late quaternary depositional and glacial history of the arliss plateau off the east siberian margin in the western arctic ocean abstract sedimentary stratigraphy and facies analysis along with seismostratigraphic and multibeam bathymetry data are used to reconstruct the last glacial impact on the arliss plateau ap and attendant sedimentation in the adjacent chukchi basin cb in the western arctic ocean off the east siberian margin sediment core ara02b 16b gc from the ap lower slope captures glacier related depositional history during the last estimated ca 100 ka marine isotope stage mis 1 to 5c based on regional lithostratigraphic correlation the sedimentary record shows distinguishable interglacial interstadial and glacial stadial patterns the identified sedimentary facies reflect several modes of glaciogenic deposition by drifting icebergs suspension settling from turbid meltwater plumes and or detached underflows and turbidity currents based on strong seismic reflectors related to lithological boundaries a downslope subbottom profile from ap to cb is divided into seismostratigraphic units ssu 1 and 2 corresponding in the core record to mis 1 3 and mis 3 5c respectively an acoustically transparent lens within ssu 2 correlates on the upper slope to debris lobes downslope from the ap top covered by megascale glacial lineations this geomorphic sedimentary pattern indicates a glacial erosional impact on the ap and proglacial deposition of eroded sediments on the slope and in the basin based on the developed sediment stratigraphy and facies analysis the last debris lobe horizon was deposited in glacial deglacial environments during late mis 4 to early mis 3 the absence of similar glaciogenic debris lobes within ssu 1 indicates no direct glacial impact on the ap during the last glacial maximum lgm these results suggest that the last glacial erosion of the ap occurred during or immediately after mis 4 possibly related to major glaciation in northern siberia at 50 70 ka

initial tectonothermal evolution within the scandinavian caledonide accretionary prism constraints from40ar 39ar mineral ages within the seve nappe complex sarek mountains sweden in the caledonide orogen of northern sweden the seve nappe complex is dominated by rift facies sedimentary and mafic rocks derived from the late proterozoic baltoscandian miogeocline and offshore continent iapetus transition metamorphic breaks and structural inversions characterize the nappe complex within the sarek mountains the sarektjakka nappe is composed of c 600 ma old dolerites with subordinate screens of sedimentary rocks these lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels the sarektjakka nappe is situated between eclogite bearing nappes mikka and tsakkok nappes which underwent high p metamorphism at c 500 ma during westward subduction of the baltoscandian margin 40ar 39ar mineral ages of c 520 500 ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the sarektjakka nappe plateau ages of 500 ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c 510 490 ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets hornblende cooling ages of 470 460 ma are recorded from massive dyke protoliths within the sarektjakka nappe these are similar to ages reported from the seve nappe complex in the central scandinavian caledonides probably these date imbrication and uplift related to early ordovician arrival of outboard terranes e g island arc sequences represented by structurally lower horizons of the koli nappes metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various seve structural units are themselves internally imbricated and were individually tectonically uplifted through argon closure temperatures during assembly of the seve nappe complex the cooling ages of 520 500 ma recorded within seve terranes and along terrane boundaries of the sarek mountains provide evidence of significant accretionary activity in the northern scandinavian caledonides in the late cambrian early ordovician

evidence for ophiolite obduction terrane accretion and polyorogenic evolution of the north scandinavian caledonides abstract recent work on the thrust sheets of the ofoten troms region north norwegian caledonides has resulted in revision of the tectonostratigraphic correlations providing new constraints on models for terrane accretion and poly orogenic cambro ordovician and siluro devonian development previous studies on late ordovician early silurian fossils in metasediments unconformably overlying the lyngen ophiolite complex within one of the troms nappes documented a period of ophiolite obduction prior to the major siluro devonian scandian orogenic event correlation of these units and underlying and overlying thrusts sheets southward into the ofoten area 150 km to the south is demonstrated this implies that an extensive completely transported upper ordovician lower silurian basin developed on top of the obducted ophiolite composes the bedrock of much of northern norway metamorphism and deformation within these basinal rocks resulted from the scandian phase of the caledonian orogeny the relatively weak metamorphic and deformational imprint on the ophiolitic basement provides a rare opportunity to evaluate the pre scandian tectonic evolution of northern scandinavia poly orogenic evolution a controversial subject in northern norway is documented the character and relative extent of cambro ordovician and siluro devonian orogenic zones in northern norway are elucidated the balsfjord ofoten unconformity formed approximately concomitant with other post ophiolite obduction unconformities in central and southern norway implying an orogen wide late cambrian early ordovician event

mis 3 to mis 1 temporal and lgm spatial variability in arctic ocean sea ice cover reconstruction from biomarkers using the sea ice proxy ip25 and phytoplankton derived biomarkers brassicasterol and dinosterol arctic sea ice conditions were reconstructed for marine isotope stage mis 3 to 1 with special emphasis on the last glacial maximum lgm in sediment cores from the northern barents sea continental margin across the central arctic ocean to the southern mendeleev ridge our results suggest more extensive sea ice cover than present day during latter part of mis 3 increasing sea ice growth during mis 2 and decreased sea ice cover during the last deglacial the summer ice edge remained north of the barents sea even during extremely cold i e last glacial maximum lgm as well as warm periods i e bolling allerod during the lgm the western svalbard margin and the northern barents sea margin areas were characterized by high concentrations of both ip25 and phytoplankton biomarkers interpreted as a productive ice edge situation caused by the inflow of warm atlantic water in contrast the lgm central arctic ocean north of 84 n was covered by thick permanent sea ice throughout the year with rare break up indicated by zero or near zero biomarker concentrations the spring summer sea ice margin significantly extended southwards to the laptev sea shelf southern lomonosov ridge and southern mendeleev ridge during the lgm our proxy reconstructions are very consistent with published model results based on the north atlantic arctic ocean sea ice model naosim

interaction of lateral baroclinic forcing and turbulence in an estuary 1 observations of density and velocity in a channel in northern san francisco bay show that the onset of vertical density stratification during flood tides is controlled by the balance between the cross channel baroclinic pressure gradient and vertical mixing due to turbulence profiles of velocity salinity temperature and suspended sediment concentration were measured in transects across suisun cutoff in northern san francisco bay on two days over the 12 5 hour tidal cycle during flood tides an axial density front developed between fresher water flowing from the shallows of grizzly bay into the northern side of suisun cutoff and saltier water flowing up the channel north of the front transverse currents were driven by the lateral salinity gradient with a top to bottom velocity difference greater than 30 cm s south of the front the secondary circulation was weak and along channel velocities were greater than to the north the gradient richardson number shows that stratification was stable north of the front while the water column was turbulently mixed south of the front time series measurements of velocity and salinity demonstrate that the front develops during each tidal cycle in estuaries longitudinal dynamics predict less stratification during flood than ebb tides these data show that stratification can develop during flood tides due to a lateral baroclinic pressure gradient in estuaries with complex bathymetry

strong middepth warming and weak radiocarbon imprints in the equatorial atlantic during heinrich 1 and younger dryas we present a benthic foraminiferal multiproxy record of eastern equatorial atlantic eea middepth water 1295m covering the last deglacial we show that eea middepth water temperatures were elevated by 3 90 5 degrees c and 5 21 2 degrees c during heinrich event 1 h1 and younger dryas yd respectively the radiocarbon content of the eea middepth during h1 and yd is relatively low and comparable to the values of the pre h1 episode and bolling allerod respectively a transient earth system model simulation which mimics the observed deglacial atlantic meridional overturning circulation amoc history qualitatively reproduces the major features of the eea proxy records the simulation results suggest that fresh water induced weakening of the amoc leads to a vertical shift of the horizon of southern ocean sourced water and a stronger influence of eea sea surface temperatures via mixing our findings reaffirm the lack of a distinctive signature of radiocarbon depletion and therefore do not support the notion of interhemispheric exchanges of strongly radiocarbon depleted middepth water across the tropical atlantic during h1 and yd our temperature reconstruction presents a critical zonal and water depth extension of existing tropical atlantic data and documents a large scale and basin wide warming across the thermocline and middepth of the tropical atlantic during h1 and yd significant difference in the timing and pace of h1 middepth warming between tropical atlantic and north atlantic likely points to a limited role of the tropical atlantic middepth warming in the rapid heat buildup in the north atlantic middepth

drivers of residual estuarine circulation in tidally energetic estuaries straight and irrotational channels with parabolic cross section the generation of residual circulation in a tidally energetic estuary with constant longitudinal salinity gradient and parabolic cross section is examined by means of a two dimensional cross sectional numerical model neglecting river runoff and stokes drift it is shown how the longitudinal and lateral residual circulation can be decomposed into contributions from various processes such as tidal straining circulation gravitational circulation advectively driven circulation and horizontal mixing circulation the sensitivity of the residual circulation and its components from various processes to changes in forcing is investigated by varying the simpson number nondimensional longitudinal buoyancy gradient and the unsteadiness parameter nondimensional tidal frequency as well as the bed roughness and the width of the estuary for relatively weak salinity gradient forcing the tidal straining circulation dominates the residual exchange circulation in support of classical estuarine circulation up estuary flow near the bed and down estuary flow near the surface the strength of the longitudinal estuarine circulation clearly increases with increased salinity gradient forcing however when the simpson number exceeds 0 15 the relative contributions of both gravitational circulation and advectively driven circulation to estuarine circulation increase substantially lateral residual circulation is relatively weak for small simpson numbers and becomes flood oriented divergent flow near the bed and convergent flow near the surface for larger simpson numbers because of increasing contributions from gravitational and advectively driven circulation increasing the unsteadiness number leads to decreased longitudinal and lateral residual circulation although changes in bed roughness result in relatively small changes in residual circulation results are sensitive to the width of the estuary mainly because of changes in residual exchange circulation driven by tidal straining

middle to late pleistocene arctic paleoceanographic changes based on sedimentary records from mendeleev ridge and makarov basin abstract three sediment cores from the makarov basin and mendeleev ridge off the siberian margin were analyzed for a number of litho bio and magnetostratigraphic proxies correlation to previously studied arctic records from mendeleev to lomonosov ridges enables paleoceanographic reconstructions for the middle to late pleistocene estimated 0 8 ma on orbital time scales cyclic changes in lithology corroborated by multiple stratigraphic constraints were tuned to global climate variations based on the manganese content that increases during interglacials while calcium and zircon were used as proxies for the laurentide and eurasian ice sheet inputs the combined evaluation of glacial and interglacial proxy variabilities provides a coherent picture of the western arctic ocean development in the pleistocene we suggest that major cyclic changes were driven by summer processes such as sea ice retreat and enhanced biological production vs suppressed ice melt in warmer and colder summers respectively a potential link to low latitudes via atmospheric moisture and heat transport is indicated by similarities in the western arctic ocean and east asian monsoon records long term cooling is signified by biogenic proxies related to sea ice expansion over the western arctic ocean with step changes at mis 11 and 7 0 4 and 0 2 0 3 ma this transition also features better fossil preservation indicating less corrosive oxidative bottom waters probably resulting from a decrease in organic export production and or weakening of bottom water ventilation under perennial sea ice the latter was possibly enhanced by atmospheric cooling and increased meltwater fluxes with the growth of circum arctic ice sheets

shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

controls on 231pa and 230th in the arctic ocean the distributions of 230th and 231pa in the arctic ocean are not well understood in order to examine the arctic 231pa 230th system and therefore to shed light on the future use of arctic sedimentary 231pa 230th we developed a 2 d scavenging model modified from an atlantic model tuned with reasonable parameters that are consistent with eurasian basin geographic settings the model can reproduce most of the features of the 230th and 231pa water column profiles as well as the sedimentary 231pa 230th distribution patterns and suggests that the sedimentary 231pa 230th in the eurasian basin is mainly controlled by the deep water circulation in our attempt to reproduce the sedimentary 231pa 230th patterns during the last glacial we found that circulation strength in the eurasian basin at shallower depths may have been stronger than today

residual transport of suspended material by tidal straining near sloping topography abstracttidal straining is known to have an important impact on the generation of residual currents and the transport of suspended material in estuaries and the coastal ocean essential for this process is an externally imposed horizontal density gradient typically resulting from either freshwater runoff or differential heating here it is shown that near sloping topography tidal straining may effectively transport suspended material across isobaths even if freshwater runoff and differential heating do not play a significant role a combined theoretical and idealized modeling approach is used to illustrate the basic mechanisms and implications of this new process the main finding of this study is that for a wide range of conditions suspended material is transported upslope by a pumping mechanism that is in many respects similar to classical tidal pumping downslope transport may also occur however only for the special cases of slowly sinking material in the vicinity of slopes with a slope angle la

particle trapping in estuarine tidal flows particle trapping in estuarine turbidity maxima etm is caused primarily by convergent mean and or tidal fluxes of sediment the result is an approximately bell shaped along channel distribution of vertically integrated tidal cycle mean suspended sediment concentration observations from the columbia river estuary suggest that 1 strong two layer or internal along channel residual and overtide flows are generated by time varying stratification and 2 correlations between the near bed velocity and the suspended sediment fields at these frequencies are important in landward transport of sediment a new spatially and temporally integrated form of the sediment conservation equation has been derived to analyze this trapping process time changes in tidally averaged sediment concentration between two estuarine cross sections can be shown to be related to the divergence of the seaward river flow transport the divergence of velocity shear sediment stratification correlations for the mean flow and each tidal constituent and net erosion or deposition at the bed vertically integrated variables other than seaward river transport are absent from this integrated balance analysis of sediment fluxes using this balance supports the idea that internal residual and overtide circulations are primarily responsible for the landward sediment transport on the seaward side of etm found near the upstream limits of salinity intrusion the balance also shows that attempts to represent fluxes causing trapping of sediment in an etm as a product of a time mean vertically integrated along channel gradient and a diffusivity inevitably lead to the appearance of countergradient transport and thus a negative diffusivity on the seaward side of the etm this result occurs because the trapping process is inherently nonlinear and at least two dimensional and because a one dimensional representation is physically unrealistic

water mass properties and exchange between the nordic seas and the northern north atlantic during the period 23 6 ka benthic oxygen isotopic evidence 1 twenty benthic oxygen isotope records from different water depths in the nordic seas and the north atlantic are compared during the last glacial maximum brine formation on continental shelves produced brine shelf water bsw sinking below 1500 m in the nordic seas open ocean convection in the nordic seas produced glacial north atlantic intermediate water gnaiw gnaiw overflowed the greenland scotland ridge and entrained depths above and at least partly below 2000 m in the north atlantic during the early deglaciation bsw enriched intermediate water masses in the nordic seas were formed these overflowed the greenland scotland ridge and influenced the north atlantic intermediate and deepwater masses in the bolling allerod ba open ocean convection increased and produced intermediate water in the nordic seas with outflow to the north atlantic however deep water with modern characteristics did not entrain water below 2000 m in the north atlantic in similar amounts as during the holocene a new period of brine formation during the younger dryas transported bsw to intermediate water masses in the norwegian sea there was also open ocean convection and meridional overturning in the nordic seas but it was probably reduced compared to the ba in the early holocene and mid holocene meridional overturning appears similar to that of today potential locations for large scale formation of bsw might have been broad and shallow 200 m areas of the north sea northeast of greenland and north of east siberia these settings should be favorable for bsw formation during cold periods

ice shelf collapse from subsurface warming as a trigger for heinrich events episodic iceberg discharge events from the hudson strait ice stream hsis of the laurentide ice sheet referred to as heinrich events are commonly attributed to internal ice sheet instabilities but their systematic occurrence at the culmination of a large reduction in the atlantic meridional overturning circulation amoc indicates a climate control we report mg ca data on benthic foraminifera from an intermediate depth site in the northwest atlantic and results from a climate model simulation that reveal basin wide subsurface warming at the same time as large reductions in the amoc with temperature increasing by approximately 2 c over a 1 2 kyr interval prior to a heinrich event in simulations with an ocean model coupled to a thermodynamically active ice shelf the increase in subsurface temperature increases basal melt rate under an ice shelf fronting the hsis by a factor of approximately 6 by analogy with recent observations in antarctica the resulting ice shelf loss and attendant hsis acceleration would produce a heinrich event

a dynamic equation for the potential energy anomaly for analysing mixing and stratification in estuaries and coastal seas abstract in this paper a time dependent dynamic equation for the potential energy anomaly is rigorously derived from dynamic equations for potential temperature and salinity the continuity equation and the equation of state for sea water the terms locally changing are a the advection b the depth mean straining c the non mean straining d the vertical advection e the vertical mixing f surface and bottom density fluxes g inner sources of density e g due to absorption of solar radiation and the non linearity of the equation of state and h horizontal divergence of horizontal turbulent density fluxes in order to derive the equation in concise form a vertical velocity linearly varying with depth with respect to depth proportional vertical coordinates had to be defined the evaluation of the terms in the equation is then carried out for a one dimensional tidal straining study and a two dimensional estuarine circulation study comparisons to empirical estimates for these terms are made for the one dimensional study it is concluded that the equation provides a general reference for empirical bulk parameterisations of stratification and mixing processes in estuaries and coastal seas and that it is a tool for complete analysis of the relevant terms from numerical models

impact of the depth to width ratio of periodically stratified tidal channels on the estuarine circulation the dependency of the estuarine circulation on the depth to width ratio of a periodically weakly stratified tidal estuary is systematically investigated here for the first time currents salinity and other properties are simulated by means of the general estuarine transport model getm in cross sectional slice mode applying a symmetric gaussian shaped depth profile the width is varied over four orders of magnitude the individual along channel circulation contributions from tidal straining gravitation advection etc are calculated and the impact of the depth to width ratio on their intensity is presented and elucidated it is found that the estuarine circulation exhibits a distinct maximum in medium wide channels intermediate depth towidth ratio depending on various parameters which is caused by a maximum of the tidal straining contribution this maximum is related to a strong tidal asymmetry of eddy viscosity and shear created by secondary strain induced periodic stratification 2sips in medium channels transverse circulation generated by lateral density gradients due to laterally differential longitudinal advection induces stable stratification at the end of the flood phase which is further increased during ebb by longitudinal straining sips thus eddy viscosity is low and shear is strong in the entire ebb phase during flood sips decreases the stratification so that eddy viscosity is high and shear is weak the circulation resulting from this viscosity shear correlation the tidal straining circulation is oriented like the classical gravitational circulation with riverine outflow at the surface and oceanic inflow close to the bottom in medium channels it is about 5 times as strong as in wide quasi onedimensional channels in which 2sips is negligible

the dynamic arctic research campaigns over the last decade have yielded a growing stream of data that highlight the dynamic nature of arctic cryosphere and climate change over a range of time scales as a consequence rather than seeing the arctic as a near static environment in which large scale changes occur slowly we now view the arctic as a system that is typified by frequent large and abrupt changes the traditional focus on end members in the system glacial versus interglacial periods has been replaced by a new interest in understanding the patterns and causes of such dynamic change instead of interpreting changes almost exclusively as near linear responses to external forcing e g orbitally forced climate change research is now concentrated on the importance of strong feedback mechanisms that in our palaeo archives often border on chaotic behaviour the last decade of research has revealed the importance of on off switching of ice streams strong feedbacks between sea level and ice sheets spatial and temporal changes in ice shelves and perennial sea ice as well as alterations in ice sheet dynamics caused by shifting centres of mass in multi dome ice sheets recent advances in dating techniques and modelling have improved our understanding of leads and lags that exist in different arctic systems on their interactions and the driving mechanisms of change future arctic research challenges include further emphases on rapid transitions and untangling the feedback mechanisms as well as the time scales they operate on

stratigraphic constraints on late pleistocene glacial erosion and deglaciation of the chukchi margin arctic ocean at least two episodes of glacial erosion of the chukchi margin at water depths to 450 m and 750 m have been indicated by geophysical seafloor data we examine sediment stratigraphy in these areas to verify the inferred erosion and to understand its nature and timing our data within the eroded areas show the presence of glaciogenic diamictons composed mostly of reworked local bedrock the diamictons are estimated to form during the last glacial maximum lgm and an earlier glacial event possibly between ois 4 to 5d both erosional events were presumably caused by the grounding of ice shelves originating from the laurentide ice sheet broader glaciological settings differed between these events as indicated by different orientations of flutes on eroded seafloor postglacial sedimentation evolved from iceberg dominated environments to those controlled by sea ice rafting and marine processes in the holocene a prominent minimum in planktonic foraminiferal 18 o is identified in deglacial sediments at an estimated age near 13 000 cal yr bp this 18 o minimum also reported elsewhere in the amerasia basin is probably related to a major laurentide meltwater pulse at the younger dryas onset the bering strait opening is also marked in the composition of late deglacial chukchi sediments

pathways and modification of the upper and intermediate waters of the arctic ocean 1 the purpose of this study is to investigate the pathways and the ventilation of source water masses of the upper and intermediate waters of the arctic ocean for the arctic and subarctic domain a coupled ice ocean general circulation model is set up to be integrated for several decades it is driven by a climatological seasonal cycle of monthly mean atmospheric data from 1980 to 1989 and by restored sea surface salinities passive tracers are used to visualize and interpret the modeled flow and to compare it with observations the model is able to reproduce known features of the arctic ocean circulation like the inflow of two branches of atlantic origin via the fram strait and the barents sea and their subsequent passage at middepths in several cyclonic circulation cells the fate of these atlantic source water masses river water and bering strait inflow water in the model are studied the branch crossing the barents sea is subject to an intense heat loss and ice formation as a result water of this branch leaves the shelf toward the central part of the arctic ocean not only at the surface but also in denser varieties which finally feed the central arctic at halocline and middepths the lightest part turns northward and finally westward joining the transpolar drift the densest part 200 1000 m depth moves eastward along the continental slope a similar path is taken by the atlantic water branch from the fram strait the inflowing branch over the barents sea turns out to be the dominant source for the lower atlantic water layer in the arctic ocean in this investigation atlantic tracers starting in fram strait need 6 years to reach the northern laptev sea slope travel times to return to fram strait are 15 20 years along the lomonossov ridge and about 30 years along the continental slope of the canadian basin tracers that mark the pacific water or the mackenzie river water flow eastward and leave the arctic mainly via the canadian archipelago the siberian river water tracers at the surface penetrate far into the canadian basin before they join the transpolar drift the travel times of the river water from the river mouths are 2 3 years to the shelf edge and 12 14 years to fram strait

an arctic ocean ice shelf during mis 6 constrained by new geophysical and geological data the hypothesis of floating ice shelves covering the arctic ocean during glacial periods was developed in the 1970s in its most extreme form this theory involved a 1000 m thick continuous ice shelf covering the arctic ocean during quaternary glacial maxima including the last glacial maximum lgm while recent observations clearly demonstrate deep ice grounding events in the central arctic ocean the ice shelf hypothesis has been difficult to evaluate due to a lack of information from key areas with severe sea ice conditions here we present new data from previously inaccessible unmapped areas that constrain the spatial extent and timing of marine ice sheets during past glacials these data include multibeam swath bathymetry and subbottom profiles portraying glaciogenic features on the chukchi borderland southern lomonosov ridge north of greenland morris jesup rise and yermak plateau sediment cores from the mapped areas provide age constraints on the glaciogenic features combining these new geophysical and geological data with earlier results suggests that an especially extensive marine ice sheet complex including an ice shelf existed in the amerasian arctic ocean during marine isotope stage mis 6 from a conceptual oceanographic model we speculate that the cold halocline of the polar surface water may have extended to deeper water depths during mis 6 inhibiting the warm atlantic water from reaching the amerasian arctic ocean and thus creating favorable conditions for ice shelf development the hypothesis of a continuous 1000 m thick ice shelf is rejected because our mapping results show that several areas in the central arctic ocean substantially shallower than 1000 m water depth are free from glacial influence on the seafloor

thermobarometric profile through the caledonian nappe stack of western ofoten north norway p t estimates from pelite thermobarometry along a transect through the caledonian nappe stack in ofoten north norway 68 30 n plot as a trend in p t space ranging from 443 c 6 9 kb to 688 c 10 6 kb the p t range for each nappe overlaps the others except for those from the lowest unit narvik group which has significantly higher values within the p t ranges for each nappe the position on the trend of a given sample is independent of its tectonic position results support our earlier conclusion based on structural and petrographic grounds that the regional metamorphic peak was reached during or after stacking of the nappes in the siluro devonian scandian orogeny

north atlantic deepwater temperature change during late pliocene and late quaternary climatic cycles variations in the ratio of magnesium to calcium mg ca in fossil ostracodes from deep sea drilling project site 607 in the deep north atlantic show that the change in bottom water temperature during late pliocene 41 000 year obliquity cycles averaged 1 5 c between 3 2 and 2 8 million years ago ma and increased to 2 3 c between 2 8 and 2 3 ma coincidentally with the intensification of northern hemisphere glaciation during the last two 100 000 year glacial to interglacial climatic cycles of the quaternary bottom water temperatures changed by 4 5 c these results show that glacial deepwater cooling has intensified since 3 2 ma most likely as the result of progressively diminished deepwater production in the north atlantic and of the greater influence of antarctic bottom water in the north atlantic during glacial periods the ostracode mg ca data also allow the direct determination of the temperature component of the benthic foraminiferal oxygen isotope record from site 607 as well as derivation of a hypothetical sea level curve for the late pliocene and late quaternary the effects of dissolution on the mg ca ratios of ostracode shells appear to have been minimal

an arctic perspective on dating mid late pleistocene environmental history to better understand pleistocene climatic changes in the arctic integrated palaeoenvironmental and palaeoclimatic signals from a variety of marine and terrestrial geological records as well as geochronologic age control are required not least for correlation to extra arctic records in this paper we discuss from an arctic perspective methods and correlation tools that are commonly used to date arctic pleistocene marine and terrestrial events we review the state of the art of arctic geochronology with focus on factors that affect the possibility and quality of dating and support this overview by examples of application of modern dating methods to arctic terrestrial and marine sequences event stratigraphy and numerical ages are important tools used in the arctic to correlate fragmented terrestrial records and to establish regional stratigraphic schemes age control is commonly provided by radiocarbon luminescence or cosmogenic exposure ages arctic ocean deep sea sediment successions can be correlated over large distances based on geochemical and physical property proxies for sediment composition patterns in palaeomagnetic records and increasingly biostratigraphic data many of these proxies reveal cyclical patterns that provide a basis for astronomical tuning recent advances in dating technology calibration and age modelling allow for measuring smaller quantities of material and to more precisely date previously undatable material i e foraminifera for c 14 and single grain luminescence however for much of the pleistocene there are still limits to the resolution of most dating methods consequently improving the accuracy and precision analytical and geological uncertainty of dating methods through technological advances and better understanding of processes are important tasks for the future another challenge is to better integrate marine and terrestrial records which could be aided by targeting continental shelf and lake records exploring proxies that occur in both settings and by creating joint research networks that promote collaboration between marine and terrestrial geologists and modellers

creation of residual flows in a partially stratified estuary the creation of residual flows in estuaries is examined using acoustic doppler current profiler data sets from northern san francisco bay the data sets are analyzed using principal component analysis to examine the temporal variability of the flows which create the residual circulation it is seen that in this periodically and partially stratified estuary the residual flows are created through a series of pulses with strong variability at the 24 hour timescale through the interaction of shear stratification and mixing this interaction is captured through the use of a dimensionless number the horizontal richardson number rix which is developed to examine the local balance between the stratifying and destratifying forces at the tidal timescale it is seen that rix is a valuable parameter in predicting the onset of the residual creating events with a threshold value of 3 on ebb tides this critical value is argued to be a threshold above which the stratification and shear flow create a feedback effect each further intensifying the other this feedback results in a highly variable exchange flow which creates the estuarine residual in intermittent pulses rather than as a steady flow although typically attributed to baroclinic forcing an argument is made that these pulses of residual creating exchange flow could be created by barotropic forcing in the presence of variable stratification which is asymmetric between flood and ebb tides this result poses a great challenge for turbulence modeling as the timing and magnitude of stratification and shear must be correctly simulated on the tidal timescale in order to reproduce the effects seen in the data sets presented

deglacial holocene svalbard paleoceanography and evidence of meltwater pulse 1b abstract better understanding of deglacial meltwater pulses mwps is imperative for future predictions of human induced warming and abrupt sea level change because of their potential for catastrophic damage however our knowledge of the second largest meltwater pulse mwp 1b that occurred shortly after the start of the holocene interglacial remains very limited here we studied fossil ostracods as paleoenvironmental indicators of water depth salinity and temperature in two marine sediment cores from storfjorden svalbard margin the arctic ocean to investigate near field i e areas located beneath continental ice sheets at the last glacial maximum evidence of mwp 1b the depositional environment changed from a cold bathyal environment to a warmer bathyal environment at 11 300 yr bp indicating incursion of warm atlantic water into the nordic seas and eventually to a cold neritic environment by 11 000 yr bp because of melting of the svalbard barents sea ice sheet and resultant isostatic rebound this process corresponds to rapid relative sea level fall of 40 80 m of mwp 1b from 11 300 to 11 000 yr bp

water mass exchange between the nordic seas and the arctic ocean on millennial timescale during mis 4 mis 2 1 the climate of the last glaciation circa 65 000 25 000 years ago was interrupted by about 15 abrupt temperature fluctuations the so called dansgaard oeschger events consisting of warm interstadials and cold stadials recorded in greenland ice cores the largest fluctuations occur in the north atlantic region but they have been registered over the most of the world the events are linked to changes in deep water formation in the nordic seas and north atlantic disrupting the thermohaline circulation yet dansgaard oeschger events have so far not been recorded north of the convection areas in the central nordic seas and it is not known if they affected the water exchange between the nordic seas and the arctic ocean in this study we analyze core jm05 31gc from the northern fram strait at the very entrance to the arctic ocean the core contains sediments from marine isotope stages miss 4 2 the results show millennial timescale shifts in all the investigated proxies including the distribution of planktonic and benthic foraminifera planktonic and benthic oxygen and carbon isotopes and several sedimentological parameters in jm05 31gc the interstadials are characterized by relatively high surface and low bottom water temperatures low content of ice rafted debris and well ventilated bottom water stadials are characterized by the presence of icebergs and decreasing surface water and increasing bottom water temperatures due to increased inflow of atlantic subsurface water ventilation decreased during heinrich events and most stadials the results show that the dansgaard oeschger events strongly affected the water exchange between the nordic seas and the arctic ocean

abrupt shifts of productivity and sea ice regimes at the western barents sea slope from the last glacial maximum to the bolling allerod interstadial abstract advanced knowledge of spatio temporal constraints on the barents sea ice sheet during the late weichselian glaciation overshadows relatively limited understanding of seasonal sea ice experiencing an annual advance retreat cycle and primary productivity trends accompanying massive abrupt climate changes during glacial deglacial cycles such paleo reconstructions are crucial prerequisites for improved comprehension and prediction of current and future climate change here we investigate sea ice and phytoplankton biomarker distributions in a barents sea sediment core covering ca 25 8 15 4 cal kyr bp to elucidate abrupt shifts of spring summer sea ice concentrations and relative sympagic pelagic productivity trends at the southwestern continental slope despite significant presence of seasonal sea ice the last glacial maximum lgm and initial shelf edge deglaciation sedg at the core site are characterised by occurrence of productive coastal polynya adjacent to the maximum ice sheet extent the onset of perennial i e multi year ice cover and near zero productivity during heinrich stadial 1 hs1 ca 18 0 16 3 cal kyr bp accompanies significant meltwater fluxes from ice sheet debuttressing and the consequent stagnation of thermohaline circulation rapid sea ice retreat and unprecedented pelagic productivity observed after 16 3 cal kyr bp coincides with areal ice sheet deglaciation and is potentially linked to the release of sub surface heat and nutrient reservoirs together with reinvigorated deep water circulation following millennial heating of the deep ocean during hs1 we find that a multivariate fingerprinting approach involving assessment of both downcore and surface biomarker distributions is able to distinguish relative ice algal and pelagic diatom productivity driven by sea ice dynamics

axial convergence in a well mixed estuary abstract transverse secondary circulations involving surface convergence observed in a well mixed estuary in north wales are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary the circulation and axial convergence however are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross section produced by non uniform advection of the longitudinal gradient through the channel a diagnostic model using measured mean distributions of cross sectional density indicates surface transverse velocities 0 1 ms 1 similar to those observed in the estuary the model further predicts appreciable transverse divergent currents at a fractional depth of 0 75 a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators

ice events along the east siberian continental margin during the last two glaciations evidence from clay minerals abstract a large ice sheet or ice shelf may have developed repeatedly along the east siberian continental margin since the middle pleistocene in combination with ice rafted debris major elements and foraminifera clay mineral records from four gravity sediment cores are used to trace ice events along the east siberian continental margin during the last two glaciations the clay mineral assemblages show a strong link to the extent of the east siberian ice sheet esis rather than the ice shelf or icebergs sourced from the canadian arctic during the last two glaciations illite rich sediment from east siberia was transported mainly toward the deep basin whereas kaolinite rich sediment from the canadian arctic was mostly blocked from the east siberian continental margin we speculate that during marine isotope stage 2 mis2 the southern mendeleev ridge chukchi basin and chukchi rise may have been covered by an esis extended ice shelf however the chukchi plateau and northwind ridge were dominated by a laurentide sourced ice shelf during mis4 the esis may have undergone a marked expansion with an extended ice shelf occupying all of the chukchi plateau and northwind ridge thereby restricting iceberg discharge from other ice sheets

lateral circulation generates flood tide stratification and estuarine exchange flow in a curved tidal inlet abstractcross channel transect measurements of microstructure and velocity in a well mixed and curved tidal inlet in the german wadden sea show the occurrence of significant late flood stratification this stratification is found to be a result of lateral straining this study observes a strong single cell lateral circulation which is strongly pronounced at late flood and absent during most of ebb this tidal asymmetry is caused by a systematic interplay between centrifugal forcing and the lateral baroclinic pressure gradient during flood a positive feedback between the terms generates strong lateral circulation whereas during ebb a negative feedback leads to a suppression of the cross channel exchange a theoretical framework based on vorticity is developed which allows lateral and longitudinal circulation to be studied in a consistent way with this framework it is possible to show that the tidal asymmetry of the lateral flow is a major driver of residual longitudinal estuarine circulation here ide

radiogenic isotopic compositions of low concentration dust and aerosol from the gisp2 ice core abstract we have produced a record of radiogenic isotope compositions of particulate and soluble dust fractions from the gisp2 ice core from basal ice to the surface for each respective dust fraction we measured sr and nd isotope values to constrain the provenance of mineral dust and sea salt aerosol transported to the greenland ice sheet we have produced high precision measurements of end values on sub ng quantities of nd in ice core samples via tims with 1013 ohm resistors the range of measured end values of soluble material 12 9 2 6 to 7 3 0 9 is interpreted to represent sea salt aerosol based on sr isotope compositions that resemble seawater values 0 711007 0 000030 to 0 709416 0 000072 therefore the end potentially provides important insights to the atmospheric circulation patterns at high latitudes at least through mis 5 we compare end values of surface seawater from ocean basins and deltas at high latitudes to alternating relatively radiogenic and unradiogenic end at gisp2 we suggest these changes represent transitions between a climate system dominated by enhanced and persistent arctic oscillation positive anomalies as determined from sea level and ocean bottom pressure measurements and a state representative of modern atmospheric circulation which typically exhibits either negative anomalies or none altogether

ostracode mg ca paleothermometry in the north atlantic and arctic oceans evaluation of a carbonate ion effect 1 the reconstruction of deep sea bottom water temperature bwt is important to assess the ocean s response to and role in orbital and millennial scale climate change deep sea paleothermometry employs magnesium to calcium mg ca ratios in calcitic benthic microfaunas foraminifera ostracodes as a primary proxy method mg ca paleothermometry may however be complicated by bottom water carbonate ion chemistry which might affect mg ca ratios in shells to address temperature and carbonate ion influence on mg ca ratios we studied mg ca ratios in the benthic ostracode genus krithe in the north atlantic and arctic oceans using a 686 specimen core top collection including 412 previously unpublished analyses mg ca ratios are positively correlated to temperature in multiple species from the north atlantic bwt 0 885 mg ca 5 69 r2 0 73 and for k glacialis in the arctic ocean and nordic seas bwt 0 439 mg ca 5 14 r2 0 50 consistent with previously published calibrations we found no evidence for a relationship between krithe mg ca and carbonate ion saturation in the north atlantic ocean nordic seas and arctic ocean supporting the use of krithe mg ca for reconstructing past bwt

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

the influence of lateral advection on the residual estuarine circulation a numerical modeling study of the hudson river estuary abstract in most estuarine systems it is assumed that the dominant along channel momentum balance is between the integrated pressure gradient and bed stress scaling the amplitude of the estuarine circulation based on this balance has been shown to have predictive skill however a number of authors recently highlighted important nonlinear processes that contribute to the subtidal dynamics at leading order in this study a previously validated numerical model of the hudson river estuary is used to examine the forces driving the residual estuarine circulation and to test the predictive skill of two linear scaling relationships results demonstrate that the nonlinear advective acceleration terms contribute to the subtidal along channel momentum balance at leading order the contribution of these nonlinear terms is driven largely by secondary lateral flows under a range of forcing conditions in the model runs the advective acceleration terms nearly always act in concert with the baroclinic pressure gradie

high resolution record of late glacial and deglacial sea ice changes in fram strait corroborates ice ocean interactions during abrupt climate shifts the transition from last glacial to deglacial and subsequently to modern interglacial climate conditions was accompanied by abrupt shifts in the palaeoceanographic setting in the subpolar north atlantic knowledge about the role that sea ice coverage played during these rapid climate reversals is limited since most marine sediment cores from the higher latitudes provide only a coarse temporal resolution and often poorly preserved microfossils here we present a highly resolved reconstruction of sea ice conditions that characterised the eastern fram strait a key area for water mass exchange between the arctic ocean and the north atlantic for the past 30 ka bp this reconstruction is based on the distribution of the sea ice biomarker ip25 and phytoplankton derived biomarkers in a sediment core from the continental slope of western svalbard during the late glacial 30 ka to 19 ka bp recurrent advances and retreats of sea ice characterised the study area and point to a hitherto less considered oceanic and or atmospheric variability a long lasting perennial sea ice coverage in eastern fram strait persisted only at the very end of the last glacial maximum i e from 19 2 to 17 6 ka bp and was abruptly reduced at the onset of heinrich event 1 coincident with or possibly even inducing the collapse of the atlantic meridional overturning circulation amoc further maximum sea ice conditions prevailed during the younger dryas cooling event and support the assumption of an amoc reduction due to increased formation and export of arctic sea ice through fram strait a significant retreat of sea ice and sea surface warming are observed for the early holocene

three dimensional residual tidal circulation in an elongated rotating basin abstract the three dimensional residual circulation driven by tides in an elongated basin of arbitrary depth is described with a small amplitude constant density model on the f plane the inclusion of rotation fundamentally alters the residual flow with rotation fluid is drawn into the basin on the right side of an observer looking toward the closed end in the northern hemisphere and the return flow is on the opposite side a lateral circulation is superposed on the axial flow with upwelling over the deeper part of each section and downwelling near the sides the residual flow is driven by a combination of advective terms including the lateral advection of axial momentum associated with the coriolis acceleration and stokes forcing tidally averaged fluid parcel trajectories are determined by integrating the lagrangian mean velocities with or without rotation these trajectories vary considerably depending on small differences in initial position as well as on basin shape and other parameters of the

a sub millennial sediment record of ice stream retreat and meltwater storage in the baltic ice lake during the bolling allerod interstadial abstract the rapid retreat of the baltic ice stream and the development of the baltic ice lake is assessed using data from sediment cores retrieved from three sub basins in the southern baltic sea hydraulic piston coring by the international ocean discovery program iodp recovered for the first time intact glacial lake sequences overlying diamictons and other ice contact deposits at sites m0063 m0064 and m0065 based on the particle size and bulk sediment chemical composition the glacial lake sediments were subdivided into a proximal and a distal varve sequence the origin of a dark lithologically distinct horizon between the proximal and distal varves is attributed to a lake drainage event following the opening of a spillway in central sweden available age constraints suggest that the baltic ice lake developed during the bolling allerod interstadial and reached its maximum size at 13 ka ice retreat was forced by surface melt and amplified by calving in the upstream deepening lake environment furthermore rapid ice retreat and glacio isostatic processes allowed for the storage of substantial amounts of meltwater in the baltic ice lake during the allerod warm period subsequent lake drainage into the north atlantic took place through a conduit at higher latitude than previous drainage pathways the pronounced changes in meltwater storage and routing caused by the rapid retreat of the baltic ice stream may have contributed to abrupt climate change through the effects of changing freshwater supply on atlantic overturning circulation

quantifying the contributions of tidal straining and gravitational circulation to residual circulation in periodically stratified tidal estuaries abstract this numerical modeling study quantifies for the first time the contribution of various processes to estuarine circulation in periodically stratified tidal flow under the impact of a constant horizontal buoyancy gradient the one dimensional water column equations with periodic forcing are first cast into nondimensional form resulting in a multidimensional parameter space spanned by the modified inverse strouhal number and the modified horizontal richardson number as well as relative wind speed and wind direction and the residual runoff the along tide momentum equation is then solved for the tidal mean velocity profile in such a way that it is equated to the sum of the contributions of tidal straining resulting from the temporal correlation between eddy viscosity and vertical shear gravitational circulation resulting from the depth varying forcing by a constant horizontal buoyancy gradient wind straining and depth mean residual flow resulting from net freshwater runoff this definitio

a new concept for the paleoceanographic evolution of heinrich event 1 in the north atlantic new records of planktonic foraminiferal 18o and lithic and foraminiferal counts from eirik drift are combined with published data from the nordic seas and the ice rafted debris ird belt to portray a sequence of events through heinrich event 1 h1 these events progressed from an onset of meltwater release at not vert similar19 ka bp through the conventional h1 ird deposition phase in the ird belt starting from not vert similar17 5 ka bp to a final phase between 16 5 and not vert similar15 ka bp that was characterised by a pooling of freshwater in the nordic seas which we suggest was hyperpycnally injected into that basin after not vert similar15 ka bp this freshwater was purged from the nordic seas into the north atlantic which preconditioned the nordic seas for convective deep water formation this allowed an abrupt re start of north atlantic deep water nadw formation in the nordic seas at the bolling warming 14 6 ka bp in contrast to previous estimates for the duration of h1 i e 1000 years to only a century or two the total combined composite h1 signal presented here had a duration of over 4000 yrs not vert similar19 14 6 ka bp which spanned the entire period of nadw collapse it appears that deep water formation and climate are not simply controlled by the magnitude or rate of meltwater addition instead the location of meltwater injections may be more important with nadw formation being particularly sensitive to surface freshening in the arctic nordic seas

three dimensional tidal flow around headlands field measurements of tidal flow around a headland indicate secondary circulation induced by flow curvature the secondary flow defined to be the flow in the plane normal to the direction of the vertically averaged current is directed toward the headland near the bottom and seaward near the surface consistent with theoretical predictions the strength of the secondary flow varies from 5 to 15 of the streamwise flow it is strongest when the water column is stratified because of both the enhanced shear of the streamwise flow and the reduced frictional damping of the secondary flow the transverse exchange accomplished by the secondary flow significantly influences the structure of the streamwise flow causing a broadening of the transverse shear and changing the horizontal and vertical distributions of momentum it may also have a significant influence on horizontal dispersion in the vicinity of headlands

the importance of tidal and lateral asymmetries in stratification to residual circulation in partially mixed estuaries measurements collected in the york river estuary virginia demonstrate the important impact that tidal and lateral asymmetries in turbulent mixing have on the tidally averaged residual circulation a reduction in turbulent mixing during the ebb phase of the tide caused by tidal straining of the axial density gradient results in increased vertical velocity shear throughout the water column during the ebb tide in the absence of significant lateral differences in turbulent mixing the enhanced ebb directed transport caused by tidal straining is balanced by a reduction in the net seaward directed barotropic pressure gradient resulting in laterally uniform two layer residual flow however the channel shoal morphology of many drowned river valley estuaries often leads to lateral gradients in turbulent mixing tidal straining may then lead to tidal asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well mixed as a result the largest lateral asymmetries in turbulent mixing occur at the end of the ebb tide when the channel is significantly more stratified than the shoals the reduced friction at the end of ebb delays the onset of the flood tide increasing the duration of ebb in the channel conversely over the shoal regions where stratification is more inhibited by tidal mixing there is greater friction and the transition from ebb to flood occurs more rapidly the resulting residual circulation is seaward over the channel and landward over the shoal the shoal channel segregation of this barotropically induced estuarine residual flow is opposite to that typically associated with baroclinic estuarine circulation over channel shoal bathymetry

modeling the lateral circulation in straight stratified estuaries the dynamics of lateral circulation in an idealized straight estuary under varying stratification conditions is investigated using a three dimensional hydrostatic primitive equation model in order to determine the importance of lateral circulation to the momentum budget within the estuary for all model runs lateral circulation is about 4 times as strong during flood tides as during ebbs this flood ebb asymmetry is due to a feedback between the lateral circulation and the along channel tidal currents as well as to time varying stratification over a tidal cycle as the stratification is increased the lateral circulation is significantly reduced because of the adverse pressure gradient set up by isopycnals being tilted by the lateral flow itself when rotation is included a timedependent cross channel ekman circulation is driven and the tidally averaged bottom lateral circulation is enhanced toward the right bank when looking toward the ocean in the northern hemisphere this asymmetry in the tidally averaged bottom lateral circulation may lead to asymmetric sediment transport leading to asymmetric channel profiles in straight estuaries for the weakly stratified model run advection due to lateral currents is a dominant term in both the along channel and cross channel momentum equations over a tidal cycle and for the tidally averaged momentum equations in the tidally averaged along channel momentum equation lateral advection acts as a driving term for the estuarine exchange flow and can be larger than the along channel pressure gradient therefore it should not be ignored when estimating momentum budgets in estuaries

differences between the last two glacial maxima and implications for ice sheet δ18o and sea level reconstructions studies of past glacial cycles yield critical information about climate and sea level ice volume variability including the sensitivity of climate to radiative change and impacts of crustal rebound on sea level reconstructions for past interglacials here we identify significant differences between the last and penultimate glacial maxima lgm and pgm in terms of global volume and distribution of land ice despite similar temperatures and radiative forcing our analysis challenges conventional views of relationships between global ice volume sea level seawater oxygen isotope values and deep sea temperature and supports the potential presence of large floating arctic ice shelves during the pgm the existence of different glacial modes calls for focussed research on the complex processes behind ice age development we present a glacioisostatic assessment to demonstrate how a different pgm ice sheet configuration might affect sea level estimates for the last interglacial results suggest that this may alter existing last interglacial sea level estimates which often use an lgm like ice configuration by several metres likely upward

three dimensional salt dynamics in well mixed estuaries influence of estuarine convergence coriolis and bathymetry abstracta semianalytical three dimensional model is set up to dynamically calculate the coupled water motion and salinity for idealized well mixed estuaries and prognostically investigate the influence of each physical mechanism on the residual salt transport as a study case a schematized estuary with an exponentially converging width and a channel shoal structure is considered the temporal correlation between horizontal tidal velocities and tidal salinities is the dominant process for the landward residual salt transport the residual salt transport induced by residual circulation is locally significant but the induced salt transport integrated over the cross section is small the impacts of the estuarine geometry coriolis force and bathymetry on the salt dynamics are studied using three dedicated experiments in which the impact of each of these factors is studied separately to assess the impact of width convergence a convergent estuary without bathymetric variations or coriolis force is conside

convergence of lateral flow along a coastal plain estuary a set of velocity profiles obtained in the james river estuary with an acoustic doppler current profiler was used in combination with the results of an analytic tidal model to depict the appearance of surface lateral flow convergences v y during both flood and ebb stages of the tidal cycle the bathymetry of the estuary was characterized by a main channel and a secondary channel separated by relatively narrow shoals lateral surface flow convergences appeared over the edges of the channels and were produced by the phase lag of the flow in the channel relative to the shoals flood convergences developed in the late tidal stages and ebb convergences appeared soon after maximum currents most of these convergences caused fronts in the density field and flotsam lines that also appeared over the edges of the channel and that lasted 2 hours the transverse flows associated with the convergences were mostly in the same direction throughout the water column in fact the vertically averaged flow produced the same convergence patterns as those near the surface the analytic tidal model reproduced well the timing and location of the convergences as observed in the james river model results with different bathymetry emulated the results in other estuaries e g axial convergence in an estuary with a channel in the middle this work showed that the strength of lateral convergences along the estuary was proportional to the tidal amplitude and the channel steepness it also suggested that the convergences were produced mainly by the tidal flow interacting with the channel shoal bathymetry i e that they did not require the presence of density gradients however the analytic model underestimated the magnitude of the convergences and did not account for vertical circulations associated with fronts the formation of fronts resulted from the interaction of the tidal flow with the bathymetry and the density field

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

rapid changes in the mechanism of ocean convection during the last glacial period high amplitude rapid climate fluctuations are common features of glacial times the prominent changes in air temperature recorded in the greenland ice cores1 2 are coherent with shifts in the magnitude of the northward heat flux carried by the north atlantic surface ocean3 4 changes in the ocean s thermohaline circulation are a key component in many explanations of this climate flickering5 here we use stable isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate change events deep water was generated more or less continuously in the nordic seas during the latter part of the last glacial period 60 to 10 thousand years ago but by two different mechanisms the deep water formation occurred by convection in the open ocean during warmer periods interstadials but during colder phases stadials a freshening of the surface ocean reduced or stopped open ocean convection and deep water formation was instead driven by brine release during sea ice freezing these shifting magnitudes and modes nested within the overall continuity of deep water formation were probably important for the structuring and rapidity of the prevailing climate changes

the interaction between sea ice and salinity dominated ocean circulation implications for halocline stability and rapid changes of sea ice cover changes in the sea ice cover of the nordic seas have been proposed to play a key role for the dramatic temperature excursions associated with the dansgaard oeschger events during the last glacial in this study we develop a simple conceptual model to examine how interactions between sea ice and oceanic heat and freshwater transports affect the stability of an upper ocean halocline in a semi enclosed basin the model represents a sea ice covered and salinity stratified nordic seas and consists of a sea ice component and a two layer ocean the sea ice thickness depends on the atmospheric energy fluxes as well as the ocean heat flux we introduce a thickness dependent sea ice export whether sea ice stabilizes or destabilizes against a freshwater perturbation is shown to depend on the representation of the diapycnal flow in a system where the diapycnal flow increases with density differences the sea ice acts as a positive feedback on a freshwater perturbation if the diapycnal flow decreases with density differences the sea ice acts as a negative feedback however both representations lead to a circulation that breaks down when the freshwater input at the surface is small as a consequence we get rapid changes in sea ice in addition to low freshwater forcing increasing deep ocean temperatures promote instability and the disappearance of sea ice generally the unstable state is reached before the vertical density difference disappears and the temperature of the deep ocean do not need to increase as much as previously thought to provoke abrupt changes in sea ice

ventilation changes in intermediate water on millennial time scales in the se nordic seas 65 14 kyr bp 1 a glacial epibenthic 13c record from 773 m water depth in the se nordic seas reveals moderate but distinct changes in the ventilation 65 000 14 000 years ago the last glacial maximum the warm interstadials and the shorter stadials are characterized by high 13c values indicating well ventilated intermediate water masses in the nordic seas decreasing 13c values during the cold heinrich events signify a reduction in intermediate water ventilation we attribute the reduction to the development of a halocline causing a stop in convection and outflow from the nordic seas the well ventilated outflow water is replaced by warmer atlantic water which due to the stratification is isolated from the atmosphere its initial high 13c values are reduced due to ageing we ascribe the lack of response in the subsurface atlantic water during the stadials to the smaller geographical extent of these events as compared to the heinrich events

a younger thinner arctic ice cover increased potential for rapid extensive sea ice loss 1 satellite derived estimates of sea ice age and thickness are combined to produce a proxy ice thickness record for 1982 to the present these data show that in addition to the well documented loss of perennial ice cover as a whole the amount of oldest and thickest ice within the remaining multiyear ice pack has declined significantly the oldest ice types have essentially disappeared and 58 of the multiyear ice now consists of relatively young 2 and 3 year old ice compared to 35 in the mid 1980s ice coverage in summer 2007 reached a record minimum with ice extent declining by 42 compared to conditions in the 1980s the much reduced extent of the oldest and thickest ice in combination with other factors such as ice transport that assist the ice albedo feedback by exposing more open water help explain this large and abrupt ice loss

deep arctic ocean warming during the last glacial cycle in the arctic ocean the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier atlantic layer by a halocline ongoing sea ice loss and warming in t

observations of turbulence in a partially stratified estuary the authors present a field study of estuarine turbulence in which profiles of reynolds stresses were directly measured using an adcp throughout a 25 h tidal day the dataset that is discussed quantifies turbulent mixing for a water column in northern san francisco bay that experiences a sequence of states that includes a weak ebb and flood that are stratified followed by a strong and eventually unstratified ebb and flood these measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification examination of individual reynolds stress profiles along with profiles of richardson number and turbulent froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects using the measured turbulence production rate p the dissipation rate e is estimated the observed turbulence had values of e nn 2 20 all of the time and e nn 2 200 most of the time suggesting that the observed motions were buoyancy affected turbulence rather than internal waves however at times turbulent froude numbers in much of the upper water column were less than one indicating important stratification effects taken as a whole the data show that stratification affects the turbulent velocity variance q2 most severely that is observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes finally the dataset is compared to predictions made using the popular mellor yamada level 2 5 closure these comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions the length scale does not appear to compensate for these errors and as a result similar errors are seen in the eddy viscosity predictions it is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self transport from the near bed region to the overlying stratification

the alteration of lateral circulation under the influence of human activities in a multiple channel system changjiang estuary abstract extensive coastal engineering activities including channel dredging and constructions of large infrastructure have greatly altered hydrodynamics in the changjiang estuary while the changes in longitudinal circulation have been well documented limited studies have focused on the alteration in lateral circulation and its impact on estuarine circulation in the changjiang estuary lateral circulation in an estuary plays an important role in modulating estuarine circulation sediment transport and morphological change different from many of previous numerical studies we utilize an unstructured grid numerical model configured in a way to allow inundation over dikes and tidal flats numerical model experiments show significant changes in lateral circulation and stratification along the north passage in response to the altered geometry and bathymetry due to the channel deepening and the dike construction both the channel deepening and the dike construction led to an enhanced stratification further analysis suggests that the along channel the lateral and the vertical including up and downwelling and vertical mixing effects are all important in modulating the intra tidal stratification destratification process the channel deepening enhances both the lateral and the vertical effects around the high water slack the construction of dikes that completely blocks the lateral water exchange between the north passage and surrounding tidal flats changes the system from a multiple channel system into a single channel system with dike construction the estuarine circulation tends to have a horizontally sheared pattern with weaker friction and a stronger stratification

deglacial floods in the beaufort sea preceded younger dryas cooling a period of cooling about 13 000 years ago interrupted about 2 000 years of deglacial warming known as the younger dryas yd the event is thought to have resulted from a slowdown of the atlantic meridional overturning circulation in response to a sudden flood of laurentide ice sheet meltwater that reached the nordic seas oxygen isotope evidence for a local source of meltwater to the open western north atlantic from the gulf of st lawrence has been lacking here we report that the eastern beaufort sea contains the long sought signal of 18o depleted water beginning at 12 94 0 15 thousand years ago oxygen isotopes in the planktonic foraminifera from two sediment cores as well as sediment and seismic data indicate a flood of meltwater ice and sediment to the arctic via the mackenzie river that lasted about 700 years the minimum in the oxygen isotope ratios lasted 130 years we suggest that the floodwater travelled north along the canadian archipelago and then through the fram strait to the nordic seas where freshening and freezing near sites of deep water formation would have suppressed convection and caused the yd cooling by reducing the meridional overturning

a multiproxy reconstruction of the evolution of deep and surface waters in the subarctic nordic seas over the last 30 000yr on the basis of various lithological mircopaleontological and isotopic proxy records covering the last 30 000 calendar years cal kyr the paleoenvironmental evolution of the deep and surface water circulation in the subarctic nordic seas was reconstructed for a climate interval characterized by intensive ice sheet growth and subsequent decay on the surrounding land masses the data reveal considerable temporal changes in the type of thermohaline circulation open water convection prevailed in the early record providing moisture for the fennoscandian barents ice sheets to grow until they reached the shelf break at 26 cal kyr and started to deliver high amounts of ice rafted debris ird into the ocean via melting icebergs low epibenthic 18o values and small sized subpolar foraminifera observed after 26 cal kyr may implicate that advection of atlantic water into the nordic seas occurred at the subsurface until 15 cal kyr although modern like surface and deep water conditions first developed at 13 5 cal kyr thermohaline circulation remained unstable switching between a subsurface and surface advection of atlantic water until 10 cal kyr when ird deposition and major input of meltwater ceased during this time two depletions in epibenthic 13c are recognized just before and after the younger dryas indicating a notable reduction in convectional processes despite an intermittent cooling at 8 cal kyr warmest surface conditions existed in the central nordic seas between 10 and 6 cal kyr however already after 7 cal kyr the present day situation gradually evolved verified by a strong water mass exchange with the arctic ocean and an intensifying deep convection as well as surface temperature decrease in the central nordic seas this process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal kyr and eventually to today s steep east west surface temperature gradient the time discrepancy between intensive vertical convection after 5 cal kyr but warmest surface temperatures already between 10 and 6 cal kyr strongly implicates that widespread postglacial surface warming in the nordic seas was not directly linked to the rates in deep water formation

interglacial climates and the atlantic meridional overturning circulation is there an arctic controversy arctic palaeorecords are important to understand the natural range of forcing and feedback mechanisms within the context of past and present climate change in this temperature sensitive region a wide array of methods and archives now provide a robust understanding of the holocene climate evolution by comparison rather little is still known about older interglacials and in particular on the effects of the northward propagation of heat transfer via the atlantic meridional ocean circulation amoc into the arctic terrestrial records from this area often indicate a warmer and moister climate during past interglacials than in the holocene implying a more vigorous amoc activity this is in conflict with marine data although recognized as very prominent interglacials in antarctic ice cores cross latitudinal surface ocean temperature reconstructions show that little of the surface ocean warmth still identified in the northeast atlantic during older interglacial peaks e g mis5e 9 11 was further conveyed into the polar latitudes and that each interglacial developed its own specific palaeoclimate features interactive processes between water mass overturning and the hydrological system of the arctic and how both developed together out of a glacial period with its particular ice sheet configuration and relative sea level history determined the efficiency of an evolving interglacial amoc because of that glacial terminations developed some very specific water mass characteristics which also affected the climate evolution of the ensuing interglacial periods moreover the observed contrasts in the arctic directed meridional ocean heat flux between past interglacials have implications for the palaeoclimatic evaluation of this polar region crucial environmental factors of the arctic climate system such as the highly dynamical interactions between deep water mass flow surface ocean temperature salinity sea ice and atmosphere exert strong feedbacks on interglacial climate regionality that goes well beyond the arctic a sound interpretation of such processes from palaeoarchives requires a good understanding of the applied proxies fossils in particular are often key to the reconstruction of past conditions but the tremendously flexible adaptation strategies of biota sometimes hampers further in depth interpretations especially when considering their palaeoenvironmental meaning in the context of rapid palaeoclimatic changes and long term pleistocene evolution

the role of advection straining and mixing on the tidal variability of estuarine stratification abstractdata from the hudson river estuary demonstrate that the tidal variations in vertical salinity stratification are not consistent with the patterns associated with along channel tidal straining these observations result from three additional processes not accounted for in the traditional tidal straining model 1 along channel and 2 lateral advection of horizontal gradients in the vertical salinity gradient and 3 tidal asymmetries in the strength of vertical mixing as a result cross sectionally averaged values of the vertical salinity gradient are shown to increase during the flood tide and decrease during the ebb only over a limited portion of the cross section does the observed stratification increase during the ebb and decrease during the flood these observations highlight the three dimensional nature of estuarine flows and demonstrate that lateral circulation provides an alternate mechanism that allows for the exchange of materials between surface and bottom waters even when direct turbu

quaternary sea ice history in the arctic ocean based on a new ostracode sea ice proxy paleo sea ice history in the arctic ocean was reconstructed using the sea ice dwelling ostracode acetabulastoma arcticum from late quaternary sediments from the mendeleyev lomonosov and gakkel ridges the morris jesup rise and the yermak plateau results suggest intermittently high levels of perennial sea ice in the central arctic ocean during marine isotope stage mis 3 25 45 ka minimal sea ice during the last deglacial 16 11 ka and early holocene thermal maximum 11 5 ka and increasing sea ice during the mid to late holocene 5 0 ka sediment core records from the iceland and rockall plateaus show that perennial sea ice existed in these regions only during glacial intervals mis 2 4 and 6 these results show that sea ice exhibits complex temporal and spatial variability during different climatic regimes and that the development of modern perennial sea ice may be a relatively recent phenomenon

numerical analysis of stratification and destratification processes in a tidally energetic inlet with an ebb tidal delta stratification and destratification processes in a tidally energetic weakly stratified inlet in the wadden sea south eastern north sea are investigated in this modeling study observations of current velocity and vertical density structure show strain induced periodic stratification for the southern shoal of the tidal channel in contrast to this in the nearby central region of the channel increased stratification is already observed directly after full flood to investigate the processes leading to this different behavior a nested model system using getm is set up and successfully validated against field data the simulated density development along a cross section that includes both stations shows that cross channel stratification is strongly increasing during flood such that available potential energy is released in the deeper part of the channel during flood an analysis of the potential energy anomaly budget confirms that the early onset of vertical stratification during flood at the deeper station is mainly controlled by the stratifying cross channel straining of the density field in contrast to this in the shallow part of the channel the relatively weak cross channel straining is balanced by along channel straining and vertical mixing an idealized analytical model confirms the following hypothesis the laterally convergent flood current advecting laterally stratified water masses from the shallow and wide ebb tidal delta to the deep and narrow tidal channel has the tendency to substantially increase cross channel density gradients in the tidal channel this process leads to stratification during flood

a 600 ka arctic sea ice record from mendeleev ridge based on ostracodes arctic paleoceanography and sea ice history were reconstructed from epipelagic and benthic ostracodes from a sediment core hly0503 06jpc 800 m water depth located on the mendeleev ridge western arctic ocean the calcareous microfaunal record ostracodes and foraminifers covers several glacial interglacial cycles back to estimated marine isotope stage 13 mis 13 500 ka with an average sedimentation rate of 0 5 cm ka for most of the stratigraphy mis 5 13 results based on ostracode assemblages and an unusual planktic foraminiferal assemblage in mis 11 dominated by a temperate water species turborotalita egelida show that extreme interglacial warmth high surface ocean productivity and possibly open ocean convection characterized mis 11 and mis 13 400 and 500 ka respectively a major shift in western arctic ocean environments toward perennial sea ice occurred after mis 11 based on the distribution of an ice dwelling ostracode acetabulastoma arcticum spectral analyses of the ostracode assemblages indicate sea ice and mid depth ocean circulation in western arctic ocean varied primarily at precessional 22 ka and obliquity 40 ka frequencies

arctic ocean glacial history while there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the arctic ocean a holistic view on the northern hemisphere s late quaternary ice sh

abrupt climatic change during the last interglacial glacial cycle in the polar north atlantic abstract we have investigated five sediment cores from the margin and abyssal plain off western svalbard barents sea in the polar north atlantic we discuss the palaeoceanography and its relationship to glacial history during the last interglacial glacial cycle based on detailed stratigraphic analyses of planktic and benthic foraminiferal fauna oxygen and carbon isotopes on foraminifera ice rafted debris grains 500 m and radiocarbon dates the sediment cores combined cover the last interglacial glacial cycle including marine isotope stage mis 5 1 sedimentation rates are generally high ranging from 3 to 100 cm 1000 years atlantic water repeatedly reached the polar north atlantic during the last interglacial glacial cycle and was present for at least 50 of the total time interval periods having atlantic water advection termed hp zones hp high productivity are characterised by high planktic foraminiferal production a small increase of benthic and subpolar planktic foraminifera benthic foraminiferal and stable isotope data indicate active deep water formation during the hp zones in substage 5e there are indications of sea surface temperatures similar to those of today the major glaciations on svalbard during the last interglacial glacial cycle are related to hp zones some hp zones correspond to an early build up phase of the ice sheets peak glacials all correspond to one hp zone that subsequently is terminated by a meltwater event however periods with advection of atlantic water during mid mis 3 do not correlate to a glacial phase on svalbard but rather to the western fennoscandian ice sheet the records in the present study are correlated to the northeast atlantic the arctic ocean and to the greenland ice core record and show parallel millennial scale variability

coesite in the straumen kyanite eclogite pod norway the occurrence of coesite in the straumen kyaniteeclogite pod in the western gneiss region wgr norway is described although the microtextures of coesite and of its partial or complete transformation into quartz are similar to those described at the first coesite occurrence in norway the grytting dolomite eclogite pod there are some significant differences at this second locality these include the host mineral of the sio2 inclusions being garnet instead of clinopyroxene and also occasionally kyanite an initial coesite eclogite sub facies paragenesis was followed by a quartz eclogite sub facies paragenesis which in addition to rutile and dolomite or calcite contains the highest number 7 of silicate phases known in any norwegian eclogite garnet clinopyroxene kyanite quartz phengite zoisite and clinoamphibole this second occurrence of definite coesite i consolidates the very high pressury estimates about 30 kbar minimum previously deduced for certain eclogite pods occurring in the wgr and ii justifies the recognition of the norwegian coesite eclogite province constituted of a nappe terrane of about 30 kbar parageneses now residing in the wgr

a numerical model for the entire wadden sea skill assessment and analysis of hydrodynamics a baroclinic three dimensional numerical model for the entire wadden sea of the german bight in the southern north sea is first assessed by comparison to field data for surface elevation current velocity temperature and salinity at selected stations and then used to calculate fluxes of volume and salt inside the wadden sea and the exchange between the wadden sea and the adjacent north sea through the major tidal inlets the model is simulating the reference years 2009 2011 an overview of tidal prisms and residual volume fluxes of the main inlets and their variability is given in addition data from an intensive observational campaign in a tidal channel south of the island of spiekeroog as well as satellite images and observations of sea surface properties from a ship of opportunity are used for the skill assessment finally the intensity of estuarine overturning circulation and its variability in the tidal gullies are quantified and analyzed as function of gravitational and wind straining using various estimates including total exchange flow tef regional differences between the gullies are assessed and drivers of the estuarine circulation are identified for some inlets the longitudinal buoyancy gradient dominates the exchange flow for some others wind straining is more important also the intensity of tidal straining scaled covariance of eddy viscosity and vertical shear depends on buoyancy gradient and wind forcing in different ways depending on local topography orientation toward the main wind direction and influence by freshwater run off inside or outside the tidal basin

paleoceanography of northeastern fram strait since the last glacial maximum palynological evidence of large amplitude changes abstract sea surface conditions in northeastern fram strait since the last glacial maximum lgm were reconstructed from cores msm5 5 712 2 and ps2863 1 2 based on palynological assemblages ecological preferences of dinocysts and application of the modern analog technique dinocyst in lgm sediments are sparse but their assemblages reflect mild summer conditions given the regional context and evidence from other tracers the dinocyst assemblages of the lgm could relate to regional fluxes of dinocysts during exceptional mild summers from 19 to 14 7 ka dinocyst data suggest very cold conditions with extensive sea ice cover while abundant reworked palynomorphs indicate intense glacial erosion an abrupt transition at 14 7 14 5 ka was marked by a peak in summer temperatures coinciding with a rapidly deposited sediment layer related to a regional meltwater plume event in western svalbard from 14 7 to 12 6 ka large seasonal temperature contrasts with mild summers and cold winters together with low salinity indicate continuous melting of the svalbard barents sea ice sheet fostered by warm climate at 12 6 ka the regional onset of the younger dryas was marked by cooling and increased salinity on a regional scale the 12 6 12 ka interval corresponds to an important transition involving enhanced circulation of arctic waters around svalbard and establishment of coastal fronts along its northern and western margins modern like oceanic conditions with relatively high salinity and low seasonal temperature contrast developed at about 7 6 ka since then a slight cooling is observed especially in winter this study offers a comprehensive picture of the deglacial phases in eastern fram strait with unique data on the sea surface salinity which controls surface water stratification and plays an important role in ocean circulation

influence of brine formation on arctic ocean circulation over the past 15 million years the early oceanographic history of the arctic ocean is important in regulating and responding to climatic changes however constraints on its oceanographic history preceding the quaternary the past 1 8 myr have become available only recently because of the difficulties associated with obtaining continuous sediment records in such a hostile setting here we use the neodymium isotope compositions of two sediment cores recovered near the north pole to reconstruct over the past approx15 myr the sources contributing to arctic intermediate water a water mass found today at depths of 200 to 1 500 m we interpret high neodymium ratios for the period between 15 and 2 myr ago and for the glacial periods thereafter as indicative of weathering input from the siberian putoranan basalts into the arctic ocean arctic intermediate water was then derived from brine formation in the eurasian shelf regions with only a limited contribution of intermediate water from the north atlantic in contrast the modern circulation pattern with relatively high contributions of north atlantic intermediate water and negligible input from brine formation exhibits low neodymium isotope ratios and is typical for the interglacial periods of the past 2 myr we suggest that changes in climatic conditions and the tectonic setting were responsible for switches between these two modes

sedimentation controls on methane hydrate dynamics across glacial interglacial stages an example from international ocean discovery program site u1517 hikurangi margin