change in climate ocean and ice sheet conditions in the ross embayment antarctica at 6 ka evidence from the ross embayment antarctica suggests an abrupt cooling and a concomitant increase in sea ice cover at about 6000 sp 6 ka stable isotope delta d concentrations in the taylor dome ice core at the western edge of the ross embayment decline rapidly after 6 ka and continue to decline through the late holocene methanesulfonic acid concentrations at taylor dome show opposite trends to delta d sediment cores from the western ross sea show a percentage minimum for the sea ice diatom fragilariopsis curta between 9 and 6 ka when taylor dome delta d values are highest followed by an increase through the late holocene radiocarbon dates from raised beach deposits indicate that the retreat of ice shelves in the ross embayment ceased atabout 6 ka coincident with the environmental changes inferred from the sediment and ice core records the similarity in timing suggests an important role for climate in controlling the evolution of ice shelf margins following the end of the last 9 glaciation

timing of millennial scale climate change at siple dome west antarctica during the last glacial period using atmospheric methane and the isotopic composition of o2 as correlation tools we place the dd record of ice from the siple dome west antarctica ice core on a precise common chronology with the gisp2 greenland ice core for the period from 9 to 57 ka the onset of major millennial warming events in siple dome preceded major abrupt warmings in greenland and the pattern of millennial change at siple dome was broadly similar though not identical to that previously observed for the byrd ice core also in west antarctica the addition of siple dome to the database of well dated antarctic paleoclimate records supports the case for a coherent regional pattern of millennial scale climate change in antarctica during much of the last ice age and glacial interglacial transition

paleogene seafloor spreading in the southeast tasman sea magnetic anomalies 11 18 have been identified in the southeast tasman sea the area of ocean crust southwest of fiordland new zealand west of the puysegur trench the present plate boundary and southeast of the older 80 58 ma ocean crust of the tasman sea basin structures associated with two changes in spreading direction are preserved in the area spreading between the australian and pacific plates began in the eocene about 40 ma in this area orthogonal to cretaceous paleocene spreading between them in the tasman sea the boundary between the two ages of ocean crust is abrupt and associated with what we have interpreted as marginal uplift blocks and rift basins of the resolution ridge system rifting appears to have propagated northeast along a fracture zone the northern blocks of the resolution ridge system may be isolated fragments of continental crust of the campbell plateau between anomalies 18 and 11 40 30 ma the spreading rate was about 1 5 cm yr swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11 about 30 ma and led to a change from tension to transcurrent motion along the plate boundary the timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

extension along the australian pacific transpressional transform plate boundary near macquarie island the australian pacific transform plate boundary fault zone along the macquarie and mcdougall segments of the macquarie ridge complex mrc south of new zealand is characterized by dominantly normal faults and pull apart basins in apparent conflict with the regional transpressional tectonic setting we propose that present day curvature of the transform is inherited from a preexisting divergent plate boundary and that the overall extensional kinematics shown by faults along the main plate boundary trace and exposed on macquarie island result from local stresses related to right lateral right stepping en echelon plate boundary faults and not to the current transpressional setting transpression along the australian pacific transform plate boundary has resulted in uplift along the 1500 km long macquarie ridge complex macquarie island the only subaerial exposure of the complex sits atop a 5 km high 50 km wide submarine ridge of oceanic crust and lies 4 5 km east of the major active plate boundary fault zone thus macquarie island and the surrounding seafloor provide a unique opportunity to study an active oceanic transform fault using complementary marine geophysical and land based geological data mapping of recent faults affecting the topography of macquarie island shows that the island is extensively cut by high angle normal faults forming pull apart basins furthermore evidence for reverse motion is rare using marine geophysical data including swath bathymetry reflectivity and seismic reflection data collected along the australian pacific plate boundary north and south of the island we have defined a 5 15 km wide plate boundary zone a series of right stepping en echelon faults within this zone lies along the main plate boundary trace at the right stepping fault terminations elongate depressions 10 km wide and 1 2 km deep parallel the plate boundary which we interpret as extensional relay zones or pull apart basins we propose that transpression is partitioned into en echelon strike slip faults at the plate boundary and a convergent component that flexes the crust causing the anomalous bathymetric ridge and trough morphology of the mcdougall and macquarie segments of the mrc

the oligocene miocene pacific australia plate boundary south of new zealand evolution from oceanic spreading to strike slip faulting abstract since the eocene the pacific australia plate boundary south of new zealand has evolved from a spreading system into a transform boundary swath data acquired in the southeast tasman oceanic crust between the macquarie ridge complex and the resolution ridge system show that the spreading fabric changes orientation southwards along the puysegur trench striking successively n60 e n85 e and n120 e this reflects the reorganisation of the plate boundary in response to changes in relative plate motion a comparison of these orientations with the positions of the pacific australia relative poles of rotation enables us to estimate the age of stoc where there are no identified magnetic anomalies the youngest age of the oceanic crust is ca 12 ma at the south end of the puysegur trench this age is consistent with spreading rates and the amount of crust generated since 31 ma curved fracture zones on either side of the macquarie ridge complex suggest a continuous reorientation of transform faults between 31 ma and ca 15 ma small scale seafloor morphology shows a 13 change of orientation in the l atalante fracture zone that indicates incremental rather than continuous changes in azimuth of the transform faults patterns of fanning ridges indicate that periods of asymmetric spreading accompanied the spreading segment reorientations using swath data and plate reconstruction models we infer that between 31 and 12 ma the plate boundary reorganisation resulted in a continuous increase in the ratio of the cumulative length of transform faults over the cumulative length of spreading segments along the whole plate boundary this indicates that since 14 15 ma the plate boundary has become progressively predominantly transcurrent allowing strike slip motion to develop along a line of merging transform faults that connected to the intracontinental alpine fault

base up growth of ocean crust by multiple phases of magmatism field evidence from macquarie island macquarie island preserves largely in situ miocene oceanic crust and mantle formed at a slow spreading ridge the crustal section on the island does not conform to a simple layer cake pseudo stratigraphy but is the result of multiple magmatic episodes macquarie island crust did not grow by top down cooling but rather from the base up peridotites cooled first and formed the basement into which gabbro plutons were intruded this was followed by cooling and deformation and by intrusion of dykes that fed a sheeted dyke basalt complex finally lava filled grabens were formed these relative age relations rule out simple co genetic relations between rock units

seismic and bathymetric evidence for four different episodes of iceberg scouring on the new jersey outer shelf possible correlation to heinrich events abstract re analysis of dense chirp seismic surveys on the outer new jersey shelf reveals two sets of shallowly buried incisions the incisions are linear to slightly sinuous oriented ne sw and exhibit cross cutting relationships we interpret both sets as paleo iceberg scours one set cuts into the r horizon a regional unconformity that formed 45 40 ka and which was subsequently buried by the finer grained outer shelf sediment wedge osw the second set is part of an erose boundary between a layered lower unit of the osw and an upper more transparent unit the erose boundary scours are deeper and have steeper slopes than the r horizon scours the latter often exhibit bermed edges whereas the former do not these differences in cross sectional morphology are likely determined by the material properties of the sediments being scoured sand at the r horizon and clay at the erose boundary the orientation of both sets of scours is consistent with previously recognized iceberg scours in stratigraphically younger seafloor on the outer new jersey shelf in 110 150 m water depths located 40 km to the northeast we argue that heinrich events recurring periods of massive iceberg flux into the north atlantic represent the most likely times for transporting icebergs to these latitudes based on stratigraphic relationships a recently published ambient sea level curve and radiocarbon age constraints we attribute the r horizon scours to heinrich event h4 37 cal ka the erose boundary scours to h3 30 cal ka and the seafloor scours to h2 23 cal ka and or h1 17 cal ka facies changes immediately following the r horizon and erose boundary scours may be related to heinrich event forced climate changes perhaps the onset of interstadial conditions and associated melt water pulses floods these pulses floods repeatedly carried fine grained rock flour produced by widespread glacial erosion to the new jersey shelf

major features of glaciochemistry over the last 110 000 years in the greenland ice sheet project 2 ice core major chemical species cl no3 so42 na nh4 k mg2 ca2 and 18o covering the last 110 000 years from the greenland ice sheet project 2 gisp2 ice core were utilized in this study in order to reconstruct the soluble chemistry of the atmosphere over greenland and interpret major climate events that have affected the region during the holocene the major chemical species and 18o do not display any significant relationship however a strong inverse correlation was found between concentrations of the major chemical species and 18o a proxy for temperature during the last glacial period suggesting that in general during periods of decreased temperature there is an increase in atmospheric chemical loading examination of changes in major chemical composition over the last 110 000 years of the gisp2 ice core reveals that during the holocene the atmosphere was acidic during inters t adials the atmosphere was neutral or alkalescent and during stadials the atmosphere was alkaline in addition the relative abundance of major chemical species varied during the holocene stadials and interstadials during the holocene nh4 and no3 are the dominant cations and anions while ca2 and so42 are the dominant cations and anions during the stadials and inters t adials this suggests that source regions or types differed between the holocene and the last glacial period in addition changes in chemical composition and changes in chemical ratios also indicate that source regions differed during the holocene stadials and interstadials twenty four previously identified dansgaard oeschger stadial interstadial events dansgaard et al 1993 were in the gisp2 chemical series the duration of the stadials is inversely correlated with variations in sea level over the last glacial period i e the more extensive the northern hemisphere ice sheet the longer the duration of the stadial there is also a close correspondence between the duration of interstadials and the timing of heinrich events massive icebergs discharged into the ocean in the gisp2 ice core long up to 2000 years warm periods follow each heinrich event suggesting perhaps that enhanced deep water circulation is re initiated following heinrich events

twentieth century constraints on sea level change and earthquake deformation at macquarie island summary through the combination of rare historical sea level observations collected during sir douglas mawson s 1911 1914 australasian antarctic expedition aae together with modern sea level data space geodetic estimates of crustal displacement and modelling of coseismic and post seismic earthquake deformation we present a contemporary analysis to constrain sea level and land level change over the twentieth century at macquarie island 54 30 s 158 57 e we combine 9 months of 1912 1913 sea level data with intermediate observations in 1969 1971 1982 and 1998 2007 to estimate sea level rise relative to the land at 4 8 0 6 mm yr 1 combined with estimates of global mean sea level rise this value supports the geologically surprising notion of land subsidence conflicting with longer term geological evidence that suggests uplift at 0 8 mm yr 1 over the last 400 300 kyr we investigate the current tectonic evolution of the island through analysis of global positioning system gps solutions that utilize data over the period 2000 2009 importantly this provides an opportunity to refine the source parameters of the m w 8 0 great earthquake of 2004 december 23 using estimates of coseismic displacements at regional gps sites we use the estimated earthquake source and gps observations of four years of post seismic deformation at macquarie island to infer the rheology of the oceanic upper mantle we find that an asthenosphere bounded by stronger material above and below is required to produce the observed post seismic deformation particularly in the vertical component assuming a maxwell rheology the best fit is given by an asthenospheric viscosity of 3 10 19 pa s the inferred rheology determined from the 2004 earthquake is used to model long period post seismic deformation from m w 8 0 earthquakes of 1989 and 1924 the 1924 earthquake is the closest of the three great earthquakes to macquarie island and our modelling suggests that the majority of the vertical deformation at the tide gauge over the subsequent 80 years is related to ongoing viscoelastic relaxation from this thrust earthquake that ruptured south from an epicentre south of macquarie island assimilated time series of land level change from the earthquake modelling suggesting ongoing subsidence constrained by the gps estimate of vertical velocity of 2 46 0 64 mm yr 1 combine with the relative sea level time series to yield an estimate of absolute sea level change of 2 0 0 8 mm yr 1 over the twentieth century we conclude this is consistent with the upper bound of the global average rate of absolute sea level rise over the same period this represents one of few estimates of observed sea level change in the southern ocean re emphasizing the importance of historical data and continued geodetic and oceanographic observation in remote areas

changes in the position of the subtropical front south of new zealand since the last glacial period this study fills an important gap in our understanding of past changes in the southern subtropical front s stf in the southwest pacific ocean paleo sea surface temperatures sst were estimated from planktic foraminiferal census counts from cores straddling the modern s stf in the solander trough south of new zealand the estimated sst were compared for 6 time slices glacial period 25 21 ka last glacial maximum lgm 21 18 ka early deglaciation 18 16 ka late deglacial early holocene period 14 8 ka mid holocene period 8 4 ka and late holocene period 4 0 ka the position of the s stf was determined by two methods 1 the location of the 10 degrees c isotherm and 2 the location of the highest sst gradients these new results suggest that the s stf was not continuous between east and west of new zealand during the glacial period steep sst gradients indicate that a strong s stf rapidly shifted south during the lgm and early deglaciation during the late deglacial and holocene periods the position of the s stf differs between the two methods with reduced sst gradients suggesting amore diffuse s stf in the solander trough at this time the glacial sst data suggest that the s stf shifted north to the west of new zealand while to the east there was a stronger sst gradient across the front this was possibly the result of an increased wind stress curl which could have been caused by stronger or more northerly southern hemisphere westerly winds shww or a merging of the shww split jet in this region

intraplate deformation adjacent to the macquarie ridge south of new zealand the tectonic evolution of a complex plate boundary abstract the distribution of relocated seismicity and the evolving shape of fracture zones through time in the oceanic crust of the australian plate adjacent to the australia pacific plate boundary south of new zealand are used to constrain the deformation of this region of the australian plate here called the puysegur block relocated seismicity reveals a broad distribution of earthquakes in the puysegur block on both inter and intraplate structures including two great m8 earthquakes in the region over the past twenty years one of which occurred over 130 km from the plate boundary plate reconstructions from the late oligocene through early mid miocene allow us to determine the undeformed shape of fracture zones in the puysegur block formed during the paleogene when the plate boundary was dominantly a divergent mid ocean ridge system comparing these reconstructions to the present day shape of the fracture zones allows us to map the deformation that has occurred within the puysegur block since the fracture zones formed these two sets of independent observations delineate a broad zone of deformation extending 150 km into the plate interior from the macquarie ridge complex the modern plate boundary structure through the region the persistence of this deformation through time indicates a link with the evolution of the plate boundary over the past 25 ma from divergence to translation and subduction of the australian plate further north at the puysegur trench we infer that this deformation may be a result of stresses in the puysegur block resulting from the impingement of the subducting plate on the thickened lithosphere of southern new zealand such a collision may resist subduction and if resistance remains substantial further deformation internal to the puysegur block may lead to a southward migration of the australia pacific subduction interface and the capturing of this section of lithosphere onto the pacific plate

evolution of macquarie ridge complex seamounts implications for volcanic and tectonic processes at the australia pacific plate boundary south of new zealand abstract the macquarie ridge complex mrc coincides with the australia pacific plate boundary south of new zealand and is characterised by a rugged bathymetry that comprises numerous seamounts along its length tectonic plate reconstructions show that the australia pacific plate boundary evolved from a spreading centre to a transpressional boundary from ca 40 to 6 ma however limited sampling and high resolution mapping of the region have meant that the composition and morphology of the mrc remain relatively unknown here we present results from multibeam mapping photographic investigation and petrologic and geochemical analyses from 10 seamounts along the mrc dredge samples collected from the seamounts are alkaline to sub alkaline basalts containing 45 1 50 8 wt sio2 3 5 12 0 wt mgo and 1 9 4 9 wt total alkalis all samples are enriched mid ocean ridge basalts that have k ti 0 15 which originated as low degree partial melts during the late stages of mid ocean ridge volcanism this oceanic crust has been sheared and accreted along the plate boundary since ca 6 ma by cumulative transpressional relative plate motion such that lavas from distinct magma chambers have been juxtaposed at individual seamounts along the ridge mrc seamounts have formed as discrete elevations as a result of dip slip and strike slip faulting of the ridge axis three guyot type seamounts have flat top summit plateaux that were presumably formed by wave erosion via relative sea level lowering due to uplift of the ridge these seamounts have since subsided due to extension associated with the step over fault geometry of the transform plate boundary along the mcdougall and macquarie ridge segments while the seamounts are of volcanic origin they are manifestations of tectonic and geomorphic processes seamounts distal from the ridge axis however are not subjected to plate boundary deformation and instead have morphologies formed through volcanic processes

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

transition from strike slip faulting to oblique subduction active tectonics at the puysegur margin south new zealand abstract south of new zealand the pacific australia pac aus plate boundary runs along the intracontinental alpine fault the puysegur subduction front and the intraoceanic puysegur fault the puysegur fault is located along puysegur ridge which terminates at ca 47 s against the continental puysegur bank in a complex zone of deformation called the snares zone at puysegur trench the australian plate subducts beneath puysegur bank and the fiordland massif east of fiordland and puysegur bank the moonlight fault system mfs represents the eocene strike slip plate boundary interpretation of seafloor morphology and seismic reflection profiles acquired over puysegur bank and the snares zone allows study of the transition from intraoceanic strike slip faulting along the puysegur ridge to oblique subduction at the puysegur trench and to better understand the genetic link between the puysegur fault and the mfs seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 geodynz survey and single beam echo soundings acquired by the nz royal navy the snares zone is the key transition zone from strike slip faulting to subduction it divides into three sectors namely east nw and sw sectors a conspicuous 3600 m deep trough the snares trough separates the nw and east sectors the east sector is characterised by the ne termination of puysegur ridge into right stepping en echelon ridges that accommodate a change of strike from the puysegur fault to the mfs between 48 s and 47 s in the nw sector and the snares trough a series of transpressional faults splay northwards from the puysegur fault between 49 50 s and 48 s thrusts develop progressively at puysegur trench into a decollement north of 48 s the snares trough develops between two splays of the puysegur fault indicating superficial extension associated with the subsidence of puysegur ridge seismic reflection profiles and bathymetric maps show a series of transpressional faults that splay northwards across the snares fault and terminate at the top of the puysegur trench slope between ca 48 s and 46 30 s the relative plate motion appears to be distributed over the puysegur subduction zone and the strike slip faults located on the edge of the upper plate conversely north of ca 46 s a lack of active strike slip faulting along the mfs and across most of puysegur bank indicates that the subduction in the northern part of puysegur trench accounts for most of the oblique convergence hence active transpression in the snares fault zone indicates that the relative pac aus plate motion is transferred from strike slip faulting along the puysegur fault to subduction at puysegur trench the progressive transition from thrusts at puysegur trench and strike slip faulting at the puysegur fault to oblique subduction at puysegur trench suggests that the subduction interface progressively developed from a western shallow splay of the puysegur fault it implies that the transfer fault links the subduction interface at depth a tectonic sliver is identified between puysegur trench and the puysegur fault its northwards motion relative to the pacific plate implies that is might collide with puysegur bank

influence of cumulative convergence on lithospheric thrust fault development and topography along the australian pacific plate boundary south of new zealand the development of faulting and topography resulting from initial convergence within oceanic lithosphere is largely unknown we explore relationships among convergence structural development and topography along 1500 km of the submarine australian pacific plate boundary south of new zealand the macquarie ridge complex mrc due to the variable orientation of the boundary and close proximity of the australian pacific poles of rotation individual segments of the plate boundary have experienced different convergence histories since 10 95 ma chron 5o because interaction along the oceanic extent of the boundary involves oceanic lithosphere of broadly the same age and therefore thermal structure structural and morphologic differences can be attributed primarily to variations in angles and rates of convergence with respect to the plate boundary orientation since 10 95 ma we relate plate boundary normal convergence determined from stage pole rotations to structural development focusing on transitions from purely strike slip faulting to partitioned thrust and strike slip faulting between 47 s and 60 s since 10 95 ma our results indicate that boundary normal convergence of 100 km marks the transition from strike slip dominated faulting to partitioned underthrusting and strike slip faulting incipient subduction establishment of subduction at the puysegur trench and incipient subduction at the hjort trench corresponds to convergence at rates between 2 and 4 cm yr angles 30 and with durations of at least 10 m y resulting in 100 km of boundary normal convergence anomalous topographic volumes resulting from tectonic deformation are quantified from swath bathymetry and compared to convergence history results from analysis of the central mrc suggest that prior to lithospheric scale thrust faulting 100 km of boundary normal convergence can be accommodated by crustal deformation 6 km relief from ridge crest to adjacent trough and strike slip faulting our research supports induced forced intraoceanic initiation of subduction

bipolar changes in atmospheric circulation during the little ice age annually dated ice cores from siple dome west antarctica and central greenland indicate that meridional atmospheric circulation intensity increased in the polar south pacific and north atlantic at the beginning 1400 a d of the most recent holocene rapid climate change event the little ice age lia as deduced from chemical concentrations at these core sites the lia was characterized by substantial meridional circulation strength variability and this variability persists today despite strong evidence for an end to lia cooling thus increased late 20th century storm variability may be in part a result of the continuation of these climatic fluctuations

silicic acid leakage from the southern ocean a possible explanation for glacial atmospheric pco2 1 using a simple box model we investigate the effects of a reduced si n uptake ratio by antarctic phytoplankton on the marine silica cycle and atmospheric pco2 recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added hutchins and bruland 1998 takeda 1998 franck et al 2000 the southern ocean may have supported diatoms with reduced si n uptake ratios compared to today during the dustier glacial times petit et al 1999 a similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as phaeocystis our model shows that reduced si n export ratios in the southern ocean create excess silicic acid which may then be leaked out to lower latitudes any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes caco3 production and therefore diminishes the carbonate pump in our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric co2 from the interglacial 277 to 230 242 ppm depending on where the excess silicic acid is consumed by comparison the atmospheric pco2 sensitivity of general circulation models to carbonate pump forcing is 3 5 fold greater which combined with carbonate compensation can account for peak glacial atmospheric pco2 we discuss the importance of the initial rain ratio of caco3 to organic carbon on atmospheric pco2 and relevant sedimentary records that support and constrain this silicic acid leakage scenario

volcanic development of small oceanic central volcanoes on the flanks of the east pacific rise inferred from narrow beam echo sounder surveys abstract narrow beam bathymetric surveys of about 30 volcanoes on young crust of the cocos plate were collected in order to help determine the origin and evolution of oceanic central volcanoes the volcanoes are located on ocean crust 1 5 7 5 m y in age and range in size from to 600 km 3 these volcanoes differ greatly in shape development of linear features small scale morphologic characteristics and crater development we conclude that these large differences in morphologic characteristics are due to the complex interaction of many factors in their development such as conduit geometry magma ascent rates eruption rate history tectonic activity crater formation and filling and others elongation directions of individual volcanoes and clear linear bathymetric features near these volcanoes are among the lines of evidence that strongly suggest these volcanoes originate and evolve on and near oceanic fracture zones their origin and morphologic evolution is thus closely tied to the tectonic processes and thermal regimes at fractures we speculate that these volcanoes on fast spreading lithosphere are generated by episodic pulses of melt production beneath ridge crests in the vicinity of fracture zones petrologic evidence shows that these volcanoes cannot be fed from sub axial magma chambers beneath the east pacific rise instead they tap the same mantle source as the ridge crest at mantle depth

glacial interglacial variations in methanesulfonate msa in the siple dome ice core west antarctica methanesulfonate msa in the siple dome ice core is a record of the deposition of biogenic sulfur to the west antarctic ice sheet covering the past 100 kyr siple dome msa levels were low during the last glacial maximum and increased to higher holocene levels with a several kyr lag relative to the deglacial warming the positive correlation between msa and temperature at siple dome is similar to that in greenland ice cores renland gisp2 and grip and stands in contrast to the negative correlation observed at vostok east antarctica the siple dome msa data suggest that the sign of the high latitude dust sulfur climate feedback is negative at least for the pacific sector of the high latitude southern ocean these results challenge the idea that fertilization by increased dust deposition led to widespread increased dms emissions from this region of the glacial southern ocean

ice sheets and sea level of the last glacial maximum this paper outlines the general issues regarding ice sheets and sea level of the last glacial maximum lgm which formed the basis of an epilog project workshop papers in this special issue of quaternary science reviews provide a comprehensive assessment of these issues from the perspective of geological reconstructions of ice sheet extent records of sea level change ice sheet modelling geophysical models of glacial isostatic adjustment and geochemical proxies of ice volume this new assessment has substantially narrowed the uncertainties in the total changes in ice sheets and sea level and their proxies suggesting a net decrease in eustatic sea level at the lgm ranging from 120 to 135 m r 2001 elsevier science ltd all rights reserved

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

evidence for late paleozoic and mesozoic non dipole fields provides an explanation for the pangea reconstruction problems abstract paleomagnetic pole positions have long been calculated with the assumption that the ancient geomagnetic field was purely that of a geocentric dipole but in recent years it has become a concern that this assumption is not always valid because of long term contributions from non dipole fields in particular octupole fields we tested for the presence of a zonal octupole field in the late carboniferous permian mesozoic and early tertiary by comparing the observed paleomagnetic paleolatitude distributions for the laurentian north america and greenland and european landmasses with those predicted from the mean paleopoles such a test is possible for a good sized continent not subsequently deformed when numerous and reliable paleomagnetic results exist from sites that cover a wide range of paleolatitudes if the field were purely dipolar coeval results should form a coherent dataset such that a regression line through the individual points in an observed versus predicted paleolatitude plot has a slope of 1 if the slopes of regression lines are systematically different from 1 0 for successive time intervals and granted that the earth s radius stayed the same then it is likely that non dipole fields can be held responsible regression line slopes standard errors for three intervals analyzed are 0 78 0 04 200 300 ma 0 93 0 11 120 200 ma and 0 82 0 06 40 120 ma as might be expected with a non dipole contribution to the total geomagnetic field paleomagnetic results from the most northerly areas of laurentia and europe i e ellesmere greenland and svalbard show clearly anomalous paleolatitudes because of the agreement between the equatorial locations determined independently by paleomagnetism and equatorial rain forest occurrences d v kent p e olsen earth planet sci lett 179 2000 311 324 we argue that long term quadrupole fields are less likely than octupole fields to be the cause of the paleolatitude discrepancies estimates of the magnitude of the octupole dipole field ratio are not very precise but center around 0 1 which could cause errors in conventional paleopole determinations using the dipole formula of 7 5 because of the antisymmetry of octupole fields a comparison of paleomagnetic poles from mid northern and mid southern hemisphere locations could thus be off by as much as 15 the well known misfit between the paleomagnetic results from the laurentia european and gondwana continents in a classical pangea a configuration could be explained by such errors due to octupole fields this explanation would negate the need to seek tectonic pangea b type solutions for the misfit these solutions have generally remained unacceptable in terms of the geology of the gondwana laurussia borderzone

a middle late quaternary age for the adakitic arc volcanics of hautere solander island southern ocean abstract hautere solander island and five nearby mainly submerged volcanic centres represent the only known subduction related volcanism on the australian pacific plate boundary south of new zealand two new ar ar biotite ages from hautere indicate magmatism they were active between 150 and 400ka middle to late quaternary this contrasts with the previously proposed pliocene to early quaternary age range based on less well constrained k ar dating the new radiometric ages are consistent with revised pollen ages new trace element analyses confirm the subduction related and adakitic nature of the igneous rocks

the antarctic ice sheet during the last glacial maximum and its subsequent retreat history a review abstract an emerging body of evidence from studies of the last glacial interglacial cycle suggests that the east and west antarctic ice sheets have not advanced and retreated in concert the west antarctic ice sheet advanced to the outer shelf in most regions during the last glacial maximum lgm the retreat from the shelf commenced shortly after the lgm and continued into the late holocene the west antarctic ice sheet in ross and amundsen seas slid across a deforming bed at least during the final phases of extended glaciation this implies that at this time the ice sheet had a low profile differences in the number and locations of grounding zone wedges and smaller grounding zone features from trough to trough imply that individual west antarctic ice sheet ice streams retreated episodically details concerning the expansion retreat and behavior of the east antarctic ice sheet eais are more sparse the picture emerging is that the eais did not expand to the continental shelf edge during the lgm rather it achieved a maximum extent of a mid shelf position in some locations while in other areas the ice terminus was situated near its present location the timing of retreat along sectors within the eais appears diachronous and in places occurred prior to the lgm the antarctic peninsula shelf contained considerably more ice during the lgm than previously proposed the results presented in this paper support more recent published ice sheet models that call for greater contributions of melting ice from west antarctica including the antarctic peninsula to the post glacial rise in sea level

methanesulfonate in the greenland ice sheet project 2 ice core in this paper we present measurements of methanesulfonate in the greenland ice sheet project 2 gisp2 ice core methanesulfonate is an atmospheric oxidation product of dimethylsulfide the gisp2 methanesulfonate record contains information about the atmospheric loading of biogenic sulfur over the past 110 kyr and its relationship to climate change the gisp2 data set supports the inferences made from the renland ice core from greenland that the glacial atmosphere over greenland had reduced concentrations of biogenic sulfur compared with the present day hansson and saltzman 1993 we conclude that the flux of biogenic sulfur from the north atlantic ocean must have been lower during glacial times and speculate that this decrease may have been related to differences in phytoplankton speciation the data suggest that changes in direct radiative forcing from biogenic sulfur aerosols would act as negative feedback to the glacial interglacial climate cycles in this region

late quaternary ice rafting events in the sw pacific ocean off eastern new zealand new records of ice rafted debris ird in eight cores from campbell plateau reveal periodic incursions of icebergs over the last 200 kyr observations of modern iceberg trajectories and provenance studies show the icebergs originated mainly in antarctica and traveled east and north to the plateau margins via the antarctic circumpolar current there local currents and winds dispersed icebergs across the plateau and adjacent regions as far north as chatham rise at 43 s the shallow rise crest along rise circulation and strong thermal gradient associated with the nearby subtropical front inhibited further northward transport although ird concentrations are very low 26 grains g downcore profiles and a 18o chronology reveal distinct peaks during i the transition from marine isotope stages mis 7 to 6 ii late interglacial mis 5 and iii glacial mis 2 in addition smaller peaks occur in mis 4 and 3 this generalised pattern is also evident off antarctica indicating that periodic destabilisation of ice shelves was the main driving force behind the campbell plateau ird events however there are significant differences between antarctic and new zealand ird records that probably reflect palaeoceanographic influences on iceberg dispersal the mis 5 ird event is better represented off antarctica confirming that last interglacial changes in sea level and temperatures encouraged iceberg discharge and proximal melting conversely the mis 2 event is relatively better shown on campbell plateau implying that last glacial temperatures inhibited proximal iceberg melting and together with a more vigorous oceanic circulation allowed more icebergs to reach the distant plateau a tentative correlation with records from the southeast atlantic and southwest indian oceans suggests that the ird events were felt over much of the southern ocean for at least the last 70 kyr

paleoproterozoic positive δ13ccarb excursion in the northeastern sino korean craton evidence of the lomagundi event abstract the 2 33 2 06 ga positive 13 c carb excursion associated with environmental change and the breakup of the kenorland or superia supercontinent is called the lomagundi or jatulian event or great oxidation event and has been reported in many early precambrian cratons but not yet in the sino korean craton the guanmenshan formation of the liaohe group occurs in the northeastern part of the sino korean craton 13 c carb and 18 o values in 42 samples from this formation range from 3 5 5 9 v pdb and 15 4 24 8 v smow respectively showing a clear positive 13 c carb excursion that characterizes the lomagundi event thirty five of the 42 samples with less hydrothermal alteration have higher 13 c carb and 18 o carb values than the other 7 samples obviously affected by fluid flow confirming that it was fluid flow that reduced the 13 c carb and 18 o carb values this positive 13 c carb excursion places deposition of the guanmenshan formation within the age range of 2 33 2 06 ga

characteristics of water soluble inorganic and organic ions in aerosols over the southern ocean and coastal east antarctica during austral summer 1 to characterize the concentrations and size distributions of water soluble organic and inorganic aerosol species including na non sea salt sulfate nss so42 methane sulfonate msa oxalate and succinate over the southern ocean so and coastal east antarctica cea bulk and size segregated aerosols were collected from 40 s 100 e to 69 s 76 e and between 69 s 76 e and 66 s 110 e during a cruise from november 2010 to march 2011 results show that sea salt was the major component of the total aerosol mass accounting for 72 over the so and 56 over cea the average concentrations of nss so42 varied from 420 ng m 3 over the so to 480 ng m 3 over cea the concentrations of msa ranged from 63 to 87 ng m 3 over the so and from 46 to 170 ng m 3 in cea the average concentrations of oxalate were 3 8 ng m 3 over the so and 2 2 ng m 3 over cea the concentrations of formate acetate and succinate were lower than those of oxalate a bimodal size distribution of aerosol mass existed over cea peaking at 0 32 0 56 m and 3 2 5 6 m msa was accumulated in particles of 0 32 0 56 m over cea high chloride depletion was associated with fine mode particles enriched with nss so42 msa and oxalate higher cation to anion and nh4 nss so42 ratios in aerosols over cea compared to that over the so imply the higher neutralization capacity of the marine atmosphere over cea

near ridge seamount chains in the northeastern pacific ocean high resolution bathymetry and side scan data of the vance president jackson and taney near ridge seamount chains in the northeast pacific were collected with a hull mounted 30 khz sonar the central volcanoes in each chain consist of truncated cone shaped volcanoes with steep sides and nearly flat tops several areas are characterized by frequent small eruptions that result in disorganized volcanic regions with numerous small cones and volcanic ridges but no organized truncated conical structure several volcanoes are crosscut by ridge parallel faults showing that they formed within 30 40 km of the ridge axis where ridge parallel faulting is still active magmas that built the volcanoes were probably transported through the crust along active ridge parallel faults the volcanoes range in volume from 11 to 187 km3 and most have one or more multiple craters and calderas that modify their summits and flanks the craters 1 km diameter range from small pit craters to calderas as large as 6 5 8 5 km although most are 2 4 km across crosscutting relationships commonly show a sequence of calderas stepping toward the ridge axis the calderas overlie crustal magma chambers at least as large as those that underlie kilauea and mauna loa volcanoes in hawaii perhaps 4 5 km in diameter and 1 3 km below the surface the nearly flat tops of many of the volcanoes have remnants of centrally located summit shields suggesting that their flat tops did not form from eruptions along circumferential ring faults but instead form by filling and overflowing of earlier large calderas the lavas retain their primitive character by residing in such chambers for only short time periods prior to eruption stored magmas are withdrawn probably as dikes intruded into the adjacent ocean crust along active ridge parallel faults triggering caldera collapse or solidified before the next batch of magma is intruded into the volcano probably 1000 10 000 years later the chains are oriented parallel to subaxial asthenospheric flow rather than absolute or relative plate motion vectors the subaxial asthenospheric flow model yields rates of volcanic migration of 3 4 3 3 and 5 9 cm yr 1 for the vance president jackson and taney seamounts respectively the modeled lifespans of the individual volcanoes in the three chains vary from 75 to 95 kyr these lifespans coupled with the geologic observations based on the bathymetry allow us to construct models of magma supply through time for the volcanoes in the three chains

ice core evidence for antarctic sea ice decline since the 1950s the instrumental record of antarctic sea ice in recent decades does not reveal a clear signature of warming despite observational evidence from coastal antarctica here we report a significant correlation p

neotectonics of the macquarie ridge complex australia pacific plate boundary new marine geophysical data along the macquarie ridge complex the australia pacific plate boundary south of new zealand illuminate regional neotectonics we identify tectonic spreading fabric and fracture zones and precisely locate the australia pacific plate boundary along the macquarie ridge complex we interpret a 5 10 km wide macquarie fault zone between the two plates along a bathymetrie high that extends nearly the entire length of the australia pacific plate boundary south of new zealand we conclude that this is the active australia pacific strike slip plate boundary arcuate fracture zones become asymptotic as they approach the plate boundary a broad zone of less intense deformation associated with the plate boundary extends 50 km on either side of the macquarie fault zone marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike slip deformation although teleseismic evidence overwhelmingly indicates strike slip motion along the entire surveyed boundary today the mcdougall and southernmost puysegur segments show no evidence for past underthrusting whereas data from the macquarie and hjort segments strongly suggest past convergence the present day strike slip plate boundary along the macquarie ridge complex coincides with the relict spreading center responsible for australia pacific crust in the region our conceptual model for the transition from seafloor spreading to strike slip motion along the macquarie ridge complex addresses the decreasing length of spreading center segments and spacing between fracture zones as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary

ree geochemistry of carbonates from the guanmenshan formation liaohe group ne sino korean craton implications for seawater compositional change during the great oxidation event abstract the worldwide 2 33 2 06 ga positive 13 c carb excursion has been correlated with the great oxidation event goe and termed as the lomagundi event the 2 3 1 85 ga guanmenshan formation in the liaohe group of the northeastern sino korean craton is characterized by positive 13 c carb excursion and is a potential candidate to evaluate the lomagundi event using rey rare earth element and yttrium ree y chemical fingerprints here we present major and trace element analysis of 42 samples from the guanmenshan formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2 3 1 85 ga 15 least altered dolomicrite samples 600 m strata have ree values of 0 739 4 175 ppm 2 414 1 184 ppm and the y ho ratios of 34 5 56 6 44 1 5 7 they show uniform positive la sn la sn 1 04 0 27 and gd sn gd sn 1 64 0 40 anomalies and notable lree depletions indicated by nd sn yb sn values of 0 24 0 92 average 0 56 0 19 these features are consistent with the geochemistry of well oxygenated shallow ambient seawater and suggest that these samples provide a robust record of the primary rey signature of seawater during the lomagundi event the rey patterns of 15 silicified dolomites marbles locally with veinlets from the pb zn mining camps in the region with average eu cn eu cn 1 56 0 95 are identical to those of high temperature hydrothermal fluids 250 c characterized by a flat pattern and marked positive eu anomalies indicating that these rocks were subjected to metasomatism by hydrothermal fluids the guanmenshan formation shows average ce sn ce sn of 0 93 0 09 and sm cn yb cn of 1 which are higher than those of the archean 2 33 ga chemical sediments generally f o 2 and p co 2 the rey in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering and also from seafloor hydrothermal processes on a subordinate scale the guanmenshan dolomicrites have eu sn eu sn values of 1 34 2 55 i e around 1 53 indicating that they were deposited during 2 33 2 06 ga as the eu sn eu sn 1 53 can be used as a proxy for the 2 33 2 06 ga marine chemical sediments our study shows that the guanmenshan formation was formed at a critical turning point in earth history when the global atmosphere hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions

reconstructions of the new zealand region for the neogene abstract the major structural elements of the present day plate boundary zone through new zealand are a subduction zone under the northern half of the country linked by a wide transform zone to the southward extension of the plate boundary through fiordland and the macquarie ridge using finite rotations of the pacific australian plate pair a sequence of reconstructions of the relative positions of the plates is presented reconstructions of the plate boundary zone based on paleomagnetic data involve essentially the same structural elements as today but with large clockwise changes in their trend with time

formation and tectonic evolution of the khondalite series at the southern margin of the north china craton geochronological constraints from a 1 85 ga mo deposit in the xiong ershan area abstract the taihua complex is an important metamorphic rock suite at the southern margin of the trans north china orogen and hosts important mineral deposits its protolith and metamorphic ages are still controversial the 1 85 ga longmendian mo deposit in the xiong ershan is hosted by the khondalite series of the duangou formation taihua complex three kinds of rocks are collected for shrimp cameca u pb dating including the meta sedimentary rock that occurred as wallrock the mo mineralized meta gabbro that intruded the meta sedimentary rock and the leucosome that cut both the meta sedimentary rock and the meta gabbro zircon u pb analyses reveal that the precursor of the meta sedimentary rock was crystallized between 2200 4 and 2742 18 ma with most before 2500 ma whereas the protolith of the meta gabbro was formed at 2178 6 ma anatexis zircons in the leucosome yield a weighted mean age of 1943 8 ma that is indistinguishable with the metamorphic age of the meta gabbro 1952 4 ma in the intensively altered meta gabbro hydrothermal titanites were formed by at least two pulses of crystallization at 1940 7 ma and 1916 9 ma respectively the new data reveal that the khondalite series of the duangou formation were formed between 2200 ma and 2178 ma the results are inconsistent with previous subdivision of the taihua complex into the upper taihua and lower taihua subgroup instead they fit well with the tripartitite divison into the beizi group 3 0 2 55 ga the dangzehe group 2 55 2 3 ga and the shuidigou group 2 3 2 1 ga compiling zircon u pb ages indicate that in the trans north china orogen the 1 95 ga and 1 85 ga metamorphic events are widespread and prominent

dating the siple dome antarctica ice core by manual and computer interpretation of annual layering the holocene portion of the siple dome antarctica ice core was dated by interpreting the electrical visual and chemical properties of the core the data were interpreted manuallyandwith acomputeralgorithm the algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the gisp2 greenland ice sheet project 2 ice core 10 be stratigraphic ties to the dendrochronology 14 c recordandthedatedvolcanic stratigraphy thealgorithm interpretation ismorecon sistent andbetter quantifiedthanthe tedious and subjective manual interpretation

a switch from si oh 4 to no 3 depletion in the glacial southern ocean 1 phytoplankton in the antarctic deplete silicic acid si oh 4 to a far greater extent than they do nitrate no3 this pattern can be reversed by the addition of iron which dramatically lowers diatom si oh 4 no3 uptake ratios higher iron supply during glacial times would thus drive the antarctic towards no3 depletion with excess si oh 4 remaining in surface waters new 30si and 15n records from antarctic sediments confirm diminished si oh 4 use and enhanced no3 depletion during the last three glaciations the present low si oh 4 water is transported northward to at least the subtropics we postulate that the glacial high si oh 4 water similarly may have been transported to the subtropics and beyond this input of si oh 4 may have caused diatoms to displace coccolithophores at low latitudes weakening the carbonate pump and increasing the depth of organic matter remineralization these effects may have lowered glacial atmospheric pco2 by as much as 60 ppm

quantification of submarine lava flow morphology through analog experiments we developed a technique for determining paleoeffusion rates and emplacement times for submarine lava flows using observations of flow morphology and estimates of eruption volume eruption temperature lava viscosity and preflow topography using laboratory simulations and correlating these results with sea floor observations four submarine lava flow morphologies are considered to be diagnostic of specific effusion rates jumbled folded and lineated sheets and striated pillows we applied this approach to the coaxial flow emplaced in less than 14 days on the juan de fuca ridge in early summer 1993 and we calculated an effusion rate of 100 m3 s giving an emplacement time of 10 days

underthrusting at the hjort trench australian pacific plate boundary incipient subduction 1 the southernmost portion of the macquarie ridge complex mrc comprises the hjort ridge trench and plateau the topographic expression of the australian pacific plate boundary south of new zealand on the basis of marine geophysical swath bathymetry reflectivity seismic reflection gravity magnetic data teleseismic data and gravity modeling we argue that the australian plate is actively underthrusting the pacific plate along the hjort trench but self sustaining subduction does not yet appear to have commenced we interpret a crustal discontinuity in the trench as a shallowly dipping thrust splay off a sub vertical transform fault in the crest of the hjort ridge hjort fault the hjort fault separates oceanic crust generated at the southeast indian ridge seir crust located to the south from oceanic crust generated at the extinct australian pacific spreading center located to the north mrc crust oblique convergence on the transform has initiated a significant plate boundary decollement in the trench but our estimate of 50 km of underthrusting does not support the existence of an eastwardly dipping australian slab below 20 km depth for the length of the hjort ridge and trench system oblique convergence is partitioned between the decollement in the trench and the hjort fault south of 57 5 s the trench decollement accommodates thrusting north of 57 5 s the boundary parallel component of convergence becomes dominant the trench gradually shallows and the trench decollement evolves into a low angle oblique slip fault the trench decollement and strike slip system in the hjort ridge are structural boundaries that contain tectonic slivers of inferred seir crust of the western flank of the hjort ridge that currently belong to neither the australian nor the pacific plates

the geology and geochemistry of the palaeoproterozoic makganyene diamictite abstract the palaeoproterozoic earth experienced a global glacial event at 2400 ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans the transvaal supergroup in the griqualand west basin south africa hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes the focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic makganyene formation in order to constrain its palaeoenvironmental settings the makganyene formation forms the base of the postmasburg group and has been regarded as resting on an erosive regional unconformity throughout the northern cape province systematic regional field observations and regional mapping carried out during this study demonstrate that this stratigraphic relationship is not universal the makganyene formation is in fact conformable with underlying formations of the koegas subgroup in the deep southern prieska basin and rests on an unconformity only on the shallow ghaap platform to the north east the makganyene formation displays lateral facies changes that reflect the palaeogeography of the study area and the advance and retreat of ice sheets shelves geochemical investigations of glacial strata of the makganyene formation demonstrate that underlying banded iron formations of the transvaal supergroup acted as the main clastic source for the diamictite detritus geographic variations in bulk composition of the diamictites correlate well with field observations and show that sorting processes were controlled largely by the morphology of the palaeobasin carbon isotope results emphasize the transitional nature of the makganyene formation in terms of the environmental conditions that resulted in widespread global glaciation in the palaeoproterozoic on the basis of the above geological evidence it is proposed that the transvaal supergroup in the northern cape province represents a continuous depositional event that lasted approximately 250 ma and hence provides a unique opportunity for assessing the transitional changes experienced by the palaeoproterozoic earth

macquarie island s finch langdon fault a ridge transform inside corner structure macquarie island consists of uplifted oceanic crust uniquely situated in the ocean basin where it formed thus allowing onshore structures to be placed into their regional oceanic tectonic context the finch langdon fault the most significant spreading related structure on the island juxtaposes upper crust rocks against lower crust and upper mantle rocks it consists of dominantly oblique strike slip northwest west northwest and north northeast striking fault segments that bear hydrothermal mineralization indicative of faulting during seafloor spreading talus breccias and graywackes overlain by volcanic flows proximal to the fault indicate a long lived submarine fault scarp that exposed diabase dikes and gabbros during volcanism swath reflectivity and bathymetry reveal ridge parallel spreading fabric and perpendicular fracture zones the closest 7 km east of the island on the basis of field and swath data we propose that this fault zone formed near the inside corner of a ridge transform intersection and that structures on the island are conformable with those in the surrounding seafloor

evidence that three seamounts off southern california were ancient islands abstract many seamounts world wide exhibit features that suggest they were once islands rodriguez and san juan seamounts and northeast bank within 250 km of the continental margin of the western us were explored with the rov tiburon and found to have guyot morphology beach deposits and erosional features degassed volcanic lavas and pyroclasts and cooling and alteration patterns unusual for submarine erupted lavas these features are evaluated as evidence that the volcanoes stood above sea level the summit of rodriguez seamount is a smooth gently domed platform rough hills stand above the platform and consist of dense degassed a a flows erupted subaerially bedded sandstones and rounded cobbles interpreted to be beach deposits occur near the top of a major break in slope at about 700 m depth the summit of northeast bank is also a smooth gently domed platform beach deposits were observed just below the pronounced break in slope at about 500 m the summit of san juan seamount is a northeast southwest trending series of rough ridges rather than a dome however subaerially exposed a a like lava flows were found above 700 m oxidized olivine degassed volcanic glass and secondary phosphatization of the lavas are found both shallower and deeper than the breaks in slope and their presence may not be robust indicators of subaerial exposure rodriguez stood at least 70 m above sea level and formed a small island 6 8 km 2 in area prior to subsiding at least 700 m northeast bank was a large island perhaps 200 m above sea level and 90 km 2 in area the summit ridge of san juan emerged as a line of eight small islands with a total area of about 2 8 km 2 and the tallest island rose at least 140 m above sea level the subsidence of northeast bank within the california borderland rodriguez seamount on the continental slope and san juan seamount on the adjacent ocean crust occurred since they formed 7 to 11 million years ago the minimum subsidence is consistent with cooling and contraction of the underlying lithosphere

discovery of the huronian glaciation event in china evidence from glacigenic diamictites in the hutuo group in wutai shan abstract paleoproterozoic glacigenic diamictites have provided key evidence for the great oxidation event goe at 2 3 ga associated with the widespread huronian glaciation event hge here we report for the first time paleoproterozoic glacigenic diamictite from the sijizhuang formation of the hutuo group in wutai shan china formed during the late stage of hge the sijizhuang formation comprises diamictite sandstone and siltstone mudstone facies conforming to a glaciomarine association the diamictites show weak sorting and roundness and carry a few striated and faceted boulders typical dropstones and associated structures are present in the sandstone and siltstone mudstone facies diamictite facies which characterizes the lower portion of the sijizhuang formation displays an upward decrease with a concomitant increase in the proportions of sandstone and siltstone mudstone facies the lithofacies associations of diamictite with sandstone and siltstone mudstone indicate a frequently changing hydrodynamic environment the petrochemical compositions of different lithofacies show that the clasts were multi sourced and generated by limited weathering in keeping with a cold climate available isotope age data constrain the glacigenic deposition of the sijizhuang formation to between 2 40 and 2 13 ga and most likely from 2 2 to 2 1 ga this estimate compares well with the 2 4 2 1 ga age of the widespread hge but the sijizhuang diamictite appears to be younger than the last glacial unit gowganda formation in the type area of the hge in canada and suggests a locally protracted glacial event

the first greenland ice core record of methanesulfonate and sulfate over a full glacial cycle methanesulfonate msa in ice cores has attracted attention as a possible tracer of past oceanic emissions of dimethylsulfide dms after sulfate msa is the second most prevalent aerosol oxidation product of dms but in contrast to sulfate dms oxidation is the only known source of msa the hypothesis by charlson et al 1987 of a climate feedback mechanism with sulfur emissions from marine phytoplankton influencing the cloud albedo adds to the interest in establishing long records of msa and non seasalt sulfate spanning large climatic changes records of msa and non seasalt sulfate covering time periods from a few years to thousands of year have been extracted from antarctic ice cores ivey et al 1986 saigne and legrand 1987 legrand and feniet saigne 1991 mulvaney et al 1992 but only the record from the vostok ice core legrand et al 1991 covers a full glacial cycle the concentrations of msa and non seasalt sulfate in antarctica have been found to increase under glacial conditions here we present the first northern hemisphere record of msa and the first continuous record of non seasalt sulfate both extracted from the renland ice core east greenland the records are extending from the holocene to the eem interglacial 130 000 years b p the contrast to the southern hemisphere records is striking with a decreasing concentration of msa with the advance of glaciation but an increasing concentration of non seasalt sulfate a strong linear relationship is found in the renland ice core between the ratio of msa to non seasalt sulfate and the temperature with higher ratios associated with warmer climatic stages while the opposite relationship to temperature is found in the vostok ice core a more complicated picture is emerging of the use of msa in ice cores as a quantitative tracer which suggests that previous interpretations can have been overly simplistic

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

seamount morphology in the bowie and cobb hot spot trails gulf of alaska full coverage multibeam bathymetric mapping of twelve seamounts in the gulf of alaska reveals that they are characterized by flat topped summits rarely with summit craters and by terraced or step bench flanks these summit plateaus contain relict volcanic features e g flow levees late stage cones and collapse craters and as such must have been constructed by volcanic processes such as lava ponding above a central vent rather than by erosion above sea level the terraced flanks are composed of a sequence of stacked lava deltas and cones probably tube fed from a central lava pond a morphology which is suggestive of long lived stable central lava sources and low to moderate eruption rates indicative of significant time spent above a hot spot outlet most of these seamounts have summit plateaus surrounded and cut into by amphitheater headwall scarps and flanks that are scarred by debris chutes but lack visible debris accumulations at their base we interpret the lack of blocky debris fields as evidence that the slope failures are mainly small scale debris flows rather than large scale flank collapses however we cannot rule out the possibility that large flank collapse blocks from early in the histories of these seamounts are now hidden beneath the thick glacio fluvial fan deposits that cover the gulf of alaska seafloor these slope failure features become smoother and longer and increase in size and abundance with increasing age of a seamount suggesting that slope failure processes continue long after volcanic activity ceases

ridge reorientation mechanisms macquarie ridge complex australia pacific plate boundary the macquarie ridge complex portion of the australia pacific plate boundary south of new zealand provides a unique complete record of a 60 90 change in spreading direction since 40 ma that resulted in the transition from a spreading center to a transform plate boundary marine geophysical data show that during reorientation most ridge segments completely disappeared and all shortened additionally modification of newly created crust caused differences in widths of correlative spreading segment corridors on the two plates we propose two models for ridge reorientation that explain the observed spreading fabric and arcuate fracture zone relationships nonrigid plate deformation was accommodated by failing and propagating spreading ridge segments rifts and transfer of crust between plates during the gradual reorientation of the spreading axes

time dependence of the stress shadowing effect and its relation to the structure of the lower crust great earthquakes like the 1906 san francisco earthquake perturb the regional stress field and can generate stress shadows that delay the occurrence of subsequent m 5 5 events we investigate how time dependent postseismic stress transfer between the three subparallel strike slip faults in northern california the san andreas hayward and calaveras faults and their northern extensions depends on the presence and geometry of lower crustal shear zones models incorporating maxwell viscoelastic relaxation of the mantle a lower crustal detachment surface and vertical shear zones beneath the seismogenic faults are compared short term relaxation of lower crustal shear zones enhances the amplitude and lateral extent of the stress shadow conversely longer period mantle relaxation reloads the entire crust as a consequence after a 1906 type event on the san andreas fault the stress shadow on the hayward fault will be greater in magnitude and duration if the faults are connected via lower crustal structures with time dependent rheologies in addition our results demonstrate that for certain geometries the total stress tectonic plus postseismic perturbation may actually continue to decrease during the decades immediately following a great earthquake this effect has not been documented in prior time dependent models of stress accumulation during the earthquake cycle

age petrogenesis and tectonic setting of the thessalon volcanic rocks huronian supergroup canada abstract the 2 5 ga huronian supergroup volcanic rocks in northern ontario represent one of the better preserved paleoproterozoic continental flood basalt sequences on earth the youngest age of huronian supergroup volcanism is refined to 2452 5 6 2 ma based on new u pb zircon analyses of the copper cliff rhyolite recent mapping and geochemical analyses of thessalon formation volcanic rocks define a well developed chemostratigraphy a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt andesite volcanic series these two mafic volcanic series are separated by felsic volcanic rocks rhyolite and dacite the mafic lavas are low ti tholeiites dominated by basalt and andesite and are characterized by enriched light rare earth element profiles and high th contents up to 12 ppm most basalt andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and in some cases limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle all magma batches display prominent negative nb ta and ti anomalies on primitive mantle normalized multi element diagrams interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism at least three distinct mantle source compositions are required to explain the range in the observed lava compositions the most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high th yb 10 and low zr nb 11 this reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward neoarchean subduction along the southern superior craton margin