building of the amsterdam saint paul plateau a 10 myr history of a ridge hot spot interaction and variations in the strength of the hot spot source the amsterdam saint paul plateau results from a 10 myr interaction between the south east indian ridge and the amsterdam saint paul hot spot during this period of time the structure of the plateau changed as a consequence of changes in both the ridge hot spot relative distance and in the strength of the hot spot source the joint analysis of gravity derived crust thickness and bathymetry reveals that the plateau started to form at 10 ma by an increase of the crustal production at the ridge axis due to the nearby hot spot this phase which lasted 3 4 myr corresponds to a period of a strong hot spot source maybe due to a high temperature or material flux and decreasing ridge hot spot distance a second phase between 6 and 3 ma corresponds to a decrease in the ridge crustal production during this period the hot spot center was close to the ridge axis and this reduced magmatic activity suggests a weak hot spot source at 3 ma the ridge was located approximately above the hot spot center an increase in the hot spot source strength then resulted in the building of the shallower part of the plateau the variations of the melt production at the ridge axis through time resulted in variations in crustal thickness but also in changes in the ridge morphology the two periods of increased melt production correspond to smooth ridge morphology characterized by axial highs while the intermediate period corresponds to a rougher rift valley morphology these variations reveal changes in axial thermal structure due to higher melting production rates and temperatures

fe si oxyhydroxide deposits at a slow spreading centre with thickened oceanic crust the lilliput hydrothermal field 9 33 s mid atlantic ridge diffuse and focused low temperature fluids emanate at 9 33 s mid atlantic ridge and precipitate fe sioxyhydroxides that form chimneys mounds and flat lying deposits this extensive vent field named lilliput lies at the axial zone of a spreading segment with a significantly thickened crust 11 km theoretically much more heat needs to be removed from a thick crust spreading center compared to a spreading center with typical thickness of 6 km therefore settings with thickened crust should be favourable for supporting very powerful hydrothermal systems capable of producing large mineral deposits this is the first report on the composition of seafloor hydrothermal deposits at abnormally thickened oceanic crust due to hotspot ridge interaction our studies revealed that generally the lilliput hydrothermal deposits are very similar in morphology structure composition and lateral extent to other low temperature hydrothermal deposits of mid ocean ridges and intraplate volcanoes deposits at the lilliput vent field are composed of si containing goethite and ferrihydrite have very low contents of a number of transition and rare earth elements and show ree distribution patterns with negative ce and eu anomalies the speciation and precipitation of the main deposit forming elements fe and si at the hydrothermal field appear to be partially controlled by live microbes and exuded organic compounds the 18o values of the precipitated silica containing feoxyhydroxides point to low temperature formation and sr nd pb isotope variations suggest that the hydrothermal precipitates scavenged metals predominantly from the ambient seawater these findings are in agreement with the biogeochemical scenario for their precipitation

low temperature fluid flow through sulfidic sediments from tag modification of fluid chemistry and alteration of mineral deposits low temperature fluid flow has been identified as an important component of the heat mass and chemical flux at sites of active venting one way to study the movement of water through hydrothermal deposits is to analyse the pore fluids extracted from sulfidic sediments the composition of diffuse flow fluids in one sediment core from the south of the active tag mound reflects the dilution of high temperature black smoker fluids with modification inside the mound during ongoing circulation fluid modification includes conductive cooling removal of si from solution and entrainment and removal of u from seawater which can represent significant sinks of these elements precipitation of clay minerals and fe oxides is currently occurring within this sediment core generating ochreous type material subsurface extensive low temperature alteration of basaltic material at the core top is apparent in conjunction with supergene enrichment of cu and mn

breakup between australia and antarctica a brief review in the light of new data abstract the arguments justifying the revised timing of breakup between australia and antarctica cande and mutter 1982 and the reconstruction of broken ridge and kerguelen plateau mutter and cande 1983 are reviewed and considered with respect to new subsidence data the age of breakup was revised from anomaly 22 time 55 my b p to anomaly 34 time 85 my b p the rough topography of the diamantina zone can be attributed to very slow spreading 5 mm yr beginning between the times of anomaly 34 and anomaly 19 the reconstruction of broken ridge and kerguelen plateau at anomaly 34 time shows overlap of these two features but the overlap problem is nearly resolved by anomaly 18 time 42 my b p normal seafloor spreading rates 22 mm yr commenced at anomaly 19 time 43 my b p subsidence patterns calculated from biostratigraphic data from wells drilled along australia s southern margin are interpreted as more consistent with the revised age of australia antarctic breakup subsidence curves systematically show rapid subsidence associated with the rift phase of margin development followed by much slower thermally controlled subsidence during the drift phase the timing of the rift to drift transition is believed to coincide with the age of breakup 60 to 110 my b p in addition the subsidence curves indicate a west to east propagation of breakup along the southern margin magnetic anomaly patterns and stratigraphie observations are consistent with this hypothesis

submarine basalts of the northern kerguelen plateau interaction between the kerguelen plume and the southeast indian ridge revealed at odp site 1140 during ocean drilling program leg 183 basaltic cores were retrieved from the northern kerguelen plateau nkp at site 1140 on the extreme north of the plateau 270 km north of the kerguelen archipelago amongst the six basement units recovered five were pillow basalts with fresh glassy rims together with flow lobes this is the first evidence for submarine eruption anywhere on the kerguelen plateau each flow unit has distinct geochemical and isotopic characteristics that span the range from southeast indian ridge seir mid ocean ridge basalt to tholeiitic transitional basalts derived from the kerguelen mantle plume relationships between element abundance ratios involving incompatible elements and isotopic ratios reflect mixing of near primary melts from an seir source and the kerguelen plume with an increasing role for an incompatible element rich end member in the order unit 1

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

boomerang seamount the active expression of the amsterdam st paul hotspot southeast indian ridge during a survey of the axis of the southeast indian ridge seir we discovered a 1100 m tall volcanically active submarine volcano boomerang seamount near the spreading center on the amsterdam st paul asp plateau the summit of the volcano is 650 m below sea level and has a 200 m deep 2 km wide circular caldera samples of very fresh volcanic glass dated by the 210po 210pb technique at 5 months old were collected from the floor of the caldera in march 1996 the volcano is 18 km northeast of amsterdam island near the intersection of a long spreading segment and the boomerang transform fault it is built on the stationary antarctic plate where widely scattered volcanic activity thickens the crust continuing to build the plateau water column profiles reveal a 1 7 c temperature anomaly and a 0 3 v stepped nephelometer water column turbidity anomaly within the caldera nearly an order of magnitude larger than other hydrothermal plume anomalies we measured these anomalies suggest hydrothermal activity within the caldera volcanic glass compositions at two sample sites on the volcano summit are similar to each other and to amsterdam and st paul island basalts but have some important differences as well k2o tio2 ratios in boomerang seamount glasses are similar to st paul island samples but differ significantly from amsterdam island samples rare earth element patterns in lavas from boomerang amsterdam and st paul are similar sr nd and pb isotope ratios in samples from the boomerang caldera floor are similar to samples from amsterdam island however another sample from boomerang seamount deviates from a seir st paul amsterdam mixing trend and shows evidence of mixing with a kerguelen like source component the geochemical complexity of these three closely spaced volcanic edifices on the asp plateau suggests that the boomerang seamount source is heterogeneous on a very small scale

210po 210pb dating of recent volcanic eruptions on the sea floor the globe encircling mid ocean ridge system accounts for most of the earth s volcanism and influences its heat budget morphology and the composition of the oceans it is therefore important to constrain the size frequency and compositional variability of ridge eruptions but this is made difficult by the remoteness of much of the sea floor and the paucity of applicable radioactive chronometers remote monitoring has provided indirect evidence for active ridge volcanism1 3 which in some cases has been strengthened by submersible observations4 5 but it has not been possible to date these eruptions directly here we present a new chronometer based on 210po 210pb radioactive disequilibrium which allows us to date glassy eruption products within a few years of their eruption our first results on lavas from 9 50 n on the east pacific rise confirm that the ridge erupted only months before sample collection and indicate that the eruptions extended over at least one year the chronometer begins with po volatilization during eruption all glasses analysed were 75 100 degassed a crystalline pillow interior was 20 degassed however implying that the flow surface may control po degassing during deep submarine eruptions although the 210po 210pb chronometer is useful for only a short time frame it should prove valuable for dating recent but unobserved submarine eruptions and determining details of eruptive sequences

hot spot activity and tectonic settings near amsterdam st paul plateau indian ocean 1 the amsterdam st paul asp plateau is located in the central part of the indian ocean and results from the interaction between the asp hot spot and the southeast indian ridge seir it is located near the diffuse boundary between the capricorn and australian plates the seamount chain of the dead poets cdp is northeast of the asp plateau and may represent older volcanism related to the asp hot spot this chain consists of two groups of seamounts 1 large flat topped seamounts formed 8 10 ma and 2 smaller conical seamounts formed during the last 2 myr the asp hot spot has produced two pulses of magmatism that have been ponded under the asp plateau and erupted along the divergent boundary between the capricorn and australian plates the n65 orientation of the cdp as well as the seamount s elongated shapes support an opening motion between the capricorn and australian plates along a suture oriented in the n155 direction this motion compared to the antarctic plate amounts to an apparent velocity of 7 7 cm yr northeastward for the capricorn australian block this motion does not fit with a fixed plume model we suggest therefore that the asp plume experienced a motion of about 1 2 cm yr to the sw which is opposite to the asthenospheric flow in this region and suggests a deep seated plume

factors controlling evolution of karst conduits in sandy limestone and calcareous sandstone turnov area czech republic abstract karst potential is rarely studied in calcareous sandstones and sandy limestones the minimum content of carbonate necessary for the development of conduits is unknown and the mechanisms of the origins of conduits in these rocks are poorly understood this article focuses upon these questions in an important regional aquifer samples of limestones and sandstones were taken from outcrops in vertical profiles and were subjected to a rapid dissolution test with 10 hcl the degree of disintegration of the samples after dissolution of the carbonate was observed in order to estimate which portions of the rock enabled formation of karst conduits carbonate content was measured by calcimetry distribution of calcite cement and other sources of cohesion were studied by scanning electron microscopy tracer tests in the conduits were evaluated calcite is the dominant carbonate mineral and main cementing agent in the jizera fm aquifer mean content 51 however calcite content was found to be a weak predictor of karst potential most of the samples disintegrated completely if the calcite content exceeded 55 on the other hand some rocks with a calcite content up to 70 did not disintegrate due to microcrystalline amorphous silica which bound the quartz grains together and or formed a foam like supporting structure after in situ calcite dissolution the rock in karst prone zones is turned to residuum which consequently is washed out by fast flow 0 01 0 2 m s based on tracer tests microscopic study of cementing agents of the siliciclastic carbonate sediments and dissolution tests are essential for understanding the mechanisms of conduit origin in ghost rock karst

condor seamount azores ne atlantic a morpho tectonic interpretation abstract high resolution datasets collected by multibeam and acoustic backscatter surveys were used to produce fine scale seafloor nature and morpho tectonic interpretations of the condor seamount condor constitutes an elongated volcanic ridge that extends for 39 km and rises more than 1800 m from the surrounding seafloor constructive morphologies include i linear eruptive centres ii volcanic cones with or without summit depressions iii lava flows and iv hummocky sectors eruptive type is interpreted to vary with depth on the deeper seamount extremities the predominance of highly acoustically backscattering volcanic cones and hummocky terrain is interpreted to result from effusive eruptions not yet covered by sediment deposits in contrast the smoother relief of the central seamount flanks is interpreted as draping and infilling of the underlying effusive relief by i primary volcaniclastic deposits produced by explosive eruptions on the shallowest parts of the ridge together with ii secondary volcanigenic sediments resulting from truncation of the seamount top by swell erosion and iii sediments resulting from biogenic production a set of wnw ese to nw se trending volcano tectonic structures are shown to control most of the fissural volcanism that formed the ridge a network of nnw sse trending faults is identified on the sea floor around condor but they show little relation with the distribution of volcanic edifices or with post emplacement dismantling of the seamount these fault sets are related to the transtensional regime acting on the azorean segment of the eurasia nubia plate boundary erosional features include i palaeo wave cut platforms on the seamount summit ii landslide scars produced by lateral collapses of the ne and sw facing flanks iii gullies and turbidity current channels and iv mass wasting deposits iceberg drag and bump marks are also identified on the seamount upper flanks representing the first reference to such features in the azores and an additional low latitude record given the lack of major erosional and tectonic dismantling condor is suggested to be a relatively young seamount a revised factoring of eustatic erosional and isostatic processes does not exclude that the summit may have been eroded as late as the last glacial maximum

geodynamic implications of moving indian ocean hotspots abstract the reconstructed positions of many of the indian ocean intraplate volcanic features are inconsistent with the present positions of the hotspots to which they have been ascribed the discrepancy can be explained by the past motion of the hotspots themselves and in the errors introduced in an absolute plate model based on fixed hotspots previous models for the motion of hotspots in the indian ocean e g steinberger and o connell in the history and dynamics of global plate motions 2000 pp 377 398 and geophys j int 132 1998 412 434 antretter et al earth planet sci lett 203 2002 635 650 have had success in matching paleolatitudes of the kerguelen hotspot but not the marion or reunion hotspots we calculate the motion of the indian ocean hotspots using a method described in steinberger and o connell op cit employing a plate motion model revised using identification of mesozoic magnetic anomalies in the enderby basin gaina et al in the evolution and dynamics of the australian plate in press we find a motion of 7 south for the kerguelen plume similar to previous models antretter et al op cit this motion alleviates the discrepancies in the position of the kerguelen plume and its purported involvement in the formation of the ninetyeast ridge rajmahal traps bunbury basalts and assorted volcanics of the western australian margin we find a motion of 5 north and 7 south for the reunion and marion plumes respectively the motion of the reunion hotspot is consistent with paleolatitude estimates the apparent fixity of the marion hotspot torsvik et al earth planet sci lett 164 1998 221 232 is not well constrained but may be due to the combination of hotspot motion and true polar wander acting in opposing directions we present revised absolute finite rotations for the indian plate for the last 65 ma based on the motion of these hotspots which place india farther north in the past than for fixed hotspot models

geochronology and geochemistry of lavas from the 1996 north gorda ridge eruption abstract radiometric dating of three north gorda ridge lavas by the 210 po 210 pb method confirms that an eruption occurred during a period of increased seismic activity along the ridge during late february early march 1996 these lavas were collected following detection of enhanced t phase seismicity and subsequent ocean bottom photographs documented the existence of a large pillow mound of fresh appearing lavas 210 po 210 pb dating of these lavas indicates that an eruption coinciding with this seismicity did occur within analytical error and that followup efforts to sample the recent lava flows were successful compositions of the three confirmed young lavas and eleven other samples of this contiguous new flow sequence are distinct from older lavas from this area but are variable at a level outside analytical uncertainty these intraflow variations can not easily be related to a single common parent magma compositional variability within the new flow is compared to that of other recently documented individual flow sequences and this comparison reveals a strong positive correlation of compositional variance with flow volumes spanning a range of 2 orders of magnitude the geochemical heterogeneity in the north gorda new flow probably reflects incomplete mixing of magmas generated from a heterogeneous mantle source or from slightly different melting conditions of a single source the compositional variability range in sample ages up to 6 weeks and range in active seismicity 4 weeks imply that this relatively large flow was erupted over an interval of several weeks

contrasted hydrothermal activity along the south east indian ridge 130 e 140 e from crustal to ultramafic circulation using a combined approach of seafloor mapping mapr and ctd survey we report evidence for active hydrothermal venting along the 130 140 e section of the poorly known south east indian ridge seir from the australia antarctic discordance aad to the george v fracture zone fz along the latter we report eh and ch4 anomalies in the water column above a serpentinite massif which unambiguously testify for ultramafic related fluid flow this is the first time that such circulation is observed on an intermediate spreading ridge the ridge axis itself is characterized by numerous off axis volcanoes suggesting a high magma supply the water column survey indicates the presence of at least ten distinct hydrothermal plumes along the axis the ch4 mn ratios of the plumes vary from 0 37 to 0 65 denoting different underlying processes from typical basalt hosted to ultramafic hosted high temperature hydrothermal circulation our data suggest that the change of mantle temperature along the seir not only regulates the magma supply but also the hydrothermal activity the distribution of hydrothermal plumes from a ridge segment to another implies secondary controls such as the presence of fractures and faults along the axis or in the axial discontinuities we conclude from these results that hydrothermal activity along the seir is controlled by magmatic processes at the regional scale and by the tectonics at the segment scale which influences the type of hydrothermal circulation and leads to various chemical compositions such variety may impact global biogeochemical cycles especially in the southern ocean where hydrothermal venting might be the only source of nutrients

seafloor age dependence of rayleigh wave phase velocities in the indian ocean variations in the phase velocity of fundamental mode rayleigh waves across the indian ocean are determined using two inversion approaches first variations in phase velocity as a function of seafloor age are estimated using a pure path age dependent inversion method second a two dimensional parameterization is used to solve for phase velocity within 1 25 1 25 grid cells rayleigh wave travel time delays have been measured between periods of 38 and 200 s the number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22 272 paths at a period of 40 s at periods 100 s the phase velocity variations are strongly controlled by seafloor age and shown to be consistent with temperature variations predicted by the half space cooling model for a mantle potential temperature of 1400 c the inferred thermal structure beneath the indian ocean is most similar to the structure of the pacific upper mantle where phase velocities can also be explained by a half space cooling model the thermal structure is not consistent with that of the atlantic upper mantle which is best fit by a plate cooling model and requires a thin plate removing age dependent phase velocity from the 2 d maps of the indian ocean highlights anomalously high velocities at the rodriguez triple junction and the australian antarctic discordance and anomalously low velocities immediately to the west of the central indian ridge

madagascar basalts tracking oceanic and continental sources extensive upper cretaceous volcanism in southern madagascar was fed in part by mantle sources resembling those expressed today in the indian ocean at marion and prince edward islands and on the central southwest indian ridge in addition very low end t to 17 4 high 87sr 86sr t to 0 72126 tholeiites in southwestern madagascar were variably but highly contaminated by ancient continental material broadly like that affecting the bushe and poladpur formations of the later deccan traps in india alkalic dikes in southwestern madagascar have a rough analogue in the mahabaleshwar formation of the deccan in that they document the influence of a low 206pb 204pb negative end relatively low 87sr 86sr reservoir a very similar reservoir is manifested at present in mid ocean ridge basalts on the central southwest indian ridge near 40 e the original location of this end member appears likely to have been in the madagascan lithospheric mantle a portion of which may have been removed in the middle cretaceous by the action of the marion hotspot or the rifting of indo madagascar an origin within the hotspot itself also may be possible however recent products of the hotspot appear to lack completely the necessary low 206pb 204pb low end signatures

temporal evolution of the kerguelen plume geochemical evidence from 38 to 82 ma lavas forming the ninetyeast ridge abstract basaltic basement has been recovered by deep sea drilling at seven sites on the linear ninetyeast ridge in the eastern indian ocean studies of the recovered lavas show that this ridge formed from 82 to 38 ma as a series of subaerial volcanoes that were created by the northward migration of the indian plate over a fixed magma source in the mantle the sr nd and pb isotopic ratios of lavas from the ninetyeast ridge range widely but they largely overlap with those of lavas from the kerguelen archipelago thereby confirming previous inferences that the kerguelen plume was an important magma source for the ninetyeast ridge particularly important are the 81 ma ninetyeast ridge lavas from dsdp site 216 which has an anomalous subsidence history coffin 1992 these lavas are feti rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic upper miocene lavas in the kerguelen archipelago the isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the kerguelen plume weis et al 1993a b despite the overlap in isotopic ratios there are important compositional differences between lavas erupted on the ninetyeast ridge and in the kerguelen archipelago the ninetyeast ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios such as la yb and zr nb which are intermediate between those of indian ocean morb mid ocean ridge basalt and the transitional to alkalic basalts erupted in the kerguelen archipelago these compositional differences reflect a much larger extent of melting for the ninetyeast ridge lavas and the proximity of the plume to a spreading ridge axis this tectonic setting contrasts with that of the recent alkalic lavas in the kerguelen archipelago which formed beneath the thick lithosphere of the kerguelen plateau from 82 to 38 ma there was no simple systematic temporal variation of sr nd and pb isotopic ratios in ninetyeast ridge lavas therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume although class et al 1993 propose that some of the isotopic variations reflect such aging we infer that most of the isotopic heterogeneity in lavas from the ninetyeast ridge and kerguelen archipelago can be explained by mixing of the kerguelen plume with a depleted morb like mantle component however with this interpretation some of the youngest 42 44 ma lavas from the southern ninetyeast ridge which have206pb 204pb ratios exceeding those in indian ocean morb and kerguelen archipelago lavas require a component with higher206pb 204pb such as that expressed in lavas from st paul island

rare earth element geochemistry of hydrothermal deposits from the active tag mound 26 n mid atlantic ridge the rare earth element ree geochemistry of various phases from the active tag hydrothermal mound has been examined and related to their mineralogy and fluid chemistry the mound deposits range from black and white smoker chimneys massive anhydrite sulphide mixtures oxides and ochres all phases except black smoker chimney anhydrite demonstrate a positive eu anomaly when normalised to chondrite ree values ree substitution into sulphide and sulphate phases appears to be strongly influenced by crystallographic control for all ree other than eu precipitation of anhydrite within the tag mound is the major mechanism for removal of ree during mound circulation and 0 15 0 35 g anhydrite is inferred to precipitate from every kg of fluid venting from the white smoker chimneys oxides from the mound fall into three different categories with distinct ree patterns oxide rims on sulphides atacamite bearing oxides and silica rich fe oxides and ochres the oxide rim phases contain sulphide and seawater derived rees whereas the atacamite bearing oxides and the ochreous material exhibit no seawater signature which suggests precipitation from or alteration by a modified hydrothermal fluid

global gravity bathymetry and the distribution of submarine volcanism through space and time 1 the seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin relatively few of these features 0 1 however have been dated and so little is known about their tectonic setting one parameter that is sensitive to whether a seamount formed on near or far from a mid ocean ridge is the elastic thickness te which is a proxy for the long term strength of the lithosphere most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of te and then comparing the calculated and observed gravity anomaly the problem with such an approach is that bathymetry data are usually limited to single beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape we therefore use the satellite derived gravity anomaly to predict the bathymetry for different values of te by comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the wessel global seamount database we have obtained 9758 te estimates from a wide range of submarine volcanic features in the pacific indian and atlantic oceans comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate te and its upper and lower bounds te at sites where there is both a sample and crustal age show considerable scatter however and there is no simple relationship between te and age nevertheless we are able to tentatively assign a tectonic setting to each te estimate the most striking results are in the pacific ocean where a broad swath of on ridge volcanism extends from the foundation seamounts and ducie island easter island ridge in the southeast across the equator to the shatsky and hess rises in the northwest interspersed among the on ridge volcanism are flank ridge and off ridge features the indian and atlantic oceans also show a mix of tectonic settings off ridge volcanism dominates in the eastern north atlantic and northeast indian oceans while flank ridge volcanism dominates the northeastern indian and western south atlantic oceans we have been unable to assign the flank ridge and off ridge estimates an age but the on ridge estimates generally reflect we believe the age of the underlying oceanic crust we estimate the volume of on ridge volcanism to be 1 1 106 km3 which implies a mean seamount addition rate of 0 007 km3 yr 1 rates appear to have varied through geological time reaching their peak during the late early cretaceous and then declining to the present day

detection of hydrothermal plumes along the southeast indian ridge near the amsterdam st paul plateau a new self contained profiling instrument attached to 91 lowerings of waxcores and dredges along a 1600 km stretch of the southeast indian ridge found six sites having optical back scatter anomalies indicative of hydrothermal plumes from high temperature seafloor venting at five other sites back scatter anomalies were less distinct and at the remaining sites anomalies were absent these are the first documented hydrothermal sites along this intermediate rate spreading center and are among the very few yet discovered in the indian ocean basin some profiles exhibit complex back scatter anomalies suggestive of multiple sources of seafloor discharge at one site near bottom temperature anomalies of 0 1 c and the recovery of a vent specific barnacle provide the first precise seafloor location of active venting in the deep indian ocean although this study is only reconnaissance in nature venting along this portion of the seir appears to be significantly less than predicted by global correlations of plume incidence as a function of spreading rate nevertheless these sites may be important as biological connections between distinct faunal assemblages of atlantic and pacific vent fields

vailulu u undersea volcano the new samoa 1 vailulu u seamount is identified as an active volcano marking the current location of the samoan hotspot this seamount is located 45 km east of ta u island samoa at 169 03 5 w 14 12 9 s vailulu u defines the easternmost edge of the samoan swell rising from the 5000 m ocean floor to a summit depth of 590 m and marked by a 400 m deep and 2 km wide summit crater its broad western rift and stellate morphology brand it as a juvenile progeny of ta u seven dredges ranging from the summit to the se rift zone at 4200 m recovered only alkali basalts and picrites isotopically the volcano is strongly em2 in character and clearly of samoan pedigree 87sr 86sr 0 7052 0 7067 143nd 144nd 0 51267 0 51277 206pb 204pb 19 19 19 40 the 210po 210pb data on two summit basalts indicate ages younger than 50 years all of the recovered rocks are extremely fresh and veneered with glass an earthquake swarm in early 1995 may attest to a recent eruption cycle a detailed nephelometry survey of the water column shows clear evidence for hydrothermal plume activity in the summit crater the water inside the crater is very turbid nephelometric turbidity unit ntu values up to 1 4 and a halo of smog several hundred meters thick encircles and extends away from the summit for at least 7 km the turbid waters are highly enriched in manganese up to 7 3 nmol kg providing further evidence of hydrothermal activity vailulu u is similar to loihi hawaii in being an active volcanic construct at the eastern end of a hotspot chain it differs importantly from the hawaiian model in its total lack of tholeiitic basalt compositions

taphonomy of the lamy amphibian quarry a late triassic bonebed in new mexico u s a located in santa fe county new mexico usa the lamy amphibian quarry is a late triassic adamanian bonebed stratigraphically low in the garita creek formation of the chinle group well known for its mass accumulation of metoposaurid amphibians it was initially interpreted as a drought induced death assemblage based on microstratigraphic and sedimentological studies additional and extensive collecting at the quarry and a revised understanding of the bonebed we provide a detailed taphonomic analysis of the lamy amphibian quarry that identifies it as a low diversity multitaxic and monodominant bonebed in pedogenically modified floodplain mudstone the lamy bonebed shows no evidence of drought and is characterized by a high density of completely dissociated bones that show clear alignment by current and sorting enrichment of voorhies group ii and iii elements the bones show no significant abrasion or weathering stage 0 preserve virtually no evidence of scavenging and show no evidence of trampling based on skull lengths the metoposaurid assemblage has a type i survivorship curve and lacks juveniles we thus posit that the following sequence of events formed the lamy amphibian bonebed 1 aggregation cause unknown of a large number of metoposaurid amphibians at a site different from the location of the bonebed though not distant 2 catastrophic mass mortality 3 complete disarticulation and disassociation of the skeletons and 4 rapid transport of the disarticulated bones onto a floodplain surface that was undergoing pedogenesis the lamy amphibian bonebed is representative of the late triassic metoposaurid bonebeds from morocco and the western usa which are monodominant and nearly monotaxic they indicate that aggregation probably of breeding populations and mass death of metoposaurids were relatively common across the riverine floodplains of late triassic pangea

southeast indian ridge between the rodriguez triple junction and the amsterdam and saint paul islands detailed kinematics for the past 20 m y the southeast indian ridge has the fastest spreading rates of the three mid oceanic ridge systems of the indian ocean and has recorded the movements of antarctica relative to australia and india since the late cretaceous new bathymetric and magnetic data have been collected by the r v marion dufresne 1983 and the r v jean charcot 1984 on the western part of this ridge between the rodriguez triple junction 25 5 s 70 0 e and the amsterdam and saint paul islands 38 s 78 e these data bring additional information on the seafloor magnetic pattern produced by the southeast indian ridge during the past 20 m y a new tectonic chart is proposed for the area around the amsterdam and saint paul islands we have mapped 17 isochrons ranging from anomalies 6 to 1 20 5 0 7 ma based on the compilation of all the data available in this area 25 cruises their distribution clearly shows asymmetric features reconstructions at short time intervals show that stage poles of rotation describe oscillatory movements along a direction parallel to the southeast indian ridge axis observed changes in spreading rates and the stability of the spreading directions since the miocene support this result

deformation of oceanic lithosphere near slow spreading ridge discontinuities abstract transform and non transform discontinuities that offset slow spreading mid ocean ridges involve complex thermal and mechanical interactions the truncation of the ridge axis influences the dynamics of spreading and accretion over a certain distance from the segment end likewise the spreading system is expected to influence the lithospheric plate adjacent to the ridge end opposite of the discontinuity tectonic effects of the truncated ridge are noticeable in for example the contrast between seafloor topography at inside corners and outside corners along axis variations in rift valley depth style of crustal accretion and ridge segment retreat and lengthening along such slow spreading discontinuities and their fossil traces oceanic core complexes or mega mullion structures are rather common extensional tectonic features in an attempt to understand deformation of oceanic lithosphere near ridge offsets the evolution of discontinuities and conditions that may favor oceanic core complex formation a three dimensional thermo mechanical model has been developed the numerical approach allows for a more complete assessment of lithosphere deformation and associated stress fields in inside corners than was possible in previous 3 d models the initial suite of results reported here focuses on deformation when axial properties do not vary along strike or with time showing the extent to which plate boundary geometry alone can influence deformation we find that non transform discontinuities are represented by a wide oblique deformation zone that tends to change orientation with time to become more parallel to the ridge segments this contrasts with predicted deformation near transform discontinuities where initial orientation is maintained in time the boundary between the plates is found to be vertical in the center of the offset and curved at depth in the inside corners near the ridge transform intersection ridge normal tensile stresses concentrate in line with the ridge tip extending onto the older plate across the discontinuity and high stress amplitudes are absent in the inside corners during the magmatic accretionary phase simulated by our models with the tested rheology and boundary conditions inside corner formation of oceanic core complexes is predicted to be unlikely during magmatic spreading phases additional modeling studies are needed for a full understanding of extensional stress release in relatively young oceanic lithosphere

petrogenesis of the flood basalts forming the northern kerguelen archipelago implications for the kerguelen plume the thick 20 km crust of the kerguelen archipelago formed as the tectonic setting of the kerguelen plume changed from an oceanic ridge centered location at 43 ma to its present location beneath the antarctic plate the uppermost crust is dominantly flood basalt with a thickness of up to 10 km inverse isochron 40ar 39ar ages for upper and lower lavas in a 630 m section of basalt flows from mont bureau are 30 4 and 29 0 ma re os isotopic systematics are consistent with this age most of the lavas in two stratigraphic sections mont bureau and mont rabouillere from the northern part of the archipelago have sr nd and pb isotopic characteristics similar to the youngest upper miocene to pleistocene lavas erupted in the southeast part of the archipelago i e initial 87sr 86sr 0 7050 143nd 144nd 10 mgo alkalic lavas in the southeast province of the archipelago in these northern sections it is confined to transitional lavas with

geochemical characteristics of the youngest volcano mount ross in the kerguelen archipelago inferences for magma flux lithosphere assimilation and composition of the kerguelen plume mount ross is the youngest 1 0 1 ma volcanic edifice recognized in the kerguelen archipelago which is located on the northern part of the submarine kerguelen plateau lava types range from basaltic trachyandesite to trachyte and extensive glaciation has exposed an intrusive core ranging from gabbro to syenite mount ross rocks are not as silica undersaturated as the basanite to phonolite upper miocene lavas erupted in the southeast province of the archipelago however both lava suites are characterized by relatively high 87sr 86sr 0 7051 0 7054 and 0 7054 0 7058 respectively and low 206pb 204pb 18 02 18 14 and 18 06 18 27 respectively the abundant trachytes and phonolites in the age range from lower miocene to pleistocene on the kerguelen archipelago indicate that the supply of basaltic magma has been low for the last 20 m y despite the prolonged time period required for extensive mineral fractionation there is no evidence for assimilation of geochemically distinctive lithosphere i e most of the trachytes and syenites have initial isotopic ratios within the narrow range of the more mafic rocks volcanic and plutonic rocks of diverse composition and age 0 1 30 ma in the kerguelen archipelago have similar initial isotopic ratios consequently we infer that the kerguelen plume is characterized by 87sr 86sr 0 7051 0 7058 143nd 144nd 0 51263 0 51249 and 206pb 204pb 18 02 18 27

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

seafloor spreading on the amsterdam st paul hotspot plateau the amsterdam st paul asp platform on the intermediate rate southeast indian ridge seir is the only oceanic hotspot plateau outside the atlantic ocean containing an active mid ocean ridge spreading axis because the asp hotspot is small and remotely located it has been relatively unstudied and the ridge axis location in many places near the asp plateau was previously unknown or ambiguous we mapped the seir out to 1 ma crust jaramillo anomaly both on and near the asp platform we located the spreading center to within a few kilometers based on side scan sonar reflectivity recent off platform magnetic anomalies and lineated abyssal hill topography are consistent with a simple spreading history off platform full spreading rates increase from 63 km myr on segment h to the north of the platform to 65 5 km myr on segment k to the south in contrast inversions of seafloor magnetization based on uniform and variable thickness magnetic source layers reflect a complex on platform tectonic history with ridge jumps off axis volcanism and propagating rifts on one section of the asp plateau the spreading location has stabilized and is beginning to rift the plateau apart generating symmetric magnetic anomalies and lineated topography for the last several hundred thousand years the larger more stable spreading segments of the asp platform are aligned with major volcanic edifices suggesting that along axis magma flow away from plume fed centers is an important influence on spreading geometry many complex tectonic features observed on the asp plateau such as ridge jumps en echelon oblique spreading centers and transforms oblique to the spreading direction are comparable to features observed on iceland the similarities suggest that moderate crustal thickening at an intermediate rate spreading center may have similar effects to pronounced thickening at a slow rate spreading center

correlated patterns in hydrothermal plume distribution and apparent magmatic budget along 2500 km of the southeast indian ridge multiple geological processes affect the distribution of hydrothermal venting along a mid ocean ridge deciphering the role of a specific process is often frustrated by simultaneous changes in other influences here we take advantage of the almost constant spreading rate 65 71 mm yr along 2500 km of the southeast indian ridge seir between 77 e and 99 e to examine the spatial density of hydrothermal venting relative to regional and segment scale changes in the apparent magmatic budget we use 227 vertical profiles of light backscatter and on 41 profiles oxidation reduction potential along 27 first and second order ridge segments on and adjacent to the amsterdam st paul asp plateau to map ph the fraction of casts detecting a plume at the regional scale venting on the five segments crossing the magma thickened hot spot plateau is almost entirely suppressed ph 0 02 conversely the combined ph 0 34 from all other segments follows the global trend of ph versus spreading rate off the asp plateau multisegment trends in ph track trends in the regional axial depth high where regional depth increases and low where it decreases at the individual segment scale a robust correlation between ph and cross axis inflation for first order segments shows that different magmatic budgets among first order segments are expressed as different levels of hydrothermal spatial density this correlation is absent among second order segments eighty five percent of the plumes occur in eight clusters totaling 350 km we hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the seir

anomalous seafloor spreading of the southeast indian ridge near the amsterdam st paul plateau the amsterdam st paul plateau is bisected by the intermediate rate spreading southeast indian ridge and numerous geophysical and tectonic anomalies arise from the interactions of the amsterdam st paul hotspot and the spreading center the plate boundary geometry on the hotspot platform evolves rapidly on timescales 1 myr off axis volcanism is abundant the seafloor does not deepen away from the axis and transform faults do not have fracture zone extensions away from the hotspot platform the ridge transform geometry is typical of mid ocean ridges globally in contrast the amsterdam st paul plateau spreading segments are shorter they often overlap each other significantly and the intervening discontinuities are smaller more ephemeral and more migratory abyssal hills are smaller and less uniform on the hotspot platform than on neighboring spreading segments from gravity and isostasy analysis the average thickness of the platform crust is 10 km approximately 50 thicker than that of typical oceanic crust most of the isostatic compensation of the hotspot plateau occurs at the moho or within the lower crust and the effective elastic thickness of the plateau lithosphere is 1 6 km less than half that of adjacent spreading segments away from the platform some transform faults contain intratransform spreading centers on the platform the two transform faults have valleys which may be depocenters for abundant axial or off axis volcanism and mass wasting although not wellconstrained by magnetics coverage the amsterdam st paul hotspot appears to have been captured by the southeast indian ridge enhancing crustal production at the ridge since about 3 5 ma prior to this time the hotspot formed a line of smaller isolated volcanoes on older australian plate the underlying cause for the present day crustal accretion anomalies is the effect of melt generation from separate sources of mantle upwelling due to plate spreading and the hotspot which has a consequent effect of weakening the lithosphere

the early opening between broken ridge and kerguelen plateau abstract reconstructions to total closure of the australia antarctic continents causes an unacceptable overlap of broken ridge and kerguelen plateau this has been partially resolved in the past by supposing that the northern part of kerguelen that is principally involved in the overlap is younger than the remainder we have revised the early reconstructions using a newly proposed breakup chronology of australia and antarctica which suggests that opening began at least 90 m y b p at an initially slow rate this eliminates the overlap problem without invoking major age differences within the kerguelen plateau we also suggest that the northeastern flank of kerguelen plateau may be underlain by the missing westward continuation of the diamantina zone it may have been isolated on the antarctic plate by a ridge crest jump at about anomaly 24 time that also formed the ob trench

mono lake or laschamp geomagnetic event recorded from lava flows in amsterdam island southeastern indian ocean summary we report a survey carried out on basalt flows from amsterdam island southeastern indian ocean in order to check the presence of intermediate directions interpreted as belonging to a geomagnetic field excursion within the brunhes epoch completing this palaeomagnetic record with palaeointensity determinations and radiometric dating because the palaeomagnetic sampling was done over a few hours during the resupply of the french scientific base martin du viviers by the marion dufresne vessel we could collect only 29 samples from four lava flows the directional results corroborate the findings by watkins nougier normal polarity is found for two units and an intermediate direction with associated virtual geomagnetic poles vgps close to the equator for the other two units a notable result is that these volcanic rocks are well suited for absolute palaeointensity determinations 50 per cent of the samples yields reliable intensity values with high quality factors an original element of this study is that we made use of the thermomagnetic criterion ptrm tail test of shcherbakova et al to help in the interpretation of the palaeointensity measurements doing this only the high temperature intervals beyond 400 c were retained to obtain the most reliable estimate of the strength of the ancient magnetic field however not applying the ptrm tail test does not change the flow mean values significantly because the samples we selected by conventional criteria for estimating the palaeointensity carry only a small proportion of their remanence below 400 c the normal units yield virtual dipole moments vdm of 6 2 and 7 7 10 22 am 2 and the excursional units yield values of 3 7 and 3 4 10 22 am 2 these results are quite consistent with the other thellier determinations from brunhes excursion records all characterized by a decrease of the vdm as the vgp latitude decreases 40 ar 39 ar isotopic age determinations provide an estimate of 26 15 and 18 9 kyr for the transitional lava flows which could correspond to the mono lake excursion however the large error bars associated with these ages do not exclude the hypothesis that this event is the laschamp

stratigraphic framework of upper pennsylvanian and lower permian marine to continental transition wichita falls lawton and sherman quadrangles north central texas approximately 1 900 ft 580 m of continental rocks of latest virgilian wolfcampian and early leonardian age are exposed in an area of about 3 800 mi2 9 900 km2 between the brazos and red rivers of north central texas the stratigraphic complexity of these strata has impeded internal correlation and mapping since the rocks were first described by w f cummins in the late 19th century precise correlation of this discontinuously stratified fluvial sequence with well defined limestone bounded fluvial deltaic and marine formations to the south has been hampered because of 1 the pronounced change in lithology that accompanies this marked facies transition 2 a shift in strike of approximately 65 that coincides with a change in facies t act thus amplifying the stratigraphic complexity of the region and 3 generally poor exposure of the gently inclined strata the continental sequence is composed of approximately 25 major and numerous minor upward fining principally fluviogenic cycles sandstone units 5 60 ft or 1 5 18 m thick mark the bases of these cycles and occur as regionally persistent zones with multistory and multilateral geometry these generally resistant cuesta forming units interfinger with limestone bearing strata or are separated from them along strike by intervening zones of red mudstone the fine grained upper portion of the cycles 10 to 100 ft or 3 to 30 m thick is predominantly concretionary red mudstone although gray and variegated claystone lenses thin siltstone and sandstone beds and lenticular and channel fill conglomerates are characteristic precise mapping of sandstone units and correlation with prominent limestone pinch outs have permitted a stratigraphic tie with the marine section of the colorado and brazos river valleys continental rocks are divided into the bowie and wichita groups equivalent marine strata are divided into the cisco and albany groups formations have been defined in each group to allow a maximum degree of intergroup correlation end of article last page 145

submarine magma degassing and explosive magmatism at macdonald tamarii seamount macdonald seamount is an active volcanic centre located at 29 98 s 140 25 w in the south central pacific since its discovery1 in 1968 a number of expeditions have surveyed and dredged rocks from the seamount during a scripps institution of oceanography expedition in october 1987 the first direct observation of volcanic activity at macdonald was made when a small eruption occurred spewing forth scoriaceous basaltic rocks which briefly floated to the sea surface the rocks were accompanied by huge bubbles of volcanic gas that created large sea surface slicks h craig personal communication similar activity at macdonald was observed by a joint french german team in january 1989 using the submersible cyana2 here we look at the radio geochemical features of the lone rock recovered from the october 1987 eruption we present analyses of u series nuclides from the first observed eruption of macdonald also known as tamarii seamount with interpretations focusing on the timescales of mag matic events and the implications of uranium series data for magma degassing this highly gas charged magma lost all of its volatile 210po upon eruption based on this estimates of the fluxes of other volatile elements to sea water which may contribute significantly to their oceanic budget are calculated high 230th 232th and 87sr 86sr ratios common signatures of altered crust suggest that the explosive nature of the macdonald volcan ism results partly from the sealing of magma conduits by seawater circulation

degassing of metals and metalloids from erupting seamount and mid ocean ridge volcanoes observations and predictions recently it has been reported that the element polonium degasses from mid ocean ridge and seamount volcanoes during eruptions published and new observations on other volatile metal and metalloid elements can also be interpreted as indicating significant degassing of magmatic vapors during submarine eruptions this process potentially plays an important role in the net transfer of chemical elements from erupting volcanoes to seawater in addition to that arising from sea floor hydrothermal systems in this paper a framework is constructed for predicting and assessing semiquantitatively the potential magnitude and chemical fingerprints in the water column of metal and metalloid degassing using 1 predictions from a summary of element volatilities during mafic subaerial volcanism worldwide and 2 limited data from submarine volcanic effusives the latter include analyses of polonium and trace metals in near volcano water masses sampled following a submarine eruption at loihi seamount hawaii 1000 m bsl in 1996 the element volatility predictions and observations show good agreement considering the limited dataset some of the highest volatility main group and transition element enrichments in seawater over loihi are predicted by the degassing mass transfer model i present when expanded to cover all submarine volcanic activity it is predicted that exit fluxes of these elements are up to 102 103 greater by degassing than by normal mor hydrothermalism in contrast mor exit fluxes of low volatility alkali and alkaline earth elements are likely 102 106 greater from hydrothermal inputs degassing inputs to the ocean are probably highly episodic occurring almost entirely during eruptions these are times of enhanced and abnormal hydrothermalism as well although major hydrothermal and degassing events may not be chemically recognizable in real water masses as wholly distinct entities it is nevertheless possible to predict to what extent each process flavors the effluents of the other degassing at mid ocean ridges may explain a variety of observations previously ascribed to complexities occurring during hydrothermal venting and or fluid ascent in the buoyant hydrothermal plumes above ridges

antipodal hotspots and bipolar catastrophes were oceanic large body impacts the cause abstract one aspect of the hotspot distribution that has received little attention is its antipodal character of 45 primary hotspots found in most hotspot compilations 22 49 form antipodal pairs within observed hotspot drift limits 20 mm yr in addition the available ages or possible age ranges for both hotspots of an antipodal pair tend to be similar 10 myr difference or overlap monte carlo simulations indicate that the antipodal primary hotspots locations and ages are not due to chance at the 99 confidence level p

seafloor seismicity antarctic ice sounds cetacean vocalizations and long term ambient sound in the indian ocean basin this paper presents the results from the deflo hydroacoustic experiment in the southern indian ocean using three autonomous underwater hydrophones complemented by two permanent hydroacoustic stations the array monitored for 14 months from november 2006 to december 2007 a 3000 x 3000 km wide area encompassing large segments of the three indian spreading ridges that meet at the indian triple junction a catalogue of 11 105 acoustic events is derived from the recorded data of which 55 per cent are located from three hydrophones 38 per cent from 4 6 per cent from five and less than 1 per cent by six hydrophones from a comparison with land based seismic catalogues the smallest detected earthquakes are m b 2 6 in size the range of recorded magnitudes is about twice that of land based networks and the number of detected events is 5 16 times larger seismicity patterns vary between the three spreading ridges with activity mainly focused on transform faults along the fast spreading southeast indian ridge and more evenly distributed along spreading segments and transforms on the slow spreading central and ultra slow spreading southwest indian ridges the central indian ridge is the most active of the three with an average of 1 9 events 100 km month along the sunda trench acoustic events mostly radiate from the inner wall of the trench and show a 200 km long seismic gap between 2 degrees s and the equator the array also detected more than 3600 cryogenic events with different seasonal trends observed for events from the antarctic margin compared to those from drifting icebergs at lower up to 50 degrees s latitudes vocalizations of five species and subspecies of large baleen whales were also observed and exhibit clear seasonal variability on the three autonomous hydrophones whale vocalizations dominate sound levels in the 20 30 and 100 hz frequency bands whereas earthquakes and ice tremor are a dominant source of ambient sound at frequencies 20 hz

recent ph and co2 profiles at lakes nyos and monoun cameroon implications for the degassing strategy and its numerical simulation abstract in situ ph profiles are reported for the first time for lakes nyos and monoun the ph profiles were converted to co2 profiles using hco3 profiles calculated from conductivity data recent observations 1993 1996 at lake nyos indicates that co2 still accumulates below 180 m depth at a rate of 125 mmol year 1 at lake monoun the majority of co2 is present below a depth of 60 m only 25 m below the saturation depth consequently a potential danger of gas explosion is high at both lakes and artificial degassing of the lakes should be performed as soon as possible a system for industrial degassing of the lakes is proposed the system based on the self sustained gas lift principle consists of multiple pipes 14 cm in diameter with different intake depths 12 pipes for lake nyos four each at 185 195 and 205 m and three pipes for lake monoun at 70 80 and 90 m the stepped degassing at different depths is intended to keep the maximum stability of the lakes the proposed degassing operation was simulated using the dyresm code for both lakes in 5 years approximately 50 of currently dissolved co2 in lake nyos and 90 in lake monoun will be removed the expected changes in the thermal and chemical structures of the lakes as degassing proceeds will be most easily monitored with a carefully calibrated ctd equipped with a ph sensor the simulation indicates that the discharged degassed water will sink to a level of neutral buoyancy i e to a maximum of 70 m at lake nyos and 35 m at lake monoun there would be no possibility of triggering a gas explosion by this plunge of discharged water because the water present there would have already been replaced by water at lower co2 concentration during the degassing from shallower pipes

taphonomy of the lower permian geraldine bonebed in archer county texas abstract lower permian admiral formation redbeds in north texas are famous for their vertebrate fauna this paper provides a detailed taphonomic analysis of the geraldine bonebed in central archer county texas one of the most important fossil vertebrate occurrences in the region the geraldine bonebed is situated in the floodbasin facies red and gray mudstones with abundant psaronius roots of a small meandering river system such basins were covered with a dense swamp forest with a high diversity of vertebrates the geraldine bonebed has yielded from an area of 30 m 2 44 partly articulated skeletons of four genera of tetrapods and the remains of another eight vertebrate taxa the bones lie on a layer of fern and conifer foliage and wood intermingled with fossil charcoal the sediment the occurrence of wasserleichen and the skeleton orientation indicate post mortem transportation and deposition in water a single mass death event which could have been a forest fire drove the animals into a pond where they perished carcasses floating in water together with plant debris were concentrated by wind induced drift into this exceptional fossil deposit a recent example of this process of bonebed formation after mass death was described by weigelt 1927 from the texas gulf coast