Contribution of atmospherically transported trace metals to South Pacific sediments

Gleochimica et Oomochimlca Acta,1970, Vol. 34, pp. 509 to 614. Per&mm Press. Printed fn NorthernIreland

NOTES

Contributionof atmospherically transportedtrace metals to South Pacific sediments HERBERT L. WINDOM Skid&wayInstitute of Oceanography, Savannah, Georgia (Received9 September1969; accepted i~ revised form 13 Ncverraber 1989) A~~~Aecumul~tion rates of the trace metals Mu, Cu, Ni, Cr, Co aud SF, calculated for Australian dust samples indioete that sediment in areas of the South Pacitlc a&&cd by dust falls from this source cau accumulate these metals at rates similar to sediment in areas of the Sout,hPacific where little d&&al material is received. INTRODUCTION

WHILE atmospheric transport has been recognized as a rne~h~~rn for transporting trace metals from the continents to the marine environment (TUREKIU, 1968), its ultimate effect on the distribution and deposition .of trace metals in marine sediments has received little attention. Because preliminary investigations have shown that atmospheric transport is an important mechanism in the accumulation of deep sea sediments (DELANEY et al., 1967; GRIFFIN et d., 1968; WINDOM, 1969; PROSPEBO and BONATTI, 1969), it is appa~nt that the effect of this mechanism on the distribution and accumulation of trace metals deserves ‘further attention. The present study which stems from previous work done on atmospheric transport by the author (WINDOM, 1969), is therefore an initial entry into the study of atmospheric transport of trace metals to marine sediments. Because there is an almost complete lack of rivers and streams emptying into the South Pacific from adjacent continents, sediment and chemical supply from rivers and streams can be considered negligible. The Antarctic continent which supplies sediment to the South Pacific from glacial runoff and ice rafting has its greatest effect on areas immediately surrounding that continent. From descriptions of the possible sources and transportation mechanisms operative in this area, it would appear that in situ de~adation of volcanic materials and inorganic and organic precipitation from sea water are the major contributors to the marine sediments of the South Pacific. Recently, however, several investigations have suggested that the Australian deserts may be important sources of sediment for the South Pacific (WINDOM, 1969; GRIFFIN et al., 1968; ARRHENIUS, 1966). On the basis of different lines of information, WINDOM (1969) proposed that this dust accounts for approximately 60 per cent of the sediment in the western South Pacific Ocean. This conclusion is substantiated by the work of GRIFFIN et al. (1968) and ARRHENIUS (1966). It is therefore reasoned that the trace metal composition of the dust when compared to that of the sediments in the South Pacifio should reveal much about the sources and accumulation of trace metals in South 50%

510

Notes

Pacific marine sediments. Because accumulation rates of dust are known ( WINDOM, 1969) accumulation rates for the different trace metals can be easily computed, so that their total supply to the area affected by the dust can be established. In this study six trace metals were considered along with iron, magnesium, and calcium. The trace metals are manganese, copper, nickel, cobalt, strontium and chromium, Manganese, copper, nickel, cobalt and chromium were considered because of their chemical similarities to iron, which is important in the dust samples. Iron oxide forms a coating on most of the mineral grains as well as being present as amorphous iron compounds in the finer fractions of the dust. It is presumably derived from the ancient lateritic soils of the Australian deserts. The elements were also selected because of the attention they have received from other investigators who have been concerned with volcanically derived material in South Pacific sediments (ARRHENIUS et al., 1964; BONATTIand JOENSUU,1966). Analyses of these elements were compared to those in marine sediments from the western South Pacific, an area presumably affected by the dust (WINDOM,1969; GRIFFIN et al., 1968) and an area of the northern South Pacific which has been shown to contain very little detrital material (GRIFFINet al., 1968). From these analyses the adequacy of the dust for supplying trace metals to South Pacific sediments could be assessed and compared to what might be supplied by marine vulcanism or transport from the Antarctic continent. METHODS Both dust and sedimentsamples were digestedusing hydrofluoricacid and were treated with perchloricacid to destroy organicmatter. After digestion, samples were brought to dryness and picked up in a minimum amount of nitric acid and brought to volume for atomic absorption spectrophotometricanalysis. For the sediment samples approximately O-5 g of material which had been dried overnight at 105°C was used for the analysis. Dust samples were limited to O-l-O.2 g. These were treated in the same way as the sediment samples but brought to smaller volumes so that concentrationsof the elementsin solutionwere sufficientlyhigh to permit assay. Elemental analyses were made on a Beckman Model 979 atomic absorption system. Due to the small amount of sample available in the dust the precisionof the analysescan only be estimated from the precisionof the analyses for the sediment samples. These would thereby be approximately 5 per cent of the stated value for trace elements (manganese,copper, nickel, cobalt, strontium and chromium) and approximately 3 per cent of the stated value of the major elements (oalcium, magnesium and iron). RESULTS The dust samples listed in Table 1 represent samples collected on both sides of the main divide of the Southern Alps of New Zealand. Samples were collected from the surface of the Franz Josef and the Tasman snowfields which are separated by approximately 15 km. This dust was also found to be ubiquitous to both North and South Islands of New Zealand (WINDOM, 1969). Samples taken at greater depths in the Tasman snowfield which represent dust falls which have occurred over many years, suggest that the origin of this surface dust is similar to the dust at greater depths in the snowfield leading to the conclusion that the dust falls which are recorded here represent a continual occurrence and an important source of material for marine sediments in the adjacent South Pacific.

511

Notes

Table 1 lists the concentration of the selected elements in the dust samples. Analyses were made on the different size fractions of the dust to get an idea of where or in what phases the particular metals were concentrated. Mineralogy of the different size fractions listed differs somewhat. As can be seen from this table, the majority of the dust is concentrated in sizes less than 15 ,u. Material greater than 15 p is composed primarily of quartz with minor amounts of albite plagioclase. The fine fraction (less than 2 ,usize fraction) consists of illite, chlorite and kaolinite in the proportions 50,23, and 27 per cent, respectively, and amounts of amorphous iron compounds which give no diffraction pattern and which account for the higher iron concentration in this fraction when compared to the other fractions. In the Table 1. Elemental composition of New Zealand dust samples Sample*

Size fraction

FJ-1

-w4

FJ-1 FJ-1

2-5 5-15

TAS-1

<2p

TAS-1 TAS-1

2-6 5-15

wt. %

sample*

PPm

Ca

Mg

Fe

Mn

Cu

Ni

co

cr

81

35 17 29

0.24 0+31 0.47

1.20 0.78 0.94

6.3 4.4 3.0

470 560 260

230 110 50

160 ND ND

ND ND ND

350 310 160

160 165 220

Average

0.40

1.01

4.7

410

140

160

-

280

180

26

0.21

1.69

% of total

14 25 Averene

Average dust oomposition-

6.8

380

300

99

46

ND

o-44 l-06 O-89 0.36 Ct.62 1.08

4.7 3*1 4-Q

400 310 380

210 80 190

77 48 73

?D 37

340 ND 340

ND ND 240 240

0.46

4.8

396

165

37

310

210

1.06

116

FJ-Franz Joaaf Snowfield. TAS-TasSnow&Id. ND-Not determined due to laok of sample. Averages for each location is based on the contribution from each eize fraction. In the oaae where analyaea were made on only one size fraotion, that value was taken aa the average.

l

fraction, the concentration of clay minerals has decreased with a concurrent increase in felspar and quartz. This continues to be true in the 5-15 ,u fraction where quartz and felspar predominate over clay minerals. In both these fractions some amorphous iron compounds or discrete iron oxide phases occur with iron oxide coatings existing on the visible grains of quartz, felspar and mica. The concentration of calcium, magnesium and iron would agree with the mineralogy if the calcium and magnesium are tied up in mineral phases while the iron is associated primarily with the amorphous phases and oxide coatings. A general trend toward greater concentrations in the less than 2 p fraction is shown for copper, nickel, chromium and possibly cobalt, suggesting that these elements are associated with the amorphous iron phases, or possibly with the clay minerals as well. Manganese, however, shows a greater association with the 2-5 ~1size fractions. It may be that the manganese is associated with chlorite which has its maximum absolute concentration in the 2-5 ,u size fraction (WINDOM, 1969). Strontium is more concentrated in the coarser fractions of the samples suggesting that it is associated with crystalline phases of plagioclase substituting possibly for calcium in plagioclase lattice. In order to determine the significance of this dust to the trace metal content of marine sediments in the South Pacific, analyses were made on cores taken from two general locations. The results of these analyses are listed in Table 2. The four cores listed in this table represent two sediment provinces (GRIFFIN et al., 2-5 p

G.C.A.

Vol. 3414 F

Notes

512

Table 2. Elementalcompositionof South Pacific sediment samples wt. % Core

I&.

Ce

Long.

Mg

Fe

Mn

Cu

Ni

PPm

co

Cr

Br.

420 390

Western

South Paoi50 MSN 104

MSN 109

4V28’S. 4tv30’8.

Northern South Pacifio MSN 131 CAP 31*

l

8917’8. 17 29%.

176”20%. 176’46’W. Average

3.6 2-6 3.05

1.20 3.8 I.44 3.4 1-32 3.6

llw34’W. 4.1 %46 Ii.2 168’4O’W. 4.2 2.10 6.7 4.16 2.28 6.96 Avanrge

l-bsuka taken from #OIDBEBQ

680 686 630

80 11 46

70 70 70

28 21 26

280 240 266

406

6,500 620 17,000 640 11,250 680

470 190 330

190 170 180

360 110 230

250 350 300

and ABREEN7.us(1958).

19gs). Samples from the western South Pacific represent the area where the influence of dust fallout from Australia is observed on the basis of mineral distributions in the sediment (GRIFFIN et al., 1968; ARRHENIUS, 1966). These cores would represent sediments which should contain a component resulting from the dust fallout. Cores ~present~g the northern South Pacific are taken in an area which is characterized by a high montmor~onite concentration and a high concentration of authigenio phases such as phillipsite and barite. This area has been discussed by GRIFFIN et al. (1968), who found it to be little affected by detrital components. The trace metal composition of sediments in this region would be a function of injections due to submarine vulcanism with subsequent precipitation or direct precipitation out of sea water. Analyses of these sediments are presented in order that a comparison can be made between the sediments that accumulat+n the ocean where effects of continental injections are minimum and those areas where the dust component should be most prevalent. It is clear that the trace metal composition of these sediments in the northern South Pacific reflect the high concentration of volcanic material and volcanically derived phases. Both iron and manganese are considerably higher in northern South Pacific sediments than those of the western South Pacific. This is expected if these elements are considered to be associated with submarine vulcanism as suggested by ARRHENIUS et al. (1964). The trace metals copper, nickel, and cobalt, also follow this trend while chromium appears to be concentrated in approximately the same way in both western South Pacific samples and those of the northern South Pacific. Strontium, on the other hand, is more associated with the western South Pacific samples than those of the northern South Pacific. This presumably results from the occurrence of greater amounts of acidic detrital minerals in the former. DISCUSSION

The rate of accumulation of the dust in the New Zealand snowfields was calculated by WINDOM (1969) to be 120 mgfem2 . lo5 yr. Using this rate, the rate of accumulation of the trace metals can be determined. GRIFFIN et al. (1968) reviewed the accumulation rate data for the world ocean and from this it can be seen that the rates of accumulation for the sediments in the area of MSN 131 and

513

Notes

CAP 31 average about O-4 mm/l@ yr or approximately 60 mg/cm2 . lo3 yr. This rate can be used with the data on the trace metal concentrations of these cores to give an accumulation rate for these elements. It is interesting to compare the rates of accumulation of the metals resulting from dust fallout with the accumulation of these elements from ilz situ formation of volcanic degradation products and precipitation products to see the relative importance of these particular mechanisms for the accumulation of trace elements in the South Pacific. Table 3 lists the rates for the dust and the sediments of the northern South Pacific. From the table it can be seen that manganese, copper, nickel and cobalt accumulate at a faster rate with the sediments than they do with the dust. However iron, chromium and strontium accumulate at a faster rate in the dust than they do in the sediment. Even in the case of copper, nickel and cobalt the accumulation rates are not greatly different. This indicates that areas affected by the dust fallout can accumulate these trace metals at rates comparable to that of marine sediments which contain little continental detrital material. Table 3. Rate of accumulation of metals in Central Pacific sediments and dusts collected in New Zealand Rates (mg/oma . 103 yr.)

nust Northern South Pacific sediments

Fe

MIl

5.8

4.7. 10-Z

2.0.

3.5

1.02

3.4. 10-s

CU

10-p

Ni 1.4. 10-a 2.0.

lo-’

CO

Cr

Sr

.

10-z

3.6. 10-a

2.6

1.1. 10-p

1.4. 10-a

1.8. 10-e

0.4.

10-p

Accumulation rate data for the sediments in the area of MSN 104 and 109 do not exist, but it can be assumed that the sedimentation rate in these areas is much more rapid than those of the northern South Pacific where influence of detrital runoff is minimum. WINDOM (1969) has suggested that approximately 60 per cent of these sediments are the result of atmospheric transport from the Australian deserts. Therefore, if it is assumed that this component of the sediment is similar to the dust listed in Table 1, then the resulting concentration of the trace metals due to the dust alone can be calculated on the basis of this component representing 60% of the total sediment. When this is done, the concentrations of the trace metals are as follows: 246 ppm manganese, 100 ppm copper, 71 ppm nickel, 22 ppm cobalt, 185 ppm chromium and 127 ppm strontium. Comparing this to the average composition of these metals in the western South Pacific sediments, it can be seen that manganese, chromium, and strontium cannot be accounted for by the dust; whereas copper, nickel, and cobalt are adequately explained by the dust injection. WINDOM (1969) has suggested that the material in the deposits of the western South Pacific Ocean not accounted for by the Australian dust component could be derived from transport of material injected from the Antarctic continent due to glacial runoff. The non-eolian component as described by WINDOM (1969) would have a mineralogical composition containing a high amount of chlorite and illite which would be consistent with the type of material that would be expected to be injected from the Antarctic continent. This material would account for 40 per cent of the western South Pacific sediments. If this conclusion is valid, then the trace metal composition of sediments near Antarctica, and presumably derived from there due to continental runoff, should have characteristics compatible with 6

Notes

614

similar material being a component of the sediments in MSN 104 and 109. Since the dust was shown to be inadequate to explain manganese, chromium, and strontium, the sediments derived from Antarctica must make up the remainder. ANGINO (1966) has determined the concentration of the trace metal considered here in Antarctic Ocean sediments. These sediments contain 1000 ppm manganese, 100 ppm copper, 35 ppm nickel, 85 ppm chromium and 25 ppm cobalt. From this it would appear reasonable that a material having a composition similar to that stated for the Antarctic sediment samples could supply the additional component to the western South Pacific Ocean sediments necessary to explain the trace metal concentrations found there. CONCLUSIONS

Results of the present investigation indicate that the trace metal composition and concentrations of dust derived from Australia and transported to the South Pacific sediments is adequate to explain the trace metal composition of the detrital sediments of this area with small amounts presumably associated with detrital phases being brought in by transport in the ocean itself, presumably from the Antarctic continent. The rates of accumulation of the trace metals in the dust when compared to those of sediments in the deep ocean basin containing small concentrations of detrital phases suggest that the accumulation of continentally derived trace metals is rapid enough to explain the concentration of these elements in detrital sediments without invoking a mechanism of volcanic injection into the aqueous media with subsequent dispersal and precipitation in areas where detrital phases predominate. These rates also indicate that the trace metal accumulation in South Pacific deposits is as rapid in areas affected by dust fallout as those where submarine vulcanism predominates as the major source of sediment. Acknowledgement-The

author greatly appreciates the assistance of Mr. RALPH SMITH in this

research. REFERENCES ANQINO

E. E. (1966) Geochemistry

of Antarctic pelagic sediments.

Geochim. Coemochim.

Acta

30, 939-961. ARRHENIUS G. (1966) Sedimentary record of long-period phenomena. In Advances in Earth Science, (editor R. M. Hurley), pp. 155-174. M.I.T. Press. ARRRENIUS G., MERO J. and KORKISCH J. (1964) Origin of oceanic manganese minerals. Science 144, 170-172. BONA~I E. and JOENSW 0. (1966) Deep-sea iron deposit from the South Pacific. Science 154, 643-645. DELANY A. C., DELANY AUDREY CLAIRE, PARKIN D. W., GRIFFIN J. J. GOLDBERG E. D. and REIMANN B. E. F. (1967) Airborne dust collected at Barbados. Cfeochim. Cosmochim. Acta 31, 885-909. GOLDBERGE. D. and ARRHENIUS G. 0. S. (1958) Chemistry of Pacific pelagic sediments. Beochim. Cosmochim. Acta 13, 153-212. GRIFFIN J. J., WINDOM H. and GOLDBERG E. D. (1968) The distribution of clay minerals in the World Ocean. Deep-sea Res. 15, 433-459. PROSPERO J. M. and BONATTI E. (1969) Continental dust in the atmosphere of the Eastern Equatorial Pacific. J. Geophys. Res. 74, 3362-3371. TVREKIAN K. K. (1968) Deep-sea deposition of barium, cobalt and silver. Geochim. Coemochim. Acta 32, 603-612. WINDOM H. L. (1969) Atmospheric dust records in permanent snowfields: Implications to marine sedimentation. Geol. Sot. Amer. Bull. 80, 761-782.