circulation and transport of water along the western weddell sea margin ocean current temperature and salinity data obtained from the western weddell sea during the austral winter 1992 u s russian drifting ice station experiment ice station weddell 1 isw 1 are used to describe water circulation and transport surface to bottom baroclinic currents were computed by applying the geostrophic approximation to derived density data these were corrected using current measurements obtained from drifting current meter arrays and the resulting total currents were vertically integrated to obtain volume transports transport was found to be northward in the region which encompassed the western boundary current of the cyclonic weddell sea gyre this northward transport increased from south to north by more than a factor of 2 from about 12 106m3 s 1 in the southwestern weddell to about 28 106m3s 1 farther north the increase in northward transport was compensated for by westward flow from the interior of the gyre into the western boundary region about 5 6 106m3s 1 of the northward transport was contained in a 300 500 m thick bottom layer of cold water this layer whose transport increased by about 1 106 m3 s 1 from south to north was identifiable by its water mass characteristics as weddell sea bottom water originating on the southwestern and western shelf regions its north flowing volume was consistent with past estimates of a 1 5 2 106 m3 s 1 production rate coupled with a 300 400 transport increase due to entrainment during downslope flow from the shelves to the deep basin the maximum northernmost northward transport less the bottom water transport is consistent with previous estimates for wind driven transport in the weddell gyre provided that bottom friction and the sea ice influence on wind forcing are taken into consideration

circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

shock induced microdeformations in quartz and other mineralogical indications of an impact event at the cretaceous tertiary boundary abstract the event terminating the cretaceous period and the mesozoic era caused massive extinctions of flora and fauna worldwide theories of the nature of this event can be classed as endogenic volcanic climatic etc or exogenic extraterrestrial causes mineralogical evidence from the boundary clays and claystones strongly favor the impact of an extraterrestrial body as the cause of this event nonmarine k t boundary claystones are comprised of two separate layers an upper layer composed of high angle ejecta material shocked quartz altered glass and spinel and a basal kaolinitic layer containing spherules clasts and altered glass together with some shocked grains recognition of this dual layered nature of the boundary clay is important for the determination of the timing and processes involved in the impact event and in the assignment and interpretation of geochemical signatures multiple sets of shock induced microdeformations planar features in quartz grains separated from k t boundary clays provide compelling evidence of an impact event this mineralogical manifestation of shock metamorphism is associated worldwide with a large positive anomaly of iridium in these boundary clays which has also been considered indicative of the impact of a large extraterrestrial body global distributions of maximum sizes of shocked quartz grains from the boundary clays and the mineralogy of the ejecta components favor an impact on or near the north american continent spinel crystals magnesioferrite occur in the boundary clays as micrometer sized octahedra or skeletal forms their composition differs from that of spinels found in terrestrial oceanic basalts magnesioferrite crystals are restricted to the high angle ejecta layer of the boundary clays and their small size and skeletal morphology suggest that they are condensation products of a vaporized bolide hollow spherules ranging up to 1 mm in size are ubiquitously associated with the boundary clays in nonmarine sections where a high angle ejecta layer and an underlying kaolinitic layer can be distinguished the spherules are found only in the kaolinitic layer the morphologies and surface features of these spherules suggest that they are original forms and not secondary growths or algal bodies these impact spherules closely resemble microtektites in size and shape all of these features of the boundary clay are uniquely associated with impact and cannot have been formed by volcanic or other terrestrial processes

selected plant microfossil records of the terminal cretaceous event in terrestrial rocks western north america abstract terrestrial or nonmarine rocks of western north america preserve a record of major disruption and permanent alteration of plant communities precisely at the k t boundary in the same rocks that preserve geochemical and mineralogical evidence of the terminal cretaceous impact event plant microfossil records from many localities show abrupt disappearance of pollen species plant extinctions closely associated with impact ejecta deposits containing iridium and shocked quartz localities discussed in detail in this review are starkville south clear creek north old raton pass and sugarite in the raton basin of colorado and new mexico west bijou in the denver basin colorado sussex in the powder river basin wyoming and pyramid butte and mud buttes in the williston basin north dakota

thermoremanent magnetization in monodomain monoclinic pyrrhotite fe7s8 the intensity and stability of thermoremanent magnetization trm acquired by fine particles 1 m of synthetic monoclinic pyrrhotite were determined a specific trm magnetization per unit inducing field of 32 with a median destructive field of 36 ka m 1 was observed the median destructive field of saturation isothermal remanence is 42 ka m 1 and the coercive force is 62 ka m 1 the intensity of trm has been compared to that predicted by the classical monodomain model adapted to the magnetic symmetry of pyrrhotite if the optically determined particle size is incorporated into the model the agreement between model and observation is poor a factor of 100 difference if a grain size 0 05 m inferred from the measurement of coercive force is adopted model and observation are in agreement within experimental error

the low temperature besnus magnetic transition signals due to monoclinic and hexagonal pyrrhotite kung suan ho and wei teh jiang kindly supplied monoclinic pyrrhotite crystals from museum and ore mineral collections respectively c s h acknowledges funding from the central geological survey and ministry of science and technology r o c through grants 104 5226904000 02 01 and most 105 2116 m 001 018 respectively a p r acknowledges funding from the australian research council through grant dp160100805

variability of the antarctic slope current system in the northwestern weddell sea the dense water outflow from the antarctic continental shelf is closely associated with the strength and position of the antarctic slope front this study explores the short term and spatial variability of the antarctic slope front system and the mechanisms that regulate cross slope exchange using highly temporally and spatially resolved measurements from three ocean gliders deployed in 2012 the 22 sections along the eastern antarctic peninsula and west of the south orkney islands are grouped regionally and composited by isobaths there is consistency in the front position around the powell basin varying mostly between the 500 and 800 m isobaths in most of the study area the flow is bottom intensified the along slope transport of the antarctic slope current upper 1000 m varies between 0 2 and 5 9 sv 1 sv 10 6 m 3 s 1 and does not exhibit a regional pattern the magnitude of the velocity field shows substantial variability up to twice its mean value higher eddy kinetic energy 0 003 m 2 s 2 is observed in sections with dense water possibly because of baroclinic instabilities in the bottom layer distributions of potential vorticity show an increase toward the shelf along isopycnals and also in the dense water layer glider sections located west of the south orkney islands indicate a northward direction of the flow associated with the weddell front which differs from previous estimates of the mean circulation this study provides some of the first observational confirmation of the high frequency variability associated with an active eddy field that has been suggested by recent numerical simulations in this region

antarctic bottom water from the adelie and george v land coast east antarctica 140 149 e 1 we report on observations of dense shelf water overflows and antarctic bottom water aabw formation along the continental margin of the adelie and george v land coast between 140 e and 149 e vertical sections and bottom layer water mass properties sampled during two rvib nathaniel b palmer hydrographic surveys nbp00 08 december 2000 january 2001 and nbp04 08 october 2004 describe the spreading of cold dense shelf water on the continental slope and rise from two independent source regions the primary source region is the adelie depression exporting high salinity dense shelf water through the adelie sill at 143 e an additional eastern source region of lower salinity dense shelf water from the mertz depression is identified for the first time from bottom layer properties northwest of the mertz sill and mertz bank 146 e 148 e that extend as far as the buffon channel 144 75 e in summer regional analysis of satellite derived ice production estimates over the entire region from 1992 to 2005 suggests that up to 40 of the total ice production for the region occurs over the mertz depression and therefore this area is likely to make a significant contribution to the total dense shelf water export concurrent time series from bottom mounted microcats and adcp instruments from the mertz polynya experiment april 1998 to may 1999 near the adelie sill and on the upper continental slope 1150 m and lower continental rise 3250 m to the north describe the seasonal variability in downslope events and their interaction with the ambient water masses the critical density for shelf water to produce aabw is examined and found to be 27 85 kg m 3 from the adelie depression and as low as 27 80 kg m 3 from the mertz depression this study suggests previous dense shelf water export estimates based on the flow through the adelie sill alone are conservative and that other regions around east antarctica with similar ice production to the mertz depression could be contributing to the total aabw in the australian antarctic basin

variable defect structures cause the magnetic low temperature transition in natural monoclinic pyrrhotite non stoichiometric monoclinic 4c pyrrhotite fe7s8 is a major magnetic remanence carrier in the earth s crust and in extraterrestrial materials because of its low temperature magnetic transition around 30 k also known as besnus transition which is considered to be an intrinsic property this mineral phase is easily detectable in natural samples although the physical properties of pyrrhotite have intensively been studied the mechanism behind the pronounced change in magnetization at the low temperature transition is still debated here we report magnetization experiments on a pyrrhotite crystal fe6 6s8 that consists of a 4c and an incommensurate 5c superstructure that are different in their defect structure the occurrence of two superstructures is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 k the disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition from this it follows that the besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4c superstructure and therefore the physics behind it is in fact different from that of the well known verwey transition

thermal alteration of pyrite to pyrrhotite during earthquakes new evidence of seismic slip in the rock record seismic slip zones convey important information on earthquake energy dissipation and rupture processes however geological records of earthquakes along exhumed faults remain scarce they can be traced with a variety of methods that establish the frictional heating of seismic slip although each has certain assets and disadvantages here we describe a mineral magnetic method to identify seismic slip along with its peak temperature through examination of magnetic mineral assemblages within a fault zone in deep sea sediments cored from the japan trench one of the seismically most active regions around japan during the integrated ocean drilling program expedition 343 the japan trench fast drilling project fault zone sediments and adjacent host sediments were analyzed mineral magnetically supplemented by scanning electron microscope observations with associated energy dispersive x ray spectroscopy analyses the presence of the magnetic mineral pyrrhotite appears to be restricted to three fault zones occurring at 697 720 and 801 m below sea floor in the frontal prism sediments while it is absent in the adjacent host sediments elevated temperatures and coseismic hot fluids as a consequence of frictional heating during earthquake rupture induced partial reaction of preexisting pyrite to pyrrhotite the presence of pyrrhotite in combination with pyrite to pyrrhotite reaction kinetics constrains the peak temperature to between 640 and 800 c the integrated mineral magnetic microscopic and kinetic approach adopted here is a useful tool to identify seismic slip along faults without frictional melt and establish the associated maximum temperature

cooling and ventilating the abyssal ocean the abyssal ocean is filled with cold dense waters that sink along the antarctic continental slope and overflow sills that lie south of the nordic seas recent inte grations of chlorofluorocarbon 11 cfc measurements are similar in antarctic bottom water aabw and in lower north atlantic deep water nadw but antarctic inputs are 2 c colder than their northern counterparts this indicates comparable ventilation rates from both polar re gions and accounts for the southern ocean dominance over abyssal cooling the decadal cfc based estimates of recent ventilation are consistent with other hydrographic observa tions and with longer term radiocarbon data but not with hypotheses of a 20 th centuryslowdown in the rate of aabw formation significant variabilityis not precluded bythe available ocean measurements however and interannual to decadal changes are increasinglyevident at high latitudes

comparative mineralogical and geochemical clay sedimentation in the betic cordilleras and basque cantabrian basin areas at the cretaceous tertiary boundary abstract the mineralogy and geochemistry of the bulk rock and clay fraction of sediments deposited at the cretaceous tertiary transition and in the ktb layer have been investigated by various methods in two sections from the betic cordilleras agost caravaca and five sections from the basque cantabrian basin monte urko sopelana zumaya hendaye biarritz active terrigenous supply prevailed in both regions uring the latest maastrichtian and earliest paleocene very different clay associations occur in the regions smectite being dominant to the south and illite to the north the betic cordilleras series mainly reflects climate and weathering conditions as well as volcanic activity with the ktb layer displaying a specific source and diagenesis control expressed as increased amounts of smectite the sequences cropping out in the basque cantabrian basin mainly reflect tectonic activity and burial diagenesis the effects of which differ according to the palaeogeographical location the clay sedimentation patterns in the north and south iberian sections across the cretaceous tertiary passage are controlled overwhelmingly by regional or local geodynamic and diagenetic phenomena thus any global namely extraterrestrial influence cannot be identified

on the fate of the antarctic slope front and the origin of the weddell front 1 data from the deep ocean ventilation through antarctic intermediate layers dovetail and antarctic large scale box analysis and the role of the scotia sea albatross projects are combined in the region of the antarctic peninsula and the south scotia ridge to determine the paths and transports associated with the antarctic coastal current the antarctic slope front and the weddell front the antarctic coastal current flows over the antarctic continental shelf and is not tied to a particular isobath it transports 1 sv westward to the bransfield strait its subsequent course is uncertain but we speculate that it may continue southwest along the western side of the antarctic peninsula close to the coast the antarctic slope front present almost all the way around antarctica tied to the continental slope has not previously been mapped after it reaches the tip of the antarctic peninsula we show that the frontal jet splits into two branches in the northwest powell basin where the isobaths diverge the shoreward portion of the water associated with the front lying above isobaths shallower than 1500 m is able to cross the south scotia ridge and head northwestward and retains many of the properties of the antarctic slope front it is associated with a transport of 7 sv the portion of the front constrained to lie above isobaths deeper than 1500 m becomes the weddell front this takes a path around the northern powell basin and south of the south orkney islands of the 13 sv that circulates around the southern flank of the south orkney plateau 8 sv of weddell sea deep water and warm deep water leaves the front to continue west the remaining shallow and intermediate water retroflects and returns south associated with the weddell front through the orkney passage approximately 5 7 sv is transported eastward associated with the weddell front tied closely to the southern flanks of the bruce and discovery banks above the 2000 2500 m isobaths and forming the southern boundary of the weddell scotia confluence

low temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the besnus transition monoclinic pyrrhotite fe7s8 owes its ferrimagnetism to an ordered array of fe vacancies its magnetic properties change markedly around 30 k in what is known as the besnus transition plausible explanations for the besnus transition are either due to changes in crystalline anisotropy from a transformation in crystal symmetry or from the establishment of a two phase system with magnetic interaction between the two phases to help resolve this discrepancy we measured hysteresis loops every 5a degrees and backfield curves every 10a degrees in the basal plane of an oriented single crystal of monoclinic pyrrhotite at 300 k and every 2 k from 50 k through the besnus transition until 20 k between 50 and 30 k hysteresis loops possess double inflections between crystallographic a axes and only a single inflection parallel to the a axes magnetization energy calculations and relative differences of the loops show a sixfold symmetry in this temperature range we propose that the inflections stem from magnetic axis switching which is both field and temperature dependent in a manner somewhat analogous to an isotropic point where magnetocrystalline constants change their sign the besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6a degrees temperature span 28 34 k with a maximum rate of change at 30 k a surprising yet puzzling finding is that the coercivity ratio becomes less than unity below the transition when fourfold symmetry arises because the changes in magnetic parameters are linked to the crystal structure we conclude the besnus transition owes its origin to a distortion of the crystallographic axes below 30 k rather than an apparition of a two phase system an isothermal magnetization of natural pyrrhotite cycled from room temperature to successively lower temperatures through the besnus transition decreases 2 4 times less than equivalent grain sizes of magnetite with less than a 10 per cent loss in remanence between 300 and 150 k in pseudo single domain psd pyrrhotite as psd monoclinic pyrrhotite carries the magnetic remanence in some meteorites it is likely that low temperature cycling in space then warming to ambient conditions at the earth s surface will have only a minor influence on palaeointensity values derived from those meteorites

bottom water export from the western ross sea 2007 through 2010 bottom water export from the ross sea february 2007 to january 2011 exhibits seasonal and interannual variability temperature minima coupled to salinity maxima in late austral summer into the fall indicate input from high salinity shelf water hssw secondary temperature minima lacking the high salinity trait characteristic of low salinity shelf water lssw appear in the spring warmer bottom water similar to modified circumpolar deep water mcdw is observed in winter and in early summer the lssw and mcdw may be drawn from the drygalski basin as the hssw pool retreats poleward from the shelf break in response to increased winter polar easterlies allowing these less dense overlying waters to spill into the deep ocean within the benthic layer bottom salinity decreased from 2007 to 2011 by 0 007 year 1 significantly higher than regional decadal trends which we propose is a result of hssw retreat induced by strengthening polar easterlies

high resolution velocity and hydrographic observations of the drygalski trough gravity plume during the second cruise of the anslope project the hydrography and velocity fields within a descending gravity plume in the northwestern ross sea shelf area were sampled at high resolution the observations give insights into small scale fluctuations of the three dimensional velocity field in and around the core of a strong gravity current associated with peak velocities close to 2 m s there are two distinct layers of high acoustic backscatter at 300 khz near the seabed associated with increased velocity variance and richardson numbers near 0 25 suggesting that the high backscatter is caused by increased levels of turbulence resulting in increased acoustic bragg scattering c 2008 elsevier ltd all rights reserved

authigenic or detrital origin of pyrrhotite in sediments resolving a paleomagnetic conundrum monoclinic pyrrhotite fe7s8 is widely claimed to carry magnetizations acquired during early diagenesis in anoxic sedimentary environments in contrast geochemical literature indicates that pyrrhotite formation is extremely slow below 180 c which makes it a highly unlikely carrier of early diagenetic remanences in sediments this view is confirmed by the occurrence of late diagenetic fe7s8 in ancient sediments and the general lack of fe7s8 in modern sediments horng et al c s horng m torii k s shea s j kao inconsistent magnetic polarities between greigite and pyrrhotite magnetite bearing marine sediments from the tsailiao chi section southwestern taiwan earth planet sci lett 164 1998 467 481 documented the presence of fe7s8 that carries a magnetic signal indistinguishable from that of detrital magnetite in pleistocene marine sediments from taiwan we tested the possibility that the fe7s8 could have a detrital origin by conducting a source to sink study and found fe7s8 in metamorphic rocks of the taiwan central range and in material eroded from these rocks in the transportation pathway and in the depositional sink this confirms that the fe7s8 has a detrital origin rapid transportation from source to sink e g by typhoon associated flood events probably assists preservation of the fe7s8 which might otherwise oxidize during transportation the widespread occurrence of exhumed metamorphic rocks in orogenic belts around the world makes them a likely source of fe7s8 in marginal sedimentary basins detrital fe7s8 should therefore be more routinely considered to be responsible for paleomagnetic records when it is present in sediments with partial metamorphic provenance

strong remanent magnetization in pyrrhotite a structurally controlled example from the paleoproterozoic tanami orogenic gold province northern australia abstract rocks associated with an orogenic gold system in the paleoproterozoic granites tanami inlier of northern australia exhibit strong natural remanent magnetism identifiable in regional aeromagnetic data petrographical analysis indicates the two dominant magnetic minerals in the rocks of this region are multidomain magnetite and monoclinic pyrrhotite chemical analysis using an electron microprobe has determined that the magnetite and pyrrhotite are stoichiometrically pure without significant elemental substitution alternating field demagnetization curie temperature and hysteresis analysis indicates that the remanently magnetized phase is monoclinic pyrrhotite with low magnetic susceptibility in contrast multidomain magnetites lack remanence and have high magnetic susceptibilities pyrrhotite is preserved in the rock mass along with other sulfide minerals as millimeter scale veins parallel to regionally developed penetrative mylonitic shear bands c planes formed during sulfide remobilization in the waning cooling stages of hydrothermal au deposition in contrast magnetite grains exhibit porphyroblastic textures consistent with formation during prograde to peak metamorphism indicating formation prior to pyrrhotite earlier in the orogenic cycle because of the low curie temperature ca 325 c of pyrrhotite it is therefore likely that remanent magnetization effects observed in aeromagnetic images of the tanami inlier are associated with structures intimately related to the late stages of au mineralization during retrograde greenschist metamorphism after the peak thermal point of the orogenic cycle

fire across the k t boundary initial results from the sugarite coal new mexico usa abstract it has been hypothesized that major global fires occurred following a bolide impact at the cretaceous tertiary boundary evidence for this has been mainly from soot or black carbon at a number of marine boundary sites as well as the occurrence of inertinites fusinite in coals above the boundary in addition the occurrence of fossil charcoal inertinites in potential tsunami deposits has been used to strengthen this idea however fire is known to have been widespread throughout the cretaceous based on the distribution of fossil charcoal and it is necessary to evaluate the claims for a global fire at the k t boundary within the context of this more extensive record of ancient fires the occurrence of the k t boundary within the sugarite coal sequence in the raton basin new mexico offers the opportunity to assess the incidence of fire in local peat forming vegetation in the latest cretaceous across the cretaceous tertiary boundary interval and in the earliest tertiary the distribution of fire products i e fossil charcoal is assessed using a combination of inertinite group macerals in polished blocks of coal and plant particles released by chemical maceration of coal inertinite group macerals fusinite semifusinite inertodetrinite which represent fossil charcoal and particles of charcoal in maceration residues are abundant throughout the sugarite coal sequence both before during and after the k t boundary event samples from below the boundary yield inertinites that constitute more than 20 of the coal three horizons are inertinite rich 50 fire was obviously an important element of the terrestrial environment during the latest cretaceous high inertinite values and abundant charcoal particles characterize the latest cretaceous part of the coal the carbonaceous shale just below the boundary the boundary interval itself and tertiary coals and carbonaceous shales above the boundary charcoal in mineral rich units of the latest cretaceous immediately below the boundary in the boundary interval and in the tertiary is dominated by small inertodetrinite particles these might have been wind blown from regional fires or they may reflect reworking during erosion which led to sediment input into the mire in either case the charcoal signature in these units is the same irrespective of their position in the sequence our data combined with that from two other sites in terrestrial sequences in north america show that fires were an integral part of mire ecosystems through the latest cretaceous and into the early tertiary therefore evidence for distinctive global wildfires at the cretaceous tertiary boundary would have to be sought from careful consideration of other aspects of charcoal deposition including flux rates for charcoal and soot

cross shelf exchange in a model of the ross sea circulation and biogeochemistry abstract transport of warm nutrient rich circumpolar deep water cdw onto antarctic continental shelves and coastal seas has important effects on physical and biological processes the present study investigates the locations of this transport and its dynamics in the ross sea with a high resolution three dimensional numerical model the model circulation is forced by daily wind stress along with heat and salt fluxes calculated from atmospheric climatologies by bulk formulae all surface fluxes are modified by an imposed climatological ice cover waters under the ross ice shelf are not included explicitly but their effect on temperature and salinity is imposed in a buffer zone at the southern end of the model domain a simple nutrient uptake is calculated based on the climatological chlorophyll distribution and monod uptake kinetics model circulation is strongly affected by bottom topography due to weak stratification and agrees with schematics of the general flow and long term current measurements except near the southern boundary the sea surface temperature is similar to satellite estimates except that the warmest simulated temperatures are slightly higher than observations there is a significant correlation between the curvature of the shelf break and the transport across the shelf break a momentum term balance shows that momentum advection helps to force flow across the shelf break in specific locations due to the curvature of the bathymetry that is the isobaths curve in front of the flow for the model to create a strong intrusion of cdw onto the shelf it appears two mechanisms are necessary first cdw is driven onto the shelf at least partially due to momentum advection and the curvature of the shelf break then the general circulation on the shelf takes the cdw into the interior

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

role of tides on the formation of the antarctic slope front at the weddell scotia confluence the structure of the antarctic slope front asf and the associated antarctic slope current asc on the scotia sea side of the weddell scotia confluence wsc is described using data from a hydrographic survey and three 1 year long moorings across the continental slope the asc in this region flows westward along isobaths with an annual mean speed of 0 2 m s 1 with time variability dominated by the k1 and o1 tidal diurnal constituents a narrowband oscillation with 2 week period attributable to the spring neap tidal cycle and seasonal variability realistic and idealized high resolution numerical simulations are used to determine the contribution of tides to the structure of the asf and the speed of the asc two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured asf asc cross slope structure the time variability at our moorings and the reduced stratification within the wsc two idealized simulations run with tide only forcing one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous show that tides can generate the asc and asf through volume flux convergence along the slope initiated by effects including the lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior climate models that exclude the effects of tides will not correctly represent the asf and asc or their influence on the injection of intermediate and dense waters from the wsc to the deep ocean

on the total input of antarctic waters to the deep ocean a preliminary estimate from chlorofluorocarbon measurements 1 deep ocean inventories of dissolved chlorofluorocarbon 11 cfc 11 along representative sections off antarctica provide the first estimate of the overall strength of all dense water sources in the southern ocean their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the thermohaline circulation thc the bottom layer is supplied via sinking of antarctic bottom water aabw produced at a few continental shelves the middle layer receives the offshore injection of ventilated modified circumpolar deep water mcdw produced along much of the lengthy antarctic slope front the top layer is ventilated by northward export of antarctic surface water into the upper circumpolar deep water of the antarctic circumpolar current average southern ocean inputs to the upper two layers of the deep ocean for the 1970 1990 period are derived on the basis of the cfc 11 distributions along meridional sections the mean cfc 11 saturations of all water mass ingredients and the inferred mixing leading to production of dense source waters about 5 4 1 7 sv 1 sv 10 6 m 3 s 1 of near freezing shelf water ventilate the bottom layer and 4 7 1 7 sv and 3 6 1 3 sv of cold antarctic surface water ventilate the middle and top layers therefore the total contribution of ventilated southern ocean waters to the lower limb of the global thc is about 14 sv this is close to the about 17 sv estimated for north atlantic near surface sources from cfc 11 inventories their entrainment of 7 4 2 4 sv of cfc poor subsurface lower circumpolar deep water during the formation and sinking of aabw and mcdw raises the total southern ocean input to the deep ocean to about 21 sv

mineralogy and petrology of the cretaceous tertiary boundary clay bed and adjacent clay rich rocks raton basin new mexico and colorado abstract the cretaceous tertiary k t boundary is preserved in a sequence of coal bearing fluvial rocks in the lower part of the raton formation late cretaceous and paleocene at several sites in the east central part of the raton basin the k t boundary occurs at the top of a kaolinitic claystone layer commonly referred to as the boundary clay layer in an interval of coal and carbonaceous shale the boundary is defined by the disappearance of certain fossil pollen taxa the boundary clay layer also contains shocked quartz grains and abundance anomalies of iridium chromium and other elements each of these characteristics support the hypothesis of an asteroid impact at the end of the cretaceous small goyazite spheres were also found in the boundary clay bed however their origin is ncertain x ray powder diffraction xrd scanning electron microscopy sem and thin section petrography have shown that the boundary clay bed exhibits unique mineralogic and petrologic characteristics not possessed by other clay rich beds in coal bearing rocks of the raton basin the boundary clay bed is composed primarily of well crystallized kaolinite but also contains subordinate randomly interstratified mixed layer illite smectite both of which appear to have been formed by the alteration of vitreous material in a coal swamp environment the bulk mineralogy of the boundary clay bed is similar to that of other kaolinite rich clay beds tonsteins that altered in coal swamps from airfall volcanic ash the alteration process and diagenetic products of the host material in the boundary cl y bed however differed from those of the tonsteins resulting in a unique petrographic fabric the boundary clay bed is characterized by an overall imbricate fabric that may reflect an original vitreous material that included shards bubbles and spheres the original fallout material of the boundary clay bed first altered to cabbage like microspherules indicating a halloysite or allophane precursor and then to well crystallized kaolinite the well crystallized kaolinite is the diagenetic stable phase for these rocks at their maximum burial conditions in comparison the vitreous components of the tonsteins were altered directly to coarse vermicular and platey kaolinite alteration of the original fallout material in the boundary bed was probably rapid leading to hydrated disordered fine grained clay minerals that formed into curled spherical or tubular shapes the facto s that produced these unique fabrics in the boundary clay bed existed at all k t sites in the raton basin and were much different from those that formed the coarse platey and vermicular fabrics of ordinary tonsteins the distinctive character of the boundary clay may be due to the unique nature and chemistry of the fallout material and or physical or chemical conditions existing shortly after its deposition

a new tide model for the antarctic ice shelves and seas we describe a new tide model for the seas surrounding antarctica including the ocean cavities under the floating ice shelves the model uses data assimilation to improve its fit to available data typical peak to peak tide ranges on ice shelves are 1 2 m but can exceed 3 m for the filchner ronne and larsen ice shelves in the weddell sea spring tidal ranges are about twice these values model performance is judged relative to the 5 10 cm accuracy that is needed to fully utilize ice shelf height data that will be collected with the geoscience laser altimeter system scheduled to be launched on the ice cloud and land elevation satellite in late 2002 the model does not yet achieve this level of accuracy except very near the few high quality tidal records that have been assimilated into the model some improvement in predictive skill is expected from increased sophistication of model physics but we also require better definition of ice shelf grounding lines and more accurate water column thickness data in shelf seas and under the ice shelves long duration tide measurements bottom pressure gauge or gps in critical data sparse areas particularly near and on the filchner ronne and ross ice shelves and pine island bay are required to improve the performance of the data assimilation model

unusual magnetic properties of sedimentary pyrrhotite in methane seepage sediments comparison with metamorphic pyrrhotite and sedimentary greigite

rock magnetism in the global palaeomagnetic database gpmdb abstract the iaga global palaeomagnetic database initiated in 1988 is supposed to contain directional and statistical palaeomagnetic data and additional information on rock magnetic measurements for assessment of the reliability of the palaeomagnetic results the rock magnetic information content of the database for 1969 1988 was analysed as a main result it could be shown that at present only about half of the palaeomagnetic results are supported by rock magnetic measurements however there is a tendency towards the improvement of this figure the increase of investigations on sediments is clearly associated with the introduction of more sensitive magnetometers since the early 1970s trends could also be found for some rock magnetic techniques e g isothermal remanent magnetization acquisition curves

magnetic characterization of non ideal single domain monoclinic pyrrhotite and its demagnetization under hydrostatic pressure up to 2 gpa with implications for impact demagnetization here we present a comprehensive magnetic characterization of synthesized non ideal single domain sd monoclinic pyrrhotite fe 7 s 8 the samples were in the form of a powder and a powder dispersed in epoxy non ideal refers to a powder fraction of predominantly sd size with a minor contribution of small pseudo single domain grains such non ideal sd pyrrhotite was found to be a remanence carrier in several types of meteorites carbonaceous chondrites snc which justifies the usage of synthetic compositions as analogous to natural samples data were collected from 5 to 633 k and include low field magnetic susceptibility 0 thermomagnetic curves major hysteresis loops back field remanence demagnetization curves first order reversal curves forcs alternating field and pressure demagnetization of saturation isothermal remanent magnetization sirm low temperature data such as zero field cooled and field cooled remanence datasets together with room temperature sirm cooling warming cycles as well as xrd and mossbauer spectra the characteristic besnus transition is observed at 33 k forc diagrams indicate interacting sd grains the application of hydrostatic pressure up to 2 gpa using nonmagnetic high pressure cells resulted in the demagnetization of the sample by 32 38 repeated cycling from 1 8 gpa to atmospheric pressure and back resulted in a total remanence decrease of 44 after 3 cycles pressure demagnetization experiments have important implications for meteorite paleomagnetism and suggest that some published paleointensities of meteorites with non ideal sd monoclinic pyrrhotite as remanence carrier may be lower limits because shock demagnetization was not accounted for

a model for the structure of the antarctic slope front the antarctic slope front is an oceanic front that appears to be a semi permanent feature situated above or near long sections of the shelf break of the antarctic continental shelf these sections include the shelf break of the ross and weddell seas it is distinguished by a v shaped structure unique to coastal antarctica in which isopycnals slope downward from near the surface on both the shallow continental shelf and the deep continental slope sides toward the bottom near the upper part of the slope in the regions where it is observed the overflow of dense water from the shelf down the slope is observed or presumed to occur such overflows entrain fluid from the overlying stratified environment into the downflow and this entrainment acts as a distributed sink of the overlying fluid a model of this frontal structure is constructed here consisting of the downslope flow modelled as a plume and the overlying fluid governed by quasi geostrophic dynamics with the effect of the plume on the latter being represented by a suitable distribution of sinks the alongslope currents follow from geostrophy and potential vorticity conservation two distinct flow regimes are found in the first regime a realistic depiction of the two sided v shaped structure is obtained where most of the fluid flowing off the shelf enters the downflow at the top of the slope constituting the shallow side of the v and fluid from offshore enters the deep side of the v alongslope flow in this region is mostly eastward in the second regime where most of the fluid flowing off the shelf passes over the downflow only the shallow side of the v is present and alongslope flow is westward near the shelf break and eastward further offshore

quality of ptrm acquisition in pyrrhotite bearing contact metamorphic limestones possibility of a continuous record of earth magnetic field variations abstract the rockmagnetic and palaeomagnetic signal in pyrrhotite bearing limestones of different contact metamorphic settings were investigated related to intrusions ranging from small sills to large magmatic complexes magnetic susceptibility the pyrrhotite magnetite ratio and thermal modelling serve as an investigative tool to define three metamorphic zonations a contact zone of a mixed magnetic assemblage and low susceptibilities a pyrrhotite bearing transitional zone where full thermoremanent magnetisations trms are acquired due to temperatures above the curie temperature of pyrrhotite t c po and a marginal zone containing pyrrhotite and magnetite generated at temperatures below t c po the fact that trms can consist of independent ptrms is successfully tested by modified thellier experiments it is shown that a metamorphic environment with low fluid circulation provides a scenario for the recording of independent ptrms multicomponent behaviour of the nrm residing in samples from the transitional zone can therefore be interpreted as a continuous record of earth magnetic field variations

late pleistocene to holocene palaeoenvironmental variability in the north west spanish mountains insights from a source to sink environmental magnetic study of lake sanabria we present a source to sink environmental magnetic study of a sediment core from lake sanabria north west iberian peninsula and rocks of its catchment the results indicate the occurrence of magnetite and probably also pyrrhotite in sediments accumulated between ca 26 and 13 cal ka bp in a proglacial lake environment these minerals also appear to dominate the magnetic assemblage of palaeozoic rocks from the lake catchment this indicates that sedimentation was then driven by the erosion of glacial flour which suffered minimal chemical transformation due to a rapid and short routing to the lake a sharp change in magnetic properties observed in the lake sediments between 13 and 12 6 cal ka bp reflects the rapid retreat of glaciers from the lake catchment sediments from the upper half of the studied sequence accumulated after 12 6 cal ka bp in a lacustrine environment with strong fluvial influence contain magnetite and smaller amounts of maghemite and greigite we suggest that greigite grew authigenically under anoxic conditions caused by enhanced accumulation of organic matter into the lake the occurrence of maghemite in these sediments suggests pedogenic activity in the then deglaciated lake catchment before the erosion and transportation of detrital material into the lake

magnetic anisotropy behaviour of pyrrhotite as determined by low and high field experiments summary here we report on the sources of magnetic anisotropy in pyrrhotite an iron sulphide present in many rocks as an important carrier of the natural remanent magnetization while the magnetic hysteresis parameters of pyrrhotite are well known the existing database concerning its anisotropy behaviour is patchy and ambiguous therefore a collection of 11 seemingly single crystals of natural pyrrhotite was scrutinized before embarking on the anisotropy determinations the set of single crystals was extensively characterized rock magnetically by measuring curie temperatures hysteresis loops irm acquisition curves and forc diagrams the latter three all at room temperature first the variation of the low field susceptibility as function of applied field and grain size was evaluated for fields ranging from 1 to 450 a m 1 existing grain size dependent data and the present larger crystals show a logarithmic grain size dependence this enables estimating the grain size for unimodal pyrrhotite distributions in rocks measured trends are better fitted with an exponential function than with a rayleigh law style function based on the rock magnetic characterization and the behaviour of the anisotropy of magnetic susceptibility six samples of the original 11 were selected for the high field anisotropy determinations within the basal plane those data were acquired with a torque cantilever type magnetometer as expected most single crystals showed a pure 6 curve within their basal plane because of the easy axis configuration in some crystals however lower harmonic terms overlapped the 6 term this may be the dominant source of the observed variation in magnetic anisotropy properties torque data of three of the six samples were of sufficient quality to allow evaluation of k 1 re evaluation of existing torque data and including the present newly derived determinations yields for the anisotropy constant of pyrrhotite within the basal plane k 1 2 7 0 2 10 4 jm 3 this is over an order of magnitude more precise than the sparse existing k 1 data only the value reported by mikami and co authors in 1959 agrees with the new determination with this firmly established k 1 value meaningful anisotropy models are now possible for pyrrhotite bearing rocks

evidence for aqueous activity on comet 81p wild 2 from sulfide mineral assemblages in stardust samples and ci chondrites abstract the discovery of nickel copper and zinc bearing iron sulfides from comet 81p wild 2 wild 2 represents the strongest evidence in the stardust collection of grains that formed in an aqueous environment we investigated three microtomed tem sections which contain crystalline sulfide assemblages from wild 2 and twelve thin sections of the hydrothermally altered ci chondrite orgueil detailed structural and compositional characterizations of the sulfide grains from both collections reveal striking similarities the stardust samples include a cubanite cufe 2 s 3 grain a pyrrhotite fe ni 1 x s pentlandite fe ni 9 s 8 assemblage and a pyrrhotite sphalerite fe zn s assemblage similarly the ci chondrite sulfides include individual cubanite and pyrrhotite grains cubanite pyrrhotite assemblages pyrrhotite pentlandite assemblages as well as possible sphalerite inclusions within pyrrhotite grains the cubanite is the low temperature orthorhombic form which constrains temperature to a maximum of 210 c the stardust and orgueil pyrrhotites are the 4c monoclinic polytype which is not stable above 250 c the combinations of cubanite and pyrrhotite as well as pyrrhotite and pentlandite signify even lower temperatures the crystal structures compositions and petrographic relationships of these sulfides constrain formation and alteration conditions taken together these constraints attest to low temperature hydrothermal processing our analyses of these minerals provide constraints on large scale issues such as heat sources in the comet forming region aqueous activity on cometary bodies and the extent and mechanisms of radial mixing of material in the early nebula the sulfides in the wild 2 collection are most likely the products of low temperature aqueous alteration they provide evidence of radial mixing of material e g cubanite troilite from the inner solar system to the comet forming region and possible secondary aqueous processing on the cometary body

tides of the northwestern ross sea and their impact on dense outflows of antarctic bottom water abstract moored and depth profile measurements of hydrography and currents during the 2003 2005 antarctic slope anslope program on the shelf slope region near drygalski trough sill in the northwest ross sea have revealed an energetic benthic outflow of cold dense antarctic bottom water aabw produced near the shelf break this outflow couples dense water production processes on the continental shelf to ventilation of the global deep ocean we use time series of bottom pressure and currents on the anslope moorings and a high resolution three dimensional primitive equation numerical model to explore the hypothesis that the tidal currents near the shelf break have a significant impact on outflow volume transport and hydrographic properties tidal currents exceed 1 m s 1 at spring tide over the outer shelf and upper slope tidal advection at spring tide carries water parcels 20 km across slope during a half tidal cycle sufficient to advect water from northern drygalski trough across the sill to the shelf break or similarly from the shelf break 500 m water depth down to the 1600 m isobath a tidally rectified lagrangian mean flow of order 0 05 0 1 m s 1 over the outer shelf and upper slope accompanies the spring tide thickness and water mass properties of the benthic layer over the sill and upper slope exhibit pronounced tidal cycles due to advection of hydrographic gradients and to energetic mixing that can extend more than halfway up from the seabed to the ocean surface maximum parameterized diapycnal diffusivity in the benthic layer at spring tides approaches 1 m 2 s 1 the addition of tides has a profound effect on time averaged outflows of aabw primarily through an increase in benthic layer thickness during spring tide downslope volume fluxes of aabw over the slope averaged over a tidal cycle are 2 5 times larger at spring tide than at neap indicating a need to include the effect of tides in large scale ocean models attempting to represent shelf water contributions to the global ocean circulation

the three dimensional overturning circulation of the southern ocean during the woce era a box inverse model of the southern ocean during the world ocean circulation experiment is constructed to investigate the three dimensional structure of the regional overturning circulation in that era the model has many features in common with various preceding inverse studies but also contains several novel elements that make it well suited for addressing many of the significant uncertainties that surround the circulation at present the net overturning circulation of the southern ocean is found to consist of two well defined cells of similar strength the upper cell consists of a northward transport of 18 8 5 5 sv of surface mode and intermediate waters lighter than the 27 5 kg m 3 isoneutral and an equivalent southward flow in the approximate 27 5 27 9 kg m 3 neutral density range encompassing the bulk of the upper circumpolar deep water the lower cell involves the northward export of 18 6 0 9 sv of antarctic bottom water and lower circumpolar deep water denser than 28 08 kg m 3 and an opposing transport in the lighter classes of that water mass substantial structural differences between the overturning circulations of the atlantic indian and pacific basins are indicated by the model s solution overall the diagnosed southern ocean circulation shares many qualitative and some quantitative features with previous inverse estimates particularly as regards the large scale depth integrated lateral circulation and associated energy fluxes in the subtropics and in the antarctic circumpolar current and the strength of the upper overturning cell however it also suggests several significant adjustments to current views of the regional circulation most notable amongst these are the subpolar circulation of the southern ocean is more vigorous and zonally interconnected than generally thought the associated lower overturning cell is more intense than indicated by most preceding estimates contrary to common perception sub surface mixing processes play a role of comparable importance to air sea ice exchanges of buoyancy in underpinning the dianeutral closure of the southern ocean overturning even at shallow mode and intermediate water levels and the connection between north atlantic deep water formation and southern ocean upwelling is fundamentally three dimensional such that deep waters from the north atlantic must upwell dianeutrally before being returned to the permanent pycnocline of the northern oceans

paleoenvironmental significance of clay mineral associations at the cretaceous tertiary passage abstract the clay mineral stratigraphy at the cretaceous tertiary transition is considered by the study of new sections walvis ridge maud rise central italy northern tunisia which are compared with results of previous investigations increased contents of smectite and or kaolinite or substrate derived species e g illite group depending on the areas investigated are observed near the boundary these diversified modifications indicate increased erosion and predominance of local detrital supply including that of volcanic rocks over general related causes most probably these modifications of clay mineral associations express a major geodynamic disruption characterized by tectonic instability and sea level drop those events began prior to the cretaceous tertiary boundary i e prior to the major iridium anomaly recorded in sediments this could explain why a catastrophic event at the cretaceous tertiary boundary is hardly evidenced in the clay sedimentological record which is extremely sensitive to the global changes in the earth s terrestrial environment

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

terrestrial maria the origins of large basalt plateaus hotspot tracks and spreading ridges large lava plateaus which form abruptly within plates without apparent tectonic cause appear to be the terrestrial equivalents of lunar maria deep erosion of lava plateaus reveals that their magma chambers are gabbro and granophyre complexes the classic lopoliths the close association of the deccan plateau with the timing of the terminal cretaceous boundary clay suggests that it is an impact crater large enough to cause pressure relief melting in the asthenosphere basalt then flooded the crater to form a lava lake the terrestrial equivalent of a lunar mare part of the crater rim survives in the amirante ridge the carlsberg oceanic ridge and chagos laccadive hotspot track formed simultaneously with the deccan plateau several other mesozoic and cenozoic lava plateaus are parts of similar arrays of simultaneous features one of those is the southern oregon part of the columbia plateau with its associated continental rifting in the northern basin and range and the yellowstone hotspot track along t

overflow dynamics and bottom water formation in the western ross sea influence of tides 1 the tidal impact on overflow dynamics and bottom water production in the western ross sea is studied with the finite element ocean model which allows embedding a mesh with 0 5 km resolution in a coarse resolution 30 km setup without nesting the simulated overflow properties inside and downstream of the western ross sea are described the overflow exhibits pronounced variability at both daily and spring neap tidal time scales in the western ross sea tides increase mixing over both the outer shelf and upper slope there plume jets are shaped by tidal currents at a bathymetric bend west of the drygalski trough mouth descending rapidly and supplying the bottom water a fraction of shelf water remains over the shelf and propagates westward from the ross sea but it does not contribute significantly to bottom water formation because of energetic mixing over the upper slope compared to a simulation without tidal forcing tides with the major k1 and o1 constituents increase the outflow rate over the continental slope off cape adare by about 70 a set of sensitivity experiments show that the rate of bottom water production is not a monotonic function of the tidal currents amplitude tidal forcing with intermediate strength leads to the most efficient bottom water formation

watermass properties of the antarctic slope front a simple model abstract in the antarctic dense shelf water is formed in coastal polynyas and is differentiated from the fresher surface water by the wind induced ice motion that displaces offshore the ice melt from production zones where the shelf water discharges into the deep ocean the antarctic slope front asf is v shaped separating the shelf and surface waters referred to as frontal waters from the circumpolar deep water cdw to elucidate basic constraints on frontal properties a minimal model of homogeneous water masses forced by offshore wind and freshwater input in a perpetual winter is considered with the surface water stirred by and hence aligned with the ice cover there is little leakage of ice or meltwater from the frontal system so ice production and melt merely redistribute heat and salt between frontal waters as such the heat loss to the atmosphere needs to be supplied by entraining cdw which then necessitates the shelf water discharge on account of the mass balance because of the fresh

mixing in flows down gentle slopes into stratified environments observations of the flow of dense fluid into uniformly density stratified environments down plane slopes with small inclination to the horizontal 20 are described and a quantitative model for such flows is presented in these experiments the dense fluid is released at the top of the slope for a finite period of time the resulting downslope gravity current or downflow has uniform thickness with a distinct upper boundary until it approaches its level of neutral density where the fluid leaves the proximity of the slope turbulent transfers of mass and momentum occur across the upper boundary causing a continuous loss of fluid from the downflow in most cases and associated loss of momentum the flow may be characterized by the local values of the richardson number ri the reynolds number re generally large and of m qn 3 g 2 where q is the two dimensional volume flux n the buoyancy frequency and g the negative buoyancy of the dense fluid the model for the downflow describes the turbulent transfers in terms of entrainment detrainment and drag coefficients e e e d and k respectively and the observations enable the determination of these coefficients in terms of the local values of m and ri the model may be regarded as an extension of that ellison turner 1959 to stratified environments describing the consequent substantial changes in mixing and distribution of the inflow it permits the modelling of the bulk properties of these flows in geophysical situations including shallow and deep flows in the ocean

connecting antarctic cross slope exchange with southern ocean overturning previous idealized investigations of southern ocean overturning have omitted its connection with the antarctic continental shelves leaving the influence of shelf processes on antarctic bottom water aabw export unconsidered in particular the contribution of mesoscale eddies to setting the stratification and overturning circulation in the antarctic circumpolar current acc is well established yet their role in cross shelf exchange of water masses remains unclear this study proposes a residual mean theory that elucidates the connection between antarctic cross shelf exchange and overturning in the acc and the contribution of mesoscale eddies to the export of aabw the authors motivate and verify this theory using an eddy resolving process model of a sector of the southern ocean the strength and pattern of the simulated overturning circulation strongly resemble those of the real ocean and are closely captured by the residualmean theory over the continental slope baroclinic instability is suppressed and so transport by mesoscale eddies is reduced this suppression of the eddy fluxes also gives rise to the steep v shaped isopycnals that characterize the antarctic slope front in aabw forming regions of the continental shelf furthermore to produce water on the continental shelf that is dense enough to sink to the deep ocean the deep overturning cell must be at least comparable in strength to wind driven mean overturning on the continental slope this results in a strong sensitivity of the deep overturning strength to changes in the polar easterly winds

trajectories and distribution of material ejected from the chicxulub impact crater implications for postimpact wildfires 1 the trajectories of low and high energy ejecta from the chicxulub impact crater have been computed using a numerical code and compared with analyses of debris deposited in k t boundary sections these calculations indicate that low energy ejecta produces centimeter to meter thick deposits within 4000 km of the point of impact although its azimuthal distribution depends on the incident angle of the projectile with the surface of the earth the high energy ejecta or vapor plume rises through the atmosphere and isotropically expands at the top of the atmosphere the velocity of this ejecta increases as it rises approximately 12 of the high energy ejecta is lost because it reaches escape velocities while 25 of the material reaccretes within 2 hours 55 reaccretes within 8 hours and 85 reaccretes within 72 hours this ejecta is distributed globally but it is concentrated around the chicxulub impact site and at the antipode corresponding to india and the indian ocean 65 million years ago it is also slightly smeared in a longitudinal direction because of earth s rotation because this debris does not reaccrete all at once the collisional shock heating of the atmosphere will be drawn out over a period of a few days and will occur in pulses this could be an important factor when calculating postimpact chemical reactions in the stratosphere the calculations also indicate the ejecta would have ignited wildfires on several continents around the world although the distribution of those fires depends partly on the trajectory of the projectile

observations of the antarctic slope undercurrent in the southeastern weddell sea 1 the antarctic slope front presents a dynamical barrier between the cold antarctic shelf waters in contact with ice shelves and the warmer subsurface waters offshore two hydrographic sections with full depth current measurements were undertaken in january and february 2009 across the slope and shelf in the southeastern weddell sea southwestward surface intensified currents of 30 cm s 1 and northeastward undercurrents of 6 9 cm s 1 were in thermal wind balance with the sloping isopycnals across the front which migrated offshore by 30 km in the time interval between the two sections a mid depth undercurrent on february 23 was associated with a 130 m uplift of the main pycnocline bringing warm deep water closer to the shelf break this vertical displacement comparable to that caused by seasonal variations in wind speed implies that undercurrents may affect the exchanges between coastal and deep waters near the antarctic continental margins

electrical and magnetic properties of sulfides the metal sulfides exhibit a great diversity of electrical and magnetic properties with both scientific interest and practical applications these properties apply major constraints on models of the electronic structure or chemical bonding in sulfides vaughan and rosso 2006 this volume the pure and doped synthetic equivalents of certain sulfide minerals have actual or potential applications in the electronics industries optical devices photovoltaics photodiodes and magnetic recording devices sulfides are also components of many thin film devices and have been extensively investigated as part of the nanotechnology revolution certain electrical and magnetic properties of sulfide minerals mean they contribute to geomagnetism and paleo magnetism and provide the geophysical prospector with exploration tools for metalliferous ore deposits to the mineral technologist these same properties provide methods for the separation of the metal bearing sulfides from associated waste minerals after mining and milling and before extraction of the metal by pyrometallurgical or hydrometallurgical treatment in this chapter the theory and measurement of electrical and magnetic properties are outlined along with spectroscopic and diffraction studies that can provide insights into magnetic behavior are discussed a brief review of electrical and magnetic studies of major sulfide minerals includes some examples of the applications of sulfide electrical and magnetic properties including special consideration of the properties of sulfide nanoparticles most of the available data presented are for pure synthetic binary and ternary sulfides as very small concentrations of impurities can dramatically affect electrical properties leading to problems of interpreting data from natural samples although data for several of the commonly found sulfides are discussed in this chapter no attempt is made at a comprehensive coverage the section below on theory and measurement of electrical and magnetic properties draws on the account given in vaughan and craig 1978 to which readers are referred for further details it

magnetic properties of k t and e o microspherules origin by combustion abstract magnetic spherules and spheroids are found in association with ir anomalies at the cretaceous tertiary k t boundary and near to the eocene oligocene e o boundary detailed magnetic analyses have been performed on these particles and on bulk k t boundary clay from marine and continental sections coercivity spectra and blocking and curie temperatures of the spheroids and k t boundary clays have been determined and natural remanence and susceptibility measured for the bulk clay and associated strata marine k t boundary clays are strongly magnetic with remanent coercivities similar to those of the spheroids in contrast the continental clays are diamagnetic except for the thin overlying impact layer which contains abundant paramagnetic jarosite along with shocked quartz and feldspar the morphologies mineralogies and magnetic properties of the k t magnetic spheroids are closely analogous to magnetic fly ash spherules this similarity may be related to the presence of carbonaceous soot particles in some marine k t boundary clays and the recent identification of plerospheres and cenospheres in continental boundary clay in contrast tektites and microtektites differ from the k t magnetic spherules in a number of fundamental ways the late eocene particles are magnetically distinct from the k t spheroids both in their coercivity and thermal spectra inaa results on upper cretaceous combustible shales indicate that the anomalous abundances of certain chalcophile and lithophile elements at the k t boundary may be explained in terms of the natural combustion of bituminous sediments

magnetic properties of synthetic analogues of pyrrhotite ore in the grain size range 1 24 µm summary iron and sulphur in proportions appropriate to fe 7 s 8 were reacted in evacuated quartz tubes for 24 hr at 500 c quenching produces predominantly hexagonal pyrrhotite whereas further prolonged annealing at 250 c produces monoclinic pyrrhotite we mainly report the properties of material cooled from 500 ca t 10 c min 1 which is predominantly monoclinic but probably contains hexagonal pyrrhotite together with pyrite fes 2 this material which we refer to as non ideal pyrrhotite may be a better analogue of pyrrhotite ore than ideal synthetic monoclinic pyrrhotite the samples were characterized by x ray mossbauer evect and thermomagnetic analyses the slow cooled non ideal monoclinic pyrrhotite exhibits thermal stability intermediate between monoclinic and hexagonal pyrrhotites one striking observation is that the deviation from ideal monoclinic pyrrhotite increases with decreasing particle size particles about 1 mm in size have a low saturation magnetization value of about 6 am2 kg 1 indicating a lower concentration of ferrimagnetic monoclinic pyrrhotite saturation magnetization 18 am2 kg 1 than particles of about 6 mm and above saturation magnetization 12 am2 kg 1 on the other hand magnetization process parameters coercive force ratio of saturation remanence to saturation magnetization coercive force of remanence median destructive and inductive fields the magnetic susceptibility follow well behaved power law dependences on particle size similar to monoclinic pyrrhotite magnetic domain patterns in the slow cooled non ideal pyrrhotite are similar in type and style to those observed in synthetic monoclinic pyrrhotite and in natural pyrrhotite particles in rocks dynamic observations of domain walls in changing external alternating and direct magnetic fields are consistent with macroscopic obser vations of magnetization change with field the critical size for the monodomain multidomain transition in synthetic monoclinic pyrrhotite at about 1 mm is essentially the same as that previously determined for natural pyrrhotite grains in rocks the observations suggest that the domain walls lie normal to the c axis of the structure consistent with the known magnetocrystalline anisotropy symmetry of monoclinic pyrrhotite the type and style of the magnetic microstructures of hexagonal pyrrhotite are quite diverent from those of monoclinic pyrrhotite the domain patterns are on a much smaller scale wavy and sometimes resemble those of a hexagonal material such as magnetoplumbite or cobalt

is there a distinct continental slope fauna in the antarctic abstract the antarctic continental slope spans the depths from the shelf break usually between 500 and 1000 m to 3000 m is very steep overlain by warm 2 2 5 c circumpolar deep water cdw and life there is poorly studied this study investigates whether life on antarctica s continental slope is essentially an extension of the shelf or the abyssal fauna a transition zone between these or clearly distinct in its own right using data from several cruises to the weddell sea and scotia sea including the andeep antarctic benthic deep sea biodiversity colonisation history and recent community patterns i iii biopearl biodiversity phylogeny evolution and adaptive radiation of life in antarctica 1 and easiz ecology of the antarctic sea ice zone ii cruises as well as current databases sombase scar marbin four different taxa were selected i e cheilostome bryozoans isopod and ostracod crustaceans and echinoid echinoderms and two areas the weddell sea and the scotia sea to examine faunal composition richness and affinities the answer has important ramifications to the link between physical oceanography and ecology and the potential of the slope to act as a refuge and resupply zone to the shelf during glaciations benthic samples were collected using agassiz trawl epibenthic sledge and rauschert sled by bathymetric definition these data suggest that despite eurybathy in some of the groups examined and apparent similarity of physical conditions in the antarctic the shelf slope and abyssal faunas were clearly separated in the weddell sea however no such separation of faunas was apparent in the scotia sea except in echinoids using a geomorphological definition of the slope shelf slope abyss similarity only changed significantly in the bryozoans our results did not support the presence of a homogenous and unique antarctic slope fauna despite a high number of species being restricted to the slope however it remains the case that there may be a unique antarctic slope fauna but the paucity of our samples could not demonstrate this in the scotia sea it is very likely that various ecological and evolutionary factors such as topography water mass and sediment characteristics input of particulate organic carbon poc and glaciological history drive slope distinctness isopods showed greatest species richness at slope depths whereas bryozoans and ostracods were more speciose at shelf depths however significance varied across weddell sea and scotia sea and depending on bathymetric vs geomorphological definitions whilst the slope may harbour some source populations for localised shelf recolonisation the absence of many shelf species genera and even families in a poorly dispersing taxon from the continental slope indicate that it was not a universal refuge for antarctic shelf fauna

end cretaceous mass extinction event argument for terrestrial causation the end cretaceous mass extinctions were not a geologically instantaneous event and were selective in character these features are incompatible with the original alvarez hypothesis of their being caused by a single asteroid impact that produced a world embracing dust cloud with devastating environmental consequences by analysis of physical and chemical evidence from the stratigraphic record it is shown that a modified extraterrestrial model in which stepwise extinctions resulted from encounter with a comet shower is less plausible than one intrinsic to the earth involving significant disturbance in the mantle

domain structure of natural fine grained pyrrhotite in a rock matrix diabase the domain structure of natural pyrrhotite grains in a devonian diabase was studied with the bitter pattern technique examples are shown of the domain structure of large d 100 m true multidomain grains intermediate 10 d 40 m pseudo single domain grains and ore grains with less than four domains down to the transition from the two domain to the single domain state plate shaped domains prevail for all sizes of ore grains closure domains are only rarely formed non magnetic inclusions and surface irregularities impede the domain wall mobility as expected resulting in an often quite considerable spread of muscopic coercive forces in the large grains intergrowths of chemically only slightly different phases of pyrrhotite which cannot be distinguished in a reflected light muscope seem to be responsible for entirely stationary pseudo domain walls in low dc fields domain wall displacements as well as spin rotations within the basal plane of easy magnetization are probably the dominant magnetization processes in pseudo single domain and multidomain particles

antarctic bottom water production and export by tides in the ross sea 1 current measurements in the western ross sea show that antarctic bottom water is produced at the outer edge of the continental shelf by tidal stirring of antarctic surface water circumpolar deep water and shelf water current amplitudes during spring tides are greater than 1 m s sufficient to carry circumpolar deep water onto the shelf from offshore and stir at least the lower portion of the water column to a nearly homogeneous mixture the antarctic bottom water produced every day during spring tides does not appear to have a seasonal signal its export rate is estimated to be 1 95 1 85 10 6 m 3 s

rapid subsurface warming and circulation changes of antarctic coastal waters by poleward shifting winds the southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s this wind trend is projected to persist under continued anthropogenic forcing but the impact of the changing winds on antarctic coastal heat distribution remains poorly understood here we show that a poleward wind shift at the latitudes of the antarctic peninsula can produce an intense warming of subsurface coastal waters that exceeds 2 c at 200 700 m depth the model simulated warming results from a rapid advective heat flux induced by weakened near shore ekman pumping and is associated with weakened coastal currents this analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around antarctica with potentially significant ramifications for global sea level rise

energetic plumes over the western ross sea continental slope rapid descent of dense drygalski trough western ross sea antarctica shelf water over the continental slope within 100 to 250 m thick benthic plumes is described speeds of up to 1 0 m s are recorded flowing at an average angle of 35 to the isobaths entraining ambient lower circumpolar deep water en route this process is predominant in determining the concentration and placement of the shelf water injected into the deep sea as a precursor antarctic bottom water nonetheless a 4 hour duration pulse of undiluted shelf water was observed at depth 1407 m directly north of the drygalski trough moving at around 90 degrees to isobaths and at a speed of 1 4 m s thus the export of ross sea shelf water to the deep sea is accomplished within plumes descending at moderate angle to isobaths punctuated by rapid downhill cascades

on the circulation and water masses over the antarctic continental slope and rise between 80 and 150 e abstract the circulation and water masses in the region between 80 and 150 e and from the antarctic continental shelf to the southern boundary of the antarctic circumpolar current acc 62 s are described from hydrographic and surface drifter data taken as part of the multi disciplinary experiment baseline research oceanography krill and the environment broke two types of bottom water are identified adelie land bottom water formed locally between 140 and 150 e and ross sea bottom water ross sea bottom water is found only at 150 e whereas adelie land bottom water is found throughout the survey region the bottom water mass properties become progressively warmer and saltier to the west suggesting a westward flow all of the eight meridional ctd sections show an antarctic slope front of varying strength and position with respect to the shelf break in the water formation areas between 140 and 150 e and 104 e the antarctic slope front is more v shaped while elsewhere it is one sided the shape of the slope front and the presence or absence of water formation there are consistent with other meridional sections in the weddel sea and simple theories of bottom water formation gill 1973 deep sea research 20 111 140 whitworth et al 1998 in jacobs and weiss eds ocean ice and atmosphere interactions at the antartic continental margin antarctic research series american geophysical union washington pp 1 27 adcp surface velocities and buoy drift tracks show a strong westward flow over the shelf and slope regions in the region 90 100 e there is a strong eastward flow of the waters just south of the southern boundary of the acc suggesting a recirculation of the westward slope current and the presence of a weak cyclonic gyre using the adcp velocities as a reference for the ctd data the average westward transport in this region is 29 4 14 7 sv

the antarctic slope current in a changing climate the antarctic slope current asc is a coherent circulation feature that rings the antarctic continental shelf and regulates the flow of water towards the antarctic coastline the structure and variability of the asc influences key processes near the antarctic coastline that have global implications such as the melting of antarctic ice shelves and water mass formation that determines the strength of the global overturning circulation recent theoretical modeling and observational advances have revealed new dynamical properties of the asc making it timely to review earlier reviews of the asc focused largely on local classifications of water properties of the asc s primary front here we instead provide a classification of the current s frontal structure based on the dynamical mechanisms that govern both the along slope and cross slope circulation these two modes of circulation are strongly coupled similar to the antarctic circumpolar current highly variable motions such as dense overflows tides and eddies are shown to be critical components of cross slope and cross shelf exchange but understanding of how the distribution and intensity of these processes will evolve in a changing climate remains poor due to observational and modeling limitations results linking the asc to larger modes of climate variability such as el nino show that the asc is an integral part of global climate an improved dynamical understanding of the asc is still needed to accurately model and predict future antarctic sea ice extent the stability of the antarctic ice sheets and the southern ocean s contribution to the global carbon cycle

magnetic inclusions in diamonds abstract we demonstrate that the pyrrhotite that commonly occurs as inclusions in diamonds is capable of carrying strong and stable remanent magnetizations this suggests that with suitable samples it may be possible to use such inclusions to obtain information about the geomagnetic field such as the paleointensity during key intervals of geologic time the pyrrhotite inclusions examined here exhibit a strong and pronounced anisotropy of magnetic remanence that is coupled to the crystal structure of the diamond while this anisotropy of remanence may complicate efforts to obtain paleointensity data we propose that it may provide a valuable way to uniquely identify individual diamonds that contain even minute inclusions of pyrrhotite

the impact of open oceanic processes on the antarctic bottom water outflows abstractthe impact of open oceanic processes on the antarctic bottom water aabw outflows is investigated using a numerical model with a focus on outflows that occur through deep channels a major branch of the aabw outflow is known to occur as an overflow from the filchner depression to the weddell sea through a deep channel a few hundred kilometers wide and a sill roughly 500 m deep when this overflow enters the weddell sea it encounters the antarctic slope front asf at the shelf break a density front commonly found along the antarctic continental shelf break the presence of an aabw outflow and the asf create a v shaped isopycnal structure across the shelf break indicating an interaction between the overflow and oceanic processes model experiments show the overflow transport to increase significantly when an oceanic wind stress increases the depth of the asf this enhancement of overflow transport occurs because the channel walls allow a pressure gradient in the along slope direction to exist a

domain wall dynamics in 4c pyrrhotite at low temperature summary monoclinic 4c pyrrhotite fe7s8 is ferrimagnetic due to an ordered defect structure with alternating vacancy and vacancy free sublattices its low temperature magnetic transition near 35k is characterized by the distinct increase in coercivity and remanent magnetization the increase of these parameters has been attributed to changes in the domain wall structure we present static and dynamic magnetization data of a powder sample to study the domain wall dynamics across the low temperature transition the amplitude dependent ac susceptibility and the ferromagnetic resonance spectroscopy indicate that the hardening of the domain wall pinning at the transition occurs simultaneously with the decrease in initial saturation remanent magnetization these two effects are explained by the enhanced inhomogeneity of the bulk material caused by the persistency of the ordered vacancies and by newly formed defects due to localized distortion of fe ii sites in the vacancy free sublattice the generated localized defects are the link between the domain wall dynamics and the low temperature transition in 4c pyrrhotite

western ross sea continental slope gravity currents antarctic bottom water of the world ocean is derived from dense shelf water that is carried downslope by gravity currents at specific sites along the antarctic margins data gathered by the anslope and clima programs reveal the presence of energetic gravity currents that are formed over the western continental slope of the ross sea when high salinity shelf water exits the shelf through drygalski trough joides trough immediately to the east offers an additional escape route for less saline shelf water while the glomar challenger trough still farther east is a major pathway for export of the once supercooled low salinity ice shelf water that forms under the ross ice shelf the drygalski trough gravity currents increase in thickness from 100 to 400 m on proceeding downslope from 600 m the shelf break to 1200 m upper slope sea floor depth while turning sharply to the west in response to the coriolis force during their descent the mean current pathway trends 35 downslope from isobaths benthic layer current and thickness are correlated with the bottom water salinity which exerts the primary control over the benthic layer density a 1 year time series of bottom water current and hydrographic properties obtained on the slope near the 1000 m isobath indicates episodic pulses of shelf water export through drygalski trough these cold 34 75 pulses correlate with strong downslope bottom flow extreme examples occurred during austral summer fall 2003 comprising concentrated high salinity shelf water 1 9 c 34 79 and approaching 1 5 m s 1 at descent angles as large as 60 relative to the isobaths such events were most common during november may consistent with a northward shift in position of the dense shelf water during austral summer the coldest saltiest bottom water was measured from mid april to mid may 2003 the summer fall export of high salinity shelf water observed in 2004 was less than that seen in 2003 this difference if real may reflect the influence of the large iceberg c 19 over drygalski trough until its departure in mid may 2003 when there was a marked decrease in the coldest saltiest gravity current adjacent to drygalski trough northward transport of cold saline recently ventilated antarctic bottom water observed in march 2004 off cape adare was 1 7 sv including 0 4 sv of high salinity shelf water

three dimensional interpretation of the ktb gravity and magnetic anomalies a combination of a closely spaced surface gravity survey with a high resolution helicopter aeromagnetic survey as well as borehole gravity and magnetometer measurements allowed a detailed three dimensional 3 d modeling of the anomalies at the ktb drill site the models could be constrained by new evidence from a 3 d seismic survey and by structural geology and petrophysical data from drill cores and cuttings the source body for the positive gravity anomaly consist of high density metabasite the vertical derivative of the bouguer anomaly does not resemble the aeromagnetic anomaly in all areas indicating that parts of the metabasites are more or less nonmagnetic surprisingly and confirming the observation in other deep drill holes into continental crystalline basement rocks pyrrhotite is the dominant magnetic mineral below a depth of about 300 m magnetite mainly occurs in the depth intervals 360 520 m and 7300 7900 m the lower interval causes the anomalous vertical gradient of 60 nt km for the geomagnetic field the occurrence of strongly magnetic minerals in the borehole down to about 3000 m correlates with the lithology while in the deeper parts it is more related to fissures and fault zones where chemical processes reduction oxidation are active

ice shelf water overflow and bottom water formation in the southern weddell sea 1 cold shelf waters flowing out of the filchner depression in the southern weddell sea make a significant contribution to the production of weddell sea bottom water wsbw a precursor to antarctic bottom water aabw we use all available current meter records from the region to calculate the flux of cold water 1 9 c over the sill at the northern end of the filchner depression 1 6 0 5 sv and to determine its fate the estimated fluxes and mixing rates imply a rate of wsbw formation referenced to 0 8 c of 4 3 1 4 sv we identify three pathways for the cold shelf waters to enter the deep weddell sea circulation one path involves flow constrained to follow the shelf break the other two paths are down the continental slope resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36 w and 37 w mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m probably aided by thermobaricity the plume water at this depth can flow at a speed approaching 1 m s 1 implying that the flow is occasionally supercritical we postulate that such supercriticality acts to limit mixing between the plume and its environment the transition from supercritical to slower more uniform flow is associated with very efficient mixing probably as a result of hydraulic jumps