inter monsoonal changes in the t s properties of the near surface waters of the northern arabian sea the arabian sea experiences extremes in atmospheric forcing that lead to the greatest seasonal variability in any ocean during 1995 6 cruises were carried out within the northern arabian sea the data collected represent the first consistent dataset covering an entire monsoonal cycle ocean soundings of temperature and salinity from these cruises are used to characterize variability of water masses in the surface layers associated with monsoonal forcing this paper documents the role of advection versus local water mass modification on seasonal extremes in t s in near surface waters of the arabian sea seasonal changes are associated with seasonal heating advection of waters from the oman upwelling zone varying of mixed layer depth due to ekman pumping in the central basin and mixing of high salinity waters from persian gulf

seasonal variation of hydrographic and nutrient fields during the us jgofs arabian sea process study abstract between september 1994 and december 1995 the us jgofs arabian sea process experiment collected extensive high quality hydrographic data temperature salinity dissolved oxygen and nutrients during all seasons in the northern arabian sea an analysis of this unique data suite suggests the presence of many features that are described in the canonical literature but these new data provided the following insights 1 although the seasonal evolution of mixed layer depths was in general agreement with previous descriptions the deepest mixed layer depths in our data occurred during the late ne monsoon instead of the sw monsoon 2 the region exhibits considerable mesoscale variability resulting in extremely variable temperature salinity ts distributions in the upper 1000 db this mesoscale variability is readily observed in satellite imaging in the high resolution data taken by a companion onr funded project and in underway adcp data 3 the densest water reaching the sea surface during coastal upwelling appeared to have maximum offshore depths of 150 m and s close to the core value 25 for the saline arabian sea water asw but salinities in these upwelling waters were relatively low the densest water found at the sea surface during late ne monsoon conditions has s 24 8 and relatively high salinities suggesting that they are a source for the asw salinity maximum 4 persian gulf water pgw with a core of 26 6 forms a widespread salinity maximum despite the considerable extent of this feature persian gulf outflow water with a salinity 4 of 39 at its source can only be a minor contributor within the standard us jgofs sampling grid maximum salinities on this surface are 36 8 at stations near the gulf falling to values as low as 35 3 at the stations farthest removed from its influence even at our standard stations closest to the gulf n 1 and n 2 the high salinity low nutrient persian gulf water has only a modest direct effect on nutrient concentrations this pgw salinity maximum is associated with the suboxic portions of the arabian sea s oxygen minimum zone 5 the salinity maximum associated with red sea water rsw core 27 2 in the jgofs study region is clearly evident at the southermost sampling site at 10 n s 15 elsewhere this signal is weak or absent and salinity on the 27 2 surface tends to increase towards the persian gulf suggesting that the disappearance of this salinity maximum is due at least in part to the influence of the persian gulf outflow 6 inorganic nitrogen to phosphate ratios were lower frequently much lower than the standard redfield ratio of 15 1 16 1 by atoms at all times and all depths suggesting that inorganic nitrogen was more important than phosphate as a limiting nutrient for phytoplankton growth and that the effects of denitrification dominated the effects of nitrogen fixation 7 the water upwelling off the omani coast during the sw monsoon has inorganic nitrogen to silicate ratios that were higher 2 1 than the 1 1 ratio often assumed as the ratio of uptake during diatom growth 8 the temporal evolution of inorganic nitrogen to silicate ratios suggests major alteration by diatom uptake only during the late sw monsoon cruise tn050 in august september 1995 9 widespread moderate surface layer nutrient concentrations occurred during the late ne monsoon 10 a zone of high offshore nutrient concentrations was encountered during the sw monsoon but instead of being associated with offshore upwelling it may represent offshore advection from the coastal upwelling zone the influence of an eddy or both 11 although our data do not contradict previous suggestions that the volume of subtoxic water may be reduced the sw monsoon they suggest a weaker re oxygenation than indicated by some previous work similarly they do not confirm results suggesting that secondary nitrite maxima may be common in waters with oxygen concentrations 5 m

seasonality in the hydrography and biogeochemistry across the pakistan margin of the ne arabian sea abstract a series of cruises were conducted across the pakistan continental shelf edge and upper slope in the ne arabian sea between march and october 2003 coinciding with spring inter monsoon and late to post sw monsoon conditions water column hydrographic profiling together with nutrient chemistry show the upper boundary of the oxygen minimum zone omz rose during the sw monsoon by approximately 80 m leading to an inundation of a substantial area of the continental shelf with low oxygen water while the lower boundary showed only slight movement the core of the omz identified by the presence of a secondary nitrite maximum and high nitrate deficits rose by approximately 200 m nitrite concentrations within the secondary nitrite maximum approximately doubled over the period of study but this was not matched with an increase in the nitrate deficit dissolved oxygen concentrations in the core of the omz showed little or no substantial change during the course of the study hydrographic profiling showed greatest change in water masses around the 200 600 m depth range probably resulting from the northward movement of the west india undercurrent

late holocene primary productivity and sea surface temperature variations in the northeastern arabian sea implications for winter monsoon variability variability in the oceanic environment of the arabian sea region is strongly 34 influenced by the seasonal monsoon cycle of alternating wind directions prominent and well studied is the summer monsoon but much less is known about late holocene changes in winter monsoon strength with winds from the northeast that drive convective mixing and highsurface ocean productivity in the northeastern arabian sea to establish the first high resolution record of winter monsoon variability for the late holocene we analyzed alkenone derived sea surface temperature sst variations and proxies of primary productivity organic carbon and 15n in a well laminated sediment core from the pakistan continental margin increased summer monsoon and weak winter monsoon intensities off pakistan are indicated from 400 b c to 700 a d by reduced productivity and relatively high sst at about 700 a d the intensity of the winter monsoon increased off pakistan as indicated by a trend to lower sst we infer that winter monsoon was still weak from 700 to 1400 a d because primary production did not increase despite decreasing sst declining sst and elevated biological production from 1400 to 1900 a d suggest invigorated convective winter mixing by strengthening winter monsoon circulation most likely a regional expression of colder climate conditions during the little ice age on the northern hemisphere the comparison of winter monsoon intensity with records of summer monsoon intensity suggests that an inverse relationship between summer and winter monsoon strength exists in the asian monsoon system during the late holocene effected by shifts in the intertropical convergence zone

catastrophic debris avalanche from ancestral mount shasta volcano california a debris avalanche deposit extends 43 km northwestward from the base of mount shasta across the floor of shasta valley california where it covers an area of at least 450 km 2 the surface of the deposit is dotted with hundreds of mounds hills and ridges all formed of blocks of pyroxene andesite and unconsolidated volcaniclastic deposits derived from an ancestral mount shasta individual hills are separated by flat topped laharlike deposits that also form the matrix of the debris avalanche and slope northwestward about 5 m km radiometric ages of rocks in the deposit and of a postavalanche basalt flow indicate that the avalanche occurred between about 300 000 and 360 000 yr ago an inferred average thickness of the deposit plus a computed volume of about 4 km 3 for the hills and ridges indicate an estimated volume of about 26 km 3 making it the largest known quaternary landslide on earth

hydrothermal fluid circulation through the sediment of crater lake oregon pore water and heat flow constraints we present evidence for pore water flow through the sediment of crater lake oregon based on systematic variations in pore water chemical compositions and thermal gradients pore water was extracted from sediment by centrifugation and diffusive exchange using a gravity corer deployed from a surface vessel and a box corer and peepers deployed from the submersible deep rover in a known geologic context depth profiles of sediment temperature were measured using two probes deployed from the submersible one probe was connected to the submersible whereas the other was self contained and deployed for 7 days on the basis of measured and calculated depth profiles of pore water na ca mg k li and temperature we show that pore water upwells in zones of focused upflow at speeds of meters to hundreds of meters per year these zones of focused flow are patchy and usually cover several square meters to hundreds of square meters and are marked by iron manganese rich crusts bacterial mats and saline pools in contrast most of the lake floor consists of sediment derived from the caldera walls and has a low heat flow with pore water velocities of millimeters per year the chemical composition of the pore water that upwells through the sampled section of the sediment column differs from core to core this difference results from mixing a hydrothermal fluid in igneous basement below the lake with lake water before the final ascent through the sediment column elemental ratios of this thermally and chemically altered fluid in basement match those calculated based on mass balance considerations calculation of mass balance and geothermometry constrain the temperature in basement and ultimately the power output which is about 30 mw this power output is in agreement with two other estimates that were calculated using temperature data from the water column and measurements of sediment heat flow

morphology volcanism and mass wasting in crater lake oregon crater lake was surveyed nearly to its shoreline by high resolution multibeam echo sounding in order to define its geologic history and provide an accurate base map for research and monitoring surveys the bathymetry and acoustic backscatter reveal the character of landforms and lead to a chronology for the concurrent filling of the lake and volcanism within the ca 7700 calibrated yr b p caldera the andesitic wizard island and central platform volcanoes are composed of sequences of lava deltas that record former lake levels and demonstrate simultaneous activity at the two vents wizard island eruptions ceased when the lake was 80 m lower than at present lava streams from prominent channels on the surface of the central platform descended to feed extensive subaqueous flow fields on the caldera floor the wizard island and central platform volcanoes andesitic merriam cone and a newly discovered probable lava flow on the eastern floor of the lake apparently date from within a few hundred years of caldera collapse whereas a small rhyodacite dome was emplaced on the flank of wizard island at ca 4800 cal yr b p bedrock outcrops on the submerged caldera walls are shown in detail and in some cases can be correlated with exposed geologic units of mount mazama fragmental debris making up the walls elsewhere consists of narrow talus cones forming a dendritic pattern that leads to fewer wider ridges downslope hummocky topography and scattered blocks up to 280 m long below many of the embayments in the caldera wall mark debris avalanche deposits that probably formed in single events and commonly are affected by secondary failures the flat floored deep basins contain relatively fine grained sediment transported from the debris aprons by sheet flow turbidity currents crater lake apparently filled rapidly ca 400 750 yr until reaching a permeable layer above glaciated lava identified by the new survey in the northeast caldera wall at 1845 m elevation thereafter a gradual climatically modulated rise in lake level to the present 1883 m produced a series of beaches culminating in a modern wave cut platform commonly 40 m wide where suitable material is present the new survey reveals landforms that result from intermediate composition volcanism in rising water delineates mass wasting and sediment transport into a restricted basin and yields a more accurate postcaldera history leading to improved assessment of volcanic hazards

oxygen and organic matter thresholds for benthic faunal activity on the pakistan margin oxygen minimum zone 700 1100 m abstract a transition from fully laminated to highly bioturbated sediments on continental margins is thought to derive from increased animal activity associated with increasing bottom water oxygen concentration we examined faunal community responses to oxygen and organic matter gradients across the lower oxygen minimum zone omz on the bathyal pakistan margin where sediments grade from fully laminated sediment at 700 m 0 12 ml l 1 o 2 5 m to highly bioturbated sediment at 1100 m 0 23 ml l 1 o 2 10 m high resolution sampling of the seafloor every 50 m water depth was conducted along a single transect during inter and post monsoon periods in 2003 to address a the existence of oxygen thresholds regulating macrofaunal abundance composition diversity and lifestyles b the interactive effects of organic matter quantity and quality c associated community effects on sediment structure and d potential seasonality in these processes macrofaunal biomass and bioturbation depth were positively correlated with organic matter availability which peaked at 850 950 m 3 39 3 53 org c in contrast macrofaunal diversity h dominance r1d and burrow number exhibited threshold responses at oxygen concentrations of 0 12 0 20 ml l 1 5 9 m with few animals and highly laminated sediments present below this concentration and most taxa present in fully bioturbated sediments above it the highly mobile burrowing amphinomid polychaete linopherus sp exhibited almost complete dominance and high density at 750 850 m 0 12 0 14 ml l 1 o 2 5 6 m but despite its activity sediment laminae remained faintly visible formation of permanent burrows and detritivory were dominant macrofaunal lifestyles within the omz allowing laminae to persist at surprisingly high animal density and biomass results reflect a shift from organic matter to oxygen regulation of body size and biogenic structures following the monsoon this study suggests that for assemblages evolving under permanent severe hypoxia food availability remains a significant determinant of animal abundance and biogenic structure depth oxygen influences patterns of diversity and dominance and interacts with organic matter to generate abrupt faunal transitions on the pakistan margin

nitrogen biogeochemical cycling in the northwestern indian ocean abstract the vertical distribution and fine scale structure of nitrate no 3 nitrite no 2 nitrous oxide n 2 o phosphate po 4 oxygen o 2 and chlorophyll chl were determined in the north western indian ocean nwio along a meridional section 67 e from the equator to the gulf of oman using an autoanalyser for micromolar levels of nutrients and chemiluminescence and gas chromatographic methods for nanomolar levels of no 3 and no 2 and n 2 o respectively three biogeochimically contrasting regimes were investigated 1 the highly oligotrophic nutrient depleted subtropical gyre 2 the nonsoonal upwelling of nutrient rich intermediate waters of the southeastern arabian coast and 3 the denitrifying o 2 depleted zone odz ca 150 1200 m depth in the arabian sea concentrations of no 3 and no 2 were severely depleted in surface oligotrophic waters from the equator average 43 and 3 6 nm respectively to the subtropical gyre 12 15 n average 13 3 and 2 0 nm respectively with similar levels in the more stratified gulf of oman upwelling waters off southern arabia had three orders of magnitude higher no 3 levels and throughout the nwio the calculated no 3 fuelled primary production appeared to be regulated by no 3 concentration existing redfield o 2 no 3 regeneration ratios 9 1 previously derived for the deep indian ocean were confirmed 9 35 within the oxic upper layers of the nwio the no potential temperature relationship broecker 1974 earth and planetary science letters 23 100 107 needed for the derivation of expected no 3 and no 3 deficits within the denitrifying odz were refined using an isopycnal binary mixing model along the 26 6 density layer to take into account the inflowing contribution of no 3 depleted persian gulf water vertically integrated no 3 deficits increased northwards from 0 8 mol no 3 n m 2 at sta 2 04 n up to 6 49 mol no 3 n m 2 at sta 9 at the mouth of the gulf of oman then decreased to 4 10 moles no 3 n m 2 toward sta 11 near the straits of hormuz when averaged for the denitrification area of the arabian sea this corresponds to a deficit of 118 tg no 3 n adopting a recent freon 11 based estimate of water residence time of 10 years olson et al 1993 deep sea research ii 40 673 685 for the o 2 depleted layer we calculate an annual net denitrification flux of 11 9 tg n to the atmosphere or approximately 10 of the global water column denitrification rates supersaturated n 2 o concentrations were found in both surface oxic and upwelling waters up to 246 and peaked at the base of the odz up to 1264 in the northern arabian sea both nitrification in oxic waters and denitrification in hypoxic layers can be invoked as sources of n 2 o the inventory of excess n 2 o amounted to 2 55 1 3 tg n 2 o n corresponding to annual production of 0 26 0 13 tg from denitrification this is comparable to earlier law and owens 1990 nature 346 826 828 estimates of the ventilation flux of n 2 o 0 22 0 39 tg yr 1 from the upwelling region of the arabian sea the decadal response times for circulation deoxygenation denitrification and ventilation of the odz derived n 2 o and co 2 greenhouse gases and their monsoonal coupling implies the arabian sea is a sensitive oceanic recorder of global climate change

megafaunal responses to strong oxygen gradients on the pakistan margin of the arabian sea the arabian sea oxygen minimum zone omz which intersects the continental margin between approximately 100 and 1200 m is one of the world s largest deep water oxygen deficient water masses we analysed megafaunal organisms seen in images obtained using a wide angle survey photographic wasp system at nine sites 140 1850 m water depth across the omz on the pakistan margin during the late monsoon period august september 2003 the visible megafauna comprised 1 the megabenthos sensu strictu s s 2 large polychaetes and 3 the benthopelagic megafauna fish natant decapods and octopods large protozoans mainly the foraminiferan pelosina sp were counted but not included in the megafauna the megabenthos s s were rare at the seasonally hypoxic 140 m site o2 0 11 ml l 1 entirely absent in the omz core and most of the lower transition zone 300 900 m o2 0 12 0 15 ml l 1 but peaked in abundance 27 94 indiv m 2 at 1000 m o2 0 16 ml l 1 densities were much lower at 1100 and 1200 m 0 52 0 69 indiv m 2 o2 0 25 0 38 ml l 1 and declined to minimal values 0 01 indiv m 2 at 1850 m o2 1 68 ml l 1 there was no correlation with depth dissolved oxygen concentration or sediment organic chemistry variables corg total n c n 13c 15n pelosina sp was the only strictly benthic organism visible at 400 and 700 m fish and natant decapods were fairly common at 300 m and fish were the only metazoans seen in photographs from 700 m large polychaetes almost certainly linopherus sp were very abundant in photographs from 900 m where megabenthos s s were absent and somewhat less abundant at 1000 m suspension feeding cnidarians and tunicates were abundant at 1100 and 1200 m respectively the number of megabenthos s s species visible at each site ranged from six 1000 and 1850 m to 11 1100 and 1200 m diversity h loge was the lowest at 1000 and 1850 m and the highest at 1100 m with intermediate values at 140 and 1200 m dominance was the highest 99 at 1000 m high 87 at 1200 m and lower 32 50 at 140 1100 and 1850 m benthopelagic megafauna appeared more tolerant of dysoxia than the megabenthos s s although densities peaked at 1100 m slightly deeper than for the megabenthos s s the presence of uneaten carrion dead fish and natant decapods between 300 and 900 m suggests that scavengers were either not active or not present within the omz the megabenthos s s appeared to respond to oxygen concentrations above a threshold value 0 15 0 16 ml l 1 the ophiuroid dominated abundance peak at 1000 m and the abrupt changes in megabenthic assemblage composition between 1000 and 1200 m were probably expressions of an edge effect known from other omzs and believed to reflect a threshold release from physiological oxygen limitation accompanied by an abundant food supply

holocene organic geochemical record from the western indus continental shelf northern arabian sea primary production on the western indus continental shelf has been linked to the large quantities of nutrients delivered to the shelf by the indus river multiple geochemical tracers and biomarker

upper ocean currents in the northern arabian sea from shipboard adcp measurements collected during the 1994 1996 u s jgofs and onr programs abstract a large upper ocean velocity data set was obtained using a shipboard acoustic doppler current profiler adcp on seventeen rv t g thompson cruises during the jgofs and onr expedition to the northern arabian sea from september 1994 through january 1996 seven of the cruises followed a large area survey track centered over the arabian basin four cruises conducted seasoar surveys on either side of the findlater jet axis and six cruises were for the deployment and maintenance of moored instrumentation together providing some 380 cruise days and 96 000 track kilometers of coverage the adcp data extended over the upper 250 400 m of the water column depending upon the temporal spatial distributions of acoustic scatterers the velocity data revealed several items that differed significantly from the historical perspective maximum current magnitudes in this area varied from more than 1 m s 1 along the arabian coast to 10 20 cm s 1 well offshore perhaps the most important result was the complete dominance of the velocity field by eddies that had offshore correlation length scales of roughly 100 km a spectral peak at around 300 km and kinetic energies that ranged from 70 to more than 90 of the total kinetic energy the total kinetic energy was highest within about 300 km of the shore and decreased significantly in magnitude and vertical extent offshore within the coastal region the temporal variability was such that currents of 50 cm s 1 or more could completely reverse within a two week period seemingly independent of any local atmospheric forcing mean and seasonal velocities also differed from historical results there was a large anti cyclonic feature located for most of the year south of ras ash shabatat 58 e which intensified during the southwest monsoon there was a strong jet like current off ras al hadd 22 n which also intensified during the southwest monsoon although itflowed southwestward against the wind in contrast to the historical ship drift data which indicated that the surface currents followed the monsoonal winds the adcp data over the upper 200 400 m was highly variable with overall seasonal means that were often directed against the wind except within 100 200 km of the arabian coast the mean currents were generally toward the southwest with a mean transport in the upper 150 m between the coast and 10 n of approximately 9 10 6 m 3 s 1

sedimentary denitrification rates across the arabian sea oxygen minimum zone abstract sedimentary denitrification was measured at sites across the pakistan margin oxygen minimum zone omz during cruises before and after the 2003 southwest sw monsoon denitrification rates were calculated using nitrate fluxes and n 2 production rates in shipboard whole core incubations and benthic lander deployments we observed generally good agreement between the sedimentary denitrification rates obtained from nitrate consumption and n 2 production results of these investigations indicate that sedimentary denitrification is a persistent process within the arabian sea omz and the regions proximal to the permanent omz e g 140 and 1200 m sedimentary denitrification in the region intensified during the sw monsoon as noted by the increase in associated rates between the inter monsoon april may 2003 and post monsoon september october 2003 assessments sedimentary denitrification was observed at all stations during the inter monsoon period nitrate consumption rates ranged from 0 40 to 3 17 mmol n m 2 d 1 during the post monsoon assessment apparent sedimentary denitrification rates increased in the shallow sites 140 and 300 m likely in response to a shoaling of the omz but were not definitively observed at the deepest site 1850 m where observed post monsoon sedimentary denitrification rates increased to 0 66 3 78 mmol n m 2 d 1 these rates compare favorably to sedimentary denitrification rates obtained in other shelf environments and are higher than abyssal sedimentary denitrification rates by extrapolating these rates to the entire pakistan margin omz we estimate a net annual sedimentary denitrification flux of 1 1 10 5 tg n yr 1 moderately higher than current estimates of sedimentary denitrification in this region and in line with estimates of regional sedimentary denitrification in other sites

an estimate of the efficiency of the iron and manganese driven dissolved inorganic phosphorus trap at an oxic euxinic water column redoxcline 1 geochemical records suggest the ocean has undergone periods of at least partial deeper ocean anoxia or euxinia two counteracting feedback loops involving redox control of the dynamics of the phytoplankton nutrient dissolved inorganic phosphorus dip might coexist helping to stabilize the redox state of the atmosphere and oceans this concept implies that during deeper ocean anoxia the dip transfer from the deep anoxic into the oxic surface ocean is uninhibited by processes taking place at the redoxcline this implicit assumption requires testing because iron fe and manganese mn dynamics at oxic anoxic water column redoxclines have the potential to form a dip trap inhibiting dip transport from anoxic deep into oxic surface waters using a time series data set of fe mn dip and dissolved oxygen distributions in the eastern gotland basin of the baltic sea we provide estimates of the efficiency of this fe and mn driven dip trap this efficiency was estimated by calculating the ratios of 1 the downward flux of dip adsorbed onto and or coprecipitated into the settling authigenic fe and mn rich particles just above the redoxcline and 2 the upward turbulent diffusive dip flux across the redoxcline depending on the assumed particle densities we find average 1 sd trapping efficiencies of 0 38 0 29 and 0 63 0 45 the efficiencies are significant in that they seem to impact cyanobacterial dynamics in the central baltic sea we discuss possible implications of the trapping mechanism for and propose two hypotheses relating to the potential importance of fe controlled dip trapping at redoxclines during ocean anoxic events

carbon and nitrogen elemental and stable isotopic compositions of surficial sediments from the pakistan margin of the arabian sea abstract the factors controlling sedimentary organic matter distributions are the subject of ongoing debate we report here the carbon and nitrogen contents and stable isotopic compositions for surface sediments and downcore profiles from sites ranging in water depth from 140 to 3200 m located across the indus margin of the arabian sea the sites spanned the intense midwater oxygen minimum zone omz and showed dramatic differences in bottom water redox conditions benthic communities and degrees of bioturbation five primary stations were studied at 140 300 940 1200 and 1850 m water depth with sampling in march may intermonsoon and august october late to postmonsoon 2003 high resolution sampling was also conducted across the upper 140 275 m and lower omz 600 1200 m boundaries the sampling strategy was selected to help clarify the roles of oxygen availability and other potential environmental factors in determining sediment organic matter content and composition the extent of organic matter cycling within the sediments under contrasting depositional conditions and the fate of organic matter deposited during the southwest monsoon as previously noted on this and other omz margins there was a roughly inverse spatial relationship between bottom water oxygen and sedimentary organic matter concentrations however variability in organic carbon content within the omz that was unrelated to oxygen and the fact that maximal organic carbon contents occurred under relatively oxidizing conditions towards the lower omz boundary rather than at the omz core indicate that factors other than oxygen contribute significantly as controls on carbon distributions despite a 3 fold range in organic carbon content carbon to nitrogen ratios showed no systematic trends across the omz with values reflecting uniform predominantly marine organic matter inputs nitrogen isotopic signatures 15n values in the range of 6 5 8 5 also did not show clear cross omz trends and are attributed to a 14n depleted nitrate pool created by pelagic denitrification there was little downcore change in organic matter content at any site suggesting either that organic matter decay occurs largely in the water column or that decay within the sediments at oxygenated sites is masked by bioturbation seasonal variability in organic matter content and composition was restricted to the most surficial sediments and changes were ephemeral suggesting rapid turnover of organic matter deposited during the southwest monsoon on timescales of weeks stable c isotopic compositions trended towards more negative values in surficial sediments at hypoxic sites within the omz core this is attributed to a chemosynthetic bacterial imprint

sedimentary oxygen consumption and microdistribution at sites across the arabian sea oxygen minimum zone pakistan margin abstract biogeochemical processes and fluxes occurring across the sediment water interface on continental margins impacted by oxygen minimum zones omzs are important to bioelement cycles ocean inventories and productivity the nature and magnitude of these processes depend heavily on spatial and temporal variability in dissolved o 2 concentrations in bottom waters and porewaters in 2003 four research cruises to the indus margin of the arabian sea pakistan were undertaken to survey the benthic biogeochemical processes in the resident omz before and after the southwest monsoon sediment o 2 microdistribution and consumption rates were measured at five stations along a depth transect 140 1850 m across the omz during the spring intermonsoon and the late to post southwest monsoon periods using in situ benthic research platforms landers lander o 2 electrode data show that the intermonsoon and late to postmonsoon bottom water o 2 levels had little to no variation 300 m no change 940 m 1 7 2 8 m 1200 m 10 2 12 6 m and 1850 m 82 80 m in contrast at the shallowest station 140 m a large fluctuation occurred between the intermonsoon o 2 44 5 m and the late to postmonsoon o 2 1 m due to monsoon forced shoaling of the upper omz boundary oxygen did not penetrate into the sediments at the 300 m site during either sampling season during the intermonsoon season at the 140 m site o 2 penetrated to a depth of 3 mm but no measurable o 2 penetration occurred after the monsoon at the 940 and 1200 m sites o 2 penetration into the sediments was small ca 1 2 mm at both sites and did not measurably change between the two sampling seasons in contrast at the 1850 m site o 2 penetration decreased after the monsoon 18 12 mm calculated late to postmonsoon o 2 consumption rates were generally similar to or lower than intermonsoon values 0 vs 2 22 mmol m 2 d 1 at 140 m 0 37 vs 0 31 mmol m 2 d 1 at 1200 m and 0 73 vs 1 01 mmol m 2 d 1 at 1850 m the relatively small seasonal signal suggests that organic matter delivered during the monsoon period may have already been largely remineralized by the late to postmonsoon sampling period modelling of porewater o 2 profiles indicates that subsurface o 2 consumption associated oxidation of reduced inorganic species makes a significant contribution to total o 2 consumption at some sites similarly differences in o 2 consumption rates determined by porewater profile modelling and whole core incubations at some sites indicate significant contributions associated with bioturbation and bioirrigation

benthic biological and biogeochemical patterns and processes across an oxygen minimum zone pakistan margin ne arabian sea abstract oxygen minimum zones omzs impinging on continental margins present sharp gradients ideal for testing environmental factors thought to influence c cycling and other benthic processes and for identifying the roles that biota play in these processes here we introduce the objectives and initial results of a multinational research program designed to address the influences of water depth the omz 150 1300 m and organic matter om availability on benthic communities and processes across the pakistan margin of the arabian sea hydrologic sediment and faunal characterizations were combined with in situ and shipboard experiments to quantify and compare biogeochemical processes and fluxes om burial efficiency and the contributions of benthic communities across the omz in this introductory paper we briefly review previous related work in the arabian sea building the rationale for integrative biogeochemical and ecological process studies this is followed by a summary of individual volume contributions and a brief synthesis of results five primary stations were studied at 140 300 940 1200 and 1850 m water depth with sampling in march may intermonsoon and august october late to postmonsoon 2003 taken together the contributed papers demonstrate distinct cross margin gradients not only in oxygenation and sediment om content but in benthic community structure and function including microbial processes the extent of bioturbation and faunal roles in c cycling hydrographic studies demonstrated changes in the intensity and extent of the omz during the sw monsoon with a shoaling of the upper omz boundary that engulfed the previously oxygenated 140 m site oxygen profiling and microbial process rate determinations demonstrated dramatic differences in oxygen penetration and consumption across the margin and in the relative importance of anaerobic processes but surprisingly little seasonal change a broad maximum in sediment om content occurred on the upper slope roughly coincident with the omz but the otherwise poor correlation with bottom water oxygen concentrations indicated that other factors are important in determining sediment om distributions downcore profiles generally showed little clear evidence of in situ om alteration and there was little sign of om enrichment resulting from the southwest monsoon in sediments collected in the late to postmonsoon sampling this is interpreted to be due to rapid cycling of labile om organic geochemical studies confirmed that sediment om is overwhelmingly of marine origin across the margin but also that it is heavily altered with only small changes in degradation state across the omz more negative stable c isotopic compositions in surficial sediments at hypoxic sites within the omz core are attributed to a chemosynthetic bacterial imprint dramatic changes in benthic community structure occurred across the lower omz transition apparently related to om availability and quality as well as to do concentrations high resolution sampling biomarkers and isotope tracer studies revealed that oxygen availability appears to exert threshold type controls on benthic community structure and early faunal c processing biomarker studies also provided evidence of faunal influence on sediment om composition together the results offer strong evidence that benthic fauna at sites across the margin play important roles in the early cycling of sediment om through differential feeding and bioturbation activities

manganese iron and sulphur cycling on the pakistan margin abstract benthic biogeochemical processes in the oceans oxygen minimum zones omzs are of global significance yet they remain poorly defined here sedimentary manganese iron and sulphur cycling are studied at sites above within and below the omz on the pakistan margin of the arabian sea before and immediately after the southwest monsoon mechanisms and rates of sedimentary mn fe and s cycling demonstrated down slope variation according to the position of the omz no significant seasonal change was evident except at one station on the upper margin where bottom waters became hypoxic during the late to post monsoon sampling due to the shoaling of the upper omz boundary within the omz mn 4 reduction within the water column limited supply of reactive mn to the underlying sediments the majority of reactive mn reaching the sedimentary environment is reduced abiotically via reaction with fe 2 sediments within the omz act as a source of mn 2 to the overlying water below the omz mn 2 is re oxidised within the water column and is deposited on the surface of the underlying sediments manganese is then reduced microbially and re oxidised within the oxygenated surface sediments uniform surface sediment fe al ratios across the margin suggest that fe 3 reduction is lacking within the water column omz in contrast fe is actively cycled within the sediments with fe 3 reduction occurring via dissimilatory pathways an absence of potential fe 2 oxidants within the omz results in an efflux of fe 2 from the sediments to the water column below the omz greater oxidant availability ensures that fe is retained within the sediments measured sulphate reduction rates at all sites across the margin were surprisingly low 0 0 45 mmol m 2 d 1 relative to rates observed on other omz margins the abundance of fe oxides in the sedimentary environment appears to suppress sulphate reduction

the ras al hadd jet remotely sensed and acoustic doppler current profiler observations in 1994 1995 abstract the existence of a surface barotropic front jet system at the confluence region off the eastern tip of oman ras al hadd or rah is documented for 1994 1995 through advanced very high resolution radiometer avhrr and acoustic doppler current profiler adcp observations the thermal signature of this confluence is visible in 1995 between early may and the end of october i e throughout the sw monsoon and into the transition period between sw and ne monsoons the thermal characteristics are those of a ne oriented front between cooler water of southern upwelled origin and warmer waters of northern gulf of oman origin during the period when the thermal front is absent adcp data suggest that the confluence takes a more southward direction with gulf of oman waters passing rah into the southeastern oman coastal region the thermal gradient is initially small june july but later increases august october into a front that exhibits small scale instabilities surface current velocities within the jet estimated by tracking these features in consecutive satellite images are 0 5 0 7 m s 1 and in remarkable agreement with concurrent adcp retrievals in which the seasonal maximum in velocity is 1 m s 1 adcp observations collected during several us jgofs cruises reveal a weakly baroclinic current in the confluence region that drives the waters into the offshore system the fully developed jet describes a large meander that demarcates two counter rotating eddies cyclonic to the north and anticyclonic to the south of the jet of approximately 150 200 km diameter the southern eddy of this pair is resolved by the seasonally averaged sea level anomaly derived from topex poseidon observations during the sw monsoon the rah jet advects primarily cold waters along its path but as soon as the wind system reverses with the transition to the intermonsoonal period a warm current is rapidly established that advects the surface coastal waters of the gulf of oman offshore in accordance with the interannual variation of the wind forcing phase the reversal of the currents from ne to sw occurred earlier in 1994 than in 1995 confirming that the rah jet is integral part of the east arabian current the transport of the jet estimated by combining sst information on the width with adcp data on the velocity s vertical structure is found to fluctuate between 2 8 106 m3 s 1 and its thickness between 150 400 m these significant fluctuations are due to the time variable partition of horizontal transport between eddies and the rah jet and are potentially important to the nutrient and phytoplankton budgets of the arabian sea

porewater nutrient concentrations and benthic nutrient fluxes across the pakistan margin omz abstract porewater concentrations and benthic fluxes of phosphate silicate ammonia nitrate and nitrite were measured at five sites spanning the pakistan margin oxygen minimum zone omz in order to characterise the biogeochemical processes occurring and to assess whether oxygen concentration and a seasonal pulse of organic matter are controlling factors typical concentrations of 1 70 m 50 250 m 0 270 m 30 cm in the sediment these processes are all redox sensitive and their intensities varied across the margin suggesting that oxygen concentration exerts a strong influence over nutrient concentrations and cycling variation in nutrient concentrations and fluxes before and after the summer monsoon was limited to an oxygen driven change to the po43 profile at one site indicating that either nutrient profiles do not generally alter on seasonal timescales or that any impact of the monsoon had subsided before the post monsoon sampling period porewater profile modelling tended to underestimate the magnitude of fluxes but was in general agreement with the directions of measured fluxes and in situ and shipboard flux measurements also generally agreed phosphate and h4sio4 concentrations and benthic fluxes on the pakistan margin were similar to those reported at abyssal sites from around the world while nh4 and no3 concentrations and fluxes were comparable to shallower more productive and or hypoxic marine settings

faunal responses to oxygen gradients on the pakistan margin a comparison of foraminiferans macrofauna and megafauna the pakistan margin is characterised by a strong mid water oxygen minimum zone omz that intercepts the seabed at bathyal depths 150 1300 m we investigated whether faunal abundance and diversity trends were similar among protists foraminiferans and gromiids metazoan macrofauna and megafauna along a transect 140 1850 m water depth across the omz during the 2003 intermonsoon march may and late post monsoon august october seasons all groups exhibited some drop in abundance in the omz core 250 500 m water depth o2 0 10 0 13 ml l 1 4 46 5 80 m but to differing degrees densities of foraminiferans 63 m were slightly depressed at 300 m peaked at 738 m and were much lower at deeper stations foraminiferans 300 m were the overwhelmingly dominant macrofaunal organisms in the omz core macrofaunal metazoans reached maximum densities at 140 m depth with additional peaks at 850 940 and 1850 m where foraminiferans were less abundant the polychaete linopherus sp was responsible for a macrofaunal biomass peak at 950 m apart from large swimming animals fish and natant decapods metazoan megafauna were absent between 300 and 900 m o2 0 2 ml l 1 8 92 m the progressively deeper abundance peaks for foraminiferans 63 m linopherus sp and ophiuroids probably represent lower omz boundary edge effects and suggest a link between body size and tolerance of hypoxia macro and megafaunal organisms collected between 800 and 1100 m were dominated by a succession of different taxa indicating that the lower part of the omz is also a region of rapid faunal change species diversity was depressed in all groups in the omz core but this was much more pronounced for macrofauna and megafauna than for foraminiferans oxygen levels strongly influenced the taxonomic composition of all faunal groups calcareous foraminiferans dominated the seasonally and permanently hypoxic sites 136 300 m agglutinated foraminiferans were relatively more abundant at deeper stations where oxygen concentrations were 0 13 ml l 1 5 80 m polychaetes were the main macrofaunal taxon within the omz calcareous macrofauna and megafauna molluscs and echinoderms were rare or absent where oxygen levels were lowest the rarity of larger animals between 300 and 700 m on the pakistan margin compared with the abundant macrofauna in the omz core off oman is the most notable contrast between the two sides of the arabian sea this difference probably reflects the slightly higher oxygen levels and better food quality on the western side

an 1899 glacier inventory for nordland northern norway produced from historical maps glaciers depicted on old maps reveal their historical extents before the advent of aerial and satellite remote sensing digital glacier inventories produced from these maps can be employed in assessments of centennial scale glacier change this study reconstructs the 1899 covering the period 1882 1916 glacier extent in nordland northern norway from historical gradteigskart maps with an emphasis on examining the accuracy of the mapped glaciers glacier outlines were digitised from georectified scans of the analogue maps in a raster graphics editor and were subsequently inventoried in a gis the accuracy of the historical glacier extent was established from written descriptions and landscape photographs created during the original field surveys and further validated against independent glacier outlines of 1 the maximum little ice age extent derived from geomorphological evidence and 2 the 1945 extent derived from vertical aerial photographs an overall uncertainty of 17 is associated with our inventory nordland s glaciers covered an area of 1712 291 km2 in 1899 by 2000 total ice cover had decreased by 47 807 137 km2 at a rate of 6 10 a 1 80 14 km2 10 a 1 the approach presented here may serve as a blueprint for future studies intending to derive glacier inventories from historical maps