reconstruction of surface water conditions in the central region of the okhotsk sea during the last 180 kyrs abstract core gc9a a 6 7 m long gravity core collected from the central region of the okhotsk sea during cruise yk0712 on r v yokosuka jamstec was used to reconstruct the changes in surface water conditions by measuring biogenic components biogenic opal caco3 total organic carbon and 15n of sediment organic matter of sediment samples the age of core gc9a was determined indirectly by graphic correlation comparing the b psychometric yellow blue chromaticness values with those of well dated core md01 2415 with complement to the tephra layer k3 50 ka the bottom age of core gc9a was estimated to be about 180 kyr therefore it provides the history of surface water conditions from mis 1 to mis 6 the biogenic opal caco3 and toc contents were high during the interglacial periods as expected indicating enhanced surface water production under warm climatic conditions this condition resulted from sufficient nutrient supply to the surface waters by active vertical mixing which was validated by low 15n values of the sediment organic matter in contrast surface water productivity was depressed during the colder glacial periods probably due to the expanded sea ice distribution and limited nutrient supply however the glacial sediments had moderately high 15n values indicating enhanced nitrate utilization resulting from the limited nutrient supply caused by strong stratification of the surface water high 15n values were also observed during the deglaciation which was attributed to the increased nitrate utilization during enhanced surface water productivity however the low 15n values during the glacial and deglacial periods may be attributed to the increased supply of terrestrial organic matter diatom production was primarily responsible for surface water paleoproductivity during the interglacial periods rather than coccolithophores however the succession of glacial to early deglacial coccolithophore production and late deglacial to interglacial diatom production was remarkable corresponding to the present day seasonal phytoplankton succession such an advanced coccolithophore production relative to diatom production might be attributed to the degree of nutrient availability associated with surface water conditions on the basis of variations in the 15n value finally the opal and toc contents decreased abruptly in conjunction with a gradual decrease in caco3 content from about 2 ka which seems to implicate a late holocene sudden decrease in paleoproductivity in the central region of the okhotsk sea according to the increase in 15n values during this interval nutrient availability appears to be poor which is likely attributed to the resumed strong stratification that occurred due to the southward shift of the aleutian low atmospheric pressure system

evolution and thickness of the lithosphere beneath coastal california many of the tectonic features of the california coast ranges are directly related to the northward migration of the mendocino triple junction along the western edge of north america during the past 20 to 30 m y at the triple junction the western edge of the north american plate slides off the relative northward moving and subducting gorda plate leaving the thin western edge in direct contact with asthenosphere upwelling to fill the space vacated by the underlying gorda plate two dimensional time dependent thermal modeling of this process constrained by teleseismic delay studies is used to construct a map of lithospheric thickness of coastal california among the implications of this map are that 1 the high heat flow in the coast ranges can be almost entirely accounted for by the asthenospheric upwelling associated with the migrating mendocino triple junction 2 the general elevation and the late cenozoic volcanism in the california coast ranges are responses to a zone of unusually thin 20 to 45 km lithosphere that extends southward behind the mendocino triple junction and 3 the course of some segments of the san andreas fault in both central and southern california appear to deviate from a deeper more fundamental transform boundary separating the pacific and north american plates

fission track analysis reveals character of collisional tectonics in new zealand during the late cenozoic the pacific plate has been obliquely converging with the australia plate in south island new zealand this has resulted in westward obduction of continental crust of the pacific plate and consequently a major mountain range the southern alps has formed east of the alpine fault and within the leading edge of the pacific plate we have applied the apatite and zircon fission track dating technique to a suite of outcrop samples from three transects across the southern alps starting at the alpine fault to better understand the uplift age and structure of the continental collision the results show the following 1 uplift was underway at 7 ma in the south at haast pass and by 5 ma in the north at arthur s pass 2 profiles of the total amount of late cenozoic uplift show an exponential increase toward the alpine fault adjacent to the alpine fault there has been 14 5 to 21 5 km of uplift in the vicinity of haast pass 14 5 to 21 km near mt cook and 12 5 to 15 5 km in the arthur s pass transect adjacent to the alpine fault in the mount cook region the average uplift rate from 5 5 ma to 1 ma was 2 3 to 2 5 mm yr between 1 ma and the present it has averaged 6 2 mm yr the strongly asymmetric uplift pattern is responsible for the exposure of a 13 25 km wide belt of greenschist to midamphibolite facies rocks immediately east of the alpine fault known as the alpine schists 3 late cenozoic uplift of torlesse greywackes and alpine schists was preceeded during the jurassic early cretaceous by 5 8 km of uplift the otago schist belt to the south was uplifted much more during this interval and in addition western parts of the otago schists were uplifted 5 6 km during the late cretaceous 4 the fission track data do not support the notion of significant late cenozoic shear heating about the alpine fault as previously suggested from spatial variations in k ar ages of the otago and alpine schists

evidence from diatom bound nitrogen isotopes for subarctic pacific stratification during the last ice age and a link to north pacific denitrification changes higher diatom bound d 15 n 70 wt lower opal content and 1200 ppm lower biogenic barium taken together and with constraints on sediment accumulation rate these results suggest a reduced supply of nitrate to the surface due to stronger stratification of the upper water column of the bering sea during glacial times with more complete nitrate consumption resulting from continued iron supply through atmospheric deposition this finding extends the body of evidence for a pervasive link between cold climates and polar ocean stratification in addition we hypothesize that more complete nutrient consumption in the glacial age subarctic pacific contributed to the previously observed ice age reduction in suboxia and denitrification in the eastern tropical north pacific by lowering the nutrient content of the intermediate depth water formed in the subpolar north pacific in the deglacial interval of the bering sea record two apparent peaks in export productivity are associated with maxima in diatom bound and bulk sediment d 15 n the high d 15 n in these intervals may have resulted from greater surface nutrient consumption during this period however the synchroneity of the deglacial peaks in the bering sea with similar bulk sediment d 15 n changes in the eastern pacific margin and the presence of sediment lamination within the bering sea during the deposition of the productivity peaks raise the possibility that both regional and local denitrification worked to raise the d 15 n of the nitrate feeding bering sea surface waters at these times

chronostratigraphy of sediment cores from the bering sea and the subarctic pacific based on paleomagnetic and oxygen isotopic analyses the chronostratigraphy of six piston cores collected from the bering sea and one piston core from the subarctic pacific is reported age models are primarily based on oxygen isotope data from cores bow 8a and es in conjuction with relative paleointensity proxy records biogenic opal data and a radiolarian datum from other cores whose sediments mainly consist of diatom microfossils and poorly preserved calcium carbonate the records the first paleointensity proxy reported from the bering sea show that characteristic paleointensity lows at 40 60 and 100 ka which have been well documented in other paleointensity records those features can be correlated to standard paleointensity curves with the aid of biogenic opal data and a radiolarian datum the mean sedimentation rates of the diatom rich cores collected in the eastern bering sea range from 10 to 20 cm kyr 1 in contrast the mean sedimentation rate of core bow 8a from the western edge of the bowers ridge is only 3 cm kyr 1 which is comparable to core es from the pelagic realm in the subarctic pacific these results suggest that biological productivity has been quite high during the late pleistocene in the eastern part of the bering sea but low over the western edge of the bowers ridge r 2005 elsevier ltd all rights reserved

productivity changes in the bering sea during the late quaternary changes in biological productivity of the bering sea have been evaluated for the late quaternary based on biogenic opal calcium carbonate and microfossils in two piston cores biological productivity increased during marine isotope stage mis 1 after the last glaciation during the last deglaciation two pronounced peaks of calcium carbonate content were observed at ca 2400 m water depth which can be explained by the caco3 compensation hypothesis and coccolithophore blooms simultaneous with the caco3 peaks oxygen poor deep water was apparently distributed in the aleutian basin based on benthic foraminiferal assemblages the low o2 events seem to be related to deep water circulation and or elevated productivity in the northwest pacific after the caco3 peak events biogenic opal contents and diatoms increased gradually associated with enhanced vertical mixing with an inflow of the alaskan stream through the eastern aleutian arc passes r 2005 elsevier ltd all rights reserved

diatoms silicoflagellates and ebridians at site u1341 on the western slope of bowers ridge iodp expedition 323 abstract the biostratigraphy and datum levels of diatoms silicoflagellates and ebridians are presented at site u1341 drilled by the iodp expedition 323 on the western slope of bowers ridge in the bering sea identified biostratigraphic diatom zones were from the neodenticula seminae zone to the thalassiosira oestrupii subzone silicoflagellate and ebridian zones were from the distephanus octangulatus zone to the distephanus jimlingii zone the bottom age in the deepest hole u1341b is estimated between 4 1 ma and 5 0 ma based on the succession of microfossil assemblages throughout the studied holes the changes in paleoceanographic conditions are outlined

orbital scale benthic foraminiferal oxygen isotope stratigraphy at the northern bering sea slope site u1343 iodp expedition 323 and its pleistocene paleoceanographic significance abstract a continuous composite oxygen isotope 18o stratigraphy from benthic foraminifera in the bering sea was reconstructed in order to provide insight into understanding sea ice evolution in response to northern hemisphere glaciation oxygen isotope records from multiple species of benthic foraminifera at integrated ocean drilling program iodp expedition 323 site u1343 54 33 4 n 176 49 0 e water depth 1950 m yield a highly refined orbital scale age model spanning the last 1 2 ma and a refined age model between 1 2 and 2 4 ma an inter species calibration was used to define species offsets and to successfully obtain a continuous composite benthic 18o record correlated with the global composite benthic 18o stack curve lr04 to construct an orbital scale age model the consistency of the benthic 18o stratigraphy with biostratigraphy and magnetostratigraphy confirms the reliability of both methods for constraining age the time difference between cyclic changes in sedimentary physical properties and glacial interglacial cycles since 0 8 ma is notable and suggests that physical properties alone cannot be used to construct an orbital scale age model amplitude changes in physical properties and a significant drop in the linear sedimentation rate during glacials after 0 9 ma indicate that the glacial sea ice edge extended beyond the bering sea slope site u1343 at this time

evidence for limited zonation in silicic magma systems taupo volcanic zone new zealand smith 1979 suggested that all pyroclastic eruptions that exceed 1 km 3 will show pronounced systematic compositional zonation since then chemical and thermal zonation in large silicic magma bodies has become the expected condition in rhyolitic magma bodies however trace element chemistry and fe ti oxide geothermometry of quaternary rhyolitic plinian tephras from two volcanic centers in the taupo volcanic zone new zealand suggest that no strong systematic chemical or thermal zonation was present in either magma system of the plinian tephra units erupted over the past 50 ka from the taupo and okataina volcanoes 28 were sampled and most were analyzed for th pb rb sr y zr and nb as well as fe ti oxide composition taupo volcano tephras erupted over the past 10 ka have uniform compositions for all elements except sr and all these samples fall on the same t f o 2 buffer trend however pre 22 ka taupo tephras are mineralogically and chemically distinct from the younger group and fall on a different t f o 2 buffer trend this may be a result of the large eruption 155 km 3 of magma from the taupo volcano at 22 5 ka and subsequent reequilibration of the magmatic system the okataina volcano tephras erupted over the past 31 ka show slight systematic and considerable nonsystematic trace element variation but all fall on a single t f o 2 buffer trend this may indicate slight preeruptive alteration of a single okataina magma body

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

orbital and millennium scale environmental changes in the southern bering sea during the last glacial holocene geochemical and paleontological evidence the d 18 o benthic foraminiferal curve and ams 14 c radiocarbon data provide a representative age model for a southern bering sea sediment core gc 11 downcore profiles of d 18 oo fn pachyderma s and u auberiana carbonate and organic carbon contents species changes in the planktonic foraminifera assemblages and some diatom species show orbital mis 1 2 and 3 and millennium scale variability influenced by the alaska current flowing into the studied region three warmer episodes in the southern bering sea environment were assumed to be synchronous with do interstadials 8 12 and 14 in gisp 2 chronology during mis 3 more severe climate and environmental conditions in the southern bering sea during mis 2 occurred synchronously with the lgm within a 17 19 ka time span two well pronounced environmental warming events productivity spikes and changes in the pore water geochemistry separated by younger dryas cooling were observed at the base of mis 1 coeval with mwp 1a and 1b regional glacial primary productivity based on the calculation of the organic carbon mar similar to that found in the far northwestern pacific exceeded the late holocene values under conditions close to the present day r 2005 elsevier ltd all rights reserved

the role of the alaskan stream in modulating the bering sea climate 1 a numerical ocean circulation model with realistic topography but with an idealized forcing that includes only lateral transports is used to study the role of the alaskan stream as in modulating the bering sea bs variability sensitivity experiments each one with a different strength of the as transport reveal a nonlinear bs response an increase of as transport from 10 to 25 sv causes warming 0 25 c mean 0 5 c maximum and sea level rise in the bs shelf due to increased transports of warmer pacific waters through the eastern passages of the aleutian islands but an increase of as transport from 25 to 40 sv had an opposite impact on the bs shelf with a slight cooling 0 1 c mean 0 5 c maximum as the as transport increases flows through passages farther downstream in the western aleutian islands are affected and the variability in the entire bs is reduced transport variations of 0 1sv in the bering strait are found to be correlated with mesoscale variations of the as and associated transport variations in the aleutian islands passages these results have important implications for understanding the observed variations in the bering strait and potential future climate variations in the arctic ocean

granulite formation during continental extension in fiordland new zealand the formation of the high pressure high temperature rocks known as granulites has traditionally been attributed to metamorphism in the deeper parts of continental collision zones and magmatic arcs1 3 more recently greater consideration has been given to the possibility of granulite formation during continental rifting4 6 and in particular4 to whether granulites are forming today beneath the north american basin and range province here we report thermobarometric data and single zircon u pb ages for an analogous but much more deeply exhumed extensional terrain in fiord land new zealand7 8 where early cretaceous granulites underwent renewed granulite facies metamorphism at significantly shallower crustal levels following the extensional collapse of magmatically thickened continental crust our data show that granulite facies metamorphism can take place during continental extension and demonstrate that in western new zealand it was accompanied at higher structural levels by the emplacement of one or more metamorphic core complexes7 9 in keeping with the model postulated4 for the basin and range province

the lithospheric geodynamics of plate boundary transpression in new zealand initiating and emplacing subduction along the hikurangi margin and the tectonic evolution of the alpine fault system abstract in contrast to the normal wilson cycle sequence of subduction leading to continental collision and associated mountain building the evolution of the new zealand plate boundary in the neogene reflects the converse initially a period of continental convergence that is followed by the emplacement of subduction plate reconstructions allow us to place limits on the location and timing of the continental convergence and subduction zones and the migration of the transition between the two plate boundary regimes relative plate motions and reconstructions since the early to mid miocene require significant continental convergence in advance of the emplacement of the southward migrating hikurangi subduction a sequence of tectonism seen in the present plate boundary geography of hikurangi subduction beneath north island and convergence in the southern alps along the alpine fault in contrast to a transition from subduction to continental convergence where the leading edge of the upper plate is relatively thin and deformable the transition from a continental convergent regime with its associated crustal and lithospheric thickening to subduction of oceanic lithosphere requires substantial thinning removal of upper plate continental lithosphere to make room for the slab the simple structure of the wadati benioff zone seen in the present day geometry of the subducting pacific plate beneath north island indicates that this lithospheric adjustment occurs quickly associated with this rapid lithospheric thinning is the development of a series of ephemeral basins younging to the south that straddle the migrating slab edge based on this association between localized vertical tectonics and slab emplacement the tectonic history of these basins records the effects of lithospheric delamination driven by the southward migrating leading edge of the subducting pacific slab although the new zealand plate boundary is often described as simply two subduction zones linked by the transpressive alpine fault in actuality the present is merely a snapshot view of an ongoing and complex evolution from convergence to subduction

biogenic opal production changes during the mid pleistocene transition in the bering sea iodp expedition 323 site u1343 abstract biogenic opal content and mass accumulation rate mar at iodp expedition 323 site u1343 were found to fluctuate consistently generally being high under warm conditions and low under cold conditions during the last 2 4 ma continuous wavelet transform analysis of the normalized biogenic opal content indicates that export production in the bering sea varied predominantly at 41 ka periodicity before 1 25 ma and shifted to 100 ka periodicity at the onset of the mid pleistocene transition mpt 1 25 0 7 ma the 100 ka cycles dominated until the holocene export production in the bering sea decreased markedly in the bering sea two times during the mpt the first occurred at the beginning of the mpt 1 25 ma and the second in the middle of the mpt 0 9 ma these decreases coincided with both increases in the relative abundance of sea ice diatoms and decreases in the warm water diatom species neodenticula seminae indicating that reductions in export production in the bering sea during the mpt were associated with climate cooling decreases in export production in the bering sea during the mpt were most likely associated with the increased influence of polar arctic domains on the high latitude north pacific

teleseismic p wave delays and modes of shortening the mantle lithosphere beneath south island new zealand a high speed zone in the mantle directly beneath the southern alps of new zealand is required by the recorded pattern of teleseismic p waves two parallel lines of 80 seismographs spaced at 2 km intervals recorded three earthquakes from the western pacific with epicentral distances of 52 53 and 78 azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines differences between measured delays and those predicted from the crustal structure reach 0 8 s along one line and 1 0 s along the other with the rays for the earliest arriving signals passing the depth of 120 km beneath the center of the island assuming these early arrivals are due to structure within the mantle shallower than 200 km they imply that the core of the high speed zone lies beneath the thickest crust which has been shortened by 100 km of convergence during the past 6 7 myr although the shape and position of the high speed body cannot be fixed uniquely a roughly symmetric body centered about a depth of 120 km 80 100 km wide with a depth extent of 100 km and with a maximum speed advance of 7 satisfies the observations the pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other rather the residuals favor thickening of mantle lithosphere by a more homogenous straining of it as if mantle lithosphere beneath continental crust behaved as a continuum an excess mass in the mantle is also required by the observed gravity anomalies once allowance is made for the seismically determined crustal thickness this high density mantle anomaly provides sufficient force per unit length to maintain the crustal root which is approximately twice as thick as that necessary to support the topography

strain partitioning between structural domains in the forearc of the hikurangi subduction zone new zealand the pacific plate obliquely converges with the australian plate at latitude 39 50 s along the hikurangi margin off the east coast of the north island of new zealand an extensive and youthful subaerially exposed forearc on the east coast of the north island in the hawke s bay area provides the opportunity to document contemporaneous forearc deformation in this obliquely convergent margin setting geologic mapping and analysis of strain at both mesoscale and megascale indicates that strain is partitioning into domains of extension contraction and strike slip the domains are elongate and trend parallel to the margin measurements of net shortening and transcurrent slip in the forearc show that the obliquely convergent motion is transferred across the plate interface deformation rates calculated for the past 1 2 m y for structures in all six forearc domains account for 50 70 of the margin parallel motion required by pacific australian plate convergence and about 6 of the plate motion perpendicular to the plate boundary at the surface in the forearc this obliquely convergent motion is manifest not by transpressional faults but rather by paired structural domains that consist of a strike slip fault zone and an accompanying contractional fault and fold zone on the trenchward side there are two such transcurrent faulting and contraction couplets one where the backstop daylights at the arcward edge of the forearc and another couplet trenchward of a relatively undisturbed forearc basin the small amount of shortening relative to strike slip in the onshore part of the forearc suggests that shortening perpendicular to the plate boundary may be concentrated offshore and that most of the component of plate motion perpendicular to the plate boundary may be accommodated by slip along the subduction zone megathrust

the mid cenozoic challenger rift system of western new zealand and its implications for the age of alpine fault inception analysis of the structure and sedimentary geology of western new zealand has identified a middle eocene to early miocene continental rift system 1 200 km long and 100 200 km wide named here the challenger rift system four phases of rift development occurred 1 infra rift subsidence 2 active axial trough subsidence 3 expanded rift subsidence involving collapse of the rift shoulders and 4 incipient sea floor spreading the spatial and temporal distribution of these phases identifies a north island and a south island rift segment and shows that rifting propagated toward the center of the rift from both ends the northern segment shows a simple pattern of rifting that is comparable with vink9s model of rift propagation the southern segment with locked zones and rift nucleation segments is comparable to courtillot9s model of rift propagation the sea floor spreading history of the southwest pacific shows that the northern rift segment probably linked with a sea floor spreading center in the norfolk basin and the southern segment linked with the southeast indian ridge this is corroborated by the good correlation between the ages of sea floor magnetic anomaly lineations that are aligned with the rift and the biostratigraphic ages of rifting the probable continuity of the rift system in its early development precludes pre miocene transcurrent displacement on the alpine fault an early miocene 23 m y b p age of alpine fault inception is indicated by the age and pattern of rift disruption attributed to compression that originated at the australia pacific plate boundary the modern australia and pacific plates were not discrete entities therefore until the early miocene

intermediate depth earthquakes beneath nelson new zealand and the southwestern termination of the subducted pacific plate 1 there has been debate on how convergence of mantle lithosphere is accommodated during continental collision in the central south island of new zealand whether by intracontinental subduction or continuous deformation of mantle lithosphere intermediate depth earthquakes are sparse beneath the central south island and abundant seismicity 130 230 km deep within the hikurangi subduction zone to the northeast ends abruptly beneath the nelson region of the northern south island this termination of seismicity has previously been interpreted as marking the southwestern edge of the subducted slab here we investigate the stress and strain regime at this termination of activity and find that it is very similar to that within the subducted plate both updip and along strike towards the northeast our results are consistent with subduction continuing further southwest albeit largely aseismically as suggested by shallow earthquakes in the northeastern south island

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

biogenic opal indicating less productive northwestern north pacific during the glacial ages 1 biogenic opal and ice rafted detritus ird data from sediments in the okhotsk sea and the neighboring north pacific revealed the remarkable reduction in opal production and southward advancement of sea ice covered area during the last glacial maximum resulting also southward shift of high biological productive area in the northwestern north pacific it implies that the substantial reduction in outflux of co2 to the atmosphere in northwestern north pacific and the pronounced increase in co2 sequestering in temperate north pacific this could be an additional co2 reduction mechanism of atmospheric co2 in the last glacial period

influence of the mendocino triple junction on the tectonics of coastal california abstract the migration of the mendocino triple junction through central and northern california over the past 25 30 million years has led to a profound change in plate interactions along coastal california the tectonic consequences of the abrupt change from subduction plate interactions north of the triple junction to the development of the san andreas transform system south of the triple junction can be seen in the geologic record and geophysical observations the primary driver of this tectonism is a coupling among the subducting juan de fuca gorda north american and pacific plates that migrates with the triple junction this coupling leads to ephemeral thickening of the overlying north american crust associated uplift and subsequent subsidence and a distinctive sequence of fault development and volcanism

late neogene exhumation patterns in taranaki basin new zealand evidence from offset porosity depth trends taranaki basin new zealand is located adjacent to the australian pacific plate boundary where the tectonic regime changes from dominantly subduction related to the north to transpression related along the alpine fault to the south during the neogene burial and exhumation varied extensively in both time and space in response to subsidence and uplift along this evolving plate boundary zone the basin can be divided into two regions 1 the western platform outside the plate boundary deformation zone where no uplift nor exhumation has occurred and 2 the eastern mobile belt which lies inside the deformation zone and has been variably uplifted and exhumed exhumation magnitudes for sedimentary deposits of the basin are estimated from offset porosity depth trends the analysis is based on correlating sonic log travel times with compensated density logs permitting sonic travel time to be used as a proxy for porosity twelve western platform wells are used to define a standard exponential porosity depth trend with an extrapolated surface porosity of 50 and an exponential decay factor of 2265 m valid for mudstone shale sections spanning a depth range of 300 3000 m this curve is a calibration curve against which offset porosity depth trends from wells in exhumed regions can be compared to determine apparent exhumation amounts porosity depth trends for 43 eastern mobile belt wells are offset 0 2800 m shallower relative to the western platform trend but generally are parallel to it in the southern region of the eastern mobile belt net exhumation amounts which are the sum of the porosity depth trend offset and depth to unconformity range from 850 to 3000 m most of this exhumation occurred on discrete contractional structures in late miocene to early pliocene time probably associated with an increased rate of convergence across the alpine fault system to the south in the eastern region of the eastern mobile belt net exhumation progressively increases from 0 m in the south to 1800 m in the northeast part of the basin consistent with the 2 4 sw structural tilt of onshore strata the regional and consistent pattern of exhumation in the eastern region contrasts with the discrete structure related exhumation in the southern region and is consistent with models of thermal and flexural uplift

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

changes in biogenic opal productivity with milankovitch cycles during the last 1 3 ma at iodp expedition 323 sites u1341 u1343 and u1345 in the bering sea abstract sediments from three sites u1341 u1343 and u1345 of iodp expedition 323 located in the bering sea were analyzed for variation of biogenic opal representing surface water paleoproductivity initial age models of three sites bottom age u1341 4 3 ma u1343 2 4 ma u1345 0 5 ma were established onboard by biostratigraphic and paleomagnetic investigations the initial age models were tuned by 18 o stratigraphy which was established on shore biogenic opal contents of sediments were analyzed for the interval of 1 3 ma to the present generally the biogenic opal values at site u1341 bowers ridge were significantly higher than those at the beringian slope sites u1343 and u1345 higher natural gamma radiation ngr counts and magnetic susceptibility ms at the beringian slope sites suggested a greater contribution of lithogenic matter than at site u1341 on bowers ridge the percentages of sea ice diatoms at the beringian slope sites are also higher than those at bowers ridge sites this is primarily attributed to the fact that the sediments from the beringian slope area were significantly diluted by lithogenic matter derived from the alaskan continent and were largely affected by sea ice cover the cyclicity of biogenic opal accumulation rates at site u1341 is determined to be about 160 40 and 30 ky whereas that at site u1343 is determined to be about 330 100 40 and 20 ky if these had been reflecting a global climate change the cyclicity of biogenic opal contents at all sites would be consistently the same however the results do not show such a consistency obliquity is observed at both sites of the bowers ridge and beringian slope a strong eccentricity was observed only at u1343 but no signature for eccentricity was detected at u1341 therefore the milankovitch cycle with 100 ky periodicity was only observed at the beringian slope area the results suggest that the biogenic opal at site u1341 indicated a local characteristic for biogenic productivity changes thus the absence of the 100 ky cycle of biogenic opal at site u1341 seems to be due to local phenomena such as water circulation although the changes in biogenic opal productivity at site u1341 are attributed to summer insolation

lithospheric behavior with triple junction migration an example based on the mendocino triple junction abstract the northward migration of the mendocino triple junction along the western margin of the north american plate produces a region of thin lithosphere in association with the growth of the san andreas transform numerical modeling of the resultant thermal regime in conjunction with modeling of the mechanical response of the region indicate the role of lithospheric thinning in the tectonism along the western margin of north america the geometry of the region of thin lithosphere is inherited from the previous subduction regime with resulting implications for the mechanical response the elevation of the california coast ranges is maintained primarily by flexure from buoyant loading from below additionally sporadic volcanism associated with the passage of the triple junction is a likely consequence of super solidus mantle emplaced at shallow depths after the removal of the subducting slab

cretaceous high p granulites at milford sound new zealand metamorphic history and emplacement in a convergent margin setting granulite facies orthogneiss of the arthur river complex arc at milford sound western fiordland records a complex early cretaceous magmatic and orogenic history for the pacific gondwana margin that culminated in the emplacement and burial of a dioritic batholith the western fiordland orthogneiss wfo enstatite bearing mafic to intermediate protoliths of the arc and wfo intruded the middle to upper crust the early deformation history of the arc is preserved in the pembroke granulite where two pyroxene s1 assemblages that reflect p 750 c were only patchily recrystallized during later deformation s1 is cut by garnet bearing leucogabbroic to dioritic veins which are cut by distinctive d2 fractures involving anorthositic veins and garnet diopside plagioclase bearing reaction zones these zones are widespread in the arc and wfo and record conditions of p 14 kbar and t 750 c garnet clinopyroxene bearing corona reaction textures that mantle enstatite in both the arc and wfo reflect early cretaceous burial by approximately 25 km of continental crust most of the arc is formed from the milford and harrison gneisses which contain steeply dipping s4 assemblages that envelop the pembroke granulite and involve garnet hornblende diopside clinozoisite rutile and plagio clase with or without kyanite the p t history of rocks in western fiordland reflects pronounced early cretaceous convergence related tectonism and burial possibly related to the collision and accretion of island arc material onto the pacific gondwana margin

the tectonic evolution of the southern alps new zealand insights from fully thermally coupled dynamical modelling summary we present here the results of numerical models of the southern alps in which the thermal and dynamical developments of the orogen are fully coupled to one another this interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models in particular the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the alpine fault zone in these models this thermal weakening of the crust also produces a zone of high strain antithetic to the alpine fault in the upper crust that is comparable to the main divide fault zone within the southern alps the introduction of orographic rainfall and erosional processes into the model leads to the development of high asymmetric topography comparable to that of the southern alps this topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side due to the time required to accumulate suycient topography for this rain capture evect to become significant the establishment of this high asymmetric topography lags the initiation of surface uplift by several million years comparison of the observed thermal history of the southern alps with that seen in models based on divering hypotheses of the tectonic evolution of the south island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 ma with relative stability of the tectonic regime since that time the variation in isotopic ages along the southern alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the australian and pacific plates

relationship between δ15n values of bulk sediments and total organic carbon concentration in response to orbital scale biogenic opal production in the bering slope area over the last 600 kyrs abstract biogenic opal production in the surface water of the bering sea shows clear orbital scale variations such as interglacial highs and glacial lows in this study we examined the variation patterns in 15 n of bulk sediment 15 n bulk reflecting nitrate utilization and total organic carbon toc concentration in terms of organic carbon oc production in response to the orbital scale biogenic opal production during the last 600 kyrs at site u1343 iodp exp 323 in the bering slope area at site u1343 15 n bulk value and toc concentration do not show the distinct glacial interglacial change moreover contrary to the weak correlation between toc and biogenic opal concentrations toc concentration is positively correlated with 15 n bulk value indicating the strong control of oc production on nitrate utilization it should be noted that glacial terminations are characterized by high 15 n bulk value high toc and low biogenic opal concentration whereas the following full interglacials are characterized by low 15 n bulk value low toc and high biogenic opal concentration during the last 600 kyrs in other words glacial terminations are characterized by complete nitrate utilization associated with high oc production under low biogenic opal production whereas the following full interglacial periods are characterized by decreased nitrate utilization associated with low oc production under high biogenic opal production such different glacial interglacial variation patterns of oc production and nitrate utilization in response to biogenic opal production can best be explained by variable si c or si n uptake ratio by diatoms possibly owing to fe availability the glacial fe input increase at site u1343 is thought to be ascribed to more dust input or large discharge from the glacial yukon river these findings imply that fe transport and contribution may play a significant role in governing the nutrient cycles in the bering slope region of the northern hemisphere

environmental change in the sea of okhotsk during the last 1 1 million years on the basis of two sedimentary records from the central sea of okhotsk we reconstruct the closely coupled glacial interglacial changes in terrigenous flux marine productivity and sea ice coverage over the past 1 1 myr the correspondance of our sedimentary records to the china loess grain size record china loess particle timescale chiloparts suggests that environmental changes in both the sea of okhotsk area and in se asia were closely related via the siberian atmospheric high pressure cell during full glacial times our records point to a strong siberian high causing northerly wind directions the extension of the sea ice cover and a reduced amur river discharge deglacial maxima of terrigenous flux were succeeded by or synchronous to high productivity events marine productivity was strengthened during glacial terminations because of an effective nutrient utilization at times of enhanced water column stratification and high nutrient supply from fluvial runoff and sea ice thawing during interglacials se monsoonal winds prevailed analogous to today s summer situation of a pronounced mongolian heat low and a strong hawaiian high strong freshwater discharge induced by high precipitation rates in the amur drainage area and a seasonally reduced and mobile sea ice cover favored marine productivity although being considerably lower than during the terminations and a lowered flux of ice rafted detritus

subduction platform subsidence and foreland thrust loading the late tertiary development of taranaki basin new zealand borehole seismic and gravity data are used to investigate deformation of continental lithosphere at a miocene collisional zone deformation is manifested in the three following principal forms a long wavelength 500 km platform subsidence ascribed to mantle convection flexural deformation on a scale of 100 200 km due to crustal thrusting at the eastern boundary of the taranaki basin and a ductile thickening evident on the deep seismic section of taranaki basin that occurs on a scale of 10 km evidence for flexural deformation principally comes from the deep seismic section that shows a 150 km wavelength bending of the moho down toward the major zone of thrusting within the taranaki fault zone paleowater depths however provide evidence for an initial early miocene regional subsidence that is too long in wavelength to be explained by flexure induced from thrust sheet loading instead we propose that this broad platform subsidence was driven by loading from a deep source probably subduction induced flow in the mantle by 22 19 ma 1 2 km of water existed over most of the area now occupied by south taranaki basin by 19 17 ma the water depth in the zone east of the taranaki basin the taranaki fault zone had been replaced by rock due to submarine thrusting and crustal thickening this build up of submarine topography in the taranaki fault zone constitutes part of the load 25 8 mpa that created and maintains south taranaki basin gravity data place further constraints on loading at the thrust front and point to an additional intracrustal loading equivalent to 15 7 mpa over a 50 km wide zone this intracrustal load is explained as being due to thick skinned thrusting bringing denser lower crustal rocks nearer to the surface in the thrust zone the complete load on the taranaki foreland is therefore in three parts the submarine topographic load the intracrustal load and the loading of infilling sediments

thermochronology of the torlesse accretionary complex wellington region new zealand the torlesse complex comprises several mesozoic accretionary prism complexes together forming continental basement over large parts of new zealand this study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern north island that crosses the structural grain of the torlesse complex including its older and younger parts zircon fission track ft ages for the late triassic rakaia terrane which is the most inboard of the accretionary complexes are partially annealed some possibly reset and may indicate early cretaceous 134 10 ma cooling from maximum temperatures tmax probably related to imbrication of younger complexes of the pahau terrane numerical modeling of the zircon ft ages and published 40ara 39ar muscovite and biotite ages for the rakaia terrane suggest tmax values of 265 310 c and exhumation from depths of 10 12 km the rocks underlying the aorangi range and involving the youngest accretionary complex have experienced much lower tmax values of 210 and 110 c bracketed by reset apatite ft ages and detrital zircon ft ages the occurrence of a circa 100 ma component of zircon ft ages in both the weakly and highly indurated rocks beneath the aorangi range as well as in remnants of an overlying albian accretionary slope basin whatarangi formation imply multistorey accretion and incorporation of sediment into the youngest prism this circa 100 ma zircon ft age component also places a maximum age on the termination of mesozoic subduction beneath the new zealand region the occurrence of reset apatite ft ages across the whole of the wellington transect indicates that at least 4 km of exhumation occurred during the late miocene

plate boundary deformation in south island new zealand is related to inherited lithospheric structure abstract the alpine fault is the main active structure in the oblique continental collision zone of south island new zealand it is continuous at the surface for 800 km and accommodates 70 of current plate motion a 460 km offset of basement rocks suggests it has accommodated 50 of plate displacement since 45 ma geophysical geological and contemporary kinematic data are successfully modeled by slip on a narrow alpine fault zone extending into the lower crust the data are consistent with plate boundary deformation occurring on either northwest or southeast dipping shear zones in the upper mantle or with widely distributed strain in the upper mantle we propose that the position of the alpine fault and any associated lithospheric discontinuity is controlled by an eocene passive margin that separated palaeozoic continental lithosphere of the challenger plateau from much younger oceanic lithosphere strike slip motion since 25 ma has translated the passive margin into the continental collision zone resulting in subduction of oceanic lithosphere beneath south island and progressive localisation of shear strain near the alpine fault the eocene rift boundary formed by exploiting cretaceous oceanic transform faults and an older discontinuity within the new zealand continent we propose that a precursor to the alpine fault could be as old as palaeozoic inherited structure has clearly controlled the first order deformation pattern through new zealand since at least 100 ma and demonstrates that ancient faults can play an important role in determining lithospheric scale patterns of deformation at continental plate boundaries

ephemeral crustal thickening at a triple junction the mendocino crustal conveyor as the north american crust interacts with the migrating mendocino triple junction the crust is first significantly thickened and then equivalently thinned over a distance of a few hundred kilometers within a time frame of 5 m y or less this process of ephemeral crustal thickening is proposed to result from viscous coupling between the northward migrating gorda slab and the base of north america south of the triple junction a time dependent thermal mechanical finite element model is developed to test this hypothesis of plate boundary tectonics results of the numerical simulations show patterns of crustal deformation consistent with the mapped sequence of folding and faulting in the area the observed crustal structure and triple junction regional seismicity and localized regions of crustal extension coincident with the position of a hypothesized lower crustal melt zone

reduced upwelling of nutrient and carbon rich water in the subarctic pacific during the mid pleistocene transition abstract reduction in atmospheric pco2 has been hypothesised as a causal mechanism for the mid pleistocene transition mpt which saw global cooling and increased duration of glacials between 0 6 and 1 2 ma sea ice modulated high latitude upwelling and ocean atmospheric co2 flux is considered a potential mechanism for pco2 decline although there are no long term nutrient upwelling records from high latitude regions to test this hypothesis using nitrogen isotopes and opal mass accumulation rates from 0 to 1 2 ma we calculate a continuous high resolution nutrient upwelling index for the bering sea and assess possible changes to regional co2 fluxes and to the relative control of sea ice sea level and glacial north pacific intermediate water gnpiw on deep mixing and nutrient upwelling in the region we find nutrient upwelling in the bering sea correlates with global ice volume and air temperature throughout the study interval from 1 ma and particularly during the 900 ka event suppressed nutrient upwelling would have lowered oceanic fluxes of co2 to the atmosphere supporting a reduction in global pco2 during the mpt this timing is consistent with a pronounced increase in sea ice during the early pleistocene and restriction of flow through the bering strait during glacials after 900 ka both of which would have acted to suppress upwelling we suggest that sea level modulated gnpiw expansion during glacials after 900 ka was the dominant control on subarctic pacific upwelling strength during the mid late pleistocene while sea ice variability played a secondary role

a multiproxy study of past environmental changes in the sea of okhotsk during the last 1 5 ma abstract long chain diols have been detected in a wide range of environments and have been used to reconstruct past environmental changes however only a few long term records exist to date here we reconstructed past environmental changes in the central sea of okhotsk over the last 1 5 million years covering the mid pleistocene transition mpt sea surface temperatures sst reconstructed using the long chain diol index ldi reflects glacial interglacial changes however when compared with other organic paleothermometers uk 37 and texl86 the ldi sst is lower during interglacials and similar or higher during glacials possibly suggesting a shift of diol production season during interglacials the ldi sst does not change in periodicity around the mpt as observed for the texl86 likely due to this seasonal shift diatom productivity as recorded by 1 14 diols and biogenic opal content increased during the main deglaciations with a succession from proboscia diatoms to diatoms with a more heavily silicified shell confirming that primary productivity in the central sea of okhotsk is driven by sea ice progress and retreat in contrast to the ldi sst the 1 14 diols record shows a change in periodicity around the mpt from 41 to 100 kyr cycle suggesting an influence of orbital parameters on diatom productivity in the central sea of okhotsk the relative amount of c32 1 15 diol fc32 1 15 a proxy for riverine input correlates with sea level change with more riverine derived material reaching the core site when the amur river mouth is closer at lower sea levels in agreement fc32 1 15 shows a change in periodicity during the mpt with the appearance of a 100 kyr cycle our results show that the long chain diols can provide important paleoceanographic information in subpolar environments over long time scales but that temperature reconstructions can be severely impacted by changes in seasonality

continental subduction and three dimensional crustal structure the northern south island new zealand the three dimensional vp and vp vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment and 3146 earthquakes have been relocated between 40 km and 100 km depth the subducted plate is imaged as a relatively low velocity feature in the uppermost mantle reflecting the continental nature of the subducted crust in this region an increase in amplitude of this low velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction velocity variations within the subducted and overlying plates show some spatial correlation this suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath marlborough is currently not accommodating much active subduction in the overlying plate the awatere fault is a major structural feature associated with a low velocity zone extending to 23 km depth there is a marked change in structure near this fault with seismic velocities being lower to the southeast a relatively high level of seismicity occurs in this region of lower seismic velocities suggesting a relationship between the two a possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust the low velocity region in the overlying plate coincides with the region of most intense active deformation suggesting it is relatively weak

diatoms as paleoenvironmental proxies for seasonal productivity sea ice and surface circulation in the bering sea during the late quaternary abstract diatom assemblages in suspended particulate matter spm surface sediments and piston core samples in the southern bering sea and the western subarctic pacific were examined the distribution of diatom assemblages in the southern bering sea shows not only differences in species composition between the spm and surface sediment samples but also preferential dissolution of different species in the upper water column or surface sediments furthermore we present the downcore profiles of selected diatom taxa for four piston cores to indicate changes in paleoceanographic conditions e g seasonal productivity sea ice and surface water circulation of the glacial and deglaciation periods during the late quaternary in particular we propose five models for the annual cycling of the water masses in the western subarctic pacific and the southern bering sea during the late quaternary significant regional differences in diatom accumulation rates and taxonomic composition between the southern bering sea and the western subarctic pacific reflect spatial shifts in water masses caused by the variable extent of sea ice cover and cold low salinity surface waters in these regions

late pliocene to early pleistocene 2 4 1 25 ma paleoproductivity changes in the bering sea iodp expedition 323 hole u1343e abstract late pliocene to early pleistocene paleoproductivity changes in the bering sea were reconstructed using geochemical concentrations and mass accumulation rates mars of caco 3 biogenic opal and total organic carbon toc and sedimentary nitrogen isotope ratios 15 n at iodp expedition 323 hole u1343e drilled in the northern slope area 1956 m deep of the bering sea caco 3 concentration is generally low but prominent caco 3 peaks occur intermittently due to subseafloor authigenic carbonate formation rather than biogenic accumulation regardless of glacial interglacial variations biogenic opal concentrations reflect orbital scale glacial interglacial variations however toc concentration did not show clear glacial interglacial variation probably due to poor preservation the sedimentary 15 n values vary synchronously with biogenic opal concentration on orbital timescales the co varying pattern of opal productivity and 15 n values at hole u1343e is a result of nutrient utilization controlled by diatom productivity in the bering slope area where fe is not a limiting factor biogenic opal and toc mars showed a temporal shift at around 1 9 ma from a high productivity period under nutrient enriched conditions to a low productivity period under relatively nutrient depleted conditions high diatom productivity with low 15 n values before 1 9 ma is associated with abundant nutrient supply by upwelling in relation to strong surface current system this productivity decrease at about 1 9 ma was also found in the southern bering sea site u1341 and may be related to global opal reorganization

late quaternary paleoceanographic changes in the bering sea and the western subarctic pacific based on radiolarian assemblages seven piston cores were obtained from the bering sea and the western subarctic pacific during cruise kh99 3 of r v hakuho maru in august 1999 cores bow 8a bow 9a bow 12a ab umk 3a gat 3a in the bering sea and core es in the western subarctic pacific the quantitative high resolution analyses of radiolarians in the cores revealed significant variations in both their faunal assemblages and accumulation rates ar a trend of conspicuous turn over of the surface dwellers and intermediate dwellers increase of one at expense of the other after the last glacial maximum lgm is clearly visible in the bering sea data set however this turn over is not present at site es in the western subarctic pacific core these changes in radiolarian assemblages around the lgm suggest that the surface waters had extremely low temperature and low salinity and that the subsurface dichothermal layer temperature minimum layer was thickened all the way across the entire east west span of the bering sea furthermore sea ice coverage extended to the central aleutian basin around the lgm this suggests that the water mass structure in the western subarctic pacific around the lgm was basically the same as that of the eastern bering sea based on the species composition of the surface dwellers however the detailed development of intermediate water masses differed from that of the eastern bering sea during marine isotope stage mis 5 to 3 such species turn over events did not occur and species abundance stayed relatively uniform in the studied cores therefore the present water mass structure is interpreted to have been formed after the lgm based on the relative abundances of cycladophora davisiana the most dominant species dwelling in the intermediate water and a useful tracer of cold and well oxygenated intermediate water in the north pacific the past north pacific intermediate water npiw had the following source regions during the last 100 kyr 1 both the okhotsk sea and the bering sea during mis 5 to 3 2 the bering sea around the lgm and 3 shifting from the bering sea to the okhotsk sea after the lgm relatively low radiolarian ar at site bow 8a which is the shallowest site studied here present water depth 884 m were observed during the glacial periods this is due to decrease of radiolarian production in the thinned upper water layer above the dicothermal layer caused by sea level drop cycladophora davisiana probably lived deeper during the glacials than today as it was absent in the shallow piston core r 2005 elsevier ltd all rights reserved

dependence of wind driven current on wind stress direction in a small semienclosed homogeneous rotating basin abstract the dependence of wind driven current wdc on wind stress direction in a small semienclosed homogeneous rotating basin is investigated using a linear steady state analytical model based on ekman solutions the model is applicable to the middle of the basin midbasin and the current is driven by a constant wind stress of an arbitrary direction the wdc is made up of wind stress driven current wsdc and pressure driven current pdc components the laterally varying water depth of the basin confines the total volume transport in the longitudinal direction while the wind stress driven volume transport changes direction according to the wind stress direction therefore the pressure driven volume transport or equivalent the pressure gradient depends on the wind stress direction the relationship between the pressure gradient and the wind stress is anisotropic as a result the midbasin wdc is also dependent on the wind stress direction the dependence varies according to the lateral position a

pliocene exhumation and landscape evolution of central north island new zealand the role of the upper mantle 1 mudstone porosity analyses show that over 2 km of exhumation has occurred in central north island new zealand since 5 ma this combined with a coeval surface uplift of 500 m indicates a maximum 2 5 km of rock uplift in central north island the magnitude and wavelength 400 km of the rock uplift is similar to some topographic doming observed in continental areas associated with mantle upwelling yet central north island is recognized as a region of back arc extension marine terraces 100 150 km from the center of maximum exhumation show uplift rates of 0 2 0 3 mm yr 1 consistent with about 1 km of exhumation if averaged over 4 5 myr which compares well with mudstone porosity analyses seismic attenuation and slow pn velocities indicate that a thermal anomaly in the upper mantle is the source of this rock uplift the rock uplift may be explained by the replacement of a 100 km thick mantle lid by asthenosphere with a density 66 kg m 3 less than the lid about half this density contrast can be explained by thermal expansion and the rest is ascribed to partial melts in the upper mantle southward migration for the locus of maximum rock uplift is consistent with the zone of maximum incision erosion being 50 km south of the southernmost volcano such a migration is in keeping with other geological markers of volcanic and thermal phenomena and further strengthens the link of landscape evolution at active continental margins with processes in the mantle

paleoceanography of the last 500 kyrs in the central okhotsk sea based on geochemistry abstract analyses of geochemical constituents organic carbon nitrogen calcium carbonate and biogenic opal on core yk07 12 pc3b core pc3b provided detailed and useful information in biological productivity and paleoceanographic changes which occurred in the central okhotsk sea during the last 500 kyrs an age model for core pc3b was established based on changes in benthic foraminiferal 18 o the trend in organic carbon oc content represented the temporal change that can be correlated with the 18 o curve in core pc3b high during the interglacial periods mis 1 5 9 11 and 13 the trend in caco 3 content represented is similar to the trend seen in the oc content the high correlation between oc and caco 3 contents indicates an increase in coccolithophorid productivity during the early deglaciations the comparison between the changes in caco 3 and biogenic opal indicates that the dominant phytoplankton group was coccolithophorids during the early deglaciations which was subsequently replaced by diatoms during the late deglaciations it appears that such a temporal phytoplankton succession was caused by an increase in dissolved silicon supply to the euphotic layer possibly associated with a change in surface hydrography that subsequently caused an upward expansion of the intermediate layer such ecological changes probably played an important role in the uptake of atmospheric co 2

two sided orogen collision and erosion from the sandbox to the southern alps new zealand in continental collision zones the indentor is of approximately the same vertical dimension as the material being deformed thereby allowing material to move over the indentor and out of the deforming orogen this simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite dipping topographic slopes these two wedges are mechanically coupled and the mechanics are strongly influenced by the erosion conditions imposed on the wedge where the growing mountains perturb prevailing moisture laden winds the erosion pattern is highly asymmetric erosional work being concentrated at the toe of the steep wedge facing the indentor the inboard wedge concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision these include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity the outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the southern alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units as in the western alps of europe the other wedge the outboard wedge undergoes relatively little erosion is broad with a more gentle surface slope and maintains an outcrop pattern consisting of upper crustal material

authigenic nd isotope record of north pacific intermediate water formation and boundary exchange on the bering slope abstract the bering sea is a potential location for the formation of the north pacific intermediate water npiw which drives the global ocean circulation as a counterpart to the north atlantic deep water nadw to evaluate the npiw nadw seesaw hypothesis we reconstructed the long term variation of the bottom water nd isotopic composition at site u1345 on the bering slope by extracting authigenic fe mn oxyhydroxide from bulk sediments we examined six different extractions in order to ensure that authentic seawater composition is recovered for bering slope sediments whose typical carbonate content is less than 5 average 2 the most reliable results are obtained if the decarbonation step is omitted and a low reagent to sediment ratio is adopted the reconstructed authigenic end record for the last 520 kyr exhibits large temporal variations depending on whether the npiw formation or the boundary exchange process is dominant periods of radiogenic end can be attributed to npiw formation triggered by brine rejection as evidenced by the difference in 18o of benthic foraminifera between sites 18obf high sea ice related diatoms and low abundance of bulimina aff exilis low oxygen deep fauna diminished supply of unradiogenic nd from boundary exchange seems to intensify these radiogenic peaks on the other hand the unradiogenic end intervals can be attributed to stagnant bottom water conditions as can be deduced from the 18obf values low sea ice related diatoms abundant b aff exilis and laminations when there is no npiw formation the continental margin sediments are exposed to boundary exchange for a longer period of time leading to release of unradiogenic nd the mid mis 6 and mid mis 5 are exceptions in that npiw formation occurred yet the end compositions are unradiogenic npiw formation and cold climate closed bering strait are not always correlated comparison against end records of the south atlantic suggests only an ambiguous npiw nadw seesaw for the last 200 kyr

early pleistocene glacial cycles and the integrated summer insolation forcing long term variations in northern hemisphere summer insolation are generally thought to control glaciation but the intensity of summer insolation is primarily controlled by 20 000 year cycles in the precession of the equinoxes whereas early pleistocene glacial cycles occur at 40 000 year intervals matching the period of changes in earth s obliquity the resolution of this 40 000 year problem is that glaciers are sensitive to insolation integrated over the duration of the summer the integrated summer insolation is primarily controlled by obliquity and not precession because by kepler s second law the duration of the summer is inversely proportional to earth s distance from the sun

the response of the bering sea gateway during the mid pleistocene transition abstract the mid pleistocene transition and the response of sea ice in the subarctic north pacific are examined using a diatom record from the north eastern bering sea slope iodp core u1344a high percentages of diatom sea ice species in the assemblage foster the development of groupings specific to differing ice conditions thalassiosira antarctica var borealis emerges as a key diatom species that delineates the development of thick multiannual sea ice and marks a series of advancements from 0 9 to1 2 ma after which it dominates the assemblage until 0 6 ma the decline of neodenticula seminae indicative of alaskan stream water inflow is in agreement with neighbouring core u1343e confirming the reduction of bering sea circulation after 0 9 ma we propose that during the mpt a stepped advancement of sea ice occurs in the bering sea as a response to increased moisture delivery to the high latitudes an extended period of diatom productivity between 0 75 and 0 65 ma marks major deglacial conditions towards the end of the mpt by 0 6 0 55 ma the work highlights the important role played by sea ice and supports the contention of a colder mpt in the subarctic pacific

stable isotope and lithologic evidence of late glacial and holocene oceanography of the northwestern pacific and its marginal seas abstract stable isotopes geochemical lithological and micropaleontological results from cores from the far northwest fnw pacific and the okhotsk and bering seas are used to reconstruct the regional environment for the last glaciation the deglacial transition and the holocene 18 o records of planktonic foraminifera of the region show two light shifts during deglacial time provoked by the freshening of the surface water and climate warming these north pacific terminal events t1anp and t1bnp with ages of 12 500 and 9300 yr b p respectively occur almost simultaneously with two episodes of accelerated glacier melting around the north atlantic along with the isotopic shifts the caco 3 content in regional sediments increased abruptly 1a and 1b carbonate peaks probably due to changes of productivity and pore water chemistry of surface sediments organic matter and opal concentration increased during the transition between t1anp and t1bnp events in the sediments of the fnw pacific and the southern part of the bering sea and opal content increased in the holocene in the bering and okhotsk seas 13 c records of cores from the okhotsk and bering seas and the fnw pacific do not contradict the hypothesis of increased intermediate water formation in the region during glaciation during deglaciation accumulation of the coarse terrigenous component decreased in sediments of the bering sea and the fnw pacific before the t1anp event probably as a result of rising sea level and opening of the bering strait

the deglacial history of surface and intermediate water of the bering sea the lithology of deglacial sediments from the bering sea includes intervals of laminated or dysaerobic sediments these intervals are contemporaneous with the occurrence of laminated sediments from the california margin and gulf of california which suggests widespread low oxygen conditions at intermediate depths in the north pacific ocean the cause could be reduced intermediate water ventilation increased organic carbon flux or a combination of the two we infer abrupt decreases of planktonic foraminifer d 18 o at 14 400 and 11 650 yr bp which may be a combination of both freshening and warming on the shirshov ridge the abundance of sea ice diatoms of the genus nitzschia reach local maxima twice during the deglaciation the latter of which may be an expression of the younger dryas these findings expand the extent of the expression of deglacial millennial scale climate events to include the northernmost pacific r 2005 elsevier ltd all rights reserved