Isotopic versus micrometeorologic ocean CO 2 fluxes: A serious conflict

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JOURNALOF GEOPHYSICALRESEARCH,VOL. 91, NO. C9, PAGES 10,517-10,527,

SEPTEMBER15, 1986

ISOTOPIC VERSUS MICROMETEOROLOGICOCEAN CO2 FLUXES: A

SERIOUS

CONFLICT

WallaceS. Broecker, 1 JamesR. Ledwell,1 Taro Takahashi, 1 RayWeiss,2 Liliane Merlivat,3 LaurentMemery, 3 Tsung-Hung Peng, • BerndJahne,5 andKarl Otto Munnich 5 Abstract.

Eddy correlation

measurements

portant than that driven by molecular through the diffusive boundary layer. not the first time the eddy correlation has yielded gas fluxes widely different

over

tbe ocean give CO2 fluxes an order of magnitude or more larger than expected from mass balance measurements using radiocarbon and radon 222. In particular, Smith and Jones (1985) reported large upward and downward fluxes in a surf zone at supersaturations of 15% and attributed them to the equilibration of bubbles at elevated pressures. They argue that even on the open ocean such bubble injection may create steady

state

CO2 supersaturations

transport This is method from

isotope studies. Jones and Smith [1977], Weseley et al. [1982], Bingham [1982], and Greenhut et al. [1983] have reported CO2 flux estimates far

greater

carbon.

than

Before

presenting

CO2 fluxes

and that inferences

those

based on radon

and radio-

ß

our case in

determined

the

that

not

wish

topic means: prebomb radiocarbon inventories; global surveys of mixed layer radon deficits; and oceanic uptake of bomb-produced radiocarbon. We argue that laboratory and lake data do not lead one to expect fluxes as large as reported from the eddy correlation technique; that the radon method of determining exchange velocities

daylight

showing that the energy loss estimates based on the net sensible heat flux (measured micrometeorologically) and net water flux (measured micrometeorologically) balance the net solar in-

is indeed useful

put (corrected

CO2 fluxes;

ing all tion

thesis

that

Smith

and

Jones

that

advance

that

neither

the

observed

the hypo-

concentrations

A strong

case has been made for

of CO2 fluxes

hours

challeng-

measured during

over vegetated

land

surfaces

for

soil

heat storage),

workers

carrying out these measurements clearly have established that the basic technique is valid for heat and water. By showing that the micro-

and

meteorologically

derived CO2 fluxes

agree with

independent agricultural estimates of net photosynthesis, these investigators make a convincing case for the validity of their CO2 fluxes. The

that the pCO2 values reported by Smith and Jones are likely to be systematically much too high. The CO2 fluxes for the ocean measured to date by the micrometeorological method can be reconciled with

technique.

we are

measured by the eddy correla-

[Alvo et al., 1984; Anderson et al., 1984; Ohtaki, 1984; Verma and Anderson, 1984]. By

cannot

account for the fluxes that they report;

CO2 fluxes

the reliability

supersaturations of CO2 due to bubble injection on the open ocean are negligible;

clear

of

methods we

of fluxes based on air-sea pCO2 differences and radon exchange velocities must be made with caution. We defend the global average CO2 exchange rate determined by three independent radioiso-

for estimating

to make it

defense

by isotopic

net CO2 fluxes

so measured range from 100 to

1000toolm-2 yr-1.

of

workers

radioisotopes of radon and carbon in the oceans nor the tracer experiments carried out in lakes and in wind/wave tunnels.

is

about

Theerror stated by these

3%, and the detection

limit

is

about10 toolm-2 yr-1. The situation

ent. Introduction

over

the

Based on isotopic

ocean

is

studies

quite

differ-

(see below),

the

average exchange rate of CO 2 int• yr andlOUt of the sea is estimated to be 20 tool m . The net --

In a recently

published

paper,

flux

Smith and

of CO2 for a specific

Jones [1985] cast doubt on the validity of air-

order

sea CO2 flux

the fractional

estimates

derived

from the radon

defined

method. Their conclusions are based on co2 flux measurementsmade by the eddy correlation technique. Their claim is that exchange driven by bubble

entrainment

is potentially'•far

of

the product

area must be of the

of this

exchange

supersaturation

rate

and

of dissolved

CO2,

as

(i)

S = ApCO2/pCO2(air)

more im-

where ApCO 2 is the excess of pCO2(sw), the CO2 pressure

1Lamont-Doherty GeologicalObservatoryof Columbia

University,

Palisades,

pCO2 (air),

California.

•Centred'Etudes,Nucleairesde Saclay, Gif-sur-Yvette,

France.

by the seawater,

over

pressure of CO2 in the

overlying air. The absolute value of S is usually much less than one so that the net is much less than the exchange rate. For

New York.

2ScrippsInstitution of Oceanography, La Jolla,

exerted

the partial

example, values



Figure for

zonal

1 shows annual average bands

in

the

world

flux

ApCO2

oceans.

With values rangingbetween-38x10-6 and

•OakRidgeNational Laboratory,OakRidge,

+50x10-6 atto, and the current meanvalue for

Tenne s see.

pCO2(air)of about335x10 -6 atto,estimatesof

5Institute fur UmweltphySik der Universitat Heidelberg, Federal RepublicOf Germany.

zonal

Copyright 1986 by the AmericanGeophysicalUnion.

Local fluxes are not likely to be very much larger except in very special circumstances. CO2 supersaturations or undersaturations of more than 25% are seldom encountered in the open

Paper number 6C0236. 0148-0227 / 86/O06C-0236 $ 05. O0

average net flux

vary

from -3 (int_o the

sea) to +3 (out of the sea) toolm-2 yr-1.

10,517

10,518

Broeckeret al.:

Isotopic VersusMicrometeorologicOceanCO 2 Fluxes

80

80# 70

7

•0 •0 40

•50

:•0 I0

I0 0

0

I0

I0 :•0

gO

•,0

$0

40

40

5O

50 6O 70

80S

80S

Fig. 1.

Thedistribution of the pCO 2 difference(ApCO 2 in 10-B atm)

between surface water and air. The values represent mean value in each zone indicated by the heavy lines.

a zonal and seasonal Positive values

(hatched areas) indicate that the net CO2 flux is from sea to air, and negative values, air to sea. The global mean ApCO2 value is about -9 •atm, which is consistent with estimates of the invasion rate of fossil fuel CO2 into the ocean. The data were obtained during various expeditions including the GEOSECS,Transient Tracers in the Ocean (TTO), Weddell Polynya Experiment, Hawaii-Tahiti Shuttle, Atlantic Long-Lines (Ajax), NOAA Acid Rain, United State Coast Guard Polar Star, HMS Bransfield, ESSO Atlantic, Trans Freight Lines Jefferson, Japan Line Ltd. Japan Alliance and Icelandic Marine institute/Lamont-Doherty Geological Observatory joint seasonal

ocean, tions

tor

study

programs.

and radon measurements have

failed

of 3 greater

conditions studies

to

exchange

[Peng et al.,

have

not

in

show much

rates

detection

limits

of

average

The first

Laboratory

ultrasensitive

the

eddy

depen-

correlation

technique stated above (see Figure 2). Furthermore, corrections to the CO2 flux due to fluctuations

associated

with

heat

and

moisture

fluxes

are of this same order, again suggesting that accurate oceanic fluxes will be very difficult to extract from eddy correlation measurements. In

the

sections

which

follow

we first

summar-

ize the results of the CO2 flux measurements made by the eddy correlation technique. Then, we defend

data;

the exchange

rates

based on isotope

we show that evidence from wind/wave tun-

nels and lakes supports the isotope-based resuits rather than the eddy correlation results; we show

that

from bubble and we review

the

freedom

injection the

of

the

effects

evidence

for

radon

is

method

an advantage;

the

effect

of

bubbles on gas supersaturations in the open ocean. Finally, we discuss the bubble entrainment mechanism proposed by Smith and Jones to explain the high flux values obtained by their method, and we discuss a problem which may have affected the pCO2(sw) measurements of Smith and Jone s.

Results

for CO 2

a fac-

dencies of gas exchange on dynamical conditions for CO2 or other gases, nor, as we argue later, are such dependencies expected from studies of bubble phenomenology. In summary, the range of net fluxes expected on the open ocean lies almost entirely within the

Summary of Ocean Eddy Correlation

condi-

than

than in

1979].

indicated

stormy

more

attempt

to measure the CO2 flux

tween the ocean and atmosphere relation

method

was

made

on

the

be-

by the eddy corbeach

at

Sable

Island, Nova Scotia, by Jones and Smith [1977]. The weather at the time of measurement (July 1976) was characterized by high humidity, fog, and relatively gentle winds. Three measurements were made over a 2-day period. The result

yielded netyrupward CO 2 fluxesrangingfrom16 to -2 25 mol m ß As the ApCO2 between air and sea was not

measured,

there

is

no way to convert

this result to an exchange rate. If for example the CO2 partial pressure in the water were 20% higher than that for the air, then their measurement would correspond to a CO2 exchange rate

in the rangeof 80-125molm-2 yr-1 (i.e.,

4-6

times the radioisotope-based mean global CO2 exchange rate). The second published CO2 net flux by the eddy correlation

method

coastal waters Miami, Florida,

was

measured

1980 [Weseley et al. ' 1982].

flux

over

shallow

above a submerged reef near during a 2-week period in March

was reported

to be 215-250

The net CO 2 2 urnward • mol m-

yr-

at

a windspeedof 10 ms-1. Again,no pCO 2 data are available. If the surface ocean pCO2 were 20% above the atmospheric value, these results

would yield CO•yr exchange ranging to 1250 mol m-1 (i.e.,rates = 50 times from the 1075 radioisotope-based mean). Bingham [1982] reported the first aircraftbased CO2 net flux measurement by the eddy cor-

Broeckeret al.: 2O

I

.028

i

Summary of Radioisotope-Based

'E 021

od

.007 LIMIT

-5

X

u

been made from radioisotope



'•

o

-.007

- I

n-

-

:D -I0 --

I-z

_15

m,m I-' z

m,mZ

n,'o

•w



o•o z _20 ø

esti-

CO2 exchange rate have inventories.

The

reader is referred to Broecker for a summary of these studies of the basic papers containinM

and Peng [1982] and for a listing the measurements.

The

the

first

estimate

of natural

LL X

,014 .J

-

near

is

based

radiocarbon.

on

distribution

The assumption

is made

steady

-,021 U

i.uz

o•-•o•

w -.028

200 400 pSEA -6• co2(io tm)

z

600

state.

In particular,

the assumption

is madethat the decayof 14Cwithin the sea was balancedby the entry of I•C atomsthroughthe air-sea boundary. Through a knowledge of the volume of the ocean, the average ZCO2 content

of

seawaterandthe averageI•C/C ratio for the CO 2

speg.ies

of •C

0

radioisotope-based

mates of the mean global

0

.ooo

M ICRO-

rr

independent

Gas Exchange Rates

that prior to 1850 the distribution of 14Cwas

METEOROLOGY

o

Three

o

DETECTION

o

10,519

Isotopic VersusMicrometeorologic OceanCO 2 Fluxes

dissolved

in

ocean water,

the

in the sea can be determined.

decay

rate

The amount

of i•C enteringthe seadepends on the CO 2 exchange rate andthe differencebetween the I•C/C ratio in atmospheric and surface ocean carbon (corrected for the equ.ilibrium isotope fraction-

ation).

The air-sea I•C/C difference has been

estimated

from

existing

measurements

to be

46+10ø/oo (seeFigure3). TheCO 2 exchan_•e -1

Fig. 2. Relationship between net CO2 flux across the air-sea interface and the CO2 partial pressure for surface water based on the radio-

rate estimated in this way is 17+4 mol m yr (the reader is reminded of the difference between exchange rate and net flux introduced earlier). This rate applies to the preanthropo-

yr

ß Also shown is the relationship

genicCO 2 pressure in the atmosphere (=280x10 -6

yr

) and the units used by Smith and Jones

isotope-based CO 2 exchange rate of 20 molm-2 -1

between

the units usedby geochemists (i.e., molm-2 -1

[1985](mgCO 2 m-2 s-l).

Although surfacewater

atm). Correcting for the anthropogenic increase in-2 atmos.pheric CO2 content this becomes 20+5 mol

m

yr -l for the period 1970-1980 whenthe atmo-

pCO 2 values as high as 550 x 10-ø atm (in upwel-

sphericpCO 2 averaged about330x10 -6 atm.

the ice edges)

of the 222Rn to 226Ra ratio in the surface ocean

ling areas)andas lowas 150x 10-6 atto(near have been observed

locally,

valuesrangeMostcommonly between 200x 10-6 and 400 x 10-6 atm.

The important point is that

the range of expected net CO2 flux corresponding to the commonlyobserved pCO2 values in global surface

limits

ocean

water

lies

within 2

of +0.01 mg CO2 m-

and Jones

s-

the

1

detection

stated by Smith

[1985].

The

July

method.

1981 off

His

measurements

San Diego,

were

California

(at

122øW). The net CO2 upward flux, for

humidity-induced

density

made

in

30øN,

uncorrected

fluctuations,

was

about100molm-2 yr-1. Again,no pCO 2 results are available. If the ocean had a CO2 pressure 20% higher than the air, this value would cur2 -1 respond to an exchange rate of 500 mol m- yr . In their most recent paper, Smith and Jones [1985] report eddy correlation measurements of CO2 made at the Sable Island beach site in late autumn. Results of their measurements range from

a maximum

net

air-to-sea

flux

of

about

32

toolm-2 yr-1 to a maximum net sea-to-air flux of about 17 mol m-2 yr-1. They reported that during the entire observation period the partial pressure of CO2 in the seawater remained above that in the atmosphere by 10-15%. Because of the rapid changes in the sign of the flux these authors do not attempt to calculate a mean CO2 flux. Hence to date, all meteorologic measurements

CO2 exchange rates estimated

from

the reported microover the ocean give

which are far

radioisotope

higher

studies.

than

estimate

is

based

on

mixed layer. For the open ocean, present in this layer is generated

measurements

the radon by the in

situ decayof 226Radissolvedin seawater. The radon content in the air over the open ocean is too small to contribute significantly to the surface water burden. By measuring the ratio of 222Rnactivity to 226Ra activity in samples from the mixed layer, the fraction of radon which escapes to the atmosphere and, in turn, the evasion

relation

second

rate

station where made that the

of

radon

can

be

assessed.

At

each

this is done the assumption is system is at steady state on a

time scale of several days (the half-life of Rn is 3.82 days). However, as both the mixed layer thickness and radon evasion rate vary with meteorological conditions, this assumption is rarely fulfilled. The argument is that if the results from enough stations (or times) are averaged, the biases resulting from departures from steady state will cancel. 222

Penget al. [1979]use the 222Rnand226Ra results from stations world ocean to obtain

located throughout the an average radon evasion

rate

If

(see Figure

4).

this

average

is used to

obtaina meanglobalCO•exchange rate, the re-

suit is about 16molm6 yr-1 fortheatmospheric pCO2 of 330x10- atm. The reader unfamiliar

with

the

models

used

to

convert

from

exchange rate for one gas to that of another is referred to Broecker and Peng [1982] and Jahne [1985] for discussions of this procedure. While it

is

difficult

to

estimate

the

error

with this result, it is likely about Because the higher-latitude portions

associated

+25%. of GEOSECS

10,520

Broecker et al.:

0

l.p

I

IsotopicVersusMicrometeorologic Ocean CO 2 Fluxes

I

: *

I

I

-

tweenthe bombi•C to C ratio for atmospheric CO2 and surface water ZC02, prior to this As shown by Broecker et al. [1980; 1985],

time. the

given

the

•" '•%", ONSET - water columninventoryof bombi•C atomsat any 14CFROM '

-or

-..:-%... ,/_

ocean

station

can be estimated

from

profiles of i•C/C, ZCO 2 and 3Hmeasured during the GEOSECS programand the i•C/C for thesestations

estimated

from measurements on prebomb

samples. Thehistory of the 14C/Cfor atmo-

•--•-40

x x

-=____._ß: __ .. .-... -60

ß

spheric

CO2 is well

documented from direct

meas-

urements. A complete history for the I•C/C for CO2 dissolved in the surface ocean is available from measurements on corals in a few places. However, as shown by Broecker et al. [1985], adequate estimates of this history can be made at

all

other

places

in

the

ocean based

on the

prebomb i•C/C value and the i•C/C value measured 1850

I

I

I

I

1875

1900

1925

1950

x

YEAR

Fig. 3. Summary of radiocarbon an estimate of the preindustrial

data yielding (i.e., presum-

2 duringthe GEOSECS survey. In this wa{, mean

CO2 exchange rates

of 19.2 mol m-

yr

for

the

Indian Ocean 19.4 mo12m-2 Pacific Ocean, and 2•.3 mol m- yr-{r-1 forfor thethe Atlantic Ocean

are

estimated.

From

these

an

ocean

aver-

ably steady state) difference betweenthe 14C/C

age close to 20 molm-2 yr-1 is obtainedin

ratio

natural i•C discussedabove. The error in this average is probably no more than 3 mol m-2 yr-1.

in atmospheric CO2 and surface ocean ZCO2

(corrected

for

the equilibrium

ation). The small atmosphere obtained

isotope

fraction-

dots show the trend for the from measurements on tree

close

agreement

with

the

estimate

based on

rings [Stuiver and Quay, 1981]. As the 14C/C ratio in the atmosphere is geographically uniform, this record is typical of the entire atmosphere. The large dots show the trend for surface

water

from

the

Florida

Straits

from measurements on growth r•ng

as

I

obtained

dated corals

[Druffel and Linick, 1978]. As the 14C/Cratio in surface location,

ocean carbon varies this

record

cannot

with

geographic

be taken

as the

ocean average. Only a limited number of prenuclear water samples were collected and analyzed for radiocarbon. Most of these were in

the Atlantic

[Broecker

et al.,

1960].

the

data

for

more sparse, significantly

Straits

site

becomes 43O/oo

between 40øN and 40øS. the

Pacific

there

and

different

from

those

I I I

III

THICKNESS

• 5 SPEED o.o

for

oceans

that for

ocean was 46O/oo (i.e.,

are

the At-

Two pieces

of

information

the

were wind likely

distribu-

are

needed:

the inventory of bombI•C atomsin the oceanfor one time

(in

this

case

survey);

and the history

the

time

of

¬

-

0.4

0.6

0.8

t.o

I

I

I

I

40øS) shows lower values in all three oceans. We estimate that the global average pre-

and surface

-

Although

Indian

is no evidence

I



½r' ='l.7cm/hr rn _ MEAN WIND

These

of 46O/oo obtained for the 1850-1900 period at Florida

I

:• =0.77 I-- 15 - MEANMIXEDLAYER 0 I0-VELOCITY =Z.er./•oy

value for 41 such samples collected between 40øN and 40øS in the Atlantic Ocean is -53O/oo as compared to-56O/oo for four Florida Straits coral samples from the same time interval (i.e., 1955-1957). Thus the preindustrial difference the

I

O3 MEAN PISTON

results are shown by crosses. TheaverageA14C

the Atlantic

I

(/320 - MEAN 222Rn-Z26Ra ACTIVITITY RATIO •1

the

GEOSECS

of the difference

be-

4.

2

4 K(m6/d(3y) 8 I0 12

In the top panel

is shown a histogram

of the activity ratios of 222Rnto 226Rafound

in the mixed layer for 90 stations occupied during the GEOSECSexpeditions [Peng et al., 1979]. The average ratio is 0.77 with no values below 0.50. In the lower panel, a histogram for the iston velocities (k) calculated from the

22Rn/226Ra activity ratios andmixedlayer thicknesses

(h) are shown.

velocity is 2.8 m d-1 (i.e.

values higherthan7.0md-•

The average piston

11 7 cmh-l).

'

were

observed.

No

Broeckeret al.:

What Would the Radioisotope Distribution Like if the Gas Exchange Rate Were 10 Times Larger? Starting with the radon method, let sider how the radioisotope distributions

3.5

Look

N•3.0

us conwould

,

ß

ß ,

•t

10 times the best estimatesß This

rate corresponds to net CO 2 fluxesof order30

• 2.0

toolm-2 yr- at 15%supersaturation,still at the low end of fluxes reported from the eddy correlation techniqueß For the average mixed

layer

O'

•_ 2.5

differ if the CO 2 exchange rate were200toolm-2 yr-1 i e

10,521

IsotopicVersusMicrometeorologic OceanCO 2 Fluxes

thickness

(ißeß,

, •



= 54 m) and the average

0)



radioisotope exchange (i.e. a piston velocity of 2.b•sed md-l), therate 222Rn to 2•BRa activity

ratio

is 0.77.

Were the piston

veloc-

ity instead10 timesas great (ißeß, 28 md-l), then the expected ratio would drop to 0.25. Ninety radon stations were taken as part of the GEOSECSAtlantic and Pacific programsß No ratio

less that 0ß50 was found (see Figure 41• •/C The mean preanthropogenic air-sea

15

Fig. 6.

curves) and of the mean surface water bomb 1•C

In addi-

result obtainedusing the meanbomb1•C excess

summary), more recent high-quality measurements on ring-dated corals exclude the possibility that this difference was so small (see Figure

in surface

Finally, if the CO•exchange rate were 200

rather than 20 toolm- yr-1, the increase in

from the distribution

the atmospheric

14C. At the time of the GEOSECS survey the atmospherici•C/C ratio was400O/oohigher than its

prebomb value,

while

the surface

than its

(160ø/oo,

see Figure

5) and

obtained by Broecker et al. [1985] for the time of the GEOSECS survey. •e mean oceanic vertical mixing rate obtained by Li et al. [1984] is given for comparison. This diagram demonstrates the sensitivity of the CO2 exchange rate derived

surface ocean 14C due to the bomb tests would

was only 160ø/oo higher

water

the total bomb1•C inventoryof 2.9x1028 atoms

3).

behind

curves) at the time of

the GEOSECS surveys for various combinations of the mean ocean CO2 exchange rate and the mean ocean vertical •xing rate. •e results were calculated for a one-dimensional ocean [see Broecker et al., 1980]. The black dot shows the

tion to several tens of measurements on prenuclear surface water (see Broecker [1963] for

so far

30

Plot showingconto•s of the ocean

excess (in O/oo, light

exchange ra•e200toolm -2 yr-1 rather than20 would be

not have lagged

25

inventoryof bomb 1•C (in 10'• atoms,heavy

ratio

difference (corrected for equilibrium isotope fractionation) is 46+10ø/oo. Were the true

tool m- yr- , the expected difference one-tenth this value (ißeß, 5ø/oo)ß

20

CO• EXCHA•E RATE(toolm'•yr-I)

ties

in

of bomb1•C to uncertain-

the mean surface

water

excess

(i.e.,



+15ø/oo•sand in theThe oceanic inventory (i.e., is •0.2x10 atoms). range of uncertainty

ocean ratio

prebomb value

sho•

by the dashed ellipse.

As can be seen,

the average CO2 exchange rate is tightly

con-

strained. 3o

I

o_ 20
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