the distribution of fe in the antarctic circumpolar current abstract the large scale distributions of dissolved and total fe in surface and deep waters of the antarctic circumpolar current exhibit strong relationships with hydrography and biological processes the mean dissolved fe concentrations are low in surface waters of the antarctic circumpolar current 0 31 0 49 nm with a minimum of 0 17 nm and higher averaging 1 11 9 nm in the polar frontal region enhanced dissolved surface water concentrations in the polar frontal region are attributed to input from the continental shelf and coincide with phytoplankton spring blooms of large diatoms the effects of sea ice melting and iceberg melting on the fe concentrations were relatively small dissolved deep water concentrations 400 m in the antarctic circumpolar current ranged from 0 4 to 2 8 nm circumpolar deep water has relatively high dissolved fe concentrations in the polar frontal region 0 4 2 8 nm compared with deep waters further to the south 0 6 1 1 nm similarly total dissolvable unfiltered fe concentrations in the upper circumpolar deep water tend to decrease southward from the polar frontal region in the lower circumpolar deep water total dissolvable fe concentrations are higher than in the upper circumpolar deep water due to the existing nepheloid layer and sources on the mid atlantic ridge dissolved and total dissolvable fe concentrations in the antarctic bottom water are higher than those of other water masses in the antarctic circumpolar current consistent with the nepheloid layer as well as diagenetic input from shelf sediments the high nutrient low chlorophyll areas of the antarctic ocean and northeast pacific ocean have different major fe input sources of similar magnitude in the antarctic circumpolar current upward transport of fe is the main input source whereas in the north pacific ocean aerosols are the dominant source

carbon to phosphorus ratios in sediments implications for nutrient cycling defining the transformations of phosphorus p in sediments is critical to assessing the geochemical behavior of sedimentary p and the behavior of reactive p preactive poxide associated pauthigenic porganic relative to organic carbon corganic this study includes the determination of p components and corganic concentrations on samples from sites in the california current region ocean drilling program odp leg 167 sites 1010 1011 1012 1014 1016 and 1021 in the benguela current region odp leg 175 sites 1082 1084 and 1085 and on blake nose in the western atlantic odp leg 17 1b sites 1050 and 1052 sample depths range up to 650 m and ages range up to 65 ma our data demonstrate the transformation of labile forms of p porganic and poxide associated to pauthigenic throughout the sediment column in all redox states and at all sedimentation rates investigated until a substantial portion of reactive p is in the authigenic form 80 c p organic ratios are always greater than or equal to the redfield ratio in all of the sediments we studied sediments with corganic concentrations 2 wt have corganic preactive ratios less than the redfield ratio and corganic preactive ratios increase linearly with corganic consistent with degradation of corganic with age in general corganic preactive ratios for sediments with corganic concentrations 2 wt from a variety of redox states and sedimentation rates center around the redfield ratio although some ratios are as high as 3 times the redfield ratio however corganic preactive ratios up to 400 do not approach the high ratios observed for c p organic ratios up to 4500 because of the efficient transfer of labile forms of p to pauthigenic corganic preactive ratios better describe the geochemical behavior of sedimentary p

late quaternary variations of elemental ratios c si and n si in diatom bound organic matter from the southern ocean le cycle du carbone de l ocean austral est certainement un des facteurs majeurs controlant les concentrations atmospheriques passees en co 2 cependant les taux d accumulation de l opale biogene ne sont pas lineairement lies aux taux d accumulation du carbone organique et ne peuvent donc pas etre directement utilises en tant que traceur de la paleoproductivite nous montrons ici qu il est possible de mesurer les teneurs en carbone et azote dans la matiere organique intrinseque des diatomees c diat and n diat respectivement et que les signaux fossiles ne sont pas le resultat d artefacts analytiques l analyse de deux carottes provenant des secteurs atlantique et indien de l ocean austral indique que les c diat and n diat ont respectivement varie de 30 40 et de 120 175 au cours des cycles glaciaires interglaciaires les rapports c n ont en consequence varie de 3 a 7 au cours des cycles glaciaires interglaciaires si les variations enregistrees dans la matiere organique intrinseque sont representatives des changements du bulk des diatomees nous aurons un nouvel outil pour documenter le cycle oceanique du carbone et pour determiner son role dans les variations passees de pco 2 atmospheriques des experiences en laboratoire sur des cultures de diatomees sont cependant necessaires afin de valider l outil c diat and n diat comme traceur des conditions physiologiques des diatomees et des flux de carbone exportes depuis la couche euphotique

intraplate deformation and closure of the australia antarctica africa plate circuit to determine the current motion between the australian antarctic and african plates and to test whether this plate circuit obeys closure all plate motion data available along the southeast southwest and central indian ridges are analyzed and reduced to 67 spreading rates 38 transform fault azimuths and 135 earthquake slip vectors carlsberg ridge data were excluded because they record india africa not australia africa motion new data include 10 new transform azimuths along the southwest indian ridge many slip vectors and a dense aeromagnetic survey along the southeast indian ridge all published and many unpublished magnetic profiles are modeled to determine rates consistently over a 3 m y time averaging interval for three reasons 1 magnetic profiles from the southeast and central indian ridges suggest recent spreading rate changes 2 some published rates differ for identical profiles and 3 prior studies have not used identical criteria and time scales to determine spreading rates the new rates differ from published rates by as much as 5 mm yr transform fault azimuths are estimated from bathymetry seasat altimetric data epicenter distributions and offsets of magnetic lineations azimuths of plate motion are also estimated from the horizontal projections of slip vectors from the centroid moment tensor solutions and other focal mechanism studies our new australia antarctica and australia africa euler vectors differ from all prior euler vectors at the 95 confidence level along the southeast and central indian ridges our model gives rates differing by 4 7 mm yr from those of prior models from a systematic analysis of the plate motion data we find no evidence for a nubia somalia antarctica triple junction along the southwest indian ridge we also find no evidence for a triple junction previously proposed to be at 80 e along the southeast indian ridge any deformation within this seismically active region of the australian plate adds up to less than a few millimeters per year along each boundary data are fit well by a single euler vector except for a 5 systematic misfit to azimuthal data along the eastern southeast indian ridge near a seismically active region of the australian plate south of tasmania although seismicity and the azimuthal misfit suggest the australian plate is deforming measurably possibly by distributed deformation or by the westward motion of a small microplate southeast of tasmania the significance of the misfit is marginal and is small enough that systematic errors may cause it these alternative explanations could be distinguished by surveying these transform faults with modern seafloor mapping techniques prior studies found that enforcing plate circuit closure causes systematic misfits to rates along the southeast indian ridge here we find that enforcing closure causes no systematic misfits to any data moreover only insignificant nonclosure is found by an f ratio test which numerical experiments suggest could detect deformation exceeding 2 mm yr we conclude that indian ocean plate circuit nonclosure and the deformation it suggests are much smaller than thought before the absence of significant nonclosure argues against the usefulness of a model of deformation distributed throughout an indo australian plate but favors a model in which the significant deformation occurs in a diffuse plate boundary along the equatorial indian ocean between the central indian ridge and the sumatra trench

new constraints on the tectonic evolution of the eastern indian ocean abstract from marine magnetic anomaly studies a fossil spreading ridge is identified beneath the nicobar fan in the northwestern wharton basin several north south trending transform faults offset this ridge left laterally east of the 86 e transform fault our findings show that this ridge which was part of the plate boundary between the indian and australian plates ceased its spreading shortly after formation of magnetic anomaly 20 45 6m y b p since the breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p we suggest that the indian australian and antarctic plates were moving relative to one another from about 90 to 45 m y b p a triple junction would have existed in the southeastern indian ocean during that period of time at anomaly 19 time 45m y b p the junction became inactive and australia and india became a single plate the northwest southeast trending southeast indian ridge was formed by connecting the india antarctica spreading center with the australia antarctica spreading center its activity has continued to the present time

location of the nubia somalia boundary along the southwest indian ridge the northern boundary between the nubian west african and somalian east african plates is marked by the existence of the east african rift which is the locus of roughly east west stretching due to the separation of these plates south of 20 s any expression of deformation or seismicity due to the relative motion of these two presumably distinct plates vanishes or is extremely subtle although the boundary must continue until it intersects another plate boundary until recently there were no observational limits on the location of that intersection and the recent limits were very broad being thousands of kilometers wide here from an analysis of the locations of the old edge of magnetic anomaly 5 which is over 11 ma seafloor we show that the main locus of deformation between nubia and somalia near the southwest indian ridge over the past 11 m y can be no wider than a few hundred kilometers from 100 km west of the du toit fracture zone to 50 km east of the andrew bain fracture zone complex deformation is most likely concentrated along a closely spaced set of several fracture zones known collectively as the andrew bain fracture zone complex which is only 100 km wide the displacement along the nubia somalia boundary over the past 11 m y is 23 km 6 km 95 confidence limits indicating a displacement rate of 2 mm yr much slower than typical rates of seafloor spreading and subduction but comparable to that along the owen fracture zone and dalrymple trough which separate the arabian plate from the indian plate and along the azores gibraltar line which separates the eurasian plate from the nubian plate

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

the zn abundance and isotopic composition of diatom frustules a proxy for zn availability in ocean surface seawater we have developed cleaning methods for extracting diatomopal from bulk marine sediment samples for measurement of both zinc zn abundance and isotope composition this cleaning technique was then applied to a set of holocene core top samples from the southern ocean the measured 66zn reported relative to the jmclyon standard and zn si ratios from the southern ocean diatomopal samples range from 0 7 to 1 5 and from 14 to 0 9 mol mol respectively the zn abundance and isotope composition data show a clear correlation with opal burial rates and other oceanographic parameters in common with previous work we interpret the systematic changes in the zn si ratio to be linked to the variability in the concentrations of bioavailable zn in the ambient surface seawater where the diatom opal is formed this variability is likely to be primarily controlled by the degree to which zn is taken up into phytoplankton biomass the observed systematic pattern in the 66zn compositions of the diatomopal core top samples is similarly likely to reflect changes in the 66zn composition of the ambient zn in the surface waters above the core top sites which is progressively driven towards isotopically heavier values by preferential incorporation of the lighter isotopes into phytoplankton organic material thus the systematic relationship between zn isotopes and abundance observed in the core top diatomopal samples suggests a potential tool for investigating the biogeochemical cycling of zn in the past surface ocean for down core diatomopal material in this respect it may be possible to test hypotheses that attribute variations in atmospheric co2 on glacial interglacial timescales to the degree to which trace metals limited primary productivity in hnlc zones

benthic remineralization and burial of biogenic sio2 caco3 organic carbon and detrital material in the southern ocean along a transect at 170 west abstract we investigated the composition recycling and mass accumulation rates of sediments along a transect in the southern ocean located from 66 s to 57 s at 170 w this transect also corresponds to the location of a sediment trap mooring line the sediments at the seven sites studied range from largely terrigenous material to nearly pure 90 biogenic silica caco3 is a minor but persistent component at most sites mass accumulation rates have been determined on the basis of excess 230th in the sediments i e 230th normalized accumulation rates the influence of redistribution of sediments on the sea floor has been estimated from 14c analyses the recycling of material delivered to the sediments has been characterized on the basis of pore water studies that make extensive use of both in situ sampling and shipboard extractions the influence of the highly variable rates of input of particulate matter that characterize much of the southern ocean upon pore water gradients and fluxes across the sediment water interface has been considered we find only poor correspondence between bsio2 burial fraction burial particulate flux a quantifiable measure of preservation efficiency and bsio2 particulate rain along the transect however preservation does appear to be closely linked to a combination of sedimentation rate and particulate rain the burial fraction of bsio2 is small relative to benthic rain 5 19 despite the small fraction buried burial flux normalized to sedimentation rate 1 2 appears to provide a very consistent means of predicting benthic particulate rain over a large range of rain rates including data from a number of different studies and environments at sites with bsio2 rain 250 mmol m 2 yr 1 the average difference between predicted and observed rain is 25 30 such rain rates occur in many marine areas particularly the southern ocean with the result that this relationship potentially provides a means of estimating bsio2 benthic rain over prolonged periods in the past on the basis of readily measured sediment parameters at the southern most deep ocean station the particulate flux was characterized by an extremely high corg caco3 ratio 10 but this high ratio does not appear to have a substantial influence on caco3 burial caco3 is preserved in the sediments at this site despite a particulate flux with a 10 fold excess of corg above that required for complete dissolution in the sediments the unexpectedly high preservation of caco3 is due largely to the very steep corg oxidation rate profile at this site as a result a large fraction of the organic matter oxidized in the sediments does so in close proximity to the sediment water interface where most of the metabolic co2 is neutralized by co32 from the overlying water rather than by the dissolution of sedimentary caco3 diagenetic modeling indicates that at several of the stations the remineralization fluxes of carbonate species across the sediment water interface may not have been at steady state as a result of the highly pulsed nature of particulate rain in this environment we estimate that at the time of our sampling it is possible that near interface fluxes could have been a factor of 1 6 2 times the annual average at every site on the transect the burial fluxes of detrital material are substantially greater than the detrital particulate rain measured in the sediment traps by as much as a factor of 40 detrital burial is bimodal being greatest at the southern and northern extremes of the transect we postulate that the excess of burial over particulate rain in the south reflects the contribution of ice rafted debris at these high latitudes increases in the supply at the northern stations must have a different source we believe that the excess at these stations is material eroded from the sea floor to the west possibly on the campbell plateau and advected by currents to the northern portion of the transect at depths below the shallow traps

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

controls on the carbon isotopic composition of southern ocean phytoplankton carbon isotopic compositions of suspended organic matter and biomarker compounds were determined for 59 samples filtered from southern ocean surface waters in january 1994 along two north south transects woce sr3 from tasmania to antarctica and across the princess elizabeth trough pet east of prydz bay antarctica along the sr3 line bulk organic matter show generally decreasing 13c contents southward which are well correlated with increasing dissolved molecular carbon dioxide concentrations co2 aq this relationship does not hold along the pet transect using concentrations and isotopic compositions of molecular compounds we evaluate the relative roles of several factors affecting the 13c of southern ocean suspended particulate organic matter along the woce sr3 transect the concentration of co2 aq plays an important role it is well described by a supply versus demand model for the extent of cellular co2 utilization and its associated linear dependence of isotopic fractionation ep on the reciprocal of co2 aq an equally important factor appears to be changes in algal assemblages along the sr3 transect with their contribution to isotopic fractionation also well described by the supply and demand model when formulated to include the cell surface volume control of supply changes in microalgal growth rates appear to have a minor effect on ep along the pet transect algal assemblage changes and possibly changes in microalgal growth rates appear to strongly affect the carbon isotopic variations of suspended organic matter these results can be used to improve the formulation of modern carbon cycle models that include phytoplankton carbon isotopic fractionation

orbital and millennial antarctic climate variability over the past 800 000 years a high resolution deuterium profile is now available along the entire european project for ice coring in antarctica dome c ice core extending this climate record back to marine isotope stage 20 2 800 000 years ago experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation we assessed the general correspondence between dansgaard oeschger events and their smoothed antarctic counterparts for this dome c record which reveals the presence of such features with similar amplitudes during previous glacial periods we suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

morphometric variability in the diatom fragilariopsis kerguelensis implications for southern ocean paleoceanography abstract diatoms play a central role in the ecosystem of the southern ocean where they represent the main producers and carriers of organic carbon and dissolved silicon towards the deep ocean variability in space and time of the size of the most abundant species fragilariopsis kerguelensis may directly impact the way this ecosystem functions and also affect the nutrient balance of the global ocean as important water masses e g the antarctic intermediate water get their nutrient signature in this area we used a biometric approach and analyzed the size variability of f kerguelensis valves in sixty four surface sediment samples from the southern ocean in sediment trap samples from a mooring site pf3 at the antarctic polar front apf and along a nearby piston core ps1654 2 the average length and valve area of f kerguelensis displayed a distinct increase in the proximity of the apf and lower values to the north and south of it and also changed markedly over seasonal cycles as diatom blooms and nutrient availability waxe and wane the last glacial to interglacial transition also witnesses important changes in the general shape and valve size of f kerguelensis with specimens from glacial intervals being ca 30 larger than their interglacial counterparts 195 versus 150 m 2 valve area a sharp peak in average area and a very specific valve morphology correspond to the first signs of deglaciation and concurring sst sea surface temperature increase and sea ice retreat the latter together with melting icebergs may have contributed dissolved iron to surface waters the highest average sizes during glacial intervals are higher than what found anywhere in the southern ocean today the newly proposed proxy valve area and shape of f kerguelensis may thus prove useful for the reconstruction of the past position and nutrient characteristics of the apf and opal belt a region characterized by high production and export of biogenic silica in a particularly sensitive and dynamic area such as the southern ocean

accumulation of biogenic and lithogenic material in the pacific sector of the southern ocean during the past 40 000 years the 230th normalization technique was applied to surface sediments and down core records from the pacific sector of the southern ocean between 120 w and 170 w in order to reconstruct vertical rain rates corrected for lateral redistribution of sediment preserved rain rates of opal organic carbon and biogenic ba all indicate that biological productivity south of the antarctic polar front apf was much lower than today during the last glacial maximum lgm probably in response to increased ice cover however by analogy with modern opal fluxes south of the acc we show that year round ice cover is not a prerequisite for very low rates of opal accumulation available records from north of the apf indicate little to no change in opal export in contrast to records from the indian and atlantic sectors which show enhanced opal export north of the apf during the lgm glacial records from north of 66 s in the pacific all show lithogenic fluxes that were greater than today but significantly lower than lithogenic fluxes in the glacial atlantic and indian sectors together these new records from the pacific sector show that during the lgm the three basins of the southern ocean were significantly more differentiated than today the observed inter basin distribution of lithogenic fluxes and opal burial and the presumed geographical distribution of increased dust input to the southern ocean during the lgm are consistent with an atlantic indian pacific gradient in the degree of iron stimulated diatom productivity within the zone lying between the apf and the subantarctic front previous studies which focused primarily on the atlantic sector may have overestimated the glacial increase in the strength of the biological pump in the southern ocean

growth rate and size effect on carbon isotopic fractionation in diatom bound organic matter in recent southern ocean sediments carbon isotopic fractionation during photosynthesis epep is used to reconstruct past co2 and phytoplankton growth rates typically by measuring the 13c of biomarkers produced by coccolithophorids however organic molecules bound within diatom frustules represent another phase for measurement of 13c and offer the opportunity to obtain epep for specific diatom sizes and geometries here from core top sediments covering a strong productivity gradient in the southern ocean we present determinations of 13c and epep from frustule bound organic matter from a fine opal fraction dominated by pennate diatoms and a coarse opal fraction dominated by larger centric diatoms the 13c of the pennate diatom fraction is typically 2 8 more positive than that of the centric fraction both fractions show a comparable range of 9 10 over the core top transect epep is lowest 6 3 in pennate fraction between the polar front pf and southern antarctic circumpolar current front saccf and increases both to the north and south with maximum values at greatest distance from the pf 18 in the pennate fraction these spatial changes in epep are too large to arise from the rather modest variation in dissolved co2 in surface waters across the core top transect we suggest instead that the maximum epep reflects higher diatom growth rates and in the case of pennate diatom f kerguelensis also an increase in the frustule width and volume to surface area ratio both processes may result from enhanced fe supply due to upwelling of circumpolar deep water between the pf and saccf farther south diatom growth is strongly fe limited and farther north it is fe and si co limited the optima of growth rates between the pf and saccf appears to be a general feature in all sectors of the southern ocean such growth rate induced changes in diatom epep allow us to resolve a 5 northward displacement of the pf during glacial times compared to interglacial times by estimating co2 aq in equilibrium with the holocene ice core atmospheric co2 concentrations we quantify this growth rate effect and document that it is strongly correlated with indicators of trace metal supply such as frustule zn content as well as indicators of diatom productivity such as opal and opal accumulation rates in sediments and sediment traps these relationships may be applied to constrain the effect of growth rate variations on epep and more accurately derive co2 variations from epep during periods prior to ice core co2 proxy records

refining our estimate of atmospheric co2 across the eocene oligocene climatic transition abstract the eocene oligocene transition eot followed by oligocene isotope event 1 oi 1 is a dramatic global switch in climate characterized by deep sea cooling and the first formation of permanent antarctic ice models and proxy evidence suggest that declining partial pressure of atmospheric carbon dioxide co 2 atm below a threshold may explain the onset of global cooling and associated ice formation at oi 1 however significant uncertainty remains in the estimated values and salient features of reconstructed co 2 atm across this interval in this study we present novel carbon isotope records from size separated diatom associated organic matter 13 c diatom preserved in silica frustules physical preservation of this material allows concurrent investigation of isotopic and cell size information providing two input parameters for biogeochemical models and the reconstruction of co 2 atm we estimate co 2 atm in two ways first we use size and reaction diffusion kinetics of a cell to calculate a co 2 atm threshold second we use the calibrated relationship between e p diatom and carbon dioxide from culture and field studies to create a record of co 2 atm prior to and across the transition our study from site 1090 in the atlantic sector of the southern ocean shows co 2 atm values fluctuating between 900 and 1700 100 p p m v across the eot followed by a drop to values in the order of 700 to 800 100 p p m v just prior to the onset of oi 1 our values and magnitude of co 2 atm change differ from previous estimates but confirm the overall trends inferred from boron isotopes and alkenones including a marked rebound following oi 1 due to the intricate nature of the climate system and complexities in constraining paleo proxies this work emphasizes the importance of a multi proxy approach to estimating of co 2 atm in order to elucidate its role in the emplacement of antarctic ice sheets at the eot

nutrient regime at the siliceous belt of the atlantic sector of the southern ocean during the past 660 ka 1 sediment cores from ocean drilling program sites 1094 and 1093 hold the most extensive and most detailed records of carbon and nitrogen isotopes of diatom bound organic matter in the atlantic sector of the southern ocean over the past 660 ka these records were combined with summer sea surface temperatures and winter sea ice that were deduced from diatom transfer functions to reconstruct the nutrient regime at the northern and southern boundaries of the siliceous belt at latitudes 53 s and 50 s respectively the strong coupling between carbon and nitrogen isotope records at site 1093 suggests that the diatom productivity was influenced mainly by changes in nutrient availability the anticorrelation between the carbon and nitrogen isotopes at site 1094 suggests that the diatom productivity was closely linked to global temperature changes sea ice coverage and dissolved co2 concentration a detailed comparison of the nitrogen isotope records between the two sites shows three time intervals that represent different nutrient regimes the nutrients at site 1093 which is close to the polar front were dominated by eddies and meanders that sustained relatively high productivity at times of stratification or reduced ventilation whereas at site 1094 the nutrients were influenced by ventilation processes

dissolved fe in the upper waters of the pacific sector of the southern ocean the concentration of dissolved fe was determined in upper ocean waters along 170 degreesw between 53 and 72 degreess during four cruises between october 1997 and march 1998 as part of the us jgofs southern ocean antarctic polar frontal zone cruises fe concentrations were extremely variable in both space and time as a result of advection of meso scale eddys through the region observed values ranged from a maximum of 0 34 nm at 64 degreess in november next to the retreating ice edge to 0 075 nm in march at 71 degreess in the northern ross sea gyre in general the highest average mixed layer fe concentrations and the largest seasonal changes were observed in the two frontal zones at similar to 60 degreess and similar to 64 degreess formation of deep mixed layers during winter and the entrainment of sub surface waters enriched in fe is the primary source of this element to surface waters in this region of the southern ocean additionally upwelling of circumpolar deep water in the circumpolar current along its northern boundary at the polar front and its southern boundary at the southern antarctic circumpolar current front is an important mechanism supplying fe to surface waters in these areas throughout the summer comparison of integrated fe concentrations and th derived poc export and n drawdown all suggest that the maximum changes in these properties are comparable to those that would be predicted by fe availability fe concentrations determined by shipboard flow injection analysis measures et al mar chem 50 1995 3 were significantly higher than those found in replicate samples determined by shored based flameless atomic absorption spectrophotometry after preconcentration by chelation solvent extraction johnson et al mar chem 57 1997 137 the concentration differences which appear to be restricted to the upper 200 300 m do not appear to arise from different sampling or handling processes but instead are believed to result from the different sensitivity of the methods to organic fractions of fe in seawater c 2001 elsevier science ltd all rights reserved

iron and silicic acid concentrations regulate si uptake north and south of the polar frontal zone in the pacific sector of the southern ocean abstract we investigated the relative roles of fe and silicic acid availabilities in regulating si uptake rates across the polar frontal zone in the pacific sector of the southern ocean 59 68 s 170 w during the us jgofs antarctic environment southern ocean process study aesops meridional gradients in silicic acid concentration si oh 4 of about 0 25 0 56 m km 1 were observed in this area during austral spring and summer 1997 1998 with si oh 4 ranging from 32 si oh 4 and 15 no 3 uptake rates biogenic silica concentrations and si oh 4 no3 uptake ratios fe addition had little or no effect on si uptake rates in enrichments conducted in the low si waters north of the si gradient however fe addition increased si uptake rates 3 5 times over controls in enrichments conducted in the high si waters south of the gradient in both spring and summer fe addition decreased si oh 4 no3 uptake ratios by 2 5 times largely due to stimulation of no3 uptake rates zn addition had no effect on si oh 4 and no3 uptake rates short term 24 h si additions had varying effects on si uptake rates depending on season and location in spring additions of 40 m si to water from bottle enrichments conducted north of the si gradient in situ si oh 4 15 m did not increase si uptake rates initially but did increase uptake rates after 8 days in the summer enrichment north of the si gradient in situ si oh 4 5 m 50 m si additions doubled in situ si uptake rates in the initial water collected for the enrichment and increased si uptake rates as much as 16 fold during the experiment south of the si gradient where in situ si oh 4 was 40 m in both spring and summer si addition had no effect on in situ si uptake rates in the initial enrichment water nor on any si uptake rates measured during the experiment our results indicate that both fe and si availabilities regulate si uptake rates and silica production in the southern ocean along 170 w fe limitation appears to restrict si uptake rates south of the si gradient and plays a role in preventing si depletion south of the acc where ambient si oh 4 never fell below 40 m during 1997 1998 our experiments in the seasonal ice zone at 62 suggest that si uptake in this area switched from being fe limited in the spring when in situ si oh 4 was 40 m to si limited in the summer when si oh 4 was

revisiting nutrient utilization in the glacial antarctic evidence from a new method for diatom bound n isotopic analysis holocene in the indian sector is smaller than in previous measurements these data suggest no change in the degree of nitrate utilization in the atlantic sector and at most a 20 increase from 25 to 45 in the indian sector the new measurements suggest that during the last ice age in the atlantic sector of the antarctic the atmospheric source of biologically available iron was not so great as to become significant relative to the iron supply from below given the apparent spatial variability in the degree of nitrate drawdown more work is required to develop an adequate picture of the glacial antarctic nutrient field index terms 4806 oceanography biological and chemical carbon cycling 4845 oceanography biological and chemical nutrients and nutrient cycling 4894 oceanography biological and chemical instruments and techniques 9310 information related to geographic region antarctica

evolution of the carlsberg ridge between 60 and 45 ma ridge propagation spreading asymmetry and the deccan reunion hotspot despite the availability of numerous geophysical data no convincing magnetic anomaly identification and structural map of a major part of the arabian and eastern somali basins have been published to date i propose a new interpretation of the available magnetic data based on the identification of tiny wiggles i e singular second order patterns of the magnetic signal only one fracture zone and a set of propagating rifts existed between the owen fracture zone and the chagos laccadive ridge between anomalies 26 and 20 at anomalies 26 25 at least two eastward propagating rifts are identified while at anomalies 24 20 seven westward propagating rifts are confidently recognized the major consequence of this systematic ridge propagation is a tremendous regional asymmetry between anomalies 26 and 25 about 65 of the crust formed at the carlsberg ridge was accreted to the african plate while at anomalies 24 20 more than 75 benefited to the indian plate i suggest that these asymmetries result from the relative position of the carlsberg ridge and the nearby deccan reunion hotspot the ridge tending to remain near the hotspot

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

paleogeographic and paleotectonic evolution of the himalayan range as reflected by detrital modes of tertiary sandstones and modern sands indus transect india and pakistan detrital modes of sandstones derived from the himalayan suture belt record the history of the mountain range since initial collision between india and asia which began in latest paleocene time tertiary clastic wedges deposited in fore arc foreland and remnant ocean basins and exposed along the indus transect from northernmost india to the arabian sea represent the best opportunity to study sedimentary responses to successive tectonic events during continental collision quartzose continental block and feldspatholithic magmatic arc sandstones were deposited respectively on the passive indian tethys himalayan succession and active asian indus group continental margins during late cretaceous paleocene time closure of the neotethys was marked by sudden arrival of volcanic and ophiolitic detritus on the passive continental margin of the indian plate during deposition of sediments dated at foraminiferal zones p6 pakistan to p8 india starting in early eocene time deposition of chulung la formation and murree supergroup volcanic and ophiolitic to metasedimentary detritus was accumulated in rapidly subsiding piggy back and foreland basins homogeneous petrographic composition within the eocene lower miocene murree supergroup with only slight progressive increase of detrital feldspars suggests erosion of largely supracrustal rocks involved in thrusting in the north in middle miocene time marked enrichment in medium to high grade metamorphic detritus in foreland sandstones siwalik group reflects rapid uplift of a warm wedge of indian crust which was carried southward along the main central thrust this major paleogeographic change was recorded also by quartzolithic remnant ocean turbidites which were fed great distances along transverse fracture zones and later accreted in the coastal makran subduction complex panjgur association and makran group recycled orogen detritus derived from the elevated himalayan chain is still accumulating today in the indus fan enrichment in feldspars with respect to ancient sandstones reflects deep erosion levels into mid crustal rocks along the core of the growing orogen

holocene glacial discharge fluctuations and recent instability in east antarctica article history antarctica holds the largest ice sheet in the world the east antarctic ice sheet eais and plays a significant role in both local and global climate through the interactions between ice sheets ocean sea ice and atmosphere our understanding of east antarctica holocene climate variability relies mainly on ice cores that however do not document glacial discharge history here we present the first high resolution 18 odiatom record derived from two marine sediment cores retrieved on the east antarctic continental shelf to reconstruct glacial discharge off adelie land and george v land al gvl over the last 11 000 years from decadal to centennial resolution our results suggest multi centennial glacier advances and retreats until 2000 cal yr bp followed by a period of relative instability marked by two major glacial retreats centered at 1700 cal yr bp and 1980 ce we suggest that the multi centennial oscillations during the early mid holocene reflect glacier fluctuations in response to long term local seasonal insolation and short term solar variability we also propose that 18

the motion and boundary between the capricorn and australian plates the motions between the somalian antarctic and australian plates the three plates believed to meet at the rodrigues triple junction in the indian ocean are inconsistent with the assumption that all three plates are rigid the discrepancy is best explained if the australian plate contains two component plates thus the traditionally defined indo australian plate consists of three component plates and multiple diffuse plate boundaries the pattern of present deformation indicates that the boundaries between the three component plates are two unconnected zones accommodating divergence and a larger zone which we interpret as three diffuse convergent plate boundaries and a diffuse triple junction

coccolithophore cell size and the paleogene decline in atmospheric co2 article i nfo alkenone based cenozoic records of the partial pressure of atmospheric carbon dioxide pco2 are founded on the carbon isotope fractionation that occurred during marine photosynthesis ep37 2 however the magnitude of ep37 2 is also influenced by phytoplankton cell size a consideration lacking in previous alkenone based co2 estimates in this study we reconstruct cell size trends in ancient alkenone producing coccolithophores the reticulofenestrids to test the influence that cell size variability played in determining ep37 2 trends and pco2 estimates during the middle eocene to early miocene at the investigated deep sea sites the reticulofenestrids experienced high diversity and largest mean cell sizes during the late eocene followed by a long term decrease in maximum cell size since the earliest oligocene decreasing haptophyte cell sizes do not account for the long term increase in the stable carbon isotopic composition of alkenones and associated decrease in ep37 2 values during the paleogene supporting the conclusion that the secular pattern of ep37 2 values is primarily controlled by decreasing co2 concentration since the earliest oligocene further given the physiology of modern alkenone producers and considering the timings of coccolitho phorid cell size change extinctions and changes in reconstructed pco2 and temperature we speculate that the selection of smaller reticulofenestrid cells during the oligocene primarily reflects an adaptive response to increased co2 aq limitation

particle fluxes to the interior of the southern ocean in the western pacific sector along 170 w abstract an array of five bottom tethered moorings with 19 parflux time series sediment trap at three depths 1 and 2 km below the surface and 0 7 km above the sea floor was deployed in the western pacific sector of the southern ocean along 170 w the five stations were selected to sample settling particles in the main hydrological zones of the southern ocean the sampling period spanned 425 days november 28 1996 january 23 1998 and was divided into 13 or 21 synchronized time intervals a total of 174 sequential samples were recovered and analyzed to estimate fluxes of total mass tmf organic carbon carbonate biogenic silica and lithogenic particles the fluxes of biogenic material were higher than anticipated challenging the notion that the southern ocean is a low productivity region organic carbon fluxes at 1 km depth within the polar frontal zone and the antarctic zone were relatively uniform 1 7 2 3 g m 2 yr 1 and about twice the estimated ocean wide average ca 1 g m 2 yr 1 carbonate fluxes were also high and uniform between the subantarctic front and ca 64 s 11 13 g m 2 yr 1 a large fraction of the carbonate flux in the antarctic zone was due to the presence of pteropod shells coccoliths were found only to the north of the polar front and calcium carbonate became the dominant phase in the subantarctic zone in contrast carbonate particles were nearly absent near 64 s latitudinal variations in biogenic silica fluxes were substantial the large opal flux 57 g m 2 yr 1 measured in the antarctic zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the polar front unlike biogenic material fluxes of lithogenic particles were among the lowest measured in the open ocean 0 12 0 05 g m 2 yr 1 reflecting a very low dust input

quantifying the opal belt in the atlantic and southeast pacific sector of the southern ocean by means of 230th normalization 1 a set of 114 samples from the sediment surface of the atlantic eastern pacific and western indian sectors of the southern ocean has been analyzed for 230th and biogenic silica maps of opal content th normalized mass flux and th normalized biogenic opal flux into the sediment have been derived significant differences in sedimentation patterns between the regions can be detected the mean bulk vertical fluxes integrated into the sediment in the open southern ocean are found in a narrow range from 2 9 g m 2 yr 1 eastern weddell gyre to 15 8 g m 2 yr 1 indian sector setting upper and lower limits to the vertically received fraction of open ocean sediments the silica flux to sediments of the atlantic sector of the southern ocean is found to be 4 2 1 4 1011 mol yr 1 just one half of the last estimate this adjustment represents 6 of the output term in the global marine silica budget

diatom δ13c δ15n and c n since the last glacial maximum in the southern ocean potential impact of species composition measurements of delta c 13 delta n 15 and c n on diatom bound organic matter were made over the holocene and last glacial maximum lgm from three sediment cores in the southern ocean one each from the atlantic indian and pacific sectors the site in the scotia sea atlantic sector differed considerably from the other two sites by having markedly lower delta c 13 more variable delta n 15 and c n ratios and a sedimentary diatom assemblage that was never dominated by fragilariopsis kerguelensis although environmental parameters certainly have a strong impact on the isotope ratios delta c 13 is also correlated to the proportion of f kerguelensis in the three cores investigated here r 2 0 8 extreme values of delta c 13 delta n 15 and c n at the last glacial maximum were also related to the abundance of winter stages of eucampia antarctica these results suggest that diatom specific isotope records should be interpreted in conjunction with information on the species composition of the samples

a diatom record of co 2 decline since the late miocene abstract extratropical sea surface temperature records from alkenones record a dramatic cooling of up to 17 c over the last 14 ma but the relationship between this cooling and greenhouse gas forcing has been elusive due to sparse and contrasting reconstructions of atmospheric co2 for the time period alkenone carbon isotopic fractionation during photosynthesis has previously been used to estimate changes in pco2 over this interval but is complicated by significant changes in cell size of the alkenone producing coccolithophorids over this time period in this study we reconstruct carbon isotopic fractionation during photosynthesis e p using organic compounds trapped within the frustules of pennate diatoms in sediments from the eastern equatorial pacific ocean at ocean drilling program site 846 over the last 13 ma physical separation of pennate diatoms prior to measuring carbon isotopic fractionation enables us to obtain a record with constant cell geometry eliminating this factor of uncertainty in our pco2 reconstruction in the past 11 ma e p declines from 15 5 to 10 3 using the classic diffusive model and taking into account variations in opal content alkenone concentration and coccolith sr ca as indicators of past productivity and growth rate and sea surface temperature records from the site we estimate a decline in pco2 from 454 41 to 250 15 ppmv between 11 and 6 ma models accounting for changing the significance of active carbon uptake for photosynthesis which likely produce more accurate co2 estimates suggest a significant larger pco2 decline of up to twice that shown by the classic diffusive model in average from 794 233 ppmv at 11 ma to 288 25 ppmv at 6 ma considering growth rates varying between 0 5 and 1 7 day 1 large uncertainties in the pco2 estimated between 8 and 11 ma using the active uptake model are related to the growth rate used for calculations together these results suggest co2 forcing for this period of steep decline in temperatures

southern hemisphere westerly wind changes during the last glacial maximum paleo data synthesis changes in the strength and position of southern hemisphere westerly winds during the last glacial cycle have been invoked to explain both millennial and glacial interglacial climate fluctuations however neither paleo models nor paleodata agree on the magnitude or even the sign of the change in wind strength and latitude during the most studied glacial period the last glacial maximum lgm compared to the recent past this paper synthesizes paleo environmental data that have been used to infer changes in lgm winds data compilations are provided for changes in terrestrial moisture dust deposition sea surface temperatures and ocean fronts and ocean productivity and existing data on southern hemisphere ocean circulation changes during the lgm are summarized we find that any hypothesis of lgm wind and climate change needs to provide a plausible explanation for increased moisture on the west coast of continents cooler temperatures and higher productivity in the subantarctic zone and reductions in agulhas leakage around southern africa our comparison suggests that an overall strengthening an equatorward displacement or no change at all in winds could all be interpreted as consistent with observations if a single cause related to the southern westerlies is sought for all the evidence presented then an equatorward displacement or strengthening of the winds would be consistent with the largest proportion of the observations however other processes such as weakening or poleward shifts in winds a weakened hydrological cycle extended sea ice cover and changed buoyancy fluxes cannot be ruled out as potential explanations of observed changes in moisture surface temperature and productivity we contend that resolving the position and strength of westerly winds during the lgm remains elusive based on data reconstructions alone however we believe that these data reconstructions of environmental conditions can be used in conjunction with model simulations to identify which processes best represent westerly wind conditions during the lgm

the uncertainties of finite rotations in plate tectonics incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates a reconstruction may be represented by the finite rotation that describes one configuration relative to another in this paper a method of reconstruction is provided that reflects the uncertainties in the data the method minimizes a weighted least squares measure of fit as a function of the rotation parameters for a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points representing corresponding former plate margin segments from a common plate margin the common margin is estimated from the locations of both fixed and rotated data points the estimated margin consists of a set of great circle arcs it is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation these techniques are then used to study the uncertainties of reconstructions in the south pacific for the times of anomalies 13 and 18 the best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work however the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigators

dissolution and preservation of antarctic diatoms and the effect on sediment thanatocoenoses abstract comparison of southern ocean diatom populations from i surface ocean production ii underlying antarctic sediments and iii laboratory dissolution experiments demonstrates that dissolution can account for the temporal and spatial variations in sedimentary diatom assemblages observed in southern ocean sediments increasing dissolution causes relative depletions in n kerguelensis k enrichments in t lentiginosa l and slight enrichments in e antarctica a this reflects the relative susceptibility to dissolution of the three species that dominate antarctic sediments we have devised a preservation index for the southern ocean based on the ratio k k l to estimate relative extents of dissolution and applied it to natural assemblages holocene southern ocean sediments display increasing opal preservation toward higher latitudes but south of the antarctic polar front preservation decreases in the order well preserved se indian s atlantic sw indian se pacific poorly preserved dissolution also accounts for the pattern of diatom assemblages in the last glacial maximum lgm sediments of the indian and pacific sectors but in the atlantic increased e antarctica abundances at lgm must have resulted from an increase in surface ocean production of this species holocene and lgm diatoms in atlantic and pacific sector sediments are equally well preserved but in the indian sectors holocene sediments are better preserved than those of lgm age paleoceanographic and paleoclimatic transfer functions derived from factor analyses of variations in the sedimentary abundances of these three diatoms have ignored the effects of differential dissolution on thanatocoenosis and thus should be interpreted with caution

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos