jebar bottom pressure torque and gulf stream separation abstract a diagnostic finite element barotropic ocean model has been used to simulate the mean circulation in the north atlantic with the inclusion of the joint effect of baroclinicity and relief jebar the gulf stream is found to separate at the correct latitude 35 n off cape hatteras results suggest that the jebar term in three key regions offshore of the separation point in the path of the main jet along the slope region of the north atlantic bight and in the central irminger sea is crucial in determining the separation point the transport driven by the bottom pressure torque component of jebar dominates the solution except in the subpolar gyre and is also responsible for the separation of the gulf stream excluding high latitudes in the deep water formation regions density variations in the upper 1000 m of the water column govern the generation of the necessary bottom pressure torque in our model examination of results from the world ocean circulation experiment community modelling

dynamics of decadal variability in the atlantic subpolar gyre a stochastically forced oscillator internal variability of the atlantic subpolar gyre is investigated in a 600 years control simulation of a comprehensive coupled climate model the subpolar gyre shows irregular oscillations of decadal time scale with most spectral power between 15 and 20 years positive and negative feedback mechanisms act successively on the circulation leading to an internal oscillation this involves periodically enhanced deep convection in the subpolar gyre center and intermittently enhanced air sea thermal coupling as a result anomalies of the large scale atmospheric circulation can be transferred to the ocean on the ocean s intrinsic time scale exciting the oscillator stochastically a detailed understanding of oscillatory mechanisms of the ocean and their sensitivity to atmospheric forcing holds considerable potential for decadal predictions as well as for the interpretation of proxy data records

decadal variability of subpolar gyre transport and its reverberation in the north atlantic overturning 1 analyses of sea surface height ssh records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the north atlantic subpolar gyre during the 1990s here we report hindcast simulations with high resolution ocean circulation models that demonstrate a close correspondence of the ssh changes with the volume transport of the boundary current system in the labrador sea the 1990s decline of about 15 of the long term mean appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the north atlantic oscillation nao the changes in the subpolar gyre as manifested in the deep western boundary current off labrador reverberate in the strength of the meridional overturning circulation moc in the subtropical north atlantic suggesting the potential of a subpolar transport index as an element of a moc monitoring system

combined influences of seasonal east atlantic pattern and north atlantic oscillation to excite atlantic multidecadal variability in a climate model the physical processes underlying the internal component of the atlantic multidecadal variability amv are investigated from a 1 000 yr pre industrial control simulation of the cnrm cm5 model the low frequency fluctuations of the atlantic meridional overturning circulation amoc are shown to be the main precursor for the model amv the full life cycle of amoc amv events relies on a complex time evolving relationship with both north atlantic oscillation nao and east atlantic pattern eap that must be considered from a seasonal perspective in order to isolate their action the ocean is responsible for setting the multidecadal timescale of the fluctuations amoc rise leading to a warm phase of amv is statistically preceded by wintertime nao and eap from lag 40 20 yrs associated wind stress anomalies induce an acceleration of the subpolar gyre spg and enhanced northward transport of warm and saline subtropical water concurrent positive salinity anomalies occur in the greenland iceland norwegian seas in link to local sea ice decline those are advected by the eastern greenland current to the labrador sea participating to the progressive densification of the spg and the intensification of ocean deep convection leading to amoc strengthening from lag 10 yrs prior an amoc maximum opposite relationship is found with the nao for both summer and winter seasons despite negative lags nao at that time is consistent with the atmospheric response through teleconnection to the northward shift intensification of the inter tropical convergence zone in link to the ongoing warming of tropical north atlantic basin due to amoc rise amv build up nao acts as a positive feedback for the full development of the model amv through surface fluxes but at the same time prepares its termination through negative retroaction on amoc relationship between eap and amoc is also present in summer from lags 30 10 yrs while winter eap is favored around the amv peak based on additional atmospheric forced experiments both are interpreted as the local seasonal dependent atmospheric response to warmer north atlantic finally advection of fresher water from the tropical basin created by local atmosphere ocean anomalous circulation on one hand and from the arctic on the other hand due to large scale sea ice melting leads to decrease of density in the spg and contributes terminating the model amoc amv events all together the combined effects of nao and eap their intertwined seasonal forcing forced role upon by the ocean and the primary role of salinity anomalies associated with oceanic dynamical changes acting as an integrator are responsible in cnrm cm5 for an irregular and damped mode of variability for amoc amv that takes about 35 40 15 20 years to build up dissipate

constraints on magma genesis behind the neogene cascade arc evidence from major and trace element variation of high alumina and tholeiitic volcanics of the bear creek area lava flows and dikes from the bear creek area within the neogene blue mountains province bmp were erupted in an extensional tectonic environment major and trace element variations in the bear creek suite constrain models for the petrogenesis of these rocks bear creek basalts basaltic andesites and andesites fall into two compositional groupings high alumina and tholeiitic the most primitive flow is high alumina in composition with an mg 0 69 various phase equilibria tests suggest that magma of this composition may be a hybrid of an eclogite derived melt and a peridotite contaminant or it could be a melt derived wholly from a peridotite source large ion lithophile element enrichment and high field strength element depletion especially nb in this sample are consistent with its derivation from a fluid metasomatized peridotite source an alternative source is required for three bear creek tholeiites and high alumina basalts habs that differ from the other bear creek samples primarily in having larger nb contents relative to other incompatible trace elements this source can be modeled as partial melts of a peridotite which has been contaminated by a pelagic sediment presumably from the subducted slab least squares mixing calculations with high pressure mineral phases from peridotites indicate that high pressure fractionation may have generated the more primitive high alumina types in the bear creek suite however using the computer program silmin ghiorso 1985 to semiquantitatively model shallow magmatic processes demonstrates that assimilation of greywacke and rhyolite as a partial melt of the greywacke and fractional crystallization afc at 1 kbar pressure better explain the major element variation in the high alumina types the tholeiites appear to be derived from high alumina parents via fractional crystallization at shallower levels in the crust semiquantitative modeling using trace element distributions supports a model in which the hab suite was derived from peridotite and underwent afc in the upper crust and the tholeiites were derived via rayleigh fractionation from hab parents the trace element modeling also indicates that the range of trace element abundances cannot be explained by mixing of melts derived from eclogite and peridotite therefore a model in which dehydration of the farallon plate metasomatizes mantle wedge peridotite and where pelagic sediment is incorporated into the peridotite which subsequently melt to yield magmas that are modified by contamination and fractionation in the upper crust explains all of the geochemical variation in the bear creek area rocks this model may in part be applicable to the origin of the picture gorge and slide creek volcanic sequences in the bmp

a petrologic re investigation of the adak volcanic center central aleutian arc alaska the adak volcanic center is located in the central part of the aleutian arc and consists of three main volcanic vents andrew bay volcano the oldest center has been mostly removed by erosion the next youngest vent mount adagdak was built in three major volcanic stages whereas mount moffett the largest volcanic edifice consists of a main cone and a parasitic cone each with several magmatic phases adak is unique compared to other modern aleutian volcanic centers in that it contains two xenolith suites conrad and kay 1984 debari et al 1987 one suite consisting predominantly of mafic xenoliths occurs on mount moffett whereas an assemblage of ultramafic and mafic xenoliths is found on mount adagdak lavas erupted at adak span the compositional range from 48 4 to 65 0 wt sio2 and are characterized by significant variations in al2o3 mgo sr ni and cr on harker diagrams this variability produces compositional trends with significant scatter the adak suite has total ree contents that vary from 32 to 154 ppm but do not correlate systematically with silica layb n ratios range from 2 41 to 21 72 with the majority of lavas between 2 41 and 6 06 on process identification diagrams the adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other aleutian centers measured isotopic ranges are large and nearly equal to those for the entire arc although they span similar silica ranges subtle geochemical and isotopic differences distinguish the different volcanic vents of adak on mount moffett a geochemically and isotopically distinct group of andesites 55 5 57 9 sio2 the mafic andesites occur on its ne flank these lavas have elevated mgo ni and cr but are depleted in al2o3 relative to other mount moffett andesites with similar silica they also have more heterogeneous ree abundances and isotopic ratios than most of the other andesites significant compositional differences exist between adak and the other volcanic centers of the central aleutian arc although these differences are characteristic of all geochemical systems they are greatest for major and rare earth elements and isotopic ratios the lack of coherent relationships on major and trace element harker diagrams the isotopic variability as well as the steeply sloping trends on ree process identification diagrams suggest that the adak volcanic suite was not formed predominantly by closed system crystal fractionation but must be the product of a complex open system process es the significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite ree data are also suggestive of a role for magma mixing such a complex petrologic evolution is consistent with an immature lithospheric plumbing system based on ree systematics the xenolith suites of adak cannot as previously proposed be related to the host lavas or the rest of the adak suite through crystal fractionation schemes rather they are probably accidental fragments derived from various depths along lithospheric conduits in light of their relation to xenolith bearing units the mafic andesites of adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock they are therefore characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas because of the close proximity of the vents and the subtle compositional differences between their lavas the adak volcanic center was probably supplied by a single deep lithospheric plumbing system that fed separate crustal magma chambers the absence of historic volcanic activity on adak suggests this plumbing system was abandoned before complete conduit development this decline in magmatism may reflect a re adjustment of volcano spacing within this part of the aleutian arc

on the role of wind driven sea ice motion on ocean ventilation abstract simulations with a coupled ocean atmosphere sea ice model are used to investigate the role of wind driven sea ice motion on ocean ventilation two model experiments are analyzed in detail one including and the other excluding wind driven sea ice transport model simulated concentrations of chlorofluorocarbons cfcs are compared with observations from the weddell sea the southeastern pacific and the north atlantic the authors show that the buoyancy fluxes associated with sea ice divergence control the sites and rates of deep and intermediate water formation in the southern ocean divergence of sea ice along the antarctic perimeter facilitates bottom water formation in the weddell and ross seas neglecting wind driven sea ice transport results in unrealistic bottom water formation in drake passage and too strong convection along the southern ocean sea ice margin whereas convection in the weddell and ross seas is suppressed the freshwater fluxes implicitly associated with sea ice export also

transition from mildly tholeiitic to calc alkaline suite the case of chichontepec volcanic centre el salvador central america abstract the chichontepec volcano is a plio pleistocene composite volcano that erupted lavas ranging from high alumina basalts to dacites it experienced a caldera forming paroxysmal eruption during the early pleistocene pre caldera lavas are mildly tholeiitic and they evolved mainly by low pressure crystal fractionation notwithstanding the fact that most mafic lavas low mgo high alumina basalts retain traces of polybaric evolution conversely post caldera lavas which are mainly pyroxene andesites are clearly calc alkaline having evolved by open system crystal fractionation sr nd isotopic data and trace elements characteristics indicate that the same mantle source was involved in the petrogenesis of these series modelling the afc process showed that it did not play any role in the petrogenesis of these rocks a crystal fractionation model is considered to be more relevant a slight variation in the fractionating assemblage could have caused the transition from an early mildly tholeiitic trend to a late calc alkaline one mineralogical evidence mass balance calculations and elemental chemistry support this hypothesis assuming that the greater amount of pyroxene on the liquidus is at the expense of plagioclase this would have prevented the trend in iron enrichment

fe mn nodules in a southern indiana loess with a fragipan and their soil forming significance abstract little is known about how fe mn nodules vary in relation to pedogenetic horizons in soils derived from loess in this investigation nodules were collected according to soil genetic horizons from a fragiudalf in loess at southeast purdue agricultural center sepac in indiana and physical chemical micro and macromorphological features of nodules and their soil matrix were determined as a function of soil depth the quantity chemical composition and micromorphology of the nodules varied with genetic horizons the ap horizon was found to contain the largest amount by weight of nodules 25 43 g kg and this was likely due to soil erosion after the ap horizon the 2btx1 and 2btx2 horizons had the greatest content of fe mn nodules by weight nodules had lower concentrations of sio2 al2o3 k2o tio2 mgo na2o and cao in contrast fe2o3 mno and p2o5 in nodules showed clearly greater concentrations than that in the corresponding soil material the sio2 content decreased with nodule size whereas fe2o3 and mno increased the least depletion of sio2 in nodules of 1 2 mm n2 and 0 5 1 mm n1 was in the bt2 horizon and was greatest in the 2btx2 the al2o3 had accumulated in the n2 and n1 nodules within the ap horizon and in the n2 within the 2btx2 while other horizons were depleted of al2o3 brown nodules had a small mn fe ratio 1 the internal morphology of the nodules also varied with mn fe ratios the fungal hyphae or bacteria resulted in honeycomb cell cone or dendritic structures the cell and cone morphology required large mn fe ratios low ph and high content of ald the scanning electron microscopy sem analysis illustrated an elemental distribution trend such that the mn was centralized in the nodule internal structure whereas fe was concentrated towards the exterior the band structure of nodules likely formed in response to seasonal wetting and drying as an accretionary process three basic formation stages for a well developed nodule can be inferred which correspond to different soil forming environments 1 fe enrichment for nucleus formation in wet soil 2 mn enrichment in an intense dry soil and 3 fe banding enrichment in a slightly dry soil nodule formation occurred synchronously with soil pedogenesis and can therefore be used as a proxy for understanding soil pedogenesis and the environment under which it occurred

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

the 8 2 ka event abrupt transition of the subpolar gyre toward a modern north atlantic circulation 1 climate model simulations of the 8 2 ka event show an abrupt strengthening of the atlantic subpolar gyre that allows us to connect two major but apparently contradictory climate events of the early holocene the freshwater outburst from proglacial lakes and the onset of labrador sea water formation the 8 2 ka event is the largest climatic signal of our present interglacial with a widespread cooling in the north atlantic region about 8200 years before present it coincides with a meltwater outburst from north american proglacial lakes that is believed to have weakened the atlantic meridional overturning circulation and northward heat transport followed by a recovery of the deep ocean circulation and rising temperatures after a few centuries marine proxy data however date the onset of deep water formation in labrador sea to the same time the subsequent strengthening of the slope current system created a regional signal recorded as an abrupt and persistent surface temperature decrease although similarities in timing are compelling a mechanism to reconcile these apparently contradictory events was missing our simulations show that an abrupt and persistent strengthening of the atlantic subpolar gyre provides a plausible explanation the intense freshwater pulse triggered a transition of the gyre circulation into a different mode of operation stabilized by internal feedbacks and persistent after the cessation of the perturbation as a direct consequence deep water formation around its center intensifies this corresponds to the modern flow regime and stabilizes the meridional overturning circulation possibly contributing to the holocene s climatic stability

a 20 year coupled ocean sea ice atmosphere variability mode in the north atlantic in an aogcm in order to understand potential predictability of the ocean and climate at the decadal time scales it is crucial to improve our understanding of internal variability at this time scale here we describe a 20 year mode of variability found in the north atlantic in a 1 000 year pre industrial simulation of the ipsl cm5a lr climate model this mode involves the propagation of near surface temperature and salinity anomalies along the southern branch of the subpolar gyre leading to anomalous sea ice melting in the nordic seas which then forces sea level pressure anomalies through anomalous surface atmospheric temperatures the wind stress associated to this atmospheric structure influences the strength of the east greenland current across the denmark strait which in turn induces near surface temperature and salinity anomalies of opposite sign at the entrance of the labrador sea this starts the second half of the cycle after approximatively 10 years the time scale of the cycle is thus essentially set by advection of tracers along the southern branch of the subpolar gyre and by the time needed for anomalous east greenland current to accumulate heat and freshwater anomalies at the entrance of the labrador sea the atlantic meridional overturning circulation amoc does not play a dominant role in the mode that is confined in the subpolar north atlantic but it also has a 20 year preferred timescale this is due to the influence of the propagating salinity anomalies on the oceanic deep convection the existence of this preferred timescale has important implications in terms of potential predictability of the north atlantic climate in the model although its realism remains questionable and is discussed

an evaluation of geochemical weathering indices in loess paleosol studies abstract applying geochemical proxies as measure for the weathering intensity of paleosols and sediments such as loess the quaternary scientist is confronted with various element ratios that have been proposed in literature this paper gives an overview on the principle of geochemical weathering indices different types of indices are evaluated with respect to the suitability for loess paleosol sequences regarding the special characteristics of this type of sediments and paleosols case examples in this study are key sections in southeastern and eastern europe the loess paleosol sequences batajnica stari slankamen serbia mircea voda romania and stary kaydaky ukraine which represent archives of the late and mid pleistocene climate change of the region considering element behavior during weathering or diagenesis the chemical proxy of alteration cpa i e the molar ratio al2o3 al2o3 na2o 100 is proposed as the most appropriate index for silicate weathering the cpa was evaluated against commonly used weathering indices including the chemical index of alteration cia the chemical index of weathering ciw the plagioclase index of alteration pia the index b of kronberg and nesbitt and the ba sr and rb sr ratio depth profiles of sr type indices e g ba sr rb sr are likely to be influenced by the dynamics of secondary carbonate on the other hand common na type indices e g cia pia ciw may suffer from uncertainties in separating carbonate ca from silicate ca or from biases due to k fixation illitization the cpa is insensitive against such effects additionally using the cpa as with other na type indices provides the possibility to evaluate the homogeneity of the parent material regarding the relevant host minerals via the a cn k diagram

the dependence of southern ocean meridional overturning on wind stress an eddy resolving numerical model of a zonal flow meant to resemble the antarctic circumpolar current is described andanalyzedusing the framework of j marshallandt radko in additionto windand buoyancy forcing at the surface the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation moc to exist at depth the strength of the residual moc is diagnosed for different strengths of surface wind stress it is found that the eddy circulation largely compensates for the changes in ekman circulation the extent of the compensation and thus the sensitivity of the moc to the winds depend on the surface boundary condition a fixed heat flux surface boundary severely limits the ability of the moc to change an interactive heat flux leads to greater sensitivity to explain the moc sensitivity to the wind strength under the interactive heat flux transformed eulerian mean theory is applied in which the eddy diffusivity plays a central role in determining the eddy response a scaling theory for the eddy diffusivity based on the mechanical energy balance is developed and tested the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress the moc sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments

reversed north atlantic gyre dynamics in present and glacial climates the dynamics of the north atlantic subpolar gyre spg are assessed under present and glacial boundary conditions by investigating the spg sensitivity to surface wind stress changes in a coupled climate model to this end the gyre transport is decomposed in ekman thermohaline and bottom transports surface wind stress variations are found to play an important indirect role in spg dynamics through their effect on water mass densities our results suggest the existence of two dynamically distinct regimes of the spg depending on the absence or presence of deep water formation dwf in the nordic seas and a vigorous greenland scotland ridge gsr overflow in the first regime the gsr overflow is weak and the spg strength increases with wind stress as a result of enhanced outcropping of isopycnals in the centre of the spg as soon as a vigorous gsr overflow is established its associated positive density anomalies on the southern gsr slope reduce the spg strength this has implications for past glacial abrupt climate changes insofar as these can be explained through latitudinal shifts in north atlantic dwf sites and strengthening of the north atlantic current regardless of the ultimate trigger an abrupt shift of dwf into the nordic seas could result both in a drastic reduction of the spg strength and a sudden reversal in its sensitivity to wind stress variations our results could provide insight into changes in the horizontal ocean circulation during abrupt glacial climate changes which have been largely neglected up to now in model studies

bistability of the atlantic subpolar gyre in a coarse resolution climate model 1 high resolution models indicate that the dynamics of the nordic seas and the subpolar gyre is crucial for deep water formation and overturning circulation in the atlantic during the last decades significant changes in ocean properties have been observed in this region we show that large scale dynamics as captured by coarse resolution climate models allows for a bistability of the atlantic subpolar gyre we suggest three positive feedbacks which yield the necessary nonlinearity for this behavior transitions between the two states can be triggered by small fluctuations in surface freshwater flux to the nordic seas only slightly larger than natural variability presently observed in the region

sea ice induced changes in ocean circulation during the eemian we argue that arctic sea ice played an important role during early stages of the last glacial inception two simulations of the institut pierre simon laplace coupled model 4 are analyzed one for the time of maximum high latitude summer insolation during the last interglacial the eemian and a second one for the subsequent summer insolation minimum at the last glacial inception during the inception increased arctic freshwater export by sea ice shuts down labrador sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15 respectively a positive feedback of the atlantic subpolar gyre enhances the initial freshening by sea ice the reorganization of the subpolar surface circulation however makes the atlantic inflow more saline and thereby maintains deep convection in the nordic seas these results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes

decline of subpolar north atlantic circulation during the 1990s observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid to high latitude north atlantic ocean topex poseidon altimeter data show that subpolar sea surface height increased during the 1990s and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation combining the data from earlier satellites we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s direct current meter observations in the boundary current of the labrador sea support the weakening circulation trend of the 1990s and together with hydrographic data show that the mid to late 1990s decline extends deep in the water column analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations a weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation

aleutian tholeiitic and calc alkaline magma series i the mafic phenocrysts diagnostic mafic silicate assemblages in a continuous spectrum of aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the aleutian arc crust tectonic segmentation of the arc exerts a primary control on the variable mixing fractional crystallization and possible assimilation undergone by the magmas end members of the continuum are termed calc alkaline ca and tholeiitic th ca volcanic rocks e g buldir and moffett volcanoes have low feo mgo ratios and contain compositionally diverse phenocryst populations indicating magma mixing their ni and cr rich magnesian olivine and clinopyroxene come from mantle derived mafic olivine basalts that have mixed with more fractionated magmas at mid to lower crustal levels immediately preceding eruption high al amphibole is associated with the mafic end member in contrast th lavas e g okmok and westdahl volcanoes have high feo mgo ratios and contain little evidence for mixing evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma these lavas contain groundmass olivine fo 40 50 and lack ca poor pyroxene aleutian volcanic rocks with intermediate feo mgo ratios are termed transitional tholeiitic tth and calc alkaline tca tca magmas are common e g moffett adagdak great sitkin and kasatochi volcanoes and have resulted from mixing of high al basalt with more evolved magmas they contain amphibole high and low al or orthopyroxene or both and are similar to the japanese hypersthene series tth magmas e g okmok and westdahl contain orthopyroxene or pigeonite or both and show some indication of upper crustal mixing they are mineralogically similar to the japanese pigeonite series high al basalt lacks mg rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite the low pressure mineral assemblage of high al basalt results from crystallization at higher crustal levels

formation and pathways of north atlantic deep water in a coupled ice ocean model of the arctic north atlantic oceans we investigate the formation process and pathways of deep water masses in a coupled ice ocean model of the arctic and north atlantic oceans the intent is to determine the relative roles of these water masses from the different source regions arctic ocean nordic seas and subpolar atlantic in the meridional overturning circulation the model exhibits significant decadal variability in the deep western boundary current and the overturning circulation we use detailed diagnostics to understand the process of water mass formation in the model and the resulting effects on the north atlantic overturning circulation particular emphasis is given to the multiple sources of north atlantic deep water the dominant deep water masses of the world ocean the correct balance of labrador sea greenland sea and norwegian sea sources is difficult to achieve in climate models owing to small scale sinking and convection processes the global overturning circulation is described as a function of potential temperature and salinity which more clearly signifies dynamical processes and clarifies resolution problems inherent to the high latitude oceans we find that fluxes of deep water masses through various passages in the model are higher than observed estimates despite the excessive volume flux the nordic seas overflow waters are diluted by strong mixing and enter the labrador sea at a lighter density through strong subpolar convection these waters along with other north atlantic water masses are converted into the densest waters similar density to antarctic bottom water aabw in the north atlantic we describe the diminished role of salinity in the labrador sea where a shortage of buoyant surface water or excess of high salinity water leads to overly strong convection the result is that the atlantic overturning circulation in the model is very sensitive to the surface heat flux in the labrador sea and hence is correlated with the north atlantic oscillation as strong subpolar convection is found in other models we discuss broader implications

andesites from northeastern kanaga island aleutians kanaga island is located in the central aleutian island arc northeastern kanaga is a currently active late tertiary to recent calc alkaline volcanic complex basaltic andesite to andesite lavas record three episodes series of volcanic activity series i and series ii lavas are all andesite while series iii lavas are basaltic andesite to andesite four series ii andesites contain abundant quenched magmatic inclusions ranging in composition from high mgo low alumina basalt to low mgo highalumina basalt the spectrum of lava compositions is due primarily to fractional crystallization of a parental low mgo high alumina basalt but with variable degrees of crustal contamination and magma mixing the earliest series i lavas represent mixing between high alumina basalt and silicic andesite with maximum sio2 contents of 65 67 wt later series i and all series ii lavas are due to mixing of andesite magmas of similar composition the maximum sio2 content of the pre mixed andesites magmas is estimated at 60 63 wt the youngest lavas series iii are all non mixed and have maximum estimated sio2 contents of 59 wt the earliest series i lavas contain a significant crustal component while all later lavas do not it is concluded that the maximum sio2 contents of silicic magmas the contribution of crustal material to silicic magma generation and the role of magma mixing all decrease with time furthermore silicic magmas generated by fractional crystallization at this volcanic center have a maximum sio2 content of 63 wt all of these features have also been documented at the central aleutian cold bay volcanic center brophy 1987 based on data from these two centers a model of aleutian calc alkaline magma chamber development is proposed the main features are 1 a single low pressure magma chamber is continuously supplied by primitive low alumina basalt 2 non primary high alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase 3 sidewall crystallization accompanied by crustal melting produces buoyant silicic 63 wt sio2 liquids that pond at the top of the chamber and 4 continued sidewall crystallization now isolated from the chamber wall produces silicic liquids with 63 wt sio2 that increase the thickness and lowers the overall sio2 content of the upper silicic zone it is suggested that the maximum sio2 content of 63 imposed on fractionation generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal liquid mush along the chamber wall

simultaneous evolution of gyre and atlantic meridional overturning circulation anomalies as an eigenmode of the north atlantic system abstractthe authors identify an interdecadal oscillatory mode of the north atlantic atmosphere ocean system in a general circulation model gfdl cm2 1 via a linear inverse model lim the oscillation mechanism is mostly embedded in the subpolar gyre anomalous advection generates density anomalies in the eastern subpolar gyre which propagate along the mean gyre circulation and reach the subpolar gyre center around 10 years later when associated anomalous advection of the opposite sign starts the other half cycle as density anomalies reach the labrador sea deep convection region atlantic meridional overturning circulation amoc anomalies are also induced both the gyre and amoc anomalies then propagate equatorward slowly following the advection of density anomalies the oscillation is further demonstrated to be more likely an ocean only mode while excited by the atmospheric forcing in particular it can be approximated as a linearly driven damped oscillator that is partly excited by the north atla

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

surface wind stress threshold for glacial atlantic overturning 1 using a coupled model of intermediate complexity the sensitivity of the last glacial maximum lgm atlantic meridional overturning circulation amoc to the strength of surface wind stress is investigated a threshold is found below which north atlantic deep water formation dwf takes place south of greenland and the amoc is relatively weak above this threshold dwf occurs north of the greenland scotland ridge leading to a vigorous amoc this nonlinear behavior is explained through enhanced salt transport by the wind driven gyre circulation and the overturning itself both pattern and magnitude of the nordic sea s temperature difference between strong and weak amoc states are consistent with those reconstructed for abrupt climate changes of the last glacial period our results thus point to a potentially relevant role of surface winds in these phenomena

loess is not just the accumulation of dust most regrettably there is much misuse of terminology in connection with the concept of loess and the interpretation of its origin people often speak and write incorrectly about loess accumulation even loess specialists who are aware of the fact that loess as a concept has to satisfy at least 10 criteria loess is not simply dust carried and deposited by wind dust only becomes loess after the passage of a certain amount of time in a given geographical zone i e only through diagenesis in certain ecological environments to state that loess is of aeolian origin is an oversimplification and an incorrect definition because an aeolian origin applies only to the dust from which the loess has been formed we are aware of the fact that airborne dust cannot be transformed into loess in every geographical environment but only under those conditions typical of semi arid grassland or steppe or forested steppe the process primarily occurs where the rate of dust accumulation exceeds sheet wash or weathering soil formation rates if the rate of dust accumulation is less than that of surface erosion or of soil biogenetic processes the dust then develops into soil or through intensive weathering and increased precipitation into loam or clay only part of the aeolian dust transported and deposited in a zone suitable for loess formation remains there permanently and is transformed in situ into loess dust not affected by diagenesis is usually further transported by snowmelt or rainwash and is only transformed into loess after it has been redeposited the loess itself however is easily erodible and its minerals are readily reworked and reaccumulated and given the appropriate conditions it readily undergoes diagenesis again we do not always have sufficient evidence to determine whether a given loess body is of primary or secondary origin traditionally the minerals making up primary loess have been regarded as originating in dust accumulated by aeolian processes secondary loess by contrast is different from typical loess in many ways and it is not unusual to find various loess series in which superimposed dust fractions have been transformed into loess by different processes recently we have observed that the origin of various types of loess is governed by differences in litho ecological conditions rather than by the way in which the mineral material from which the loess is derived dust has been primarily accumulated loessification is therefore determined by environmental conditions

mineralogy of fe mn nodules in albeluvisols in the carpathian foothills poland fe mn nodules are common components occurring in various soils and especially in soils exhibiting imperfect drainage the formation distribution chemical composition size and shape of fe mn nodules are the subject of many studies and the research literature contains a lot of information concerning these issues on the other hand data regarding the mineral composition of fe mn nodules in the soil environment are limited to only a few studies the main aim of the present study was to determine the mineral composition of the fe mn nodules found in albeluvisols in the carpathian foothills in poland using quantitative x ray diffraction qxrd fourier transform infrared spectroscopy ftir and mossbauer spectroscopy ms the obtained results show that fe mn nodules from albeluvisols in the carpathian foothills in poland are composed mainly of silicates quartz k feldspars plagioclases and phyllosilicates dioctahedral mica smectite chlorite and kaolinite which are impregnated with iron and manganese oxides iron minerals are represented by goethite and probably lepidocrocite both iron minerals are characterized by very small particles and poor crystallinity goethite occurring in all of the studied fe mn nodules clearly prevails over lepidocrocite which exhibits a higher concentration only in nodules from the lower part fragipan of the soil profile located on a footslope and showing the strongest stagnic color pattern manganite and todorokite represent manganese minerals however todorokite was observed only in nodules obtained from illuvial horizons btx btg of the studied albeluvisols the fe mn nodules obtained from eluvial horizons eg show higher amounts of silicates quartz k feldspars plagioclases than the nodules obtained from illuvial horizons on the other hand fe mn nodules obtained from illuvial horizons fragipan and argillic exhibit higher amounts of phyllosilicates i e clay minerals in comparison with the eluvial horizon the differences in the quantitative mineral composition of the fe mn nodules between eluvial and illuvial horizons are related to the translocation of clay minerals from upper soil horizons into the lower part of the soil profiles this indicates that the nodules from e horizons were formed after translocation of major portion of clay fraction down the soil profile the similar mineral composition of the fe mn nodules and bulk soil material indicates that the nodules were formed in situ because of cyclic reduction and oxidation of fe and mn oxides

late eemian warming in the nordic seas as seen in proxy data and climate models 1 we analyze a transient simulation of the last glacial inception in a climate model of intermediate complexity focusing on sea ice ocean circulation dynamics in the north atlantic and nordic seas as northern high latitude summer insolation decreases toward the end of the eemian interglacial arctic sea ice export to the north atlantic increases this surface fresh water transport weakens deep water formation in the north atlantic and the near surface circulation of the subpolar gyre as a consequence the relative contribution of subpolar gyre waters to the atlantic inflow into the nordic seas is reduced giving way to more warm and saline subtropical waters from the north atlantic current we thus find an episode of relatively high heat and salt transport into the nordic seas during the last glacial inception between 119 000 and 115 000 years before present this stabilizes deep ocean convection in the region and warms scandinavia during a phase of low insolation these findings are in good agreement with proxy data from the nordic seas and north atlantic at the end of the warm interval sea surface temperature drops by about 3 c marking the onset of large scale glacier growth over scandinavia

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

multiple equilibria as a possible mechanism for decadal variability in the north atlantic ocean decadal climate variability in the north atlantic has received increased attention in recent years because modeling results suggest predictability of heat content and circulation indices several years ahead however determining the applicability of these results in the real world is challenging because of an incomplete understanding of the underlying mechanisms here the authors show that recent attempts to reconstruct the decadal variations in one of the dominant circulation systems of the region the subpolar gyre spg are not always consistent a coherent picture is partly recovered by a simple conceptual model solely forced by reanalyzed surface air temperatures this confirms that surface heat flux indeed plays a leading role for this typeofvariability ashasbeensuggestedinpreviousstudies theresultsfurthersuggestthatlargevariationsin the spg correspond to the crossing of a bifurcation point that is predicted from idealized experiments and an analytical solution of the model used herein performance of this conceptual model is tested against a statistical stochastic model hysteresis and the existence of two stable modes of the spg circulation shape its response to forcing by atmospheric temperatures the identification of the essential dynamics and the reduction to a minimal model of spg variability provide a quantifiable basis and a framework for future studies on decadal climate variability and predictability

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

the obduction of equatorial 13 c water in the pacific identified by a simulated passive tracer the obduction of equatorial 13 degrees c water in the pacific is investigated using a simulated passive tracer of the consortium for estimating the circulation and climate of the ocean ecco the result shows that the 13 degrees c water initialized in the region 8 degrees n 8 degrees s 130 degrees 90 degrees w enters the surface mixed layer in the eastern tropical pacific mainly through upwelling near the equator in the costa rica dome and along the coast of peru approximately two thirds of this obduction occurs within 10 years after the 13 degrees c water being initialized with the upper portion of the water mass reaching the surface mixed layer in only about a month the obduction of the 13 degrees c water helps to maintain a cool sea surface temperature year round equivalent to a surface heat flux of about 6 0 w m 2 averaged over the eastern tropical pacific 15 degrees s 15 degrees n 130 degrees w eastern boundary for the period of integration 1993 2006 during el nino years when the thermocline deepens as a consequence of the easterly wind weakening the obduction of the 13 degrees c water is suppressed and the reduced vertical entrainment generates a warming anomaly of up to 10 w m 2 in the eastern tropical pacific and in particular along the coast of peru providing explanations for the warming of sea surface temperature that cannot be accounted for by local winds alone the situation is reversed during la nina years

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

dilution of the northern north atlantic ocean in recent decades declining salinities signify that large amounts of fresh water have been added to the northern north atlantic ocean since the mid 1960s we estimate that the nordic seas and subpolar basins were diluted by an extra 19 000 5000 cubic kilometers of freshwater input between 1965 and 1995 fully half of that additional fresh water about 10 000 cubic kilometers infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year patterns of freshwater accumulation observed in the nordic seas suggest a century time scale to reach freshening thresholds critical to that portion of the atlantic meridional overturning circulation

rapid switches in subpolar north atlantic hydrography and climate during the last interglacial mis 5e plateauing just below early mis 5e values a planktonic d 18 o minimum during the cooling event indicates that marked freshening of the surface waters accompanied the cooling we suggest that switches in the subpolar gyre hydrography occurred during a warmer climate involving regional changes in freshwater fluxes balance and east greenland current influence in the study area the nature of these hydrographic transitions suggests that they are most likely related to large scale circulation dynamics potentially amplified by gis meltwater influences

mid pleistocene basalt from the seguam volcanic center central aleutian arc alaska local lithospheric structures and source variability in the aleutian arc recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern aleutian arc and the sources and processes that produce the broad spectrum of aleutian basaltic lavas mid pleistocene basalt from the seguam island volcanic center in the central aleutian arc erupted through strongly extended arc crust contains 4 8 mgo is low in k2o 18 and is distinguished from aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements ree and low sr and pb content the ba la 47 72 and pb la 0 7 1 5 ratios are high incompatible element ratios are variable and ree patterns cross one another the 87sr 86sr ratio 0 70365 0 00003 is the highest value obtained from the aleutian islands also 206pb 204pb 18 91 0 02 207pb 204pb 15 60 0 02 and 208pb 204pb 38 51 0 04 ratios are among the most radiogenic values known whereas the 143nd 144nd ratio 0 51297 0 00003 is among the lowest measured low abundances of incompatible elements high ba la and pb la ratios uniform sr nd and pb isotope ratios and high eruptive temperatures 1160 c suggest that basalt erupted at seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics trace element abundances rule out an eclogite source and suggest instead a peridotite dominated source with 5 10 of a subducted sediment component this peridotite may reside in the mantle wedge or in the amlia fracture zone afz of the pacific plate that was subducted beneath seguam 1 ma in addition the afz possibly enhanced the subduction of terrigenous sediment into the source region beneath seguam and this extra terrigenous component may explain the ba la pb la and sr nd and pb isotope ratios the distinctive composition of seguam basalt reflects a complex set of physical conditions including fracture zone subduction and intraarc extension that affected both the source region and thermo chemical history of magma ascent local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the aleutian arc

atlantic salinity budget in response to northern and southern hemisphere ice sheet discharge the impact of an idealised scenario of future mass release of major ice sheets on the atlantic ocean is studied a freshwater forcing is applied to the ocean surface in a coupled climate model forced in accordance with a high end future climate projection for mass loss from the greenland and antarctic ice sheet together with the rcp8 5 emission scenario the added freshwater dilutes the entire ocean by increasing total volume but changes in freshwater budget are non linear in time especially in the atlantic ocean in the atlantic the initial dilution mainly comes from greenland freshwater but the increase in mass is counteracted by the mass flux across the boundaries of the atlantic with the outflow into the southern ocean becoming larger than the inflow through bering strait associated with this mass divergence salt is exported to the southern ocean by the barotropic flow further freshening is associated with more freshwater import by the atlantic meridional overturning circulation across the southern boundary of the atlantic also the subtropical gyre exports salt and imports freshwater across the atlantic s southern boundary especially when freshwater from the antarctic ice sheet arrives at the boundary of the basin it appears that the response to northern hemisphere nh sources the greenland ice sheet and southern hemisphere sh sources the antarctic ice sheet are opposite in the case of nh only freshwater forcing sea surface height ssh increases in the arctic causing a reduction of the ssh gradient over the bering strait and hence the barotropic throughflow across the arctic atlantic basin reduces in case of sh only freshwater forcing ssh increases in the pacific enhancing the barotropic throughflow in the arctic atlantic when both nh and sh freshwater forcings are present the response in the atlantic is dominated by nh forcing changes in overturning transport to either nh or sh forcing counteract the response to changes in barotropic transport these changes are not due to volume transport but mainly due to salinity changes in particular across the southern boundary of the atlantic only when both sh and nh freshwater forcing are present changes in barotropic transport and overturning transport reinforce each other the barotropic transport more strongly reacts to nh forcing while the overturning transport reacts more strongly to sh forcing

last interglacial mis5e hydrographic shifts linked to meltwater discharges from the east greenland margin abstract proximal evidence of the surface ocean response to size reduction of the greenland ice sheet gis during the last interglacial mis5e and preceding glacial termination t2 remains largely elusive using a new sediment record from the western iceland sea the behavior of the northeastern gis and its relation to the subpolar north atlantic surface hydrography is examined extremely light oxygen isotopic 18o values are found off central east greenland during early mis5e and point to enhanced meltwater release potentially from the northeastern sector of the gis data from downstream the cold east greenland current egc and its eastward branches suggest a far reaching effect of this meltwater not only in the nordic seas but also in the se labrador sea in particular whereas an early mis5e warming at 128 5 126 5 ka in the two regions coincided with the relative reduction of meltwater input into the egc the subsequent cooling noted at 126 5 ka followed a renewed major freshwater event off central east greenland our data further indicate persistent freshwater influence from the east greenland margin over the entire mis5e interval and in addition also reveal a late mis5e meltwater event the latter event occurred just prior to the last glacial inception and emphasizes the importance of greenland meltwater as forcing factor on interglacial climates

stochastically driven multidecadal variability of the atlantic meridional overturning circulation in ccsm3 the atlantic meridional overturning circulation amoc in the last 250 years of the 700 year long present day control integration of the community climate system model version 3 with t85 atmospheric resolution exhibits a red noise like irregular multi decadal variability with a persistence longer than 10 years which markedly contrasts with the preceding 300 years of very regular and stronger amoc variability with 20 year periodicity the red noise like multi decadal amoc variability is primarily forced by the surface fluxes associated with stochastic changes in the north atlantic oscillation nao that intensify and shift northward the deep convection in the labrador sea however the persistence of the amoc and the associated oceanic anomalies that are directly forced by the nao forcing does not exceed about 5 years the additional persistence originates from anomalous horizontal advection and vertical mixing which generate density anomalies on the continental shelf along the eastern boundary of the subpolar gyre these anomalies are subsequently advected by the mean boundary current into the northern part of the labrador sea convection region reinforcing the density changes directly forced by the nao as no evidence was found of a clear two way coupling with the atmosphere the multi decadal amoc variability in the last 250 years of the integration is an ocean only response to stochastic nao forcing with a delayed positive feedback caused by the changes in the horizontal ocean circulation

aleutian lead isotopic data additional evidence for the evolution of lithospheric plumbing systems abstract lead isotopic ratios and concentrations have been measured in lavas from the aleutian volcanic centers of adak 12 and atka 12 whereas lavas from atka have very small pb isotopic ranges 206 pb 204 pb 18 78 18 86 207 pb 204 pb 15 55 15 62 208 pb 204 pb 38 31 38 52 those from adak have ranges nearly twice as large 206 pb 204 pb 18 34 18 86 207 pb 204 pb 15 47 15 63 208 pb 204 pb 37 81 38 56 on lead lead isotopic diagrams atka data form tight clusters whereas those from adaak define a well developed linear trend extending from the atka field toward less radiogenic values at a given 206 pb 204 pb the lavas from both islands plot in the morb trend for 208 pb 204 pb but atka lavas plot above the morb 207 pb 204 pb 206pb 204pb trend and those from adak fall both within and above the trend lead contents in lavas from atka increase four fold 5 21 ppm over the compositional range of the volcanic suite 49 68 sio2 in contrast actak lavas 49 65 have concentration levels of 0 6 13 ppm and display no simple correlation with sio2 the lead isotopic data alone can be explained by three different processes model 1 assigns lead isotopic differences to original magma source heterogeneity according to model 2 the isotopic ratios of a primary non radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab derived component in contrast model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non radiogenic lithospheric component during conduit formation because these models all adequately explain the lead isotopic data supplementary geologic petrographic geochemical and isotopic data must be used to select the most likely model careful consideration of the evidence suggests model 3 best explains our extensive lead isotopic data as well as other characteristics of aleutian lavas our study suggests detailed isotopic studies of individual volcanic centers arc or otherwise can be extremely useful in understanding the complex processes of magma generation extraction ascent and evolution

eruptive history and petrology of mount drum volcano wrangell mountains alaska mount drum is one of the youngest volcanoes in the subduction related wrangell volcanic field 80x200 km of southcentral alaska it lies at the northwest end of a series of large andesite dominated shield volcanoes that show a northwesterly progression of age from 26 ma near the alaska yukon border to about 0 2 ma at mount drum the volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka cone lavas range from basaltic andesite to dacite in composition ring domes are dacite to rhyolite the last constructional activity occured in the vicinity of snider peak on the south flank of the volcano where extensive dacite flows and a dacite dome erupted at about 250 ka the climactic explosive eruption that destroyed the top and a part of the south flank of the volcano produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows the mount drum rocks have medium k calc alkaline affinities and are generally plagioclase phyric silica contents range from 55 8 to 74 0 wt with a compositional gap between 66 8 and 72 8 wt all the rocks are enriched in alkali elements and depleted in ta relative to the lree typical of volcanic arc rocks but have higher mgo contents at a given sio2 than typical orogenic medium k andesites strontium isotope ratios vary from 0 70292 to 0 70353 the compositional range of mount drum lavas is best explained by a combination of diverse parental magmas magma mixing and fractionation the small but significant range in 87sr 86sr ratios in the basaltic andesites and the wide range of incompatible element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas the lavas show abundant petrographic evidence of magma mixing such as bimodal phenocryst size resorbed phenocrysts reaction rims and disequilibrium mineral assemblages in addition some dacites and andesites contain mg and ni rich olivines and or have high mgo cr ni co and sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite but the rhyolites have k ba th and rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks limited sr isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks

response of southern ocean circulation to global warming may enhance basal ice shelf melting around antarctica we investigate the large scale oceanic features determining the future ice shelf ocean interaction by analyzing global warming experiments in a coarse resolution climate model with a comprehensive ocean component heat and freshwater fluxes from basal ice shelf melting ism are parameterized following beckmann and goosse ocean model 5 2 157 170 2003 melting sensitivities to the oceanic temperature outside of the ice shelf cavities are varied from linear to quadratic holland et al in j clim 21 2008 in 1 per year co2 increase experiments the total freshwater flux from ism triples to 0 09 sv in the linear case and more than quadruples to 0 15 sv in the quadratic case after 140 years at which 4 280 ppm 1 120 ppm was reached due to the long response time of subsurface temperature anomalies ism thereafter increases drastically if co2 concentrations are kept constant at 1 120 ppm varying strength of the antarctic circumpolar current acc is crucial for ism increase because southward advection of heat dominates the warming along the antarctic coast on centennial timescales the acc accelerates due to deep ocean warming north of the current caused by mixing of heat along isopycnals in the southern ocean so outcropping regions in contrast to previous studies we find an initial weakening of the acc during the first 150 years of warming this purely baroclinic effect is due to a freshening in the so which is consistent with present observations comparison with simulations with diagnosed ism but without its influence on the ocean circulation reveal a number of ism related feedbacks of which a negative ism feedback due to the ism related local oceanic cooling is the dominant one

boundary layer crystallization in a basaltic magma chamber evidence from rishiri volcano northern japan the evolution of basaltic magma by mushy boundary layer processes introduction in a magma chamber is documented for an alkali basalt kutsugata the evolution of magma chambers is of major petrologic lava from rishiri volcano northern japan on the basis of zoning interest and has been studied extensively experimental patterns of plagioclase phenocrysts and their spatial distribution in and theoretical studies that have investigated the cooling the lava plagioclase phenocrysts with complex zoning patterns can of magma chambers have demonstrated that a magma be divided into four types type 1 is characterized by an extremely chamber solidifies through formation of a mushy boundan rich core an71 90 which is commonly corroded and filled with ary layer composed of crystals and interstitial melt along sodic plagioclase type 2 has an ab rich core surrounded by a calcic the chamber walls e g brandeis jaupart 1986 turner mantle characteristic of partial dissolution type 3 is characterized by et al 1986 marsh 1988 tait jaupart 1992 layered igneous complexes e g morse 1969 shirley 1987 an ab rich core that is commonly surrounded by a reversely zoned mcbirney 1996 provide useful information on how slightly calcic mantle type 4 has an ab rich core without a calcic mafic magma chambers evolve and consolidate but they mantle the an rich cores of the type 1 plagioclase are too calcic do not offer much insight into the instantaneous state of to have crystallized from a liquid represented by any whole rock a chamber because those intrusions integrate processes composition at the estimated temperatures of the kutsugata magma that occurred over long solidification times mcbirney system the an rich cores are inferred to have been formed in a noyes 1979 tait 1988 volcanic products such as mushy boundary layer along the wall of a magma chamber where lava flows and tephra deposits can sample the inthe magma was relatively cool and rich in water because of significant stantaneous state of a magma chamber there are a few olivine fractionation and possibly because of addition of water studies along this line of approach tait 1988 de silva expelled from the chilled margin and or groundwater from the 1989 nakada et al 1994 these studies focused on surrounding crust the ab rich cores of types 2 4 plagioclase crystal aggregates in volcanic products and concluded phenocrysts are interpreted to have crystallized in the main magma that the crystal formation in a mushy boundary layer body of the reservoir the types 2 and 3 ab rich cores were brought plays a dominant role in magmatic evolution in a magma to the mushy layer by convection the temporal and spatial variations reservoir however these studies were mostly on silicic of zoning patterns of plagioclase phenocrysts in basaltic lava flows magmas and there are significant differences in physical give useful information on the evolution of mafic magma chambers and chemical features between silicic and mafic magma chambers in this paper the magmatic evolution of an alkali basalt kutsugata lava from rishiri volcano northern japan is studied the kutsugata lava is divided into three lava flows on the basis of whole rock chemical

mineralogy geochemistry and petrogenesis of kurile island arc basalts whole rock major and trace element and mineral chemical data are presented for basaltic rocks from the main evolutionary stages of the kurile island arc nw pacific an outer inactive arc contains a cretaceous lower tertiary sequence of tholeiitic calcalkaline and shoshonitic basalts the main arc miocene quaternary is dominated by weakly tholeiitic with lesser alkalic basalts the mineralogy of kuriles basalts is characterised by an rich plagioclases a continuous transition from chromites to titanomagnetites pyroxenes with low fe3 contents and without strong fe enrichment abundance of groundmass pigeonites and the absence of amphiboles there is an increase in k2o contents both along arc northwards and towards the reararc side the basalts show an exceptionally wide but continuous range of k2o contents 0 1 4 7 which correlate with other lil element contents tholeiitic basalts with low lil element contents la yb and th u but high k rb p2o5 la and zr nb were derived from depleted lherzolitic mantle which had suffered fluid metasomatism by k rb cs sr ba pb and h2o only alkali basalts are also thought to be derived from depleted mantle but melt metasomatism involved addition of all lil elements to a garnet lherzolite mantle the kuriles basalts and their mantle sources range continuously between these two end member compositions the metasomatic fluids melts were probably released by early dehydration and later melting within subducted oceanic lithosphere though the process is not adequately constrained

rapid freshening of the deep north atlantic ocean over the past four decades the overflow and descent of cold dense water from the sills of the denmark strait and the faroe shetland channel into the north atlantic ocean is the principal means of ventilating the deep oceans and is therefore a key element of the global thermohaline circulation most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse gas concentrations predict a weakening thermohaline circulation in the north atlantic as the subpolar seas become fresher and warmer1 2 3 and it is assumed that this signal will be transferred to the deep ocean by the two overflows from observations it has not been possible to detect whether the ocean s overturning circulation is changing but recent evidence suggests that the transport over the sills may be slackening4 here we show through the analysis of long hydrographic records that the system of overflow and entrainment that ventilates the deep atlantic has steadily changed over the past four decades we find that these changes have already led to sustained and widespread freshening of the deep ocean

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

influence of the atlantic subpolar gyre on the thermohaline circulation during the past decade record high salinities have been observed in the atlantic inflow to the nordic seas and the arctic ocean which feeds the north atlantic thermohaline circulation thc this may counteract the observed long term increase in freshwater supply to the area and tend to stabilize the north atlantic thc here we show that the salinity of the atlantic inflow is tightly linked to the dynamics of the north atlantic subpolar gyre circulation therefore when assessing the future of the north atlantic thc it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account

pedogenic iron manganese nodules in vertisols a new proxy for paleoprecipitation the total fe content of pedogenic iron manganese fe mn nodules taken from a vertisol climosequence on the texas gulf coastal plain correlates with mean annual precipitation map r 2 0 92 no significant trend of total fe fetot with depth was noted in profiles using the regression developed from modern vertisol data fetot contents of paleozoic paleo vertisol fe mn nodules yielded map regimes comparable to previously inferred paleoenvironmental interpretations paleoprecipitation estimates derived from fe mn nodules for an uneroded late mississippian paleo vertisol are very close to estimates determined from a depth to pedogenic carbonate horizon dch proxy determined from the modern vertisol climosequence the lack of soil depth dependence of the fe mn nodule proxy provides a consistent paleoprecipitation estimate even in eroded paleo vertisols and in combination with the dch may be useful in determining original paleosol thickness

are arc type rocks the products of magma crystallisation observations from a simple oceanic arc volcano raoul island kermadec arc sw pacific raoul island is the emergent summit of a large intra oceanic strato volcano in what is globally one of the simplest of subduction settings in this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high mg numbers and high ni and cr expected of primitive mantle derived melts the lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene olivine orthopyroxene and spinel phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition indicating a decoupling of melt and crystal components in the system a consistent model for the raoul magmatic system is that primitive high mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low mg magmas by fractional crystallisation within the lower crust low mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone these processes are essentially those that characterise continental subduction related magmatic systems but differ only in the absence of an evolved crustal component

on the na2o content of convergent zone high alumina basalts abstract high alumina basalts in the convergent zone are classified into high medium and low na type the high na high alumina basalts occur in arcs without low alkali tholeiites where younger and warmer oceanic lithospheres are subducting the low na high alumina basalts occur in arcs where on the contrary older and cooler plates are descending and occur also in arcs with extensional tectonics even though younger lithospheres are underthrusting a good correlation is observed between the na2o content of high alumina basalts and the age of the subducting lithosphere this suggests that generation of high alumina basalt magma is controlled by the nature of the descending plate

a generalized transport theory water mass composition and age abstract a general theory to describe and understand advective and diffusive ocean transport is reported it allows any passive tracer field with an atmospheric source to be constructed by superposing sea surface contributions with a generalized green s function called the boundary propagator of the passive tracer equation the boundary propagator has the interpretation of the joint water mass and transit time distribution from the sea surface the theory thus includes the classical oceanographic idea of water mass analysis and extends it to allow for a distribution of transit times from the sea surface the joint water mass and transit time distribution contains complete information about the transport processes in the flow it captures this information in a more accessible way than using velocity and diffusivity fields however at least for the case of sequestration and transport of dissolved material by the ocean circulation the boundary propagator is thus the natural quantity to consider when discus

coherent surface subsurface fingerprint of the atlantic meridional overturning circulation 1 satellite altimeter data shows a weakening of the north atlantic subpolar gyre during the 1990s which is thought as an indicator of a slowdown of the atlantic meridional overturning circulation amoc however whether the recent slowing subpolar gyre is a decadal variation or a long term trend remains unclear here i show that altimeter data is highly correlated with instrumental subsurface ocean temperature data in the north atlantic and both show opposite signs between the subpolar gyre and the gulf stream path such a dipole pattern is a distinctive fingerprint of amoc variability as shown for the first time by a 1000 year coupled ocean atmosphere model simulation the results suggest that contrary to previous interpretations the recent slowdown of the subpolar gyre is a part of a multidecadal variation and suggests a strengthening of the amoc the ongoing satellite and subsurface temperature measurements could be used to monitor future amoc variations sensitively

strontium isotopic and selected trace element variations between two aleutian volcanic centers adak and atka implications for the development of arc volcanic plumbing systems major and trace element concentrations and initial 87sr 86sr ratios of lavas from the aleutian volcanic centers of adak and atka have been used to study the evolution of their respective lithospheric plumbing systems the centers are within 150 km of one another and show similar overall silica ranges 47 67 but adak 40 km3 is smaller than atka 200 km3 adak s lavas are chemically and isotopically heterogeneous 87sr 86sr 0 70285 0 70330 and two units contain lithospheric xenoliths the lavas of the much larger atka on the other hand have much less variability in major and trace elements as well as 87sr 86sr 0 70320 0 70345 we suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers because aleutian volcanic centers often remain fixed for relatively long periods of time 5 m y once established magmatic passageways are repeatedly used young plumbing systems are relatively cool and contain large amounts of wallrock contaminant and ascending magmas undergo contamination as well as concurrent crystallization and fractionation with time however heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas in effect the plumbing system matures the chemical heterogeneity displayed by young dirty systems like adak reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation thus it is the petrologic data of mature clean systems like atka that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution

primitive calc alkaline and alkaline rock types from the western mexican volcanic belt since the early pliocene small volumes of alkaline magmas have erupted in the western mexican volcanic belt mvb in close association with the volumetrically dominant calc alkaline magmas both suites include relatively rare primitive types characterized by high mg cr and ni contents primitive hypersthene normative basalts parental to the calc alkaline suite are found along the volcanic front but are absent at greater distances from the trench the volcanic front calc alkaline suites are typically hornblende bearing indicating relatively high water contents associated primitive alkaline magmas at volcanic front locations also contain hydrous minerals these nepheline normative suites include basanites phlogopite bearing minettes and other hornblende bearing lamprophyres these are probably the youngest and freshest lamprophyres yet discovered on earth and ideal samples for addressing the origin of this exotic class of rocks on extended morb normalized elemental plots all lamprophyres show patterns similar to the calc alkaline basalts but have overall enrichments of 2 to 25x in ti k p ba sr light rare earth elements and other incompatible elements the lamprophyres show the same strong relative enrichments of ba k and sr and depletions in ti and nb that characterize the calc alkaline rocks of western mexico and all subduction zones these similarities in relative element abundances support a common source region for all primitive magmas of the western mvb the primitive calc alkaline and alkaline rocks of western mexico show narrow overlapping ranges in sr nd and pb isotopic ratios also consistent with derivation of all magmas from a common source the presence of the relatively cold subducted cocos and rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km these magmas are enriched in ba k sr and other elements probably transported by hydrous fluids rising from the subducted slab several lines of evidence support the presence of phlogopite in the source region of western mvb magmas probably in the form of phlogopite rich veins cutting asthenospheric peridotite the first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres these lamprophyric melts rarely erupt in arc settings more commonly forming dikes in the arc crust eruption of lamprophyric magmas in the western mvb is favored by through crustal extensional fracture systems related to active rifting of the jalisco block from the n american plate larger volumes of calc alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock

two stable equilibria of the atlantic subpolar gyre the cyclonic circulation of the atlantic subpolar gyre is a key mechanism for north atlantic climate variability on a wide range of time scales it is generally accepted that it is driven by both cyclonic winds and buoyancy forcing yet the individual importance and dynamical interactions of the two contributions remain unclear the authors propose a simplified four box model representing the convective basin of the labrador sea and its shallow and deep boundary current system the western subpolar gyre convective heat loss drives a baroclinicflow of relatively light water around the dense center eddy salt flux from the boundary current to the center increases with a stronger circulation favors the formation of dense waters and thereby sustains a strong baroclinic flow approximately 10 25 of the total in contrast when the baroclinic flow is not active surface waters may be too fresh to convect and a buoyancy driven circulation cannot develop this situation corresponds to a second stable circulation mode a hysteresis is found for variations in surface freshwater flux and the salinity of the near surface boundary current an analytical solution is presented and analyzed

late cretaceous crustal growth in the gangdese area southern tibet petrological and sr nd hf o isotopic evidence from zhengga diorite gabbro abstract recent studies of gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern tibet during the jurassic early eocene however the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes here we report on the zhengga diorite gabbro suite from the gangdese area they consist of plagioclase labradorite and anorthite amphibole clinopyroxene biotite and minor magnetite epidote zircon and apatite la icp ms zircon u pb dating for two samples gives a ca 94 ma age for the zhengga intrusive rocks i e the late cretaceous apart from one high sio 2 52 2 wt diorite sample with slightly high k 2 o 1 72 wt and initial 87 sr 86 sr 0 7068 and low e nd t 5 6 values the gabbro samples are geochemically characterized by low sio 2 39 8 50 1 wt and k 2 o 0 3 1 1 wt strongly negative nb ta and positive sr anomalies and uniform initial 87 sr 86 sr 0 7043 0 7048 the gabbros can be divided into two groups group i gabbros with relatively low total rare earth element ree rb and sio 2 contents and positive eu anomalies and group ii gabbros with slightly higher total ree rb and sio 2 contents and negative eu anomalies the group i gabbros have e nd t 1 7 to 4 1 and zircon e hf t 6 5 to 11 1 and 18 o 5 89 to 7 24 values which are slightly different to those of the group ii gabbros 0 2 to 2 0 2 9 to 6 5 and 6 24 to 7 05 trace element compositions of amphibole and clinopyroxene grains suggest that the zhengga mafic magmas contained a significant fluid transported component probably released from subducted oceanic lithosphere we suggest that the parental magmas of the zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts fluids the group i gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas with insignificant crustal contamination whereas the group ii gabbros were probably produced by assimilation and fractional crystallization afc processes from mafic magmas that were geochemically similar to the group i gabbros pre collisional underplating of mantle derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late late cretaceous cenozoic granitoids of the gangdese batholiths this study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust

enso suppression due to weakening of the north atlantic thermohaline circulation changes of the north atlantic thermohaline circulation thc excite wave patterns that readjust the thermocline globally this paper examines the impact of a freshwater induced thc shutdown on the depth of the pacific thermocline and its subsequent modification of the el nino southern oscillation enso variability using an intermediate complexity global coupled atmosphere ocean sea ice model and an intermediate enso model respectively it is shown by performing a numerical eigenanalysis and transient simulations that a thc shutdown in the north atlantic goes along with reduced enso variability because of a deepening of the zonal mean tropical pacific thermocline a transient simulation also exhibits abrupt changes of enso behavior depending on the rate of thc change the global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales simulated multidecadal global sea surface temperature sst patterns show a large degree of similarity with previous climate reconstructions suggesting that the observed pan oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections

isolating the role of mesoscale eddies in mixing of a passive tracer in an eddy resolving model 1 this study examines the role of mesoscale eddies in distribution of chlorofluorocarbon cfc in a numerical model of the north atlantic our main conclusions are based on a comparison of two eddy resolving experiments one with and one without eddy advection of cfc the cfc distributions in these two experiments exhibit substantial differences within the subpolar gyre in particular in the presence of eddies the horizontal distribution of cfc in the upper 1000 m is more uniform and the cfc concentration is lower in the lower 1500 m our analysis demonstrates that these basin wide differences are caused by the interactions of eddy advection with convection and mean advection in several isolated locations the corresponding eddy flux divergence terms are spatially non uniform in magnitude their effects are often nonlocal and sometimes work against the spreading of cfc being carried out by the mean advection these conclusions are further confirmed by a third experiment in which the eddy advection was replaced by a down gradient diffusive parameterization our results demonstrate that advection by mesoscale eddies is crucial to the mixing of atmospheric gasses into the ocean and outline difficulties in representing eddies by the down gradient diffusion

lack of bipolar see saw in response to southern ocean wind reduction 1 a cessation of the atlantic meridional overturning circulation amoc significantly reduces northward oceanic heat transport in response to anomalous freshwater flux this leads to the classic bipolar see saw pattern of northern cooling and southern warming in surface air and ocean temperatures by contrast as shown here in a coupled climate model both northern and southern cooling are observed for an amoc reduction in response to reduced wind stress in the southern ocean so for very weak so wind stress not only the overturning circulation collapses but sea ice export from the so is strongly reduced consequently sea ice extent and albedo increase in this region the resulting cooling overcompensates the warming by the reduced northward heat transport the effect depends continuously on changes in wind stress and is reversed for increased winds it may have consequences for abrupt climate change the last deglaciation and climate sensitivity to increasing atmospheric co2 concentration

the cold bay volcanic center aleutian volcanic arc the cold bay volcanic center has experienced two major stages of eruptive activity early m series acitivity produced bimodal hi alumina basalt and calc alkaline andesite lavas while later fpk series activity produced only calc alkaline andesite the spectrum of basalt compositions is believed to be due to high pressure 8 kb fractionation at or near the base of the crust abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites inspection of the mineralogical data has shown that the earliest m series andesites were produced by mixing of basalt 53 wt sio2 and silicic andesite 60 5 to 62 5 wt while later fpk series andesites resulted from the mixing of basaltic andesite 53 to 56 wt and less silicic andesite 58 5 to 60 0 wt the major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions the complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and or assimilation role in the origin of the silicic endmembers many of the features of all andesites including the important long term convergence of endmember magma compositions are consistent with the process of liquid fractionation accompanied by large scale magma mixing a deduced upper limit of 62 5 wt sio2 for the silicic endmember magmas suggests that liquid fractionation in the absence of major crustal melting cannot produce more silicic magmas a possible explanation is the presence of a rheological barrier based on the concept of critical crystallinity marsh 1981 which prohibits more silicic liquids from being extracted from a crystal liquid suspension

on the origin of high alumina arc basalt and the mechanics of melt extraction most models of high alumina arc basalt petrogenesis rely heavily on the supposition that the abundances of certain trace elements in particular the relatively unfractionated rare earth element ree patterns and the unusually high concentrations of k rb sr and ba are incompatible with a garnet bearing subducted oceanic crustal quartz eclogite source rock we have carefully examined this apparently unequivocal evidence in light of recent progress on the physics of melt extraction and the heat transfer and mechanics of magma ascent the weakest element of all trace element models involving a quartz eclogite source is the assumption that the element concentrations are fixed at the source and only later modified in the near surface environment we expand on such models by monitoring the concentrations of r ee and major and trace elements during initial melting ascent and extraction of magma this is done by combining calculated cooling curves for ascending magmatic bodies with high pressure phase equilibria the amount that each phase contributes to the melt is monitored along with the composition of the melt and residual solids with quartz eclogite initial melting initiates gravitational instability of the entire source material melt plus solids before melt extraction can occur during ascent of this mush melting increases until the solids can be repacked to free the melt this extraction takes place some 15 20 km above the slab after about 50 per cent melting at which point the melt has a pattern of ree and other trace element concentrations almost identical to those observed in high alumina arc basalts assuming an initial composition equivalent to altered oceanic crust plus 5 per cent pelagic sediment sr abundances are the only ones which are not well matched by this process the major element concentrations of the extracted melt also closely match those of high alumina arc basalt a similar but less detailed evaluation of both fertile and depleted peridotite source rocks yields good agreement for the ree and other trace element concentrations assuming a lree enriched source rock strongly enriched in k rb sr and ba ni cr and co abundances are satisfied only through substantial low pressure fractionation of mafic phases in particular olivine though not rigorously tested such a process may be compatible with the observed major element concentrations of high alumina basalt however the experimentally verified fact that high alumina basalts could never have been in equilibrium with either an olivine beari ng magma or source rock eliminates this possibility altogether thus the simultaneous consideration of the mechanics of ascent and melt extraction along with phase equilibria clearly shows that partial melting of quartz eclogite best satisfies the chemical constraints of major trace and ree characteristics of high alumina arc basalts

indications for a north atlantic ocean circulation regime shift at the onset of the little ice age a prominent characteristic of the reconstructed northern hemisphere temperature signal over the last millennium is the transition from the medieval climate anomaly to the little ice age lia here we report indications for a non linear regime shift in the north atlantic ocean circulation at the onset of the lia specifically we apply a novel statistical test based on horizontal visibility graphs to two ocean sediment august sea surface temperature records from the norwegian sea and the central subpolar basin and find robust indications of time irreversibility in both records during the lia onset despite a basin wide cooling trend we report an anomalous warming in the central subpolar basin during the lia that is reproduced in ensemble simulations with the climate model of intermediate complexity climber 3 as a result of a non linear regime shift in the subpolar north atlantic ocean circulation the identified volcanically triggered non linear transition in the model simulations provides a plausible explanation for the signatures of time irreversibility found in the ocean sediment records our findings indicate a potential multi stability of the north atlantic ocean circulation and its importance for regional climate change on centennial time scales