structural geology and regional tectonic significance of the ramgarh thrust himalayan fold thrust belt of nepal 1 the ramgarh thrust is one of the major fault systems of the himalayan thrust belt in nepal and northern india the ramgarh thrust sheet is 0 2 2 0 km thick and can be traced along strike the entire length of the himalaya in nepal the fault generally places the oldest paleoproterozoic rocks in the lesser himalayan series upon younger lesser himalayan rocks or lower miocene foreland basin deposits regional balanced cross sections suggest that the ramgarh thrust had at least 120 km of initial south vergent displacement subsequently the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying lesser himalayan rocks ramgarh hanging wall strata are greenschist grade phyllite quartzite and augen gneiss all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top to the south sense of shear structural fabrics in the ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet regional and local mapping of the ramgarh thrust in nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat except where local lateral ramps complicate its geometry similarly the structurally overlying main central thrust always places a hanging wall flat in greater himalayan series rocks upon the regionally flat ramgarh thrust sheet these geometric relationships preclude kinematic and thermal models that elevate greater himalayan and lower lesser himalayan rocks along high angle thrust ramps in the vicinity of the present traces of the ramgarh and main central thrust faults instead the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the main central thrust the present steep dips of the ramgarh and main central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the lesser himalayan zone the ramgarh thrust sheet overlaps a bed length of at least 100 km in lower miocene foreland basin deposits indicating that a significant amount of displacement on the thrust must have occurred after 15 ma growth of the lesser himalayan duplex and additional slip on the frontal part of the ramgarh thrust occurred from 12 to 5 ma the presence of a major greenschist grade metasedimentary thrust sheet composed of lesser himalayan rocks directly below the main central thrust suggests that the famous inverted metamorphism in this region is a result of structural inversion similarly the concept of a broad zone of intense shear strain related exclusively to emplacement of the main central thrust sheet is probably invalid in nepal

radon 222 as a tracer for mixing in the water column and benthic exchange in the southern california borderland the concentrations of222rn and226ra in the water column and in the sediments of santa barbara and san nicolas basins have been measured semi annually over the last four years approximately one third of excess radon profiles obtained in the water column in these basins can be adequately fit with a one dimensional eddy diffusion decay model exponential profiles in the center of san nicolas basin yield a vertical eddy diffusivity of 26 16 cm2 s and 3 4 1 0 cm2 s for santa barbara basin the application of a two dimensional eddy diffusion decay model to profiles obtained in the center and on the margins of san nicolas basin produces a better fit than is found using a one dimensional vertical eddy diffusivity the two dimensional model for san nicolas basin predicts a vertical eddy diffusivity of 17 cm2 s and a horizontal eddy diffusivity of 105 cm2 s these values are in reasonable agreement with those predicted from the vertical buoyancy gradient and the horizontal length scale the vertically integrated radon excess standing crop in the water column of santa barbara basin averages 53 23 atoms m2 s this is in good agreement with the flux across the sediment water interface of 60 15 atoms m2 s calculated by measuring radon emanation in the sediments as a function of depth and applying a molecular diffusion reaction model hence one dimensional molecular diffusion accurately predicts the flux of radon from the laminated santa barbara basin sediments in san nicolas basin the integrated radon excess in the water column is 376 143 atoms m2 s but the diffusive randon flux from san nicolas basin sediments averages only 190 53 atoms m2 s this descrepancy indicates that a non diffusive process probably macrofaunal irrigation supplies much of the flux of radon from san nicolas basin sediments

advances in marine chemistry 1979 1982 these four years saw advances in many areas further developments in sampling and analytical techniques have opened new avenues of exploration for trace elements and particulate matter the impact of the geosecs expedition was further felt in the field of marine chemistry as the results were still being published a major field effort occurred as a follow on to the geosecs work tto and intensive sampling was performed in the newly discovered submarine hydrothermal vent fields an important and unresolved controversy has arisen regarding the ability to measure preformed co2 in the seawater system in many respects the field of marine chemistry appears to be gaining a momentum spurred by both improvements in analytic techniques and an increasing social and economic relevance

remobilization of authigenic uranium in marine sediments by bioturbation uranium behaves as a nearly conservative element in oxygenated seawater but it is precipitated under chemically reducing conditions that occur in sediments underlying low oxygen bottom water or in sediments receiving high fluxes of particulate organic carbon sites characterized by a range of bottom water oxygen bwo and organic carbon flux ocf were studied to better understand the conditions that determine formation and preservation of authigenic u in marine sediments our study areas are located in the mid latitudes of the northeast pacific and the northwest atlantic oceans and all sites receive moderate 0 5 g cm2 kyr to high 2 8 g cm2 kyr ocf to the sediments bwo concentrations vary substantially among the sites ranging from 3 to 270 m a mass balance approach was used to evaluate authigenic u remobilization at each site within each region studied the supply of particulate nonlithogenic u associated with sinking particles was evaluated by means of sediment traps the diffusive flux of u into sediments was calculated from pore water u concentration profiles these combined sources were compared with the burial rate of authigenic u to assess the efficiency of its preservation a large fraction one third to two thirds of the authigenic u precipitated in these sediments via diffusion supply is later regenerated even under very low bwo concentrations 15 m bioturbating organisms periodically mix authigenic u containing sediment upward toward the sediment water interface where more oxidizing conditions lead to the remobilization of authigenic u and its loss to bottom waters

sediment fluxes re suspension and accumulation rates at two wind exposed coastal sites and in a sheltered bay abstract sediment fluxes measured by sediment traps placed at different heights above the sea bed at two near coastal sites and in a sheltered bay are analysed in relation to wave and current conditions traps were deployed for 13 15 coastal sites and 6 months and recovered with a mean period of 16 6 14 0 and 30 days respectively the traps with an aspect ratio of 5 were placed at 0 3 0 5 0 8 1 0 2 0 4 0 6 0 8 0 and 10 0 m above the sea bed at one location whereas an aspect ratio of 2 3 was used at the second coastal location with similar trap intervals the trap in the sheltered bay was placed at 1 0 m above the sea bed results showed that sediment fluxes varied between 505 0 and 38 9 g m 2 day 1 at 0 3 m above the sea bed at a coastal site whereas only small flux variations occurred in the sheltered bay accumulation rates were measured by 210 pb at the three sites and was 2 4 g m 2 day 1 at the same site where fluxes varied between 505 0 and 38 9 g m 2 day 1 at 0 3 m above the sea bed this shows that re suspension here is high and frequent a similar pattern was found at the other coastal site whereas accumulation rate and sediment flux were very similar in the sheltered bay analyses showed that re suspension was induced both by surface wave activity related to periods of high wind speeds and by current shear stress in more calm periods re suspension is quantified as a time series by applying the introduced depth integrated re suspension flux dirf high dirf occurred during periods of high surface wave activity and one re suspension event accounted for about 25 of the total re suspension flux at the site although the event had a very low frequency of occurrence less than 5 of the time about 50 of the total re suspension flux occurred in the interval between 0 0 and 20 0 g m 2 day 1 comparison of different time scales for measuring sediment fluxes and re suspension using sediment traps shows that a time scale of 24 12 h gives an adequate resolution

budgets and behaviors of uranium and thorium series isotopes in santa monica basin sediments abstract nine natural decay series isotopes were measured in six box cores collected from a transect across the santa monica basin the 210 pb derived sedimentation rate decreases from 80 mg cm 2 yr at the slope to 20 mg cm 2 yr in the deep central basin sediment mixing prevails in sites underlying oxic waters but is subdued in the anoxic deep basin below the sill depth uranium contents in sediments are controlled by levels of authigenic u which are higher in the more reduced condition in the deep basin most of the authigenic u results from precipitation within the sediments the 232 th 228 th disequilibrium in sediments indicates that 228 ra is lost from the sediments from a depth of 10 cm upward modelling the distribution of excess 228 th and 234 th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0 2 cm 2 yr and a sedimentation rate close to that based on 210 pb there is no evidence of changing sedimentation rate in the central basin during the past century fluxes of excess 210 pb 230 th and 231 pa to the central santa monica basin sediments are much higher than what can be predicted from local supply advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause

sediment cd and mo accumulation in the oxygen minimum zone off western baja california linked to global climate over the past 52 kyr 1 concentrations of organic carbon orgc cadmium cd and molybdenum mo were measured in two sediment cores raised from depths of 430 and 700 m within the oxygen minimum zone omz off southern baja california at a temporal resolution of 0 5 kyr over the past 52 kyr these records are supplemented with diffuse spectral reflectance dsr measurements obtained on board ship soon after collection at a resolution of 0 05 kyr in the core from 700 m depth a component extracted from the dsr data and the three geochemical proxies generally vary in concert with each other and over a wide range 4 22 orgc 1 40 mg kg cd 5 120 mg kg mo intervals of increased orgc cd and mo accumulation generally correspond to warm periods recorded in the oxygen isotopic composition of greenland ice with the exception of the bolling allerod which is only weakly expressed off baja california concentrations of the biogenic proxies are higher in the core from 430 m depth but erratic sediment accumulation before 15 ka precludes dating of the older intervals that are laminated and contain elevated orgc cd and mo concentrations the new data provide further evidence of an intimate teleconnection between global climate and the intensity of the omz and or productivity along the western margin of north america on the basis of a comparison with cd and mo records collected elsewhere in the region we conclude that productivity may actually have varied off southern baja california by no more than a factor of 2 over the past 52 kyr

the contrasting biogeochemistry of iron and manganese in the pacific ocean abstract vertical and horizontal distributions of dissolved and suspended particulate fe and mn and vertical fluxes of these metals obtained with sediment traps were determined throughout the pacific ocean in general dissolved fe is low in surface and deep waters 0 1 to 0 7 nmol kg with maxima associated with the intermediate depth oxygen minimum zone 2 0 to 6 6 nmol kg vertical distributions of dissolved mn are similar to previous reports exhibiting a surface maximum a subsurface minimum a mn maximum layer coincident with the oxygen minimum zone and lowest values in deep waters near shore removal processes are more intense for dissolved fe than for dissolved mn dissolved mn in the surface mixed layer remains elevated much farther offshore than dissolved fe elevated near surface dissolved mn concentrations occur in the north pacific equatorial current suggesting transport from the eastern boundary near surface mixed layer dissolved mn concentrations are higher in the north pacific gyre reflecting enhanced northern hemisphere aeolian sources residence time estimates for the settling of refractory paniculate fe and mn from the upper water column are 62 220 days fe and 105 235 days mn in contrast residence times for the scavenging of dissolved fe and mn are 2 13 years fe and 3 74 years mn scavenging residence times for dissolved mn based on horizontal mixing in the surface mixed layer of the northeast pacific are 0 4 years nearshore to 19 years 1000 km offshore there is no evidence for in situ fe redox dissolution within sub oxic waters in the eastern tropical north pacific dissolved fe appeared to be controlled by dissolution from sub oxic sediments with oxidative scavenging in the water column or upper sediment layers however in situ mn dissolution within the oxygen minimum zone was evident

the effect of early diagenesis on the fe isotope compositions of porewaters and authigenic minerals in continental margin sediments abstract iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the california continental margin where isotope compositions range from 56fe 3 0 to 0 4 at one site near monterey canyon off central california organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction dir and bacterial sulfate reduction whereas at our other site in the santa barbara basin dir appears to be comparatively small and production of sulfides fes and pyrite was extensive the largest range in fe isotope compositions is observed for fe ii aq in porewaters which generally have the lowest 56fe values minimum 3 0 near the sediment surface and increase with burial depth 56fe values for fes inferred from hcl extractions vary between 0 4 and 0 4 but pyrite is similar at both stations where an average 56fe value of 0 8 0 2 was measured we interpret variations in dissolved fe isotope compositions to be best explained by open system behavior that involves extensive recycling of feflux this study is the first to examine fe isotope variations in modern marine sediments and the results show that fe isotopes in the various reactive fe pools undergo isotopic fractionation during early diagenesis importantly processes dominated by sulfide formation produce high 56fe values for porewaters whereas the opposite occurs when fe iii oxides are present and dir is a major pathway of organic carbon respiration because shelf pore fluids may carry a negative 56fe signature it is possible that the fe isotope composition of ocean water reflects a significant contribution of shelf derived iron to the open ocean such a signature would be an important means for tracing iron sources to the ocean and water mass circulation

nicolas and eel submarine fans california continental borderland nicolas and eel submarine fans occur in the san nicolas basin an outer basin of the california continental borderland that has a low sedimentation rate nicolas fan lies southeast of san nicolas island and the broad san nicolas bank the upper fan is characterized by numerous channels the midfan region may be divided into three distinct areas a central midfan and two subfans the central midfan depositional system is typical of normark s suprafan the subfans are essentially flat sandy lobes eel fan lies west of san clemente island and is fed by an erosional valley its midfan region may also be characterized as a flat sandy lobe box core data show that holocene turbidity currents have occurred on the central nicolas fan whereas the subfans and eel fan are nearly nactive the local tectonic regime influences these fans by determining slope trends creating bathymetric obstacles controlling canyon location and triggering mass movements sea level changes affect sedimentation patterns of the fans by increasing the mean grain size and the amount of sediment delivered to the fan during lowstands these changes may in turn affect the morphology of the fan the characteristics of these fans represent variations of the generalized fan models described in the literature

role of large particles in the transport of elements and organic compounds through the oceanic water column abstract during the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea this finding has important implications for understanding the transfer distribution and fate of elements and organic compounds in marine waters the large 100 m detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small discrete fecal pellets and plankton hard parts to large amorphous aggregates or snow which contain both organic and inorganic constituents depending on size shape and density these particles sink at rates ranging from 1000 m day 1 several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport flux studies have highlighted the importance of marine heterotrophs in packaging fine suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements radionuclides and organic compounds natural and anthropogenic in various oceanic regimes pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested

fluxes of uranium and thorium series isotopes in the santa barbara basin abstract samples from the manop santa barbara basin sediment trap intercomparison were analyzed for the isotopes of uranium thorium radium lead and polonium all of the traps showed approximately the same compositions and isotopic ratios indicating that they trapped similar materials the234th flux via falling particles was very close to the flux predicted from the production and scavenging rates of234th from the water column the210pb content of the trapped particles and the surface sediments were the same however the measured flux of210pb was seven times greater than the predicted flux predicted and measured fluxes of228th and210po were similarly out of balance to explain this apparent inconsistency we suggest as others have done that the santa barbara basin is an area where scavenging from the water column is intensified and where sediments deposited initially on the margins may be physically remobilized on a short time scale these two effects increase the apparent area from which the basin derives the longer lived isotopes but does not increase significantly the supply of the short lived234th

profiles of dissolved and particulate thorium isotopes in the water column of coastal southern california abstract profiles of 234 th 232 th and 230 th in the santa monica basin were measured 234 th in the upper part of the water column suggests that the residence time of dissolved 234 th with respect to adsorption onto and removal by particles increases with depth from 3 4 weeks at the surface to 5 months below the euphotic zone the partition of 234 th between dissolved and particulate forms points to a residence time of 4 7 days for particulate 234 th in the surface euphotic zone rapid removal of particulate 234 th from the surface water precludes the possibility of an adsorption desorption equilibrium state between dissolved and particulate 234 th based on the 230 th data the overall residence time of dissolved th in the water column is estimated at 5 6 years both dissolved 230 th and 232 th profiles exhibit higher concentrations at the surface than immediately underneath suggesting a terrestrial input for them from the surface to the bottom of the water column the concentration of dissolved 232 th decreases by a factor of 3 4 and the dissolved 230 th 232 th activity ratio increases by a factor of about five from 2 3 to 9 12 this reflects the surface source of the terrestrial end member and its removal at depth on the average about 6 of the 234 th 25 of the 230 th and 50 of the 232 th were found in the particulate form which is as expected and relates to their respective sources to the difference in radioactive decay half life and to the kinetics of the scavenging process

a sediment trap intercomparison study in the santa barbara basin four sediment traps of radically different design were deployed in the santa barbara basin for approximately 45 days the measured fluxes ranged from 370 to 774 g m 2 yr 1 for the different designs these values lie within flux measurements previously determined for the basin compared to the 25 year record 920 g m 2 yr 1 however all fluxes determined in this experiment are somewhat low because this experiment was conducted during a general period of high storm activity and runoff measurement of greater than average flux was expected it is probable that the higher flux recorded by the sediments results from a significant input of detritus into the basin by near bottom transport the chemical composition of trapped material was nearly identical in all four trap designs the deep cone design however had a significantly lower mn content since this trap was the only one in which reducing conditions were produced in the sample container reduction and mobilization of manganese after collection is believed to have occurred in spite of the very different designs tested the factor of two agreement in flux determination and the compositional similarity of the material collected is encouraging for future attempts to directly measure the flux of particulates in the ocean

the trace metal composition of suspended particles in the oceanic water column near bermuda trace metal partitioning between seawater and particles both suspended and sinking must be determined to understand and model oceanic trace metal scavenging processes in order to improve the oceanic particulate trace metal data base a newin situ pump was developed and deployed to collect suspended particles from 0 4000 m in the sargasso sea near bermuda these particles were analyzed for al fe mn co zn cu ni cd and pb major particulate carrier phases were estimated from ca opaline si and p cd is the only trace element of this group with a particulate maximum in near surface waters the other metals have low particulate concentrations mol l in near surface waters and increase with depth into the upper thermocline particulate al and fe are then uniform with depth below about 1000 m until increasing in the bottom nepheloid layer the remaining elements mn co zn cu ni and pb decrease from the mid thermocline values to lower and relatively uniform concentrations in deep waters the similarities among the vertical profiles for these metals suggest that authigenic mn oxides influence their uptake from the dissolved pool although an important or dominant role for other host phases specifically particulate organic matter is not ruled out by the data mean intermediate and deep water concentrations are al 3600 fe 1200 mn 146 co 1 5 zn 7 4 cu 15 ni 6 0 cd 0 06 and pb 2 1 pmol l these particulate al and mn values agree with previous measurements in this region but the other elements are lower by a factor of 3 fe co to 30 zn suggesting that earlier data were compromised by contamination the suspended metal data are used in a simple steady state two flux model to evaluate the role of suspended particles in determining the vertical flux of metals in the sargasso sea the model calculations indicate that removal of suspended particles in the deep 500 m water column contributes less than half of the total deep flux for all of the elements investigated except for cd this result implies that uptake of these metals onto particles in the deep central ocean is a minor factor in their oceanic mass balance if this finding is generally true for oligotrophic ocean environments whole ocean residence times for these elements must be governed largely by processes occurring either in near surface waters or at ocean margins

anomalous isotopic concentrations in the sea off southern california measurements are reported of the 10be concentrations in 15 sediments from the sea off southern california the sedimentation patterns are modelled with a two component system one component having the high 10be concentration of pelagic sediment and the other the low concentration of continental runoff the pelagic component brought to the region by the california current with the pacific ocean as reservoir is very likely the source not only of anomalously high 10be deposition but also of similar effects with 2101pb 239 240pu and 241pu

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

fluxes and budget of on the continental margin of the bay of biscay northeastern atlantic abstract as part of the ecofer experiment ecomarge program jgofs france concentrations of the natural radionuclide 210 pb were measured in particulate matter collected by sediment traps deployed for a 14 month period june 1990 august 1991 on two mooring lines within the cap ferret canyon on the french continental slope of the bay of biscay northeastern atlantic 210 pb concentrations range 63 218 dpm g 1 and fluxes range 0 1 11 2 dpm cm 2 yr 1 displayed typical gradients increasing with depth at both mooring sites also at equivalent trap depths 210 pb activities were systematically higher on the deep slope mooring 3000 m compared to the shallow one 2300 m fluxes of 210 pb generally decreased seaward and exhibited temporal variations but with no marked seasonal trend essentially matching variations in total mass flux various correlations between 210 pb activities and major constituents of trapped particles were examined 210 pb activities were independent of the particle composition indicating that in this area scavenging of this nuclide from the water column was not directly tied to the quality of settling particles the 210 pb budget as determined by the balance of local supply flux and fluxes to sediment traps and sediments implies important lateral inputs to both mooring sites 65 89 of the flux measured by traps a feature typical of continental margin environments on the basis of annual mean 210 pb activity in traps of the two mooring sites two particle sources are hypothesized the first source located in shallow waters of the shelf and upper slope supplies the entire slope region the second source probably composed of resuspended particles from upper slope sediments feeds essentially the upper and intermediate slope region maximum sedimentation rates determined on seven box cores decreased with bottom depth from 0 3 to 0 4 cm yr 1 at the canyon heads down to 0 06 cm yr 1 at 3000 m a good agreement was observed at both mooring sites between 210 pb fluxes registered by the near bottom traps and excess 210 pb inventories within the underlying sediments suggesting that nuclide fluxes measured by traps during this experiment are representative of fluxes on a longer time scale 10 100 yr nevertheless total mass fluxes measured by traps were 3 4 times lower than 210 pb determined sediment accumulation rates a discrepancy that may be more apparent than real given the possible errors in the latter due to bioturbation

non annual laminations and expansion of anoxic basin floor conditions in santa monica basin california borderland over the past four centuries sediments from box cores in the deep portion of santa monica basin have been examined by x radiography and the upper 10 20 cm are characterized by laminations 1 5 mm in thickness age dating of 9 cores by 210pb analysis has shown that accumulation rates on the central basin floor have been remarkably constant during the past century averaging 16 0 0 4 mg cm2 yr densitometer analysis of the x radiography films from 7 cores and spectral analysis of the resulting time series indicates that most lamination couplets have a periodicity in the 3 7 year range a range matching that of the el nino southern oscillation periodicity the median periodicity appears to have decreased slightly during the past 3 centuries from 7 0 years during 200 300 yrs b p to 5 2 yrs during 0 100 yrs b p this trend could be due to a decreasing sedimentation rate an increasing forcing frequency or an increasingly faithful response of the basin in more recent years multiple processes have contributed to creation of the laminations in santa monica basin the principal factors appear to be 1 variations in porosity due to the relative abundance of organic filaments probably formed by beggiatoa mats and 2 compositional variations that reflect variations in carbonate lithogenous ratios and in grain size both of these are expected to respond to enso events the presence of near anoxic bottom waters has prevented bioturbation permitting laminations to be preserved basin bottom waters became nearly anoxic about 350 years ago at the onset of warming as the little ice age waned expansion of the near anoxic area during the past century may have been augmented by 20th century anthropogenic effects the style of these non annual laminations in santa monica basin matches that of the annual laminations in the adjacent santa barbara basin and demonstrates the need for caution in interpreting all laminations as representing annual varves

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

influence of boundary scavenging and sediment focusing on 234th 228th and 210pb fluxes in the santa barbara basin abstract sediment trap samples were collected from the santa barbara basin to examine the flux of 234 th 228 th and 210 pb mean ratios of measured to predicted fluxes are 2 10 and 21 for 234 th 228 th and 210 pb respectively boundary scavenging may supply a portion of all three radionuclides however sediment focusing supplies additional 228 th and 210 pb but little 234 th sediment temporarily deposited on the shelf around the basin contains little excess 234 th due to its short half life 228 th and 210 pb fluxes to the sediments in the santa barbara basin are several times greater than predicted from their supply in the overlying water column these enhanced fluxes require lateral transport of 228 th and 210 pb to the basin through boundary scavenging and sediment focusing 234 th flux is poorly correlated with 228 th and 210 pb fluxes because individual radionuclide sources to the basin differ i e water column production boundary scavenging and or sediment focusing correlations between 228 th and 210 pb fluxes are high due to the strong influence of sediment focusing on both radionuclides there is a strong correlation between both the 228 th and 210 pb flux and total mass flux and lithogenic flux but a weak correlation with the other major sediment components e g organic carbon carbonate and biogenic opal in the santa barbara basin a large fraction of the total mass flux is lithogenic material 50 80 hence the strong correlation between 228 th and 210 pb fluxes and both total mass and lithogenic fluxes 234 th flux is poorly correlated with total mass and major sediment component fluxes

thermal model for the zanskar himalaya abstract crustal thickening along the northern margin of the indian plate following the 50 ma collision along the indus suture zone in ladakh caused widespread high temperature medium pressure barrovian facies series metamorphism and anatexis in the zanskar himalaya metamorphic isograds are inverted and structurally telescoped along the main central thrust mct zone at the base of the high himalayan slab along the zanskar valley at the top of the slab isograds are the right way up and are also telescoped along northeast dipping normal faults of the zanskar shear zone zsz which are related to culmination collapse behind the miocene himalayan thrust front between the mct and the zsz a metamorphic anatectic core within sillimanite grade rocks contains abundant leucogranite granite crustal melts of probable himalayan age a thermal model based on a crustal scale cross section across the zanskar himalaya suggests that m1 isograds developed during early himalayan barrovian metamorphism were overprinted during high grade mct related anatexis and folded around a large scale recumbent fold developed in the hanging wall of the mct

defining the himalayan main central thrust in nepal an inverted metamorphic field gradient associated with a crustal scale south vergent thrust fault the main central thrust has been recognized along the himalaya for over 100 years a major problem in himalayan structural geology is that recent workers have mapped the main central thrust within the greater himalayan sequence high grade metamorphic sequence along several different structural levels some workers map the main central thrust as coinciding with a lithological contact others as coincident with the kyanite isograd up to 1 3 km structurally up section into the tertiary metamorphic sequence without supporting structural data some workers recognize a main central thrust zone of high ductile strain up to 2 3 km thick bounded by an upper thrust mct 2 vaikrita thrust and a lower thrust mct 1 munsiari thrust some workers define an upper lesser himalaya thrust sheet that shows similar p t conditions to the greater himalayan sequence others define the main central thrust either on isotopic nd sr differences differences in detrital zircon ages or as being coincident with a zone of young

particulate iron and manganese in the santa barbara basin california abstract particulate fe and mn may be important trace metal scavengers in the water column as well as being probable indicators of biologically mediated redox processes a study has been made of suspended particulate composition in the santa barbara basin a shallow near shore basin off southern california with sub oxic conditions below sill depth observations have revealed several interesting phenomena relating to the geochemistry of fe and mn most striking is a profound enrichment of particulate fe in samples from the bottom two hundred meters these particulates have a constant fe p mole ratio of about three and may originate at the sediment water interface or may be transported to the basin from local marshes for particulate mn enrichments are observed both in the sub sill waters and near the base of the euphotic zone a consideration of particle removal rates suggests that the sub photic zone enrichment has a biogenic origin in the sub sill waters enrichment in mn is apparently due to the precipitation of dissolved mn diffusing from the anoxic basin sediments a simple mass balance suggests that most of the mn lost from the sediments is transported from the santa barbara basin in dissolved form

copper isotope signatures in modern marine sediments abstract the development of metal stable isotopes as tools in paleoceanography requires a thorough understanding of their modern marine cycling to date no cu isotope data has been published for modern sediments deposited under low oxygen conditions we present data encompassing a broad spectrum of hydrographic and redox regimes including continental margin and euxinic sulphide containing settings taken together with previously published data from oxic settings these data indicate that the modern oceanic sink for cu has a surprisingly homogeneous isotopic composition of about 0 3 65cu relative to nist srm976 we suggest that this signature reflects one of two specific water column processes 1 an equilibrium isotope fractionation between soluble isotopically heavy cu complexed to strong organic ligands and an isotopically light pool sorbed to particles that deliver cu to the sediment or 2 an equilibrium isotope fractionation between the same isotopically heavy ligand bound pool and the particle reactive free cu2 species with the latter being scavenged by particulates and thereby delivered to the sediment an output flux of about 0 3 into sediments is isotopically light relative to the known inputs to the ocean at around 0 6 and the seawater value of 0 6 to 0 9 suggesting the presence of an as yet unidentified isotopically light source of cu to the oceans we hypothesize that this source may be hydrothermal or may result from the partial dissolution of continentally derived particles

authigenic molybdenum formation in marine sediments a link to pore water sulfide in the santa barbara basin pore water and sediment mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the santa barbara basin to examine the connection between authigenic mo formation and pore water sulfide concentration only at the deepest site 580 m where pore water sulfide concentrations rise to 0 1 mm right below the sediment water interface was there active authigenic mo formation at shallower sites 550 430 and 340 m where pore water sulfide concentrations were consistently 0 05 mm mo precipitation was not occurring at the time of sampling a sulfide concentration of 0 1 mm appears to be a threshold for the onset of mo fe s co precipitation a second threshold sulfide concentration of 100 mm is required for mo precipitation without fe possibly as mo s or as particle bound mo mass budgets for mo were constructed by combining pore water and sediment results for mo with analyses of sediment trap material from santa barbara basin as well as sediment accumulation rates derived from 210 pb the calculations show that most of the authigenic mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters there is however a non lithogenic particulate mo associated with sinking particles that contributes 15 to the total authigenic mo accumulation analysis of sediment trap samples and supernant brine solutions indicates the presence of non lithogenic particulate mo a large fraction of which is easily remobilized and perhaps associated with mn oxides our observations show that even with the very high flux of organic carbon reaching the sediment of santa barbara basin active formation of sedimentary authigenic mo requires a bottom water oxygen concentration below 3 mm however small but measurable rates of authigenic mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mm indicating that the formation of authigenic mo occurred in the recent past but not at the time of sampling copyright 2000 elsevier science ltd

normal faulting in the 1923 berdun earthquake and postorogenic extension in the pyrenees the 10 july 1923 earthquake near berdun spain is the largest instrumentally recorded event in the pyrenees we recover old analog seismograms and use 20 hand digitized waveforms for regional moment tensor inversion we estimate moment magnitude mw 5 4 centroid depth of 8 km and a pure normal faulting source with strike parallel to the mountain chain n292 e dip of 66 and rake of 88 the new mechanism fits into the general predominance of normal faulting in the pyrenees and extension inferred from global positioning system data the unique location of the 1923 earthquake near the south pyrenean thrust front shows that the extensional regime is not confined to the axial zone where high topography and the crustal root are located together with seismicity near the northern mountain front this indicates that gravitational potential energy in the western pyrenees is not extracted locally but induces a wide distribution of postorogenic deformation

southern california inner basin sediment trap calibration the deployment of particle interceptor traps pits in the three inner basins of the southern california bight santa barbara santa monica and san pedro where preserved laminated bottom sediments occur provides a natural calibration between the parameters determined with the pits and those derived from the historical sedimentary deposits the accumulation rates and chemical composition of the pit materials compare favorably with these recently deposited bottom sediments the ratio of the measured particle collection rate to the sediment accumulation rate for these three basins averaged0 93 0 20 radionuclide 210pb 228th 232th and trace element fe cu zn cd pb compositions of the pit materials and surface sediments agree within generally 30 in each basin an application of pits as a sampling system in an outer basin san nicolas where the sedimentary record has been obscured by bioturbation is also presented to further demonstrate the utility of pits for spatial and temporal studies of various aspects of particle transport and sediment deposition based on the calibration of the pits in the three inner basins we conclude from the comparison of the radionuclide and trace element compositions between the pit materials and surface sediments from the san nicolas basin that recently deposited sedimentary material has been mixed by bioturbation with older material from the last several hundred years excess210pb and the228th 232th activity ratio in the pit material are 6 and 15 times higher than in the surface sediment total pb and1n hno3 leachable pb are 4 and 8 fold higher in the pit material

radionuclide concentrations fluxes and residence times at santa monica and san pedro basins our participation in the california basin study cabs in 1986 1988 has produced a radionuclide data base that allows us to trace the particle and water movement in the santa monica and the san pedro basins these data enable us to calculate the radionuclide inventories in the three compartments of the basin the water column the settling particles and the sediments we have also determined the fluxes of several radionuclides associated with the sinking particles the residence time in the water column and the rate of deposition in the sediments the biogeochemical environment of the santa monica basin is uniquely suited to the application of tracer techniques to study the sedimentation processes subtle but measurable changes in the distribution of fallout of 137cs from 1977 to 1988 were noted in the water column compared to the decay corrected measurement in 1973 1977 at the geosecs and farallon islands stations off the coast of california 137cs inventory in the water column in santa monica basin has not changed significantly for the past decade increases in the deep water concentration profiles of 137cs particle transport and diagenetic release of cesium in the deep water the major 137cs inventory 70 resides in the upper 400m of water with a mean residence time of 480 90 years less than 11 of the total 137cs inventory was found in the sediments at water depths of 900m in 1987 unlike 137cs more than 80 of the fallout 239 240pu has been deposited in the santa monica basin sediments a mean residence time of 13 2 years was measured for 239 240pu in the basin water the ubiquitous 239 240pu subsurface concentration maximum observed in the pacific oceanic waters at about 350m depth is also evident in the basin water we have no definitive explanation for the persistence of the subsurface maximum except that it appears to be correlated with the density gradient at 26 5 to 26 9 sigma t the 239 240pu flux for settling particles correlates remarkably well with mass flux and it increases with depth in the water column plutonium deposition in anoxic sediments also appears to be useful as a time marker for age dating of sediments and estimating the sediment accumulation rates the sedimentation rates determined by plutonium are shown to be comparable to those estimated by excess 210pb in the cores radionuclide fluxes measured from sediment trap particles in 1986 1988 were lower by a factor of 0 6 0 2 compared to the average fluxes determined from the box core samples radionuclides such as 210pb 137cs and 239 240pu have been shown to be useful tools in the study of biogeochemical processes in the santa monica basin

modeling studies of dissolved organic matter cycling in santa barbara basin ca usa sediments abstract here we describe new reaction transport models for the cycling of dissolved organic matter dom both dissolved organic carbon doc and dissolved organic nitrogen don in anoxic marine sediments and apply these models to data from santa barbara basin sediment cores maximum depth of 4 6 m model results show that most organic carbon and nitrogen flow in the sediments occurs through reactive dom intermediates that turn over rapidly to produce inorganic remineralization end products refractory dom is also produced and the vast majority of this refractory dom is not remineralized and either escapes as a benthic flux across the sediment water interface or is buried except near the sediment surface refractory dom represents 95 of the total pore water dom pore water dom appears to be consistently depleted in nitrogen as compared to its source organic matter which may be the result of differential production of carbon versus nitrogen containing refractory dom during remineralization refractory doc docr in santa barbara basin sediment pore waters is largely produced from degradation of sediment particulate organic carbon poc in addition there is an upward basal flux of docr that is strongly depleted in 14c 810 the 14c value of docr varies according to its source ranging from 60 a component of surface sediment poc enriched with radiocarbon from nuclear weapons testing in the 1960 s to 810 the basal doc flux each contributes to the docr benthic flux which has a weighted average 14c value of 40 the model determined docr benthic flux is roughly half of the total doc benthic flux consistent with observations in the literature that sediments are a source of both labile and refractory doc to bottom waters these results support previous arguments that sediment benthic fluxes represent an important source of refractory doc to the oceans the benthic flux of refractory doc from these sediments may also contribute pre aged doc to the water column if the different sub components of the anoxic pore water docr pool with differing radiocarbon ages have differing reactivities in the oxic marine water column

mixing and cycling of uranium thorium and 210pb in puget sound sediments abstract activity profiles of excess 234 th excess 210 pb 232 th 230 th 234 u and 238 u and 228 232 th ratios determined in eight box cores of sediment from six sites in central puget sound provide new insights into the dynamic nature of solid phase mixing in surface sediments the exchange of 228 ra and other soluble species across the sediment water interface and the cycling of u th and 210 pb in this coastal zone comparison of excess 234 th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central puget sound water column surface sediment horizons with excess 234 th have no excess 228 th which might be used to ascertain sediment accumulation rates over the past decade instead deficiencies of 228 th due to loss of soluble 228 ra from pore water to the overlying water persist to 20 30 cm revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of puget sound solid phase u isotope concentrations tend to increase by up to a factor of two with depth in sediments as a result of dissolved u being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing 232 th and 230 th activities and 230 232 th ratios are constant with depth in sediments indicating constant detrital phase compositions and essentially no authigenic 230 th steady state 210 pb depositional activities in and fluxes to puget sound sediments average only about onehalf those for sediments of the open washington coast north of the columbia river mouth primarily because of a much lower supply of dissolved 210 pb in sea waters adverting into puget sound excess 234 th profiles in sediments reveal much more detail about the depth dependency dynamic nature and recent history of solid phase mixing processes than excess 210 pb profiles at least six of eight 234 th profiles show that mixing within the 210 pb defined surface mixed layer is depth dependent in three profiles 234 th derived mixing rates are fastest several centimeters below the sediment water interface indicating greater macro benthic activity at these depths depth dependent mixing coefficients derived from the best fit of a four layer advection diffusion decay model to the 234 th data are consistent with 210 pb profiles determined for the same sediments strongly suggesting that 234 th and 210 pb are mixed equivalently and in a multilayered manner

el nino in santa barbara basin diatom radiolarian and foraminiferan responses to the 1983 el nino event abstract santa barbara basin box cores collected in august of 1985 were sub sampled and processed in search of microfossil evidence of the 1983 el nino microplankton and microbenthos indicate the presence of that el nino bounded by non el nino conditions specifically the event is recognized by an increase in diatom diversity and the presence of several warm water taxa e g coscinodiscus plicatoides hemidiscus cuneiformis nitzschia kolaczekii pseudoeunotia doliolus thalassiosira lineata thalassiosira nodulolineata an increase in polycystine radiolarian flux and the presence of the eastern tropical pacific species spongaster pentas an increase in the diversity of planktonic foraminifera and the occurrence of the warm water species globoturborotalita rubescens there is also evidence for an oxygenation of the basin during the 1983 el nino as suggested by the increase in the abundance of the benthic foraminifera bolivina vaughani and by the decrease in the preservation of phaeodarian radiolarian spines which are best preserved under anoxic conditions a new onboard procedure for sampling of box cores and freezing of subcores is described

sensitivity of the phosphate cadmium carbon isotope relation in the ocean to cadmium removal by suboxic sediments reconstructions of past deep ocean nutrient concentrations rely largely on the carbon isotopic composition and cadmium content of shells of benthic foraminifera when comparing records of these two paleonutrient tracers a relation similar to the phosphate cadmium carbon isotope relation in the water column today is generally assumed we present new water column sediment and pore water data from the california continental margin which demonstrate that cd accumulation is enhanced in suboxic sediments these results suggest that the oceanic cd inventory is sensitive to changes in the areal extent and redox intensity of suboxic sediments and thus that the phosphate cadmium relation in the ocean could have been different in the past the magnitude of this effect and its evolution through time is presently hard to quantify because the oceanic cd budget is poorly constrained

variabilities of particulate flux and 210pb in the southern east china sea and western south okinawa trough abstract variabilities of the particulate mass flux and associated 210 pb activity were examined on samples collected from time series sediment traps deployed in the continental slope area of the east china sea off northeast taiwan and in the western south okinawa trough sot as a part of the keep program the particulate flux decreases laterally from the continental slope toward the trough increases vertically toward the bottom at most of the mooring sites and shows fairly synchronous temporal variations among the traps in each mooring for most of the deployments the temporal variation of 210 pb is generally smaller than that of the particulate flux in terms of relative change as expressed in standard deviation of the mean of each trap in the lower slope area the mean particulate flux varies between 9 5 and 79 g m 2 d and the mean 210 pb varies between about 44 and 81 dpm g in the trough the mean particulate flux has the smallest values from 0 06 to 5 7 g m 2 d while the mean 210 pb has the highest from 82 to 192 dpm g the particulate flux decreases while the 210 pb activity increases from the slope toward the trough in the trough 210 pb increases with depth while the particulate flux may show a mid depth maximum of a nepheloid layer although the 210 pb activity increases from the slope toward the trough the 210 pb flux decreases toward the trough because of much greater particulate flux in the slope area within the trough the 210 pb flux as determined from the traps is comparable to that determined from the underlying sediments but on the lower slope the 210 pb flux determined from the traps is much higher than that obtained from the underlying sediments thus a large portion of the particulates and the associated 210 pb may have been transported laterally from elsewhere to the slope area in transit without depositing onto the underlying sediments plots of the mean 210 pb flux versus the mean mass flux for each of the traps show positive linear correlation with four distinctive slopes each representing a mean 210 pb activity of a particular area or a group of traps in the mien hua canyon the mean activity is about 40 dpm g in the sot the mean value is about 130 dpm g on the lower slope the mean values fall in between those of the canyon and the trough in the middle traps where the mass flux is higher in a nepheloid layer the mean activity is about 47 dpm g while the remaining traps yield a mean of about 65 dpm g taking the mean activity of the canyon and that of the trough as the two end member values the slope with a nepheloid layer is a mixture of about 90 canyon and 10 trough materials indicating the canyon as the major particulate provider for the nepheloid layer the slope without a nepheloid layer is a mixture of about 70 canyon and 30 trough materials