how paleosols influence groundwater flow and arsenic pollution a model from the bengal basin and its worldwide implication in the bengal basin the land surface exposed during the last lowstand of sea level around 20 ka and now buried by holocene sediment is capped by an effectively impermeable clay paleosol that we term the last glacial maximum paleosol lgmp the paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern west bengal and by implication in shallow aquifers across the bengal basin and as polluted deltaic aquifers worldwide the presence of the lgmp defines paleointerfluvial areas it is absent from paleochannel areas a paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic while that beneath paleointerfluvial areas is not paleointerfluvial aquifers are unpolluted because they are protected by the lgmp from downward migration of arsenic and from downward migration of organic matter that drives as pollution via reductive dissolution of as bearing iron oxyhydroxides horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers in which sorption of arsenic minimizes the risk of pollution

trace element signatures of chilean upwelling sediments at 36 s abstract with this contribution we continue the discussion on trace element te cycling in chilean upwelling sediments from within and below the oxygen minimum zone omz boning et al 2005 marine geology 220 1 21 low authigenic i e non lithogenic contents of te ag cd mo re and se were interpreted to result from the distinct seasonality in productivity induced organic flux bottom water oxygenation as well as dilution by terrigenous matter while all sediments were reducing enough to become depleted in mn highest enrichments in mo cd and re occur in sediments in contact with the omz here for a meaningful discussion total contents of as bi co cr cu ni pb sb tl v u and zn were corrected for the lithogenic background by determining te al ratios from local river samples the latter showed significantly lower te al ratios than average shale crust or andesite which are classically used for background correction hence the precise assessment of the lithogenic te contribution for the calculation of authigenic te contents allows to examine the source sink relationship of individual authigenic te for chilean sediments four te groups may be distinguished i mo re u and sb likely originate from sea water and accumulate via diffusive supply across the swi ii cd cr cu ni and zn require biologically induced pre concentration within biogenic material iii ag as se tl and v show an intermediate behaviour and iv bi and pb are contributed by anthropogenic activity highest enrichments in ni cu u as and v occur in sediments in contact with the omz the distinct depletion in co along with mn indicates the overall reducing but non sulfidic character of the chilean omz the degree of sedimentary sulfate reduction seems to be the most important control on chilean and peruvian near coastal enrichment of cd mo and u

redox chemistry of west antarctic peninsula margin surface sediments abstract continental margin sediments are commonly studied using trace metal enrichments as proxies for characterizing their modern redox conditions and tracking past changes in bottom water ventilation and marine primary productivity currently little is known about the sedimentary redox history of the continental shelf west of the antarctic peninsula yet environmental conditions in the antarctic peninsula region have changed rapidly in recent decades in response to global warming paleoclimate archives including bulk sediment trace metal enrichments can provide insight into both past and future environmental changes in this climatically sensitive region and characterization of redox conditions in surface modern sediments is essential for establishing a framework for paleoredox interpretations in this study we measured concentrations of trace metals ag cd re and mo and productivity proxies total ba organic carbon and biogenic silica in surface sediments from the marguerite bay gerlache strait and bransfield strait areas proxy concentrations suggest that sediments are generally suboxic due to seasonally high export of organic carbon from surface waters comparison of west antarctic peninsula trace metal al ratios to trace metal data from continental margins outside the antarctic region demonstrates that although the west antarctic peninsula water column differs from other locations e g it lacks a strong oxygen minimum zone sedimentary redox chemistry is similarly controlled by organic matter decomposition through slow recycling of highly seasonal export production falling through a cold water column

glacial interglacial variability in denitrification in the world s oceans causes and consequences the late quaternary history of water column denitrifcation of the eastern pacific margins and the arabian sea is reconstructed using sedimentary 15n measurements the 15n values in six piston cores raised from these regions show remarkably similar cyclic variations being heavy 9 10 5 during the interglacials and 2 3 lighter during the glacials this implies that denitrification in these regions decreased substantially during the glacial periods the glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone since water column denitrification in these areas accounts for about half of the fixed nitrogen loss in the modern ocean the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods because nitrate is a limiting nutrient oceanic productivity and attendant changes in co2 may therefore have been modulated on glacial interglacial timescales by variations in the oceanic no3 content

ice sheet control of continental erosion in central and southern chile 36 41 s over the last 30 000 years bulk sediment chemistry from three chilean continental margin ocean drilling program sites constrains regional continental erosion over the past 30 000 years sediments from thirteen rivers that drain the mostly igneous andes and the mostly metamorphic coast range along with existing rock chemistry datasets define terrestrial provenance for the continental margin sediments andean river sediments have high mg al relative to coast range river sediments near 36 s marine sediments have high mg al i e more andean sources during the last glacial period and lower mg al less andean sources during the holocene near 41 s a ti rich source likely from coast range igneous intrusions is prevalent during holocene time whereas high mg al andean sources are more prevalent during the last glacial period we infer that there is a dominant ice sheet control of sediment sources at 36 s andean sourced sediment decreased as andean mountain glaciers retreated after w17 6 ka coincident with local oceanic warming and southward retreat of the patagonian forest and by inference westerly winds at 41 s andean sediment dominance peaks and then rapidly declines at w19 ka coincident with local oceanic warming and the earliest deglacial sea level rise we hypothesize that this decreased flux of andean material in the south is related to rapid retreat of the marine based portion of the patagonian ice sheet in response to global sea level rise as the resulting flooding of the southern portion of the central valley created a sink for andean sediments in this region reversal of the decreasing deglacial mg al trend at 41 s from 14 5 to 13 0 ka is consistent with a brief re advance of the patagonian ice sheet coincident with the antarctic cold reversal

late quaternary sedimentary environments and landforms in the ganges delta abstract late quaternary sediments deposited since the lowest stand of sea level during the last glacial maximum in the ganges delta and its surrounding region were classified into five units according to their sedimentary facies these are the lowest unit of sandy gravels the lower unit of sand with a few gravels and the middle upper and uppermost units consisting mainly of sand and silt with occasional peat layers the contact between the lower and middle units is fairly sharp and the upper part of the lower unit is oxidized in some places this suggests a period of subaerial exposure after the deposition of the lower unit during the low stand of sea level about the time of the last glacial maximum rivers in the ganges delta and its surrounding region dissected the surfaces of the region and deposited the lowest unit on the valley floors the lower unit was deposited over the lowest unit during an early stage of the post glacial transgression between ca 12 000 yr bp and 10 000 yr bp the surface of the delta was slightly dissected and the top of the lower unit was weathered and oxidized following this period the middle and upper units of alluvial or deltaic sand and silt with occasional peat were deposited the coastline in the early holocene retreated towards the central part of the present ganges delta in the middle and late holocene the silt and clay with occasional peat layers of the uppermost unit indicate that the lowland gradually became marshy and poorly drained as the rate of transgression became slower

rhenium and molybdenum enrichments in sediments as indicators of oxic suboxic and sulfidic conditions of deposition abstract the trace elements re and mo both behave conservatively in seawater yet are strongly enriched in reducing sediments their potential for authigenic enrichment above crustal concentrations is greater than for many other elements due to the high ratio of metal sw metal crust we present sedimentary re and mo data from box and multi cores spanning a range of redox conditions from well oxygenated sites to locations with substantial sulfide concentrations at the oxic sites re and mo as expected accumulate at concentrations close to their crustal abundances re shows substantial enrichment in suboxic absence of o 2 and h 2 s sediments of the sea of japan in sediments within the oxygen minimum of the pakistan margin as well as in sediments underlying the sulfide bearing waters of the black sea and saanich inlet re enrichment occurs in these cores just below the depths of fe and u reduction only in the sediments underlying the sulfidic waters of the black sea and saanich inlet is there substantial authigenic mo accumulation absence of re and mo enrichments in sediment trap samples from the sulfide bearing basins suggests that the addition of both metals occurs at or below the sediment water interface the re and mo concentration profiles are modeled to hindcast the removal depth of re and mo in the sediments which along with the re mo ratio determine whether oxic suboxic or sulfidic conditions were present in the water column or in the sediments in the past using this approach historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals

tropical cooling at the last glacial maximum and extratropical ocean ventilation1 1 the ncar csm is used to simulate the climate at last glacial maximum the csm simulates a tropical sst cooling of 2 1 c it is shown that about half of this tropical cooling is associated with the upper ocean circulation especially the ventilation of thermocline and intermediate waters from the mid high latitude south pacific

impact of local recharge on arsenic concentrations in shallow aquifers inferred from the electromagnetic conductivity of soils in araihazar bangladesh 1 the high degree of spatial variability of dissolved as levels in shallow aquifers of the bengal basin has been well documented but the underlying mechanisms remain poorly understood we compare here as concentrations measured in groundwater pumped from 4700 wells 22 m 75 ft deep across a 25 km2 area of bangladesh with variations in the nature of surface soils inferred from 18 500 measurements of frequency domain electromagnetic induction a set of 14 hand auger cores recovered from the same area indicate that a combination of grain size and the conductivity of soil water dominate the electromagnetic signal the relationship between pairs of individual em conductivity and dissolved as measurements within a distance of 50 m is significant but highly scattered r2 0 12 n 614 concentrations of as tend to be lower in shallow aquifers underlying sandy soils and higher below finer grained and high conductivity soils variations in em conductivity account for nearly half the variance of the rate of increase of as concentration with depth however when the data are averaged over a distance of 50 m r2 0 50 n 145 the association is interpreted as an indication that groundwater recharge through permeable sandy soils prevents as concentrations from rising in shallow reducing groundwater

increased glacial age ventilation of the chilean margin by antarctic intermediate water antarctic intermediate water is an oxygen rich water mass that spreads throughout the southern hemisphere oceans marine sediment chemistry indicates that the water mass was produced at higher rates and had higher oxygen concentrations during the last glacial maximum antarctic intermediate water is at present a water mass that brings oxygen to intermediate depths throughout the southern hemisphere oceans models have suggested that intermediate waters had higher concentrations of oxygen during the last glacial period1 2 consistent with globally reduced denitrification3 and increased production of antarctic intermediate water4 however some palaeoceanographic reconstructions5 6 have indicated that production decreased in the southeast pacific ocean at this time here we analyse the concentrations of re and mn the sedimentary concentrations of which are controlled by the amount of dissolved oxygen at the sea floor from three sediment cores located along the chilean margin for the past 30 000 years our results from the cores which bracket the present day water column extent of antarctic intermediate water show that the depth range of well oxygenated antarctic intermediate water increased off chile during the last glacial maximum dissolved oxygen content began to decrease approximately 17 000 years ago coincident with rapid antarctic warming and a poleward shift of the southern westerly winds7 our estimates of productivity from accumulation rates of organic carbon and opal do not co vary with the seafloor oxygen variations ruling out local control of seafloor oxygenation we conclude that the data are best explained by a combination of increased oxygenation and increased flux of antarctic intermediate water during the last glacial maximum

deglacial pattern of circulation and marine productivity in the upwelling region off central south chile abstract a high resolution sea surface temperature and paleoproductivity reconstruction on a sedimentary record collected at 36 s off central south chile geob 7165 1 36 33 s 73 40 w 797 m water depth core length 750 cm indicates that paleoceanographic conditions changed abruptly between 18 and 17 ka comparative analysis of several cores along the chilean continental margin 30 41 s suggests that the onset and the pattern of deglacial warming was not uniform off central south chile due to the progressive southward migration of the southern westerlies and local variations in upwelling marine productivity augmented rather abruptly at 13 14 ka well after the oceanographic changes we suggest that the late deglacial increase in paleoproductivity off central south chile reflects the onset of an active upwelling system bringing nutrient rich oxygen poor equatorial subsurface water to the euphotic zone and a relatively higher nutrient load of the antarctic circumpolar current during the last glacial maximum when the southern westerlies were located further north productivity off central south chile in contrast to off northern chile was reduced due to direct onshore blowing winds that prevented coastal upwelling and export production

mineralogy of the ganges and brahmaputra rivers implications for river switching and late quaternary climate change late quaternary sediments of the bengal basin contain a history of river switching and climate change as revealed from sand and clay size mineralogy of boreholes and modern riverbed grabs epidote to garnet ratios e g in sand fraction sediments are diagnostic of source with high 1 e g indicating brahmaputra provenance and low 1 e g indicating ganges provenance in the clay fraction of surficial sediments smectite is diagnostic with high values 39 in the ganges and low values 3 in the brahmaputra in contrast the brahmaputra contains more kaolinite 29 vs 18 more illite 63 vs 41 and more chlorite 3 vs 1 than the ganges analysis of mineralogic and stratigraphic data indicates that the two rivers have changed position several times during the holocene extended periods of mixed river inputs appear to be isolated to the early holocene suggesting rapidly migrating braided channels during sea level lowstand tectonically driven accommodation in the sylhet basin may have contributed to the favored easterly course of the brahmaputra during much of the holocene relative abundances of illite and chlorite ic vs smectite and kaolinite sk record varying degrees of physical and chemical weathering respectively high ic values in early post glacial deposits suggest a dominance of physical weathering at that time however a general increase in sk concentrations throughout the holocene appears to reflect enhanced chemical weathering under increasingly warmer and more humid conditions notably a peak in sk concentrations that corresponds to an early holocene warm period 10 7 ka suggests that weathering patterns in the catchment respond quickly to climatic shifts

sr and nd isotopes in river sediments from the ganga basin sediment provenance and spatial variability in physical erosion 1 sr and nd concentrations and isotope compositions in sediment of the ganga river from gangotri to rajmahal and its tributaries have been measured to determine provenance and the spatial variability in physical erosion among the ganga subbasins sr and nd in silicates range from 37 to 138 and from 10 to 36 g g with 87sr 86sr and nd of 0 7474 0 8428 and 25 5 to 15 5 respectively the results suggest that 65 of ganga mainstream sediments are derived from the higher himalayan crystallines highlighting intense physical erosion in this region the 87sr 86sr values of sediments in the gangetic plain show nearly identical trends during two seasons with a sharp and significant decrease at barauni downstream of gandak confluence this brings out the major impact of the sediment contribution from the gandak to the ganga mainstream model calculation suggests that about half of the ganga sediment at rajmahal is sourced from the gandak the erosion rates in the himalayan subbasins of the ganga range between 0 5 and 6 mm a where a is years with the gandak having the highest erosion rate high relief and intense precipitation over the headwater basins of the gandak appear to drive the rapid and focused erosion of this basin the results of this study and those in literature suggest that the eastern syntaxis brahmaputra the western syntaxis indus and the gandak have much higher physical erosion rates than the other himalayan basins focused erosion in the hot spots of these river basins contributes significantly to the global riverine sediment budget and influence regional tectonics

response of the ganges dispersal system to climate change a source to sink view since the last interstade the forcing of sediment dispersal systems by climate is widely accepted but there remains disagreement on the general responses to climate change such as how resulting signals propagate downstream and ultimately how they are preserved in the stratigraphic record towards a better understanding of these issues this paper presents a comprehensive overview of ganges dispersal system behavior since the last interstade which reveals major coeval responses to changes in the south asian monsoon at 3000 km from its mountain headwaters to deep sea fan the ganges dispersal system is immense and yet shows tight coupling between source area catchment basin and coastal and marine depocenters furthermore system wide responses and downstream transfer of the signal appear to occur at least as fast as current temporal resolution of 1 2 ky this tight linkage of source to sink components is considered a function of the southwest summer monsoon s overwhelming control on regional hydrology about 80 of the ganges discharge and 95 of its sediment load are delivered to the margin during only 4 months making the system extremely sensitive to this seasonal forcing in addition the regional scale and distribution of the monsoon weather system means that changes in atmospheric circulation affect the entire drainage basin rather than local subcomponents finally despite its present intensity strength of the summer monsoon has varied significantly over the past 150 ky under orbitally driven changes in insolation and global glacial boundary conditions the resulting changes in precipitation both well above and below modern values have forced system wide responses ranging from glacier advance retreat to sediment fluxes to the margin and deep sea fan the overall conclusions are 1 that this immense dispersal system responds to multimillennial scale 104 years climate change in a system wide and largely contemporaneous manner and 2 that major sedimentary signals can be transferred rapidly from source to sink with little apparent attenuation furthermore these acute responses to climate change have produced sedimentary stratigraphic features that diverge from traditional sequence models in their nature and timing

different mechanisms of silicic acid leakage and their biogeochemical consequences in the modern ocean silicic acid is effectively trapped in the southern ocean according to the silicic acid leakage hypothesis a loosening of this trapping may have contributed to low atmospheric co2 during glacial times using a model with dynamical feedbacks in ocean physics and biology we explore three distinct mechanisms to loosen the trapping and trigger silicic acid leakage from the southern ocean sea ice expansion weaker winds and iron addition the basic idea of the iron addition mechanism was previously explored using a simple box model here we confirm the main results of the earlier work and demonstrate further that sea ice expansion and weaker southern westerlies can also trigger silicic acid leakage the three mechanisms are not mutually exclusive and their biogeochemical consequences are dissimilar in terms of the spatial patterns of si n uptake and the relative changes in opal and particulate organic carbon fluxes both in and outside the southern ocean while it is not entirely clear how sea ice winds and iron deposition were different during the last glacial period compared to today we examine the synergistic effects of these three triggers in one simulation in this simulation the combination of sea ice and iron reproduces the north south dipole of productivity recorded in export production proxies but effects of the iron perturbation seem to dominate the overall biogeochemical response

evidence for enhanced convection of north pacific intermediate water to the low latitude pacific under glacial conditions we provide high resolution foraminiferal stable carbon isotope 13c records from the subarctic pacific and eastern equatorial pacific eep to investigate circulation dynamics between the extratropical and tropical north pacific during the past 60 kyr we measured the 13c composition of the epibenthic foraminiferal species cibicides lobatulus from a shallow sediment core recovered from the western bering sea so201 2 101kl 58 52 52 n 170 41 45 e 630 m water depth to reconstruct past ventilation changes close to the source region of glacial north pacific intermediate water gnpiw information regarding glacial changes in the 13c of subthermocline water masses in the eep is derived from the deep dwelling planktonic foraminifera globorotaloides hexagonus at odp site 1240 00 01 31 n 82 27 76 w 2921 m water depth apparent similarities in the long term evolution of 13c between gnpiw intermediate waters in the eastern tropical north pacific and subthermocline water masses in the eep suggest the expansion of relatively 13c depleted nutrient enriched and northern sourced intermediate waters to the equatorial pacific under glacial conditions further it appears that additional influence of gnpiw to the tropical pacific is consistent with changes in nutrient distribution and biological productivity in surface waters of the glacial eep our findings highlight potential links between north pacific mid depth circulation changes nutrient cycling and biological productivity in the equatorial pacific under glacial boundary conditions

glacial interglacial modulation of the marine nitrogen cycle by high latitude o2 supply to the global thermocline 1 an analysis of sedimentary nitrogen isotope records compiled from widely distributed marine environments emphasizes the global synchrony of denitrification changes and provides evidence for a strong temporal coupling of these variations to changes in nitrogen fixation as previously inferred we explain the global coherence of these records by a simple physical control on the flux of dissolved oxygen to suboxic zones and the coupling to fixation via the supply of phosphorus to diazotrophs in suitable environments according to our hypothesis lower glacial stage sea surface temperature increased oxygen solubility while stronger winds in high latitude regions enhanced the rate of thermocline ventilation the resultant colder rapidly flushed thermocline lessened the spatial extent of denitrification and consequently n fixation during warm periods sluggish circulation of warmer less oxygen rich thermocline waters caused expansion of denitrification zones and a concomitant increase in n fixation local fluctuations in export productivity would have modulated this global signal

northeastern pacific oxygen minimum zone variability over the past 70 kyr impact of biological production and oceanic ventilation 1 during the last glacial period the oxygen minimum zone omz within the northeastern pacific ocean strengthened and weakened on a millennial time scale demonstrating a tight linkage with northern high latitude climate although the precise mechanisms responsible remain unknown core md02 2508 retrieved off baja california was analyzed for major and trace elements br ca ti fe mn and sr using a xrf scanner and redox sensitive trace elements cu ni cd as v cr mo and u using the icp ms the trace element content the fe ti ratio and br based organic carbon exhibit higher values during the holocene and during warm dansgaard oeschger events than during the last glacial maximum lgm stadials and heinrich h events a principal component analysis of the element al ratio indicated that the following two main factors controlled the chemical composition of the sediments 1 export production as represented by organic carbon that was lower during cold periods and 2 regional intermediate water oxygenation as represented by u and mo variability that was not supported by a change in export production the latter suggests that intermediate water oxygenation improved during h events but slightly deteriorated during late marine isotope stage mis 3 and mis 2 a local biogeochemical effect forced by atmospheric processes impacted the lgm and h events in the same manner whereas regional intermediate oceanic circulation varied in an opposite manner during the lgm and h events possibly as a result of the global reorganization of intermediate water circulation during the lgm

millennial scale variations in hydrography and biogeochemistry in the eastern equatorial pacific over the last 100 kyr abstract in this study we use records of nitrogen isotope ratios 15n uk 37 temperature estimates organic carbon and opal percentages from high resolution sediment cores located in the eastern equatorial pacific eep to explore the mechanisms linking millennial scale changes in low latitude sea surface temperature water column denitrification and surface productivity to the timing of northern or southern polar climate during the last 100 000 yr our results support a hypothesis that the southern hemisphere and its connection to the low latitudes via shallow subsurface ocean circulation has a primary influence on the biogeochemistry of the eep in addition our results suggest that during the last glacial stage denitrification rates fluctuated on millennial timescales in response to water column ventilation rather than upstream oxidant demand in intermediate depth waters however due to the poor age constraints available for marine isotopic stage mis 3 the eep sedimentary data presented here could support two conflicting mechanisms one driven by enhanced intermediate overturning circulation in the southern ocean during heinrich events antarctic warm events implying that subsurface flow rates control thermocline ventilation and a second one consistent with more sluggish intermediate circulation during antarctic warm events and giving a central role to the temperature control on oxygen solubility in southern ocean surface waters

reduced oxygenation at intermediate depths of the southwest pacific during the last glacial maximum to investigate changes in oxygenation at intermediate depths in the southwest pacific between the last glacial maximum lgm and the holocene redox sensitive elements uranium and rhenium were measured in 12 sediment cores located on the campbell and challenger plateaux offshore from new zealand the core sites are currently bathed by subantarctic mode water samw antarctic intermediate water aaiw and upper circumpolar deep water ucdw the sedimentary distributions of authigenic uranium and rhenium reveal reduced oxygen content at intermediate depths 800 1500 m during the lgm compared to the holocene in contrast data from deeper waters 1500 m indicate higher oxygen content during the lgm compared to the holocene these data together with variations in benthic foraminiferal 13c are consistent with a shallower aaiw ucdw boundary over the campbell plateau during the lgm whilst aaiw continued to bathe the intermediate depths 1500 m of the challenger plateau during the lgm the data suggest that the aaiw at these core sites contained less oxygen compared to the holocene these results are at odds with the general notion that aaiw was better oxygenated and expanded deeper during the lgm due to stronger westerlies and colder temperatures these findings may be explained by an important change in aaiw formation and circulation

last glacial sea surface temperatures and sea ice extent in the southern ocean atlantic indian sector a multiproxy approach 1 on the basis of the quantitative study of diatom radiolarian and planktic foraminiferal assemblages we estimated summer sea surface temperature ssst and sea ice extent at 50 sediment core localities in the atlantic and western indian sector of the southern ocean to reconstruct the last glacial environment at the glamap 18 to 15 ka equal to 21 500 to 18 000 calendar cal years bp and epilog 19 5 to 16 0 ka equal to 23 000 to 19 000 cal years bp time slices stratigraphic identification of the time slices was accomplished by a combination of ams 14c measurements benthic isotope and siliceous microfossil abundance records while the ssst estimates reveal greater surface water cooling than reconstructed by climap 1981 reaching a maximum in the area of the present subantarctic zone the sea ice reconstruction indicates that climap overestimated the expansion of the antarctic sea ice field especially for austral summer during winter the sea ice field was expanded by 60 70 compared to the present last glacial summer sea surface isotherms indicate a northward shift of the zonal bands of the antarctic circumpolar current and a relative expansion of the cold water realm south of the subantarctic front by 5 in latitude this coincides with a northward displacement of the zone of enhanced biogenic silica deposition and iceberg occurrence as a result of northward expansion of antarctic cold waters and a relatively small displacement of the subtropical front thermal gradients were steepened during the last glacial in the area of the present subtropical front the northward displacement of antarctic cold waters and the related deflection of southern ocean waters along the eastern boundary of south america may have resulted in a weakened cold water route across the drake passage in contrast the transport of warm and salty surface water from the indian into the atlantic ocean via the warm water route was not blocked allowing continuous but reduced import of heat into the south atlantic

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

piecing together the ganges brahmaputra meghna river delta use of sediment provenance to reconstruct the history and interaction of multiple fluvial systems during holocene delta evolution three main rivers the ganges brahmaputra and meghna coalesce in the bengal basin to form the world s largest delta system which serves as filter and gateway between the himalayan collision and vast bengal fan repository new insights into the holocene construction of the ganges brahmaputra meghna delta with a focus on river sedimentation channel migration and avulsion history are presented here using the sr geochemistry of bulk sediments as a provenance tracer the sediment load of each river transmits a distinct sr signature owing to differences in source rocks from the himalaya tibet and local regions allowing for effective tracking of river channels and stratigraphic development within the delta in the early holocene vigorous delta aggradation occurred under rapid sea level rise and high river discharge and supported the construction of sand dominated stratigraphy by laterally mobile braided stream channels however the vertically i e temporally uniform but geographically distinct sr signatures from these deposits indicate that the ganges brahmaputra and meghna fluvial systems remained isolated from one another and apparently constrained within their lowstand valleys by the mid holocene though delta stratigraphy records spatially and temporally nonuniform sr signatures that hallmark the onset of avulsions and unconstrained channel migration like those that characterize the modern ganges and brahmaputra fluvial systems such mobility developed in the mid holocene despite declining discharge and sea level rise suggesting that earlier channel behavior had been strongly influenced by antecedent topography of the lowstand valleys it is only after the delta had aggraded above the valley margins that the fluvial systems were able to avulse freely resulting in numerous channel reorganizations from mid holocene to present these channel system behaviors and their role in delta evolution remain coarsely defined based only on this initial application of sr based provenance tools but the approach is promising and suggests that a more complete understanding can be achieved with continued study

the three dimensional upb method generalized models and implications for upb two stage systematics four forms of the three dimensional u pb method established so far are physically generalized within the framework of a u pb two stage evolution such a generalization provides an explicit insight into the principle of conventional u pb discordia dating its implications for both u pb geochronology and pb isotopic geology are illustrated for whole rock systems when applying the three dimensional data processing method to the experimentally determined results on the u and pb concentrations and pb isotopic composition of a given suite of cogenetic samples one cannot only obtain a unique solution to timing of two episodic events i e ages t1 and t2 directly but also shed some light on the feature of the source region of initial pb incorporated at time t1 and on the nature of u pb differentiation at the two episodic events common pb for correction in the discordia dating is clarified to be responsible for the initial pb at t1 rather than t2 such a clarification would have an impact on the conventional u pb geochronology that makes assumptions about the isotopic composition of common pb in calculating the discordias the three dimensional u pb method has advantages over the two dimensional ones u pb discordias or pb pb isochron for treating the results of u pb isotopic measurements on a suite of cogenetic samples because it takes into account three variables instead of two variables in data processing and thus enables straightforward calculation of ages responsible for two episodic events without a priori assumptions this is a particular advantage for treating the u pb isotopic data of whole rock samples and therefore for discovering possible low temperature u pb differentiation responsible for terrane uplift

denitrification under glacial and interglacial conditions a physical approach 1 recent analysis of sedimentary 15n records over the last 200 000 years has shown an expansion of water column denitrification zones during warmer periods and a shrinkage of these oxygen poor regions during glacial periods two different mechanisms could be responsible for driving the changes in the denitrification records variations in local productivity leading to a change in remineralization rates and or changes in the ocean circulation and ocean temperature resulting in variations of the physical supply of dissolved oxygen here we focus on the supply mechanism by using a fully coupled atmosphere ocean sea ice land surface scheme model the university of victoria earth system climate model to examine oxygen supply under varying physical conditions while maintaining an invariant biological oxygen utilization scheme we show that circulation changes can be the cause for the observed changes in dissolved oxygen in the glacial ocean lower temperatures and enhanced formation of antarctic intermediate water and north pacific intermediate water during glacial periods increase the physical supply of oxygen and therefore decrease water column denitrification in our last glacial maximum simulations the change in water characteristics in the eastern tropical pacific is important enough to reduce denitrification by 46 to 65 compared to present day conditions depending on the wind fields used as boundary conditions the consequences of our findings could be substantial for the near future with a warming climate denitrification zones could expand leading to changes in the biological pump and the flux of n2o into the atmosphere

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

sand petrology and focused erosion in collision orogens the brahmaputra case abstract the high relief and tectonically active himalayan range characterized by markedly varying climate but relatively homogeneous geology along strike is a unique natural laboratory in which to investigate several of the factors controlling the composition of orogenic sediments coupling of surface and tectonic processes is most evident in the eastern namche barwa syntaxis where the tsangpo siang brahmaputra river draining a large elevated area in south tibet plunges down the deepest gorge on earth here composition of river sands changes drastically from lithic to quartzofeldspathic after confluence with the lohit river draining the transhimalayan equivalent mishmi arc batholiths sediment composition remains remarkably constant across assam indicating subordinate contributions from himalayan tributaries independent calculations based on petrographical mineralogical and geochemical data indicate that the syntaxis representing only 4 of total basin area contributes 35 6 to the total brahmaputra sediment flux and 20 of total detritus reaching the bay of bengal such huge anomalies in erosion patterns have major effects on composition of orogenic sediments which are recorded as far as the bengal fan in the brahmaputra basin in spite of very fast erosion and detrital evacuation chemical weathering is not negligible sand sized carbonate grains are dissolved partially in mountain reaches and completely in monsoon drenched assam plains where clinopyroxenes are selectively altered plagioclase instead is preferentially weathered only in detritus from the shillong plateau which is markedly enriched in microcline most difficult to assess is the effect of hydraulic sorting in bangladesh where quartz garnet and epidote tend to be sequestered in the bedload and trapped on the coastal plain whereas cleavable feldspars and amphiboles are concentrated in the suspended load and eventually deposited in the deep sea high resolution petrographic and dense mineral studies of fluvial sands provide a basis for calculating sediment budgets for tracing patterns of erosion in mountain belts and for better understanding the complex dynamic feedback between surface processes and crustal scale tectonics

latitudinal variations in intermediate depth ventilation and biological production over northeastern pacific oxygen minimum zones during the last 60 ka abstract mechanisms affecting past variability in the oxygen minimum zone omz in the eastern tropical north pacific etnp are poorly known we analyzed core md02 2524 obtained from the nicaragua margin in the present etnp omz for major and minor elements titanium ti brome br silicon si potassium k and calcium ca using an x ray fluorescence xrf core scanner and redox sensitive trace elements uranium u molybdenum mo and nickel ni determined by icp ms the u and mo content was higher during the deglaciation than during the holocene and the last glacial maximum whereas enrichment was not observed for ni an element closely associated with organic matter high resolution xrf scanning indicated that the ca based carbonate content had millennial scale variability inversely correlated with br based organic matter and si k based opal content during the last glacial period the available data suggest no clear regional trend in biological productivity during the last deglaciation but significant local variability in the coastal eastern equatorial pacific the trace element enrichment and the lack of a concomitant increase in biogenic phases indicated that an enhanced etnp omz at least between 15 n and 12 n at a water depth of 500 900 m was principally caused by a reduced oxygen supply driven by oceanic circulation to the nicaragua basin during the deglaciation the observed patterns can be interpreted as the distinct changes in the oxygenation state of northern and southern water masses at intermediate depths we also found evidence for a decoupling between local productivity and pore water oxygenation for several millennial scale events during marine isotopic stage 3 indicating that remote oxygen consumption and or oceanic ventilation impacted omz intensity multi millennial scale variations of the productivity at papagayo upwelling cell displayed an opposite trend from productivity at the costa rica dome in relation with the latitudinal shift of the inter tropical convergence zone itcz latitudinal variations of the omz intensity reflected the relative influence of the northern and southern intermediate water masses whose oxygenation was driven by high latitude climates and productivity over the intermediate water pathway

southern ocean and glaciogenic nutrients control diatom export production on the chile margin abstract biogenic particle flux was reconstructed using 230 thorium normalization at two sites on the southern chile margin odp site 1233 at 41 s 838 m depth is at the southern limit of the peru chile upwelling system where the northern extent of the antarctic circumpolar current impinges on the south american continental margin odp site 1234 at 36 s 1014 m depth is located within the core of the coastal upwelling system near the mouths of the bio bio and itata rivers at 41 s opal lithogenic and carbonate fluxes are greatest during the last glacial interval 26 20 ka carbonate has a secondary peak during the mid holocene 8 ka and organic carbon fluxes increase slightly from 17 ka to the present at 36 s large lithogenic fluxes are observed both during the last glacial interval and the holocene and a maximum in organic carbon flux is observed during the late holocene 5 ka without an accompanying peak in opal flux these reconstructed fluxes at 36 s and 41 s fit within a larger latitudinal pattern of a poleward increase in the magnitude of opal flux during the glacial period the pattern of normalized opal flux opal mass accumulation rate and opal carbonate ratios is consistent with either i enhanced supply of si from the southern ocean as proposed by the silicic acid leakage hypothesis or ii enhanced si and fe delivery from land driven by glacial erosion the pattern of reconstructed export production supports our view that the appearance of more reducing conditions in the sediments upon deglaciation was most likely driven by decreased ventilation rather than increased local productivity

changes in the advection of antarctic intermediate water to the northern chilean coast during the last 970 kyr 1 the antarctic intermediate water aaiw is a key player in global scale oceanic overturning processes and an important conduit for heat fresh water and carbon transport the aaiw past variability is poorly understood mainly due to the lack of sedimentary archives at intermediate water depths we present records of benthic stable isotopes from sediments retrieved with the seafloor drill rig marum mebo at 956 m water depth off northern chile geob15016 27 29 48 s 71 07 58 w that extend back to 970 ka the sediments at this site are presently deposited at the boundary between aaiw and pacific deep water pdw for previous peak interglacials our results reveal similar benthic 13c values at site geob15016 and of a newly generated stack of benthic 13c from various deep pacific cores representing the average pdw this suggests unlike today the absence of aaiw at the site and the presence of nearly pure pdw in contrast more positive 13c values at site geob15016 compared to the stack imply a considerable aaiw contribution during cold phases of interglacials and especially during glacials besides we used three short sediment cores to reconstruct benthic 13c values from the aaiw core during the last glacial and found a 13c signature similar to today s assuming that this was the case also for the past 970 kyr we demonstrate that sea level changes and latitudinal migrations of the aaiw formation site can only account for about 50 of the full range of past 13c increases at site geob15016 during cold periods other processes that could explain the remaining of the positive 13c anomalies are increases in glacial aaiw production and or deeper convection of the aaiw with respect to preceding interglacials

changes in dissolved oxygen in the southern ocean with climate change 1 climate models predict that increasing greenhouse gases in the atmosphere will change our climate however the range of observations available to assess these model predictions is limited we have utilized an ocean biogeochemical model to project the impact of climate change on the oxygen cycle in the ocean and the air sea exchange of oxygen with climate change our model predicts an outgassing of oxygen from the ocean in the model the southern ocean is the region that displays the largest change in the air sea exchange of oxygen with climate change the reduced oxygen uptake in the southern ocean is dominated by a reduction in convective mixing and in the subduction rate of bottom water which decreases the amount of dissolved oxygen in the southern ocean our model results suggest that dissolved oxygen is sensitive to changes in southern ocean circulation revealing the potential to use dissolved oxygen to test model predictions of climate change from southern ocean observations collected 28 years a part we observed a decrease in dissolved oxygen that was consistent with model climate change simulation this provides credibility to the climate model predictions additional analysis and observations of dissolved oxygen are needed to substantiate our climate model simulations

alternating influence of northern versus southern sourced water masses on the equatorial pacific subthermocline during the past 240 ka the eastern equatorial pacific eep is a key area to understand past oceanic processes that control atmospheric co2 concentrations many studies argue for higher nutrient concentrations by enhanced nutrient transfer via southern ocean intermediate water soiw to the low latitude pacific during glacials recent studies however argue against soiw as the primary nutrient source at least during early marine isotope stage 2 mis 2 as proxy data indicate that nutrients are better utilized in the southern ocean under glacial conditions new results from the subarctic pacific suggest that enhanced convection of nutrient rich glacial north pacific intermediate water gnpiw contributes to changes in nutrient concentrations in equatorial subthermocline water masses during mis 2 however the interplay between soiw versus gnpiw and its influence on the nutrient distribution in the eep spanning more than one glacial cycle are still not understood we present a carbon isotope 13c record of subthermocline waters derived from deep dwelling planktonic foraminifera globorotaloides hexagonus in the eep which is compared with published 13c records around the pacific results indicate enhanced influence of gnpiw during mis 6 and mis 2 compared to today with largest contributions of northern sourced intermediate waters during glacial maxima these observations suggest a mechanistic link between relative contributions of northern and southern intermediate waters and past eep nutrient concentrations a switch from increased gnpiw decreased soiw to diminished gnpiw enhanced soiw influence on equatorial subthermocline waters is recognized during glacial terminations and marks changes to modern like conditions in nutrient concentrations and biological productivity in the eep

similar glacial and holocene southern ocean hydrography we present new holocene and glacial d 18 o data measured on planktonic foraminifera neogloboquadrina pachyderma and benthic foraminifera cibicidoides species from the atlantic and pacific sectors of the southern ocean in order to better understand its glacial hydrography combined with previously published data the latitudinal d 18 o distributions of these foraminifera show no appreciable difference during the holocene and the last glacial maximum this suggests that the basic southern ocean hydrography including stratification and the mean position of the antarctic circumpolar current frontal system remained largely unchanged although the small number of pacific data render any conclusion in that sector preliminary the southern ocean is a critical part of the global climate system the continental shelves of the weddell and ross seas are sites of antarctic bottom water aabw formation the highly barotropic antarctic circumpolar current acc reaches the ocean floor and is able to mix efficiently the north atlantic deep water nadw and deep waters from the indian and pacific oceans the mixture of these deep waters the circumpolar deep water cdw then spreads back into the other ocean basins in one form or another the formation of antarctic intermediate water aaiw by ekman pumping occurs in the convergence zone at the polar front and to the north in the subantarctic zone the northward advection of temperatures and salinities of aabw cdw and aaiw from the southern ocean to other ocean basins has a profound effect on the global ocean circulation and thus on global climate in addition the surface waters of the southern ocean are key in controlling the atmospheric co2 content because they link the atmosphere and the large carbon reservoir of the world deep ocean the southern ocean surface waters have thus been a focus of attention in connection with the investigations of low atmospheric co2 content during the last glacial maximum lgm barnola et al 1987 as well as with the forecasts of future atmospheric pco2 levels e g sarmiento et al 1998 specifically the relatively abundant surface macronutrients of the modern southern ocean provide an opportunity for enhanced biological pump export of surface nutrients and co2 to the deep ocean to reduce the atmos phericco2content martinandfitzwater 1988 also reducingthe vertical mixing of southern ocean surface waters can lower the atmospheric pco2 by limiting the transfer of co2 from the deep ocean to the atmosphere ennever and mcelroy 1985 toggweiler and sarmiento 1985 wenk and siegenthaler 1985 in order to better understand the glacial southern ocean hydrog raphy near the surface and at depth we examine the latitudinal distributions of d 18 o in the calcite test of planktonic foraminifera neogloboquadrina pachyderma and benthic foraminifera cibici doides species their d 18 o reflects the seawater temperature and salinity in which they calcify their shells the planktonic fora miniferal carbon isotope ratios associated with this data set are discussed extensively by kohfeld 1998 also benthic foramini feral carbon isotope ratios in the southern ocean have been discussed recently at length matsumoto and lynch stieglitz 1999 ninnemann and charles 1997 oppo and horowitz 2000 our new data in combination with those in the literature show that the spatial distribution ofd 18 o in planktonic and benthic foraminiferal tests remained nearly the same during the lgm as during the holocene a conclusion similar to that reached by charles and fairbanks 1990 on the basis of a subset of the planktonic foraminiferal d 18 o data shown here the additional work in the southern ocean since that of charles and fairbanks 1990 more strongly constrains the lgm southern ocean hydrog raphy and we also infer that the circulation was similar during the holocene and the lgm

antarctic timing of surface water changes off chile and patagonian ice sheet response marine sediments from the chilean continental margin are used to infer millennial scale changes in southeast pacific surface ocean water properties and patagonian ice sheet extent since the last glacial period our data show a clear antarctic timing of sea surface temperature changes which appear systematically linked to meridional displacements in sea ice westerly winds and the circumpolar current system proxy data for ice sheet changes show a similar pattern as oceanographic variations offshore but reveal a variable glacier response time of up to 1000 years which may explain some of the current discrepancies among terrestrial records in southern south america

disparities in glacial advection of southern ocean intermediate water to the south pacific gyre highlights surface mg ca temperatures and salinities show comparable glacial conditions subsurface mg ca temperatures suggest that lgm is 3 to 2 c colder than mis 6 relative to holocene subsurface salinities suggest fresher lgm and saltier mis 6 thermal gradients suggest stratified lgm and well mixed mis 6 water column contrasting glacial scenarios suggest variable formation and advection of soiws abstract the intermediate waters formed in the southern ocean are critical for ventilating the thermocline in the southern hemisphere gyres and transporting climatic signals from high to low latitudes on glacial interglacial time scales despite the importance of the southern ocean intermediate waters soiws information on past changes in soiws formation is fragmentary and its impact on the south pacific gyre spg s thermocline largely unknown here we present a 200 kyr record of paired mg ca ratios and stable oxygen isotope from surface and deep dwelling planktonic foraminifera from the spg on average the globigerina bulloides mg ca derived sea surface temperatures show similar conditions during the lgm and marine isotope stage mis 6 9 4 c versus 9 8 c in contrast the subsurface temperatures derived from the mg ca values of globorotalia inflata and globorotalia truncatulinoides suggest that lgm is 3 to 2 c colder than mis 6 furthermore at subsurface depths the reconstructed 18osw ivc record proxy for relative local salinity changes suggests opposite glacial conditions with slightly saltier than holocene waters during mis 6 and fresher than holocene waters during lgm contrasting glacial scenarios plausibly due to changes in the presence of soiws at the study site suggest variable formation and or advection of soiws to the spg during different glacial stages the variability in soiws is probably driven by the changes in the intensity of the southern westerly winds

paleoproductivity in the southern peru chile current through the last 33 000 yr abstract based on a multiparameter approach including organic carbon biogenic opal carbonate and the species composition of planktic foraminifera this paper provides the first qualitative assessment of the history of paleoproductivity on glacial interglacial time scales in the southern peru chile current pcc off chile which belongs to the least studied parts of the world ocean during the last glacial maximum lgm highest relative paleoproductivity of the last 33 kyr indicated by high accumulation rates of organic carbon biogenic opal and carbonate has been found contrasted by lowest values during the early and middle holocene this shift from high to low productivity is accompanied by a major change in the species composition of planktic foraminifera from a dominance of neogloboquadrina pachyderma sin to the dominance of n pachyderma dex the temporal pattern of paleoproductivity off chile is very similar to the history of the continental paleoclimate in the region which is supposed to be driven by the position of the southern westerlies this observation points to a functional relationship between the position of the southern westerlies and the history of paleoproductivity off chile assuming that atmospheric and oceanographic circulation are closely linked a northward displacement of the antarctic circumpolar current during the lgm in line with the northward movement of the southern westerlies would bring the main nutrient source closer to our core sites resulting in increased productivity however based on the available data it is not clear if the higher productivity in the southern pcc during the lgm reflects generally higher productivity during this time or only a regional displacement of the main productivity centers

a 70 kyr sea surface temperature record off southern chile ocean drilling program site 1233 1 we present the first high resolution alkenone derived sea surface temperature sst reconstruction in the southeast pacific ocean drilling program site 1233 covering the major part of the last glacial period and the holocene the record shows a clear millennial scale pattern that is very similar to climate fluctuations observed in antarctic ice cores suggesting that the southern hemisphere high latitude climate changes extended into the midlatitudes involving simultaneous changes in air temperatures over antarctica sea ice extent extension of the antarctic circumpolar current and westerly atmospheric circulation a comparison to other midlatitude surface ocean records suggests that this antarctic millennial scale pattern was probably a hemisphere wide phenomenon in addition we performed sst gradient reconstructions over the complete latitudinal range of the pacific eastern boundary current system for different time intervals during the last 70 kyr the main results suggest an equatorward displaced subtropical gyre circulation during marine isotope stages 2 and 4

rapid coastal subsidence in the central ganges brahmaputra delta bangladesh since the 17th century deduced from submerged salt producing kilns the densely populated low lying ganges brahmaputra delta is highly vulnerable to global sea level rise in order to estimate the rate of subsidence of the delta we examined submerged salt producing kiln sites in the coastal sundarbans a huge unesco protected mangrove forest these kilns were built just above the winterly spring high tide level of the time but their bases are currently located 155 cm below the corresponding modern level according to optically stimulated luminescence osl dating the kilns were last fired 300 yr ago and salt production was terminated by a catastrophic event that affected the kiln sites at different levels and locations 14 c ages of charcoal at the kilns bases and associated mangrove stump horizons support the osl dates based on the elevations and ages the 300 yr average rate of sinking of the outer delta is 5 2 1 2 mm yr which includes 0 8 mm yr of eustatic sea level rise with the expectation of further acceleration of sea level rise the already present problematic situation will be aggravated and only prudent control of sediment accretion will keep southern bangladesh above sea level

suboxic trace metal geochemistry in the eastern tropical north pacific abstract we analyzed al ti fe mn cu ba cd u mo v and re in water column settling particulate and sediment 0 to 22 cm samples from the intense oxygen minimum zone omz of the eastern tropical north pacific near mazatlan mexico the goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique suboxic geochemical signature the water column was characterized by a mn maximum reaching 8 nmol kg 1 at 400 m concentrations of cu ba cd mo re u and v were unaffected by the low o2 conditions and were comparable to those of the open ocean sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin al ti and fe were mostly at least 79 lithogenic about 75 of the mn was nonlithogenic significant amounts at least 58 of cu ba cd and mo were nonlithogenic sediment geochemistry varied across the continental shelf and slope cadmium u and re have prominent maxima centered at 310 m with 12 3 ppm 10 9 ppm and 68 3 ppb respectively at the core top high values of mo averaging 6 8 ppm and v averaging 90 ppm are seen in omz surface sediment additional down core enrichment occurs for all redox sensitive elements in the top 10 cm for u mo v and re surface sediments are a poor indicator of metal enrichment comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition especially for cd u and mo we evaluated re mo and cd u ratios as tracers for redox environments rhenium and mo concentrations and re mo ratios do not lead to consistent conclusions concurrent enrichments of re and mo are an indicator of an anoxic depositional environment in contrast high re mo ratios are an indicator of suboxic conditions cadmium is enriched in surface sediments while u has considerable down core enrichment the concentrations of cd and u and the cd u ratio do not follow patterns predicted from thermodynamics though the water column is suboxic these four redox sensitive elements indicate that the sediments are anoxic the implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column

antarctic and southern ocean influences on late pliocene global cooling the influence of antarctica and the southern ocean on late pliocene global climate reconstructions has remained ambiguous due to a lack of well dated antarctic proximal paleoenvironmental records here we present ice sheet sea surface temperature and sea ice reconstructions from the andrill and 1b sediment core recovered from beneath the ross ice shelf we provide evidence for a major expansion of an ice sheet in the ross sea that began at 3 3 ma followed by a coastal sea surface temperature cooling of 2 5 c a stepwise expansion of sea ice and polynya style deep mixing in the ross sea between 3 3 and 2 5 ma the intensification of antarctic cooling resulted in strengthened westerly winds and invigorated ocean circulation the associated northward migration of southern ocean fronts has been linked with reduced atlantic meridional overturning circulation by restricting surface water connectivity between the ocean basins with implications for heat transport to the high latitudes of the north atlantic while our results do not exclude low latitude mechanisms as drivers for pliocene cooling they indicate an additional role played by southern high latitude cooling during development of the bipolar world

intense molybdenum accumulation in sediments underneath a nitrogenous water column and implications for the reconstruction of paleo redox conditions based on molybdenum isotopes the concentration and isotope composition of molybdenum mo in sediments and sedimentary rocks are widely used proxies for anoxic conditions in the water column of paleo marine systems while the mechanisms leading to mo fixation in modern restricted basins with anoxic and sulfidic euxinic conditions are reasonably well constrained few studies have focused on mo cycling in the context of open marine anoxia here we present mo data for water column particulate matter modern surface sediments and a paleo record covering the last 140 000 years from the peruvian continental margin mo concentrations in late holocene and eemian penultimate interglacial shelf sediments off peru range from 70 to 100 g g 1 an extent of mo enrichment that is thought to be indicative of and limited to euxinic systems to investigate if this putative anomaly could be related to the occasional occurrence of sulfidic conditions in the water column overlying the peruvian shelf we compared trace metal mo vanadium uranium enrichments in particulate matter from oxic nitrate reducing nitrogenous and sulfidic water masses coincident enrichments of iron fe oxyhydr oxides and mo in the nitrogenous water column as well as co variation of dissolved fe and mo in the sediment pore water suggest that mo is delivered to the sediment surface by fe oxyhydr oxides most of these precipitate in the anoxic nitrogenous water column due to oxidation of sediment derived dissolved fe with nitrate as a terminal electron acceptor upon reductive dissolution in the surface sediment a fraction of the fe and mo is re precipitated through interaction with pore water sulfide the fe and nitrate dependent mechanism of mo accumulation proposed here is supported by the sedimentary mo isotope composition which is consistent with mo adsorption onto fe oxyhydr oxides trace metal co variation patterns as well as mo and nitrogen isotope systematics suggest that the same mechanism of mo delivery caused the anomalously high interglacial mo accumulation rates in the paleo record our findings suggest that fe and nitrate dependent mo shuttling under nitrogenous conditions needs to be considered a possible reason for sedimentary mo enrichments during past periods of widespread anoxia in the open ocean

the impact of ocean deoxygenation on iron release from continental margin sediments in the oceans high nitrate low chlorophyll regions such as the peru humboldt current system and the adjacent eastern equatorial pacific1 primary productivity is limited by the micronutrient iron within the peruvian upwelling area bioavailable iron is released from the reducing continental margin sediments2 the magnitude of this seafloor source could change with fluctuations in the extension or intensity of the oxygen minimum zones3 4 here we show that measurements of molybdenum uranium and iron concentrations can be used as a proxy for sedimentary iron release and use this proxy to assess iron release from the sea floor beneath the peru upwelling system during the past 140 000 years we observe a coupling between levels of denitrification as indicated by nitrogen isotopes trace metal proxies for oxygenation and sedimentary iron concentrations specifically periods with poor upper ocean oxygenation are characterized by more efficient iron retention in the sediment and a diminished iron supply to the water column we attribute efficient iron retention under more reducing conditions to widespread sulphidic conditions in the surface sediment and concomitant precipitation of iron sulphides we argue that iron release from continental margin sediments is most effective in a narrow redox window where neither oxygen nor sulphide is present we therefore suggest that future deoxygenation in the peru upwelling area would be unlikely to result in increased iron availability whereas in weaker oxygen minimum zones partial deoxygenation may enhance the iron supply

carbon isotope evidence for changes in antarctic intermediate water circulation and ocean ventilation in the southwest pacific during the last deglaciation 1 deep sea sediment core fr1 97 gc 12 is located 990 mbsl in the northern tasman sea southwest pacific where antarctic intermediate water aaiw presently impinges the continental slope of the southern great barrier reef analysis of carbon 13c and oxygen 18o isotope ratios on a suite of planktonic and benthic foraminifera reveals rapid changes in surface and intermediate water circulation over the last 30 kyr during the last glacial maximum there was a large 13c offset 1 1 between the surface dwelling planktonic foraminifera and benthic species living within the aaiw in contrast during the last deglaciation termination 1 the 13cplanktonic benthic offset reduced to 0 4 prior to an intermediate offset 0 7 during the holocene we suggest that variations in the dominance and direction of aaiw circulation in the tasman sea and increased oceanic ventilation can account for the rapid change in the water column 13cplanktonic benthic offset during the glacial interglacial transition our results support the hypothesis that intermediate water plays an important role in propagating climatic changes from the polar regions to the tropics in this case climatic variations in the southern hemisphere may have led to the rapid ventilation of deep water and aaiw during termination 1 which contributed to the postglacial rise in atmospheric co2

controls on facies distribution and stratigraphic preservation in the ganges brahmaputra delta sequence abundant sediment supply and accommodation space in the bengal basin have led to the development of a major late quaternary delta sequence this sequence has formed in a tectonically active setting and represents an important example of a high energy marine and fluvial high yield continental margin deposit recent studies have detailed the delta s stratigraphy and development noting that tectonics and sediment supply control the ganges brahmaputra more significantly than in many other delta systems these ideas are developed here through a discussion of the effects that spatial and temporal variations in tectonics and sediment supply have had on deltaic processes and sequence character unique and differing stratigraphies are found within thedeltasystem suchthatfine grainedsedimentpreservationisfavoredinareasofactivetectonicprocessessuchasfolding block faulting and subsidence coarse grained deposits dominate the stratigraphy under the control of high energy fluvial processes and mixed fine coarse stratigraphies are found in areas dominantly influenced by eustatic sea level change overlaid upon these spatially varying stratigraphic patterns are temporal patterns related to episodic events e g earthquakes and rivers avulsions and long termchangesinclimate andsediment supply modelingisalsousedtoinvestigatethe influenceofavariable sedimentsupply on sequence character results show that the timing and magnitude of sediment input relative to sea level rise is a significant control on the subaerial extent of the delta and the relative dominance of alluvial and marine facies within the sequence d 2002 elsevier science b v all rights reserved

glacial interglacial changes in nutrient supply and stratification in the western subarctic north pacific since the penultimate glacial maximum abstract in piston cores from the open subarctic pacific and the okhotsk sea diatom bound 15n 15ndb biogenic opal calcium carbonate and barium were measured from coretop to the previous glacial maximum mis 6 glacial intervals are generally characterized by high 15ndb 8 and low productivity whereas interglacial intervals have a lower 15ndb 5 7 6 3 and indicate high biogenic productivity these data extend the regional swath of evidence for nearly complete surface nutrient utilization during glacial maxima consistent with stronger upper water column stratification throughout the subarctic region during colder intervals an early deglacial decline in 15ndb of 2 at 17 5 ka previously observed in the bering sea is found here in the open subarctic pacific record and arguably also in the okhotsk and a case can be made that a similar decrease in 15ndb occurred in both regions at the previous deglaciation as well the early deglacial 15ndb decrease best explained by a decrease in surface nutrient utilization appears synchronous with southern hemisphere associated deglacial changes and with the heinrich 1 event in the north atlantic this 15ndb decrease may signal the initial deglacial weakening in subarctic north pacific stratification and or a deglacial increase in shallow subsurface nitrate concentration if the former it would be the north pacific analogue to the increase in vertical exchange inferred for the southern ocean at the time of heinrich event 1 in either case the lack of any clear change in paleoproductivity proxies during this interval would seem to require an early deglacial decrease in the iron to nitrate ratio of subsurface nutrient supply or the predominance of light limitation of phytoplankton growth during the deglaciation prior to bolling allerod warming

glacial differences of southern ocean intermediate waters in the central south pacific abstract southern ocean intermediate waters soiws such as antarctic intermediate water and subantarctic mode water play a key role in modulating the global climate on glacial interglacial time scales they link the southern ocean and the tropics via mechanisms such as oceanic tunneling that transport climatic signals across latitudes despite their importance the past evolution of the soiws in the central south pacific is largely unknown here we compare paired mg ca temperature stable carbon 13c and oxygen 18o isotope records from surface dwelling and deep dwelling planktic foraminifera to infer changes in the water column structure for the last 260 ka in the central south pacific 54 s our study focuses on the subsurface oceanographic variability controlled by soiws which are subducted at the subantarctic front our data show that the subsurface ocean in the central south pacific was colder and fresher during glacial stages than during the holocene 0 10 ka bp suggesting a general glacial enhanced presence of antarctic intermediate water in agreement with previous studies from the eastern equatorial pacific and the southeast pacific however the subsurface ocean salinity differs for both glacial stages with fresher condition during the last glacial maximum lgm 26 5 19 ka bp and more saline condition during the penultimate glacial maximum pgm 155 140 ka bp the 13c data also show contrasting conditions for both glacial time windows in the upper water column with a large depletion of 0 37 in 13c from the lgm values suggests a larger contribution of old low 13c deep waters at intermediate depths at the study site during the pgm plausibly due to stronger upwelling in high southern latitudes the dissimilar scenarios between the lgm and the pgm may have been caused by processes that are analogous to the phase switch in modern day southern annular mode

centennial to millennial scale changes in oxygenation and productivity in the eastern tropical south pacific during the last 25 000 years oxygen minimum zones omz have expanded in all tropical oceans during the last 50 years resulting in habitat contraction and considerable changes in marine biogeochemistry however for a better understanding of the omz dynamics under the current climate change two questions are relevant 1 how do the magnitude and temporal changes in oceanic dissolved oxygen of the last few decades compare to the natural variability on longer timescales and 2 what were the local and remote factors driving omz changes in the past in the present study we use a stacked record covering the last 25 kyr from the eastern tropical south pacific etsp omz to reconstruct changes in oxygenation and productivity we use a suite of proxies including the presence of laminations redox sensitive metals u mo re ni and cu total organic carbon and 15 n measurements water column denitrification and sediment redox conditions show pronounced centennial to millennial scale variability during the last 25 kyr with oxygenation levels as low as at present global cold periods at different timescales such as the last glacial maximum 23 19 kyr bp and the little ice age 1500 1850 ad were associated with a weak omz and low export production while warm intervals such as the deglaciation part of the medieval climate anomaly and the last 100 years are associated with a stronger omz and high export production water column denitrification and sediment redox conditions were strongly coupled during the last 25 kyr bp apart from one remarkable exception during the antarctic cold reversal sediments were less reducing but the water column denitrification was high resulting in a strong but shallow omz this may have been produced by an enhanced antarctic intermediate water flow contrary to our expectations and modeling predictions for the next few decades we observe a weak etsp omz during the warm mid holocene which may have been the result of a stronger walker circulation that brought oxygen rich waters to intermediate depths off peru via equatorial undercurrents in combination with other paleoceanographic reconstructions our results show that oxygenation variability in the etsp omz was influenced by ocean circulation changes in the tropical pacific high latitude oceanographic and climatic changes and local productivity

large climate driven changes of oceanic oxygen concentrations during the last deglaciation during the last glacial termination the solubility of gases in the ocean decreased as ocean temperatures rose however marine sediments have not unanimously recorded ocean deoxygenation throughout this time some records show increasing oxygenation since the last glacial maximum particularly in the deep sea while many document abrupt oxygenation changes often associated with apparent changes in the formation rate of north atlantic deep water here we present a global compilation of marine sediment proxy records that reveals remarkable coherency between regional oxygenation changes throughout deglaciation the upper ocean generally became less oxygenated but this general trend included pauses and even reversals reflecting changes in nutrient supply respiration rates and ventilation the most pronounced deoxygenation episode in the upper ocean occurred midway through the deglaciation associated with a reinvigoration of north atlantic deep water formation at this time the upper indo pacific ocean was less oxygenated than today meanwhile the bulk of the deep ocean became more oxygenated over the deglaciation reflecting a transfer of respired carbon to the atmosphere the observed divergence from a simple solubility control emphasizes the degree to which oxygen consumption patterns can be altered by changes in ocean circulation and marine ecosystems

radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230 th 234 u 238 u and 14 c dates on pristine corals abstract radiocarbon dating is the most widely used dating technique in the world recent advances in accelerator mass spectrometry ams and sample preparation techniques have reduced the sample size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes today it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates the production of 14 c in the atmosphere varies through time due to changes in the earth s geomagnetic field intensity and in its concentration which is regulated by the carbon cycle as a result of these two variables a radiocarbon age is not equivalent to a calendar age four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12 000 calendar years before present this paper presents high precision paired 230 th 234 u 238 u and 14 c age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12 000 to 50 000 years before present we developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years taking full account of combined errors in input ages and calibration uncertainties our radiocarbon calibration program is publicly accessible at http www radiocarbon ldeo columbia edu along with full documentation of the samples data and our statistical calibration model

movement of deep sea coral populations on climatic timescales during the past 40 000 years global climate has moved into and out of a full glacial period with the deglaciation marked by several millennial scale rapid climate change events here we investigate the ecological response of deep sea coral communities to both glaciation and these rapid climate change events we find that the deep sea coral populations of desmophyllum dianthus in both the north atlantic and the tasmanian seamounts expand at times of rapid climate change however during the more stable last glacial maximum the coral population globally retreats to a more restricted depth range holocene populations show regional patterns that provide some insight into what causes these dramatic changes in population structure the most important factors are likely responses to climatically driven changes in productivity o 2 and co 3 2

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

changes in the intermediate water mass formation rates in the global ocean for the last glacial maximum mid holocene and pre industrial climates 1 the paleoclimate version of the national center for atmospheric research community climate system model version 3 ncar ccsm3 is used to analyze changes in the water formation rates in the atlantic pacific and indian oceans for the last glacial maximum lgm mid holocene mh and pre industrial pi control climate during the mh ccsm3 exhibits a north south asymmetric response of intermediate water subduction changes in the atlantic ocean with a reduction of 2 sv in the north atlantic and an increase of 2 sv in the south atlantic relative to pi during the lgm there is increased formation of intermediate water and a more stagnant deep ocean in the north pacific the production of north atlantic deep water nadw is significantly weakened the nadw is replaced in large extent by enhanced antarctic intermediate water aaiw glacial north atlantic intermediate water gnaiw and also by an intensified of antarctic bottom water aabw with the latter being a response to the enhanced salinity and ice formation around antarctica most of the lgm intermediate mode water is formed at 27 4 29 0 kg m3 while for the mh and pi most of the subduction transport occurs at 26 5 27 4 kg m3 the simulated lgm southern hemisphere winds are more intense by 0 2 0 4 dyne cm2 consequently increased ekman transport drives the production of intermediate water low salinity at a larger rate and at higher densities when compared to the other climatic periods

impact of climate change on the magnetic mineral assemblage in marine sediments from izu rear arc nw pacific ocean over the last 1 myr abstract a rock magnetic study was conducted on upper pleistocene marine sediments from international ocean discovery program expedition 350 site u1437 in order to highlight the paleoenvironmental changes in the nw pacific ocean influenced by the east asian monsoon regime remanent magnetization analyses hysteresis properties first order reversal curves and low temperature magnetic measurements were carried out along with electron microscope observations the results indicate that coarse grained titano magnetite is the dominant magnetic phase in the magnetic mineral assemblage time series analysis supports that this assemblage is modulated by global climate changes over the last 1 myr during the interglacial stages magnetic minerals are more abundant and are dominated by coarse grained titano magnetite of both terrigenous likely from mainland china and volcanic izu arc front japan origin during the glacial stages the magnetic mineral content is lower probably reflecting partial dissolution of ti magnetite and the magnetic assemblage is composed of terrigenous coarse grained titano magnetite and of higher coercivity presumably finer eolian particles likely hematite as a result of the enhancement of the winter monsoon in continental asia the magnetic mineral assemblage reflects a superimposition of volcanic and global climate signals bulk organic geochemical analyses for total and organic carbon as well as nitrogen and sulfur contents confirm a climatic signature in the composition of the sediments with more oxygenated water masses being present during glacial periods additional x ray fluorescence measurements on bulk samples indicate various origins of the sediment particles with both proximal and distal sources

the acceleration of oceanic denitrification during deglacial warming the marine nitrogen cycle was altered during the transition from glacial to interglacial conditions an analysis of 15n records throughout the world s oceans suggests that rates of denitrification in the water column accelerated during the last deglaciation

revision of the global carbon budget due to changing air sea oxygen fluxes 1 carbon budgets inferred from measurements of the atmospheric oxygen to nitrogen ratio o2 n2 are revised considering sea to air fluxes of o2 and n2 in response to global warming and volcanic eruptions observational estimates of changes in ocean heat content are combined with a model derived relationship between changes in atmospheric o2 n2 due to oceanic outgassing and heat fluxes to estimate ocean o2 outgassing the inferred terrestrial carbon sink for the 1990s is reduced by a factor of two compared with the most recent estimate by the intergovernmental panel on climate change ipcc this also improves the agreement between calculated ocean carbon uptake rates and estimates from global carbon cycle models which indicate a higher ocean carbon uptake during the 1990s than the 1980s the simulated decrease in oceanic o2 concentrations is in qualitative agreement with observed trends in oceanic o2 concentrations