Geochemical Journal, Vol. 24, pp. 371 to 378, 1990

Anomalous Hg contents in soils of Niue Island, South Pacific

NEIL E. WHITEHEAD', JOHN BARRIE2 and PETER RANKIN3

Nuclear Sciences Group, Division of Physical Sciences, D.S.I.R. P.O.Box 31-312, Lower Hutt, New Zealand', Avian Mining Ltd., 24 Jindvik Place, Canberra, A.C.T., Australia2 and Division of Land and Soil Sciences, D.S.I.R., Private Bag, Taita, New Zealand3

(Received September 10, 1990; Accepted December 29, 1990)

Niue Island, a raised coralline atoll in the South Pacific, has soils that have long been known to have strongly anomalous radioactivity. We now show that there is also a highly anomalous Hg content in the soils. It is associated with the radioactivity and the goethite/gibbsite content and the values are as high as those in soils over known Hg-mineralisation in volcanic settings, though no mineralisation is known on Niue and such an occurrence on this coral island would be geochemically unusual.

INTRODUCTION GEOLOGY AND SOILS OF NIUE ISLAND Niue Island in the South Pacific is a raised A detailed description of the geological set coral atoll, located at 19° S and 169° W. The in ting of Niue Is. may be found in Schofield (1959) terior of the island is dolomitised, and is covered and a summary follows. by reddish-brown soils rich in Fe, Al and Niue Island is a raised coral atoll consisting phosphate in the form of goethite, gibbsite and of seaward cliffs rising steeply from the sea but crandallite respectively (the mean soil P205 is girded by a terrace on which sits Alofi, the 4%; Whitehead et al. in press). The 230Th/234U capital. The rim of the island rises to 60 m above ratios are 20-30, hence relatively little parent sea level and the interior shallow basin is about 234Uis present . The 230Thcontent corresponds to 40 m above sea level. The interior basin has been an equilibrium 238Ucontent of about 1000 mg extensively eroded into pinnacles. It is also kg-1, but 238Usoil contents are only about 30 mg covered with forest and access is difficult. kg-'. The soils have recently been shown Dolomite is known from many parts of the (Whitehead et al., in press) to contain a 211Pa island, mainly the interior basin. From the anomaly with "'Pa /231U ratios about 7. Radio magnetic measurements of Hill (1980) and nuclide soil contents so much in excess of bathymetry measurements, it is inferred that equilibrium values are unique, but we report there is a thickness of 300 m of coral, and here the presence of an additional geochemical dolomite, on top of a caldera-like structure, anomaly consisting of high values of Hg. The ex probably volcanic. Two drill-holes (Barrie, 1979) planation for these various anomalies is only penetrated to about 280 m below the surface and partly clear. This presentation is preliminary, encountered carbonates of Miocene age, but fail but changes in research funding mean that fur ed to reach any volcanic rock. All limestone en ther work will probably be impossible, so the countered was clean and white and showed no results are offered as a progress report. signs of mineralisation, but in the deepest regions, unlike the surface regions, thermally mature kerogen was found, showing geothermal

371 372 N. E. Whitehead et al. heat had affected the lower layers. Fossil remains original atoll topography is well preserved in from the surface of the control lagoon corres dicating only minor lowering of the land surface pond to a Plio-Pleistocene age (approximately by weathering. 1.8 Ma B.P.). Dacitic ash is found in a layer in the nearby Evidence from Uranium/Thorium series ra seabed (Glasby et al. in press) sufficiently thick dionuclides in the soils (Whitehead et al. submit to account for a substantial fraction of the ted to Isotope Geoscience) shows that the soils thickness of Niue Is. soils if ash was deposited were in contact briefly with seawater at 470 ka also on Niue Is. and subaerial weathering occur B.P. and absorbed U. The time was brief red. The ash has been linked to a Pleistocene because no large coral masses of that date have eruption on the Tongan island of Tofua. been found. The radionuclide ratios in the soils The soils are lateritic and contain 30-40% of also show that they were strongly leached some both Fe and Al (expressed as oxides). They con time in the interval 70-100 ka B.P. but not by tain a mean value of 1000 mg kg-' Cr, and 2400 seawater. The evidence is therefore that since mg kg-' Sr, but only 0.8% Si (as oxide), 1.7 mg 470 ka B.P. the soils have been above sea level. kg-' Th and 80 mg kg-' Zr. The latter three The island is in a tectonically active region, and values are very low for such normally leach-resis the causes of exposure to seawater and subaerial tant elements in lateritic soils and suggest very exposure are not simply changes in sealevel asso strong leaching has taken place. In contrast the ciated with glacial and interglacial periods but ris mean P (as oxide) is 4%, and since phosphate is ing and falling of the island itself as it slowly relatively easily leached, such values imply moves towards the Tonga Trench (Recy and Du phosphate was added to the soils well after initial pont, 1982, Dupont and Herzer, 1985). formation. In a few locations P values are as All known rock on the island is carbonate. high as 18%, but the origin of the P is not clear. The-soil survey of Blakemore et al. (1979) which There are no large contemporary bird popula examined in excess of 100 sites chosen to be tions or obvious guano deposits on the island. representative of different types of terrain on the island, collected samples of underlying rock EXPERIMENTAL which was always aragonite, calcite or dolomite, not volcanic rock, and no alteration or Many soil samples have been gathered from mineralisation was visually observed, nor Niue Is. for analysis of major and trace elements detected on analysis of the immediately underly and the results are given elsewhere (Blakemore et ing rock. Most of Niue Is. has therefore been al., 1979; Whitehead, et al., in press). For this surveyed but no mineralisation noticed below study, soil samples from 47 soil A & B horizons the soils or deeper in the holes drilled. were analysed for mercury, using a gold film Conventional hydrothermal phenomena such mercury analyser manufactured by the Jerome as springs are not present, nor has the island sur Instrument Corporation. Eight of the soils face water such as streams, because the coral is were independently analysed at the Chemistry extremely porous and rainwater sinks straight Division of the New Zealand D.S.I.R. by photo into it. acoustic analysis and the values found were The spaces between pinnacles in the interior generally within 10% of the other method. basin are filled with coral debris and soil as thick as 1 m. In areas where pinnacles are most sparse RESULTS the soils are only about 15-25 cm thick. The origin of the soils is unlikely to be simple erosion For all results combined, the mean and stan of the coral, since calculations show for most dard deviation were 980 ± 880µg kg-' . The soil components the required erosion would be distribution is log-normal; the minimum value hundreds of meters, which is unlikely, and the was 70 pg kg-' and the maximum 3680µg kg-'. Hg anomaly of Niue Island, S. Pacific 373

NIUE ISLAND

23'20 2000

1000-N-12.60] N .' 730 220 270

410 100 1 420 .\21 PO 17 2 •54 , . -, _000 '~ 0 I It 111480 l 1810 705% h` 2600' I 355 \ 11 . 300 2 iIt I 11785 1890 !~ 365 1577 t Y 106~l~ 240

1025 257

570 200 070 1990 216

Q 5 10 km Fig. 1. Contour map of Hg in Niue Island soils. Dots represent sampling points, but for clarity not all detailed results are shown. Superimposed are dashed contours for alpha activity. The lowest contour is 2 counts per minute (100 mg sample as measured in a Beckman Widebeta instrument) and thereafter the contour interval is 1 count per minute.

Lower and upper quartiles were 260 and 1560 pg results are very similar to the alpha activity con .kg-'. tours. Figure 1 shows a map of Niue Is. with con tours drawn in for Hg. The values are all higher than the global 50,ug kg-' mean quoted by Stock DISCUSSION and Cucuel, (1934), and show a concentration No mineralisation was found near the sur towards the middle of the island. Superimposed face on Niue Island, though the soil surveys car are dotted lines showing the contours for the ried out have been the equivalent of an extensive alpha activity. The highest Al and Fe contour geochemical exploration, yet if the island is 374 N. E. Whitehead et al.

Regression of Zn on m 300

...... 250 *

* 200 *......

*

f * N 150 * * * *

100 ...... * * * * * ** * * *

50 *

* * * I I I I I 0 0 10 20 30 40 50 60 m

Fig. 2. Regression for Zn (ppm) on depth (m) for samples taken from the Fonuakula well. Taken from the raw data of Rodgers et al. (1982).

really not mineralised the results for Hg in the soils Fe and Al contents. There was no relationship of are exceptional. They are typical of those found Hg content to the organic content of the soils. in soil over mineralised ground (Flanagan et al., An origin of Hg from limestone is im 1982), for example, the results in the Hg-rich probable. The contents of Hg in limestone are area at Ngawha, New Zealand (Davey and Van low (Wedepohl, 1969-1978), universally about Moort, 1986). 33 yg kg-', and it may be calculated that The mean and standard deviations for Hg in weathering of about 50 m of coral would be need the B horizon is 1060 ± 890 pg kg-'. The median ed to produce the 1 pg kg-' mean Hg content in is 750 pg kg-'. Statistical examination shows the soils which seems unlikely. It is also known that the highest values for Hg are found when from personal communications that surveys for the Al or Fe content (as oxide) is 30-40%, and Hg have been carried out on various Pacific the radioactivity levels are highest. In fact the islands by mining companies, but in no case have correlation was better between the alpha activity such striking anomalies been found. The and the Hg content than between the alpha activi phenomenon is therefore unusual and not a ty and any other element measured. Below levels universal result of the weathering of coral. of 30-40% Al there are lower but nearly con Relict minerals cannot be the origin of the stant values of 100-200µg kg-' Hg. The highest Hg. The only resistant mineral persistently values for Hg are therefore associated with high found in quantity on Niue Is. is magnetite Hg anomaly of Niue Island, S. Pacific 375

Regression of Fe on m (X 10600)

*

5

4

CU L 3

* 2

*

I ...... *. , *......

II *

0 * 10 20 •• 30 40 50 60 m

Fig. 3. Regression of Fe (ppm) on depth (m) for samples taken from the Fonuakula well. Taken from the raw d ata of Rodgers et al. (1982).

(Fieldes et al., 1960), which although typically absorbed trace Hg from sea-water giving rise to containing 500-700,ug kg-' Hg (Aidinyan et al., excess Hg contents. More analyses of gibbsite 1965), is only a minor component in the soils and from localities on other islands known to have cannot therefore account for the Hg found. been submerged would test this hypothesis. Although a global mean for Hg in soils A second possible origin for the Hg is the (Stock and Cucuel, 1934) is 50,ug kg-', lateritic leaching of phosphate deposits and concentra soils probably contain more typically 200,ug tion of Hg on the lateritic soils. Some of the kg-' (Flanagan et al., 1982). Gibbsite (taking highest contemporary phosphate concentrations bauxite as a surrogate mineral) contains 200 (18%) occur on the seaward ledges of the island, 700,ug kg-' (Flanagan et al., 1982; Abdullaev, but do not usually correspond with the highest 1970; Yakhnin et al., 1978; Miloslavskaya and Hg concentrations in the soils. The rest of the Memedov, 1984). The authors of those papers high P values are clustered in the centre, as are suggested adsorption of Hg had taken place the radioactivity and the Hg (Figure 1 shows the from groundwater. coincidence of the highest alpha activity contour Since it is already known from the plots and the highest Hg contour plots). There is Uranium/Thorium series analyses that the soils still debate about the precise origin of island were exposed to seawater at 470 ka B.P., this sug phosphates, but they contain about 8 times gests by analogy that gibbsite on Niue may have greater concentrations of Hg than other 376 N. E. Whitehead et al.

Regression of Fen on in 40 1 I I l V I I I V I I I I 1 1 1 T ~ I I I I I 1 1 1 I

30

c * 20

* 10 . . . * . *. .* *

* * * * * • * * N * * * 1 *I *I I *I*1* I I I i * I* I I I I I 1 1 I 1 1 I I I I 0 Ix0 , I "I I 10 I -i71 1 1 1 20 1I i* 1' 1 30 1 1 1 1 1 40 1 1 i i 50 I 60 M Fig. 4. Regression of Fel Zn on depth (m) for samples taken from the Fonuakula well. Taken from the raw data of Rodgers et al. (1982).

phosphate sources, supporting an origin from coral are present and the island has a thickness guano since the Hg content would reflect com of at least 300 m of coral on top of the now position of organisms from the top of the marine submerged volcano (Hill, 1980). However as food chain. Calculation shows that Niue Island noted earlier the deepest coral from the drill soils contain about 60 tonne Hg, equivalent to holes shows signs of heating, and a geothermal 700,ug kg-' Hg in the phosphate before temperature gradient has therefore existed at one leaching. Mercury concentrations for Ocean time. This confirms the suppositions of Aharon Island, Makatea Island, and Nauru Island et al., (1987) who postulated a geothermal gra phosphate are 440, 770 and 570 pg kg-', respec dient strong enough to draw sea-water inwards tively (Weissberg and Singers, 1982), hence such into the base of Niue Island through the porous an origin is feasible. The origin of the Hg is un coral, and make it rise until it encountered the likely to be marine phosphorite which contains soils. Other evidence for the ascension of slightly only 55-80pg kg-' (Weissberg and Singers, heated water comes from examination of the 1982; Altschuler, 1980). In a previous paper data of Rodgers et al. (1982). In Figs. 2 and 3 are (Whitehead et al., in press), the leaching of shown the distribution of Fe and Zn with depth phosphate was suggested to be the origin of the and the regression lines show there is an increase radioactivity. with depth for both elements. This evidence by A third possible origin is endothermal. No itself is ambiguous, and might arise from either known surface rocks except those derived from upward or downward. movement within the

U Hg anomaly of Niue Island, S. Pacific 377

porous rock. However in Fig. 4 is plotted the CONCLUSIONS Fe / Zn ratio with depth. 'It will be noted that the ratio also increases with depth, showing there is In summary therefore, the Hg is associated relatively more Fe at depth than Zn. Each of with the Fe and Al-rich portion of the soils and these regression lines is highly significant correlates with the radioactive fraction, but it is statistically. Now from :the data in I.A.E.A. not clear whether it owes its origin to direct ad (1985)it can be shown that Fe will move much sorption from the sea during a period of partial slowerthrough prorous calcite than Zn. Since Fe submergence, phosphate weathering or endother concentrations are relatively greater at depth in mal processes. Present data do not enable us to Niue Island, this is consistent only with addition decide conclusively which is more likely to be cor from beneath. rect, but endothermal-like processes have cer If the waters contained Hg and reached the tainly taken place. . The implications of the present findings soils, the Hg would be adsorbed. Such waters .are might be volcanic, but if so would have to be a intriguing but not clear. For example, we have different type of phenomenon from the seafloor no information about .the distribution of the Hg hydrothermal activity reported elsewhere associ in the biosphere of Niue Island. Samples should ated with enhanced Hg concentrations (e.g. obviously be collected and analysed to confirm Sarano et al., 1989). The temperatures would that Hg concentrations in food are below max have to be quite low, because otherwise there imum permissible levels. would be distinctive features on the island due to In view of the lack of information generally remobilisation and redeposition of carbonate about Hg in gibbsite or bauxite, more analyses caused by the thermal waters. Volcanic-related should be carried out, specifically more analyses processes by themselves are only rarely associ from Pacific islands. It also seems that the coex ated with high Hg levels (e.g. Christensen et al., istence of such a strong radioactive anomaly 1983, Davey and Van Moort, 1986). with such a strong Hg anomaly in soils is unique. Alternatively the origin would be endother mal, and both Hg and U would be derived from Acknowledgments-We would like to thank J. Pater seawater, rather than volcanic fluids. Under son of Chemistry Division, D.S.I.R. for the con firmatory mercury analyses. either of these hypotheses the simultaneous oc currence of Hg and U daughter products would be due to simultaneous transport of both to the REFERENCES soils, presumably not later than 470 ka BY when Abdullaev, A. U. (1970) Mercury in the Paleozoic the soils were exposed to U solutions. After bauxites and country rocks of Turkesten, Alai. Rtut build-up of daughter products the parent U was Osad. Poradakh Tyan-Shanya 54-73. leached away some time 70-100 ka B.P. but Hg Aharon, P., Socki, R. A. and Chan, L. (1987) remained in the soils. Dolomitization of atolls by seawater convection flow: test of a hypothesis at Niue, South Pacific. J. The soils contain a negative Ce anomaly. Geol. 95, 187-203. This is evidence against a volcanic origin, since Aidinyan, N. K., Shilin, L. L. and Belavskaya, G. A. such anomalies are associated with an origin (1965) Concerning the distribution of mercury in from seawater (e.g. Goldberg et al. 1963) and the rocks and minerals of the Khibinsky Massif. Tr. negative Ce anomalies associated with material Inst. Geol. Rudn. Mestorozhd. Petrog. Mineralog. originating from igneous activity are unusual. Geokhim. 99, 16-25. Cited in Wedepohl (1969 1978). The results of deep drilling planned for 1991 Altschuler, Z. S. (1980) The geochemistry of trace are awaited with interest. elements in marine phosphorites. Part I. Character istic abundances and enrichment. SEPM special publication 29, 19-30. Barrie, J. (1979) Geology and Mineral potential of

378 N. E. Whitehead et al.

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