circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

a model data comparison of δ13c in the glacial atlantic ocean 1 we compare a compilation of 220 sediment core 13c data from the glacial atlantic ocean with three dimensional ocean circulation simulations including a marine carbon cycle model the carbon cycle model employs circulation fields which were derived from previous climate simulations all sediment data have been thoroughly quality controlled focusing on epibenthic foraminiferal species such as cibicidoides wuellerstorfi or planulina ariminensis to improve the comparability of model and sediment core carbon isotopes the model captures the general 13c pattern indicated by present day water column data and late holocene sediment cores but underestimates intermediate and deep water values in the south atlantic the best agreement with glacial reconstructions is obtained for a model scenario with an altered freshwater balance in the southern ocean that mimics enhanced northward sea ice export and melting away from the zone of sea ice production this results in a shoaled and weakened north atlantic deep water flow and intensified antarctic bottom water export hence confirming previous reconstructions from paleoproxy records moreover the modeled abyssal ocean is very cold and very saline which is in line with other proxy data evidence

late pleistocene evolution of the ocean s carbonate system abstract we demonstrate that the carbonate record from a single site ocean drilling program site 1089 in the deep south atlantic represents a qualitative high resolution record of the temporal evolution of the carbonate saturation state of the deep sea the record is especially notable because it is free from many of the complications that limit other records low sedimentation rates blurring by chemical erosion bioturbation etc the pattern of carbonate variability is characteristic of indo pacific cores with high carbonate glacials and low carbonate interglacials wt carbonate lags changes in benthic 18 o by an average of 7 6 kyr and carbonate variations are in phase with the rate of change first derivative of benthic 18 o intense dissolution occurs at the transition from interglacial to glacial periods and increased preservation occurs during deglaciations these observations represent two fundamentally different responses of the marine carbonate system the lagged response of carbonate to 18 o reflects a steady state mass balance process whereby the lysocline adjusts to maintain alkalinity balance between riverine input and marine burial the site 1089 carbonate signal is remarkably similar to inferred changes in the sr ca of seawater for the past 250 kyr which implies that both carbonate dissolution and seawater sr ca may be controlled by sea level induced changes in the location of carbonate deposition shelf basin fractionation during glacial to interglacial cycles the transient change in preservation during the transitions into and out of glacial stages reflects a response of the carbonate system to a redistribution of alkalinity and dic in the ocean i e carbonate compensation comparison of the site 1089 carbonate and vostok p co 2 records suggests a role of deep sea co 3 2 variations for governing at least some second order features of the atmospheric p co 2 signal in order to quantify this role however measurement of indices of dissolution along a true depth transect will be required to estimate the magnitudes of changes in deep sea co 3 2

strong thermodynamic imprint on recent bottom water and epibenthic δ13c in the weddell sea revealed implications for glacial southern ocean ventilation abstract paleonutrient proxies are widely used to reconstruct the geometry of deep water masses during the last glacial maximum lgm epibenthic 13 c provides best spatial coverage and artifacts are well investigated discrepancies between reconstructed lgm circulation patterns derived from models or different benthic nutrient proxies can partly be resolved by varying air sea signatures of 13 c i e 13 c as however there are very few data available to calculate a 13 c as of modern bottom water dissolved inorganic carbon dic 13 c and to document how this signal is recorded in benthic foraminiferal 13 c here i show that today bottom water in the atlantic sector of the southern ocean is 13 c enriched with 13 c dic values between 0 4 and 1 0 and 13 c as values 0 4 and that this signal is recorded in live and dead epibenthic 13 c this is in contrast to a uniform modern antarctic circumpolar 13 c dic of rather 0 4 which hitherto is used as modern framework to compare to low lgm 13 c dic of southern sourced bottom water and glacial inter basin differences i conclude that a potential reduction of the strong recent thermodynamic imprint during bottom water generation in glacial times could explain depleted circum antarctic 13 c dic without associated co 2 enrichment and anoxia in antarctic bottom waters the present synoptic compilation of 13 c dic and live benthic foraminifera 13 c is in support of hypotheses that explain low lgm 13 c by a depletion of southern end member 13 c dic due to extensive sea ice formation with low 13 c as brine rejection and diminished air sea gas exchange

changes in the mode of southern ocean circulation over the last glacial cycle revealed by foraminiferal stable isotopic variability benthic foraminiferal oxygen and carbon isotopic records from southern ocean sediment cores show that during the last glacial period the south atlantic sector of the deep southern ocean filled to roughly 2500 m with water uniformly low in 13c resulting in the appearance of a strong mid depth nutricline similar to those observed in glacial northern oceans concomitantly deep water isotopic gradients developed between the pacific and atlantic sectors of the southern ocean the 13c of benthic foraminifera in pacific sediments remained significantly higher than those in the atlantic during the glacial episode these two observations help to define the extent of what has become known as the southern ocean low 13c problem one explanation for this glacial distribution of 13c calls upon surface productivity overprints or changes in the microhabitat of benthic foraminifera to lower glacial age 13c values we show here however that glacial interglacial 13c shifts are similarly large everywhere in the deep south atlantic regardless of productivity regime or sedimentary environment furthermore the degree of isotopic decoupling between the atlantic and pacific basins is proportional to the magnitude of 13c change in the atlantic on all time scales thus we conclude that the profoundly altered distribution of 13c in the glacial southern ocean is most likely the result of deep ocean circulation changes while the characteristics of the southern ocean 13c records clearly point to reduced north atlantic deep water input during glacial periods the basinal differences suggest that the mode of southern ocean deep water formation must have been altered as well

on the total input of antarctic waters to the deep ocean a preliminary estimate from chlorofluorocarbon measurements 1 deep ocean inventories of dissolved chlorofluorocarbon 11 cfc 11 along representative sections off antarctica provide the first estimate of the overall strength of all dense water sources in the southern ocean their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the thermohaline circulation thc the bottom layer is supplied via sinking of antarctic bottom water aabw produced at a few continental shelves the middle layer receives the offshore injection of ventilated modified circumpolar deep water mcdw produced along much of the lengthy antarctic slope front the top layer is ventilated by northward export of antarctic surface water into the upper circumpolar deep water of the antarctic circumpolar current average southern ocean inputs to the upper two layers of the deep ocean for the 1970 1990 period are derived on the basis of the cfc 11 distributions along meridional sections the mean cfc 11 saturations of all water mass ingredients and the inferred mixing leading to production of dense source waters about 5 4 1 7 sv 1 sv 10 6 m 3 s 1 of near freezing shelf water ventilate the bottom layer and 4 7 1 7 sv and 3 6 1 3 sv of cold antarctic surface water ventilate the middle and top layers therefore the total contribution of ventilated southern ocean waters to the lower limb of the global thc is about 14 sv this is close to the about 17 sv estimated for north atlantic near surface sources from cfc 11 inventories their entrainment of 7 4 2 4 sv of cfc poor subsurface lower circumpolar deep water during the formation and sinking of aabw and mcdw raises the total southern ocean input to the deep ocean to about 21 sv

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

the southern oscillation in the stratosphere abstract the signal of the southern oscillation in the lower half of the northern hemisphere stratosphere in winter appears to be as follows in the extreme of the southern oscillation when the trade winds are comparatively weak in the south pacific ocean stratospheric geopotential heights and temperatures tend to be higher over the arctic and lower in middle latitudes than in the opposite extreme at the same time the polar night stratospheric jetstream tends to be weaker and the subtropical westerlies to be stronger the conclusions are based on 11 extremes within a 15 year period and on data at standard pressure levels as high as 10 mb

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

temporal relationships of carbon cycling and ocean circulation at glacial boundaries evidence from high sedimentation rate south atlantic deep sea cores indicates that global and southern ocean carbon budget shifts preceded thermohaline circulation changes during the last ice age initiation and termination and that these were preceded by ice sheet growth and retreat respectively no consistent lead lag relationships are observed during abrupt millennial warming events during the last ice age allowing for the possibility that ocean circulation triggered some millenial climate changes at the major glacial interglacial transitions the global carbon budget and thermohaline ocean circulation responded sequentially to the climate changes that forced the growth and decline of continental ice sheets

pacific atlantic circumpolar deep water coupling during the last 500 ka investigating the interbasin deepwater exchange between the pacific and atlantic oceans over glacial interglacial climate cycles is important for understanding circum antarctic southern ocean circulation changes and their impact on the global meridional overturning circulation we use benthic foraminiferal 13c records from the southern east pacific rise to characterize the 13c composition of circumpolar deep water in the south pacific prior to its transit through the drake passage into the south atlantic a comparison with published south atlantic deepwater records from the northern cape basin suggests a continuous water mass exchange throughout the past 500 ka almost identical glacial interglacial 13c variations imply a common deepwater evolution in both basins suggesting persistent circumpolar deep water exchange and homogenization by contrast deeper abyssal waters occupying the more southern cape basin and the southernmost south atlantic have lower 13c values during most but not all stadial periods we conclude that these values represent the influence of a more southern water mass perhaps antarctic bottom water aabw during many interglacials and some glacial periods the gradient between circumpolar deep water and the deeper southern cape basin bottom water disappears suggesting either no presence of aabw or indistinguishable 13c values of both water masses

new evidence for changes in plio pleistocene deep water circulation from southern ocean odp leg 177 site 1090 abstract changes in atlantic deep water circulation were reconstructed by comparing the benthic foraminiferal 13c record at odp site 1090 in the south atlantic with similar records from the north atlantic sites 982 607 925 929 and deep pacific site 849 oceans important deep water circulation changes occurred in the early pleistocene at 1 55 myr and during the mid pleistocene transition at 0 9 myr at 1 55 myr glacial 13c values in the southern ocean became significantly lower than those in the deep pacific establishing a pattern that persisted throughout the late pleistocene we propose that the lowering of 13c values of southern component water scw at this time resulted from expansion of sea ice and reduced ventilation of deep water during glacial periods after marine isotope stage 52 accompanying this change in southern ocean deep water circulation was enhanced interhemispheric coupling between the north and south atlantic after 1 55 myr at 0 9 myr the magnitude of glacial to interglacial variability in 13c increased and shifted to a longer frequency 100 kyr along with oceanic 18o ice volume calculation of percent northern component water ncw using site 1090 as the scw end member yielded 20 30 less reduction of ncw during glacial periods of the late pleistocene also a trend toward reduced glacial suppression of ncw during the past 400 kyr is not evident the apparent decoupling of ice volume and deep water circulation reported previously may be an artifact of using a pacific rather than a southern ocean carbon isotopic record to calculate past mixing ratios of ncw and scw

similar glacial and holocene deep water circulation inferred from southeast pacific benthic foraminiferal carbon isotope composition we present holocene and last glacial maximum lgm oxygen and carbon isotope measurements on planulina wuellerstorfi in six southeast pacific cores sedimentation rates are low in this part of the ocean and measurements were made on individual foraminiferal shells in order to identify the holocene and glacial individuals on the basis of their extreme 18o the new 13c data were combined with previous p wuellerstorfi data for interpretation of global thermohaline circulation data from the southern ocean were examined closely for regional coherency and a few anomalous 13c values suspected of having productivity overprint were removed the resulting global 13c distributions and gradients indicate that the deep water circulation was similar during the holocene and lgm this interpretation brings 13c data to a better agreement with cd ca data and marks a sharp contrast with a widely held view based on 13c measurements that the glacial southern ocean was the terminus of the thermohaline circulation the proposed presence of glacial north atlantic deep water does not necessarily contradict the postulated presence of glacial north atlantic intermediate water

quantifying antarctic bottom water and north atlantic deep water volumes 1 a near global census of antarctic bottom water aabw and north atlantic deep water nadw is essayed through a nonnegative least squares analysis of conservative and quasi conservative seawater properties aabw thickness generally decreases from south to north modulated by ocean bathymetry likewise nadw thickness generally decreases from north to south in the atlantic ocean nadw dominates below the thermocline of the atlantic ocean at least as far south as the subtropical gyre of the south atlantic with a lesser but still significant influence around the entire antarctic circumpolar current in the indian ocean and in the pacific ocean however in the pacific and indian oceans aabw dominates below the thermocline in addition measurable quantities of aabw reach into the abyssal north atlantic on both sides of the mid atlantic ridge the census results suggest that aabw occupies roughly twice the volume of nadw in the three main oceans and that aabw is in contact with roughly twice the area of the deep main ocean floor compared with nadw however these results are somewhat sensitive to choices of water masses their values of seawater properties and the weightings of the seawater properties used in the analysis

refining the stable isotope budget for antarctic bottom water new foraminiferal data from the abyssal southwest atlantic 1 stable isotope tracer budget results suggest the transport to vertical diffusivity ratio for antarctic bottom water aabw in the atlantic was higher at the last glacial maximum lgm reduced mixing across the upper boundary of aabw is consistent with movement of this surface away from the seafloor and may be a factor in sequestering co2 in the abyssal atlantic two key unknowns in the budget are the isotopic composition of aabw and the spatial representativeness of isolated vertical profiles of 18o and 13c due to a lack of core material below 3 km water depth lund et al 2011 based their holocene budget on water column data and their lgm budget on extrapolation of isotopic trends from shallower cores here we determine 18o and 13c for aabw using new isotopic records from 3 to 4 km water depth at the brazil margin the core top data yield tracer budget parameters consistent with water column data in the broader southwest atlantic at the lgm benthic 18o reaches 4 9 at 4 km water depth the highest lgm 18o value in the published literature the corresponding 13c of 0 2 is less depleted than expected and 0 5 greater than 13c in the southeast atlantic our peclet number estimates suggest 13c acted conservatively during both the holocene and lgm both 18o and 13c imply the transport to vertical diffusivity ratio for aabw was an order of magnitude larger during the lgm due to enhanced aabw transport or reduced mixing across its upper boundary

the influence of air and sea exchange on the carbon isotope distribution in the sea we explore here the influence of the temperature dependence of isotope fractionation between atmospheric co2and ocean co2 on the distribution of carbon isotopes in the ocean this is accomplished by an analysis of departures from the expected redfield tie between po4 and 13c we find that for the surface ocean the temperature influence largely compensates for the biologic influence in the deep ocean the temperature influence imprinted at the sites of deepwater formation reduces somewhat the biologically induced difference between the carbon isotope ratios for deep waters produced in the northern atlantic and in the antarctic these same features are reproduced in the hamburg ocean model in order to assess the impact of changes in the ratio of ocean mixing rate to wind speed we have made a model run in which co2 exchange rates between air and sea were everywhere doubled as expected the influence of the thermodynamic effect on the oceanic carbon isotope distribution is magnified

increased dust deposition in the pacific southern ocean during glacial periods dust deposition in the southern ocean constitutes a critical modulator of past global climate variability but how it has varied temporally and geographically is underdetermined here we present data sets of glacial interglacial dust supply cycles from the largest southern ocean sector the polar south pacific indicating three times higher dust deposition during glacial periods than during interglacials for the past million years although the most likely dust source for the south pacific is australia and new zealand the glacial interglacial pattern and timing of lithogenic sediment deposition is similar to dust records from antarctica and the south atlantic dominated by patagonian sources these similarities imply large scale common climate forcings such as latitudinal shifts of the southern westerlies and regionally enhanced glaciogenic dust mobilization in new zealand and patagonia

variability in the deep and intermediate water circulation of the atlantic ocean during the past 25 000 years northern hemisphere modulation of the southern ocean the relative flux of north atlantic deep water nadw out of the atlantic can be monitored in the southern ocean where high 13c nadw mixes rapidly with low 13c recirculated pacific water before forming antarctic bottom water aabw by using 13c measured in benthic foraminifera as a water mass tracer it is shown that the southern ocean was dominated by pacific water with little nadw contribution during the last glaciation these tracer results document the direct physical connection between insolation controlled northern hemisphere oscillations and changes in the chemical and physical properties of southern ocean water the 13c records from the deep tropical atlantic document the transition from an atlantic dominated by southern ocean water during the last glaciation to an atlantic dominated by nadw during the holocene with more southern ocean water in the deep atlantic during the last glaciation the boundary between nadw and southern ocean water was north of its modern position allowing more southern ocean water to enter the eastern atlantic through the romanche fracture zone carbon isotope records from the caribbean and mediterranean seas suggest that the influence of mediterranean overflow water mow extended to the caribbean during the last glaciation and therefore was a volumetrically much more important water mass in the intermediate depth atlantic than it is today

lower friction of the xianshuihe xiaojiang fault system and its effect on active deformation around the south eastern tibetan margin we use three dimensional mechanical modelling with fault as coulomb type frictional surface to explore the active deformation of the xianshuihe xiaojiang fault system in south eastern tibet crustal rheology is simplified as an elastoplastic upper crust and a viscoelastic lower crust far field gps velocities and quaternary fault slip rates are used to constrain the model results numerical experiments show that effective fault friction lower than 0 1 0 08 leads to a high slip rate that fits well with geological estimates of the slip rate on the fault system associating with the modelled fault slip rate strain in the surrounding crust distributes broadly and is partitioned into strike slip and thrust senses this means that in the indianeurasia convergence accommodation of the large fraction of sinistral motion on the fault system is achieved mainly due to its lower fault friction this in turn affects crustal deformation around the south eastern tibetan margin resulting in negligible compression across the longmen shan

abyssal atlantic circulation during the last glacial maximum constraining the ratio between transport and vertical mixing transport to vertical diffusivity in the deep atlantic our calculations suggest that the ratio was at least twice as large at the lgm we speculate that the primary cause was reduced mixing between northern and southern component waters associated with movement of this water mass boundary away from the zone of intense mixing near the seafloor the shallower water mass boundary yields an order of magnitude increase in the volume of southern component water suggesting its residence time may have increased substantially our analysis supports the idea that an expanded volume of antarctic bottom water and limited vertical mixing enhanced the abyssal ocean s ability to trap carbon during glacial times