coexisting pseudobrookite ilmenite and titanomagnetite in hornblende andesite of the coleman pinnacle flow mount baker washington evidence for a highly oxidized arc magma pseudobrookite microphenocrysts occur in cognate inclusions in the 305 ka coleman pinnacle hornblende andesite flow from the mount baker volcanic field washington pseudobrookites are associated with hornblende phenocrysts and glomerophyric clusters of orthopyroxene clinopyroxene plagioclase ilmenite titanomagnetite apatite and zircon in a matrix of fresh rhyolitic glass grains of pseudobrookite are rimmed by or intergrown with ilmenite these textures are analogous to those observed between armalcolite and ilmenite in high ti lunar basalts in a unique occurrence pseudobrookite and titanomagnetite form a symplectitic intergrowth surrounding a core of ilmenite mass balance calculations show that the pseudobrookite titanomagnetite assemblage is not an isochemical decomposition of ilmenite in the tio 2 feo fe 2 o 3 system mg free pseudobrookite and titanomagnetite solid solutions do not coexist however all three fe ti oxides in the symplectitic assemblage contain significant amounts of mg in the tio 2 mgo feo feo 1 5 system at high oxygen fugacities the mg rich pseudobrookite titanomagnetite assemblage is stable relative to the conjugate pair of mg bearing ilmenite solid solutions at lower f o 2 fe 2 increases mg mg fe 2 mg no decreases and the conjugate ilmenite pair becomes the stable assemblage at mg no t 900 1000 c and f o 2 nno 1 5 to 1 75 one of the highest redox states on record for arc magmas the calculated f o 2 range is consistent with the composition of the ilmenite in equilibrium with pseudobrookite rutile and with fe 3 rich cores in hornblende phenocrysts

integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the mekong delta abstract arsenic is contaminating the groundwater of holocene aquifers throughout south and southeast asia to examine the biogeochemical and hydrological processes influencing dissolved concentrations and transport of as within soils sediments in the mekong river delta a 50 km2 field site was established near phnom penh cambodia where aqueous as concentrations are dangerously high and where groundwater retrieval for irrigation is minimal dissolved as concentrations vary spatially ranging up to 1300 g l in aquifer groundwater and up to 600 g l in surficial clay pore water groundwaters with high as concentrations are reducing with negligible dissolved o2 and high concentrations of fe ii nh 4 and dissolved organic c within near surface environments these conditions are most pronounced in sediments underlying permanent wetlands often found within oxbow channels near the mekong river there labile c co deposited with as bearing fe hydr oxides under reducing conditions drives the reductive mobilization inclusive of fe and as reduction of as here conditions are described under which as is mobilized from these sediments and near surface as release is linked to aquifer contamination over long time periods 100s to 1000s of years site biogeochemistry is coupled with extensive hydrologic measurements and accordingly a comprehensive interpretation of spatial as release and transport within a calibrated hydraulic flow field is provided of an as contaminated aquifer that is representative of those found throughout south and southeast asia

age and petrology of alkalic postshield and rejuvenated stage lava from kauai hawaii at the top of the waimea canyon basalt on the island of kauai rare flows of alkalic postshield stage hawaiite and mugearite overlie tholeiitic flows of the shield stage these postshield stage flows are 3 92 ma and provide a younger limit for the age of the tholeiitic shield stage the younger koloa volcanics consist of widespread alkalic rejuvenated stage flows and vents of alkalic basalt basanite nephelinite and nepheline melilitite that erupted between 3 65 and 0 52 ma all the flows older than 1 7 ma occur in the west northwestern half of the island and all the flows younger than 1 5 ma occur in the east southeastern half the lithologies have no spatial or chronological pattern the flows of the koloa volcanics are near primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet bearing source that has about two thirds the concentration of p2o5 rare earth elements and sr of the source of the honolulu volcanics on the island of oahu the same lithology in the koloa and honolulu volcanics is generated by similar degrees of partial melting of distinct source compositions the lavas of the koloa volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt respectively phases that remain in the residue of the honolulu volcanics such as rutile and phlogopite are exhausted during formation of the koloa volcanics at all but the smallest degrees of partial melting the mantle source for kauai lava becomes systematically more depleted in 87sr 86sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage at the same time the lavas become systematically more enriched in incompatible trace elements on a shorter timescale the lavas of the koloa volcanics display the same compositional trends but at a lower rate of change the source characteristics of the koloa volcanics considered along with those of the honolulu volcanics support a mixing model in which the source of rejuvenated stage lava represents large percent melts of a plume source mixed with small amounts of small percent melts of a heterogeneous mid ocean ridge source

worldwide initiation of holocene marine deltas by deceleration of sea level rise radiocarbon dated deltaic sequences of holocene age from different parts of the world began to accumulate within a restricted time range from about 8500 to 6500 years ago evaluation of major delta processes indicates that deceleration in sea level rise was the key factor in holocene delta formation within many deltas there is as much as a 2000 year age range between basal deposits in seaward and landward cores this age difference records the progressive landward migration of near mean sea level depositional environments during the lower to mid holocene establishment of a chronostratigraphic framework for holocene delta development provides a fundamental global baseline for distinguishing sea level change from vertical land motion by tectonism and isostasy and for evaluating rates of future marine incursion into low lying deltas

estimates of p t ph2o and fo2 for lavas from patmos greece and implications for magmatic evolution abstract various geothermometers and geobarometers are used to estimate the conditions at which magmas erupted on patmos crystallized equilibrium phenocryst compositions are used as input data for the calculations and previously reported activity composition relationships for minerals and melts are used the results constitute an internally consistent set of estimates of t p p h 2 o and f o 2 but there are large auncertainties in values of p 2 kbar p h 2 o up to 0 7 kb and f o 2 up to 1 4 log units the ne trachybasalt hy trachybasalt hy trachyandesite q trachyte lavas of the main volcanic series mvs crystallized over a temperature interval of 1173 920 c at 2 4 kbar water contents calculated from p h 2 o ranged from 2 5 in the least evolved magmas to 5 6 in the q trachytes oxygen fugacity about 3 log units above values for the fmq buffer during crystallization of the ne trachybasalt was similar to values for alkaline lavas from arcs and from oceanic islands calculated oxygen fugacities for crystallization of the evolved mvs magmas are lower within about 1 log unit of the fmq buffer and this partly reflects uncertainties in calculation of the activity of annite the ne trachybasalts of the young volcanic series yvs crystallized at 1141 1121 c at values of f o 2 about 2 log units above those defined by the fmq buffer these magmas contained 2 2 3 8 h 2 o prior to eruption crystallization of the yvs magmas was polybaric and occurred over the pressure range 7 2 kbar the higher pressure indicates that magma evolution began in chambers sites at or near the base of the crust about 28 km the results place constraints on models for the evolution of the magmas erupted on patmos the data are consistent with eruption of the mvs magmas from a single chamber sited at a depth of 11 5 km or from several chambers sited over a depth interval of 8 5 to 14 5 km the f o 2 t data do not necessarily indicate that the trachyandesites and q trachytes are unrelated to the ne trachybasalts other factors such as assimilation and degassing prior to eruption can lead to a lowering of f o 2 during evolution polybaric crystallization of the yvs magmas as they traversed crust heated by previous magmatic activity is consistent with evolution via assimilation coupled with fractionation the preservation of xenocrysts of mantle olivine in the primitive mvs and yvs ne trachybasalts requires magma ascent times of less than about 78 days calculated from published diffusion data it is suggested that the yvs ne trachybasalts experienced fractional crystallization in the mantle prior to picking up the xenocrysts the phenocrysts in the mvs ne trachybasalt record low pressure 2 3 kbar crystallization indicating that either a these magmas represent hybrids formed by mixing of a primitive xenocryst bearing magma with a more evolved magma or b they evolved via fractional crystallization within the crust mixing is not consistent with mineralogical and geochemical data and the latter alternative is preferred crystallization occurred rapidly under supercooled conditions when magma ascent was temporarily arrested in low pressure chambers containing less dense evolved magmas hy trachybasalt to q trachyte fractionation probably reflects preferential nucleation and growth along the chamber margins it is apparent that xenocryst bearing magmas can evolve via fractional crystallization and an important implication is that xenolith xenocryst bearing magmas may also assimilate crustal material hence the trace element and isotopic signatures of such magmas do not necessarily reflect those of the upper mantle source region

transgressive wave and tide dominated barrier lagoon system and sea level rise since 8 2 ka recorded in sediments in northern bohai bay china abstract two sediment boreholes dq3 and dq4 were drilled in northern bohai bay to investigate the genetic type and sedimentary evolution process of the caofeidian barrier lagoon system and the impact of relative sea level rsl rise the core sediment was tested for grain size microfossils foraminifera and ostracods and 14c dating the holocene formation of the delta coast morphology and sedimentary types was closely associated with different phases of rsl rise fluvial facies palaeosols and limnetic facies formed successively during the latest deglaciation but did not exhibit a direct relationship with rsl rise these sediment facies were immediately followed by the littoral limnetic facies that formed during 9 5 8 2 cal ka bp when the holocene sea level reached the study area then offshore sand bars formed on the southern side and tidal sand ridges formed on the eastern and western sides forming a barrier island which suggest that the caofeidian barrier lagoon system began to form under wave and tide dominated ocean dynamics at 8 2 7 8 cal ka bp as the sea level rose at a rate of ca 11 5 mm a 1 the caofeidian barrier lagoon system reached the maximum extent and was mainly influenced by wave dominated ocean dynamics at 7 8 7 5 cal ka bp during which the sea level rose at a rate of ca 9 0 mm a 1 the caofeidian barrier lagoon system began to erode and deteriorate at 7 5 6 7 cal ka bp with a sea level rise rate of ca 2 6 mm a 1 tidal sand ridges as barrier islands formed on the shore side under the influence of tide dominated ocean dynamics rsl rise was estimated at 2 3 m over the past 6 7 ka over this period the sandy barrier islands were eroded by tides and the lagoon areas transitioned into an intertidal zone our reconstruction of rsl changes showed that the rsl rose abruptly in four stages increasing by 1 7 m 11 0 m 1 7 m and 2 3 m at 9 5 8 7 8 7 7 5 7 5 6 7 and 6 7 0 cal ka bp respectively

delta progradation and chenier formation in the huanghe yellow river delta china abstract to clarify holocene development of the huanghe yellow river delta and the relationship between delta progradation and chenier formation detailed sediment analyses and high resolution radiocarbon dating were done on borehole samples taken from two sites on the present huanghe h9601 latitude 37 40 5 n and longitude 118 28 7 e with an altitude of 5 5 m and h9602 lat 37 47 8 n and long 118 54 3 e 4 8 m downcore changes of sediment facies and accumulation rate show that delta progradation occurred at least twice during ca 2 6 1 2 c 14 yrbp and 1855 present at the two borehole sites these phases of progradation correlate with superlobes 6 and 7 and superlobe 10 respectively of ten superlobes composing the holocene huanghe delta shown by c xue historical changes in the yellow river delta china marine geology 113 321 329 1993 the boundary of these sediments at h9602 is sharp and erosional and correlates with cheniers located between borehole sites on the delta plain the diastem period between these phases of progradation almost coincides with the periods when the river mouth of the huanghe was located in the northwestern part of the bohai sea during 1048 1128 ad superlobe 8 and the jiangsu region facing the yellow sea during 1128 1855 superlobe 9 this suggests that chenier formation and delta progradation are linked and controlled by sediment supply and river course shifts of the lower reaches of the huanghe

late quaternary floodplain development along the stung sen river in the lower mekong basin cambodia abstract the stung sen river the biggest tributary to lake tonle sap in the lower mekong basin in cambodia is characterized by large seasonal changes of water discharge under the asian monsoon climate and seasonal changes in water level that reach at least 7 m and are controlled by the water level of the lake the stung sen river floodplain consists of two geomorphic units meander belt along the river channel and backmarsh coupled observations of outcrops along the river channel and arrays of sediment cores across the floodplain north of kampong chheuteal village and kampong thom city c 150 km and c 70 km respectively reveal that floodplain environmental changes at c 11 ka were possibly associated with the holocene onset of the southeast asian monsoon and probably with the emergence of lake tonle sap these observations also show that the present backmarsh meander belt system was established about 5 5 ka along with the unique mekong tonle sap connection characterized by a reversal in flow direction during the monsoon season the meander belt materials are replaced as the river channel shifts on a decadal to centennial timescale backmarsh sediments at sites kampong chheuteal and kampong thom had a constant accumulation rate of about 0 5 mm y during the holocene contrasting with rates of 0 1 mm y during the late pleistocene at around 11 ka a sand layer was deposited over all of the valley around kampong chheuteal while wetlands enlarged around kampong thom probably because of increased rainfall triggered by an enhancement of the asian summer monsoon this 11 ka horizon has since been covered by clayey sediments keeping pace with the accumulation of lacustrine sediments in lake tonle sap

mesoscale convective complexes and persistent elongated convective systems over the united states during 1992 and 1993 abstract large long lived convective systems over the united states in 1992 and 1993 have been classified according to physical characteristics observed in satellite imagery as quasi circular mesoscale convective complex mcc or elongated persistent elongated convective system pecs and cataloged the catalog includes the time of initiation maximum extent termination duration area of the 52 c cloud shield at the time of maximum extent significant weather associated with each occurrence and tracks of the 52 c cloud shield centroid both mcc and pecs favored nocturnal development and on average lasted about 12 h in both 1992 and 1993 pecs produced 52 c cloud shield areas of greater extent and occurred more frequently compared with mccs the mean position of initiation for pecs in 1992 and 1993 followed a seasonal shift similar to the climatological seasonal shift for mcc occurrences but was displaced eastward of the mean position of mcc initiation in 1992 and 1993 the spatial distribution

evolution of the coastal depositional systems of the changjiang yangtze river in response to late pleistocene holocene sea level changes abstract the paleo changjiang yangtze incised valley fills approximately 80 90 m thick provide an opportunity to document the evolution of coastal depositional systems with large sediment supply in response to late pleistocene holocene sea level fluctuations on time scales of 103 to 104 years the sedimentary facies of the incised valley fills record three main depositional systems fluvial tide dominated estuary and tide dominated delta radiocarbon ages for the incised valley fills suggest that these depositional systems developed before about 11 ka between 11 and 8 ka and after approximately 8 ka respectively by applying sequence stratigraphic concepts the evolution of the depositional systems can be divided into three systems tracts a lowstand systems tract lst a transgressive systems tract tst and a highstand systems tract hst sea level changes on a 104 year time scale controlled the basic architecture of the sequence of the incised valley fill on the other hand sea level changes on a millennial time scale affected the stacking pattern of the systems tracts in particular the continuous sea level rise with episodic rapid rises during the last deglaciation affected the stacking pattern of the tst which is characterized by a combination of aggradation and backstepping the aggradation of fluvial and estuarine systems was dominant and the shoreline migrated only gradually landward under the relatively slow rise in sea level and a very rapid sea level rise around 12 and 10 ka caused the system to migrate abruptly landward unlike the transgressive estuarine phase the stacking pattern of the regressive tide dominated delta hst which developed within the almost filled incised valley and on the surrounding interfluve zones was characterized by seaward progradation with clinoform architecture it was initiated with aggradational and progradational stacking about 8 ka during the last phase of decelerated sea level rise and was followed by a progradational phase after the highest sea level about 6 ka

tracing organic matter composition and distribution and its role on arsenic release in shallow cambodian groundwaters biogeochemical processes that utilize dissolved organic carbon are widely thought to be responsible for the liberation of arsenic from sediments to shallow groundwater in south and southeast asia the accumulation of this known carcinogen to hazardously high concentrations has occurred in the primary source of drinking water in large parts of densely populated countries in this region both surface and sedimentary sources of organic matter have been suggested to contribute dissolved organic carbon in these aquifers however identification of the source of organic carbon responsible for driving arsenic release remains enigmatic and even controversial here we provide the most extensive interrogation to date of the isotopic signature of ground and surface waters at a known arsenic hotspot in cambodia we present tritium and radiocarbon data that demonstrates that recharge through ponds and or clay windows can transport young surface derived organic matter into groundwater to depths of 44 m under natural flow conditions young organic matter dominates the dissolved organic carbon pool in groundwater that is in close proximity to these surface water sources and we suggest this is likely a regional relationship in locations distal to surface water contact dissolved organic carbon represents a mixture of both young surface and older sedimentary derived organic matter ground surface water interaction therefore strongly influences the average dissolved organic carbon age and how this is distributed spatially across the field site arsenic mobilization rates appear to be controlled by the age of dissolved organic matter present in these groundwaters arsenic concentrations in shallow groundwaters 20 m groundwaters we suggest that while the rate of arsenic release is greatest in shallow aquifer sediments arsenic release also occurs in deeper aquifer sediments and as such remains an important process in controlling the spatial distribution of arsenic in the groundwaters of se asia our findings suggest that any anthropogenic activities that alter the source of groundwater recharge or the timescales over which recharge takes place may also drive changes in the natural composition of dissolved organic carbon in these groundwaters such changes have the potential to influence both the spatial and temporal evolution of the current groundwater arsenic hazard in this region

physical processes involved in the 1988 drought and 1993 floods in north america abstract an analysis of the spring summer 1988 drought and 1993 floods over north america reveals a reversal in the sign of anomalies in several fields large sea surface temperature anomalies of opposite signs existed in the tropical pacific with strong la nina conditions in 1988 and a mature el nino in 1993 the distribution of tropical convection in the convergence zones and associated latent heating of the atmosphere were correspondingly altered implying a large scale switch in the anomalous tropical heating and forcing of extratropical quasi stationary waves in the atmosphere influencing the subtropical jet stream over the north pacific and across north america in 1988 the jet stream and the closely related storm track of high frequency disturbances in the upper troposphere were displaced into canada well north of the normal location the farthest north of any year from 1979 to 1993 in 1993 a broader jet stream and the storm track were displaced well south of normal to a more springlike location a

compositional variation of coexisting olivine orthopyroxene and fe mg ferrite as a function of t and f_ o_2 a geothermometer and oxygen barometer a thermodynamic model is developed that describes the compositional variation of coexisting olivine orthopyroxene and ferrite fe 3o 4 mgfe 2o 4 as a function of f o 2 and t the ferrite phase has a cation distribution which varies from nearly inverse to nearly random with increasing t and is described with a model in which the number of sites per formula unit on which mixing occurs varies from 1 67 to 2 0 given this model and the equilibrium phase composition data for coexisting olivine and ferrite at 1 300 c of jamieson and roeder 1984 the ferrite solution is described to an excellent approximation by a symmetric regular solution model with w ft 14 0 0 3 kj mole orthopyroxene and olivine non ideality are also considered the t dependence of the equilibrium constant for the oxidation reaction 6fs 2mt 6fa o 2 and the two fe mg exchange reactions between olivine ferrite and olivine orthopyroxene are used to calculate the compositional variation of coexisting phases as a function of f o 2 and t the results are summarized on an isobaric 1 bar f o 2 1 tplot with the compositional variation of olivine ferrite and orthopyroxene shown by sets of isopleths the ferrite isopleths intersect those of olivine and orthopyroxene at sufficiently high angles for this assemblage to serve as a sensitive geothermometer and oxygen barometer the model is applied to orthopyroxene ferrite symplectite in coronas around olivine in a metamorphosed gabbro to olivine hosted orthopyroxene ferrite symplectite in unmetamorphosed gabbros and norites and to olivine hosted orthopyroxene ferrite symplectites developed within the rims of lherzolite xenoliths

fine sediment dynamics in the mekong river estuary vietnam field studies of fine sediment transport were carried out in the mekong river estuary vietnam in the high flow season of november 1993 in the freshwater region erosion and deposition of suspended sediment occurred at tidal frequency with a strong down river transport at a mean velocity of 1 m s 1and mean suspended solid concentration of about 250 mg l 1 also the suspended sediment was mostly fine silt and the clay fraction accounted for only 15 the suspended sediment was transported either as individual particles or agglomerated with organic detritus at the mouth of the estuary a salt wedge was present but was flushed out of the estuary at low tide flow reversal occurred across the pycnocline a turbidity maximum zone was present at the toe of the salt wedge at flood tide most of the suspended sediment was coagulated with little organic matter the bulk of the suspended sediment was exported to coastal waters but some sediment returned to the estuary in the salt wedge data for the low flow season are sparse but suggest that partially well mixed estuarine conditions prevail with salinity penetrating about 40 km inland carrying fine sediment up river to a turbidity maximum zone most of this sediment would have been deposited in shallow coastal waters in the previous high flow seasons the construction of large hydro electric dams further upstream on the mekong river may exacerbate siltation patterns in the estuary

stratigraphic sequence and episodes of the ancient huanghe delta along the southwestern bohai bay since the lgm abstract the sedimentary evolution of the ancient huanghe yellow river delta in china is not well documented due to frequent river avulsions and limited deep cores in this paper 22 cores with lengths of 20 m at the coastal plain along the southwestern bohai bay were used the sedimentary facies were determined by grain size and microfossil analysis and carbon 14 dating the strata in the study area since the last glacial maximum lgm are divided into lowstand system tract lst transgressive system tract tst highstand system tract hst and retrograding system tract rst the lst which formed during the lgm to 10 8 ka bp consists of hard mud layer and overlying interfluvial lowland and flood plain deposits the tst that formed during 10 8 8 5 ka bp mainly include coastal marsh onshore sand bars and filled incised valleys the hst that formed during 8 5 2 6 ka bp mainly includes flood plain river channel delta front mouth bar and prodelta the rst that formed after 2 6 ka bp mainly includes crevasse splay interfluvial lowlands and distributary channel about eight episodes of the ancient huanghe delta are observed during the holocene and the names of the deltas are given by corresponding location names of cheniers 1 during 10 8 8 5 ka bp around ys3 and np1 cores without formation of cheniers during the transgression 2 during 8 5 7 4 ka bp around ys1 ys4 and np3 cores the dongsun shuiyicheng delta 3 during 7 4 6 1 ka bp around ck1 core the mashanzi delta 4 during 6 9 5 7 ka bp around gz3 core the miaozhuang zhaizhuang delta 5 during 4 6 3 5 ka bp around ys4 core the wuditai delta 6 during 3 0 2 6 ka bp around hh4 and hh1 cores the qikou delta 7 during 2 6 1 9 ka bp around ys7 and np1 cores the nanpaihe delta and 8 during 1034 bc 1099 bc around wangzidao and dakouhe the wangzidao delta

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

compositional variability of terrestrial mantle apatites thermodynamic modeling of apatite volatile contents and the halogen and water budgets of planetary mantles abstract apatites in ultramafic xenoliths from hawaii kamchatka nunivak island lunar crater volcanic field and the navajo volcanic field have variable volatile f cl oh compositions that correlate with their petrogenesis apatites in metasomatized lherzolite harzburgite xenoliths are relatively cl rich whereas apatites in igneous pyroxenite and amphibolite xenoliths are relatively f rich our results support the generalization of o reilly and griffin 2000 that metasomatic apatites tend to be cl rich and magmatic apatites tend to be f rich mantle metasomatism may largely be the consequence of devolatilization of cl rich fluids from subducted lithosphere however the occurrence of metasomatic xenoliths far removed from subduction zones e g bullenmerri as well as the geochemistry of cl in some plume basalts and diamonds indicates that not all metasomatism is subduction triggered thermodynamic modeling of the influence on apatite compositions of variable f cl h2o and p abundances as well as temperature is consistent with the idea that terrestrial mantle fluids are h2o rich and that halogen enrichment in terrestrial apatites need not represent large absolute halogen abundances in the fluids the models indicate that the occurrence of volatile poor calcium phosphate merrillite whitlockite in the martian and terrestrial mantles is facilitated by halogen and phosphorus rich and water poor environments the rarity of this assemblage in the terrestrial mantle compared to mars is consistent with the relatively wet and halogen and phosphorus poor nature of the terrestrial mantle compared to that of mars

song hong red river delta evolution related to millennium scale holocene sea level changes abstract the song hong red river delta occurs on the northwest coast of the south china sea its evolution in response to holocene sea level changes was clarified on the basis of sedimentary facies and 14 radiocarbon dates from the 40 m long duy tien core from the delta plain and using previously reported geological geomorphological and archaeological data the delta prograded into the drowned valley as a result of early holocene inundation from 9 to 6 cal kyr bp as sea level rise decelerated the sea level highstand at 2 3 m from 6 to 4 cal kyr bp allowed widespread mangrove development on the delta plain and the formation of marine notches in the ha long bay and ninh binh areas during sea level lowering after 4 cal kyr bp the former delta plain emerged as a marine terrace and the delta changed into the present tide and wave influenced delta with accompanying beach ridges delta morphology depositional pattern and sedimentary facies are closely related to holocene sea level changes in particular falling sea level at 4 cal kyr bp had a major impact on the evolution of the song hong delta and is considered to be linked to climate changes

luminescence dating of beach ridges for characterizing multi decadal to centennial deltaic shoreline changes during late holocene mekong river delta abstract empirical understanding of decadal to centennial scale deltaic shoreline changes in the past is essential for understanding the fate of coasts in the coming decades and centuries we tested the effectiveness of quartz optically stimulated luminescence osl dating of beach ridges to constrain shoreline changes of the mekong river delta over the last 3500 years forty seven osl ages have been analyzed from the tra vinh delta plain central mekong river delta the tra vinh beach ridges are recurved and branching showing the hierarchy of shoreline changes which include discontinuous shifts c 5 km seawards caused by the emergence and elongation of a delta front bar island and subsequent downdrift accretion of spits the spit accretion is interrupted by changes of shoreline orientation each of the discontinuous shifts resulted in a cluster of beach ridges which is referred to as ridgeset the beach ridge sediments were found to have excellent luminescence properties resulting in low age uncertainties of c 5 the osl chronology agrees well with shoreline changes over recent decades and with radiocarbon ages of tidal flat sediment underlying the beach ridges the chronology clearly illustrates the coastal progradation from 3500 years relative to ad 2010 onwards two main types of beach ridge are present recurved and trunk ridges osl ages of recurved ridges are consistently younger both downdrift and seawards documenting decadal to centennial scale shoreline migration especially over the last 1500 years trunk ridges in contrast have less systematic ages because a trunk ridge is formed where the sediment is likely to have been reworked by waves ages of updrift trunk ridges characteristically show relatively young ages in each ridgeset representing the extensive erosion and resedimentation of the updrift coast major changes in shoreline orientation occurred around 400 500 years suggesting strengthening of the northeasterly winter monsoon at the beginning of the little ice age the rate of progradation of the tra vinh delta plain is described in two ways frequency of the discontinuous shift and growth rate of the delta plain area both suggest the coast has expanded regularly over the last 1000 years a decrease in sand supply to the coast in the last few decades due to river dam construction and fluvial sand dredging is inferred possibly affecting the behavior of the modern and future shorelines which can be compared with the less human influenced centennial to decadal scale past shoreline changes reconstructed in this study

sedimentary facies and holocene progradation rates of the changjiang yangtze delta china abstract the changjiang yangtze river one of the largest rivers in the world has formed a broad tide dominated delta at its mouth during the holocene sea level highstand three boreholes cm97 js98 and hq98 were obtained from the changjiang delta plain in 1997 1998 to clarify the characteristics of tide dominated delta sediments and architecture based on sediment composition and texture and faunal content core sediments were divided into six depositional units in ascending order they were interpreted as tidal sand ridge prodelta delta front subtidal to lower intertidal flat upper intertidal flat and surface soil deposits the deltaic sequence from the prodelta deposits to the delta front deposits showed an upward coarsening succession overlain by an upward fining succession from the uppermost part of the delta front deposits to the surface soil thinly interlaminated to thinly interbedded sand and mud sand mud couplets and bidirectional cross laminations in these deposits show that tide is the key factor affecting the formation of changjiang deltaic facies sediment facies and their succession combined with ams 14c dating revealed that isochron lines cross unit boundaries clearly and delta progradation has occurred since about 6000 to 7000 years bp when the rising sea level neared or reached its present position the average progradation rate of the delta front was approximately 50 km kyear over the last 5000 years the progradation rate however increased abruptly ca 2000 years bp going from 38 to 80 km kyear the possible causes for this active progradation could have been an increase in sediment production in the drainage basin due to widespread human interference and or decrease in deposition in the middle reaches related to the channel stability caused by human activity and climatic cooling after the mid holocene

sedimentary facies diatom and foraminifer assemblages in a late pleistocene holocene incised valley sequence from the mekong river delta bentre province southern vietnam the bt2 core abstract a detailed description of sediment facies diatom mollusca and foraminifer assemblages and 14 c ages of the mekong river delta southern vietnam is presented in this paper a 71 m long core recovered in 1997 at the bentre province in the eastern part of the mekong river delta is divided into eight units and nine sediment facies on the basis of sedimentary properties diatom and foraminifer assemblages and 14 c ages these units show facies changes from transgression to regression in relation to the late pleistocene holocene sea level changes the postglacial transgression caused by a sea level rise led to infilling of the incised valley and formation of estuarine sediments and open bay muddy sediments in ascending order estuarine sediments consist of slightly oxidized yellowish grey silty sand and intercalated greenish grey and stiffly brownish grey sandy silt and silty clay the muddy facies 13 5 m thick is characterized by abundant marine planktonic diatoms and open sea foraminifers this facies indicates the maximum holocene marine influence was around 5300 cal yr bp at the bt2 site the regressive succession is composed of deltaic sediments from prodelta delta front sub to inter tidal flat and beach ridge in ascending order 14 c ages indicate that the delta front passed the core site at approximately 4000 3000 cal yr bp the coastal progradation rate was 17 18 m yr 1 during the 5300 3500 cal yr and decreased to 13 14 m yr 1 during the last 3500 cal yr

early growth stage of a large delta transformation from estuarine platform to deltaic progradational conditions the northeastern mekong river delta vietnam abstract the initial growth stage of a river delta can be defined either as the beginning geomorphologic expansion of the river dominated coastal lowland or as the starting basinward directed progradation of the geological stratigraphic delta body differentiating between these two perspectives is necessary for understanding the sensitive reaction of a deltaic system to dynamic changes in sea level and sediment availability the holocene evolution of the northeastern part of the mekong river delta southern vietnam offers a great chance to depict these two different definitions and relate them genetically to each other since the transition between an old abrasion platform inner delta and the continental shelf basin outer delta is well developed in this marginal part of the delta the early stage of delta growth taking place during transgression related inundation between 8 cal ka bp maximum flooding and 5 7 6 0 4 8 cal ka bp sea level highstand was characterized by tide and saltwater influenced nearshore conditions allowing extensive mangrove and tidal flat deposits to aggradate on the wide abrasion platform with the onset of regression around 4 8 cal ka bp the depositional center shifted immediately beyond the seaward margin of this estuarine platform and thus true deltaic progradation could develop leading to rapid construction of a delta body and in addition to subsidence tilting of the inner delta zone directly at the transition of these two deposystems remains of shore related deposits indicate the sudden breakdown of the estuarine platform conditions around 4 6 cal ka bp

early holocene initiation of the mekong river delta vietnam and the response to holocene sea level changes detected from dt1 core analyses abstract the early holocene initiation of the mekong river delta vietnam was investigated based on sedimentary facies microfossil analyses and radiocarbon dating of dt1 core taken from the upper delta plain the dt1 core 105 38 51 e 10 17 02 n elevation 2 m is 51 5 m long and located in fill of a buried incised valley this incised valley of the paleo mekong river formed during the last glacial maximum which is 20 30 km wide has been traced spatially from the cambodian border to the south china sea coast by compilation of existing core data the dt1 core records the history of the estuarine channel and delta systems of the mekong river during the early to middle holocene the sedimentary facies determined particularly from sedimentary structures diatom and foraminifera assemblages grain size variation and the accumulation rate clearly show an upward deepening succession in the lower part of the core from marsh or intertidal flat to subtidal inner bay facies and an overlying upward shallowing succession from prodelta to intertidal flat floodplain the boundary between the two successions indicating the maximum flooding surface occurred at approximately 8 0 ka at the time that the mekong river delta was initiated the delta initiation and sediment facies succession are closely linked to holocene sea level changes particularly a standstill from 8 0 to 7 5 ka after a rapid rise of sea level from 8 8 to 8 2 ka during the further sea level rise from 7 5 to 7 0 ka sediments aggraded forming topset sedimentary deposits

holocene sea levels in southeast asia maldives india and sri lanka the seamis database abstract we assembled a database of holocene relative sea level index points n 213 and marine n 211 and terrestrial n 122 limiting points for the broader south and southeast asian region including the maldives india and sri lanka the standardized review of published age elevation information from corals deltaic estuarine and mangrove deposits beachrocks and tidal notches yielded a new suite of relative sea level index and limiting points produced according to a standardized protocol expected spatial variability in holocene relative sea level change due to glacial isostatic adjustment was accounted for by first subdividing the study area into ten geographic sub regions from the central indian ocean to the western tropical pacific and second by comparing sub regional relative sea level data to model predictions of glacial isostatic adjustment results show that some of the regionally constrained relative sea level data are characterized by significant inconsistencies that cannot be explained by glacial isostatic adjustment such inconsistencies of standardized relative sea level data become particularly obvious in areas around the red river delta in vietnam the gulf of thailand the northwest coast of malaysia and the spermonde archipelago in indonesia based on a critical evaluation of the reviewed relative sea level indicators we discuss possible sources of local divergence and identify regions where data are currently insufficient to constrain glacial isostatic adjustment predictions the remaining quality controlled and consistent relative sea level data show that glacial isostatic adjustment and syn post formational influences such as tectonic uplift subsidence and compaction were the dominant local drivers of holocene relative sea level change collectively the results of this review suggest that holocene sea levels in south and southeast asia and surrounding regions have been controlled by a variety of global and local drivers and imply that additional index points from the java sea in indonesia would be valuable to better assess the spatial variability and to calibrate geophysical models of glacial isostatic adjustment

delta initiation and holocene sea level change example from the song hong red river delta vietnam abstract delta initiation and subsequent delta growth were investigated by using radiocarbon dated borehole cores from the song hong red river delta vietnam the sediments underlying the present delta plain record three depositional systems since the last glacial maximum fluvial estuarine and deltaic in ascending order the landward part of the initial delta was characterized by an active tide influenced accumulation of sandy sediments the seaward part of the initial delta is composed of muddy prodelta and seafloor sediments that accumulated slowly radiocarbon ages of molluscan shells and organic materials suggest that the transition from estuary to delta occurred at approximately 8500 cal years bp the transition was induced by deceleration of the postglacial sea level rise which was coeval with the delta initiation the stacking pattern of the delta system is very different from that of the estuary system the sediments of the estuary that developed during the rapid rise in sea level between 11000 and 8500 cal years bp display a retrogradational stacking pattern forming a thick transgressive systems tract on the other hand aggradational as well as progradational stacking with clinoform architecture characterized the delta system between 8500 and 6500 cal years bp as the sea level continued its slow rise progradation has been dominant during the last 6500 years because the cessation of sea level rise resulted in little accommodation being added the results of this study are similar to those for other major delta systems such as the changjiang and the ganges brahmaputra deltas despite large differences in sediment discharge tectonics basin size and coastal environment this implies that the postglacial sea level rise and the change in the rise rate markedly influenced the evolution of depositional systems at river mouths

depositional facies and radiocarbon ages of a drill core from the mekong river lowland near phnom penh cambodia evidence for tidal sedimentation at the time of holocene maximum flooding abstract the depositional facies and radiocarbon ages of a long holocene sediment core ks from cambodia are reported here to clarify the sedimentary environments of the mekong river delta system at the time of holocene maximum flooding by the sea the 30 7 m long ks core from the upper mekong river lowland about 20 km southeast of phnom penh penetrated five depositional facies a to e in ascending order facies a is cross laminated fluvial sand facies b is laminated very fine sand with mud drapes facies c to e are salt marsh flood plain and natural levee deposits characterized by a succession of peat organic clay and reddish brown silt facies b dates to 9 0 7 5 ka and is interpreted as aggradational tidal deposits 16 5 m thick deposited during a rapid rise of sea level these deposits became overlain by an accumulation of salt marsh and flood plain deposits during the subsequent period of slowly rising and then falling sea level the maximum flooding surface lies in the facies b interval the presence of tidal deposits implies that the core site about 230 km inland from the present river mouth was near the shoreline and experienced strong tidal influences during the early holocene

stratigraphy and holocene evolution of the mud dominated chao phraya delta thailand abstract the central plain of thailand was formed over the last 8 7 kyr mainly by the deltaic processes of two major rivers the chao phraya and the mae klong the delta plain is the third largest delta plain in southeast asia after that of the mekong and the irrawaddy on the basis of sedimentological and paleontological analyses of samples from three boreholes and an open pit survey the evolution of the delta was clarified with high resolution 14 c dating after the maximum transgression at between 8 and 7 cal kyr bp the delta system migrated southward into the paleo gulf of ayutthaya a large mud shoal the sananivate mud shoal formed near the mouth of the paleo gulf between 7 and 3 cal kyr bp and facilitated its infilling as a result the delta has prograded rapidly particularly during the last 2 kyr deltaic sediment volume for the last 7 5 0 5 kyr shows that the average rate of sedimentation was 23 1 3 6 million t yr which is nearly the same as the present total sediment discharge from both rivers the comparatively wide delta plain relative to sediment discharge resulted from the stable sea level the shallow paleo gulf acting as a receiving basin and effective sediment trapping because of the mud shoal

a highly oxidized ferrian salite kennedyite forsterite and rhoenite bearing alkali gabbro from kauai hawaii and its mantle xenoliths three types of rock each with unusually oxidized mineral assemblages are found in close association as part of a probable feeder pipe to the alkalic lavas on kauai hawaii the first type is an alkali gabbro consisting ofpotassic oligoclase 66voby volume ferrian salite 27vo kennedyite 3vo magnesioferrite 3 and trace amounts of rh6nite kaersutite apatite hematite and nearly pure forsterite electron microprobe analyses of these phases show them to be exceptionally enriched in mg and ferric iron these data combined with textural observations demonstrate that the alkali gabbro melt existed in a highly oxidized state prior to the onset of crystallization and that this condition prevailed throughout the course of crystallization the second lilhlogic type consists of an assortment of mantle xenoliths mostly spinel iherzolites that were entrained by the alkali gabbro at depth prior to its crystallization there was extensive development of oxidation symplectites within the primary grains in the outer two cm of the xenoliths original mantle olivine foss sr has oxidized to form bleb like intergrowths of orthopyroxene enq and magnesioferrite mtse in a more magnesian foe5 olivine host comparison of these data with relevant i atm experimental data in the system mgo feo fe2o3 sio2 indicates conditions of l0r o2 and 1165 c for the formation of these symplectites the th ird lithologic type is referred to here as the fine grained groundmass like the alkali gatbro the fine grained groundmass has a high fe3 fe2 but consists only offerrian salite 62 potassic oligoclase l7vo forsterite 14 magnesioferrite 7 and trace amounts of apatite the gradational contacts of the fine grained groundmass with the xenoliths and comparative microprobe analyses suggest that the groundmass liquid was derived in part from the xenoliths by partial melting and the subsequent crystallization of that liquid was complete prior to the introduction of the alkali gabbro the formation of the oxidation symplectites is closely associated with the fine grained groundmass and hence the melting event and appears to have been prior to alkali gabbro intrusion

late pleistocene holocene seismic stratigraphy of nha trang shelf central vietnam the late pleistocene holocene stratigraphic architecture on the steep and narrow shelf off nha trang central vietnam has been explored by high resolution seismic profiles integrated with sediment core data sequence stratigraphic results reveal five major seismic units and three bounding surfaces which are composed of two distinctive sequences those sequences are bounded by two regional unconformities sb1 sb2 which have been formed in respond to different sea level regimes the revealed relict beach ridge deposits at water depth of about similar to 130 m below the present water depth indicate that the last glacial lowstand lgm sea level in this area was lower than in neighboring areas and it probably resulted from subsidence due to high sedimentation rate and or neotectonic movements of the east vietnam fault system the late pleistocene high amplitude of sea level change during a long fourthorder and superimposed by shorter fifth order cycle is the principal factor in reorganizing the formation of the nha trang continental shelf sequence other local controlling factors as fluctuations in sediment supply morphological variations of the lgm surface subsidence rate and hydrodynamic conditions provided the distinctive features of the nha trang shelf sequence stratigraphic model in comparison with neighboring other areas c 2014 elsevier ltd all rights reserved

peat formation concentrates arsenic within sediment deposits of the mekong delta abstract mekong river delta sediment bears arsenic that has been released to groundwater under anaerobic conditions over the past several thousand years the oxidation state speciation and distribution of arsenic and the associated iron bearing phases are crucial determinants of as reactivity in sediments peat from buried mangrove swamps in particular may be an important host source or sink of arsenic in the mekong delta the total concentration speciation and reactivity of arsenic and iron were examined in sediments in a mekong delta wetland by x ray fluorescence spectrometry xrf x ray absorption spectroscopy xas and selective chemical extractions total solid phase arsenic concentrations in a peat layer at a depth of 6 m below ground increased 10 fold relative to the overlying sediment extended x ray absorption fine structure exafs spectroscopy revealed that arsenic in the peat was predominantly in the form of arsenian pyrite arsenic speciation in the peat was examined further at the micron scale using xrf and x ray absorption near edge structure xanes spectroscopy coupled with principal component analysis the multiple energy xrf mapping and xanes routine was repeated for both iron and sulfur phase analyses our xrf xanes analyses confirm arsenic association with pyrite a less reactive host phase than iron hydr oxides under anaerobic conditions the arsenian pyrite likely formed upon deposition formation of the peat in a past estuarine environment 5 5 ka bp a process that is not expected under current geochemical conditions presently arsenian pyrite is neither a detectable source nor a sink for aqueous arsenic in our sediment profile and under present geochemical conditions represents a stable host of as under the reducing aquifer conditions of the mekong delta furthermore organic carbon within the peat is unable to fuel fe iii reduction as noted by the persistence of goethite which can be reduced microbially with the addition of glucose

use of lithium tracers to quantify drilling fluid contamination for groundwater monitoring in southeast asia abstract drilling is widely used in groundwater monitoring and many other applications but has the inherent problem of introducing some degree of external contamination into the natural systems being monitored contamination from drilling fluid is particularly problematic for i wells with relatively low water flow rates which are difficult to flush and for ii hydrogeochemical research studies of groundwaters hosted by incompletely consolidated shallow sediments which are widely utilized as sources of drinking water and irrigation water across many parts of asia here we develop and evaluate a method that can be simply used to quantify the extent of drilling fluid contribution to a water sample either to optimize sample collection for reduced contamination or to allow a correction for contamination to be made we report the utility of lithium chloride tracers using both field and laboratory analytical techniques to quantitatively evaluate and correct for drilling fluid contamination of casing waters through an investigation of 15 sites in kandal province cambodia high analytical errors limit the practicality and resolution of field based lithium ion selective electrode measurements for purposes other than broad estimates of gross contamination however when laboratory analysis is integrated with the method e g via inductively coupled plasma atomic emission spectrometry analysis lithium tracers can provide a robust and accurate method for evaluating drilling related contamination if appropriate samples are collected casing water is susceptible to contamination from drilling fluid which was shown to be significantly reduced within two to three well volumes of flushing but can still persist above background for greater than seven well volumes of flushing a waiting period after drilling and prior to water sampling was shown to further decrease contamination due to dilution from the surrounding aquifer particularly in more permeable wells contamination values were generally

late holocene sedimentary and environmental development of the northern mekong river delta vietnam abstract a sedimentological and palynological study of three sediment cores from the northern mekong river delta shows the regional sedimentary and environmental development since the mid holocene sea level highstand a sub to intertidal flat deposit of mid holocene age is recorded in the northernmost core shoreline deposits in all three cores show descending ages from n to s documenting 1 the early stages of the late holocene regression and 2 the subsequent delta progradation the delta plain successions vary from floodplain deposits with swamp like elements to natural levee sediments the uppermost sediments in all cores show human disturbance to varying degrees the most intense alteration is recorded in the northernmost core where the palynological signal together with a charcoal peak indicates the profound change of the environment during the modern land reclamation the sediments from at least one of the three presented cores do not show a true delta facies succession but rather estuary like features as also observed in records from southern cambodia this absence is probably due to lack of accommodation space during the initial phase of rapid delta progradation which impeded the development of true delta successions as shown in cores from the southern mekong river delta

preservation and grain size trends of holocene wave dominated facies successions in eastern japan implications for high resolution sequence stratigraphic analysis vertical grain size trends provide significant information for the sequence stratigraphic analysis of siliciclastic wave dominated facies successions this paper discusses the relationships between the preservation timing and the grain size trends of holocene wave dom inated successions in two contrasting coastal plains the kujukuri and sendai using high resolution ams 14 c dating depositional facies and grain size modal grain size is proposed as a good criterion for eval uating qualitative changes of depositional depth the kujukuri and sendai successions are both fining upward to coarsening upward wave dominated successions that have developed upon ravinement surfaces the fining upward part of the kujukuri succession formed in regressive deposits during a highstand period whereas that of the sendai formed during transgression a difference in the timing of the generation of upward fining is due to the varying preservation potential of transgressive shoreface deposits on the ravi nement surface in the sendai area the potential was much higher than in the kujukuri area because the sendai ravinement surface had re ceived greater sediment supply from the adjacent rivers during trans gression a differing rate of tectonic uplift between the areas was not critical to the preservation potential because of the short time scale the successions show bimodal grain size distributions in the basal parts and unimodal distributions elsewhere the bimodality is consid ered to be the result of mixing of a coarse transgressive lag with finer sediment derived from external sources the finer mode forms a con sistent trend with unimodal grain sizes in overlying deposits the trend is of a simple upward coarsening in the kujukuri and an upward fining to coarsening in the sendai showing distinctive correlation with chang es of depositional depth thus the vertical trend of the modal grain size and not mean or median is best for evaluating changes of de positional depth in ancient wave dominated facies successions

metasomatised lower crustal and upper mantle xenoliths from north queensland chemical and isotopic evidence bearing on the composition and source of the fluid phase a diverse range of ultramafic and mafic granulite xenoliths from cone 32 north queensland display mineralogical and chemical evidence for enhanced fluid activities in the lowermost crust and uppermost mantle metasomatism of spinel lherzolite garnet and spinel websterite and garnet granulite produced variable amounts of amphibole mica locally resulting in development of hornblendite as the end product of reaction between spinel websterite and c o h fluids in contrast associated felsic and 2 pyroxene granulite xenoliths have remained relatively unaffected the garnet granulites and spinel websterites are interpreted as products of underplating of the lower crust by cenozoic magmatic activity whereas nd sr isotope systematics for the 2 pyroxene and felsic granulite xenoliths suggest that they may be of proterozoic age the development of amphibole and mica in the mafic and ultramafic xenoliths was accompanied by increases in tio2 na2o k2o a12o2 sr ba zr nb lree and fe2o3 feo and decreased mgo sio2 cr and ni virtually anhydrous spinel websterite xenoliths have 87sr86sr and 143nd144nd almost identical to amphibole rich variants and hornblendite and very similar values to the host nepheline mugearite and an amphibole megacryst the chemical and isotopic data suggest that the metasomatism is a relatively young feature caused by oxidized fluids exsolved from alkaline magmas similar to the host as they ascended through the lithosphere this style of metasomatism is considered a relatively localized phenomenon which is unlikely to provide an important contribution to the source regions of alkali basaltic magmas

groundwater flow in an arsenic contaminated aquifer mekong delta cambodia abstract to advance understanding of hydrological influences on as concentrations within groundwaters of southeast asia the flow system of an as rich aquifer on the mekong delta in cambodia where flow patterns have not been disturbed by irrigation well pumping was examined monitoring of water levels in a network of installed wells extending over a 50 km2 area indicates that groundwater flow is dominated by seasonally variable gradients developed between the river and the inland wetland basins while the gradient inverts annually net groundwater flow is from the wetlands to the river hydraulic parameters of the aquifer k 10 4 ms 1 and overlying clay aquitard k 10 8 ms 1 were determined using grain size permeameter and slug test analyses when coupled with observed gradients they indicate a net groundwater flow velocity of 0 04 0 4 ma 1 downward through the clay and 1 13 ma 1 horizontally within the sand aquifer producing aquifer residence times on the order 100 1000 a the results of numerical modeling support this conceptual model of the flow system and when integrated with observed spatial trends in dissolved as concentrations reveal that the shallow sediments upper 2 10 m of fine grained material are an important source of as to the underlying aquifer

physical mechanisms of the 1993 summer floods abstract the physical mechanisms responsible for the onset and maintenance of the 1993 summer floods were examined using the localized eliassen palm flux diagnostics and solutions of a single level primitive equation model linearized about a meridionally varying basic state the unusually long persistent summer pattern is linked with the marked transient eddy activity in late may and june the feedback of eddies in the time mean flow caused a strengthening and eastward extension of the pacific jet and a strengthening of the jet over north america results from the model suggest that the summer pattern may be interpreted as that of a lee trough forced by the rocky mountains in the presence of a strong westerly mean flow maintained by the eddies upstream composites from cases similar to that of the 1993 summer exhibit strong low level southerly flow cast of the rockies and suggest that the low level jet may be an important mechanism to sustain the anomalous rainfall it is concluded that the effect of the

large scale meteorological conditions associated with midlatitude mesoscale convective complexes abstract objective analyses of composited meteorological conditions attending ten mesoscale couvective complexes mcc reveal a number of distinctive characteristics and important interactions with the large scale environment the systems appear to be linked to eastward progression of a weak middle tropospheric short wave trough initial thunderstorms develop within a region of mesoscale convergence and lifting that is apparently forced primarily by low level warm advection the mcc system acquires mesoscale organization while it moves eastward ahead of the short wave trough diabatic heating eventually produces a system that is warm core in the middle troposphere and cold core in upper levels the mature mcc exhibits many similarities to tropical convective systems although it occurs within a considerably different large scale setting inflow within the lower half of the troposphere feeds convection within a region characterized by significant net upward mass flux and widespread precipitation thickne

electrical resistivity tomography determines the spatial distribution of clay layer thickness and aquifer vulnerability kandal province cambodia despite being rich in water resources many areas of south east asia face difficulties in securing clean water supply this is particularly problematic in regions with a rapidly growing population in this study the spatial variability of the thickness of a clay layer controlling surface groundwater interactions that affect aquifer vulnerability was investigated using electrical resistivity tomography ert data were acquired along two transects showing significant differences in the imaged resistivities borehole samples were analyzed regarding particle density and composition and linked to their resistivity the obtained relationships were used to translate the field electrical resistivities into lithologies those revealed considerable variations in the thickness of the clay layer ranging from 0 m up to 25 m geochemical data highlighting zones of increased ingress of surface water into the groundwater confirmed areas of discontinuities in the clay layer which act as preferential flow paths the results may guide urban planning of the phnom penh city expansion in order to supply the growing population with safe water the presented approach of using geophysics to estimate groundwater availability accessibility and vulnerability is not only applicable to kandal province cambodia but also to many other areas of fast urbanization in south east asia and beyond

mesoscale circulation growth under conditions of weak inertial instability abstract the hypothesis that inertial instability plays a role in the upscale development of mesoscale convective systems mcss is explored by sampling environments that supported the growth of mcss in the preliminary regional experiment for storm stormscale operational and research meteorology pre storm network with high quality special soundings secondary circulations that occurred in the presence of inertial instabilities were analyzed and documented using rawinsonde data with high spatial and temporal resolution from the pre storm field program additional examples of mcs environments were examined using data from the mesoscale analysis and prediction system results show strong divergence and cross stream accelerations occurred at upper tropospheric levels where inertial instabilities were present these accelerations were not uniform over the domain but were focused in the regions of instability also the analyses of these data showed that regions of inertial instability may be more commonpla

monsoon influenced variations in morphology and sediment of a mesotidal beach on the mekong river delta coast abstract a mesotidal beach on the mekong river delta coast exhibits unique patterns of river sediment discharge and experiences reversals of ocean wave directions in response to the summer and winter monsoons we analyzed long term changes of the shoreline since 1936 from past topographic maps and satellite images and short term changes in geomorphology and sedimentology by undertaking repeated field surveys between november 2005 and february 2008 along six shore normal beach transects the shoreline of the mekong river delta coast has changed asymmetrically over the last 70 years in response to net southwestward sediment transport related to dry northeasterly winter monsoons during the summer rainy season large volumes of sediments are discharged from the river at this time wave direction is reversed in response to the relatively weak southwesterly summer monsoon mud and very fine sand in the surface sediments of the northeastern updrift beach transects tend to be removed during winter suggesting that the sediment supplied from the river during summer is temporarily deposited near the river mouth and later transported southwestward during the winter monsoon the relief of intertidal bars on the beach increased during winter in response to higher waves however previous studies have suggested that higher waves flatten bars we hypothesize that the increase in relief that we observed is related to a wide and shallow subtidal delta front platform that attenuates ocean waves at all times other than at high tide the beach profile varies longshore the accreting to stable beach dips seaward at a gradient of about 1 80 with up to three longshore intertidal bars classified here as either slip face bars or low amplitude ridges in contrast the eroding beach profile is much steeper 1 20 1 40 and linear without intertidal bars and is similar in form to a low tide terrace because there is little variation of wave size and grain size within the study area we consider that the spatial variation of the beach profile is related to the availability of sediment accumulated on the beach

generation of forsteritic olivine fo99 8 by subsolidus oxidation in basaltic flows we identify olivine grains with compositions up to fo99 8 which are found in multiple primitive basaltic lava flows from a monogenetic volcano in the big pine volcanic field california usa in this study we show that the forsterite in these basalts formed by subsolidus recrystallization in a high fo2 environment olivine compositions are bimodal with flows having either all normal compositions fo74 9 94 4 or highly forsteritic fo97 2 99 8 compositions in many grains the subhedral forsteritic olivine has a hematite and clinopyroxene rim and internal parallel oriented planes of hematite clinopyroxene and orthopyroxene results of isotopic chemical crystallographic petrographic and mineralogical analyses show that the forsterite formed through subsolidus oxidation of olivine phenocrysts the forsteritic olivines generally occur in the thinner flows we infer that a rapidly emplaced sequence of thin vesicular spatter fed flows allowed the original olivine phenocrysts to become repeatedly reheated while exposed to air our study required sampling each flow so it was difficult to avoid the altered portions of the thinner flows other studies would tend to avoid such flows which may account for why such forsteritic olivines have not been more widely recognized

a method for simulating effects of quasi stationary wave anomalies on regional climate abstract a combined observational and numerical modeling methodology was employed to examine the interaction of large scale quasi stationary waves with regional forcing in the drought of 1988 and flood of 1993 in the north central united states the study implements and tests a new lateral boundary condition closure approach enabling regional climate modeling sensitivity studies to account for anomalies external to the simulated domain thirty day regional simulations of the continental united states using the mm5 regional model successfully reproduced the anomalous drought and flood conditions in an additional set of simulations the observed large scale quasi stationary wave anomalies in the dynamic and thermodynamic fields were filtered out through adjustment of the observationally based lateral boundary conditions in these filtered simulations the simulated meteorological fields over the drought flood areas for both years tended toward those for normal years suggesting that the proposed filtering te

effects of quasi stationary large scale anomalies on some mesoscale features associated with the 1993 flood a regional model simulation we investigate the effect of quasi stationary large scale anomalies on mesoscale factors contributing to the 1993 u s midwest flood by performing 30 day simulations with a mesoscale model the contributions of anomalous mesoscale features to the flood were quantified by contrasting model integrations forced by initial and lateral boundary conditions with and without large scale anomalous stationary waves that are believed to have been responsible for the flood supplementing previous investigations the present study finds the following 1 the large scale anomalies produced environmental conditions that favored a generation of mcc like systems rather than individual convective cells or squall lines that would have produced less precipitation over large areas ambient conditions favoring mesoscale convective complex mcc include an intensified east west orientation of quasistationary fronts weaker upper tropospheric inertial stability and a strong low level jet llj 2 the anomalous ageostrophic wind component to the far south of the flood region texas area and geostrophic wind component over the flood region bridged together to form an extended moisture conveyer that sustained the long lasting 1993 flood

a composite model of mesoscale convective complexes abstract a composite analysis technique is used to investigate the evolution of mesoscale features of mesoscale convective complexes mccs the early stage of the mcc lifecycle is characterized by convergence vertical motion and heating being centered in the lower troposphere as the mcc matures the level of peak upward motion and heating shifts to the upper troposphere the system achieves and maintains its maximum divergence upward motion and anticyclonic vorticity in the upper troposphere during the latter half of the life cycle this is in contrast to gate tropical clusters where the maximum divergence upward motion and anticyclonic vorticity occurred at the mature stage of the cluster and then weakened this difference might be explained by an mcc being an inertially stable form of mesoscale convective system whose radius exceeds the rossby radius of deformation the mcc is shown to be an efficient rain producer exhibiting a precipitation efficiency exceeding 100 at the mature stage due to th

multicomponent cubic oxide exsolution in synthetic basalts temperature dependence and implications for magnetic properties 1 although the compositional unmixing of cubic structured iron oxides has profound effects on the magnetic properties of rocks that contain them a basic understanding of the kinetics and thermodynamics of this process has not been achieved in experimental studies due to sluggish reaction rates in binary oxide phases exploiting the fact that many natural fe oxides contain multiple additional cations including ti mg and al we perform novel forward laboratory experiments in which cubic cubic phase exsolution proceeds from initially homogeneous multicomponent oxides a variety of fe ti mg al cubic iron oxides were nucleated and grown in synthetic multicomponent basalt under different fo2 environments and annealed at temperatures ranging from 590 790 c for up to 88 days fine scale lamellar intergrowths of fe ti al mg oxides interpreted to represent cubic phase exsolution were observed in seven samples one that was synthesized and annealed at approximately constant fo2 the quartz fayalite magnetite or qfm buffer and six that were synthesized at very oxidizing conditions qfm 6 log units and then annealed at moderately oxidizing qfm conditions results demonstrate that the consolute temperature of the multicomponent system is significantly higher than anneal temperatures and curie temperatures suggesting that samples that undergo this type of exsolution can carry a total thermal remanent magnetization exsolved samples are characterized by a dramatic increase in magnetization and coercivity and a shift in curie temperature s confirming predictions that this type of exsolution exerts strong control on the strength and stability of magnetization

geochemical sourcing of rock specimens and stone artifacts from nihoa and necker islands hawai i we report geochemical analysis of seven rock specimens and six stone artifacts from nihoa and necker islands hawai i the analyzed rock specimens exhibit chemistry consistent with published data sets and establish a major element distinction between nihoa and necker their trace element data indicate that the analyzed rocks and artifacts are local in origin and they reliably discriminate from sources on kaua i these preliminary results support emory s thesis that early archaic hawaiian colonists on nihoa and necker relied on their local basaltic stone to manufacture stone tools containers and other portable objects

late holocene depositional environments and coastal evolution of the mekong river delta southern vietnam abstract the mekong river delta in southern vietnam is a tide dominated delta and has been formed by the mekong river system the sediments are dominantly fine grained and were deposited in the receiving basin with slight inclination of pre existing deposits in the south china sea and gulf of thailand around 6000 to 5000 yr bp maximum holocene transgression occurred and seawaves surrounded uplands of the late pleistocene terrace basement rock and weathered land in the northern parts of the delta double notches were undercut by seawaves on the limestone hill in hatien at about 4 5 and 2 5 m above present sea level marine deposits prevailed throughout the delta during the highstand and regressions of relative sea level over the last 4550 yr bp delta progradation has produced a great flat plain of 62 520 km 2 this extremely fast progradation could be due to a very high sediment supply depositing in the slight inclination of the receiving basin b neotectonic movements and relative sea level changes c widespread mangrove forests playing an important role to enhance sediment accumulation former mangrove marshy deposits were significantly developed in the plain of reeds and longxuyen quadrangle mangrove marshes have also been developing well in the camau peninsula and present coastlines moreover regressions over the last 4550 yr bp have produced many distinct sets of relict beach ridges which are useful in distinguishing the former coastlines and evolutionary history of the delta

holocene delta evolution and sediment discharge of the mekong river southern vietnam abstract evolutionary changes delta progradation and sediment discharge of the mekong river delta southern vietnam during the late holocene are presented based on detailed analyses of samples from six boreholes on the lower delta plain sedimentological and chronostratigraphic analyses indicate clearly that the last 3 kyr were characterized by delta progradation under increasing wave influence southeastward sediment dispersal decreasing progradation rates beach ridge formation and steepening of the face of the delta front estimated sediment discharge of the mekong river for the last 3 kyr based on sediment volume analysis was 144 36 million t yr 1 on average or almost the same as the present level the constant rate of delta front migration and stable sediment discharge during the last 3 kyr indicate that a dramatic increase in sediment discharge owing to human activities as has been suggested for the yellow river watershed did not occur although southeast asian rivers have been considered candidates for such dramatic increases in discharge during the last 2 kyr the mekong river example although it is a typical large river of this region does not support this hypothesis therefore estimates of the millennial scale global pristine sediment flux to the oceans must be revised

mid holocene mangrove succession and its response to sea level change in the upper mekong river delta cambodia abstract middle holocene vegetation and mangrove successions are clearly evident in the palynological records of two cores from the upper mekong river delta in cambodia spanning from 9 4 to 6 3 cal ka bp the cores mainly record a transgressive sequence from floodplain freshwater marsh to tidal flat which was overlain by mangrove corresponding to the decelerated sea level rise at 8 3 cal ka bp pioneer mangrove species sonneratia alba and sonneratia caseolaris appeared in the sediments and then was replaced by regressive mangrove succession containing upward increasing abundances of rhizophora apiculata and bruguiera spp high salinity and flooding tolerant community s alba was developed at the western core site psg at 8 2 cal ka bp and the eastern core site pk at 7 5 cal ka bp the time difference of s alba appearance between the two sites might be resulted from the complexity of sedimentary environment where a higher sediment supply was provided to the western floodplain than to the eastern floodplain after 7 5 cal ka bp aggradational stacking of intertidal sediments of which the thickness is larger than the present maximum tidal range may have resulted from continuous sea level rise during 7 5 7 0 cal ka bp

spectral nudging to eliminate the effects of domain position and geometry in regional climate model simulations 1 it is well known that regional climate simulations are sensitive to the size and position of the domain chosen for calculations here we study the physical mechanisms of this sensitivity we conducted simulations with the regional atmospheric modeling system rams for june 2000 over north america at 50 km horizontal resolution using a 7500 km 5400 km grid and ncep ncar reanalysis as boundary conditions the position of the domain was displaced in several directions always maintaining the u s in the interior out of the buffer zone along the lateral boundaries circulation biases developed a large scale structure organized by the rocky mountains resulting from a systematic shifting of the synoptic wave trains that crossed the domain the distortion of the large scale circulation was produced by interaction of the modeled flow with the lateral boundaries of the nested domain and varied when the position of the grid was altered this changed the large scale environment among the different simulations and translated into diverse conditions for the development of the mesoscale processes that produce most of precipitation for the great plains in the summer season as a consequence precipitation results varied sometimes greatly among the experiments with the different grid positions to eliminate the dependence of results on the position of the domain we used spectral nudging of waves longer than 2500 km above the boundary layer moisture was not nudged at any level this constrained the synoptic scales to follow reanalysis while allowing the model to develop the small scale dynamics responsible for the rainfall nudging of the large scales successfully eliminated the variation of precipitation results when the grid was moved we suggest that this technique is necessary for all downscaling studies with regional models with domain sizes of a few thousand kilometers and larger embedded in global models