The loss of dissolved oxygen in Nansen bottle samples from the deep Atlantic Ocean

Deep-SeaResearch.Vol.29, No. IOA.Pp. 1259to 1266. 1982. PrintedinGreat Britain.

0198-0149/82/101259-08 S03.00/0 ~ 1982PergamonPressLtd.

NOTE The loss of dissolved oxygen in Nansen bottle samples from the deep Atlantic Ocean* L. V. WORTHINGTON t

(Receiped24 January 1982; accepted 2 April 1982) Abstract--In the tropical and subtropical Atlantic Ocean loss of dissolved oxygen nearly always takes place within Nansen bottles between the time they dose, in deep water, and the time the samples are drawn for analysis.The principalcause of this oxygenloss,in routinestations, is shown to be dqassing; deep samples, rich in oxygen,are heated to above their 8aturatloa imint as they are brought tip through the themtodi~ and surface lay~$. Niskin bo(xle~which are better insulated, are immune from the fault. Examplesof oxygen loss due to chemical~ within the Nansen bottle and loss due, possibly,to bacterialrespiration,are also described.

ON 3 October 1958, while W. O. METCALF(1960) was making a transatlantic hydrographic section at 240S, the winch broke down on CrawfordSta. 420 (24 ° 15'S, 38022'W) when the deep east was down. The Nansen bottles were retrieved by manpower; the ship's wheel was removed from the steering column and attached to the gypsyhead on the winch. This was a slow process. The crew started hauling at 1700 (+3 time); the first bottle (true depth 755 m, wire out 1036 m) was recovered at 2300 h at an average speed of 2.9 m rain-l. The last bottle (true depth 2415 m, wire out 3036 m) was recovered at 0517 on 4 October at an average speed o f 4.5 m min -l. Dissolved oxygen-depth plots for CrawfordSta. 420 and its two nearest neighbors are plotted in Fig. 1. It was clear that samples trapped in Nansen bottles for long periods lost large quantities of oxygen and, by inference, that the oxygen loss in routine deep stations where samples were trapped for about I h could also be unacceptable. As a result of this station and through the initiative o f V. T. Bowen the Woods Hole Oceanographic Institution had all its Nansen bottles coated with Teflon on the inside to avoid contamination of the samples by contact with the brass wall. In June 1959 Crawfordvisited Bermuda shortly before making a m c r ~ o n a l section at 57 ° 30'W from 31 °N to the Canadian continental shelf. D. W. Mc:tzel asked me to take some of the brand new, uncoated Nansen bottles belonging to the Bermuda Biological Station back to Woods Hole to be coated with Teflon. I agreed and obtained his permission to use two o f the bottles in the deep water on the meridional section to compan: their performance with that of coated bottles. For comparison, the uncoated bottles were interpolated into the deep cast in * Contribution No. 5053 from the Woods Hole Oceanographic Institution. t 561 West Falmouth Highway, Faknouth, MA 02540, U.S.A. 1259

1260

L.v. WORTHINGTON 0

X

Y

G E N

(ml/l)

"4•

500

•

am4i9

.,

• ,~o

o~

~.421

-

em ;

-/

i

e A

,e

"i! mA

2

Fig. 1. Dissolved oxygen vs depth at CrawfordSt& 420, in which the Nahum bottles were closed up to 7 h, compared with normal, adjacent Stas 419 and 421.

the following order: W.H.O.I. bottle 15 coated, B.B.S. bottle I l uncoated, W.H.O.I. bottle 5 coated, B.B.S. bottle 20 uncoated, W.H.O.I. bottle 17 coated. The mean depth of bottle 11 was 3100 m (s.d. 250 In) and that of bottle 20, 3940 m (s.d, 311 m). The recruitso f the comparison are shown in Fig. 2. The diagram represents the difference (rot l-l) in the oxygen concentration of samples drawn from uncoated bottle 11 and the average from coated bottles 15 and 5 (circles) and the same for uncoated bottle 20 and the average of coated bottles 5 and 17 (triangles). If we except a single value of oxygen loss of 0.98 roll -~ at Sta. 571, the uncoated bottles appeared to lose oxygen at an increasing rate for the first three days of exposure (17 to 19 June) until the averageamount of oxygen lost was about 0.5 ml I-a. Therenfler, the amount of oxygen lost ~ steadily until the last day (24 June) when the difference betwce~ and uncoated bottles was almost zero. Station 585 ('Cold Wall') marks an abrupt change into much colder water (WogTmNOTON, 1964, Fig. 1). In July 1977, a similar hydrographic section was occupied in K n o t t from 33 to 42°N at 55°W (MCCAgrNEY, WOartm~TON and RAYwn, 1980). In the section stations u~_'n~a CTD with 22 Niskin bottles mounted in a rosette were interpolated between stations udng Tefloncoated Nansen bottles. Most of these were the same Nansen bottles that had been coated about 19 years previously and visual inspection showed that their c o ~ were extemively pitted. There were 6 deep CTD stations that were closely (,-,20 km) bracketed by 10 Nansen stations. It was immediately apparent that on these stations the dissolved oxygen concentrations in deep samples from Nansen bottles were generally lower than in deep samples drawn from Niskin bottles. The differences, with standard deviations, are listed in Table 1. The

1261

Loss of dissolved oxygen from the Atlantic Ocean

CRAWFORD 568 I 0~-

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570 ._1

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Difference in dissolved oxygen between Teflon coated and new uncoated Nansen bottles on a Crawford hydi'ographic section from 32 to 44ON at 57030'W.

samples are grouped by 0.5°C potential temperature intervals from 4.5 to 2.0°C and by 0.2°C intervals below 2.0°C. In the temperature ranges 3.5 to 4.0°C and 4.0 to 4.5°C, samples taken on the shallow cast have been separated from those taken in the deep cast; the former samples spent 30 to 40 rain after closing until the samples were drawn and the latter between 45 and 60 rain. No water colder than 3.5°C was sampled on the shallow cast. The oxygen difference was nearly negligible in the shallow cast but became increasingly significant with decreasing temperaturemand increasing depthmin the deep cast. Table 1. Comparison of dissolved oxygen valuesfrom N~kln and Nansen bottles in the deep North Atlantic near 370N, $SeW

0 (°C Range)

N

4.0-4.5 3.5-4.0

16 12

0 (°C Range)

N

4.0-4.5 3.5--4.0 3.0-3.5 2.5-3.0 2.0-2.5 1.8-2.0 ! .6-1.8

1 13 13 !5 22 27 17

Nansen (shallow cast) 0 2 ml I-' Mean s.d. 5.98 6.18

0.11 0.07

Nansen (deep cast) 02 ml !-I Mean s.d. 5.99 6.12 6.10 6.08 6.20 6.12 5.95

0.06 0.07 0.11 0.06 0.08 0.07

N 9 9

Y 9 9 7 9 10 17 lI

Niskin (shallow cast) 0 2 ml I-' Mean s.d. 6.02 6.18

0.10 0.05

Niskin (deep cast) 02 ml 1-1 Mean s.d. 6.02 6.18 6.21 6.25 6.32 6.30 6.12

0.10 0.05 0.06 0.07 0.07 0.09 0.06

Niskin-Nansea O: ml I-s 0.04 0.00 Niskin-Nansen 02 ml 1-t 0.03 0.06 0.11 0.17 0.12 0.18 0.17

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L.V. WORTHINGTON

Since the earfier Crawford section suggested that the lack of bottle coating may not have been the sole cause of the loss of oxygen in mature Nansen bottles, it seemed possible that the temperature of the samples might have been a contributing factor. The walls of the Niskin bottles used in the Knorr cruise are 9.5-ram thick polyvinylchloride, a poor conductor of heat. The walls of the Nansen bottles, on the other hand, are 1.65 mm thick and brass, an excellent conductor of heat. (The Teflon coating is only 0.1 mm thick.) If a sample is trapped in a Nansen bottle at a depth at which the oxygen concentration is high its temperature can subsequently be raised to above its oxygen saturation point as it is brought up through the thermocline and the warm surface layers. During November to December 1978, Wm~ltF.~D and WORTmNGTON(1982) were conducting an investigation into the flux of Antarctic Bottom Water into the North Atlantic. On the cruise (Oceanus cruise 52) Niskin bottles were attached 2 or 3 m below the top and the bottom Nansen bottles in the deep cast. Samples for Winkler titration were drawn from the Nansen and Niskin bottles into 125-ml Erlenmeyer flasks. Before sampling, a thermometer was inserted into each flask and it remained in the flask while the sample was drawn. It was read and removed while the sample water was still overflowing the flask. After the thermometer was removed the fu'st two reagents (manganous chloride and sodium iodidehydroxide) were added and the flasks were stoppered, shaken and stored to await the addition of sulphuric acid. 02, ml/I, Niskin-N0nsen --0.1

0.0

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Differcnc~ in d L s ~

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oxygen b~w'~n NU*kinmid Nm-lsenbottl~ as a function of'excess' oxygen (see t e x t ) .

Loss of dissolved oxygen from the Atlantic Ocean

1263

The Nansen and Niskin bottles at the top of the deep east closed at an average in situ temperature of 3.2°C and those at the bottom of the cast at between 2.0 and i.2°C depending on the extent to which cold, Antarctic Bottom Water was present. The sample-drawing temperature difference between the Nansen and Niskin bottle samples was large. The Niskin samples, when drawn, averaged 6.3°C with an extreme range of 5.3 to 8.1°C. The Nansen samples from the top of the cast averaged 15. I°C and those from the bottom of the cast 17.8°C; although initially colder, the latter spent more time in the thermocline and surface layers because the winch was stopped periodically to remove the Nansen bottles above them. None of the Niskin bottles was warmed to anywhere near the temperature at which it could become supersaturated with oxygen, but nearly all the Nansen bottle samples were warmed to above their saturation temperature and degassing should have taken place. Oxygen loss did, in fact, take place (Fig. 3). In Fig. 3 the ordinate scale is the amount of supersaturation of oxygen due to the sampledrawing temperature of the Nansen bottle samples; it is the difference between the oxygen concentration in the Niskin bottle samples and the oxygen solubility value of the Nansen bottle samples at the temperature at which they were drawn in the laboratory, according to the nomogram of Gp.Em¢and C^ggrrr (1967). It is assumed the Niskin bottle samples lost no oxygen between the time that they closed at depth and the time the samples were drawn. It is further assumed that the Niskin and Nansen bottles had the same temperature and the same concentration of dissolved oxygen at the time they closed, 2 or 3 m apart. The abscissa scale is the difference in oxygen concentration between the Niskin and Nansen bottle samples. A least squares regression line is drawn through the points; the standard deviation is +0.052 ml 1-t and the correlation coefficient is 0.44, which indicates a < 1% chance that the ordinate and abscissa are uncorrelated. DISCUSSION

The loss of oxygen in Nansen bottles can be the result of at least three causes: (1) chemical reaction of the oxygen in seawater with the wall of the bottle; (2) degassing, due to the raising of the temperature of the trapped samples to above their saturation temperature; and (3) bacterial respiration. It is thought that the data presented here contain examples of all three causes.

1. Chemical reaction The loss of oxygen from new, uncoated bottles in the Crawford section in 1959 (Fig. 2) can clearly be attributed to this cause. Samples trapped for only about I h on routine stations lost about 0.5 rnl 1-s. It is probable, though not proved, that uncoated Nansen bottles 'matured' by developing an oxide coating a/ter repeated exposure to seawater. If the Crawford hydrographic section had been run in the opposite direction (north to south) and had shown the same results the evidence for 'maturing' of Nansen bottles would have been more convincing. During the last three days of the comparison, the whole water column was extremely cold (WORTHINGTON, 1964) and any chemical reaction should have been greatly inhibited.

2. Degassing The oxygen losses from Nansen bottle samples in the 1977 Knott cruise (Table 1) and from the 1978 Oceanus cruise (Fig. 3) were, presumably, the result of degassing~ Most of the samples were warmed to well above their saturation temperatures. Sample temperatures were

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L.V. WoRTHINGTON

not taken on the Knorr cruise (Table 1), but the temperatures of all the samples were probably higher than those on the Oceanus cruise because the samples were taken from the center of the Gulf Stream system where the thermocline is deepest. The data shown in Fig. 3 from the Oceanus 1978 cruise were obtained over a wide range of latitude (0 to 28°N). Near the equator the thetmocline is steep and shallow and the Nansen bottle samples were not always warmed to above their saturation temperatures. The large scatter of the data points in Fig. 3 was alarming and it seemed desirable to determine the amount of natural scatter in the ocean. This was done by establishing the relationship between potential temperature and dissolved oxygen concentration in the Antarctic Bottom Water over a fairly short range of latitude (0 to 12°N) where no geographical differences in this relationship could be discerned in the Oceanus data. For comparison, GEOSECS data from the western Atlantic section over a similar latitude range were included. the 0 - O z relationships for the two data sets are plotted in Fig. 4. There is a distinct change in slope at I A ° C , and separate least squares regression lines have been calculated above and below 1.4°C for both data sets. The shaded envelopes represent standard deviations; _+0.022 ml 1-= for the GEOSECS data (59 samples) and +0.036 ml 1-~ for the Oceanus data (135 samples). The greater precision of the GEOSECS data is evidently due to editing;

Dissolved 5.2

5.4 ~

2.

Oxygen,

5.6 •

OCEANUS 1978 St. Dev. 0 . 0 3 6

1.8

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5.8 I

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1.6 "GEOSECS" KNORR 1972 St. Oev. 0.022 m l / I

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Fig. 4. Dissolved oxygen-potential temperature rclatinnship in Antarctic Bottom Water in the equatorial North Atlantic (0 to 12°N); Shaded envelopes represent standard deviation. Data from an Oceanus cruise in 1978 are compared with the GEOSECS western Atlantic section.

Loss of dissolvedoxygenfrom the AtlanticOcean

1265

unedited data from the original GEOSECS cruise reports gave a standard deviation of +0.038 mi !-t, virtually identical to my own, unedited result. In view of a natural scatter of +0.036 ml l-t the greater scatter in Fig. 3, +0.052 ml I-t, seems reasonable because new sources of error were introduced. Among these are that the Niskin and Nansen bottles did not sample identical parcels of water--they were attached to the wire 2 or 3 m apart to permit each messenger to gather sufficient momentum to trip the lower of the two bottles. Also, the Niskin bottles had no reversing thermometers; their closing temperatures were assumed to be the same as the Nansen closing temperatures. A slight difference in standing time (before adding reagents) must result from the necessity of drawing two samples instead of one. The GEOSECS samples (Fig. 4) were taken from Niskin bottles and their generally higher oxygen concentration (,~0.1 ml I -t) than the Oceanus samples is consistent with the Oceanus experiment (Fig. 4). The experiment suggests that it is unwise to use Nansen bottles for sampling oxygen in the deep tropical and subtropical Atlantic Ocean where the oxygen concentrations are high and the upper layers are warm. Oxygen loss by degassing may range from 0. l 0 ml l-l near the equator (Fig. 4) to 0.17 ml l- t in the subtropical gyre (Table l). This warning does not apply, for example, to the subtropical gyre of the North Pacific where the deep oxygen concentration is about 3.75 rail -I (WORTHINGTON and KAWAI, 1972). One would have to warm such water to about 37°C before degassing could take place.

3. Bacterial respiration The oxygen loss in Crawford Sta. 420 (Fig. 1) cannot be attributed to degassing due to warming of the samples. The surface layer temperature at the station was about 23.5°C (23.67°C at 0 m, 23.48°C at 40 m). In the salinity range sampled in the deep cast in question (34.36 to 34.95 u 10-3) the oxygen solubility at 23.5°C is about 4.85 ml 1-i (GREEU and CARRITT, 1967). In all but the deepest four bottles the oxygen loss took place at concentrations below this value (Fig. 1). Although chemical reaction cannot be ruled out in this case, it was thought that the large oxygen losses were the result of bacterial respiration because the Nansen bottles were old and oxide coatings must have been formed years previously. I tried to reproduce the conditions that might encourage bacterial growth and reproduction by repeatedly idling, letting stand and emptying an old, coated Nansen bottle with more or less polluted seawater from Eel Pond, in Woods Hole. At the end of a week of this (on 22 June 1981) the Nansen bottle was closed in the (relatively) clean water of Buzzards Bay and allowed to stand at a temperature of about 22°C for 7 h. For control, a sterile Nansen bottle was also filled from Buzzards Bay and allowed to stand for the same period. The ambient temperature and oxygen were taken from a third Nansen bottle from which the samples were drawn immediately after closing. The values were T = 19.7°C, O 2 = 5.40mil -t (salinity estimated to be 31.9 u 10-3). At the conclusion of the 7-h period (at T = 22°C) the oxygen concentrations were 5.17mil -I for the sterile bottle and 5.13 for the 'dirty' bottle. The oxygen solubility of seawater at 22°C and 32 x 10-3 is about 5.10 ml 1-1 according to GREEN and CAsm'rr (1967); it appears that degassing (due to warming) from the original concentrations of 5.40 ml 1-! took place but nothing else. The data further argue against chemical reaction as a cause of oxygen loss in routine stations where the samples are trapped for 1 h or less. On 22 July 1981 the experiment was repeated, but the sterile and 'dirty' Nansen bottles were allowed to stand for 24 h instead of 7 h. The samples were drawn from Vineyard Sound where the ambient values were T = 23.1°C, S = 31.96 x 10 -3, 0 2 -----6.23 ml 1-1. The oxygen value seemed a little high--it was 125% of saturation--but it was the average of 8 titrations (two

1266

L.V. WORTHINGTON

each from four samples d r a w n from the third N a n s e n bottle; s.d. + 0.03 ml l-l). After the 24-h standing period in the l a b o r a t o r y at a t e m p e r a t u r e o f 2 3 o C , the oxygen concentration in the sterile bottle h a d been reduced to 4.99 ml i-1, o r 100% o f saturation. The oxygen concentration in the ' d i r t y ' bottle was actually less r e d u c e d - - - t o 5.33 ml 1-1, o r 107% o f saturation. A t this point the experiments were a b a n d o n e d . The reduction o f dissolved oxygen concentration was not observed in either the sterile or the "dirty' Nansen bottle to below saturation value. There was no evidence o f bacterial growth, o r for that matter, o f chemical reaction in Nansen bottles over either a 7 or a 24-h period. The experiment did conf'wm the conviction that neither o f these is likely to cause oxygen loss in routine stations where the samples are trapped for 1 h or less.

Acknowledgemeats--My tlmaks to Dr. CINDY LEE for sterilizing the Nansen bottles used in the experiment described in the final pnrallt~hs. The re~at~h was supported by the Office of Naval ReNarch under Contract N00014-79-C-0071, NR 083-004, and by the National Science Foundation under Grant OCE77-07507. REFERENCES GREEN E.J. and D.E. CAgRITT (1967) New tables for oxyge~ saturation of seawater. Journal of Marine Research, 25, 146. MCCARTNEYM. S., L. V. WORTmNOTONandM. E. RAYMEK(1980) Anomalous water mass di~ributions at 55W in the North Atlantic in 1977.JournalqfMartneRaearch, 38, 147-172. Ms'rc.m~ W. O. (1~0) ~ data from ~ ~ mcdo~ at the equator and 24° South. Crawford Cruise 22 for the I n t e r n ~ ~ Year of 1957-58. W.H.O,L Rcf. No. 60.3, 152 pp. Warr~eAD J. A . JR. and L.V. W O l t ~ (1982) The flux and mixing rates of Antarctic Bottom Water within the North Atlatic. J m m m / q f ~ t k ~ i Reszarch, subaC~_. WORTHINGTONL. V. (1964) ~ S ~ in the Slope Water Area in 1959. Journal of the Fisheries Research Board ~'Canada, 21, 327-333. WORTHINGTON L.V. and H. KAWAI(1972) Comparison between deep sections across the Kuro~,io and the Florida Current and Gu~ SUumL In: Kurmhto, Its physical mpet~, H. STOMMELand K. YOSmDA,editors, University of Tokyo Press, pp. 371-385.