a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

tectonic modification of axial crustal structure evidence from spectral analyses of residual gravity and bathymetry of the mid atlantic ridge flanks abstract bathymetry and gravity data of the northern atlantic suggest that oceanic crustal structure on the flanks of the mid atlantic ridge is strongly modified by tectonic extension at spreading ridge segments the seafloor along the zero age isochron is systematically more elevated and the crust thicker near the segment midpoint than at the ends suggesting focused magmatic accretion at segment centers in contrast seafloor older than 2 3 ma is usually the shallowest at inside corners of ridge offset intersections where positive residual gravity anomalies indicate tectonically thinned crust coherence between residual gravity and bathymetry for zero age crust is found at wavelength greater than 20 km which reflects the characteristic length of ridge segments and is consistent with an effective elastic plate thickness of 1 3 km no significant coherence however is observed on isochron profiles at ages of 2 5 ma and older at all wavelengths flow line profiles display coherence at wavelength greater than 5 km yielding an estimated effective elastic plate thickness of only 0 3 0 5 km synthetic fault models suggest that the flow line coherence may be controlled by the relatively close spacing of fault generated abyssal hills rather than reflecting the true elastic thickness of unfaulted lithosphere together these results indicate that the crustal structure of the zero age crust is modified strongly by tectonic extension along the rift valley walls of the slow spreading mid atlantic ridge

submarine groundwater discharge driven nitrogen fluxes to long island sound ny terrestrial vs marine sources abstract bottom waters in smithtown bay long island sound ny are subject to hypoxic conditions every summer despite limited nutrient inputs from waste water and riverine sources while modeling estimates of groundwater inputs are thought to be insignificant terrestrial and marine fluxes of submarine groundwater discharge sgd were quantified to smithtown bay using mass balances of 222 rn 224 ra 226 ra and 228 ra during the spring and summer of 2014 2015 in order to track this seasonal transition period intertidal pore waters from a coastal bluff terrestrial sgd and from a barrier beach marine sgd displayed substantial differences in n concentrations and sources traced using a multi isotope approach 222 rn ra 15 n no 3 18 o no 3 no 3 in terrestrial sgd did not display any seasonality and was derived from residential septic systems and fertilizer marine sgd n concentrations varied month to month because of mixing between oxic seawater and hypoxic saline pore waters n concentrations were greatest during the summer when no 3 was derived from the remineralization of organic matter short lived 222 rn and 224 ra sgd fluxes were used to determine remineralized n loads along tidal recirculation flow paths while long lived 228 ra was used to trace inputs of anthropogenic n in terrestrial sgd 228 ra derived terrestrial n load estimates were between 20 and 55 lower than 224 ra derived estimates excluding spring 2014 228 ra may be a more appropriate tracer of terrestrial sgd n loads terrestrial sgd no 3 derived from 228 ra to smithtown bay varied from 1 40 12 8 10 6 mol n y 1 with comparable marine sgd no 3 fluxes of 1 70 6 79 10 6 mol n y 1 derived from 222 rn and 224 ra remineralized n loads were greater during the summer compared with spring and these may be an important driver toward the onset of seasonal hypoxic conditions in smithtown bay and western long island sound seawater recirculation through the coastal aquifer can rival the n load from terrestrial sgd from a heavily polluted aquifer

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

dynamics of submarine groundwater discharge and associated fluxes of dissolved nutrients carbon and trace gases to the coastal zone okatee river estuary south carolina abstract multiple techniques including thermal infrared aerial remote sensing geophysical and geological data geochemical characterization and radium isotopes were used to evaluate the role of groundwater as a source of dissolved nutrients carbon and trace gases to the okatee river estuary south carolina thermal infrared aerial remote sensing surveys illustrated the presence of multiple submarine groundwater discharge sites in okatee headwaters significant relationships were observed between groundwater geochemical constituents and 226 ra activity in groundwater with higher 226 ra activity correlated to higher concentrations of organics dissolved inorganic carbon nutrients and trace gases to the okatee system a system level radium mass balance confirmed a substantial submarine groundwater discharge contribution of these constituents to the okatee river diffusive benthic flux measurements and potential denitrification rate assays tracked the fate of constituents in creek bank sediments diffusive benthic fluxes were substantially lower than calculated radium based submarine groundwater discharge inputs showing that advection of groundwater derived nutrients dominated fluxes in the system while a considerable potential for denitrification in tidal creek bank sediments was noted in situ denitrification rates were nitrate limited making intertidal sediments an inefficient nitrogen sink in this system groundwater geochemical data indicated significant differences in groundwater chemical composition and radium activity ratios between the eastern and western sides of the river these likely arose from the distinct hydrological regimes observed in each area groundwater from the western side of the okatee headwaters was characterized by higher concentrations of dissolved organic and inorganic carbon dissolved organic nitrogen inorganic nutrients and reduced metabolites and trace gases i e methane and nitrous oxide than groundwater from the eastern side differences in microbial sulfate reduction organic matter supply and or groundwater residence time likely contributed to this pattern the contrasting features of the east and west sub marsh zones highlight the need for multiple techniques for characterization of submarine groundwater discharge sources and the impact of biogeochemical processes on the delivery of nutrients and carbon to coastal areas via submarine groundwater discharge

swash zone moisture dynamics and unsaturated infiltration in two sandy beach aquifers the intertidal zone of coastal aquifers is a dynamic region of mixing between saline surface water and fresh groundwater groundwater circulation in this zone is affected by complex forcing mechanisms that operate on a range of timescales and can regulate chemical fluxes to marine environments we evaluated wave swash induced infiltration and associated flow dynamics in the unsaturated region of two sandy beach aquifers with differing wave conditions and beach morphologies moisture and pressure sensors were used to measure fluctuations in water content water table elevation and hydraulic gradients at high frequencies 5 hz in the swash zone at mean lower low water mean sea level and mean higher high water water content in the unsaturated region of the swash zone responded to wave overtopping and swash infiltration with a rapid rise in water content followed by a slower decline swash induced unsaturated infiltration rates calculated from water content response were lowest near low tide and increased up the beachface with inflow highest near high tide at both sites consistent with an increase in water table depth up the beach unsaturated infiltration was 1 6 m 3 m per tidal cycle at the wavedominated beach and 0 4 m 3 m per tidal cycle at the tide dominated beach saturated pore pressure measurements show that a water table mound formed as a consequence of swash infiltration that migrated up the beach during rising tide leading to divergent seaward landward groundwater flow the results demonstrate the significant and spatially variable effects of wave swash on moisture dynamics in the unsaturated zone of beach aquifers and show that these effects depend on wave conditions and beach characteristics results have implications for understanding transport and reaction of solutes in this biogeochemically active zone

oxygen transport and reactivity within a sandy seepage face in a mesotidal lagoon ria formosa southwestern iberia sandy sediments are now recognized as sites of high carbon turnover enhanced by porewater flow however it is unclear how coupling between porewater advection and benthic metabolism might permit o2 saturated areas and suboxic environments to coexist in close proximity field sampling campaigns flow through reactor ftr experiments and diagenetic modeling were used to study benthic o2 dynamics in surface sediments of a permeable intertidal seepage face ria formosa southwestern iberia field results demonstrated that the pressure gradient imposed by tidal oscillation at the seepage face permits seawater infiltration at high tide and hence resupplies the benthos in o2 and organic matter om significant aerobic respiration rates 12 1 26 8 nmol cm 3 bulk sediment bs h 1 were derived from vertical o2 gradients taken from the top 28 cm of the sediment during active seepage ftr experiments showed a vertical zonation of respiration rates following benthic om availability from 8 63 1 88 nmol cm 3 bs h 1 at 12 32 cm depth to 53 55 7 93 nmol cm 3 bs h 1 at 0 2 cm depth advection enhances o2 consumption rates but respiration is ultimately limited by om availability at high seepage rates diagenetic modeling of o2 distribution in the porewater suggests that even within a well oxygenated beach aquifer local respiration is sufficiently high to lower the porewater o2 concentration near the sediment surface during active seepage our results indicate that high seepage rates om and o2 loading promoted by tidally driven seawater infiltration control the o2 consumption at the sediment surface thus favoring the occurrence of suboxic biogeochemical processes near the sediment surface

nitrogen biogeochemistry of submarine groundwater discharge to investigate the role of the seepage zone in transport chemical speciation and attenuation of nitrogen loads carried by submarine groundwater discharge we collected nearshore groundwater samples n 5 328 and examined the distribution and isotopic signature d15n of nitrate and ammonium in addition we estimated nutrient fluxes from terrestrial and marine groundwater sources we discuss our results in the context of three aquifer zones a fresh groundwater zone a shallow salinity transition zone stz and a deep stz groundwater plumes containing nitrate and ammonium occurred in the freshwater zone whereas the deep stz carried almost exclusively ammonium the distributions of redox cycled elements were consistent with theoretical thermodynamic stability of chemical species with sharp interfaces between water masses of distinct oxidation reduction potential suggesting that microbial transformations of nitrogen were rapid relative to dispersive mixing in limited locations in which overlap occurs between distribution of nitrate with that of ammonium and dissolved fe2 changes in concentration and in d15n suggest loss of all species concurrent removal of no and nh z both in freshwater and the deep stz might occur through a range of mechanisms including heterotrophic or autotrophic denitrification coupled nitrfication denitrification anammox or mn oxidation of nh z loss of nitrogen was not apparent in the shallow stz perhaps because of short water residence time despite organic cpoor conditions the nearshore aquifer and subterranean estuary are biogeochemically active zones where attenuation of n loads can occur extent of attenuation is controlled by the degree of mixing of biogeochemically dissimilar water masses highlighting the critical role of hydrogeology in n biogeochemistry mixing is related in part to thinning of the freshwater lens before discharge and to dispersion at the fresh saline groundwater interface features common to all submarine groundwater discharge zones

geochronology and geochemistry of the triassic bimodal volcanic rocks and coeval a type granites of the olzit area middle mongolia implications for the tectonic evolution of mongol okhotsk ocean abstract the olzit volcanism in middle mongolia comprises a bimodal suite of basalts and peralkaline rhyolites adjacent to the main mongolia lineament the basalts are characterized by enrichment in lile and lree and depletion in hfse with typical sr nd isotopic signatures end t 2 50 to 0 38 and 87sr 86sr i 0 7058 0 7063 indicating they were likely derived from partial melting of an enriched lithospheric mantle modified by subducted slab derived fluids the rhyolites show a close affinity to a type granites with enrichment in lile and lree and depletion in nb ta and ti they also show a significant negative eu anomaly and have end t values ranging from 0 50 to 1 38 and initial 87sr 86sr ratios ranging from 0 7022 to 0 7200 suggesting the rhyolites stem from partial melting of crustal rocks rather than fractional crystallization of the basaltic melt the rhyolite porphyry yields a shrimp zircon u pb age of 207 2 ma mswd 1 42 indicating the bimodal volcanic suite formed in the late triassic the miarolitic per alkaline granite and biotite bearing granite which are associated with the bimodal volcanic rocks show typical a type granitic geochemical affinity with end t 0 89 0 91 and 87sr 86sr i 0 7021 0 7043 indicating they are likely generated by partial melting of crustal rocks similar to the rhyolitic end member of bimodal suite the miarolitic per alkaline granite and biotite bearing granite yielded shrimp zircon u pb ages of 209 2 ma mswd 0 91 and 213 3 ma mswd 1 65 respectively which are nearly coeval with the age of the bimodal volcanic suites in view of the new geochemical and chronological data in this study we suggest the olzit late triassic bimodal volcanic rocks together with the coeval a type granites represent a back arc basin extensional environment which probably related to the roll back of mongol okhotsk oceanic plate during the southward subduction under the central mongolia microcontinent

uranium and barium cycling in a salt wedge subterranean estuary the influence of tidal pumping abstract the contribution of submarine groundwater discharge sgd to oceanic metal budgets is only beginning to be explored here we demonstrate that biogeochemical processes in a northern florida subterranean estuary ste significantly alter u and ba concentrations entering the coastal ocean via sgd tidal pumping controlled the distribution of dissolved metals in shallow beach groundwater hourly observations of intertidal groundwaters revealed high u and low ba concentrations at high tide as a result of seawater infiltration into the coastal aquifer during ebb tide u decreased and ba increased due to freshwater dilution and more importantly biogeochemical reactions that removed u and added ba to solution u removal was apparently a result of precipitation following the reduction of u vi to u iv a significant correlation between ba and dissolved organic carbon doc in shallow beach groundwaters implied a common source likely the mineralization of marine particulate organic matter driven into the beach face by tidal pumping in deeper groundwaters where the labile organic matter had been depleted ba correlated with mn we estimate that net sgd fluxes were 163 and 1660 mol m 1 d 1 for u and ba respectively or 1 and 8 mol m 2 d 1 if a 200 m wide seepage area is considered our results support the emerging concept that subterranean estuaries are natural biogeochemical reactors where metal concentrations are altered relative to conservative mixing between terrestrial and marine endmembers these deviations from conservative mixing significantly influence sgd derived trace metal fluxes

stratigraphic controls on fluid and solute fluxes across the sediment water interface of an estuary shallow stratigraphic features such as infilled paleovalleys modify fresh groundwater discharge to coastal waters and fluxes of saltwater and nutrients across the sediment water interface we quantify the spatial distribution of shallow surface water groundwater exchange and nitrogen fluxes near a paleovalley in indian river bay delaware using a hand resistivity probe conventional seepage meters and pore water samples in the interfluve region outside the paleovalley most nitrate rich fresh groundwater discharges rapidly near the coast with little mixing of saline pore water and nitrogen transport is largely conservative in the peat filled paleovalley fresh groundwater discharge is negligible and saltwater exchange is deep 1 m long pore water residence times and abundant sulfate and organic matter promote sulfate reduction and ammonium production in shallow sediment reducing iron rich fresh groundwater beneath paleovalley peat discharges diffusely around paleovalley margins offshore in this zone of diffuse fresh groundwater discharge saltwater exchange and dispersion are enhanced ammonium is produced in shallow sediments and fluxes of ammonium to surface water are large by modifying patterns of groundwater discharge and the nature of saltwater exchange in shallow sediments paleovalleys and other stratigraphic features influence the geochemistry of discharging groundwater redox reactions near the sediment water interface affect rates and patterns of geochemical fluxes to coastal surface waters for example at this site more than 99 of the groundwater borne nitrate flux to the delaware inland bays occurs within the interfluve portion of the coastline and more than 50 of the ammonium flux occurs at the paleovalley margin

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

intercomparison of submarine groundwater discharge estimates from a sandy unconfined aquifer summary recent studies suggest that chemical loading from submarine groundwater discharge sgd may rival other major sources such as rivers in many coastal areas sgd can occur as terrestrially derived typically fresh groundwater discharge or by seawater circulation through aquifer sediments because both terrestrial and recirculated seawater sgd can be significant sources of chemical loading to coastal waters appropriate methods are needed to properly assess and quantify the rates and distribution of these different inputs although many techniques exist to quantify sgd each method samples different components of sgd e g fresh versus saline discharge over different temporal and spatial scales the result is that confusion exists about how to use the different methods in this study we investigate the applicability of a variety of techniques for estimating sgd from an unconfined sandy coastal aquifer physical methods are first used to understand the spatial variability of discharge this information is then used with physical and geochemical data to estimate sgd investigative methods include direct measurement via seepage meters hydrogeologic estimation using darcy s law and tracer based estimates using radon and radium isotopes at our field site on cape cod massachusetts nearshore landward topography appears to exert a significant control on the spatial variability of fresh groundwater discharge presumably through its effects on evapotranspiration and hence recharge rates the darcy estimate of fresh sgd 4 0 m 3 m 1 d 1 is similar to a previous estimate made using a seepage meter transect that spans the seepage face radon serves as a valuable estimator of total sgd fresh plus saline 5 6 m 3 m 1 d 1 the difference between the total and fresh sgd estimates is in good agreement with the radium based sgd estimate 0 56 m 3 m 1 d 1 which primarily measures saline circulation through coastal sediments thus we conclude that the hydrogeologic estimate and radon and radium techniques are complimentary for estimating different components of total sgd at our field site

submarine groundwater discharge as a source of dissolved nutrients to an arid coastal embayment la paz mexico submarine groundwater discharge sgd was investigated in the southeastern portion of la paz bay baja california mexico using radon 222rn as a natural tracer in the absence of permanent surface flows in this arid region we hypothesize sgd is a major regional source of dissolved nutrients four spatial surveys showed higher radon and nutrient nh4 no3 no2 po4 and sio4 values in the winter than summer lagging rainfall by 3 4 months the surveys revealed two sites balandra and merito with a stronger radon signal intensive time series 12 24 h measurements at those sites were used to estimate sgd fluxes using a radon mass balance approach in balandra sgd was estimated to be 0 18 m3 m 2 day 1 and significant correlations between radon and nutrients no3 and sio4 were observed in merito sgd rates were estimated to be 0 10 m3 m 2 day 1 and no correlations between nutrients and 222rn were observed the difference between the two sites was interpreted to be related to different components dominating sgd i e fresh sgd in balandra and saline sgd in merito the estimated sgd derived nutrients fluxes were 2 52 0 04 0 94 7 164 mmol m2 day 1 for dissolved inorganic nitrogen phosphate and silicate respectively these fluxes could explain between 5 and 20 of the regional marine primary productivity values

the marine permian of east and northeast asia an overview of biostratigraphy palaeobiogeography and palaeogeographical implications abstract the permian marine biostratigraphy faunal successions and mutual correlations where possible throughout east and northeast asia are synthesized region by region based on both published literature and the author s field observations in certain parts of the region the correlation of the permian marine successions of ne asia with the permian international timescale and in particular with gondwanan permian marine sequences remains a major challenge due to profound marine provincialism during the permian however by employing biogeographically mixed faunas from east asia se mongolia ne china south primorye of far east russia and the south kitakami terrane of japan as biostratigraphic gateways coupled with some bipolarly and bi temperately shared permian marine taxa and faunas it has been possible to correlate with reasonable confidence some of the high palaeolatitude permian marine rock units and faunas of ne asia with those of the tethyan region and gondwana palaeobiogeographically the permian marine faunas of east and ne asia are assigned to four major provinces verkolyman sino mongolian japanese cathaysian and panthalassan provinces on the basis of their palaeogeographical distribution patterns and characteristics of faunal assemblages of these the sino mongolian japanese province has considerable significance for regional palaeogeographical plate tectonic and palaeoceanographical reconstructions during the middle permian because of its conspicuously mixed cool and warm water marine biota the origin of this biogeographically mixed marine biota is interpreted to have resulted from a combination of some key factors including the increased tectonic convergence between the bureya jiamusi terrane and the sino korean platform during the permian and the intermingling of both warm and cold water ocean currents off the eastern coastal areas of the bureya jiamusi terrane and the sino korean platform during the middle permian

binary sources of loess on the chinese loess plateau revealed by u pb ages of zircon the age distribution of detrital zircon has been used to trace sediment sources existing datasets show great similarity of zircon ages between the loess on the chinese loess plateau clp and the sediments from the north tibetan plateau ntp implying that eolian dust is delivered from the ntp to the clp by westerly winds or via the yellow river however ntp dust can also be transported by northwesterly winds from the alxa arid lands aals where materials are received from both the ntp and the gobi altay mountains gams here we report u pb zircon ages for aals sands and ntp and clp loess the results show that the zircons in the aals are mixed from ntp and gams zircons ntp loess is mainly derived from local sources mixing of materials from the ntp and gams defines the zircon ages of the loess on the clp better than the pure ntp source no temporal and spatial heterogeneities of zircon ages have been observed for the loess on the clp which suggests that the well mixed materials in the aals likely have an eolian source

two phase southward subduction of the mongol okhotsk oceanic plate constrained by permian jurassic granitoids in the erguna and xing an massifs ne china abstract geodynamics of the mongol okhotsk oceanic plate southward subduction are still pending problems this paper presents new zircon la icp ms u pb age and whole rock geochemical data for the middle permian to middle jurassic granitoids in the western erguna and central xing an massifs 267 264 ma 241 ma and 173 ma i type granites and 216 ma a type granites were identified in the erguna and xing an massifs ne china the i type granites were produced by partial melting of the lower mafic crust the 216 ma a type granites were derived from partial melting of crustal materials with tonalitic to granodioritic compositions the 267 264 ma and 241 ma i type granites were generated in an andean type arc setting wheras the 216 ma a type and 173 ma granites were formed in supra subduction extensional setting we summarized previous age data of the middle permian to middle jurassic magmtaic rocks in the erguna and xing an massifs and identified two isolated phases of magmatic activity including the ca 267 225 ma and ca 215 165 ma periods with a significant magmatic gap at ca 225 215 ma these middle permian to middle jurassic magmatic rocks are closely related to the southward subduction of the mongol okhotsk ocean a two stage tectonic evolutionary model was proposed to account for these geological observations in the erguna and xing an massifs involving permian to middle triassic continuous southward subduction of the mongol okhotsk oceanic plate and late triassic to jurassic slab rollback and supra subduction extension

groundwater flow and salt transport in a subterranean estuary driven by intensified wave conditions a numerical study based on a density dependent variably saturated groundwater flow model was conducted to investigate flow and salt transport in a nearshore aquifer under intensified wave conditions caused by offshore storms temporally varying onshore hydraulic gradients due to wave setup were determined as the seaward boundary condition for the simulated aquifer the results showed a rapid increase in influxes across the aquifer ocean interface in response to the wave event followed by a more gradual increase in effluxes the upper saline plume first widened horizontally as the wave setup point moved landward it then expanded vertically with recirculating seawater pushed downward by the wave induced hydraulic gradient the time for the salt distribution to return to the prestorm condition was up to a hundred days and correlated strongly with the time for seawater to recirculate through the aquifer the pathways of recirculating seawater and fresh groundwater were largely modified by the wave event these pathways crossed through the same spatial locations at similar times indicating significant salt freshwater mixing the flow and salt transport dynamics were more responsive to wave events of longer duration and higher intensity especially in more permeable aquifers with lower fresh groundwater discharge despite their larger response aquifers with higher permeability and beach slope recovered more rapidly postevent the rapid recovery of the flows compared with the salinity distribution should be considered in field data interpretation due to their long lasting impact wave events may significantly influence the geochemical conditions and the fate of chemicals in a subterranean estuary key points intensified waves perturb flow and transport in a subterranean estuary lengthy period months for salinity distribution to recover from a wave event exchange fluxes and flows respond and recover rapidly following a wave event

meso cenozoic building of the northern central asian orogenic belt thermotectonic history of the tuva region abstract the tuvinian and west sayan mountain ranges form part of the central asian orogenic belt caob more specifically they align along the altai sayan hangay zone its precambrian paleozoic basement has been subjected to mesozoic and cenozoic tectonic reactivation two north south transects across the mountain belts and intervening basins of tuva were sampled for apatite fission track aft thermochronology in order to elucidate the thermal history of the tuvinian basement in relation to the mesozoic and cenozoic reactivation of the caob most aft ages are late cretaceous and range between 55 and 115 ma mean lengths of confined fission tracks are relatively long with most values between 13 and 14 m thermal history modeling shows a rapid late jurassic cretaceous cooling for the sampled tuvinian crystalline rocks related to exhumation of the paleozoic basement this exhumation is possibly related to the building and subsequent orogenic collapse of the mongol okhotsk orogen that formed between the siberian and north china mongolian sino korean or amurian continental blocks during the late mesozoic far field effects of this orogeny and its collapse might have affected the baikal altai and sayan units of the central asian orogenic belt including the tuvinian basement also at the mesozoic southern eurasian margin growth of the asian continent continued and several collision accretion events asserted distal tectonic influence into the caob after a paleogene period of stability thermal history models for some samples hint at a renewed period of basement cooling during the neogene in support of this neogene event a single sample from the main west sayan fault zone contains an apatite population with 2 ma reset aft ages this is interpreted in the framework of ongoing building of the modern central asian orogens and associated fault movements and exhumation of the basement presumably related with the ongoing india eurasia convergence

nitrate reduction in an unconfined sandy aquifer water chemistry reduction processes and geochemical modeling nitrate distribution and reduction processes were investigated in an unconfined sandy aquifer of quaternary age ground water chemistry was studied in a series of eight multilevel samplers along a flow line deriving water from both arable and forested land results show that plumes of nitrate contaminated groundwater emanate from the agricultural areas and spread through the aquifer the aquifer can be subdivided into an upper 10 to 15 m thick oxic zone that contains o2 and no3 and a lower anoxic zone characterized by fe2 rich waters the redox boundary is very sharp which suggests that reduction processes of o2 and no3 occur at rates that are fast compared to the rate of downward water transport nitrate contaminated groundwater contains total contents of dissolved ions that are two to four times higher than in groundwater derived from the forested area the persistence of the high content of total dissolved ions in the no3 free anoxic zone indicates the downward migration of contaminants and that active nitrate reduction is taking place nitrate is apparently reduced to n2 because both nitrite and ammonia are absent or found at very low concentrations possible electron donors in the reduced zone of the aquifer are organic matter present as reworked brown coal fragments from the underlying miocene and small amounts of pyrite at an average concentration of 3 6 mmol kg electron balances across the redoxcline based on concentrations of o2 no3 so42 and total inorganic carbon tic indicate that pyrite is by far the dominant electron donor even though organic matter is much more abundant groundwater transport and chemical reactions were modeled using the code phreeqm which combines a chemical equilibrium model with a one dimensional mixing cell transport model only the vertical component of the water transport was modeled since in contrast to rates along flow lines the vertical rates are close to constant as required by the one dimensional model average vertical transport rates of water in the saturated zone were obtained by tritium dating the modeling process is a two step procedure first the sediment column is initialized with natural water containing only oxygen as electron acceptor and subsequently agricultural waters containing both oxygen and nitrate are fed into the column the nitrate concentration of agricultural waters entering the saturated zone varies with time and an input function was therefore constructed by linear mixing of natural waters and agricultural waters this input function was fed into the column initialized with natural water and the model run forward in time to the year 1988 where field data are available comparison with field data shows that the variation in groundwater chemistry is well described by the model when reduction of oxygen and reduction of nitrate by pyrite oxidation are the only redox reactions occurring finally predictions are made for the distribution of water chemistry in the year 2003 downward progression of the redoxcline is accelerated by a factor of five due to nitrate pollution of the aquifer but absolute rates remain small of the order of a few centimeters per year the controlling factor for nitrate migration through the aquifer once it has reached the anoxic zone is the concentration and distribution of pyrite in the sediments

dynamic groundwater flows and geochemistry in a sandy nearshore aquifer over a wave event dynamic coastal forcing influences the transport of pollutants in nearshore aquifers and their ultimate flux to coastal waters in this study field data are presented that show for the first time the influence of a period of intensified wave conditions wave event on nearshore groundwater flows and geochemistry in a sandy beach field measurements at a freshwater beach allow wave effects to be quantified without other complex forcing that are present along marine shorelines e g tides pressure transducer data obtained over an isolated wave event reveal the development of transient groundwater flow recirculations the groundwater flows were simulated in feflow using a phase averaged wave setup approach to represent waves acting on the sediment water interface comparison of measured and simulated data indicates that consideration of wave setup alone is able to adequately capture wave induced perturbations in groundwater flows while prior studies have shown sharp ph and redox spatial zonations in nearshore aquifers this study reveals rapid temporal variations in conductivity ph and redox orp in shallow sediments up to 0 5 m depth in response to varying wave conditions comparison of head gradients with calculated conductivity and ph mixing ratios indicates the controlling effect of the wave induced water exchange and flows in driving the observed geochemical dynamics while we are not able to conclusively determine the extent to which temporal variations are caused by conservative mixing versus reactive processes the ph and orp variations observed will have significant implications for the fate of reactive pollutants discharging through sandy nearshore aquifers

pathways and processes associated with the transport of groundwater in deltaic systems summary rivers are considered to be the primary means driving hydrological and geochemical fluxes between the continents and the ocean however it is unclear how well surface water fluxes represent total fluxes or whether more diffuse subterranean fluxes of river water to the coastal ocean occur this question is important in light of research demonstrating that submarine groundwater discharge sgd is important for geochemical and hydrological fluxes here we examine the pathways and potential magnitude of the role that sgd plays in the mississippi river delta mrd the largest delta in north america we present multiple independent lines of evidence demonstrating a hydrological connection between the mississippi river mr and the mrd evidence includes hydrological budgets demonstrating downstream water losses from the mr which are unexplained and are of the same magnitude as water sources to adjacent coastal bays of the mrd well data indicating a correlation between groundwater height and the stage of the mr and excess 222 rn inventories exceeding that expected from in situ production implying an advective i e groundwater source to coastal bays sgd likely flows from the mr to its delta via paleochannels and other buried sand bodies seismic data indicates that such features are common whereas resistivity data suggest the intrusion of low salinity water to coastal bays adjacent to the river these results may be applicable to other deltas worldwide as many of the world largest rivers have deltas with numerous abandoned distributaries that could act as conduits for groundwater

paleozoic to mesozoic micro block tectonics in the eastern central asian orogenic belt insights from magnetic and gravity anomalies abstract the ne china region involves a complex accretionary belt formed by multiple stages of assembly of micro blocks from the paleozoic to mesozoic the tectonics of which provides important clues on the evolution of the east asian continental margin with a view to evaluating the micro block evolution in ne china we used gravity and magnetic methods to analyze the geophysical characteristics of the micro blocks and their boundaries combining with regional geology paleomagnetism and paleontology we reconstructed the micro block assembly history in the eastern central asian orogenic belt five paleozoic to mesozoic micro blocks are recognized in ne china the erguna xing an songnen jiamusi and nadanhada and their boundaries show significant differences in gravity and magnetic fields the boundary between the erguna block eb and the xing an block xb shows as a continuous tectonic line in the gravity field whereas the one between the xb and the songnen block sb exhibits a rather discrete but strong magnetic anomaly belt the boundary between the sb and the jiamusi block jb is characterized by both distinct gravity and magnetic anomalies the nadanhada micro block nmb appears as an independent micro block in the fourth order detail field of the bouguer gravity anomaly where it shows as a short axis or circinate structure in this anomaly field the paleozoic to mesozoic tectonic evolution of these micro blocks in ne china is closely related to five sutures in the eastern central asian orogenic belt the xinlin xiguitu suture was first formed at 500 ma leading to the assembly of the eb and the xb the heihe hegenshan suture was formed at 320 290 ma marking the amalgamation of the xb and the sb the mudanjiang yilan suture was formed at 200 ma marking the collision of the jb and the sb the paleo asian ocean closed during 250 230 ma in a scissor style the subduction of the paleo pacific ocean was initiated at 200 ma based on the deep structural architecture imaged from gravity and magnetic anomalies we propose an evolutionary model of the micro blocks in the ne china which provides important insights into the tectonic evolution of the ne china region during the paleozoic to mesozoic

carbonate system biogeochemistry in a subterranean estuary waquoit bay usa abstract quantifying carbon fluxes associated with submarine groundwater discharge sgd remains challenging due to the complex biogeochemistry of the carbonate system in the subterranean estuary ste here we conducted time series measurements of total alkalinity talk and dissolved inorganic carbon dic in a well studied coastal aquifer waquoit bay massachusetts usa groundwater samples were collected monthly from may 2009 to june 2010 across the freshwater saltwater mixing zone of the waquoit bay wb ste the concentrations of both talk and dic in zero salinity groundwater were variable but were lower than those in the bay water s 28 dic underwent slightly non conservative mixing between low and intermediate salinities while there was an apparent additional dic source at high salinity 20 in all seasons talk concentrations exhibited even stronger variations with evidence of both production and consumption in high salinity zones and consistent talk consumption at intermediate salinity in summer and fall june december 2009 the increases in dic and talk at high salinity were attributed to aerobic respiration and denitrification in wb sediments during bay water recharge of the ste we infer that the loss of talk at intermediate salinity reflects h production as reduced compounds e g fe2 are oxidized within the ste in terms of impacts on surface water inorganic carbon budgets the sgd derived dic flux was mainly controlled by seasonal changes in sgd while a combination of talk concentration variability and sgd drove the talk flux sgd derived dic aqueous co2 and h fluxes to the bay were 40 50 higher in summer vs in winter a result of enhanced marine groundwater flux and significant talk removal proton addition during periods of high seawater intrusion furthermore the sgd derived dic flux was consistently greater than talk flux regardless of season indicating that sgd serves to reduce the co2 buffering capacity of surface water our results highlight the importance of seasonality and subsurface biogeochemical processes on the subterranean estuary carbonate system and the resulting impact on sgd derived talk dic aqueous co2 and h fluxes to the coastal ocean

large scale natural gradient tracer test in sand and gravel cape cod massachusetts 1 experimental design and observed tracer movement a large scale natural gradient tracer experiment was conducted on cape cod massachusetts to examine the transport and dispersion of solutes in a sand and gravel aquifer the nonreactive tracer bromide and the reactive tracers lithium and molybdate were injected as a pulse in july 1985 and monitored in three dimensions as they moved as far as 280 m down gradient through an array of multilevel samplers the bromide cloud moved horizontally at a rate of 0 42 m per day it also moved downward about 4 m because of density induced sinking early in the test and accretion of areal recharge from precipitation after 200 m of transport the bromide cloud had spread more than 80 m in the direction of flow but was only 14 m wide and 4 6 m thick the lithium and molybdate clouds followed the same path as the bromide cloud but their rates of movement were retarded about 50 relative to bromide movement because of sorption onto the sediments

advective transport affecting metal and nutrient distributions and interfacial fluxes in permeable sediments abstract our laboratory flume experiments demonstrate that advective porewater flows produce biogeochemical reaction zones in permeable sediments leading to specific and reproducible complex patterns of fe mn and nutrients oxygenated water forced into the sediment when boundary flows were deflected by protruding sediment structures generated distinct zones of nitrification and ferric iron precipitation this inflow was balanced by ammonium rich porewater ascending from deeper sediment layers thereby creating an anoxic channel where dissolved fe2 and mn2 could reach the surface between the zones of ferric iron precipitation and fe2 upwelling a layer with increased manganese oxide and solid phase fe ii concentrations formed indicating redox reaction between these components the establishment of topography on the previously smooth sediment surface reversed the net interfacial flux of solutes while the smooth control core was found to be a sink for metals and nutrients the sediment with mounds acted as a source for these substances our experiments show that in sandy sediment with an oxidised surface layer reduced metal species can be released to the water column by flow topography interactions we conclude that advective transport processes constitute an important process controlling biogeochemical zonations and fluxes in permeable sea beds

the south siberia central mongolia transect abstract studies of geological and geophysical data on the south siberia central mongolia geoscience transect which runs approximately along 100 e show that the asian continent was formed in the phanerozoic by accretion of terranes most of which were microcontinents with precambrian basement suture zones separating the terranes are deformed and deeply eroded magmatic arcs of different widths likewise classified as specific terranes in these sutures magmatic arc rocks are locally associated with fore and back arc basin series most of the oceanic crust has been subducted the observed ophiolite belts are as a rule attributed to the base of oceanic island arcs the fragment of the asian continent traversed by the transect was formed through early middle and late paleozoic stages of accretion

accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield kroner a windley b f badarch g tomurtogoo o hegner e jahn b m gruschka s khain e v demoux a and wingate m t d 2007 accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield in hatcher r d jr carlson m p mcbride j h and martinez catalan j r eds 4 d framework of continental crust geological society of america memoir 200 p 181 209 doi 10 1130 2007 1200 11 for permission to copy contact editing geosociety org 2007 the geological society of america all rights reserved kroner kroener mail uni mainz de windley brian windley btinternet com tomurtogoo igmr magicnet mn hegner hegner lmu de jahn jahn earth sinica edu tw khain khain ginras ru demoux demoux uni mainz de wingate mwingate tsrc uwa edu au

trace element cycling in a subterranean estuary part 1 geochemistry of the permeable sediments abstract subterranean estuaries are characterized by the mixing of terrestrially derived groundwater and seawater in a coastal aquifer subterranean estuaries like their river water seawater counterparts on the surface of the earth represent a major but less visible hydrological and geochemical interface between the continents and the ocean this article is the first in a two part series on the biogeochemistry of the subterranean estuary at the head of waquoit bay cape cod ma usa the pore water distributions of salinity fe and mn establish the salt and redox framework of this subterranean estuary the biogeochemistry of fe mn p ba u and th will be addressed from the perspective of the sediment composition a second article will focus on the groundwater and pore water chemistries of fe mn u and ba three sediment cores were collected from the head of waquoit bay where the coastal aquifer consists of permeable sandy sediment a selective dissolution method was used to measure the concentrations of p ba u and th that are associated with amorphous hydr oxides of iron and manganese and crystalline fe and mn hydr oxides the deeper sections of the cores are characterized by large amounts of iron hydr oxides that are precipitated onto organic c poor quartz sand from high salinity pore waters rich in dissolved ferrous iron unlike fe hydr oxides which increase with depth the mn hydr oxides display midcore maxima this type of vertical stratification is consistent with redox controlled diagenesis in which mn hydr oxides are formed at shallower depths than iron hydr oxides p and th are enriched in the deep sections of the cores consistent with their well documented affinity for fe hydr oxides in contrast the downcore distribution of ba especially in core 3 more closely tracks the concentration of mn hydr oxides even though mn hydr oxides are 200 300 times less abundant than fe hydr oxides in the cores mn hydr oxides are known to have an affinity for ba which is many orders of magnitude greater than iron hydr oxides hence the downcore distribution of ba in fe hydr oxide rich sediments is most probably controlled by the presence of mn hydr oxides u is enriched in the upper zones of the cores consistent with the formation of highly reducing near surface sediments in the intertidal zone at the head of the bay hence the recirculation of seawater through this type of subterranean estuary coupled with the abiotic and or biotic reduction of soluble u vi to insoluble u iv leads to the sediments acting as a oceanic net sink of u these results highlight the importance of permeable sediments as hosts to a wide range of biogeochemical reactions which may be impacting geochemical budgets on scales ranging from coastal aquifers to the continental shelf

submarine groundwater discharge revealed by aerial thermal infrared imagery a case study on jeju island korea submarine groundwater discharge sgd is a global phenomenon that carries large volumes of groundwater and dissolved chemical species such as nutrient metals and organic compounds to coastal zones we report the influence of sgd on the coastal waters of jeju island korea using high resolution aerial thermal infrared tir mapping techniques and field investigations an aircraft based system was implemented using a cost effective tir camera for aerial tir mapping ground based calibrations and system integration with gps imu global positioning system inertial measurement unit were performed for the aerial systems the aerial surveys showed distinct low temperature signatures of sgd along the coasts of jeju island revealing large groundwater inputs from the coastal aquifers to the ocean multiple aerial surveys over a range of seasons and tidal stages revealed that sgd rates dynamically affect the sea surface temperature sst of the coastal zone the in situ measurements supported that sgd has a substantial influence on the coastal water chemistry as well as sst our observations highlight the extent to which aerial based tir mapping can serve as a powerful tool for studying sgd and other coastal processes copyright 2016 john wiley sons ltd

upper silurian and devonian pelagic deep water radiolarian chert from the khangai khentei belt of central mongolia evidence for middle paleozoic subduction accretion activity in the central asian orogenic belt abstract recent mapping projects undertaken in central mongolia have revealed the widespread occurrence of radiolarian chert within a paleozoic accretionary complex we present the results of the first detailed tectonostratigraphic and radiolarian biostratigraphic investigations of the gorkhi formation in the khangai khentei belt of the central asian orogenic belt the gorkhi formation consists of sandstone shale alternating sandstone and shale of turbidite affinity and chert with small amounts of siliceous shale basalt limestone and clast bearing mudstone radiolarian chert that is completely devoid of terrigenous clastic material is commonly associated with underlying basalt sedimentary contact and with conformably overlying siliceous shale and turbidite deposits the tectonic stacking of basalt chert and chert turbidite successions is the most remarkable structural feature of the formation the recovery of moderately well preserved radiolarians and conodonts from red chert led to the recognition of four radiolarian assemblages that have a combined age range from the latest silurian pridolian to the late devonian frasnian no age control exists for the siliceous shale shale and sandstone although they are considered to be latest devonian or slightly younger on the basis of stratigraphic relationships with underlying chert the gorkhi formation has previously been interpreted as a thick sedimentary basin deposit overlying an unexposed archean neoproterozoic basement however the stratigraphy within individual tectonic slices clearly corresponds to that of an ocean plate stratigraphy of an accretionary complex generated by the trenchward movement of an oceanic plate from the lowermost to uppermost units the stratigraphy comprises ocean floor basalt pelagic deep water radiolarian chert hemipelagic siliceous shale and terrigenous turbidite deposits the biostratigraphic data obtained in the present study provide corroborating evidence for the existence of an extensive deep water ocean that enabled the continuous sedimentation of pelagic chert over a period of nearly 50 million years these data together with structural data characterized by tectonic repetition of the stratigraphy indicate that these rocks formed as an accretionary wedge along an active continental margin possibly that of the angara craton the mid oceanic chert was probably deposited in the northern hemisphere portion of the paleo pacific ocean that faced the angara craton and the north china tarim blocks thus we propose that subduction accretion processes along the paleo pacific rim played an important role in the accretionary growth of the active continental margin of the angara craton directly influencing the evolution of the central asian orogenic belt

science society and the coastal groundwater squeeze coastal zones encompass the complex interface between land and sea understanding how water and solutes move within and across this interface is essential for managing resources for society the increasingly dense human occupation of coastal zones disrupts natural groundwater flow patterns and degrades freshwater resources by both overuse and pollution this pressure results in a coastal groundwater squeeze where the thin veneers of potable freshwater are threatened by contaminant sources at the land surface and saline groundwater at depth scientific advances in the field of coastal hydrogeology have enabled responsible management of water resources and protection of important ecosystems to address the problems of the future we must continue to make scientific advances and groundwater hydrology needs to be firmly embedded in integrated coastal zone management this will require interdisciplinary scientific collaboration open communication between scientists and the public and strong partnerships with policymakers

shape of the shallow aquifer at the fresh water sea water interface on a high energy sandy beach abstract inland aquifers are connected to the ocean through permeable coastal sediments the mixing zone between sea water and fresh groundwater forms a so called subterranean estuary the dynamics and the shape of this interface influence fluxes of fresh water to the coastal zone on tidal sandy beaches tide and waves actions lead to the formation of an intertidal saline circulation cell that complements the structure of the subterranean estuary between sea water and fresh water the upper beach corresponds to the recharge zone where large volumes sea water penetrate the sediment at high tide whereas the lower beach corresponds to the discharge zone of older pore water that remains in the sand for several tidal cycles the objective of our study was to characterize the shape and the evolution of this saline cell in a high energy macro tidal beach the truc vert beach sw france in order to assess the distribution of saline brackish and fresh water in this high energy beach aquifer we conducted electrical resistivity tomography ert measurements from the lower beach face to the sand dune in winter 2012 2013 we also measured water table elevation and salinity in three piezometers situated at the base of the dune and in pore waters at low tide on tidal cross shore transects results show that the intertidal saline plume can be identified and localized with the ert method the fresh saline water interface is almost vertical in the upper beach the saline circulation cell occupies the upper part of the tidal beach over a thickness of 5 m deep below this saline cell a brackish water flows to the ocean about 30 of fresh water was found in pore water in the lower part of the beach shifting of the vertical front between saline and fresh waters of about 2 5 m occurred during the spring neap tidal cycle this migration is small relative to the beach size suggesting that the transition zone is relatively stable the variations in salinity of the circulation cell suggest that the position of the saline plume is controlled by the spring tide level our results differ slightly from studies based on modelling and piezometric measurement made in less exposed micro tidal sandy beaches our data would allow to better constrain numerical models that would enable to quantify water residence time and solute fluxes in subterranean estuaries of high energy beaches

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

segmentation and crustal structure of the western mid atlantic ridge flank 25 25 27 10 n and 0 29 m y we conducted a detailed geological geophysical survey of the west flank of the mid atlantic ridge between 25 25 n and 27 10 n and from the ridge axis out to 29 ma crust acquiring hydrosweep multibeam bathymetry hawaii mr1 sidescan sonar imagery gravity magnetics and single channel seismic reflection profiles the survey covered all or part of nine spreading segments bounded by mostly nontransform right stepping discontinuities which are subparallel to flow lines but which migrated independently of one another some discontinuities alternated between small right and left stepping offsets or exhibited zero offset for up to 3 4 m y despite these changes the spreading segments have been long lived and extend 20 m y or more across isochrons a large shift 9 in relative plate motion about 24 22 ma caused significant changes in segmentation pattern the nature of this plate boundary response together with the persistence of segments through periods of zero offset at their bounding discontinuities suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt within segments there are distinct differences in seafloor depth morphology residual mantle bouguer gravity anomaly and apparent crustal thickness between inside corner and outside corner crust this demands fundamentally asymmetric crustal accretion and extension across the ridge axis which we attribute to low angle detachment faulting near segment ends cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal thickness changes of at least 1 2 km every 2 3 m y these are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension controlled by retention and quasiperiodic release of melt from the upwelling mantle detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes creating crustal domes analogous to turtleback metamorphic core complexes that are formed by low angle detachment faulting in subaerial extensional environments

hydrogeochemical characteristics in coastal groundwater mixing zone abstract this study presents the hydrogeochemical analysis in coastal groundwater mixing zone of a sandy aquifer ting kok hong kong groundwater samples collected along a sampling transection that covers the salinity transition zone were analyzed for spatial and temporal distribution of major ions cl so 4 2 na k mg 2 and ca 2 and ancillary parameters such as salinity ph temperature and total alkalinity the seawater freshwater interface sfi based on different ions ionic delta of major ions phreeqc based saturation index si of minerals are used to systematically investigate geochemical processes such as cation exchange precipitation dissolution of minerals and sulfate reduction under the effect of seasonal hydrologic variation compared to cl based sfi the landward movements of cation based sfi and seaward movement of so 4 2 based sfi indicate that the coastal aquifer serves as the source of cations and sink of so 4 2 for groundwater the sfi is sensitive to seasonal hydrologic variation due to the variation of land sea hydraulic gradient the cation exchange and precipitation dissolution of minerals are dynamic and complex in coastal groundwater mixing zone the simultaneous enrichment of all cations is found in the aquifer due to external input of a specific cation the sulfate reduction is responsible for so 4 2 depletion and also the enrichment of total alkalinity in coastal groundwater mixing zone because gypsum precipitation does not occur according to mineral si analysis the cation exchange is very sensitive to the seasonal hydrologic variation while the sulfate reduction is not obviously influenced

a reevaluation of submarine groundwater discharge along the southeastern coast of north america 1 new data of fluxes of radium isotopes and concentrations of radium in coastal groundwater enable an estimation of fluxes of submarine groundwater discharge sgd to a large section of the south atlantic bight sab the new fluxes are considerably greater than sgd fluxes to this region estimated earlier the annual average total sgd fluxes which represent a range of salinity are 3 times greater than the river fluxes the highest fluxes of radium and sgd occur in the summer off the coast of georgia when scaled per kilometer of shoreline the sgd flux is the same for the coastlines of onslow bay nc 140 km a section of the south atlantic bight from onslow bay nc to cresent beach florida united states 600 km and the entire atlantic ocean 75 000 km the fact that each independent estimate is based on different methods and assumptions gives confidence to the results these sgd fluxes are not restricted to the shoreline but occur throughout the continental shelf they are important for supplying not only radium but also nutrients carbon metals and freshwater to these coastal waters citation moore w s 2010 a reevaluation of submarine groundwater discharge along the southeastern coast of north america global biogeochem cycles 24 gb4005 doi 10 1029 2009gb003747

late jurassic to early early cretaceous tectonic nature on the ne asian continental margin constraints from mesozoic accretionary complexes abstract it is widely accepted that subduction of the paleo pacific plate beneath eurasia began in the early mesozoic however the tectonic nature and processes of the ne asian continental margin in the late jurassic to early early cretaceous remain unclear in this contribution lithological provenance and metamorphic deformation data are used to constrain the formation of mesozoic accretionary complexes ac along the ne asian continental margin and determine the tectonic relationship between the ne asian continental margin and the paleo pacific plate in the late jurassic to early early cretaceous the mesozoic ac formed during the late jurassic including the raohe ac in ne china the khabarovsk samarka and taukha acs in the russian far east and the mino tamba ac in japan and early cretaceous including the shimanto ac in japan and the kiselevka manoma ac in the russian far east the jurassic ac underwent a complex evolution including the formation of seamounts middle late jurassic terrigenous clastic sedimentation adjacent to the trench collision of seamounts with eurasia northward migration of the ac early early cretaceous terrigenous clastic sedimentation a second period of accretion and final emplacement the earliest mesozoic ac arrived at the trench at ca 195 ma combined with the occurrence of early mesozoic calc alkaline igneous rocks along the ne asian continental margin we suggest that subduction of the paleo pacific plate beneath eurasia began in the early jurassic jurassic ac in japan was affected by a regional high p t metamorphic event at 160 ma which might have been related to subduction collision of seamounts with eurasia at the trench ages and hf isotopic compositions of detrital zircon grains from the middle upper jurassic and cretaceous ac sandstones together with paleontological and paleomagnetic data indicate that jurassic clastic sediments were sourced mainly from the south china block scb whereas the lower cretaceous sandstones were sourced from the eastern central asian orogenic belt caob the inferred northward migration of jurassic ac in the late jurassic to early early cretaceous indicates that the boundary between the ne asian continental margin and the paleo pacific plate was characterized by strike slip tectonics at this time likely due to a small angle of obliquity in the subduction of the paleo pacific plate beneath eurasia in the late early cretaceous westward subduction of the paleo pacific plate beneath eurasia resulted in the emplacement of the raohe and khabarovsk acs at 137 130 ma and the formation of acs in the russian far east and japan

relating estuarine geology to groundwater discharge at an oyster reef in copano bay tx abstract coastal bays with fresh submarine groundwater discharge sgd have been found to provide a favorable environment to shellfish e g oysters significant variability in the spatial distribution and magnitude of sgd may occur because of subsurface heterogeneity we investigated the relationship between stratigraphic controls i e paleovalley systems and the spatial distribution of sgd in relation to oyster reefs in copano bay south texas major oyster reefs have been shown to be oriented perpendicular to shore along interfluves and near paleovalley margins a novel combination approach of radioactive geochemical tracers stable isotopes geophysical and petrophysical techniques were used to identify estuarine depositional environments for individual hydrogeologic characterization and evaluation of sgd distribution with implications to ecological features such as oyster reefs a 1 km2 area within a paleovalley system in copano bay yielded discernible differences in sediment type apparent resistivity and sgd rates among four different depositional environments paleovalley paleovalley margin oyster reef and interfluve electrical resistivity tomography revealed a direct connection between the oyster reef and the underlying more permeable interfluve and the presence of a fresher sgd component in the subsurface measurements of sgd from stationary and mobile continuous radon 222 surveys across each depositional environment showed that by and large areas in proximity to reefs have larger sgd rates 62 6 11 3 cm d in january and 28 7 10 8 cm d in july when compared to paleovalleys 31 9 8 9 cm d in january and 19 5 7 2 cm d in july the role of depositional environments as hydrologic controls on sgd to oyster reefs revealed in this study is further confirmed by significant differences in stable and radioactive isotopes observed over small spatial scales offshore when compared to the entire estuary although constant mixing and radon 222 degassing of bay waters are likely altering the relationships between the different depositional environments and magnitude of sgd this study provides an improved understanding of possible pathways of freshwater and solute inputs to oyster reefs for more informed decisions on reef restoration efforts

the mongol okhotsk belt in mongolia an appraisal of the geodynamic development by the study of sandstone provenance and detrital zircons abstract the mongol okhotsk belt formed in a late stage of jurassic orogeny in the composite central asian orogenic belt the present paper investigates the late palaeozoic mesozoic sandstones associated with the belt in mongolia aimed at reconstructing the time and mode of ocean formation subduction and collision we apply provenance analysis including 1 heavy mineral and sandstone framework grain analysis 2 u pb laser ablation icp ms dating of detrital zircons for identifying contemporaneous volcanic arc activity and the recycling of older crustal material and 3 on dated zircon grains we analyze trace element contents and hf isotopic ratios in order to characterize the rock types in which they crystallized and the magma sources respectively the investigated samples are derived from 1 the adaatsag and doschgol terranes which represent the suture zone 2 the hangai hentei belt to the northwest and 3 the ereendavaa terrane and the middle gobi volcanic belt to the southeast of the suture the latter two are concurrent with the northern and southern margins of the mongol okhotsk ocean tectono stratigraphic arguments suggest that the mongol okhotsk ocean opened during the silurian within the early palaeozoic collage in the suture zone permian syn sedimentary zircon trace element contents confirm mafic rock sources and the mantle involvement in the magmatism epsilon hf t from 13 up to 20 n and s directed bi vergent subduction developed as revealed by contemporaneous zircons 1 along the northern margin hangai hentei from silurian early carboniferous subduction and accretion prevailed epsilon hf t from 3 up to 12 in associated zircons which was re initiated during the permian and 2 the contemporaneous silurian devonian southern margin ereendavaa middle gobi still represented an extensional continental margin showing reworking of neoproterozoic early palaeozoic zircons from the basement it turned into an active continental margin with starting arc magmatism in the carboniferous zircon mean epsilon hf t 5 75

climate driven sea level anomalies modulate coastal groundwater dynamics and discharge 1 to better understand the physical drivers of submarine groundwater discharge sgd in the coastal ocean we conducted a detailed field and modeling study within an unconfined coastal aquifer system we monitored the hydraulic gradient across the coastal aquifer and movement of the mixing zone over multiple years at our study site sea level dominated over groundwater head as the largest contributor to variability in the hydraulic gradient and therefore sgd model results indicate the seawater recirculation component of sgd was enhanced during summer while the terrestrial component dominated during winter due to seasonal changes in sea level driven by a combination of long period solar tides temperature and winds in one year sea level remained elevated year round due to a combination of enso and nao climate modes hence predicted changes in regional climate variability driven sea level may impact future rates of sgd and biogeochemical cycling within coastal aquifers

age of palaeozoic granites and metamorphism in the tuvino mongolian massif of the central asian mobile belt loss of a precambrian microcontinent abstract the tuvino mongolian massif tmm was previously interpreted as a precambrian block within the central asian mobile belt according to this idea it consists of tectonic slices composed of metamorphic rocks of pre mesoproterozoic basement that experienced two episodes of regional metamorphism and mesoproterozoic cover rocks that were reworked together with the basement during high grade metamorphism zircon u pb dating of granitoids from all metamorphic complexes demonstrates that the earliest metamorphic event occurred at 536 6 ma significantly later than the deposition of the cover rocks regional upper amphibolite facies metamorphism which affected all metasedimentary units of the tmm occurred in the short time interval of 497 4 to 489 3 ma we propose that there is no simple basement cover relationship in the tuvino mongolian massif instead the massif consists of tectonic domains composed of heterogeneous metasedimentary successions with distinct pre metamorphic and pre tectonic histories that were juxtaposed by thrusting prior to 497 ma and then metamorphosed under upper amphibolite facies conditions we suggest that this thrusting event was associated with early palaeozoic collisional processes related to amalgamation of the central asian mobile belt our new model for the evolution of the tmm implies derivation of neoproterozoic terrigeneous metasediments from 800 900 ma granitoids formed in an andean type active continental margin setting

segmentation in gravity and magnetic anomalies along the u s east coast passive margin implications for incipient structure of the oceanic lithosphere segmentation is a characteristic feature of seafloor spreading along the global mid ocean ridge system while segmentation of active spreading centers has been the focus of much recent research the process by which a rifted continental margin develops into a segmented mid ocean ridge is still poorly understood in this study we investigate the segmentation character of the u s east coast margin through a modeling study of the margin basement structure magnetics and gravity anomalies the east coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust presumably formed during rifting the east coast magnetic anomaly ecma a distinct total field magnetic high running offshore along the margin is commonly located seaward of the thickest sections of the high velocity crust and displays segmentation on length scales 100 120 km similar to the segmentation observed at the mid atlantic ridge mar isostatic gravity anomalies were calculated by removing from free air gravity the predicted effects of seafloor sediments and a crust mantle model assuming local isostatic compensation the resultant residuals show a corridor of high anomaly running along the margin situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines reduction to the pole r t p of the total field magnetic anomaly shows that after the removal of skewness from the ecma the location of the isostatic gravity high is closely correlated to the ecma the isostatic gravity high is also segmented but in two distinct wave bands 100 150 km and 300 500 km the short wavelength 100 150 km segmentation in the r t p magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle bouguer anomaly observed along the present day mar the 300 500 km segmentation in the along margin isostatic gravity anomaly is similar in length scale to both intermediate wavelength tectonic segmentation observed in the south atlantic and variations in lithospheric strength observed along the african margin furthermore two of the intermediate wavelength 300 500 km isostatic gravity lows correspond to the early traces of the kane and atlantis fracture zones suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the north atlantic we hypothesize that the direct cause of the intermediate wavelength segmentation may be along margin variations in both the amount of underplated igneous crust and the strength of the lithosphere although the relative importance of these two effects remains unresolved our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting

the effect of salinization and freshening events in coastal aquifers on nutrient characteristics as deduced from field data summary the effect of seawater intrusion and freshening events in coastal aquifers on nutrient dissolved inorganic nitrogen species phosphate and silica dynamics across the fresh saline groundwater interface fsi were quantified using field data seasonal vertical profiles revealed a clear transition between nutrient species across the fsi which is also an oxycline in view of the results of our experimental simulations it is clear that the major process controlling the nutrient dynamics at the fsi besides the mixing that takes place between the two different water bodies is the seasonal variation between seawater intrusion salinization in summer and flushing of the aquifer freshening in winter aquifer salinization during the summer shifts the fsi and the anaerobic depth location upwards and leads to the enrichment of nh4 po43 and dsi dissolved silica in the saline groundwater nh4 and po43 are enriched due to ion exchange and dsi is enriched either by ion exchange as po43 or as a result of dissolution of biogenic silica denitrification occurs at the base of the fsi as indicated by the slight no3 depletion and the enrichment in 15n of no3 aquifer freshening during the winter shifts the fsi downward and the water becomes suboxic with the influence of the oxic fresh groundwater this leads to nitrification of the nh4 enrichment of no2 and depletion of 15n in the residual no3 in the fsi these cyclic processes generate a certain depletion of n and enrichment of p in the saline groundwater circulation of the saline groundwater below the fsi back to the sea can serve as a partial counterbalance to the high anthropogenic load of n impacting the coastal groundwater system

discussion on switch of ne asia from extension to contraction at the mid cretaceous a tale of the okhotsk oceanic plateau from initiation by the perm anomaly to extrusion in the mongol okhotsk ocean reply abstract we disagree the comment by zyabrev and abrajevitch 2020 because neither the data nor the ideas they presented are new we stress that the diachronous closure of the mongol okhotsk ocean should be highlighted that the existence and timing of the early cretaceous okhotsk oceanic plateau are sound that the motion of the okhotsk oceanic plateau towards the pacific subduction zone is reasonable and that an island arc bound by a paleo pacific subduction zone since 140 ma is highly probable

trace element cycling in a subterranean estuary part 2 geochemistry of the pore water abstract submarine groundwater discharge sgd is an important source of dissolved elements to the ocean yet little is known regarding the chemical reactions that control their flux from sandy coastal aquifers the net flux of elements from sgd to the coastal ocean is dependent on biogeochemical reactions in the groundwater seawater mixing zone recently termed the subterranean estuary this paper is the second in a two part series on the biogeochemistry of the waquoit bay coastal aquifer subterranean estuary the first paper addressed the biogeochemistry of fe mn p ba u and th from the perspective of the sediment composition of cores charette et al charette m a sholkovitz e r hansell c m 2005 trace element cycling in a subterranean estuary part 1 geochemistry of the permeable sediments geochim cosmochim acta 69 2095 2109 this paper uses pore water data from the subterranean estuary along with bay surface water data to establish a more detailed view into the estuarine chemistry and the chemical diagenesis of fe mn u ba and sr in coastal aquifers nine high resolution pore water groundwater profiles were collected from the head of the bay during july 2002 there were non conservative additions of both ba and sr in the salinity transition zone of the subterranean estuary however the extent of sr release was significantly less than that of its alkaline earth neighbor ba pore water ba concentrations approached 3000 nm compared with 25 50 nm in the surface waters of the bay the pore water sr salinity distribution suggests a 26 elevation in the amount of sr added to the subterranean estuary the release of dissolved ba to the mixing zone of surface estuaries is frequently attributed to an ion exchange process whereby seawater cations react with ba from river suspended clay mineral particles at low to intermediate salinity results presented here suggest that reductive dissolution of mn oxides in conjunction with changes in salinity may also be an important process in maintaining high concentrations of ba in the pore water of subterranean estuaries in contrast pore water u was significantly depleted in the subterranean estuary a result of sgd driven circulation of seawater through reducing permeable sediments this finding is supported by surface water concentrations of u in the bay which were significantly depleted in u compared with adjacent coastal waters using a global estimate of sgd we calculate u removal in subterranean estuaries at 20 106 mol u y 1 which is the same order of magnitude as the other major u sinks for the ocean our results suggest a need to revisit and reevaluate the oceanic budgets for elements that are likely influenced by sgd associated processes

seasonal enhancement of submarine groundwater discharge sgd derived nitrate loading into the ria formosa coastal lagoon assessed by 1 d modeling of benthic no3 profiles the role of benthic sandy ecosystems in mitigating no3 loads carried by submarine groundwater discharge sgd to coastal marine ecosystems is uncertain benthic biogeochemical mediation of no3 rich submarine groundwater discharge was studied at the seepage face of a barrier island site in the ria formosa coastal lagoon southern portugal preliminary analysis of no3 porewater distributions at the seepage face during discharge indicated that benthic biogeochemical processes could significantly affect the fluxes of groundwater borne no3 into the lagoon in order to discriminate between the relative contribution of transport and reaction processes to shape and concentration range evidenced by in situ porewater no3 gradients an advection dispersion reaction adr model of no3 diagenesis was applied to describe no3 porewater profiles obtained in march june september and december 2006 good agreement between modeled and measured profiles was obtained model derived apparent benthic nitrification and no3 reduction rates ranged from 0 01 to 5 2 mmol m 2 h 1 sufficient to explain gross observed changes in no3 fluxes arriving at the seepage face up to 70 within the surficial 20 cm depth layer results of the analysis indicated that the upper limit of the seepage face promoted mitigation of no3 fluxes to the lagoon throughout the year in contrast the lower limit of the seepage area promoted net amplification of the no3 fluxes into the lagoon in june and september these results will help constrain further work aiming to clarify the role of permeable sediments in mitigating nitrogen loading of coastal ecosystems

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

carbon nutrient and trace metal cycling in sandy sediments a comparison of high energy beaches and backbarrier tidal flats abstract in order to evaluate the importance of coastal sandy sediments and their contribution to carbon nutrient and metal cycling we investigated two beach sites on spiekeroog island southern north sea germany and a tidal flat margin located in spiekeroog s backbarrier area we also analyzed seawater and fresh groundwater on spiekeroog island to better define endmember concentrations which influence our study sites intertidal sandy flats and beaches are characterized by pore water advection seawater enters the sediment during flood and pore water drains out during ebb and at low tide this pore water circulation leads to continuous supply of fresh organic substrate to the sediments remineralization products of microbial degradation processes i e nutrients and dissolved trace metals from the reduction of particulate metal oxides are enriched in the pore water compared to open seawater concentrations the spatial distribution of dissolved organic carbon doc nutrients po43 no3 no2 nh4 si oh 4 and total alkalinity trace metals dissolved fe and mn as well as sulfate suggests that the exposed beach sites are subject to relatively fast pore water advection which leads to organic matter and oxygen replenishment frequent pore water exchange further leads to comparatively low nutrient concentrations sulfate reduction does not appear to play a major role during organic matter degradation high nitrate concentrations indicate that redox conditions are oxic within the duneward freshwater influenced section while ammonification denitrification manganese and iron reduction seem to prevail in the ammonium dominated seawater circulation zone in contrast the sheltered tidal flat margin site exhibits a different sedimentology coarser beach sands versus finer tidal flat sands and nutrients dissolved manganese and doc accumulate in the pore water ammonium is the dominant pore water nitrogen species and intense sulfate reduction leads to the formation of sulfide which precipitates dissolved iron as iron sulfide these findings are due to slower advective pore water exchange in the tidal flat sediments this study illustrates how different energy regimes affect biogeochemical cycling in intertidal permeable sediments

geology mineralogy and stable isotope geochemistry of the dzuunmod area in northern mongolia constraints for gold ore genesis and sources abstract the dzuunmod area located in the north khentii gold belt nkgb of northern mongolia includes lode gold deposits such as gatsuurt sujigtei and boroo with several minor gold deposits and occurrences they show similar hydrothermal alteration assemblages sericitic siliceous and potassic and ore mineral assemblages pyrite and arsenopyrite with minor amount of galena sphalerite and chalcopyrite gold occurs as native form and invisible gold in pyrite and arsenopyrite the major sulfide minerals are separated into earlier non auriferous stage and later auriferous grains containing invisible gold native gold postdates the major sulfide mineralization gold and arsenic content of pyrite grains indicates that gold exists mainly as solid solution form au 1 in the gatsuurt and boroo deposit whereas gold nanoparticle au0 is present in the sujigtei deposit high co ni and mo ni ratios of pyrite grain suggest a post sedimentary or hydrothermal origin and the ore forming fluid was significantly affected by fluid host rock interactions during mineralization processes large variation of 34s values of pyrite and arsenopyrite from 2 6 to 17 2 indicates that sulfur seems to be mainly derived from a source with heterogeneous sulfur isotope composition even though the role of magmatic sulfur as one of possible sulfur sources cannot be ruled out consistent with geological evidence relatively positive 34s values suggest that sulfidation plays an important role for gold and sulfide precipitation the calculated 18o values of hydrothermal fluid from the measured 18o values of quartz samples from 14 7 to 17 7 indicate a metamorphic derivation of ore forming fluid gold mineralization processes in the dzuunmod area seem to occur several times by multiple input of hydrothermal fluid and fluid host rock interactions the gold deposits in the dzuunmod area are considered to be orogenic gold type influenced by fluid host rock interactions in the deposit area

geodynamics of the western part of the mongolia okhotsk collisional belt trans baikal region russia and mongolia abstract after the western edge of the mongolian microcontinent joined the siberian continent in the region of central mongolia in the earliest permian these two continental blocks remained turned at an angle of about 120 with respect to each other and separated on greater extent of their present day boundary by an enormous gulf of the paleopacific called the mongolia okhotsk ocean closure of this ocean at the early middle jurassic boundary led to the complete collision of siberia and mongolia which by then had already become part of the mongolia north china continent this main collisional episode which lasted through the middle and late jurassic involved thrusting folding and magmatism and produced the mongolia okhotsk belt the onon island arc which was located in the mongolia okhotsk ocean was squeezed between the two major continents inasmuch as the third element the island arc was involved in the collision it is reasonable to distinguish two branches of the mongolia okhotsk suture these branches control the spatial distribution of gold mineralization in the trans baikal region on the southeastern periphery of siberia the crust thickened considerably after the collision and a plateau like uplift formed in the early cretaceous when compression ceased the collisional uplift collapsed and the thrusts were transformed into low angle normal faults the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes

saltwater freshwater mixing dynamics in a sandy beach aquifer over tidal spring neap and seasonal cycles the biogeochemical reactivity of sandy beach aquifers is closely linked to physical flow and solute transport processes thus a clearer understanding of the hydrodynamics in the intertidal zone is needed to accurately estimate chemical fluxes to the marine environment a field and numerical modeling study was conducted over a 1 year timeframe to investigate the combined effects of tidal stage spring neap variability in tidal amplitude and seasonal inland water table oscillations on intertidal salinity and flow dynamics within a tide dominated microtidal sandy beach aquifer measured and simulated salinities revealed an intertidal saline circulation cell with a structure and cross sectional mixing zone area that varied over tidal spring neap and seasonal time scales the size of the circulation cell and area of the mixing zone were shown for the first time to be most affected by seasonal water table oscillations followed by tidal amplitude and tidal stage the intertidal circulation cell expanded horizontally and vertically as the inland water table declined displacing the fresh discharge zone and lower interface seaward over monthly spring neap cycles the center of the circulation cell shifted from beneath the backshore and upper beachface to the base of the beach salinity variations in the intertidal zone over semidiurnal tidal cycles were minimal the dynamics of the circulation cell were similar in simulations with and without a berm the highly transient nature of intertidal salinity over multiple time scales may have important implications for the types and rates of chemical transformations that occur in groundwater prior to discharge to the ocean

nutrient inputs to the coastal ocean from submarine groundwater discharge in a groundwater dominated system relation to land use kona coast hawaii u s a we evaluated the magnitude and composition in terms of salinity 223ra and 224ra activity and nutrient concentrations of submarine groundwater discharge sgd fluxes at 11 sites on the kona coast of hawaii discharging groundwater was typically brackish to saline and had high ra activity relative to ocean water and fresher inland groundwater brackish sgd fluxes estimated using a mass balance approach based on excess coastal ocean 224ra activity ranged from 3 to 1300 l m21 of coastline min21 fluxes of the freshwater component of sgd calculated based on salinity ranged from 2 to 310 l m21 min21 nutrient fluxes into coastal waters from sgd varied by up to three orders of magnitude among sites nitrate nitrite n n phosphate and silica concentrations showed strong inverse linear correlations with salinity in coastal seawater and groundwater these correlations were consistent with previous work that documented conservative mixing between fresh highnutrient groundwater and saline low nutrient seawater at a few coastal ocean sites we extend this conservative relationship to more coastal ocean sites and to groundwater in the coastal aquifer n n concentrations in the fresh component of discharging groundwater were higher at sites near golf courses and those with a greater percentage of bare land within a 5 km radius and silica concentrations in the fresh component of discharging groundwater were higher at sites with more bare land and lower population densities however neither urbanization nor agriculture was significantly correlated with groundwater nutrient concentrations on this relatively undeveloped coast

timing of closure of the eastern mongol okhotsk ocean constraints from u pb and hf isotopic data of detrital zircons from metasediments along the dzhagdy transect abstract the mongol okhotsk belt a major structural element of east asia is probably the youngest orogenic segment within the central asian orogenic belt however the timing of final closure of the mongol okhotsk ocean remains unresolved here we present detrital zircon u pb hf isotopic data and whole rock geochemical data major and trace elements and sm nd isotopes for the metasedimentary rocks from the un ya bom terrane dzhagdy terrane and the eastern part of the tukuringra terrane our new zircon u pb ages suggest that all sedimentary formations along the dzhagdy transect are early mesozoic in age rather than paleozoic as previously thought the detrital zircons from the metasedimentary rocks in the un ya bom terrane the dzhagdy terrane and the eastern part of the tukuringra terrane yielded the youngest concordant ages of 194 4 193 2 and 171 2 ma respectively moreover we note that the so called sedimentary formations of these terranes are not single sedimentary sequences as previously suggested but a set of an olistostrome or tectonic melanges composed of rocks of different ages and origins these sedimentary formations are probably relics of the mongol okhotsk remnant basin that formed in the gaps between the southern margin of the north asian craton and the amur block during their collision the absence of detrital zircons younger than 171 ma in the sedimentary rocks of the mongol okhotsk basin implies that the final closure of this basin could have taken place at the boundary of the early and middle jurassic as a result of the collision or the development of the mongol okhotsk orogenic belt in this region after that the mongol okhotsk belt underwent intense deformation related to within plate strike slip faulting which could be attributed to the late mesozoic rotation of the north asian craton relative to the continental massifs of east asia

geochronology and geochemistry of early jurassic volcanic rocks in the erguna massif northeast china petrogenesis and implications for the tectonic evolution of the mongol okhotsk suture belt abstract the mongol okhotsk suture belt played an important role in the tectonic evolution of northeast asia during the mesozoic however few studies have examined the influence of this tectonic belt on the geological evolution of northeast china in this paper we present zircon u pb geochronology major and trace element geochemistry and zircon hf o isotopic data for early jurassic volcanic rocks in the erguna massif of northeast china with the aim of constraining the evolution of the mongol okhotsk suture belt and its influence on the tectonic history of china during the early jurassic zircon u pb dating indicates that the trachybasalt and basaltic andesite in the study area were erupted between 193 5 ma and 181 9 ma i e in the early jurassic these early jurassic volcanic rocks belong to the high k calc alkaline series and are enriched in large ion lithophile elements and light rare earth elements as well as being depleted in heavy rare earth elements and high field strength elements such as nb and ta the rocks show a small negative eu anomaly the zircon ehf 182 ma values of the volcanic rocks range from 1 9 to 5 1 corresponding to t dm1 values of 640 901 ma and t dm2 values of 901 1345 ma zircons from two volcanic rocks yield 18 o values of 7 2 1 5 n 19 and 6 6 0 7 n 35 geochemically these early jurassic volcanic rocks are similar to those from active continental margin settings and their primary magmas could have been derived from the partial melting of a lithospheric mantle wedge modified by fluid from a subducted slab the discovery of early jurassic calc alkaline volcanic rocks in the erguna massif together with the coeval porphyry cu mo deposits indicates that an active continental margin existed in the erguna area during the early jurassic taken together we conclude that southward subduction of the mongol okhotsk oceanic plate took place beneath the erguna massif during the early jurassic

discharge of nitrate containing groundwater into a coastal marine environment the discharge of nitrate containing fresh groundwater from a sandy coastal aquifer and into the adjacent shallow marine environment was investigated near esbjerg at the northern end of the wadden sea in denmark the geological structure of the coastal site was determined from drilling data and geoelectrical methods such as multi electrode profiling mep the distribution of shoreface sediment containing freshwater was mapped using mep underwater multi electrode profiling umep and the electrical conductivity measured on pore waters freshwater discharge fluxes were measured using seepage meters and estimated from observed head gradients and measured hydraulic conductivities in the aquifer sediments the nitrate distribution in the coastal sediments was obtained from water samples a groundwater flow model was established to quantify the groundwater flow and travel times and the distribution of freshwater discharge along the coastline results show that on the watershed scale the fresh groundwater discharge is controlled by sand filled buried channels that connect the aquifer with the shoreface sediments the freshwater discharge along the coastline is at this scale probably best estimated by the groundwater flow model however at a more detailed scale the distribution of freshwater discharge in the intertidal zone is controlled by small scale geological heterogeneity and models are unable to predict what happens at a small scale on the beach and offshore for that purpose umep seepage meters pore water sampling and local hydraulic gradients are more useful these measurements indicate that the freshwater discharge occurs in distinct zones and that the highest discharge is near the high tide line decreasing rapidly in a seaward direction nitrate is abundant in the shallow groundwater of the coastal aquifer and is also present in the discharging fresh groundwater at certain patches along the coast however on average very little nitrate is observed in the freshwater discharging at the coast the maximum travel time of groundwater through the aquifer until it discharges at the coast is 100 years and since nitrate leaching from soils has only taken place during the last 40 50 years part of the fresh groundwater discharging at the coast must be free of nitrate

geologic effects on groundwater salinity and discharge into an estuary summary submarine groundwater discharge sgd can be an important pathway for transport of nutrients and contaminants to estuaries a better understanding of the geologic and hydrologic controls on these fluxes is critical for their estimation and management we examined geologic features porewater salinity and sgd rates and patterns at an estuarine study site seismic data showed the existence of paleovalleys infilled with estuarine mud and peat that extend hundreds of meters offshore a low salinity groundwater plume beneath this low permeability fill was mapped with continuous resistivity profiling extensive direct sgd measurements with seepage meters n 551 showed fresh groundwater discharge patterns that correlated well with shallow porewater salinity and the hydrogeophysical framework small scale variability in fresh and saline discharge indicates influence of meter scale geologic heterogeneity while site scale discharge patterns are evidence of the influence of the paleovalley feature beneath the paleovalley fill fresh groundwater flows offshore and mixes with saltwater before discharging along paleovalley flanks on the adjacent drowned interfluve where low permeability fill is absent fresh groundwater discharge is focused at the shoreline shallow saltwater exchange was greatest across sandy sediments and where fresh sgd was low the geologic control of groundwater flowpaths and discharge salinity demonstrated in this work are likely to affect geochemical reactions and the chemical loads delivered by sgd to coastal surface waters because similar processes are likely to exist in other estuaries where drowned paleovalleys commonly cross modern shorelines the existence and implications of complex hydrogeology are important considerations for studies of groundwater fluxes and related management decisions

the timing of final closure along the changchun yanji suture zone constraints from detrital zircon u pb dating of the triassic dajianggang formation ne china abstract the dajianggang formation is located in the changchun yanji suture zone of central jilin province and unconformably overlies the changchun yanji accretionary complex cyac which is a melange resulting from subduction of the jiamusi khanka block jkb beneath the north china craton ncc la mc icp ms u pb dating of detrital zircon from four samples of the formation yields ages of 2516 to 216 ma zircons with u pb ages at 2516 2501 ma and 1897 1832 ma indicate a provenance from precambrian basement rocks of the ncc the 525 482 ma ages indicate a provenance from metamorphic rocks of late pan african age in the jkb that have a tectonic affinity to the central asian orogenic belt caob zircon grains with ages of 383 314 ma and 275 250 ma were likely derived from the underlying cyac the youngest population has a peak age of ca 225 ma which together with late triassic fossils suggests that deposition of the dajianggang formation was late triassic or younger this result supports the view that the final collision of the jkb and ncc along the changchun yanji suture took place before the late triassic furthermore this closure time is at least 10 20 ma later than closure along the solonker xar moron changchun suture in the late permian we thus establish that the changchun yanji suture is not related to the collision between the siberia craton sc and the ncc but was instead related to the paleo pacific plate subduction consequently the changchun yanji suture is not the eastward extension of the solonker xar moron changchun suture as previously considered but the southern margin of the jilin heilongjiang high pressure metamorphic belt ji hei hp belt and resulted from westward subduction of the paleo pacific ocean thus the late triassic marked the switch in subduction from the paleo asian ocean to the paleo pacific ocean in ne china

the most ancient ophiolite of the central asian fold belt u pb and pb pb zircon ages for the dunzhugur complex eastern sayan siberia and geodynamic implications abstract ophiolitic rocks with a zircon age of 1020 ma occur in the dunzhugur complex of east sayan siberia and are part of a neoproterozoic to early palaeozoic segment of the central asian fold belt the most spectacular suite is exposed along the oka and bokson rivers where a complete ophiolite sequence with mantle tectonites a layered sequence composed of dunite wehrlite and pyroxenite a gabbro section a sheeted diabase dyke complex and basaltic pillow lavas are exposed petrologic and geochemical data suggest that all members of the ophiolite originally belonged to the same cogenetic mafic ultramafic crustal section and support a supra subduction zone setting in a fore arc rifting environment for its origin two multigrain zircon size fractions from a plagiogranite are both slightly discordant but yielded a combined mean 207pb 206pb age of 1020 10 ma evaporation of three additional fractions of three to four grains each from the same sample produced a mean 207pb 207pb age of 1019 9 0 7 ma that we consider to most closely reflect the time of igneous crystallization of the plagiogranite this is the oldest ophiolite so far dated from the central asian fold belt the southern margin of the siberian craton and the palaeo asian ocean were established at the end of the mesoproterozoic at least 1000 ma ago during the time interval 1000 570 ma one or several large ocean basins existed between baltica siberia kazakhstan tarim and northern china and these blocks are therefore unlikely to have been part of the supercontinent rodinia rifting initiation of subduction and marginal basin formation began prior to 1000 ma and continued through 570 ma the dunzhugur ophiolite of eastern sayan provides evidence for the early opening of the palaeo asian ocean not later than 1000 ma ago

coupled surface subsurface hydrologic measurements reveal infiltration recharge and discharge dynamics across the swash zone of a sandy beach swash groundwater interactions affect the biogeochemistry of beach aquifers and the transport of solutes and sediment across the beachface improved understanding of the complex coupled dynamics of surface and subsurface flow processes in the swash zone is required to better estimate chemical fluxes to the sea and predict the morphological evolution of beaches simultaneous high frequency measurements of saturation water table elevation and the cross shore locations of runup and the boundary between the saturated and unsaturated beachface surface saturation boundary were collected on a sandy beach to link groundwater flow dynamics with swash zone forcing saturation and lysimeter measurements showed the dynamic response of subsurface saturation to swash events and permitted estimation of infiltration rates surface and subsurface observations revealed a decoupling of the surface saturation boundary and the intersection between the water table and the beachface surface measurements alone were insufficient to delineate the infiltration and discharge zones which moved independently of the surface saturation boundary results show for the first time the motion and areal extent of infiltration and recharge zones and constrain the maximum size of the subaerial discharge zone over swash and tidal time scales the width of the infiltration zone was controlled by swash processes and subaerial discharge was controlled primarily by tidal processes these dynamics reveal the tightly coupled nature of surface and subsurface processes over multiple time scales with implications for sediment transport and fluid and solute fluxes through the hydrologically and biogeochemically active intertidal zone of sandy beaches

geoelectrical signals of geologic and hydrologic processes in a fringing reef lagoon setting summary coastal groundwater may discharge into nearshore and offshore waters forced by terrestrial fluxes controlled by local geology and modulated by the hydrodynamics of littoral water we investigated the electrical signature of these features with a dense multiscale network of electrical resistivity tomography ert surveys in the muri lagoon of rarotonga cook islands the ert surveys spanned from onshore to 400 m into the lagoon and used standard electrodes on land and across the foreshore submerged electrodes in the shallow subtidal zone and floating electrodes towed throughout the reef lagoon by a boat ert surveys on land mapped a typical freshwater lens underlain by a saltwater wedge but with possible deviations from the classical model due to an adjacent tidal creek further inland ert surveys imaged a layer of lava flow deposits that is potentially a confining hydrogeologic unit this unit was used to constrain the expected electrical resistivity of these deposits below the lagoon ert surveys across the intertidal zone and into the lagoon indicated fresh groundwater and porewater salinity patterns consistent with previous small scale studies including the seaward extension of fresh groundwater pathways to the lagoon electrical resistivity er variations in the lagoon subsurface highlighted heterogeneities in the lagoon structure that may focus submarine groundwater discharge sgd through previously unknown buried lava flow deposits in the lagoon a transition to higher er values near the reef crest is consistent with the er signature of porosity reduction due to ongoing differential cementation of reef deposits across the lagoon the imaged coastal hydrostratigraphic heterogeneity may thus control terrestrial and marine porewater mixing support sgd and provide the pathways for groundwater and the materials it transports into the lagoon this hydrogeophysical investigation highlighted the spatial heterogeneity of submarine coastal geology and its hydrogeologic control in a reef lagoon setting but is likely to occur in many similar coastal settings ignoring geologic complexity can result in mischaracterization of sgd and other coastal groundwater processes at many spatial scales

the mongol okhotsk ocean subduction related permian peraluminous granites in northeastern mongolia constraints from zircon u pb ages whole rock elemental and sr nd hf isotopic compositions abstract northeastern mongolia represents a unique area that experienced evolution of both the mongol okhotsk ocean moo and the central asian orogenic belt caob in order to better understand the evolution of moo and its effect on the caob we performed zircon u pb dating and hf isotope and whole rock elemental and sr nd isotopic analyses on three permian peraluminous granitic plutons in northeastern mongolia zircon dating result revealed that granitoids of east monhhaan south monhhaan and tuvshinshiree plutons were emplaced at 254 3 265 5 and 273 3 ma respectively the result provides permian ages for the permian triassic volcanic plutonic belt in northeastern mongolia geochemical analysis suggests that all the granitoids are of peraluminous s type granites elemental and sr nd hf isotopic data indicate that these s type granites were generated from mainly clay poor greywacke with some contribution of juvenile mantle component the permian s type granites emplaced likely in an active margin related to the subduction of the mongol okhotsk ocean the protracted intermittent extension during the long lived compression was considered as a likely geodynamic mechanism for the upwelling of mantle derived basaltic magma and melting of crustal materials