model and field observations of effects of circulation on the timing and magnitude of nitrate utilization and production on the northern gulf of alaska shelf abstract the globec program was tasked with understanding the mechanistic links between climate forcing and the ocean ecosystem response on the northern gulf of alaska goa shelf to address this task samples were collected five to six times times annually along the seward line between 1998 and 2004 however interpreting seward line field observations in space and time is complicated by the complex circulation on the goa shelf the alaska current alaskan stream and alaska coastal current produce eddies and meanders which mix the iron limited small cell oceanic community with the iron rich large cell coastal community thus observations at any point in space and time are the result of the degree of mixing of the oceanic and coastal water masses the roms circulation model with an embedded ecosystem model was used to extend globec observations in space and time on the goa shelf the timing of the spring bloom in simulations was related to shallowing of the pycnocline the spring bloom began in late march april on the inner shelf and in may on the mid and outer shelf the simulations suggest that the magnitude of shelf production is a balance between the amount of iron from freshwater runoff and nitrate with iron limitation on the outer shelf and adjacent ocean and nitrate limitation on the inner shelf simulated shelf break eddies form near yakutat have elevated iron concentrations relative to surrounding waters and propagate westward influencing production and nitrate concentrations on the outer shelf and in the adjacent ocean during spring and summer simulated primary production in the seward line region was about 100 130 g cm 2 y 1 but production of up to 300 g cm 2 y 1 is predicted for regions in lower cook inlet and around kodiak

linking the 8 2 ka event and its freshwater forcing in the labrador sea 1 the 8 2 ka event was the last deglacial abrupt climate event a reduction in the atlantic meridional overturning circulation amoc attributed to the drainage of glacial lake agassiz may have caused the event but the freshwater signature of lake agassiz discharge has yet to be identified in 18o of foraminiferal calcite records from the labrador sea calling into question the connection between freshwater discharge to the north atlantic and amoc strength using mg ca paleothermometry we demonstrate that 3 c of near surface ocean cooling masked an 1 0 decrease in western labrador sea 18o of seawater concurrent with lake agassiz drainage comparison with north atlantic 18o of seawater records shows that the freshwater discharge was transported to regions of deep water formation where it could perturb amoc and force the 8 2 ka event

ice sheet sources of sea level rise and freshwater discharge during the last deglaciation we review and synthesize the geologic record that constrains the sources of sea level rise and freshwater discharge to the global oceans associated with retreat of ice sheets during the last deglaciation the last glacial maximum 26 19 ka was terminated by a rapid 5 10 m sea level rise at 19 0 19 5 ka sourced largely from northern hemisphere ice sheet retreat in response to high northern latitude insolation forcing sea level rise of 8 20 m from 19 to 14 5 ka can be attributed to continued retreat of the laurentide and eurasian ice sheets with an additional freshwater forcing of uncertain amount delivered by heinrich event 1 the source of the abrupt acceleration in sea level rise at 14 6 ka meltwater pulse 1a 14 15 m includes contributions of 6 5 10 m from northern hemisphere ice sheets of which 2 7 m represents an excess contribution above that derived from ongoing ice sheet retreat widespread retreat of antarctic ice sheets began at 14 0 15 0 ka which together with geophysical modeling of far field sea level records suggests an antarctic contribution to this meltwater pulse as well the cause of the subsequent younger dryas cold event can be attributed to eastward freshwater runoff from the lake agassiz basin to the st lawrence estuary that agrees with existing lake agassiz outlet radiocarbon dates much of the early holocene sea level rise can be explained by laurentide and scandinavian ice sheet retreat with collapse of laurentide ice over hudson bay and drainage of lake agassiz basin runoff at 8 4 8 2 ka to the labrador sea causing the 8 2 ka event

the 8 2 ka event from greenland ice cores we present a collection of high resolution chemistry and stable isotope records from the plateau of the greenland ice cap during the cold event 8200 yr ago using a composite of four records the cold event is observed as a 160 5 yr period during which decadal mean isotopic values were below average within which there is a central event of 69 yr during which values were consistently more than one standard deviation below the average for the preceding period four cores in north south and central greenland show differences at decadal and shorter timescales it is not yet clear if this represents significant spatial differences in response the results show clear evidence for colder temperatures and a decrease in snow accumulation rate however the changes in chemical concentrations for the ions looked at here are small suggesting only minor changes in atmospheric circulation for this event apart from the decrease in methane concentration greenland ice cores give only weak evidence for effects outside the north atlantic region

size fractionated iron distribution on the northern gulf of alaska 1 concentrations of soluble 0 4 m and colloidal 0 025 0 4 m fe size fractions were removed from surface waters within the inner and mid shelf as a result the contribution of soluble fe to the total fe concentration increased from the inner shelf to the shelf break slope waters surface water dissolved fe concentrations on the northern goa continental slope were higher than those previously observed in the central goa gyre variations in surface water fe concentrations from spring through summer appear to result from the changes in freshwater discharge and physical processes on the shelf

glacial influence on the geochemistry of riverine iron fluxes to the gulf of alaska and effects of deglaciation riverine iron fe derived from glacial weathering is a critical micronutrient source to ecosystems of the gulf of alaska goa here we demonstrate that the source and chemical nature of riverine fe input to the goa could change dramatically due to the widespread watershed deglaciation that is underway we examine fe size partitioning speciation and isotopic composition in tributaries of the copper river which exemplify a long term goa watershed evolution from one strongly influenced by glacial weathering to a boreal forested watershed iron fluxes from glacierized tributaries bear high suspended sediment and colloidal fe loads of mixed valence silicate species with low concentrations of dissolved fe and dissolved organic carbon doc iron isotopic composition is indicative of mechanical weathering as the fe source conversely fe fluxes from boreal forested systems have higher dissolved fe concentrations corresponding to higher doc concentrations iron colloids and suspended sediment consist of fe hydr oxides and organic complexes these watersheds have an iron isotopic composition indicative of an internal chemical processing source we predict that as the goa watershed evolves due to deglaciation so will the source flux and chemical nature of riverine fe loads which could have significant ramifications for alaskan marine and freshwater ecosystems

trace metal distributions within a sitka eddy in the northern gulf of alaska in the northern gulf of alaska mesoscale anticyclonic eddies have been implicated as a mechanism of the cross shelf exchange of iron fe replete coastal waters with fe deplete subarctic alaskan gyre waters based on existing hydrography and macronutrient distributions a sitka eddy sampled during august 2007 is divided into a surface eddy core a shallow subsurface eddy core and a deeper subsurface eddy core the distributions of aluminum al manganese mn and fe in the eddy are examined and compared to distributions at shelf stations similar to where the eddy formed as well as basin stations representative of the fe limited subarctic alaskan gyre relative to basin stations dissolved and particulate al and dissolved mn were elevated in eddy core waters reactive fe concentrations within the shallow subsurface eddy core were nearly seven times greater than reactive fe concentrations at similar densities at the basin stations this shallow subsurface eddy core is likely mixed into the euphotic zone during storm induced mixing as well as mixing during isopycnal relaxation as an eddy dies out the flux of reactive fe to surface waters from the shallow subsurface core of eddies in the gulf of alaska is a significant source compared to both remote and local atmospheric dust deposition it is hypothesized that fe supply from eddies in the eastern gulf of alaska shifts the core of the high nutrient lower than expected chlorophyll water farther west in conjunction with an eddy desert in that same region

geochemistry of crustally derived leucocratic igneous rocks from the ulugh muztagh area northern tibet and their implications for the formation of the tibetan plateau this is the published version copyright 2012 american geophysical union all rights reserved

routing of western canadian plains runoff during the 8 2 ka cold event 1 the collapse of the laurentide ice sheet over hudson bay 8 47 ka allowed the rapid drainage of glacial lake agassiz into the labrador sea an event identified as causing a reduction in atlantic meridional overturning circulation amoc and the 8 2 ka cold event atmosphere ocean models simulations based on this forcing however fail to reproduce several characteristics of this event particularly its duration here we use planktonic foraminifera u ca records to document the routing of western canadian plains runoff that accompanied ice sheet collapse geochemical modeling of the 7 nmol mol increase in u ca at the opening of hudson bay indicates an increase in freshwater discharge of 0 13 0 03 sverdrups 106 m3 s 1 from routing a sufficient magnitude to cause an amoc reduction we suggest that this routing event suppressed amoc strength for several centuries after the drainage of lake agassiz explaining multi centennial climate anomalies associated with the 8 2 ka cold event

the influence of net community production and phytoplankton community structure on co2 uptake in the gulf of alaska 1 biological productivity is a key factor controlling the ocean s ability to take up carbon dioxide from the atmosphere however the ecological dynamics that drive regions of intense productivity and carbon export are poorly understood in this study we present high spatial resolution estimates of air sea co2 flux net community production ncp rates calculated from o2 ar ratios and phytoplankton population abundances determined by continuous underway measurements on a cruise across the gulf of alaska in may 2010 the highest rates of ncp 79 2 13 mmol c m 2 d 1 and oceanic co2 uptake air sea flux of 14 7 2 4 mmol c m 2 d 1 were observed across a transition zone between the high nitrate low chlorophyll hnlc waters of the alaskan gyre and the coastal waters off the aleutian islands while the transition zone comprises 20 of the total area covered in crossing the gulf of alaska it contributed 49 of the total ncp and 48 of the total co2 uptake observed along the cruise track a corresponding transition zone phytoplankton bloom was dominated by two small celled 20 m phytoplankton communities which were distinct from the phytoplankton communities in the surrounding alaskan gyre and coastal waters we hypothesize that mixing between iron rich coastal waters and iron limited alaskan gyre waters stimulated this bloom and fueled the high ncp and co2 export observed in the region

glacial flour dust storms in the gulf of alaska hydrologic and meteorological controls and their importance as a source of bioavailable iron 1 iron is an essential micronutrient that limits primary productivity in much of the ocean including the gulf of alaska goa however the processes that transport iron to the ocean surface are poorly quantified we combine satellite and meteorological data to provide the first description of widespread dust transport from coastal alaska into the goa dust is frequently transported from glacially derived sediment at the mouths of several rivers the most prominent of which is the copper river these dust events occur most frequently in autumn when coastal river levels are low and riverbed sediments are exposed the dust plumes are transported several hundred kilometers beyond the continental shelf into iron limited waters we estimate the mass of dust transported from the copper river valley during one 2006 dust event to be between 25 80 ktons based on conservative estimates this equates to a soluble iron loading of 30 200 tons we suggest the soluble fe flux from dust originating in glaciofluvial sediment deposits from the entire goa coastline is two to three times larger and is comparable to the annual fe flux to goa surface waters from eddies of coastal origin given that glaciers are retreating in the coastal goa region and in other locations it is important to examine whether fluxes of dust are increasing from glacierized landscapes to the ocean and to assess the impact of associated fe on marine ecosystems

marine evidence for the last glacial advance across eastern hudson strait eastern canadian arctic data from accelerator mass spectrometer radiocarbon dated sediment cores and huntec high resolution seismic profiles were used to investigate the age and origin of the sediments in the eastern basin of hudson strait the data indicate that the ice contact and glacial marine sediments on the basin flanks and much of the upper sequence in the deep floor of the basin were produced during the noble inlet advance 8 9 to 8 4 ka the last northward expansion of the labrador dome on to southeastern baffin island on the northern flank of eastern basin one sequence of ice contact sediments and glacial marine deposits overlies bedrock the glacial marine sediments are transitional upslope to ice contact sediments and form at least two successive ice sheet grounding zones the earliest abundance peaks of benthic foramininfera in glacial marine sediments date ca 8 6 and 8 4 ka and correlate to sediments near the base of the 58 m thick glacial marine section in the deepest part of eastern basin this correlation suggests that noble inlet ice was grounded throughout eastern basin during the early part of its advance in later stages the thinning ice produced grounding zones on the basin flanks while glacial marine sediments were deposited in the deep basin 1998 john wiley sons ltd

distribution and lability of dissolved iron in surface waters of marginal seas in southeastern asia abstract more than eighty surface water samples from marginal seas in southeastern asia including the south china sea east china sea western philippine sea and the taiwan strait were collected in order to assess the sources and transport of iron in this region dissolved fe chelex labile and total dissolved concentrations were determined additionally samples were collected from near shore waters along the west coast of taiwan during two different seasons and analyzed for potential lability of fe although total dissolved fe concentrations were in general replete in coastal waters and decreasing seaward the proportions of the dissolved labile chelex extractable and non labile fe fractions showed substantial spatial variability results from dissolved fe distributions indicate that fe in estuarine and coastal regimes forms highly stable non labile complexes which are removed quickly resulting in moderate to low dissolved concentrations offshore most of the fe replete river plumes contain a large proportion of dissolved non labile fe species that also show a low potential bio reactivity and seasonally variable concentrations making biological production in the coastal and adjacent marginal seas temporally dependent on the potential chemical reactivity of the different fe fractions in the source waters regional flow patterns in the study area led to localized supplies of fe enriched waters in the northern taiwan strait and southern east china sea waters

the behavior of u and th series nuclides in the estuarine environment rivers carry the products of continental weathering and continuously supply the oceans with a broad range of chemical constituents this erosional signature is however uniquely moderated by biogeochemical processing within estuaries estuaries are commonly described as complex filters at land sea margins where significant transformations can occur due to strong physico chemical gradients these changes differ for different classes of elements and can vary widely depending on the geographic location u and th series nuclides include a range of elements with vastly different characteristics and behaviors within such environments and the isotopic systematics provide methods for investigating the transport of these nuclides and other analog species across estuaries and into the coastal ocean there are numerous types and definitions of estuaries see dyer 1973 in this paper it is simply considered to be a region where freshwater and seawater mix to emphasize the chemical focus of the issues involved here this region can either be contained within a river channel or extend onto the shelf with effects often extending well into an adjoining ocean basin fairbridge 1980 these are complex ecological biogeochemical and hydrodynamic systems with strong gradients in the concentrations and composition of micro and macro particulate matter as well as dissolved organic and inorganic species particle reactive radionuclides and trace elements being transported across estuaries are subject to partitioning between the dissolved colloidal and particulate phases the variable chemical nature of the u and th series radionuclides is evident in their distributions in an estuarine environment such radionuclides can effectively serve as tracers to identify the sources fate and transport of particles and colloids as well as other pollutants that behave similarly to these nuclides these nuclides also have a wide range of half lives that can be used to examine processes over various time scales for example th and ra daughter isotopes have

10be dating of former glacial lake naskaupi quebec labrador and timing of its discharges during the last deglaciation abstract the last deglaciation of the laurentide ice sheet in northern quebec and labrador led to the formation of several glacial lakes that drained into ungava bay and the nearby labrador sea assessing the potential impact of their drainage on ocean surface conditions and climate however is limited by the few existing age constraints on ice retreat and associated evolution of these lakes here we report 21 10be ages from shorelines and an outburst flood landform formed by lake naskaupi one of the largest glacial lakes in this region the results indicate that the lake drained from its full extent at 8300 300 a suggesting that it may have contributed to the freshwater forcing initiated by the drainage of lake agassiz ojibway that caused the 8 2 ka cold event additionally the results provide important constraints on the position of the ice margin of the labrador sector during its retreat across this region

effects of seawater carbonate ion concentration and temperature on shell u mg and sr in cultured planktonic foraminifera we investigate the sensitivity of u ca mg ca and sr ca to changes in seawater co3 and temperature in calcite produced by the two planktonic foraminifera species orbulina universa and globige rina bulloides in laboratory culture experiments our results demonstrate that at constant temperature u ca in o universa decreases by 25 7 per 100 mol co3 k g 1 as seawater co3 increases from 110 to 470 mol kg 1 results from g bulloides suggest a similar relationship but u ca is consistently offset by 40 at the same environmental co3 in o universa u ca is insensitive to temperature between 15 c and 25 c applying the o universa relationship to three u ca records from a related species globigerinoides sacculifer we estimate that caribbean and tropical atlantic co3 was 110 70 mol kg 1 and 80 40 mol kg 1 higher respectively during the last glacial period relative to the holocene this result is consistent with estimates of the glacial interglacial change in surface water co3 based on both modeling and on boron isotope ph estimates in settings where the addition of u by diagenetic processes is not a factor down core records of foraminiferal u ca have potential to provide information about changes in the ocean s carbonate concentration below ambient ph ph 8 2 mg ca decreased by 7 5 o universa t o 16 6 g bulloides per 0 1 unit increase in ph above ambient ph the change in mg ca was not significant for either species this result suggests that mg ca based paleotemperature estimates for the quaternary during which surface ocean ph has been at or above modern levels have not been biased by variations in surface water ph sr ca increased linearly by 1 6 0 4 per 0 1 unit increase in ph shell mg ca increased exponentially with temperature in o universa where mg ca 0 85 exp 0 096 t whereas the change in sr ca with temperature was within the reproducibility of replicate measurements copyright 2004 elsevier ltd

response of atmospheric co2 to the abrupt cooling event 8200 years ago atmospheric co2 records for the centennial scale cooling event 8200 years ago 8 2 ka event may help us understand climate carbon cycle feedbacks under interglacial conditions which are important for understanding future climate but existing records do not provide enough detail here we present a new co2 record from the siple dome ice core antarctica that covers 7 4 9 0 ka with 8 to 16 year resolution we observe a small about 1 2 ppm increase of atmospheric co2 during the 8 2 ka event the increase is not significant when compared to other centennial variations in the holocene that are not linked to large temperature changes our results do not agree with leaf stomata records that suggest a co2 decrease of up to 25 ppm and imply that the sensitivity of atmospheric co2 to the primarily northern hemisphere cooling of the 8 2 ka event was limited

the 8 2 ka cooling event caused by laurentide ice saddle collapse abstract the 8 2 ka event was a period of abrupt cooling of 1 3 c across large parts of the northern hemisphere which lasted for about 160 yr the original hypothesis for the cause of this event has been the outburst of the proglacial lakes agassiz and ojibway these drained into the labrador sea in 0 5 5 yr and slowed the atlantic meridional overturning circulation thus cooling the north atlantic region however climate models have not been able to reproduce the duration and magnitude of the cooling with this forcing without including additional centennial length freshwater forcings such as rerouting of continental runoff and ice sheet melt in combination with the lake release here we show that instead of being caused by the lake outburst the event could have been caused by accelerated melt from the collapsing ice saddle that linked domes over hudson bay in north america we forced a general circulation model with time varying meltwater pulses 100 300 yr that match observed sea level change designed to represent the hudson bay ice saddle collapse a 100 yr long pulse with a peak of 0 6 sv produces a cooling in central greenland that matches the 160 yr duration and 3 c amplitude of the event recorded in ice cores the simulation also reproduces the cooling pattern amplitude and duration recorded in european lake and north atlantic sediment records such abrupt acceleration in ice melt would have been caused by surface melt feedbacks and marine ice sheet instability these new realistic forcing scenarios provide a means to reconcile longstanding mismatches between proxy data and models allowing for a better understanding of both the sensitivity of the climate models and processes and feedbacks in motion during the disintegration of continental ice sheets

a strong temperature effect on u ca in planktonic foraminiferal carbonates three planktonic foraminiferal species globigerina bulloides neogloboquadrina pachyderma d and globorotalia inflata collected from core tops spanning 35 to 65 n in the north atlantic were used for u ca and mg ca and foraminiferal shell weight analyses except for u ca in g bulloides calcified under warm conditions 13 c u ca ratios in all three studied species increase with decreasing latitude and show strong positive correlations with mg ca ratios a dissolution effect on planktonic u ca is suggested by decreased shell weight and u ca and mg ca ratios for shells from very deep water depth 4 4 km along the latitudinal transect g bulloides from down core samples in the north atlantic show low u ca ratios during the last glacial and high ratios during the holocene similar to the mg ca evolution trend in general our data indicate that the u incorporation into planktonic foraminiferal carbonates is strongly influenced by calcification temperature although u ca in g bulloides may be affected by seawater carbonate ion concentration under warm conditions and or other factors

ensemble water isotope enabled coupled general circulation modeling insights into the 8 2 ka event 1 freshwater forcing has long been postulated as a catalyst for abrupt climate change because of its potential to interfere with thermohaline circulation thc the most recent example may have occurred about 8 2 ka ago with the sudden drainage of glacial lakes agassiz and ojibway into the hudson bay we perform an ensemble of simulations for this freshwater release using the fully coupled atmosphere ocean general circulation model goddard institute for space studies modele r in all cases simulated effects include reduced ocean heat transport and enhanced atmospheric heat transport in the atlantic increased surface albedo through greater low cloud and sea ice cover and local cooling of up to 3 c our suite of ensemble experiments allows us to examine the importance of the initial ocean state in particular the presence or absence of labrador sea water in controlling the magnitude and length of the climate response water isotope tracers included in this model provide an improved means for direct comparisons of the modeled tracer response to water isotope based climate proxy data comparison of model simulations to data implies that there was an abrupt approximate halving of atlantic thc hence providing strong support for the hypothesis that the 8 2 ka event was caused by an abrupt release of fresh water into the north atlantic

how flat is tibet high resolution digital topography three arc second grid for most of tibet provides new information to characterize the relief of the highest and largest plateau on earth the arid to semiarid central and northern part of the plateau interior has low relief average slopes of over 250 m windows and a mean elevation of 5023 m above sea level at moderate wavelengths of m relief is or less for most of tibet as opposed to the much higher relief of up to 6 km on the plateau edges where glacial and fluvial dissection is greater because of higher levels of precipitation the only faults manifesting significant topographic relief are the relatively small scale generally north trending graben systems primarily in southern tibet and several large scale fault systems near the edges of tibet the flatness of tibet implies that 1 there has been little deformation especially shortening of the uppermost crust north of the graben systems during the late cenozoic and 2 shallow crustal isostatic compensation has been acting to level the surface of the plateau

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

large sensitivity to freshwater forcing location in 8 2 ka simulations the 8 2 ka event is a key test case for simulating the coupled climate response to changes in the atlantic meridional overturning circulation amoc recent advances in quantifying freshwater fluxes at 8 2 ka from the proxy record have improved the realism of the forcing magnitude in model simulations yet this forcing is still generally applied in an unrealistic geographic manner across most of the labrador sea rather than just along the labrador coast previous simulations with eddy resolving ocean models have come to conflicting conclusions regarding the ability of such a coastally confined flow to impact the amoc these simulations have also not incorporated full atmosphere models nor have they used the new meltwater forcing values for 8 2 ka we use the community climate system model version 3 with an ocean model resolution only slightly coarser than that used in previous eddy resolving simulations to test the sensitivity to freshwater forcing location when revised freshwater forcing is applied across the labrador sea the amoc is reduced by 40 and climate anomalies compare well with proxy records of the 8 2 ka event in terms of magnitude and duration when the forcing is added just along the labrador coast however most meltwater joins the subtropical gyre and travels to the subtropics with minor impact to the amoc 10 decrease it is likely that model biases in the placement of the north atlantic current remain an important limitation for correctly simulating the 8 2 ka event

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

coastal fresh water discharge in the northeast pacific very high annual rates of precipitation in the coastal mountains that border the northeast pacific ocean produce large fresh water discharges 23000 m3 s 1 this discharge has been ignored previously since it does not enter the ocean in the form of large rivers but instead the water enters by way of numerous small rivers and streams thus it acts as a line source instead of a point source this coastal discharge contributes at least 40 of the fresh water that enters the northeast pacific from the atmosphere the discharge is comparable to the mean annual discharge of the mississippi river system the fresh water creates a cross shelf density gradient that drives an alongshore baroclinic jet the width of this jet is less than 25 km with speeds in excess of 100 cm s 1 it extends along the coast from southeast alaska to at least kodiak island apparently the flow is maintained as a narrow current adjacent to the coast by wind stress that causes downwelling conditions here throughout most of the year

the 8 2 ka bp event in north eastern north america first combined oxygen and hydrogen isotopic data from peat in newfoundland finding direct evidence for atmospheric circulation change in terrestrial records of holocene climate variability remains a fundamental challenge here we present the first combined stable oxygen and hydrogen isotopic palaeorecord from a peatland core in newfoundland canada sphagnum cellulose samples were isolated from a core from nordan s pond bog newfoundland and analysed for d values combined with existing 18o data the resulting d 18o bi plot correlates directly with existing measurements of the modern late 20th century isotopic composition of precipitation from gnip stations in nova scotia and labrador implying a close relationship between the estimated isotopic composition of source water used by the mosses and that of the source precipitation we use the relative variations between the two isotope records to test the hypothesis that atmospheric circulation changed in the millennium following the 8 2 ka bp climate event the data reveal a secondary complex isotopic response 200 years 8250 8050 a bp after a primary oxygen isotopic event that is widespread in the north atlantic region this secondary event is characterized by a divergence in oxygen and hydrogen isotope records that can most plausibly be explained by the augmentation of precipitation moisture from a more distant and more continental vapour source

some characteristics of yakutat eddies propagating along the continental slope of the northern gulf of alaska abstract data from array for real time geostrophic oceanography argo profiling floats oil tanker thermosalinographs shipboard adcp and towed ctd surveys and satellite altimetry are used to examine properties of two 200 km diameter anticyclonic yakutat eddies that propagated westward at 1 5 km day 1 along the continental slope of the northern gulf of alaska goa in 2001 and 2003 the eddies had lifetimes of up to 5 years remained close to the shelfbreak and had relatively constant size and strength until they encountered the alaskan stream where they appeared to spawn smaller shorter lived anticyclones the azimuthal velocity field was vigorous 25 40 cm s 1 and in gradient wind balance with rossby numbers of 0 05 conservation of salt and azimuthal mass transports between 20 m and the depth of the 32 2 isohaline from shipboard surveys in may and august 2003 suggest little mass exchange occurred between the surface layers of the eddy and ambient waters chlorophyll concentrations were greater in the eddy than in ambient waters in both may and august in may the chlorophyll was patchily distributed while in august dense chlorophyll concentrations occurred in and beneath the seasonal thermocline within 50 km of the eddy center the high august chlorophyll concentrations might have been fostered by a broad and shallow it appears likely that as yakutat eddies approach the shelfbreak non linear processes will modify the slope flow field and the stability and structure of the shelfbreak front leading to cross slope flows and flow reversals this interaction may induce 30 km wide streamers of shelf water to flow around the trailing edge of the anticyclone the role of streamers in the freshwater and nutrient budgets of the goa shelf and basin is unknown but their contribution to these budgets will depend on the trajectory of a yakutat eddy especially its proximity to the shelfbreak as the eddy propagates along the goa continental slope

the importance of freshwater to spatial variability of aragonite saturation state in the gulf of alaska high latitude and subpolar regions like the gulf of alaska goa are more vulnerable than equatorial regions to rising carbon dioxide co2 levels in part due to local processes that amplify the global signal recent field observations have shown that the shelf of the goa is currently experiencing seasonal corrosive events carbonate mineral saturation states 1 including suppressed in response to ocean acidification as well as local processes like increased low alkalinity glacial melt water discharge while the glacial discharge mainly influences the inner shelf on the outer shelf upwelling brings corrosive waters from the deep goa in this work we develop a high resolution model for carbon dynamics in the goa identify regions of high variability of and test the sensitivity of those regions to changes in the chemistry of glacial melt water discharge results indicate the importance of this climatically sensitive and relatively unconstrained regional freshwater forcing for variability in the nearshore the increase was nearly linear at 0 002 per 100 mol kg increase in alkalinity in the freshwater runoff we find that the local winds biological processes and freshwater forcing all contribute to the spatial distribution of and identify which of these three is highly correlated to the variability in given that the timing and magnitude of these processes will likely change during the next few decades it is critical to elucidate the effect of local processes on the background ocean acidification signal using robust models such as the one described here

a study of azimuthal p residuals and shear wave splitting across the kunlun range northern tibetan plateau abstract the kunlun fault system is an east west transcurrent fault bordering the tibetan plateau that has experienced left lateral motion since 3 ma a linear array of one component and three component seismological stations was deployed across the kunlun range for 4 months p travel time residuals define three basic units along the profile the north kunlun unit the south kunlun unit and the bayan har unit p residuals are a function of azimuth p residuals are fast in the southern stations for southern azimuths and in the northern stations for northern azimuths measurements of shear wave splitting of the s scs and sks phases have been performed on deep events delay times and fast polarization directions have been computed by finding the maximum of the cross correlation between the two horizontal components rotated at different angles delay times from 0 40 s to 2 52 s remain in the range of those found by mcnamara et al 1994 j geophys res 99 13655 13665 at two sites along the same profile p residual travel times and the splitting of s waves are discussed in relation to models of the geological evolution of this region the northern tibetan plateau appears as a juxtaposition of different terranes the azimuthal p residual pattern implies that p wave anisotropy is about 3 in the lithosphere on both sides of the kunlun fault and that fast velocities are oriented outwards from the kunlun fault plane s wave splitting is observed far from the fault and implies that the lithosphere beneath northern tibet has undergone an important deformation

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

crustal structure and variability of the himalayan border of tibet reversed critical wide angle reflection profiles of the crust mantle boundary south of the yarlung zangbo suture in tibet reveal a deep 70 km moho extending north from the high himalayas whilst to the south the moho is 15 km higher even further south reflections from 35 km depth may correspond to the moho of peninsular india

model support for forcing of the 8 2 ka event by meltwater from the hudson bay ice dome previous model experiments of the 8 2 ka event forced by the drainage of lake agassiz often do not produce climate anomalies as long as those inferred from proxies in addition to the agassiz forcing there is new evidence for significant amounts of freshwater entering the ocean at 8 2 ka from the disintegration of the laurentide ice sheet lis we use the community climate system model version 3 ccsm3 to test the contribution of this additional meltwater flux similar to previous model experiments we find that the estimated freshwater forcing from lake agassiz is capable of sustaining ocean and climate anomalies for only two to three decades much shorter than the event duration of 150 years in proxies using new estimates of the lis freshwater flux 0 13 sv for 100 years from the collapse of the hudson bay ice dome in addition to the agassiz drainage the ccsm3 generates climate anomalies with a magnitude and duration that match within error those from proxies this result is insensitive to the duration of freshwater release a major uncertainty if the total volume remains the same an analysis of the modeled north atlantic freshwater budget indicates that the agassiz drainage is rapidly transported out of the north atlantic while the lis contribution generates longer lasting freshwater anomalies that are also subject to recirculation by the subtropical gyre back into the north atlantic thus the meltwater flux originating from the lis appears to be more important than the agassiz drainage in generating 8 2 ka climate anomalies and is one way to reconcile some model data discrepancies

iron transport by mesoscale haida eddies in the gulf of alaska abstract a two year study of the iron distributions in anticyclonic mesoscale haida eddies of the eastern north pacific ocean has shown that such eddies are a major source of iron to the high nitrate low chlorophyll hnlc waters of the central gulf of alaska basin these eddies which are typically about 100 km in diameter form off the west coast of canada in winter and then track roughly westward into the open ocean therefore they carry large quantities of iron rich coastal waters into iron limited waters of the oligotrophic gyre when the eddies had first formed their dissolved iron concentrations were nearly two orders of magnitude higher than is typically observed at station papa 50 n 145 w 3 n m versus less than 0 05 n m and the total iron in the eddies surface waters 10 40 m was more than 13 n m higher than open ocean waters of the alaska gyre 14 n m versus 0 5 n m while the overall iron content of the eddies decreased rapidly during the first year after they formed the eddy we were able to track the longest still contained 1 5 2 times more iron than the surrounding waters 16 months after its formation therefore although iron concentrations in the surface mixed layers of the eddies drop to levels observed outside very rapidly within 4 months upward transport along isopycnals and upwelling due to eddy decay as well as vertical advective and diffusive transports typical of north pacific waters probably provide steady fluxes of iron into the euphotic zone from the still iron rich eddy core waters throughout the lifetime of the eddy

the 8k event cause and consequences of a major holocene abrupt climate change abstract a prominent abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern hemisphere regions especially in wintertime in response to a very large outburst flood that freshened the north atlantic changes were much larger than typical climate variability before and after the event with anomalies up to many degrees contributing to major displacement of vegetative patterns this 8k event provides a clear case of cause and effect in the paleoclimatic realm and so offers an excellent opportunity for model testing the response to north atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following north atlantic freshening and so greatly strengthens the case that those older events also reflect north atlantic changes the north atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming caused ice melt and hydrologic cycle intensification at high latitudes lead to major changes in north atlantic circulation few model experiments have directly addressed the 8k event and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies so much work remains to be done

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

evidence from gravity and topography data for folding of tibet bouguer gravity and topography data from tibet suggest that the surface of the plateau and the subsurface density interfaces are warped into two series of ridges and troughs trending parallel to the collision zone with wavelengths of 150 and 500 km these folds are superimposed on an overall state of isostatic compensation owing to crustal thickening such folding is predicted from models of compressional shortening of a rheologically layered plate the results thus suggest the presence of a weak decoupling zone between the tibetan crust and upper mantle

the alaska coastal current continuity of transport and forcing current moorings were deployed on the continental shelf at 13 locations in the northwest gulf of alaska they measured the alaska coastal current at three positions along the coast during april october 1991 the strongest currents were in the confined region of shelikof strait mean daily transport was as large as 2 5 106 m3 s 1 mean transport over the 6 month period ranged from 0 85 106 m3 s 1 at the easternmost line off gore point to 0 64 106 m3 s1 at the westernmost section shelikof strait indicating significant transport to the south of kodiak island the transports were well correlated r 0 8 and in phase with each other and were also correlated with wind r 0 6 data suggest that in shelikof strait and off gore point baroclinic flow is 75 and 40 respectively of total transport

a no lid zone in the central chang thang platform of tibet evidence from pure path phase velocity measurements of long period rayleigh waves pure path phase velocities of fundamental mode rayleigh waves in the period range 30 80s are obtained for different paths across the tibet plateau using a two event method with events located in tibet and aligned with world wide standard seismograph network stations at teleseismic distance evidence is found for significant lateral variations of structure of the crust and uppermost mantle in tibet in particular phase velocities obtained on paths contained in the chang thang platform are higher than average in tibet at periods shorter than 60s while remaining very low at longer periods when interpreting these phase velocities in terms of simple layered s velocity models it is found that contrary to other areas in tibet they are incompatible with the existence of a high velocity lid s velocities of 4 4 4 5 km s are found immediately beneath the moho and the crust is somewhat thinner 50 60 km than average in tibet this indicates a possible upwelling of hot asthenospheric material compatible with landsat evidence for subrecent basaltic volcanism and independent seismic evidence such as inefficient transmission of sn waves on the other hand a 70 km thick crust and very high upper mantle s velocities are found on the western border of tibet karakorum hindu kush compatible with previous finer scale results concerning the associated zone of intermediate depth seismicity

the configuration of asia prior to the collision of india cretaceous paleomagnetic constraints paleomagnetic data from central asia show that 1700 610 km of shortening of southern asia since cretaceous time have been absorbed by distributed deformation between southern tibet and the siberia craton this result is based on a compilation of cretaceous poles from the junggar tarim tibet indochina south china north china and mongolia blocks complementing the recent compilation of enkin et al 1992a we propose a paleogeographic reconstruction of asia in the cretaceous in which the position of siberia is derived from the synthetic apparent polar wander path of besse and courtillot 1991 the resulting map which likely represents asia as it remained throughout the cretaceous until the collision with india began features an unbent tibet with an east west trending andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from sichuan to tarim through tibet the map allows one to estimate continental shortening and rotations between the blocks which are attributed to the collision despite large uncertainties these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of avouac 1991 appendix is available with entire paper on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b93 007 2 50 payment must accompany order

seismic tomography of northern tibet and kunlun evidence for crustal blocks and mantle velocity contrasts although the crust and mantle of the tibet plateau reveal vital information for understanding the interplay of dynamic processes that has governed its recent uplift and growth their deep physical and thermal structure remains poorly understood in order to throw light on the structure and hence to constrain the processes and models tied to them we performed a teleseismic experiment on a 600 km long profile across the northern part of the plateau the kunlun range and the qaidam basin the 400 km deep tomographic image we obtain has a resolution 50 km over one order of magnitude better than achieved in previous broader scale studies at relatively shallow depth 100 km the tomographic cross section clearly reveals that the mid lower crust and upper lithosphere of northern tibet is an assemblage of blocks with different velocities and thicknesses hence different natures histories and ages the crust of the qiantang block appears to be thickest 70 km with the lowest velocity along the north edge of that block the jinsha suture although not remarkable in the local geology and topography stands as a particularly sharp and prominent crustal boundary the crust and lithosphere of the region between that suture and the qaidam bayan har songpan south kunlun appear to be thinnest 50 km and with the highest velocity at greater depth 150 km our experiment confirms the existence of a bulky low velocity zone in the mantle beneath the northernmost reaches of the plateau the tomogram helps assess with unprecedented accuracy the location and shape of this low velocity anomaly which forms a 250 300 km wide dome rising to about 150 200 km south of the kunlun range roughly coincident with the region where the strongest shear wave splitting hence horizontal anisotropy has recently been found that dome lies between two relatively high velocity zones with opposite dips one towards the north under the qaidam basin the other towards the south beneath the quangtang platform the implications that this new detailed image of the deep structure of northern tibet has upon the uplift and thermo mechanical evolution of the plateau are discussed

amplitude and timing of temperature and salinity variability in the subpolar north atlantic over the past 10 k y paired planktic foraminiferal 18 o and mg ca data reveal trends of increasing temperatures 3 c and salinities in the subpolar north atlantic over the course of the holocene which were punctuated by abrupt events the trends likely refl ect an insolation forced northward retreat of the boundary between polar and north atlantic subsurface waters the superimposed variability does not appear to be periodic but tends to recur within a broad millennial band the records provide convincing evidence of open ocean cooling nearly 2 c and freshening during the 8 2 ka event and suggest similar conditions at 9 3 ka however the two largest temperature oscillations in our record 2 c occurred during the past 4 k y suggesting a recent increase in temperature variability relative to the mid holocene perhaps in response to neoglaciation which began at about this time

paleohydraulics of the last outburst flood from glacial lake agassiz and the 8200 bp cold event abstract during the last deglaciation of north america huge proglacial lakes formed along the southern margin of the laurentide ice sheet the largest of these was glacial lake agassiz which formed about 11 7 14 c kyr and drained into hudson bay about 7 7 14 c kyr 8 45 cal kyr overflow from these lakes was variably directed to the mississippi st lawrence and mackenzie drainage systems and it is thought that switches in routing were accompanied by a response in ocean circulation that produced abrupt climate events when the ice dam across hudson bay finally was breached a massive flood drained lake agassiz which was routed through hudson strait to the labrador sea in terms of stored water volume the largest reservoir was associated with the kinojevis level of lake agassiz for this maximum filling the impounded water volume available to produce floods is estimated as 40 000 151 000 km 3 depending on the location of the ice margin and route used the timing of this rapid release of stored freshwater just precedes the early holocene cooling event at 8 2 cal kyr bp we use the spring hutter theory to simulate flood hydrographs for floods that originate in subglacial drainage conduits and find that flood magnitude and duration are 5 sv and 0 5 yr multiple fillings and floods are possible as are single floods having a complex multipulse structure modelling results suggest that the outburst flood from lake agassiz may have terminated before the lake surface elevation dropped to sea level and that the flood ended when a stable drainage channel was established connecting lake agassiz to the tyrrell sea

early holocene simulations using different forcings and resolutions in awi esm the earliest part of the holocene from 11 5k to 7k k 1000 years before present is a critical transition period between the relatively cold last deglaciation and the warm middle holocene it i

crust and upper mantle tomography in tibet using surface waves we have obtained maps of lateral variations of structure in the crust and lithosphere in tibet and surrounding regions by inversion of single station measurements of fundamental mode rayleigh wave phase velocities in the period range 25 100 sec for earthquakes located in central asia and observed at teleseismic digital stations in eurasia phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries the lateral variations of s velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the chang tang region north central tibet this zone is bounded by rather steep velocity gradients to the south and east about 250 and 400 km north and west of the frontal thrusts of the himalayas and lungmen shan respectively the northern boundary of the zone crosses the northern limit of the tibet plateau altyn tagh and kunlun extending into the southwest tarim and qaidam basins the low crustal velocities are consistent with a thick hot crust in the central part of tibet consistent with quaternary volcanism in chang tang the steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south and suggest that a similar situation might exist in the north

final laurentide ice sheet deglaciation and holocene climate sea level change abstract despite elevated summer insolation forcing during the early holocene global ice sheets retained nearly half of their volume from the last glacial maximum as indicated by deglacial records of global mean sea level gmsl partitioning the gmsl rise among potential sources requires accurate dating of ice sheet extent to estimate ice sheet volume here we date the final retreat of the laurentide ice sheet with 10 be surface exposure ages for the labrador dome the largest of the remnant laurentide ice domes during the holocene we show that the labrador dome deposited moraines during north atlantic cold events at 10 3 ka 9 3 ka and 8 2 ka suggesting that these regional climate events helped stabilize the retreating labrador dome in the early holocene after hudson bay became seasonally ice free at 8 2 ka the majority of laurentide ice sheet melted abruptly within a few centuries we demonstrate through high resolution regional climate model simulations that the thermal properties of a seasonally ice free hudson bay would have increased laurentide ice sheet ablation and thus contributed to the subsequent rapid labrador dome retreat finally our new 10 be chronology indicates full laurentide ice sheet had completely deglaciated by 6 7 0 4 ka which re quires that antarctic ice sheets contributed 3 6 6 5 m to gmsl rise since 6 3 7 1 ka

the deglacial transition on the southeastern alaska margin meltwater input sea level rise marine productivity and sedimentary anoxia 1 oxygen isotope data from planktonic and benthic foraminifera on a high resolution age model 44 14c dates spanning 17 400 years document deglacial environmental change on the southeast alaska margin 59 33 32 n 144 9 21 w 682 m water depth surface freshening i e 18o reduction of 0 8 began at 16 650 170 cal years b p during an interval of ice proximal sedimentation likely due to freshwater input from melting glaciers a sharp transition to laminated hemipelagic sediments constrains retreat of regional outlet glaciers onto land circa 14 790 380 cal years b p abrupt warming and or freshening of the surface ocean i e additional 18o reduction of 0 9 coincides with the bolling interstade of northern europe and greenland cooling and or higher salinities returned during the allerod interval coincident with the antarctic cold reversal and continue until 11 740 200 cal years b p when onset of warming coincides with the end of the younger dryas an abrupt 1 reduction in benthic 18o at 14 250 290 cal years b p likely reflects a decrease in bottom water salinity driven by deep mixing of glacial meltwater a regional megaflood event or brine formation associated with sea ice two laminated opal rich intervals record discrete episodes of high productivity during the last deglaciation these events precisely dated here at 14 790 380 to 12 990 190 cal years b p and 11 160 130 to 10 750 220 cal years b p likely correlate to similar features observed elsewhere on the margins of the north pacific and are coeval with episodes of rapid sea level rise remobilization of iron from newly inundated continental shelves may have helped to fuel these episodes of elevated primary productivity and sedimentary anoxia

geochemical proxies of north american freshwater routing during the younger dryas cold event the younger dryas cold interval represents a time when much of the northern hemisphere cooled from 12 9 to 11 5 kiloyears b p the cause of this event which has long been viewed as the canonical example of abrupt climate change was initially attributed to the routing of freshwater to the st lawrence river with an attendant reduction in atlantic meridional overturning circulation however this mechanism has recently been questioned because current proxies and dating techniques have been unable to confirm that eastward routing with an increase in freshwater flux occurred during the younger dryas here we use new geochemical proxies mg ca u ca and 87sr 86sr measured in planktonic foraminifera at the mouth of the st lawrence estuary as tracers of freshwater sources to further evaluate this question our proxies combined with planktonic 18oseawater and 13c confirm that routing of runoff from western canada to the st lawrence river occurred at the start of the younger dryas with an attendant increase in freshwater flux of 0 06 0 02 sverdrup 1 sverdrup 106 m3 s 1 this base discharge increase is sufficient to have reduced atlantic meridional overturning circulation and caused the younger dryas cold interval in addition our data indicate subsequent fluctuations in the freshwater flux to the st lawrence river of 0 06 0 12 sverdrup thus explaining the variability in the overturning circulation and climate during the younger dryas

centennial scale climate cooling with a sudden cold event around 8 200 years ago the extent of climate variability during the current interglacial period the holocene is still debated temperature records derived from central greenland ice cores show one significant temperature anomaly between 8 200 and 8 100 years ago which is often attributed to a meltwater outflow into the north atlantic ocean and a slowdown of north atlantic deep water formation this anomaly provides an opportunity to study such processes with relevance to present day freshening of the north atlantic anomalies in climate proxy records from locations around the globe are often correlated with this sharp event in greenland but the anomalies in many of these records span 400 to 600 years start from about 8 600 years ago and form part of a repeating pattern within the holocene more sudden climate changes around 8 200 years ago appear superimposed on this longer term cooling the compounded nature of the signals implies that far field climate anomalies around 8 200 years ago cannot be used in a straightforward manner to assess the impact of a slowdown of north atlantic deep water formation and the geographical extent of the rapid cooling event 8 200 years ago remains to be determined

vegetation responses to climatic variability in the swiss southern alps during the misox event at the early mid holocene transition sedimentary pollen charcoal and plant macrofossil analyses with high resolution and precision suggest a strong shift in vegetation composition during the early to mid holocene transition in the upper mountain belt at piano mire 1439m above sea level a s l ticino switzerland forests were dominated by abies alba during the early holocene prior to ca 8000cal a bp abrupt collapses of a alba at ca 7800 7400cal a bp enabled the expansion of the light demanding pioneer betula afterwards a alba populations regained their previous abundance in the forests within the dating uncertainties of our record we assume that a unique combination of wet and cold years between 8400 and 7500cal a bp led to repeated lethal disadvantages for abies our record of abies oscillations is in good biostratigraphic agreement with the record that has been used to define the misox cold event pian disignano 1540m a s l whichhas been previouslycorrelated withthe8200cal a bp event giventhe ageestimatesoftheabiescollapsesinourwell datedrecord ourresultssuggestthatadditionaleffortsare needed to understand the linkage between the misox and the 8200cal a bp event they imply a high sensitivity of mountain vegetation far below the tree line 800m to holocene climatic changes of about 28c in annual air temperature copyright 2010 john wiley sons ltd

uranium in foraminiferal calcite as a recorder of seawater uranium concentrations abstract we present results of an investigation of uranium calcium ratios in cleaned foraminiferal calcite as a recorder of seawater uranium concentrations for accurate reconstruction of past seawater uranium content shell calcite must incorporate uranium in proportion to seawater concentration and must preserve its original uranium composition over time laboratory culture experiments with live benthic amphistegina lobifera and live planktonic globigerinella calida foraminifera show that the u ca ratio of cleaned calcite tests is proportional to the concentration of uranium in solution after correcting results for the presence of initial calcite the apparent distribution coefficient d u ca calcite u ca solution 10 6 0 3 10 3 for a lobifera and d 7 9 0 1 10 3 for g calida u ca ratios in planktonic foraminifera from core tops collected above 3900 m in the equatorial atlantic and above 2100 m in the pacific ocean show no significant difference among the species analyzed d estimated from core top samples ranges from 7 6 0 4 10 3 for o universa to 8 4 0 5 10 3 for g ruber in benthic species c wuellerstorfi d 7 0 0 8 10 3 u ca and mg ca in g tumida and g sacculifer from core tops taken near and below the regional lysocline decrease with water depth smaller decreases in u ca and mg ca with depth were observed in c wuellerstorfi in the planktonic species we believe that u ca and mg ca are lower in the more dissolution resistant fraction of calcite leading to lower u ca in more highly dissolved samples

a regional stratigraphic isochron ca 800014c yr b p from final deglaciation of hudson strait sedimentologic rock magnetic and x ray fluorescence data from two marine sediment cores in hudson strait suggest that a red hematite rich clay layer was deposited throughout the strait during the final collapse of the laurentide ice sheet in the vicinity of northern hudson bay and western hudson strait this layer which can be recognized by its reddish pink color 10yr6 2 to 5yr4 2 and relatively high hematite proportions low magnetic susceptibility and magnetite to hematite ratio is dated from 8000 to 790014c yr b p at both ends of the strait the dubawnt group a proterozoic bedrock unit in northern hudson bay is the most likely source of this stratigraphic isochron in eastern hudson strait the recognition of this red unit and other distal glaciomarine sediments from 8400 to 790014c yr b p indicates that little sediment from the nearby labrador dome reached eastern hudson strait during this 500 yr interval this time interval immediately postdates the noble inlet advance a northward flow of labrador ice across eastern hudson strait onto southern baffin island from ca 8900 to 840014c yr b p one explanation for the lack of labrador sediments is that the northern margin of the labrador dome was cold based for up to 500 yr following the noble inlet advance

qp melting temperature relation in peridotite at high pressure and temperature attenuation mechanism and implications for the mechanical properties of the upper mantle the anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures to 1280 c and pressures to 0 73 gpa to warrant comparison with seismic measurements of the earth a substantial decrease of qp is observed at temperatures well below the onset of partial melting qp systematically increases with increasing pressure over the entire temperature range of major significance is the finding that qp is dependent on the ratio of the temperatures to the melting solidus temperature i e qp depends on the homologous temperature the pressure dependence of qp appears through the pressure dependence of the solidus of the peridotite within the uncertainties of measurement of both qp and the phase diagram it appears that melting and high temperature anelastic properties have a common origin in peridotite the homologous temperature dependence of qp suggests that we may estimate the temperature and pressure dependence of qp for peridotites of different compositions and possibly even for hydrous peridotites if solidus temperatures are known as a function of pressure a far easier measurement than elastic and anelastic properties the activation volume of qp is greatly reduced at high pressure since the slope of solidus versus pressure rapidly decreases with increasing pressure pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature the qp homologous temperature relation suggests a connection between qp and the properties of the grain boundaries that is the major loss of seismic energy occurs at the grain boundaries grain boundary relaxation or high temperature background attenuation is a possible mechanism for the grain boundary damping no frequency dependence of qp is resolved 0 0 2 in qp f over the pressure temperature and frequency ranges of the measurement the present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic q that is applicable to the earth experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle implications of the results are as follows 1 seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90 100 of the solidus temperature 2 partial melting may not produce any abrupt change of seismic velocity and q rather elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus 3 there may be no sharp mechanical boundary between the lithosphere and the asthenosphere

impact of floods versus routing events on the thermohaline circulation 1 the last deglaciation was interrupted by three major cooling events the younger dryas the preboreal oscillation and the 8 2 ka cold event as the laurentide ice sheet retreated different outlets from lake agassiz became available causing the proglacial lake to drain to a new level in a rather short timescale flood and divert continental runoff over hundreds of years routing here we use an earth system model to investigate the impact of floods short term high amplitude versus routing events long term low amplitude on the thermohaline circulation and the resulting climate change we show that the initial outbursts of freshwater are not strong enough to influence large scale climate and can therefore not be the cause of millennial scale climate variability however routing events have the potential of weakening the thermohaline circulation and therefore causing a cool event in the northern hemisphere

dissolved iron distribution in the western and central subarctic pacific hnlc water formation and biogeochemical processes it is well known that phytoplankton growth is broadly limited by iron fe availability in the subarctic pacific to investigate which fe sources control the amplitude of seasonal variation in biogeochemical parameters in the subarctic pacific we examined the spatial variation in the west to east distribution of dissolved fe dfe across the western and central subarctic pacific through the japanese geotraces program the vertical section profile of the western subarctic pacific gyre showed high dissolved fe concentrations from the bottom of the surface mixed layer to as deep as approximately 3000 m suggesting that fe rich intermediate water is transported laterally and distributed across the western subarctic gyre over 2000 km the section data also indicate that the influence of the western fe rich intermediate water does not reach the alaskan gyre fe index dfe observed po4 observed rfe p distribution clearly indicates that the western fe rich intermediate water is well explained by external sedimentary fe sources and water transport systems from a subpolar marginal sea the sea of okhotsk and the continental margin the spatial pattern of fe to nutrient stoichiometry supplied from the intermediate water to the surface in comparison with reported fe and nutrient demand in surface phytoplankton quantitatively explains the differences in surface macronutrient consumption between the western and eastern gyre as well as the formation of the high nutrient and low chlorophyll region in the whole subarctic pacific

freshwater forcing control on early holocene south american monsoon abstract climate anomalies due to lake agassiz outbursts and hudson bay ice dome melting are commonly considered triggers of north american atmospheric cooling however in the southern hemisphere these freshwater fluxes are mostly associated with increased precipitation and a possible intensification of the south american monsoon system sams here we tested how the sams responded to early holocene meltwater events based on both proxy data and simulations we find that sea surface temperatures ssts and precipitation indicate a freshwater driven strengthening of the sams due to a weakening of the south atlantic subtropical dipole simulated sams strengthening accounts for up to 50 of the variance in early holocene precipitation in south america in turn changes in the south atlantic subtropical dipole accounts for up to 31 of the variance in south atlantic ssts additionally we propose that the stronger sams in the early holocene might have been due to a freshwater driven weakening of the southeasterly trade winds slower trade winds weaken the zonal and meridional surface water transport concentrating warm waters in the northeastern south atlantic

estuarine removal of glacial iron and implications for iron fluxes to the ocean while recent work demonstrates that glacial meltwater provides a substantial and relatively labile flux of the micronutrient iron to oceans the role of high latitude estuary environments as a potential sink of glacial iron is unknown here we present the first quantitative description of iron removal in a meltwater dominated estuary we find that 85 of dissolved fe is removed in the low salinity region of the estuary along with 41 of total dissolvable iron associated with glacial flour we couple these findings with hydrologic and geochemical data from gulf of alaska goa glacierized catchments to calculate meltwater derived fluxes of size and species partitioned fe to the goa iron flux data indicate that labile iron in the glacial flour and associated fe minerals dominate the meltwater contribution to the fe budget of the goa as such goa nutrient cycles and related ecosystems could be strongly influenced by continued ice loss in its watershed

deep structure of the northeastern japan arc and its relationship to seismic and volcanic activity evidence for possible deep seated magmatic activity beneath northeastern japan can be obtained from seismic observations tomographic inversions of p wave velocity data show low velocity zones distributed in the crust and upper mantle beneath active volcanoes in or around these zones anomalously low frequency micro earthquakes perhaps caused by magmatic activity are found at depths of 25 40 km distinct s wave reflectors corresponding to the upper surfaces of magma bodies are found at depths of 10 18 km large crustal earthquakes also occur around the low velocity zones these observations reveal the distribution of magma reservoirs at depths and elucidate its relation with shallow seismicity beneath volcanic arcs

synchronizing a sea level jump final lake agassiz drainage and abrupt cooling 8200 years ago abstract freshwater pulses draining into the north atlantic ocean are commonly hypothesized to have perturbed the atlantic meridional overturning circulation moc triggering abrupt climate changes such as heinrich events the younger dryas and the 8 2 ka event however dating uncertainties have prevented causal links between freshwater pulses and climate events from being firmly established here we report a high resolution relative sea level record from the mississippi delta that documents a sea level jump that occurred within the 8 18 to 8 31 ka 2 time window and is attributed to the final drainage of proglacial lake agassiz ojibway lao this age is indistinguishable from the onset of the 8 2 ka climate event consistent with a nearly immediate ocean atmosphere response to the freshwater perturbation this constitutes a rare currently available example of a major abrupt climate cooling that can be directly linked to a well documented freshwater source with a temporal resolution on the order of a century the total inferred eustatic sea level rise associated with the very final stage of lao drainage at 8 2 ka ranges from 0 8 to 2 2 m considerably higher than previous estimates these new constraints on the timing and amount of final lao drainage permit significantly improved quantitative analysis of the sensitivity of moc to freshwater perturbation a crucial step toward understanding abrupt climate change

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

the high k2o volcanism of northwestern tibet geochemistry and tectonic implications cenozoic volcanic rocks sampled during the 1989 sino french kunlun geotraverse have been analyzed to constrain the origin of widespread tertiary to neogene tibetan basic magmatism samples ranging from phonotephrites to trachyandesites are high k lavas k na always greater than 1 which exhibit typical calc alkaline characteristics although they are highly enriched in mobile trace elements and ree 600 times chondrites for la isotopic compositions exhibit radiogenic sr and pb 87sr 86sr 0 708 208pb 204pb 39 39 4 207pb 204pb 15 6 15 7 206pb 204pb 18 7 and unradiogenic nd 143nd 144nd 0 5123 0 5124 constancy of sr and nd isotopic values contrasts with variations in pb isotopes and highlights the problem of crustal contamination afc and simple mixing models are compatible with isotopic variations but were probably buffered by elemental concentration in sr and nd while pb isotopic values for the most primitive magmas are probably uncontaminated isotopic features are therefore proposed to be representative of a mantle source in which heterogeneity could be highlighted in the pb isotopic values such a source highly enriched in trace elements and radiogenic sr and pb is proposed to lay in the subcontinental mantle isolated from the overall convection movements the calc alkaline characteristics can date from the succession of enrichment phases linked to the numerous subduction events that have affected this part of asia while the isotopic characteristics are proposed to be the result of the aging of the concomitant lile enrichments the broad geographical geochemical and timing aspects of recent basic cenozoic magmatism are discussed in the light of our new data and we suggest that two mechanisms are acting together 1 a limited delamination of the base of the lithosphere or a thermal reequilibration of isotherms subsequent to thickening and 2 intracontinental subduction under the kunlun such mechanisms appear the most probable as they respect at the same time recent geophysical data and other geochemical studies

comparison of the upper mantle shear wave velocity structure of the indian shield and the tibetan plateau and tectonic implications summary travel times and waveforms of long period sh waves recorded at distances of 10 30 and some ss waveforms are used to constrain the upper mantle velocities down to a depth of 400km beneath both the indian shield and the tibetan plateau the shear velocity in the uppermost mantle beneath both the indian shield and the tibetan plateau is high and close to 4 7 km s 1 the indian shield has a fairly thick high velocity lid and the mean velocity between 40 and 250 km is between 4 58 and 4 68 km s 1 in contrast s wave travel times and waveforms of s waves as well as a few for ss show that the mean velocity between 70 and 250km beneath the central and northern part of the tibetan plateau is slower by 4 per cent or more than that beneath the indian shield and probably is between 4 4 and 4 5km s 1 no large differences in the structure of the two areas below 250 km are required to explain both the arrival times and the waveforms of sh phases crossing tibet or the indian shield these results show that the structure of tibet is not that of a shield and imply that the indian plate is not underthrusting the whole of the tibetan plateau at the present time

seasonal and spatial variabilities in northern gulf of alaska surface water iron concentrations driven by shelf sediment resuspension glacial meltwater a yakutat eddy and dust phytoplankton growth in the gulf of alaska goa is limited by iron fe yet fe sources are poorly constrained we examine the temporal and spatial distributions of fe and its sources in the goa based on data from three cruises carried out in 2010 from the copper river ak mouth to beyond the shelf break april data are the first to describe late winter fe behavior before surface water nitrate depletion began sediment resuspension during winter and spring storms generated high total dissolvable fe tdfe concentrations of 1000 nmol kg 1 along the entire continental shelf which decreased beyond the shelf break in july high tdfe concentrations were similar on the shelf but more spatially variable and driven by low salinity glacial meltwater conversely dissolved fe dfe concentrations in surface waters were far lower and more seasonally consistent ranging from 4 nmol kg 1 in nearshore waters to 0 6 1 5 nmol kg 1 seaward of the shelf break during april and july despite dramatic depletion of nitrate over that period the reasonably constant dfe concentrations are likely maintained during the year across the shelf by complexation by strong organic ligands coupled with ample supply of labile particulate fe the april dfe data can be simulated using a simple numerical model that assumes a dfe flux from shelf sediments horizontal transport by eddy diffusion and removal by scavenging given how global change is altering many processes impacting the fe cycle additional studies are needed to examine controls on dfe in the gulf of alaska

freshwater forcing of abrupt climate change during the last glaciation large millennial scale fluctuations of the southern margin of the north american laurentide ice sheet occurred during the last deglaciation when the margin was located between about 43 and 49 n fluctuations of the ice margin triggered episodic increases in the flux of freshwater to the north atlantic by rerouting continental runoff from the mississippi river drainage to the hudson or st lawrence rivers we found that periods of increased freshwater flow to the north atlantic occurred at the same time as reductions in the formation of north atlantic deep water thus providing a mechanism for observed climate variability that may be generally characteristic of times of intermediate global ice volume