nd sr isotopic geochemical and petrographic stratigraphy and paleotectonic analysis mesozoic great valley forearc sedimentary rocks of california measurements of nd sr isotopes major and trace elements and model mineralogy were made on upper jurassic and cretaceous great valley forearc sedimentary rocks to test models for the temporal and spatial evolution of sierra nevada arc sources isotopes and major and trace elements are sensitive provenance indicators because of the large west east isotopic geochemical and age gradients in the plutonic rocks of the sierra nevada batholith and because petrographic models indicate that source areas moved east during the cretaceous isotopic and chemical variations are correlated in the forearc sandstone as nd decreases th u la nb zr hf pb rb sio2 and k2o concentrations increase and feo mgo tio2 ni and cr concentrations decrease this relation is the same as that observed in the plutonic rocks and indicates that the arc was the primary source of sediment and that the sandstone chemistry was not disturbed by sedimentary processes the nd sr relation of san joaquin valley sandstone is the same as the plutonic rocks but sacramento valley sandstone is elevated in sr because of seawater exchange weathering and diagenesis whole rock sandstone decreases in nd from 7 to 5 and increases in 87sr 86sr from 0 7045 to 0 7073 with decreasing stratigraphic age the nd sr isotopic composition is correlative with the plagioclase to feldspar ratio and indicates that source areas moved inland during the cretaceous upper cretaceous san joaquin valley shale is similar in nd to the sandstone indicating that sandstone and shale were derived from the same source and that the nd isotopic composition is independent of grain size the shale is higher in 87sr 86sr than the sandstone possibly due to concentration of biotite in the fine fraction during transport and subsequent rb loss during diagenesis nd sr isotopes were used to construct models to locate source areas parameters include lithology drainage basin geometry and erosion rate the age and isotopic compositions of the calculated igneous component of the sandstone correspond to the age and isotopic compositions of the plutonic rocks of the batholith this correspondence indicates that 1 the isotopic composition of the plutonic rocks and the coeval volcanic cover were similar 2 the volcanic front was denuded within a few million years and 3 the sediment was derived from the head of the drainage basin located at the migrating volcanic front

finite element study of uplift and strain across vancouver island geological evidence for sudden coastal subsidence along the west coast of southern vancouver island points to the occurrence of great prehistorical subduction earthquakes contemporary uplift and crustal shortening patterns in southern vancouver island appear to indicate that the subduction megathrust fault is currently locked to understand better the dynamics of the observed surface deformation we develop a finite element model of earthquake cycles for the northern cascadia subduction zone across southern vancouver island using a linear viscoelastic rheology the model consists of the continental and oceanic lithospheres the asthenospheric mantle with a viscosity of 5 1019 pa s and a low viscosity 1018 pa s mantle wedge between the subducted oceanic plate and the overlying continental plate the shallow geometry of the subducted juan de fuca plate is well defined by the results of various geophysical surveys and the deep geometry is constrained by the results of seismic tomography the model me

north atlantic versus southern ocean contributions to a deglacial surge in deep ocean ventilation past glacial interglacial climate transitions were accompanied by millennial scale pulses in atmospheric co2 that are widely thought to have resulted from the release of co2 via the southern ocean however direct proxy evidence for a southern ocean role in regulating past ocean atmosphere co2 exchange is scarce here we use combined radiocarbon and neodymium isotope measurements from the last deglaciation to confirm greatly enhanced overturning and or air sea exchange rates relative to today in particular during the bolling allerod warm interval we show that this deglacial pulse in ocean ventilation was not driven by the north atlantic overturning alone and must have involved an increase in the ventilation of southern sourced deep waters our results thus confirm the removal of a physical and or dynamical barrier to effective air sea co2 exchange in the southern ocean during deglaciation and highlight the antarctic region as a key locus for global climate carbon cycle feedbacks

evaluating zr rb ratio from xrf scanning as an indicator of grain size variations of glaciomarine sediments in the southern ocean the ln zr rb count ratio derived from x ray fluorescence xrf core scanning holds potential as a high resolution tracer for grain size variations of glaciomarine sediments to evaluate this appro

geologic and geophysical investigation of two fine grained granites sierra nevada batholith california evidence for structural controls on emplacement and volcanism the johnson granite porphyry and the silver pass porphyry are two late stage porphyritic intrusions in the cretaceous sierra nevada batholith california both intrusions are characterized by fine grained textures miarolitic cavities pegmatite veins and abundant xenoliths microstructures in the johnson granite porphyry record magmatic flow magnetic fabrics in the johnson granite porphyry determined by anisotropy of magnetic susceptibility show that the poles to magnetic foliations define a nne ssw girdle and magnetic lineations plunge shallowly to moderately to the wnw and ese a gravity survey and three dimensional inversion indicate that the johnson granite porphyry is shallow and tabular extending to 1 5 km depth beneath the southwestern margin a second gravity anomaly was delineated southeast of the johnson granite porphyry suggesting the intrusion continues to depth the silver pass porphyry is deformed by the dextral rosy finch shear zone and records a gradient of high to low temperature solid state fabrics field and magnetic foliations in the silver pass porphyry are subvertical and lineations are shallowly plunging we propose that regional shear zones controlled the emplacement of these intrusions the johnson granite porphyry was emplaced in a three dimensional tension gash associated with the termination of the bench canyon shear zone the rosy finch shear zone facilitated the emplacement of the silver pass porphyry probably as a tension fracture associated with the shear zone the field observations and magnetic fabrics suggest that the johnson granite porphyry may record an extrusive event suggesting that volcanism may be related to deformation along active shear zones

initiation of strike slip faults serpentinization and methane the nootka fault zone the juan de fuca explorer plate boundary the nootka fault zone is a ridge trench trench transform fault that was initiated 4 ma when the explorer ridge became independent of the juan de fuca ridge multibeam data around the fault zone and a compilation of several seismic reflection surveys provide insight into initiation of strike slip faults previous interpretations assumed that the two faults seen cutting the seafloor are subparallel to shear between the explorer and juan de fuca plates and formed instantaneously at 4 ma increased data density shows that these faults are subparallel to seafloor magnetic anomalies and appear to have utilized extensional faults formed at the ridge they are surrounded by numerous buried steeply dipping small offset growth faults at least some of which are likely still active our observations corroborate analogue models of strike slip fault initiation that predict formation of riedel like shears within a zone of faulting and that displacement localizes over time the existence of several long subparallel faults and a very wide zone of faulting has been predicted by models of distributed shear at depth along the nootka fault zone basement has risen by several hundred meters and bright reversed polarity reflectors some of which are interpreted to be methane hydrate reflectors are common hydration likely as serpentinization of the upper mantle could explain both sets of observations serpentinization can result in a 30 50 volume expansion and methane is observed in vents driven by this process biogenic sources of methane are likely to be present and concentrated by currently active fluid flow in the faulted sediments

the geochemistry of deep sea coral skeletons a review of vital effects and applications for palaeoceanography deep sea corals were discovered over a century ago but it is only over recent years that focused efforts have been made to explore the history of the oceans using the geochemistry of their skeletal remains they offer a promising archive of past oceanic environments given their global distribution layered growth patterns longevity and preservation as well as our ability to date them using radiometric techniques this paper provides an overview of the current state of the art in terms of geochemical approaches to using deep sea coral skeletons to explore the history of the ocean deep sea coral skeletons have a wide array of morphologies e g solitary cup corals branching colonial corals and materials calcite aragonite and proteins as such their biomineralization strategies are diverse leading to complex geochemistry within coral skeletons notwithstanding these complications progress has been made on developing methods for reconstructing the oceanographic environment in the past using trace elements and isotopic methods promising approaches within certain coral groups include clumped isotopes and mg li for temperature reconstructions boron isotopes and radiocarbon for carbon cycling end and radiocarbon for circulation studies and 15 n p ca and ba ca for nutrient tracer studies likewise there is now a range of techniques for dating deep sea corals skeletons e g u series radiocarbon and determining their growth rates e g radiocarbon and 210 pb dating studies on historic coral populations in the atlantic southern ocean and pacific point to climate and environmental changes being dominant controls on coral populations over millennial and orbital timescales this paper provides a review of a range of successes and promising approaches it also highlights areas in which further research would likely provide new insights into biomineralization palaeoceanography and distribution of past coral populations

middle cretaceous ash flow tuff and caldera collapse deposit in the minarets caldera east central sierra nevada california a 2 3 km section of ash flow tuff and associated caldera collapse deposit representing the extrusive facies of part of the sierra nevada batholith is totally exposed from its floor to its top in the minarets caldera east central sierra nevada rapid burial and subsequent hornblende hornfels facies metamorphism resulted in remarkable preservation of primary textures and structures despite the development of cleavage domains in parts of the caldera fill late tertiary uplift and quaternary erosion have produced a rugged terrain where every meter of section is available for study large scale caldera filling eruption of ash flow tuff was interrupted by emplacement of a wedge shaped mass of caldera collapse deposit as much as 2 km thick whose volume exceeded 70 km 3 individual clasts in the caldera collapse deposit range to as much as 1 8 km across and include a wide variety of andesitic to rhyoliic lavas and related volcaniclastic rocks remnants of a precaldera volcanic field that was probably much more extensive than the caldera itself the caldera fill sequence rests with angular unconformity on a rugged surface eroded into older volcanic rocks the sequence is capped by bedded volcaniclastic rocks including delicately laminated tuffs of probable caldera lake origin the total aerial extent of the minarets caldera is not known but the area studied plus scattered pendants of ash flow tuff and associated volcaniclastic rocks to the west defines a 30 x 22 km elliptical area that may approximate its original shape the caldera fill is invaded by a body of quartz monzonite porphyry locally miarolitic that was probably emplaced during an episode of caldera resurgence

growth rates of the deep sea scleractinia desmophyllum cristagalli and enallopsammia rostrata with uranium rich skeletons and density bands similar to their surface coral counterparts deep sea scleractinia are a promising archive of past climate to improve the utility of fossil samples as monitors of deep ocean variability we have measured 210 pb and 226 ra activities in a variety of modern specimens to constrain the range of growth rates mechanical and chemical cleaning of each sample are required to isolate the radionuclides trapped in the coral skeleton from surface contaminants however in many cases mechanically cleaned samples show the same overall growth rate as parallel transects of samples subjected to the full chemical and mechanical cleaning but with much higher overall activities three samples of desmophyllum cristagalli show a range of vertical extension rates from 0 5 mm yr to 2 mm yr consistent with previous estimates a single enallopsammia rostrata from the north atlantic is over 100 years old its average radial growth rate is 0 07 mm yr and the clear banding in this direction is not consistent with annual periodicity a minimum vertical extension rate of 5 mm yr is estimated from the 210 pb data both of these species are found in the fossil record and with the growth rates determined here can record about 100 years of climate change the growth rates will allow the reconstruction of climate at subdecadal resolution in d cristagalli and even higher resolution in e rostrata d 2004 elsevier b v all rights reserved

minimal change in antarctic circumpolar current flow speed between the last glacial and holocene relatively little is known about the dynamics of the antarctic circumpolar current during the last glacial period estimates of current speeds over the past 20 000 years based on sediment grain size suggest that average flow speeds during the last glacial were comparable to modern speeds but not in the areas with overlying winter sea ice

plate boundary deformation between the pacific and north america in the explorer region abstract one of the consequences of plate tectonics is that a spreading ridge will eventually approach a subduction zone the problem whether the possible break up of the approaching ridge will lead to the development of independent micro plates or not is still unresolved some 4 million years ago the interaction between the juan de fuca ridge and the cascadia subduction zone resulted in ridge fragmentation in the explorer region there are two proposed post miocene kinematic models one that proposes the presence of a micro plate and the other that treats the region as a transform deformation zone or so called pseudo plate we use earthquake strain rates derived from 74 events since 1948 to estimate a long term velocity field for the region by comparing this result with the predicted velocity fields for both models we try to discriminate between the two the earthquake strain rates indicate the presence of a transform deformation zone between the north american nam and pacific pac plates the velocity field derived from the inversion of the earthquake strain rates indicates that seismic activity takes up 50 30 1 of the pac nam relative motion nuvel 1a demets et al 1994 effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions geophys res lett 21 2191 2194 within the seismogenic layer of this zone and is indistinguishable in direction from the nuvel 1a pac nam model the presence of this explorer transform zone is consistent with the strain rate and velocity field for the pseudo plate model and indicates that seismicity defines a new plate boundary zone between the pacific and north american plates earthquake derived strain rates are low along the nootka transform which accommodates relative motion between the juan de fuca and north american plates the cause for the absence of significant seismic slip along this transform is unclear and may be closely linked to the fact that the adjacent cascadia subduction zone is locked the micro plate model which we reject predicts sw ne convergence in the eastern explorer region and this is inconsistent with the earthquake strain rates as well as with surface fault observations

great earthquakes on canada s west coast a review this article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent geological survey of canada contribution 20090196 the first lithoprobe transect in 1984 across vancouver island had primary objectives to define the structure associated with subduction and constraints on the potential for great thrust earthquakes

precise interpolar phasing of abrupt climate change during the last ice age a new ice core from west antarctica shows that during the last ice age abrupt northern hemisphere climate variations were followed two centuries later by a response in antarctica suggesting an oceanic propagation of the climate signal to the southern hemisphere high latitudes

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

the southern ocean s role in carbon exchange during the last deglaciation changes in the upwelling and degassing of carbon from the southern ocean form one of the leading hypotheses for the cause of glacial interglacial changes in atmospheric carbon dioxide we present a 25 000 year long southern ocean radiocarbon record reconstructed from deep sea corals which shows radiocarbon depleted waters during the glacial period and through the early deglaciation this depletion and associated deep stratification disappeared by 14 6 ka thousand years ago consistent with the transfer of carbon from the deep ocean to the surface ocean and atmosphere via a southern ocean ventilation event given this evidence for carbon exchange in the southern ocean we show that existing deep ocean radiocarbon records from the glacial period are sufficiently depleted to explain the 190 per mil drop in atmospheric radiocarbon between 17 and 14 5 ka

the presence of polynyas in the weddell sea during the last glacial period with implications for the reconstruction of sea ice limits and ice sheet history cores from a contourite drift on the continental slope in the nw weddell sea antarctica contain up to four sandy foraminifer rich layers wa1 4 with dispersed ice rafted debris ird interbedded with silty clayey sands and sandy muds accelerator mass spectrometry ams 14c dating on neogloboquadrina pachyderma from the foraminifer rich layers wa2 and wa3 yielded ages spanning the last glacial maximum lgm and marine isotope stage mis 2 20 319 28 543 cal yr bp and the middle part of mis 3 41 349 43 242 cal yr bp respectively we attribute the high abundances of planktonic and benthic foraminifera to deposition in a seasonally open marine environment given that the nw weddell sea shelf was covered by grounded ice at the lgm and probably during mis 2 and mis 3 and that diatom based reconstructions place the northern limit of winter and summer sea ice coverage to the north of the core sites during the last glacial period our results document the presence of a seasonal or perennial polynya in the nw weddell sea the polynya may have been formed by katabatic winds blowing off the antarctic peninsula ice sheet apis when it was grounded at the continental shelf break although polynya formation through upwelling of deep water cannot be ruled out in addition we argue that previously published data from the southern southeastern and southwestern weddell sea as well as the ross sea may indicate the widespread occurrence of polynyas along the antarctic continental margin during the last glacial period the widespread occurrence of polynyas could help explain how i aabw formation through brine rejection continued throughout the lgm and ii antarctic shelf benthos survived glacial periods the presence of polynyas through glacial periods also has implications for sea ice reconstructions from ice core records possibly biasing the signal towards a reduced sea ice cover finally we propose that the fringes of the apis which probably influence the presence or absence of polynyas in the nw weddell sea are sensitive to changes in antarctic temperature that may be related to a reduction in oceanic heat transport from the north atlantic

antarctic sea ice control on the depth of north atlantic deep water abstractchanges in deep ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide

weak overturning circulation and high southern ocean nutrient utilization maximized glacial ocean carbon abstract circulation changes have been suggested to play an important role in the sequestration of atmospheric co2 in the glacial ocean however previous studies have resulted in contradictory results regarding the strength of the atlantic meridional overturning circulation amoc and three dimensional quantitative reconstructions of the glacial ocean constrained by multiple proxies remain scarce here we simulate the modern and glacial ocean using a coupled physical biogeochemical global three dimensional model constrained simultaneously by 13 c radiocarbon and 15 n to explore the effects of amoc differences and southern ocean iron fertilization on the distributions of these isotopes and ocean carbon storage we show that 13 c and radiocarbon data sparsely sampled at the locations of existing glacial sediment cores can be used to reconstruct the modern amoc accurately applying this method to the glacial ocean we find that a surprisingly weak 6 9 sv or about half of today s and shallow amoc maximizes carbon storage and best reproduces the sediment isotope data increasing the atmospheric soluble iron flux in the model s southern ocean intensifies export production carbon storage and further improves agreement with 13 c and 15 n reconstructions our best fitting simulation is a significant improvement compared with previous studies and suggests that both circulation and export production changes were necessary to maximize carbon storage in the glacial ocean

dynamical reconstruction of the global ocean state during the last glacial maximum the global ocean state for the modern age and for the last glacial maximum lgm was dynamically reconstructed with a sophisticated data assimilation technique a substantial amount of data including global seawater temperature salinity only for the modern estimate and the isotopic composition of oxygen and carbon only in the atlantic for the lgm were integrated into an ocean general circulation model with the help of the adjoint method thereby the model was optimized to reconstruct plausible continuous fields of tracers overturning circulation and water mass distribution the adjoint based lgm state estimation of this study represents the state of the art in terms of the length of forward model runs the number of observations assimilated and the model domain compared to the modern state the reconstructed continuous sea surface temperature field for the lgm shows a global mean cooling of 2 2 k and the reconstructed lgm ocean has a more vigorous atlantic meridional overturning circulation shallower north atlantic deep water nadw equivalent stronger stratification and more saline deep water

antarctic sea ice control on ocean circulation in present and glacial climates in the modern climate the ocean below 2 km is mainly filled by waters sinking into the abyss around antarctica and in the north atlantic paleoproxies indicate that waters of north atlantic origin were instead absent below 2 km at the last glacial maximum resulting in an expansion of the volume occupied by antarctic origin waters in this study we show that this rearrangement of deep water masses is dynamically linked to the expansion of summer sea ice around antarctica a simple theory further suggests that these deep waters only came to the surface under sea ice which insulated them from atmospheric forcing and were weakly mixed with overlying waters thus being able to store carbon for long times this unappreciated link between the expansion of sea ice and the appearance of a voluminous and insulated water mass may help quantify the ocean s role in regulating atmospheric carbon dioxide on glacial interglacial timescales previous studies pointed to many independent changes in ocean physics to account for the observed swings in atmospheric carbon dioxide here it is shown that many of these changes are dynamically linked and therefore must co occur

origin damping and pattern of development of faults in granite observations of brittle fractures in the granite of the massif de la borne as well as in other rocks allow us to discuss the different hypotheses regarding the origin of faults it seems that all faults originate by shear along a preexisting plane which can be an important fracture joint former fault or a microflaw pore grain boundary the shear on the plane gives rise to an array of en echelon cracks that makes the rock less resistant and permits the lengthening of the fault we also found an arrangement of horsetail fractures damping the movements on the faults we then propose a pattern for the development of the faults they originate on preexisting plane and propagate by initiating en echelon cracks the movements are damped by horsetails which permit by faults connection in relays the creation of larger faults

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

magnitudes of large shallow earthquakes from 1904 to 1980 abstract in order to obtain a uniform magnitude catalogue surface wave magnitudes ms and broad band body wave magnitudes mb have been determined for large shallow earthquakes from 1904 to 1980 in making the catalogue homogeneous the author consistently adheres to the original definitions of ms and mb given by gutenberg 1945 and gutenberg and richter 1956 the determinations of ms and mb are all based on the amplitude and period data listed in gutenberg and richter s unpublished notes bulletins from stations worldwide and other basic information mb is measured on broad band instruments in periods of 8 s consistency of the magnitude determinations from these different sources is carefully checked in detail more than 900 shallow shocks of magnitude 7 and over are catalogued the meaning of the magnitude scales in various catalogues is examined in terms of ms and mb most of the magnitudes listed by gutenberg and richter 1954 in their seismicity of the earth are basically ms for large shocks shallower than 40 km but are basically mb for large shocks at depths of 40 60 km the surface wave magnitudes given by earthquake data reports are higher than ms by 0 2 unit whether the combined horizontal amplitude or the vertical amplitude is used mb and the currently used 1 s body wave magnitude are measured at different periods and should not be directly compared

extreme deepening of the atlantic overturning circulation during deglaciation glacial terminations during the late pleistocene epoch are associated with changes in insolation they are also punctuated by millennial scale climate shifts characterized by a weakening and subsequent strengthening of the atlantic meridional overturning circulation this ubiquitous association suggests that these oscillations may be a necessary component of deglaciation model simulations have suggested that the period of weakened circulation during these terminal oscillations would be followed by an overshoot of the circulation on its resumption but this phenomenon has not yet been observed here we use radiocarbon measurements of benthic foraminifera and carbonate preservation indices to reconstruct ventilation changes in the deep south atlantic ocean over the past 40 000 years we find evidence for a particularly deep expansion of the atlantic overturning cell directly following the weak mode associated with heinrich stadial 1 our analysis of an ocean general circulation model simulation suggests that north atlantic deep water export during the expansion was greater than that of interglacial conditions we find a similar deep expansion during dansgaard oeschger interstadial event 8 38 000 years ago which followed heinrich stadial 4 we conclude that the rise in atmospheric co2 concentrations and resultant warming associated with an especially weak overturning circulation are sufficient to trigger a switch to a vigorous circulation but a full transition to interglacial conditions requires additional forcing at an orbital scale

sequestration of carbon in the deep atlantic during the last glaciation atmospheric co2 concentrations declined markedly about 70 000 years ago when the earth s climate descended into the last glaciation much of the carbon removed from the atmosphere has been suspected to have entered the deep oceans but evidence for increased carbon storage remains elusive here we use the b ca ratios of benthic foraminifera from several sites across the atlantic ocean to reconstruct changes in the carbonate ion concentration and hence the carbon inventory of the deep atlantic across this transition we find that deep atlantic carbonate ion concentration declined by around 25 mol kg 1 between 80 000 and 65 000 years ago this drop implies that the deep atlantic carbon inventory increased by at least 50 gt around the same time as the amount of atmospheric carbon dropped by about 60 gt from a comparison with proxy records of deep circulation and climate model simulations we infer that the carbon sequestration coincided with a shoaling of the atlantic meridional overturning circulation we thus conclude that changes in the atlantic ocean circulation may have played an important role in reductions of atmospheric co2 concentrations during the last glaciation by increasing the carbon storage in the deep atlantic

late quaternary sea ice history in the indian sector of the southern ocean as recorded by diatom assemblages abstract a modern analog technique mat5201 31 has been applied to fossil diatom assemblages to provide down core estimates of february sea surface temperatures ssts and of sea ice duration over the past 220 000 years at 56 40 s 160 14 e at the core location sea ice progression lagged the sst drop by 1 ka at interglacial glacial transitions and sea ice retreat was almost synchronous to the sst increase at glacial interglacial terminations sea ice increased continuously during glacial periods to reach its maximum extent at the end of glacial times although ssts were almost constant during glacials this indicates that ssts are the major parameter determining the advance and retreat of sea ice at transitions but that the sea ice advance during glacial conditions may be related to positive feedbacks of the ice on albedo air temperature and meridional wind stress the strong correlation r 0 75 between sea ice duration at the core location and the vostok co2 record argues for a control of antarctic sea ice extent on atmospheric co2 concentration via the modification of the ocean to atmosphere gas balance

flushing of the deep pacific ocean and the deglacial rise of atmospheric co 2 concentrations during the last deglaciation 19 000 9 000 years ago atmospheric co2 increased by about 80 ppm understanding the mechanisms responsible for this change is a central theme of palaeoclimatology relevant for predicting future co2 transfers in a warming world deglacial co2 rise hypothetically tapped an accumulated deep pacific carbon reservoir but the processes remain elusive as they are underconstrained by existing tracers here we report high resolution authigenic neodymium isotope data in north pacific sediment cores and infer abyssal pacific overturning weaker than today during the last glacial maximum but intermittently stronger during steps of deglacial co2 rise radiocarbon evidence suggestive of relatively old deglacial deep pacific water is reinterpreted here as an increase in preformed 14c age of subsurface waters sourced near antarctica consistent with movement of aged carbon out of the deep ocean and release of co2 to the atmosphere during the abyssal flushing events the timing of neodymium isotope changes suggests that deglacial acceleration of pacific abyssal circulation tracked southern hemisphere warming sea ice retreat and increase of mean ocean temperature the inferred magnitude of circulation changes is consistent with deep pacific flushing as a significant and perhaps dominant control of the deglacial rise of atmospheric co2

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

enhanced δ13c and δ18o differences between the south atlantic and south pacific during the last glaciation the deep gateway hypothesis enhanced vertical gradients in benthic foraminiferal 13c and 18o in the atlantic and pacific during the last glaciation have revealed that ocean overturning circulation was characterized by shoaling of north atlantic sourced interior waters nonetheless our understanding of the specific mechanisms driving these glacial isotope patterns remains incomplete here we compare high resolution depth transects of cibicidoides spp 13c and 18o from the southwest pacific and the southwest atlantic to examine relative changes in northern and southern sourced deep waters during the last glacial maximum lgm and deglaciation during the lgm our transects show that water mass properties and boundaries in the south atlantic and pacific were different from one another the atlantic between 1 0 and 2 5 km was more than 1 enriched in 13c relative to the pacific and remained more enriched through the deglaciation during the lgm atlantic 18o was 0 5 more enriched than the pacific particularly below 2 5 km this compositional difference between the deep portions of the basins implies independent deep water sources during the glaciation we attribute these changes to a deep gateway effect whereby northern sourced waters shallower than the drake passage sill were unable to flow southward into the southern ocean because a net meridional geostrophic transport cannot be supported in the absence of a net east west circumpolar pressure gradient above the sill depth we surmise that through the lgm and early deglaciation shoaled northern sourced waters were unable to escape the atlantic and contribute to deep water formation in the southern ocean

multistage emplacement of the mount givens pluton central sierra nevada batholith california the ca 90 ma mount givens pluton is one of the largest granodioritic to granitic intrusions in the sierra nevada batholith of california emplacement of the pluton occurred during a critical time in the tectonic evolution of the central sierra nevada magmatic arc marked by a transition from regional contraction to dextral transcurrent shear a model for the emplacement of the intrusion is developed based on detailed mapping of the pluton and its wall rocks and characterization of its internal structure by measurements of the anisotropy of magnetic susceptibility ams at 351 stations one of the key results of the study is documentation of a strong correlation between petrologic and structural fabrics in the pluton and determination that these fabrics reflect internal magma chamber dynamics more than regional tectonic strain the 80 km long 15 30 km wide pluton crystallized from a multiphase three segment magma chamber marked by a bulbous northern lobe and linear central and southern segments the pluton is interpreted to be tabular in shape with a thickness of 5 km most of the space for the pluton was created by piecemeal block downdrop of the magma chamber floor along three principal fracture sets the most important of which were steeply dipping northwest trending fractures formed parallel to the structural grain of the arc and vertical north trending extension fractures formed in response to a component of arc parallel dextral shear some of these fractures acted as magma conduits episodically filling the pluton as source rocks became depleted in melt an initial voluminous intrusive event stage 1 quickly filled the southern chamber with granodiorite magma but only partially filled the northern and central chambers stage 2 magmatism involved underplating of megacrystic granite in the northern chamber and lateral flow of a large batch of this magma from the northern to the central chamber the latter delineated by a 20 km long belt of megacrystic granite containing subhorizontal magnetic lineations that connects the pluton segments floor downdrop eventually ceased to be an effective space making process in the northern lobe and renewed magmatism stage 3 led to expansion and doming of the chamber as the northern lobe cooled a ring fault ruptured within the viscoelastic stage 1 2 carapace allowing ring dike intrusion stage 4 and sinking of a central flap of consolidated material the temporal and spatial variations in emplacement mechanisms demonstrated for the mount givens pluton i e fracture generation floor downdrop underplating inflation ring diking suggest that end member models e g fracture vs diapir are oversimplifications of the pluton assembly process

antarctic sea ice and the control of pleistocene climate instability a hypothesis is presented for the origin of pleistocene climate instability based on expansion of antarctic sea ice and associated changes in the oceans salinity structure the hypothesis assumes that thermohaline overturning is dominated by the reconfigured conveyor of toggweiler and samuels 1993b in which deepwater upwelling is restricted to high southern latitudes the reconfigured conveyor is shown to be potentially stabilized in an on mode by precipitation at high southern latitudes and potentially destabilized into on and off modes by the counteracting influence of antarctic sea ice the mechanism is clarified by the use of a hydraulic analogue we hypothesize that this mechanism accounts for dominant patterns of thermohaline overturning and climate instability between pleistocene warm and cold periods the hypothesis is shown to be consistent with a range of paleoceanographic evidence and to potentially account for details of observed rapid climate changes during glacial and interglacial periods including aspects of interhemispheric timing

ventilation of the deep southern ocean and deglacial co2 rise past glacial interglacial increases in the concentration of atmospheric carbon dioxide co2 are thought to arise from the rapid release of co2 sequestered in the deep sea primarily via the southern ocean here we present radiocarbon evidence from the atlantic sector of the southern ocean that strongly supports this hypothesis we show that during the last glacial period deep water circulating around antarctica was more than two times older than today relative to the atmosphere during deglaciation the dissipation of this old and presumably co2 enriched deep water played an important role in the pulsed rise of atmospheric co2 through its variable influence on the upwelling branch of the antarctic overturning circulation

assembly of a dike fed magma chamber the jackass lakes pluton central sierra nevada california the mechanisms of ascent assembly and emplacement of granitic magma in the crust are critical to understanding the dynamics of continental margin growth yet these mechanisms remain controversial and poorly understood detailed study of structural and petrologic fabrics in the middle cretaceous jackass lakes pluton wall rock system central sierra nevada california coupled with u pb geochronology indicates that the pluton formed via sheet like assembly of a dike fed magma chamber final emplacement of the pluton was facilitated by multiple brittle and ductile mechanisms that were active at different times and places within the system this supports hybrid viscoelastic emplacement models as realistic alternatives to end member models i e dike versus diapir fracture propagation which initiated 40 of the space required for emplacement may have been facilitated by a small component of arc parallel dextral shear that produced north northwest striking tension gashes a combination of ductile wall rock shortening during lateral expansion of sheets and return flow of elongate strongly deformed wall rock septa produced an additional 25 of the space required other mechanisms including coeval formation of the overlying minarets caldera and stoping in the subvolcanic part of the magma chamber must account for the remaining 35 space implying that vertical transfer of material is an important emplacement mechanism at shallow crustal levels

deep sea coral aragonite as a recorder for the neodymium isotopic composition of seawater deep sea corals have been shown to be useful archives of rapid changes in ocean chemistry during the last glacial cycle their aragonitic skeleton can be absolutely dated by u th data freeing radiocarbon to be used as a water mass proxy for certain species of deep sea corals the growth rate allows time resolution that is comparable to ice cores an additional proxy is needed to exploit this opportunity and turn radiocarbon data into rates of ocean overturning in the past neodymium isotopes in seawater can serve as a quasi conservative water mass tracer and initial results indicate that deep sea corals may be reliable archives of seawater nd isotopes here we present a systematic study exploring nd isotopes as a water mass proxy in deep sea coral aragonite we investigated five different genera of modern deep sea corals caryophyllia desmophyllum enallopsamia flabellum lophelia from global locations covering a large potential range of nd isotopic compositions comparison with ambient seawater measurements yields excellent agreement and suggests that deep sea corals are reliable archives for seawater nd isotopes a parallel study of nd concentrations in these corals yields distribution coefficients for nd between seawater and coral aragonite of 1 10 omitting one particular genus enallopsamia the corals and seawater did however not come from exactly the same location and further investigations are needed to reach robust conclusions on the incorporation of nd into deep sea coral aragonite lastly we studied the viability of extracting the nd isotope signal from fossil deep sea corals by carrying out stepwise cleaning experiments our results show that physical removal of the ferromanganese coating and chemical pre cleaning have the highest impact on nd concentrations but that oxidative reductive cleaning is also needed to acquire a seawater nd isotope signal

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

the 1918 and 1957 vancouver island earthquakes since 1918 six significant earthquakes 5 2 m 7 2 have occurred in the region of central vancouver island where the juan de fuca explorer and america plates interact in this study two of the largest earthquakes are examined in detail the 1918 ms 7 and the 1957 ms 6 events the preferred location of the 1918 earthquake is on vancouver island at 49 44 n 126 22 w with a focal depth of 15 km magnitudes determined are ms 6 9 0 3 and mb 7 2 0 4 analysis of surface waves suggests this is a predominantly strike slip earthquake occurring along either a nnw or an ene striking fault the seismic moment is estimated as 7 40 1025 dyne cm and the stress drop to be 122 bars the 1957 earthquake has been relocated on the continental shelf west of vancouver island at 49 64 n 127 00 w with a focal depth of 30 km magnitudes determined are ms 5 9 0 2 and mb 6 3 0 3 surface wave and p nodal analyses indicate that this is a predominantly strike slip earthquake either dextral along an nnw striking fault or sinistral along a ene striking fault the seismic moment is estimated as 8 14 1024 dyne cm and the stress drop to be 36 bars the 1918 earthquake appears to be a crustal intraplate event occurring in the lithosphere of the america plate resulting from the complicated interaction of the explorer juan de fuca and america plates the preferred epicenter depth focal mechanism and stress drop for the 1957 earthquake are consistent with the left lateral motion between the juan de fuca and explorer plates along the subducted extension of the nootka fault zone this earthquake is identical within uncertainties to events occurring in 1972 and 1986 we believe that these three earthquakes provide the best definition to date of both the position of the subducting portion of the nootka fault zone west of vancouver island and the direction of relative motion along this fault

a revised magnitude recurrence relation for shallow crustal earthquakes in southwestern british columbia considering the relationships between moment magnitude and regional magnitudes an important component of seismic hazard analysis is the magnitude recurrence relation which provides the cumulative rate of occurrence of earthquakes within a seismic source zone as a function of magnitude in the national seismic hazard maps developed for the 2005 national building code of canada the magnitude recurrence relation for the cascade mountains zone of southwestern british columbia casr the seismic zone in which vancouver and victoria are located is based mainly on earthquakes measured on the local scale m l richter scale but other scales including body wave magnitude m b coda duration magnitude m c and moment magnitude m have been used for some of the events the mixture of magnitude types is a potential source of bias in the relations by using data from recently calculated regional moment tensor solutions a local magnitude moment magnitude relationship is developed for magnitudes of 2 9 m l 6 0 in the casr zone this relation along with relations from other sources for m b and m c and specific moment magnitude values where available are used to develop a homogeneous moment magnitude catalog for use in the formulation of a new casr zone magnitude recurrence relation the use of a homogeneous moment magnitude scale produces a significant change in the shape of the magnitude recurrence curve in the casr zone perhaps more importantly the use of a homogeneous catalog reduces the uncertainty in the recurrence rates of moderate to large earthquakes in casr by nearly an order of magnitude we conclude that the use of a homogeneous magnitude scale is critical to producing unbiased magnitude recurrence relations

crustal earthquake triggering by pre historic great earthquakes on subduction zone thrusts triggering of earthquakes on upper plate faults during and shortly after recent great m 8 0 subduction thrust earthquakes raises concerns about earthquake triggering following cascadia subduction zone earthquakes of particular regard to cascadia was the previously noted but only qualitatively identified clustering of m 6 5 crustal earthquakes in the puget sound region between about 1200 900 cal years b p and the possibility that this was triggered by a great cascadia thrust subduction thrust earthquake and therefore portends future such clusters we confirm quantitatively the extraordinary nature of the puget sound region crustal earthquake clustering between 1200 900 cal years b p at least over the last 16 000 we conclude that this cluster was not triggered by the penultimate and possibly full margin great cascadia subduction thrust earthquake however we also show that the paleoseismic record for cascadia is consistent with conclusions of our companion study of the global modern record outside cascadia that m 8 6 subduction thrust events have a high probability of triggering at least one or more m 6 5 crustal earthquakes

northern cascadia episodic tremor and slip a decade of tremor observations from 1997 to 2007 1 we analyze continuous seismic and gps records collected in the last decade 1997 2007 to establish the most comprehensive observational basis for northern cascadia episodic tremor and slip ets events a simple ets scale system using a combination of a letter and a digit is proposed to quantitatively characterize the spatial and temporal dimensions of ets events clear correlation between gps and tremor signals is observed for all a b class episodes but the gps signature is less obvious for minor ones regular ets recurrence can be established only for a b class episodes in southern vancouver island halting and jumping are very common in ets migration patterns and along strike migration can happen in both directions a prominent tremor gap is observed in midisland around 49 5 n this gap coincides with the epicenters of the only two large earthquakes beneath vancouver island ets tremors also tend to occur in places where the local seismicity is relatively sparse the tremor depth distribution shows a peak in the 25 35 km range where strong seismic reflectors i e the e layer are documented detailed waveform analysis confirms the existence of shallow tremors above the currently interpreted plate interface our results suggest that a significant portion of the tremor activity and perhaps associated shearing are taking place along well developed structures such as the e layer while fewer tremor bursts are generated elsewhere in response to the induced stress variation throughout the source volume

structural eruptive and intrusive evolution of the grizzly peak caldera sawatch range colorado volcanic and shallow intrusive features of the deeply dissected 34 ma grizzly peak caldera in west central colorado record evolution of the magmatic center from pre caldera through post resurgent stages pre caldera dikes zones of hydrothermally altered rocks and lava flows formed along a circular swarm of cone sheet fractures around the site of the future caldera early largely rhyolitic uppercrustal magmatism culminated in the caldera forming eruption of the grizzly peak tuff intracaldera tuff is zoned from high silica rhyolite at the base to low silica rhyolite at the eroded top and further contains dacite to mafic latite pumice lumps in two heterogeneous tuff layers in the upper third of the preserved section half of the erupted tuff ponded in the 17 by 23 km 600 km 3 caldera filling the asymmetric depression to a compacted thickness locally 2 7 km including intercalated rock avalanche megabreccias shed from ring fault scarps the asymmetric caldera has an inner ring fracture zone that separates two structural segments that collapsed to different depths caldera fill buried collapse structures as they formed the inner ring fracture zone is a growth or syn depositional fault in the single cooling unit tuff welded tuff ring dikes are locally exposed at erosion levels below the caldera fill these dikes are remnants of fissure vents in the outer ring fracture zone following collapse the grizzly peak caldera was uplifted forming a complexly faulted resurgent dome resurgence resulted partly from emplacement of a composite granodiorite laccolith now exposed in the eroded core of the dome a belt of mafic latite to rhyolite porphyry dikes and small stocks formed across the center of the domed caldera during the waning of the magmatic center latite intrusions in this suite represent the penetration of relatively mafic magma to the surface following solidification of the felsic subcaldera batholith

interhemispheric controls on deep ocean circulation and carbon chemistry during the last two glacial cycles changes in ocean circulation structure together with biological cycling have been proposed for trapping carbon in the deep ocean during glacial periods of the late pleistocene but uncertainty remains in the nature and timing of deep ocean circulation changes through glacial cycles in this study we use neodymium nd and carbon isotopes from a deep indian ocean sediment core to reconstruct water mass mixing and carbon cycling in circumpolar deep water over the past 250 thousand years a period encompassing two full glacial cycles and including a range of orbital forcing building on recent studies we use reductive sediment leaching supported by measurements on isolated phases foraminifera and fish teeth in order to obtain a robust seawater nd isotope reconstruction neodymium isotopes record a changing north atlantic deep water nadw component in the deep indian ocean that bears a striking resemblance to northern hemisphere climate records in particular we identify both an approximately in phase link to northern hemisphere summer insolation in the precession band and a longer term reduction of nadw contributions over the course of glacial cycles the orbital timescale changes may record the influence of insolation forcing for example via nadw temperature and or antarctic sea ice extent on deep stratification and mixing in the southern ocean leading to isolation of the global deep oceans from an nadw source during times of low northern hemisphere summer insolation that evidence could support an active role for changing deep ocean circulation in carbon storage during glacial inceptions however mid depth water mass mixing and deep ocean carbon storage were largely decoupled within glacial periods and a return to an interglacial like circulation state during marine isotope stage mis 6 5 was accompanied by only minor changes in atmospheric co2 although a gradual reduction of nadw export through glacial periods may have produced slow climate feedbacks linked to the growth of northern hemisphere ice sheets carbon cycling in the glacial ocean was instead more strongly linked to southern ocean processes

crustal scale en echelon p shear tensional bridges a possible solution to the batholithic room problem the apparent paradox of voluminous granitoid emplacement in an overall compressional magmatic are has been somewhat alleviated by the idea of pluton emplacement in tension cracks and dilational jogs within arc parallel strike slip fault systems as an alternative hypothesis we propose that an en echelon p shear array provides a continuous batholith scale zone of dilation along the trend of active magmatism and that it is favored by transpressional tectonics in the sierra nevada california the late cretaceous granitoids of the cathedral range intrusive epoch are consistent with emplacement into bridges between p shears because 1 they are elongated oblique to the magmatic trend consistent with p shear orientation within a dextral system 2 they were all emplaced within 5 10 m y along a 300 km trend resulting in a displacement rate of 1 cm yr across the arc and 3 syn to late magmatic dextral shear zones bound and crosscut the plutons as in a p shear tensional bridge

rapid cross density ocean mixing at mid depths in the drake passage measured by tracer release diapycnal mixing across density surfaces is an important process in the global ocean overturning circulation1 2 3 mixing in the interior of most of the ocean however is thought to have a magnitude just one tenth of that required to close the global circulation by the downward mixing of less dense waters4 some of this deficit is made up by intense near bottom mixing occurring in restricted hot spots associated with rough ocean floor topography5 6 but it is not clear whether the waters at mid depth 1 000 to 3 000 metres are returned to the surface by cross density mixing or by along density flows7 here we show that diapycnal mixing of mid depth 1 500 metres waters undergoes a sustained 20 fold increase as the antarctic circumpolar current flows through the drake passage between the southern tip of south america and antarctica our results are based on an open ocean tracer release of trifluoromethyl sulphur pentafluoride we ascribe the increased mixing to turbulence generated by the deep reaching antarctic circumpolar current as it flows over rough bottom topography in the drake passage scaled to the entire circumpolar current the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups 1 sv 106 m3 s 1 upwell in the southern ocean with cross density mixing contributing a significant fraction 20 to 30 per cent of this total and the remainder upwelling along constant density surfaces the great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean floor topography

submillennial variations in ocean conditions during deglaciation based on diatom assemblages from the southwest atlantic we present a high resolution paleoceanographic record of deglaciation based on diatom assemblages from a core located just south of the polar front in the southwest atlantic core kc073 is from a sediment drift at the mouth of the falkland trough and contains sediments from the last glacial maximum lgm to present dated using radiocarbon dates on bulk organic matter and radiolarian stratigraphy the site lies along the path of the antarctic circumpolar current acc and immediately downstream of where north atlantic deep water nadw is entrained into the acc significant variations in ocean conditions are reflected in high amplitude changes in diatom concentrations and assemblage composition the diatom assemblage at the lgm indicates that winter sea ice extent was at least 5 farther north than present until at least 19 0 ka calendar years and summer sea ice may have occasionally extended over the site but for the most part it lay to the south during deglaciation chaetoceros resting spores crs dominate the diatom assemblage with valve concentrations in excess of 500 106 valves per gram submillennial scale variations in the numbers of crs and thalassiosira antarctica occur throughout the late deglacial and dominate the changes in diatom concentration we propose that the influx of crs is controlled by the flow of nadw over the falkland plateau as such our data provide unique evidence that nadw impacted on this sector of the southern ocean during deglaciation during the holocene the sedimentation rate dramatically reduced we suggest that the acc flow increased over the site and inhibited settling and winnowed the surface sediments

exposure of a late cretaceous layered mafic felsic magma system in the central sierra nevada batholith california new u pb zircon ages for the lamarck granodiorite associated synplutonic gabbro and diorite plutons and two large mafic intrusive complexes that underlie them in the sierra nevada batholith are 92 1 ma these ages establish the late cretaceous as a period of extensive mafic felsic magmatism in the central part of the batholith and confirm the significance of mafic magmatism in the evolution of the voluminous silicic plutions in the sierran arc the lack of significant zircon inheritance in any of the units analyzed supports isotopic evidence that the lamarck and other late cretaceous sierran plutons were derived predominantly from young crust recognition of an extensive mafic felsic magma system in the sierra nevada batholith emphasizes the importance of basaltic liquids in the evolution of continental crust in arc settings

seismotectonics of the explorer region 1 the explorer region offshore western canada is a tectonically complex area surrounded by the pacific north america and juan de fuca plates existing tectonic models for the region differ fundamentally proposed plate configurations range from multiple independent plate fragments to an explorer plate now fused to north america along the continental margin and cut by pacific north america transform faults in the west we present new seismological data constraining the region s current tectonics we use three component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4 combined with 34 harvard centroid moment tensor solutions they represent the region s largest earthquake source parameter data set obtained by robust waveform modeling techniques in addition we perform joint epicenter determination to relocate larger earthquakes recorded since 1918 the source parameters and improved locations provide a consistent tectonic picture earthquake slip vector azimuths along the pacific plate boundary change smoothly and are significantly less northerly oriented than the pacific north america plate motion direction requiring an independent explorer plate the present day pacific explorer boundary is formed by transform faults subparallel to the revere dellwood wilson fault plate motion vectors indicate that the winona block is part of the explorer plate current explorer motion is more northerly than indicated by magnetic anomalies prior to 2 ma implying a recent change possibly coinciding with a northwestward ridge jump near explorer plate s northern end transferring the winona block from the pacific to the explorer plate in response to these plate motion changes the region north of the western sovanco fracture zone was assimilated into the pacific plate the region around the eastern sovanco fracture zone characterized by broadly distributed seismicity is composed of well defined sets of conjugate faults bounding rotating crustal blocks earthquake fault strikes agree with the dominant northwest southeast fault sets however the conjugate sets must be also active to fully accommodate present day explorer plate motion the sw portion of the strike slip nootka fault zone the explorer juan de fuca plate boundary is well defined by focused seismicity however its full extent under nootka island remains unresolved the explorer north america boundary shows sporadic low magnitude seismicity our explorer north america rotation pole predicts convergence varying from negligible at the boundary s northwest end to 2 cm yr at the se end this convergence can be accommodated either by subduction or by crustal thickening extending to the north american continent we favor subduction based on low deformation rates observed by onshore gps sites the present explorer plate system configuration is a result of stepwise reorientation of the explorer ridge system each step successively reducing the subduction rate relative to north america

carbon dioxide effects of antarctic stratification north atlantic intermediate water formation and subantarctic nutrient drawdown during the last ice age diagnosis and synthesis in a geochemical box model in a box model synthesis of southern ocean and north atlantic mechanisms for lowering co2 during ice ages the co2 changes are parsed into their component geochemical causes including the soft tissue pump the carbonate pump and whole ocean alkalinity when the mechanisms are applied together their interactions greatly modify the net co2 change combining the antarctic mechanisms stratification nutrient drawdown and sea ice cover within bounds set by observations decreases co2 by no more than 36 ppm a drawdown that could be caused by any one of these mechanisms in isolation however these antarctic changes reverse the co2 effect of the observed ice age shoaling of north atlantic overturning in isolation the shoaling raises co2 by 16 ppm but alongside the antarctic changes it lowers co2 by an additional 13 ppm a 29 ppm synergy the total co2 decrease does not reach 80 ppm partly because antarctic stratification antarctic sea ice cover and the shoaling of north atlantic overturning all strengthen the sequestration of alkalinity in the deepest ocean which increases co2 both by itself and by decreasing whole ocean alkalinity increased nutrient consumption in the sub antarctic causes as much as an additional 35 ppm co2 decrease interacting minimally with the other changes with its inclusion the lowest ice age co2 levels are within reach these findings may bear on the two stepped co2 decrease of the last ice age

short term variations of deep water masses in drake passage revealed by a multiparametric analysis of the ant xxiii 3 bottle data short term variations of deep water masses in drake passage revealed by a multiparametric analysis of the ant xxiii 3 bottle data j sudre v garcon c provost n sennechael o huhn and m lacombe doi 10 1016 j dsr2 2011 01 005 a multiparametric analysis is applied on the ant xxiii 3 bottle data to determine the spreading and mixing of water masses across drake passage during january february 2006 focusing our interest on the flow of the deep water masses mixing proportions are quantified along both the southward and northward journeys between the tip of south america and the antarctic peninsula the method is sensitive enough to detect a marked variability in the flow pathways of the circumpolar deep water the southeast pacific deep water spdw and the weddell sea deep water at the intersection of the shackleton fracture zone sfz and the west scotia ridge wsr between the southward and northward journeys indeed the presence of a sharp meander of the southern branch of the polar front pfs together with the displacement of the northern branch of the southern antarctic circumpolar current front saccfn above the complex bathymetric structure of the wsr induced a strong perturbation in the deep flow field during the southward journey the height of the sfz crests clearly sets the lower limit of the spdw in the ona basin an enhanced 3 he signature is confined to the north of the polar front between 55 5 s and 56 5 s in the 1500 2200 m depth range it clearly indicates the signature of the core of the southeast pacific deep slope water the entry pathway in the acc of the spdsw during ant xxiii 3 was farther to the south off the continental slope of south america than it was during the woce a21 expedition in 1990 probably due to the sharp changes in the slope orientation at the extreme tip of the south america

reconstruction of changes in the weddell sea sector of the antarctic ice sheet since the last glacial maximum the weddell sea sector is one of the main formation sites for antarctic bottom water and an outlet for about one fifth of antarctica s continental ice volume over the last few decades studies on glacial geological records in this sector have provided conflicting reconstructions of changes in ice sheet extent and ice sheet thickness since the last glacial maximum lgm at ca 23 19 calibrated kiloyears before present cal ka bp terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the weddell sea ice sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding line advance at the lgm other marine geological and geophysical studies concluded that subglacial bedforms mapped on the weddell sea continental shelf subglacial deposits and sediments over compacted by overriding ice recovered in cores and the few available radiocarbon ages from marine sediments are consistent with major ice sheet advance at the lgm reflecting the geological interpretations different ice sheet models have reconstructed conflicting lgm ice sheet configurations for the weddell sea sector consequently the estimated contributions of ice sheet build up in the weddell sea sector to the lgm sea level low stand of 130 m vary considerably in this paper we summarise and review the geological records of past ice sheet margins and past ice sheet elevations in the weddell sea sector we compile marine and terrestrial chronological data constraining former ice sheet size thereby highlighting different levels of certainty and present two alternative scenarios of the lgm ice sheet configuration including time slice reconstructions for post lgm grounding line retreat moreover we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research the aim of our study is to provide two alternative interpretations of glacial geological datasets on antarctic ice sheet history for the weddell sea sector which can be utilised to test and improve numerical ice sheet models

last glacial holocene climate variability in the atlantic sector of the southern ocean abstract the southern ocean plays a major role in the glacial interglacial global carbon cycle however there is a substantial lack of information from its antarctic zone south of the polar front pf to understand key climate processes e g sea ice variability productivity changes co2 source region shifts of the southern westerly wind active in this region during the glacial interglacial transition due to the limited high resolution sediment records from this area to close this gap we investigated high resolution diatom records from a series of sediment cores from the atlantic and western indian sectors of the southern ocean between the modern pf and the winter sea ice wsi edge summer sea surface temperature ssst and sea ice information spanning the past 30 thousand years were derived from diatom transfer functions and indicators which augment comprehensive information on past surface ocean conditions and related ocean and atmospheric circulation as well as opal deposition these complementary lines of evidences also provide important environmental boundary conditions for climate simulations understanding the past climate development in the high latitudes southern ocean our reconstructions show that the last glacial lg sssts south of the modern pf are 1 3 c colder than modern conditions wsi expanded to the modern pf our data suggests effective carbon export in the antarctic zone during the lg deglacial two steps of warming support the bipolar seesaw mechanism antarctic zone is an important source region for the co2 deglacial increase the warming was more suppressed towards south due to continuous ice discharge from antarctica the sssts exceeded modern values during the early holocene optimum when wsi extent probably retreated south of its modern position the southern boundary of maximum opal deposition zone may have shifted to south of 55 s in the bouvet island area at this time the mid late holocene cooling with wsi re expanding to the bouvet island area probably related to enhanced cold water export by the weddell gyre from the developing cavity under the west antarctic ice shelf the cooling also suggests a northward shift of the southern westerly wind at least its southern boundary

oscillating glacial northern and southern deep water formation from combined neodymium and carbon isotopes abstract while ocean circulation is driven by the formation of deep water in the north atlantic and the circum antarctic the role of southern sourced deep water formation in climate change is poorly understood here we address the balance of northern and southern sourced waters in the south atlantic through the last glacial period using neodymium isotope ratios of authigenic ferromanganese oxides in thirteen deep sea cores from throughout the south atlantic the data indicate that northern sourced water did not reach the southern ocean during the late glacial and was replaced by southern derived intermediate and deep waters the high resolution neodymium isotope record 300 yr sample spacing from two spliced deep cape basin sites indicates that over the last glacial period northern sourced water mass export to the southern ocean was stronger during the major greenland millennial warming intervals and southern hemisphere cool periods and particularly during the major interstadials 8 12 and 14 northern sourced water mass export was weaker during greenland stadials and reached minima during heinrich events the benthic foraminiferal carbon isotopes in the same cape basin core reflect a partial control by southern hemisphere climate changes and indicate that deep water formation and ventilation occurred in the southern ocean during major greenland cooling intervals stadials together neodymium isotopes and benthic carbon isotopes provide new information about water mass sourcing and circulation in deep southern ocean waters during rapid glacial climate changes combining carbon and neodymium isotopes can be used to monitor the relative proportion of northern and southern sourced waters in the cape basin to gain insight into the processes which control the carbon isotopic composition of deep waters in this study we show that deep water formation and circulation was more important than biological productivity and nutrient regeneration changes for controlling the carbon isotope chemistry of antarctic bottom water during millennial scale glacial climate cycles this observation also lends support to the hypothesis that ocean circulation is linked to interhemispheric climate changes on short timescales and that ventilation in the glacial ocean rapidly switched between the northern and southern hemisphere on millennial timescales

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

reconstructing deglacial north and south atlantic deep water sourcing using foraminiferal nd isotopes during the last few decades neodymium isotopes have been increasingly used as a paleoceanographic and paleoclimate proxy on million year through millennial time scales the widespread use of the nd isotope proxy depends on whether the nd isotopic composition of past seawater can be reliably extracted from authigenic sediment phases here we show evidence that planktonic foraminifera with authigenic coatings preserve a deep water nd isotopic signature and thus can be used to reconstruct past deep water nd isotopes applying this method downcore on south atlantic deep cape basin core tno57 21 41 1 s 7 9 e 4981 m the nd isotopic composition of the planktonic foraminiferal fe mn oxide coatings is identical with and confirms that of the reductively leached bulk sediment fraction piotrowski et al 2004 2005 2008 as reflecting changes in bottom water sourcing in the case of northeastern north atlantic core bofs 8k 52 5 n 22 1 w 4045 m the planktonic foraminiferal fe mn oxide coatings have different nd isotopes from reductive sediment leaches clearly acid reductive leaching must be tested in more detail we observe that the nd isotopic composition of the deep northeastern atlantic core shifted during the deglaciation from glacial values similar to the south atlantic cape basin towards holocene values similar to the bermuda rise roberts et al 2010 taken as a whole the nd isotope record from foraminiferal authigenic coatings suggests large scale changes in the proportion of northern and southern deep water sources occurred throughout the atlantic ocean with substantial chemical gradients at times during the last deglaciation

shear zone development during magmatic arc construction the bench canyon shear zone central sierra nevada california the bench canyon shear zone is a 20 km long moderately to steeply dipping zone of ductile to brittle strain that cuts cretaceous plutonic and volcanogenic rocks located in the central sierra nevada california this zone is one of several cretaceous age ductile shear zones that record fluctuating strain fields associated with the tectonic magmatic evolution of the central sierra nevadan magmatic arc as determined by field relations fabric analysis and u pb and 40 ar 39 ar geochronology the bench canyon shear zone underwent an episodic deformation history involving both contractional and extensional strain over a period of 17 m y or longer deformation is divided into early ca 101 95 ma main ca 95 90 ma and late ca 90 78 ma phases temperature during main phase deformation followed a retrograde t t path of 600 300 c after emplacement of multiple plutons main phase deformation involved ductile thrust movement in both the 95 1 ma red devil lake pluton and lower to mid cretaceous volcanic rocks where thrusting was facilitated by heat and fluids associated with plutonism fingerlike sills associated with the red devil lake pluton were emplaced syntectonically with respect to main phase deformation but cooled rapidly undergoing heterogeneous strain more typical of pre tectonic emplacement the ca 90 ma mount givens pluton cuts the shear zone fabric in the wall rock limiting the bulk of deformation to have occurred prior to that time domainal and diachronous late phase reactivation s along the zone involved weak fluid enhanced ductile 500 t 300 c deformation and generation of abundant cataclasite and pseudotachylyte the long lived episodic and diachronous deformation history of the bench canyon shear zone illustrates the complexities to be expected within shear zones during continental magmatic arc evolution

variability of neodymium isotopes associated with planktonic foraminifera in the pacific ocean during the holocene and last glacial maximum abstract the deep pacific ocean holds the largest oceanic reservoir of carbon which may interchange with the atmosphere on climatologically important timescales the circulation of the deep pacific during the last glacial maximum lgm however is not well understood neodymium nd isotopes of ferromanganese oxide coatings precipitated on planktonic foraminifera are a valuable proxy for deep ocean water mass reconstruction in paleoceanography in this study we present nd isotope compositions e nd of planktonic foraminifera for the holocene and the lgm obtained from 55 new sites widely distributed in the pacific ocean the holocene planktonic foraminiferal e nd results agree with the proximal seawater data indicating that they provide a reliable record of modern bottom water nd isotopes in the deep pacific there is a good correlation between foraminiferal e nd and seawater phosphate concentrations r 2 0 80 but poorer correlation with silicate r 2 0 37 our interpretation is that the radiogenic nd isotope is added to the deep open pacific through particle release from the upper ocean during deep water mass advection and aging the data thus also imply the nd isotopes in the pacific are not likely to be controlled by silicate cycling in the north pacific the glacial nd isotopic compositions are similar to the holocene values indicating that the nd isotope composition of north pacific deep water npdw remained constant 3 5 to 4 during the lgm the southwest pacific cores throughout the water column show higher e nd corroborating previous studies which suggested a reduced inflow of north atlantic deep water to the pacific however the western equatorial pacific deep water does not record a corresponding radiogenic excursion implying reduced radiogenic boundary inputs during the lgm probably due to a shorter duration of seawater particle interaction in a stronger glacial deep boundary current a significant negative glacial e nd excursion is evident in mid depth 1 2 km cores of the eastern equatorial pacific eep which may suggest a stronger influence of npdw return flow to the core sites and decreased local input in the eep taken together our nd records do not support a dynamically slower glacial pacific overturning circulation and imply that the increased carbon inventory of pacific deep water might be due to poor high latitude air sea exchange and increased biological pump efficiency in glacial times

sequence of events during the last deglaciation in southern ocean sediments and antarctic ice cores variation depends on the age model as 14 c determinations cannot be obtained for the time interval of 29 5 14 5 ka assuming a constant sedimentation rate for this interval our data suggest that sea ice and nutrient changes at about 19 ka b p lead the increase in atmospheric pco2 by approximately 2000 years our diatom based sea ice record is in phase with the sodium record of the vostok ice core which is related to sea ice cover and similarly leads the increase in atmospheric co2 if gas exchange played a major role in determining glacial to interglacial co2 variations then a delay mechanism of a few thousand years is needed to explain the observed sequence of events otherwise the main cause of atmospheric pco2 change must be sought elsewhere rather than in the southern ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4267 oceanography general paleoceanography 4845 oceanography biological and chemical nutrients and nutrient cycling 4870 oceanography biological and chemical stable isotopes 9325 information related to geographic region atlantic ocean keywords biogenic opal stable isotopes diatoms ird last glacial southern ocean

antarctic density stratification and the strength of the circumpolar current during the last glacial maximum the interaction between ocean circulation and biological processes in the southern ocean is thought to be a major control on atmospheric carbon dioxide content over glacial cycles a better understanding of stratification and circulation in the southern ocean during the last glacial maximum lgm provides information that will help us to assess these scenarios first we evaluate the link between southern ocean stratification and circulation states in a suite of climate model simulations while simulated antarctic circumpolar current acc transport varies widely 80 350 sverdrup sv it co varies with horizontal and vertical stratification and the formation of the southern deep water we then test the lgm simulations against available data from paleoceanographic proxies which can be used to assess the density stratification and acc transport south of australia the paleoceanographic data suggest a moderate increase in the southern ocean stratification and the acc strength during the lgm even with the relatively large uncertainty in the proxy based estimates extreme scenarios exhibited by some climate models with acc transports of greater than 250 sv and highly saline antarctic bottom water are highly unlikely

strong and deep atlantic meridional overturning circulation during the last glacial cycle extreme abrupt northern hemisphere climate oscillations during the last glacial cycle 140 000 years ago to present were modulated by changes in ocean circulation and atmospheric forcing1 however the variability of the atlantic meridional overturning circulation amoc which has a role in controlling heat transport from low to high latitudes and in ocean co2 storage is still poorly constrained beyond the last glacial maximum2 3 4 here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle dominating ocean circulation in the atlantic whereas a shallower glacial mode with southern sourced waters filling the deep western north atlantic prevailed during glacial maxima3 5 our results are based on a reconstruction of both the strength and the direction of the amoc during the last glacial cycle from a highly resolved marine sedimentary record in the deep western north atlantic parallel measurements of two independent chemical water tracers the isotope ratios of 231pa 230th and 143nd 144nd 6 7 8 which are not directly affected by changes in the global cycle reveal consistent responses of the amoc during the last two glacial terminations any significant deviations from this configuration resulting in slowdowns of the amoc were restricted to centennial scale excursions during catastrophic iceberg discharges of the heinrich stadials severe and multicentennial weakening of north atlantic deep water formation occurred only during heinrich stadials close to glacial maxima with increased ice coverage probably as a result of increased fresh water input in contrast the amoc was relatively insensitive to submillennial meltwater pulses during warmer climate states and an active amoc prevailed during dansgaard oeschger interstadials greenland warm periods

neodymium isotope analyses after combined extraction of actinide and lanthanide elements from seawater and deep sea coral aragonite isotopes of the actinide elements protactinium pa thorium th and uranium u and the lanthanide element neodymium nd are often used as complementary tracers of modern and past oceanic processes the extraction of such elements from low abundance matrices such as seawater and carbonate is however labor intensive and requires significant amounts of sample material we here present a combined method for the extraction of pa th and nd from 5 to 10 l seawater samples and of u th and nd from 1 g carbonate samples neodymium is collected in the respective wash fractions of pa th and u th anion exchange chromatographies regardless of the original sample matrix nd is extracted during a two stage ion chromatography followed by thermal ionization mass spectrometry tims analysis as ndo using this combined procedure we obtained results for nd isotopic compositions on two geotraces consensus samples from bermuda atlantic time series bats which are within error identical to results for separately sampled and processed dedicated nd samples e nd 9 20 0 21 and 13 11 0 21 for 15 and 2000 m water depths respectively intercalibration results from 14 laboratories e nd 9 19 0 57 and 13 14 0 57 furthermore nd isotope results for an in house coral reference material are identical within analytical uncertainty for dedicated nd chemistry and after collection of nd from u th anion exchange chromatography our procedure does not require major adaptations to independently used ion exchange chromatographies for u pa th and nd and can hence be readily implemented for a wide range of applications

neodymium isotopes and concentrations in aragonitic scleractinian cold water coral skeletons modern calibration and evaluation of palaeo applications cold water corals cwcs are unique archives of mid depth ocean chemistry and have been used successfully to reconstruct the neodymium nd isotopic composition of seawater from a number of species high and variable nd concentrations in fossil corals however pose the question as to how nd is incorporated into their skeletons we here present new results on modern specimens of desmophyllum dianthus balanophyllia malouinensis and flabellum curvatum collected from the drake passage and madrepora oculata collected from the north atlantic all modern individuals were either collected alive or uranium series dated to be 500 years old for comparison with local surface sediments and seawater profiles modern coral nd isotopic compositions generally agree with ambient seawater values which in turn are consistent with previously published seawater analyses supporting small vertical and lateral nd isotope gradients in modern drake passage waters two balanophyllia malouinensis specimens collected live however deviate by up to 0 6 epsilon units from ambient seawater we therefore recommend that this species should be treated with caution for the reconstruction of past seawater nd isotopic compositions seventy fossil drake passage cwcs were furthermore analysed for their nd concentrations revealing a large range from 7 3 to 964 5 ng g samples of the species d dianthus and caryophyllia spp show minor covariation of nd with 232th content utilised to monitor contaminant phases in cleaned coral aragonite strong covariations between nd and th concentrations are however observed in the species b malouinensis and g antarctica in order to better constrain the source and nature of nd in the cleaned aragonitic skeletons a subset of sixteen corals was investigated for its rare earth element ree content as well as major and trace element geochemistry our new data provide supporting evidence that the applied cleaning protocol efficiently removes contaminant lithogenic and ferromanganese oxyhydroxide phases mass balance calculations and seawater like ree patterns rule out lithogenic and ferromanganese oxyhydroxide phases as a major contributor to elevated nd concentrations in coral aragonite based on mass balance considerations geochemical evidence and previously published independent work by solid state nuclear magnetic resonance nmr spectroscopy we suggest authigenic phosphate phases as a significant carrier of skeletal nd such a carrier phase could explain sporadic appearance of high nd concentrations in corals and would be coupled with seawater derived nd isotopic compositions lending further confidence to the application of nd isotopes as a water mass proxy in cwcs

the neodymium isotopic composition of waters masses in the eastern pacific sector of the southern ocean the antarctic circumpolar current is one of the key components of ocean circulation and a knowledge of its isotopic composition is essential to the use of neodymium nd isotopes to trace circulation now and in the past here we present 57 new analyses of the nd isotopic composition of the water column in the eastern pacific sector of the southern ocean documenting both the variation in three dimensions as well as the controls on that variability nd isotopic data for the middle of the water column demonstrate the homogeneity of circumpolar deep water cdw at an end value of 8 7 0 1 this homogeneity reflects the large nd inventory in the acc flow and the degree to which this large inventory buffers the nd characteristics of the acc against extra oceanic inputs from the continents either via dust from the atmosphere or through dissolved and particulate material from the adjacent continents cdw upwells onto the amundsen sea shelf and even here its nd isotopic properties are close to conserved in the middle of the water column end 8 0 0 2 at 600 m at the top and bottom of the shelf water column however the nd isotopic and concentration characteristics are strongly modified to end as high as 4 5 all the shelf water column data obtained here are consistent with net addition of nd to bottom and surface waters with a contrasting isotopic composition that is matched by local sediment end 1 to 2 followed by conservative mixing of that water into intermediate levels mixing with this shelf composition also leads to significant modification of open ocean surface water nd isotopic shift around 1 epsilon unit in the acc as it flows eastwards modification of open ocean bottom waters by interaction with sediment is more subtle but there is marked non conservative removal of nd accompanied by significant changes in isotopic composition in waters within 10 m of the seabed the new data demonstrate the conservativity of nd in the middle water column especially for such large volume flows as the acc though boundary exchange type processes are clearly important in this region and their imprint on both the shelf and open ocean surface water is significant there is no observable impact on the main core of cdw this finding augurs well for the use of nd isotopes as a conservative water mass tracer now and in the past for example these data suggest that the only likely control on the temporal variability of southern component deep water exported northwards into the atlantic through the last glacial cycle is variations in the input of north atlantic deep water in the atlantic sector on the other hand the data also suggest caution in the use of sedimentary archives of bottom water nd as records of deep water nd isotopic characteristics given the dramatic modification of bottom waters on the shelf and the more subtly non conservative behaviour in the open ocean bottom water that are both suggestive of modification of the nd characteristics of bottom waters by sediment

current deformation and the width of the seismogenic zone of the northern cascadia subduction thrust evidence has been obtained for the accumulation of elastic strain across the northern cascadia subduction zone that may be released in a future very large subduction thrust earthquake vertical and horizontal strain rates across the southern vancouver island region have been determined through 1 long term trends in tide gauge data 2 changes in repeated accurate leveling surveys 3 changes in repeated high accuracy gravity profiles and 4 horizontal shortening observed in repeated precise positioning surveys the outer coast is uplifting at a rate of a few millimeters per year decreasing landward and shortening is occurring across the 100 km wide coastal region at a rate of about 0 1 microstrain per year mm km 1yr 1 the results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault the location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern vancouver island and northern washington provided the locked portion is restricted to a 60 km wide band offshore beneath the continental shelf and slope this conclusion also results from modeling the coseismic subsidence on the outer coast of vancouver island about 300 years ago deduced from paleoseismicity data the unusually narrow downdip extent of the subduction thrust seismogenic zone that extends little if at all beneath the coast is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments the high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km the distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of vancouver and victoria from this source the narrow width may also limit the earthquake size however events of magnitude well over 8 are possible

interhemispheric ice sheet synchronicity during the last glacial maximum the timing of the last maximum extent of the antarctic ice sheets relative to those in the northern hemisphere remains poorly understood we develop a chronology for the weddell sea sector of the east antarctic ice sheet that combined with ages from other antarctic ice sheet sectors indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of northern hemisphere ice sheets surface climate forcing of antarctic mass balance would probably cause an opposite response whereby a warming climate would increase accumulation but not surface melting our new data support teleconnections involving sea level forcing from northern hemisphere ice sheets and changes in north atlantic deep water formation and attendant heat flux to antarctic grounding lines to synchronize the hemispheric ice sheets

southern ocean evidence for reduced export of north atlantic deep water during heinrich event 1 deep sea corals form unique high resolution archives of ocean circulation that can be dated using the decay of uranium to thorium they are abundant in the southern ocean and can provide unprecedented insights into ocean circulation and ocean chemistry on sub millennial time scales in areas where application of traditional paleoceanographic proxies is difficult here we present the first coupled neodymium nd isotope and radiocarbon data from deep sea corals in the drake passage southern ocean adding new constraints on ocean circulation during the last heinrich event h1 16 7 ka the modern day drake passage water column is homogeneous with respect to nd isotopes expressed in epsilon units nd its isotopic value of close to 9 is largely controlled by the mixture of north atlantic deep water and pacific deep water the aragonite of modern drake passage corals reflects this water column value whereas a fossil coral from h1 is significantly higher at 6 4 0 4 we interpret this 2 5 unit shift as a reduction in the influence of north atlantic sourced nd in the southern ocean during h1 this interpretation is supported by a series of radiocarbon analyses on the same sample and is consistent with a twofold or greater reduction in export of north atlantic waters from the atlantic basin combining analyses of radiocarbon and nd isotopes on u series dated deep sea coral skeletons holds great potential for quantification of past ocean ventilation rates

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

abyssal atlantic circulation during the last glacial maximum constraining the ratio between transport and vertical mixing transport to vertical diffusivity in the deep atlantic our calculations suggest that the ratio was at least twice as large at the lgm we speculate that the primary cause was reduced mixing between northern and southern component waters associated with movement of this water mass boundary away from the zone of intense mixing near the seafloor the shallower water mass boundary yields an order of magnitude increase in the volume of southern component water suggesting its residence time may have increased substantially our analysis supports the idea that an expanded volume of antarctic bottom water and limited vertical mixing enhanced the abyssal ocean s ability to trap carbon during glacial times

sea ice control on deglacial lower cell circulation changes recorded by drake passage deep sea corals abstract the sequence of deep ocean circulation changes between the last glacial maximum and the holocene provides important insights for understanding deglacial climate change and the role of the deep ocean in the global carbon cycle although it is known that significant amounts of carbon were sequestered in a deep overturning cell during glacial periods and released during deglaciation the driving mechanisms for these changes remain unresolved southern ocean sea ice has recently been proposed to play a critical role in setting the global deep ocean stratification and circulation and hence carbon storage but testing such conceptual and modelling studies requires data constraining past circulation changes to this end we present the first deglacial dataset of neodymium nd isotopes measured on absolute dated deep sea corals from modern lower circumpolar deep water depths in the drake passage our record demonstrates deglacial variability of 2 5 end units with radiogenic values of up to e nd 5 9 during the last glacial maximum providing evidence for a stratified glacial circulation mode with restricted incorporation of nd from north atlantic deep water in the lower cell during the deglaciation a renewed atlantic influence in the deep southern ocean is recorded early in heinrich stadial 1 coincident with antarctic sea ice retreat and is followed by a brief return to more pacific like values during the antarctic cold reversal these changes demonstrate a strong influence of southern ocean processes in setting deep ocean circulation and support the proposed sea ice control on deep ocean structure furthermore by constraining the nd isotopic composition of lower circumpolar deep water in the southern ocean our new data are important for interpreting deglacial circulation changes in other ocean basins and support a spatially asynchronous return of north atlantic deep water to the deep southeast and southwest atlantic ocean

crustal structure of the cascadia subduction zone southwestern british columbia from potential field and seismic studies the northern cascadia subduction zone is a region of convergence between the oceanic explorer and northern juan de fuca plates and the continental north american plate potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2 5 and 3 dimensional interpretations the lateral extent and thickness of the accreted wedge vary significantly along the zone the narrow metasedimentary pacific rim terrane lies immediately west of wrangellia and extends the length of vancouver island consistent with its emplacement by strike slip faulting following the accretion of wrangellia the ophiolitic crescent terrane is a narrow slice lying seaward of the pacific rim terrane but not extending northward beyond the juan de fuca plate in this region the crescent terrane was emplaced in a strike slip or obliquely convergent style

breakup of last glacial deep stratification in the south pacific stratification of the deep southern ocean during the last glacial maximum is thought to have facilitated carbon storage and subsequent release during the deglaciation as stratification broke down contributing to atmospheric co2 rise here we present neodymium isotope evidence from deep to abyssal waters in the south pacific that confirms stratification of the deepwater column during the last glacial maximum the results indicate a glacial northward expansion of ross sea bottom water and a southern hemisphere climate trigger for the deglacial breakup of deep stratification it highlights the important role of abyssal waters in sustaining a deep glacial carbon reservoir and southern hemisphere climate change as a prerequisite for the destabilization of the water column and hence the deglacial release of sequestered co2 through upwelling

revision of the epica dome c co2 record from 800 to 600 kyr before present the european project for ice coring in antarctica dome ice core from dome c edc has allowed for the reconstruction of atmospheric co2 concentrations for the last 800 000 years here we revisit the oldest part of the edc co2 record using different air extraction methods and sections of the core for our established cracker system we found an analytical artifact which increases over the deepest 200 m and reaches 10 1 2 4 ppm in the oldest deepest part the governing mechanism is not yet fully understood but it is related to insufficient gas extraction in combination with ice relaxation during storage and ice structure the corrected record presented here resolves partly but not completely the issue with a different correlation between co2 and antarctic temperatures found in this oldest part of the records in addition we provide here an update of 800 000 years atmospheric co2 history including recent studies covering the last glacial cycle

seismic structure across the active subduction zone of western canada the vancouver island seismic project was conducted in 1980 to study the structure of the subducting oceanic juan de fuca plate and the overriding continental america plate the principal seismic refraction line line i was a 350 km onshore offshore profile perpendicular to the continental margin an array of 32 receivers was located on the america plate on the mainland and across vancouver island and extended offshore with three ocean bottom seismometers obs s two shots were fired at the eastern end of the line and 17 shots were located along the westernmost 100 km of the profile control for the interpretation of the onshore offshore profile was provided by a reversed refraction profile along the length of vancouver island and by a marine refraction profile recorded on the obs s modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two dimensional media the major features of the refraction structural model are that 1 the oceanic lithosphere dips at 3 or less beneath the continental slope so the bend in the subducting slab occurs landward of the foot of the slope 2 the oceanic lithosphere dips at 14 16 beneath the continental shelf until it passes beneath the continental moho at 37 km depth below western vancouver island 3 an upper mantle reflector may correspond to the base of the subducting lithosphere and 4 a segment of high velocity material above the downgoing crust with velocity 7 7 km s and depth range 20 25 km may represent a remnant of subducted lithosphere perhaps detached when the subduction zone jumped westward to its present position

synchronous centennial abrupt events in the ocean and atmosphere during the last deglaciation antarctic ice core data reveal that the atmosphere experienced abrupt centennial increases in co2 concentration during the last deglaciation 18 thousand to 11 thousand years ago establishing the role of ocean circulation in these changes requires high resolution accurately dated marine records here we report radiocarbon data from uranium thorium dated deep sea corals in the equatorial atlantic and drake passage over the past 25 000 years two major deglacial radiocarbon shifts occurred in phase with centennial atmospheric co2 rises at 14 8 thousand and 11 7 thousand years ago we interpret these radiocarbon enriched signals to represent two short lived less than 500 years overshoot events with atlantic meridional overturning stronger than that of the modern era these results provide compelling evidence for a close coupling of ocean circulation and centennial climate events during the last deglaciation