fertilizing potential of volcanic ash in ocean surface water the fertilization potential of newly erupted and well preserved ash from the 2000 hekla eruption in iceland was measured for the first time by flow through experiments as previously shown 1 the north atlantic ocean including the subarctic seas surrounding iceland is the largest net sink of the world s oceans for atmospheric co 2 owing to biological drawdown during summer 2 almost complete consumption of phosphate in chlorophyll rich areas of the north atlantic ocean might limit primary production and 3 in the southern pacific ocean and parts of the equatorial pacific ocean iron might limit primary production we found through laboratory experiments that volcanic ash exposed to seawater initially releases large amounts of adsorbed phosphate 1 7 mmol g 21 h 21 iron 37 0 mmol g 21 h 21 silica 49 5 mmol g 21 h 21 and manganese 1 7 mmol g 21 h 21 dissolution of acid aerosols adsorbed to the surface of the ash caused the high initial release of major and trace elements because of the instantaneous dissolution of adsorbed components when newly erupted volcanic ash comes in contact with the ocean surface water macronutrients and bioactive trace metals are released fast enough to become available to support primary production

the adiabatic pole to pole overturning circulation abstractthe adiabatic pole to pole cell of the residual overturning circulation roc is studied in a two hemisphere semienclosed basin with a zonally reentrant channel occupying the southernmost eighth of the domain three different models of increasing complexity are used a simple analytically tractable zonally averaged model a coarse resolution numerical model with parameterized eddies and an eddy resolving general circulation model two elements are found to be necessary for the existence of an adiabatic pole to pole cell 1 a thermally indirect wind driven overturning circulation in the zonally reentrant channel analogous to the deacon cell in the antarctic circumpolar current acc region and 2 a set of outcropping isopycnals shared between the channel and the semienclosed region of the northern hemisphere these points are supported by several computations varying the domain geometry the surface buoyancy distribution and the wind forcing all three models give results that are qualitat

rapid stepwise onset of antarctic glaciation and deeper calcite compensation in the pacific ocean the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth at present this depth is 4 500 m with some variation between and within ocean basins the calcite compensation depth is linked to ocean acidity which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate1 geological records of changes in the calcite compensation depth show a prominent deepening of more than 1 km near the eocene oligocene boundary 34 million years ago 2 when significant permanent ice sheets first appeared on antarctica3 4 5 6 but the relationship between these two events is poorly understood here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical pacific ocean that cover the eocene oligocene boundary we find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40 000 years each synchronous with the stepwise onset of antarctic ice sheet growth the glaciation was initiated after climatic preconditioning7 by an interval when the earth s orbit of the sun favoured cool summers the changes in oxygen isotope composition across the eocene oligocene boundary are too large to be explained by antarctic ice sheet growth alone and must therefore also indicate contemporaneous global cooling and or northern hemisphere glaciation

the late cretaceous to recent tectonic history of the pacific ocean basin abstract a vast ocean basin has spanned the region between the americas asia and australasia for well over 100 myr represented today by the pacific ocean its evolution includes a number of plate fragmentation and plate capture events such as the formation of the vancouver nazca and cocos plates from the break up of the farallon plate and the incorporation of the bellingshausen kula and aluk phoenix plates which have been studied individually but never been synthesised into one coherent model of ocean basin evolution previous regional tectonic models of the pacific typically restrict their scope to either the north or south pacific and global kinematic models fail to incorporate some of the complexities in the pacific plate evolution e g the independent motion of the bellingshausen and aluk plates thereby limiting their usefulness for understanding tectonic events and processes occurring in the pacific ocean perimeter we derive relative plate motions with 95 uncertainties for the pacific farallon vancouver kula pacific bellingshausen pacific and early pacific west antarctic spreading systems based on recent data including marine gravity anomalies well constrained fracture zone traces and a large compilation of magnetic anomaly identifications we find our well constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the north and south pacific in conjunction with recently published and well constrained relative plate motions for other pacific spreading systems e g aluk west antarctic pacific cocos recent pacific west antarctic spreading we explore variations in the age of the oceanic crust seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern pacific asymmetries in crustal accretion within the overall pacific basin where both flanks of the spreading system are preserved have typically deviated less than 5 from symmetry and large variations in crustal accretion along the southern east pacific rise i e pacific nazca farallon spreading appear to be unique to this spreading corridor through a relative plate motion circuit we explore the implied convergence history along the north and south america s where we find that the inclusion of small tectonic plate fragments such as the aluk plate are critical for reconciling the history of convergence with onshore geological evidence

sensitivity of the atlantic meridional overturning circulation to south atlantic freshwater anomalies the sensitivity of the atlantic meridional overturning circulation amoc to changes in basin integrated net evaporation is highly dependent on the zonal salinity contrast at the southern border of the atlantic biases in the freshwater budget strongly affect the stability of the amoc in numerical models the impact of these biases is investigated by adding local anomaly patterns in the south atlantic to the freshwater fluxes at the surface these anomalies impact the freshwater and salt transport by the different components of the ocean circulation in particular the basin scale salt advection feedback completely changing the response of the amoc to arbitrary perturbations it is found that an appropriate dipole anomaly pattern at the southern border of the atlantic ocean can collapse the amoc entirely even without a further hosing the results suggest a new view on the stability of the amoc controlled by processes in the south atlantic

the key role of the western boundary in linking the amoc strength to the north south pressure gradient abstracta key idea in the study of the atlantic meridional overturning circulation amoc is that its strength is proportional to the meridional density gradient or more precisely to the strength of the meridional pressure gradient a physical basis that would indicate how to estimate the relevant meridional pressure gradient locally from the density distribution in numerical ocean models to test such an idea has been lacking however recently studies of ocean energetics have suggested that the amoc is driven by the release of available potential energy ape into kinetic energy ke and that such a conversion takes place primarily in the deep western boundary currents in this paper the authors develop an analytical description linking the western boundary current circulation below the interface separating the north atlantic deep water nadw and antarctic intermediate water aaiw to the shape of this interface the simple analytical model also shows how available potential energy is converted into

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

an indicator of the multiple equilibria regime of the atlantic meridional overturning circulation recent model results have suggested that there may be a scalar indicator s monitoring whether the atlantic meridional overturning circulation moc is in a multiple equilibrium regime the quantity s is based on the net freshwater transport by the moc into the atlantic basin it changes sign as soon as the steady atlantic moc enters the multiple equilibrium regime because of an increased freshwater input in the northern north atlantic this paper addresses the issue of why the sign of s is such a good indicator for the multiple equilibrium regime changes in the atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model the authors show that the net anomalous freshwater transport into or out of the atlantic resulting from the interactions of the velocity perturbations and salinity background field is coupled to the background steady state state freshwater budget and hence to s the sign of s precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the moc therefore it can indicate whether the moc is in a single or multiple equilibrium regime

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

circum pacific arc flare ups and global cooling near the eocene oligocene boundary explosive eruptions from subaerial arc volcanoes can have significant environmental impact because of the discharge of ash and volatiles directly into the atmosphere and oceans the link between climate cooling and voluminous volcanic eruptions has remained speculative due to a lack of supporting evidence a compilation of 2814 k ar and 40 ar 39 ar age determinations from four circum pacific arcs indicates that periods of high volcanic output i e flare ups have been episodic and in some cases synchronous peak periods and subsequent lulls in arc magmatism over the past 50 m y have occurred coeval with major fluctuations in global climate including the eocene oligocene transition one of the most prominent global climate reorganizations in the cenozoic hundreds of intermediate to silicic eruptions occurred during an extremely vigorous period of circum pacific volcanism beginning in the late eocene which likely led to the production of sulfur aerosols in the stratosphere and fertilization of surface waters of the pacific ocean we provide a mechanism that may have been partly responsible for the climatic preconditioning that must have preceded and ultimately promoted antarctic ice sheet growth 34 m y ago

lead isotope provinciality of central north pacific deep water over the cenozoic understanding the pre anthropogenic pb cycle of central north pacific deep water has attracted a lot of attention in recent years partly because of its unique geographical location in that it is a remote gyre system characterized by high dust fluxes and sluggish overturning circulation however the factors controlling pb isotope evolution in this area over the cenozoic are still controversial and various mechanisms have been proposed in previous studies here we report new pb and nd isotope time series of four ferromanganese crusts two from the western pacific near the mariana arc and the other two from the central pacific together with previously published records we discuss for the first time the significance of a persistent and systematic pb isotopic provinciality recorded by central north pacific crusts over the cenozoic we propose that globally well mixed stratosphere volcanic aerosols could contribute pb but have not been the major factors controlling the pb isotope distribution in the central north pacific over time island arc input and probably enhanced hydrothermal input between about 45 and 20 ma likely controlled the pb isotope provinciality and evolution prior to 20 ma when coeval pb isotope records in different crusts showed large differences and atmospheric silicate dust flux was extremely low after the eocene in particular after 20 ma asian dust input has become an isotopically resolvable source while island arc derived pb has remained important to balance the dust input and to produce the observed pb isotope distribution in the central north pacific during this period

the stability of the moc as diagnosed from model projections for pre industrial present and future climates the stability of the atlantic meridional overturning circulation moc is investigated for various climate scenario runs using data from the cmip3 archive of coupled atmosphere ocean models apart from atmospheric feedbacks the sign of the salt flux into the atlantic basin that is carried by the moc determines whether the moc is in the single or multiple equilibria regime this salt advection feedback is analyzed by diagnosing the freshwater and salt budgets for the combined atlantic and arctic basins consistent with the finding that almost all coupled climate models recover from hosing experiments it is found that most models feature a negative salt advection feedback in their pre industrial climate freshwater perturbations are damped by this feedback excluding the existence of a stable off state for the moc all models feature enhanced evaporation over the atlantic basin in future climates but for a moderate increase in radiative forcing b1 and 2 co2 scenarios there is a decrease of the fresh water flux carried by the moc into the atlantic the deficit is made up by increased fresh water transport by the gyre circulation in this forcing regime the salt advection feedback becomes less negative for three models from an ensemble of eight it is positive in a 2 co2 climate while two models feature a positive feedback in the pre industrial climate for even warmer climates a1b equilibrium and 4 co2 the salt feedback becomes more negative damping again it is shown that the decrease in northward fresh water transport at 34 s by the moc in b1 equilibrium and 2 co2 is due to a reduction of the inflow of intermediate waters relative to thermocline waters associated with a robust shoaling of the moc in future warmer climates in a1b and 4 co2 climates northward freshwater transport increases again the moc keeps shoaling but both intermediate and thermocline water masses freshen

geochemical confirmation of the kula farallon slab window beneath the pacific northwest in eocene time plate tectonic models indicate subduction of the kula farallon spreading ridge and thus imply formation of the kula farallon slab window beneath western north america from late cretaceous to middle eocene time seafloor magnetic anomalies however pro vide few constraints on the configuration of the subducted ridge and the location of the slab window we use geochemical data from eocene igneous rocks to provide an estimate of the slab window s position at 50 ma contouring of the trace element ratios k 2o sio2 rb zr and ta ce for lavas of the eocene challis kamloops volcanic belt demonstrates a southward trend toward enriched geochemical character this trend is consistent with a pacific northwest position for the slab window this is supported by the presence of adak ites slab melts near the canada united states border and by ca 51 ma forearc intru sions and volcanic rocks on vancouver island that are regarded as products of ridge subduction a new reconstruction of the slab window s geometry over time indicates that its chemically defined position is kinematically viable

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

thermohaline circulation hysteresis a model intercomparison we present results from an intercomparison of 11 different climate models of intermediate complexity in which the north atlantic ocean was subjected to slowly varying changes in freshwater input all models show a characteristic hysteresis response of the thermohaline circulation to the freshwater forcing which can be explained by stommel s salt advection feedback the width of the hysteresis curves varies between 0 2 and 0 5 sv in the models major differences are found in the location of present day climate on the hysteresis diagram in seven of the models present day climate for standard parameter choices is found in the bi stable regime in four models this climate is in the mono stable regime the proximity of the present day climate to the stommel bifurcation point beyond which north atlantic deep water formation cannot be sustained varies from less than 0 1 sv to over 0 5 sv

burial dominated cementation in non tropical carbonates of the oligocene te kuiti group new zealand abstract cementation of bryozoan echinoid benthic foraminiferal temperate shelf carbonates of the oligocene te kuiti group north island new zealand occurred mainly during subsurface burial the calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan given an iron supply and compared to the skeletal material with normal marine 18o values from 2 to 1 more depleted in 18o with depth of burial the 18o composition of bulk cement samples ranging from 1 to 7 these trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures 20 50 c respectively the 13c values 0 to 3 of the cements remain the same as the host marine shells suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution two gradational burial diagenetic environments influenced by marine derived porewaters are arbitrarily distinguished shallow burial phase and moderate burial phase during the shallow burial phase down to 500 600 m sub bottom depth the carbonates lost at least 25 of their original porosity by mechanical compaction and were selectively cemented by non ferroan or usually ferroan variably luminescent slightly 18o depleted sparry calcite cement 18o 2 to 4 mainly as syntaxial rims about echinoid grains these shallow burial cements form less than about 10 of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction during the moderate burial phase between about 600 and 1100 m sub bottom depth porosity loss continued typically to about 70 of its original value as a result of pressure solution of calcitic bioclasts associated with more advanced stages of chemical compaction this involved development of a wide variety of non sutured and microstylolitic solution seams including both single and composite wispy or continuous bedding parallel types and non parallel reticulate forms the released carbonate was precipitated as ferroan or non ferroan where iron supply was negligible dull luminescent strongly 18o depleted 18o 4 to 7 mainly equant calcite spar cement occluding available pore space in the limestones

where do winds drive the antarctic circumpolar current the strength of the antarctic circumpolar current acc is believed to depend on the westerly wind stress blowing over the southern ocean although the exact relationship between winds and circumpolar transport is yet to be determined here we show based on theoretical arguments and a hierarchy of numerical modeling experiments that the global pycnocline depth and the baroclinic acc transport are set by an integral measure of the wind stress over the path of the acc taking into account its northward deflection our results assume that the mesoscale eddy diffusivity is independent of the mean flow while the relationship between wind stress and acc transport will be more complicated in an eddy saturated regime our conclusion that the acc is driven by winds over the circumpolar streamlines is likely to be robust

sulfur and chlorine in late cretaceous deccan magmas and eruptive gas release large volume p hoehoe lava flows erupted 67 to 65 million years ago forming the deccan traps india the impact of these flood basalt eruptions on the global atmosphere and the coeval end cretaceous mass extinction has been uncertain to assess the potential gas release from this volcanism we measured sulfur and chlorine concentrations in rare glass inclusions inside crystals and on glassy selvages preserved within lavas concentrations range from 1400 parts per million of s and 900 parts per million of cl in inclusions down to a few hundred parts per million in the lava these data indicate that eruptions of deccan lavas could have released at most 0 103 weight of s yielding up to 5 4 teragrams of so2 per cubic kilometer of lava a more conservative estimate is 0 07 weight of s and 0 04 weight of cl yielding 3 5 teragrams of so2 and 1 teragram of hcl for every cubic kilometer of lava erupted the flows were very large in volume and these results imply that huge amounts of s and cl gases were released the environmental impact from even individual eruptions during past flood basalt activity was probably severe

multiple regimes and low frequency variability in the quasi adiabatic overturning circulation 2015 american meteorological society when interior mixing is weak the ocean can support an interhemispheric overturning circulation on isopycnals that outcrop in both the northern hemisphere and a high latitude southern circumpolar channel this overturning cell participates in a salt advection feedback that counteracts the precipitation induced surface freshening of the northern high latitudes the net result is an increase in the range of isopycnals shared between the two hemispheres which strengthens the overturning circulation however if precipitation in the northern hemisphere sufficiently exceeds that in the southern hemisphere the overturning cell collapses and is replaced by a cell circulating in the opposite direction whose southern end point is equatorward of the channel this reversed cell is shallower and weaker than its forward counterpart and is maintained diffusively for a limited range of parameters freshwater hysteresis occurs and multiple overturning regimes are found for the same forcing these multiple regimes are by definition unstable with regard to finite amplitude disturbances since a sufficiently large perturbation can affect a transition from one regime to the other both overturning regimes show pronounced nearly periodic thermohaline variability on multidecadal and multicentennial time scales the multidecadal oscillation is expressed in the north hemisphere gyre and driven by a surface thermohaline instability the multicentennial oscillation has the character of an interhemispheric loop oscillation these oscillations mediate transitions between overturning regimes by providing an internal source of finite amplitude disturbances as the diffusivity is reduced the reverse cell becomes weaker and thus less stable to a given perturbation amplitude this causes the width of the hysteresis to decrease with decreasing diffusivity

evaluation of cold water carbonates as a possible paleoclimatic indicator abstract the common belief currently shared by many geoscientists concerning the climatic interpretation of limestones is that a warm water environment is essential this concept is not necessarily true because the rate and extent of terrigenous sediment dilution rather than water temperature is the primary factor determining whether or not a limestone forms at nearshore or continental shelf depths because carbonate productivity is lowest in cold climates however caco3 abundance and the thickness of carbonate accumulations tend to be least at high latitudes in this regard present day continental shelves and beaches offer a poor model for comparing cold water and warm water carbonates because of the generally emergent continental tectonic framework recent eustatic sea level changes and the presence of ice caps at the modern poles typically the influence of climate on non reef continental shelf and beach environments cannot be clearly distinguished by the presence or absence of major taxonomic groups faunal diversity and equitability are more sensitive in this regard the absence of shelf depth inorganic carbonate precipitates micrite envelopes and peloids may also point to the cold water origin of a rock skeletal mineralogy and oxygen isotopes of certain unrecrystallized carbonates may be good paleoclimatic indicators however trace elements and physical textural attributes of the carbonate fraction are probably temperature insensitive previous studies of high latitude continental shelves have concentrated merely on the abundance of calcareous material and there is seemingly a disproportionate amount of information with respect to low latitude carbonate studies further research on cold water carbonates may open up new avenues for alternative paleoenvironmental and paleoclimatic interpretations

temperate skeletal carbonate sediments on scott shelf northwestren vancouver island canada abstract scott shelf 2000 km 2 centred at scott islands northwestern vancouver island is mainly shallower than 150 m topographically diversified and floored by bedrock outcrops and terrigenous lithic gravels and sands left stranded following the post glacial rise in sea level 13 000 yrs b p it lies in a zone of vigorous wind wave currents and strong tidal flows but is largely starved of modern terrigenous sediment as a consequence it is slowly accumulating a thin discontinuous blanket of clean skeletal carbonate sands and gravels admixed to varying degrees with the underlying terrigenous deposits principal skeletal contributors are infaunal bivalves on coarse sandy and gravelly substrates barnacles on low amplitude gravelly ridges bryozoans on bedrock outcrops and boulders and benthic foraminifera on fine sands in deep 100 m waters south of the scott islands living carbonate benthos are scattered and generally sparse occupying specific ecologic niches and the shelf wide rate of carbonate production is low skeletons are fragmented transported and mixed during storms and are concentrated within bedrock hollows and crevices and shallow depressions between gravel ridges where infaunal bivalves are abundant the carbonates are dominated by aragonite but otherwise the skeletal hashes are predominantly calcitic many grains and especially aragonitic ones are corroded and weakened by epilithic and endolithic bioerosion and probably also by marine and dissolution the most corroded shells have ages of only about 1000 yrs so that their preservation potential is low the character of scott shelf skeletal carbonate deposits reflects their temperate latitude cold water heritage

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

on the motion of hawaii and other mantle plumes abstract the hawaiian hotspot and its prominent bend at 47 ma have figured prominently in the development of ideas on the nature of mantle plumes and plate motion here i review paleomagnetic analyses associated with ocean drilling program odp leg 197 which together with studies of plate circuits and geodynamic modeling results indicate southward motion of the hawaiian hotspot during formation of the emperor seamounts these analyses confirm the long posited idea that mantle plumes should be influenced by mantle flow moreover these results indicate that late cretaceous early paleogene hotspot motion was the dominant factor in forming the emperor track and thus the famous bend morphology an important corollary of this finding on hotspot motion is that long term polar wander of earth which has been inaccurately assessed by viewing paleomagnetic data in a fixed hotspot reference frame has been far less than previously thought salient points for future research raised by the odp leg 197 paleomagnetic results include the scale of mantle flow recorded by hotspot track segments and the underlying processes that cause track segments to be dominated by mantle flow rather than plate motion for given time intervals

momentum balance of the wind driven and meridional overturning circulation abstractwhen a force is applied to the ocean fluid parcels are accelerated both locally by the applied force and nonlocally by the pressure gradient forces established to maintain continuity and satisfy the kinematic boundary condition the net acceleration can be represented through a rotational force in the rotational component of the momentum equation this approach elucidates the correspondence between momentum and vorticity descriptions of the large scale ocean circulation if two terms balance pointwise in the rotational momentum equation then the equivalent two terms balance pointwise in the vorticity equation the utility of the approach is illustrated for three classical problems barotropic rossby waves wind driven circulation in a homogeneous basin and the meridional overturning circulation in an interhemispheric basin in the hydrostatic limit it is shown that the rotational forces further decompose into depth integrated forces that drive the wind driven gyres and overturning forces

how did the summer go the cold summer of 1601 resulted from a spectacular eruption of the volcano huaynaputina in peru in february 1600 analysis of tree growth has shown that the summer of 1601 was the coldest of the past 600 years and from a reconstruction of the deposits of volcanic ash around the volcano it can be calculated that the volcano ejected about 10 km3of molten rock other cold summers have been linked with volcanic eruptions but in some cases it is not certain which eruptions caused the climate change

meridional density gradients do not control the atlantic overturning circulation a wide body of modeling and theoretical scaling studies support the concept that changes to the atlantic meridional overturning circulation amoc whether forced by winds or buoyancy fluxes can be understood in terms of a simple causative relation between the amoc and an appropriately defined meridional density gradient mdg the mdg is supposed to translate directly into a meridional pressure gradient here two sets of experiments are performed using a modular ocean model coupled to an energy moisture balance model in which the positive amoc mdg relation breaks down in the first suite of seven model integrations it is found that increasing winds in the southern ocean cause an increase in overturning while the surface density difference between the equator and north atlantic drops in the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment dt that ranges from 238 to 98c an increase in dt results in increased sensitivity of density to temperature gradients the amoc in these model integrations drops as the mdg increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean traditional scaling analysis can only produce this weaker amoc if the scale depth decreases enough to compensate for the stronger mdg five estimates of the depth scale are evaluated and it is found that the changes in the amoc can be derived from scaling analysis when using the depth of the maximum overturning circulation or estimates thereof but not from the pycnocline depth these two depth scales are commonly assumed to be the same in theoretical models of the amoc it is suggested that the correlation between the mdg and amoc breaks down in these model integrations because the depth and strength of the amoc is influenced strongly by remote forcing such as southern ocean winds and antarctic bottom water formation

reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria it has recently been suggested that the structure and strength of the meridional overturning circulation in the global ocean is governed by the input of mechanical energy to the system by winds and tides however it is not clear how this suggestion relates to the existence of multiple equilibria of the meridional overturning circulation which depends on thermohaline feedbacks and is more consistent with a buoyancy driven view of the circulation both theories have been illustrated by box models in the past stommel in tellus 13 224 230 1961 gnanadesikan in science 283 2077 2079 1999 here we incorporate these two theories into a single box model in an attempt to reconcile the roles of mechanical and buoyancy forcing in driving the meridional overturning circulation the box model has two equilibrium solutions one with sinking at high northern latitudes as in the present day atlantic and one without the circulation is mechanically driven but the northern sinking can be thought of as a release valve which acts as a sink of potential energy when the surface water at high northern latitudes is dense enough to convect while the source of energy comes from mechanical forcing the presence or otherwise of multiple equilibria is therefore determined by thermohaline feedbacks in some areas of parameter space an oscillation between the model s two circulation regimes occurs reminiscent of a bipolar seesaw

atlantic pycnocline theory scrutinized using a coupled climate model simulations with changed southern ocean wind stress oceanic vertical mixing surface freshwater forcing and global warming confirm the basic equations of gnanadesikan s 1999 theory for the atlantic one vertical scale the pycnocline depth d contributes inversely proportional to low latitudinal upwelling and linearly to southern ocean eddy transport the maximum atlantic overturning is confirmed to be quadratic in d but is also shown to be linear in a meridional density difference our simulations strongly suggest that the theory needs to be complemented by a dynamical equation for since changes in both d and are significant and mutually independent while under global warming d varies strongly and is practically constant the situation is reversed for altered surface freshwater fluxes similarly variations in the meridional length scale of southern ocean outcropping require a dynamical equation in order to capture the fundamental behavior of the atlantic meridional overturning circulation

extinction and environmental change across the eocene oligocene boundary in tanzania the eocene oligocene transition between ca 34 and 33 5 ma is the most profound episode of lasting global change to have occurred since the end of the cretaceous diverse geological evidence from around the world indicates cooling ice growth sea level fall and accelerated extinction at this time turnover in the oceanic plankton included the extinction of the foraminifer family hantkeninidae which marks the eocene oligocene boundary in its type section another prominent extinction affected larger foraminifera which resulted in the loss of some of the world s most abundant and widespread shallow water carbonate secreting organisms however problems of correlation have made it difficult to relate these events to each other and to the global climate transition as widely recorded in oxygen and carbon isotope records from deep sea cores here we report new paleontological and geochemical data from hemipelagic sediment cores on the african margin of the indian ocean tanzania drilling project sites 11 12 and 17 the eocene oligocene boundary is located between two principal steps in the stable isotope records the extinction of shallow water carbonate producers coincided with an extended phase of ecological disruption in the plankton and preceded maximum glacial conditions in the early oligocene by 200 k y

eddy driven buoyancy gradients on eastern boundaries and their role in the thermocline abstract it is demonstrated that eddy fluxes of buoyancy at the eastern and western boundaries maintain alongshore buoyancy gradients along the coast eddy fluxes arise near the eastern and western boundaries because on both coasts buoyancy gradients normal to the boundary are strong the eddy fluxes are accompanied by mean vertical flows that take place in narrow boundary layers next to the coast where the geostrophic constraint is broken these ageostrophic cells have a velocity component normal to the coast that balances the geostrophic mean velocity it is shown that the dynamics in these thin ageostrophic boundary layers can be replaced by effective boundary conditions for the interior flow relating the eddy flux of buoyancy at the seaward edge of the boundary layers to the buoyancy gradient along the coast these effective boundary conditions are applied to a model of the thermocline linearized around a mean stratification and a state of rest the linear model parameterizes the eddy fluxes of buoya

subduction zone volcanic ash can fertilize the surface ocean and stimulate phytoplankton growth evidence from biogeochemical experiments and satellite data volcanoes confront earth scientists with new fundamental questions can airborne volcanic ash release nutrients on contact with seawater thereby excite the marine primary productivity mpp and most notably can volcanoes through oceanic fertilization affect the global climate in a way that is so far poorly understood here we present results from biogeochemical experiments showing that 1 volcanic ash from subduction zone volcanoes rapidly release an array of nutrients co limiting algal growth in vast oceanic areas 2 at a speed much faster minute scale than hitherto known and that marine phytoplankton from low iron oceanic areas can swiftly within days utilize iron from volcanic sources we further present satellite data possibly indicating an increase of the mpp due to the seaward deposition of volcanic particulate matter our study supports the hypothesis that oceanic iron fertilization with volcanic ash may play a vital role for the development of the global climate

boundary currents and watermass transformation in marginal seas the properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed the net heat loss within the marginal sea is ultimately balanced by lateral advection from the open ocean in a narrow boundary current that flows cyclonically around the basin heat loss in the interior is offset by lateral eddy fluxes originating in the boundary current the objectives of this study are to understand better what controls the density of waters formed within the marginal sea the temperature of the outflowing waters the amount of downwelling and the mechanisms of heat transport within the marginal sea the approach combines heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system the theory compares well to a series of eddy resolving primitive equation model calculations the downwelling is concentrated within the boundary current in both a diffusive boundary layer near topography and an eddy driven region on the offshore edge of the boundary current for most high latitude regions the horizontal gyre is expected to transport more heat than does the overturning gyre

eocene volcanism above a depleted mantle slab window in southern alaska the caribou creek volcanic fi eld lies along the continent side edge of forearc basin rocks in south central alaska and consists of over 1000 m of shallow dipping basalt and andesite lavas with minor mafi c pyroclastic deposits dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas the basalts are tholeiitic and strongly depleted in the light rare earth elements la yb 0 18 1 5 with concentrations of high fi eld strength elements e g zr hf ti y similar to mid ocean ridge basalt and with variable enrichment in fl uid mobile elements e g cs ba and pb intermediate and felsic rocks show enrichment in the rare earth elements and fl uid mobile elements plus rb and k but retain low la yb ratios 0 48 3 6 a few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites e g sr y up to 52 ten 40 ar 39 ar ages for the caribou creek volcanic rocks range from 49 4 2 2 to 35 6 0 2 ma an adakite like tuff beneath the other volcanic rocks yields an age of 59 0 0 4 ma caribou creek basalts were derived from mid ocean ridge like depleted mantle that was emplaced beneath the southern margin of alaska through a slab window following spreading ridge subduction caribou creek volcanism was coeval with oblique subduction oroclinal bending and right lateral strike slip faulting in south central alaska all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas the basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of jurassic arc basement rocks to form the intermediate and felsic magmas enrichment of the basaltic parent magmas in fl uid mobile elements occurred by contamination from the jurassic arc rocks and or by contamination with metasomatic mantle remnant from preceding subduction high heat fl ow through the slab window induced partial melting of garnet bearing mafi c parts of the jurassic arc basement to form the adakite like rocks the caribou creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime e g crustal extension and that the infl uence of a slab window on continental margin magmatism can be long lived 20 m y

rapid onset of glacial conditions in the subarctic north pacific region at 2 67 ma clues to causality north pacific deep sea sediments recovered by ocean drilling leg 145 constrain the timing of the late pliocene major intensification of northern hemisphere continental glaciation a change in sediment character from nonglacial to glacial specifically abrupt increases in the amount of terrigenous clastic sediment and ice rafted debris occurred all across the north pacific at about 2 67 ma this change occurred very rapidly perhaps within 2000 yr too quickly to be a direct response to either tectonic or orbital forcing as the number and thickness of volcanic ash layers increased 10 fold at this same time we suggest that rapid intensification of glaciation may be associated with the widespread episode of explosive volcanism that began then

ocean gyres and abrupt change in the thermohaline circulation a conceptual analysis the implications are investigated of representing ocean gyre circulations by a diffusion term in the stommel and rooth box models of the thermohaline circulation thc in one and two hemispheres respectively the approach includes mostly analytical solution and study of the bifurcation structure but also numerical integration and feedback analysis sufficient diffusion gyre strength eliminates multiple equilibria from either model highlighting the need for accurate gyre circulation strength in general circulation models gcms when considering the potential for abrupt climate change associated with thc shutdown with diffusion steady state flow strength in the rooth model depends on freshwater forcing i e implied atmospheric water vapor transport in both hemispheres not only on that in the upwelling hemisphere as in the nondiffusive case with asymmetric freshwater forcing two solutions strong stable and weak unstable are found with sinking in the hemisphere with stronger forcing and one solution with sinking in the other hemisphere under increased freshwater forcing the two solutions in the hemisphere with stronger forcing meet in a saddle node bifurcation if diffusion is sufficiently strong to prevent a subcritical hopf bifurcation first followed by flow reversal thus the bifurcation structure with respect to freshwater forcing of the diffusive rooth model of two hemisphere thc is similar to that of the stommel model of single hemisphere thc albeit with a very different dynamical interpretation gyre circulations stabilize high latitude sinking in the stommel model in the rooth model gyre circulations only stabilize highlatitude sinking if the freshwater forcing is weaker in the sinking hemisphere than in the upwelling hemisphere by an amount that increases with diffusion the values of diffusion and freshwater forcing at which qualitative change in behavior occurs correspond to the range of the values used in and obtained with gcms suggesting that this analysis can provide a conceptual foundation for analyzing the stability of the interhemispheric thc and also for the potential of the atlantic thc to undergo abrupt change

palaeotectonic implications of increased late eocene early oligocene volcanism from south pacific dsdp sites late eocene early oligocene 42 35 myr sediments cored at two dsdp sites in the south west pacific contain evidence of a pronounced increase in local volcanic activity particularly in close association with the eocene oligocene boundary this pulse of volcanism is coeval with that in new zealand and resulted from the development of an indo australian pacific plate boundary through the region during the late eocene the late eocene earliest oligocene was marked by widespread volcanism and tectonism throughout the pacific and elsewhere and by one of the most important episodes of cenozoic climatic cooling

meridional overturning circulation stability and ocean feedbacks in a box model a box model of the inter hemispheric atlantic meridional overturning circulation is developed including a variable pycnocline depth for the tropical and subtropical regions the circulation is forced by winds over a periodic channel in the south and by freshwater forcing at the surface the model is aimed at investigating the ocean feedbacks related to perturbations in freshwater forcing from the atmosphere and to changes in freshwater transport in the ocean these feedbacks are closely connected with the stability properties of the meridional overturning circulation in particular in response to freshwater perturbations a separate box is used for representing the region north of the antarctic circumpolar current in the atlantic sector the density difference between this region and the north of the basin is then used for scaling the downwelling in the north these choices are essential for reproducing the sensitivity of the meridional overturning circulation observed in general circulation models and therefore suggest that the southernmost part of the atlantic ocean north of the drake passage is of fundamental importance for the stability of the meridional overturning circulation with this configuration the magnitude of the freshwater transport by the southern subtropical gyre strongly affects the response of the meridional overturning circulation to external forcing the role of the freshwater transport by the overturning circulation m ov as a stability indicator is discussed it is investigated under which conditions its sign at the latitude of the southern tip of africa can provide information on the existence of a second permanently shut down state of the overturning circulation in the box model m ov will be an adequate indicator of the existence of multiple equilibria only if salt advection feedback dominates over other processes in determining the response of the circulation to freshwater anomalies m ov is a perfect indicator if feedbacks other than salt advection are negligible

the atlantic freshwater budget as a diagnostic for the existence of a stable shut down of the meridional overturning circulation 1 which physical processes effectively determine the stability regime of the atlantic meridional overturning circulation moc is not yet fully understood we investigate the role of the oceanic freshwater transport into the basin employing a coupled model of intermediate complexity by modifying the longitudinal variation of surface salinities near the southern border at 33 s the amount of salt flowing out of the atlantic via the brazil current can be regulated in turn this will influence whether the moc exports or imports salt water the latter is associated with a basin scale salinity overturning feedback which can be either positive or negative pulse experiments strongly suggest that its sign determines the existence of a monostable or bistable regime in our model

a theory of the interhemispheric meridional overturning circulation and associated stratification abstracta quantitative theoretical model of the meridional overturning circulation and associated deep stratification in an interhemispheric single basin ocean with a circumpolar channel is presented the theory includes the effects of wind eddies and diapycnal mixing and predicts the deep stratification and overturning streamfunction in terms of the surface forcing and other parameters of the problem it relies on a matching among three regions the circumpolar channel at high southern latitudes a region of isopycnal outcrop at high northern latitudes and the ocean basin between the theory describes both the middepth and abyssal cells of a circulation representing north atlantic deep water and antarctic bottom water it suggests that although the strength of the middepth overturning cell is primarily set by the wind stress in the circumpolar channel middepth stratification results from a balance between the wind driven upwelling in the channel and deep water formation at high northern latitudes d

maceration of calcareous skeletons in the december issue of sedimentology 1979 26 two papers on the disintegration of calcareous skeletons appeared in both articles the authors alexandersson 1979 fitzgerald parmenter milliman 1979 describe skeletal degradation producing fragments and even single calcium carbonate crystallites in the present marine environment at least part of this maceration alexandersson 1978 is attributed to the breakdown of the skeletal organic matrix on the sea bottom both studies were carried out on samples from the sediment surface of marine cold water regions there another destructive process is active etching and dissolution of the calcareous particles by the ambient carbonate under saturated water e g alexandersson 1975 1978 the possible coexistence of these biological and physicochemical destructive processes and the lack of information from cores have left open questions in the above mentioned two studies some of these questions were discussed by me lewy 1975 my observations and conclusions may now have a wider significance

volcanic ash inputs enhance the deep sea seabed metal biogeochemical cycle a case study in the yap trench western pacific ocean abstract although volcanic ash deposition into oceans has been a frequent phenomenon for much of geologic history the potential effects on deep sea seabed metal biogeochemical cycles remain enigmatic in this study we analyzed mineral compositions microbial communities and metal related functional genes in a sediment core with volcanic ash layers from the yap trench in the western pacific ocean the mineralogical analysis showed that volcanic activity introduced large amounts of fe rich igneous minerals such as olivine pyroxene hornblende grossularite and biotite into the trench seabed sedimentary environments furthermore some differentiation in microbial communities was observed and potential fe metabolizing bacteria such as pseudomonas alteromonas marinobacter halomonas pseudoalteromonas and erythrobacter were dominant in the volcanic ash layers moreover compared with the background sediment metagenome the volcanic ash metagenomes were enriched in pathways for siderophore synthesis and uptake and metal resistant functional genes these results suggest that volcanic ash can lead to changes in trench sediment based microbial communities and stimulate microbial fe acquisition and other metal related metabolism in response microorganisms could promote decomposition of the metal rich volcanic minerals and metal release rates from them subsequently enhancing the deep sea metal biogeochemical cycle

timing constraints on building an intermediate plutonic arc crustal section u pb zircon geochronology of the sierra valle fertil la huerta famatinian arc argentina 1 the sierra valle fertil range in northwestern argentina exposes a tilted crustal section through the ordovician famatinian arc from 25 km to shallow crustal paleodepths fourteen new u pb zircon crystallization ages of magmatic rocks from sierra valle fertil area show that this section of the arc was built over a short time interval during the ordovician between 485 and 465 ma zircon rim ages demonstrate that high grade metamorphism and migmatization were synchronous with magmatic emplacement inherited ages in some of the plutonic rocks as well as detrital zircons in the metasedimentary framework suggest that the famatinian arc was emplaced into a thick miogeoclinal cover to the thinned margin of the proto south american continent in the ordovician which represents a part of gondwana docking of the precordilleran terrane outboard of proto south america led to the cessation of arc magmatism in the valle fertil area and preservation of the arc in its early stages after 60 myr of subduction during the pampean and famatinian magmatic stages our age data from the sierra valle fertil indicate that wet mafic magmas were emplaced into the section at a rate typical for modern island arcs 30 km3 km 1 myr 1 the production of intermediate to felsic magmas in the upper plate was a result of partial melting of the metasedimentary framework and hybridization with mantle derived melts and resulted in the generation of tonalites granodiorites and granites in the upper crust at a rate of 100 km3 km 1 myr 1 similar to major magmatic flare ups in the mature arcs of north american cordillera the sierra valle fertil arc section is a type example of an immature cordilleran arc before major crustal thickening and crustal overturn takes place in the upper plate the famatinian arc section may also be an equivalent to the crustal architecture of more mature modern island arcs such as parts of the aleutians caribbean and japan

precise timing of the late ordovician sandbian super eruptions and associated environmental biological and climatological events two of the largest known eruptions in the phanerozoic produced the ordovician millbrig k bentonite of north america and the kinnekulle k bentonite of scandinavia which have been previously suggested to be coeval the millbrig k bentonite from kentucky usa and the kinnekulle k bentonite from bornholm denmark yielded chemical abrasion thermal ionization mass spectrometry u pb zircon dates of 452 86 0 29 and 454 41 0 17 ma 2 analytical uncertainty respectively thus showing significant age differences contrary to what is generally held these data and four additional newly dated k bentonites directly establish the first radioisotopic age constraints for the ordovician katian sandbian global stage boundary refine global stratigraphic correlations date associated chemostratigraphic events and suggest an alternative volcanic climate hypothesis for the late ordovician supplementary material u pb radioisotopic data table and analytical methods are available at www geolsoc org uk sup18636

astronomically forced climate cooling across the eocene oligocene transition in the pearl river mouth basin northern south china sea abstract during the late eocene early oligocene major changes in the global climate and environment occurred including decreases in global temperature and the appearance of antarctic glaciation these changes marked the transition from early cenozoic greenhouse conditions to icehouse conditions research on this cooling trend in the atlantic and high latitude regions has increased but few reports have studied low latitude regions e g the northern south china sea and it remains unclear whether sediment records in these areas preserve signals related to this global cooling event in this study we studied records from a lacustrine well in the wenchang sag pearl river mouth basin prmb northern south china sea we conducted palynological and cyclostratigraphic analyses to reconstruct the depositional environment and assess the influence of orbital forcing the cyclostratigraphic analysis showed that depositional cycles including fining upwards and coarsening upwards sequences were controlled mainly by long eccentricity forcing the expansion and contraction of the lake system at 1 2 myr time intervals in addition changes in the pollen spore assemblage before and after the eocene oligocene e o transition show a dramatic shift from a moderately deep lake to a shallow lake with delta facies as recorded by the wenchang to enping formations in the wenchang sag prmb these findings suggest that the climate deteriorated in response to global cooling the amplification of obliquity forcing and the coevolution between obliquity power and global pco2 during the e o transition suggest that the low latitude sedimentary records from china contain a high latitude signal furthermore the low amplitudes of the precession obliquity and eccentricity cycles are coupled with the initial decreases in the global sea level and pco2 and a positive bias in the carbon oxygen isotopic compositions at 34 3 ma these results suggest that orbital forcing played a role in elevating pco2 across a critical threshold during the e o transition 33 9 ma

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

temporal variability of the atlantic meridional overturning circulation at 26 5 n the vigor of atlantic meridional overturning circulation moc is thought to be vulnerable to global warming but its short term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain we combine continuous measurements of the moc beginning in 2004 using the purposefully designed transatlantic rapid climate change array of moored instruments deployed along 26 5 n with time series of gulf stream transport and surface layer ekman transport to quantify its intra annual variability the year long average overturning is 18 7 5 6 sverdrups sv range 4 0 to 34 9 sv where 1 sv a flow of ocean water of 106 cubic meters per second interannual changes in the overturning can be monitored with a resolution of 1 5 sv

seawater nd isotope variation in the western pacific ocean since 80 ma odp 807 ontong java plateau abstract this study reports neodymium isotopic end variability at a time resolution of 0 5 to 3 ma since the late cretaceous as recorded in a marine sedimentary core from the western pacific odp 807 3 36 n 156 3 e ontong java plateau our core is mainly fine grained and composed of continuous sequences of nannofossil oozes the end measured in the carbonate fraction was used as a proxy of end of seawater of the western pacific on a long term our results indicate a general increase in end of seawater by 4 5 end units from the late cretaceous end 6 to modern times end 1 7 this pattern was related to the emergence of the west pacific margin and the progressive isolation of the pacific ocean from the other oceanic basins resulting in its progressive shift to more radiogenic values through the cenozoic this long term pattern is in accordance with previously published fe mn crusts data from the same study area nonetheless by being at higher time resolution our data records additional sharp and pseudo cyclic variations 7 11 ma periods superimposed on this long term pattern from 40 ma to modern times these oscillations might reflect the alternating dominance of the two main deep water masses npdw and ucdw bathing our study area in the same core we also measured the end in the detrital fraction in order to trace back the local terrigenous inputs the terrigenous record shows a significant variability up to 12 end units this was linked to the emergence of the west pacific subduction zone 50 ma ago causing a higher input of radiogenic isotopes in conclusion the large variability observed in both seawater and detrital end records most probably result from a major tectonic and oceanic circulation reorganization of the pacific ocean

revised age of aleutian island arc formation implies high rate of magma production radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the aleutian island arc provides insight into the timing of arc formation in the middle eocene twenty eight 40 ar 39 ar ages constrain the duration of arc magmatism to the last 46 m y basaltic lavas from the finger bay volcanics the oldest exposed rocks in the arc gave an isochron age of 37 4 0 6 ma which is 12 17 m y younger than a widely cited age of 55 50 ma three main pulses of arc wide magmatism occurred at 38 29 16 11 and 6 0 ma which coincide with periods of intense magmatism in other western pacific island arcs using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m y we calculate a time averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude