subtropical atlantic salinity variability and atlantic meridional circulation during the last deglaciation during the last deglaciation ca 21 10 ka freshening of the north atlantic surface likely caused reductions in atlantic meridional overturning circulation amoc the mechanisms related to amoc recovery remain poorly understood here we present three new deglacial surface temperature and 18oseawater 18osw reconstructions from the western subtropical north and south atlantic similarities to tropical caribbean and western atlantic 18osw records suggest that a salty surface water mass accumulated in the western atlantic from 27 s to 33 n during periods of reduced amoc however 18osw decreases led deep amoc resumption by hundreds of years we suggest that the northward export of salt previously trapped in the western atlantic resulted in the early establishment of a shallow overturning circulation that eventually culminated in deep amoc resumption implying that amoc may constitute a self limiting system

the u series toolbox for paleoceanography the geochemistry of marine sediments is a major source of information about the past environment of the many measurements that provide such information those of the u series nuclides are unusual in that they inform us about the rate and timescales of processes oceanic processes such as sedimentation productivity and circulation typically occur on timescales too short to be assessed using parent daughter isotope systems such as rb sr or sm nd so the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides principally 14c or the u series nuclides this makes the u series nuclides powerful allies in the quest to understand the past ocean climate system and has led to their widespread application over the last decade as in other applications of the u series those in paleoceanography rely on fractionation of the nuclides away from secular equilibrium in the oceanic setting this fractionation is generally due to differences in the solubility of the various nuclides the behavior of the u series nuclides in the ocean environment was widely researched in the middle decades of the twentieth century this work established knowledge of the concentrations of the nuclides in the various compartments of the ocean system and of their fluxes between these compartments such understanding was comprehensively summarized in the ivanovich and harmon u series volume 1992 particularly by cochran 1992 understanding of the behavior of the u series nuclides has not advanced very dramatically in the decade since that summary but a major theme of research has been the use of this geochemical understanding to develop u series tools to assess the past environment table 1 view this table table 1 summary of seawater data for u series nuclides with paleoceanographic applications s soluble i insoluble full descriptions of the paleoceanographic uses and references are provided in the text further details of the half

231pa 230th fractionation by ocean transport biogenic particle flux and particle type 231pa and 230th are removed from the water column by a process of reversible scavenging which quickly removes 230th to the sediment 231pa is less efficiently scavenged onto particles than 230th and is therefore more effectively transported via advection and diffusion before it reaches the ocean sediment this study combines particle fields dust opal caco3 poc derived from observations with the bern3d intermediate complexity ocean model and an equilibrium scavenging model for isotopes the equilibrium partition coefficient for particulate versus dissolved isotope activity is varied with particle type the model can explain many of the features of the global 231pa and 230th distribution the success of such a simple model at representing the global pattern of 231pa 230th activity ratio supports the use of this proxy in paleoceanographic studies we use the model to address the controversy concerning which particle types are dominant in fractionating 231pa 230th in the ocean the lithogenic dust flux is found to be unimportant for 231pa 230th fractionation the ocean fractionation of 231pa 230th is dominated by the distribution of the caco3 and opal flux we also confirm that opal is a weak scavenger of 230th

constant bottom water flow into the indian ocean for the past 140 ka indicated by sediment 231pa 230th ratios a down core 231pa 230th record has been measured from the southwestern indian ocean to reconstruct the history of deep water flow into this basin over the last glacial interglacial cycle the 231paxs 230thxs 0 ratio throughout the record is nearly constant at approximately 0 055 significantly lower than the production ratio of 0 093 indicating that the proxy is sensitive to changes in circulation and or sediment flux at this site the consistent value suggests that there has been no change in the inflow of antarctic bottom water to the indian ocean during the last 140 ka in contrast to the changes in deep circulation thought to occur in other ocean basins the stability of the 231paxs 230thxs 0 value in the record contrasts with an existing sortable silt ss record from the same core the observed ss variability is attributed to a local geostrophic effect amplifying small changes in circulation a record of authigenic u from the same core suggests that there was reduced oxygen in bottom waters at the core locality during glacial periods the consistency of the 231paxs 230thxs 0 record implies that this could not have arisen by local changes in productivity thus suggesting a far field control either globally reduced bottom water oxygenation or increased productivity south of the opal belt during glacials

boundary scavenging at the east atlantic margin does not negate use of 231pa 230th to trace atlantic overturning abstract the 231 pa 230 th method is a promising tool to reconstruct ocean circulation over the past glacial interglacial cycle however marine particle flux may constrain the applicability of this ratio as a direct quantitative proxy for the strength of atlantic meridional overturning circulation amoc by influencing the fractionation between the in situ produced 231 pa and 230 th in ocean water here we present 231 pa 230 th down core profiles from high particle flux areas off namibia and senegal covering the past 35 ka the 231 pa 230 th profiles at these sites show very different responses to temporal variations of particle fluxes and to changes in water masses our results show that sedimentary 231 pa 230 th in the eastern atlantic margin is linked to particle flux but controlled primarily by the mode of the amoc our data suggest that during the past 30 ka the high productivity eastern margin was not capable of importing and storing significant amounts of 231 pa from the open ocean consequently the applicability of the 231 pa 230 th proxy to reconstruct past ocean circulation is not hampered by this potentially additional 231 pa sink

quantifying export production in the southern ocean implications for the ba xs proxy the water column and sedimentary baxs distribution around the crozet plateau is used to decipher the controls and timing of barite formation and to evaluate how export production signals are recorded in sediments underlying a region of natural fe fertilization within the fe limited southern ocean export production estimated from preserved vertical sedimentary baxs accumulation rates are compared with published export fluxes assessed from an integrated study of the biological carbon pump to determine the validity of baxs as a quantitative proxy under different fe supply conditions typical of the southern ocean detailed assessment of the geochemical partitioning of ba in sediments and the lithogenic end member allows appropriate correction of the bulk ba content and determination of the baxs content of sediments and suspended particles the upper water column distribution of baxs is extremely heterogeneous spatially and temporally organic carbon baxs ratios in deep traps from the fe fertilized region are similar to other oceanic settings allowing quantification of the inferred carbon export based on established algorithms there appears to be some decoupling of poc and ba export in the fe limited region south of the plateau the export production across the crozet plateau inferred from the baxs sedimentary proxy indicates that the fe fertilized area to the north of the plateau experiences enhanced export relative to equivalent southern ocean settings throughout the holocene and that this influence may also have impacted the site to the south for significant periods this interpretation is corroborated by alternative productivity proxies opal accumulation 231paxs 230thxs baxs can be used to quantify export production in complex settings such as naturally fe fertilized volcanoclastic areas providing appropriate lithogenic correction is undertaken and sediment focusing is corrected for along with evaluation of barite preservation

influence of particle size and type on 231pa and 230th simulation with a global coupled biogeochemical ocean general circulation model a first approach 1 the oceanic distributions of 231pa and 230th are simulated with the global coupled biogeochemical ocean general circulation model nemo pisces these natural nonconservative tracers which are removed from the water column by reversible scavenging processes onto particles have been used to study modern and past ocean circulation our model includes three different types of particles particulate organic matter pom calcium carbonate caco3 and biogenic silica bsi it also considers two particle classes small particles pom that sink slowly 3 m d and large particles pom caco3 bsi that sink much more rapidly 50 m d to 200 m d in the water column 231pa and 230th are simulated with a reversible scavenging model that uses partition coefficients between dissolved and particulate phases that depend on particle type and size model results are then compared with 231pa and 230th observations in the water column and modern sediments a preliminary evaluation of the particle fields simulated by the pisces model has revealed that particle concentrations are reasonable at the surface but largely underestimated in the deep ocean largely to compensate for this we find it necessary to use partition coefficients that vary as a function of particle size by significantly more that observed to obtain relatively realistic results in the water column 231pa and 230th fluxes are mainly controlled by the slowly sinking particles and partition coefficients need to be parameterized as a function of particle flux as suggested by observations considering discrepancies between the modeling particle fields and those observed we were forced to use exaggerated values for partition coefficients in order to get realistic tracer distributions these 231pa and 230th simulations have provided an opportunity to propose some future developments of the pisces model in order to make progress in the simulation of trace elements assigning calcium carbonate biogenic silica and aluminosilicates to the small particle pool represents a credible approach to increase its concentration and subsequently simulate realistic tracer distributions in the water column using reasonable values for the partition coefficients as well as a realistic fractionation in the sediments at all depths

measurement of adsorbed and total 232th 230th ratios from marine sediments the predictable in situ production of 230th from the decay of uranium in seawater and its subsequent removal by scavenging onto falling particles provides a valuable tool for normalizing fluxes to the seafloor we describe a new application determination of the 232th that dissolves in the water column and is removed to the seafloor 232th is supplied to the ocean in continental minerals dissolution of which leads to a measurable standing stock in the water column sedimentary adsorbed 232th 230th ratios have the potential to provide a proxy for estimating the amount of dissolved material that enters the ocean both today and in the past ten core top samples were treated with up to eight different leaching techniques in order to determine the best method for the separating adsorbed from lattice bound thorium in addition separate components of the sediments were analyzed to test whether clay dissolution was an important contribution to the final measurement there was no systematic correlation between the strength of acid used in the leach and the measured 232th 230th ratios in all cases clean foraminifera produced the same ratio as leaches on bulk sediment in three out of five samples leaches performed on non carbonate detritus in the 63 mu m size fraction were also identical without additional water column data it is not yet clear whether there is a simple one to one correlation between the expected deep water 232th 230th and that produced by leaching especially in carbonate rich sediments however higher ratios and associated high 232th adsorbed fluxes were observed in areas with high expected detrital inputs the adsorbed fraction was 35 50 of the total 232th in seven out of ten samples our 230th normalized 232th fluxes are reasonable by comparison to global estimates of detrital inputs to the ocean in nine cases out of ten the total 230th normalized 232th flux is greater than predicted from the annual dust fall at each specific location but lower than the average global detrital input from all sources

modeling the relationship between 231pa 230th distribution in north atlantic sediment and atlantic meridional overturning circulation down core variations in north atlantic 231paxs 230thxs have been interpreted as changes in the strength of the atlantic meridional overturning circulation amoc this modeling study confirms that hypothetical changes in the amoc would indeed be recorded as changes in the distribution of sedimentary 231paxs 230thxs at different sites in the north atlantic the changes in sedimentary 231pa 230th that we simulate are diverse and do not reflect a simple tendency for 231paxs 230thxs to increase toward the production ratio 0 093 when the amoc strength reduces but instead are moderated by the particle flux in its collapsed or reduced state the amoc does not remove 231pa from the north atlantic instead 231pa is scavenged to the north atlantic sediment in areas of high particle flux in this way the north atlantic 231paxs 230thxs during amoc shutdown follows the same pattern as 231paxs 230thxs in modern ocean basins with reduced rates of meridional overturning i e pacific or indian oceans we suggest that mapping the spatial distribution of 231paxs 230thxs across several key points in the north atlantic is an achievable and practical qualitative indicator of the amoc strength in the short term our results indicate that additional north atlantic sites where down core observations of 231paxs 230thxs would be useful coincide with locations which were maxima in the vertical particle flux during these periods reliable estimates of the north atlantic mean 231paxs 230thxs should remain a goal in the longer term our results hint at a possible seesaw like behavior in 231pa 230th in the south atlantic

heat sources for tertiary metamorphism and anatexis in the annapurna manaslu region central nepal following the collision of india with southern tibet crustal rocks of the leading edge of india 1 underwent regional metamorphism to upper amphibolite grade 2 melted locally to produce anatectic granitoids and 3 were sheared and thrust onto lower grade rock along the main central thrust yielding an inverted metamorphic sequence this sequence is exemplified in the annapurna manaslu region we use simple physical calculations to examine the heat sources involved in the different phases of metamorphism the regional metamorphism apparently is due to the burial of the northern edge of india beneath the accretionary prism along the southern edge of tibet the observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation during the 10 30 m y before slip on the main central thrust began of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mw m 2 the release of water from the footwall of the main central thrust apparently facilitated melting of the overlying crust in the second phase such melting could have occurred in the first million years or so of thrusting if warm 550 650 c crust in the footwall contained the necessary water if melting did not occur in the earliest stages of slip on the main central thrust dissipative heating with shear stresses of 10 to 100 mpa is required for temperatures near the main central thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material the thickness 6 8 km of the zone of inverted isograds associated with the fault if undisrupted and due solely to thermal diffusion implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30 40 km was 4 8 m y the apparent inverted temperature gradients about 10 25 c km in this zone can be understood as the combined result of two processes diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10 20 c km the peak temperatures in such a zone however would not have exceeded about 350 c without an additional source of heat dissipative heating at shear stresses of about 100 mpa can account for peak temperatures in excess of 600 c during slip on the fault and would have contributed as much as 13 c km to the inverse gradient although inversion of metamorphic isograds could have occurred as a result of deformation within the main central thrust zone the high temperatures during slip on this zone still require dissipative heating unless the duration of slip exceeded 25 m y

230th and 232th distributions in mid latitudes of the north pacific ocean effect of bottom scavenging abstract vertical distributions of total 230 th and dissolved 232 th were determined in the mid latitudes of the north pacific ocean 8 n 95 w 40 n 160 e the observed 232 th concentrations are some of the lowest values reported for oceans because of low biogenic particle flux high 230 th activity is exhibited in the central gyre most vertical distributions of 230 th in the pacific ocean have been described well using the reversible scavenging model however we observed the depletion of total 230 th compared with a reversible scavenging model prediction for the deep layers in the mid latitudes of the north pacific ocean considering the time scale of the scavenging residence time of th horizontal eddy diffusion from the higher particle flux region could not have a major influence on 230 th depletion in deep waters the observed depletion is partly explained by lateral advection adopting the scavenging mixing model additionally a bottom scavenging process above the seafloor and a diffusion process are considered to explain the 230 th deficit taking into account the time scale of the scavenging residence time of th in deep waters 230 th in deep water to the upper layers

comment on glacial interglacial circulation changes inferred from 231pa 230th sedimentary record in the north atlantic region by j m gherardi et al

changes in opal flux and the rain ratio during the last 50 000 years in the equatorial pacific abstract changes in the orgc caco3 ratio in particles sinking from the surface to the deep ocean have the potential to alter the atmospheric pco2 over the span of a glacial interglacial cycle recent paleoceanographic and modern observational studies suggest that silica is a key factor in the global carbon biogeochemical cycle that can influence the flux ratio especially at low latitudes through silicic acid leakage brzezinski m pride c franck m sigman d sarmiento j matsumoto k gruber n rau r coale k 2002 a switch from si oh 4 to no 3 depletion in the glacial southern ocean geophysical research letters 29 5 to test this hypothesis we reconstruct biogenic fluxes of caco3 orgc and si for three equatorial pacific cores we find evidence that a floral shift from a sio2 based community to a caco3 based occurred starting in mid marine isotope stage mis 3 24 59 cal ka and declining toward mis 2 19 24 cal ka this could reflect the connection of the peru upwelling system to the subantarctic region and we postulate that excess silica was transported from the subantarctic via the deep equatorial undercurrent to the eastern equatorial pacific in the eastern equatorial pacific only we document a significant decrease in rain ratio starting mid mis 3 toward mis 2 this decrease is concomitant with a significant decrease in silica accumulation rates at the seabed this pattern is not observed in the pacific influenced by equatorial divergence and shallow upwelling where all reconstructed fluxes caco3 orgc and opal increase during mis 2 we conclude that the overall calcium carbonate pump weakened in the eep under peru upwelling influence

the influence of particle composition and particle flux on scavenging of th pa and be in the ocean we have examined the relative affinity of th pa and be for sorption from seawater onto particles of variable composition opal carbonate lithogenic particles and organic carbon nuclide concentrations in particles collected from time series sediment traps were normalized by the dissolved nuclide concentration in the overlying water column in order to compute partition coefficients under conditions spanning a wide range of particle flux and particle composition our results suggest that the affinity of particles for pa and be increases with their increasing opal content and decreasing carbonate content while the affinity of particles for th increases with increasing carbonate content and decreases with increasing opal content we find no correlation between the aluminosilicate content of particles and their affinity for scavenging of any of these elements extrapolating to a pure caco3 end member the partition coefficient for th 9 0 106 g g 1 is 40 times greater than for pa and roughly 100 times greater than for be whereas for a pure opal end member the partition coefficient for th 3 9 105 g g 1 is slightly less than that for pa and be partition coefficients decrease with increasing particle flux in open ocean settings but not in an ocean margin region this kinetic effect reflects the increasing contribution of unaltered surface material reaching the sediment traps as particle flux increases the degree of fractionation between pa and th and between be and th depends on the opal carbonate rain ratio these results challenge the use of sedimentary 231pa 230th and 10be 230th ratios as simple proxies of particle flux however the strong dependence of nuclide scavenging on the opal carbonate rain ratio may provide a needed tool for reconstructing past changes in planktonic community composition

similar rates of modern and last glacial ocean thermohaline circulation inferred from radiochemical data today the ocean thermohaline circulation transports half of the 231pa produced by radioactive decay in the atlantic ocean water column to the southern ocean this export respectively imparts low and high 231pa 230th ratios to the surface sediments of these oceans ocean sediments from the last glacial maximum bear a similar isotopic fingerprint implying that advection of north atlantic deep intermediate water into the circumpolar deep water of the southern ocean occurred at a similar or slightly higher rate during the last glacial period

controversy over the relationship between major components of sediment trap materials and the bulk distribution coefficients of 230th 231pa and 10be abstract a four end member mixing model is introduced to estimate the distribution coefficients of 230th 231pa and 10be for pure end members lithogenic clays organic carbon carbonates and opal in sediment trap materials from three oceanic regions four pure end members contribute differently to the fractionation of nuclides in three regions contribution of organic matter to the bulk distribution coefficients cannot be ignored

deep water provenance and dynamics of the de glacial atlantic meridional overturning circulation abstract reconstructing past modes of ocean circulation is an essential task in paleoclimatology and paleoceanography to this end we combine two sedimentary proxies nd isotopes e nd and the 231pa 230th ratio both of which are not directly involved in the global carbon cycle but allow the reconstruction of water mass provenance and provide information about the past strength of overturning circulation respectively in this study combined 231pa 230th and e nd down core profiles from six atlantic ocean sediment cores are presented the data set is complemented by the two available combined data sets from the literature from this we derive a comprehensive picture of spatial and temporal patterns and the dynamic changes of the atlantic meridional overturning circulation over the past 25 ka our results provide evidence for a consistent pattern of glacial stadial advances of southern sourced water along with a northward circulation mode for all cores in the deeper 3000 m atlantic results from shallower core sites support an active overturning cell of shoaled northern sourced water during the lgm and the subsequent deglaciation furthermore we report evidence for a short lived period of intensified amoc in the early holocene

biogeography in 231pa 230th ratios and a balanced 231pa budget for the pacific ocean the ratio of unsupported protactinium 231 to thorium 230 in marine sediments pa th xs is potentially sensitive to several processes of oceanographic and climatological interest deep ocean circulation marine biological productivity as it relates to total particle flux and particle composition specifically biogenic opal and authigenic mn in order to attribute variations in pa th xs observed in sediment records to changes in specific processes through time a better understanding of the chemical cycling of these elements in the modern ocean is necessary to this end a survey was undertaken of pa th xs in surface sediments from the subarctic pacific so202 inopex expedition in combination with a pacific wide compilation of published data throughout the pacific pa th xs is robustly correlated with the opal content of sediments in the north and equatorial pacific simultaneous positive correlations with productivity indicators suggest that boundary scavenging and opal scavenging combine to enhance the removal of pa in the eastern equatorial pacific and subarctic pacific deep ocean water mass ageing 3 5 km 3 5 km associated with the pacific overturning appears to play a secondary role in determining the basin scale distribution of pa th xs a basin wide extrapolation of pa removal is performed which suggests that the pacific pa budget is nearly in balance we hypothesize that through time pa th xs distributions in the pacific could define the evolving boundaries of contrasting biogeographic provinces in the north pacific while the influence of hydrothermal scavenging of pa potentially confounds this approach in the south pacific

indian ocean circulation and productivity during the last glacial cycle abstract the indian ocean is an important part of the global thermohaline circulation system receiving deep waters sourced from the southern ocean and being the location of upwelling and surface ocean current flow which returns warm and salty waters to the atlantic it is also an ideal location to reconstruct the link between thermohaline circulation and deep water nutrient contents no mixing occurs between major deep water masses along flow paths within the indian ocean so changes in water mass provenance reflect changes in deep ocean circulation while nutrient contents reflect addition and dissolution of organic matter we present neodymium nd and carbon c isotope records from an equatorial indian ocean core sk129 cr2 spanning the last 150 kyr the nd isotope record shows that an increased proportion of north atlantic deep water nadw reached the indian ocean during interglacials marine isotope stages mis 1 and 5 and a reduced proportion during glacials mis 2 4 and 6 and also that changes occurred during mis 3 the magnitude and timing of deglacial and some mis 3 variability is very similar to those in the rc11 83 tno57 21 south atlantic deep cape basin nd isotope record piotrowski et al 2005 since the deep cape basin is oceanographically upstream of the indian ocean from a deep water circulation perspective the nd isotopes suggest that atlantic meridional overturning circulation changes were effectively propagated from the southeastern atlantic into the central indian ocean via the southern ocean comparison of the nd and c isotope records shows that deep ocean circulation was decoupled from nutrient content changes on glacial interglacial timescales in particular suggesting higher productivity during mis 5 increased equatorial productivity during mis 5 is supported by benthic infaunal epifaunal foraminiferal 13 c gradients as well as benthic foraminiferal 13 c gradients along deep water flow concurrent warming indicated by planktonic foraminiferal mg ca during mis 5 at the site is consistent with changing thermocline temperature and may indicate a link to surface ocean hydrographic changes

13c record of benthic foraminifera in the last interglacial ocean implications for the carbon cycle and the global deep water circulation the 13c 12c ratios of upper holocene benthic foraminiferal tests genera cibicides and uvigerina of deep sea cores from the various world ocean basins have been compared with those of the modern total carbon dioxide tco2 measured during the geosecs program the 13c difference between benthic foraminifera and tco2 is 0 07 0 04 for cibicides and 0 83 0 07 for uvigerina at the 95 confidence level 13c analyses of the benthic foraminifera that lived during the last interglaciation isotopic substage 5e about 120 000 yr ago show that the bulk of the tco2 in the world ocean had a 13c value 0 15 0 12 lower than the modern one at the 95 confidence level reflecting a depletion compared to the present value of the global organic carbon reservoir regional differences in 13c between the various oceanic basins are explained by a pattern of deep water circulation different from the modern one the antarctic bottom water production was higher than today during the last interglaciation but the eastward transport in the circumpolar deep water was lower

paired carbon isotope and cadmium data from benthic foraminifera implications for changes in oceanic phosphorus oceanic circulation and atmospheric carbon dioxide abstract the carbon isotope and cadmium content of foraminiferal shells from oceanic sediments provides information on past chemical distributions in the ocean starting from reasonable assumptions it can be shown that paired data on both tracers provides more information than does either separately the slope of paired isochronous 13c and cd ca data from fossil benthic foraminifera depends on oceanic p cd and gd 13c p ratios the zero cd ca intercept of this line depends on mean oceanic 13c p and the 13c p ratio given data sufficient to define oceanic mean 13c and cd contents it is possible to place quantitative constraints on the product of the c p redfield ratio and mean oceanic phosphorus and to estimate a circulation parameter which is independent of changes in ocean chemical inventories with the limited available data no significant changes in oceanic p cd can be detected between the holocene and most recent glacial maximum if this data is taken to be representative of the whole ocean then the p and cd contents of the ocean were about 17 higher during the last glacial maximum than at present ocean box models indicate that this change in oceanic p and a 2 c deep ocean cooling during glacials would cause a 47 ppmv reduction in atmospheric pco2 during glacial intervals

u and th concentrations and isotope ratios in modern carbonates and waters from the bahamas the short residence times of th and pa in seawater make them very responsive to changes in the ocean environment we use a new multi ion counting technique to make th and pa isotope measurements in seawaters from a near shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs the bahamas an unusual feature of the bahamas setting is the shallow depth of water residing on the bank tops these waters have significantly lower 232th 230th 10 000 than those immediately adjacent to the banks 24 000 31 000 and a 231pa 230th near the production ratio 0 1 the change in 232th 230th and 231pa 230th on the bank tops is explained by almost quantitative removal of th and pa by scavenging and their replacement with a mixture of 230th and 231pa alpha recoiled from the underlying carbonates together with th from dust dissolution analysis of a water profile in the tongue of the ocean which separates the great and little bahama banks allows us to trace the movement of bank top water to depth a distinct minimum in both 232th 230th 13 000 and 231pa 230th 0 5 is observed at 430 m and is interpreted to reflect density cascading of bank top water with entrained carbonate sediment these results suggest that th and pa can be used as water mass tracers in near shore environments uranium concentration measurements on the same waters demonstrate that u is conservative across a range in salinity of 2 psu with a concentration of 3 33 ppb at a salinity of 35 the incorporation of u and th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these bahamas waters such incorporation is critical for u th geochronology u isotope analyses demonstrate that seawater 234u averages 146 6 and does not vary by more than 2 5 o and that carbonates capture this value additional high precision measurements 1 o on modern carbonates confirm that all oceans have identical 234u modern marine carbonates are shown to have 232th 230th ratios that reflect the local seawater in which they formed

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

ocean thermohaline circulation and sedimentary 231pa 230th ratio holocene sediments from the atlantic are characterized by 231pa 230th ratios below the production ratio of the two radionuclides in the water column 0 093 whereas holocene sediments from the southern ocean have 231pa 230th 0 093 this pattern of 231pa deficit and excess was ascribed to southward 231pa export from the atlantic by the atlantic thermohaline circulation thc as pa is scavenged less efficiently by marine particles and more effectively transported by the thc than th the same pattern is observed in deposits of the last glacial maximum lgm which led to a previous contention that the thc strength did not vary markedly through the last glacial termination here we embed a description of trace metal scavenging into a zonally averaged circulation biogeochemistry ocean model to explore the sensitivity of 231pa 230th in atlantic and southern ocean sediments to thc changes our results show that the production of biogenic opal which unlike other marine particles poorly fractionates th and pa in the southern ocean water column determines the spatial pattern of the sensitivity also 231pa 230th increases in the north atlantic but changes little in the south atlantic and decreases in the southern ocean as thc is reduced the mean 231pa 230th of the whole atlantic is therefore less sensitive to thc changes than the mean 231pa 230th of the north atlantic the current uncertainties in atlantic mean 231pa 230th are too large to rule out a twofold reduction of the thc at the lgm however the increase in north atlantic mean 231pa 230th simulated in response to a twofold thc reduction is larger than the observed change in the north atlantic mean 231pa 230th from the lgm to holocene comparing this change with the modeled sensitivity of north atlantic 231pa 230th to thc variations indicates that the thc at the lgm could not have been reduced by 30 of its present strength experiments of transient thc changes indicate that high resolution 231pa 230th records from north atlantic sediments could also document thermohaline oscillations on century to millennial timescales

measurement of the sortable silt current speed proxy using the sedigraph 5100 and coulter multisizer iie precision and accuracy the weight percentage and mean size of the 10 63 m terrigenous silt fraction termed sortable silt of fine grained marine sediments have recently been argued to be proxies for near bottom palaeocurrent intensity this paper details the accuracy and precision of the sedigraph 5100 and the coulter multisizer iie in determining the weight percentage and mean size of the sortable silt at abundances characteristic of deep ocean sediments three sortable silt standards and a fine silt clay diluent were generated from natural deep sea sediment of the iceland basin these were used to produce silty clays with standard additions of sortable silt ranging from 1 to 20 of the total by weight accuracy was measured relative to the size of the pure spike measured by each instrument at sortable silt abundance 5 the sedigraph and the coulter multisizer estimate both weight percentage and mean grain size precisely and accurately while below 5 instrumental noise degrades their performance on the basis of the sedigraph results a series of percentage errors is defined which may be applied to sortable silt mean size and weight percentage estimates used in palaeoceanographic near bottom current reconstruction it is suggested that ideally palaeocurrent reconstructions should be based on sedigraph determinations of the 0 63 m grain size distribution yielding both mean and abundance or on coulter multisizer determinations of the sortable silt grain size mean the coulter multisizer may prove especially useful in regions of very depleted 5 sortable silt component

oceanic 231pa 230th ratio influenced by particle composition and remineralization abstract we have investigated the role of particle compositions in affecting the distribution of 231pa and 230th in the ocean by separating sediments collected by sediment traps and cores into three size fractions and analyzed for their excess 231pa 231paex excess 230th 230thex and major chemical components the samples came from stations along 140 w in the equatorial pacific and from the blake outer ridge in the north atlantic results indicate that aluminosilicates clays are the major carrier of 231pa and 230th and the ratio 231paex 230thex in marine particulates decreases with increasing 232th concentration model simulation shows that sea floor sediments and trap collected particles have similar 231paex 230thex ratios in their biogenic components which are very close to the seawater ratio but are about one order higher than the ratios in coexisting clays preferential scavenging of 230th over 231pa by clays is primarily responsible for the 231pa 230th fractionation in the ocean the fractionation factor between 230th and 231pa fth pa varies from 10 for aluminosilicates to 1 for biogenic 232th free particles changes in fth pa as a function of particle composition exert primary controls on 231paex 230thex in marine particulates and bottom sediments and on the boundary scavenging of pa remineralization of biogenic particles in the water column may result in a decrease of 231paex 230thex our observations place an important constraint on the use of sediment 231paex 230thex as a paleoproductivity proxy namely in order to use down core variations of 231paex 230thex to assess past changes in surface productivity one must either verify or assume that the lithogenic particle flux and water column particle remineralization have remained essentially constant at the core site

high precision radiogenic strontium isotope measurements of the modern and glacial ocean limits on glacial interglacial variations in continental weathering existing strontium radiogenic isotope 87sr 86sr measurements for foraminifera over quaternary glacial interglacial climate cycles provide no evidence for variations in the isotope composition of seawater at the 9 13 ppm level of precision however modelling suggests that even within this level of uncertainty significant up to 30 variations in chemical weathering of the continents are permitted accounting for the longer term rise in 87sr 86sr over the quaternary and the apparent imbalance of sr in the oceans at the present day this study presents very high precision 87sr 86sr isotope data for modern seawater from each of the major oceans and a glacial interglacial seawater record preserved by planktic foraminifera from ocean drilling program odp site 758 in the north east indian ocean strontium isotope 87sr 86sr measurements for modern seawater from the atlantic pacific and indian oceans are indistinguishable from one another 87sr 86sr 0 7091792 0 0000021 n 17 at the level of precision obtained in this study 4 9 ppm 2 this observation is consistent with the very long residence time of sr in seawater and underpins the utility of this element for high precision isotope stratigraphy the 87sr 86sr seawater record preserved by planktic foraminifera shows no resolvable glacial interglacial variation 87sr 86sr 0 7091784 0 0000035 n 10 and limits the response of seawater to variations in the chemical weathering flux and or composition to 4 9 ppm or less calculations suggest that a variation of 12 around the steady state weathering flux can be accommodated by the uncertainties obtained here the new data cannot accommodate a short term weathering pulse during de glaciation although a more a diffuse weathering pulse accompanying protracted ice retreat is permissible however these results still indicate that modern weathering fluxes are potentially higher than average over the quaternary and such variations through glacial cycles can also account for the longer term rise in 87sr 86sr over this time interval the very high precision measurements made for the marine 87sr 86sr record in this study place clear limits on the magnitude and timing of changes in the chemical weathering flux during glacial interglacial cycles further constraints must be sought from even higher precision measurement or elements with shorter residence times in the ocean such as osmium os that have the capacity to respond to short term variations in input

atlantic ocean circulation changes preceded millennial tropical south america rainfall events during the last glacial during the last glacial period greenland s climate shifted between cold stadial and warm interstadial phases that were accompanied by ocean circulation changes characterized by reduced atlantic meridional overturning circulation amoc during stadials here we present new data from the western tropical atlantic demonstrating that amoc slowdowns preceded some of the large south american rainfall events that took place during stadials based on 231pa 230th and ti ca measurements in the same sediment core we determine that the amoc started to slowdown 1420 250 and 690 180 1 years before the onset of two large precipitation events associated with heinrich stadials our results bring unprecedented evidence that amoc changes could be at the origin of the large precipitation events observed in tropical south america during heinrich stadials in addition we propose a mechanism explaining the differences in the extent and timing of amoc slowdowns associated with shorter and longer stadials

causes and consequences of protracted melting of the mid crust exposed in the north himalayan antiform gneiss domes exposed by the north himalayan antiform in southern tibet provide evidence for protracted melting of the mid crust during convergent tectonics anatexis was initiated during underthrusting and recurred during the southwards extrusion of a low viscosity crustal sheet sr nd isotope systematics establish that the cores of the domes dominated by cambrian gneiss form a window of the high himalayan crystalline series hhcs indian plate combined trace element and isotopic analyses from 4 two mica granites that intrude both core and the metasedimentary mantle of the domes indicate an origin from melting of the high himalayan crystalline series by the incongruent breakdown of muscovite and to a lesser extent biotite at temperatures from 680 to 760 c u pb dating of accessory phases extends the known period of granite magmatism from the late oligocene to mid miocene a ca rich garnet bearing leucogranite of oligocene age 27 5 ma provides a rare insight into the pre miocene history of the underthrust indian plate reflecting prograde melting at greater depths during a period of crustal thickening the origin of the miocene granites 15 10 ma can be modeled from the exhumation of an extruding channel at a rate of ca 1 mm year constrained by ar isotope data melting over a protracted time scale along the leading edge of the indian plate less than 50 km from the suture accelerated horizontal shortening within the collision zone and provided a positive feedback mechanism between magmagenesis and deformation

dynamic intermediate ocean circulation in the north atlantic during heinrich stadial 1 a radiocarbon and neodymium isotope perspective the last deglaciation was characterised by a series of millennial scale climate events that have been linked to deep ocean variability while often implied in interpretations few direct constraints exist on circulation changes at mid depths here we provide new constraints on the variability of deglacial mid depth circulation using combined radiocarbon and neodymium isotopes in 24 north atlantic deep sea corals their aragonite skeletons have been dated by uranium series providing absolute ages and the resolution to record centennial scale changes while transects spanning the lifetime of a single coral allow sub centennial tracer reconstruction our results reveal that rapid fluctuations of water mass sourcing and radiocarbon affected the mid depth water column 1 7 2 5 km on timescales of less than 100 years during the latter half of heinrich stadial 1 the neodymium isotopic variability 14 5 to 11 0 ranges from the composition of the modern northern sourced waters towards more radiogenic compositions that suggest the presence of a greater southern sourced component at some times however in detail simple two component mixing between well ventilated northern sourced and radiocarbon depleted southern sourced water masses cannot explain all our data instead corals from 15 0 ka and 15 8 ka may record variability between southern sourced intermediate waters and radiocarbon depleted northern sourced waters unless there was a major shift in the neodymium isotopic composition of the northern endmember in order to explain the rapid shift towards the most depleted radiocarbon values at 15 4 ka we suggest a different mixing scenario involving either radiocarbon depleted deep water from the greenland iceland norwegian seas or a southern sourced deep water mass since these mid depth changes preceded the bolling allerod warming and were apparently unaccompanied by changes in the deep atlantic they may indicate an important role for the intermediate ocean in the early deglacial climate evolution

testing the 231pa 230th paleocirculation proxy a data versus 2d model comparison 1 variations of the atlantic meridional overturning circulation amoc are believed to have crucially influenced earth s climate due to its key role in the inter hemispheric redistribution of heat and carbon to assess its past strength the sedimentary 231pa 230th proxy has been developed and improved but also contested due to its sensitivity to other factors beyond ocean circulation in order to provide a better basis for the understanding of the atlantic 231pa 230th system and therefore to shed light on the controversy we compare new measurements of holocene sediments from the north brazilian margin to water column data and the output of a two dimensional scavenging circulation model based on modern circulation patterns and reversible scavenging parameters we show that sedimentary 231pa 230th data from one specific area of the atlantic are in very good agreement with model results suggesting that sedimentary 231pa 230th is predominantly driven by the amoc therefore 231pa 230th represents an appropriate method to reconstruct past amoc at least qualitatively along the western margin

evolution of north himalayan gneiss domes structural and metamorphic studies in mabja dome southern tibet field structural and metamorphic petrology investigations of mabja gneiss dome southern tibet suggest that contractional extensional and diapiric processes contributed to the structural evolution and formation of the domal geometry the dome is cored by migmatites overlain by sillimanite zone metasedimentary rocks and orthogneiss metamorphic grade diminishes upsection and is defined by a series of concentric isograds evidence for three major deformational events two older penetrative contractional and extensional events and a younger doming event is preserved metamorphism migmitization and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid crustal levels metamorphic temperatures and pressures range from 500 8c and 150 450 mpa in chloritoid zone rocks to 705 65 8c and 820 100 mpa in sillimanite zone rocks we suggest that adiabatic decompression during extensional collapse contributed to development of migmatites diapiric rise of low density migmatites was the driving force at least in part for the development of the domal geometry the structural and metamorphic histories documented in mabja dome are similar to kangmar dome suggesting widespread occurrence of these events throughout southern tibet q 2004 elsevier ltd all rights reserved

assessing subsidence rates and paleo water depths for tahiti reefs using u th chronology of altered corals we present uranium thorium chronology fora 102 m core through a pleistocene reef at tahiti french polynesia sampled during iodp expedition 310 tahiti sea level we employ total and partial dissolution procedures on the older coral samples to investigate the diagenetic overprint of the uranium thorium system although alteration of the u th system cannot be robustly corrected diagenetic trends in the u th data combined with sea level and subsidence constraints for the growth of the corals enables the age of critical samples to be constrained to marine isotope stage 9 we use the ages of the corals together with o based sea level histories to provide maximum constraints on possible paleo water depths these depth constraints are then compared to independent depth estimates based on algal and foraminiferal assemblages microbioerosion patterns and sedimentary facies confirming the accuracy of these paleo water depth estimates we also use the fact that corals could not have grown above sea level to place a maximum constraint on the subsidence rate of tahiti to be 0 39 m ka with the most likely rate being close to the existing minimum estimate of 0 25 m ka

deep sea coral evidence for rapid change in ventilation of the deep north atlantic 15 400 years ago coupled radiocarbon and thorium 230 dates from benthic coral species reveal that the ventilation rate of the north atlantic upper deep water varied greatly during the last deglaciation radiocarbon ages in several corals of the same age 15 41 0 17 thousand years and nearly the same depth 1800 meters in the western north atlantic ocean increased by as much as 670 years during the 30 to 160 year life spans of the samples cadmium calcium ratios in one coral imply that the nutrient content of these deep waters also increased our data show that the deep ocean changed on decadal centennial time scales during rapid changes in the surface ocean and the atmosphere

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

a thermal model for the distribution in space and time of the himalayan granites abstract in southern tibet crustal thickening due to the india asia collision has led to the formation of two granite belts one is located at the southern edge of the accretionary wedge of tethyan sedimentary rocks close to the contact with basement gneisses of the tibetan slab the other is found within the wedge itself close to the kangmar thrust trace available ages suggest that the granites appeared first in the southern belt and then in the kangmar belt this sequence seems to violate the chronology of thrusting another feature of the himalayas is that melting started only about 20 ma after the onset of thickening which is much less than the thermal time constant of thick crust we give a thermal model based on the assumption of conductive heat transfer which explains these features the model relies on the geometry of a sedimentary accretionary wedge bounded by low angle thrust faults and on the existence of a thermal conductivity contrast between old basement and young sedimentary rocks the wedge of sedimentary rocks acts as an insulating cap and its southern edge heats up along the contact with basement rocks on a horizontal cross section there is a temperature maximum along this southern edge which explains why melting starts there the early thermal evolution is sensitive to local conditions and granites first appear in the vicinity of the most radiogenic parts of the basement the distribution of granites in space and time is seemingly random reflecting different melting events in different radiogenic environments in the heterogeneous basement this model predicts a relationship between radioactivity and age which is compatible with available data the results emphasize that there are large horizontal temperature variations across a thickened region and that granite ages are not related simply to the timing of tectonic phases

when did the roof collapse late miocene north south extension in the high himalaya revealed by th pb monazite dating of the khula kangri granite th pb ion microprobe measurements made on 12 monazite grains from the khula kangri granite tibet bhutan frontier are interpreted to indicate that crystallization occurred at 12 5 0 4 ma the leucogranite is cut by the gonto la detachment part of the southern tibet detachment system that has allowed upper level north directed extension of the himalayan orogen significant orogen normal extension in southern tibet appears to have continued 8 10 m y later than previously recognized this is the first reported crystallization age for a leucogranite east of the yadong cross structure an apparent 70 km offset of the high himalaya and southern tibet detachment west of the yadong cross structure reliable ages for high himalaya events major main central thrust slip granite generation and emplacement attainment of critical topography and major detachment extension group between ca 24 and 19 ma we interpret the west to east change across the yadong cross structure to be due to either 1 an abrupt 10 m y younging of principal high himalayan events or 2 a deeper thus younger exposed part of the footwall of the southern tibet detachment near khula kangri the southern tibet detachment is cut by the highly oblique yadong gulu rift a manifestation of tibet plateau east west extension integrated estimates of magnitude and rate of detachment displacement suggest that the observed postcrystallization north directed extension lasted for 1 3 m y after which time the yadong gulu rift formed this interpretation is consistent with initiation of east west extension of tibet at ca 8 ma

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

glacial interglacial circulation changes inferred from 231pa 230th sedimentary record in the north atlantic region 1 studies from the subtropical western and eastern atlantic ocean using the 231pa 230th ratio as a kinematic proxy for deep water circulation provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation moc over the last deglaciation in this study we present a compilation of existing and new sedimentary 231pa 230th records from north atlantic cores between 1710 and 4550 m water depth comparing sedimentary 231pa 230th from different depths provides new insights into the evolution of the geometry and rate of deep water formation in the north atlantic during the last 20 000 years the 231pa 230th ratio measured in upper holocene sediments indicates slow water renewal above 2500 m and rapid flushing below consistent with our understanding of modern circulation in contrast during the last glacial maximum lgm glacial north atlantic intermediate water gnaiw drove a rapid overturning circulation to a depth of at least 3000 m depth below 4000 m water renewal was much slower than today at the onset of heinrich event 1 transport by the overturning circulation declined at all depths gnaiw shoaled above 3000 m and significantly weakened but did not totally shut down during the bolling allerod ba that followed water renewal rates further decreased above 2000 m but increased below our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the lgm but less accentuated moc during the younger dryas appears very similar to ba down to 2000 m and slightly slower below

reversed flow of atlantic deep water during the last glacial maximum the meridional overturning circulation moc of the atlantic ocean is considered to be one of the most important components of the climate system this is because its warm surface currents such as the gulf stream redistribute huge amounts of energy from tropical to high latitudes and influence regional weather and climate patterns whereas its lower limb ventilates the deep ocean and affects the storage of carbon in the abyss away from the atmosphere despite its significance for future climate the operation of the moc under contrasting climates of the past remains controversial nutrient based proxies1 2 and recent model simulations3 indicate that during the last glacial maximum the convective activity in the north atlantic ocean was much weaker than at present in contrast rate sensitive radiogenic 231pa 230th isotope ratios from the north atlantic have been interpreted to indicate only minor changes in moc strength4 5 6 here we show that the basin scale abyssal circulation of the atlantic ocean was probably reversed during the last glacial maximum and was dominated by northward water flow from the southern ocean these conclusions are based on new high resolution data from the south atlantic ocean that establish the basin scale north to south gradient in 231pa 230th and thus the direction of the deep ocean circulation our findings are consistent with nutrient based proxies and argue that further analysis of 231pa 230th outside the north atlantic basin will enhance our understanding of past ocean circulation provided that spatial gradients are carefully considered this broader perspective suggests that the modern pattern of the atlantic moc with a prominent southerly flow of deep waters originating in the north atlantic arose only during the holocene epoch

scavenging of thorium in the ocean the distributions of th isotopes in both suspended participate matter and sea water were measured in the western north pacific and japan sea suspended particulate matter was collected simultaneously on four 142 mm diameter nuclepore filters 1 0 m in pore size by using an in situ deep sea pumping system particle associations were found to be 6 40 for 228th 9 52 for 230th and 9 72 for 232th with mean values of 26 32 for all three isotopes in order to explain the observed th isotope distributions in the pacific a reversible exchange model was developed that includes two classes fine and large of particle population in the ocean the fine particles are defined as those collected by filtration and comprising most of the standing crop of particles in seawater with no significant settling flux the large particles on the other hand are defined as those collected via sediment traps and represent a negligible standing stock in the water but play a predominant role in the vertical flux the model calculation suggests that the transfer of th from solution to fine particles is controlled by an adsorption desorption process which depends upon the chemical reactivity of elements aggregation of fine particles to fast sinking large particles was estimated to occur throughout the water column on a time scale of one to five months the large aggregates seem to disintegrate on a time scale of less than a few days suggesting their extremely fragile nature

deep circulation changes in the central south atlantic during the past 145 kyrs reflected in a combined231pa 230th neodymium isotope and benthic δ13c record abstract previous work showed that south atlantic sediments have lower glacial than holocene 231pa 230th which was attributed to a switch in the flow direction of atlantic deep water debate exists however as to the degree to which two processes circulation and scavenging determine sedimentary 231pa 230th making this interpretation contentious here we address this issue using 145 kyr records of paleocirculation proxies benthic foraminiferal c 13 neodymium isotopes end and sedimentary 231pa 230th were all measured in a single sediment core from the south atlantic subtropical gyre this site largely excludes the influence of local productivity changes on 231pa 230th records measured 231pa 230th ranges between 0 041 during glacials and 0 055 during interglacial periods and is consistently lower than the production ratio indicating export of 231pa from the central south atlantic for the entire duration of the record the lower glacial 231pa 230th is regionally consistent suggesting that basin scale oceanographic processes cause the decrease in turn less radiogenic end and lower benthic c 13 confirm the classical picture of an increase in southern component water scw influence in the atlantic during glacial periods and point to a circulation control on the observed 231pa 230th decrease rather than a local productivity change we suggest that associated with this change in water mass distribution the dominant sink for 231pa shifted from the margins of the south atlantic and or the southern ocean during interglacials to the north atlantic during glacial periods indeed elevated 231pa 230th in the deep north atlantic during glacials supports this mechanism of northward transport of 231pa by scw

seawater 234 u 238 u during the last 800 thousand years abstract constraining the history of seawater 234u 238u is important because this ratio is used to assess the validity of u th ages and because it provides information about the past rate of physical weathering on the continents this study makes use of u rich slope sediments from the bahamas in an attempt to reconstruct seawater 234u 238u for the last 800 kyr for the last 360 kyr u th dating of these sediments provides ages and initial 234u 238u values sixty seven samples largely from marine highstands have initial 234u 238u which scatter somewhat about the modern seawater value 1 145 but neither this scatter nor the average value increases with age of sample these data contrast with published coral data and suggest that seawater 234u 238u has remained within 15 of the modern value for the last 360 kyr this confirms the rejection of coral u th ages where the initial 234u 238u is significantly different from modern seawater data from older highstands dated with 18o stratigraphy or by the presence of the brunhes matuyama b m reversal at 780 kyr allow seawater 234u 238u to be assessed prior to the range of the 230th chronometer unfortunately diagenetic scatter in the data between the b m reversal and 360 kyr is rather large probably relating to low u concentrations for these samples but there is no indication of a trend in seawater 234u 238u with age high u samples from close to the b m reversal show less diagenetic scatter and an initial 234u 238u that averages 1 102 this lower value can be explained by lower seawater 234u 238u at the time of the b m reversal or by progressive loss of 234u from the sediment by recoil a simple box model is presented to illustrate the response of seawater 234u 238u to variations in riverine input such as might be caused by changes in continental weathering comparison of the bahamas 234u 238u data with model results indicates that riverine 234u 238u has not varied by more than 65 for any 100 kyr period during the last 360 kyr it also indicates that the ratio of physical to chemical weathering on the continents has not been higher than at present for any extended period during the last 800 kyr

advection and scavenging effects on 230th and 231pa distribution off southwest africa we investigated the controls of hydrography and of scavenging on the distribution of the particle reactive radionuclides 231pa and 230th in the water column and in surface sediments off southwest africa angola and cape basins based on a vertical section of total 230thex concentrations in the water column we show that small differences in the salinity between the north atlantic deep water nadw in the angola basin and the nadw in the cape basin as well as the advection of nadw associated with the namib col current are reflected in total 230thex concentrations these variable total concentrations are believed to reflect the flow path and mixing history of nadw with the nadw in the angola basin being relatively older and 230th enriched compared to the nadw in the cape basin in the area investigated we found high 231paex 230thex ratios 231paex 230thex 0 093 in surface sediments at the continental margin and lower ones 231paex 230thex 0 093 in the open ocean such a distribution is normally interpreted to result from high particle flux at ocean margins boundary scavenging however the lack of any significant depletion of dissolved 230th and 231pa in the water column does not indicate extensive scavenging at the continental margin high 231paex 230thex ratios are constrained to shallow waters depths 2000 m only and coincide with low fractionation between 231pa and 230th indicating that preferential scavenging of 231pa on opal may have caused high 231paex 230thex ratios in the sediments the observed close negative correlation r2 0 82 between 231paex 230thex ratios in sediments and water depths is believed to reflect changes in the particle composition i e a decrease in opal content with water depth in the angola and cape basins the total 231paex concentrations in nadw were the highest observed so far in the atlantic ocean and they are attributed to the meridional export of 231pa from the north atlantic this caused the average dissolved 231pa 230th in the southeast atlantic to be about a factor 2 higher when compared to the north atlantic labrador sea these differences in the dissolved 231pa 230th were not reflected in 231pa 230th ratios of surface sediments because the fractionation is lower in the labrador sea compared to the southeast atlantic i e fractionation counteracts changes in the dissolved 231pa 230th this suggests that fractionation is more important for the determination of 231paex 230thex ratios in sediments than the meridional export of 231pa from the north atlantic

a model for the origin of himalayan anatexis and inverted metamorphism the origin of the paired granite belts and inverted metamorphic sequences of the himalaya has generally been ascribed to development of the main central thrust mct although a variety of models have been proposed that link early miocene anatexis with inverted metamorphism recent dating studies indicate that recrystallization of elements of the mct footwall occurred in the central himalaya as recently as 6 ma the recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the high himalayan leucogranites and north himalayan granites which differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active thrust that cuts through indian supracrustal rocks that had previously experienced low degrees of partial melting numerical simulations assuming a shear stress of 30 mpa indicate that continuous slip on the himalayan decollement beginning at 25 ma could trigger partial melting reactions leading to formation of the high himalayan granite chain between 25 and 20 ma and the north himalayan belt between 17 and 8 ma the ramp flat geometry we apply to model the himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle miocene late miocene out of sequence thrusting within the broad shear zone beneath the mct provides a mechanism to bring these rocks to the surface in their present location i e well to the north of the present tectonic front and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the mct we envision that formation of the mct zone involved successive accretion of tectonic slivers of the lesser himalayan formations to the hanging wall and incorporate these effects into the model the model predicts continued anatexis up to 400 km north of the himalayan range consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south tibetan rift

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

investigating the use of 232th 230th as a dust proxy using co located seawater and sediment samples from the low latitude north atlantic abstract the thorium isotope ratio 232 th 230 th can be measured in seawater and sediment samples and has been used as a proxy to reconstruct lithogenic fluxes to the oceans for the modern day and the pleistocene there has not yet been a systematic study testing the proxy using the 232 th 230 th ratio in seawater and the ratio recorded in the underlying sediment in this study we use co located core top sediments and seawater samples from five seamount sites spanning the tropical north atlantic to investigate the link between seawater and sediment 232 th 230 th ratios across a range of water depths our results indicate that a broad correlation exists between seawater and sedimentary 232 th 230 th ratios both seawater and sedimentary 232 th 230 th ratios record a signal consistent with decreasing lithogenic input east to west from africa to south america however calculated 232 th fluxes for the core top sediment samples indicate a strong dependence on depth with up to a factor of 4 difference from shallow 2900 m this depth dependence is likely caused by either a deficit of 230 th burial at depth compared to the production in the overlying water column through addition of 232 th or by a combination of the two by comparing seawater and sedimentary 232 th 230 th ratios we derive an apparent fractional solubility of 232 th of 29 3 in reasonable agreement with the upper end of existing estimates

new insights into cycling of 231pa and 230th in the atlantic ocean abstract we use the bern3d model of intermediate complexity to examine the marine cycle of isotopes 231pa and 230th and the relationship between the particle bound ratio pa p th p and changes in the formation of the north atlantic deep water nadw model parameters describing reversible scavenging of isotopes by organic particles opal calcite and resuspended sediments were systematically varied and alternative sink parametrisations explored it proves difficult to simultaneously achieve a good agreement with observations of dissolved and particle associated concentrations of 231pa and 230th pa d th d pa p th p as well as the particle bound ratio pa p th p within the classical concept of reversible scavenging alone agreement between simulated and observed pa d th d and estimates of mean ocean residence times is improved by taking into account simplified representations of additional sinks at the sea floor bottom scavenging and at continental boundaries boundary scavenging we also find improved agreement between model and data by increasing lateral advection in particular for pa d these results point to the importance of sink processes that act in addition to reversible scavenging to shape the steady state distribution of 231pa and to a lesser degree of 230th in transient experiments in which the strength of the atlantic meridional overturning circulation amoc is periodically turned on and off we find a strong statistical relationship between variations in amoc strength and pa p th p at great depths in the northwest atlantic region these conclusions are robust across the range of sink parametrisations that are consistent with estimates in the mean ocean residence time of 231pa and 230th our results indicate that the relationship between pa p th p and amoc strength may not be fundamentally affected by uncertainties in sink processes at least on the large spatial and temporal scale considered here and support the idea that changes in pa p th p in sediments of the northwest atlantic are indicative of changes in amoc strength taking into account our simplified approach our results indicate that the relationship between pa p th p and amoc strength in the deep northwest atlantic is not affected by boundary scavenging or bottom scavenging our results thus support the idea that changes in pa p th p in sediments of the northwest atlantic are indicative of changes in amoc strength

dynamic storage of glacial co2 in the atlantic ocean revealed by boron co32 and ph records abstract the origin and carbon content of the deep water mass that fills the north atlantic basin either antarctic bottom water aabw or north atlantic deep water nadw is suggested to influence the partitioning of co2 between the ocean and atmosphere on glacial interglacial timescales fluctuations in the strength of atlantic meridional overturning circulation amoc have also been shown to play a key role in global and regional climate change on timescales from annual to millennial the north atlantic is an important and well studied ocean basin but many proxy records tracing ocean circulation in this region over the last glacial cycle are challenging to interpret here we present new b ca co 3 2 and boron isotope ph data from sites spanning the north atlantic ocean from 2200 to 3900 m and covering the last 130 kyr from both sides of the mid atlantic ridge these data allow us to explore the potential of the boron based proxies as tracers of ocean water masses to ultimately identify the changing nature of atlantic circulation over the last 130 kyr this possibility arises because the b ca and boron isotope proxies are directly and quantitatively linked to the ocean carbonate system acting as semi conservative tracers in the modern ocean yet the utility of this approach has yet to be demonstrated on glacial interglacial timescales when various processes may alter the state of the deep ocean carbonate system we demonstrate that the deep 3400 m north atlantic ocean exhibits considerable variability in terms of its carbonate chemistry through the entirety of the last glacial cycle our new data confirm that the last interglacial marine isotope stage mis 5e has a similar deep water geometry to the holocene in terms of the carbonate system in combination with benthic foraminiferal 13c and a consideration of the co 3 2 of contemporaneous southern sourced water we infer that aabw influences the eastern abyssal north atlantic throughout the whole of the last glacial mis2 through 4 whereas only in the coldest stages mis2 and mis4 of the last glacial cycle was aabw an important contributor to our deep sites in both north atlantic basins taken together our carbonate system depth profiles reveal a pattern of changing stratification within the north atlantic that bears strong similarities to the atmospheric co2 record evidencing the important role played by ocean water mass geometry and the deep ocean carbonate system in driving changes in atmospheric co2 on these timescales

geochemistry of granitic melts produced during the incongruent melting of muscovite implications for the extraction of himalayan leucogranite magmas granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium both between restite and melt and within individual phases during crystallization miocene leucogranites from the himalayas exemplify melt formation from a pelitic source through muscovite breakdown trace elements rb sr and ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling equilibrium between melt and restite was established for sr under fluid phase absent conditions but some disequilibrium is suggested by rb abundances consistent with known diffusion rates for these elements abundances of zr and ce in the granites which provide essential structural constituents of the accessory phases zircon and monazite indicate that the himalayan granite melts were saturated in these elements if formed at temperatures of 670 700 c but slightly undersaturated if formed at somewhat higher temperatures ion microprobe traverses of y and heavy rare earth element abundances across single garnets from himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth thus constraining the evolution of magmatic compositions during melt formation and crystallization in some cases progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant indicating that minimum solidus temperatures were maintained throughout the melting event this suggests that thermal buffering rather than melt extraction provided the rate determining step in magma evolution

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

deep flow in the madagascar mascarene basin over the last 150 000 years the sw indian ocean contains at least four layers of water masses with different sources deep antarctic lower circumpolar deep water flow to the north midwater north indian deep water flow to the south and upper circumpolar deep water to the north meridional convergence of intermediate waters at 500 1500 m and the shallow south equatorial current flowing west sedimentation rates in the area are rather low being less than 1 cm ka 1 on madagascar ridge but up to 4 cm ka 1 at amirante passage bottom flow through the madagascar mascarene basin into amirante passage varies slightly on glacial interglacial time scales with faster flow in the warm periods of the last interglacial and minima in cold periods far more important are the particularly high flow rates inferred from silt grain size which occur at warm to cold transitions rather than extrema this suggests the cause is changing density gradient driving a transiently fast flow corroboration is found in the glacial interglacial range of benthic d18o which is ca 2 suggesting water close to freezing and at least 1 2 more saline and thus more dense glacial bottom waters than present significant density steps are inferred in isotope stage 6 the 5e 5d and 5a 4 transitions oxygen isotope data suggest little change by mixing in glacial bottom water on their northward path benthic carbon isotope ratios at amirante passage differ from glacial southern ocean values due possibly to absence of a local productivity effect present in the southern ocean

himalayan metamorphism and deformations in the north himalayan belt southern tibet china abstract the kangmar massif which has been attributed to the north himalayan belt of two mica granites in southern tibet china is shown to be an orthogneiss in the core of a dome which displays an intermediate high pressure type metamorphism himalayan in age 40 to 20 ma and above the main central sheet this metamorphism took place during high shear strain deformation and it is not recorded to the west where north himalayan plutons have developed contact metamorphism

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

interpretation of the 231 pa 230 th paleocirculation proxy new water column measurements from the southwest indian ocean measurements of 231 pa 230 th and 232 th concentrations have been made on five water column profiles along the western margin of the madagascar and mascarene basins in the southern indian ocean these measurements help to fill a significant gap in the global coverage of water column 232 th 230 th and 231 pa data 232 th concentrations vary but generally increase with depth suggesting higher particle loading in deeper waters and the presence of a significant dissolved fraction of 232 th 230 th concentrations increase with depth and profiles are similar to the average of existing data from other regions 231 pa concentrations on the other hand show significant depth structure apparently reflecting the various water masses sampled at this location the modified remnants of north atlantic deep water are found at a depth of 62000 m and exhibit elevated 231 pa concentrations exported from the south atlantic antarctic intermediate and bottom waters have lower 231 pa probably due to scavenging onto opal particles during transit from the southern ocean the differences between water masses raises a question which water mass is important in controlling the 231 pa 230 th ratio in underlying sediments a simple one dimensional model is used to demonstrate that the 230 th and 231 pa exported to sea floor sediments last equilibrates with waters close to the seafloor within 61000 m rather than averaging the whole water column these findings suggest that 231 paxs 230 thxs in sediments provides information primarily about deep water masses in this region sedimentary records will therefore provide information about the past flow of antarctic bottom water into the indian ocean interpretation of data from other regions such as the north atlantic where this proxy has most successfully been applied requires careful consideration of regional oceanography and knowledge of the composition of the water masses being investigated d 2005 elsevier b v all rights reserved

enhanced scavenging of 231pa relative to 230th in the south atlantic south of the polar front implications for the use of the 231pa 230th ratio as a paleoproductivity proxy abstract the fractionation of 230th and 231pa was investigated throughout the atlantic sector of the southern ocean published scavenging models generally assume that the 231pa 230t ratio of surface sediments is primarily determined by the mass flux of particles this relationship holds north of the polar front where low primary productivity coincides with ratios of unsupported 231pa 230th xs 231pa 230th in surface sediments below the production ratio of both radionuclides in the water column however we observed high xs231pa 230th ratios conventionally interpreted as a high productivity signal in surface sediments south of the polar front especially throughout the weddell sea in contradiction with the low particle flux of this region measurements of both dissolved and particulate fractions of 231pa and 230th in the water column revealed a strong n s decrease in the th pa fractionation factor from typical open ocean values around 10 north of the polar front to values between 1 and 2 south of 60 s this observation clearly indicates that the high xs231pa 230th ratios in surface sediments south of the antarctic circumpolar current are produced by a n s increase in the relative scavenging efficiency of 231pa relative to 230th most probably due to a change in the chemical composition of particulate matter and not by a high mass flux it is speculated that biogenic opal suggested not to significantly fractionate231pa and 230th may explain the enhanced scavenging of 231pa to the south this assumption is further supported by extremely high 231pa 230th ratios up to 0 34 in material collected with sediment traps south of the polar front where fluxes are primarily determined by biogenic opal based on these results we conclude that in regions where the sedimenting flux is dominated by biogenic opal the 231pa 230th ratio is not a reliable indicator for the mass flux of particles thus limiting its use as a paleoproductivity proxy in the southern ocean

the role of deep ocean circulation in setting glacial climates the glacial cycles of the pleistocene involve changes in the circulation of the deep ocean in important ways this review seeks to establish what were the robust patterns of deep sea water mass changes and how they might have influenced important parts of the last glacial cycle after a brief review of how tracers in the modern ocean can be used to understand the distribution of water masses i examine the data for biogeochemical circulation rate and conservative tracers during glacial climates some of the robust results from the literature of the last 30 years include a shoaled version of northern source deep water in the atlantic expanded southern source water in the abyss and deep ocean salt rather than heat stratification of the last glacial maximum lgm deep sea and several lines of evidence for slower overturning circulation in the southern deep cell we combine these observations into a new idea for how the ocean atmosphere system moves from interglacial to glacial periods across a single cycle by virtue of its influence on the melting of land based ice around antarctica cooling north atlantic deep water nadw leads to a cold and salty version of antarctic bottom water aabw this previously underappreciated feedback can lead to a more stratified deep ocean that operates as a more effective carbon trap than the modern helping to lower atmospheric co 2 and providing a mechanism for the deep ocean to synchronize the hemispheres in a positive feedback that drives the system to further cooling

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

opal burial in the equatorial atlantic ocean over the last 30 ka implications for glacial interglacial changes in the ocean silicon cycle 1 the silicic acid leakage hypothesis salh suggests that during glacial periods excess silicic acid was transported from the southern ocean to lower latitudes which favored diatom production over coccolithophorid production and caused a drawdown of atmospheric co2 downcore records of 230th normalized opal fluxes and 231pa 230th ratios from seven equatorial atlantic cores were used to reconstruct diatom productivity over the past 30 ka where a is years and to test the salh downcore records of 231pa 230th ratios and opal fluxes are highly correlated suggesting that they constitute a production based record of opal flux opal flux records support the salh in that glacial opal burial exceeded holocene burial by 1 8 gt opal ka in the area 0 40 w and 5 n 5 s earlier results from the eastern equatorial pacific ocean showed the opposite trend with greater holocene than glacial opal burial but approximately the same magnitude of difference between glacial and interglacial opal burial we suggest four nonexclusive scenarios to explain the data from both basins 1 increased upwelling in the equatorial atlantic and el nino like conditions suppressing upwelling in the eastern equatorial pacific altered the overall nutrient supply to both basins 2 si leaked from the southern ocean because of fe fertilization but was prevented from upwelling in the equatorial pacific because of el nino like conditions during the lgm 3 si leaked from the southern ocean because diatom productivity was limited by increased sea ice extent and was again prevented from upwelling in the equatorial pacific or 4 changes in ocean circulation related to the decreased input of agulhas water to the glacial south atlantic provided excess dissolved si to the equatorial atlantic ocean a deglacial opal pulse is seen in both the atlantic and pacific oceans all four scenarios and the presence of the common deglacial opal pulse suggest a common driver in the southern ocean

uranium series and radiocarbon geochronology of deep sea corals implications for southern ocean ventilation rates and the oceanic carbon cycle we present new uranium series and radiocarbon measurements for deep sea corals from the southern ocean these data are used to reconstruct ventilation ages both at present and at the end of the last glacial period approximately 16 500 years ago we apply an improved two component mixing approach to correct uranium series dates for contaminant thorium and protactinium present in oxide coatings calculated seawater radiocarbon values for contemporary samples decrease with depth in the water column and agree with direct seawater radiocarbon measurements for this area this indicates that deep sea corals can accurately record seawater radiocarbon distributions two of three glacial samples experienced open system uranium series systematics however a third sample from the drake passage yields concordant thorium and protactinium dates as well as seawater values for initial 234u 238u this coral yields a ventilation age that is approximately 20 40 greater than modern values for its location this increase is consistent with published deep sea coral and calibrated planktonic benthic foraminifera radiocarbon data suggesting that the glacial oceans as a whole may have been substantially less ventilated presumably due to decreased formation of north atlantic deep water an overall decrease in oceanic mixing rates could have contributed to lower dissolved carbon in surface ocean water and lower atmospheric pco2 during the past glacial period

distribution of 230th in the labrador sea and its relation to ventilation abstract measurements of dissolved and particulate230th and232th by thermal ionization mass spectrometry were made on samples collected from the labrador sea and the denmark strait and iceland scotland overflow waters the large scale feature of low and invariant230th evident in deep waters of the labrador sea and previously observed in the deep northern atlantic can be reproduced using a reversible scavenging model that includes the effect of ventilation advective transport also must be important in the redistribution of230th in other regions of the atlantic and for other long lived tracers such as231pa 26al and10be

controls on seawater 231pa 230th and 232th concentrations along the flow paths of deep waters in the southwest atlantic measurements of dissolved th 230 pa 231 and th 232 were made for twelve full depth profiles along a southwest atlantic section during geotraces cruise ga02s sampling captures all the main atlantic deep water masses along their meridional flow paths and allows insight into the control on th and pa in a setting where waters are flowing in opposing directions with direct relevance to understanding the use of pa 231 th 230 as an ocean circulation proxy water column th 230 increases linearly with depth in line with expected reversible scavenging models pa 231 increases from the surface to similar to 1200 1500 m but is invariant or decreases with greater depth deviating from the behavior expected for reversible scavenging dissolved pa 231 th 230 ratios display a mid water column maximum at similar to 1000 2000 m which is broadly coincident with upper circumpolar deep water below 2000 m nuclide distributions and ratios exhibit no dependence on water mass nor any indication of progressive change within a water mass challenging the use of pa 231 th 230 as a past circulation tracer in the south atlantic calculation of horizontal transport of th 230 and pa 231 by ocean circulation indicates a net southward export out of the atlantic of 19 of the pa 231 and 3 of the th 230 produced in that ocean this removal is all from the north atlantic while in the south atlantic removal to sediment equals production simple one dimensional modeling can simulate th 230 profiles but not the mid water column maximum observed in pa 231 profiles suggesting an additional source of pa 231 perhaps lateral transport from the margin or removal at depth due to bottom scavenging near seafloor minima in concentrations indicates bottom scavenging of th 230 and 231 pa which is enhanced in the presence of nepheloid layers particularly for 231pa this additional scavenging fractionates th 230 and pa 231 and in the presence of nepheloid layers may lead to an increase in sedimentary pa 231 th 230 ratios th 232 concentrations were paired with th 230 derived residence times in the upper 250 m of the water column to test the application of th as a tracer of dust deposition maxima in th 232 indicate high dust input from the african and possibly south american continents

constraints on deep water age and particle flux in the equatorial and south atlantic ocean based on seawater 231pa and 230th data high precision measurements of 231 pa and 230 th in filtered seawater and suspended particulate matter samples are reported for the equatorial and south atlantic distributions of 231 pa and 230 th clearly indicate the influence of advection as evidenced by departures from scavenging models that predict a linear increase with depth for these tracers application of a scavenging mixing model implies a deep water transit time of 60 100 years from the northern source water regions the average particulate 231 pa 230 th activity ratio is 0 0498 0 0160 a factor of 2 lower than the 231 pa 230 th production ratio of 0 093 and in agreement with reported excess 231 pa 230 th ratios of 0 06 0 004 in holocene sediments north of 50 s in the atlantic these water column data further suggest that lateral eddy diffusive transport combined with enhanced scavenging in high particle flux marginal regions boundary scavenging is weakly expressed in the atlantic particle fractionation of these tracers is also indicated by the elevated fractionation factors of f th pa 4 32 24 04 ave 9 97 4 98 compared to values of 1 4 in the southern ocean

early arrival of southern source water in the deep north atlantic prior to heinrich event 2 the atlantic meridional overturning circulation amoc plays an important role in the northern hemisphere climate system significant interest went into the question of how excessive freshwater input through melting of continental ice can affect its overturning vigor and hence heat supply to higher northern latitudes such forcing can be tested by investigating its behavior during extreme iceberg discharge events into the open north atlantic during the last glacial period the so called heinrich events he here we present neodymium nd isotope compositions of past seawater a sensitive chemical water mass tag extracted from sediments of ocean drilling program site 1063 in the western north atlantic bermuda rise covering the period surrounding he 2 the last glacial maximum and the early deglaciation these data are compared with a record of the kinematic circulation tracer 231pa 230th xs extracted from the same sediment core both tracers indicate significant circulation changes preceding intense ice rafting during he 2 by almost 2 kyr moreover the nd isotope record suggests the presence of deeply ventilating north atlantic deep water early during marine isotope stage 2 until it was replaced by southern source water at 27 ka the early switch to high pa th xs and radiogenic nd in relation to intensified ice rafting during he 2 suggests that ice rafting into the open north atlantic during major he 2 was preceded by an early change of the amoc this opens the possibility that variations in amoc contributed to or even triggered the ice sheet instability rather than merely responding to it

variability in mid depth ventilation of the western atlantic ocean during the last deglaciation negative stable carbon isotopic excursions have been observed throughout most of the mid depth 1000 3000 m atlantic ocean during heinrich stadial 1 hs1 and the younger dryas yd although there is an agreement that these mid depth excursions were in some way associated with a slowdown of the atlantic meridional overturning circulation amoc there is still no consensus on the precise mechanism s here we present benthic stable carbon and oxygen isotopic 13c and 18o records from five cores from the western equatorial atlantic wea together with published benthic isotopic records from nearby cores we produced a wea depth transect 800 2500 m we compare hs1 and yd data from this transect with data from previously published north and south atlantic cores and demonstrate that the largest negative 13c excursions occurred in the wea during these times moreover our benthic 18o records require the presence of two water masses flowing from the southern ocean bisected by a northern component water ncw given that 18o is a conservative water mass tracer we suggest that 13c was decoupled from water mass composition and does not correspond to simple alternations between northern and southern sourced waters instead 13c behaved non conservatively during hs1 and the yd consistently with our new 231pa 230th record from the wea transect that allowed the reconstruction of amoc strength we hypothesize that the negative 13c excursions reflect an increase in the residence time of ncw in response to a weakened amoc allowing for a marked accumulation of 13c depleted respired carbon at the mid depth wea

distribution of 230th and 231pa in the water column in relation to the ventilation of the deep arctic basins distribution of 230th and 231pa in the water column was measured in the eurasian basins of the arctic ocean i e the makarov amundsen and nansen basins the profiles obtained were compared to one previously published from the alpha ridge bacon et al earth and planetary science letters 95 1989 15 22 the amundsen and nansen basins are similar in their 23oth and 231pa distribution whereas in the makarov basin and at the alpha ridge these radionuclides are comparatively enriched in the deep basin this may be due to low particle flux in conjunction with low water mass renewal rates the 230th concentration in water depth 1000 m in the makarov basin is relatively low compared with the alpha ridge which may be caused by an exchange of water masses between the amundsen and makarov basins in the eurasian basin deep water 230th profiles can be explained both by a scavenging model and by a mixing scavenging model enhanced particle load caused by resuspension may explain the increased scavenging in the eurasian basin bottom water the differences in scavenging between the norwegian greenland sea and the eurasian basins at present are not large enough to cause significant fractionation between 230th and 231pa

yadong cross structure and south tibetan detachment in the east central himalaya 89 90 e the yadong cross structure ycs occurring at 89 30 east longitude in the himalaya is the largest across strike discontinuity in the geologic structure and topography of the high himalaya between the himalayan syntaxes it is manifest by a plan view left offset of the topographic crest of the range and a coincident apparent left strike offset of the south tibetan detachment system stds of about 70 km new field mapping indicates that the stds intersects the west side of the ycs at zherger la and further suggests that the ycs is the surface expression of a large north northeast striking west facing lateral ramp in the himalayan thrust system the greater himalayan allochthon is apparently draped across this lateral ramp resulting in a north northeast striking monoclinal flexure of the allochthon manifest in the chomolhari range superimposed steep west northwest directed normal shear appears to have obliterated earlier northerly directed stds shear fabric along along the ycs segment of the tethyan belt greater himalayan belt contact recent seismicity suggests that the greater himalayan allochthon is still moving southward along the lateral ramp u pb monazite dates on leucogranites in the footwall of the stds on opposite sides of the ycs are suggestive of a south to north decrease in the crystallization age of these granites the age pattern is consistent with southward extrusion of the greater himalayan allochthon and suggests the possibility of determining an average slip rate for the stds by dating granites in the immediate footwall of the stds that are substantially separated in north south section an average slip rate of about 7 mm yr between 23 and 12 ma is suggested by the data presented here

combining seawater 232th and 230th concentrations to determine dust fluxes to the surface ocean abstract aeolian dust is a significant source of iron and other nutrients to the surface ocean but constraining the size of this flux is difficult chemical tracers such as al are useful but are limited by lack of information about their rate of removal from surface seawater in this study we test the use of th isotopes as an improved tracer of dust input by measuring 232 th and 230 th concentrations of upper ocean seawater at six stations along a meridional atlantic section that spans the saharan dust plume open ocean 232 th like al is dominantly derived from dust dissolution while 230 th is produced in seawater by u decay and provides an assessment of th removal rates the highest 232 th concentrations and 232 th 230 th ratios particularly in the surface waters are observed at stations expected to be influenced by input of saharan dust the pattern of 232 th concentration is in agreement with dissolved al distributions and seawifs aerosol imagery supporting the use of 232 th as a proxy for short term dust addition deeper in the water column 232 th concentration may also reflect longer term dust inputs in addition lateral input of 232 th from mediterranean outflow water is detected at 1000 m in the north atlantic surface water residence times are calculated separately from 230 th and 234 th using a simple scavenging model and values from the two isotopes are generally consistent with one another th 230 residence times appear to reflect the longer term value however rather than exhibiting the strong seasonal influence seen for 234 th these th residence times allow dust flux to the surface ocean to be calculated using the known concentration of th in dust and assumed values for the solubility of dust th in seawater resulting dust fluxes show good agreements with models of dust deposition if th solubility is 1 to 5 in the north atlantic yielding fluxes of 0 4 19 2 g m 2 yr 1 and 5 to 10 in the south atlantic fluxes of 0 2 0 7 g m 2 yr 1 despite the need for better constraints on the fractional solubility of dust th this study demonstrates the power of combining th isotope measurements to constrain dust fluxes to the surface ocean

meridional circulation across the antarctic circumpolar current serves as a double 231pa and 230th trap abstract upwelling of circumpolar deep water in the weddell gyre and low scavenging rates south of the antarctic circumpolar current acc cause an accumulation of particle reactive nuclides in the weddell gyre a ventilation reversible scavenging model that successfully described the accumulation of 230th in this area was tested with other particle reactive nuclides and failed to adequately describe the depth distributions of 231pa and 210pb we present here a modified model that includes a nutrient like accumulation south of the antarctic polar front in an upper meridional circulation cell as well as transport to a deep circulation cell in the weddell gyre by scavenging and subsequent release at depth the model also explains depletion of 231pa and 230th in weddell sea bottom water wsbw by ventilation of newly formed deep water on a timescale of 10 years but this water mass is too dense to leave the weddell gyre in order to quantify the processes responsible for the 231pa and 230th composition of newly formed antarctic bottom water aabw we present a mass balance of 231pa and 230th in the atlantic sector of the southern ocean based on new data from the geotraces program the acc receives 6 0 1 5 10 6 dpm s 1 of 230th from the weddell sea similar in magnitude to the net input of 4 2 3 0 10 6 dpm s 1 from the north for 231pa the relative contribution from the weddell sea is much smaller only 0 3 0 1 10 6 compared to 2 7 1 4 10 6 dpm s 1 from the north weddell sea deep water wsdw leaving the weddell gyre northward to form aabw is exposed in the acc to resuspended opal rich sediments that act as efficient scavengers with a th pa fractionation factor f 1 hydrothermal inputs may provide additional removal with low f scavenging in the full meridional circulation across the opal rich acc thus acts as a double 231pa and 230th trap that preconditions newly formed aabw