silicic acid dynamics in the glacial sub antarctic implications for the silicic acid leakage hypothesis 1 the silicic acid leakage hypothesis salh purports that changes in silicon and nitrogen depletion ratios in the glacial antarctic created a large pool of unused silicic acid that was transported to lower latitudes in subantarctic mode water samw where it enhanced diatom productivity lowering atmospheric pco2 however increased opal accumulation beneath the sub antarctic during glacial times implies significant consumption of silicic acid in subantarctic surface waters that may have significantly diminished or eliminated si leakage to test how nutrient dynamics in the sub antarctic affected the si oh 4 content of samw during the last glacial period we produced 30si opal records for cores from the subantarctic and subtropical zones of the indian ocean spanning the last 50 000 years comparison with diatom bound 15n records shows that subantarctic surface waters were enriched in si relative to n during the last glaciation consistent with the salh the record from the subtropics does not show this enrichment in si during the last glacial period suggesting that subantarctic surface waters were mainly incorporated into samw rather than being transported across the subtropical front isotope mass balance calculations were used to test for si leakage from the sub antarctic the results show that silicic acid concentration in samw would more than double during the last glaciation if upwelling and northward ekman drift in the antarctic were similar to the present day circulation calculations that assume increased stratification in the glacial antarctic eliminate si leakage but they do not produce the known increase in opal burial in the glacial sub antarctic reconciling the isotope data and opal burial records with a highly stratified antarctic requires the addition of a large local source of silicic acid in the glacial sub antarctic that is inconsistent with present day circulation and nutrient distributions including such a source in our calculations results in significant opal burial in the sub antarctic but it does not enhance si leakage over that occurring in the holocene resolving past changes in the circulation of the southern ocean is clearly vital to future tests of the salh

controls on the silicon isotope distribution in the ocean new diagnostics from a data constrained model the global distributions of the silicon isotopes within silicic acid are estimated by adding isotope fractionation to an optimized data constrained model of the oceanic silicon cycle that is embedded in a data assimilated steady circulation including fractionation during opal dissolution improves the model s ability to capture the approximately linear relation between isotope ratio 30si and inverse silicic acid concentration observed in the deep atlantic to quantify the importance of hydrographic control on the isotope distribution 30si is partitioned into contributions from preformed and regenerated silicic acid further partitioned according to euphotic zone origin we find that the large scale features of the isotope distribution in the atlantic basin are dominated by preformed silicic acid with regenerated silicic acid being important for setting vertical gradients in the pacific and indian oceans preformed and regenerated silicic acid make roughly equally important contributions to the pattern of the isotope ratio with gradients of the preformed and regenerated contributions tending to cancel each other in the deep pacific the southern ocean euphotic zone is the primary origin of both the preformed and regenerated contributions to 30si nearly the entire preformed part of 30si is of southern ocean and north atlantic origin the regenerated part of 30si in the atlantic basin also has a contribution of central atlantic 40 s 40 n origin that is comparable in magnitude to the north atlantic contribution in other basins the central pacific and indian ocean are the second largest contributors to the regenerated part of 30si

modelling twentieth century global ocean circulation and iceberg flux at 48 n implications for west greenland iceberg discharge abstract we have used a coupled ocean iceberg model to study the variation in global ocean circulation and north atlantic iceberg flux from 1900 to 2008 the latter component of the study focused particularly on greenland icebergs feeding into the labrador current and past newfoundland the model was forced with daily heat freshwater and wind fluxes from the twentieth century reanalysis the reanalysis heat fluxes were shown to be offset from the shorter ncep reanalysis and a grid point correction was applied to this component of the forcing the model produces a generally realistic ocean circulation although with an enhanced atlantic meridional overturning largely due to the forcing the modelled iceberg flux at 48 n is well correlated with the long term observed flux when using a modelled iceberg discharge that varies in a similar fashion to the highly variable observed flux at 48 n from this model we infer changes in the spatial and temporal variability of iceberg calving from western greenland during the first third of the twentieth century the majority of modelled icebergs reaching 48 n derive from southern greenland while only after 1930 is the traditional perspective of a majority of such icebergs originating from baffin bay consistent with model results decadal scale changes in the dominant regional sources are found with oscillations between western greenland and northern baffin bay the latter origin was modelled to be most important in the last third of the twentieth century although west greenland sources have increased in importance in recent years the model correctly reproduces the pronounced late spring peak in flux at 48 n for southern greenland icebergs but has an approximately six month offset for icebergs from baffin bay most likely due to resolution issues leading to model icebergs not being delayed in shallow coastal waters whereas in reality they may be grounded for some time or trapped in coastal sea ice

relevance of silicon isotopes to si nutrient utilization and si source assessment in antarctic waters 1 we analyzed 29si of dissolved silicate for eight water column profiles across the southern ocean south of australia in spring 2001 from the seasonal ice zone siz north to the subantarctic zone saz including the first isotopic compositions measured for si depleted seawaters all profiles display mixed layer enrichments in heavy si isotopes relative to deep water in accordance with preferential uptake of the light isotope by diatoms as silicate levels decrease from the siz northward across the polar front zone pfz to the saz surface and mesopelagic 29si signatures generally become progressively heavier but the most si depleted saz waters do not exhibit 29si values heavier than in the pfz this intricacy appears to derive from variations in the vertical and horizontal supply of silicate to surface waters and by applying a steady state open system model we estimate a fractionation factor 29 between diatoms and seawater of 0 45 0 17 independently of zones and phytoplankton community though encouraging these results are related to latitudinal changes in mesopelagic 29si values complexity in surface silicate 29si correlations and differences from previous studies which underline the need for caution in the use of silicon isotopes in paleoceanographic studies until systematic efforts have been undertaken to better understand modern variations

southern ocean control of silicon stable isotope distribution in the deep atlantic ocean the fractionation of silicon si stable isotopes by biological activity in the surface ocean makes the stable isotope composition of silicon delta si 30 dissolved in seawater a sensitive tracer of the oceanic biogeochemical si cycle we present a high precision dataset that characterizes the delta si 30 distribution in the deep atlantic ocean from denmark strait to drake passage documenting strong meridional and smaller but resolvable vertical delta si 30 gradients we show that these gradients are related to the two sources of deep and bottom waters in the atlantic ocean waters of north atlantic and nordic origin carry a high delta 30 sisignature of 1 7 parts per thousand into the deep atlantic while antarctic bottom water transports si with a low delta si 30 value of around 1 2 parts per thousand the deep atlantic delta si 30 distribution is thus governed by the quasi conservative mixing of si from these two isotopically distinct sources this disparity in si isotope composition between the north atlantic and southern ocean is in marked contrast to the homogeneity of the stable nitrogen isotope composition of deep ocean nitrate delta n 15 no3 we infer that the meridional delta si 30 gradient derives from the transport of the high delta si 30 signature of southern ocean intermediate mode waters into the north atlantic by the upper return path of the meridional overturning circulation moc the basin scale deep atlantic delta si 30 gradient thus owes its existence to the interaction of the physical circulation with biological nutrient uptake at high southern latitudes which fractionates si isotopes between the abyssal and intermediate mode waters formed in the southern ocean

silicon isotope fractionation during nutrient utilization in the north pacific the distribution of silicon in the north pacific is controlled by the utilization of silicic acid by diatoms a process that fractionates silicon si isotopic compositions silicon isotope variations are presented for six water column profiles from the surface mixed layer down to the deep waters of the north pacific ocean although the observed si isotopic variations display an apparently simple inverse relationship with dissolved nutrient concentrations in fact they reflect mixing of surface waters undergoing active si isotope fractionation and deep waters with more uniform concentrations and isotope compositions samples from the surface of the subtropical gyre have the lowest dissolved si concentrations and heaviest si isotope compositions of marine waters measured thus far fractionation in the surface waters follows a typical rayleigh type distillation curve for a closed surface water reservoir resulting from stratification of the surface layer in the subarctic region in contrast an open system prevails within the subtropical gyre where there is significant recycling of silicic acid in the upper water column and lateral transport of silicon within surface currents for deep waters the si isotope composition distinguishes between the northern north pacific deep water npdw and the southerly derived bottom water the relatively light si isotope compositions measured from waters within the subarctic gyre provides evidence for isolation of the nutrient pool in the north pacific

sea ice and iceberg dynamic interaction 1 a model of iceberg motion has been implemented in the los alamos sea ice model cice individual bergs are tracked under the influence of winds currents sea surface tilt coriolis and sea ice forcing in turn sea ice is affected by the presence of icebergs primarily as obstacles that cause the sea ice to ridge on the upstream side or create open water on the downstream side of the bergs open water formed near icebergs due to sea ice ridging and blocking of sea ice advection increases level and ridged ice downstream of the bergs through increased frazil ice formation resulting anomalies in sea ice area and thickness compared with a simulation without icebergs are transported with the sea ice flow expanding over time although local changes in the sea ice distribution may be important for smaller scale studies these anomalies are small compared with the total volume of sea ice and their effect on climate scale variables appears to be insignificant

factors controlling the silicon isotope distribution in waters and surface sediments of the peruvian coastal upwelling we present the first systematic study of the silicon isotope composition in the water column 30sisi oh 4 and in diatoms 30sidiatom from the underlying surface sediments in a coastal upwelling region the surface waters upwelling on the shelf off peru are mainly fed by southward flowing subsurface waters along the coast which show a mean 30sisi oh 4 of 1 5 the concentration of dissolved silicic acid si oh 4 increases towards the south in these waters and with increasing water depth suggesting lateral mixing with water masses from the south and intense remineralisation of particulate biogenic silica bsio2 in the water column and in the surface sediments surface waters in the realm of the most intense upwelling between 5 s and 15 s have only marginally elevated 30sisi oh 4 values 30sisi oh 4 1 7 with respect to the source si isotope composition whereas further north and south where upwelling is less pronounced surface waters are more strongly fractionated 30sisi oh 4 up to 2 8 due to the stronger utilisation of the smaller amounts of available si oh 4 the degree of si oh 4 utilisation in the surface waters along the shelf estimated from the si oh 4 concentration data ranges from 51 to 93 the 30sidiatom values of hand picked diatoms in the underlying surface sediments vary from 0 6 to 2 0 which is within the range of the expected fractionation between surface waters and diatoms the fractionation signal in the surface waters produced during formation of the diatoms is reflected by the 30sidiatom values in the underlying sediments with the lowest 30sidiatom values in the main upwelling region the silicon isotope compositions of bsio2 30sibsio2 from the same surface sediment samples are generally much lower than the 30sidiatom signatures indicating a significant contamination of the bsio2 with biogenic siliceous material other than diatoms such as sponge spicules this shift towards lighter 30sibsio2 values by up to 1 3 compared to 30sidiatom signatures for the same surface sediment samples potentially biases the interpretation of 30si paleorecords from sediments with low bsio2 concentrations and thus the reconstruction of past si oh 4 utilisation in surface waters

prediction of iceberg trajectories for the north atlantic and arctic oceans icebergs are a well known hazard for shipping their study also provides information about diverse geophysical processes as varied as ocean circulation air sea fluxes calving rates of glaciers or the mass balance of ice sheets as a first step to obtaining this information from iceberg data we have developed a model of iceberg drift driven by ocean and atmospheric forcing derived from general circulation models we have applied the drift model to a distribution of typical icebergs released from the main tidewater glaciers of the north atlantic and arctic oceans we demonstrate that the main driving force of iceberg motion is rooted in the unsteady component of oceanic advection from simulated trajectories we are able to reproduce the observed southwards limit of iceberg penetration and demonstrate sometimes surprising geographical links between iceberg origin and ultimate melting zones

an analytical model of iceberg drift abstractthe fate of icebergs in the polar oceans plays an important role in earth s climate system yet a detailed understanding of iceberg dynamics has remained elusive here the central physical processes that determine iceberg motion are investigated this is done through the development and analysis of an idealized model of iceberg drift the model is forced with high resolution surface velocity and temperature data from an observational state estimate it retains much of the most salient physics while remaining sufficiently simple to allow insight into the details of how icebergs drift an analytical solution of the model is derived which highlights how iceberg drift patterns depend on iceberg size ocean current velocity and wind velocity a long standing rule of thumb for arctic icebergs estimates their drift velocity to be 2 of the wind velocity relative to the ocean current here this relationship is derived from first principles and it is shown that the relationship holds in the limit of

simulating the modern δ30si distribution in the oceans and in marine sediments the 30si of biogenic silica inline image in marine sediments is a promising proxy for the reconstruction of silicic acid utilization by diatoms in the geological past the application of this proxy however requires an understanding of the modern 30si distributions and their controlling mechanisms here we present results from a modern climate simulation with a coupled ocean sediment model that includes a prognostic formulation of biogenic silica production with concurrent silicon isotopic fractionation in agreement with previous studies biological fractionation combined with physical transport and mixing determines the oceanic distribution of simulated 30si a new finding is a distinct seasonal cycle of 30si in the surface ocean which is inversely related to that of silicic acid concentration and mixed layer depth we also provide the first simulation results of sedimentary 30si which reveal that 1 the inline image distribution in the surface sediment reflects the exported inline image signal from the euphotic zone and 2 the dissolution of biogenic silica in the sediment acts as a source of relatively light 30si into the bottom waters of the polar oceans while it is a source of heavier 30si to the subtropical south atlantic and south pacific

source to sink statistical identification of regional variations in the mineralogy of surface sediments in the western nordic seas 58 n 75 n 10 w 40 w the non clay and clay mineralogies of the 2 mm sediment fraction from 157 seafloor samples in the western nordic seas is calculated by quantitative x ray diffraction analysis our sample coverage includes the iceland shelf the ice impacted east greenland margin and the greenland iceland and irminger seas the bedrock geology of source areas includes precambrian basement rocks devonian red beds and younger marine and fluvial sediments and large outcrops of early tertiary to recent flood basalts and other volcanic facies sediment erosion and transport in the area are associated with ice sea ice and icebergs and glacial meltwater and fluvial sediment plumes we use principal component analysis fuzzy k means clustering and discriminant function analysis to reduce the original 31 minerals 157 sites matrix to more manageable 5 regional coherent clusters of sites which are well discriminated by the weight of six minerals namely quartz pyroxene illites magnetite and maghemite amorphous silica e g glass and muscovite the close spatial relationship between bedrock geology and discrete cluster membership stresses the importance of this variable and indicates that far distant sediment transport by sea ice icebergs or other mechanisms although occurring does not radically change the mineral composition of present day sea floor sediments our improved knowledge of the regional mineral assemblages allows for tracking distinct eastern greenland and icelandic sources

variations in felsic versus mafic sources in the western nordic seas during mis 1 to mis 4 abstract the goal of this paper is to employ quantitative methods to first describe the spatial variations in the quantitative non clay and clay mineral composition of seafloor surface sediments secondly use those data to describe the temporal variations in mineral compositions of nine cores that extend variously between the present day and 85 ka bp and thirdly describe regional variations in the transport of felsic versus mafic rich glacial marine sediments at four sites close to denmark strait k mean clusters of recent seafloor samples n 156 distinguished four mineral facies which characterize a range of mafic to felsic compositions these samples serve as a training set to identify the down core mineral facies in eight cores n 1053 the degree of provenance mixing is measured by fuzzy clustering on an unsupervised cluster analysis classification decision trees validate the mineral facies only 6 misclassified and indicate that variations in the weight of pyroxene are of primary importance in defining cluster membership mineral compositions identify two end member core sites jm07 174gc greenland sea and lo9 18 reykjanes ridge while the cores north and south of the greenland faroes ridge cores ps2644 md99 2274 2280 and 2323 are a mixture of mafic rich and felsic rich mineral facies variations in magnetic susceptibility and quartz wt exhibit coherent runs of values above or below their medians in three of the cores occurred indicating that these cannot be ascribed to random variations p

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

relationship between tidewater glacier calving velocity and water depth at the calving front an analysis of the relationship between iceberg calving rates and water depth has been completed for 22 tidewater glaciers a linear relationship provides reasonable accuracy with a correlation coefficient of 0 85 for all tidewater glaciers examined whether they be polar or temperate the polar glaciers have a slightly lower calving rate for a given water depth this relationship indicates a lower calving rate for water depths over 50 m than determined by brown and others 1982 it is based only on glaciers or ice streams and cannot be applied to ice shelves

a comparison between multiproxy and historical data ad 1990 1840 of drift ice conditions on the east greenland shelf 66 n the reduction in sea ice along the se greenland coast during the last century has severely impacted ice rafting to this area in order to reconstruct ice rafting and oceanographic conditions in the area of denmark strait during the last 150 years we conducted a multiproxy study on three short 20 cm sediment cores from outer kangerdlugssuaq trough 300 m water depth the proxy based data obtained have been compared with historical and instrumental data to gain a better understanding of the ice sheet ocean interactions in the area a robust chronology has been developed based on 210pb and 137cs measurements on core po175gkc 9 66 2 n 32 w and expanded to the two adjacent cores based on correlations between calcite weight percent records our proxy records include sea ice and phytoplankton biomarkers and a variety of mineralogical determinations based on the 2 mm sediment fraction including identification with quantitative x ray diffraction ice rafted debris counts on the 63 150 m sand fraction

the geochemistry of early diagenetic dolostones from a low salinity jurassic lagoon in the middle jurassic great estuarine group of nw scotland argillaceous dolostones and dolomitic limestones occur interbedded with limestones and mudstones of low salinity 9closed lagoon9 palaeoenvironments these microcrystalline dolostones are imperfectly ordered and calcium rich and have escaped pervasive dissolution reprecipitation processes during burial diagenesis geochemical studies show that the dolostones retain a significant early diagenetic memory 18 o dol values 0 o are the heaviest reported from the great estuarine group and probably formed by evaporative fractionation from meteoric water 13 c dol values 2 o may represent precipitation from a standing body of meteoric water partially equilibrated with atmospheric co 2 calcite coexisting with the dolomite is lighter in both 13 c and 18 o by 0 8 and 3 7 o respectively it is suggested that the minerals were not co precipitates from one solution strontium is preferentially concentrated in the dolomite with 87 sr 86 sr values between 0 7095 and 0 7099 these values which are slightly more radiogenic than jurassic seawater may originate from the continental weathering of ancient limestones mixed with seawater derived from the compaction of underlying sediments sm nd model ages are consistent with weathering of dalradian rocks in the drainage hinterland dolomite formation proceeded by dolomitization of a caco 3 precursor in a warm seasonal climate during dry arid seasons the lagoon water evaporated concentrating 18 o raising mg ca and sr ca ratios and thereby promoting dolomitization during humid wet periods sr poor caco 3 precipitated with lighter 18 o and 13 c values

coupling of the distribution of silicon isotopes to the meridional overturning circulation of the north atlantic ocean abstract the distribution of silicon isotopes within silicic acid 30 si oh 4 was examined along a section in the north atlantic from the cape verde islands off africa to cape cod massachusetts in north america surface water displayed elevated 30 si oh 4 associated with biological fractionation of si during silica production below 300 m variations in 30 si oh 4 were closely tied to the distribution of water masses as diagnosed through optimum multiparameter analysis confirming a tight relationship between 30 si oh 4 and the meridional overturning circulation in the atlantic a linear relationship between 30 si oh 4 and the inverse of silicic acid concentration supported control of si isotope distribution by conservative mixing of end member water masses of different isotopic composition in the atlantic there was a suggestion of a weak local minimum in 30 si oh 4 in deep waters above the trans atlantic geotraverse hydrothermal zone on the mid atlantic ridge consistent with the light 30 si oh 4 of hydrothermal waters the lightest 30 si oh 4 values were observed in the deep western and deep eastern basins where antarctic bottom water aabw dominated the heaviest values in subsurface waters occurred in north atlantic deep water due to strong ventilation and the contribution of heavy northern source waters that are influenced by the arctic ocean the concept of a silicon isotope bipole is introduced to explain how the isotopic differences between the northern and southern end member water masses arise and how they influence si isotope distributions northern end member water masses are heavy due to the influence of the arctic ocean bottom topography prevents light deep waters from entering the arctic and the further removal of light isotopes through local biological productivity results in extremely heavy 30 si oh 4 within the arctic light aabw dominates the southern end member the southern ocean silicic acid trap distills heavier isotopes of si out of the southern ocean as preformed silicic acid within antarctic mode waters while retaining light isotopes that become incorporated into aabw the influence of the silicon isotope bipole is predicted to be strongest in the atlantic as the net flow of waters through the arctic ocean is from the pacific to the atlantic

deconvolving the controls on the deep ocean s silicon stable isotope distribution abstract we trace the marine biogeochemical silicon si cycle using the stable isotope composition of si dissolved in seawater expressed as 30 si open ocean 30 si observations indicate a surprisingly strong influence of the physical circulation on the large scale marine si distribution here we present an ocean general circulation model simulation that deconvolves the physical and biogeochemical controls on the 30 si distribution in the deep oceanic interior by parsing dissolved si into its preformed and regenerated components we separate the influence of deep water formation and circulation from the effects of biogeochemical cycling related to opal dissolution at depth we show that the systematic meridional 30 si gradient observed in the deep atlantic ocean is primarily determined by the preformed component of si whose distribution in the interior is controlled solely by the circulation we also demonstrate that the 30 si value of the regenerated component of si in the global deep ocean is dominantly set by oceanic regions where opal export fluxes to the deep ocean are large i e primarily in the southern ocean s opal belt the global importance of this regionally dynamic si cycling helps explain the observed strong physical control on the oceanic 30 si distribution since most of the regenerated si present within the deep atlantic and indo pacific oceans is in fact transported into these basins by deep waters flowing northward from the southern ocean our results thus provide a mechanistic explanation for the observed 30 si distribution that emphasizes the dominant importance of the southern ocean in the marine si cycle

a robust multisite holocene history of drift ice off northern iceland implications for north atlantic climate an important indicator of holocene climate change is provided by evidence for variations in the extent of drift ice a proxy for drift ice in iceland waters is provided by the presence of quartz q

fractionation of silicon isotopes during biogenic silica dissolution abstract silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization primarily by diatoms in surface waters of the ocean interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean as driven by biological fractionation during the production of biogenic silica bsio2 or opal by diatoms however other factors which may influence the silicon isotope composition of diatom opal particularly post formational aging and maturation processes remain largely uninvestigated here we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bsio2 suspended in seawater under closed conditions this fractionation acts counter to that occurring during bsio2 production and at about half its absolute magnitude with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor edsi bsi of 0 55 corresponding to a fractionation factor 30 28 of 0 99945 the enrichment factor did not vary with source material indicating the lack of a significant species effect or with temperature from 3 to 20 c thus the dissolution of bsio2 produces dissolved silicon with a 30si value that is 0 55 more negative than its parent bsio2 an effect that must be accounted for when interpreting oceanic 30si distributions the 30si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed implying a measurable 0 1 change in the relative 30si of opal samples whenever the difference in preservation efficiency between them is 20 this effect could account for 10 30 of the difference in diatom 30si values observed between glacial and interglacial conditions it is unlikely however that the inferred maximum possible change in b30sio2 of 0 55 would be manifested in situ as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage

a switch from si oh 4 to no 3 depletion in the glacial southern ocean 1 phytoplankton in the antarctic deplete silicic acid si oh 4 to a far greater extent than they do nitrate no3 this pattern can be reversed by the addition of iron which dramatically lowers diatom si oh 4 no3 uptake ratios higher iron supply during glacial times would thus drive the antarctic towards no3 depletion with excess si oh 4 remaining in surface waters new 30si and 15n records from antarctic sediments confirm diminished si oh 4 use and enhanced no3 depletion during the last three glaciations the present low si oh 4 water is transported northward to at least the subtropics we postulate that the glacial high si oh 4 water similarly may have been transported to the subtropics and beyond this input of si oh 4 may have caused diatoms to displace coccolithophores at low latitudes weakening the carbonate pump and increasing the depth of organic matter remineralization these effects may have lowered glacial atmospheric pco2 by as much as 60 ppm

the sizes frequencies and freeboards of east greenland icebergs observed using ship radar and sextant the scoresby sund fjord system east greenland contains the most productive fast flowing outlet glaciers draining east from the greenland ice sheet calving 18 km3 a 1 of icebergs the sizes frequencies and freeboards of 1900 icebergs were measured from f s polarstern using ship x band radar and sextant radar beam spreading exaggerates iceberg width by 60 m per nautical mile of range beyond the first mile data sets on iceberg size e g that collated for antarctic icebergs collected using ship radars which do not take this effect into account will overestimate iceberg dimensions significantly the location and concentration of icebergs within the fjord complex can be explained by 1 the locations of the principal source glaciers and 2 fjord topography and bathymetry iceberg concentration maximum 0 6 icebergs km 2 declines with distance from the major iceberg sources we found that 69 of icebergs within the fjord system are 1 km in length the largest is 2 7 km long icebergs become spread more evenly over the range of size classes in the outer fjord and shelf modal iceberg keel depth calculated from freeboard measurements is 4 500 m in the inner fjords shifting to lower values in the outer fjords reflecting shallower bathymetry radar measurements of iceberg width cannot be used to infer keel depths accurately because width and keel depth are only weakly correlated comparison between freeboards and keel depths for icebergs from east greenland and the barents sea indicates that the iceberg source i e floating or grounded exerts a fundamental control on iceberg dimensions the drift pattern of icebergs is from the head to the mouth of the fjord system although fjord bifurcations bathymetry and currents provide additional complications the trends in observed iceberg size and frequency and in inferred keel depth in the scoresby sund region are likely to be applicable to other fjords and shelves around greenland

thermal energy conservation in icebergs and tracking by temperature calculations using a two dimensional numerical model which simulates the heat balance and temperature distribution of icebergs show that the temperature in the central region of an iceberg is almost unaffected by the thermal conditions imposed on its boundary hence the original temperature of the iceberg at the time of calving is retained in its core owing to the insulating quality of the ice when correlated with glacioclimatic conditions this temperature conservation may be used to backtrack an iceberg to its parent ice cliff nine different icebergs were manually temperature profiled in the barents sea during late winter 1988 1990 a scenario is considered wherein one of these icebergs under the forces of winds and currents is assumed to set off from franz josef land in late autumn 1987 the numerical heat balance model is used to compute the temperature distribution heat balance and ablation of this iceberg as it drifts through the barents sea the calculations show that the heat balance of the part of the iceberg exposed to air has marginal influence on the ablation and thus on the expected lifetime of the iceberg ablation of the submerged body also occurs during winter when the water temperature is well below the melting point of glacial ice this is caused by freezing point depression leading to an ablation of about 0 5 m per month of the underside and sidewalls of the submerged body for water temperatures above 0 6 c the ablation results in a steep temperature gradient reaching the core temperature 3 m into the ice with a tolerance of 0 4 c the temperature is virtually free of any thermal boundary influence 12 m into the ice the simulated and measured temperature profiles of the iceberg are compared with measurements of the thermal regime of austfonna nordaustlandet and the churlenis ice dome on franz josef land the model calculations indicate that the ice cliffs on svalbard can be excluded as parent ice cliffs for these icebergs and that the core temperature of the bergs appears to be a conservative tracer

a model of icebergs and sea ice in a joint continuum framework the ice melange a mixture of sea ice and icebergs often present in front of outlet glaciers in greenland or ice shelves in antarctica can have a profound effect on the dynamics of the ice ocean system the current inability to numerically model the ice melange motivates a new modeling approach proposed here a continuum sea ice model is taken as a starting point and icebergs are represented as thick and compact pieces of sea ice held together by large tensile and shear strength selectively introduced into the sea ice rheology in order to modify the rheology correctly an iceberg tracking procedure is implemented within a semi lagrangian time stepping scheme designed to exactly preserve iceberg shape through time with the proposed treatment sea ice and icebergs are considered a single fluid with spatially varying rheological properties mutual interactions are thus automatically included without the need for further parametrization an important advantage of the presented framework for an ice melange model is its potential to be easily included within sea ice components of existing climate models

ocean circulation at the last glacial maximum a combined modeling and magnetic proxy based study 1 formation of north atlantic deep water nadw is an important component of the ocean thermohaline circulation but debate exists over the ocean circulation state during glacial stages some geological and modeling studies suggest decreased nadw and increased formation of southern ocean deep water during the last glacial maximum lgm others indicate similar or higher rates of nadw advection here we test three different potential lgm ocean states by comparing the modeled iceberg trajectories each produces with magnetically mapped patterns and sources of lgm ice rafted debris ird the three lgm states are characterized by vigorous nadw formation deepwater production in the southern ocean and a third intermediate state with southern ocean deepwater formation but also some north atlantic intermediate water formation cluster analysis of sediment magnetic properties was used to characterize north atlantic ird patterns and sources which match most closely iceberg trajectories arising from some combination of the southern sinking and intermediate ocean circulation states the magnetic data indicate two major ird sources fennoscandia and greenland iceland and one minor source the st lawrence region the model and magnetic data suggest that the lgm north atlantic circulation was dominated by a cyclonic central north atlantic gyre separated from the north atlantic current which was displaced south of 42 n

biological fractionation of silicon isotopes in southern ocean surface waters 1 the fractionation of silicon isotopes by diatoms during silicification i e opaline cell wall formation provides a new tool for paleoceanographic studies of the silicon cycle here we examine the natural variations of the 30si 28si ratio in silicic acid si oh 4 and biogenic silica bsio2 in surface waters of the antarctic circumpolar current acc along 170 w the results provide direct evidence of biologically mediated fractionation of silicon isotopes with an enrichment factor of between 1 1 and 1 9 depending on the model employed comparison of the mass flux of bsio2 captured in sediment traps deployed in the study area with the silicon isotopic composition of that material establishes a direct linkage between diatom dynamics in surface waters and the isotopic signature of exported particles we calculated si oh 4 no3 utilization ratios from variations in silicon and nitrogen isotopic ratios in surface waters and sediment traps that agree well with direct observations of 4 1 in this region this work supports the use of variations in silicon isotopes in sedimentary opal as a proxy for relative si oh 4 utilization in surface waters

the silicon isotopic composition of surface waters in the atlantic and indian sectors of the southern ocean abstract we report here the silicon isotopic composition 30si of dissolved silicon dsi from 42 surface water samples from the drake passage the weddell gyre other areas south of the southern boundary of the antarctic circumpolar current acc and the acc near the kerguelen plateau taken between the beginning of february and the end of march 2007 from the beginning to end of the cruise antxxiii 9 dsi diminished in the antarctic by 50 mol l 1 while concentrations of nitrate nitrite and phosphate showed no net decline indicating that the high seasonal si n removal ratios well known for the southern ocean may be more related to the strength of the silicate pump in the southern ocean than to the instantaneous si n uptake ratio of diatoms the 30si of dsi in samples containing more than 20 m dsi were strongly negatively correlated to dsi concentrations supporting the use of 30si as a proxy for dsi removal the open system fractionation observed e 1 2 0 11 agrees well with results from previous work in other areas and the estimate of the initial 30si of dsi of 1 4 is not far off observations of the 30si of dsi in winter water ww in this area results were used to model dsi draw down in the past from the 30si of sediment cores although isotopic fractionation during silica dissolution appeared to influence the 30si of some surface water samples inviting further study of this phenomenon

sources and biological fractionation of silicon isotopes in the eastern equatorial pacific abstract silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4 n and 3 s latitude at 110 w longitude in the eastern equatorial pacific silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation the value of e estimated by applying a steady state isotope fractionation model to data from all stations between 4 n and 3 s was 0 77 0 12 std err when the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the equatorial undercurrent euc the resulting value of e was 1 08 0 27 std err similar to the value established in culture studies 1 1 when the non steady state rayleigh model was applied to the same restricted data set the resulting value of e was significantly more positive 0 61 0 16 std err to the extent that the equatorial system approximates a steady state these results support a value of 1 1 for the fractionation factor for isotopes of si in the sea without the assumption of steady state the value of e can only be constrained to be between 0 6 and 1 1 silicic acid in equatorial pacific deep water below 2000 m had a near constant 30 si of 1 32 0 05 that value is significantly more positive than obtained for north pacific deep water at similar depths at stations to the northwest of our study area 0 9 1 0 and it is slightly less positive than new measures of the 30 si of silicic acid from the silicic acid plume centered over the cascadia basin in the northeast pacific si oh 4 180 m 30 si 1 46 0 12 sd n 4 we show that the data from the equator and cascadia basin fit a general trend of increasing 30 si oh 4 with increasing silicic acid concentration in the deep sea but that the isotope values from the northeast pacific are anomalously light the observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution confirmation of such high variability in deep water 30 si oh 4 within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea

3d seismic evidence of buried iceberg ploughmarks from the mid norwegian continental margin reveals largely persistent north atlantic current through the quaternary over 7500 buried linear and curvilinear depressions interpreted as iceberg ploughmarks were identified within the quaternary naust formation from an extensive three dimensional seismic dataset that covers 40 000 km2 of the mid norwegian continental margin the morphology and net orientation of ploughmarks were mapped and analysed these features are up to 28 km long 700 m wide and are incised up to 31 m deep on average ploughmarks are incised 5 m deep with median width of 185 m and median lengths ranging from 1 2 to 2 7 km for individual palaeo surfaces width to depth ratio ranges from 8 1 to 400 1 and is on average 36 1 the presence of ploughmarks buried deeply within some palaeo slope surfaces implies the occasional presence of very large icebergs since the middle quaternary suggesting that thick ice sheet margins with fast flowing ice streams were present in order to calve icebergs of such dimensions into the norwegian sea the wide geographical distribution of ploughmarks suggests unrestricted iceberg drift and an open norwegian sea during the periods of iceberg calving since the early quaternary ploughmark trajectory analysis demonstrates that the ocean current circulation now dominated by the northeasterly flowing norwegian atlantic current nwac has largely persisted throughout the quaternary despite the overall strikingly consistent pattern of iceberg drift ploughmark mapping also shows evidence for short lived nwac reductions possibly related to major phases of iceberg discharge and or meltwater pulses from the fennoscandian ice sheet during the middle and late quaternary

quantifying flow regimes in a greenland glacial fjord using iceberg drifters funded by the national science foundation grant numbers oce 1130008 arc 0909274 and the university of oregon

the influence of water mass mixing on the dissolved si isotope composition in the eastern equatorial pacific silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon si in this study the distribution of the si isotope composition of dissolved silicic acid 30si oh 4 was analyzed in the water column of the eastern equatorial pacific eep where one of the globally largest oxygen minimum zones omzs is located samples were collected at 7 stations along two meridional transects from the equator to 14 s at 85 50 w and 82 00 w off the ecuadorian and peruvian coast surface waters show a large range in isotope compositions 30si oh 4 2 2 to 4 4 with the highest values found at the southernmost station at 14 s this station also revealed the most depleted silicic acid concentrations 0 2 mol kg which is a function of the high degree of si utilization by diatoms and admixture with waters from highly productive areas samples within the upper water column and the omz at oxygen concentrations below 10 mol kg are characterized by a large range in 30si oh 4 which mainly reflects advection and mixing of different water masses even though the highly dynamic hydrographic system of the upwelling area off peru does not allow the identification of clear si isotope signals for distinct water masses therefore we cannot rule out that also dissolution processes have an influence on the 30si oh 4 signature in the subsurface water column deep water masses 2000 m in the study area show a mean 30si oh 4 of 1 2 0 2 which is in agreement with previous studies from the eastern and central pacific comparison of the new deep water data of this study and previously published data from the central pacific and southern ocean reveal substantially higher 30si oh 4 values than deep water signatures from the north pacific as there is no clear correlation between 30si oh 4 and silicic acid concentrations in the entire data set the distribution of 30si oh 4 signatures in deep waters of the pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct si isotope signatures including lower circumpolar deep water lcdw and north pacific deep water npdw

surface sea floor and near surface box cores sediment mineralogy in baffin bay as a key to sediment provenance and ice sheet variations to better understand the glacial history of the ice sheets surrounding baffin bay and to provide information on sediment pathways samples from 82 seafloor grabs and core tops and from seven box cores were subjected to quantitative x ray diffraction weight percent wt analysis of the 2000 m all show an abrupt drop in calcite wt post 5 cal ka bp following a major peak in detrital carbonate mainly dolomite this dolomite rich detrital carbonate dc event in jr175bc06 is possibly coeval with the younger dryas cold event four possible glacial sourced end mem bers were employed in a compositional unmixing algorithm to gain insight into down core changes in sediment provenance at the deep central basin estimates of the rates of sediment accumulation in the central basin are only in the range of 2 to 4 cm cal ka surprisingly low given the glaciated nature of the surrounding land

the impact of a downslope water transport parametrization in a global ocean general circulation model the impact of a downslope water transport parametrization on the circulation and water mass characteristics of a global depth level ocean general circulation model is investigated the spreading of dense water from the formation regions into the deep ocean is known to be poorly represented in depth level models with no bottom boundary layer resolved or attached the new scheme is simple and intends to parametrize the effects of various oceanographic processes rather than the processes themselves that help dense water to descend topographic slopes by which the formation regions are separated from the world ocean the new scheme significantly improves the large scale properties of the north atlantic deep water changes in the north atlantic circulation however are rather small in the southern ocean the exchange between the dense water formation regions on the continental shelves and the deep ocean is strengthened at the expense of deep water mass formation by open ocean convection in all three ocean basins the density of the deep and bottom water is higher with the new parametrization which brings the simulations closer to observations in the atlantic and indian oceans in the pacific ocean however where the density has already been well reproduced without the downslope transport it becomes slightly too high the results are in agreement with those from other model studies

modeling the modern marine δ30si distribution 1 the silicon isotopic composition of opal sediments can be used as a proxy for past si utilization and hence paleoproductivity for a fixed ocean circulation interpretation of this si utilization proxy is limited by our understanding of the present marine si isotope distribution observations show significant isotopic variations is silicic acid between deep water masses that are not captured by a previous global climate model gcm here simple box models of the global oceans for the si cycle are presented which can account for the observed 0 3 o difference between atlantic and pacific deep water 30 si values using typical opal export fluxes the 30 si gradient results from both a high 30 si of dissolved si overturning in the north atlantic and the dissolution of low 30 si of opal in the deep waters around the southern ocean that feed into the pacific

mechanisms controlling silicon isotope distribution in the eastern equatorial pacific abstract the distribution of silicon isotopes along a meridional transect at 140 w longitude in the eastern equatorial pacific was used to test the hypothesis that 30 si of silicic acid in surface waters should correlate with net silica production rates gross silica production minus silica dissolution rather than rates of gross silica production due to the opposing si isotope fractionations associated with silica production and silica dissolution variations in 30 si appeared significantly correlated with net silica production rates in equatorial surface waters and not with gross production rates around the equator values of 30 si as low as deep water values occurred in the upper mesopelagic in a zone of net silica dissolution and high detrital biogenic silica content where the release of low 30 si silicic acid from opal dissolution would be expected to decrease 30 si the 30 si of the deep water at 140 w appears constant for depths 2000 m and is similar to the deep water at 110 w this study brings to light the importance of considering si fractionation during diatom silica dissolution the biological fractionation during silica production and physical factors such as currents and mixing with adjacent water masses when interpreting silicon isotope distributions

holocene ice rafting and sediment transport from the glaciated margin of east greenland 67 70 n to the n iceland shelves detecting and modelling changing sediment sources abstract we examine variations in the ice rafted sources for sediments in the iceland east greenland offshore marine archives by utilizing a sediment unmixing model and link the results to a coupled iceberg ocean model surface samples from around iceland and along the e ne greenland shelf are used to define potential sediment sources and these are examined within the context of the down core variations in mineralogy in the