the evolution of oceanic and atmospheric anomalies in the northeast pacific during the el nino and la nina events of 1995 2001 abstract from late 1995 through early 2001 three major interannual climate events occurred in the tropical pacific the 1995 97 la nina ln 1997 98 el nino en and 1998 2001 ln we analyze atmospheric and upper oceanic anomalies in the northeast pacific nep during these events and compare them to anomalies both elsewhere in the north and tropical pacific and to typical en and ln anomaly patterns the atmospheric and oceanic anomalies varied strongly on intraseasonal and interannual scales during the 1995 97 ln and 1997 98 en the northeast pacific was dominated by negative slp and cyclonic wind anomalies and by upper ocean temperature and sea surface height ssh anomalies the latter were positive along the north american west coast and in the nep thermal anomaly pool between hawaii vancouver island and baja california and negative in the central north pacific this atmospheric oceanic anomaly pattern is typical of en an eastward shift in the atmospheric teleconnection from east asia created en like anomalies in the nep during the 1995 97 ln well before the 1997 98 en had begun the persistence of negative sea level pressure slp and cyclonic wind anomalies in the nep during the 1997 98 en intensified pre existing upper oceanic anomalies atmospheric anomalies were shifted eastward during late 1996 early 1998 leading to a similar onshore shift of oceanic anomalies this produced exceptionally strong positive upper ocean temperature and ssh anomalies along the west coast during the 1997 98 en and explains the unusual coastal occurrences of several species of large pelagic warm water fishes the growth and eastward shift of these pre existing anomalies does not appear to have been linked to tropical pacific en anomalies until late 1997 when a clear atmospheric teleconnection between the two regions developed prior to this remote atmospheric impacts on the nep were primarily from east asia as the 1998 2001 ln developed nep anomalies began reversing toward the typical ln pattern this led to predominantly negative slp and cyclonic wind anomalies in the nep and upper ocean temperature and ssh anomalies that were mainly negative along the west coast and positive in the central north pacific the persistence of these anomalies into mid 2001 and a number of concurrent biological changes in the nep suggest that a decadal climate shift may have occurred in late 1998 during 1995 2001 nep oceanic anomalies tracked the overlying atmospheric anomalies as indicated by the maintenance of a characteristic spatial relationship between these anomalies in particular wind stress curl and ssh anomalies in the nep maintained an inverse relationship that strengthened and shifted eastward toward the west coast during late 1996 early 1998 this consistent relationship indicates that anomalous ekman transport driven by regional atmospheric forcing was an important contributor to temperature and ssh anomalies in the nep and ccs during the 1997 98 en other studies have shown that coastal propagations originating from the tropical pacific also may have contributed to coastal nep anomalies during this en our results indicate that at least some of this coastal anomaly signal may have been generated by regional atmospheric forcing within the nep

reply to the comments of d l pinti r mineau and v clement and of a o marshall and c p marshall on biogenicity of earth s earliest fossils a resolution of the controversy by j william schopf and anatoliy b kudryavtsev gondwana research 22 2012 761 771 abstract the abundant and diverse assemblage of filamentous microbial fossils permineralized in the 3465 ma apex chert of northwestern australia among the oldest records of life are arguably the best studied by the most workers using the most advanced techniques in the history of science despite the extensive body of data establishing the biogenicity of the demonstrably cellular carbonaceous apex fossils pinti et al 2013 and marshall and marshall 2013 have raised issues regarding the interpretation of their studies of the apex chert presented in our recent review article schopf and kudryavtsev 2012 we agree with the assessment of both of the relevant papers by pinti et al 2009 2013 the observations they report do not apply to the bona fide microscopic fossils of the apex chert similarly like the minute objects reported by pinti et al 2009 2013 the quartz and haematite filled fractures discussed by marshall and marshall 2013 are mineralic pseudofossils that are not relevant to interpretation of the apex fossil microbes and their suggestion that multiple populations of carbonaceous material may be a wide spread issue through out the precambrian is without merit

new constraints on the evolution of the mallina basin and their bearing on relationships between the contrasting eastern and western granite greenstone terranes of the archaean pilbara craton western australia abstract the northeast trending mallina basin in the central part of the archaean pilbara craton separates western and eastern granite greenstone terranes with significantly different structural histories and age distribution patterns the basin which is up to 70 km wide and has a strike length of more than 150 km is mainly composed of metamorphosed turbidites assigned to the de grey group in the north these rocks are in faulted contact with metamorphosed volcanosedimentary rocks of the whim creek group new shrimp u pb zircon dates show that the turbidite sequence and the whim creek group were deposited within the same period between ca 3010 and 2940 ma and that the basement to the mallina basin is as young as ca 3015 ma geochronological structural and stratigraphic data now mitigate against previous models which suggested that the whim creek group was significantly younger than the turbidite sequence and that the whim creek group evolved in a late ensialic pull apart basin the mallina basin reinterpreted to include at least part of the whim creek group is now believed to be an extensional basin containing an overlap sequence an ensialic setting is preferred in which the basement to the basin consisted of ca 3170 3020 ma granites and greenstones rifted by major northeast trending faults a continental block to the south and east of which the presently exposed east pilbara granite greenstone terrane is a remnant influenced sedimentation in most of the basin previous tectonostratigraphic models that correlated across and beneath the mallina basin and interpreted the entire granite greenstone terrane of the pilbara craton in terms of evolution during two cycles of arc accretion are not compatible with the new geochronological data and stratigraphic interpretations

the 1997 1998 oceanic el nino signal along the southeast and northeast pacific boundaries an altimetric view changes in the sea surface heights ssh and geostrophic currents along the eastern boundaries of the pacific north central and south america are examined during the 1997 1998 el nino using altimeter data and proxy winds these show that symmetric ssh signals left the equator and propagated into both hemispheres in two episodes with primary periods of high equatorial ssh during may july and october december 1997 these are the distant signals from the mid latitude perspective as the signals spread poleward in each hemisphere their loss of symmetry demonstrates the degree to which they were altered by topographic features local winds and or local currents the first four eofs are calculated for 2 d ssh fields in 10 wide strips along the eastern margins 60 n 60 s and extending out along the equator from the coast to 110 w these account for approximately 40 of the overall variability and represent the main features of the seasonal cycles and el nino interannual variability snapshots of the 2 d ssh fields depict the structure of the el nino signal at different phases of its evolution

sea level fluctuations in central california at subtidal to decadal and longer time scales with implications for san francisco bay california abstract sea level elevations from near the mouth of san francisco bay are used to describe the low frequency variability of forcing of the coastal ocean on the bay at a variety of temporal scales about 90 of subtidal fluctuations in sea level in san francisco bay are driven by the sea level variations in the coastal ocean that propagate into the bay at the estuary mouth we use the 100 year sea level record available at san francisco to document a 1 9 mm yr mean sea level rise and to determine fluctuations related to el nino southern oscillation enso and other climatic events at time scales greater than 1 year enso dominates the sea level signal and can result in fluctuations in sea level of 10 15 cm alongshore wind stress data from central california are also analyzed to determine the impact of changes in coastal elevation at the mouth of san francisco bay within the synoptic wind band of 2 30 days at least 40 of the subtidal fluctuations in sea level of the bay are tied to the large scale regional wind field affecting sea level variations in the coastal ocean with little local direct wind forcing of the bay itself the majority of the subtidal sea level fluctuations within the bay that are not related to the coastal ocean sea level signal are forced by an east west sea level gradient resulting from tidally induced variations in sea level at specific beat frequencies that are enhanced in the northern reach of the bay river discharge into the bay through the sacramento and san joaquin river delta also contributes to the east west gradient but to a lesser degree

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

a three dimensional microgeodynamic model of melt geometry in the earth s deep interior 1 this article presents a three dimensional microgeodynamic model of grain melt geometry in partially molten rocks the isotropic unit cell of the partially molten rock is characterized by a face centered cubic symmetry consisting of rhombic dodecahedral grains the variation of surface tension between grain grain and grain melt contacts excites a coupled viscous flow within grains and the interstitial melt leading to a steady state grain melt geometry we obtain the fractional area of intergranular contact contiguity from these models as a function of melt volume fraction between melt fractions of 0 05 and 0 25 comparison with previous results indicates that the contiguity in three dimensional models is lower than in two dimensional models the contrast between two and three dimensional values of contiguity increases at high melt volume fractions we apply our model to the ultralow velocity zones ulvzs and the very low velocity province vlvp in the earth s core mantle boundary the observed seismic signature of the ulvz and vlvp can be explained by peridotite melt volume fractions between 0 08 and 0 12 and between 0 01 and 0 05 respectively in a matrix of elastic properties similar to the preliminary earth reference model

filamentous microfossils in a 3 235 million year old volcanogenic massive sulphide deposit the record of archaean microfossils is sparse1 of the few bona fide fossil assemblages most are from shallow water settings and they are typically associated with laminated stromatolitic sedimentary rocks2 3 4 microfossils from deep sea hydrothermal systems have not been reported in precambrian rocks 544 million years old although thermophilic microbes are ubiquitous in modern sea floor hydrothermal settings5 6 and apparently have the most ancient lineages7 8 here i report the discovery of pyritic filaments the probable fossil remains of thread like microorganisms in a 3 235 million year old deep sea volcanogenic massive sulphide deposit from the pilbara craton of australia from their mode of occurrence the micro organisms were probably thermophilic chemotropic prokaryotes which inhabited sub sea floor hydrothermal environments they represent the first fossil evidence for microbial life in a precambrian submarine thermal spring system and extend the known range of submarine hydrothermal biota by more than 2 700 million years9 such environments may have hosted the first living systems on earth consistent with proposals for a thermophilic origin of life10 11 12 13

buoyant melting instabilities beneath extending lithosphere 1 numerical models buoyant decompression melting instabilities in regions of partially molten upper mantle have been proposed to be an important process that might account for some characteristics of intraplate volcanism on earth and other terrestrial planets the instability is driven by variations in the melting rate within a partially molten layer whenever a relative decrease in density accompanies decompression melting of ascending mantle here the development of buoyant decompression melting instabilities in a plane layer of passively upwelling and partially melting mantle beneath diffusely extending lithosphere is studied using numerical convection models covering a wide range of physical parameters we find that the occurrence and nature of these instabilities in such a scenario is strongly affected by the rate of extension and melt percolation as well as depth distribution of solid density variations arising from melt depletion in some cases instabilities do not occur during extension but only develop after extension has slowed or stopped completely this behavior creates two pulses of magma generation due to passive upwelling accompanying extension followed by the subsequent instability and is favored by a faster rate of extension higher mantle viscosity higher rate of melt percolation and smaller amount of solid residuum depletion derived buoyancy larger degrees of solid density changes accompanying melt depletion can enhance the instability of partially molten mantle during extension but decrease the cumulative volume of generated melt this kind of behavior modifies the conventional expectation of spatially and temporally correlated volcanism and extension and may lend insight into the observed increase in localized volcanic activity following miocene basin and range extension in the western united states

insights on upper mantle melting rheology and anelastic behavior from seismic shear wave tomography in seismic tomography we map the wave speed structure inside the earth but we ultimately seek to interpret those images in terms of physical parameters this is challenging because many parameters can trade off with each other to produce a given wave speed the problem is compounded by the convention of mapping seismic structures as perturbations relative to a 1 d reference model rather than absolute wave speeds using a full waveform tomography model of europe as a case study we quantify the extent to which thermochemical and dynamic properties can be constrained using only s wave speed expressed in absolute values the wave speed distributions of this tomography model are compared with 4 million thermochemical models whose seismic properties are computed via thermodynamic modeling these models sample the full range of realistic mantle compositions including variable water and melt contents and mineral intrinsic anelasticity is taken into account intrinsic anelasticity causes waves to travel more slowly at higher temperatures leading to seismic attenuation but the sensitivity of the wave speed reduction to temperature is in turn controlled by the wave frequency global studies of surface waves indicate an anticorrelation between s wave speed and attenuation we therefore only retain thermochemical models satisfying this anticorrelation our study indicates that the frequency dependence of anelasticity depends on temperature or rheology with 0 1 being most appropriate in cold or lithospheric mantle and 0 3 in warmer regions i e the asthenosphere additionally the slowest regions require specific compositions and or a velocity weakening mechanism such as partial melting elastically accommodated grain boundary sliding or water

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

volcanic degassing hydrothermal circulation and the flourishing of early life on earth a review of the evidence from c 3490 3240 ma rocks of the pilbara supergroup pilbara craton western australia abstract new data gathered during mapping of c 3490 3240 ma rocks of the pilbara supergroup in the pilbara craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism for most cherts fossil hydrothermal fluid pathways are preserved as silica barite fe bearing veins that cut through the footwall and up to the level of individual bedded chert units but not above indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events silica barite fe bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units and occurred under alternating high sulphidation and low sulphidation conditions these veins provided pathways to the surface for elements leached from the footwall e g si ba fe and volcanogenic emissions from underlying felsic magma chambers e g co 2 h 2 s hs so 2 stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of warrawoona group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and or degassing of these magmas heat from these intrusions drove episodes of hydrothermal circulation listric normal faulting during caldera collapse produced basins with restricted circulation of seawater eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components thereby providing habitats suitable for early life forms fossil stromatolites from two distinct stratigraphic units in the north pole dome grew in shallow water conditions but in two very different geological settings with different morphologies stratiform and domical stromatolites in the stratigraphically lower c 3490 ma dresser formation of the warrawoona group are intimately associated with barite and chert precipitates from hydrothermal vents suggesting that component microbes may have been chemoautotrophic hyperthermophiles evidence of shallow water to periodically exposed conditions active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water tectonically active caldera and concentrated therein to produce an extreme habitat for early life widespread conical and pseudocolumnar stromatolites in the c 3400 ma strelley pool chert at the base of the unconformably overlying kelly group occur in shallow marine platform carbonates silicification was the result of later hydrothermal circulation driven by heat from the overlying newly erupted euro basalt the markedly different morphology and geological setting of these only slightly younger stromatolites compared with the dresser formation suggests a diversity of microbial life on early earth the biogenicity of putative microfossils from this and younger hydrothermal silica veins in the warrawoona group remains controversial and requires further detailed study

an upper mantle seismic discontinuity beneath the galapagos archipelago and its implications for studies of the lithosphere asthenosphere boundary an upper mantle seismic discontinuity the gutenberg or g discontinuity at which shear wave velocity decreases with depth has been mapped from s to p conversions in radial receiver functions recorded across the galapagos archipelago the mean depth of the discontinuity is 91 8 km beneath the southeastern archipelago and 72 5 km beneath surrounding regions the discontinuity appears deeper beneath the portion of the nazca plate that we infer passed over the galapagos mantle plume than elsewhere in the region we equate the depth of the g discontinuity to the maximum depth extent of anhydrous melting which forms an overlying layer of dehydrated and depleted mantle we attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the galapagos spreading center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 30 c the g discontinuity lies within a high seismic velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile bearing mantle material at the depth of the solidus for anhydrous mantle material removal of remaining water creates a sharp decrease in velocity with depth this discontinuity may also mark a site of melt accumulation results from seismic imaging the compositions of galapagos lavas and rare earth element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer we conclude that the g discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere

rapid in situ silicification of microbes at loburu hot springs lake bogoria kenya rift valley microbial mats located along the margins of hot spring pools and outflow channels at lake bogoria kenya are commonly silicified forming friable laminated crusts columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray silicification of the microbes involves impregnation of organic tissue by very fine amorphous silica particles and encrustation by small 2 m silica spheroids rapid silicification of the microbes which may begin while some are still alive can preserve sheaths and in some examples the filaments capsules and cells although this provides evidence of their general morphology the biological features that are required for taxonomic identifications are commonly poorly preserved the silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes almost all the silica at the loburu springs nucleates on microbial substrates this affinity of silica for functional groups on microbial surfaces contributes to the rapid silicification of the microbes and their preservation in modern and ancient cherts

origin and global tectonic significance of early archean cherts from the marble bar greenstone belt pilbara craton western australia abstract five sections of bedded chert in mafic ultramafic rocks of the archean warrawoona group in the marble bar greenstone belt pilbara craton were analyzed in order to understand their depositional environment and to provide some constraints on early archean tectonics the sections are divisible into two types based on their field occurrence mineralogy and geochemistry thicker ones a and b that overlie fe rich low k tholeiites and thinner ones c1 c2 and c3 overlying komatiitic basalts the thickest ferruginous section a 45 m thick is the marble bar chert of the towers formation and is interpreted to have been an in situ precipitate derived from a high t hydrothermal solution emanating from a mid oceanic ridge mor the following geochemical features are similar to those of modern hydrothermal iron rich sediments at a mor i p v zn and y are positively correlated with fe ii a positive eu anomaly normalized to chondrite decreases from 6 6 to 1 3 and the magnitude of a negative ce anomaly decreases from 0 6 to 1 0 as ree and lree hree increase the 13 m thick b section in the apex basalt dominated by sio2 and containing significant amounts of ba up to 4330 ppm originated from a low t mor hydrothermal solution this section is characterized by an association with massive black gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center geochemical evidence from the greenstones underlying the b section indicates that they are of morb origin in contrast to a and b c1 c2 and c3 sections are intercalated conformably with komatiitic basalts of the apex basalt and are 3 6 m thick the geochemical signatures of these three sections suggest that they were most likely formed by low t weak hydrothermal activity that may have been associated with hot spot volcanism they show strong enrichment of cr and ni reflecting a significant contribution of komatiitic basaltic detritus during sedimentation the presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity among the three sections the uppermost c3 exhibits greater enrichment in zr nb hf and th and higher th sc and la yb n relative to the lower c1 and c2 sections this could mean that the depositional environment of c3 was relatively closer to a continent or to island arcs composed of granitoids and or their volcanic equivalents siliceous mudstones and sandstones of the uppermost clastic rocks t section panorama formation have geochemical signatures analogous to those of felsic plutonic volcanic rocks high th sc negative eu anomalies and high ree in some siliceous mudstones from this unit imply that differentiated granitoids or volcanic equivalents were already exposed erupted at least locally in the pilbara at this time this study shows these warrawoona group cherts were deposited in a variety of environments ranging from a mid oceanic spreading center to a convergent plate boundary via a hot spot this variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the early archean

creation of residual flows in a partially stratified estuary the creation of residual flows in estuaries is examined using acoustic doppler current profiler data sets from northern san francisco bay the data sets are analyzed using principal component analysis to examine the temporal variability of the flows which create the residual circulation it is seen that in this periodically and partially stratified estuary the residual flows are created through a series of pulses with strong variability at the 24 hour timescale through the interaction of shear stratification and mixing this interaction is captured through the use of a dimensionless number the horizontal richardson number rix which is developed to examine the local balance between the stratifying and destratifying forces at the tidal timescale it is seen that rix is a valuable parameter in predicting the onset of the residual creating events with a threshold value of 3 on ebb tides this critical value is argued to be a threshold above which the stratification and shear flow create a feedback effect each further intensifying the other this feedback results in a highly variable exchange flow which creates the estuarine residual in intermittent pulses rather than as a steady flow although typically attributed to baroclinic forcing an argument is made that these pulses of residual creating exchange flow could be created by barotropic forcing in the presence of variable stratification which is asymmetric between flood and ebb tides this result poses a great challenge for turbulence modeling as the timing and magnitude of stratification and shear must be correctly simulated on the tidal timescale in order to reproduce the effects seen in the data sets presented

hydrothermal alteration and microfossil artefacts of the 3 465 million year old apex chert the earliest fossils of cyanobacteria have been controversially reported from the 3 465 million year old apex chert in western australia mineralogical analyses of the rock near the fossil site indicate that the site experienced repeated alterations at low and medium temperatures and is therefore unlikely to have preserved any early forms of life the 3 465 million year old apex chert of the pilbara craton in western australia has been controversially reported to contain some of the oldest fossil evidence for life tiny carbonaceous filaments interpreted as cyanobacteria1 yet on the basis of the presence of barite and native metals it has been suggested that the chert formed from hydrothermal fluids at temperatures greater than 250 c refs 2 3 an unlikely environment for cyanobacteria here we use scanning electron microscopy to assess the mineralogy and depositional setting of the chert veins surrounding the nodes that contain the microfossils in addition to rare native metals we find an assemblage of iron oxides and clay minerals which we interpret to have formed during repeated pulses of hydrothermal alteration at low to medium temperatures we also find micrometre sized silica structures that resemble microbial exopolymers4 and textures formed by the partial dissolution of tubular minerals that look similar to fossilized microbial mats5 other branched microstructures suggest post depositional colonization of microcracks and fissures by microbes5 these observations are not directly applicable to the carbonaceous filaments purported to be the earliest fossil evidence of life as we did not recover any in our sample however because of the observed hydrothermal and groundwater alteration we conclude that the apex chert is unlikely to have preserved any early forms of life

the generation and composition of partial melts in the earth s mantle abstract a set of equations is presented which combines the constraints of fluid dynamics and multicomponent phase equilibrium to provide a unified description of partial melting in the earth s mantle the equations are applied to a one dimensional model for pressure release melting of a simplified mantle material which contains only two chemical components exhibiting either a complete solid solution or b a binary eutectic in both cases melting occurs over a range of depths the unmelted crystalline residue matrix is modeled as a saturated porous medium through which the melt can migrate because of its differential buoyancy since melt interacts continuously with the matrix during ascent melting occurs by equilibrium rather than fractional fusion this equilibrium fusion is not the same as batch fusion however since material elements are quickly dispersed by migration of melt relative to the matrix to a first approximation the temperature profiles adiabats in the partially molten zone are independent of melt migration the slope of the adiabats varies in inverse proportion to the number of degrees of freedom which characterizes the melting melting of a complete solid solution occurs along a wet adiabat whose slope is controlled by absorption of latent heat melting of a eutectic system occurs along a steeper univariant adiabat until one solid phase is exhausted and subsequently along a wet adiabat the velocity of melt migration can exceed the mantle upwelling velocity by an order of magnitude or more the volume fraction of melt present is always less than the fraction of the material which has melted and is unlikely to exceed a few percent for a wide range of initial conditions melting of a eutectic system produces erupted melts having constant major element composition and widely varying trace element composition this result may provide a partial explanation for the characteristic major and lil element patterns observed in morb liquid compositional paths during pressure release melting cannot be determined from isobaric sections of phase diagrams and can only be calculated by solving the energy equation subject to the appropriate depth dependent phase equilibrium constraints

a small unextractable melt fraction as the cause for the low velocity zone abstract throughout the oceanic asthenosphere there exists a zone of seismic velocities that are lower than would be predicted the cause of this low velocity zone lvz whether partial melt or solid state mechanisms has been debated for decades we investigate the lvz by considering seismic and magnetotelluric data from tectonically stable 70 myr old lithosphere in the central pacific ocean we utilise recent experimental advances on the influence of partial melt and seismic attenuation results show that the lvz is characterised by a small volume of interconnected melt and by low hydrogen contents beneath the lvz the asthenosphere does not contain partial melt but has high hydrogen contents and low attenuation values indicating large grain sizes and or low oxygen fugacities to explain these observations we propose that a small amount of unextractable melt is trapped in the asthenosphere after melting at mid ocean ridges

low viscosity of the bottom of the earth s mantle inferred from the analysis of chandler wobble and tidal deformation abstract viscosity of the d layer of the earth s mantle the lowermost layer in the earth s mantle controls a number of geodynamic processes but a robust estimate of its viscosity has been hampered by the lack of relevant observations a commonly used analysis of geophysical signals in terms of heterogeneity in seismic wave velocities suffers from major uncertainties in the velocity to density conversion factor and the glacial rebound observations have little sensitivity to the d layer viscosity we show that the decay of chandler wobble and semi diurnal to 18 6 years tidal deformation combined with the constraints from the postglacial isostatic adjustment observations suggest that the effective viscosity in the bottom 300 km layer is 1019 1020 pa s and also the effective viscosity of the bottom part of the d layer 100 km thickness is less than 1018 pa s such a viscosity structure of the d layer would be a natural consequence of a steep temperature gradient in the d layer and will facilitate small scale convection and melt segregation in the d layer

the criteria for the biogeneicity of microbially induced sedimentary structures miss in archean and younger sandy deposits abstract the identification of fossils or biogenic sedimentary structures in rocks of archean age is difficult because similar lithological features could rise from purely physical or chemical processes alone therefore it is important to define criteria that serve the secure definition of a fossil or structure in question as of biological origin such criteria have been established for stromatolites and microfossils this contribution discusses the 6 criteria of biogeneicity of microbially induced sedimentary structures miss those structures are found in sandy deposits of early archean age to the present and rise from the interaction of benthic microbiota with physical sediment dynamics the six criteria for their biogeneicity are i miss occur in rocks of not more than lower greenschist facies ii in stratigraphic sections miss correlate with turning points of regression trangressions iii miss correlate with a characteristic depositional facies that enhances the development and the preservation of microbial mats iv the distribution of miss correlates with the ancient average hydraulic pattern v the geometries and dimensions of fossil miss correspond to that of the modern ones vi the miss include at least one of 9 specific microtextures

sensitivity of glacial isostatic adjustment to a partially molten layer at 410 km depth we present a sensitivity analysis aimed at testing whether observables related to glacial isostatic adjustment can support or refute the occurrence of a low viscosity melt rich layer mrl above the mantle transition zone as required by the transition zone water filter model bercovici karato 2003 in total 1600 model runs were performed sampling a range of mrl thicknesses 1 10 and 20 km and viscosities 10 15 10 19 pa s plausible viscosity values in the upper and lower mantle regions and four distinct ice histories to determine decay time constraints we consider relative sea level rsl data from two sites angerman river ar sweden and richmond gulf rg canada and use a new method of observational sea level data correction comparing model output of postglacial decay times and j 2 to observational constraints we find numerous possible solutions largely as a result of parameter trade off the investigated observables are sensitive to the existence of an mrl and reasonable variations in its thickness and viscosity the magnitude and nature of this sensitivity varies between the two data types as well as the adopted background viscosity structure decay time results from either considered location do not strictly support or exclude mrl existence however both locations offer mrl viscosity requirements for given thicknesses with ar being more restrictive rg constraints allow mrl viscosities as low as 10 16 pa s 10 km and 10 17 pa s 20 km ar results narrow these permitted viscosity ranges to 10 18 pa s or greater for both 10 and 20 km mrl thicknesses in the case of a 1 km thick mrl ar constraints permit the viscosity to be as low as 10 17 pa s whereas those of rg permit any mrl viscosity the decay time observations are satisfied by only a small subset of background mantle viscosities regardless of the mrl properties none of which support a spherically symmetric solution of earth viscosity finally comparing model output to the observed j 2 value did not provide any constraints on mrl properties however our results show that this observable has a strong preference for viscosity values in the lower mantle that are equal to or greater than 10 22 pa s

early biosphere magnifying ancient microstructures purported 3 465 million year old microfossils from australia have been the subject of considerable debate a method to distinguish between pristine fossils mineral artefacts and subsequent microbial contamination will aid the search for ancient biogenic material

diverse microstructures from archaean chert from the mount goldsworthy mount grant area pilbara craton western australia microfossils dubiofossils or pseudofossils abstract a diverse assemblage of indigenous carbonaceous microstructures classified here as highly probable microfossils to pseudomicrofossils is present in the ca 2 97 ga farrel quartzite gorge creek group at mount grant and mount goldsworthy pilbara craton western australia the microstructures are an integral part of the primary sedimentary fabrics preserved in black chert beds the interbedding of chert with layers of large silicified crystal pseudomorphs and fine to coarse grained volcaniclastic clastic beds indicate deposition in a partially evaporitic basin with terrigenous clastic and volcaniclastic input similar associations of microstructures are present at the same stratigraphic level in outcrops more than 2 km apart four major microstructural types are present thread like film like spheroidal and lenticular to spindle like each of which can be further subdivided into several sub types most of the microstructures were deposited as part of an assemblage of chemical and clastic sediments although there are some thread like microstructures present for which a synsedimentary origin cannot be confirmed many specimens appear to have originally had flexible but breakable walls and some occur in colony like aggregations size distributions for the four major types are generally narrow a feature typical of biogenic structures the microstructures are composed of disordered carbon as revealed by raman spectroscopy and the bulk isotopic composition of the carbon is 13 c the combined morphological and geological evidence suggests that the film like structures small spheres associated with films large spheroids and spindle like structures are probable to highly probable fossil remains of microorganisms the morphological variety among the microstructures suggests that a diverse microbial ecosystem flourished in the pilbara region during the archaean

a simple model of estuarine subtidal fluctuations forced by local and remote wind stress observations of estuarine low subtidal sea level and current fluctuations have often shown domination by the remote effects of the wind acting on the adjacent coastal ocean over the local surface stress acting on the estuary itself the remote effects are transmitted to the estuary by impressment on its mouth of sea level change indtlced by the onshore component of coastal ekman transport and become increasingly dominant as the frequency decreases a simple barotropic model is developed to investigate the joint action of these two wind forcing mechanisms the relative shortness of most estuaries relative to low subtidal estuarine wavelengths explains the dominance of the remote effect for both sea level and barotropic current fluctuations in the estuary for the same reason however surface slope is dominated by local wind setup for an estuary with axis nearly parallel to the coast the two effects will operate either in concert or in opposition depending on hemisphere and orientation of the estuary axis relative to the coast for the geometries of both chesapeake bay and the delawm e estuary the model predicts opposition with the remote effect dominant at lower frequencies consistent with recent observations

viscosity of oceanic asthenosphere inferred from remote triggering of earthquakes a sequence of large interplate earthquakes from 1952 to 1965 along the aleutian arc and kurile kamchatka trench released accumulated stresses along nearly the entire northern portion of the pacific plate boundary the postseismic stress evolution across the northern pacific and arctic basins calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10 17 pascal seconds is consistent with triggering of oceanic intraplate earthquakes temporal patterns in seismicity at remote plate boundaries and space based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30 year period after the sequence

high and highly anisotropic electrical conductivity of the asthenosphere due to hydrogen diffusion in olivine experimental studies on electrical conductivity in hydrated olivine had been made only at relatively low temperatures the extrapolation of these data to asthenosphere temperatures shows that explaining high and highly anisotropic conductivity by hydrogen is difficult we present new experimental results on the electrical conductivity in hydrated olivine single crystals measured under a broader temperature range than before t temperature 573 1373 k at p pressure 4 gpa the new results agree well with the previous results at low temperatures t 1000 kt 1000 k substantial deviations from the previous results are observed i anisotropy in conductivity increases with temperature and ii activation energies at high temperature regime are substantially higher than those at lower t the high temperature behavior is consistent with a model invoking diffusion of two protons at m site vacancy whereas the low temperature behavior suggests conduction by more mobile species such as free proton the present results i explain the discrepancy between different previous studies conducted under different temperature regimes and ii show that most of geophysical observations on the electrical conductivity in the asthenosphere including the high and highly anisotropic conductivity can be explained by high water content in the asthenosphere that is consistent with the geochemical model combining with a previous model of low seismic wave velocity of the asthenosphere a subsolidus model invoking the role of hydrogen provides a unified explanation for the geophysical anomalies of the asthenosphere

evidence of archean life stromatolites and microfossils abstract fossil evidence of the existence of life during the archean eon of earth history 2500 ma is summarized data are outlined for 48 archean deposits reported to contain biogenic stromatolites and for 14 such units that contain a total of 40 morphotypes of described microfossils among the oldest of these putatively microfossiliferous units is a brecciated chert of the 3465 ma apex basalt of western australia the paleoenvironment carbonaceous composition mode of preservation and morphology of the apex microbe like filaments backed by new evidence of their cellular structure provided by two and three dimensional raman imagery support their biogenic interpretation such data together with the presence of stromatolites microfossils and carbon isotopic evidence of biological activity in similarly aged deposits indicate that the antiquity of life on earth extends to at least 3500 ma

dynamics and structure of a stirred partially molten ultralow velocity zone abstract the seismic properties of thin 5 40 km thickness patches exhibiting low seismic velocities termed ultralow velocity zones or ulvz just above the core mantle boundary cmb might be explained by the presence of partially molten rock where a liquid phase occupies interstices within a skeletal network of solid grains however a key problem with this explanation is that in the absence of improbably strong surface tension effects partial melt is expected to drain by percolation over geological time scales to form a dense melt rich layer at the cmb with physical properties that are inconsistent with seismic and other geophysical constraints here we consider whether stirring within ulvz driven by viscous coupling to convective motions in the overlying mantle can inhibit the production of such stratification and maintain a partially molten region with a structure and constitution comparable to what is inferred seismically we use two dimensional numerical simulations of the response of a melt solid mixture to stirring imposed from above and scaling analysis to identify conditions leading to melt separation retention and drainage over a broad range of parameters we find that melt migration at plausible ulvz conditions is governed predominantly by dynamic pressure gradients arising from the viscous deformation related to mantle stirring rather than by the buoyancy effects driving melt percolation in particular dense melt that would otherwise drain downward and accumulate at the cmb is expected to remain in suspension as a result of the stirring driven within ulvz in addition our model predicts that partially molten ulvz patches will be characterized by a positive gradient in seismic shear velocity i e increasing with depth consistent with seismic inferences and may persist in this state over geological time scales

a wet heterogeneous lunar interior lower mantle and core dynamo evolution while recent analyses of lunar samples indicate the moon had a core dynamo from at least 4 2 3 56 ga mantle convection models of the moon yield inadequate heat flux at the core mantle boundary to sustain thermal core convection for such a long time past investigations of lunar dynamos have focused on a generally homogeneous relatively dry moon while an initial compositionally stratified mantle is the expected consequence of a postaccretionary lunar magma ocean furthermore recent re examination of apollo samples and geophysical data suggests that the moon contains at least some regions with high water content using a finite element model we investigate the possible consequences of a heterogeneously wet compositionally stratified interior for the evolution of the moon we find that a postoverturn model of mantle cumulates could result in a core heat flux sufficiently high to sustain a dynamo through 2 5 ga and a maximum surface dipolar magnetic field strength of less than 1 t for a 350 km core and near 2 t for a 450 km core we find that if water was transported or retained preferentially in the deep interior it would have played a significant role in transporting heat out of the deep interior and reducing the lower mantle temperature thus water if enriched in the lower mantle could have influenced core dynamo timing by over 1 0 gyr and enhanced the vigor of a lunar core dynamo our results demonstrate the plausibility of a convective lunar core dynamo even beyond the period currently indicated by the apollo samples

ridge asymmetry and deep aqueous alteration at the trench observed from rayleigh wave tomography of the juan de fuca plate using rayleigh wave tomography of noise removed ocean bottom seismometer obs data from the cascadia initiative we illuminate the structure of the upper mantle beneath the juan de fuca plate beneath the juan de fuca ridge there is strong asymmetry with a pronounced low velocity zone in the 25 65 km depth range extending to the west from the spreading axis this anomaly has velocities low enough to indicate the presence of melt the asymmetry in velocity structure and the much greater abundance of seamounts on the west flank of the ridge suggest that dynamic buoyant upwelling is important perhaps triggered by thermal or compositional anomalies beneath axial seamount in contrast there is no evidence for asymmetry in the axial zone or lower than expected velocities beneath the gorda ridge on the eastern flank of the juan de fuca ridge the shear velocity in the 25 65 depth range is higher than expected the lithosphere appears to be colder and thicker than predicted by standard plate cooling models perhaps caused by the downwelling counterpart of the upwelling on the west side of the ridge close to the trench there is a sharp decrease in shear velocity we interpret this as aqueous alteration caused by hydrothermal circulation through deep normal faults associated with bending of the plate beneath the astoria and nitinat fans where abyssal plain sediment is thickest the velocity decrease is much smaller which is consistent with a thick sediment cap that prevents hydrothermal alteration of the plate

thermal structure radial anisotropy and dynamics of oceanic boundary layers defining the oceanic lithosphere as a thermal boundary layer allows to explain to first order age dependent bathymetry and isotropic wave speeds in contrast ss precursors and receiver functions suggest a subhorizontal interface within this layer on top of a radially anisotropic zone comparing a suite of geodynamic scenarios against surface wave dispersion data and seismic discontinuities we find that only weak age dependency of the radially anisotropic zone is compatible with observations we show that this zone is confined from below by a second weaker seismic interface while observed azimuthal anisotropy is consistent with lattice preferred orientation of olivine due to asthenospheric flow underneath the lithosphere radial anisotropy requires additional contributions perhaps from petrological fabrics or melt ponding this implies that seismic reflectors previously associated with the base of the lithosphere are instead associated with preserved structures embedded in it they carry information about plate formation but have little control on plate deformation

shear wave velocity and radial anisotropy structures beneath the central pacific from surface wave analysis of obs records abstract the central pacific plate is an ideal area to study the dynamics of the lithosphere asthenosphere system as this region is far away from other dynamics systems such as subduction zones and volcanic hotspots of mantle plumes and has experienced simple oceanic evolution in this study we collect three component continuous seismic data and differential pressure gauge data recorded by 15 broadband ocean bottom seismometers obss from the nomelt experiment we process continuous seismic noise data to obtain phase velocities of the fundamental 6 30 s and first high 5 11 s mode rayleigh waves and fundamental mode love waves 5 12 s we apply a two plane wave method to teleseismic rayleigh waves to obtain the fundamental mode phase velocities at 20 150 s periods and apply high resolution linear radon transform to teleseismic love waves to obtain phase velocities for the fundamental mode love waves at 28 48 s periods then we jointly invert the average phase velocities of rayleigh and love waves to construct 1 d isotropic shear wave velocity and radial anisotropy from the seafloor to 150 km depth beneath the nomelt region our 1 d model shows a low velocity zone lvz beneath a high velocity lid by comparing our results with the predictions of partial melt models we suggest the lvz is attributed to the presence of a small amount of melt in the asthenosphere strong radial anisotropy 5 6 is observed in the crust but the mantle lithosphere is characterized by weak radial anisotropy

restricted shallow water sedimentation of early archean stromatolitic and evaporitic strata of the strelley pool chert pilbara block western australia abstract the 3 4 ga old strelley pool chert is a 25 m thick sedimentary unit near the top of the predominantly volcanic warrawoona group in greenstone belts of the eastern pilbara block western australia it is here subdivided into 5 members containing 13 lithofacies the basal member i is composed of quartzose sandstone deposited in a high energy wave or tide dominated shallow water system overlying this are members ii and iii which make up the bulk of the formation and were deposited in a low energy partially restricted hypersaline basin they record a predominantly regressive succesion of deposits including subaqueous laminite stromatolite and evaporite stromatolite carbonaceous laminite black and white banded chert evaporite and intraformational detrital units deposited under intermittently to predominantly exposed conditions and subaerially deposited windblown sand evaporite and evaporite solution layers members iv and v record the progradation of a volcaniclastic alluvial fringe the strelley pool chert represents an association of sedimentary environments directly comparable to that observed in modern low energy shallow marine carbonate evaporite systems such as along the trucial coast of the persian gulf and abundantly developed in phanerozoic carbonate platform deposits there is no evidence however that uniquely identifies the environment as having been marine deposition may have taken place in either a large hypersaline lake or a restricted marine basin evidence of predominantly low energy depositional conditions and a paucity of terrigenous detritus indicate that sedimentation was dominated by orthochemical and biological processes silicified evaporites including coarsely crystalline layers resembling messinian selenite are widespread and similar to younger evaporite deposits they clearly indicate that evaporites were common within shallow water archean sequences the presence of an assemblage of biogenic deposits including organic laminite stromatolites encrusting carbonaceous mats carbonaceous granules and oncolites deposited under conditions ranging from fully subaqueous to nearly subaerial and locally evaporitic points to the existence of an ecologically and probably biologically diverse microbial community 3 4 ga ago

early life recorded in archean pillow lavas pillow lava rims from the mesoarchean barberton greenstone belt in south africa contain micrometer scale mineralized tubes that provide evidence of submarine microbial activity during the early history of earth the tubes formed during microbial etching of glass along fractures as seen in pillow lavas from recent oceanic crust the margins of the tubes contain organic carbon and many of the pillow rims exhibit isotopically light bulk rock carbonate 13 c values supporting their biogenic origin overlapping metamorphic and magmatic dates from the pillow lavas suggest that microbial life colonized these subaqueous volcanic rocks soon after their eruption almost 3 5 billion years ago

asthenospheric upwelling beneath northeastern margin of ordos block constraints from rayleigh surface wave tomography abstract phase velocities at periods of 20 143 s are inverted from rayleigh surface waves recorded at 201 stations of permanent and temporary seismic network deployed in the northern ordos block we obtained the shear wave velocities in the upper mantle from phase velocities our velocity models clearly reveal a striking low velocity anomaly zone in the upper mantle beneath the northeastern margin of the ordos block the low velocity zone at periods of 50 100 s extends from the north end of the luliangshan mountain to the north border of the western north china craton the minimum shear wave velocity is approximately 4 2 km s and the minimum lithospheric thickness is approximately 80 km in the low velocity anomaly zone our shear wave velocity results indicate that lithospheric thinning asthenospheric upwelling and partial melting may occur in the region during the cenozoic high temperature and thinned lithosphere may be significant factors that cause partial melting in the asthenospheric mantle during the cenozoic asthenospheric upwelling may be associated with the lithospheric stretching caused by the extension stress in the northeastern margin of the ordos block the extension stress may be contemporaneous with the regional extension of various stages which is related to the pacific subduction and the far field effect of the indian eurasian collision in the cenozoic

seismic evidence for a global low velocity layer within the earth s upper mantle above the 410 km discontinuity in the upper mantle an intermittent layer characterized by low seismic wave velocities is thought to represent partial melting in areas of subduction or mantle plume activity seismic data now show that the low velocity layer extends globally with no affinity to a particular tectonic environment

successive geothermal volcanic hydrothermal and contact metasomatic events in cenozoic volcanic arc basalts south shetland islands antarctica hydrothermal alteration in volcanic arcs occurs in many settings and may involve magmatic marine lacustrine or groundwaters driven by magmatic tectonic or thermal events king george island part of the south shetland island cenozoic volcanic arc contains an 80 km long zone of propylitized volcanic rocks with numerous occurrences of quartz veining silicic sericitic argillic and advanced argillic alteration on barton peninsula a basaltic lava sequence 49 44 ma intruded by a small high level granodiorite pluton 42 ma contains these alteration types previously interpreted as a single porphyry copper system in this study we report three possibly four distinct fossil hydrothermal episodes 1 banded chalcedonic quartz quartz sericite and propylitic alteration occurs along ese faults and as reworked clasts in nearby tuffs drusy quartz calcite veins with silicic sericitic argillic and propylitic wallrocks may represent feeders to the near surface silicification these characteristics and anomalous ag pb sb au plus te se zn as suggest a neutralph geothermal system that was active during volcanism 2 the lavas and banded quartz rocks were brecciated veined and replaced by alunite native sulphur pyrite and pyrophyllite quartz pyrite zunyite diaspore assemblages with anomalous hg se as bi au tl sb cu such advanced argillic alteration is diagnostic of degassing of a felsic magma into shallow 500 m meteoric groundwaters rhyolite tuffs previously not reported on king george island may represent leakage of this magma to the surface 3 subsequent burial to 3 km was followed by emplacement of a granodiorite pluton and formation of a silicic contact metasomatic aureole containing muscovite biotite actinolite magnetite k feldspar and tourmaline disseminated andalusite corundum also formed in areas previously affected by the advanced argillic alteration iron copper sulphide veinlets are locally abundant but a porphyry style geochemical signature is not present early cretaceous ar ar ages near the intrusive contact indicate flow of an excess ar bearing hydrothermal plume up the contact finally isolated areas of propylitic alteration in the lavas nearby may be related either to quartz veins of episode 1 at depth or to 4 continued circulation of heated groundwaters around the cooling pluton

mantle dynamics beneath the east pacific rise at 17 s insights from the mantle electromagnetic and tomography melt experiment 1 the electromagnetic data from the mantle electromagnetic and tomography melt experiment are inverted for a two dimensional transversely anisotropic conductivity structure that incorporates a correction for three dimensional topographic effects on the magnetotelluric responses the model space allows for different conductivity values in the along strike cross strike and vertical directions along with imposed constraints of model smoothness and closeness among the three directions anisotropic models provide a slightly better fit to the data for a given level of model smoothness and are more consistent with other geophysical and laboratory data the preferred anisotropic model displays a resistive uppermost 60 km thick mantle independent of plate age except in the vicinity of the ridge crest in most inversions a vertically aligned sheet like conductor at the ridge crest is especially prominent in the vertical conductivity its presence suggests that the melt is more highly concentrated and connected in the vertical direction immediately beneath the rise axis the melt zone is at least 100 km wide and is asymmetric having a greater extent to the west off axis and to the east of the ridge the mantle is more conductive in the direction of plate spreading at depths greater than 60 km the flat resistive conductive boundary at 60 km agrees well with the inferred depth of the dry solidus of peridotite and the deeper conductive region is consistent with the preferred orientation of olivine inferred from seismic observations this suggests that the uppermost 60 km represents the region of mantle that has undergone melting at the ridge and has been depleted of water dissolved hydrogen by contrast the underlying mantle has retained a significant amount of water

alongshore wind forcing of coastal sea level as a function of frequency abstract the amplitude of the frequency response function between coastal alongshore wind stress and adjusted sea level anomalies along the west coast of the united states increases linearly as a function of the logarithm log10 of the period for time scales up to at least 60 and possibly 100 days the amplitude of the frequency response function increases even more rapidly at longer periods out to at least 5 yr at the shortest periods the amplitude of the frequency response function is small because sea level is forced only by the local component of the wind field the regional wind field which controls the wind forced response in sea level for periods between 20 and 100 days not only has much broader spatial scales than the local wind but also propagates along the coast in the same direction as continental shelf waves hence it has a stronger coupling to and an increased frequency response for sea level at periods of a year or more observed coastal sea level fluctuations are not only forced b

late pleistocene and holocene sea level change in the australian region and mantle rheology summary spatial and temporal variations in sea level are produced by the melting of the late pleistocene ice and by the earth s response to the redistribution in surface loads by examining different parts of the sea level curves of the past 20 000 yr from geographically widely distributed regions it becomes possible to constrain models of the melting history of the ice sheets and of the earth s rheology observations from sites away from the former arctic ice sheets such as the australian and south pacific region are particularly important for constraining the total meltwater volumes added into the oceans in the past 20 000 yr and the rates at which this occurred these observations indicate that the antarctic ice sheets provided a significant contribution to the sea level rise at a rate that was approximately synchronous with the melting of the laurentide ice sheet except for the interval 9000 6000 yr ago when it may have lagged behind minor melting of the antarctic ice sheet appears to have continued throughout the late holocene differential observations of the late holocene sea level change recorded at sites in the same region are particularly useful for estimating parameters describing the earth s non elastic response to surface loading the effective parameters used here are a lithospheric thickness an upper mantle viscosity and a lower mantle viscosity describing the response below 670 km depth with the observations used here it is not possible to separate the lithospheric thickness h from the upper mantle viscosity and the viscosity results are based on the assumption that 50 h 100 km neither do these observations provide a resolution of the depth dependence of viscosity in the upper mantle and the resulting estimates are effective parameters only differential sea levels along continental margins and along the shores of large gulfs and bays constrain the effective upper mantle viscosity to be about 1 2 1020 pa s 1 while differential values from islands of different sizes are suggestive of a somewhat lower value the lower mantle taken to be below 670 km depth viscosity is about two orders of magnitude greater than this the estimated ice and rheological models explain many of the holocene sea level observations throughout the australian and southern pacific region the tilting of continental margins as exemplified by observations of variable amplitudes of the holocene high stands and the variable times at which sea levels first reached their present level along the north queensland coast and great barrier reef is well represented by these models differential holocene sea levels observed along the narrow spencers gulf of south australia are also well explained by the models and in neither case is it necessary to invoke tectonic motions predicted late holocene sea levels at small to medium sized islands are characterized by small amplitude high stands that reached their maximum values about 4000 2000 yr ago consistent with observations from the society cook and tuamotu islands

electromagnetic evidence for volatile rich upwelling beneath the society hotspot french polynesia we have conducted a seafloor magnetotelluric survey that images for the first time three dimensional electrical conductivity structure in the upper mantle beneath the society hotspot a striking feature in our model is a high conductivity anomaly a few hundred kilometers in diameter which is continuous from the lowest part of the upper mantle to a depth of approximately 50 km below sea level using theoretical and experimental results from mineral physics we interpret the high conductivity anomaly as evidence of the melt fraction up to 2 2 vol which is robust regardless of assumed temperature and the existence of carbonated silicate melt beneath the hotspot our results suggest that the society hotspot is a pathway for ascending volatiles from the deeper part of the upper mantle to the surface

buoyant melting instabilities beneath extending lithosphere 2 linear analysis 1 in a companion paper numerical models reveal that buoyant melting instabilities can occur beneath extending lithosphere for a sufficiently small mantle viscosity extension rate and rate of melt percolation however in some cases instabilities do not develop during extension but only occur after extension slows or stops these results are suggestive of a critical behavior in the onset of these kinds of instabilities and motivate a linear analysis to study the onset of instability in a partially melting passively upwelling plane layer of mantle beneath extending lithosphere the model we employ includes the effects of buoyancy arising from thermal expansion the presence of a retained fraction of partial melt and depletion of the solid by melt extraction we find a critical behavior in the onset of instability controlled by melt retention buoyancy that is characterized by a rayleigh number m such that m must exceed some critical value mcrit which depends on the efficiency of stokes rise of a partially molten body relative to the rate of background percolation comparison of this theory to the numerical results in the companion paper yields a close quantitative agreement we also find that solid depletion buoyancy can either stabilize or destabilize a partially melting layer depending upon both the distribution of preexisting depletion and the magnitude of density changes with depth this theory is compared with previous studies of buoyant melting instabilities beneath mid ocean ridges where similar behavior was reported and it suggests that the stability of passively upwelling partially melting mantle underlying both narrow and wide rift settings is controlled by similar processes

sea level response to enso along the central california coast how the 1997 1998 event compares with the historic record abstract long term monthly sea level and sea surface temperature sst anomalies from central california show that during winter months positive anomalies are associated with el nino events and the negative ones with la nina events there is no significant impact on monthly mean anomalies associated with pacific decadal oscillations although there is a tendency for more extreme events and greater variance during positive decadal oscillations the very strong 1997 1998 el nino was analyzed with respect to the long term historic record to assess the forcing mechanisms for sea level and sst beginning in the spring of 1997 we observed several long period 30 days fluctuations in daily sea level with amplitudes of over 10 cm at san francisco california fluctuations of poleward long period alongshore wind stress anomalies awsa are coherent with the sea level anomalies however the wind stress cannot entirely account for the observed sea level signals the sea level fluctuations are also correlated with sea level fluctuations observed further south at los angeles and tumaco columbia which showed a poleward phase propagation of the sea level signal we suggest that the sea level fluctuations were to a greater degree forced by the passage of remotely generated and coastally trapped waves that were generated along the equator and propagated to the north along the west coast of north america however both local and remote awsa can significantly modulate the sea level signals the arrival of coastally trapped waves began in the spring of 1997 which is earlier than previous strong el nino events such as the 1982 1983 event

does partial melting explain geophysical anomalies abstract the existence of partial melt is frequently invoked to explain geophysical anomalies such as low seismic wave velocity and high electrical conductivity i review various experimental and theoretical studies to evaluate the plausibility of this explanation in order for a partial melt model to work not only the presence of melt but also the presence of appropriate amount of melt needs to be explained using the mineral physics observations on the influence of melt on physical properties and the physics and chemistry of melt generation and transport i conclude that partial melt model for the asthenosphere with homogeneous melt distribution does not work one needs to invoke inhomogeneous distribution of melt if one wishes to explain observed geophysical anomalies by partial melting however most of models with inhomogeneous melt distribution are either inconsistent with some geophysical observations or the assumed structures are geodynamically unstable and or implausible therefore partial melt models for the geophysical anomalies of the asthenosphere are unlikely to be valid and some solid state mechanisms must be invoked the situation is different in the deep upper mantle where melt could completely wet grain boundaries and continuous production of melt is likely by dehydration melting at around 410 km in the ultralow velocity zone in the d layer where continuous production of melt is unlikely easy separation of melt from solid precludes the partial melt model for low velocities and high electrical conductivity unless the melt density is extremely close to the density of co existing solid minerals or if there is a strong convective current to support the topography of the ulvz region compositional variation such as fe enrichment is an alternative cause for the anomalies in the d layer

comment on biogenicity of earth s earliest fossils a resolution of the controversy by j william schopf and anatoliy b kudryavtsev gondwana research 22 2012 761 771 recently schopf and kudryavtsev 2012 have published an exhaustive review on the biogenicity of the most controversial evidence of ancient life on earth to date an assemblage of carbonaceous filaments found in the 3465 ma apex chert of northwestern australia schopf and packer 1987 schopf 1993 the morphology of the filaments their carbon isotopic composition and the sedimentary environment interpreted as shallow marine suggested to schopf 1993 that these filaments were fossilized cyanobacteria this discovery sparkled a large controversy among the scientific community that lasted for more than 25 years see pinti and altermann 2011 for a review on the subject in the synopsis of the controversy page 762 schopf andkudryavtsev 2012 wrote the apex fossils have subsequently been suggested to be inorganic silica or clay mineral needle like crystals pinti et al 2009 again at page 769 the authors reported that needles composed of halloysite al2si2o5 oh 4 a clay mineral and of quartz sio2 have been compared with the apex fossils pinti et al 2009 these are two serious misstatements because we never compared any of the pseudofossils that we observed in apex chert to the microfossils of schopf 1993 schopf and kudryavtsev 2012 themselves report at

complete wetting of olivine grain boundaries by a hydrous melt near the mantle transition zone many physical properties of liquid bearing rocks in the deep earth s interior are strongly controlled by its wetting behavior we report experimental results on the variation of dihedral angle and microstructures in the forsterite h2o system at 1200 c and pressures ranging from 1 to 13 gpa the dihedral angle of the system decreases systematically with pressure and above the mantle transition zone 400 km depth it becomes approximately 0 corresponding to completely wetted grain boundaries this condition is probably reached because of a decrease in the solid liquid interfacial energy with pressure due to the dramatically increased solubility of silicates in the liquid these results suggest that the presence of partial melt would have drastic influences on physical properties of upper mantle around the transition zone even if the melt fraction is very low low velocity regions just above the mantle transition zone may result from the presence of hydrous melt 2007 elsevier b v all rights reserved

electrical conductivity of basaltic and carbonatite melt bearing peridotites at high pressures implications for melt distribution and melt fraction in the upper mantle abstract electrical impedance measurements were performed on two types of partial molten samples with basaltic and carbonatitic melts in a kawai type multi anvil apparatus in order to investigate melt fraction conductivity relationships and melt distribution of the partial molten mantle peridotite under high pressure the silicate samples were composed of san carlos olivine with various amounts of mid ocean ridge basalt morb and the carbonate samples were a mixture of san carlos olivine with various amounts of carbonatite high pressure experiments on the silicate and carbonate systems were performed up to 1600 k at 1 5 gpa and up to at least 1650 k at 3 gpa respectively the sample conductivity increased with increasing melt fraction carbonatite bearing samples show approximately one order of magnitude higher conductivity than basalt bearing ones at the similar melt fraction a linear relationship between log conductivity bulk and log melt fraction can be expressed well by the archie s law archie 1942 bulk melt c n with parameters c 0 68 and 0 97 n 0 87 and 1 13 for silicate and carbonate systems respectively comparison of the electrical conductivity data with theoretical predictions for melt distribution indicates that the model assuming that the grain boundary is completely wetted by melt is the most preferable melt geometry the gradual change of conductivity with melt fraction suggests no permeability jump due to melt percolation at a certain melt fraction the melt fraction of the partial molten region in the upper mantle can be estimated to be 1 3 and 0 3 for basaltic melt and carbonatite melt respectively

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

subtidal sea level and current variations in the northern reach of san francisco bay abstract analyses of sea level and current meter data using digital filters and a variety of statistical methods show a variety of phenomena related to non local coastal forcing and local tidal forcing in the northern reach of san francisco bay a partially mixed estuary low frequency variations in sea level are dominated by non local variations in coastal sea level and also show a smaller influence from tidally induced fortnightly sea level variations low frequency currents demonstrate a gravitational circulation which is modified by changes in tidal current speed over the spring neap tidal cycle transients in gravitational circulation induce internal oscillations with periods of two to four days

partial melting in a thermo chemical boundary layer at the base of the mantle abstract seismological detections of complex structures in the lowermost mantle boundary layer the d region motivate a conceptual model of a compositionally stratified thermo chemical boundary layer tcbl within which lateral temperature variations sustained by large scale mid mantle flow cause variations of partial melt fraction partial melt fractions of from 0 to 30 in the tcbl occur due to the eutectic of the boundary layer material lying below the high temperature at the core mantle boundary cmb coupled with the presence of steep thermal gradients across the boundary layer regions of the tcbl with the highest temperatures have extensive partial melting producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics this tcbl concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as predominance of large scale volumetric elastic wave velocity heterogeneity in the boundary layer laterally extensive but intermittent abrupt shear and compressional velocity increases and decreases at the top of the d region small scale topography of d velocity discontinuities thin ultra low velocity zones at the cmb widespread shear wave anisotropy in d bulk sound velocity anomalies detected in low shear velocity regions of d and large scale upwellings from the most extensively melted regions of the boundary layer neutral or negative buoyancy of the partial melt in the tcbl is required along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle more complex models in which the partially melted chemical boundary layer is laterally displaced by mid mantle downwellings are also viable but these appear to require additional special circumstances such as a phase change efficient segregation of slab crustal material or abrupt onset of anisotropy in order to account for rapid velocity increases at the top of d the precise nature of the compositional anomaly or anomalies in d and the eutectic composition of this zone are yet to be determined but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity increased resolution geodynamic calculations of dense boundary layer structures with partial melting and attendant viscosity reductions and chemical heterogeneity are needed to quantify this tcbl concept

the 154 year record of sea level at san francisco extracting the long term trend recent changes and other tidbits a data adaptive method called ensemble empirical mode decomposition eemd is used to examine the 154 year record of monthly sea level at san francisco the mode that is lowest in frequency corresponds to the long term trend the next highest mode corresponds to an oscillation with a period of 100 years and may be related to solar variability when this mode is combined with the long term trend the rate of increase in sea level starts to decrease by 1980 the next lower mode corresponds to interdecadal time scales and thus includes the pacific decadal oscillation when combined with the two lower modes sea level itself starts to decrease by the mid 1990s these results are consistent with the most recent results from the intergovernmental panel on climate change ipcc and may be the first obtained from a tidal record prior to conducting eemd corrections for glacial isostatic adjustment gia and the inverse barometer ib effect were applied the effect of applying the gia correction was relatively small but the ib correction reduced the slope of the long term trend in sea level by almost 15 this reduction is due to a long term increase in the variance of sea level pressure to determine if the 10 15 year enso modulation cycle could be detected from the decomposition we first compared the envelope from the mode associated with enso with the two adjacent modes that were lower in frequency spectral analysis revealed no significant maxima in the enso mode envelope but a major peak in the spectrum for the two adjacent modes with a period of 12 8 years this is consistent with a local response to el nino warming for the enso mode but a non local response for the two adjacent modes a similar analysis was performed for the southern oscillation index and a spectral maximum was found between 12 and 16 years consistent with our non local interpretation of the previous two modes

estimation of regional sea level change in the pearl river delta from tide gauge and satellite altimetry data the present study proposes a reconstruction of regionally consistent sea level anomalies in the pearl river delta prd over the period 1959 2011 spatial empirical orthogonal functions eofs derived from satellite altimetry dataset and the corresponding time series of tide gauge records were combined to generate regional sea level anomalies based on these datasets regionally consistent sea level anomalies rcsla are reconstructed using a dimension reducing method known as principal components analysis the results show that the accuracy of reconstruction is sensitive to the number of the available tide gauge records however no significantly effect of the length of the records is observed the results also indicate that the eof reconstruction method addresses issues such as the relatively short term coverage of satellite altimetry data and the sparse and discontinuous nature of tide gauge records demonstrating the applicability of this technique in investigation of long term sea level change both river flow and el nino event have considerable impacts on sea level variability in the prd c 2014 elsevier ltd all rights reserved

geophysical constraints on partial melt in the upper mantle this paper addresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical anomaly typical observed anomalies are high electrical conductivity of the order of 0 1 s m or greater velocity decreases of 7 10 seismic q values less than 100 and a frequency band for seismic effects in the region near 1 hz existing theories of electrical conduction in partial melts and of frequency dependent seismic properties together with recent measurements of melt electrical conductivity viscosity and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods from electrical anomalies mainly sensitive to melt volume and its interconnection one can require a minimum melt fraction of several percent at temperatures close to the solidus 1150 1300 c however seismic models demand only a small volume in very flattened shapes aspect ratio 0 001 melt fraction 0 1 further if melt configuration permits seismic dissipation in bulk that is there exist flattened voids intersecting more or less equant voids then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies observed equilibrium textures of partly melted peridotite together with inferred melt solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern thus if partial melt causes mantle geophysical anomalies it should exist in a variety of void shapes and probably of sizes while the association of electrical and elastic anomalies with indications of reduced density volcanism and high heat flow makes the hypothesis of partial melting an attractive explanation the minimum physical requirement is for existence of relatively high temperature