a deep tow magnetic survey of middle valley juan de fuca ridge 1 we report here results from a deep tow magnetic survey over middle valley juan de fuca ridge a series of track lines are combined to generate a high resolution map of the magnetic field anomaly within a 10 12 km region surrounding the bent hill massive sulfide bhms deposit a uniformly magnetized body 5 a m with a cross section approximating the body inferred from ocean drilling program odp drilling can account for the observed near bottom magnetic anomaly amplitude assuming this magnetization is entirely induced the average susceptibility 0 11 si corresponds to 3 5 magnetite pyrrhotite by volume consistent with the abundance of these phases observed in drill core samples however this uniform magnetization model significantly underestimates the magnetic anomaly measured a few meters above the seafloor by submersible indicating that the upper portion of the sulfide mound must have a significantly higher magnetization 10 magnetite pyrrhotite than at deeper levels on a larger scale the near bottom magnetic anomaly data show that basement magnetizations are not uniformly near zero as had been inferred from analysis of the sea surface anomaly pattern we interpret this heterogeneity as reflecting primarily differences in the degree of hydrothermal alteration our results highlight the potential of magnetic anomaly data for characterizing hydrothermal deposits where extensive drill core sampling is not available

hydrothermal alteration within the basement of the sedimented ridge environment of middle valley northern juan de fuca ridge the middle valley segment at the northern end of the juan de fuca ridge is a deep extensional rift blanketed with 200 500 m of pleistocene turbiditic sediment sites 857 and 858 were drilled during ocean drilling program leg 139 to determine whether these two sites were hydrologically linked end members of an active hydrothermal circulation system site 858 was placed in an area of active hydrothermal discharge with fluids up to 270 c venting through anhydrite bearing mounds on top of altered sediment the shallow basement of fine grained basalt that underlies the vents at site 858 is interpreted as a seamount that was subsequently buried by turbidites site 857 was placed 1 6 km south of the site 858 vents in a zone of high heat flow and numerous seismically imaged ridge parallel faults drilling at site 857 encountered sediments that are increasingly altered with depth and that overlie a series of mafic sills at depths of 460 940 m below sea floor sill margins and adjacent baked sediment are highly altered to magnesian chlorite and crosscut with veins filled with quartz chlorite sulfides epidote and wairakite the sill interiors vary from slightly altered with unaltered plagioclase and clinopyroxene in a mesostasis replaced by chlorite to local zones of intense alteration and brecciation in these latter zones the sill interiors are pervasively replaced by chlorite epidote quartz pyrite titanite and rare actinolite the most complete replacement is associated with brecciated horizons with low recovery and slickensides on fracture surfaces which we interpret as intersections between faults and the sills geochemically the alteration of the sill complex is reflected in significant whole rock depletions in ca sr and na with corresponding enrichments in mg al and most metals the latter results from the formation of conspicuous sulfide poikiloblasts in contrast metamorphism of the site 858 seamount includes incomplete albitization of plagioclase phenocrysts and replacement of sparse mafic phenocrysts much of the basement alteration at site 858 is confined to crosscutting veins except for a highly altered and veined horizon at the contact between basaltic basement and the overlying sediment the sill complex at site 857 is more highly depleted in 18 o 18 o 2 4 4 7 and more pervasively replaced by secondary minerals relative to the extrusives at site 858 18 o 4 5 5 5 there is no evidence of significant albitization of the plagioclase at site 857 suggesting high ca na in the pore fluids fluid inclusion data from hydrothermal minerals in altered mafic rocks and veins at sites 857 and 858 show a consistency of homogenization temperatures varying from 245 to 270 c which is within the range of temperatures observed for the fluids venting at site 858 the consistency of the fluid inclusion temperatures the lack of albitization within the site 857 sills and the apparently low water rock ratio collectively suggest that the sill complex at site 857 is in thermal equilibrium and being altered by a highly evolved ca rich fluid similar to the fluids now venting at site 858 the alteration evident in these two deep crustal drillsites is a result of the ongoing hydrothermal circulation and is consistent with downhole logging results instrumented borehole results and hydrothermal fluid chemistry the pervasive alteration of the laterally extensive sill sediment complex at site 857 determines the chemistry of the fluids that are venting at site 858 the limited alteration of the site 858 lavas suggests that this basement edifice acts as a penetrator or ventilator for the regional hydrothermal reservoir with much of the flow focussed at the highly altered and veined sediment basalt contact

why are marine magnetic anomalies suppressed over sedimented spreading centers the absence of lineated marine magnetic anomalies over sedimented recent spreading centers has been observed in several areas we propose that magnetic anomalies can be suppressed as a result of pervasive hydrothermal reactions underneath thick blankets of sediment the relatively impermeable sediment cover produces comparatively closed hydrothermal systems which increase the residence time of the hot fluids in basaltic layer 2 causing more thorough leaching of the remanence carrying iron titanium oxides and diminishing the marine magnetic anomalies this contrasts with lesser alteration at sparsely or unsedimented zones of extension with more open circulation conversely spreading ridges and oceanic crust characterized by absent or subdued lineated anomalies may signal sites of extensive hydrothermal mineralization this process might also provide an explanation for some magnetic quiet zones such as in the gulf of california the gulf of aden and the northern red sea

vegetation zones and diagnostic pollen profiles of a coastal peat swamp bocas del toro panama abstract a survey of the dominant vegetative cover of a large domed coastal swamp near changuinola in the province of bocas del toro panama has been undertaken as an initial step in reconstructing the holocene history of peat accumulation on this coast seven phasic communities of peat forming vegetation are defined and mapped 1 rhizophora mangle mangrove swamp 2 mixed back mangrove swamp 3 raphia taedigera palm swamp 4 mixed forest swamp 5 campnosperma panamensis forest swamp 6 sawgrass stunted forest swamp 7 myrica cyrilla bog plain pollen extracted from surface peat samples and collected from dominant vegetation at representative sites is used to prepare a pollen profile of each phasic community these profiles are then compared to pollen distribution in 2 peat cores one from the deep central part of the deposit and the second from a site near the marine margin in order to construct a history by floral succession of the 4000 year evolution of the deposit the changuinola mire originated as freshwater palm and hardwood forest swamps that developed in close proximity to both the changuinola river mouth probably behind a barrier bar and freshwater lagoon system adjacent to a low energy mangrove dominated bay the early swamp was likely drained to the southeast by brackish blackwater creeks much as it is today and formerly extended considerably farther in the direction of almirante bay the palm swamp was succeeded by hardwood forest swamp dominated by a very limited number of specialized species only one of which campnosperma panamensis is prone to forming monospecific stands increasing accumulation of woody peat promoted by the everwet climate impeded drainage of the mire leading to doming increased oligotrophy and establishment of bog plain conditions in a manner similar to that described by the anderson model of succession in the coastal swamps of malaysia and indonesia development of the changuinola mire did not require the initial mangrove phase which is common to the peat swamps of southeast asia as the palm raphia taedigera is able to colonise and institute peat accumulation in a variety of freshwater and brackish environments

heat flow variations correlated with buried basement topography on the juan de fuca ridge flank systematic heat flow variations are commonly observed on sedimented flanks of mid ocean ridges these variations have been attributed to the convective circulation of pore fluids within the oceanic crust and have been used to estimate the scale and pattern of this circulation here we report similar systematic variations observed in a detailed heat flow and seismic reflection survey over sediment covered 3 myr old sea floor on the eastern flank of the juan de fuca ridge we find that heat flow peaks are located above buried basement ridges and adjacent to two minor basement outcrops heat flow lows are found where thick sediments fill basement valleys the simple inverse relationship between heat flow and sediment thickness defined by the data indicates that hydrothermal circulation sealed within the permeable portion of the oceanic crust is vigorous enough to maintain approximately isothermal 10 k conditions at the sediment crust interface as no systematic variations in estimated basement temperatures are observed it seems that at this location the heat flow variations result only from sediment thickness variations and do not reflect a particular convective pattern

mineralogy of the ganges and brahmaputra rivers implications for river switching and late quaternary climate change late quaternary sediments of the bengal basin contain a history of river switching and climate change as revealed from sand and clay size mineralogy of boreholes and modern riverbed grabs epidote to garnet ratios e g in sand fraction sediments are diagnostic of source with high 1 e g indicating brahmaputra provenance and low 1 e g indicating ganges provenance in the clay fraction of surficial sediments smectite is diagnostic with high values 39 in the ganges and low values 3 in the brahmaputra in contrast the brahmaputra contains more kaolinite 29 vs 18 more illite 63 vs 41 and more chlorite 3 vs 1 than the ganges analysis of mineralogic and stratigraphic data indicates that the two rivers have changed position several times during the holocene extended periods of mixed river inputs appear to be isolated to the early holocene suggesting rapidly migrating braided channels during sea level lowstand tectonically driven accommodation in the sylhet basin may have contributed to the favored easterly course of the brahmaputra during much of the holocene relative abundances of illite and chlorite ic vs smectite and kaolinite sk record varying degrees of physical and chemical weathering respectively high ic values in early post glacial deposits suggest a dominance of physical weathering at that time however a general increase in sk concentrations throughout the holocene appears to reflect enhanced chemical weathering under increasingly warmer and more humid conditions notably a peak in sk concentrations that corresponds to an early holocene warm period 10 7 ka suggests that weathering patterns in the catchment respond quickly to climatic shifts

alteration processes at deep sea drilling project ocean drilling program hole 504b at the costa rica rift implications for magnetization of oceanic crust magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504b during ocean drilling program leg 111 our results combined with those from previous studies show abrupt first order changes in magnetic properties at alteration boundaries within the 504b crustal section changes in style and degree of alteration fall near the boundaries of the three well defined lithologic units the extrusive basalts the transition zone and the sheeted dike complex this postemplacement alteration heavily influences magnetic properties and is observed to change in both style and degree with depth within each lithologic unit our results indicate that at 504b the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration the upper more permeable basalts above this critical depth have not experienced this reheating within the sheeted dike complex the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth the changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth overall these observations suggest that the effective penetration of water into layer 2c decreased with increasing depth and resulted in a lower rate of convective cooling magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration the upper dikes have become magnetically more stable than the lower dikes a review of all magnetic properties indicate that the dike section at 504b carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict we suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions despite a magnetization which is lower than expected it appears that the sheeted dike complex at hole 504b is capable of making a substantial contribution to the overlying marine magnetic anomaly because of the systematic decrease in hydrothermal alteration of magnetic minerals the ability of the dike section to contribute decreases as a function of depth

taphonomic modes on fluvial deposits of the monte hermoso formation early pliocene buenos aires province argentina abstract the sedimentary deposits of the monte hermoso formation outcropping at its type locality farola monte hermoso buenos aires province argentina accumulated through fluvial processes acting on highly sinuous rivers the monte hermoso formation comprises a rich vertebrate fauna corresponding to the montehermosan stage age early pliocene which includes numerous and diverse vertebrate remains comparative taphonomic analysis indicated that the faunal assemblages coming from two different fossiliferous levels fl 1 and fl 2 recognized in floodplain deposits and channel deposits respectively are composed of remains affected by diverse taphonomic processes and therefore representing distinct taphonomic histories the variations recorded between both fossiliferous levels with respect to their taphonomic attributes and sedimentary context allowed the recognition of two distinct taphonomic modes floodplain in the fl 1 and channel lag in the fl 2 these taphonomic modes represent different conditions within the range of taphonomic histories for the monte hermoso formation assemblage formed by attritional accumulation of remains at the moment continental systems linked to fluvial environments have been poorly analyzed for the south american neogene this study contributes new information on taphonomic and paleoenvironmental characteristics of this type of deposit

a towed marine gravity meter for near bottom surveys gravity is measured presently on the sea surface and on the sea floor surface gravity suffers from loss of resolution over the deep ocean because the perturbing source masses are far from the observer bottom measurements recover this resolution but suffer from poor coverage because of the time needed for each measurement we have constructed a gravimetry system that combines the rapid data collection capability of a moving platform with the high resolution gained by locating the observations near the bottom this gravity sensor is tethered to a ship and towed just above the sea floor the instrument consists of a lacoste and romberg shipboard gravity meter modified to fit inside a pressure case that is mounted on a platform designed for towing stability we have tested it in a survey in the san diego trough a 1000 m deep sedimented valley in the pacific ocean in the california continental borderlands multiple gravity tracklines collected there at a depth of 935 m show a resolution of a few tenths of a mgal the new instrument will be useful for surveys of features whose lateral extent is equal to or less than the ocean depth

magnetic properties and opaque mineralogy of rocks from selected seafloor hydrothermal sites at oceanic ridges magnetic properties natural remanent magnetization nrm susceptibility curie point temperature tc saturation isothermal remanent magnetization irms and koenigsberger ratio q and opaque mineralogy were determined for basalts diabases gabbros peridotites and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites 15 n 17 n 23 n the snake pit hydrothermal field 26 n the tag hydrothermal field on the mid atlantic ridge and 42 n the sea cliff hydrothermal field on the northern gorda ridge the magnetomineralogy is interpreted in terms of deuteric hydrothermal metasomatic and ambient seawater alteration evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases gabbros and ultramafic rocks this lithologiec distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low temperature alteration in young surface basalts to more pervasive high temperature alteration in diabases gabbros and peridotites the studies reveal distinct correlations between magnetization intensity thermomagnetic behavior and magnetic mineralogy grain size style and intensity of alteration and rock type basalts have the highest nrm intensities and the lowest curie points 105 294 c basalts with nrm irms ratios less than 10 2 may have been remagnetized and have nrm intensities and q values much lower one third and one half respectively than basalts with nrm irms ratios of 10 2 or greater in the oceanic lithospheric suite examined remanent q 1 magnetization is present in basalts and induced magnetization q 1 occurs in metabasalts and the other rock types layer 2a basalts are the source of median valley magnetic anomalies the magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2b dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites this shift is influenced by fluctuations in the curie isotherm related to the duration of active magma chambers and convective hydrothermal cells the magnetic and mineralogie properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere magmatic cooling history and hydrothermal circulation

geophysical observations of the northern juan de fuca ridge system lessons in sea floor spreading by virtue of its proximity to the coastline of north america and to numerous oceanographic institutions the juan de fuca ridge has been the focus of a large number of marine geological geochemical and geophysical investigations systematic studies began in the early 1960 s with the geophysical survey of a d raff and r g mason which provided much of the foundation for the development of the extraordinarily successful paradigms of sea floor spreading and plate tectonics subsequent systematic and detailed studies of the plates and plate boundaries of the area by investigators from many academic industrial and government agencies including the geological survey of canada have provided the basis for much of the fundamental understanding we now have of global plate motions and the processes that are involved in the creation of new oceanic crust at sea floor spreading centres much of the success of these studies can be attributed to the geological diversity found along the juan de fuca ridge clear

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

reduced crustal magnetization beneath the active sulfide mound tag hydrothermal field mid atlantic ridge at 26 n abstract a detailed near bottom magnetic field survey was carried out by the submersiblealvin over the actively venting mound located within the tag trans atlantic geotraverse hydrothermal field on the mid atlantic ridge at 26 08 n 44 49 w three dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound this magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure in contrast the sea surface magnetic anomaly is associated with a broad 2 8 km magnetization low elongated along the axis that includes both the active and inactive mounds the short wavelength 250 m near bottom magnetic anomaly over the active mound is far too small to produce the long wavelength 8 km sea surface magnetic anomaly at tag however and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly other hypotheses including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely the existence of vigorous hydrothermal activity at tag and the lack of microseismic activity in the tag area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly the thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long wavelength sea surface anomaly pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long wavelength magnetic signal could be preserved in the crust the overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites these studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late stage intrusive bodies 1 while detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones only drilling will ultimately test these hypotheses

deep sea magnetic vector anomalies over the hakurei hydrothermal field and the bayonnaise knoll caldera izu ogasawara arc japan 1 we conducted deep sea magnetic measurements using autonomous underwater vehicles in the bayonnaise knoll caldera the izu ogasawara island arc which hosts the large hakurei hydrothermal field we improved the conventional correction method applied for removing the effect of vehicle magnetization thus greatly enhancing the precision of the resulting vector anomalies the magnetization distribution obtained from the vector anomaly data shows a 2 km wide belt of high magnetization trending nnw sse going through the caldera and a low magnetization zone 300 m by 500 m in area extending over the hakurei site comparison between the results obtained using the vector anomaly and the total intensity anomaly shows that the magnetic field is determined more accurately especially in areas of sparse data distribution when the vector anomaly rather than the total intensity anomaly is used we suggest a geologically motivated model that basaltic volcanism associated with the back arc rifting occurred after the formation of the caldera resulting in the formation of the high magnetization belt underneath the silicic caldera the hakurei hydrothermal field lies in the intersection of the basaltic volcanism belt and the caldera wall fault suggesting a mechanism that hot water generated by the heat of the volcanic activity has been spouting out through the caldera wall fault the deposit apparently extends beyond the low magnetization zone climbing up the caldera wall this may indicate that hot water rising from the deep through the alteration zone is transported laterally when it comes near the seafloor along fissures and fractures in the caldera wall

pollen distribution in marine sediments from the south eastern indonesian waters abstract pollen analysis on box core sediments from the south eastern indonesian waters provide information on trends in pollen transport and on pollen distribution in the marine surface sediments pollen and at a lower rate pteridophyta spore concentration values tend to decrease with increasing distance from the shoreline as a result pteridophyta percentages tend to increase with increasing distance from the shoreline mangrove pollen shows very little export from its distribution area while pollen from the mid upper montane group shows a substantial export from its onshore distribution area in general despite the complex nature of vegetation patterns and pollen transport in this region the onshore distribution of individual taxa and major vegetation types is well reflected in the pollen distribution in the marine sediments

high resolution magnetic signature of active hydrothermal systems in the back arc spreading region of the southern mariana trough high resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back arc spreading region of the southern mariana trough using shinkai 6500 a deep sea manned submersible a new 3 d forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization the results revealed that magnetic anomaly derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region the distribution of magnetic anomaly derived absolute magnetization suggests that all five andesite hosted hydrothermal fields are associated with a lack of magnetization as is generally observed at basalt hosted hydrothermal sites furthermore both the pika and urashima sites were found to have their own distinct low magnetization zones which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension the spatial extent of the resulting low magnetization is approximately 10 times wider at off axis sites than at on axis sites possibly reflecting larger accumulations of nonmagnetic sulfides stockwork zones and or alteration zones at the off axis sites

the holocene history and development of the tonle sap cambodia abstract the tonle sap the great lake of central cambodia is the central component of wetland ecosystems in the lower mekong river basin and is of enormous conservation value the lake s unusual hydraulic relationship with the mekong river and its consequent sensitivity to monsoon variability makes the tonle sap sensitive to climate change exploring the dynamics and development of this system under different climate regimes of the past offers a perspective on possible future impacts which is critical for sound management biostratigraphic and sedimentological data derived from cores of lake sediment indicate that during the period 7000 to ca 5500 14 c years before present the lake was less variable than present in terms of depth during the annual cycle of flood and may have been strongly influenced by saline tidal waters associated with higher than present seas levels as regional environments became drier and more seasonal in the late holocene more sediment was re suspended during the increasingly marked dry season lake level minimum lowering the effective sediment accumulation rate contrary to current interpretations of the history of the lake and associated wetland ecosystems the data presented here imply that regional hydraulic connections between the lake and the mekong river existed from at least the early holocene

taphonomy of the late cretaceous dinosaur bearing beds of the lano quarry iberian peninsula abstract the lano quarry located in the northern part of the iberian peninsula basque cantabrian region has yielded a diverse continental vertebrate assemblage from the late cretaceous late campanian to early maastrichtian the fossil assemblage consists of almost 40 species including bony fish amphibians lizards snakes turtles crocodilians dinosaurs pterosaurs and mammals lano is the most productive european vertebrate locality hitherto known from the late cretaceous the sedimentology of the lano section shows that the three associated fossiliferous beds known as l1a l1b and l2 were deposited in an alluvial system composed primarily of fluvial sands and silts the sedimentary structures are consistent with channel areas within an extensive braided river the l1a level was excavated in detail and mapped following a square metre grid system the main features of the multispecies vertebrate assemblage are as follows wide range of body size including microfossils large proportion of adult individuals rare articulated skeletal parts size sorting and dominance of small elements high spatial density long bones showing some preferential orientation variable dip variable degree of breakage and large proportion of splintering wide range of weathering and abrasion no evidence of predatory activity or chemical alteration on the basis of these features we suggest that the lano association lies between the taphonomic modes for attritional vertebrate assemblages in fluvial channels though it tends more toward the channel fill than the channel lag mode the l1a sample of lano is a heterogeneous assemblage of elements ranging from isolated bones and teeth to articulated skeletons with allochthonous and para autochthonous characteristics it consists of a mixture of vertebrates from different palaeocommunities and includes aquatic or semi aquatic elements and remains transported from a variety of habitats on the alluvial plain the dinosaur bones are commonly broken and show a greater degree of abrasion than those of freshwater vertebrates indicating that they may be allochthonous however actinopterygians amphibians pleurodiran turtles and eusuchian crocodilians are interpreted as being para autochthonous palaeobatrachid frogs bothremydid turtles and alligatoroid crocodilians are the most relatively abundant vertebrates of the lano fossil assemblage the occurrence of pelomedusid turtles and crocodilians is indicative of an intertropical warm climate

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

taphonomic modes in the maastrichtian continental deposits of the haţeg basin romania palaeoecological and palaeobiological inferences abstract the uppermost cretaceous continental deposits of the ha eg basin sw romania have yielded a high diversity vertebrate assemblage including the dwarf insular dinosaurs of nopcsa in 1902 franz nopcsa was the first to comment on the preservation patterns of vertebrate fossils suggesting that the most important fossil accumulations which he simply referred to as fossiliferous pockets were the result of the predatory activity of crocodilians crocodilian feeding grounds recent investigations of the fossil occurrences within the ha eg basin revealed a much wider range of taphonomic modes from microfossil bonebeds to isolated partially articulated skeletons than previously believed the survey of the vertebrate accumulation types and their sedimentary context documents a wide range of processes responsible for their genesis operating within a fluvial dominated upland setting study of the individual fossil accumulations yields important information on the palaeoecology composition of local biocenoses trophic interactions and palaeobiology social behaviour habitat preferences of the ha eg fossil assemblage

magnetization of axial lavas from the southern east pacific rise 14 23 s geochemical controls on magnetic properties although the spatial association of iron rich lavas and high amplitude magnetic anomalies is well documented a causal link between enhanced iron content and high remanent magnetization has been difficult to establish here we report magnetic data from approximately 250 samples with 8 16 feo total iron as feo from the southern east pacific rise epr that provide strong support for the presumed geochemical dependence of remanent intensity the limited age range 0 6 ka of axial lavas from this ultrafast spreading ridge 150 mm yr full rate effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low temperature alteration systematic sampling relative to the chilled margin illustrates that substantial grain size related variations in magnetic properties occur on a centimeter scale both microprobe data and curie temperatures suggest that the average groundmass titanomagnetite composition in the southern epr samples is approximately constant modal modified ulv6spinel content 0 67 over a wide range of lava compositions saturation magnetization and saturation remanence are highly correlated with feo r 0 73 and 0 83 respectively indicating that more iron rich lavas have higher abundances of otherwise similar titanomagnetite we show that there is a good correlation between natural remanent magnetization nrm and feo provided that sufficient specimens are used to determine the average nrm of a sample r 0 63 because the range of iron contents in mid ocean ridge basalts is limited the best fit slope 4 44 a m per feo in an ambient field of 0 030 mt should provide reasonable bounds on the equatorial magnetization of submarine lavas 10 a m at 8 5 feo and 50 a m at 16 feo finally we demonstrate that along axis variations in nrm closely parallel geochemical changes along the southern epr where magnetization values deviate significantly from those predicted from the range of measured feo contents these discrepancies may reflect additional unrecognized geochemical variability

multiple scales of hydrothermal circulation in middle valley northern juan de fuca ridge physical constraints and geologic models a synthesis of data including seafloor heat flow water column thermal anomalies shallow pore pressures pore water chemical profiles paired borehole observations and 3 5 khz records help to characterize distinct hydrothermal circulation systems within and surrounding two active vent areas in middle valley a sedimented rift on northern juan de fuca ridge we estimate the total heat and fluid budgets for a 260 km 2 area encompassing 17 km of ridge to be 274 mw 16 mw km 1 of ridge and 0 157 m 3 s 1 respectively about half of the seafloor heat loss is conductive requiring efficient lateral fluid and heat transport in basement lateral head gradients fluid driving forces in basement between the primary fluid discharge site and the surrounding formation are very low simple mass balance and flow calculations indicate that the majority of the recharge to the primary hydrothermal reservoir cannot occur through sediments immediately around the vent area recharge is probably focused along valley bounding normal 10 12 2 faults crustal scale lateral permeability in basement must be 10 to 10 m in order to allow rapid flow and maintain nearly isothermal temperatures over many kilometers on a smaller scale heat flow patterns within the main vent area are controlled by the depth to the primary hydrothermal reservoir proximity to active vents and local recharge of a secondary circulation system shallow underpressures and isolated zones of very low heat flow result from vigorous secondary circulation driven by pressure gradients associated with venting models of secondary circulation suggest that most of the overpressure observed in hydrothermal basement is lost when the fluid enters the vents and that cooler sediment pore fluids flow toward and into the vents as a result of shallow underpressuring secondary circulation results in significant heat flow reduction at the seafloor but adds minor sediment pore fluid to vent discharge

vertebrate preservation in fluvial channels abstract two taphonomic modes for attritional vertebrate assemblages in channels are proposed based on the sedimentary context of the vertebrate remains and taphonomic features of the bones themselves the channel lag mode includes bones that are buried with coarse lithologies near the bases of active channels the channel fill mode occurs in fine grained to mixed fills of abandoned channels the extreme for a channel lag assemblage would be a cluster of allochthonous abraded unidentifiable fragments and the extreme for a channel fill assemblage would be a cluster of autochthonous unbraded complete skeletons between these extremes there is a broad spectrum of possible taphonomic histories for bones in channels but distinct channel lag vs channel fill modes can be recognized in fluvial deposits in different tectonic and climatic settings throughout the phanerozoic physical and biological processes that affect the different modes produce different samples of vertebrate paleocommunities with bones in the channel lag mode representing transported remains from a variety of habitats whereas channel fill assemblages are more autochthonous and habitat specific channel facies channel pattern and alluvial architecture are used to develop hypotheses concerning how the taphonomic modes relate to different scales of fluvial processes fluvial systems with numerous abandoned channels provide more sites for preservation of relatively complete fossil vertebrates in channel fills while systems that continually rework sediments by lateral migration preserve more vertebrate remains as channel lags large scale physical controls on channel pattern and fluvial architecture probably have had significant effects on the quality and quantity of the vertebrate record throughout the history of land vertebrates taphonomic modes provide a basis for comparing faunas with similar preservational histories throughout the geologic record and they can help to minimize biases in important paleobiological parameters such as diversity estimates and the timing of appearance and extinction events

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

holocene uplift derived from relative sea level records along the coast of western kobe japan abstract mid to late holocene sea level observations have been obtained from tamatsu and tarumi in western kobe japan in this tectonically active area we evaluate the local tectonic effect during the holocene by comparison between observations and predictions of sea level variations for the purpose of this study we analysed the diatom assemblages and carried out sulphur analysis of sediments these were used along with radiocarbon dates to derive relative sea level variations for the sites a rate of tectonic uplift of between 0 1 and 0 3 mm per year corrected for the prediction fits well with relative sea level rsl index points of tamatsu at 6415 5320 cal bp depending on the altitude of the rsl index point at about 5700 cal bp the best estimate of the rate of tectonic uplift for tamatsu is calculated as 0 11 0 25 mm yr over the period concerned a rate of tectonic uplift between 0 3 and 0 5 mm per year is derived for tarumi at about 7300 and 3835 3365 cal bp these values may explain the height distributions of the late pleistocene marine terraces along the coast of the western kobe providing a first approximation for rates of local crustal movements during the late quarternary

fine scale magnetic anomaly field over the southern juan de fuca ridge axial magnetization low and implications for crustal structure sea surface deep tow and submersible magnetic surveys show that the spreading axis of the cleft segment of the southern juan de fuca ridge sjdf is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent vance segment and most mid ocean ridge spreading centers forward iterative thickness modeling based on the near bottom magnetic data shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the cleft axial valley thick crust is required to produce the magnetic anomaly high over the vance axial valley this magnetic modeling demonstrates that variations in the base of the magnetic source layer which is assumed to be equivalent to the base of the extrusive lavas can produce a significant proportion of the observed short wavelength magnetic anomaly signal these magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2a thins beneath the axial valley of the cleft and is thicker beneath the vance axial valley recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading which could produce significant magnetic contrasts the results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration first order change in upper crustal thickness is one of the most important variables to consider in the production of fine scale magnetic anomalies in young ocean crust

variable ti content and grain size of titanomagnetite as a function of cooling rate in very young morb transmission electron microscopy observations and rock magnetic measurements on a pillow from the new flow extruded in 1993 at the juan de fuca ridge demonstrate that ti content in large up to 40 m titanomagnetite grains varies as a function of the cooling rate large grains in the interior of the pillow have a narrow composition range of approximately x 0 6 where x represents the ulvospinel content in the solid solution series with magnetite in contrast the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range average x 0 45 within 0 5 cm of the rim a significant portion of the grains is single domain sd to superparamagnetic sp and appears to have little or no ti this lower ti content provides a ready explanation for the higher curie temperature near the glassy margin of the pillow moreover determination of the oxidation state of the titanomagnetite does not show low temperature oxidation anywhere within the pillow indicating no rapid alteration to titanomaghemite as previously suggested for this very young mid ocean ridge basalt morb in order to explain the curie temperatures submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variable x values these grains make contributions to susceptibility that show up as high up to 580 c curie temperatures in thermomagnetic analyses and they appear to be sd to sp the variation of x values for the titanomagnetite in the morb is consistent with thermodynamic and kinetic considerations because crystallization of titanomagnetite in the morb is controlled by processes depending on cooling rate and crystal melt fractionation

what causes low magnetization at basalt hosted hydrothermal sites insights from inactive site krasnov mar 16 38 n high resolution magnetic surveys acquired near the seafloor show that active basalt hosted hydrothermal sites are associated with zones of lower magnetization this observation may reflect the thermal demagnetization of a hot hydrothermal zone the alteration of basalt affected by hydrothermal circulation and or the presence of thick nonmagnetic hydrothermal deposits in order to discriminate among these inferences we acquired vector magnetic data 50 m above inactive hydrothermal site krasnov using the remotely operated vehicle rov victor this deep hydrothermal site located 7 km east of the mid atlantic ridge mar axis at 16 38 n is dissected by major normal faults and shows no evidence of recent hydrothermal activity it is therefore a perfect target for investigating the magnetic signature of an inactive basalt hosted hydrothermal site krasnov exhibits a strong negative magnetic anomaly which implies that the lower magnetization observed at basalt hosted hydrothermal sites is not a transient effect associated with hydrothermal activity but remains after activity ceases thermal demagnetization plays only a secondary role if any in the observed magnetic low forward models suggest that both the nonmagnetic hydrothermal deposits and an altered zone of demagnetized basalt are required to account for the observed magnetic low the permanence of this magnetic signature makes it a useful tool to explore midocean ridges and detect inactive hydrothermal sites

fine scale heat flow shallow heat sources and decoupled circulation systems at two sea floor hydrothermal sites middle valley northern juan de fuca ridge fine scale heat flow patterns at two areas of active venting in middle valley a sedimented rift on the northern juan de fuca ridge provide thermal evidence of shallow hydrothermal reservoirs beneath the vent fields the extreme variability of heat flow is explained by conductive heating immediately adjacent to vents and shallow circulation within sediments above the reservoir this secondary circulation is hydrologically separated from the deeper system feeding the vents by a shallow conductive lid within the sediments a similar separation of shallow and deep circulation may also occur at sediment free ridge crest hydrothermal environments

abrupt changes in the asian southwest monsoon during the holocene and their links to the north atlantic ocean during the last ice age the indian ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial scale climate events in the north atlantic region1 2 3 suggesting a mechanistic link in the holocene epoch which had a more stable climate the amplitude of abrupt changes in north atlantic climate was much smaller and it has been unclear whether these changes are related to monsoon variability here we present a continuous record of centennial scale monsoon variability throughout the holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic arabian sea our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the north atlantic region4 including the most recent climate changes from the medieval warm period to the little ice age and then to the present we therefore suggest that the link between north atlantic climate and the asian monsoon is a persistent aspect of global climate

a 40 000 year palynological record from north east thailand implications for biogeography and palaeo environmental reconstruction abstract there are few palaeoenvironmental data from the indo chinese peninsula that document environmental and climatic conditions prior to the holocene this study of pollen and spores from a small peat swamp in north east thailand provides a record of vegetation change covering ca 40 000 years b p before present these data indicate that the region supported a fagaceous coniferous forest similar to contemporary vegetation described from south west china climatic conditions were cooler and probably drier than present day climates in thailand levels of biomass burning are relatively high throughout this period tropical broad leaf deciduous forest becomes dominant at the pleistocene holocene boundary with the fragmentation of fagaceous coniferous forest in response to increased temperatures

crustal magnetization and the subseafloor structure of the ashes vent field axial seamount juan de fuca ridge implications for the investigation of hydrothermal sites

morphology and tectonics of the andaman forearc northeastern indian ocean summary the andaman sea has developed as the result of highly oblique subduction at the western sunda trench leading to partitioning of convergence into trench perpendicular and trench parallel components and the formation of a northward moving sliver plate to accommodate the trench parallel motion the andaman forearc contains structures resulting from both components of motion the main elements of the forearc are the accretionary prism and outerarc ridge a series of forearc basins and major n s faults the accretionary prism is an imbricate stack of fault slices and folds consisting of ophiolites and sediments scrapped off the subducting indian plate the western outer slope of the accretionary prism is very steep rising to depths of 1500 2000 m within a distance of 30 km there is a difference in the short wavelength morphology between the western and eastern portions of the accretionary prism the outer portion consists of a series of faulted anticlines and synclines with amplitudes of a few 100 to 1000 m and widths of 5 15 km resulting from ongoing deformation of the sediments the inner portion is smoother with lower slopes and forms a strong backstop the width of the deforming portion of the accretionary prism narrows from 80 to 100 km in the south to about 40 km between 10 n and 11 30 n it remains at about 40 km to 14 40 n north of there the inner trench wall becomes a single steep slope up to the myanmar shelf the eastern edge of the outerarc ridge is fault bounded and north of the nicobar islands a forearc basin is located immediately to the east a deep gravity low with very steep gradients lies directly over the forearc basin the west andaman fault waf and or the seulimeum strand of the sumatra fault system form the boundary between the burma and sunda plates south of the andaman spreading centre the waf is the most prominent morphologic feature of the andaman sea and divides the sea into a shallow forearc and a deeper backarc region the diligent fault runs through the forearc basin east of little andaman island although it has the general appearance of a normal fault multichannel seismic data show that it is a compressional feature that probably resulted from deformation of the hanging wall of the eastern margin fault this could occur if the forearc basins were formed by subduction erosion of the underlying crust rather than by east west extension

seafloor gravity evidence for hydrothermal alteration of the sediments in middle valley juan de fuca ridge abstract gravity data collected at two different levels sea surface and seafloor are compared and interpreted to characterize the effect of hydrothermal circulation on the sediment density in middle valley a deeply sedimented spreading center on the juan de fuca ridge the sea surface gravity data constrain density variations at depth beneath the seafloor whereas sea bottom measurements are more sensitive to shallow sources at least two different types of hydrothermal signatures in the sediments can be distinguished from the gravity data short wavelength anomalies associated with sulfide deposits and broader anomalies associated with areas of lithified sediments in middle valley three distinct gravity anomalies were identified a an anomaly over a sulfide mound south of bent hill shows that the sulfide body extends to depths of 120 to 180 m and has been fed by several near surface conduits b an anomaly at the base of the fault bordering the valley to the east is interpreted as a regional anomaly combined with the local effect of lithified sediments and possibly sulfide deposits c an anomaly paralleling an intra valley fault that limits the deepest part of the graben is interpreted as indicating lithification of the upper sediment layer a high heat flow anomaly is located 1 to 2 km east of this fault suggesting that sediment lithification occurred in a wide band above the fault and eastward to the current high heat flow area due to the progressive migration of the hot fluid circulation

holocene monsoon changes inferred from lake sediment pollen and carbonate records northeastern cambodia major holocene monsoon changes in continental southeast asia are reconstructed from analysis of 14c dated changes in pollen and organic inorganic carbon in sediment cores taken from permanent closed basin volcanic lakes in ratanakiri province northeastern cambodia analysis focuses on the nature and timing of monsoon changes inferred from changes in vegetation and lake conditions these data provide the first well dated palynological record covering most of the holocene and continuous up to the present from a terrestrial site in mainland southeast asia the record from a 15 m core retrieved from kara lake representing the last 9300 years shows that the late glacial conditions ended about 8500 14c yr b p more than 1000 years later than sites in southwest china summer monsoon intensity increased over the period ca 8400 5300 14c yr b p similar to most other sites in the asian monsoon region a subsequent expansion of secondary forests at the expense of dense semievergreen forests suggest a drier climate leading to more frequent fire disturbance after ca 3500 14c yr b p disturbance frequency may have increased further with increasing seasonality from ca 2500 14c yr b p to the present dense forest has recovered in a mosaic with annually burned dry forest but climate may not be the main control on local vegetation dynamics in the late holocene

vertebrate taphonomy and paleoecology in an upper pleistocene tank deposit of paraiba brazil taphonomic modes evidence of temporal and spatial resolutions and paleoecological patterns of the brazilian intertropical region abstract curimatas paleontological site cps preserved one of the most iconic fossil assemblages of late pleistocene vertebrates of brazil fossils were collected in a tank deposit a singular type of quaternary vertebrate bearing deposit of south america in this work a detailed taphonomic study of the tank assemblage of cps is presented this analysis also provides taphonomic and paleoecological data obtained through the application of common multivariate techniques and provides estimates of time and spatial resolution for tank deposits the results show that although the cps assemblage has experienced an array of taphonomic processes the long distance transport was the main process controlling the taphonomic pathway of the thanatocoenosis during the biostratinomic phase and was responsible for the co occurrence of other processes such as abrasion fragmentation and bioclastic sorting the origin of the taphonomic modes of vertebrates in the tank deposit of cps may have been controlled by three major taphonomic phenomena i a taphonomic bias towards the large bodied size ii long time span of transportation towards the final burial environment and iii some degree of reworking furthermore the analysis of these taphonomic modes supports a previous hypothesis although taphonomic biases affect the origin of tank assemblages paleoecological patterns can be preserved with considerable fidelity the time resolution in tank accumulations is of the order of 10 3 to 104 years and their spatial resolution seems to be from local to regional

investigating the history of east asian monsoon and climate during the last glacial interglacial period 0 140 000 years mineralogy and geochemistry of odp sites 1143 and 1144 south china sea abstract monsoon climate is an important component of the global climatic system a comprehensive understanding of its variability over glacial interglacial time scales as well as of its effects on the continent and in the ocean is required to decipher links between climate continental weathering and productivity a detailed multiproxy study including bulk and clay mineralogy grain size analysis phosphorus geochemistry sedex extraction organic matter characterization and nitrogen stable isotopes was carried out on samples from odp sites 1143 and 1144 leg 184 south china sea covering the past 140 000 years we tentatively reconstruct the complex sedimentation and climatic history of the region during the last glacial interglacial cycle when sea level variations linked to the growth and melting of ice caps interact with monsoon variability during interglacial periods of high sea level summer monsoon was strong and humid and warm climate characterized the adjacent continent and islands clay minerals bear signals of chemical weathering during these intervals high calcite and reactive phosphorus mass accumulation rates mars indicate high productivity especially in the southern region of the basin during glacial intervals strong winter monsoon provided enhanced detrital input from the continent as indicated by high detrital mar glacial low sea level resulted in erosion of sediments from the exposed sunda shelf to the south and clay mineral variations indicate that warm and humid conditions still prevailed in the southern tropical areas enhanced supply of nutrients from the continent both by river and eolian input maintained high primary productivity reduced circulation during these periods possibly induced active remobilization of nutrients such as phosphorus from the sediments intense and short cold periods recorded during glacial and interglacial stages correlate with loess records in china and marine climatic records in the north atlantic confirming a teleconnection between low and high latitude climate variability

ages and magnetic structures of the south china sea constrained by deep tow magnetic surveys and iodp expedition 349 combined analyses of deep tow magnetic anomalies and international ocean discovery program expedition 349 cores show that initial seafloor spreading started around 33 ma in the northeastern south china sea scs but varied slightly by 1 2 myr along the northern continent ocean boundary cob a southward ridge jump of approximate to 20 km occurred around 23 6 ma in the east subbasin this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the southwest subbasin which propagated for about 400 km southwestward from approximate to 23 6 to approximate to 21 5 ma the terminal age of seafloor spreading is approximate to 15 ma in the east subbasin and approximate to 16 ma in the southwest subbasin the full spreading rate in the east subbasin varied largely from approximate to 20 to approximate to 80 km myr but mostly decreased with time except for the period between approximate to 26 0 ma and the ridge jump approximate to 23 6 ma within which the rate was the fastest at approximate to 70 km myr on average the spreading rates are not correlated in most cases to magnetic anomaly amplitudes that reflect basement magnetization contrasts shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers in addition to large vertical intensity variations these complexities are caused by late stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0 5 which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization the primary contribution to magnetic anomalies of the scs is not in the top 100 m of the igneous basement

late quaternary peat formation and vegetation dynamics in a lowland tropical swamp nee soon singapore abstract cores of sediment from nee soon a peat forming freshwater swamp located in the perimarine zone of singapore have yielded a record of vegetation history that comprises parts of the last glacial and holocene periods the evidence includes two conventional and four ams radiocarbon dates the oldest of which is 23 050 330 b p uncalibrated radiocarbon years and variations in the abundances of sub fossil pollen and spores and charcoal the results of a study of the relationship between contemporary vegetation and modern pollen accumulation facilitated interpretation of sub fossil tree pollen data peat formation occurred at nee soon during the last glacial when sea levels on the sunda continental shelf were well below those at present the occurrence of montane pollen types notably podocarpaceae in sediment samples dated to the late glacial and early holocene suggests either temperatures substantially lower than those of the present and possibly humid conditions or cold seasonally dry climates and reduced levels of atmospheric co 2 the abundance of charcoal indicates the widespread occurrence of fires throughout the entire period represented by the cores for the mid holocene the evidence indicates an increased saline influence presumably as a result of relatively high sea levels falling sea levels during the late holocene appear to have preceded the onset of the most recent period of peat formation at nee soon the evidence confirms that long last glacial interglacial records of lowland vegetation history are available from terrestrial locations in the humid core of southeast asia and suggests that the complexity of perimarine swamp dynamics is not captured fully by anderson s model which assumes a close relationship between peat formation and sea level

origin of magnetic highs at ultramafic hosted hydrothermal systems insights from the yokoniwa site of central indian ridge high resolution vector magnetic measurements were performed on an inactive ultramafic hosted hydrothermal vent field called yokoniwa hydrothermal field yhf using a deep sea manned submersible shinkai6500 and an autonomous underwater vehicle r2d4 the yhf has developed at a non transform offset massif of the central indian ridge dead chimneys were widely observed around the yhf along with a very weak venting of low temperature fluids so that hydrothermal activity of the yhf was almost finished the distribution of crustal magnetization from the magnetic anomaly revealed that the yhf is associated with enhanced magnetization as seen at the ultramafic hosted rainbow and ashadze 1 hydrothermal sites of the mid atlantic ridge the results of rock magnetic analysis on seafloor rock samples including basalt dolerite gabbro serpentinized peridotite and hydrothermal sulfide showed that only highly serpentinized peridotite carries high magnetic susceptibility and that the natural remanent magnetization intensity can explain the high magnetization of yokoniwa these observations reflect abundant and strongly magnetized magnetite grains within the highly serpentinized peridotite comparisons with the rainbow and ashadze 1 suggest that in ultramafic hosted hydrothermal systems strongly magnetized magnetite and pyrrhotite form during the progression of hydrothermal alteration of peridotite after the completion of serpentinization and production of hydrogen pyrrhotites convert into pyrite or nonmagnetic iron sulfides which considerably reduces their levels of magnetization our results revealed origins of the magnetic high and the development of subsurface chemical processes in ultramafic hosted hydrothermal systems furthermore the results highlight the use of near seafloor magnetic field measurements as a powerful tool for detecting and characterizing seafloor hydrothermal systems

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

formation of magnetic single domain magnetite in ocean ridge basalts with implications for sea floor magnetism although magnetic data are the primary evidence for ocean floor spreading the processes by which magnetic phases in ocean floor basalts are formed remain poorly constrained scanning transmission electron microscopic observations show that magnetic single domain magnetite in sheeted dike basalts of deep sea drilling project hole 504b formed through oxidation exsolution of ilmenite exsolution of ulvospinel lamellae and recrystallization of end member magnetite by interaction with convecting fluids the data suggest that the sheeted dike basalts with single domain magnetite as an efficient and stable magnetic carrier contribute significantly to sea floor magnetism

massive sulfides in a sedimented rift valley northern juan de fuca ridge a number of mounds each several hundred meters across and up to sixty meters high have been observed with seamarc ii acoustic imagery and seabeam bathymetry in the sediment filled axial valley at the northern end of the juan de fuca ridge the mounds are located a few kilometers west of the eastern valley bounding normal fault scarp where the local sediment fill is approximately 300 m thick all of the mounds are believed to be of hydrothermal origin and one is associated with anomalously high heat flow in excess of 1 w m 2 a piston core collected from that mound comprises coarse clastic sulfide units interbedded with sulfidic muds hydrothermal minerals present in the 2 3 m section include pyrrhotite pyrite marcasite sphalerite chalcopyrite iss intermediate solid solution in the cufezns system chalcopyrrhotite galena talc barite and amorphous silica mineral fabrics of the clasts indicate that the material was precipitated at or near the sea floor by mixing of hot hydrothermal fluids with cold seawater low concentrations of zn cu cd and ag relative to those found in unsedimented ridge hydrothermal deposits and the presence of pyrrhotite as an early phase mineral indicates that the vent fluids have been modified by reaction with sediments beneath the mound rapid sedimentation in a rift valley is clearly conducive to the formation of large hydrothermal mineral deposits like those believed to be present within and beneath these mounds the relatively impermeable sediment cover insulates the crust inhibits groundwater recharge promotes long lived discharge at a restricted number of sites provides a substrate for the efficient subsurface precipitation of minerals and through continued sedimentation protects surficial deposits from the corrosive effects of seawater no reliable estimate of the bulk composition of the mounds can be made with existing data but their size is comparable to major hydrothermal mineral deposits found on land ancient settings in which many land deposits formed are in many ways similar to the one in which the features described here are currently forming

climate variability in central china over the last 1270 years revealed by high resolution stalagmite records oxygen and carbon isotopic variations in the upper section of a stalagmite sf 1 from buddha cave 33 40 n 109 05 e dated by 230th 234u 210pb and lamination counting to a time resolution as fine as 1 3 years have recorded climate changes in central china for the last 1270 years the changes include those corresponding to the medieval warm period little ice age and 20th century warming lending support to the global extent of these events the isotopic records also show cycles of 33 22 11 9 6 and 7 2 years the 33 year cycle could well represent the 35 year periodicity of climate fluctuations previously recognized in china and europe cycles of 22 11 and 9 6 years have often been associated with the sunspot or lunar orbit variations the 7 2 year cycle was recently identified also in tree ring records from an area close to buddha cave these cycles suggest that external forcing e g solar irradiance may affect the summer monsoon over eastern china the general consistency between the climate characteristics inferred from the stable isotope records of sf 1 and those from other proxy records underscores the value of stalagmites as recorders of paleoclimate