pH-Temperature Relationships in the Gulf of California - Advances in

20 pH-Temperature Relationships in the Gulf of California Downloaded by CALIFORNIA INST OF TECHNOLOGY on March 30, 2017 | http://pubs.acs.org Publication Date: August 1, 1985 | doi: 10.1021/ba-1985-0209.ch020

ALBERTO ZIRINO and STEPHEN H. LIEBERMAN Marine Environment Branch, Code 522, Naval Ocean Systems Center, San Diego, CA 92152

The potential of a glass-reference combination electrode immersed in a pumped stream of seawater was measured while underway in the Gulf of California. The pH at the surface (sampling) temperature was computed from the temperature dependencies of the electrode and buffers. For most of the Gulf, pH and temperature showed a high degree of positive correlation. However, in the warmer waters of the southern Gulf and on spatial scales of less than 20 km, negative correlations were also observed; pH maxima were associated with temperature minima and chlorophyll a maxima. These observations are discussed in terms of oceanographic processes. The positive pH-temperature correlation is probably caused by mixing processes and the cumulative biological depletion of CO from surface waters. 2

AL

DIRECT RELATIONSHIP EXISTS B E T W E E N T H E p H values of the oceans sur-

face and the potential exchange of CO2 across the air-sea interface (I). Although the literature contains many discussions about the possible values of the surface p H (1-4), few measurements have been made. Studies of p H and C 0 relationships across the northern Pacific Ocean have revealed that p H , p C 0 , and temperature correlated closely (5). The p C 0 (and by inference, pH) was seldom at a value close to that suggested by equilibrium considerations; biological and physical processes strongly affected p C 0 at the surface. W i t h a simple underway technique for measuring p H , a strong correlation previously was found between p H , coastal upwelling, and the resulting planktonic production (6-8). However, discussions of these measurements were limited to changes that were greater than 0.1 p H units because, previously, p H was measured electrometrically and the electrometric measurement of p H may involve an absolute error of 0.1 p H units (9); thus, no attempt was made to look at the finer structure. 2

2

2

2

0065-2393/85/0209-0393$06.00/0 © 1985 American Chemical Society

Zirino; Mapping Strategies in Chemical Oceanography Advances in Chemistry; American Chemical Society: Washington, DC, 1985.

394

M A P P I N G STRATEGIES I N C H E M I C A L

OCEANOGRAPHY

This chapter demonstrates that p H may be measured continuously while underway to 0.01 units, and that pH-temperature relationships can be interpreted in terms of known oceanographic processes. Data from the Gulf of California are used to illustrate these points.

Downloaded by CALIFORNIA INST OF TECHNOLOGY on March 30, 2017 | http://pubs.acs.org Publication Date: August 1, 1985 | doi: 10.1021/ba-1985-0209.ch020

Experimental A l l measurements were made aboard the USNS DeSteiguer during the V A R I F R O N T III expedition of the Naval Ocean Systems Center to the Gulf of California. The expedition was carried out during November and December of 1981 i n conjunction with scientists from the Centro de Investigaciones Cientificas y Escuela Superior de Ensenada ( C I C E S E ) . Measurements were made while underway at approximately 10 knots during northerly and southerly transects of the Gulf. The data presented here deal only with the southerly transects shown in Figure 1 as A - B . p H Measurements. The p H of a flowing stream of sea water was measured in the ship's laboratory. The stream originated from a conduit mounted in the forward transducer well with the intake approximately 1 m under the bow at the keel. Approximately 50 m of Teflon-lined hose was used in the conduit between the intake and the laboratory. Determinations were made with a Corning 476055 combination electrode mounted i n a Teflon manifold (8). A single electrode, in conjunction with a p H meter (Corning Model 103), was used for all the measurements. Initially, the electrode was calibrated in the manifold by placing the appropriate buffers in the Teflon reservoir (Figure 2) and recirculating them past the electrode until a constant millivolt reading was obtained. T w o buffer solutions, made up to National Bureau of Standards (NBS) specifications (JO) (available commercially from Beckman Instruments), were used. These solutions were K H P 0 , 8.69 x 10~ M ; N a H P 0 , 3.043 x 10~ M (pH = 7.413 at 25 °C); and borax, 0.01 M (pH = 9.180 at 25 °C). A single calibration was used to compute p H over the 3-day measurement period. The manifold temperature, during calibration and during the measurements of sea water, was determined with a thermistor mounted in the manifold alongside the p H electrode. The thermistor output was read with a YSI (Yellow Springs Instrument Company) Model 47 scanning telethermometer. As an electrical check, the buffer solution was placed in contact with the seawater in the Teflon hose by using Pt wire during the standardization. A constant meter reading signified that the seawater and buffer solution were at the same ground potential. The p H at the i n situ temperature T was calculated from the following expression:

P

H

2

4

( T )

= {(E

4

cell(T)

2

- E )/S(T/TC)} 7(T)

2

4

+

B

1{T)


(1)

ZIRINO AND LIEBERMAN

pH- Temperature Relationships

395


20.

Figure 1. Cruise track of the USNS DeSteiguer superimposed on an IR image of the Gulf of California. Lighter shades denote colder temperatures.



Figure 2. Apparatus used in the underway measurement of p H , Cw, and fluorescence. (Cu data are not discussed here.) (Reproduced with permission from Ref. 8.)


20.

ZIRT

D LIEBERMAN

397

pH-Temperature Relationships

where the ratio Tl TC is in Kelvins and B is the NBS determined p H value for the p H 7 buffer at T. £ i i is given by 7

ce

£ceii

(T)

(7)

= Ecdi TAo

+

(

0.27mV/deg(T - TM)

(2)

Equation 4 defines Ej , TM is the temperature of the manifold, TC is the temperature during calibration, and 0.27 mV/deg is the whole-cell (inner reference/glass/seawater//outer reference) temperature response of the combination electrode in 34 ppt seawater at 1 atm (12). Similarly, the electrode slope S is given by


(T)

S

=

( 9(TC) ~ 7(TC)) ( 9 TC) £

£

/

5

(

7(TC))

()

B

3

where E and E are the electrode potentials generated in NBS 7 and 9 buffers at the calibration temperature (TC), respectively, and B and JB are the NBS determined p H values at TC. Finally, 7 ( T C )

9 ( T C )

7(TC)

9(TC)

E

7m

=E

7(TC)

+ F(T)

(4)

where E is the empirically determined temperature-dependent response of the combination electrode i n phosphate buffer (pH = 7.413 at 25 ° C ) . Using the method of Flynn (12), we found that between 10 and 30 °C the potential generated by this electrode d i d not differ significantly from E . In principle, Equations 1-4 should computate the p H , at any temperature T, after calibration with buffers at temperature TC. Sea Surface Temperature Measurements. Sea surface temperature was measured with a calibrated InterOcean thermistor (0.01 °C) mounted at the conduit under the bow. (Thus, i n the figures, recorded temperature slightly precedes recorded p H ) . Seawater for the determination of chlorophyll a fluorescence was pumped from the sea chest of the USNS DeSteiguer and measured separately although i n parallel with p H . Fluorescence was measured with a Turner Designs Model 10 fluorometer equipped with a flow-through cell. Chlorophyll a was calculated from fluorescence (11) by calibrating the flow-through i n vivo fluorescence with discrete acetone-extracted fluorescence measurements of particulate chlorophyll obtained from water samples that passed through the i n vivo system. The responses of the electrode, fluorometer, and thermistor were recorded on an inhouse-built data logger. Measurements were recorded every 5 s. Each time (space) series between A and B represents approximately 40,000 individual determinations. 7(T)

7{TC)

Results Figure 1 is an IR image of the G u l f of California taken by the National Oceanic and Atmospheric Administration (NOAA) 7 satellite on December



398

MAPPING STRATEGIES IN CHEMICAL OCEANOGRAPHY

10, 1981, and processed by the National Earth Satellite Service (NESS) at Redwood City, California. In Figure 1, progressively higher temperatures are shown as progressively darker areas. Physically, the Gulf can be d i vided into two distinct hydrographic regions separated by a shallow sill just south of Angel de la Guarda and Tiburon Islands. The northern, shallower portion is mixed by strong tidal currents with overall lower surface temperatures and slightly higher salinities. Conversely, the hydrographic features of the southern G u l f resemble those of the eastern tropical Pacific Ocean, and show greater stratification, higher surface temperatures, lower surface salinities, and a distinct 0 minimum from 500 to 800 m . The difference i n surface temperatures between the northern and southern portions of the Gulf can be seen i n Figure 1, which also shows that the area immediately south of Tiburon Island is a source of cold, upwelled water. This feature, which can be attributed to the combined effect of tidal mixing and shallow topography, is present year-round. Also, from November to A p r i l prevailing northerly winds form additional plumes of upwelled water that extend westward of the mainland Mexican coast. Several of these thermal features were crossed by the USNS DeSteiguer during its southern transect. Figure 3 shows a time-space series of pH(T) as computed from Equations 1-4 from the raw data collected during transect A - B . Similarly, the figure also shows plots of chlorophyll a and temperature as functions of cruise time and distance. The x-axis denotes the Julian date and the time of sampling. The straight-line distance from A to B is 850 k m ; the actual distance traveled was approximately 930 k m . Average speed was 18 km/h. Some of the corresponding positions are also indicated in Figure 1 (dotted lines). Frontal areas are denoted by an F . Temperature in the survey area increased from north to south, going from a low value of 19 °C in the upwelling area to a value of approximately 27 °C at the mouth of the Gulf. A l l of the major frontal features visible i n the IR image appear in the temperature record. Three degree fronts occurred near 344/06 (December 10, 6 A . M . ) and at 344/15. Overall, pH(T) follows temperature very closely and increases from a minimum of 8.24 at 110 km south of Tiburon Island (344/06) to approximately 8.45 at the southern portion of the Gulf. O n the whole, identifiable major patches with length scales greater than 100 km coincide in temperature and pH(T). Positive correlation between surface temperature and pH(T) is also evident on smaller scales of 20 km or less, but only i n the northern, colder portion of the G u l f (to 345/06). In the coldest waters traversed—from 343/23 to 344/06 in Figure 3—correlation is observable on scales of 3 km or less (Figure 4). O n the other hand, in the warmer portion of the G u l f and on space scales of approximately 20 km or less, T and pH(T) are not strongly correlated or are often negatively correlated. Negative correlation is often characterized by a pH(T) maximum appearing in conjunction with a T 2



Figure 3. Time (space) series of pH(T) temperature, and chlorophyll a measured in the surface waters of the Gulf of California during December 10-12, 1981.




400

8.201 2245

I 2345

| 45

| 145

| 245

I 345

I 445

L_ 545

hours Figure

4. Fine-scale

structure of pH(T) and temperature December 10, 1981.

measured on

minimum. When pH(T) maxima accompany colder surface waters the pH(T) signal is often matched by a very similar signal i n chlorophyll a. Figure 5 shows one such section. Other similar observations are visible i n Figure 3 at 344/23, 344/07, and 345/12. Large-scale correlation does not exist between chlorophyll a and pH(T) or T. Chlorophyll a concentrations, as measured by i n vivo fluorescence, average 2-3 /xg/L from Point A to the frontal area at 344/06 and decrease to between 1 and 2 /xg/L thereafter, to the location at 344/23, where an intrusion of cold water has produced a 1° warm-to-cold front. This upwelled "patch" lies at the beginning of an extensive (330 km) stretch Zirino; Mapping Strategies in Chemical Oceanography Advances in Chemistry; American Chemical Society: Washington, DC, 1985.


20.


8.25"

1

21,

:

401

pH-Temperature Relationships

1

L-

1

I

i

1

L

:

O

181 1220

I

i

1240

1300

1320 1340 hours

i

I

1400

1420

1440

Figure 5. pH(T) and chlorophyll a maxima in association with a temperature minimum (December 10, 1981).

of h i g h c h l o r o p h y l l a c o n c e n t r a t i o n , w h i c h averages b e t w e e n 4 a n d 5 ug/h;

i n d i v i d u a l peak concentrations r e a c h above 9 /xg/L. F r o m n o r t h

to s o u t h , the c h a n g e patches observed i n the c h l o r o p h y l l a c o n c e n t r a t i o n is o n a c o n s i d e r a b l y greater scale t h a n the 1 0 0 - k m scale changes observable i n both temperature a n d p H ( T ) . Discussion Sampling.

T h e measurement system is able t o detect p H changes as

s m a l l as 0 . 0 1 p H units per k i l o m e t e r ( F i g u r e 4 ) . F i g u r e 4 shows changes i n


402


pH(T) that have been scaled to resemble changes in T. The high positive correlation between pH(T) and T indicates that the pumping system is adequate for this degree of resolution. Weichart (6) and Simpson (see chapter 21) also found that their own pumping syterns were adequate. Additionally, they have both shown that when inorganic phosphate in seawater is measured in parallel with p H , time-space series are obtained in which P O 4 and p H are highly, albeit inversely, correlated. Thus, little gaseous C 0 is lost or "smeared" through pumping. pH(T) vs. T Relationships. The pH-temperature coefficient of seawater (dpH/dT) is not only a property of seawater but a quantity that is dependent on the characteristics of the test (glass) and reference electrodes, as well as on the temperature characteristics of the buffer solutions (10). A n average response for the Corning Semimicro (476055) electrode in 34.3 ppt seawater was determined previously (12) in the 10-to-30 °C range to be 0.27 ± 0.03 mV/°C. This value was not sensitive to salinity down to 30.8 ppt, nor to initial p H . When this average value is used to compute pH(T) from Equations 1-4, an average dpHldT of - 0 . 0 1 2 p H units/°C is obtained (Figure 6). (Actual dmV/dT values increased with decreasing temperature, but the deviations from the mean are small enough to be ignored i n this discussion.) This value is in excellent agreement with an average coefficient of - 0 . 0 1 [determined by Gieskes (13)] and with Ben Yaakov's computed values of dpH/dT, which were estimated from the temperature dependence of the equilibrium constants of carbonic acid in seawater (14). From the previous discussion a plot of pH(T) vs. T for the data from transects A - B would be expected to resemble Figure 6. In fact, such a plot possesses a positive slope with dpU(T)ldT = 0.034 with R = 0.88 for a linear fit to the data (Figure 7). Electrode drift during the 3 days of sampling from north to south does not account for this positive trend because other data, taken with the same electrode between December 3 and 5, going from south to north, also plot out linearly with dpH(T)/dT = 0.037 p H units/°C, albeit with less significance ( H = 0.55). In addition, the relationship between pH(T) and T demonstrated in Figure 7 is actually a curve whose slope decreases at increasing p H values. This curvature occurs because changes i n pH(T) represent actual changes in the hydrogen-ion content of the seawater, and these changes reflect the uptake of C 0 by primary producers. O u r reasoning follows. Uptake of C 0 by Marine Organisms. The p H of the surface seawater may be thought to increase because of loss of C 0 according to the following reaction: -


2

2

2

2

2

2

H C O 3 - t± C 0 ( ) + O H " 2

aq

(5)

Reaction 5 indicates that the p H of surface seawater may be raised, by loss of C 0 to photosynthetic organisms or to the atmosphere via carbonic acid, 2



pH-Temperature

Relationships


20.

(l)Hd


403

404

MAPPING STRATEGIES IN CHEMICAL OCEANOGRAPHY 8.50

8.44

••

•

•

• •

•

• • ••••••• • •••• • •• •••••••• ••• ••• • • •••••••• * •••••• • ••

••••• • •••••••• • • ••• • 8.38


• •••• 8.32

••••

••••••

8.26

•• •

•••• 8.20

18

19

20

21

22

23

24

25

26

TEMPERATURE(DEGC) Figure 7. Plot of pH(T) vs. T for Gulf of California data. pH(T) values are rounded to 0.02, and temperature is rounded to 0.1. Each circle represents a 5-min average, or 60 points.

without producing a change i n the alkalinity (the concentration of weak acid anions titratable to the carbonic acid-bicarbonate equivalence point). Although not strictly true for planktonic photosynthesis (15), it is a reasonable first-order approximation because the specific alkalinity (alkalinity/chlorinity ratio x 10 ) is nearly constant for surface waters of the world ocean (I). Prima facie evidence can be seen i n Simpson's data (see chapter 21). O f the two processes under consideration, that is, atmospheric venting and photosynthesis, evidence for the latter can be seen i n the data (Figure 3, 344/12, 344/22, 345/07, etc.) where p H maxima coincide with chlorophyll a maxima. Largely, for the Gulf area studied, no correlation exists between chlorophyll a and pH(T) i n Peruvian, and Baja and central Californian upwelling areas as observed previously (see chapter 21) (7,8). In the latter cases, much higher concentrations of chlorophyll a were measured; thus, recent in situ growth could be correlated directly to p H . A t the lower chlorophyll a levels found during this study, however, p H may more clearly reflect cumulative past growth rather than i n situ standing stock. Also, variations i n the fluorescence yield with physiological state of the organism (16), as well as variations i n the carbon-to-chlorophyll ratio (17), make the correlation between plant growth and p H less obvious at low chlorophyll concentrations. The overall increase of pH(T) with temperature can also not be attributed to loss of G 0 to the atmosphere. Surface salinities collected on the 3

2



20.

pH-Temperature

Relationships

405

cruise track indicate that the salinity did not vary greatly from 35.3 ppt. Similarly, the specific alkalinity of Gulf surface waters did not significantly depart from 0.120 (18). By using these data and an atmospheric C 0 value of 330 ppm, equilibrium p H values can be calculated for a portion of the survey area (19). These values (Table I) indicate that, under equilibrium conditions, the surface p H value of the Gulf should not vary significantly from 8.23, which is a value close to the low p H values measured at the beginning of the survey. Thus, even within the uncertainties associated with absolute p H measurements, the stepwise increase of p H is probably not due to atmospheric exchange. Finally, the rate of C 0 uptake associated with planktonic production was measured i n Stuart Channel, British Columbia (20). In this study, it was observed that C 0 was removed from the water at a rate approximately 20 times greater than the rate of C 0 invasion from the atmosphere. W h y then does pH(T) vary directly with T? Although we can only theorize at present, we can do so within a self-consistent model, the elements of which all appear i n the Gulf data. In light-poor, subsurface waters, low p H is associated with low temperatures because respiration processes dominate and C 0 is accumulated i n the water along with micronutrients, i n accordance with the Redfield ratio (21-23). During upwelling, this water is brought near the surface where it is mixed with surface water by local winds. L o w p H and the high positive correlation between pH(T) and T (Figure 4) reflect the mixing. Positive correlation is maintained as photosynthetic processes become dominant because removal of C 0 is accompanied by a warming of the water (Figure 7). Because the exchange of C 0 across the air-sea interface is very slow (24), increasing p H w i l l accompany increasing temperature. This correlation w i l l continue as long as the water stays at the surface until either convergence occurs or a


2

2

2

2

2

2

2

Table I. Equilibrium pH Computed from Temperature and Salinity a' ( x 10 )

pH

1.943

3.187

8.23

1.953

3.169

8.23

6.917

1.836

3.414

8.22

1.009

7.260

1.919

3.226

8.22

0.992

7.156

1.894

3.281

8.22

0.985

7.009

1.860

3.357

8.22

35.21

1.042

7.615

2.005

3.041

8.23

22.4

35.23

1.044

7.636

2.010

3.032

8.23

344/2247

21.1

35.15

1.023

7.403

1.955

3.141

8.23

345/0015

21.8

35.23

1.034

7.536

1.986

3.081

8.23

Time (day)

T ("C)

S(ppt)

343/2300

20.5

35.49

1.018

7.372

344/0015

20.7

35.53

1.021

7.414

344/0500

18.1

35.21

0.976

344/0630

20.1

35.26

344/1230

19.5

35.24

344/1400

18.7

35.17

344/1715

22.3

344/1805

N O T E : T h e „ c o = 3.3 x SOURCE: Reference 1. 2

10

(xlO ) 6

( x 10 ) l()

KB (Xl0' ) ]

2

~ atm and specific iilkalinity = 0.120. 4


406


w i n d event (storm) l o w e r s the p H b y m i x i n g i n m o r e subsurface T h u s , the coefficient dpH(T)/dT

water.

m a y be some measure of the net f i x a t i o n

of c a r b o n o c c u r r i n g i n the w a t e r as it resides at the surface. D e s p i t e these a r g u m e n t s , s h a r p , positive increases i n p H associated w i t h l o w e r temperatures such as those observed here a n d i n the P e r u v i a n u p w e l l i n g zone (8) are d i f f i c u l t to e x p l a i n because u p w e l l e d waters r i c h i n nutrients for g r o w t h are also r i c h i n C 0 . If the R e d f i e l d r a t i o is to be m a i n 2


t a i n e d , then a n e a r - e q u i l i b r i u m p H must also be m a i n t a i n e d . B r e w e r has used this a r g u m e n t to estimate past C 0

2

(25)

p a r t i a l pressures i n the atmos-

phere. T h u s , a n a d d i t i o n a l m e c h a n i s m f o r C 0

2

r e m o v a l is r e q u i r e d , a n d

the r a p i d r e c y c l i n g of excreted n i t r o g e n is suggested.

F i g u r e 8 shows a

3 5 - k m p a t c h f r o m the southern e n d of the G u l f c h a r a c t e r i z e d b y a p H m a x i mum

i n association w i t h a t e m p e r a t u r e m i n i m u m . T h i s negative c o r r e l a -

t i o n is characteristic of several other " g r o w t h events" a l r e a d y m e n t i o n e d a n d suggests that C 0

2

has been r e m o v e d i n excess of the C 0 - n u t r i e n t r a t i o 2

of the i n t r u d i n g c o l d w a t e r . S u c h a d e v i a t i o n f r o m the R e d f i e l d r a t i o c o u l d be

caused

by

simultaneous

zooplankters conversion

of

excreting organic

N H

3

matter

into into

n i t r o g e n r e c y c l i n g is evident i n surface waters (26).

the

water

C0 . 2

without

Indeed,

a

much

T h u s the " h o l e " i n the

c h l o r o p h y l l a p r o f i l e suggests g r a z i n g b y z o o p l a n k t o n . M a n y of the c o n cepts just discussed w e r e used i n a m u c h m o r e q u a n t i t a t i v e m a n n e r to explain 0 (27).

2

s u p e r s a t u r a t i o n i n surface waters off the northwest A f r i c a n coast

I n this area a distinct N H

3

subsurface l a y e r w a s f o r m e d w h e n u p w e l l -

i n g processes w e r e w e a k .

Conclusion B y u s i n g s i m p l e e q u i p m e n t , u n d e r w a y measurements of p H s i g n i f i c a n t to better t h a n 0.01 p H units c a n be m a d e . I n a d d i t i o n , the p H - t e m p e r a t u r e relationships o b t a i n e d f r o m u n d e r w a y measurements y i e l d d a t a that c a n be used to interpret o c e a n o g r a p h i c processes. I n the G u l f of C a l i f o r n i a , the p H of freshly u p w e l l e d waters shows a s i g n i f i c a n t positive c o r r e l a t i o n w i t h the i n situ t e m p e r a t u r e o n scales f r o m 3 k m to h u n d r e d s of k i l o m e t e r s . O l d e r surface w a t e r s m a y s h o w a negative c o r r e l a t i o n o n scales of a b o u t 20 k m . A negative c o r r e l a t i o n is u s u a l l y associated w i t h c h l o r o p h y l l m a x i m a that resemble the p H m a x i m a . A l s o , the p H at the i n situ t e m p e r a t u r e i n creases w i t h t e m p e r a t u r e b e y o n d that w h i c h is a c c o u n t a b l e to p h y s i c o c h e m i c a l processes. T h e positive p H - t e m p e r a t u r e c o r r e l a t i o n is p r o b a b l y due to the c u m u l a t i v e b i o l o g i c a l d e p l e t i o n of C 0

f r o m surface waters. C o n f i r m a t i o n of

2

this hypothesis a w a i t s the d e v e l o p m e n t of u n d e r w a y techniques f o r determ i n i n g a l k a l i n i t y (28) a n d t o t a l C 0 . 2



pi I~ Temperature

Relationships

407


20.

Figure 8. pH(T) maximum associated with temperature and chlorophyll a minima. The "hole" in chlorophyll a suggests grazing (December 22, 2982).


408


Acknowledgment W e are grateful to T . T . P a c k a r d a n d to o u r m a n y colleagues at the Scripps I n s t i t u t i o n of O c e a n o g r a p h y f o r their v a l u a b l e suggestions. W e also w i s h to t h a n k S a u l A l v a r e z - B o r r e g o , G i l b e r t o G a x i o l a , a n d the students at the G e n t r o de Investigaciones C i e n t i f i c a s y E s c u e l a S u p e r i o r de E n s e n a d a f o r m a k i n g Varifront

III

possible. T h i s w o r k w a s f u n d e d u n d e r the N O S C


IR/IED program.

Literature

Cited

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