Do diatom algae frustules accumulate uranium?

Nuclear Instruments and Methods in Physics Research A 405 (1998) 584-589

NUCLEAR INSTRUMENTS a METHODS IN PWSICS R%zH

ELSEVIER

Do diatom algae frustules accumulate E.L. Goldberg”,

uranium?

M.A. Gracheva’* V.A. Bobrovb, A.V. Bessergenevd, Ye.V. Likhoshway”

B.V. Zolotaryov’,

“Limnological Institute of the Siberian Branch of RAS, 664033 Irkutsk, Russia bUnited Institute of Geology and Geophysics of the Siberian Branch of RAS, 63oo90 Novosibirsk. Russia ‘Budker Institute of Nuclear Physics of the Siberian Branch of RAS, 630090 Novosibirsk, Russia dInstitute of Solid State Chemistry of the Siberian Branch of RAS, 630091 Novosibirsk, Russia

Abstract Neutron Activation Analysis (NAA) and Synchrotron Radiation X-Ray Fluorescent Analysis (SRXRFA) were used to measure the content of uranium and a few other trace elements in samples of bottom sediments of Lake Baikel separated into biogenic (diatom algae frustules) and elastic components by an aerodynamic method. Uranium is rejected, rather than accumulated by diatom algae frustules.

1. Introduction Studies of the bottom sediments of Lake Baikal (East Siberia) give a unique possibility to reconstruct the paleoclimates of the Asian continent because they were continuously accumulated in this rift basin during ca. 20My. Changes of the climates of Upper Pleistocene manifest themselves in sediments of Lake Baikal by accumulation of diatomaceous silts during the warm intervals, and of diatom barren clays during the cold ones (for a review, see Ref. [I]). Recent investigations [2] revealed a uranium anomaly: it was found that diatomaceous silts had a Th/U ratio equal to =l, whereas clays were characterized by the same ratio equal to -3-5, i.e. by a value that is typical of the Earth’s Crust. Gavshin et al. [2] suggested that uranium accumulated in the diatom silts due to binding with organic matter, because the high content of diatom algae frustules correlated with high content of organic carbon. This hypothesis was indirectly confirmed by Pampura et al. [3] who found that the 234U1238U ratio was the highest in the fraction of humic acids isolated from the Baikal bottom sediments. Edgington et al. [4] studied a nine-meter core of Baikal sediments, and came to the same conclusion: layers with a high content of biogenic silica and diatom algae frustules were characterized by an increased content of uranium. It was also found that diatomaceous silts were also enriched in non-equilibrium 234U and 230Th. Edgington et al. [4] proposed a model according to which dissolved total uranium and non-

* Corresponding author: Wan-Batorskaya str. 3, 664033 Irkutsk, Russia. Tel.: +7 3952 460504; fax: +7 095 4202106; e-mail: [email protected] 0168~9002/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved PII SO168-9002(96)01060-1

equilibrium 234U is carried by rivers to Baikal during the warm intervals. It is adsorbed in the lake by clay particles and finally buried on the bottom. The presence of excess 234U in sediments of Lake Baikal gives a unique possibility of their absolute dating over a time interval of some 300 Ky [ 1,4] using the thorium-uranium and the uraniumuranium chronometers [5]. In this connection, it is very important to know which fraction of the sediment absorbs dissolved uranium and non-equilibrium 234U.

2. Materials and methods 2.1. Element

analysis

Majority of these analyses were done by means of NAA. Samples were irradiated by neutrons in an experimental nuclear reactor of the Tomsk’s Institute of Nuclear Physics at a flux of lOI neutrons per second per square cm; neutrons were passed through a cadmium filter for the determination of U, no filter was used for other elements. The time of irradiation was 20 h, the mass of the sample was 20 mg. Gamma-spectra were measured using spectrometers JN-90 and JN-96 with germanium semiconductor detectors, a planar and a coaxial one. The resolution was 0.5 keV (at 122 keV), and 2.5 keV (at 666 keV). The limit of U detection was 0.1 ppm with 5-10 mg sample. Thorium and rare-earth elements (REE) were determined with an accuracy of better than 10% rel. The content of Fe, Ba, La, Ce, Nd in some samples was checked additionally by SRXRFA. The XRF station using synchrotron radiation of the storage ring VEPP-3 consists of a vacuum monochromator (pyro-graphite, or Si [OOI]), of a sample

chamber, and of a Si (Li) solid-state detector. A computer equipped by CAMAC modules operates the station. and collects data. Energy of the incident radiation can be varied over a range between 3 and 48 keV. The area of samples was 0.2 to 7 mm*. A detailed description of the station is given in Refs. [6,7}. Reference materials BIL-I and SDO-I 181 were used as standards.

exposure time was 1-3 min, number of counts per one frame was =2-3 min. Masses of samples were equal ~5-7 mg. A detailed description of the station and its oppo~unities are given in Ref. 191.

22.

Samples were taken from a core of Baikat sediments obtained by a piston corer (No. 331 PC) near the northern outlet of the Maloye Morye strait of Lake Baikal at a depth of 360 m (53.473 N, 107.78% E), one of them with a moderate content of diatom algae frustuies 16.5-180 cm, and the second 360-38Ocm below the sediment surface consisting of diatomaceous silt. A few grams of dry

Phase

~l~ul~.si~~

Semi-quantitative phase analysis was done at an X-ray diffractometer using the synchrotron radiation beam of the storage ring VEPP-3. The single-coordinate parallel detectar was employed to record diffraction pictures simultaneously in a wide range of diffraction angles. The

13.

Prepi&rution

of .sutnplrs

Fig. I. SEM photographs of typical fractions obtained by aerodynamic separation: (a) fine-grain clay particles. scale 0.001 mm: (b) “rings” (see the text). scale 0.1 mm; (c) diatom algae frustules, scale 0.1 mm; (d) sand particles. scale 0.1 mm.

VI. ELEMENT ANALYSIS

E.L. Goldberg et al. I Nucl. Instr. and Meth. in Phw. Res. A 405 (1998) 584-589

586

sediment were passed through a crude sieve. The powder was subjected to aerodynamic separation [IO] by means of an aerofuge [11], which consisted of two rotating discs, one of them with a tube attached in the center through which air was pumped into a cyclon followed by a filter. The powder was fed at the edge into the gap between the two discs. Depending on the velocity of rotation and air flow, particles were separated according to their size and form into different fractions collected in the cyclon. The first fraction consisted of particles of the smallest size (clay, size less a 4 Km). The second fraction consisted of diatom algae frustules of the greatest dimensions (50150 pm diameter) and clay aggregates (5- 10 pm). The third fraction consisted of large particles (sand). The

second fraction was thoroughly washed by distilled water through a 40 pm sieve. Diatom algae frustules collected on the sieve were washed with ethanol and dried. Changing the velocity of rotation of the aerofuge’s rotor, it was possible to separate the “biogenic” fraction into particles of different form and size. These were “rings”-fragments of values of Cyclotella baicalensis and Stephanodiscus grundis, unbroken frustules and fragments of frustule faces of different species, etc. The aerodynamic method, unlike other techniques (e.g., separation with heavy liquids), is much safer in terms of contamination, or loss of organic carbon. The quality of separation was checked by optical microscopy with polarized light, and by scanning electron microscopy (SEM).

Table 1 Results of SRX-ray phase analysis Fine-grain particles (clay)

Large particles (sand) d(A)

I (%)b

phase”

d (A)

4.955 4.693 4.553 4.447 4.243 3.536 3.364

14 5 3 5 36 5 100

HM HM HM HM Si02 HM Si02

4.95

4.44 4.25 3.54 3.36

3.266 3.231 3.21 2.951 2.584 2.557 2.484 2.453 2.407 2.28 2.236 2.167 2.13 2.093 1.98 .963

17 3 6 5 1.5 1.5 3 3 2 5 10 8 8 4 =8 -17

FS FS FS

I ,813

15

Si02

,658 ,542 ,532 1.448 1.319 1.313 I.281

35 45 10 5

Si02 Si02 HM Si02 Si02 Si02 Si02

10 5 15

Si02

Rings phase”

phase”

d (A)

I (%)

18

HM

6.94 4.94 1 4.693

21 49 20

HM HM HM

18 27 25 100

HM Si02 HM Si02

3.536

84

3.332

100

2.489

17

1.981

31

I (%)

3.254

20

FS

3.21 2.909 2.874 2.555 2.53 2.452

50 13 23 26 11 12

FS

Si02

Si02

2.13

12

Si02

1.984 Si02

1.96 1.846 1.816 1.799 1.657 1.544

1.532 1.496 1.38 1.373

-18 -18 8 24 8 20 16 7 11 15 I

Si02 Si02 Si02 Si02 HM Si02 Si02 Si02

intensity.

phase”

a’ (d;)

I(%)

HM

3.54

67

HM

HM

3.332

100

HM

1.984

19

Si02 Si02

a FS - feldspars, HM - hydromicas (illit, chlorite, etc.). Sio, _ cy-quartz. h Relative integral

Diatom algae frustules

3. Results and discussion Fig. I illustrates the composition of typical fractions obtained by aerodynamic separation. These fractions are fine-grain “clay” (Fig. la). “rings’‘-fragments of valves of c‘. huiculensi.r (Fig. I b), intact frustules and face fragments of frustules of Aulacoseira baicalensis, Cyclotellu minrrtu, Stephunodiscus grandis (Fig. 1c). coarse-grain sand (Fig. Id). It is seen that biogenic particles are not significantly contaminated by elastic material. Examination by means of optical microscopy with polarized light confirms this conclusion. Separation in the aerofuge depends on the aerodynamic form. Biogenic particles have a regular, “two-dimensional” form, and are easier drawn by how of air through the rotor, compared with “three-dimensional” particles of elastic material of the same maximum diameters. Therefore. biogenic particles pass the aerofuge together with the much smaller particles of clays. and may be subsequently obtained in a pure state by washing on a sieve. Table I shows the results of SRX-ray phase anaiysis of one of the samples. Diffractograms of sand consist of narrow reflections. and are characterized by a low background. Integral intensity of the peaks of quartz is ~7% 80% suggesting that the content of this mineral is of the same order of magnitude. Diffractograms of the fraction of tine-grain “clay” consist of a diffuse halo with wide reflections of purely crystallized phases of quartz, feldspars. hydromicas. Diffractograms of biogenic Ractions revealed that the material was X-ray amorphous, except for the small and wide peaks of hydromicas.

0.1

Fig. 2.a.b Correlation 165-180cm.

100.0 10.0 1.0 element contents in clay between

the contents

Crystalline quartz peaks were absolutely absent in them. The total intensity of the peaks of hydromicas in the biogenic fractions was equal to IO- 15% of that found for the “clay” fraction, i.e., this admixture was not negligible, although it is not revealed by SEM (cf. Fig. I ). Presumably. small particles of clays are entrapped by diatom frustules, and buried inside the matrix of opal silica. Table 2 shows the results of elemental analysis. It is seen that the biogenic fractions have a small content of a11 the tracer elements studied, including uranium. The content of practically insoluble rare-earth elements, Th. Sc is 3-6 times smaller than that found in clays. It is known that these “insoluble” elements are delivered to sediments with elastic material, and that the relations of rare-earth elements are practically the same in sandstones, carbonaceous rocks, shales. etc., and practically do not depend on the mineralogical composition [ 121. Fig. 2 illustrates correlation between the contents of rare-earth elements. Th. SC in clay. and in the biogenic fraction. This correlation suggests that the admixture of trace elements in rhe biogenic fraction is indeed due to the above-mentioned entrapment of tine-grain clay particles. The content of entrapped clay in diatom algae frustules (w) estimated as mean ratio of the concentrati~~ns of rare-earth elements, Th and SC, in the two fractions was found to be equal to l&20%-. Estimates of the cont~butions of elements absorbed by diatom algae frustules in a dissolved form from water may be obtained from mass balance: F =.fw + u( I - w),

1000.0

0.1

of REE, Th, SC in clay and in the biogenic

1.0 10.0 100.0 element contenta in clay fraction.

(a) Depth 360-?XOcm.

VI. ELEMENT

looo.o (h) depth

ANALYSIS

9.5 1.4 10 2.7 2.4 2.55

U 0.27 0.24

initial sand clay diatoms

biog. average

ratio: bioglclay diatoms rings

Th 0.18 0.14

28 9 34 6.2 4.6 5.4

3%

8.4 4,9 12 3.2 3.7 3.45

U 0.27 0.31

initial sand clay diatoms rings biog. average

ratio: biog./clay diatoms rings

*ppm, default.

U

fraction

Th 0.18 0.26

19.5 15 30 5.5 7.8 6.65

Th

Sample N2 (depth ~65-1SOcm)

riR@

U

fK&iOIb

Sample Nl (depth 360-380 cm)

La 0.24 0.31

60 60 80 19 25 22

16 14.6 15.5 5 3.4 4.2

SC 0.32 0.22

La

SC

La 0.14 0.14

82 34 92 13 13 13

16.7 11 12.3 3.5 3.5 3.5

SC 0.28 0.28

La

SC

Table 2 Cantent of elements in the different fractions”

Ce 0.23 0.28

110 I15 145 33 40 36.5

Ce

Ce 0.14 0.20

124 60 138 20 27 23.5

Ce

Nd 0.23 0.45

48 57 65 15 29 22

Nd

Nd 0.16 0.20

51 23 61 9.5 12 10.8

Nd

Eu 0.24 0.44

1.45 1.6 1.8 0.44 0.8 0.62

8.7 9 10.5 2.9 4 3.45 Sm 0.28 0.38

Eu

Sm

Eu 0.12 0.18

1.57 0.87 1.9 0.23 0.34 0.29

7.6 3.6 7.5 1.4 1.2 1.3 Sm 0.19 0.16

Eu

Sm

Yb 0.29 0.24

I.1

1

3.2 3.6 4.2 1.2

yb

Yb 0.16 0.19

3.3 1.25 3.2 OS 0.6 0.55

Yb

Lu 0.15 0.27

0.6 0.7 1.1 0.17 0.3

Lu

Lu 0.18 0.14

0.54 0.17 0.44 0.08 0.06 0.07

Lo

Tb 0.21 0.33

1.7 5.2 1.6 1.5

1.4 0.7 1.2 0.25 0.4 0.33

Fe 0.31 0.29

1.ss

Fe(%)

Tb

Fe 0.25 0.28

4.9 2.6 5.3 1.3 1.5 1.4

3 3.6 3.2 0.7 0.5 0.6 Hf 0.22 0.16

Fe(%)

Hf

Cr 0.35 0.78

180 140 230 80 180 130

Cr

0.27 0.17

Cr

630 1030 365 100 63 81.5

Cr

Co 0.29 0.32

18.5 15.5 22 6.4 7 6.7

Co

Co 0.27 0.27

z 32 8.6 8.6 8.6

Co

Cs 0.29 0.36

J” 7 2 2.5 2.25

Cs

Cs 0.36 0.35

z.7 7.8 2.8 2.7 2.15

Cs

Rb 0.32 0.21

:z 190 60 40 50

Rb

Rb 0.31 0.31

187 117 163 50 50 50

Rb

Ta 0.21 0.27

1.2 1.5 1.1 0.23 0.3 0.27

Ta

1

1

Ca

1.75 1.5 1.3 1.3 1.3 1.3

Ca(%)

0.25 0.25

Ba 0.36 0.26

540 800 700 250 180 215

Ba

Ba

1050 600 950 235 235 235

Ba

Acknowledgment

Table 3 Content of elements adsorbed by diatoms in dissolved form from water (ppm)

u*

Fe (%)

Cr

Co

Cs

Rb

Ca(%)

Ba

Th

SC REE

0.48

0.3

56

2

I

11

1.3

55

0

0

0

* 23XU content in modem diatoms is equal to 0.4ppm.

where o is the content of clay in the biogenic fraction, f the content of given element in clay, u the content of the same element absorbed by the biogenic fraction from water, and F the total content of the same element in the biogenic fraction. Results of these estimates are given in Table 3. It is seen that the content of uranium absorbed from water is equal to -0.48 ppm. It is some 20 times smaller than that in clay, and approximately the same as that found in modern diatoms collected as live phytoplankton in Lake Baikal0.3ppm (see Table 3, and cf. Ref. 131). Hence. dissolved uranium is not absorbed by diatom algae frustules neither from the water body, nor from pore water during postdepositional diagenesis. Abso~tion of uranium by the sediment is either due to direct interaction with fine-grain clay, or due to binding by organic matter associated w-ith fine-grain clay. As for the entrapment of clay particles by diatom algae frustules, the same data suggest that it occurs after the deposition, rather than during the biological growth.

The present studies were done in part due to support of the INTAS-Russian Fundamental Research Foundation IN/ RU-950646. Authors are grateful to Professor KM. Gavschin for productive discussion and to Dr. G.I. Goldenberg for electron microscopic photographs.

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4. Conclusions

E.L.

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A.V.

Golubev

and

E.A.

SB Int. Conf., Baikal as a

natural laboratory for Global Change. Vol. 2 (Lisna Irkutsk.

Diatom algae frustules do not absorb dissolved uranium from waters of Lake Baikal. The presence of ur~ium and other trace elements in frustules is due to entrapment of small clay particles by their opal silica.

M.A.

Beresikov, Abstr. of INTAS-RAS

Publ..

1994) p. 13.

1111 E.L. Goldberg. A.F. Eromin. VV. Boldyrev. V.Ya. Gololobov and %I. Petrazhizki, Pat. N 1620161. SU. 1121 S.R. Taylor and S.M. McLennan, The continental crust: its composition and evolution (Mir. Moscow. 19881 p. 383.

VI.

ELEMENT

ANALYSIS