three dimensional architecture of shelf building sediment drifts in the offshore canterbury basin new zealand abstract a grid of high resolution multichannel seismic profiles from the offshore canterbury basin new zealand reveals that at least 11 large up to 1000 m thick 50 km long along strike and 20 km wide down dip elongate sediment drifts developed within the lower miocene to recent shelf slope sediment prism the drifts overlie a condensed section of late eocene to late oligocene limestone and cover an area of 5000 km2 the drifts were deposited in water depths of 300 750 m probably by a northward flowing contour current and aggraded to shelf depths the drifts exhibit mounded morphologies with channel like moats along their landward flanks erosion of the landward flanks creates prominent unconformities these unconformities are diachronous and therefore not sequence boundaries the internal architecture of the drifts defines two end members of elongate drift which we describe as simple and complex early early to middle miocene simple drifts are small 50 km long and up to 20 km wide and occur in the northeastern part of the survey area these late simple elongate drifts are subdivided into three parts base core and crest based on seismic facies these facies form in response to progressive confinement of current flow within the moat complex drifts may be multi crested or multistage multi crested drifts form in response to rapid lateral shifts in position of the moat perhaps associated with relative sea level change modulated by paleoslope inclination and orientation such drifts together with the observation that several drifts were often active simultaneously indicate that flow patterns were complex and involved multiple pathways multistage drifts comprise superimposed subdrifts whose retrogradational and progradational stacking patterns indicate fluctuations in the rate of sediment supply complex drift formation may require intermediate shelf relief high enough to sustain drift development during changes in sea level and rate of sediment supply but low enough so that such changes are still able to influence moat position in addition coeval climatic cooling may amplify changes in sea level current intensity and sediment supply thereby contributing to complex drift formation

oceanographic and climatic changes over the past 160 000 years at deep sea drilling project site 594 off southeastern new zealand southwest pacific ocean high resolution textural carbonate microfossil and oxygen 18o and carbon 13c stable isotopic analyses are presented for the late quaternary isotopic stages 1 to 6 interval of a core at deep sea drilling project site 594 situated just south of the present subtropical convergence in northernmost subantarctic surface waters on the southern flank of chatham rise in the southwest pacific downcore alternations of pelagic and hemipelagic oozes correspond to interglacial and glacial episodes respectively interglacial oozes contain a northern subantarctic assemblage of planktonic foraminifera with rare cool subtropical species while glacial oozes are characterized by species typical of southern most subantarctic waters and include radiolaria with affinity for antarctic waters the planktonic 18o record for the site supports a 3 6 c temperature change in near surface waters between interglacial and glacial stages and indicates that during stage 5e the near surface waters were about 1 c warmer than at present a pronounced cooling during stage 5d matches that of the vostok ice core d record and negative excursions at the end of stages 2 and 6 support a southern latitude warming preceding northern hemisphere deglaciation benthic foraminifera typical of cold intermediate to deep waters can increase dramatically in abundance in the hemipelagic ooze intervals when 18o results suggest a temperature drop in glacial stage bottom waters of 2 4 c possibly a result of upward displacement of antarctic intermediate water by circumpolar deep water at the site the foraminiferal 13c records support a reduced influence of north atlantic deep water in the southwest pacific during glacial stages when nutrient enhancement occurred in both bottom and surface waters despite the pronounced changes between interglacial and glacial conditions inferred at site 594 contributed to by a substantial northward shift by at least 5 of latitude in the position of the antarctic convergence south of new zealand during glacial episodes we conclude that the subtropical convergence remained locked to chatham rise approximately 44 s throughout stages 1 to 6 major compression of the intervening belt of subantarctic water during glacial episodes and the associated very steep thermal gradients and intensified atmospheric and oceanic circulation patterns that developed in this part of the southwest pacific account for the harsh frigid environment reported for on land southern new zealand at these times

deconvolution of three component teleseismic data from southern tibet using the sva technique in this study the sv autocorrelation sva technique of dasgupta and nowack 2006 is tested using three component p wave data from selected indepth ii and cdsn stations in southern tibet the sva technique involves constructing an estimate of the source and distant earth wavelet from the autocorrelation of the sv component and using this to deconvolve the data the deconvolved vertical compo nents are then used to infer the crustal p velocity structure initial models for the in version of the deconvolved vertical components were based on the s velocity results obtained from the inversions of radial receiver functions by mitra et al 2005 the moho depths obtained from inverting the vertical components deepen from the south to the north in the study area and are comparable to the depths obtained by mitra et al 2005 to the north and similar but somewhat shallower to the south the crustal vp vs ratios are obtained from the inverted vs models from mitra et al 2005 and the inferred vp speeds obtained from this study except for station bb18 with a some what higher vp vs ratio the other stations to the north in the tethyan himalayas and the southern lhasa terrane have vp vs ratios between 1 75 and 1 80 with a mean of 1 77 a poisson s ratio of 0 265 the range of values could result from noise in the data as well as from lateral heterogeneity in the region with different piercing points at depth for the ps and pppp phases

intensified deep pacific inflow and ventilation in pleistocene glacial times the production of cold deep waters in the southern ocean is an important factor in the earth s heat budget1 the supply of deep water to the pacific ocean is presently dominated by a single source the deep western boundary current east of new zealand here we use sediment records deposited under the influence of this deep western boundary current to reconstruct deep water properties and speed changes during the pleistocene epoch in physical and isotope proxies we find evidence for intensified deep pacific ocean inflow and ventilation during the glacial periods of the past 1 2 million years the changes in throughflow may be directly related to an increased production of antarctic bottom water during glacial times possible causes for such an increased bottom water production include increasing wind strengths in the southern ocean or an increase in annual sea ice formation leaving dense water after brine rejection and thereby enhancing deep convection we infer also that the global thermohaline circulation was perturbed significantly during the mid pleistocene climate transition between 0 86 and 0 45 million years ago

old radiocarbon ages in the southwest pacific ocean during the last glacial period and deglaciation marine radiocarbon 14c dates are widely used for dating oceanic events and as tracers of ocean circulation essential components for understanding ocean climate interactions past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep water and surface water reservoirs but the apparent age of surface waters currently approximately 400 years in the tropics and approximately 1 200 years in antarctic waters might not be constant through time as has been assumed in radiocarbon chronologies and palaeoclimate studies here we present independent estimates of surface water and deep water reservoir ages in the new zealand region since the last glacial period using volcanic ejecta tephras deposited in both marine and terrestrial sediments as stratigraphic markers compared to present day values surface reservoir ages from 11 900 14c years ago were twice as large 800 years and during glacial times were five times as large 2 000 years contradicting the assumption of constant surface age furthermore the ages of glacial deep water reservoirs were much older 3 000 5 000 years the increase in surface to deep water age differences in the glacial southern ocean suggests that there was decreased ocean ventilation during this period

glacial interglacial mis 1 10 migrations of the subtropical front across odp site 1119 canterbury bight southwest pacific ocean abstract ocean drilling program odp site 1119 is located at water depth 395 m near the subtropical front stf here represented by the southland front just downslope from the shelf edge of eastern south island new zealand the upper 86 19 metres composite depth mcd of site 1119 sediment was deposited at an average sedimentation rate of 34 cm kyr during marine isotope stages mis 1 8 0 252 ka and is underlain across a 25 kyr intra mis 8 unconformity by mis 8 5 11 277 367 ka and older sediment deposited at 14 cm kyr a time scale is assigned to site 1119 using radiocarbon dates for the period back to 39 ka and prior to then by matching its climatic record with that of the vostok ice core which it closely resembles four palaeoceanographic proxy measures for surface water masses vary together with the sandy muddy glacial interglacial g i cyclicity at the site interglacial intervals are characterised by heavy 13c high colour reflectance a proxy for carbonate content low ray a proxy for clay content and light 18o conversely glacial intervals exhibit light 13c low reflectance high ray and heavy 18o signatures early interglacial intervals are represented by silty clays with 10 105 cm thick beds of sharp based chondrites burrowed shelly graded fine sand the sands are rich in foraminifera and were deposited distant from the shoreline under the influence of longitudinal flow in relatively deep water glacial intervals comprise mostly micaceous silty clay though with some thin 2 10 cm thick sands present also at peak cold periods and contain the cold water scallop zygochlamys delicatula interglacial sandy intervals are characterised by relatively low sedimentation rates of 5 32 cm kyr cold climate intervals mis 10 6 and 2 have successively higher sedimentation rates of 45 69 and 140 cm kyr counter intuitively and forced by the bathymetric control of a laterally moving shoreline during g i and i g transitions the 1119 core records a southeasterly seaward movement of the stf during early glacial periods accompanied by the incursion of subtropical water stw above the site and northwesterly landward movement during late glacial and interglacial times resulting in a dominant influence then of subantarctic surface water saw the history of passage of these different water masses at the site is clearly delineated by their characteristic 13c values the intervals of thin graded sands muds which occur within mis 2 3 6 7 4 and 10 indicate the onset at times of peak cold of intermittent bottom currents caused by strengthened and expanded frontal flows along the stf which at such times lay near site 1119 in close proximity to seaward encroaching subantarctic waters within the bounty gyre in common with other nearby southern hemisphere records the cold period which represents the last glacial maximum lasted between 23 18 ka at site 1119 during which time the stf and subantarctic front saf probably merged into a single intense frontal zone around the head of the adjacent bounty trough

current controlled sediment deposition from the shelf to the deep ocean the cenozoic evolution of circulation through the sw pacific gateway the circulation of cold deep water is one of the controlling factors of the earth s climate forty percent of this water enters the world ocean through the southwest pacific as a deep western boundary current dwbc flowing northwards at bathyal to abyssal depths east of the new zealand microcontinent south of latitude 50 s the dwbc is intimately linked with the antarctic circumpolar current acc which is the prominent force for the shallow water circulation the pacific dwbc is presently the largest single contributor of deep ocean water and deciphering its evolution is of fundamental importance to understanding ocean and climate history and global ocean hydrography the evolution of the dwbc system and of related circum antarctic currents has taken place since 30 25 ma when plate movements created the first oceanic gaps south of australia and south america the stratigraphic record preserved in sediment drifts of the southwest pacific in eastern new zealand is the best available for deciphering the neogene history of southern ocean water masses and of the circulation of the acc dwbc and their precursor systems major current activity commenced on the new zealand margin in the late eocene or early oligocene hoiho drift early acc and was widespread by the mid late oligocene marshall paraconformity and weka pass limestone drift acc during the neogene the eastern south island continental shelf built seawards by accretion at its outer edge of large miocene current drifts up to tens of kilometres long and hundreds of metres thick canterbury drifts also commencing in the mid cenozoic but in depths 2000 m the dwbc emplaced large deep water sediment drifts rates of drift deposition accelerated considerably in the late neogene when climatic change and particularly glacial sea level falls caused the delivery of large volumes of turbiditic sediment into the path of the dwbc via the bounty and hikurangi channels

sea surface temperature estimates from the southwest pacific based on planktonic foraminifera and oxygen isotopes planktonic foraminiferal census data were collected on 34 surficial sediment samples from east of new zealand sea surface temperature estimates were derived from the data using the modern analogue technique mat and compared to data from climap s fa 20 equation they were also computed from oxygen isotope analyses of globigerina bulloides the temperature estimates compared well between the three methods and were found to correlate closely with the appropriate seasonal sea surface temperature as measured by satellite the foraminiferal assemblages and derivative temperatures clearly identify the major watermasses and oceanic fronts in the region and thus can be applied to palaeoceanographic analysis of southwest pacific sediments

response of surface water masses and circulation to late quaternary climate change east of new zealand a series of cores from east of new zealand have been examined to determine the paleoceanographic history of the late quaternary in the sw pacific using planktonic foraminiferal data distinct shifts of species can be seen between glacial and interglacial times especially south of chatham rise east of south island foraminiferal fragmentation ratios and benthic planktonic foraminiferal ratios both show increased dissolution during glacials especially isotope stage 2 to the south of chatham rise the present day subtropical convergence appears to be tied to the chatham rise at 44 s but during glacial times this rise separated cold water to the south from much warmer water to the north with an associated strong thermal gradient across the rise we estimate that this gradient could have presented as much as an 8 c temperature change across 4 of latitude during the maximum of the last ice age there is only weak evidence of the younger dryas cool event but there is a clear climatic optimum between 8 and 6 4 ka with temperatures 1 2 c higher than the present day the marine changes compare well with vegetational changes on both south and north island

similar glacial and holocene deep water circulation inferred from southeast pacific benthic foraminiferal carbon isotope composition we present holocene and last glacial maximum lgm oxygen and carbon isotope measurements on planulina wuellerstorfi in six southeast pacific cores sedimentation rates are low in this part of the ocean and measurements were made on individual foraminiferal shells in order to identify the holocene and glacial individuals on the basis of their extreme 18o the new 13c data were combined with previous p wuellerstorfi data for interpretation of global thermohaline circulation data from the southern ocean were examined closely for regional coherency and a few anomalous 13c values suspected of having productivity overprint were removed the resulting global 13c distributions and gradients indicate that the deep water circulation was similar during the holocene and lgm this interpretation brings 13c data to a better agreement with cd ca data and marks a sharp contrast with a widely held view based on 13c measurements that the glacial southern ocean was the terminus of the thermohaline circulation the proposed presence of glacial north atlantic deep water does not necessarily contradict the postulated presence of glacial north atlantic intermediate water

input of river derived sediment to the new zealand continental shelf i mass abstract the input of river borne sediments to the new zealand continental shelf has been calculated for all the major rivers and basins in new zealand south island yields 284 40 106 tonnes per year of sediment from a land area of 152 977 km2 and north island yields 105 9 4 106 tonnes per year from a land area of 114 621 km2 particularly high discharges are noted off the west coast of south island and east coast of north island and result in higher offshore sedimentation there the data are compatible with measured sedimentation rates on the new zealand continental shelf the specific sediment yield from south island is amongst the highest previously recorded

glacial interglacial variations in quaternary production of marine organic matter at dsdp site 594 chatham rise southeastern new zealand margin abstract caco3 and total organic carbon concentrations organic matter c n and carbon isotope ratios and sediment accumulation rates in late quaternary sediments from dsdp site 594 provide information about glacial interglacial variations in the delivery of organic matter to the chatham rise offshore of southeastern new zealand low c n ratios and nearly constant organic 13c values of 23 indicate that marine production dominates organic matter supply in both glacial and interglacial times during oxygen isotope stages 1 through 6 0 140 ka and 17 through 19 660 790 ka increased organic carbon mass accumulation rates in isotope stages 2 4 6 and 18 record enhanced marine productivity during glacial maxima excursions of organic 13c values to ca 29 in portions of isotope stage 2 suggest that the local concentration of dissolved co2 was occasionally elevated during the last glacial maximum probably as a result of short periods of lowered sea surface temperature dilution of carbonates by clastic continental sediment generally increases at this location during glacial maxima but enhanced delivery of land derived organic matter does not accompany the increased accumulation of clastic sediments

geochronological study of zircons from continental crust rocks in the frido unit southern apennines zircon crystals have been separated from gneisses and metagranitoids of the pollino area southern apennines in order to unravel the origin of these crustal slices within the ophiolite bearing frido unit the morphology of the zircon has been investigated by sem and the internal structure was revealed by cathodoluminescence data obtained by u pb dating have been used to deduce the age and significance of the different crystallization stages of zircon connected to the evolutionary stages of the continental crust late paleozoic early mesozoic zircons in gneisses are characterized by inherited cores of magmatic origin bordered by metamorphic rims inherited zircons generally show paleoproterozoic to ordovician ages indicating the provenance of the sedimentary protolith from different sources the exclusive presence of late neoproterozoic zircon cores in leucocratic gneisses may suggest a different magmatic source possibly connected to pan african events late carboniferous early permian ages are found mainly in zircon rims of metamorphic origin these are similar to the emplacement ages of protolith of the metagranites in the middle crust portion late carboniferous early permian metamorphism and magmatism testify the extensional collapse of the hercynian belt recorded in european particularly in the corsica sardinia block and in calabria late permian triassic ages have been detected in zircon rims from gneisses and metagranitoids these younger ages appear related to deformation and emplacement of albite quartz veins in both lithologies and are related to an extensional episode predating the middle triassic to middle jurassic rifting in the tethyan domain followed by middle to late jurassic spreading

mesoscale structure of the mean east australian current system and its relationship with topography abstract we document the mean circulation of the east australian current eac system which is the major western boundary current in the southwest pacific ocean a new high resolution climatology of the region csiro atlas of regional seas cars is used to produce regional fields of steric height and sections of geostrophic velocity the realism of these fields has been enhanced by the high spatial resolution allowance for bathymetric control and the influence of land barriers the increased detail in the maps and sections reveal the major influence that the complex regional topography has on the circulation the results also provide the appropriate resolution for comparison with recent model studies of the region we have been able to both resolve well known features with greater detail and identify previously unrecognized aspects of the circulation important results include 1 the zonal inflow north of 30 s breaks into a series of individual jets after flowing through the island chains as suggested by recent model results 2 the interaction of the zonal inflow with the local topography as it undergoes bifurcation is documented 3 a detailed picture of the evolution of the eac at the western boundary is presented including surface currents and transports 4 separation and recirculation of the eac within the tasman abyssal basin are shown to occur as a double cell structure constrained by bathymetry 5 the eac outflow is resolved as a series of eastward and northeastward currents 6 the path of the tasman front is defined as it interacts with the tasman ridge systems 7 a quasi permanent eddy is identified for the first time adjacent to the norfolk ridge northwest of new zealand

origin of the first global meltwater pulse following the last glacial maximum well dated sea level records show that the glacioeustatic rise following the last glacial maximum was characterized by two or possibly three brief intervals of rapid sea level rise separating periods with much lower rates these very high rates of sea level rise indicate periods of exceptionally rapid deglaciation of remaining ice sheets the laurentide ice sheet is commonly targeted as the source of the first and largest of the meltwater pulses mwp ia between 14 200 12 200 14c years b p and 13 700 years ago 11 700 14c years b p in all oceanic records of deglaciation of the former northern hemisphere ice sheets that we review only those from the gulf of mexico and the bermuda rise show evidence of low 18o values at the time of mwp ia identifying the southern laurentide ice sheet as a potential source for mwp ia we question this source for mwp ia however because 1 ice sheet models suggest that this sector of the ice sheet contributed only a fraction 10 of the sea level needed for mwp ia 2 melting this sector of the ice sheet at the necessary rate to explain mwp ia is physically implausible and 3 ocean models predict a much stronger thermohaline response to the inferred freshwater pulse out of the mississippi river into the north atlantic than is recorded this leaves the antarctic ice sheet as the only other ice sheet capable of delivering enough sea level to explain mwp ia but there are currently no well dated high resolution records to document this hypothesis these conclusions suggest that reconstructions of the laurentide ice sheet in the ice 4g model which are constrained to match the sea level record may be too low for time periods younger than 15 000 years ago furthermore 18o records from the gulf of mexico show variable fluxes of meltwater from the southern margin of the laurentide ice sheet which can be traced to the opening and closing of eastward draining glacial lake outlets associated with surging ice sheet behavior these variable fluxes through eastern outlets were apparently sufficient to affect formation of north atlantic deep water thus underscoring the sensitivity of this process to changes in freshwater forcing

glacial to interglacial changes in non carbonate and carbonate accumulation in the sw pacific ocean new zealand abstract fluxes of carbonate and non carbonate terrigenous biogenic siliceous sediment back to a maximum of marine isotope stage mis 6 were determined for 54 cores from off eastern new zealand mass accumulation rates mars were derived using dry bulk densities measured directly from fresh cores or in the case of dry archived samples on estimates from a density carbonate relationship mars exhibit wide spatial and temporal variability in response to i the location of cores relative to the terrigenous source of new zealand current pathways and local oceanographic features and ii palaeoclimatic change for mis 1 2 non carbonate accumulated at a high rate between 27 and 18 ka calendar years due to an influx of waterborne and aeolian terrigenous detritus together with localised production of biogenic silica under a northward intrusion of subantarctic waters and an increase in wind induced upwelling by 18 12 ka non carbonate mars reduced as winds ameliorated and sea level rose to confine some of the terrigenous load to the continental shelf the holocene transgression was accompanied by warmer sea surface temperatures the formation of coastal and lacustrine sediment traps and more widespread terrestrial vegetation these conditions further reduced the deep oceanic non carbonate flux to levels about half those of glacial periods in contrast carbonate mars were lower in cold periods reflecting lower productivity increased dissolution and in the case of the generally productive waters of chatham rise to redistribution by bottom currents however off central new zealand carbonate production was higher on the continental margin possibly because of enhanced upwelling under the strong wind regime of mis 2 such localised effects produced glacial carbonate mars higher than interglacial mis 1 values for the rest of the region carbonate fluxes increased to an early to middle mis 1 maximum before generally declining in recent times

late quaternary marine stratigraphy southeast of new zealand sediment cores of late quaternary age from the continental margin and deep sea bounty trough southeast of new zealand reveal an alternating sequence of glacial and interglacial sediments during glacial episodes of lowered sea level glaciation in the southern alps was at its peak and east coast rivers delivered enormous volumes of terrigenous sediments to the shelf edge at these times sediments of the deep adjacent basins were dominated by micaceous hemipelagic deposits with a low biogenic component consisting mostly of siliceous remains radiolaria and sponge spicules planktonic foraminifera although present were much reduced in abundance because of increased dissolution as in deeper water subantarctic cores farther to the south cool water forms dominated during interglacial episodes terrigenous sediment supply to the deep basins was reduced in response to higher sea levels and as glaciers retreated and deposited much of their loads in newly formed glacial lakes and on the plains terrigenous sediment dilution of the biogenic portion was thus much reduced calcium carbonate dissolution also was reduced these processes in combination led to the deposition of foraminifera rich hemipelagic sediments siliceous biogenic productivity decreased thus the late quaternary marine sediment record in the area adjacent to southeast new zealand is dominated by paleoclimatic influences that control terrigenous input calcium carbonate dissolution biogenic productivity and the migration of planktonic assemblages

the subantarctic and polar fronts in relation to deep water masses through the southwestern atlantic the two principal fronts within the antarctic circumpolar current acc the subantarctic front saf and the polar front pf are investigated with respect to their spatial patterns and relations to deep water masses from the northern drake passage to the western and southern argentine basin observations from four recent cruises in adjoining and overlapping regions are supplemented with satellite infrared imagery and trajectories from surface drifters east of drake passage the width of the polar frontal zone pfz increases rapidly when the saf turns sharply northward as a part of the falkland malvinas current while the pf remains oriented mainly toward the northeast found within this region of the pfz are mesoscale eddies the upper layers of which consist of subantarctic mode water from the pacific the contrasts in upper layer water properties that identify the saf can be traced the entire length of the falkland current to the brazil falkland confluence zone and then southward to the southern argentine basin there the saf turns eastward and later but perhaps not always merges with the pf north of ewing bank to form a single intense current core strongly influenced by bottom topography at times this eastward current loops southward through a gap in the falkland ridge into the georgia basin thereby supplying relatively warm and salty subantarctic water to the antarctic zone east of the gap the two fronts become separated and are once again distinct features in the vicinity of the islas orcadas rise at depth within drake passage and the western scotia sea the saf and pf are not lateral boundaries between distinct water masses but are instead identified by enhanced vertical displacements of property isopleths with the northward turn of the saf east of drake passage a thick layer of circumpolar deep water cdw is advected over the falkland plateau into the argentine basin there it is joined by waters entering the argentine basin via a deep spreading route through the georgia basin denser cdw deep water from the weddell sea and episodically deep water from the southeastern pacific ocean together these waters form a northward flow off the patagonian shelf that extends from the sea surface to the bottom most of which turns back toward the south with the falkland current return also flowing south along the seaward side of the falkland current return well away from the western boundary is the main core of north atlantic deep water nadw detatched masses of nadw are observed within the cdw at the western boundary as well as within the acc south of the saf in the south central argentine basin

evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate

dolomite chimneys as possible evidence of coastal fluid expulsion uppermost otago continental slope southern new zealand abstract a suite of carbonate chimneys slabs and ridges occur on a 1 km 2 220 m deep terrace on the uppermost continental slope east of the otago peninsula new zealand carbonate content and petrographic observations suggest cementation took place just below the sediment water interface encrustation borings and the abundance of chimneys and carbonate debris observed today on the uppermost slope suggest that the chimneys have been exhumed and are relict features with a maximum radiocarbon age of 33 000 550 yr b p negative 13 c values of 4 9 to 12 8 pdb indicate that the carbon contained within the cement was derived from the biogenic degradation of organic matter a 18 o composition of 4 9 to 5 4 pdb suggests a marine origin palaeotempertures of precipitation are consistent with modern bottom water temperatures cement in the chimneys is considered to be derived from aquifer forced ground water marine in character and sourced from the exposed inner middle shelf during glacially lowered sea level this contrasts with the tectonically induced fluid flow proposed for vent sites on convergent margins