account for their larger size compared to the belgica mounds

hydrography and through flow in the north eastern north atlantic ocean the nansen project abstract the circulation and hydrography of the north eastern north atlantic has been studied with an emphasis on the upper layers and the deep water types which take part in the thermohaline overturning of the oceanic conveyor belt over 900 hydrographic stations were used for this study mainly from the 1987 1991 period the hydrographic properties of subpolar mode water in the upper layer which is transported towards the norwegian sea showed large regional variation the deep water mass was dominated by the cold inflow of deep water from the norwegian sea and by a cyclonic recirculation of lower deep water with a high antarctic bottom water content at intermediate levels the dominating water type was labrador sea water with only minor influence of mediterranean sea water in the permanent pycnocline traces of antarctic intermediate water were found geostrophic transports have been estimated and these agreed in order of magnitude with the local heat budget with current measurements with data from surface drifters and with the observed water mass modification a total of 23 sv of surface water entered the region of which 20 sv originated from the north atlantic current while 3 sv entered via an eastern boundary current of this total 13 sv of surface water left the area across the reykjanes ridge and 7 sv entered the norwegian sea while 3 sv was entrained by the cold overflow across the iceland scotland ridge approximately 1 4 sv of norwegian sea deep water was involved in the overflow into the iceland basin which with about 1 1 sv of entrained water and 1 1 sv recirculating lower deep water formed a deep northern boundary current in the iceland basin at intermediate depths where labrador sea water formed the dominant water type about 2 sv of entrained surface water contributed to a saline water mass which was transported westwards along the south icelandic slope