glacial interglacial variablity in the benguela upwelling system spatial distribution and budgets of organic carbon accumulation 1 modern sedimentary total organic carbon toc content as a proxy for surface water export production was mapped on the shelf and on the upper continental slope of the benguela upwelling system using 137 core tops shelf maxima in toc can be correlated with maxima in surface water productivity on the slope high toc contents are observed offshore from sites of strong modern upwelling estimates of modern toc mass accumulation rates mar show that approximately 85 of the total is accumulating on the shelf toc mar were calculated mapped and budgeted for the holocene and for the last glacial maximum lgm using 19 sediment cores from the continental slope during the lgm centers of deposition and production have migrated offshore with respect to their holocene positions toc accumulation on the continental slope was approximately 84 higher during the lgm than during the holocene possibly reflecting enhanced productivity the toc distribution patterns and sediment echo sounding data suggest that undercurrents strongly influence the sedimentation off namibia winnowing and focusing result in great lateral heterogeneity of sedimentation rates and sediment properties individual cores therefore do not necessarily reflect general changes in export production these results highlight the need for detailed regional studies based on a large number of sediment cores for highly heterogeneous high productivity areas in order to derive general statements on total fluxes

water column 230th systematics in the eastern equatorial pacific ocean and implications for sediment focusing 230 th and 232 th abstract in an effort to investigate the cause of higher than expected sediment inventories of 230 th in the panama basin thorium isotopes were measured in eight deep water casts within the guatemala panama and peru basins along a meridional transect at 861w between 6 51n and 8 51s dissolved 230 th concentrations increase linearly from the surface to 1000 m at each transect station below 1000 m the deep waters of the panama basin show the highest deficit 50 of 230 th assuming a reversible exchange of 230 th between dissolved and sinking particulate matter and in comparison with the globally averaged water column 230 th peru basin waters have a larger range of dissolved 230 th concentrations 7 9 16 5 fg kg than that within panama basin waters 5 7 7 1 fg kg there is a progressive decrease in average dissolved deep water 41000 m 230 th concentrations from the southernmost sites in the peru basin toward the panama basin we suggest that intense scavenging by upwelling derived productivity near the equator 7 21 and resuspension of ubiquitous mn rich particulates in panama basin sediments are the primary causes of the significant south to north lateral gradient in deep water dissolved 230 th although 230 th from peru basin waters may be transported and then scavenged and ultimately buried in the panama basin our calculations suggest that the quantity of advected 230 th is relatively small between 15 and 30 of the total 230 th being produced within water column of the panama basin itself panama basin sediment focusing factors greater than 1 3 cannot be explained by lateral export and excess scavenging of water column 230 th dissolved 232 th concentra tions in addition to being the lowest reported so far in the literature fall within a very narrow range 6 26 pg kg and are generally invariable with depth this invariability suggests the dissolution of dust in surface waters as a likely sole source of dissolved 232 th for the entire water column

increased productivity in the eastern equatorial pacific during the last glacial maximum 19 000 to 14 000 yr b p a widespread pronounced enrichment in the organic carbon content occurs at shallow depths in eastern equatorial pacific sediments the c org peak is shown by radiocarbon dating to be temporally coincident with the last stage 2 or wisconsin glacial maximum in contrast caco 3 profiles show less overall variation which appears to be partly a function of the location of the sediments relative to the lysocline consideration of the iodine c org ratio in the sediments rules out enhanced preservation as a cause of the stage 2 organic carbon enrichment the peak instead must have resulted from a significant increase in productivity during the glacial maximum

non linear inversion of resistivity profiling data for some regular geometrical bodies1 the inversion of resistivity profiling data involves estimation of the spatial distribution of resistivities and thicknesses of rock layers from the apparent resistivity data values measured in the field as a function of electrode separation the drawbacks of using traditional curve matching techniques to solve this inverse problem have been overcome by iterative linear techniques but these require good starting models even if the shape of the causative body is asssumed known in spite of the recent developments in inversion techniques no robust method exists for the inversion of resistivity profiling data for the simple model of dikes and spheres which are the classical models of geophysical prospecting we apply three different non linear inversion schemes to invert synthetic resistivity profiling data for the classical models embedded in a uniform matrix of contrasting resistivity the three non linear algorithms used are called the metropolis simulated annealing sa very fast simulated annealing vfsa and a genetic algorithm ga we compare the performance of the three algorithms using synthetic data for an outcropping vertical dike model although all three methods were successful in obtaining optimal solutions for arbitrary starting models vfsa proved to be computationally the most efficient

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

tracing dust input to the global ocean using thorium isotopes in marine sediments thoromap continental dust input into the ocean atmosphere system has significant ramifications for biogeochemical cycles and global climate yet direct observations of dust deposition in the ocean remain scarce the long lived isotope thorium 232 232th is greatly enriched in upper continental crust compared to oceanic crust and mid ocean ridge basalt like volcanogenic material in open ocean sediments away from fluvial and ice rafted sources of continental material 232th is often assumed to be of predominantly eolian origin in conjunction with flux normalization based on the particle reactive radioisotope thorium 230 230th 232th measurements in marine sediments are a promising proxy for dust accumulation in the modern and past ocean here we present thoromap a new global data compilation of 230th normalized fluxes of 232th after careful screening we derive dust deposition estimates in the global ocean averaged for the late holocene 0 4 ka and the last glacial maximum lgm 19 23 ka thoromap is compared with dust deposition estimates derived from the community climate system model ccsm3 and ccsm4 two coupled atmosphere land ocean and sea ice models model data correlation factors are 0 63 ccsm3 and 0 59 ccsm4 in the late holocene and 0 82 ccsm3 and 0 83 ccsm4 in the lgm thoromap is the first compilation that is built on a single specific proxy for dust and that exclusively uses flux normalisation to derive dust deposition rates

sediment redistribution and grainsize effects on 230th normalized mass accumulation rates and focusing factors in the panama basin abstract here we examine how redistribution of differing grain sizes by sediment focusing processes in panama basin sediments affects the use of 230th as a constant flux proxy we study representative sediments of holocene and last glacial maximum lgm time slices from four sediment cores from two different localities close to the ridges that bound the panama basin each locality contains paired sites that are seismically interpreted to have undergone extremes in sediment redistribution i e focused versus winnowed sites both holocene and lgm samples from sites where winnowing has occurred contain significant amounts up to 50 of the 230th within the 63 m grain size fraction which makes up 40 70 of the bulk sediment analyzed for sites where focusing has occurred holocene and lgm samples contain the greatest amounts of 230th up to 49 in the finest grain sized fraction kretschmer et al 2010 suggest a maximum change of about 30 in 230th derived mars and focusing factors at focused sites except for our most focused site which requires an approximate 70 correction in one sample our 230th corrected 232th flux results suggest that the boundary between hemipelagically and pelagically derived sediments falls between 350 and 600 km from the continental margin

deglacial changes in dust flux in the eastern equatorial pacific abstract atmospheric dust levels may play important roles in feedbacks linking continental source areas tropical convection marine productivity and global climate these feedbacks appear to be particularly significant in the tropical pacific where variations in local convection and productivity have been demonstrated to have impacts on climate at higher latitudes modeling of past dust levels and related feedbacks has been limited however by a paucity of observational data in this study we present a temporal and spatial survey of dust fluxes to the eastern equatorial pacific over the past 30 kyr glacial and holocene fluxes of 232th a proxy for continental material were calculated by normalization to 230th from a north south transect of cores along 110 w between 3 s and 7 n odp sites 848 853 fluxes were 30 100 higher during the last glacial suggesting increased dustiness in both hemispheres during the glacial period in both time periods dust fluxes decrease towards the south reflecting scavenging of northern hemisphere dust by precipitation at the itcz the holocene meridional dust flux gradient between 7 n and 3 s is characterized by a steep drop in dust levels at the southern edge of the modern range of the itcz while the gradient is shallower and more nearly linear during the last glacial this change may indicate that the glacial itcz in this region was a less effective barrier to inter hemispheric dust transport most likely due to a decrease in convective intensity and precipitation during the last glacial alternatively the change in gradient may be explained by increased variability in the location of the glacial itcz our data do not appear to require a mean southerly displacement of the glacial itcz as suggested by the results of other studies

northern timing of deglaciation in the eastern equatorial pacific from alkenone paleothermometry 1 the equatorial cold tongue ect of the eastern pacific is the most dynamic ocean region in the world s tropics and sets the tempo for global climate anomalies arising from el nino southern oscillation enso events this region s deglaciation history and relationship with north and south polar climates remains poorly understood impeding integration of tropical pacific ocean atmosphere dynamics and enso variability in our understanding of glacial cycles here we present alkenone reconstructions of sea surface temperature sst across the last glacial termination from five ect cores east of the galapagos islands a composite index of sst based on these demonstrates strong temporal affinity with the two step deglaciation of the northern hemisphere composed of two distinct warming steps at the beginning of the bolling and the end of the younger dryas intervals within dating uncertainty warming in the ect began in phase with the bolling excursion was followed by a 2 3 ka plateau and resumed with a second pulse at the end of the younger dryas on the basis of our reconstructions about two thirds of the warming materialized at or after the end of the younger dryas implying a marked delay in the region s response to deglaciation the results challenge the prevailing paradigm that ect deglacial history conforms to an antarctic timing commonly attributed to advection from the southern ocean through an interior oceanic link or to synchronous response of both regions to co2 forcing our results emphasize instead the role of dynamical adjustments linked to northern hemisphere processes most likely transmitted through the atmosphere

boundary scavenging in the pacific ocean a comparison of 10be and 231pa concentrations of u th 231pa and 10be were measured in holocene sediments from two cores collected off the west coast of south america two cores from the east pacific rise two from the equatorial pacific and one from the south pacific central gyre our results together with data from 5 cores reported in the literature show that boundary scavenging plays a major role in the removal of 10be from the pacific ocean deposition rates of 10be at three margin sites are more than an order of magnitude greater than at sites of red clay accumulation in the deep central pacific deposition of 231pa is 4 to 5 fold greater at the margin sites the residence time of 10be with respect to chemical scavenging defined as its inventory in the water column divided by its rate of removal to the sediments varies regionally from 1000 years at the red clay sites in the deep central pacific to 100 years at the margin sites different factors control boundary scavenging of pa and be for example scavenging of 231pa is enhanced by metal oxide coatings of particles whereas this seems to have little influence on the scavenging of 10be 1990

excess sediment 230th transport along the sea floor or enhanced water column scavenging 1 the sedimentary inventory of 230th is often employed to distinguish between areas which receive excess sediment as the result of along bottom transport from those which lose sediment as a result of winnowing this process is referred to as focusing a case is made here that at least in the western equatorial pacific the excess 230th in on equator sediment is more likely scavenged from the overlying water column by the rain of organic matter than retransported along the bottom if so lateral variations in biological productivity may well contribute to the excess 230th inventories elsewhere as well

glacial to interglacial changes in carbonate and clay sedimentation in the atlantic ocean estimated from 230th measurements abstract the cause of the climatically controlled fluctuations in the carbonate content of deep sea sediments remains the subject of uncertainty and debate three variables are involved supply of biogenic carbonate loss by dissolution and dilution by non carbonate phases it is suggested that 230 th which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale results of a preliminary analysis based on published data indicate that for depths at and above the lysocline the carbonate fluctuations observed in cores from the north atlantic ocean are due primarily to variations in the terrigenous clay input which was 2 5 times higher during glacials than during interglacials carbonate deposition appears to have been somewhat reduced during glacials but probably not by more than a factor of 2 from published 230 th 232 th profiles it appears that the south atlantic ocean also received increased inputs of terrigenous clay during glacial periods

productivity control of fine particle transport to equatorial pacific sediment accumulation rates of 3he from cosmic dust 230th produced in the water column barite produced in the water column during decay of organic matter and fe and ti arriving with wind borne dust all are positively correlated in an equatorial pacific core tt013 pc72 01 1 n 139 4 w water depth 4298 m these accumulation rates are also positively correlated with the accumulation rates of noncarbonate material they are not significantly correlated to the mass accumulation rate of carbonate which makes up the bulk of the sediment the fluctuations in accumulation rates of these various components from different sources thus must result from variations in some process within the oceans and not from variations in their original sources sediment focusing by oceanic bottom currents has been proposed as this process marcantonio et al 1996 we argue that the variations in the accumulation rates of all these components are dominantly linked to changes in productivity and particle scavenging 3he 230th fe ti by fresh phytoplankton detritus which delivers ba upon its decay in the equatorial pacific upwelling region we speculate that as equatorial pacific productivity is a major component of global oceanic productivity its variations over time might be reflected in variations in atmospheric levels of methanesulfonic acid an atmospheric reaction product of dimethyl sulfide which is produced by oceanic phytoplankton and recorded in antarctic ice cores

seismic interpretation of pelagic sedimentation regimes in the 18 53 ma eastern equatorial pacific basin scale sedimentation and infilling of abyssal valleys understanding how pelagic sediment has been eroded transported and deposited is critical to evaluating pelagic sediment records for paleoceanography we use digital seismic reflection data from an integrated ocean drilling program site survey amat03 to investigate pelagic sedimentation across the eastern central equatorial pacific which represents the first comprehensive record published covering the 18 53 ma eastern equatorial pacific our goals are to quantify 1 basin hill scale primary deposition regimes and 2 the extent to which seafloor topography has been subdued by abyssal valley filling sediments the eastern pacific seafloor consists of a series of abyssal hills and basins with minor late stage faulting in the basement ocean crust rarely outcrops at the seafloor away from the rise crest both hills and basins are sediment covered the carbonate compensation depth is identified at 4440 m by the appearance of acoustically transparent clay intervals in the seismic data overall we recognized three different sedimentation regimes depositional high sedimentation rate transitional and minimal sedimentation low sedimentation rate regimes in all areas the sedimented seafloor mimics the underlying basement topography although the degree to which topography becomes subdued varies depositional regimes result in symmetric sedimentation within basins and subdued topography whereas minimal sedimentation regimes have more asymmetric distribution of sediments within topographic lows and higher seafloor relief regardless of sedimentation regime enhanced sediment deposition occurs within basins however we observe that basin infill is rarely more than twice as thick as sediment cover over abyssal hills if this variation is due to sediment focusing the focusing factor in the basins as measured by 230th is no more than a factor of 1 3 of the total vertical particulate rain

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

removal of230th and231pa from the open ocean concentrations of230th and231pa were measured in particulate matter collected by sediment traps deployed in the sargasso sea site s2 the north equatorial atlantic site e and the north equatorial pacific site p as well as in particles collected by in situ filtration at site e concentrations of dissolved th and pa were determined by extraction onto manganese dioxide adsorbers at site p and at a second site in the sargasso sea site d dissolved230th 231pa activity ratios were 3 6 at sites p and d in contrast for all sediment trap samples from greater than 2000 m unsupported230th 231pa ratios were 22 35 average 29 7 ratios were lower in particulate matter sampled at shallower depths particles filtered at 3600 m and 5000 m at site e had ratios of 50 and 40 results show that suspended particulate matter in the open ocean preferentially scavenges th relative to pa most of the230th produced by decay of234u in the open ocean is removed by adsorption to settling particulate matter in contrast less than 50 of the231pa produced by decay of235u is removed from the water column by this mechanism mixing processes transport the remainder to other sinks

paleoclimate history of galapagos surface waters over the last 135 000 yr abstract records of planktonic and benthic foraminiferal 18 o and planktonic mg ca from core tr163 22 just northwest of the galapagos islands reveal a detailed 250 450 year resolution climate history of the region over the last 135 thousand years kyr sea surface temperatures ssts reconstructed from g ruber mg ca averaged 24 3 0 4 c during the holocene 22 6 0 6 c during marine isotope stage mis 2 3 and 4 and 26 0 0 9 c during mis 5e changes in sst lead changes in both planktonic and benthic 18 o by an average 3 kyr suggesting that sst changes in this region predated continental ice volume changes changes in sst display clear millennial scale variability especially in marine isotope stage 3 with behavior somewhat similar to antarctic proxy air temperature and south pacific sst records removal of the temperature component from the planktonic 18 o record demonstrates that glacial interglacial 18 o water changes at this site were 1 0 0 2 similar to estimates for mean ocean shifts implying that salinity changes due to regional hydrological variation between the last glacial maximum and holocene in the galapagos region were minimal comparison between the tr163 22 sst record and an sst record from core tr163 19 north of the equatorial front reveals a largely similar broad scale climate history suggesting that changes in the galapagos region were caused by large scale forcing rather than by local or regional dynamical changes changes in atmospheric greenhouse forcing are the most plausible explanation for the observed large scale climate changes in the eastern equatorial pacific

beryllium 10 thorium 230 and protactinium 231 in galapagos microplate sediments implications of hydrothermal activity and paleoproductivity changes during the last 100 000 years biogenic particle fluxes from highly productive surface waters boundary scavenging and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the galapagos microplate eastern equatorial pacific in order to evaluate the importance of these three processes throughout the last 100 kyr concentrations of the radionuclides 10be 230th and 231pa and of mn and fe were measured at high resolution in sediment samples from two gravity cores klh 068 and klh 093 high biological productivity in the surface waters overlying the investigated area has led to 10be and 231pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr however during periods of high productivity at the up welling centers off peru and extension of the equatorial high productivity zone a relative loss of 10be and 231pa may have occurred in these sediment cores because of boundary scavenging the effects of hydrothermal activity were investigated by comparing the 230thex concentrations to the mn fe ratios and by comparing the fluxes of 230th and 10be which exceed production the results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core klh 093 during isotope stages 2 and 3 the hydrothermal supply of mn was deposited elsewhere probably because of changes in current regime or deep water oxygenation a strong increase of the mn fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230thex concentration is interpreted to be an effect of mn remobilization and reprecipitation in the sediment

230th normalization an essential tool for interpreting sedimentary fluxes during the late quaternary there is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates notwithstanding its common incidence this process generally referred to as sediment focusing is often difficult to recognize if redistribution is near synchronous to deposition the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked ignoring it however can lead to serious misinterpretations of sedimentary fluxes particularly when past changes in export flux from the overlying water are inferred in many instances this problem can be resolved at least for sediments deposited during the late quaternary by normalizing to the flux of 230th scavenged from seawater which is nearly constant and equivalent to the known rate of production of 230th from the decay of dissolved 234u we review the principle advantages and limitations of this method notwithstanding its limitations it is clear that 230th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates

extraterrestrial 3 he as a tracer of marine sediment transport and accumulation the deposition rate of deep sea sediments and their focused redeposition by deep sea currents can be evaluated from analyses of sedimentary 230th with a temporal resolution limited only by bioturbation6 7 10 11 230th is produced uniformly throughout the ocean by radioactive decay of dissolved 234u and is removed sufficiently fast by sorption onto sinking particles to act as a constant flux tracer of sedimentation rates but the half life of 230th 75 kyr limits its use for this purpose to the past 200 250 kyr here we explore the use of extraterrestrial 3he from interplanetary dust particles1 4 idps as a constant flux proxy that is free from this limitation a comparison of 3he with 230th in two cores from the equatorial pacific ocean indicates that the variability in the mean flux of idps over the past 200 kyr is less than 75 but in contrast to this relatively constant rate of supply of 3he to the deep sea the local burial rates of 3he and 230th have varied by a factor of five over the past 450 and 200 kyr respect ively we interpret this variability as reflecting sediment focusing with a temporal pattern that suggests regular cycles of climate driven reorganization of near bottom currents in the deep pacific ocean

do geochemical estimates of sediment focusing pass the sediment test in the equatorial pacific 1 the paleoceanographic recording fidelity of pelagic sediments is limited by chemical diagenesis and physical mixing bioturbation and horizontal sediment transport diagenesis and bioturbation are relatively well studied but the effects of physical sedimentation have been largely ignored modeling u series isotopes e g 230th can potentially quantify horizontal sediment movement but model horizontal sediment focusing often equals or exceeds the vertical particle rain we find no evidence of this level of sediment focusing in the equatorial pacific from geophysical data or sediment core comparisons the overestimate of sediment focusing by 230th is probably caused by poor model assumptions that sediment does not fractionate does not sort according to size during transport and that 230th cannot leak from slowly accumulating sediments both assumptions are weak u series methods do hold promise to quantify sediment focusing if properly calibrated with calibration the trade offs between seeking high sedimentation rates for better time resolution and the blurring by horizontal sediment focusing can be better assessed

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

climatically forced organic carbon burial in equatorial atlantic and pacific oceans over the past ten years sedimentary geochemists have noted that climate changes associated with pleistocene glaciations have had a pronounced effect on the rate of organic carbon burial found in the tropical oceans1 4 in particular two to five times higher organic carbon accumulation rates have been observed for the 18 kyr bp glacial maximum than for the holocene in several short sediment cores from both oceans see fig 1 here i present 300 kyr time series for organic carbon mass accumulation rate from four sediment cores two each from the equatorial pacific and atlantic and show that orbital forcing of global climate has strongly affected the rate of organic carbon burial in the equatorial oceans

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

3d geoelectric tomography and archaeological applications following a previous paper in which the principles of a 3d ground surface tomographic processing of self potential data were established we extend the method to active source geoelectric surveying the main purpose of the new tomographic approach is to obtain a physical image reconstruction of the induced electric charges distributed over buried resistivity discontinuities the information is produced in a probabilistic sense as the mathematical formulation underlying the method treats only the intrinsic physical nature of the generated electric field underground and the method of its ground surface detection independently of the geometry of the unknown structures in practice a 3d tomography is realized by cross correlating a set of distributed electric field ground surface data with a scanning function representing a unit positive point charge located anywhere in the lower half space the resolution of the method is tested on the synthetic response of a 3d structural simulation of an archaeological target consisting of an infinitely resistive prismatic body immersed in a half space including surface inhomogeneities and layering finally the field response of a 3d structure consisting of a hypogeal dromos chamber tomb inside the sabine necropolis at colle del forno close to rome is presented and discussed

grain size effects on th 230 xs inventories in opal rich and carbonate rich marine sediments abstract excess thorium 230 230thxs as a constant flux tracer is an essential tool for paleoceanographic studies but its limitations for flux normalization are still a matter of debate in regions of rapid sediment accumulation it has been an open question if 230thxs normalized fluxes are biased by particle sorting effects during sediment redistribution in order to study the sorting effect of sediment transport on 230thxs we analyzed the specific activity of 230thxs in different particle size classes of carbonate rich sediments from the south east atlantic and of opal rich sediments from the atlantic sector of the southern ocean at both sites we compare the 230thxs distribution in neighboring high vs low accumulation settings two grain size fractionation methods are explored we find that the 230thxs distribution is strongly grain size dependent and 50 90 of the total 230thxs inventory is concentrated in fine material smaller than 10 m which is preferentially deposited at the high accumulation sites this leads to an overestimation of the focusing factor and consequently to an underestimation of the vertical flux rate at such sites the distribution of authigenic uranium indicates that fine organic rich material has also been re deposited from lateral sources if the particle sorting effect is considered in the flux calculations it reduces the estimated extent of sediment focusing in order to assess the maximum effect of particle sorting on we present an extreme scenario in which we assume a lateral sediment supply of only fine material comparing 230thxs normalized mass fluxes biased by sorting effects with uncorrected mass fluxes we suggest that 230thxs normalization is still a valid tool to correct for lateral sediment redistribution however differences in focusing factors between core locations have to be evaluated carefully taking the grain size distributions into consideration

glacial interglacial dynamics of the eastern equatorial pacific cold tongue intertropical convergence zone system reconstructed from oxygen isotope records record mean late holocene lh last glacial maximum lgm d 18 o amplitudes ranging between 1 0 and 1 3 we estimate that mean sea surface temperatures sst in this region during the lgm were on average 1 5 0 5 c lower than the lh larger d 18 o amplitudes are observed in sites north of the equator indicating a spatial pattern of reduced meridional sst gradient across the equator during the lgm this result is supported by comparison of mg ca sst reconstructions from two sites straddling the equator we interpret the reduction of this gradient during the lgm as evidence for a less intense cold tongue intertropical convergence zone itcz frontal system a more southerly position of the itcz and weaker southeast equatorial trades in the eep indexterms 1620 global change climate dynamics 3309 3339 meteorology and atmospheric dynamics ocean atmosphere interactions 0312 4504 3344 meteorology and atmospheric dynamics paleoclimatology 4231 oceanography general equatorial oceanography 4267 oceanography general paleoceanography keywords cold tongue itcz oxygen isotopes tropical pacific

sediment focusing creates 100 ka cycles in interplanetary dust accumulation on the ontong java plateau the accumulation of extraterrestrial 3 he a tracer for interplanetary dust particles idps in sediments from the ontong java plateau ojp western equatorial pacific ocean has been shown previously to exhibit a regular cyclicity during the late pleistocene with a period of v100 ka those results have been interpreted to reflect periodic variability in the global accretion of idps that in turn has been linked to changes in the inclination of earth s orbit with respect to the invariable plane of the solar system here we show that the accumulation in ojp sediments of authigenic 230 th produced by radioactive decay of 234 u in seawater exhibits a 100 ka cyclicity similar in phase and amplitude to that evident in the 3 he record we interpret the similar patterns of 230 th and 3 he accumulation to reflect a common origin within the ocean climate system comparing spatial and temporal patterns of sediment accumulation against regional patterns of biological productivity and against the well established pattern of caco3 dissolution in the deep pacific ocean leads to the further conclusion that a common 100 ka cycle in accumulation of biogenic authigenic and extraterrestrial constituents in ojp sediments reflects the influence of climate related changes in sediment focusing rather than changes in the rate of production or supply of sedimentary constituents 7 2002 elsevier science b v all rights reserved

export fluxes of calcite in the eastern equatorial pacific from the last glacial maximum to present 1 the eastern equatorial pacific eep is an important center of biological productivity generating significant organic carbon and calcite fluxes to the deep ocean we reconstructed paleocalcite flux for the past 30 000 years in four cores collected beneath the equatorial upwelling and the south equatorial current sec by measuring ex230th normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for fraction of calcite preserved this method produced very similar results at the four sites and revealed that the export flux of calcite was 30 50 lower during the lgm compared to the holocene the internal consistency of these results supports our interpretation which is also in agreement with emerging data indicating lower glacial productivity in the eep possibly as a result of lower nutrient supply from the southern ocean via the equatorial undercurrent however these findings contradict previous interpretations based on mass accumulation rates mar of biogenic material in the sediment of the eep which have been taken as reflecting higher glacial productivity due to stronger wind driven upwelling

glacial deep ocean sequestration of co2 driven by the eastern equatorial pacific biologic pump abstract the potential influence of low latitude ocean primary productivity on glacial atmospheric carbon dioxide levels has proven challenging to deduce using mass accumulation rates mars of biogenic particulates in deep sea sediment cores benthic foraminiferal b ca serves as a proxy for past seawater calcite saturation state and thereby provides a fresh perspective on this outstanding paleoceanographic problem here we employ cibicidoides wuellerstorfi b ca in the panama basin region of the eastern equatorial pacific eep to investigate the nature of deep tropical pacific carbon storage over the past 50 ka bp we present evidence for persistently lower deep panama basin calcite saturation state reflecting an increase in total carbon dioxide storage during the last ice age relative to the holocene these results reflect the modification of inflowing deep waters by overlying export productivity and support the concept of an invigorated glacial eep soft tissue pump possibly driven by oceanic nutrient iron and silica redistribution benthic cibicidoides spp carbon 13 is consistent with this conclusion by exhibiting substantially lighter values during glacial time reflecting the accumulation of metabolic carbon dioxide in the deep tropical pacific counterintuitively downcore application of the globorotalia menardii calcite fragmentation index mfi reveals enhanced glacial sedimentary calcite preservation in the panama basin together these results point towards a systematic decoupling of bottom water chemistry from biogenic burial fluxes the crux of the aforementioned traditional paleoproductivity problem

patterns of caco3 deposition in the eastern tropical pacific ocean for the last 150 kyr evidence for a southeast pacific depositional spike during marine isotope stage mis 2 1 we constructed biogenic mass accumulation rate mar time series for eastern pacific core transects across the equator at 105 and 85 w and along the equator from 80 to 140 w we used empirical orthogonal function eof analysis to extract spatially coherent patterns of caco3 deposition for the last 150 kyr eof mode 1 51 variance is a caco3 mar spike centered in marine oxygen isotope stage 2 mis 2 found under the south equatorial current eof mode 2 19 of variance is high north of the equator eof mode 3 9 of variance is an east west mode centered along the north equatorial counter current the mis 2 caco3 spike is the largest event in the eastern pacific for the last 150 kyr caco3 mars are 2 3 times higher at 18 ka than elsewhere in the record including mis 6 it is caused by high caco3 production rather than minimal dissolution eof 2 while it resembles deep water flow patterns nevertheless shows coherence to corg deposition and is probably also driven by caco3 production index terms 4267 oceanography general paleoceanography 1615 global change biogeochemical processes 4805 1030 geochemistry geochemical cycles 0330 3022 marine geology and geophysics marine sediments processes and transport keywords late pleistocene paleoproductivity pacific calcite biogeochemistry carbon cycle

the response of excess 230th and extraterrestrial 3he to sediment redistribution at the blake ridge western north atlantic the constant flux proxies excess 230th 230thxs and extraterrestrial 3he 3heet are commonly used to calculate sedimentary mass accumulation rates and to quantify lateral advection of sediment at core sites in settings with significant lateral input or removal of sediment these calculations depend on the assumption that concentrations of 230thxs and 3heet are the same in both advected sediment and sediment falling through the water column above the core site sediment redistribution is known to fractionate grain sizes preferentially transporting fine grains though relatively few studies have examined the grain size distribution of 230thxs and 3heet presently available data indicate that both are concentrated in fine grains suggesting that fractionation during advection may bias accumulation rate and lateral advection estimates based on these proxies in this study we evaluate the behavior of 230thxs and 3heet in holocene and last glacial samples from two cores from the blake ridge a drift deposit in the western north atlantic at the end of the last glacial period both cores received large amounts of laterally transported sediment enriched in fine grained material we find that accumulation rates calculated by normalization to 230th and 3he are internally consistent despite large spatial and temporal differences in sediment advection our analyses of grain size fractions indicate that 70 of 3heet bearing grains are in the 20 m fraction with roughly equal amounts in the 4 and 4 20 m fractions 230thxs is concentrated in 4 m grains relative to 4 to 20 m grains by approximately a factor of 2 in holocene samples and by a much larger factor averaging a factor of 10 in glacial samples despite these enrichments of both constant flux proxies in fine particles the fidelity of 230th and 3he based accumulation rate estimates appears to be preserved even in settings with extreme sediment redistribution perhaps due to the cohesive behavior of fine particles in marine settings

comment on do geochemical estimates of sediment focusing pass the sediment test in the equatorial pacific by m lyle et al accurately estimating the vertical flux of material reaching the seafloor from the overlying surface waters is essential for the paleoceanographic reconstruction of a wide variety of oceanic processes two approaches are currently being used one consists of estimating mass accumulation rates mar between dated horizons as the product of linear sedimentation rates sediment dry bulk densities and concentrations one pitfall with this approach is that sediments can be redistributed on the seafloor by bottom currents and their accumulation may not necessarily reflect the true vertical rain rate originating from the overlying water column to address this problem the method of 230th normalization was developed bacon 1984 this method is based on the assumption that the rapid scavenging of 230th produced in the water column by decay of dissolved uranium results in its flux to the seafloor always being close to its known rate of production to the extent that this assumption is correct scavenged 230th can be used as a reference to estimate the settling flux of other sedimentary constituents and to correct for sediment redistribution on the seafloor henderson and anderson 2003 francois et al 2004

glacial interglacial variability in the delivery of dust to the central equatorial pacific ocean abstract fluxes of continental mineral aerosols dust were greater during glacial periods than during interglacials throughout most regions of the earth the equatorial pacific ocean was a possible exception to this pattern in that previous studies have reported either greater dust fluxes during interglacials or no consistent glacial interglacial pattern of dust flux we have applied the 230th normalization technique to derive five new records of dust flux from central equatorial pacific ocean sediments in contrast to previous studies which relied on stratigraphic accumulation rates the 230th normalization technique produces internally consistent results revealing fluxes to this region of continental lithogenic material that were positively correlated with global ice volume throughout the past 300 000 yr maximum glacial fluxes of continental mineral aerosols exceed minimum interglacial fluxes by about a factor of 2 similar to changes found elsewhere at low and mid latitudes this amplitude of variability is substantially smaller than that seen in some recent models and these observations provide a calibration point for future model development

reply to comment by r francois et al on do geochemical estimates of sediment focusing pass the sediment test in the equatorial pacific further explorations of 230th normalization

abrupt intensification of the sw indian ocean monsoon during the last deglaciation constraints from th pa and he isotopes abstract sediments from western arabian sea core 74kl representing the last 23 ka were analyzed for helium thorium and protactinium isotopes assuming global average fluxes of extraterrestrial 3he and 230th the average 3he derived sediment mass accumulation rate mar is a factor of 1 8 higher than the average 230th derived mar 3he and 230th derived mars converge however during the younger dryas yd and during the peak of the early holocene humid interval these features not seen anywhere else in the world probably reflect a combination of climate driven changes in the flux of 230th and 3he ratios of xs231pa xs230th proxies of paleoproductivity are lowest during the last glacial maximum lgm and increase abruptly during the bolling allerod later following a sudden decrease to near lgm values during the yd they rise abruptly to maximum values for the entire record in the early holocene we hypothesize that low xs231pa xs230th ratios reflect low productivity due to the decreased intensity of the sw monsoon whereas the opposite is true for high ratios the correlation between arabian sea productivity and monsoonal upwelling on the one hand and north atlantic climate variability on the other suggests a linkage between high and low latitude climates caused by changing patterns of atmospheric circulation

near collapse of the meridional sst gradient in the eastern equatorial pacific during heinrich stadial 1 1 sea surface temperatures sst and inorganic continental input over the last 25 000 years 25 ka are reconstructed in the far eastern equatorial pacific eep based on three cores stretching from the equatorial front 0 01 n me0005 24jc into the cold tongue region 3 6 s tr163 31p and v19 30 we revisit previously published alkenone derived sst records for these sites and present a revised chronology for v19 30 inorganic continental input is quantified at all three sites based on 230th normalized fluxes of the long lived continental isotope thorium 232 and interpreted to be largely dust our data show a very weak meridional cross equatorial sst gradient during heinrich stadial 1 hs1 18 15 ka b p and high dust input along with peak export production at and north of the equator these findings are corroborated by an earth system model experiment for hs1 that simulates intensified northeasterly trade winds in the eep stronger equatorial upwelling and surface cooling furthermore the related southward shift of the intertropical convergence zone itcz during hs1 is also indicative of drier conditions in the typical source regions for dust

a pacific hydrographic section at 88 w water property distribution full depth conductivity temperature depth ctd hydrographic measurements with high horizontal and vertical resolution were made in february april 1993 along a line lying at a nominal longitude of 88 w and extending from southern chile 54 s to guatemala 14 n it crossed five major deep basins southeast pacific chile peru panama and guatemala basins east of the east pacific rise vertical sections of potential temperature salinity potential density oxygen silica phosphate nitrate and nitrite are presented to illustrate the structure of the entire water column some features of interest found in the sections are described and an attempt is made to interpret them in terms of the isopycnal property distributions associated with the large scale ocean circulation these features include various near surface waters observed in the tropical and subtropical regions and the fronts that mark the boundaries of these waters the possible importance of salt fingering to the downward salt transfer from the high salinity subtropical water a shallow thermostad pycnostad developed at 16 18 5 c in the subtropical water low salinity surface water in the subantarctic zone west of southern chile large domains of extremely low oxygen in the subpycnocline layer on both sides of the equator and a secondary nitrite maximum associated with a nitrate minimum in these low oxygen domains high salinity low oxygen high nutrient subpycnocline water that is carried poleward along the eastern boundary by the peru chile undercurrent the subantarctic mode and antarctic intermediate waters middepth isopycnal property extrema observed at the crest of the sala y gomez ridge influences of the north pacific and the north atlantic upon deep waters along the section and the characteristics and sources of the bottom waters in the five deep basins along the section

eastern pacific warm pool paleosalinity and climate variability 0 30 kyr multiproxy geologic records of 18o and mg ca in fossil foraminifera from sediments under the eastern pacific warm pool epwp region west of central america document variations in upper ocean temperature pycnocline strength and salinity i e net precipitation over the past 30 kyr although evident in the paleotemperature record there is no glacial interglacial difference in paleosalinity suggesting that tropical hydrologic changes do not respond passively to high latitude ice sheets and oceans millennial variations in paleosalinity with amplitudes as high as 4 practical salinity units occur with a dominant period of 3 5 ky during the glacial deglacial interval and 1 0 1 5 ky during the holocene the amplitude of the epwp paleosalinity changes greatly exceeds that of published caribbean and western tropical pacific paleosalinity records epwp paleosalinity changes correspond to millennial scale climate changes in the surface and deep atlantic and the high northern latitudes with generally higher lower paleosalinity during cold warm events in addition to intertropical convergence zone itcz dynamics which play an important role in tropical hydrologic variability changes in atlantic pacific moisture transport which is closely linked to itcz dynamics may also contribute to hydrologic variations in the epwp calculations of interbasin salinity average and interbasin salinity contrast between the epwp and the caribbean help differentiate long term changes in mean itcz position and atlantic pacific moisture transport respectively

sediment sorting and focusing in the eastern equatorial pacific abstract the interpretation of sedimentary records in terms of glacial interglacial changes in particle flux in the eastern equatorial pacific eep has been controversial here we analyze disaggregated inorganic grain size digs distributions of three marine sediment cores from this region focusing on the last 21 ka to investigate evidence of sediment redistribution on the sea floor grain size sorting coefficients show that sediments in the eep are moderately to well sorted indicating sediment reworking in this region due to bottom currents furthermore a systematic correlation between focusing factors and sorting coefficients at two sites shows that more focused sediments are also better sorted we conclude that grain size based sedimentary records are consistent with the 230 th based evidence of lateral sediment redistribution on the sea floor in the eep

thorium 230 normalized particle flux and sediment focusing in the panama basin region during the last 30 000 years 1 application of the 230 th normalization method to estimate sediment burial fluxes in six cores from the eastern equatorial pacific eep reveals that bulk sediment and organic carbon fluxes display a coherent regional pattern during the holocene that is consistent with modern oceanographic conditions in contrast with estimates of bulk mass accumulation rates mars derived from core chronologies two nearby sites less than 10 km apart which have different mars show nearly identical 230 th normalized bulk fluxes focusing factors derived from the 230 th data at the foot of the carnegie ridge in the panama basin are 2 in the holocene implying that lateral sediment addition is significant in this part of the basin new geochemical data and existing literature provide evidence for a hydrothermal source of sediment in the southern part of the panama basin and for downslope transport from the top of the carnegie ridge the compilation of core records suggests that sediment focusing is spatially and temporally variable in the eep during oxygen isotope stage 2 ois 2 from 13 27 ka bp focusing appears even higher compared to the holocene at most sites similar to earlier findings in the eastern and central equatorial pacific the magnitude of the glacial increase in focusing factors however is strongly dependent on the accuracy of age models we offer two possible explanations for the increase in glacial focusing compared to the holocene the first one is that the apparent increase in lateral sediment redistribution is partly or even largely an artifact of insufficient age control in the eep while the second explanation which assumes that the observed increase is real involves enhanced deep sea tidal current flow during periods of low sea level stand

timescales of lateral sediment transport in the panama basin as revealed by radiocarbon ages of alkenones total organic carbon and foraminifera abstract paired radiocarbon measurements on haptophyte biomarkers alkenones and on co occurring tests of planktic foraminifera neogloboquadrina dutertrei and globogerinoides sacculifer from late glacial to holocene sediments at core locations me0005 24jc y69 71p and mc16 from the south western and central panama basin indicate no significant addition of pre aged alkenones by lateral advection the strong temporal correspondence between alkenones foraminifera and total organic carbon toc also implies negligible contributions of aged terrigenous material considering controversial evidence for sediment redistribution in previous studies of these sites our data imply that the laterally supplied material cannot stem from remobilization of substantially aged sediments transport if any requires syn depositional nepheloid layer transport and redistribution of low density or fine grained components within decades of particle formation such rapid and local transport minimizes the potential for temporal decoupling of proxies residing in different grain size fractions and thus facilitates comparison of various proxies for paleoceanographic reconstructions in this study area anomalously old foraminiferal tests from a glacial depth interval of core y69 71p may result from episodic spillover of fast bottom currents across the carnegie ridge transporting foraminiferal sands towards the north

particle sorting during sediment redistribution processes and the effect on 230th normalized mass accumulation rates the 230th method of determining mass accumulation rates mars assumes that little to no fractionation occurs during sediment redistribution processes at the seafloor we examine 230th inventories in radiocarbon dated multicore sediments from paired winnowed and focused sites at cocos and carnegie ridges panama basin radiocarbon derived sand mars which likely represent the vertical rain of particles poorly transported by bottom currents are similar at each of the paired sites but are different using 230th normalization 230th normalized mars are about 60 lower at focused sites and likely underestimate vertical mars while the reverse is true for winnowed sites we hypothesize that size fractionation occurs most frequently at lower current velocities resulting in the coarse fraction being left behind and primarily the fine 230th rich grains being transported downslope 230th normalization works well for recording fine grained detrital and opal but not coarse grained carbonate fluxes in regions that have undergone sediment redistribution

enhanced sedimentation along the equator in the western pacific measurements on cores from the ontong java plateau demonstrate that on equator sediment accumulates more 230th than is produced in the overlying water column in contrast cores from 3 8 n accumulate an amount of excess 230th more nearly equal to that produced in the overlying water column taken together with the observation that both the caco3 and non caco3 components of the sediment accumulate at approximately twice the rate in on equator than in off equator cores broecker et al 1999 this requires either that the rain of the larger amount of organic matter generated above the equatorial upwelling plume entrains excess clay and 230th during its fall to the sea floor or that physical transport processes either in the upper ocean or near the sea bed redistribute sediments in such a way that they preferentially accumulate in a narrow 1 latitude equatorial centered belt

variations in productivity and eolian fluxes in the northeastern arabian sea during the past 110 ka abstract high resolution one to two samples ka radionuclide proxy records from core 93kl in the northeastern arabian sea provide evidence for millennial climate variability over the past 110 ka we interpret 230th normalized 232th fluxes as a proxy for eolian input and authigenic uranium concentrations as a proxy for past productivity we attribute orbital and suborbital variations in both proxies to changes in the intensity of the southwest indian ocean monsoon the highest 230th normalized 232th fluxes occur at times that are consistent with the timing of the younger dryas heinrich events 1 7 and cold dansgaard oeschger stadial events recorded in the gisp2 ice core such high dust fluxes may be due to a weakened southwest monsoon in conjunction with strengthened northwesterlies from the arabian peninsula and mesopotamia authigenic uranium concentrations on the other hand are highest during warm dansgaard oeschger interstadials when the southwest monsoon is intensified relative to the northwesterly winds our results also indicate that on orbital timescales maximum average eolian fluxes coincide with the timing of marine isotopic stage mis 2 and 4 while minimum fluxes occur during mis 1 3 and 5 although the forcing mechanism s controlling suborbital variabilities in monsoonal intensity is still debated our findings suggest an atmospheric teleconnection between the low latitude southwest monsoon and north atlantic climate

millennial scale iron fertilization of the eastern equatorial pacific over the past 100 000 years productivity in the eastern equatorial pacific is limited by the availability of iron geochemical analyses of a 100 000 year long sediment core suggest that pulses of dust deposition during heinrich stadials fuelled primary productivity

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

particulate fluxes of 230th 231pa and 10be in the northeastern pacific ocean abstract measurements of 230th 231pa and 10be in sediment trap samples and holocene sediments collected along a transect in the northeastern pacific indicate that the removal of these nuclides from the ocean is greatly influenced by boundary scavenging more than 35 of the 230th at least 70 of the 231pa and as much as 85 of the 10be fluxes collected by sediment traps at a nearshore site located 120 km off shore are supplied by lateral transport of the dissolved nuclides from the open ocean these findings are consistent with the hypothesis that boundary scavenging is proportional to the oceanic residence times of the nuclides 10 40 y for 230th 100 y for 231 pa and 1000 y for 10be results of this study indicate that particle flux is a principal factor influencing scavenging of 230th 231pa and 10be from the water column the marine geochemistry of 10be in ocean margins is fundamentally different from that in the open ocean scavenging of 10be occurs throughout the water column at the margin area in this study in contrast to the pattern of 10be being removed only in the surface waters and then remineralized at depth in the open ocean aluminosilicate may be a major phase removing 10be from the water column in the open ocean but not at ocean margins nuclide accumulation rates in the underlying holocene sediments are all higher than the corresponding fluxes collected by sediment traps several possibilities that could account for this observation include downslope sediment transport deployment of the traps during a period when the particle flux was not representative of long term average conditions and scavenging of the nuclides from near bottom waters the high accumulation rate of authigenic mn in some holocene sediments which can be attributed to redox cycling of mn may have caused near bottom scavenging of 230th and 231pa as well as certain other trace elements such as ba cu and zn

modeling the particle flux effect on distribution of 230th in the equatorial pacific 1 the normalization of sediment accumulation rates by 230th is increasingly used to constrain particle rain rates because 230th is produced at a known rate in the water column and removed relatively quickly to the sediment several recent contributions have questioned this approach and suggested that the preferential removal of 230th in areas of high particle flux the particle flux effect introduces a systematic bias in the 230th normalization technique we use a circulation model that includes a description of particle scavenging to show that the particle flux effect cannot explain observations of high 230th accumulation in equatorial pacific sediments relative to 230th production in the water column we further consider the possible variation of particle rain rates over time we find only a minimal sensitivity in the particle flux effect due to increases in the particle rain rate at the equatorial pacific by a factor of up to 10 this situation exists because the residence time of 230th in the water column is too short to permit significant lateral transport with reasonable isopycnal mixing coefficients we conclude that the increased rates of 230th accumulation found in equatorial pacific sediments deposited during the last glacial maximum cannot have been caused by the particle flux effect

sediment focusing in the panama basin eastern equatorial pacific ocean abstract age model derived sediment mass accumulation rates mars are consistently higher than 230th normalized mars in the equatorial pacific ocean during the past 25 ka the offset being highest in the panama basin suggests a significant role for deep sea sediment redistribution i e sediment focusing in this region here we test the hypothesis that downslope transport of sediments from topographically high regions that surround the panama basin is the cause of higher than expected xs230th inventories over the past 25 ka in the deeper parts of the basin we find little difference in xs230th inventories between the highest and lowest reaches of the basin furthermore there is no correlation between xs230th derived sediment focusing factors and water depth which suggests that the topographic highs do not serve as a source of xs230th a spatial analysis suggests that there may be an enhanced scavenging effect on xs230th concentrations in sediment closest to the equator where productivity is the highest although further data is necessary to corroborate this at the equator xs230th derived focusing factors are high and range from about 1 to 5 during the holocene and about 1 to 11 during the last glacial in contrast non equatorial cores show a smaller range in variability from about 0 7 to 2 8 during the holocene and from 0 7 to 3 6 during the last glacial based on 232th flux measurements we hypothesize that the location at which eolian detrital fluxes surpass the riverine detrital fluxes is approximately 300 km from the margin while riverine fluxes from coastal margins were higher during the holocene eolian fluxes were higher during the last glacial

artificial neural networks for parameter estimation in geophysics artificial neural systems have been used in a variety of problems in the fields of science and engineering here we describe a study of the applicability of neural networks to solving some geophysical inverse problems in particular we study the problem of obtaining formation resistivities and layer thicknesses from vertical electrical sounding ves data and that of obtaining 1d velocity models from seismic waveform data we use a two layer feedforward neural network fnn that is trained to predict earth models from measured data part of the interest in using fnns for geophysical inversion is that they are adaptive systems that perform a non linear mapping between two sets of data from a given domain in both of our applications we train fnns using synthetic data as input to the networks and a layer parametrization of the models as the network output the earth models used for network training are drawn from an ensemble of random models within some prespecified parameter limits for network training we use the back propagation algorithm and a hybrid back propagation simulated annealing method for the ves and seismic inverse problems respectively other fundamental issues for obtaining accurate model parameter estimates using trained fnns are the size of the training data the network configuration the description of the data and the model parametrization our simulations indicate that fnns if adequately trained produce reasonably accurate earth models when observed data are input to the fnns

glaciological reconnaissance of an ice core drilling site penny ice cap baffin island analyse climatique sur 10 4 ans d une carotte de glace de 20 m prelevee dans l ile de baffin on a etudie les teneurs en oxygene en tritium le ph la conductivite electrolytique la concentration en ions majeurs et en particules une carotte proche a ete analysee de son contenu en pollen l epaisseur et la morphologie de la couche ont ete mesurees par echo radar

is the core top modern observations from the eastern equatorial pacific abstract a compilation of ages from 67 core tops in the eastern equatorial pacific eep does not display an easily discernible regional pattern the ages range from 790 to over 15 000 years the youngest core tops with the highest sediment focusing factors are located in the panama basin there are weak but statistically significant inverse relationships between core top age and age model based mass accumulation rates bioturbation depth linear sedimentation rate and sediment focusing factors however we found no statistically significant relationship between core top age and calcite dissolution in sediments or 230th normalized mass accumulation rates we found evidence suggesting that greater amount of sediment focusing helps to preserve the carbonate fraction of the sediment where focusing is taking place when focusing factors are plotted against percent calcite dissolved we observe a strong inverse relationship and core tops younger than 4500 years tend to occur where focusing factors are high and percent calcite dissolved values are low using labile organic carbon fluxes to estimate bioturbation depth in the sediments results in the observation that where bioturbation depth is shallow

more humid interglacials in ecuador during the past 500 kyr linked to latitudinal shifts of the equatorial front and the intertropical convergence zone in the eastern tropical pacific 1 studying past changes in the eastern equatorial pacific ocean dynamics and their impact on precipitation on land gives us insight into how the intertropical convergence zone itcz movements and the el nino southern oscillation modulate regional and global climate in this study we present a multiproxy record of terrigenous input from marine sediments collected off the ecuadorian coast spanning the last 500 kyr in parallel we estimate sea surface temperatures sst derived from alkenone paleothermometry for the sediments off the ecuadorian coast and complement them with alkenone based sst data from the panama basin to the north in order to investigate sst gradients across the equatorial front near the equator today s river runoff is tightly linked to sst reaching its maximum either during the austral summer when the itcz migrates southward or during el nino events our multiproxy reconstruction of riverine runoff indicates that interglacial periods experienced more humid conditions than the glacial periods the north south sst gradient is systematically steeper during glacial times suggesting a mean background climatic state with a vigorous oceanic cold tongue resembling modern la nina conditions this enhanced north south sst gradient would also imply a glacial northward shift of the intertropical convergence zone at least in vicinity of the cold tongue a pattern that has not yet been reproduced in climate models

sediment focusing in the central equatorial pacific ocean at four sites in the central equatorial pacific ocean the flux of extraterrestrial he determined using the excess 230th profiling method is 8 10 13 cm stp cm 2 ka 1 this supply rate is constant to within 30 at these same sites however the burial rate of he determined using chronostratigraphic accumulation rates varies by more than a factor of 3 the lowest burial rates which occur north of the equator at 1 n 139 w are lower than the global average rate of supply of extraterrestrial he by 20 and indicate that sediment winnowing may have occurred the highest burial rates which are recorded at the equator and at 2 s are higher than the rate of supply of extraterrestrial he by 100 and these provide evidence for sediment focusing by analyzing several proxies measured in core pc72 sediments spanning the past 450 kyr we demonstrate that periods of maximum burial rates of 230th he 10be ti and barite with a maximum peak to trough amplitude of a factor of 6 take place systematically during glacial time however the ratio of any one proxy to another is constant to within 30 over the entire length of the records given that each proxy represents a different source 234u decay in seawater interplanetary dust upper atmosphere continental dust or upper ocean our preferred interpretation for the covariation is that the climate related changes in burial rates are driven by changes in sediment focusing

dry bulk density and caco3 relationships in upper quaternary sediments of the eastern equatorial pacific abstract results from 1110 paired caco3 and bulk density measurements from cores raised from the eastern equatorial pacific permit formulation of well constrained caco3 dbd relationships for that region the cores lie along a n s transect at 110 w from 10 n to 3 s underneath the different currents of the equatorial current system and along an e w transect from 110 to 90 w at approximately 3 s two distinct crescent shaped dry bulk density caco3 relationships are observed for equal caco3 percentages sediments from those sites at 110 w which are situated in the high productivity zone have lower dry bulk density cores raised from closer to shore have relatively greater dbd

glacial to interglacial fluctuations in productivity in the equatorial pacific as indicated by marine barite an empirical correlation between marine barite baso4 accumulation rate in core top sediment samples from two equatorial pacific transects at 140 w and 110 w and the estimated primary productivity of the overlying water column were used to evaluate glacial to interglacial changes in productivity fluctuations in barite accumulation rates down core indicate that during glacial periods of the past 450 000 years the productivity in the central and eastern equatorial pacific was about two times that during intervening interglacial periods this result is consistent with other evidence that productivity was high in the eastern and central equatorial pacific during the last glacial