Marine Chemistry, 2 (1974) 287--299 ~ Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

M E R C U R Y IN THE SOUTH P O L A R SEAS AND IN TI lE N O R T H E A S T

PACIFIC O C E A N

P. M. WILLIAMS 1 , K. J. ROBERTSON 1 , K. CHEW 2 and H. V. WEISS 3

I Institute o f Marine Resources, University o f California, San Diego, La Jolla, Calif. (U.S.A.) 2 San Diego State University, San Diego, Calif. (U.S.A.) 3 Naval Undersea Center, San Diego, Calif. (U.S.A.)

(Received June 6, 1974; accepted September 4, 1974)

ABSTRACT

Williams, P. M., Robertson, K. J., Chew, K. and Weiss, H. V., 1974. Mercury in the South Polar Seas and in the northeast Pacific Ocean. Mar. Chem., 2: 287- 299.

Seventy-nine total-mercury analyses of seawater samples, collected from the surface down to 5,700 m depth in the northeastern Pacific Ocean and in the South Polar seas, showed a homogeneous distribution of mercury with depth at all stations, although its absolute concentration in the northeastern Pacific (12--30 ng/kg) was 3 to 5 times less than that in the polar waters (50--150 ng/kg). The high concentrations are ascribed to an input of mercury resulting from submarine volcanism.

The mercury content was also determined in 8 surface-film samples, 3 sediment cores (0--30 cm), 2 pack-ice samples and I sample each of snow and sea smoke.

INTRODUCTION

There is a general lack o f data for the dis t r ibut ion of tota l mercu ry in the world oceans, in bo th the vertical and hor izonta l dimensions. This circum- stance is part icularly true for the Pacific Ocean and the South Polar seas. The available published data indicate tha t the mercu ry con t en t o f seawater is not at all un i fo rm in the oceans and may vary by a fac tor of 10, excluding obviously pol luted coastal env i ronments and o ther embaymen t s .

In the Atlant ic Ocean and cont iguous seas, surface concen t ra t ions of mercury are: 10--21 ng/kg in the English Channel and of f S o u t h a m p t o n (Bur ton and Leather land, 1971) ; 13- -18 ng/kg in the nor theas te rn Atlant ic (Leather land et al., 1971); 10- -50 ng/kg in unpo l lu ted areas of the Irish Sea (Gardner and Riley, 1973) ; 10--55 ng/kg in the Nor th Sea (Topping and Pirie, 1972) ; 64 - -140 ng/kg in the t ropical nor thwes te rn Atlant ic (Rober t son , 1970); and ca. 60 - -270 ng/kg (mean: 150) in the nor theas te rn Atlant ic (Halifax to Bermuda, Fitzgerald et al., 1974). A summary of the total- mercury con t en t o f surface waters o f the Atlant ic and Indian Oceans

2b~

(Chester et al., 19731 states a mean value of 47 ng tlg/1 for 28 samples analyzed. The mercury content reported for Atlantic deep waters are: < 3--18 ng/kg at depths of 1000--4665 m in the northeastern Atlantic (Leatherland et al., 1971 i, and ca. 50--300 ng/kg (mean: 1501 at depths of 100--4,700 m for the Halifax--Bermuda transect (Fitzgerald et al., 1971 i. Recent results from the Oeosecs (Geochemical Ocean Sections Study l show high concentrat ions of mercury in the deep and bot tom waters off the north and south Atlantic, notably in the vicinity of Iceland and South Georgia (D. E. Robertson, pers. comm., 19741. Carr et al. (1972) report 16 364 ng/kg total mercury at depths of 0--3,169 m in the Greenland Sea, where high surface values were associated with sea ice and there was no variation with depth.

In the Pacific Ocean, 60--270 ng/kg of total mercury has been reported for the northwestern Pacific, where the highest concentrations occurred at depth; for example in the Ramapo Deep (Hosohara, 1961). The vertical distri- bution of mercury at two locations in the Eastern Tropical Pacific was 22--173 ng/kg at a station 60 km off Mexico (0--4,500 m) and 12--27 ng/kg, 150 km from the coast (0--3,500 m) (Weiss et al., 1972). Both of these stations are in the vicinity of the Middle American Trench, and the high values may be associated with volcanism in this area. The mercury content of seawater from the surface to 4080 m at a station in the northeast Pacific (430 km southeast of San Diego, see Fig.l) was 45--55 ng/kg from 210--4,080 m and 270 and 96 ng/kg at depths of 10 and 100 m, respectively (Weiss and Williams, 1973). In the Beaufort Sea--Alaska North Slope area (Weiss et al., 1974), the mercury distribution in the shelf waters (0--400 m) was 5--90 ng/kg (mean: 20 ng/kg); these values are substantially lower than those reported by Carr et al. (1972) for the Greenland Sea.

METHODS

Sample collectiol~s

Tables I and lI and Figs.1 and 2 show station locations, bo t tom depths, and the sampling dates for the cruises Cato I to the northeast Pacific area and Eltanin 51 to the South Polar seas.

TABLE

Station

Station

r • •

Cato I-2 Cato I-4 Cato I-8 Cato 1-13 Cato I-A

1

locations, bottom depths and sampling dates; R/V 'Melville', cruise Cato I

number and location Bottom depth Sampling date (m)

32°28.2'N 123°32.0'W 2,196 9--VI--72 31°51.9'N 127°25.0'W 3,660 10--VI- 72 30°34.5'N 136°15.3'W 4,264 12--VI- 72 28°55.0'N 147°08.0'W 4,659 15--VI--72 30°38.9'N 155°22.5'W 5,700 25 VI--72

289

Seawater samples were collected with 30-1 Niskin bottles. The water was drawn immediately into 1-1 polyethylene containers previously leached with 50% nitric acid, and the samples were frozen at --20°C within 20 min of collection. Considerable a t tent ion has recently been paid to the storage of natural waters with respect to the adsorption of mercury by polyethylene (Carr and Wilkniss, 1973; Coyne and Collins, 1972; Feldman, 1974; Fitzgerald

TABLE II

Station locations, bottom depths and sampling dates; U.S.N.S. 'Eltanin', cruise 51

Station number and location Bottom depth Sampling date (m)

3 54°07.28'S 172°54.58'E 4,673 22--I --72 4 59°37.12tS 171°13.47'E 5,082 25--I --72 6 66°17.28'S 166°30.09'E 3,090 31--I --72 7 71°37.36'S 172°36.63'E 1,765 4--II--72 8 71°50.19'S 179°31.07'W 2,023 6--II--72 9 73°50.28'S 174°17.22'W 2,560 7--II--72

11 76°00.93'S 169°59.18'W 816 12--II--72 12 75°51.35'S 160°45.04'W 3,065 13--II--72 15 78°28.88'S 164°18.97'W 596 18--II--72 16 78°17.72'S 173°00.95'W 455 20--II--72

et al., 1974). These authors report a substantial loss (50% or more) of mercury from seawater solutions on standing, and recommend that the samples be stored with 5% nitric acid plus 0.5% chromic acid to prevent these losses. We have established that the loss by adsorption in this work is negligible, as was determined by 2°3Hg tracer studies in the laboratory.

The surface-film samples were collected from a dory, 0.5--1 mile from the ship, using a stainless-steel screen (Garrett, 1965). In this technique, the film, nominally the upper 150 t~m of the sea surface, is allowed to slide off the screen into 2.5-1 acid-washed bottles and immediately frozen.

The pack-ice samples were obtained with grappling hooks employed over the side of the ship to retrieve large chunks of ice (which in most cases showed plankton growth on the underside of the ice, as evidenced by brown coloration). The surface layers (1--5 cm) of the ice chunks were shaved off with stainless- steel knives and cleaned by washing with surface seawater. The ice chunks were then quickly melted, and refrozen at --20 ° C.

In the case of the sea-smoke, the finely divided ice crystals were deposited on the ship's superstructure, thus allowing collection of uncontaminated samples. These were also quickly melted and refrozen.

The sediments were sampled with a 2-m gravity corer and 1-cm vertical sections were taken at each desired depth. Sediment scrapings off the outside of each section were discarded and the samples frozen at --20°C.

290

Mercury analysis

The mercury analyses were pe r fo rmed by a modi f ica t ion of the neut, ron- act ivat ion p rocedure previously descr ibed {Weiss and Crozier, 19721. The changes were i n t roduced to provide for radiochemical pur i ty at an earli~,r t ime af ter the irradiat ion; accordingly, the sensitivity was appreciabl5 enhanced.

2 0 ° "" % .

i

1 6 0 ° 1 4 0 ° i l 2 0 °

Fig. 1. Station positions, R/V 'Melville', cruise Cato I. x denotes a station at 29°00.0'N, 122°31.2'W referred to in the text.

Mercury was isolated f rom seawater, sea-smoke, surface-fi lm, and pack-ice samples by coprec ip i t a t ion with copper . One mg of coppe r carrier (99 .999% pur i ty ) was added to a weighed quan t i t y of acidified seawater; coppe r was prec ip i ta ted as the sulfide and mercury coprec ip i ta ted quant i ta t ive ly . The prec ip i ta ted sulfides were dissolved in nitric acid and irradiated as descr ibed below.

In the sediment analysis, app rox ima te ly 2--3 g of f rozen sed iment was t ransfer red to i r radiat ion vials, weighed and 3 ml of concen t r a t ed nitr ic acid added pr ior to i rradiat ion. A sect ion of f rozen sed iment tha t was in juxta- posi t ion to the sample was ch ipped o f f and its wate r c o n t e n t de t e rmined by drying at l l O ° C for one hour .

Cornparator and nitric-acid blanks 291

The comparator consisted of 10 #g of mercury as the nitrate in 10 ml of concentra ted nitric acid. The nitric-acid blank was comprised of three irradiation vials each filled with 14 ml of concentrated nitric acid.

Irradiation

Samples, comparators and blanks were irradiated for 1 h at a flux of 1 .8 .10 '5 neutrons cm - ' sec - ' in a 'Lazy Susan' rotated at 1 rpm about the core of the MARK I TRIGA reactor at Gulf Energy and Environmental Systems Services, San Diego, California. The irradiations were made at 1,500--1,600 h, and the following morning samples were processed to attain radiochemical purity.

4

/

i 6°°w " I I 60°E

Fig.2. Station positions, U.S.N.S. 'Eltanin' , cruise 51.

Radiochemical purification

Sediments were digested in a nitric- and sulfuric-acid mixture as described previously (Williams and Weiss, 1973). The sediment was prepared further for

292

radiochemical purification by addition of 25 ml of concentrated ammonium hydroxide to the digest, and Lhe mixture was filtered. If, at this stage, pebble,s were detected in the residual sediment, they were removed and the sample was corrected for their weight.

Ten mg of mercury carrier was added to the water samples and tile nitric- acid blanks. The nitric-acid blank was reduced in volume to about 10 ml. Water samples and blanks both received 9 ml of concentrated ammonium hydroxide and 10 mg each of potassium and sodium chloride (these quantities of chlorides were also added prior to the succeeding precipitations).

From this point, with one exception noted later, all samples were treated similarly. To the filtrate was added 2.5 ml of freshly prepared stannous chloride. The precipitated mercury metal was collected by centrifugation. The precipitate was dissolved in 5 ml aqua regia; 5 mg of copper (as nitrate) was added and the solution filtered. The reduct ion of mercury to the metal was repeated and the solid collected by filtration. The precipitate was again dissolved with 5 ml aqua regia ( 1 or 2 drops of concentrated phosphoric acid were added to the water samples), and the solution was neutralized with 5 ml of concentrated ammonium hydroxide. Hydrogen-sulfide gas was passed through the pit-adjusted solution and the precipitate was collected by filtra- tion. The mercuric sulfide was of sufficient puri ty to permit immediate measurement. The comparator was neutralized with 9 ml of concentrated ammonium hydroxide after the addition of mercury carrier, and the mercuric sulfide was precipitated and collected for measurements.

Carrier yield determination

The processed mercury samples as well as mercury carrier standards (10 mg mercury) were re-irradiated for 5 sec. Through comparison of the activity level of the samples and standards, the carrier yield was computed and the counting rate in the original irradiation was corrected for this factor.

Measurement

The radioactive measurements were made with a sodium-iodide detec tor coupled to a 400-channel pulse-height analyzer. The counts at tr ibutable to the 77-keV radiation of 197Hg were integrated by the method of Covell (1959).

RESULTS AND DISCUSSION

The total mercury content of seawater samples and surface films collected on cruise Cato I is shown in Table III. In the waters studied from San Diego to Honolulu, the mean concentrat ion of mercury from the surface to 500 m, excluding three anomalously high values, was 24 ng/kg, as compared to a mean of 23 ng/kg in the entire water column at Station A (5,545 m depth).

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The d is t r ibu t ion of m e rcu ry in the vertical and hor izonta l dimensit~)s al(mg this 2 ,500-mile t ransec t was relat ively un i fo rm.

In Table IV. the vertical d i s t r ibu t ion of to ta l me rcu ry is given for ~h~ t ransec t f r o m New Zealand d o w n into the l~.oss Sea. Clearly the mer,-,iry concen t r a t i ons at all these s ta t ions are s ignif icant ly higher than those found in the nor theas t Pacific. Again, the vertical d i s t r ibu t ion of m e r c u r y wii:hin each water co lumn is relat ively cons tan t at all s tat ions. The m o s t s t r iking observa t ion is the a b r u p t change in the m e r c u r y c o n t e n t in the wa te r co lumn be tween Stat ions 3 and 4 ('Fable IV, Fig .2h The mean value of the m e r c u r y content~ increases a p p r o x i m a t e l y 50~:~ be tween Sta t ions 3 and 4, and the high values at S ta t ion 4 are again ref lec ted as one moves in to higher la t i tudes at

T A B L E IV

Total mercury con t en t of seawater s a m p l e s c o l l e c t e d o n cru ise Eltanin 51 (all values ng/kg)

S t a t i o n 3 Stat ion 4 Sta t ion 6

depth Hg depth Hg depth Hg (in} (m) (m)

0 53 0 107 6 105 7 81 13 90 1~ 90

13 59 22 142 ,15 132 20 75 50 105 60 103 27 71 100 101 172 150 43 57 125 116 216 86 85 75 180 104 492 79

125 80 255 128 920 103 175 70 505 122 1,875 95 250 70 1,005 87 2,5:50 _ 8:• 475 58 2,967 86 mean 103 960 50 3,96,1 105

1,897 86 4,892 . . . . . 90 2,905 74 mean 106 3,9oo 6A m e a n 68

Stat ion 7 Sta t ion 12

dep th Pig depth tig (m) (m)

0 102 0 112 65 87 242 t 24 24 ioo ~2j t@6)

249 76 mean 207 ,188 92 (118) 995 101

1,695 92 mean 93

Station 6 and 7. During this cruise, the South Polar Front (the Antarctic Convergence) was located at approximately 61 ° south latitude. This was evident from the intrusion and northward flow of the cold Ross Sea surface water. There were no significant changes in the TS diagrams between Stations 3 and 4 that could possibly explain the large differences in the mercury content throughout the water column. The TS diagrams and the distribution of temperature and salinity conform to the general pattern for this area reported by Gordon (1969). The high values in the surface waters at Station 4, 6, 7, and 12 could be attributed to generally higher concentrations of mercury in the Antarctic Circumpolar Current and, possibly, to the

295

westerly influx of the Circumpolar Deep Water over the Ross Sea continental slope (Jacobs et al., 1970). However, this does not explain the elevated concentrat ions in the deeper water (1,000--4,000 m). The mean mercury content in the water column at Station 3 {68 ng/kg) is intermediate between stations in the northeastern Pacific (see above) and at stations fur ther to the south. The mean values (20 ng/kg) in the northeastern Pacific are lower by a factor of 3.5 than those found at Station 3, and 5 times lower than at Stations 4, 6, 7, and 12. The consistent homogenei ty in the vertical distribution of mercury at all stations, both in the north and the south Pacific, is remarkable in face of the wide variations in concentrat ions that are found from one location to another. All of the samples f rom each of the two cruises were collected by identical techniques, and the irradiation and analyses were performed at the same time and wi thout variation; thus the marked differ- ences are not due to experimental error.

TABLE V

Total mercury content of surface films, pack-ice, and snow (all values ng/kg)

Station Sample Hg

3 surface film 90 7 surface film 90 8 snow 26 9 pack-ice 25

12 pack-ice 19 15 surface film 82 16 sea-smoke 165

The possible origin of the high mercury content in the South Polar waters may be the active volcanism which exists in this whole general region. Robertson observed high values in North Atlantic deep water in the vicinity of Iceland (D. E. Robertson, pers. comm., 1974), and he at tr ibuted this to the input of mercury resulting from the emergence of Surtsey Island in 1964. High values of atmospheric-mercury contents have also been observed in this area (Siegel et al., 1973), which, according to Aston et al. (1972), may explain the elevated mercury content of North Atlantic deep-sea sediments near Iceland. In the Antarctic region, Duce et al. (1973) have observed high concentrat ions of mercury in atmospheric aerosols as compared to its abundance in average crustal material, although the absolute concentrat ion of mercury in these aerosols was less than that observed in the nor thern hemisphere. It is inter- esting to note tha t the highest concentrat ions of mercury in birds and mammals have been observed in Weddell seal and penguin livers.

In an earlier report , Williams and Weiss (1973) suggested that the higher mercury content in the upper 100 m of the ocean might be at tr ibutable to

2~36

the in t roduc t ion of mercury through a tmospher ic fal lout, ttigh mercury concen t ra t ions in the euphor ic -zone waters were not found in any work repor ted here, nor by Fitzgerald et al. (197.11 for the Atlantic, henc~ * tile original suggestion appears invalid.

The mercury con t en t of the surface fihns col lected on cruise Cato 1 tTable III) was 16--93 ng/kg and two of these values (60 and 93 ng/kg) were four t imes higher than the cor responding mercury co n t en t of the 1 0-m water. The surface fihns giving these high mercury con ten t s were col lected in an area of relatively high p roduc t iv i ty as evidenced f rom ch lorophyl l r~ observations. Analyses of the surface fihns themselves showed high values of total organic carbon and inorganic nut r ients I P()4, NO3, SiO4 ), aild visually the films were well-developed as compared to those at the o ther three sampling sites. Duce et al. (1972) have observed high concen t ra t ions of o ther heavy metals (Pb, Fe, Cu, Nil in surface films in relat ion to th~ ~ under ly ing water.

The mercury con t en t of the surface films col lected on cruise Eltanin 51 (Table V I was not significantly greater than tha t in the subsurface waters. In fact, the fihns present at all of these stat ions exhibi ted ex t r em e ly low surface tension as evidenced f rom their behavior on the screens during col lec t ion and it cannot be said tha t they represent a cohe ren t surface film as has been observed in previous work (Williams, 1969) at o the r locations in the Pacific. f towever , the mercury con t en t of the pack ice and snow was substant ia l ly lower (Table V) than tha t found in bo th the surface fihns and the subsurface water. This may indicate that : (1) in the case of the pack ice, m e rcu ry is preferent ia l ly concen t r a t ed in the brine during the freezing process, or (2} tha t subsequent to the freezing of the pack ice, snow is depos i ted on the ice floes during the previous year and hence the mercu ry co n t en t o f the pack-ice samples reflects the a tmospher ic input. For example , high t r i t ium conten t s have been found in these same pack-ice samples (Michel and Williams, 1973 i.

The mercury con t en t of the snow sample (26 ng/kg) is similar to tha t found in snows col lected near Barrow, Alaska (Weiss et al., 1974) , though perhaps fo r tu i tous ly . It is, however , three t imes lower than tha t of one ice sample col lec ted at New Byrd Sta t ion in the Antarc t ic (Weiss et al., 1972) or at a s ta t ion in Greenland, where the average concen t ra t ions of m e rcu ry in prec ip i ta t ion depos i ted before 1952 averaged 60 ng/kg (Weiss et al., 1972). Sea smoke, a solidified marine vapor, contains a quan t i ty of mercu ry character is t ic of Sou th Polar Sea waters adjacent to the Ross Shelf, the environs in which the sample was collected. Thus, f rac t iona t ion of mercu ry upon vapor iza t ion of the sea water is no t suggested.

The to ta l -mercury con t en t of sed imenta ry material col lec ted at three stat ions (Table VI) is anomalous ly low, consider ing the high concen t r a t i on of mercury in the overlying waters. These values are an order of magni tude lower than those which have been r epo r t ed for deep-sea sediments in the Nor th Atlant ic (Aston et al., 1972) ; or on the East Pacific Rise (Bos t rgm

297

anti Fisher, 1969). The sediments in the Ross Sea area (Stations 9 and 15) have been termed 'iceberg sediments' by Lisitzin (1972), and are largely of terrigenous origin. They contain less than 1% calcium carbonate and very few diatom tests, the siliceous components being sponge spicules and radiolarian tests; they also contain up to 90% kerogen and shales, and appear to have an extremely high sedimentation rate (W. M. Sackett, pers. comm., 1974). The northernmost sediment sample, Station 6, is 5% calcium carbon- ate with few siliceous remains.

T A B L E VI

Total mercu ry c o n t e n t of sed imen t s ; cruise El tan in 5] .

S ta t ion Cen t ime te r s Hg below the surface (ag/g dry weight )

6 0 21 11 5 18 8

30 13 6

9 0 9 7 2.5 13 8 5 8 5

20 9 5

15 0 34 15 5 22 11

10 23 12 3O 18 10

Hg (ng/g wet weight )

There is a slight decrease in mercury content with depth in the sedimentary column at Stations 6 and 15. Conceivably, this decrease could result from upward migration and re-mobilization of mercury from the seawater-sediment interface into the water column. It is difficult to explain the low concentra- tions of mercury in the sediments as being due to analytical techniques, for the analyses in this case were identical to those of Weiss et al. (1974) for sediments collected in the Beaufort Sea and in the northeastern Pacific having higher mercury content (H. V. Weiss, pers. comm., 1974).

It is evident that the concentration of mercury in seawater, sediments and precipitation exhibits large disparities in its geographical distribution. We do not believe that these differences are due to sampling and analytical dis- crepancies, but represent the natural circumstance. We feel that the upcoming results from Geosecs expeditions and future cruises of the Eltanin may provide clues regarding the reasons for these differences.

A C K N O W L E D G E M E N T S

We extend thanks to the members of the Alpine Geophysical Scientific Support Party, and to R. Cuhel, for invaluable cooperation in the Antarctic

29S

operations. This work was supported by Grant GV-2722 from the Office of Polar Programs, National Science Foundation, and by the U.S. Atomi,. Energy Commission. Contract No. A T ( l l l )GEN 10, P.A. 20.

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