representation of the carbon cycle in box models and gcms 1 solubility pump 1 bacastow 1996 broecker et al 1999 and archer et al 2000 have called attention recently to the fact that box models and general circulation models gcms represent the thermal partitioning of co2 between the warm surface ocean and cold deep ocean in different ways they attribute these differences to mixing and circulation effects in gcms that are not resolved in box models the message that emerges from these studies is that box models have overstated the importance of the ocean s polar regions in the carbon cycle a reduced role for the polar regions has major implications for the mechanisms put forth to explain glacial interglacial changes in atmospheric co2 in parts 1 and 2 of this paper a new analysis of the ocean s carbon pumps is carried out to examine these findings this paper part 1 shows that unresolved mixing and circulation effects in box models are not the main reason for box model gcm differences the main factor is very different kinds of restrictions on gas exchange in polar areas polar outcrops in gcms are much smaller than in box models and they are assumed to be ice covered in an unrealistic way this finding does not support a reduced role for the ocean s polar regions in the cycling of organic carbon the subject taken up in part 2

changing atmospheric δ14c and the record of deep water paleoventilation ages we propose a new calculation method to better estimate the deep water ventilation age from benthic planktonic foraminifera 14 c ages our study is motivated by the fact that changes in atmospheric 14 c through time can cause contemporary benthic and planktonic foraminifera to have different initial 14 c values this effect can cause spurious ventilation age changes to be interpreted from the geologic data using a new calculation method 14 c projection ages we recalculate the data from the pacific ocean contrary to previous results we find that the pacific intermediate and deep waters were about 600 years older than today at the last glacial maximum in addition there are possible signals of ventilation age change prior to ice sheet melting and at the younger dryas however the data are still too sparse to constrain these ventilation transients

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

the southern ocean s biological pump during the last glacial maximum ice core records from antarctica show large 80 ppm and regular climate related changes in atmospheric co2 with minimum values during glacial periods and maximum values during peak interglacials the suggested role of the southern ocean in driving these changes is based on either the potential for increased utilization of surface nutrients or the potential for decreased ventilation of deep waters during glacial times several recent studies have invoked increased stratification of the southern ocean to explain lower glacial atmospheric co2 levels in terms of reduced exchange of co2 between the deep sea and the atmosphere a northward displacement and or substantial weakening of the westerly winds during glacial periods are implicit in the scenarios that invoke enhanced stratification however both circulation models and proxy results argue against a weakening of the westerlies in fact the mean flow of the antarctic circumpolar current and wind driven upwelling during the last glacial maximum lgm are thought to be at least as vigorous as those which exist today given these boundary conditions we offer two competing scenarios for ecosystem structure and export production of the glacial southern ocean the first scenario satisfies all proxy records for nutrient utilization and phytoplankton growth rate and requires increased relative to today nitrate utilization south of the antarctic polar front apf by phytoplankton other than diatoms together with a shift in the zone of maximum diatom growth from south interglacials to north glacials of the apf the second scenario has reduced growth of all phytoplankton species south of the apf during glacials and a shift in the zone of maximum export production to the north of the polar front the principal weakness of the first scenario is that there is little sedimentary evidence to support the increased export of particulate organic carbon required by the inferred increase in nitrate utilization south of the apf the principal weakness of the second scenario is that it requires a change in the isotopic fractionation of nitrogen by phytoplankton growing south of the polar front for reasons presently unknown the southern ocean s biological pump at the lgm has yet to be characterized unequivocally

evidence from diatom bound nitrogen isotopes for subarctic pacific stratification during the last ice age and a link to north pacific denitrification changes higher diatom bound d 15 n 70 wt lower opal content and 1200 ppm lower biogenic barium taken together and with constraints on sediment accumulation rate these results suggest a reduced supply of nitrate to the surface due to stronger stratification of the upper water column of the bering sea during glacial times with more complete nitrate consumption resulting from continued iron supply through atmospheric deposition this finding extends the body of evidence for a pervasive link between cold climates and polar ocean stratification in addition we hypothesize that more complete nutrient consumption in the glacial age subarctic pacific contributed to the previously observed ice age reduction in suboxia and denitrification in the eastern tropical north pacific by lowering the nutrient content of the intermediate depth water formed in the subpolar north pacific in the deglacial interval of the bering sea record two apparent peaks in export productivity are associated with maxima in diatom bound and bulk sediment d 15 n the high d 15 n in these intervals may have resulted from greater surface nutrient consumption during this period however the synchroneity of the deglacial peaks in the bering sea with similar bulk sediment d 15 n changes in the eastern pacific margin and the presence of sediment lamination within the bering sea during the deposition of the productivity peaks raise the possibility that both regional and local denitrification worked to raise the d 15 n of the nitrate feeding bering sea surface waters at these times

the exhaust valve of the north atlantic during glacial periods climate records are marked by large amplitude oscillations believed to be a result of north atlantic na freshwater anomalies which weakened the thermohaline circulation thc and introduced instabilities such oscillations are absent from the present interglacial period with the aid of a semiglobal analytical model it is proposed that the bering strait bs acts like an exhaust valve for the above na freshwater anomalies specifically it is suggested that large instabilities in the thc are only possible during glacial periods because during these periods the bs is closed during interglacial periods when the bs the exhaust valve is open low salinity anomalies are quickly flushed out of the north atlantic by the strong southern ocean winds

glacial interglacial changes in subarctic north pacific stratification since the first evidence of low algal productivity during ice ages in the antarctic zone of the southern ocean was discovered there has been debate as to whether it was associated with increased polar ocean stratification or with sea ice cover shortening the productive season the sediment concentration of biogenic barium at ocean drilling program site 882 indicates low algal productivity during ice ages in the subarctic north pacific as well site 882 is located southeast of the summer sea ice extent even during glacial maxima ruling out sea ice driven light limitation and supporting stratification as the explanation with implications for the glacial cycles of atmospheric carbon dioxide concentration

synchronous high frequency oscillations in tropical sea surface temperatures and north atlantic deep water production during the last glacial cycle stable isotopic measurements of g sacculifer and c wuellerstorfi in a core from the western equatorial atlantic imply that there are parallel suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3 low values of g sacculifer 18o accompany high values of c wuellerstorfi 13c linking warmer sea surface temperatures ssts in the tropics with increased production of lower north atlantic deep water nadw the amplitude of the 18o oscillations is 0 6 or 2 3 c which is superimposed on a glacial interglacial amplitude of about 2 1 using the g sacculifer 18o data we calculate that surface waters were colder during stage 2 than calculated by climap 1976 1981 the longer period 2 kyr oscillations in air temperature recorded in the greenland and antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial atlantic the magnitude of these oscillations in tropical sst is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone the apparent synchroneity of equatorial sst and polar air temperature changes as well as the amplitude of the sst changes at the equator are consistent with the climate effects expected from changes in the atmosphere s greenhouse gas content h2ovapor co2 and ch4

the effect of cold climate upon north atlantic deep water formation in a simple ocean atmosphere model the sensitivity of north atlantic deep water formation to variations in mean surface temperature is explored with a meridional vertical plane ocean model coupled to an energy balance atmosphere it is found that north atlantic deep water formation is favored by a warm climate while cold climates are more likely to produce southern ocean deep water or deep decoupling oscillations when the southern sinking region is halocline covered this behavior is traced to a cooling induced convective instability near the north atlantic sinking region that is to unstable horizontal spreading of a halocline that stratifies part of the region under the convective instability it is found that climate cooling is generally equivalent to increased freshwater forcing this is because in a cold climate high latitude water masses approach the temperature of maximum density and the convectiondriving upward thermal buoyancy flux induced by surface cooling becomes insufficient to overcome the stratifying effect of surface freshening a downward buoyancy flux an extensive halocline is then formed and this halocline interferes with the heat loss necessary for the steady production of north atlantic deep water

rapid climate change and conditional instability of the glacial deep ocean from the thermobaric effect and geothermal heating previous results from deep sea pore fluid data demonstrate that the glacial deep ocean was filled with salty cold water from the south this salinity stratification of the ocean allows for the possible accumulation of geothermal heat in the deep sea and could result in a water column with cold fresh water on top of warm salty water and with a corresponding increase in potential energy for an idealized 4000 dbar two layer water column we calculate that there are 10 6j m 2 0 2j kg of potential energy available when a 0 4 psu salinity contrast is balanced by a 2 c temperature difference this salt based storage of heat at depth is analogous to convectively available potential energy cape in the atmosphere the thermobaric effect in the seawater equation of state can cause this potential energy to be released catastrophically because deep ocean stratification was dominated by salinity at the last glacial maximum lgm the glacial climate is more sensitive to charging this thermobaric capacitor and can plausibly explain many aspects of the record of rapid climate change our mechanism could account for the grouping of dansgaard oeschger events into bond cycles and for the different patterns of warming observed in ice cores from separate hemispheres

sea ice switch mechanism and glacial interglacial co2 variations 1 a physical mechanism is proposed for the glacial interglacial variations in the rate of vertical mixing and in sea ice cover in the southern ocean such variations were postulated without an explanation by toggweiler 1999 and stephens and keeling 2000 who used them to explain the glacial interglacial co2 variations in the physical mechanism explored here initially given by gildor and tziperman 2001b changes in the stratification of the southern ocean due to the cooling of north atlantic deep water nadw during glacial maxima reduce the rate of vertical mixing of the surface water with the deep water the changed temperature of the nadw arriving to the southern ocean and the reduced vertical mixing there also increase the southern ocean sea ice cover during glacial maxima these vertical mixing and sea ice cover changes are shown to be a natural result of the sea ice switch mechanism of the glacial cycles a box model of the climate system is used to demonstrate the above physical mechanism and its effect on the atmospheric co2 because of the uncertainties in the exact dependence of the vertical mixing on vertical stratification it is impossible to quantify the exact separate contribution of reduced vertical mixing and larger sea ice cover to the glacial co2 variations the co2 variations are not essential to the existence of the glacial cycles in the sea ice switch mechanism yet they are shown to amplify the glacial interglacial variability consistent with the view of the role of co2 deduced from proxy observations

old radiocarbon ages in the southwest pacific ocean during the last glacial period and deglaciation marine radiocarbon 14c dates are widely used for dating oceanic events and as tracers of ocean circulation essential components for understanding ocean climate interactions past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep water and surface water reservoirs but the apparent age of surface waters currently approximately 400 years in the tropics and approximately 1 200 years in antarctic waters might not be constant through time as has been assumed in radiocarbon chronologies and palaeoclimate studies here we present independent estimates of surface water and deep water reservoir ages in the new zealand region since the last glacial period using volcanic ejecta tephras deposited in both marine and terrestrial sediments as stratigraphic markers compared to present day values surface reservoir ages from 11 900 14c years ago were twice as large 800 years and during glacial times were five times as large 2 000 years contradicting the assumption of constant surface age furthermore the ages of glacial deep water reservoirs were much older 3 000 5 000 years the increase in surface to deep water age differences in the glacial southern ocean suggests that there was decreased ocean ventilation during this period

the topsails igneous suite western newfoundland an early silurian subduction related magmatic suite the topsails igneous suite contains several late ordovician to early silurian volcanic and intrusive sequences which overlie and intrude early to middle ordovician oceanic and arc rocks the oldest components of this suite may represent calc alkaline continental arc magmatism the younger components are bimodal with felsic compositions vastly predominating and include a major 2200 km2 alkaline a type granite complex these felsic components have similarities to peralkaline suites formed in unusual subduction related settings younger mafic components resemble within plate basalts emplaced in a continental setting silurian magmatic activity in the canadian appalachians is widespread includes diverse magmatic types and has contrasting metamorphic and tectonic overprinting even in contiguous areas these features and the probability of major post silurian displacements in the orogen render correlation and interpretation difficult tectonic models that consider basin closure and major plate movem

was a change in thermohaline circulation responsible for the little ice age long term 800 year and short term 20 year estimates of the rate of deep water formation in the southern ocean are in serious disagreement this disagreement suggests to me that the rate of deep water formation may have been larger during the little ice age lia than it is now

southern ocean sea ice and radiocarbon ages of glacial bottom waters abstract analyzing simulations of glacial ocean circulation and radiocarbon distribution i show that increased sea ice cover over the southern ocean reduces ventilation and radiocarbon content of the deep ocean two simulations one present day and one glacial tuned to have similar rates of north atlantic deep water formation are used insulation from air sea gas exchange due to more extended sea ice cover increases southern ocean radiocarbon ages by more than 100 yr higher rates of sea ice formation and export from high southern latitudes lead to a salinification of antarctic bottom water aabw making it the most saline deep water mass of the glacial world oceans this increases the density of aabw and hence its rate of formation mass and radiocarbon fluxes to the deep ocean are thus decoupled both older southern ocean waters and a stronger flux of aabw increase radiocarbon ages of glacial bottom waters by up to 300 yr available reconstructions of glacial bottom water properties are broadly consistent with the simulation these results question previous inferences from radiocarbon distributions on glacial deep water formation rates

glacial ventilation rates for the deep pacific ocean 1 a key constraint in attempts to reconstruct the patterns and rates of the ocean s thermohaline circulation during the last glacial period is the difference between the 14c to c ratio in surface and deep water while imperfect it is our best index of past deep sea ventilation rates in this paper we review published ventilation rate estimates based on the measured radiocarbon age difference between coexisting benthic and planktic foraminifera from glacial age pacific sediments we also present new results from a series of eastern equatorial pacific sediment cores the conclusion is that the scatter in these results is so large that the apparent 14c age of glacial deep pacific water could lie anywhere between double and half today s further it is not clear what is responsible for the wide scatter in the radiocarbon results

expansion of pelagic denitrification during early pleistocene cooling bioavailable nitrogen is removed from the oceans in oxygen deficient benthic and pelagic environments by denitrification future warming is predicted to reduce ocean oxygenation and to cause hypoxic regions to expand potentially accelerating denitrification a compilation of high resolution sedimentary nitrogen isotope delta n 15 records from the eastern tropical pacific north pacific and the arabian sea and a global multi site survey are presented as evidence for weak pelagic denitrification at the end of the pliocene warm period mean delta n 15 values increased in the major oxygen minimum zones omzs between 2 1 and 1 5 ma pelagic denitrification strengthened during a period of long term global cooling despite solubility driven increases in initial oxygen contents of antarctic intermediate and subantarctic mode waters ventilating the omzs this trend is opposite to the predicted mean trend for a cooling ocean as well as to the observed glacial interglacial variation several alternatives to explain the shift are proposed including a rise in net respiration a progressive increase in the ventilation age of the deep ocean associated with million year scale secular cooling and a shoaling of the remotely ventilated thermocline to shallow depths corresponding to the zone of peak subsurface respiration given no evidence for a net increase in production we assert that large scale climate driven changes in ocean circulation regulate long timescale variations in the extent of pelagic denitrification additional data and modeling are required to fully explain the observations c 2013 elsevier b v all rights reserved

enhanced circulation during a warm period the early pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today to characterize thermohaline circulation during this warm period 4 4 3 1 ma we combined new benthic foraminiferal isotopic data from the southeast atlantic with published data and reconstructed pacific and atlantic ocean nutrient distributions the data indicate enhanced ventilation of the deep atlantic and intermediate pacific during the early pliocene enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of north pacific and atlantic subtropical gyres was probably also enhanced future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation

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

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

14c activity and global carbon cycle changes over the past 50 000 years a series of 14c measurements in ocean drilling program cores from the tropical cariaco basin which have been correlated to the annual layer counted chronology for the greenland ice sheet project 2 gisp2 ice core provides a high resolution calibration of the radiocarbon time scale back to 50 000 years before the present independent radiometric dating of events correlated to gisp2 suggests that the calibration is accurate reconstructed 14c activities varied substantially during the last glacial period including sharp peaks synchronous with the laschamp and mono lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14c production rate together with plausible changes in deep ocean ventilation and the global carbon cycle during glaciation

climate variability at the southern boundaries of the namib southwestern africa and atacama northern chile coastal deserts during the last 120 000 yr abstract in this study we present grain size distributions of the terrigenous fraction of two deep sea sediment cores from the se atlantic offshore namibia and from the se pacific offshore northern chile which we unmix into subpopulations and which are interpreted as coarse eolian dust fine eolian dust and fluvial mud the downcore ratios of the proportions of eolian dust and fluvial mud subsequently represent paleocontinental aridity records of southwestern africa and northern chile for the last 120 000 yr the two records show a relatively wet last glacial maximum lgm compared to a relatively dry holocene but different orbital variability on longer time scales generally the northern chilean aridity record shows higher frequency changes which are closely related to precessional variation in solar insolation compared to the southwestern african aridity record which shows a remarkable resemblance to the global ice volume record we relate the changes in continental aridity in southwestern africa and northern chile to changes in the latitudinal position of the moisture bearing southern westerlies potentially driven by the sea ice extent around antarctica and overprinted by tropical forcing in the equatorial pacific ocean

antarctic sea ice and the control of pleistocene climate instability a hypothesis is presented for the origin of pleistocene climate instability based on expansion of antarctic sea ice and associated changes in the oceans salinity structure the hypothesis assumes that thermohaline overturning is dominated by the reconfigured conveyor of toggweiler and samuels 1993b in which deepwater upwelling is restricted to high southern latitudes the reconfigured conveyor is shown to be potentially stabilized in an on mode by precipitation at high southern latitudes and potentially destabilized into on and off modes by the counteracting influence of antarctic sea ice the mechanism is clarified by the use of a hydraulic analogue we hypothesize that this mechanism accounts for dominant patterns of thermohaline overturning and climate instability between pleistocene warm and cold periods the hypothesis is shown to be consistent with a range of paleoceanographic evidence and to potentially account for details of observed rapid climate changes during glacial and interglacial periods including aspects of interhemispheric timing

radiocarbon and stable isotope constraints on last glacial maximum and younger dryas ventilation in the western north atlantic 1 foraminiferal abundance 14c ventilation ages and stable isotope ratios in cores from high deposition rate locations in the western subtropical north atlantic are used to infer changes in ocean and climate during the younger dryas yd and last glacial maximum lgm the 18o of the surface dwelling planktonic foram globigerinoides ruber records the present day decrease in surface temperature sst of 4 c from gulf stream waters to the northeastern bermuda rise if during the lgm the modern 18o salinity relationship was maintained this sst contrast was reduced to 2 c with lgm to interglacial 18o changes of at least 2 2 ssts in the western subtropical gyre may have been as much as 5 c colder above 2 3 km glacial 13c was higher than today consistent with nutrient depleted younger bottom waters as identified previously below that 13c decreased continually to 0 5 about equal to the lowest lgm 13c in the north pacific ocean seven pairs of benthic and planktonic foraminiferal 14c dates from cores 2 5 km deep differ by 1100 340 years with a maximum apparent ventilation age of 1500 years at 4250 m and at 4700 m apparent ventilation ages are presently unavailable for the lgm 2 5 km because of problems with reworking on the continental slope when sea level was low because lgm 13c is about the same in the deep north atlantic and the deep north pacific and because the oldest apparent ventilation ages in the lgm north atlantic are the same as the north pacific today it is possible that the same water mass probably of southern origin flowed deep within each basin during the lgm very early in the yd dated here at 11 25 0 25 n 10 conventional 14c kyr bp equal to 12 9 calendar kyr bp apparent ventilation ages 2 3 km water depth were about the same as north atlantic deep water today below 2 3 km four yd pairs average 1030 400 years the oldest apparent ventilation age for the yd is 1600 years at 4250 m this strong contrast in ventilation which indicates a front between water masses of very different origin is similar to glacial profiles of nutrient like proxies this suggests that the lgm and yd modes of ocean circulation were the same

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

effect of global ocean temperature change on deep ocean ventilation 1 a growing number of paleoceanographic observations suggest that the ocean s deep ventilation is stronger in warm climates than in cold climates here we use a general ocean circulation model to test the hypothesis that this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature that is at lower temperatures the surface cooling is not as effective at densifying fresh polar waters and initiating convection in order to isolate this factor from other climate related feedbacks we change the model ocean temperature only where it is used to calculate the density to which we refer below as dynamic temperature change we find that a dynamically cold ocean is globally less ventilated than a dynamically warm ocean with dynamic cooling convection decreases markedly in regions that have strong haloclines i e the southern ocean and the north pacific while overturning increases in the north atlantic where the positive salinity buoyancy is smallest among the polar regions we propose that this opposite behavior of the north atlantic to the southern ocean and north pacific is the result of an energy constrained overturning

widespread intense turbulent mixing in the southern ocean observations of internal wave velocity fluctuations show that enhanced turbulent mixing over rough topography in the southern ocean is remarkably intense and widespread mixing rates exceeding background values by a factor of 10 to 1000 are common above complex bathymetry over a distance of 2000 to 3000 kilometers at depths greater than 500 to 1000 meters this suggests that turbulent mixing in the southern ocean may contribute crucially to driving the upward transport of water closing the ocean s meridional overturning circulation and thus needs to be represented in numerical simulations of the global ocean circulation and the spreading of biogeochemical tracers

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

an inverse dislocation model of surface deformation in western washington abstract active horizontal and vertical deformation in western washington is modeled using an inverse dislocation model the modeling technique uses published horizontal crustal velocities derived from campaign and continuous gps data and uplift rates calculated from repeated historical leveling surveys the plate interface is represented by both screw and edge dislocations divided into segments the rate of slip deficit accumulation on each segment is estimated by inverting the surface deformation data using a linear least squares procedure the model predicts the data well with an overall rms residual of 0 85 mm year the horizontal residual vectors show no discernable pattern and no significant uniform regional strain and rigid rotations can be identified the estimated slip deficit accumulation rates suggest that the plate interface is locked in a broad region beneath the coastline and possibly partially locked further downdip the maximum slip deficit rate was estimated at 32 1 1 9 mm year or approximately 91 of the plate convergence rate the partially locked zone appears to be accumulating slip deficit at 8 2 1 9 mm year

north pacific seasonality and the glaciation of north america 2 7 million years ago in the context of gradual cenozoic cooling the timing of the onset of significant northern hemisphere glaciation 2 7 million years ago is consistent with milankovitch s orbital theory which posited that ice sheets grow when polar summertime insolation and temperature are low however the role of moisture supply in the initiation of large northern hemisphere ice sheets has remained unclear the subarctic pacific ocean represents a significant source of water vapour to boreal north america but it has been largely overlooked in efforts to explain northern hemisphere glaciation here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic pacific ocean indicating that 2 7 million years ago late summer sea surface temperatures in this ocean region rose in response to an increase in stratification at the same time winter sea surface temperatures cooled winter floating ice became more abundant and global climate descended into glacial conditions we suggest that the observed summer warming extended into the autumn providing water vapour to northern north america where it precipitated and accumulated as snow and thus allowed the initiation of northern hemisphere glaciation

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

characteristics of the deep ocean carbon system during the past 150 000 years σco2 distributions deep water flow patterns and abrupt climate change abstract studies of carbon isotopes and cadmium in bottom dwelling foraminifera from ocean sediment cores have advanced our knowledge of ocean chemical distributions during the late pleistocene last glacial maximum data are consistent with a persistent high co2 state for eastern pacific deep water both tracers indicate that the mid depth north and tropical atlantic ocean almost always has lower co2 levels than those in the pacific upper waters of the last glacial maximum atlantic are more co2 depleted and deep waters are co2 enriched compared with the waters of the present in the northern indian ocean 13c and cd data are consistent with upper water co2 depletion relative to the present there is no evident proximate source of this co2 depleted water so i suggest that co2 depleted north atlantic intermediate deep water turns northward around the southern tip of africa and moves toward the equator as a western boundary current at long periods 15 000 years milankovitch cycle variability is evident in paleochemical time series but rapid millennial scale variability can be seen in cores from high accumulation rate series atlantic deep water chemical properties are seen to change in as little as a few hundred years or less an extraordinary new 52 7 m long core from the bermuda rise contains a faithful record of climate variability with century scale resolution sediment composition can be linked in detail with the isotope stage 3 interstadials recorded in greenland ice cores this new record shows at least 12 major climate fluctuations within marine isotope stage 5 about 70 000 130 000 years before the present

the atlantic pacific seesaw abstract a global oceanic teleconnection of salinity meridional overturning circulation moc and climate of the north atlantic and north pacific is proposed simulations with a global climate model show that an extraction of freshwater from the pacific results not only in an increase of salinity there but also in a decrease of salinity in the atlantic as a result a pacific moc develops while the atlantic moc collapses without freshwater perturbation in the atlantic similarly an input of freshwater to the atlantic leads not only to a decrease of salinity there but also to an increase of salinity in the pacific the atlantic moc collapses whereas the pacific moc develops without freshwater perturbation in the pacific the mechanism behind this antiphase atlantic pacific relationship is the positive feedback between ocean circulation and salinity contrasts originally proposed by stommel to operate between low and high latitudes here the authors show that the same mechanism operates on the atlan

radiocarbon age of last glacial pacific deep water one of the most important projects envisaged ever since the possibility emerged of making tandem accelerator mass spectrometric radiocarbon determinations has been that of investigating changes in the radiocarbon age of the deep ocean1 initial attempts to achieve this2 were thwarted by difficulties imposed by bioturbation because these workers used core v28 238 with an accumulation rate of only 1 5 cm kyr 1 thus emphasizing the need to work in a core with a higher accumulation rate working in the glacial section of core tr163 31b with an accumulation of 10 cm kyr 1 we show here that the ventilation age of pacific deep water was about 500 years greater in last glacial times than it is today

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

reconstructing last glacial maximum bottom water salinities from deep sea sediment pore fluid profiles deep sea sediment pore fluids contain a record of past glaciations in their cl and 18 o the signal of the last glacial maximum lgm ice volume increase remains in the pore fluids as a local peak in each of these species using a one dimensional model to account for the diffusive and advective transport within the sediment column since the lgm the past bottom water salinity and 18 o seawater values can be estimated the model is most sensitive to the shape of the forcing function used to represent bottom water variations through time the effective diffusion coefficient and the scatter in the data assuming steady state compaction the model is relatively insensitive to the initial condition the bulk sedimentation rate and the assumed porosity profile though these last two are measured independently overall uncertainties in the relative cl increase at the lgm are between 0 1 and 0 5 where the mean ocean change is about 3 5

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

weaker gulf stream in the florida straits during the last glacial maximum as it passes through the florida straits the gulf stream consists of two main components the western boundary flow of the wind driven subtropical gyre and the northward flowing surface and intermediate waters which are part of the global conveyor belt compensating for the deep water that is exported from the north atlantic ocean1 the mean flow through the straits is largely in geostrophic balance and is thus reflected in the contrast in seawater density across the straits2 here we use oxygen isotope ratios of benthic foraminifera which lived along the ocean margins on the boundaries of the florida current during the last glacial maximum to determine the density structure in the water and thereby reconstruct transport through the straits using the geostrophic method a technique which has been used successfully for estimating present day flow3 our data suggest that during the last glacial maximum the density contrast across the florida straits was reduced with the geostrophic flow referenced to the bottom of the channel at only about two thirds of the modern value if the wind driven western boundary flow was not lower during the last glacial maximum than today these results indicate a significantly weaker conveyor belt component of the gulf stream compared to present day values whereas previous studies based on tracers suggested that deep waters of north atlantic origin were not widespread during glacial times indicating either a relatively weak or a shallow overturning cell our results provide evidence that the overturning cell was indeed weaker during glacial times

revisiting nutrient utilization in the glacial antarctic evidence from a new method for diatom bound n isotopic analysis holocene in the indian sector is smaller than in previous measurements these data suggest no change in the degree of nitrate utilization in the atlantic sector and at most a 20 increase from 25 to 45 in the indian sector the new measurements suggest that during the last ice age in the atlantic sector of the antarctic the atmospheric source of biologically available iron was not so great as to become significant relative to the iron supply from below given the apparent spatial variability in the degree of nitrate drawdown more work is required to develop an adequate picture of the glacial antarctic nutrient field index terms 4806 oceanography biological and chemical carbon cycling 4845 oceanography biological and chemical nutrients and nutrient cycling 4894 oceanography biological and chemical instruments and techniques 9310 information related to geographic region antarctica

simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation according to milankovitch theory the lower summer insolation at high latitudes about 115 000 years ago allowed winter snow to persist throughout summer leading to ice sheet build up and glaciation1 but attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only2 3 4 5 these results point towards the importance of feedback effects for example through changes in vegetation or the ocean circulation for the amplification of solar forcing6 7 8 9 here we present a fully coupled ocean atmosphere model of the last glaciation that produces a build up of perennial snow cover at known locations of ice sheets during this period we show that ocean feedbacks lead to a cooling of the high northern latitudes along with an increase in atmospheric moisture transport from the equator to the poles these changes agree with available geological data10 11 12 13 14 15 and together they lead to an increased delivery of snow to high northern latitudes the mechanism we present explains the onset of glaciation which would be amplified by changes in vegetation in response to weak orbital forcing

late neogene history of deepwater ventilation in the southern ocean we compiled carbon isotope records from the north atlantic south atlantic and pacific oceans to estimate changes in vertical and interbasinal 13c gradients for the last 9 myr benthic 13c values of deep water in the south atlantic decreased in a series of steps at 7 2 75 and 1 55 ma away from the north atlantic and toward the pacific the benthic 13c of intermediate water in the south atlantic evolved differently from deep waters resulting in an increase in the intermediate to deep 13c gradient 13cid the 13cid increased in steps at 2 75 and 1 55 ma marking the development and intensification of a chemical divide in the atlantic ocean between well ventilated intermediate water and poorly ventilated deep water we suggest these changes in interbasinal and vertical gradients resulted from decreasing northern component water and reduced ventilation of southern component water scw the latter was caused by several interrelated processes in antarctic sources areas including increased sea ice cover enhanced surface water stratification decreased ekman induced upwelling and reduced vertical mixing across the thermocline because antarctic surface water processes and deepwater ventilation rates ultimately influence atmospheric co2 concentration these processes may have acted as a positive feedback that contributed to the major cooling steps in late neogene climate

deep circulation of the north atlantic over the last 200 000 years geochemical evidence variations in the cadmium calcium ratio of north atlantic deep water are recorded in the fossil shells of benthic foraminifera the oceanic distribution of cadmium is similar to that of the nutrients hence the cadmium calcium ratio in shells records temporal variations in nutrient distributions data from a north atlantic sediment core show that over the past 200 000 years there has been a continuous supply of nutrient depleted waters into the deep north atlantic the intensity of this source relative to nutrient enriched southern waters diminished by about a factor of 2 during severe glaciations this evidence combined with carbon isotope data indicates that the continental carbon inventory may have been less variable than previously suggested

sudden changes in north atlantic circulation during the last deglaciation sudden changes in the flow of warm atlantic surface waters into the norwegian sea occurred frequently during the last deglaciation typically involving shifts in sea surface temperature of 5 c in fewer than 40 years these led to equally large and rapid changes in atmospheric temperatures and to shifts in atlantic deep thermo haline circulation and ice sheet melting rates

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

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte