controls on the silicon isotope distribution in the ocean new diagnostics from a data constrained model the global distributions of the silicon isotopes within silicic acid are estimated by adding isotope fractionation to an optimized data constrained model of the oceanic silicon cycle that is embedded in a data assimilated steady circulation including fractionation during opal dissolution improves the model s ability to capture the approximately linear relation between isotope ratio 30si and inverse silicic acid concentration observed in the deep atlantic to quantify the importance of hydrographic control on the isotope distribution 30si is partitioned into contributions from preformed and regenerated silicic acid further partitioned according to euphotic zone origin we find that the large scale features of the isotope distribution in the atlantic basin are dominated by preformed silicic acid with regenerated silicic acid being important for setting vertical gradients in the pacific and indian oceans preformed and regenerated silicic acid make roughly equally important contributions to the pattern of the isotope ratio with gradients of the preformed and regenerated contributions tending to cancel each other in the deep pacific the southern ocean euphotic zone is the primary origin of both the preformed and regenerated contributions to 30si nearly the entire preformed part of 30si is of southern ocean and north atlantic origin the regenerated part of 30si in the atlantic basin also has a contribution of central atlantic 40 s 40 n origin that is comparable in magnitude to the north atlantic contribution in other basins the central pacific and indian ocean are the second largest contributors to the regenerated part of 30si

contrasting styles of late jurassic syn rift turbidite sedimentation a comparative study of the magnus and oseberg areas northern north sea abstract contrasting types of sediment gravity flow and hemipelagite deposition are recorded by the late jurassic syn rift successions in the magnus and oseberg areas in the northern north sea in both areas the late jurassic rift interval was characterized by recurrent tectonic phases during the late oxfordian to early volgian the penguin halfgraben in the magnus area received a significant amount of coarse clastics and can be regarded as an example of a well fed deeper marine riftbasin periods with high rates of extensional faulting are characterized by isolated sediment gravity flow lobes which stack in an uneven albeit downdip shifting or backstepping manner the stratigraphic motif of these intervals is overall mud prone with coarse clastics represented by thick conglomerate sandstone mudstone aprons along the basin bounding masterfaults in contrast periods with low rates of extensional faulting are characterized by basinwide sheet like parallel layered sediment gravity flow packages which infill the riftbasin in a progradational manner from the middle volgian the penguin halfgraben became sediment starved and shows a similar development to the oseberg area in the oseberg area the late jurassic syn rift succession provides a typical example of a sediment starved deeper marine riftbasin sediment gravity flow deposition in this area is limited to periods of high rates of extensional faulting these coarser clastic depositional episodes mimic their counterparts in the penguin halfgraben being characterized by mud prone strata with localized isolated sediment gravity flow lobes and footwall derived clastic wedges along the basin bounding masterfaults in contrast to the well fed basins the intervening stages of tectonic quiescence in the sediment starved basins are characterized by basinwide blanketing of clay commonly of high source rock potential in both basins the rotational tilt packages can be subdivided into an early a climax and a late syn rotational interval based on the architecture of their contained facies tracts these depositional episodes reflect the temporal evolution of the riftbasins catchment areas and their linked depositional systems it is mainly the nature and amount of sediment supply during the following tectonic quiescence or post rift stages that determine whether the riftbasin remained sediment starved or became infilled simplified sequence stratigraphic models are of limited use in the analyses and lithology prediction of riftbasin infills as individual halfgrabens often have their own specific infill history and hence characteristic syn rift stratigraphic signature the main controls on the variability of these deeper marine syn rift sediment gravity flow hemipelagite packages are the style and rate of extensional faulting sediment supply the source area s sediment yield potential and calibre and the halfgraben configuration and location with respect to the main hinterland areas

relevance of silicon isotopes to si nutrient utilization and si source assessment in antarctic waters 1 we analyzed 29si of dissolved silicate for eight water column profiles across the southern ocean south of australia in spring 2001 from the seasonal ice zone siz north to the subantarctic zone saz including the first isotopic compositions measured for si depleted seawaters all profiles display mixed layer enrichments in heavy si isotopes relative to deep water in accordance with preferential uptake of the light isotope by diatoms as silicate levels decrease from the siz northward across the polar front zone pfz to the saz surface and mesopelagic 29si signatures generally become progressively heavier but the most si depleted saz waters do not exhibit 29si values heavier than in the pfz this intricacy appears to derive from variations in the vertical and horizontal supply of silicate to surface waters and by applying a steady state open system model we estimate a fractionation factor 29 between diatoms and seawater of 0 45 0 17 independently of zones and phytoplankton community though encouraging these results are related to latitudinal changes in mesopelagic 29si values complexity in surface silicate 29si correlations and differences from previous studies which underline the need for caution in the use of silicon isotopes in paleoceanographic studies until systematic efforts have been undertaken to better understand modern variations

silicon isotope fractionation during nutrient utilization in the north pacific the distribution of silicon in the north pacific is controlled by the utilization of silicic acid by diatoms a process that fractionates silicon si isotopic compositions silicon isotope variations are presented for six water column profiles from the surface mixed layer down to the deep waters of the north pacific ocean although the observed si isotopic variations display an apparently simple inverse relationship with dissolved nutrient concentrations in fact they reflect mixing of surface waters undergoing active si isotope fractionation and deep waters with more uniform concentrations and isotope compositions samples from the surface of the subtropical gyre have the lowest dissolved si concentrations and heaviest si isotope compositions of marine waters measured thus far fractionation in the surface waters follows a typical rayleigh type distillation curve for a closed surface water reservoir resulting from stratification of the surface layer in the subarctic region in contrast an open system prevails within the subtropical gyre where there is significant recycling of silicic acid in the upper water column and lateral transport of silicon within surface currents for deep waters the si isotope composition distinguishes between the northern north pacific deep water npdw and the southerly derived bottom water the relatively light si isotope compositions measured from waters within the subarctic gyre provides evidence for isolation of the nutrient pool in the north pacific

southern ocean control of silicon stable isotope distribution in the deep atlantic ocean the fractionation of silicon si stable isotopes by biological activity in the surface ocean makes the stable isotope composition of silicon delta si 30 dissolved in seawater a sensitive tracer of the oceanic biogeochemical si cycle we present a high precision dataset that characterizes the delta si 30 distribution in the deep atlantic ocean from denmark strait to drake passage documenting strong meridional and smaller but resolvable vertical delta si 30 gradients we show that these gradients are related to the two sources of deep and bottom waters in the atlantic ocean waters of north atlantic and nordic origin carry a high delta 30 sisignature of 1 7 parts per thousand into the deep atlantic while antarctic bottom water transports si with a low delta si 30 value of around 1 2 parts per thousand the deep atlantic delta si 30 distribution is thus governed by the quasi conservative mixing of si from these two isotopically distinct sources this disparity in si isotope composition between the north atlantic and southern ocean is in marked contrast to the homogeneity of the stable nitrogen isotope composition of deep ocean nitrate delta n 15 no3 we infer that the meridional delta si 30 gradient derives from the transport of the high delta si 30 signature of southern ocean intermediate mode waters into the north atlantic by the upper return path of the meridional overturning circulation moc the basin scale deep atlantic delta si 30 gradient thus owes its existence to the interaction of the physical circulation with biological nutrient uptake at high southern latitudes which fractionates si isotopes between the abyssal and intermediate mode waters formed in the southern ocean

the δ15n of nitrate in the southern ocean nitrogen cycling and circulation in the ocean interior we report analyses of the nitrogen isotopic composition of nitrate in the eastern indian and pacific sectors of the southern ocean in this paper we focus on the subsurface data as well as data from the deep waters of other ocean basins nitrate 15n is relatively invariant in much of the abyssal ocean i e below 2 5 km with a value of 4 8 0 2 observed in lower circumpolar deep water north atlantic deep water and central pacific deep water the isotopic invariance of deep ocean nitrate stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean the southern ocean surface being an important exception in the subantarctic zone north of the polar frontal zone the nitrate 15n of upper circumpolar deep water is 0 7 greater than that of lower circumpolar deep water this isotopic enrichment appears to result from denitrification in the low latitude water masses with which upper circumpolar deep water communicates the isotopic enrichment of upper circumpolar deep water is diminished in the antarctic probably because of the remineralization of sinking organic n which has a low 15n in the antarctic relative to the other water masses of the southern ocean the subantarctic thermocline has a very low nitrate 15n for its nitrate concentration because of exchange with the low latitude thermocline where this isotopic signature appears to originate this signature of the low latitude thermocline has two probable causes 1 mixing with low nitrate surface water and 2 the oxidation of newly fixed n

the petroleum geology of the unst basin north sea abstract the unst basin is situated in the northern north sea between the east shetland basin and the shetland isles the basin is essentially a three armed permo triassic fault controlled basin containing up to 3600 m of red beds this is overlain by a westerly thickening jurassic and early cretaceous sequence the stratigraphy of which is very similar to that of the east shetland basin in particular the brent group 140 m humber group 685 m and cromer knoll group 300 m are well represented as a result of laramide uplift of the area the thick upper cretaceous and palaeocene strata of the east shetland basin are absent from the unst basin this uplift resulted in substantial erosion within the unst basin providing the major source for palaeocene sands in the viking graben and the faeroes basin late palaeocene and younger tertiary strata transgress westwards across this erosion surface petroleum exploration within the basin culminated in the drilling of two exploration wells these wells encountered potential reservoir and source rocks in the jurassic section however geochemical analyses indicate these source rocks are immature for hydrocarbon generation within the unst basin it is concluded that the unst basin has a low petroleum potential

silicon dynamics within an intense open ocean diatom bloom in the pacific sector of the southern ocean abstract an intense diatom bloom developed within a strong meridional silicic acid gradient across the antarctic polar front at 61 s 170 w following stratification of the water column in late october early november 1997 the region of high diatom biomass and the silicic acid gradient propogated southward across the seasonal ice zone through time with the maximum diatom biomass tracking the center of the silicic acid gradient high diatom biomass and high rates of silica production persisted within the silicic acid gradient until the end of january 1998 ca 70 d driving the gradient over 500 km to the south of its original position at the polar front the bloom consumed 30 to 40 m si oh 4 in the euphotic zone between about 60 and 66 s leaving near surface concentrations our observations indicate that high diatom biomass and high silica production rates previously observed in the marginal seas around antarctica also occur in the deep ocean near the polar front the bloom we observed propagated across the latitudinal band overlying the sedimentary opal belt which encircles most of antarctica implying a role for such blooms in the formation of those sediments comparison of our surface silica production rates with new estimates of opal accumulation rates in the abyssal sediments of the southern ocean which have been corrected for sediment focusing indicate a burial efficiency of 4 6 for biogenic silica that efficiency is considerably lower than previous estimates for the southern ocean

simulating the modern δ30si distribution in the oceans and in marine sediments the 30si of biogenic silica inline image in marine sediments is a promising proxy for the reconstruction of silicic acid utilization by diatoms in the geological past the application of this proxy however requires an understanding of the modern 30si distributions and their controlling mechanisms here we present results from a modern climate simulation with a coupled ocean sediment model that includes a prognostic formulation of biogenic silica production with concurrent silicon isotopic fractionation in agreement with previous studies biological fractionation combined with physical transport and mixing determines the oceanic distribution of simulated 30si a new finding is a distinct seasonal cycle of 30si in the surface ocean which is inversely related to that of silicic acid concentration and mixed layer depth we also provide the first simulation results of sedimentary 30si which reveal that 1 the inline image distribution in the surface sediment reflects the exported inline image signal from the euphotic zone and 2 the dissolution of biogenic silica in the sediment acts as a source of relatively light 30si into the bottom waters of the polar oceans while it is a source of heavier 30si to the subtropical south atlantic and south pacific

the continental si cycle and its impact on the ocean si isotope budget abstract the silicon isotope composition of biogenic silica 30sibsi in the ocean is a function of the 30si of the available dissolved si dsi h2sio4 the degree of utilisation of the available dsi and for some organisms the concentration of dsi this makes 30sibsi in sediment archives a promising proxy for past dsi concentrations and utilisation at steady state mean 30sibsi must equal a weighted average of the inputs the majority of which are of continental origin variation in the functioning of the continental si cycle on timescales similar to the residence time of dsi in the ocean 10 ka may therefore contribute to downcore variability in 30sibsi on millennial or longer timescales the direction and magnitude of change in published 30sibsi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers they document glacial values that are typically 0 5 to 1 0 lower than interglacial values and together hint at coherent and predictable glacial interglacial variability in whole ocean 30si driven by a change in mean 30si of the inputs in this contribution we review the modern inputs of dsi to the ocean and the controls on their isotopic composition and assess the evidence for their variability on millennial plus timescales today 9 55 1012 mol yr 1 dsi enters the ocean of which roughly 64 and 25 are direct riverine inputs of dsi and dsi from dissolution of aeolian and riverborne sediment respectively the remainder derives from alteration or weathering of the ocean crust each input has a characteristic 30si with our current best estimate for a weighted mean being 0 74 although much work remains to be done to characterise the individual fluxes many aspects of the continental si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean 30si of dsi entering the ocean these changes relate to i a cooler drier glacial climate ii lowered sea level and the exposure of continental shelves iii the presence of large continental ice sheets and iv altered vegetation zonation using a simple box model with a monte carlo approach to parameterisation we find that a transition from a hypothesised glacial continental si cycle to the modern si cycle can drive an increase in whole ocean 30si of comparable rate and magnitude to that recorded in 30sibsi this implies that we may need to revisit our understanding of aspects of the si cycle in the glacial ocean although we focus on the transition from the last glacial our synthesis suggests that the continental si cycle should be seen as a potential contributory factor to any variability observed in ocean 30sibsi on millennial or longer timescales

evolution of the oseberg fault block in context of the northern north sea structural framework abstract the central segment 60 20 61 n of the northern north sea 170 km in width demonstrates contrasting basin geometries related to permo triassic and jurassic rift episodes in the oseberg area representing a part of this segment major faults of two generations dip in opposite directions as a consequence the e tilted oseberg fault block of jurassic origin ca 20 km in width is underlain by a wider ca 35 km w tilted permo triassic half graben the early jurassic thickness distribution was governed by a westerly palaeotilt mild extensional tectonics influenced early jurassic rhaetian aalenian deposition whereas the middle late jurassic main rift episode is expressed by an accelerated fault related subsidence from the late bajocian during the middle late jurassic the oseberg area evolved through a series of high frequency rift phases characterized by rotational tilt events separated by periods during which extension was less severe this allows the subdivision of the jurassic stretching episode into distinct rift phases 4 6 myrs which involved smaller scale units corresponding to rotational tilt events 1 2 myrs the large scale pattern of middle late jurassic syn rift infill is overall one of upwards fining reflecting a deepening trend as the rift evolved syn rotational sediment architecture related to a single rift phase shows systematic variations from a threefold shallow marine sandstone mudstone sandstone motif tarbert fm through a twofold shallow marine sandstone mudstone motif heather fm to a twofold deep marine conglomerate sandstone mudstone motif draupne fm the vertical stacking of these syn rotational packages reflects an inability of sediment supply to keep pace with the rift related subsidence and the temporal evolution from a sediment balanced through a sediment underfilled to a sediment starved rift basin

the fall dump a new perspective on the role of a shade flora in the annual cycle of diatom production and export flux investigations of diatom fluxes recorded in laminated sediments using scanning electron microscope techniques together with evidence from sediment trap studies have contributed to a reappraisal of the annual cycle of diatom production and export flux we propose that where there is a strong seasonal thermocline and nutricline a number of diatoms hitherto regarded as typical sparse summer flora characteristic of oligotrophic waters are able to generate substantial production at depth these species including rhizosolenia spp stephanopyxis palmeriana thalassiothrix spp and some coscinodiscus spp may represent a shade flora that have adapted to grow in low light conditions and or to regulate their buoyancy to move between a deep nutrient source and the euphotic zone although rates of growth and primary production are substantially lower than species characteristic of spring bloom or upwelling conditions the total primary production integrated over the several month period of summer stratification may be as significant as the spring bloom or greater the term fall or autumn bloom as a counterpart of the spring bloom is therefore a misnomer whereas the spring bloom involves a rapid burst of reproduction and sedimentation the fall dump is the sedimentation of a long lived episode of production lasting the duration of the seasonal thermocline and triggered by the fall winter mixing that breaks down stratification the fall dump may produce as much or in some cases more export production than the spring bloom the results of this study suggest that a reorientation of thinking on diatom ecology and palaeoecology may be required

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

deglacial si remobilisation from the deep ocean reveals biogeochemical and physical controls on glacial atmospheric co2 levels abstract during the last glacial period the sluggish deep ocean circulation sequestered carbon into the abyss leading to the lowering of atmospheric co2 the impact of this redistribution on biologically essential nutrients remains poorly constrained using sedimentary 30 si of diatoms and biogenic accumulation rates in the eastern equatorial pacific eep we present evidences for the remobilisation of dissolved silica dsi along with carbon from the deep ocean during the last deglaciation because dsi is essential for diatoms growing in the surface ocean its concentration in the abyss during the glacial periods amounts to a negative feedback on the oceanic co2 uptake however this effect can be muted by the increased fe inputs during glacial periods which reduces diatom si requirements in fe limited regions such as the eep our results from the eep suggest that the efficiency of the biological co2 pump and the size of the local co2 source is tightly controlled by changes in dsi utilisation driven by fe availability across the last glacial interglacial transition we use a modified pandora box model to illustrate that the inventory of dsi in the global ocean surface is controlled by fe availability in hnlc areas rather than by straightforward si supply though upwelling the holocene is characterised by a fast mode of si cycling driven by high biological requirement for si under conditions of iron limitation and efficient overturning promoting co2 outgassing and an inefficient biological c pump via the rapid exhaustion of dsi in the surface the last glacial period saw slower marine si cycling as a result of decreased dsi biological requirement under fe replete conditions in the sea surface and increased si and co2 sequestration in the abyssal ocean the switch between the two modes of si cycling happened at 15 ka bp i e mid deglaciation and resulted in contrasting biological carbon drawdown responses in the eep and globally between both phases of the deglacial co2 rise this illustrates that in addition to deep sea co2 storage and overturning the efficiency of the biological pump also plays a crucial role in determining ocean atmosphere co2 exchange and shows the dual controls of ocean circulation and fe si availability in this process

silica cycling and isotopic composition in northern marguerite bay on the rapidly warming western antarctic peninsula the southern ocean is a key region for silica si cycling and the isotopic signatures established here influence the rest of the world s oceans the climate and ecosystem of the southern ocean are changing rapidly with the potential to impact si cycling and isotope dynamics this study examines high resolution time series dataset of dissolved si concentrations and isotopic signatures particulate si concentrations and diatom speciation at a coastal site on the western antarctic peninsula wap in order to characterise changes in si cycling with respect to changes occurring in productivity and diatom assemblages dissolved and particulate si phases reflect the dominant control of biological uptake and combined with isotopic fractionation were consistent with a season of low intermediate productivity biogenic si is tightly coupled to both chlorophyll and particulate organic carbon at the sampling site consistent with diatom dominated phytoplankton assemblages along the wap variability in diatom speciation has a negligible impact on the isotopic signature of dissolved si in surface waters although this is unlikely to hold for sediments due to differential dissolution of diatom species a continued decline in diatom productivity along the wap would likely result in an increasing unused si inventory which can potentially feed back into si limited areas promoting diatom growth and carbon drawdown further afield

using silicon isotopes to understand the role of the southern ocean in modern and ancient biogeochemistry and climate abstract the growth of siliceous phytoplankton mainly diatoms in the southern ocean influences the preformed nutrient inventory in the ocean on a global scale silicic acid use by diatoms and deep circulation combine to trap dissolved si in the southern ocean resulting in high levels of silica production and expansive diatom oozes in southern ocean sediments the analysis of the silicon isotope composition of biogenic silica or opal and dissolved silicic acid provide insight into the operation of the global marine silicon cycle and the role played by the southern ocean in nutrient supply and carbon drawdown both in the modern and in the past silicon isotope studies of diatoms have provided insight into the history of silica production in surface waters while the analysis of spicules from deep sea sponges has defined both the spatial and the temporal variability of silicic acid concentrations in the water column together these and other proxies reveal variations in the northward flow of southern ocean intermediate and mode waters and how changes in southern ocean productivity altered their preformed nutrient content we present a new hypothesis the silicic acid ventilation hypothesis savh to explain the geographical variation of opal based proxy records in particular the contrasting patterns of opal burial change found in the low and high latitudes by understanding the silicon isotope systematics of opal and silicic acid in the modern we will be able to use opal based proxies to reconstruct past changes in the southern ocean and so investigate its role in global carbon cycling and climate

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

the middle jurassic oseberg delta northern north sea a sedimentological and sequence stratigraphic interpretation the aalenian oseberg formation 0 80 m thick is an important reservoir unit in the middle jurassic brent group in the northern north sea consisting of multiple sets of sandy gilbert type deltas small scale 1 5 10 m fining upward units seen in the gamma ray log correspond with individual delta sets as independently confirmed by steepening upward trends seen in the dip log within each set the steep foreset slopes typically show thinly bedded sandstone facies avalanche grain flows whereas the lower foreset slopes toesets and bottomsets are formed largely by massive sandstone facies sandy debris flows on an intermediate scale up to 40 m the gamma ray logs show both fining upward and coarsening upward trends through stacked delta sets and these trends traceable between wells are interpreted in terms of decelerating and accelerating rates of relative sea level rise respectively the relative abundance of the sandy debris flow deposits reflects a periodic and significant instability of the delta s upper foreset slope probably during times of increased water depth in front of the delta the normal progradation of individual gilbert type sets however is likely to have been along a subhorizontal topography during periods of little or no change in water depth the long term change to produce the observed vertical stacking of deltaic sets was one of a generally rising relative sea level modeling of the sea level rise in a steplike manner can account for the rarity of topsets the progressive eastward truncation of the underlying drake formation on the horda platform by the oseberg formation suggests tectonic uplift and a lowstand of sea level immediately prior to the development of the oseberg deltas the oseberg formation is thus seen as a lowstand prograding wedge that developed during an interval of relatively high sediment supply and a variably rising sea level culminating in the latest aalenian flooding seen in the base of the overlying rannoch formation the dip log data integrated with cores and other logs are critical to correctly interpreting the reservoir s internal architecture the slope readjustment model predicts that the development of stacked massive reservoir sandstones of oseberg type will occur preferentially in local areas of high subsidence or syndepositional faulting or where there is deeper water in the lee of already existing fault scarps

tectonic control on facies distribution of the camp rice and palomas formations pliocene pleistocene in the southern rio grande rift the pliocene pleistocene camp rice and palomas formations in the rio grande rift of southern new mexico provide an excellent test of the role of basin symmetry in the distribution of piedmont and axial fluvial facies in the asymmetrical palomas and northern mesilla basins the axial fluvial facies is characterized by multistory channel sands and sandstones and is concentrated near the locus of maximum subsidence within a few kilometers of the footwall scarp fanglomerate derived from the footwall uplift extends only a few kilometers or less from the footwall scarp whereas alluvial fan and alluvial flat conglomerate sand and mudstone deposited on the hanging wall dip slope occupy a much wider outcrop belt in the symmetrical hatch rincon basin the axial fluvial facies extends to within a few kilometers of both the northern and southern basin margins and has a much higher percentage of fine grained overbank deposits some of which contain calcareous paleosols in all three basins a tongue of fanglomerate as much as 30 m thick prograded over the axial fluvial facies near the end of camp rice and palomas deposition the distribution of facies of the camp rice and palomas formations supports previously published models that predict a two stage history of asymmetrically subsiding basins during tectonically active periods axial river channels preferentially avulse into the topographically lowest area of the alluvial plain which is directly above the axis of maximum subsidence during the postorogenic stage when erosion rate exceeds subsidence rate coarse transverse dispersed sediment progrades toward the center of the basin

silicon isotopic fractionation in marine sponges a new model for understanding silicon isotopic variations in sponges abstract the silicon si isotope 30 si composition of deep sea sponges from near antarctica subantarctic waters tasmania seamounts and subtropical waters north of new zealand vary widely between 0 87 and 3 40 vs nbs28 depth profiles show that sponge 30 si compositions trend to lower values with increasing depth this is exemplified by sponges from the tasmania seamounts where 30 si varies from 0 87 to 3 13 over a depth range from 100 m to 1200 m these changes in 30 si of sponges are inconsistent with a rayleigh type isotope fractionation model requiring constant 30 si fractionation between sponge and seawater we conclude that overall si isotope fractionation 30 si 30 si sponge 30 si seawater is influenced by seawater si concentration with more fractionated lower isotope values being associated with sponges collected from waters high in si we invoke and fit a model whereby the 30 si fractionation varies as a function si influx and efflux using this model it appears that 30 si fractionation during transport into the sponge is constant at 1 34 the model also shows asymptotic behaviour with 30 si trending towards a maximum of 6 02 at very high si concentrations these results suggest that the 30 si composition of fossil spicules may be useful for reconstruction paleo si concentrations during the past

tectonic and climatic control of triassic sedimentation in the beryl basin northern north sea the beryl basin embayment occupies a central position along the viking graben of the northern north sea and has been a hydrocarbon play since the early 1970s detailed analysis reveals a complex half graben that was established during a rift phase of development that occurred in the early triassic teist formation after which a long period of thermal subsidence lomvi and lunde formations provided accommodation for in excess of 1000 m of sediment the teist formation sediments are complex they include shales sands and conglomerates and their superimposition on zechstein salts is indicative of both uplift and the development of a moderate relief they give way to the comparatively clean and occasionally pebbly sands of the lomvi formation for which an analysis of both bedding and texture suggests deposition by a westerly directed river system draining the hanging wall ramp the lunde formation records the spreading influence of a bajada playa complex and its eventual conversion to a more persistent lake the growing importance and eventual dominance of the succession by lacustrine sediments gives clear indication of a change in climate and this has been attributed to the rapid northward drift of pangea during the triassic period

development of rift basins illustrated by the structural evolution of the oseberg feature block 30 6 offshore norway the mesozoic tilted fault block in norwegian north sea block 30 6 containing the giant oseberg oil and gas field is used to illustrate basin development during the jurassic and cretaceous there is seismic evidence for an early rifting phase in the northern viking graben on which the jurassic cretaceous rifting phase early bathonian to berriasian is overprinted the oseberg structure is representative of other fault blocks bordering the northern viking graben axis planar normal faults and listric faults which were restricted to the rifting phase dominate both fault types exhibit syn fault sedimentation listric faulting is recognized on seismic sections by the differential tilting reverse drag antithetic faulting and a wedge shaped syn rift reflector package the initiation of rifting is dated by baselap onto pre rift reflectors and cessation is dated by an unconformity beneath the generally onlapping post rift sediments steeply dipping planar normal faults often active along previously established fault zones played a significant part in the post rift development of the basin the last movement on planar normal faults in the oseberg fault scarp becomes younger away from the rift axis and this produced the complex fault scarp visible on many seismic lines the topography defined by the post rift unconformity is a composite surface which formed in response to subsidence processes it is not the result of orogenic events such as the kimmerian 9orogeny9 footwall uplift associated with planar normal faults caused local unconformities in complex fault scarps and on fault block crests

sensitivity of silicon isotopes to whole ocean changes in the silica cycle a 2 box model has been used to assess the impact of both long and short term budgetary imbalance in the silica cycle on the average silicon isotopic composition 30si of the ocean and marine sediments over a 100 ky time span such as a quaternary glacial cycle a sustained change in the riverine flux of silicon to the oceans could alter the average 30si of seawater and the average 30si of opal outputs by a few hundredths to a few tenths of permil this would be largely tied to a change in the 30si of silicon entering the ocean due to a shift in the proportion of riverine and non riverine sources of silicon a doubling of the riverine flux of silicon would have little impact on average marine 30si but a sustained halving of river inputs could interfere with use of 30si as a tracer of nutrient utilization studies on the longer term focussed on the transition from a high silicic acid to low silicic acid ocean associated with the rise of the diatoms this transition is marked by drop in the average 30si of seawater from greater than 1 9 down to about 0 8 the isotopic composition of diatom opal however has an isotopic composition that sticks close to the 0 8 of the inputs and is thus unlikely to provide information about the transition to the low silicic acid ocean of the modern day however the 30si of opal produced in the deep sea for example by sponges should document this transition

beachrock occurrence characteristics formation mechanisms and impacts beachrocks are hard coastal sedimentary formations consisting of various beach sediments lithified through the precipitation of carbonate cements the objectives of this contribution are to a collate and review information on the reported occurrences characteristics and formation mechanisms of beachrocks and b consider their impacts on the coastal zone the analysis of the available information has shown that a beachrock formation is a global and diachronic phenomenon and b the great majority of beachrocks are found in tropical subtropical and low temperate latitude microtidal coasts the cementing agents of beachrocks are composed predominantly of the metastable carbonate phases high magnesian calcite hmc and aragonite ar appearing in a diverse crystalline morphology it has been suggested that cement precipitation in the coastal environment is controlled by i the physicochemical conditions ii the presence of organic compounds and microbes iii the magnitude and distribution of the wave energy along the coast and iv the textural characteristics of the constituent sediments various theories have been proposed to explain beachrock formation itself linking the phenomenon to either physicochemical or biological processes these theories however do not seem to be of universal validity and acceptance as each is able to explain only some of the reported occurrences the presence of beachrocks appears to affect beach morphodynamics by i locking the beach profile ii modifying the nearshore hydrodynamics iii changing the porous character of the beach and thus its response to wave forcing and iv differential bed erosion at the margins of the beachrock outcrops that can alter significantly the long and particularly the cross shore sediment transport therefore although relict submerged beachrock outcrops may provide some coastal protection by reducing the wave energy impinging onto the coastline modern beachrocks may promote offshore loss of unconsolidated beach sediments and buried beachrock outcropping finally the presence of beachrocks may have also significant ecological impacts as the indigenous mobile substrate fauna and flora of the beach is replaced by hard substrate benthic assemblages which are commonly arranged in hydrodynamically controlled zones

coupling of the distribution of silicon isotopes to the meridional overturning circulation of the north atlantic ocean abstract the distribution of silicon isotopes within silicic acid 30 si oh 4 was examined along a section in the north atlantic from the cape verde islands off africa to cape cod massachusetts in north america surface water displayed elevated 30 si oh 4 associated with biological fractionation of si during silica production below 300 m variations in 30 si oh 4 were closely tied to the distribution of water masses as diagnosed through optimum multiparameter analysis confirming a tight relationship between 30 si oh 4 and the meridional overturning circulation in the atlantic a linear relationship between 30 si oh 4 and the inverse of silicic acid concentration supported control of si isotope distribution by conservative mixing of end member water masses of different isotopic composition in the atlantic there was a suggestion of a weak local minimum in 30 si oh 4 in deep waters above the trans atlantic geotraverse hydrothermal zone on the mid atlantic ridge consistent with the light 30 si oh 4 of hydrothermal waters the lightest 30 si oh 4 values were observed in the deep western and deep eastern basins where antarctic bottom water aabw dominated the heaviest values in subsurface waters occurred in north atlantic deep water due to strong ventilation and the contribution of heavy northern source waters that are influenced by the arctic ocean the concept of a silicon isotope bipole is introduced to explain how the isotopic differences between the northern and southern end member water masses arise and how they influence si isotope distributions northern end member water masses are heavy due to the influence of the arctic ocean bottom topography prevents light deep waters from entering the arctic and the further removal of light isotopes through local biological productivity results in extremely heavy 30 si oh 4 within the arctic light aabw dominates the southern end member the southern ocean silicic acid trap distills heavier isotopes of si out of the southern ocean as preformed silicic acid within antarctic mode waters while retaining light isotopes that become incorporated into aabw the influence of the silicon isotope bipole is predicted to be strongest in the atlantic as the net flow of waters through the arctic ocean is from the pacific to the atlantic

deconvolving the controls on the deep ocean s silicon stable isotope distribution abstract we trace the marine biogeochemical silicon si cycle using the stable isotope composition of si dissolved in seawater expressed as 30 si open ocean 30 si observations indicate a surprisingly strong influence of the physical circulation on the large scale marine si distribution here we present an ocean general circulation model simulation that deconvolves the physical and biogeochemical controls on the 30 si distribution in the deep oceanic interior by parsing dissolved si into its preformed and regenerated components we separate the influence of deep water formation and circulation from the effects of biogeochemical cycling related to opal dissolution at depth we show that the systematic meridional 30 si gradient observed in the deep atlantic ocean is primarily determined by the preformed component of si whose distribution in the interior is controlled solely by the circulation we also demonstrate that the 30 si value of the regenerated component of si in the global deep ocean is dominantly set by oceanic regions where opal export fluxes to the deep ocean are large i e primarily in the southern ocean s opal belt the global importance of this regionally dynamic si cycling helps explain the observed strong physical control on the oceanic 30 si distribution since most of the regenerated si present within the deep atlantic and indo pacific oceans is in fact transported into these basins by deep waters flowing northward from the southern ocean our results thus provide a mechanistic explanation for the observed 30 si distribution that emphasizes the dominant importance of the southern ocean in the marine si cycle

the influence of a sudden climatic change on marine deposition in the kimmeridgian of northwest europe a sudden change from humid style to semi arid style deposition markedly affected the accumulation of the upper kimmeridge clay in southern england many of the changes appear to be related to a change in sedimentation rate at this time thus softground faunas are replaced by firmground faunas diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules and depositional gradients recorded by lateral biofacies changes becomes steeper the evidence available is in accord with a decline in offshore sedimentation rates during this interval other changes such as a decline in kaolinite abundance were more directly controlled by the drying out of the hinterland similar changes elsewhere in the marine geological record could be used as climatic indicators the climatic change is part of a wider northern hemisphere dry event which affect a broad area in the late jurassic the kimmeridge clay of southern england was one of the last depositional environments to be influenced by the climatic change at this latitude

the silicon isotope composition of ethmodiscus rex laminated diatom mats from the tropical west pacific implications for silicate cycling during the last glacial maximum the cause of massive blooms of ethmodiscus rex laminated diatom mats ldms in the eastern philippine sea eps during the last glacial maximum lgm remains uncertain in order to better understand the mechanism of formation of e rex ldms from the perspective of dissolved silicon dsi utilization we determined the silicon isotopic composition of single e rex diatom frustules 30sie rex from two sediment cores in the parece vela basin of the eps in the study cores 30sie rex varies from 1 23 to 0 83 average 1 04 a range that is atypical of marine diatom 30si and that corresponds to the lower limit of reported diatom 30si values of any age a binary mixing model upwelled silicon versus eolian silicon accounting for silicon isotopic fractionation during dsi uptake by diatoms was constructed the binary mixing model demonstrates that e rex dominantly utilized dsi from eolian sources i e asian dust with only minor contributions from upwelled seawater sources i e advected from subantarctic mode water antarctic intermediate water or north pacific intermediate water e rex utilized only 24 of available dsi indicating that surface waters of the eps were eutrophic with respect to silicon during the lgm our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool thus revising previously proposed models for the formation of e rex ldms

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

biological fractionation of silicon isotopes in southern ocean surface waters 1 the fractionation of silicon isotopes by diatoms during silicification i e opaline cell wall formation provides a new tool for paleoceanographic studies of the silicon cycle here we examine the natural variations of the 30si 28si ratio in silicic acid si oh 4 and biogenic silica bsio2 in surface waters of the antarctic circumpolar current acc along 170 w the results provide direct evidence of biologically mediated fractionation of silicon isotopes with an enrichment factor of between 1 1 and 1 9 depending on the model employed comparison of the mass flux of bsio2 captured in sediment traps deployed in the study area with the silicon isotopic composition of that material establishes a direct linkage between diatom dynamics in surface waters and the isotopic signature of exported particles we calculated si oh 4 no3 utilization ratios from variations in silicon and nitrogen isotopic ratios in surface waters and sediment traps that agree well with direct observations of 4 1 in this region this work supports the use of variations in silicon isotopes in sedimentary opal as a proxy for relative si oh 4 utilization in surface waters

deep ocean nutrients during the last glacial maximum deduced from sponge silicon isotopic compositions the relative importance of biological and physical processes within the southern ocean for the storage of carbon and atmospheric pco2 on glacial interglacial timescales remains uncertain understanding the impact of surface biological production on carbon export in the past relies on the reconstruction of the nutrient supply from upwelling deep waters in particular the upwelling of silicic acid si oh 4 is tightly coupled to carbon export in the southern ocean via diatom productivity here we address how changes in deep water si oh 4 concentrations can be reconstructed using the silicon isotopic composition of deep sea sponges we report 30si of modern deep sea sponge spicules and show that they reflect seawater si oh 4 concentration the fractionation factor of sponge 30si compared to seawater 30si shows a positive relationship with si oh 4 which may be a growth rate effect application of this proxy in two down core records from the scotia sea reveals that si oh 4 concentrations in the deep southern ocean during the last glacial maximum lgm were no different than today our result does not support a coupling of carbon and nutrient build up in an isolated deep ocean reservoir during the lgm our data combined with records of stable isotopes from diatoms are only consistent with enhanced lgm southern ocean nutrient utilization if there was also a concurrent reduction in diatom silicification or a shift from siliceous to organic walled phytoplankton

interaction of permo triassic and jurassic extensional fault blocks during the development of the northern north sea the main structural elements resulting from late permian early triassic extension have been resolved across the total width of the northern north sea the northern segment of the basin north of 61 n bounded to the west by the hutton fault and to the east by the norwegian mainland was occupied by four westerly tilted half grabens 25 35 km in width prior to jurassic extension the central segment of the basin 60 20 61 n exhibits a full graben beneath the jurassic horda platform flanked by half grabens tilted towards the basin margins the change in permo triassic fault block configuration defining structural domains is largely constrained by a basement discontinuity which runs diagonally ne sw across the basin maximum jurassic extension is represented by several centres of subsidence aligned nne ssw across a n s oriented and broader permo triassic basin jurassic extension was concentrated within the viking graben proper south of 60 n which in this area is represented by single westerly tilted en echelon half graben segments 20 25 km wide the area affected by jurassic extension becomes progressively wider to the north again governed by the basement discontinuity eventually encompassing the total width of the basin c 200 km north of 61 n the central segment of the northern north sea displays the greatest contrast in permo triassic and jurassic structural styles and basin geometry jurassic stretching and faulting was negligible to the east horda platform precisely the area which had been the site of maximum permo triassic stretching and fault activity it has generally been assumed that jurassic fault configuration along the eastern margin of the central segment of the viking graben to a large extent reflects the reactivation of major permo triassic faults this study demonstrates the interference of permo triassic and jurassic basin geometries in this area due to major faults of the two generations dipping in opposite directions as a consequence the e tilted oseberg fault block of jurassic origin and c 20km in width is underlain by a wider c 35 km and w tilted permo triassic fault block

estimates of late quaternary mode and intermediate water silicic acid concentration in the pacific southern ocean the southern ocean plays a critical role in the exchange of carbon between the ocean and atmosphere over glacial interglacial timescales hypotheses used to explain late quaternary variations in atmospheric carbon dioxide co2 implicate changes in the nutrient dynamics and circulation of the southern ocean here we present silicon isotope delta si 30 records of late quaternary sponges and diatoms from the nz sector of the southern ocean analysis of our sponge delta si 30 records strongly suggests that the silicic acid concentration at mode and intermediate depths was higher during the lgm and the deglacial period compared to the present day our diatom delta si 30 record suggests biological productivity near of the polar front was greater during the deglacial period but not significantly different during the lgm compared to the present day taking our dataset in context with other regional paleoceanographic records we interpret the predicted elevation in lgm and deglacial silicic acid concentration to reflect a shoaling of water masses during the lgm and leakage of excess southern ocean dissolved silicon during the deglacial period c 2016 elsevier b v all rights reserved

distribution of 32si in the world ocean model compared to observation calculations of the 32si sio2 ratio obtained in ocean models are compared with measurements of this ratio made as part of the geosecs program somayajulu et al 1987 1991 a major difference is found while the models predict threefold to fivefold higher ratios in the deep atlantic ocean than in the deep pacific and indian oceans no such difference is seen in the measurements this points to a flaw in the measurements our modeling results suggest that the most interesting application for this 120 year half life cosmogenic isotope is as a monitor of upwelling however considering the expense and difficulty associated with 32si measurements such studies may prove impractical

heavy silicon isotopic composition of silicic acid and biogenic silica in arctic waters over the beaufort shelf and the canada basin the silicon isotopic composition of silicic acid 30si oh 4 and biogenic silica 30si bsio2 were measured for the first time in marine arctic waters from the mackenzie river delta to the deep canada basin in the late summer of 2009 in the upper 100 m of the water column 30si oh 4 signals 1 82 to 3 08 were negatively correlated with the relative contribution of mackenzie river water the biogenic si isotope fractionation factor estimated using an open system model 30e 0 97 0 17 agrees well with laboratory and global ocean estimates nevertheless the 30si dynamics of this region may be better represented by closed system isotope models that yield lower values of 30e between 0 33 and 0 41 depending on how the contribution of sea ice diatoms is incorporated in the upper 400 m 30si bsio2 values were among the heaviest ever measured in marine suspended bsio2 2 03 to 3 51 a positive correlation between 30si bsio2 and sea ice cover implies that heavy signals can result from isotopically heavy sea ice diatoms introduced to pelagic assemblages below the surface bsio2 production zone the 30si oh 4 distribution followed that of major water masses vertical 30si oh 4 profiles showed a minimum average of 1 84 0 10 in the upper halocline 125 200 m composed of modified pacific water and heavier average values 2 04 0 11 in atlantic water 300 500 m deep in the canada basin deep water below 2000 m 30si oh 4 averaged 1 88 0 12 which represents the most positive value ever measured anywhere in the deep ocean since most si oh 4 enters the arctic from shallow depths in the atlantic ocean heavy deep arctic 30si oh 4 signals likely reflect the influx of relatively heavy intermediate atlantic waters a box model simulation of the global marine 30si oh 4 distribution successfully reproduced the observed patterns with the 30si oh 4 of the simulated deep arctic ocean being the heaviest of all deep ocean basins

evolution of lower jurassic coastal plain and fan delta sediments in the beryl embayment north sea coastal plain and fan delta sediments accumulated throughout the sinemurian to late pliensbachian in the beryl embayment area north sea these sediments represent a marginal clastic fringe along the southern extension of the boreal ocean into the northern north sea two informal formations are recognized in these predominantly sandy deposits the lower formation comprises interbedded sandstones siltstones and coals of minor fluvial channel and associated coastal floodbasin marsh lake and sheetflood origin the overlying formation represents a fan delta system possibly generated as a result of footwall uplift and associated erosion this formation is dominated by probable channel sandstones with marine influenced debris flow deposits developed at a number of levels indicating an active coastal fan setting fan sediments are capped by a marine sandstone beds of which interfinger with the overlying offshore siltstones thickness variations are related to major normal faults suggesting at least a limited fault control on the deposition and distribution of facies during a time conventionally regarded as tectonically quiescent in the northern north sea there is no clear evidence for a eustatic control on sedimentation although deposition of post fan delta shallow marine siltstones may have been principally controlled by eustatic sea level rise recording the acme of the early jurassic expansion of boreal waters into the viking graben

corrigendum to riverine silicon isotope variations in glaciated basaltic terrains implications for the si delivery to the ocean over glacial interglacial intervals earth planet sci lett 369 370 2013 211 219

deep ocean biogeochemistry of silicic acid and nitrate 1 observations of silicic acid and nitrate along the lower branch of the global conveyor belt circulation show that silicic acid accumulation by diatom opal dissolution occurs at 6 4 times the rate of nitrate addition by organic matter remineralization the export of opal and organic matter from the surface ocean occurs at a si n mole ratio that is much smaller than this almost everywhere cf sarmiento et al 2004 the preferential increase of silicic acid over nitrate as the deep circulation progresses from the north atlantic to the north pacific is generally interpreted as requiring deep dissolution of opal together with shallow remineralization of organic matter broecker 1991 however sarmiento et al 2004 showed that the primary reason for the low silicic acid concentration of the upper ocean is that the waters feeding the main thermocline from the surface southern ocean are depleted in silicic acid relative to nitrate by implication the same southern ocean processes that deplete the silicic acid in the surface southern ocean must also be responsible for the enhanced silicic acid concentration of the deep ocean we use observations and results from an updated version of the adjoint model of schlitzer 2000 to confirm that this indeed the case

regional sedimentology and petroleum geology of marine late bathonian valanginian sandstone in the north sea abstract a large number of marine late bathonian valanginian sandstone units have been identified in the north sea north of the mid north sea high this paper discusses their complex areal distribution and outlines sedimentological models the sandstone formations are interpreted as shallow marine transgressive and regressive units interbedded with beach lagoonal and coastal plain deposits coarse grained scarp fed fans occur along fault controlled rift margins the total proven recoverable reserves in these sandstone reservoirs are 2 3 x 10 9 ton oil equivalent of which approximately one third is oil the play types are intimately linked with the mesozoic rift system of the viking witch ground and central grabens along this rift system the sand units provide good trapping potential and they are generally interbedded with or lie immediately below the kimmeridgian ryazanian source rocks so that migration paths are relatively simple and migration efficiency is high

modeling the modern marine δ30si distribution 1 the silicon isotopic composition of opal sediments can be used as a proxy for past si utilization and hence paleoproductivity for a fixed ocean circulation interpretation of this si utilization proxy is limited by our understanding of the present marine si isotope distribution observations show significant isotopic variations is silicic acid between deep water masses that are not captured by a previous global climate model gcm here simple box models of the global oceans for the si cycle are presented which can account for the observed 0 3 o difference between atlantic and pacific deep water 30 si values using typical opal export fluxes the 30 si gradient results from both a high 30 si of dissolved si overturning in the north atlantic and the dissolution of low 30 si of opal in the deep waters around the southern ocean that feed into the pacific

mechanisms controlling silicon isotope distribution in the eastern equatorial pacific abstract the distribution of silicon isotopes along a meridional transect at 140 w longitude in the eastern equatorial pacific was used to test the hypothesis that 30 si of silicic acid in surface waters should correlate with net silica production rates gross silica production minus silica dissolution rather than rates of gross silica production due to the opposing si isotope fractionations associated with silica production and silica dissolution variations in 30 si appeared significantly correlated with net silica production rates in equatorial surface waters and not with gross production rates around the equator values of 30 si as low as deep water values occurred in the upper mesopelagic in a zone of net silica dissolution and high detrital biogenic silica content where the release of low 30 si silicic acid from opal dissolution would be expected to decrease 30 si the 30 si of the deep water at 140 w appears constant for depths 2000 m and is similar to the deep water at 110 w this study brings to light the importance of considering si fractionation during diatom silica dissolution the biological fractionation during silica production and physical factors such as currents and mixing with adjacent water masses when interpreting silicon isotope distributions

palaeomagnetic dating of fault rocks evidence for permian and mesozoic movements and brittle deformation along the extensional dalsfjord fault western norway summary palaeomagnetic and structural studies of the dalsfjord nappe western norway show that the basal ow angle detachment dalsfjord fault is a long lived fault zone and that the most important phase of faulting was of devonian extension probably nucleated on an earlier silurian scandian thrust fault rocks produced during subsequent movements indicate that the dalsfjord fault underwent periods of brittle low angle extensional reactivation during the permian 250 260 ma and upper jurassic lower cretaceous c 150 ma corresponding with stages of major extensional movements on the continental shelf palaeomagnetic studies may be of great importance for dating faults and major movement stages in long lived fault systems the particular importance of the results is that they show that low angle normal faults can operate in a brittle upper crustal regime