vertical structure energetics and dynamics of the brazil current system at 22 s 28 s 1 we use four current meter moorings and quasi synoptic hydrographic observations in conjunction with a one dimensional quasi geostrophic linear stability model to investigate downstream changes in the brazil current bc system between 22 s and 28 s the data set depict the downstream thickening of the bc its vertical extension increases from 350 m at 22 7 s to 850 m at 27 9 s most of this deepening occurs between 25 5 s and 27 9 s and is linked to the bifurcation of the south equatorial current at intermediate depths santos bifurcation which adds the antarctic intermediate water flow to the bc geostrophic estimates suggest that the bc transport is increased by at least 4 3 sv 70 to the south of that bifurcation moreover the santos bifurcation is associated with a substantial increase in the barotropic component of the bc system on average the water column average kinetic energy ike is 70 baroclinic to the north and 54 barotropic to the south of the bifurcation additionally the bc shows conspicuous mesoscale activity off southeast brazil the water column average eddy kinetic energy accounts for 30 60 of the ike instabilities of the mean flow may give rise to these mesoscale fluctuations indeed the linear stability analysis suggests that the bc system is baroclinically unstable between 22 s and 28 s in particular the model predicts southwestward propagating fastest growing waves 190 km from 25 5 s to 27 9 s and quasi standing most unstable modes 230 km at 22 7 s these modes have vertical structures roughly consistent with the observed eddy field

atlantic meridional overturning circulation and the southern hemisphere supergyre 1 the ocean s role in climate manifests itself through its high heat capacity its own rich internal dynamics and its ability to transport vast quantities of heat and freshwater of particular interest is the global ocean circulation associated with the atlantic meridional overturning amoc because observations are sparse the detailed global structure of the amoc remains poorly understood particularly the pathways along which water returns to the atlantic to compensate the export of north atlantic deep water nadw here we provide the first quantitative 3 dimensional global view of the amoc using a lagrangian reconstruction which integrates hundred of thousands water particle trajectories in an ocean model the resulting pattern elucidates the role of the wind in structuring the pathways of the amoc in particular the lagrangian analysis reveals a strong link with the three subtropical gyres of the southern hemisphere sh which merge into a supergyre spanning the three ocean basins the coupling between the upper ocean wind driven circulation and the overturning in the model suggests that changes in the sh winds can alter the pathways of the amoc and therefore the water properties and the associated heat and freshwater transports

mechanical basis for low angle normal faulting in the basin and range province low angle normal faults seem to violate the criterion that relates fault dip to stress orientation to explain such faults one must postulate a 45 rotation of principal stress directions at mid crustal depths which is difficult to understand on the basis of previous elastic crust models such a stress rotation is shown to be a natural consequence of a low viscosity lower crust

epithermal alteration and mineralization in the comstock district nevada the precious and base metal deposits of the world renowned comstock district of western nevada constitute one of several superimposed miocene hydrothermal systems in the district mesozoic metasedimentary and igneous rocks are overlain by oligocene to miocene ash flow tuffs a thick sequence of middle miocene andesitic volcanic rocks and intrusions host the bulk of the hydrothermal alteration and ore deposits some of the magmatic events were directly associated with hydrothermal activity secondary biotite alteration locally affects preore dioritic intrusions preore barren quartz alunite pyrophyllite and kandite alteration high sulfidation style is zoned outward from mostly discontinuous crudely radial fractures associated with andesitic intrusions blanket like cristobalite alunite and kaolinite alteration exposed at the periphery of the district may be linked to preore alteration near the paleowater table later large volumes of subore grade massive quartz veins and quartz adularia stockworks were deposited in steeply to moderately east dipping sets of north and northeast striking faults that also localized the major precious metal bearing lodes quartz chlorite illite and very localized muscovite alteration deep low sulfidation developed coeval with vein deposition epidote bearing propylitic alteration forms halos around the higher temperature parts of the lodes adularia and rare bladed calcite indicate boiling during deep low sulfidation hydrothermal activity but little is apparently associated with ore deposition the main stage au ag cu zn pb ore was deposited late in the deep low sulfidation system in irregularly spaced lenticular dilatant zones or structural intersections massive nonbladed calcite was deposited just below and in the lower parts of orebodies in the comstock lode but it was deposited in the upper parts of orebodies or was barren elsewhere in the district pliocene to holocene reactivation of faults has disrupted the district redistributing the relative positions of many of the orebodies and alteration assemblages

water mass pathways to the north atlantic oxygen minimum zone the water mass pathways to the north atlantic oxygen minimum zone naomz are traditionally sketched within the cyclonic tropical circulation via the poleward branching from the eastward flowing jets that lie south of 10 n however our water mass analysis of historic hydrographic observations together with numerical lagrangian experiments consistently reveal that the potential density level of 26 8 kg m 3 26 8 approximately 300 m depth separates two distinct regimes of circulation within the central water cw stratum of the naomz in the upper cw above 26 8 and in agreement with previous studies the supply of water mainly comes from the south with a predominant contribution of south atlantic cw in the lower cw below 26 8 where minimal oxygen content is found the tropical pathway is instead drastically weakened in favor of a subtropical pathway more than two thirds of the total water supply to this lower layer takes place north of 10 n mainly via an eastward flow at 14 n and northern recirculations from the northern subtropical gyre the existence of these northern jets explains the greater contribution of north atlantic cw observed in the lower cw making up to 50 of the water mass at the naomz core the equatorward transfer of mass from the well ventilated northern subtropical gyre emerges as an essential part of the ventilation of the naomz

structural localization and origin of compartmentalized fluid flow comstock lode virginia city nevada bonanza grade orebodies in epithermal style mineral deposits characteristically occur as discrete zones within spatially more extensive fault and or fracture systems empirically the segregation of such systems into compartments of higher and lower permeability appears to be a key process necessary for high grade ore formation and most commonly it is such concentrations of metals that make an epithermal vein district world class in the world class silver and gold producing comstock mining district nevada several lines of evidence lead to the conclusion that the comstock lode is localized in an extensional stepover between right lateral fault zones this evidence includes fault geometries kinematic indicators of slip the hydraulic connectivity of faults as demonstrated by veins and dikes along faults and the opening of a normal fault bounded asymmetric basin between two parallel and overlapping northwest striking lateral to lateral oblique slip fault zones during basin opening thick generally subeconomic banded quartz adularia veins were deposited in the normal fault zone the comstock fault and along one of the bounding lateral fault zones the silver city fault as deformation continued the intrusion of dikes and small plugs into the hanging wall of the comstock fault zone may have impeded the ability of the stepover to accommodate displacement on the bounding strike slip faults through extension within the stepover a transient period of transpressional deformation of the comstock fault zone ensued and the early stage veins were deformed through boudinaging and hydraulic fragmentation fault motion inversion and high and low angle axial rotations of segments of the fault planes and some fault bounded wedges this deformation led to the formation of spatially restricted compartments of high vertical permeability and hydraulic connectivity and low lateral hydraulic connectivity bonanza orebodies were formed in the compartmentalized zones of high permeability and hydraulic connectivity as heat flow and related hydrothermal activity waned along the comstock fault zone extension was reactivated in the stepover along the occidental zone of normal faults east of the comstock fault zone volcanic and related intrusive activity in this part of the stepover led to a new episode of hydrothermal activity and formation of the occidental lodes

seasonal variability of the south equatorial current bifurcation in the atlantic ocean a numerical study abstract in this study a reduced gravity primitive equation ogcm is used to investigate the seasonal variability of the bifurcation of the south equatorial current sec into the brazil current bc to the south and the north brazil undercurrent current nbuc nbc system to the north annual mean meridional velocity averaged within a 2 longitude band off the south american coast shows that the sec bifurcation occurs at about 10 14 s near the surface shifting poleward with increasing depth reaching 27 s at 1000 m in both observations and model the bifurcation latitude reaches its southernmost position in july 17 s in the top 200 m and its northernmost position in november 13 s in the top 200 m the model results show that most of the seasonal variability of the bifurcation latitude in the upper thermocline is associated with changes in the local wind stress curl due to the annual north south excursion of the marine itcz complex as the sec bifurcation latitude moves south north the nbuc tra

circulation and water mass modification in the brazil malvinas confluence abstract the confluence between the brazil current and the malvinas current the brazil malvinas confluence bmc in the argentine basin is characterized by a complicated thermohaline structure favoring the exchanges of mass heat and salt between the subtropical gyre and the antarctic circumpolar current acc analysis of thermohaline properties of hydrographic sections in the bmc reveals strong interactions between the acc and subtropical fronts in the subantarctic front subantarctic mode water samw antarctic intermediate water aaiw and circumpolar deep water cdw warm become saltier by 0 4 0 08 0 3 0 02 and 0 6 c 0 1 respectively in the subtropical gyre aaiw and north atlantic deep water have cooled freshened by 0 4 0 07 and 0 7 c 0 11 respectively to quantify those acc subtropical gyre interactions a box inverse model surrounding the confluence is built the model diagnoses a subduction of 16 4 sv 1 sv 106 m3 s 1 of newly formed samw and aaiw under the subtro

origins and variability of the benguela current a subset of the benguela sources and transports best 1992 1993 data is analyzed to study the magnitude and variability of the large scale transports in the area the data consist of inverted echo sounder series and conductivity temperature depth stations the mean 16 month transport values for the upper 1000 m indicate that of the 13 sv northward transport within the benguela current 50 is derived from the central atlantic which from geometry may be chiefly south atlantic water 25 comes from the indian ocean which may be chiefly agulhas water and the remaining 25 may be a blend of agulhas and tropical atlantic water a simple schematic of the transport pattern with a somewhat restricted corridor for agulhas eddies translation is envisioned to the west of the eddy corridor flows the south atlantic source for the benguela current to the east is the agulhas indian ocean source the corridor is breached by south atlantic and indian water as the transient eddy field stirs these water masses

terrestrial oxygen isotope variations and their implications for planetary lithospheres oxygen isotopes provide key data on the formation and evolution of the solar system particularly for earth and other rocky bodies that are principally constituted of oxygen rich phases the dynamic interactions between earth s lithosphere hydrosphere biosphere and atmosphere are elucidated by a wealth of oxygen isotope data that in turn provide a template for predicting the types of variations that occur on other planets these isotopic variations are basically controlled by two master effects first isotopic fractionations among coexisting phases tend to be greatest at low temperatures and are also large when they involve interactions between solids or liquids and gaseous phases second material balance effects require the 18 o contents of the most abundant phases to be close to the planetary average and restrict extreme isotopic enrichments or depletions to minor phases these dual controls of fractionation and material balance work in concert with practically any other dynamic process to foster the upward enrichment and increased heterogeneity of 18 o contents on earth and by implication in planetary lithospheres in general upward 18 o enrichment in planetary lithospheres is promoted by the tendency for the least dense silicate and oxide minerals to concentrate 18 o relative to more dense minerals and also by convective transfer fractional crystallization hydrothermal alteration and other processes earth s continents and the uppermost parts of its ocean floor consequently have higher 18 o contents than average upper mantle material and normal sediments carbon dioxide and oxygen gas are all very rich in 18 o heterogeneity is evident in the large ranges of 18 o values observed in sediments at least 8 to 38 and terrestrial igneous rocks at least 10 5 to 16 and in the even larger ranges in natural waters and atmospheric gases all representing materials that constitute or interacted with oxygen reservoirs near earth s surface key identified processes include weathering precipitation from water phase changes exchange with infiltrating hydrothermal fluids and assimilation of wallrocks having disparate compositions all such processes that promote the upward enrichment and heterogeneity of 18 o complicate the deduction of the bulk planetary contents of this isotope

three dimensional oxygen isotope imaging of convective fluid flow around the big bonanza comstock lode mining district nevada oxygen isotope analyses of propylitized andesites from the con virginia and california mines allow construction of a detailed three dimensional image of the isotopic surfaces produced by the convective fluid flows that deposited the famous big bonanza orebody on a set of intersecting maps and sections the 18o isopleths clearly show the intricate and conformable relationship of the orebody to a deep 500 m gyre of meteoric hydrothermal fluid that circulated along and above the comstock fault near the contact of the davidson granodiorite the core of this gyre 18o 0 to 3 8 encompasses the bonanza and is almost totally surrounded by rocks having much lower 18o values 1 0 to 4 4 this deep gyre may represent a convective longitudinal roll superimposed on a large unicellular meteoric hydrothermal system producing a complex flow field with both radial and longitudinal components that is consistent with experimentally observed patterns of fluid convection in permeable media

response of the south atlantic circulation to an abrupt collapse of the atlantic meridional overturning circulation the south atlantic response to a collapse of the north atlantic meridional overturning circulation amoc is investigated in the echam5 mpi om climate model a reduced agulhas leakage about 3 1 sv 1 sv 106 m3 s 1 is found to be associated with a weaker southern hemisphere sh supergyre and indonesian throughflow these changes are due to reduced wind stress curl over the sh supergyre associated with a weaker hadley circulation and a weaker sh subtropical jet the northward cross equatorial transport of thermocline and intermediate waters is much more strongly reduced than agulhas leakage in relation with an amoc collapse a cross equatorial gyre develops due to an anomalous wind stress curl over the tropics that results from the anomalous sea surface temperature gradient associated with reduced ocean heat transport this cross equatorial gyre completely blocks the transport of thermocline waters from the south to the north atlantic the waters originating from agulhas leakage flow somewhat deeper and most of it recirculates in the south atlantic subtropical gyre leading to a gyre intensification this intensification is consistent with the anomalous surface cooling over the south atlantic most changes in south atlantic circulation due to global warming featuring a reduced amoc are qualitatively similar to the response to an amoc collapse but smaller in amplitude however the increased northward cross equatorial transport of intermediate water relative to thermocline water is a strong fingerprint of an amoc collapse

symmetry of flow in the comstock lode hydrothermal system evidence for longitudinal convective rolls in geologic systems 1 hydrothermal fluid flow can be visualized by three dimensional 3 d kriging and computer contouring of oxygen isotope data on propylitized andesites in the comstock lode mining district this data set consists of 18o results for 327 samples from surface outcrops drill cores and mine workings in a 3 5 km 1 7 km 1 0 km block in the hanging wall of the comstock fault encompassing the region where the most important ore bodies were discovered the 18o contours deep in the hanging wall are not simply parallel to the comstock fault or to the boundary of the davidson granodiorite stock as would be expected for a hydrothermal convection system dominated by unicellular flow instead the subsurface 18o values exhibit a high degree of symmetry about a vertical plane directed radially toward the stock with isotopic contours being predominantly perpendicular to the comstock fault and the eastern contact of the davidson granodiorite a vertical cross section along the comstock lode shows a low 18o core with two symmetrical zones of higher 18o values to either side where the contours are steeply inclined which we infer reflects downwelling of fluids shifted by water rock exchange or steam loss this 3 d visualization of 18o values provides the first evidence that longitudinal convective rolls can be significant in geologic environments in this case within a zone of high temperature gradients and permeability near the comstock fault and the davidson granodiorite stock

interannual to decadal changes in the western boundary circulation in the atlantic at 11 s the western boundary current system off brazil is a key region for diagnosing variations of the atlantic meridional overturning circulation amoc and the southern subtropical cell in july 2013 a mooring array was installed off the coast at 11 s similar to an array installed between 2000 and 2004 at the same location here we present results from two research cruises and the first 10 5 months of moored observations in comparison to the observations a decade ago average transports of the north brazil undercurrent and the deep western boundary current dwbc have not changed between the observational periods dwbc eddies that are predicted to disappear with a weakening amoc are still present upper layer changes in salinity and oxygen within the last decade are consistent with an increased agulhas leakage while at depths water mass changes are likely related to changes in the north atlantic as well as tropical circulation changes

large scale mass transports water mass formation and diffusivities estimated from world ocean circulation experiment woce hydrographic data 1 a global inversion is used to estimate the oceanic mass transports and vertical diffusivities based on a subset of hydrographic data from the world ocean circulation experiment woce and java australia dynamic experiment the analysis is based on the linear inverse box model of ganachaud and wunsch 2000 that consistently combines the transoceanic sections a globally consistent solution is obtained for a depth independent adjustment to the thermal wind field the freshwater flux divergences the ekman transport and the advective and diffusive dianeutral fluxes between layers a detailed error budget permits calculation of statistical uncertainties taking into account both the nonresolved part of the solution and the systematic errors due to the temporal oceanic variability the horizontal structure of the transports for different density classes is analyzed regionally during the woce period 1985 1996 net meridional transports are estimated with 16 2 sv 106 m3 s of north atlantic deep water nadw being produced in the northern north atlantic moving southward entraining antarctic bottom water aabw and antarctic intermediate water increasing in volume transport to 23 3 sv at 30 s in the southern ocean 21 6 sv of bottom water was formed from lower circumpolar deep water which corresponds approximately to the lower nadw density range bottom water inflows nadw aabw mixture to the atlantic indian and pacific oceans are estimated at 6 1 3 11 4 and 7 2 sv respectively in the indian and pacific oceans this water returns southward at deep and intermediate levels property anomaly conservation constraints permit estimation of dianeutral diffusivities in deep layers with a global average of 3 7 0 7 cm2s 1 north of 30 s

the shallow and deep western boundary circulation of the south atlantic at 5 11 s repeated shipboard observation sections across the boundary flow off northeastern brazil as well as acoustic doppler current profiler adcp and current meter records from a moored boundary array deployed during 2000 04 near 11 s are analyzed here for both the northward warm water flow by the north brazil undercurrent nbuc above approximately 1100 m and the southward flow of north atlantic deep water nadw underneath at 5 s the mean from nine sections yields an nbuc transport of 26 5 3 7 sv sv 106 m3 s 1 along the boundary at 11 s the mean nbuc transport from five sections is 25 4 7 4 sv confirming that the nbuc is already well developed at 11 s at both latitudes a persistent offshore southward recirculation between 200 and 1100 m depth reduces the net northward warm water flow through the 5 s section west of 31 5 w to 22 1 5 3 sv and through the 11 s section to 21 7 4 1 sv west of 32 0 w the 4 yr long nbuc transport time series from 11 s yields a seasonal cycle of 2 5 sv amplitude with its northward maximum in july interannual nbuc transport variations are small varying only by 1 2 sv during the four years with no detectable trend the southward flow of nadw within the deep western boundary current at 5 s is 25 5 8 3 sv with an offshore northward recirculation yielding a nine section mean of 20 3 10 1 sv west of 31 5 w for antarctic bottom water a net northward flow of 4 4 3 0 sv is determined at 5 s for the 11 s section the moored array data show a pronounced energy maximum at 60 70 day period in the nadw depth range which was identified in related work as deep eddies translating southward along the boundary based on a kinematic eddy model fit to the first half of the moored time series the mean nadw transfer by the deep eddies at 11 s was estimated to be about 17 sv given the large interannual variability of the deep near boundary transport time series which ranged from 14 to 24 sv the 11 s mean was considered to be not distinguishable from the mean at 5 s

fluid mineral relations in the comstock lode the comstock lode a world class precious metal vein deposit in the comstock district 15 mi 24 km southeast of reno nevada formed 13 7 ma in miocene eruptive rocks of the alta and kate peak formations and in pre tertiary metavolcanic and intrusive rocks production of 8 25 million oz 311 850 kg in gold and 192 million oz 7 26 million kg in silver came largely from bonanzas or vein segments containing hundreds of thousands to millions of tons and grading or 0 4 oz ton approximately 14 g ton gold and or 10 oz ton approximately 340 g ton silver within the 8 mi 12 9 km long comstock fault bonanza ores are distributed 3 mi 4 8 km along strike and persist from the present surface to 3 300 ft 1 007 m below the surface during the middle miocene andesitic to rhyodacitic alta and kate peak rocks formed a stratovolcano that exceeded 7 000 ft 2 134 m in elevation bonanza ores were deposited within this volcano from a 200 degrees c hydrothermal system that affected more than 37 mi 3 150 km 3 of rock at least one cretaceous and several miocene hydrothermal events were imposed on comstock district rocks the comstock lode vein assemblage which resulted from the youngest and only economically important event consists of quartz electrum silver minerals mainly acanthite pyrite base metal sulfides and minor amounts of calcite sericite chlorite k feldspar and albite andesite adjacent to veins is altered to quartz sericite and pyrite at vein contacts and quartz sericite montmorillonite chlorite and pyrite distally kaolinite is present in wall rocks adjacent to vein segments in which boiling occurred addition of the external components water co 2 and sulfur gases sufficed to produce all comstock vein alteration assemblages based on paragenetic and fluid inclusion studies multiple generations of hydrothermal fluid formed comstock district veins but the earliest and highest temperature fluid deposited ore minerals it ranged in temperature from 300 degrees c at the north end of the lode cedar hill to 6 wt percent nacl and was composed of both meteoric and magmatic water according to fluid isotopic analyses younger cospatial fluid s contained up to 2 1 wt percent co 2 the thermal center of the comstock lode hydrothermal system cedar hill is also the site of the highest measured ore fluid salinities ore fluid isotherms and to an extent salinity isopleths display reversals and convolutions that are spatially related to bonanzas these sharp temperature gradients coupled with mixing of ore fluid and at least one other high temperature fluid indicated by enthalpy salinity relationships and deuterium enrichment most likely caused precipitation of ore minerals sphalerite and electrum compositions indicate that bonanzas were deposited over a narrow range of f sub s 2 13 5 to 15 2 while sphalerites from low grade and barren vein segments indicate more widely variable f sub s 2 gold silver ratios both in electrum and bulk ore appear to be temperature controlled as higher temperatures generally correlate with high au ag ratios local boiling of 250 degrees to 285 degrees c fluid provides a paleosurface datum but vein temperatures 300 degrees c require up to 3 800 ft 1 152 m of cover over the tops of bonanzas during mineralization the distribution of both ore stage and late vein calcite partially reflects hydrothermal fluid circulation patterns in veins at district margins the abundance of late calcite marks the demise of hot water circulation and the influx of large amounts of cold miocene ground water into vein interstices kate peak intrusions between cedar hill and mount davidson share a common structural control with vein segments that comprise the comstock lode and their ages span vein ages the coeval kate peak magmas that provided heat for hydrothermal circulation formed the core of and uplifted the miocene stratovolcano which was centered over cedar hill partial symmetry of thermal compositional and kinetic gradients about cedar hill suggests that additional bonanzas await discovery along unexplored comstock fault segments within declining palcothermal gradients to the north

the intermediate depth circulation of the western south atlantic the subsurface oceanic circulation is an important part of the earth climate system subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances occasionally supplemented by current meter measurements neutrally buoyant floats however now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western south atlantic here our combined data set provides unprecedented observations and quantification of key flow patterns such as the subtropical gyre return flow 12 sv 1 sverdrup 10 6 m 3 s 1 its bifurcation near the santos plateau and the resulting continuous narrow and swift northward intermediate western boundary current 4 sv this northward flowing water passes through complex equatorial flows and finally enters into the north atlantic

effects of normal faulting on fluid flow in an ore producing hydrothermal system comstock lode nevada abstract normal faults in the comstock lode mining district nv usa are delineated by low 18o belts and strong isotopic gradients perpendicular to strike these faults perturb and also offset the regional pattern of 18o contours produced by pervasive hydrothermal alteration of the alta formation and have preserved the down dropped hanging wall rocks while exposing the footwall rocks to greater erosion fluid rock interaction was greatly enhanced within 200 m wide zones centered on the normal faults where permeability was increased and fluids were focused parallel to the fault plane the effects of displacement and permeability enhancement around the faults are investigated using oxygen isotope profiles through the comstock and occidental faults and across steeper secondary normal faults such as the coryell fault that may have formed after the principal faults were rotated to more shallow dips oxygen isotope profiles record both post hydrothermal displacement and syn deformational enhancement of fluid flow around the faults fluid focusing along normal faults perturbs the 18o values by increasing the water rock ratio this isotopic effect can be mathematically described for any distance x from the fault using the formula f a e bx 2 which is analogous to the fundamental solution for diffusion away from a plane source the maximum magnitude of 18o depletion a depends on the effects of permeability temperature mineralogy fluid isotopic composition and integrated fluid flux the constant b is related to the scale of exchange enhancement around the fault the characteristic width w 1 b of the fluid focusing effects is remarkably similar at 100 m for all three faults examined coupling this isotope exchange effect with a step function which accounts for fault displacement provides a good fit to the observed 18o fault profiles the 18o distribution around faults can be used to infer relationships and timing among the processes of structural deformation hydrothermal convection and ore deposition in the comstock district

an oxygen isotope study of hydrothermal alteration in the lake city caldera san juan mountains colorado abstract a 23 m y old fossil meteoric hydrothermal system in the lake city caldera 11 14 km has been mapped out by measuring 18o values of 300 rock and mineral samples 18o varies systematically throughout the caldera reaching values as low as 2 great topographic relief regional tilting and variable degrees of erosion within the caldera all combine to give us a very complete section through the hydrothermal system from the surface down to a depth of more than 2000 m the initial 18o value of the caldera fill sunshine peak tuff was very uniform 7 2 0 1 making it easy to determine the exact amount of 18o depletion experienced by each sample during hydrothermal alteration also we have excellent stratigraphic control on depths beneath the mid tertiary surface quantitative information on mineralogical alteration products and accurate data on the shape of the central resurgent intrusion which was the principal heat engine that drove the hydrothermal circulation major conclusions are 1 although pristine mid tertiary meteoric waters in this area had 18 o 14 these fluids were 18o shifted upward to about 18o 8 to 5 prior to entering the shallow convective system associated with the resurgent intrusive rocks although there was undoubtedly radial inflow toward the caldera from all directions the highly fractured eureka graben southwest of the caldera was probably the principal source of recharge groundwater for the lake city system 2 fluid flow within the caldera was dominated by three major categories of permeable zones the porous megabreccia units which dip outward from the resurgent dome vertical fractures and faults related to resurgence and the caldera ring fault itself all of these zones exhibit marked 18o depletions and they are also typically intensely mineralogically altered 3 the resurgent intrusive stock and its contact metamorphic aureole of hornfels both experienced water rock ratios lower than the permeable zones however they have similarly low 18o values because they were altered at higher temperatures 4 throughout the caldera the 18o of sunshine peak tuff decreases with increasing depth about 6 per mil km indicative of a shallow thermal gradient typical of a convective hydrothermal system the near surface portion of this gradient was controlled by the temperature drop associated with boiling in the uprising fluid 5 deeply circulating meteoric water rose along permeable ring fractures 3 to 5 km beneath the mid tertiary surface these fluids were drawn into the shallow convective system through the lower porous megabreccia units near the resurgent intrusions fluid flow was again directed upward where resurgence related near vertical fractures intersect the megabreccia units

taphonomic resolution and hominin subsistence behaviour in the lower palaeolithic differing data scales and interpretive frameworks at boxgrove and swanscombe uk the identification of human butchery signatures on fauna from lower palaeolithic sites is well documented and readily identifiable such bone surface modifications have the potential to provide not only information about past hominin meat procurement behaviour but address the wider issue of competition for resources with other carnivore species to understand and discuss these broader issues both hominin and natural bone surface modifications must be understood and contextualised within a site specific spatial and temporal framework this paper presents new results from faunal analysis at two key british lower palaeolithic localities boxgrove and swanscombe it illustrates that different depositional environments and excavation histories have resulted in different scales and resolutions of available data and hence in varying interpretive potentials at swanscombe the archaeological record has been disturbed by both fluvial activity and excavation history providing a coarser grained record of anthropogenic behaviour than previously acknowledged conversely at boxgrove a finer grained higher resolution record of human behaviours has been preserved this combined with both an extensive and intensive excavation strategy has allowed for a broader discussion of hominin landscape use resource competition and meat procurement behaviour this paper highlights that assessing the specific depositional environment at each site is crucial to understanding palaeolithic faunal assemblage formation and consequently the available data resolution and behavioural interpretation