S

Holocene-Late Pleistocene Climatic Ice CoreRecords from Qinghai-Tibetan Plateau

L. G. THOMPSON, E. MOSLEY-THOMPSON, M. E. DAVIS, J. F. BOLZAN,J. DAI, T. YAO, N. GUNDESTRUP, X. Wu, L. KLEIN, Z. XIE

Three ice cores to bedrock from the Dunde ice cap on the north-central Qinghai-Tibetan Plateau ofChina provide a detailed record of Holocene and Wisconsin-Wurmlate glacial stage (LGS) climate changes in the subtropics. The records reveal that LGSconditions were apparently colder, wetter, and dustier than Holocene conditions. TheLGS part of the cores is characterized by more negative 8180 ratios, increased dustcontent, decreased soluble aerosol concentrations, and reduced ice crystal sizes than theHolocene part. These changes occurred rapidly -10,000 years ago. In addition, thelast 60 years were apparently one ofthe warmest periods in the entire record, equallinglevels of the Holocene maximum between 6000 and 8000 years ago.

T HE DUNDE ICE CAP (38°06'N,96°24'E) is located in a desert envi-ronment between the highest Chi-

nese desert, the Qaidam Basin, to the south,and the Gobi Desert to the north (Fig. IA).This relatively large ice cap has a summitelevation of 5325 m and total area of 60km2. The mean annual temperature is-7.3°C. The glacier is 140 m thick and theunderlying bedrock topography is flat.About 0.4 m of water equivalent per yearaccumulates on the ice cap, and the firn-icetransition occurs at a depth of-25 m (1, 2).

Detailed meteorological and climatologi-cal data for major parts of the Qinghai-Tibetan Plateau, including the surroundingmountains, are lacking (3). Ice core recordsfrom selected ice caps on the plateau that canbe highly resolved in time and that containHolocene to Late Pleistocene ice wouldallow development of a spatially coherentclimate history. Such records would greatlyenhance our understanding of the globalclimate system because of the significance ofthis region for affecting large-scale climate.

In 1987, three ice cores were recovered tobedrock from the Dunde ice cap summit(Fig. 1): cores D-1 (139.8 m), D-2 (136.6m), and D-3 (138.4 m). The visible stratig-raphy of each core was photographicallyrecorded immediately after drilling. Core D-1 was then cut into 3585 samples that weremelted in closed containers by passive solar

L. G. Thompson, E. Mosle-Thompson, M. E. Davis, J.F. Bolzan, J. Dai, L. Klein, Byrd Polar Research Center,Ohio State University, Columbus, OH 43210.T. Yao, X. Wu, Z. Xie, Lanzhou Institute of Glaciologyand Geocrvologv, Academic Sinica, Lanzhou, China.N. Gundestrup, Isotope Laboratory, Geophysical Insti-tute, University of Copenhagen, Denmark.

heating in a laboratory tent. Samples weredivided so that microparticle concentrations(MPC) and 0 isotopic ratios (8180) couldbe measured on the same sample. The upper56 m of core D-3 were melted and bottledon site while the lower 83 m were keptfrozen. The concentrations of NO, S02-,Cl-, and pH were measured for the entirelength of D-3, and crystal sizes and cationswere measured for selected sections (4-6).

Surface and basal borehole temperatureswere -7.3°C and -4.7°C, respectively. Atthe summit ofthe ice cap, ice layers can formduring the summer as a result of the intenseradiation. Summer melt, as shown by icelayers in the firn, generally makes up less

E Radar ice thicknessdetermination sites

EL3 Survey stake networkm Ice

C1 Bedrockj Lake

E! Stream

Contour interval 100 m0 1 2 3

km

Dunde II

Fig. 1. (A) Location of Gobithe Dunde ice cap, Qai- __sdam Basin, and Loess QaidumPlateau and (B) topogra-phv of the ice cap, posi- Q i E* 2 , 1 I ~~~~~~~Qinghai-Tibetanetion of the survey net- Plateauwork, firn and equilibri-um lines, and sites whereice thickness was deter-mined.

than 5% of the annual precipitation (2).The MPC and 0 isotope stratigraphy

show an annual variation (2) that is discern-ible down to about 70 m depth. Below thislevel, the sampling interval (3 cm) was toolarge to enable resolution of the annualsignal. However, enhanced dust depositionduring the dry season produced stratigraph-ic markers that are visible deeper in the ice.These were used to identify annual layers toabout 117 m depth (Fig. 2). Counting theannual 580 and MPC peaks to 70 m andthe visible dust layers below 70 m indicatesthat the ice at 117 m depth was depositedabout 4550 years ago.The age of the deeper ice can be roughly

estimated by extrapolation of the annuallayer thickness data. The D-1 borehole waslocated close to the flow divide or center offlow. A variety of data (7-9) suggest that icedeformation as a function of depth is differ-ent near a flow divide than at distances manyice thicknesses away. Although derived froma deep temperature profile from the DomeC ice divide in East Antarctica (8), thefollowing analytical expression provides agood representation of the annual layerthickness data in the D-1 core

L(z) = b(I - zlH)P ' (1)where z and H are the ice-equivalent depthand thickness, respectively, and b is theaverage annual accumulation rate (metersper year) over the depth interval of interest;p is a constant to be determined.

Measurements of gross beta radioactivityfrom the 1987 D-1 and D-3 cores allowedidentification of the 1963 atmospheric nu-clear testing horizon; the calculated averageaccumulation rate between 1963 and 1987

SCIENCE, VOL. 246474-

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E

._

e

CL

Layer thickness (cm)20 30

Fig. 2. Average annual layer thicknesses (U)based upon the visible dust layers as a function ofdepth in core D-1. The inset shows the thick-nesses in detail from 90 m to the bottom of thecore. The curve represents the calculated variationof layer thickness with depth from Eq. 1 with theparameters p = 1.612 and b = 0.428 m per year.

was about 0.42 m/year (ice equivalent). Theaverage measured value from 31 stakes inthe summit strain network for the 1986-1987 accumulation year was 0.37 m (iceequivalent). If the 24-year average value isassumed to be characteristic of the Holo-cene, the exponent p is fixed by the require-ment that the ice at 117 m depth wasdeposited 4550 years ago. The age at theprominent stratigraphic transition (Fig. 3)at 129.2 m depth in the D-1 core (125.8-mice equivalent) is then calculated to be about

0

25

50

E00

0 100

120

125

130

135

140

Total particles0 4x105 ex105a . = I.

_

i_

I

C

Avg. = 2.9 xl105

Age0

60

145

337

850

4,200

6,600

12,300

35,500

11,950 years ago. In consideration of theuncertainty in the accumulation rate as-sumption, this age is close to the annuallydated age of 10,750 years ago for the LGS-Holocene transition in the Camp Centurycore from northern Greenland (10).We estimated the age near the bottom of

the D-1 core (Figs. 3 and 4) by fixing theage at the LGS-Holocene transition at10,750 years ago and using the dated hori-zon at 117 m depth. Although these resultsare rough because pronounced variations inaccumulation rate and shape ofthe ice sheetno doubt have occurred over long periods oftime, these estimates suggest that the recordis potentially more than 100,000 years old.

All major dust events in the glacial stageice ofcore D-1 also occur in core D-3. Thesedata indicate that the records are continuousthrough the lowest sections of both cores.As is the case for the relatively shallow icecaps in the Canadian arctic islands (11, 12),the high-altitude, subtropical ice caps of theQinghai-Tibetan Plateau can provide climat-ic records comparable in length with thoseobtained in Greenland and Antarctica, al-though the oldest sections of the record aremuch more compressed.

Late in the 1986 field season, an abruptcontact was observed between clean ice andunderlying dirty ice emerging along themargin of the Dunde ice cap. The MPC and180 analyses of two shallow cores drilled

through this transition indicated that theunderlying ice was probably of Wisconsin-Wurm LGS age (1). In core D-1 at thesummit there is also a large increase in dustand 8180 depletion (Fig. 3) at 129.2 m; theMPCs increase substantially over the Holo-cene average. The increase in dust and de-crease in the '80/160 ratio are also observed

6180 (per mil)-13 -12 -I -10 -9 -8

.I

Avg. = 10.7

N03- (ppm)

0-

25-

50-

75-

100-

120-

125-

130-

135-

140-

0 .15

-q

II

30 45 60 75

Avg. = 0.26

SO42- (ppm)00 06 2 1.8 24 30A 0 I.

i~~~~~~~~~~

_ ~~~~~~~~~~IC

Avg. = 0.91

in LGS ice from polar cores. In cores fromCamp Century (5) and Dye 3 (13) in Green-land, the transition from high dust concen-trations in LGS ice to low concentrations inHolocene ice is also abrupt. In the Dundeice cores, the transition in the dust concen-tration is completed within a 30-cm sectionof ice representing approximately 40 years.The more negative 8180 ratios in the

lower 10 m of D-1 are consistent andstrongly support the interpretation thatLGS ice is present, even though the meanchange in 5 0 of 2 per mil is substantiallyless than the change of 11 per mil at CampCentury, 8 per mil at Devon Island, and 5 to7 per mil in Antarctica (13-15). This smallchange in the D-1 core implies that thetemperature decrease during the glacialstage on the subtropical Qinghai-TibetanPlateau was less than in the polar regions.Because more than 85% ofthe annual snow-fall on the Dunde ice cap arrives during themonsoon season (June through August),the isotope record is mainly a proxy ofsummer temperature. A moderate tempera-ture decrease during the LGS at lower lati-tudes is consistent with the 1981 CLIMAP(16) sea-surface temperature reconstructionsfor approximately 18,000 years ago. A varie-ty of data suggests that large areas of thetropics and subtropics had sea-surface tem-peratures as warm as or slightly warmer thanthose today. Moreover, the 0 isotope re-cord in the Dunde ice cores was not affectedby variations in sea ice cover during glacialstages. In the polar regions, such changescan increase the distance to moisturesources, which will tend to reduce 180/160ratios in ice core records during colderclimates. Unlike the rapid transition in MPCand anion concentrations at the LGS-Holo-

cr00 06

O-

25

-U

75

00-

120

125-

130

135-

140-

(ppm)2 18 24 30i '-

Avg. = 1.15

Ice crystal sIze (mm2)0 20 40 60

-I l.

Avg. = 31.0

Fig. 3. Averages over 1-m depth intervals (0 to 120 m) and 0.5-m depthintervals (120 to 139.8 m) for total particles (diameters .2.0 ,m) per chemistry profiles are derived from analysis of 1200 individual samples, andmilliliter ofsample and 5180 in core D-1 and NO , S02-, and Cl- in core D- the ice crystal sizes are measurements on 33 selected sections of core below3. The particle and 5180 profiles are based on analysis of 3585 samples, the 50 m; avg, average; age is in years ago.

-

.I|-14 o.c

7I;.P

==IP,

i.

0-

omr,

k

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8180 (per mil) NO3- (ppm)

-12 -10 0.00 0.30

so42- (ppm)

Fig. 4. Discrete 1,000-year averages of dust concentrations (diameters .2.0 ,m) and 518(and NO, SO'-, and Cl- in core D-3 for the last 40,000 years. The average values are forthe 40,000-vear record and not for the individual 1,000-ycar averages. There are 5 m of i

the 40,000-year model cutoff shown here; ka, thousand years ago.

cene boundary in the Dunde ice cores, theincrease in 8180 in precipitation appears to bemore gradual, occurring over several centuries.

Concentrations of Cl-, Sol-, Na+, Ca2+and Mg2+ var similarlv throughout thecores. Alkaline cations (Na+, Ca2+, andMg2+) and So2-, Cl-, and HCO- (estimat-ed with pH) constitute the majority of thedissolved species in D-3. The concentrationsof anion species decrease sharply in thelower 12 m of D-3 (Fig. 3), where average

concentrations of NO, So2-, and Cl- are

30, 50, and 32% of their respective averages

for the entire core. The concentrations ofmajor cations are also lower in this part ofthe core. This decrease of the ionic concen-

trations in core D-3 is temporally correlativewith the increase in microparticle concentra-

tions below 129 m of core D-1, where thereis a similarly abrupt transition. In contrast,

LGS concentrations of soluble species

(SO4, Cl) in polar ice cores are generallyhigher than those in the Holocene (17, 18).The decrease from LGS to Holocene con-

centration in the polar cores is attributed to

increased precipitation and reduced long-distance transport of terrestrial dust and sea-

salt aerosol during the Holocene.Ice crystal sizes were measured for select-

ed sections of D-3 in ice below 56 m (Fig.3) to explore the possibility that tempera-ture (19) and dust (20) affect size, as report-ed in polar ice cores. Crystal growth rate

increases with temperature (21). Betw%een 56and 89 m, a growth rate of 0.2 mm2 per yrearwas calculated, which, if applied to theentire core, would imply, that at 128.5 m,

the crystal size would be 300 mm2. Howvev-er, crystal size decreased substantially inLGS ice where a minimum of 11.4 nu2 was

measured. Similarly, LGS ice in cores fromAntarctica (19) and Greenland (18) is char-acterized by substantially smaller crystals.On the basis of these data, we conclude

that the lower 10 to 12 m of ice in the

Dunde ice cap represent ice deFing the last glacial stage. Theconcentrations correlate closelydepletion (temperature prox),lar cores extending below thecene transition; the association ccontent and more negative 8184commonly considered the princteristic of LGS ice in polar ice cc21-26).The Dunde ice cores provide

the late glacial climatic and en'

conditions on the Qinghai-Tibe(Fig. 4). The high alkalinity andcentrations suggest that the duson the ice cap originated from thing deserts and salt deposits. Thratios among major anion and

centrations throughout the co

that the provenance of the dichanged much over the last -4(The reduced concentrations c

species in LGS ice may reflect hilitation rates during LGS than itcene. A large part of the Qaidarapparently covered by freshwatcing the LGS (27). Dessicationbegan in the latter part of t

response to reduced precipitaduced precipitation to evaporaand extensive evaporite depformed. As the Holocene is apptincreasing ion concentrations Fflect both decreased precipitaticreased dry surface area in the Isin.Enhanced dust deposition in I

have resulted from increased wi

in response to increased barocliner isobaric and isothermic gra

29) around the expanded con

sheet in the higher latitudes.LGS, high winds from the desern Asia deposited loess hundthick across central and eastern

Cr(pPm) lar deposits extend to the Pacific Basin (30)1.2 2.4 and most likely to North America. Large

dust events must have occurred over theDunde ice cap and most likely much of theextensively snow-covered and glaciated pla-teau. The rapid decrease in dust depositionat the LGS-Holocene transition (-40 years)could reflect substantial change in the pre-vailing climatic conditions that limited thecapabilitv of the atmosphere to transportdust.The 5180 record (Fig. 4) reveals a short

interval of apparent warming about 35,000

in core D- 1 years ago that is also associated with less

all samples in dust deposition. Full glacial conditions were

ce core below soon reestablished. Between 30,000 and10,000 years ago, concentrations ofCl- andso4 increased gradually. One explanationfor this increase is that conditions became

)osited dur- drier and the areal extent of salt and loesshigh dust deposits increased. About 10,750 years ago,with 5180 a marked increase occurred, probably re-

as in all po- flecting the drying of the freshwater lakes.LGS-Holo- Simultaneously, the insoluble dust de-)f high dust creased sharply (Figs. 3 and 4). The 5180O has been profile suggests that there was a gradualipal charac- warming at the boundary while the aerosolDres (11, 13, (soluble and insoluble) data suggest that

other environmental changes, including re-a record of duced deposition, were rapid as might be\ironmental produced bv a reduction in wind intensitv.:tan Plateau A striking feature of the 8180 record (Fig.I NaCl con- 3) is the extreme less negative values (whichst deposited suggest warmer temperatures) of the last 60.e surround- years; decades with the highest values aree consistent the 1940s, 1950s, and 1980s. The onlycation con- comparable values are for the Holoceneres suggest maximum, 6000 to 8000 years ago. Climateast has not model results of Hansen and others (31)0,000 years. suggest that the central part of the Asian)f dissolved continent might be strongly affected by thegher precip- anticipated greenhouse warming. Althoughn the Holo- the Dunde ice core (D-1) data suggest thatn Basin was the recent warming on the Tibetan Plateau-r lakes dur- has been substantial, a connection with theof the lakes greenhouse warming has not been estab-he LGS in lished.tion or re-

Ltion ratios,)osits were

roached, the)robablvN re-

ion and in-Qaidam Ba-

LGS ice mayind strengthicity (steep-dients) (28,tinental ice

During the-rts of west-reds of feetChina; simi-

REFERENCES AND NOTES

1. L. G. Thompson, E. Moslev-Thompson, X. Wu, Z.Xie, Ceojournial 174, 517 (1988).

2. L. G. Thompson, X. Wu, E. Moslev-Thompson, Z.Xie, Aitm. Glaciol. 10, 178 (1988).

3. M. Domros and G. Peng, Tlte Clitnate of Clhitia(Springer-Verlag, Berlin, 1988).

4. Core D- 1 was analvzed at both the Byrd PolarResearch Center (BPRC) and the Lanzhou Instituteof Glaciology and Geocrv,ologv (LIGG), core D-2 atLIGG, and core D-3 at BPRC; 18Q wvas measuredat the University of Copenhagen and all othermeasurements were made at BPRC under class 100clean room conditions. The concentrations of parti-cles with diameters .2.0 p.m per miLliliter of sample(MPC) wvere measured with model TA-II CoulterCounters (5). The high concentrations ofdust neces-

sitated diluting most of the samples 36 times (up to

7000 times in extreme cases) to avoid problems withcoincident passage through the aperture tube. Sam-ples for chemistry and 8"O were not diluted. The

SCIENCE, VOL. 246

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concentrations of seven major ions, Cl1, SO2,NO5, Na+, K+ Mg2", Ca2, were measured with aDionex Model 2010i chromatograph, equipped wthAS4A (for anions) and FAST SEP CATION I andII (for cations) columns. Samples of 200 to 250 gwere cut from both D-1 and D-3 and pumpedthrough ion-exchange filters to measure total betaradioactivity in a Tennelec LB 1000 series Alpha/Beta Counting System. Total beta radioactivity isroutinely used to identify time-stratigraphic hori-zons associated with known atmospheric thermonu-clear tests (6).

5. L. G. Thompson, Microparticles, Ice Sheets and Climate[Rep. 64, Institute of Polar Studies (now Byrd PolarResearch Center) Ohio State University, Columbus,OH, 1977].

6. G. Crozaz, C. C. Langway, Jr., E. Picciotto, U.S.Army Corps Eng. Cold Reg. Res. Eng. Lab. Res. Rep.208 (1966).

7. C. F. Raymond, J. Glaciol. 29, 357 (1983).8. J. F. Bolzan,J. Geophys. Res. 90, 8111 (1985).9. N. Reeh, J. Glaciol. 34, 46 (1988).

10. C. U. Hammer, H. B. Clausen, H. Tauber, Radiocar-bon 28, 284 (1988).

11. D. A. Fisher et al., Nature 301, 205 (1983).12. W. S. B. Paterson et al., ibid. 266, 508 (1977); R.

M. Koerner, IAHS AISH Publ. 118 (1977), p. 371.13. C. U. Hammer et al., in Greenland Ice Core: Geophys-

ics, Geochemistry, and the Environment, C. C. Langway,Jr., H. Oeschger, W. Dansgaard, Eds. (Geophys.Monogr. 33, American Geophysical Union, Washing-ton, DC, 1985), pp. 90-94.

14. S. J. Johnsen, W. Dansgaard, H. B. Clausen, C. C.Langway, Jr. Nature 235, 429 (1972).

15. C. Lorius, L. Merlivat, J. Jouzel, M. Pourchet, ibid.280, 644 (1979).

16. CLIMAP Project Members, Geol. Soc. Am. MapChart Ser. MC-36 (1981).

17. M. R. Legrand and R. J. Delmas, Ann. Glaciol. 10,116 (1988).

18. S. L. Herron and C. C. Langway, Jr., in Greenland IceCore: Geophysics, Geochemistry, and the Environment,C. C. Langway, Jr., H. Oeschger, W. Dansgaard,Eds. (Geophys. Monogr. 33, American GeophysicalUnion, Washington, DC, 1985), pp. 77-84.

19. J. R. Petit, P. Duval, C. Lorius, Nature 326, 62(1987).

20. R. M. Koerner and D. Fisher, J. Glaciol. 89, 209(1979).

21. P. Duval and C. Lorius, Earth Planet Sci. Lett. 48, 59(1980).

22. L. G. Thompson and E. Mosley-Thompson, Science212, 812 (1981).

23. W. Dansgaard, H. B. Clausen, N. Gundestrup, S.Johnsen, C. Rygner, in Greenland Ice Core: Geophys-ics, Geochemistry, and the Environment, C. C. Langway,Jr., H. Oeschger, W. Dansgaard, Eds. (Geophys.Monogr. 33, American Geophysical Union, Washing-ton, DC, 1985), pp. 71-76.

24. R. M. Koerner, D. A. Fisher, W. S. B. Paterson,Can.J. Earth Sci. 24, 296 (1987).

25. M. DeAngelis, N. I. Barkov, V. N. Petrov, Nature325, 318 (1987).

26. L. G. Thompson, E. Mosley-Thompson, J. R. Petit,

Sea Level, Ice, and Climate Change (InternationalAssociation of Hydrological Sciences, Canberra,Australia, 1979), pp. 227-234.

27. K. Chen and J. M. Bowler, Palaeogeogr. Palaeoclima-tol. Palaeoecol. 54, 87 (1985).

28. S. Manabe and D. G. Hahn, J. Geophys. Res. 82,3889 (1977).

29. W. L. Gates, Science 191, 1138 (1976).30. D. K. Rea, M. Lewen, T. R. Janecek, ibid. 227,

4688 (1985).31. J. Hansen et al., J. Geophys. Res. 93, 9341 (1988).32. Supported by the National Science Foundation Of-

fice of Climate Dynamics and the Division of PolarPrograms (ATM-8519794), the National Geo-graphic Society (3323-86), the Ohio State Universi-ty and Academia Sinica of China. We thank themany individuals who contributed to the success ofthis program, especially the 50 Chinese and Ameri-can participants. Initial support was provided by theNational Academy of Sciences' Committee forScholarly Commnunication with the People's Repub-lic of China. We thank B. Koci and the Polar IceCoring Office for drilling the cores, the F. C.Hansen Company for assistance in the modificationof a tree-ring incremental measuring device for usein ice core dating, S. Smith for illustrations, and K.Doddroe for typing. This paper is Contribution 655of the Byrd Polar Research Center, The Ohio StateUniversity.

16 June 1989; accepted 23 August 1989

Carbon Dioxide Transport by Ocean Currents at25°N Latitude in the Atlantic Ocean

PETER G. BREWER, CATHERINE GOYET, DAVID DYRSSEN

Measured concentrations of CO2, 02, and related chemical species in a section acrossthe Florida Straits and in the open Atlantic Ocean at approximately 25°N, have beencombined with estimates of oceanic mass transport to estimate both the grosstransport ofCO2 by the ocean at this latitude and the net CO2 flux from exchange withthe atmosphere. The northward flux was 63.9 x 106 moles per second (mol/s); thesouthward flux was 64.6 x 106 molls. These values yield a net CO2 flux of 0.7 x 106mol/s (0.26 ± 0.03 gigaton ofC per year) southward. The North Atlantic Ocean hasbeen considered to be a strong sink for atmospheric CO2, yet these results show thatthe net flux in 1988 across 25°N was small. For 02 the equivalent signal is 4.89 x 106mol/s northward and 6.97 x 106 mol/s southward, and the net transport is 2.08 x 106mol/s or three times the net CO2 flux. These data suggest that the North AtlanticOcean is today a relatively small sink for atmospheric CO2, in spite of its large heatloss, but a larger sink for 02 because of the additive effects of chemical and thermalpumping on the CO2 cycle but their near equal and opposite effects on the CO2 cycle.

Tt HE NORTH ATLANTIC OCEAN HAS um with the atmosphere occurs. Linkage ofbeen widely regarded as an impor- the heat and gas fluxes is a necessary compo-tant CO2 sink and heat source for the nent of carbon cycle and climate modeling,

atmosphere. The large-scale circulation con- yet calculations of these fluxes have proceed-sists of both the horizontal wind-driven gyre ed along independent paths. We have mea-circulation and the vertically overturning sured oceanic CO2 concentrations and relat-thermohaline-driven circulation. Both act in ed chemical properties (temperature, salini-concert to transport heat and trace green- ty, 02 and NO3 concentrations, and alkalin-house gases to latitudes where disequilibri- ity) in a section through the Florida Straits

at 26.5°N (Hollywood, Florida, to GreatIsaacs Rock, Bahamas) and in the openAtlantic Ocean at 25°N (Bahamas to Africa)in order to test an earlier and controversialhypothesis ofBrewer and Dyrssen (1), basedon a few data, that treatment ofCO2 data in

a manner identical to that used for heattransport would yield a very small net CO2flux.The section is the same as that used by

others for the estimate of heat flux (2), andwe made specific use of the oceanic trans-ports of water derived from these studies tocompute the chemical fluxes. The work wascarried out on the Research Vessel OceanusCruise 205 in November 1988 at five sta-tions across the Florida Straits and fourstations in the open Atlantic (Fig. 1). Timedid not permit a full oceanic section, andthus interpolation along density surfacessampled by others (2-4) was necessary.

For the North Atlantic, the earlier calcula-tions (2-4) have shown that the northwardflow of 30 Sverdrups (Sv) (5), principallythrough the Florida Straits, has a meantemperature of 18.8°C, and that the com-pensating return flow has a mean tempera-ture of 9.7°C. This difference provides theobserved net heat flux of -1.1 x 10's W.Although some uncertainty still surroundsthis estimate (6), on the basis ofatmosphericobservations, the oceanic data are compel-ling.The equivalent calculation for trace gases

is more difficult because of the technicaldifficulty of obtaining measurements andthe enrichment of gases in the cold, deepflows where mass transports are less easilydetermined. Moreover heat is an internallyconserved property of sea water, whereasthe biogenic gases are transferred both at thesea surface, because of thermal, partial pres-sure, and biogenic processes, and internally,

27 OCTOBER I989

P. G. Brewer and C. Goyet, Department of Chemistry,Woods Hole Oceanographic Institution, Woods Hole,MA 02543.D. Dyrssen, Department of Analytical and MarineChemistry, Chalmers University of Technology, S-41296 Goteborg, Sweden.

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