modern sedimentary processes in the wilmington canyon area u s east coast abstract sedimentary processes active in the wilmington canyon area during the past century are documented from geochronologic sedimentologic and radiographic analysis of sediment cores collected using primarily d s r v alvin site selection for the alvin dives and for collection of box and kasten cores was achieved using sea marc and seabeam data allowing accurate sampling of numerous subenvironments from this extensively dissected area of the continental margin 210 pb sediment accumulation rates from intercanyon areas are highest on the upper slope 1 7 mm yr 140 mg cm 2 yr and decrease with water depth to about 0 4 mm yr 36 mg cm 2 yr on the upper rise within submarine canyons accumulation rates vary dramatically from a maximum of 6 mm yr 500 mg cm 2 yr to less than 0 3 mm yr 22 mg cm 2 yr higher accumulation rates in canyons occur as a result of additional supply of older sediment probably from bioerosion of the canyon walls this is supported by observations from d s r v alvin reduced 210 pb activities and indurated mud clasts observed in several canyon cores extremely low accumulation rates in wilmington and south wilmington canyons are probably the result of strong episodic boundary currents non steady state 210 pb profiles and sedimentologic data indicate small scale mass wasting in south heyes canyon and on the lower slope and rise just north of wilmington canyon

recent geologic history of lake atitlan a caldera lake in western guatemala abstract lake atitlan a caldera lake in western guatemala was investigated for evidence of recent volcanic and tectonic activity no vents faults or folds are apparent on high resolution seismic reflection profiles of lake sediment representing at least 17 500 years and probably more than 35 000 years of deposition three post caldera stratovolcanoes san pedro toliman and atitlan have grown in southern parts of the caldera and two deltas have grown from the north shore of the lake elsewhere the caldera is occupied by lake atitlan which is more than 300 m deep and has a relatively flat floor refraction profiles suggest that the original floor of the caldera lies ca 300 m below the current lake floor but prodigious amounts of methane gas in the lake sediment attenuated seismic signals and prevented any detailed view of the original caldera floor or faults along which the floor is presumed to have collapsed piston cores from deep basins of lake atitlan record ca 2 000 years of unusually rapid sedimentation ca 0 5 cm yr only one thin silicic ash layer was penetrated and it is probably from a distant source cores contain evidence of mayan disturbance of the environment around the lake and to a lesser degree of the lake itself they might also record episodes of increased thermal activity each lasting several decades heat flow measurements inside and just outside the caldera are high 290 and 230 mw m 2 suggesting hydrothermal convection and a shallow heat source high heat flow a geological record of post caldera silicic eruptions and unexplained fluctuations of lake level episodic tumescence ofthe lake floor suggest that magma remains beneath lake atitlan and that future eruptions are possible

210pb derived sediment accumulation and mixing rates for the washington continental slope abstract total sediment accumulation rates g yr in four canyons indenting the washington continental shelf are about twice those on the nearby open slope even though canyons comprise less than 40 of the depositional area of the entire slope this is because 210 pb derived sediment accumulation rates g cm 2 yr for astoria and quinault canyons are 3 4 times greater than those for willapa and grays canyons and intermediate open slope areas supporting the hypothesis that columbia river derived solids are preferentially transported to these two canyons quinault canyon currently appears to be accumulating solids at rates much greater than its low long term accumulation rate which is attributed to turbidity currents transporting solids down this canyon roughly every 500 years the following four different approaches all show that mixing may cause 210 pb derived sediment accumulation rates for slope and canyon sediments to be 2 3 times higher than true accumulation rates 1 comparison between accumulation rates calculated from 210 pb profiles with and without surface zones in which mixing has obviously affected 210 pb distributions 2 comparison between 210 pb derived and independently determined long term accumulation rates 3 comparison between 210 pb derived accumulation and modern columbia river discharge rates 4 comparison between two and three layer steady state one dimensional diffusion advection decay models mixing coefficients for surface sediments calculated using a one dimensional two layer vertical eddy diffusion model show large differences within the region with most in the 3 30 cm 2 yr range mixing rates are several times larger in canyon sediments from 0 2 1 1 km water depth probably due to physical reworking processes erroneously neglected mixing at depth of no more than 5 of that calculated for the surface mixed layer can cause the 2 3 fold calculated overestimation of true sediment accumulation rates and 2 4 fold calculated underestimation of true mixing coefficients

observations of the internal tide in monterey canyon abstract data from two shipboard experiments in 1994 designed to observe the semidiurnal internal tide in monterey canyon reveal semidiurnal currents of about 20 cm s 1 which is an order of magnitude larger than the estimated barotropic tidal currents the kinetic and potential energy evidenced by isopycnal displacements of about 50 m was greatest along paths following the characteristics calculated from linear theory these energy ray paths are oriented nearly parallel to the canyon floor and may originate from large bathymetric features beyond the mouth of monterey bay energy propagated shoreward during the april experiment itex1 whereas a standing wave that is an internal seiche was observed in october itex2 the difference is attributed to changes in stratification between the two experiments higher energy levels were present during itex1 which took place near the spring phase of the fortnightly 14 8 days cycle in sea level while itex2 occurred close to the neap phase further evide

amatitlan an actively resurging cauldron 10 km south of guatemala city a 14 16 km diameter collapse caldera has been recognized 10 km south of guatemala city guatemala the caldera is north of the presently active volcano pacaya and west of agua a large stratovolcano the caldera was not previously recognized because its eastern and western margins coincide with faults that outline the guatemala city graben and because the northern margin of the caldera is buried by pyroclastic rocks the existence of the northern caldera margin is now established by gravity data and a variety of geological observations including circumferential faults hot springs well log data and lithological changes in sedimentary rocks a sequence of nine silicic pyroclastic deposits totaling a volume of more than 70 km 3 dense rock were erupted from the caldera the ages of these eruptions are mainly between about 300 000 years b p to less than 23 000 years b p the rocks erupted at the caldera and its associated vents consist of domes and nonwelded pyroclastic flow surge and fall deposits mainly of rhyolitic to dacitic composition successive pyroclastic eruptions have generally become smaller in volume and more silicic with time major and minor element chemistry distinguish amatitlan pyroclastics from those of other nearby calderas the caldera lies at the intersection of an offset of the volcanic chain the palin shear and the faults along the volcanic front jalpatagua fault zone the caldera has a heavily faulted resurgent dome crosscut by an impressive longitudinal graben the graben s alignment with the jalpatagua fault zone suggests a genetic relationship the longitudinal graben and resurgent dome are morphologically youthful and are the sites of many young silicic vents available seismic data show a heavy concentration of epicenters over the northern part of the resurgent dome near a young silicic intrusion the caldera is active and will probably erupt again over 1 million people live within 20 km and would be threatened in the event of a moderate eruption suggestions for future research focus on hazard assessment and forecasting

controls on platinum group elemental distributions of podiform chromitites a case study of high cr and high al chromitites from chinese orogenic belts abstract a study of podiform chromite deposits from the asiatic orogenic belt and the qilian qiangling kunlun himalaya tectonic domain provides new insights into the geochemistry of the pges in podiform chromite deposits and the genesis of the deposits themselves the bulk of deposits which occur in mantle peridotites of ophiolites have typical ophiolitic pge patterns that are depleted in pt and pd relative to the average upper mantle and have negatively sloping distributions on mantle normalized diagrams type i high cr chromitites have higher os ir ru and rh contents than type ii high al chromitites although both have similar pd and pt most of the type i and ii chromite deposits have lower pd and pt contents than the upper mantle peridotites in which they occur podiform chromitites are essentially products of melt rock interaction in the upper mantle their cr and pges were contributed by not only the invading magmas but also by the upper mantle host the chromite deposits are in part metasomatic replacement bodies the type i high cr chromitite pge patterns were produced by interaction between s undersaturated boninitic magmas and depleted harzburgites whereas the type ii high al chromitite pge patterns were formed by interaction between initially s saturated tholeiitic magmas and depleted harzburgites the low to very low pd and pt contents of both type i and type ii chromitites require that the mantle assemblage in which the chromite deposits were formed had lost their sulfides and hence pd and pt prior to formation of the chromite deposits in addition no or little pd and pt were deposited by the invading magma which either remained s undersaturated boninite or became morb s undersaturated due to interaction with the s depleted harzburgitic mantle it is suggested that the very low ir os and ru contents of boninites in general might be due to loss of ir during the formation of podiform chromitites it is suggested that podiform chromitites with type i pge patterns were formed in an island arc environment whereas those with type ii pge patterns were formed in a back arc setting

explosive eruption of el chichon volcano mexico disrupted 6th century maya civilization and contributed to global cooling a remarkably long period of northern hemispheric cooling in the 6th century ce which disrupted human societies across large parts of the globe has been attributed to volcanic forcing of climate a major tropical eruption in 540 ce is thought to have played a key role but there is no consensus about the source volcano to date here we present evidence for el chichon in southern mexico as the most likely candidate based on a refined reconstruction of the volcano s eruption history a new chronological framework derived from distal tephra deposits and the world s largest holocene beach ridge plain along the gulf of mexico enabled us to positively link a major explosive event to a prominent volcanic sulfur spike in bipolar ice core records dated at 540 ce we speculate that voluminous tephra fall from the eruption had a severe environmental impact on maya societies leading to temporary cultural decline site abandonment and migration within the core area of maya civilization

geochemistry of the platinum group elements in various types of spinels from the vourinos ophiolitic complex greece abstract samples of spinel from three different environments in the vourinos ophiolitic complex were analysed for the platinum group elements pge os ir ru pd pt and au chondrite normalized pge patterns of both spinels from dunites and massive chromites are characterized by high os ir and ru normalized values as compared to the low pd and pt values whereas spinels from harzburgite have nearly chondritic patterns the bulk chemistry of os ir ru of spinels from dunites and chromitites reflects the composition of the platinum group minerals pgm trapped in the spinels the analysed pge mostly pd and pt in spinels from harzburgites are considered to be trapped at grain boundaries as sulfides the geochemical behaviour of the pge during partial melting and crystallization is used in order to define the conditions of formation of chromitites and or pge concentrations

timing and climate forcing of volcanic eruptions for the past 2 500 years volcanic eruptions contribute to climate variability but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual to decadal temperature variability in the northern hemisphere during the past 2 500 years our results are based on new records of atmospheric aerosol loading developed from high resolution multi parameter measurements from an array of greenland and antarctic ice cores as well as distinctive age markers to constrain chronologies overall cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth century pandemics famines and socioeconomic disruptions in eurasia and mesoamerica while allowing multi millennium quantification of climate response to volcanic forcing

paleomagnetic constraints on eruption patterns at the pacaya composite volcano guatemala pacaya volcano is an active composite volcano located in the volcanic highlands of guatemala about 40 km south of guatemala city volcanism at pacaya alternates between strombolian and vulcanian and during the past five years there has been a marked increase in the violence of eruptions the volcano is composed principally of basalt flows interbedded with thin scoria fall units several pyroclastic surge beds and at least one welded tuff between 400 and 2000 years bp the w sw sector of the volcano collapsed producing a horseshoeshaped amphitheater 0 65 km3 and providing a window into the cone s infrastructure lava flows and tephra exposed in the amphitheater are more then 200 m thick and when combined with flows erupted recently represent between 30 and 40 of the cone s history pacaya is ideally suited for a paleomagnetic study into the timing and duration of eruption episodes at a large composite volcano we drilled 27 paleomagnetic sites 25 aa flows 1 dike and 1 welded tuff from four lava flow sequences with between 4 and 14 sites per sequence the four sequences represent initial through historic activity at pacaya we resolved what appear to be 22 time independent paleomagnetic sites by averaging together directions from successive sites where the sitemean directions were indistinguishable at the 95 level of confidence however mean sequence directions of individual lava flow sequences yielded unusually high fisher precision parameters k 44 224 and small circles of 63 confidence a63 1 6 6 1 suggesting as few as three or four time independent sites were collected this indicates that activity as pacaya is strongly episodic and that episodes are characterized by voluminous outpouring of lavas modelling the data using holocene psv rates confirms this and shows that differences in within sequence directions 6 11 5 are consistent with emplacement of lava flow sequences in less than 100 years to as many as 300 years relatively larger differences in directions 18 23 between subjacent lava flow sequences indicates that repose is at least 300 500 years and could be even longer

bioavailable compounds in sinking particulate organic matter blanes canyon nw mediterranean sea effects of a large storm and sea surface biological processes abstract to ascertain whether the blanes submarine canyon functions as a conduit of labile organic compounds to the deep margin we analyzed phytopigment protein carbohydrate and lipid contents of sinking particles during a 6 months period comprised between a large storm event and the spring phytoplankton bloom four sediment traps were deployed at 300 900 1200 and 1500 m depth along the axis of the canyon from november 2008 to april 2009 fluxes of all study variables organic carbon proteins carbohydrates and lipids peaked from mid to late december afterwards organic matter fluxes in the upper canyon decreased to values comparable bc1200 or much lower bc900 than those observed at the beginning of the monitoring period the algal fraction of biopolymeric c i e the percentage contribution of phytopigments to biopolymeric c utilized here as an indicator of particles freshness ranging from 14 to about 100 was generally low median value about 32 and showed the highest values from november to early december 2008 at all stations except for the station at 1200 m which peaks in april 2009 a severe storm that occurred the 26th of december 2008 determined a strong increase in the downward transport of organic matter along the blanes canyon though associated with a decrease in its nutritional quality values of the protein to carbohydrate ratio utilized here as an indicator of particles nutritional quality ranged from 0 4 to 2 0 increasing from late winter to early spring at 900 and 1200 m depth in association with the spring phytoplankton bloom in superficial waters the material collected by sediment traps in spring had a higher nutritional value than in autumn winter at both stations according to the optimal foraging theory the results of this study suggest that following winter episodic events deep sea detritus feeders would need to ingest more detritus to fulfill their requirements for labile food than in spring when fresher material is derived from sinking particles associated with phytoplankton blooms we conclude that whilst submarine canyons like the blanes canyon act as major conduits for material exported from the continental shelf after high energy episodic events the supply of labile food to the deep sea benthic ecosystem is connected to biological processes occurring at the sea surface

a geophysical and geological study of laguna de ayarza a guatemalan caldera lake abstract geologic and geophysical data from laguna de ayarza a figure 8 shaped doublecaldera lake in the guatemalan highlands show no evidence of postcaldera eruptive tectonic activity the bathymetry of the lake has evolved as a result of sedimentary infilling the western caldera is steep sided and contains a large flat floored central basin 240 m deep the smaller older eastern caldera is mostly filled by coalescing delta fans and is connected with the larger caldera by means of a deep channel seismicreflection data indicate that at least 170 m of flat lying unfaulted sediments partly fill the central basin and that the strata of the pre eruption edifice have collapsed partly along inward dipping ring faults and partly by more chaotic collapses these sediments have accumulated in the last 23 000 years at a minimum average sedimentation rate of 7 m 10 3 yr the upper 9 m of these sediments is composed of 50 turbidites interbedded with laminated clayey silts containing separate diatom and ash layers the bottom sediments have 1 organic material an average of 4 pyrite and abundant biogenic gas all of which demonstrate that the bottom sediments are anoxic although thin

a preliminary experimental study of turbidite fan deposits discussion parsons et al 2002 have discussed an experimental study of turbidite fan deposits for understanding lobe switching because of the perceived importance of turbidite lobes in hydrocarbon reservoirs shanmugam and moiola 1991 this experimental study could be of value to the oil industry also experimental studies are the primary means of unraveling the complexities of the process dynamics of turbidity currents and their deposits this is because the other two options namely 1 direct observations of turbidity currents in the deep sea e g shepard et al 1979 and 2 attempts to understand turbidity currents and their fan deposits on the basis of outcrop study on land e g mutti and ricci lucchi 1972 have proven to be unsuccessful e g mutti 1992 shanmugam 2002 therefore the experimental study by parsons et al 2002 requires close scrutiny i would like to debate the following issues discussed in the paper 1 reynolds number 2 grain size variation 3 depositional bedforms and 4 importance of mud proper scaling of experimental flows to natural turbidity currents is vital according to parsons et al 2002 p 626 the reynolds number of bottom boundary layer in natural turbidity currents is often greater than 100 whereas the reynolds number of experimental turbidity currents was always much less than unity parsons et al 2002 p 626 also acknowledged that the particle reynolds number in these experiments is not compatible with values of natural flows the authors attributed these scaling difficulties to the density of the particles and the viscosity of the fluid used in their

edifice collapse and related hazards in guatemala abstract guatemalan volcanoes have at least seven debris avalanche deposits associated with cerro quemado fuego pacaya tecuamburro and an unidentified volcano the deposits range in size from less than 0 1 to in excess of 9 km 3 and from 2 5 to in excess of 300 km 2 the avalanches traveled 3 to 50 km from their sources in the guatemalan highlands three of the avalanches occurred in late pleistocene time and four in holocene time two of them within the last 2000 years the avalanches occurred at both andesitic and basaltic stratovolcanoes and at dacitic dome complexes laterally directed phreatic or magmatic pyroclastic explosions were associated with two of the debris avalanches an evaluation of factors that might lead to an edifice collapse in guatemala is based on the case studies presented in this report and a survey of the literature edifice collapses are more apt to occur if zones of weakness exist within the volcanic edifices such as unwelded pyroclastic rocks and pervasively altered rocks further the trench ward side of volcano pairs like fuego and atitlan may be more likely to fail because it may have weak zones along the contact with the older back arc volcano the direction of failure may be influenced by regional slopes which in guatemala generally trend southward toward the oceanic trench and by such structural factors as multiple vents or overly steep slopes reflecting previous activity or erosion debris avalanches are more likely to occur in drainages which have headwaters at two or more volcanoes domes are especially apt to produce small to moderate sized debris avalanches and further if the domes form a coalescing chain are most likely to fail in a direction normal to the chain these factors are used at seventeen major volcanic centers in guatemala to assess their potential for edifice collapse and most probable direction of failure

platinum group elements in rocks from the voikar syninsky ophiolite complex polar urals u s s r analyses of platinum group elements pge in rocks collected from the voikar syninsky ophiolite in the polar urals suggest that the distribution and geochemistry of pge in this paleozoic ophiolite are similar to those in mesozoic ophiolites from elsewhere chondrite normalized pge patterns for chromitite the tectonite unit and ultramafic and mafic cumulate unit have negative slopes these results are similar to those found for chromitites from other ophiolites stratiform chromities show positive slopes if the magmas that form both types of chromitite originate from similar mantle source material with respect to pge content the processes involved must be quite different however the distinct chondrite normalized pge patterns may reflect differing source materials

composition export and faunal utilization of drift vegetation in the salt river submarine canyon abstract submarine canyons may be particularly important in the transport process of drift seagrasses and seaweeds from highly productive shallow lagoon areas to deeper water we studied the composition export and faunal utilization of shallow nearshore benthic vegetation as it was transported to offshore areas via the salt river submarine canyon on the island of st croix u s virgin islands the study was conducted using a saturation diving system nuls 1 hydrolab during two missions in april and august 1980 using bottom drifters deployed in salt river bay and the submarine canyon we recorded net benthic current flow up to 2 cm s 1 moving out of the lagoon and down the canyon to deeper water using bottom nets set up at the canyon head and at the 29 m isobar and from transect surveys and drift clump samples we determined drift plant export rates and drift clump biomass and species composition the dominant drift plants were thalassia testudinum and syringodium filiforme and algae in the genera dictyota dictyopterus and diloplus during the second mission the seagrass halophila decipiens became more abundant both in the drift and in large patches along the canyon floor in both missions more drift was collected in the nets during high wind conditions than during calmer days calculated turnover times ranged from 0 01 to 4 4 days for algae in the order dictyotales and 4 4 to 18 days for thalassia blades total exported biomass of drift vegetation varied between 1 4 to 65 1 kg wet wt day 1 samples of drift vegetation contained mostly juvenile forms of both invertebrates and fishes but in relatively low numbers faunal numbers were most strongly related to rate of drift movement

the ilopango tierra blanca joven tbj eruption el salvador volcano stratigraphy and physical characterization of the major holocene event of central america abstract the ilopango caldera is the source of the large tierra blanca joven tbj eruption that occurred about 1 5 ka years ago between ca ad270 and ad535 the eruption dispersed volcanic ash over much of the present territory of el salvador and pyroclastic density currents pdcs extended 40 km from the volcano in this study we document the physical characteristics of the deposits from all over el salvador to further constrain the eruption processes and the intensity and magnitude of the different phases of the eruption the succession of deposits generated by the tbj eruption is made of 8 units the eruption started with pdcs of hydromagmatic origin unit a0 followed by fallout deposits units a and b that are

tidal rhythmites in a deep sea environment an example from mio pliocene misaki formation miura peninsula japan abstract the mio pliocene misaki formation miura peninsula japan is characterized by alternation of mafic scoriaceous pebbly sandstone pumiceous sandstone and siltstone and mudstone formed in a fore arc basin in an arc arc collisional zone the qualitative as well as quantitative evidences of tide during the misaki sedimentation in the jogashima area miura peninsula are presented here the lunar synodic period of 28 days lunar month extracted from the misaki tidal rhythmite agrees well with the published miocene tidal rhythmite data the couplet series of the misaki tidal rhythmite however is often interrupted by downslope resedimentation via turbidity currents intense penecontemporaneous deformation and bioturbation association of deep sea turbidites mass flow deposits and tidal rhythmite suggest misaki sedimentation in the miura peninsula took place in a submarine canyon setting

new ice core evidence for a volcanic cause of the a d 536 dust veil in new zealand human cryptosporidiosis demonstrates spring and autumn peaks of incidence with the spring peak being three times greater in magnitude than the autumn peak the imbalance between the two peaks is notable and may be associated with the high livestock density in new zealand in the summer and autumn the cryptosporidiosis rate was positively associated with temperatures in the current and previous month highlighting the importance of outdoor recreation to transmission no associations between spring incidence and weather were found providing little support for the importance of drinking water pathways imported travel cases do not appear to be an important factor in the aetiology of cryptosporidiosis in new zealand

geology of great abaco submarine canyon blake plateau observations from the research submersible alvin abstract nine dives in the research submersible alvin were made into great abaco submarine canyon to depths ranging from 1850 to 3666 m our observations indicate that the walls of this canyon are distinctly terraced consisting of nearly vertical to overhanging rock cliffs and intervening less steep sediment covered slopes the wall rock consists mostly of massive shallow water limestones and dolostones of cretaceous age coated on exposed surfaces with manganese oxides these rocks are heavily jointed fractured and thus very blocky to angular in appearance with sponges and other sessile organisms commonly attached talus slopes and sedimentary breccia deposits containing angular boulders are present at the base of these steep escarpments short term bottom current measurements in the axis of the eastern part of the canyon indicate that currents are relatively weak reaching velocities of only 10 cm sec this relatively placid setting is further corroborated by the abundance of turtle grass thalassia found along the canyon axis however abundant subdued symmetrical ripple marks and large scour depressions at the base of boulders indicate that high energy events sporadically impact the canyon axis contemporary erosional activity along the axis of the western headward part of the canyon appears to be more significant as evidenced by asymmetrical ripple marks sand waves and bioerosion great abaco canyon has evolved with time via a variety of processes including 1 faulting 2 subsidence 3 defacement and 4 erosional down cutting the location orientation and initiation of this canyon appear to be structurally controlled by the great abaco fracture zone during pre santonian time regional subsidence during the mesozoic allowed the walls of great abaco canyon to build vertically by accretion of shallow water limestones whereas joint controlled defacement has widened the canyon while maintaining steep walls erosional down cutting in the canyon axis by carbonate sediment gravity flows also appears to have been important episodically particularly during the miocene and pleistocene

late holocene volcanic activity and environmental change in highland guatemala abstract we present a record of late holocene volcanic eruptions with elemental data for a sequence of sampled tephras from lake amatitlan in highland guatemala our tephrochronology is anchored by a bayesian p sequence age depth model based on multiple ams radiocarbon dates we compare our record against a previously published study from the same area to understand the record of volcanism and environmental changes this work has implications for understanding the effects of climate and other environmental changes that may be related to the emission of volcanic aerosols at local regional and global scales

terminal classic drought in the northern maya lowlands inferred from multiple sediment cores in lake chichancanab mexico we present new density records from sediment cores taken along a depth transect in lake chichancanab mexico the data reveal in great detail the climatic events that comprised the terminal classic drought tcd and coincided with the demise of classic maya civilization between ca 750 and 1050 ad in shallow water cores the tcd is marked by a condensed gypsum horizon in deeper water sections it is expanded and represented by numerous interbedded gypsum and organic rich strata the tcd was not a single two century long megadrought but rather consisted of a series of dry events separated by intervening periods of relatively moister conditions we estimate the tcd occurred between ca 770 and 1100 ad and included an early phase ca 770 870 ad and late phase ca 920 1100 ad the intervening 50 year period ca 870 920 ad was relatively moister each dry phase is represented by multiple gypsum density bands interbedded with organic rich sediment that indicate alternating dry and wet conditions spectral analysis revealed significant periods around 213 50 and 27 years despite uncertainty regarding the ages of these events their pattern is robust and generally consistent with other proxy records under the same climate regime such as the marine cariaco basin off northern venezuela science 299 2003 1731

deep marine tidal bottom currents and their reworked sands in modern and ancient submarine canyons abstract submarine canyons provide a unique setting for tidal processes to operate from shallow marine to deep marine environments in modern canyons current meter measurements at varying water depths 46 4200 m show a close correlation between the timing of up and down canyon currents and the timing of semi diurnal tides these tidal bottom currents in submarine canyons commonly attain maximum velocities of 25 50 cm s based on core and outcrop studies of modern and ancient deep marine deposits it is proposed here that sand mud rhythmites double mud layers climbing ripples mud draped ripples alternation of parallel and cross laminae sigmoidal cross bedding with mud drapes internal erosional surfaces lenticular bedding and flaser bedding can be used to interpret deposits of deep marine tidal currents this approach is an alternative to the conventional approach in which most deep water traction structures e g climbing ripples and cross bedding would be attributed to deposition from turbidity currents underwater photographs show active mass flows i e slides slumps grain flows and debris flows in modern canyons box cores taken from modern submarine canyons e g la jolla california and conventional cores and outcrops of ancient canyon fill facies oua iboe pliocene nigeria and the annot sandstone eocene oligocene se france contain deposits of both tidal processes and mass flows this facies association in the rock record can be used as a criterion for recognizing submarine canyon settings in a channel mouth environment deep marine tidal deposits are likely to develop elongate bars that are aligned parallel to the channel axis within the channel whereas turbidites are more likely to develop depositional lobes that are aligned perpendicular to channel axis turbidite depositional lobes are much larger than the channel width whereas tidal sand bars are much smaller than the channel width therefore the wrong use of a turbidite lobe model with sheet geometry in lieu of a tidal bar model with bar geometry will result in an unrealistic overestimation of sandstone reservoirs in deep water exploration

volcanic forcing of climate over the past 1500 years an improved ice core based index for climate models 1 understanding natural causes of climate change is vital to evaluate the relative impacts of human pollution and land surface modification on climate we have investigated one of the most important natural causes of climate change volcanic eruptions by using 54 ice core records from both the arctic and antarctica our recently collected suite of ice core data more than double the number of cores ever used before reduces errors inherent in reconstructions based on a single or small number of cores which enables us to obtain much higher accuracy in both detection of events and quantification of the radiative effects we extracted volcanic deposition signals from each ice core record by applying a high pass loess filter to the time series and examining peaks that exceed twice the 31 year running median absolute deviation we then studied the spatial pattern of volcanic sulfate deposition on greenland and antarctica and combined this knowledge with a new understanding of stratospheric transport of volcanic aerosols to produce a forcing data set as a function of month latitude and altitude for the past 1500 years we estimated the uncertainties associated with the choice of volcanic signal extraction criteria ice core sulfate deposition to stratospheric loading calibration factor and the season for the eruptions without a recorded month we forced an energy balance climate model with this new volcanic forcing data set together with solar and anthropogenic forcing to simulate the large scale temperature response the results agree well with instrumental observations for the past 150 years and with proxy records for the entire period through better characterization of the natural causes of climate change this new data set will lead to improved prediction of anthropogenic impacts on climate the new data set of stratospheric sulfate injections from volcanic eruptions for the past 1500 years as a function of latitude altitude and month is available for download in a format suitable for forcing general circulation models of the climate system

so2 emissions to the atmosphere from active volcanoes in guatemala and el salvador 1999 2002 ground based and aircraft correlation spectrometer cospec measurements at the principal active volcanoes in guatemala pacaya santiaguito fuego and tacana and el salvador santa ana and san miguel were carried out at intervals during the period 1999 2002 as part of an attempt to measure baseline so2 emissions of potentially dangerous volcanoes and to better understand their eruption mechanisms we discuss some of the uncertainties involved in interpreting intermittent gas data together with possible improvements other problems pertaining to current monitoring of so2 and implications for future studies are also discussed santa ana volcano is proposed to be a venting hydrothermal system while santiaguito fuego pacaya and san miguel all exhibit open vent characteristics data for tacana volcano are presented but are not enough to make descriptions of its characteristics and activity pacaya is emitting high fluxes of so2 1000 tonnes day while the other vents are much lower emitters 20 300 tonnes day in general so2 emissions at pacaya suggest a large circulating and convecting high level chamber the most recent emission rates at fuego were measured during its current active phase since january 2002 average so2 emission rates during 1999 2002 are 1350 tonnes day for pacaya 340 tonnes day for fuego 120 tonnes day for santiaguito 260 tonnes day for san miguel 140 tonnes day for santa ana and 30 tonnes day for tacana these volcanoes account for about 6 to 12 of the estimated annual global volcanic output of so2

age and archaeological implications of xitle volcano southwestern basin of mexico city the pedregal lavas are fresh well exposed basaltic flows erupted from the xitle scoria and cinder cone in the southwestern part of the basin of mexico these lavas cover an area of 70 km 2 and were emplaced over pyramids and other buildings e g cuicuilco and copilco archaeological sites today a part of mexico city including the national university is built on the flows initial strombolian activity produced an ash fallout layer which was immediately followed by effusive emplacement of lava flows the xitle cone grew on the north facing slope of ajusco volcano and lava flowed down to the n ne until it reached the basin floor more than 30 radiocarbon dates have been obtained by several workers on charcoal samples from beneath the lava and several ages for the eruption have been proposed from these dates most dated samples were not directly produced by xitle s eruption but instead are artifacts of human activity that predates the eruption thus these ages mostly about 2000 bp are older than the eruption a new age of 1670 35 years bp ad 245 315 obtained on charcoal samples collected just beneath the lavas is favored for the xitle eruption these samples originated by ignition of vegetation during the emplacement of hot scoriaceous tephra the new age is within the classic period of mesoamerican archaeology whereas the earlier reported ages are at the end of the preclassic the new age carries important implications for the timing of population shifts within the basin of mexico q 2000 elsevier science b v all rights reserved

ten turbidite myths abstract during the past 50 years the turbidite paradigm has promoted many myths related to deep water turbidite deposition john e sanders 1926 1999 a pioneering process sedimentologist first uncovered many of these turbidite myths this paper provides a reality check by undoing 10 of these turbidite myths myth no 1 turbidity currents are non turbulent flows with multiple sediment support mechanisms reality turbidity currents are turbulent flows in which turbulence is the principal sediment support mechanism myth no 2 turbidites are deposits of debris flows grain flows fluidized flows and turbidity currents reality turbidites are the exclusive deposits of turbidity currents myth no 3 turbidity currents are high velocity flows and therefore they elude documentation reality turbidity currents operate under a wide range of velocity conditions myth no 4 high density turbidity currents are true turbidity currents reality ph h kuenen 1950 introduced the concept of turbidity currents of high density based on experimental debris flows not turbidity currents high density turbidity currents are sandy debris flows myth no 5 slurry flows are high density turbidity currents reality slurry flows are debris flows myth no 6 flute structures are indicative of turbidite deposition reality flute structures are indicative only of flow erosion not deposition myth no 7 normal grading is a product of multiple depositional events reality normal grading is the product of a single depositional event myth no 8 cross bedding is a product of turbidity currents reality cross bedding is a product of traction deposition from bottom currents myth no 9 turbidite facies models are useful tools for interpreting deposits of turbidity currents reality a reexamination of the annot sandstone in se france which served as the basis for developing the first turbidite facies model suggests a complex depositional origin by plastic flows and bottom currents myth no 10 turbidite facies can be interpreted using seismic facies and geometries reality individual turbidity current depositional events commonly centimeters to decimeters in thickness cannot be resolved in seismic data all turbidite myths promote falsehood and should be abandoned

chromitites platiniferes des complexes ophiolitiques de l estrie beauce appalaches du sud du quebec des chromitites contenant du platine du palladium et du rhodium viennent d etre mises a jour dans les appalaches du sud du quebec les complexes ophiolitiques hotes de ces mineralisations sont d ge ordovicien inferieur et d affinite boninitique de plus ils sont etroitement lies a un arc insulaire les chromitites a pt pd rh se situent dans les dunites de la sequence a cumulat a leur passage aux pyroxenites le platine le palladium et le rhodium sont exprimes soit sous forme d alliages et de sulfures complexes automorphes ou soit sous forme d alliages a pt pd et d arseniures rhodiferes xenomorphes cette derniere paragenese accompagne la serpentinisation des cumulats ultramafiques

a review of human and natural changes in maya lowland wetlands over the holocene abstract in the maya lowlands of mexico belize and guatemala two main types of wetlands have played important roles in human history bajos or intermittently wet environments of the upland interior yucatan and perennial wetlands of the coastal plains many of the most important maya sites encircle the bajos though our growing evidence for human wetland interactions is still sparse the deposits of these wetlands record two main eras of slope instability and wetland aggradation the pleistocene holocene transition as rainfall increased and forests eclipsed savannas and the maya preclassic to classic as deforestation land use intensity and drying increased the ancient maya adapted with terraces around these bajo margins but retracted in the late preclassic in some areas the perennial wetlands of the coastal plains have different histories and the first conceptual model of human wetland interaction described intensive wetland agriculture in the preclassic through classic based on raised fields and canals but a second model arose that interpreted the wetland stratigraphy and canals as more indicative of natural aggradation by accelerated erosion and gypsum precipitation that buried archaic and preclassic fields and there was little classic era use we present new data on a third and fourth model in this study the third is a hybrid of the models one and two including the archaic to preclassic aggradation of the second model and the first model s classic period fields and canals as piecemeal attempts by the maya to adapt to these and other environmental changes the fourth conceptual model describes a very late terminal classic preplanned project on a floodplain these wetland fields were short lived aggraded rapidly but with some reoccupation in the postclassic all of these new models display the burgeoning evidence for intricate maya interactions with wetlands and the diversity of evidence from the relatively few studies underscores the infancy of our understanding of maya interaction with tropical wetlands

a 2400 yr mesoamerican rainfall reconstruction links climate and cultural change droughts are a recurring feature of mexican climate but few high resolution data are available to test for climate change forcing of mesoamerican civilizations we present a quantitative 2400 yr rainfall reconstruction for the basin of mexico from a precisely dated and highly resolved speleothem that documents highly variable rainfall over the past 2400 yr dry conditions peaked during a 150 yr long late classic ca 600 900 ce common era mega drought that culminated at 770 ce which followed centuries of climatic drying that spanned the fall of the city of teotihuacan ca 550 ce the wettest conditions in the 1450s ce were associated with fl ooding in the basin of mexico our data suggest that rainfall variability was likely forced by the el nino southern oscillation and impacts on spring fed irrigation agriculture may have been a stressor on mesoamerican civilizations

a reconnaissance survey of the whittard sea fan southwestern approaches british isles abstract the whittard fan is a deep sea fan 148 km long by 111 km wide located in water depths between 3600 and 4400 m at the mouth of the whittard canyon southwest of the british isles a reconnaissance survey examined fan morphology sediments and bottom currents well developed diurnal tidal cycles occur with u 100 max reaching 16 cm s sufficient to erode and transport sediment at some locations at present major fan processes are apparently inactive and sedimentation is predominantly hemipelagic but other material is supplied by slope failure off the continental slope anomalous bottom topography may be interpreted as sediment waves indicating the existence of a slope parallel bottom current which may modifiy the fan surface

south pole ice core record of explosive volcanic eruptions in the first and second millennia a d and evidence of a large eruption in the tropics around 535 a d 1 a record of explosive eruptions over the last 1830 years reconstructed from a south pole antarctica ice core extends the coverage of volcanic history to the start of the first millennium a d the ice core dating by annual layer counting carries an uncertainty of 2 of the number of years from time markers with the largest dating error of 20 years at the bottom of the 182 m core several aspects of the methodology of detecting and quantifying volcanic sulfate signals in ice cores are examined in developing this record the new record is remarkably consistent with previous south pole records a comparison with records from several antarctica locations suggests that the south pole location is among the best for ice core reconstruction of volcanic records owing to the excellent preservation of volcanic signals at the south pole the relatively low and uniform sulfate background and the moderately high snow accumulation rates which allow for dating by annual layer counting a prominent volcanic event dated at 531 15 a d along with evidence from other records indicates that an unusually large eruption took place in the tropics and was probably responsible for the mystery cloud climate episode of 536 537 a d the date of 536 is suggested for a prominent volcanic signal that appears in the first half of the sixth century a d in ice cores which can in turn be used as a time stratigraphic marker in dating ice cores by annual layer counting or by computing average accumulation rates or layer thicknesses with such markers

climate variability on the yucatan peninsula mexico during the past 3500 years and implications for maya cultural evolution climate variability on the yucatan peninsula during the past 3500 yrs is reconstructed from the measurement of 18o in monospecific ostracods and gastropods in a 6 3 m sediment core from lake punta laguna mexico this late holocene record is divided into three periods based on changes in mean 18o values from 3310 to 178514c yr b p period i low mean 18o values indicate relatively wet conditions i e low evaporation to precipitation ratio e p mean oxygen isotopic values increased 178514c yr b p and the interval between 1785 and 93014c yr b p period ii was distinctly drier than the periods before or after the climate during the latter part of period ii was persistently dry with exceptionally arid events centered at 1171 1019 and 94314c yr b p equivalent to 862 986 and 1051 a d this interval of frequent drought was recorded at several other localities in mexico and central america and coincided with the collapse of classic maya civilization following the last arid event 18o values decreased abruptly at 93014c yr b p beginning of period iii signaling a return to wetter conditions that have generally prevailed to the present with the exception of a dry episode centered at 55914c yr b p 1391 a d the paleoclimatic record from punta laguna provides evidence that multi decadal and millennial scale changes ine poccurred on the yucatan peninsula during the late holocene these wet dry episodes may have influenced cultural evolution in mesoamerica

palladium platinum rhodium ruthenium and iridium in chromitites from the massif du sud and tiebaghi massif new caledonia massive and disseminated podiform chromitite from 43 mines and other occurrences in the massif du sud and tiebaghi massif both ophiolites in new caledonia contain up to 9 ppb pd up to 45 ppb pt up to 31 ppb rh up to 410 ppb ir and up to 1 130 ppb ru the chromitites are strongly depleted in pt and pd and mildly depleted in ir and ru with respect to an average chondrite composition as are podiform chromitites from oman turkey greece and northern california stratiform chromitites from the stillwater complex montana and the bushveld complex south africa show inverse patterns of enrichment and depletion these patterns suggest that either chromitite from ophiolites represents mantle rocks depleted in pt and pd or that the magmas that formed the ophiolites were depleted in pt and pd or that the process of forming the chromitite involved the concentration of os and ir from a magma the relatively high contents of ir and ru in the new caledonian chromitite suggest the potential for the occurrence of placers that contain concentrations of os ir alloys

diversity of platinum group minerals in podiform chromitites of the late proterozoic ophiolite eastern desert egypt genetic implications abstract podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the eastern desert egypt they are in most cases hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the pan african belt of late precambrian age serpentinites are the predominant components in the ophiolitic melange either as a matrix or as variably sized blocks and are derived from harzburgite and subordinate dunite the central eastern desert ced chromitites have a wide compositional range from high cr to high al varieties whilst those of the southern eastern desert sed have a very restricted compositional range the cr ratio of spinel ranges from 0 5 up to 0 8 in the former while it is around 0 8 in the latter platinum group element pge mineralization has been recently reported in podiform chromitites from the late proterozoic pan african ophiolite of the eastern desert of egypt the populations of platinum group minerals pgm in the studied ced and sed chromitites are quite distinguishable they are mainly sulfides os rich laurite in the former and os ir alloy in the latter sulfarsenides and arsenides are also found in subordinate amounts from both chromitites the most abundant base metal sulfides bms in the eastern desert chromitites of egypt are millerite heazlewoodite pentlandite chalcopyrite and pyrite the sulfur fugacity and temperature conditions are the main controlling factors of pge mineralogy in the host chromitite at the initial stage within the upper mantle os rich laurite is stable at high sulfur fugacity and low temperature conditions whereas os ir alloy is stable at lower sulfur fugacity and higher temperature conditions the diversity of pge mineralogy combined with the differences in petrological characteristics of chromian spinels from ced chromitites to sed ones suggests different degrees of partial melting of the mantle rocks of this ophiolite which in turn may be attributed to different tectonic settings during the post magmatic processes i e serpentinization the primary pgm e g os rich laurite and os ir alloy can be modified at low temperatures to secondary pgm

deep flow variability in a deeply incised mediterranean submarine valley blanes canyon abstract deep flow variability in blanes canyon is analyzed by means of a unique dataset in which the canyon was instrumented at its axis and at the two canyon walls from march 2003 to july 2004 the mean intensity of deep currents range between 3 5 cm s 1 at the canyon axis 1500 m and 5 4 cm s 1 upstream canyon wall 900 m a wavelet analysis shows that the energy is concentrated in the 2 30 day band and it is mainly associated to single not periodical not wave related events uncoupled at the different canyon sites an empirical orthogonal function analysis shows two main patterns of current variability explaining 65 of the total variance the first mode represents intensifications of the typical along bathymetry pattern of currents over the canyon the second mode corresponds to near bottom upwelling events along the canyon axis with water exiting the canyon through the canyon head after discussing possible forcing mechanisms it is here suggested that such deep upwelling events are associated to offshore displacements of the northern current namely the interaction of the current with the particular shape of the canyon topography at large depths 1800 m would be responsible for the observed deep upwelling events

unexpectedly high pge chromitite from the deeper mantle section of the northern oman ophiolite and its tectonic implications unusually high platinum group element pge enrichments are reported for the first time in a podiform chromitite of the northern oman ophiolite the chromitite contains 1 5 ppm of total pge being highly enriched in the ipge subgroup ir os and ru and strongly depleted in the ppge subgroup rh pt and pd its platinum group minerals pgms are classified into three types arranged in order of abundance 1 sulphides os rich laurite laurite erlishmanite solid solution and an unnamed ir sulphide 2 alloys os ir alloy and ir rh alloy and 3 sulpharsenides irarsite and hollingworthite the high pge concentrations are observed only in a discordant chromitite deep in the mantle section which has high cr 0 7 spinel with an olivine matrix all the other types of chromitite in the moho transition zone mtz and concordant pods in the deeper mantle section are poor in pges and tend to have spinels with lower cr up to 0 6 this diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis in response to a switch of tectonic setting the first is residual from lower degree partial melting of peridotite which produced low cr pge poor chromitites at the moho transition zone and to a lesser extent within the mantle possibly beneath a fast spreading mid ocean ridge the second chromitite forming event involves higher degree partial melting which produced high cr pge rich discordant chromitite in the upper mantle possibly in a supra subduction zone setting

sulfides associated with podiform bodies of chromite at tsangli eretria greece small bodies of pyrrhotite chalcopyrite minor pentlandite and magnetite occur at the peripheries of podiform bodies of chromite in ultramafic ophiolitic rocks at tsangli eretria central greece banding of magnetite and sulfide within the bodies is reminiscent of magmatic banding a magmatic origin has been proposed for similar sulfide masses in the troodos ophiolite panayiotou 1980 the compositions of the host rocks chromite and of the sulfides have been investigated on average the sulfide mineralization recalculated to metal content in 100 sulfide contains 0 55 ni 5 15 cu 0 29 co 9 ppb pd 179 ppb pt 16 ppb rh 112 ppb ru 31 ppb ir 58 ppb os and 212 ppb au these metal contents particularly the high cu cu ni ratio of 0 78 and the pt pd pt ratio of 0 95 are inconsistent with the sulfides having reached equilibrium with their ni rich host rocks at magmatic temperatures and accordingly it is concluded that they are not of magmatic origin the average 34s value of the sulfide bodies is 2 while that of a sample of pyrite from country rock schist is 15 6 these values are inconclusive as to the origin of the sulfur it is suggested that the sulfides have been precipitated by hydrothermal fluids possibly those responsible for the serpentinization of the host rocks the source of the metals may have been the host rocks themselves

depositional events in santa monica basin california borderland over the past five centuries the southern california coastal climate is characterized by decadal and generational floods that discharge one to three orders of magnitude more sediment than the average annual contribution these can produce turbidity currents directly or load the canyons and slopes with sediments that then may fail and form turbidity flows seismic activity is high on this transform margin and can trigger slope failures where slope sedimentation rates are high and sediments are unstable these events produce slope failures and slides and slumps in canyon heads which evolve into turbidity currents of sufficient volume to reach the adjacent margin basin floors such events occur at approximately century frequency this paper describes six basin floor turbidites that have been mapped in box cores sampling roughly the top 40 50 cm from the basin floor or santa monica basin in the california borderland these deposits were laid down during the past 500 years of record their depositional areas have been conservatively estimated to range from 500 to almost 1500 km2 and contain masses of sediment ranging from 10 000 000 to 100 000 000 tons this compares with a long term average annual terrigenous contribution rate of about 4 000 000 tons yr from the adjacent major streams although annual rates vary from 0 to 60 000 000 tons over the last century of record three of the six events have been related to two major floods 1884 6 1969 and to one major seismic event the mission earthquakes of 1822 1812 the older turbidites were deposited before european entry to the area and cannot be assigned to an historic event but from their size were probably seismically triggered and are the largest of the six events it is likely that other smaller events are masked by the more recent turbidites particularly in the area where box cores are dominantly composed of turbidites these features exhibit average outcrop thickness of 2 to 7 cm with maxima of the order of 10 cm thickness yet represent major events and significant volumes of sediment they also document that the two major sources for such events floods and seismic events produce deposits of very different mass and can probably be separated in the record on that basis thus the frequency of major floods and of major earthquakes in the region could be defined from study of such deposits in longer cores these deposits also provide examples of continued turbidity current activity to the basin floors at a time of high sea level this is contrary to the passive models for sequence stratigraphy depositional patterns

the vertical structure of turbidity currents and a necessary condition for self maintenance a simple numerical model predicts that properties of a turbidity current can vary significantly in the vertical direction particularly when more than one sediment size is in suspension each sediment size decreases in concentration with increasing distance from the bottom boundary at a rate which depends on its settling velocity the current velocity the entrainment coefficient and the bottom slope however it is shown that the details of the vertical distribution of sediment load do not necessarily greatly influence either the vertical structure of the hydraulic properties of the flow velocity stress or the bulk properties such as the entrainment coefficient at least when only one sediment size is in suspension the ability of a turbidity current to sustain itself is strongly dependent on the sediment load and an autosuspension criterion is obtained by taking into account the vertical structure of the currents the model turbidity currents often cannot maintain their sediment in suspension even when the bottom stress is greater than the critical stress of deposition for low concentration flows when this happens the concentration near the bottom increases without limit as the turbidity current evolves with time in real turbidity currents grain grain interactions which become important at high concentrations may at times cause the current to quickly deposit its near bottom sediment load this is suggested as a possible mechanism for the formation of the poorly sorted sediments frequently found in turbidites

climate change and population history in the pacific lowlands of southern mesoamerica core man015 from pacific coastal guatemala contains sediments accumulated in a mangrove setting over the past 6500 yr chemical pollen and phytolith data which indicate conditions of estuarine deposition and terrigenous inputs from adjacent dry land document holocene climate variability that parallels the maya lowlands and other new world tropical locations human population history in this region may be driven partly by climate variation sedentary human populations spread rapidly through the estuarine zone of the lower coast during a dry and variable 4th millennium b p population growth and cultural florescence during a long relatively moist period 2800 1200 b p ended around 1200 b p a drying event that coincided with the classic maya collapse

recent sedimentation on the shelf and upper slope in the bay of anamur southern coast of turkey abstract sedimentological studies supported by uniboom profiles and sonographs of surface sediments from a part of the narrow southern shelf of anatolia indented by the head of a submarine canyon and partly covered by meadows of macrophytobenthos revealed abrupt lateral and vertical variations in the holocene sedimentary facies these variations have essentially been controlled by the migration of depocentres palaeo topography of the shelf and an overall decrease in the rate of fluviatile sediment supply in the late holocene as sea level reached approximately its present position nearly 5 ka b p the bulk of the siliciclastic input began to be trapped in the inner shelf zone most of the terrigenous mud bypasses the shelf to be trapped in the canyon head but transport of modern detrital sand and gravel does not take place across the shelf this is because of the relative weakness of the onshore offshore currents the prevailing microtidal conditions and a belt of phytobenthos between 10 and 30 m which traps the sediments the plants also create the ecological conditions favourable for biological carbonate production however some gravel and sand size relict sediments have been transported from the outer shelf into the canyon head by gravity induced or cross canyon currents the mudline occurs at about the 100 m isobath marking the deepest limit of effective bottom turbulence for the resuspension of silt and clay on the shelf

age and extent of the ilopango tbj tephra inferred from a holocene chronostratigraphic reference section lago de yojoa honduras abstract eruption of central el salvador s ilopango volcano early in the first millennium a d caused death cultural devastation and exodus of southern mesoamericans it also left a time stratigraphic marker in western el salvador and adjacent guatemala the ilopango tierra blanca joven or tbj tephra mineral suites and major element abundances identify a silicic volcanic ash in cores from lago de yojoa honduras as ilopango tbj this extends its reported range more than 150 km to the northeast analyses of glass from the tbj tephra from the chalchuapa archaeological site el salvador and from lago de yojoa honduras establish the first major element reference fingerprint for the tbj tephra the lago de yojoa cores also hold two previously undated trachyandesitic tephra layers originating from the nearby lake yojoa volcanic field one fell shortly before 11 000 14c yr b p and the other about 8600 14c yr b p

radiocarbon and geologic evidence reveal ilopango volcano as source of the colossal mystery eruption of 539 40 ce abstract ilopango volcano el salvador erupted violently during the maya classic period 250 900 ce in a densely populated and intensively cultivated region of the southern maya realm causing regional abandonment of an area covering more than 20 000 km2 however neither the regional nor global impacts of the tierra blanca joven tbj eruption in mesoamerica have been well appraised due to limitations in available volcanological chronological and archaeological observations here we present new evidence of the age magnitude and sulfur release of the tbj eruption establishing it as one of the two hitherto unidentified volcanic triggers of a period of stratospheric aerosol loading that profoundly impacted northern hemisphere climate and society between circa 536 and 550 ce our chronology is derived from 100 new radiocarbon measurements performed on three subfossil tree trunks enveloped in proximal tbj pyroclastic deposits we also reassess the eruption magnitude using terrestrial el salvador guatemala honduras and near shore marine tbj tephra deposit thickness measurements together our new constraints on the age eruption size 43 6 km3 dense rock equivalent of magma magnitude 7 0 and sulfur yield 9 90 tg along with ilopango s latitude 13 7 n squarely frame the tbj as the major climate forcing eruption of 539 or 540 ce identified in bipolar ice cores and sourced to the tropics in addition to deepening appreciation of the tbj eruption s impacts in mesoamerica linking it to the major northern hemisphere climatic downturn of the mid 6th century ce offers another piece in the puzzle of understanding eurasian history of the period

oceanographic processes and morphosedimentary products along the iberian margins a new multidisciplinary approach abstract our understanding of bottom currents and associated oceanographic processes e g overflows barotropic tidal currents including intermittent processes e g vertical eddies deep sea storms horizontal vortices internal waves and tsunamis is rapidly evolving many deep water processes remain poorly understood due to limited direct observations but can generate significant depositional and erosional features on both short and long term time scales this paper represents a review work which describes for the first time these oceanographic processes and examines their potential role in the sedimentary features along the iberian continental margins this review explores the implications of the studied processes given their secondary role relative to other factors such as mass transport and turbiditic processes and highlights three major results a contourite depositional and erosional features are ubiquitous along the margins indicating that bottom currents and associated oceanographic processes control the physiography and sedimentation b the position of interfaces between major water masses and their vertical and spatial variation in time specifically appears to exert primary control in determining major morphologic changes along the slope gradient including the contourite terraces development and c contourites deposits exhibit greater variation than the established facies model suggests therefore a consistent facies model however faces substantial challenges in terms of the wide range of oceanographic processes that can influence in their development an integrated interpretation of these oceanographic processes requires an understanding of contourites seafloor features their spatial and temporal evolution and the near bottom flows that form them this approach will synthesize oceanographic data seafloor morphology sediments and seismic images to improve our knowledge of permanent and intermittent processes around iberia and evaluate their conceptual and regional role in the margin s sedimentary evolution given their complexes three dimensional and temporally variable nature integration of these processes into sedimentary oceanographic and climatological frameworks will require a multidisciplinary approach that includes geology physical oceanography paleoceanography and benthic biology

tidal motions and tidally induced fluxes through la linea submarine canyon western alboran sea detailed observations from two mooring lines deployed in la linea submarine canyon western alboran sea are presented this is a narrow canyon in the sense that its width is always less than the internal radius of deformation tidal currents within the canyon are polarized in the along canyon direction according to its narrow nature they have considerable amplitude values of around 0 5 m s are often observed and are forced by the internal pressure gradients associated with the baroclinic tide that is generated in the surroundings subsequent amplification of onshore baroclinic currents within the canyon accounts for the large amplitude observed cross shelf exchange through the canyon due to tidal motions is different from zero despite the close to zero mean of tidal currents the explanation is based on the asymmetry of water properties flowing up canyon and down canyon some sort of tidal rectification regarding the energy flux the canyon seems to be an adequate conduit to carry energy to the shore estimations made from our observations indicate that energy input onto the shelf per unit length parallel to the shore at the canyon head is enough to maintain mixing on the shelf at intermediate depths

geochemistry of chromitites and host rocks from the pindos ophiolite complex northwestern greece abstract representative chromite samples of schlieren and massive ore types from various localities of the pindos ophiolite complex were analyzed for platinum group elements pge s ni co and cu and mineral chemistry host dunites and harzburgite were also analyzed for the chalcophile elements the chromite ores show a wide compositional variation the cr cr al ratio ranges from 0 47 to 0 83 while mg mg fe 2 ranges between 0 42 and 0 73 but two populations can be distinguished the high cr and high al types which show a more restricted variation ranges of pge contents in ppb are os 8 150 ir 4 320 ru 15 550 rh 2 82 pt 3 150 and pd 1 5 20 gold ranges between 1 and 13 ppb the pge and less chalcophile elements ni co and cu the incompatible compatible element ratios the pge s ratio and pge patterns provide valuable information for the discrimination of chromite ores with a similar major element composition a fractionation trend in both chromitites and associated dunites can be used for stratigraphic orientation in the mantle sequences of ophiolite complexes and evaluation of their chromite potential

contemporary sedimentary processes in the monterey canyon fan system abstract a study of the monterey canyon and fan was conducted to investigate contemporary sedimentary activity using a camera sled alvin dives and sea beam bathymetry physical characteristics and plan view morphology of the walls terraces thalweg floor and levees of the monterey canyon and fan as well as the gullied and non gullied regions of the adjacent continental slope have been studied the canyon floor and fan valley thalweg channel from 2900 to 3500 m exhibited properties indicative of moderate to low energy conditions weak currents smoothed sea bed deposits of disintegrated kelp bioturbated mud substrate fecal pellets absence of scour chemosynthetic communities consisting of clams and a pogonophoran were found in the fan valley from 3000 to 3600 m local areas of rockfalls and slumps from the canyon and fan valley walls were not fresh sedimented abundantly colonized by benthic biota terraces and levee crests were mud draped and bedrock exposures on canyon walls were encrusted by benthic organisms freshest disturbances were found in gullies on the adjacent continental slope where bedrock was scoured clean of sediment and loose debris a major submarine slide detached from escarpments on the lower slope and extends across the fan the slide surface was mud draped hummocky and contained bedforms with wavelengths up to 150 m and up to 10 m relief the survey was conducted shortly before the loma prieta earthquake october 17 1989 that caused substantial ground motion in on shore regions of monterey bay any substantial sub sea disturbances generated by the ensuing earthquake should be discernable from the pre earthquake state by comparison with the deep sea photographs and observations

comment on internal waves an under explored source of turbulence events in the sedimentary record by l pomar m morsilli p hallock and b badenas earth science reviews 111 2012 56 81 this discussion is an attempt to alert the sedimentologic community that there is an immediate need to carry out fundamental research on process sedimentology of internal waves and internal tides in modern oceans despite the exhaustive theoretical experimental observational and numerical analyses of modern internal waves and internal tides by oceanographers with their focus on ocean mixing and modeling there are absolutely no core based studies by sedimentologists on the origin of primary sedimentary structures formed by baroclinic currents associated with internal waves and internal tides in modern marine environments as a consequence there are no objective sedimentological criteria for distinguishing internal wave deposits amid this knowledge vacuum the temptation to interpret ancient rock record as internal wave deposits must be tempered

contemporary channel levee systems in active borderland basin plains california continental borderland abstract long range large scale side scan gloria information other seismic reflection profiling studies and data from cores in the california continental borderland have defined active levee channel systems extending basinward from the lower fan of hueneme mugu submarine fan redondo submarine fan and santa cruz canyon fan in santa monica san pedro and santa cruz basins respectively the holocene distributaries have been created by a series of turbidity current events these distributaries range in length from 10 25 km and are wide 2 5 km and low relief 1 10 m in their distal parts they are also active conduits for nepheloid flows distributions of sedimentological parameters typically mimic the pattern of levee channel systems organic carbon and biogenic carbonate content roughly outline the systems channels are incised in the upper to middle fan areas and become constructional leveed channels in the lower fan and basin plain as the channel gradient adjusts to maintain a graded profile thus sediment gravity flows are generally confined to channels in the upper fan zones but deposit both channelized and over bank deposits on the lower fan and basin floor the deposits show that the canyon fan activity has continued during a rising sea level phase it is evident that canyon headward erosion rates have been equal to or greater than the transgression rate and that the canyon fan systems have remained linked with their sediment sources frequency of events was probably higher and volumes of the events were often larger during the glacially lowered sea level episodes however turbidity currents of sufficient volume to reach the basin floors continue to occur at century or multi century intervals as one progresses headward in each system the number of flows per length of core increases small flows that do not pass beyond the distributaries are much more frequent and may be decadal in frequency or even more frequent in the santa monica basin system these california borderland basins are probably typical of narrow active margins where rate of lateral sea level transgression is less than or equal to the rate of canyon headward erosion the canyons maintain connections with sediment sources during sea level rise and the systems therefore are active during the entire sea level cycle thus sediment supply is not a simple function of eustacy this contrasts with the simplified sequence model developed on passive margins where canyons turn off as sea level rises

210pb activities in and fluxes to sediments of the washington continental slope and shelf abstract surface sediments of the washington coast have 210 pb activites which average 104 48dpm g for submarine canyon and slope regimes and 18 12dpm g for the continental shelf regime 210 pb sedimentary fluxes are also higher in canyons averaging 18 13dpm cm 2 per yr compared to 5 2 3 1 dpm cm 2 per yr for slope and 4 8 1 8dpm cm 2 per year for shelf regions these 210 pb activities and fluxes are 2 7 times greater than those reported for other coastal regions inputs from the atmosphere and the columbia river are not sufficient to supply the 210 pb but advection of seawater containing dissolved 210 pb produced in situ from 226 ra provides an input several times larger than the sedimentary fluxes the sedimentary 210 pb flux is limited by scavenging reactions rather than by supply of dissolved 210 pb calculations of maximum biological uptake and fluxes of 210 pb and selective chemical leaching experiments all show that the primary scavenging processes are due to hydrous mn and fe oxides rather than biological phases the pattern of higher 210 pb depositional fluxes in canyons than in nearby open slope areas of comparable water depth is most reasonably explained by enhanced scavenging of dissolved 210 pb near the sea floor rather than by processes operating throughout the water column relatively rapid removal of dissolved 210 pb from the near bottom nepheloid layer to slope and canyon sediments is shown by its mean residence time of less than two years in this layer

common characteristics of paired volcanoes in northern central america four pairs of active volcanoes along the northern central american volcanic front have erupted basalt andesite magmas that show consistent intrapair behavioral and compositional differences these differences are found in records of volcanic activity and complete major and minor element data on over 200 samples from northwest to southeast along the volcanic front the four volcano pairs are cerro quemado santa maria toliman atitlan acatenango fuego and santa ana izalco the volcano pair relations help explain compositional differences apart from those reflecting variation in crustal thickness of about 15 km along the volcanic front providing insight into across arc variations and closely spaced subvolcanic plumbing systems intravolcano pair spacing is less than 5 km compared with an average intervolcano spacing of 25 km along the entire volcanic front within each volcano pair the seaward volcano has had more frequent historic activity erupting magmas that are generally more mafic lower in large ion lithophile elements and higher in na2o k2o than magmas erupted from its landward counterpart each paired volcano site lies in close proximity to a rhyolitic caldera situated north or northeast of the volcano pair however rare earth element data at the toliman atitlan volcano pair imply that mixing between caldera rhyolite and the mafic magma of the paired volcanoes does not occur petrographie isotopic and other geochemical data from the toliman atitlan volcano pair suggest that separate but contemporaneous magma bodies beneath each volcano evolve and pass through the crust at different rates atitlan magmas are processed through the crust more efficiently and with greater frequency than tollman magmas which undergo longer periods of stagnation interrupted by mafic injection and rapid eruption this relation appears to hold at the other paired volcano sites and is further evidence that closely spaced volcanoes with similar subcrustal magma sources evolve over separate magmatic plumbing systems that traverse the crust the pairing pattern probably reflects the regional southward migration of the volcanic front

internal tides in monterey submarine canyon abstract the m2 internal tide in monterey submarine canyon is simulated using a modified version of the princeton ocean model most of the internal tide energy entering the canyon is generated to the south on sur slope and at the head of carmel canyon the internal tide is topographically steered around the large canyon meanders depth integrated baroclinic energy fluxes are up canyon and largest near the canyon axis up to 1 5 kw m 1 at the mouth of the upper canyon and increasing to over 4 kw m 1 around monterey and san gregorio meanders the up canyon energy flux is bottom intensified suggesting that topographic focusing occurs net along canyon energy flux decreases almost monotonically from 9 mw at the canyon mouth to 1 mw at gooseneck meander implying that high levels of internal tide dissipation occur the depth integrated energy flux across the 200 m isobath is order 10 w m 1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head where internal ti

use of paleo records in determining variability within the volcanism climate system abstract volcanic eruptions that inject large quantities of sulfur rich gases into the stratosphere have the capability of cooling global climate by 0 2 0 3 c for several years after the eruption equatorial eruptions will impact global climate whereas mid latitude eruptions can cool climate in the hemisphere of origin magnitude of cooling varies by latitude and it is possible for warming to occur in certain regions primarily during the winter although instrumental records have been used to quantify the volcanic impact on climate they are limited in their temporal and spatial coverage and the style and magnitude of eruptions occurring over the two centuries of instrumental records is limited a thorough evaluation of the range of variability in the volcanism climate system requires a multidisciplinary approach that includes the analysis of ice core records geological data atmospheric measurements and visual phenomena tree ring records and other proxy data evaluation of these longer time series indicates that multiple volcanic eruptions have the potential to force climate over decadal to multi decadal time frames especially when these eruptions enhance or extend pre existing cool conditions on the other hand a lack of climatically significant eruptions may result in warmer average temperatures over decadal time frames because the volcanic cooling component within the climate system is absent mega eruptions like the toba eruption of 71 000 73 000 yr ago may impact climate on centennial time frames through positive feedback mechanisms evidence exists which indicates that environmental changes associated with rapid climatic fluctuations such as crustal loading unloading with glaciation deglaciation and variability in glacial meltwater loading on ocean basins may cause an increase in volcanic activity

platinum group element distribution in chromite ores from ophiolite complexes implications for their exploration abstract compilation of some new data on ophiolites for greece and yugoslavia and published data from previous studies indicate that platinum group element pge and gold concentrations in chromite ores are generally low ranging from less than 100 ppb to a few hundred ppb however samples from several ophiolite complexes exhibit an enrichment of a few ppm a only in os ir and ru b only in pt and or pd or c in all pge this enrichment up to 10s ppm is mainly related with chromitites hosted in supra moho dunites and dunites of the uppermost stratigraphic levels of the mantle sequence and it seems to be local independent of the chromitite major element composition and the chromite potential of the ophiolite complexes the contents of pge combined with less chalcophile elements ni co cu the ratios of incompatible compatible elements and pge patterns provide evidence for discrimination between chromitites derived from primitive magmas and those derived from partially fractionated magmas although they have a similar major element composition thus they can be used for a stratigraphic orientation in the mantle sequence and therefore for exploration targets moreover pge data offer valuable information for the evaluation of the chromite potential in ophiolite complexes the most promising ophiolites seem to be those which apart from the petrological and geochemical characteristics indicating extensive degree of partial melting in the mantle source contain only one chromite ore type the other type being only in small proportion of limited compositional variation in both major elements and pge low ratios of pd ir while pge enriched chromitites in the mantle sequence are only occasionally present in contrast ophiolites which contain both high cr and al chromitites and where their chalcophile element data implies relatively extensive fractionation trend are not good exploration targets for chromite ores although they are related with a ssz environment

geochemistry and mineralogy of early archean spherule beds barberton mountain land south africa evidence for origin by impact doubtful abstract spherule layers in the 3 4 ga barberton greenstone belt south africa have been interpreted as being the result of large asteroid or comet impacts on the early earth this interpretation was based among other arguments on the enrichment of siderophile elements especially the platinum group elements we made a detailed mineralogical petrological and geochemical study of spherule bed samples taken from drill cores and underground exposures at the princeton mt morgan and sheba gold mines as well as surface localities the macrostructure of each sample from within different spherule layer units shows evidence for multiple more than five events over about 30 cm this would require multiple impacts within a few million years which is unlikely the mineral phases are almost exclusively of secondary origin the mineralogy provides evidence for extensive hydrothermal and metasomatic alterations of the spherule beds geochemical analyses of alternating spherule shale and chert layers show no correlation between the siderophile elements e g ir co ni and au contrary to that which would be expected if the siderophile elements had an extraterrestrial source furthermore no significant variation in the content of the siderophile elements was detected between spherule layers and shale layers however siderophile element contents are high only in layers containing abundant sulphide minerals and having high as sb se and cr contents we suggest that complex mineralizations similar to those that have formed the barberton archean gold deposits or the bon accord deposit were responsible for the siderophile element enrichments in the spherule beds the mineralogical and geochemical data provide no direct evidence in favour of their initial origin by impact nowhere else in the world have such multiple or even single spherule beds been observed and none of the numerous known impact crates or the cretaceous tertiary boundary is associated with comparable spherule beds known impact debris usually contains

comment on late holocene rupture of the northern san andreas fault and possible stress linkage to the cascadia subduction zone by chris goldfinger kelly grijalva roland burgmann ann e morey joel e johnson c hans nelson julia gutierrez pastor andrew ericsson eugene karabanov jason d chaytor jason patton and eulalia gracia goldfinger et al 2008 conclude that in the northern san andreas fault and cascadia margins holocene turbidites i e deposits of turbidity currents were triggered by earthquakes however their conclusion falters because of 1 selective data sets that were used for establishing seismicity as the sole triggering mechanism of turbidites 2 flawed sedimentological concepts and criteria that were applied for interpreting turbidites and 3 faulty methodologies that were employed for correlating turbidites over long distances this same turbidite i e sedimentological theme has also been published previously by goldfinger et al 2003a b 2006 2007 as with the recent article in bssa their previous publications on the subject matter were published in journals that are concerned primarily with geophysical topics as a result critical sedimentological conclusions which are an integral part of the articles by goldfinger et al have not been subjected to a rigorous scrutiny therefore i would like to comment on the following three issues along the washington oregon california margin potential triggering mechanisms of sediment failures and related turbidity currents are 1 earthquake adams 1990 2 tectonic oversteepening greene et al 2006 3 tsunami adams 1990 geist 2005 4 cyclone adams 1990 shanmugam 2008a fig 4 5 ebb tidal current puig et al 2003 6 submarine volcanic activity davis et al 2002 7 sediment loading adams 1990 and 8 gas hydrates chapman et al 2004 the link between gas hydrate decomposition and mass movements on the norwegian margin was discussed by mienert et al 2005 in the cascadia margin a total of 40 cyclones along the oregon coast were reported for the 1984 2006 period moritz and moritz 2006 a total of 24 tsunamis were reported along the oregon coast for the 1868 1994 period table 1 cyclones and tsunamis are powerful agents of sediment failures shanmugam 2008a on the outer continental shelf of the gulf of mexico for example the

sedimentary characteristics of late pleistocene bottom current deposits barents sea slope off northern norway abstract the barents sea slope off northern norway received large amounts of terrigenous sediments from an extensively glaciated norwegian continental margin during the late pleistocene a sedimentological study of seven gravity cores from the barents sea slope suggests that some terrigenous sediments were transported by both downslope currents and contour following bottom currents the bottom current deposits comprise four sedimentary facies 1 indistinctly laminated mud 2 layered mud 3 indistinctly layered mud and 4 bioturbated mud the indistinctly laminated mud on the mid to lower slope is characterized by a relatively low degree of bioturbation and consists of monotonous diffuse and discontinuous silt rich and silt depleted laminae 1 mm scale and or discontinuous trains of coarse grains these characteristics are suggestive of sedimentation from contour currents driven by water exchange with the north atlantic ocean these bottom current deposits can be distinguished from fine grained turbidites silt clay couplets and intensively bioturbated hemipelagites on the barents sea slope

platinum group element resources in podiform chromitites from california and oregon rock samples from about 280 podiform chromite deposits in california and oregon were analyzed for pt pd rh and ir by fire assay atomic absorption and spectrographic techniques to estimate by product platinum group element potential of chromite mining these deposits include ophiolites from the sierra nevada klamath mountains coast ranges and the canyon mountain complex the deposits vary from paleozoic to mesozoic age and vary in size shape texture and degree of metamorphism the maximum platinum group element contents in ppm measured for chromitites are pt 2 53 pd 0 2 rh 0 14 ru 4 93 and ir 2 93 correlations between pt and rh 0 60 rh and ir 0 64 rh and ru 0 71 and ir and ru 0 85 are significant at the 1 percent level relatively small but significant differences in platinum group element content exist between geologic terranes chromite production from california and oregon amounts to about 692 000 metric tons 1914 1985 the mean contained platinum group element amounts in troy ounces are pt 850 pd 100 rh 260 ru 4200 and ir 1 700 using estimated grades previous investigators have suggested that relatively few samples can represent the platinum group element content of a podiform chromite deposit therefore these analyses are assumed to approximate grades total world resources of platinum group elements in podiform chromite deposits are estimated in troy ounces to be pt 210 000 pd 25 000 rh 64 000 ru 1 000 000 and ir 420 000

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

an overview of the pge concentrations in the shetland ophiolite complex extremely anomalous platinum group element pge concentrations are known at one site cliff in the shetland ophiolite complex systematic pge analysis of all the mafic and ultramafic igneous and alteration lithologies in the ophiolite has revealed many lower grade anomalous pge occurrences throughout the cumulate ultramafic sequence and their distribution is described here more sites with pge enrichments as anomalous as those at cliff have not been found at cliff the anomalous values of over 60 ppm pt pd are restricted to a chromite rich sulphide bearing dunite forming a small part of a 200 m dunite lens in mantle harzburgite close to the basal thrust of the ophiolite lower values of up to 6 4 ppm pt pd occur in chromite poor sulphide rich dunites also within this dunite lens the only other chromite rich dunite lens in the cliff area also has detected pt and pd but with low pt pd values of 200 ppb in contrast harzburgites metasediments underlying the basal thrust serpentinites from the basal thrust and internal faults are all barren of pt and pd detection limit 20 ppb in the cliff area therefore detected pge values are restricted to magmatic chromite rich dunite lenses pge analyses of drill core from beneath the disused chromite quarries at cliff are low 227 ppb pt pd indicating that the extremely anomalous pge values are very restricted this supports the idea that pge concentrated magmatically within the dunite lens at cliff may have been remobilized locally to produce extremely anomalous pge values in an alteration zone only a few metres in diameter elsewhere in the ophiolite anomalous pt pd values of up to 4 ppm occur in chromite rich sulphide bearing dunite within the dunite unit which stratigraphically overlies the harzburgite pt pd concentrations of up to 1 ppm are present in the pyroxenites and wehrlites of the upper part of the ultramafic sequences high level wehrlites and pyroxenites within the gabbro unit representing late ultramafic differentiates contain lower levels of pge of up to 310 ppb pt pd the gabbros all have less than 20 ppb pt and pd this distribution indicates that the pge concentrations occur in specific primary magmatic lithologies and pathfinder analysis shows that they are associated with ni cu au and cr pge have not been found above detection limits in fault zones away from primary pge mineralization sites and it is concluded that pge are not significantly hydrothermally reconcentrated at cliff remobilization may have occurred over short distances of only a few metres at most

evidence for the emplacement of ca 3 0 ga mantle derived mafic ultramafic bodies in the ukrainian shield abstract the archean bedrock of the ukrainian shield is divided into western and central blocks by a n w trending tectonic zone known as the golovanevsk suture zone within this zone there are numerous mafic ultramafic bodies including the chromitite bearing lipovenki and kapitanov massifs the chromitites of these massifs possess mineralogical and textural features and platinum group element systematics similar to those typically observed in ophiolitic podiform chromitites although there is no field evidence for an ophiolitic structure the geochemical evidence strongly suggests an origin in the oceanic lithosphere the re os ratios of the chromites and whole rock chromitites are extremely low and consequently their 187 os 188 os ratios have increased little since crystallization osmium isotope model ages indicate formation at approximately 3 0 ga suggesting that the massifs represent portions of the archean oceanic lithosphere

an 85 ka record of climate change in lowland central america drill cores obtained from lake peten itza peten guatemala contain a 85 kyr record of terrestrial climate from lowland central america that was used to reconstruct hydrologic changes in the northern neotropics during the last glaciation sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions respectively from 85 to 48 ka sediments were dominated by carbonate clay indicating moist conditions during marine isotope stages mis 5a 4 and early 3 the first gypsum layer was deposited at 48 ka signifying a shift toward drier hydrologic conditions and the onset of wet dry oscillations during the latter part of mis 3 peten climate varied between wetter conditions during interstadials and drier states during stadials the pattern of clay gypsum wet dry oscillations during the latter part of mis 3 48 23 ka closely resembles the temperature records from greenland ice cores and north atlantic marine sediment cores and precipitation proxies from the cariaco basin the most arid periods coincided with heinrich events when cold sea surface temperatures prevailed in the north atlantic meridional overturning circulation was reduced and the intertropical convergence zone itcz was displaced southward a thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake and pollen accumulated during the same period indicates vegetation consisted of a temperate pine oak forest this finding contradicts previous inferences that climate was arid during the last glacial maximum lgm chronozone 21 2 ka at 18 ka peten climate switched from moist to arid conditions and remained dry from 18 to 14 7 ka during the early deglaciation moister conditions prevailed during the warmer bolling allerod 14 7 12 8 ka with the exception of a brief return to dry conditions at 13 8 ka that coincides with the older dryas and meltwater pulse 1a the onset of the younger dryas at 12 8 ka marked the return of gypsum and hence dry conditions the lake continued to precipitate gypsum until 10 3 ka when rainfall increased markedly in the early holocene

mean and tidal currents in baltimore canyon the mean and tidal current fields within baltimore canyon and on the shelf and slope around it were observed with instrumented moorings at 12 sites currents on the shelf are similar to those observed elsewhere over the northeastern u s outer shelf with mean flow southwestward at 10 to 15 cm s 1 parallel to the shelf break tidal current velocity in the direction of the major axis of the m2 ellipse over the shelf is about 9 cm s 1 currents within the canyon are complex mean flow near the bottom along the canyon axis is downcanyon at 5 cm s at a depth of 275 m but upcanyon at 7 cm s 1 at 600 m depth and negligible at depths of 1000 m and 1500 m at 50 m above the bottom mean flow at each of these sites is less than it is at 6 m above the bottom the m2 tidal currents in the canyon tend to be aligned with topography and have velocities ranging up to 22 cm s 1 maximum instantaneous current speeds were found near the bottom in the canyon axis with a peak speed for the entire survey of 106 cm s 1 near the bottom at the 275 m level

paragenesis of multiple platinum group mineral populations in shetland ophiolite chromitite 3d x ray tomography and in situ os isotopes abstract chromitite from the harold s grave locality in the mantle section of the shetland ophiolite complex is extremely enriched in ru os and ir at g g concentrations high resolution x ray computed tomography on micro cores from these chromitites was used to determine the location size distribution and morphology of the platinum group minerals pgm there are five generations of pgm in these chromitites small average 5 m in equivalent sphere diameter esd euhedral laurites often with os ir alloys are totally enclosed in the chromite and are likely to have formed first by direct crystallisation from the magma as the chromite crystallised also within the chromitite there are clusters of larger 50 m esd aligned elongate crystals of pt rh ir os and ru bearing pgm that have different orientations in different chromite crystals these may have formed either by exsolution or by preferential nucleation of pgms in boundary layers around particular growing chromite grains thirdly there is a generation of large 100 m esd composite os ir ru rich pgm that are all interstitial to the chromite grains and sometimes form in clusters it is proposed that os ir and ru in this generation were concentrated in base metal sulfide droplets that were then re dissolved into a later sulfide undersaturated magma leaving pgm interstitial to the chromite grains fourthly there is a group of almost spherical large 80 m esd laurites hosting minor os ir ru rich pgm that form on the edge or enclosed in chromite grains occurring in a sheet crosscutting a chromitite layer these may be hosted in an annealed late syn or post magmatic fracture finally a few of the pgm have been deformed in localised shear zones through the chromitites the vast majority of the pgm including small pgm enclosed within chromite larger interstitial pgm and elongate aligned pgm have os isotope compositions that give re depletion model ages approximately equal to the age of the ophiolite at 492 ma a number of other pgm not confined to a single textural group fall to more or less radiogenic values with four pgm giving anomalously unradiogenic os corresponding to an older age of 1050 ma the 187 os 188 os isotopic ratios for pgm from cliff and quoys from the same ophiolite section are somewhat more radiogenic than those at harold s grave this may be due to a distinct mantle source history or possibly the assimilation of radiogenic crustal os

abrupt onset of the little ice age triggered by volcanism and sustained by sea ice ocean feedbacks 1 northern hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes however the causes of superposed century scale cold summer anomalies of which the little ice age lia is the most extreme remain debated largely because the natural forcings are either weak or in the case of volcanism short lived here we present precisely dated records of ice cap growth from arctic canada and iceland showing that lia summer cold and ice growth began abruptly between 1275 and 1300 ad followed by a substantial intensification 1430 1455 ad intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium a transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times and that cold summers can be maintained by sea ice ocean feedbacks long after volcanic aerosols are removed our results suggest that the onset of the lia can be linked to an unusual 50 year long episode with four large sulfur rich explosive eruptions each with global sulfate loading 60 tg the persistence of cold summers is best explained by consequent sea ice ocean feedbacks during a hemispheric summer insolation minimum large changes in solar irradiance are not required

pge rich podiform chromitites in the al ays ophiolite complex saudi arabia an example of critical mantle melting to extract and concentrate pge the al ays ophiolite complex in saudi arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements pge in podiform chromitite with maximum values of 2 570 ppb pt 6 870 ppb pd 840 ppb rh 5 800 ppb ru 6 200 ppb ir and 3 300 ppb os smooth chondrite normalized pge profiles indicate igneous pge ratios these suggest that in situ alteration of the pgm caused only minor mobility of pge during secondary modification of the mineralogy thus the geochemistry of the igneous concentration processes can be examined despite the mineralogical changes caused by subsequent alteration three main types of pge mineralization are observed that are defined by their relative abundances of individual pge type 1 has ru both pt and pd with negative slope chondrite normalized profiles type 2 has ru either pt or pd pt pd ir ratios of 1 to 5 and convex upward chondrite normalized profiles type 3 has ru either pt or pd pt pd ir ratios of 5 to 60 positive slope chondrite normalized profiles and is associated with elevated cu and ni concentrations the unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition for example cr2o3 varies from 39 to 69 wt percent pge mineralization types 1 2 and 3 are related to the composition of the chromite type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent cr2o3 type 2 occurs in chromitite having a range of 53 to 61 wt percent cr2o3 and type 3 occurs in chromitite having a range of 39 to 51 wt percent cr2o3 the pge form a great variety of platinum group minerals pgm and they differ among the three types of pge mineralization type 1 is characterized by euhedral os ir and ru ipge alloys and laurite both commonly enclosed in chromite as well as members of the irarsite hollingworthite solid solution series and pt ipge bearing pgm both commonly interstitial to the chromite grains type 2 pge enrichment is characterized by ipge pt and rh bearing pgm type 3 pge enrichment hosts predominantly pd and pt bearing pgm associated with ni and cu bearing minerals where exposed to the serpentinization process the pgm are altered to alloys arsenides antimonides and oxides that form irregular shapes or may form pseudomorphs of former pgm they are commonly associated with ni and cu bearing minerals including ruthanian pentlandite millerite arsenides and pge bearing awaruite mantle melting and subsequent crystallization were at an optimum to concentrate pge in the al ays ophiolite complex crystallization of ipge commonly prior to chromite crystallization and latterly with some pt and rh occurred across the range of chromitite composition crystallization of pd with some remaining pt occurred during sulfur saturation in chromitite formed from a more evolved magma we propose that this crystallization was from a magma that was enriched in pge because the degree of mantle melting was just sufficient to extract the pge but not dilute them in a melt that includes further mantle melting this feature is likely to be common to other pge rich ophiolite complexes such as in shetland in the united kingdom leka in norway thetford in canada pindos in greece tropoja in albania and in new caledonia if equilibrium partial melting had continued in al ays then the magma would have been diluted by subsequent pge poor melt this would have prevented sulfur saturation until much higher in the sequence producing pt and pd bearing base metal sulfides in the crustal wehrlite and gabbro as has occurred in the cyprus and oman ophiolites

changes in atmospheric circulation and ocean ice cover over the north atlantic during the last 41 000 years high resolution continuous multivariate chemical records from a central greenland ice core provide a sensitive measure of climate change and chemical composition of the atmosphere over the last 41 000 years these chemical series reveal a record of change in the relative size and intensity of the circulation system that transported air masses to greenland defined here as the polar circulation index pci and in the extent of ocean ice cover massive iceberg discharge events previously defined from the marine record are correlated with notable expansions of ocean ice cover and increases in pci during stadials without discharge events ocean ice cover appears to reach some common maximum level the massive aerosol loadings and dramatic variations in ocean ice cover documented in ice cores should be included in climate modeling

the tsunamite problem the genetic term tsunamite is used for a potpourri of deposits formed from a wide range of processes overwash surges backwash flows oscillatory flows combined flows soft sediment deformation slides slumps debris flows and turbidity currents related to tsunamis in lacustrine coastal shallow marine and deep marine environments tsunamites exhibit enormous variability of features e g normally graded sand floating mudstone clasts hummocky cross stratification etc these sedimentary features may also be interpreted as deposits of turbidity currents turbidites debris flows debrites or storms tempestites however sedimentary features play a passive role when these same deposits are reinterpreted as tsunamites on the basis of historical evidence for tsunamis and their triggering mechanisms e g earthquakes volcanic explosions landslides and meteorite impacts this bipartite sedimentological vs historical approach which allows here classification of the same deposit as both turbidite and tsunamite has blurred the distinction between shallow marine and deep marine facies a solution to this problem is to classify deposits solely by a descriptive sedimentological approach the notion that tsunami waves can directly deposit sediment in the deep sea is unrealistic because tsunami waves represent transfer of energy and they are sediment starved during tsunamis and major storms submarine canyons serve as the physical link between shallow water and deep water environments for sediment transport tsunami related deposition involves four progressive steps 1 triggering stage offshore 2 tsunami stage incoming waves 3 transformation stage near the coast and 4 depositional stage outgoing sediment flows in this progression deep water deposition can commence only after the demise of incoming tsunami waves due to their transformation into outgoing sediment flows deposits of these sediment flows already have established names e g debrite and turbidite therefore the term tsunamite for these deposits is obsolete

a new bipolar ice core record of volcanism from wais divide and neem and implications for climate forcing of the last 2000 years volcanism is a natural climate forcing causing short term variations in temperatures histories of volcanic eruptions are needed to quantify their role in climate variability and assess human impacts we present two new seasonally resolved annually dated non sea salt sulfur records from polar ice cores wais divide wdc06a from west antarctica spanning 408 b c e to 2003 c e and neem neem 2011 s1 from greenland spanning 78 to 1997 c e both analyzed using high resolution continuous flow analysis coupled to two mass spectrometers the high dating accuracy allowed placing the large bi hemispheric deposition event ascribed to the eruption of kuwae in vanuatu previously thought to be 1452 1453 c e and used as a tie point in ice core dating into the year 1458 1459 c e this new age is consistent with an independent ice core timescale from law dome and explains an apparent delayed response in tree rings to this volcanic event a second volcanic event is detected in 1453 c e in both ice cores we show for the first time ice core signals in greenland and antarctica from the strong eruption of taupo in new zealand in 232 c e in total 133 volcanic events were extracted from wdc06a and 138 from neem 2011 s1 with 50 ice core signals predominantly from tropical source volcanoes identified simultaneously in both records we assess the effect of large bipolar events on temperature sensitive tree ring proxies these two new volcanic records synchronized with available ice core records to account for spatial variability in sulfate deposition provide a basis for improving existing time series of volcanic forcing

characterization of the sukinda and nausahi ultramafic complexes orissa india by platinum group element geochemistry abstract samples of 20 chromitite 14 ultramafic and mafic rock and 9 laterite and soil samples from the precambrian sukinda and nausahi ultramafic complexes orissa india were analyzed for platinum group elements pge the maximum concentrations are palladium 13 parts per billion ppb platinum 120 ppb rhodium 21 ppb iridium 210 ppb and ruthenium 630 ppb comparison of chondrite normalized ratios of pge for the chromitite samples of lower proterozoic to archean age with similar data from paleozoic and mesozoic ophiolite complexes strongly implies that these complexes represent precambrian analogs of ophiolite complexes this finding is consistent with the geology and petrology of the indian complexes and suggests that plate tectonic and ocean basin developement models probably apply to some parts of precambrian shield areas

reply to shanmugam g comment on internal waves an underexplored source of turbulence events in the sedimentary record by pomar et al earth science reviews 111 2012 56 81 earth science reviews 2012

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research