the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation the degree of similarity of the 13c records of the planktonic foraminiferal species n pachyderma and of the benthic foraminiferal genus cibicides in the high latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle whereas continuous formation of deep water is recognized in the southern ocean the norwegian sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation however deep water formed in the north atlantic south of the norwegian sea during the last climatic cycle as early as isotope substage 5d and this area was also the only active northern source during stages 4 2 a detailed reconstruction of the geographic distribution of 13c in benthic foraminifera in the atlantic ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean whereas the glacial north atlantic deep water cannot be traced south of 40 n at shallower depth an oxygenated 13c rich intermediate water mass extended from 45 n to 15 s in the pacific ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700 2600 m and is best explained by stronger formation of intermediate water in high northern latitudes

enhanced ventilation of the north atlantic subtropical gyre thermocline during the last glaciation the ocean s intermediate1 6 and deep7 8 circulation are both known to have differed during the last glaciation from those of today but little is known about the history of the subtropical gyres variations in gyre processes should be closely linked to variations in global climate as gyre circulation is driven by air sea interactions and reflects the climate at the ocean surface the gyres are also a significant reservoir of carbon and nutrients so that gyre processes affect the distribution of carbon and nutrients in the oceans and thus atmospheric co2 here we use measurements of 13c and 18o in foraminifera from the bahamas to produce a detailed reconstruction of nutrient and temperature profiles in the thermocline during the last glaciation the thermocline of the glacial north atlantic subtropical gyre lacked the oxygen minimum that is characteristic of the modern ocean and was depleted in nutrients indicating greater more uniform thermocline ventilation at that time thus not only intermediate waters1 5 but the entire upper water column within the glacial north atlantic was depleted of nutrients moreover glacial thermocline waters were cooler had a steeper temperature gradient and a shallower base both the ventilation and the thermal structure of the glacial thermocline are consistent with what is known of the glacial climate9 14

incorporation and preservation of mg in globigerinoides sacculifer implications for reconstructing the temperature and 18o 16o of seawater using bathymetric transects of surface sediments underlying similar sea surface temperatures but exposed to increasing dissolution we examined the processes which affect the relationship between foraminiferal mg ca and 18o we found that globigerinoides saccculifer calcifies over a relatively large range of water depth and that this is apparent in their mg content on the seafloor foraminiferal mg ca is substantially altered by dissolution with the degree of alteration increasing with water depth selective dissolution of the chamber calcite formed in surface waters shifts the shell s bulk mg ca and 18o toward the chemistries of the secondary crust acquired in colder thermocline waters the magnitude of this shift depends on both the range of temperatures over which the shell calcified and the degree to which it is subsequently dissolved in spite of this shift the initial relationship between mg ca and 18o determined by their temperature dependence is maintained we conclude that paired measurements of 18o and mg ca can be used for reconstructing 18owater though care must be taken to determine where in the water column the reconstruction applies

the north atlantic tritium distribution in 1972 the distribution of tritium in the north atlantic in 1972 is compared with the distribution of salinity and the first order potential voracity as mapped on six constant potential density surfaces in the north atlantic the picture presented suggests an advective transport regime which is consistent with currently developing notions of the large scale gyre structure lateral mixing along isopycnals appears to be important in the northwestern region of the subtropical gyre and vertical cross isopycnal mixing needs to be invoked only in near surface layers

3h and 3he in the beta triangle observations of gyre ventilation and oxygen utilization rates abstract isopycnal maps of 3h 3he age with about 100 km resolution have been obtained for a 1000 km scale area in the eastern subtropical north atlantic the midscale texture of the maps is consistent with isopycnal diffusivities of order 500 m 2 s 1 analysis of the distributions within the context of advection diffusion equations reveals that the larger scale 3h 3he age gradients observed within the area are not seriously affected by mixing on those surfaces that outcrop in the region of ekman downwelling thus isopycnal velocities can be estimated from 1 but record only the velocity component normal to the outcrop the velocities thus obtained agree well with geostrophic estimates although the comparison is flawed by the fundamental mismatch in timescales between the two techniques backward extrapolation of the maps to zero age outcrop positions indicate that the contribution of unventilated older recirculated water to the gyre flow above 600 m depth is small in this area no more than 20

the role of vertical chemical fractionation in controlling late quaternary atmospheric carbon dioxide the 30 decrease in atmospheric carbon dioxide during glacial maxima must be driven by some change in the chemical circulation of the ocean here a new model for late quaternary co2 variability is presented which resolves some problems occurring in previous models including the timing of carbon dioxide response and changes in the oxygen content of the deep ocean the primary driving factor in this model is a rearrangement of chemical distributions in the ocean whereby labile nutrients and metabolic co2 are concentrated in deep waters rather than in intermediate waters as observed in the modern ocean this new bottom heavy chemical structure does not affect atmospheric carbon dioxide directly instead co2 induced acidity lowers the deep ocean carbonate ion concentration and temporarily increases carbonate dissolution rates oceanic alkalinity then rises until the deep ocean carbonate ion is restored to its steady state value the resulting increase in oceanic alkalinity draws co2 out of the atmosphere into the ocean alkalinity and atmospheric co2 lag several thousand years behind the change in oceanic chemical structure this delayed response is determined by the limited rate of continental weathering and deep ocean carbonate dissolution relative to the oceanic alkalinity inventory it is proposed that characteristic deglacial and preglacial states of the ocean are leading factors driving glacial interglacial climate changes this concept reinterprets the carbon isotope contrast between surface water and deep waters 13c p b as due to a shift of light metabolic carbon from intermediate waters into deep waters the process is illustrated here by a simple equilibrium five box ocean model the observed intermediate depth nutrient depletion is not sufficient in itself to determine which of several possible mechanisms are operating and some mechanisms are not as effective in changing atmospheric co2 as others the largest response is seen from deepening the regeneration cycle of organic carbon the least response is seen from converting north atlantic deep water into intermediate water

quaternary aragonite cycles and oxygen isotope record in bahamian carbonate ooze carbonate ooze in bahamian troughs displays cyclic variation in the content of aragonite a 10 m core devoid of turbidites shows 3 5 cycles each defined by a sharp increase followed by a gradual decrease of aragonite the aragonite curve matches closely the oxygen isotope curve of planktic foraminifera from the same core aragonite highs correspond to light oxygen isotope values and thus to interglacial stages content of terrigenous material is low during interglacials and higher during glacial periods intermittent flooding and exposure of the bahama banks is most probably not the cause of the cycles because the latest increase in aragonite precedes bank flooding by 8 000 yr and because the cycles are distinctly asymmetric with rapid increase and gradual decrease of aragonitic content carbonate dissolution cycles tied to the glacial rhythm of earth9s climate are a more likely explanation of the variations in aragonite content

the dynamics of the oceanic subtropical gyres recent theoretical advances in physical oceanography have produced plausible models for the structure of the circulation in the oceans subtropical gyres these theoretical ideas are reviewed and assessed in the light of present observational evidence the dominant role of potential vorticity is emphasized in describing the strong interplay between three dimensional distributions of density and velocity in particular the complementary processes of subduction and recirculation jointly act to shape the circulation pattern of the subtropical gyre

a simulation of bomb tritium entry into the atlantic ocean abstract tritium is used in a model calibration study that is aimed at developing three dimensional ocean circulation and mixing models for climate and geochemical simulations the north atlantic tritium distribution is modeled using a three dimensional advective field predicted by a primitive equation ocean circulation model the effect of wintertime convection is parameterized by homogenizing the tracer to the observed march mixed layer depth mixing is parameterized by horizontal and vertical fickian diffusivities of 5 10 6 cm2 s 1 and 0 5 cm2 s 1 respectively the spreading of tritium in the model is dominated by advection in the horizontal and by wintertime convection and advection in the vertical the horizontal and vertical mixing provided by the model have negligible effect a comparison of the model tracer fields with observations shows that most of the basic patterns of the tritium field are reproduced the model s mean vertical penetration of 543 m in 1972 is comparable to the 592 m penetra

canyon creek a late pleistocene vertebrate locality in interior alaska the canyon creek vertebrate fossil locality is an extensive road cut near fairbanks that exposes sediments that range in age from early wisconsin to late holocene tanana river gravel at the base of the section evidently formed during the delta glaciation of the north central alaska range younger layers and lenses of fluvial sand are interbedded with arkosic gravel from canyon creek that contains tephra as well as fossil bones of an interstadial fauna about 40 000 years old solifluction deposits containing ventifacts wedge casts and rodent burrows formed during a subsequent period of periglacial activity that took place during the maximum phase of donnelly glaciation about 25 000 17 000 years ago overlying sheets of eolian sand are separated by a 9500 year old paleosol that may correlate with a phase of early holocene spruce expansion through central alaska the pleistocene fauna from canyon creek consists of rodents indicated by burrows mammuthus primigenius woolly mammoth equus lambei yukon wild ass camelops hesternus western camel bison sp cf b crassicornis large horned bison ovis sp cf o dalli mountain sheep canis sp cf c lupus wolf lepus sp cf l othus or l arcticus tundra hare and rangifer sp caribou this assemblage suggests an open landscape in which trees and tall shrubs were either absent or confined to sheltered and moist sites camelops evidently was present in eastern beringia during the middle wisconsin interstadial interval but may have disappeared during the following glacial episode the stratigraphic section at canyon creek appears to demonstrate that the delta glaciation of the north central alaska range is at least in part of early wisconsin age and was separated from the succeeding donnelly glaciation by an interstadial rather than interglacial episode

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

the significance of siderite in the sediments from lake nyos cameroon abstract small amounts 1 3 wt of endogenic siderite feco3 and traces of pyrite fes2 are present in a 1 m long sediment core from lake nyos thermodynamic modeling shows that the deep waters are supersaturated with respect to siderite and pyrite no other carbonates were found to be stable this rare occurrence of siderite is symptomatic of the build up of large concentrations of co2 and fe2 in the lake nyos waters a search for siderite in the deeper sediments could reveal the existence of previous gas burst events that have occurred at lake nyos or in other cameroon lakes

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

the subpolar mode water of the north atlantic ocean abstract the warm waters of the subtropical and subpolar basins of the north atlantic have tight regional temperature salinity relationships and are conventionally called the regional central waters a volumetric census of the temperature salinity characteristics of the north atlantic by wright and worthington 1970 shows that waters characterized by certain segments of the t s relationships have large volumes compared to those of other segments volumetric mode waters such mode waters appear as layers with increased vertical separation between isopycnals pycnostads the present study reports on the existence of pycnostads in the central and eastern north atlantic these subpolar mode waters are formed by deep winter convection in the subpolar north atlantic and participate in the upper water circulation of the northern north atlantic the seasonal outcropping of the pycnostads occurs within and adjacent to the north atlantic current the irminger current the east and west greenland currents and

changes in the distribution of δ13c of deep water σco2 between the last glaciation and the holocene carbon isotopic measurements on the benthic foraminiferal genus cibicidoides document that mean deep ocean 13c values were 0 46 lower during the last glacial maximum than during the late holocene the geographic distribution of 13c was altered by changes in the production rate of nutrient depleted deep water in the north atlantic during the late holocene north atlantic deep water with high 13c values and low nutrient values can be found throughout the atlantic ocean and its effects can be traced into the southern ocean where it mixes with recirculated pacific deep water during the glaciation decreased production of north atlantic deep water allowed southern ocean deep water to penetrate farther into the north atlantic and across low latitude fracture zones into the eastern atlantic mean southern ocean 13c values during the glaciation are lower than both north atlantic and pacific 13c values suggesting that production of nutrient depleted water occurred in both oceans during the glaciation enriched 13c values in shallow cores within the atlantic ocean indicate the existence of a nutrient depleted water mass above 2000 m in this ocean

the influence of continental ice sheets on the climate of an ice age the climate influence of the land ice that existed 18 000 years before present 18k b p is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean simulated climates are obtained from two versions of the model one with the land ice distribution of the present and the other with that of 18k b p in the northern hemisphere the tropospheric flow field is strongly influenced by the laurentide ice sheet and features a split flow straddling the ice sheet with a strong jet stream forming the southern branch the northern branch of the flow brings very cold air over the north atlantic ocean where thick sea ice is maintained the distribution of sea surface temperature sst difference between the two experiments in the northern hemisphere resembles the difference between the sst at 18k b p and at present as estimated by the climap project 1981 the 18k b p ice sheets have very little influence upon atmospheric temperature and sst in the southern hemisphere this is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation hydrologic changes in the model climate are also found with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in north america and eurasia these findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum

the position of the gulf stream during quaternary glaciations ocean general circulation theories predict that the position of the boundary between subtropical and subpolar gyres and therefore the position of the gulf stream north atlantic current system and the subpolar subtropical front is set by the line of zero ekman pumping where there is no convergence or divergence of water in the directly wind forced surface layer of the ocean in the present day north atlantic ocean this line runs southwest to northeast from off the carolinas to off ireland however during the last ice age 18 000 years ago the subpolar subtropical boundary ran more zonally directly toward gibraltar a numerical atmospheric general circulation model indicates that the field of ekman pumping 18 000 years ago was modified by the presence of a continental ice cap more than 3 kilometers thick such that the line of zero ekman pumping overlaid the paleogyre boundary these results demonstrate that the presence of a thick continental ice sheet could have caused changes in sea surface temperatures in the north atlantic during quaternary glaciations by altering wind patterns

gyres driven by combined wind and buoyancy flux abstract the combined effects of buoyancy forcing and wind in a gyre scale steady ocean circulation are modeled using discrete layers with an interfacial flux not necessarily vertical the equations for vorticity conservation of the geostrophic flow in this system are fully nonlinear involving a jacobian for the layer thicknesses these equations are written in a form which can be solved by the method of characteristics the form of these equations invites the interpretation that the geostrophic baroclinic flow is driven by buoyancy and steered by the wind two examples are solved and discussed a subtropical gyre with heating and a subpolar gyre with cooling in each case there are distinct regimes of flow depending upon whether the characteristics originate at the eastern or western boundaries of the model a simple geometrical argument illustrates that the difference between these two regimes the direct and indirect cells depends upon the sign of the true vertical velocity relative to the interf

benthic isotope evidence for changes of the mediterranean outflow during the late quaternary a suit of sediment cores close to and south of the strait of gibraltar 12 36 n 500 2800 m water depth were analyzed for stable isotopes in epibenthic foraminifers cibicidoides wuellerstorfi and planulina ariminensis during peak glacial times the data exhibit higher 13c values of up to 1 6 at intermediate depths near the strait of gibraltar 36 n the values decrease to the south as evidenced by our data but also to the north as revealed by data of intermediate depth cores north of 38 n in duplessy et al 1987 thus the distribution pattern of 13c provides crucial evidence for an increased influence of nutrient depleted mediterranean outflow water mow on the glacial northeast atlantic hydrography during oxygen isotope terminations i and ii the meridional carbon isotope gradient indicates a significantly decreased but still active mow as deduced from the 18o fluctuations the temperatures of the mow in the atlantic were lower during glacial times by as much as 5 c during glacial times and during termination i the maximum 13c values of the mow correlate with minimum values of the north atlantic deep water nadw and vice versa this inverse response to climatic change of the carbon isotope signals of both water masses indicates that the supply of saline mow to the north atlantic may be less important for the formation of nadw than previously assumed