10Be and 9Be in marine sediments and their potential for dating

GeochimR'a et Cosmt~'himica Aoa Vol. 53. PP. 443-452 Copyright(D 1989PergamonPressplc. Printed in U.S.A.

0016-7037/89/$3.00 + .00

~°Be and 9Be in marine sediments and their potential for dating D. BOURLES, G. M. RAISBECKand F. YIOU Centre de Spectrom6trie Nucl6aire et de Spectrom6trie de Masse, Batiment 108, 91405 Campus Orsay, France (Received June 14, 1988; accepted in revised form October 31, 1988)

Abstract--Using sequential leaching experiments we have measured the fraction of the cosmogenic isotope m°B¢(halflife 1.5 My), and its stable isotope 9Be, co-extracted with the following phases in marine sediments: exchangeable, calcium carbonate, Fe and Mn oxyhydroxides, organic, opal and detrital. The extraction of the authigenic phases typicallyreleases ~70% of the I°Bebut only ~40% of the 9Be. In order to test its potential for dating marine sediments, we have measured the authigenically associated *°Be/gBein 17 surface sediment samples from different ocean basins. The observed spatial variability, which appears to be influenced by proximity to continental input of 9Be, precludes the general and precise application of leached m°Beff.Befor dating. However, the relatively constant value observed in surface sediments over large areas of the pacific, Indian aild Southern Atlantic oceans shows considerable promise for approximate dating of open ocean sediments. In order to test temporal variability, a profile of the authigenically associated =°lkffBe ratio has been measured iti the independently dated Central Pacific core, RC12-65. The observed variation in ~°Ikfflk over rtfiepast nine My is approximately twice that expected on the basis of l°Be decay alone. Several possible explanations for such behavior and their potential consequences for the use of leached m°Be/gBefor dating are discussed. INTRODUCTION

homogenization of ~°Be and 9Be in ocean water. On the other hand, the lower estimates would imply incomplete mixing of the two isotopes, thus complicating their use for dating

THE DEVELOPMENTof accelerator mass spectrometry (AMS) promises to open up new, or greatly expanded, applications ofcosmogenic isotopes (RAISBECKand YIOU, 1984). One of the potentially most exciting of these applications is the possibility of using the isotope ~°Be (half-life 1.5 My) for dating marine sediments in the time range of 0 to 15 My. A general discussion of the problems involved in using cosmogenic isotopes to date marine sediments is given in RAISBECK and YIOU (1984). Some earlier attempts to test the dating potential of ~oBe, using either classical decay Counting (TANAKA el al., 1977; TANAKA and INOUE, 1979) or AMS techniques (MANGIN! et al., 1984; K u et al., !985) have relied upon measuring a =°Be profile over the whole length of the sediment core involved, and then making assumptions regarding the constancy of sedimentation and/or ~oBe deposition rate over given segments of the core. Ideally one would like to be able to establish a ~°Be "age" on the basis of a single ~°Be measurement in the sediment of interest. We investigate here the possibility of doing this by measuring the l°Be/9Be ratio of leached extracts from the sediment. After formation in the atmosphere, I°Be is transferred to the ocean mainly by precipitation, probably in soluble form like 7Be (RAMA and ZUTSCHI, 1958). The stable isotope 9Be is transported from the continents to the ocean by rivers and wind. The distribution between soluble and insoluble phases is not well known. Estimates of the '°Be residence time in the ocean have ranged from ~--400 years tO ~ 4 6 0 0 years (RAISBECK et al., 1980; MANGINI et al., 1984; YOKOYAMA et all., 1978; KUSAKABEet al., 1982; KRISHNASWAMIet al., 1982). Similarly, estimates for 9Be range from 150 to ~ 4 0 0 0 years (MERRILL et al., 1960; MEASURESand EDMOND, 1982, 1983). The distinction between soluble and insoluble phases for the 9Be estimates has not always been made clear. The longer residence time estimates cited above, being greater than the ocean mixing time of --- 1000 years, would suggest a fairly effective

purposes. The l°Bc/gB¢ratioin authigenic Mn deports, extrapolated to their surfaces (SHARMA and SOMAYAJULU, 1982; K u et al., 1982; KRISHNASWAMIel al., 1982; INOUE et al., 1983; KUSAKABEand Ku, 1984), ~appear, within the fairly large uncertainties, to be reasonab!y similar to those estimated (MEASURES and EDMOND, i982, 1983) or directly measured (KuSAKABE et al., 1987) in deep ocean water. In addition, the l°Be/9Be ratio recorded in autfiigenic Mn deposits, averaged over periods of ~ 1 My, is reported to have remained constant to +6% over the past ~ 8 My (Ku et all, 1982). By contrast, it seems obvious that the undissolved 9Be input to the ocean cannot equilibrate with ~°Be (RAISm~CKand YIOU, 1984). Our goals were thus: (i) to see whether it was possible to extract an authigenic phase from marine sediments Which represents the ~°Be/9Be ratio of soluble Be in the deep ocean at the time of deposition; and (ii) to test the uniformity Of this ratio in both space and time. To do this we have first adopted sequential leaching procedures which allow us to isolate Be associated with the following phases of marine sediment: exchangeable, calcium carbonate, iron-manganese oxyhydroxides, organic, opal and detrific. The I°BeffBe ratio Co-extracted with the authigenic phases was then measured in a series of surface sediments from different ocean basins and, in an independently dated sediment core from the central Pacific, deposited during the last ,~9 My. A more detailed description of the work summarized here can be found in BOURLES (1988). EXPERIMENTAL PROCEDURES The experimental method was established after a critical evaluation of a number of sequential extraction procedures described in the literature (CHESTER and HUGHES, 1967; GUPTA and CHEN, 1975; TESSlER eta[., 1979;PICKERING, 1981; I~"EIFFEReta/., 1982;ROnmNS et al., 1984). The adopted procedure is based mainly on the procedure described by TESSlER eta/. (1979). The principal reasons for this 443

444

D. Bourles, G. M. Raisbeck and F. Yiou

choice were: (i) in their procedure the calcium carbonate fraction is extracted before dissolution of the other acid reducible phases. If not, the carbonate content, which can represent as much as 90% of marine sediment, can buffer the extractive solutions; (ii) all the dissolution steps are executed in an acidic medium. At pH > 5, beryllium tends to precipitate or to be strongly readsorbed on the walls of the container or on the remaining particles. The procedures adopted from TESSIERel aL (1979), are designed to isolate the Be associated with the following phases (the quantities are those necessary for ~ l g of dry sediment, our typical sample size). Phase I, exchangeable: agitation for one hour at room temperature in 8 ml of I M MgCl2, at pH 7. Phase II, calcium carbonate: agitation for five hours at room temperature in 8 ml of I M NaOAc adjusted to pH 5 with HOAc. Phase lII, iron and manganese oxyhydroxides: agitation for six hours at (96 -- 5)*C in 20 ml of 0.04 M, NH2OH. HCI in 25% (V/ V) HOAc. Phase IV, organic matter: agitation for two hours at (80 _+ 5)°C in 3 ml of 0.02 M HNO3 and 5 ml of 30% H:O: (pH 2 with HNO3). Add another 3 ml of H202 (pH 2 with HNO3) and further agitation at (80 -+ 5)0C for another three hours. After cooling, addition of 5 ml of 3.2 M NH4OAc in 20% (V/V) HNO3 to prevent readsorption of extracted Be onto the oxidized sediment. Phase V, residual: complete dissolution in HF and H2SO4. The only significant difference between these procedures and those described by T~SlER et al. (1979) is our less rigorous temperature control. In addition, we investigated the effects of combining, repeating or modifying the conditions of some of the steps. For each extraction, separation was carded out by centrifuging at 5,500 rpm for 15 minutes. The supernatant was removed with a pipet and the residue washed twice with 2 ml portions of the leaching solution, which were then added to the supernatant. The phase V ofTEssmR et al. (1979) includes both alumino-silicates and biogenic opal. During the latter stages of this work we thus specifically extracted opal from three different sediments, using a procedure adapted from EC,GIMAN et al. (1980). For these experiments the complete extraction procedure was carded out on ~0. i g portions of sediment. ARer step IV, the residual was agitated for four hours in 40 ml of 2 M Na2CO3 at (100 _+ 5)*C. After centrifugation the supernatent was removed with a pipette and the residue rinsed twice in 8 ml portions of 1% HO. These rinses, which were then added to the previous supernatant, were necessary to prevent readsorption of Be on the'iemaining particles because of the basic Na2CO3 leaching solution. The residue remaining after this treatment was then treated as in step V above. The purification of the Be from each of the leaching steps was done in the following way. The solution was evaporated to dryness, dissolved in 4 ml of 14 N HNO3 and 2 ml of 12 N HC104, and evaporated again to dryness. The residue was redissolved in 4 ml of 14 N HNO3 and 2 ml of 9 N HCI. This was evaporated to a volume of ~ 2 ml, and then diluted to 20 mi with deionized water. An aliquot of this solution was taken for analysis of 9Be (and, in some cases, 27A1). A known quantity (usually 0.5 rag) of 9Be carder was then added, and the Be purified by a series of (usually two) solvent extractionS of Be acetyleacetonate in the presence of EDTA. The purified Be was dissolved in a few drops of HNO3, dried in a quartz crucible and converted to BeO by heating to ~ 1000oc over a bunsen burner. 9Be and 2:AI measurements were made by flameless atomic absorption spectrometry (Perkin Elmer model 306 spectrometer, model HGA-500 furnace, model AS.I automatic sampler) using the method of standard additions and a deuterium arc backgrOund correction. Uncertainties in 9B¢ concentrations are based on the reproducibility of measurements when more than one has been made, or estimated from the fit of the standard additions line in the case of single measurements. The ~°Be measurements were made on the Gif-sur-Yvette Tandetron AMS facility, using the procedures described by RAISBECKet al. (1984, 1987). The ~°Be uncertainties have been estimated using a 7% instrumental uncertainty (RAlsBECKet al., 1984), together with one standard deviation statistics of the number of ]°Beevents counted. Locations of the samples studied are given in Table 1 and Fig. I.

Table 1. Sample loca~i~us and water depths ........................................................ Sample

Number

Latitude

Longitude

on map ........................................................

Depth (m)

RC12-62

1

06"44'N

141"22.5'W

RC12-66

2

02"36.6'N

148"12.8'W

5 073 4 755

RC12-65

3

04"39'N

144"58'W

4 868 5 316

RC13-22

4

00"10.1'S

171"14'W

RCI0-167

5

33"02'N

150"23*g

6 092

ND77-217

6

11"56.3'S

83"00'E

4 930 4 860

ND76-104

7

35"50.6'S

58"27.6'E

AET7606

8

3g'26.2'S

61"06.5tE

5 290

I~)84-527

9

49"49.3'S

51"19.1'E

3 262

2E l

I0

49"401S

70*E

VM29-105

11

48"05.0tS

17"41.0'W

41 4 350

RC13-269

12

52"37.6'S

00"07.5'W

2 591

RCII-78

13

50°52.0'S

09"52.0'W

3 115

DSDP68-502

14

11"29.42'N

79"22.78'W

3 051

V16-205

15

15"24'N

43"24'W

4 045

K 11

i6

71"47'N

01"36'E

2 900

KET 8004

17

39"40.01'N

13"34.00'E

2 909

RESULTS Distribution o f leached WBe a n d 9Be

An example of the distribution of 1°Be and 9Be associated with the phases extracted by the different steps described in the Experimental Procedures section is shown in Table 2. As suggested by TESSiER et al. (1979), and confirmed by the study of these two samples with very different calcium carbonate contents, steps I and II are not specific, the desorption process being strongly p H dependent. Thus, step II appears to contain a significant component of strongly adsorbed Be that is not removed with step I. In addition, step I was never found to remove more than 1% of the total t°Be, or 2.3% of the 9Be. For these reasons, in most of our work we removed both the adsorbed and calcium carbonate bound Be in a single step, called (I + II), using the step II leaching procedure. It can be seen in Table 2 that the l°Be/9Be ratio associated with phase V is two to four times lower than that associated with the other (authigenic) phases. Also, for these samples, from the Indian Ocean, the l°Be/gBe ratios associated with the phases (I + II) and III are similar to those found in authigenic M n deposits (see Introduction), and coral (BOURLES et al., 1984). These results suggest a largely authigenic origin for the Be extracted by steps (I + II) and III, and a largely detrital origin for the Be extracted during step V. The l°Be/gBe ratio associated with phase IV is always significantly higher than that associated with phase V, but generally lower than those from phases (I + II) or IIL This could be due to a measurement problem for the 9Be in phase IV (the measurement usually being clone near the detection limit) or to a slight attack of detritic components by the highly oxidizing leaching agents of step IV. However, the organic phase, the extraction of which is complicated and time consuming, was never observed to contain more than 8% o f the total m°Beor 9Be. Thus for many experiments we also extracted phases IV and V in a single step, designated (IV + V), using the procedure for phase V.

Be isotopes in I

I

marine

sediments

I

445

I

I

I

16

-.

60 ° ,/

40 ( 20' 0 20'

I

40

60'

120 °

180 °

~j °

,

13

7. 9

10

12

m

!

120°

SO°

0

60 °

FIG. l. Location o f samples studied.

KUSAKABE eta[. (1987) found that in certain regions of the ocean, the loBe/~Be ratio in surface water was as much as three times larger than in deep water at the same location. Ifa significant amount of Be is incorporated into or adsorbed onto hiogenic carbonate in surface waters, one might thus imagine to find t°Be/9Be ratios extracted with the carbonate fraction to be larger than those extracted with the Fe-Mn oxyhydroxides. The results for 13 surface sediment samples from different locations are shown in Table 3. The average ratio of the two phases, R, = (step I + II/step III) is quite variable (1.12 _ 0.28). However, much of this variability is caused by the two samples DSDP68-502 and 2Et. (Although we are not sure of its significance, we note that both of these samples come from areas close to land, and have large detrital components.) Excluding these two samples, R ffi 1.02 ± 0.18, which is not significantly different from unity, and where the dispersion is only slightly larger than the expected experimental variabifity of ~ 14%. This suggests that most of the Be released in step (I + II) is probably not incorporated or adsorbed in surface waters. One finds in Table 3 that steps (I + II) and III extract 73

_ 11% of the 1°Be, but only 41 _ 10% of the 9Be. This shows that, typically, approximately half of the 9Be is contained in insoluble detritic particles. For some samples we have also measured the 27A1 associated with each phase (BOURLES, 1988). The 27Alassociated with the authigenic phases ranges from 1.8 to 17.2% ofthe total, which is consistent with the upper limit given by MOORE and MILLWARD(1984) for non-dctrital Al. The ratio 27Al/ 9Be associated with the authigenic phases is three to ten times lower than in the total sediment. This suggests that Be is considerably more soluble than Al in the erosional cycle. Since the I°Be/gBe ratios extracted with phases (I + II) and III appear to be the same, we have studied the possibility of extracting the Be associated with these phases using the step Ill procedure directly. In seven samples for which the two procedures were carried out, the fraction of '°Be and, especially 9Be, extracted in the single leach were almost always smaller than the sum of the separate steps (Table 4). As a result, the ~°Be/~Be ratio in the single leach was on average (13 _ 4)% larger than with the sum of the separate leaches. By carrying out two successive hydroxylamine leaches, it

Table 2. Distribution of 10Be and 9Be associated wlth the e x t r a c t e d p h a s e s f o r two I n d i a n Ocean s u r f a c e samples ................................................................................... $/LtCI~E

Heasured

Phase I

Phase I I

parameter Exchangeable CaC03 .................................................................................... 9Ba(ng/g) MD76.104

9Be(ng/gCaCO~)

[5XCaCO 3

10Be(107at./gCaCO~)

10Be(107a¢./g) 10Be/9Be(10-10) -

<2,5

192119

Phase I I I Phase IV

Phase V

FeO-MuO

Organic Residual

737±37

ii0±II

941147

1280±128 1.6±0.2

66.6±5.2

363±26

46.0±3.3 82.8t6.3

444±35 >957

518165

736164

625178

131112

671134

183118

1012151

378128

53t4

173113

842±76

433154

256±23

.....................................................................................

9Be(rig/g) ~77.217

10ge(107a¢./g) 2gCaCO 3

52t5

9Be(ng/gCaCO~) IOBe(107aC./gCaCO~)

301±15

15 050±750 6.3t0.6

153112 7 650±600

10Be/9ge(10-10)~ 181125 760171 .....................................................................................

446

D. Boufles, G. M. Raisbeck and F. Yiou Table 3. lOBe, 9Be, 10Be/9Be r e s u l t s

f o r 13 s u r f a c e marine s e d i m e n t s

.............................................................................................................................. SAMPLE

Number

Phase ( I + I I )

Phase I I I

Phase (IV+V)

on map ....................................................................................................... 9Be lOBe lOBe/9Be 9Be 10Be 10Be/9Be (rig/g)

(I07at/8)

(10 -10)

(rig/g)

(lolat/g)

(10 -10)

B* 9Be

(rig/g)

10Be

10Be/9Be

(107at/g)

(10 -10 )

118±10

175±18

81±7

206±20

1.03±0.14

283±27

0.80±0.11

............................................................................................................................. RC12-62

I

170±8

123±10

l 082±102

583±29

333±25

RC12-66

2

51±5

46±3

1 348±159

82±8

67±5

RC13-22

4

142±10

103±8

1 084±112

528±26

372±27

1 053±92

MD77-217

6

353±20

160±13

678±67

671±34

378±28

842±76

1 195±69

226±17

)O76-104

7

192±12 68.2±5.4

128.8±9.8

183±17

0.72±0.10

MD84-527

9

124±9

388±35

1.18±0.16

6.5±0.9

7.0±1.0

1.57±0.24

97±7

239±21

0.97±0.12 0.80±0.13

89±6

854±77

I 007±55

1.10±0.19 587±29

531±54

737±77

363±26

736±65

1 051±58

689±72

224±11

87±6

58l±52

478±24

58.1±6.6

552±28

13.7±1.2

37.1±3.8

901±82

402±20

251±18

933±83

41±3

1.27±0.17

1 221±150

2E 1

10

265±19 10.3±0.9

VM29-105

11

179±9

RC13-269

12

56±6

27±2

721±91

110±8

66±5

897±91

293±15

82±6

418±37

RC11-78

13

137±7

74.6±5.7

814±75

144±7

75.8±5.7

777±71

218±15

54.3±5.0

372±43

1.05±0.14

DSDP68-502

14

75±8

12.1±1.0

241±31

365±18

35.0±2.7

143±13

I 181±62

16,8±1.4

21±2

1.68±0.26

V16-205

15

100±7

16±1

239±25

108±7

15±1

208±22

348±17

5.6±0.6

24±8

1.15±0.17

KII

16

80±8

7.4±1.0

138±21

420±21

32.4±2.2

115±I1

1 932±102

14.7±2.1

11±21

1.20±0.21

109±8

l 392±75 608±30

* R (10Ne/9Ne)(l+ll)

(10Be/9Be)llI was found that the roBe recovery could be increased by 10 to 50% compared to a single leach (Table 5). However, once again it was observed that the l°Be/gBe ratio of the second leach was significantly smaller than the first (Table 5). Use of higher concentrations of hydroxylamine hydrochloride did not change this situation significantly. Thus, despite the lack of any evidence for significant alumino-silicate dissolution during the leaching steps, as noted above, it appears that two steps of leaching may result in the partial attack of a nonauthigenic component ofgBe. This would be consistent with the observation mentioned above that 9Be appears to reside in more readily dissolvable phases than 27A1. In situations where one is more interested in recovering a "pure" authigenic ratio, rather than maximizing the recovery of 1°Be, it may thus be preferable to use a single hydroxylamine leach. While other workers have used much stronger leaching procedures, such as concentrated HCI, to extract '°Be from marine (GOEL et aL, t957) or lacrustine (BROWN et aL, 1985) sediments, the present work suggests that those techniques will extract a significant fraction of the non-authigenic 9Be. Table 4. Comparison between the single leach (phase (I+II+III)i)

and the combined leach

The fraction of '°Be associated with phases IV + V in 17 surface sediments studied varied from I0 to 50%, with the average being (27 _ 1 0%. Since the I°Be associated with phase IV (the organic phase) never exceeded 8% in the sediments where it was studied separately, it seems reasonable to conclude that most of this '°Be was associated with the detriticfraction,or with biogenic opal. One potential source of '°Be in the detriticmaterial could be in situ production at the Earth's surface (RAISBECK and YIOU, 1984). However, estimates of this, even assuming saturation concentrations (~ 107 atoms/g; NISHIIZUMI et al., 1986), suggest that such an orion could only represent a minor fraction of that observed. It thus appeared interestingto specificallystudy the l°Bc associated with the biogenic opal phase in some of these sediments. As can be seen in Table 6, a significantfraction of the l°Be in phases (IV + V) is indeed extracted with the opal. In particular,in sample RCI 1-78, from the South Atlantic ocean, located in a region of high opal productivity (BROECKER and PENG, 1982), 77% of the l°Be in phases (IV + V) is extracted with the opal, and the m°Bc•Bc ratio is similar to those measured for the plmses (I + II)and III.The remaining 1°Bc extracted with the dRrital phase could be a mixture of"in situ" production, 1°Be ions that have diffused

(phase (I+II) + phase III) for the extraction o f the authlgenlc phase ............................................. Phase (I+II+III) 1 Sample

Phase

Table 5. Comparison between two s u c c e s s i v e hydroxylamine l e a c h e s ...............................................................

(I+II) + Phase III

(Depth i n cm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Be 98e

Sample lOBe/9Be

.............................................

(Depth i n cm)

NH2ON,HCI

Phase (I+II+III)? Phase (I÷II+III)~

concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Be 10Be 10Be/9Be

...............................................................

KII (62)

0.86

0.71

1.21

RC12-65 (19)

0.04 M

0.31

0.22

0.72

RC14-14 (862)

0.93

0.74

1.25

RC12-65 (II0)

0.04 M

0.25

0.18

0.71

RC12-65 (121)

0.94

0.81

1.16

RC12-65 (262.5)

0.04 M

0.14

0.13

0.90

RC12-65 (19)

0.88

0.73

1.21

RCII-78 (10)

0.i M

0.18

0.12

0.67

RC12-65 (110)

0.95

1.02

0.93

RC11-78 (10)

I M

0.13

0.11

0.91

RC12-65 (262.5)

0.97

0.93

1.04

RC12-65 (1227.5)

0.47

0.35

0.74

RCII-78 (i0)

0.94

0.84

1.12

RC12-65 (1227.5)

0.70

0.49

0.70

0.04 M 1 M

Be isotopes in marine sediments Table 6. lOBe, 9Be and 10Be/9Be r e s u l t s ,

447

including opal, for 3 marine

ledlments ........................................................................................

Sample

Measured

Phase (I+II)

Phase I l l

Phase IV

Opal

Detrital

( d e p t h i n cm) parameter .......................................................................................... RCI1-78

9Be (rig/g)

<34

81±8

137±7

(10)

t0ee ( 1 0 7 a t / g )

74.6±5.7

137~7

74.8±5.7

144±7

5.9±0.7

41.7±3.4

6.7±0.9

10Be/9Be (10 - 1 0 )

814±75

777±71

>259

770±100

73±11

........................................................................................ RC12-65

9Be (rig/g)

176±18

812±57

89±6

56±6

776±39

(262.5)

10Be (107at/g)

47.1±5.0

201±15

16.2±1.7

13.9±2.9

40.8±4.4

lOBe/gBe (10 -10)

400±58

371±38

272±34

371±86

79±9

105±11

130±9

417±21

9.5±1.2

30±5

13±2

........................................................................................

AET7606

9Be (rig/g)

1 046¢52*

(Surface)

lOBe ( t 0 7 a t / g )

451±35"

lOBe/9Be (10 -10) 645±59* 135~21 345±64 47±6 ......................................................................................... * phase (I+II+III)

into detrital crystal lattices, and t°Be trapped during reformarion ofdetrital-like minerals in the ocean (MACK]N, 1986).

Spatial distribution of l°Be/gBe In Table 7 we show the J°Be/gBe ratio extracted with the authigenic phases (either (I + II) + ( I I I ) or (I + II + I I I ) ) for the different sediments studied. As can be seen, there are significant variations. The ratios for samples from the North Atlantic, Central Atlantic, Caribbean and Mediterranean Sea are ~ 5 to 18 times lower than those measured for the Central Pacific. The ratio in sample 2E~, taken from a bay in the Kergnelen Islands is ~ 15 times lower than those measured for other samples from this ocean. On a more modest scale the ratio from sample RC10-167, taken from a zone in the Pacific Ocean probably strongly influenced by continental

Table 7. lOBe, a u t h i g e n i c and t o t a l

10Be/9Be f o r s u r f a c e samples ............................................................... SAMPLE

A u t h i g e n i c 10Be/9Be

T o t a l lOBe

T o t a l lOBe/9Be

(10 -9 atoal/atola) (107 a t oms / g) (10 -9 atom/atom) .............................................................. RC12-62

90±8

574~45

RC12-66

127±13

159±13

RC12-65

93±3

RC13-22

106±9

(a)

275±26

49±5 91±9 (b)

63t2

556±42

66±6

RC10-167 75±7 193±14 ...............................................................

19±2

t4D77-217

78±7

764±58

• )76-104

69±7

560~41.2

42±4

AET7606

64±6

504±40

47±4

I~384-527

61±5

252±10

2E 1 4.4±0.5 30.5±3.0 ................................................................ VH29-105

93±8

~57±33

RC13-269

84±9

175±13

51±5

48±4 2.1±0.2 57±5 57±6

RCII-78 79±7 203.7±16.4 ..............................................................

61±6

DSDP68-502 16±2 63.9±5.1 .............................................................

5.9±0.6

36. 6±2. 6

9.8±0.9

Ell 12±1 54. 5±5. 3 .........................................................

V16-205

3.3±0.4

MET 8004 6.1±0.6 47. 9±3. 6 ..........................................................

2.9±0.3

(a) e x t r a p o l a t e d

22±2

t o s u r f a c e (see f i g u r e s 4 and 6)

(b) measured a t 10 cm

(a)

input from the Asian continent, is about 30% lower than ratios from the Central Pacific. The spatial distribution of'°Be/gBe suggests that this ratio is influenced by the proximity to 9Be input from the continents. This in turn suggests that the oceanic residence time of Be is less than or comparable to the mixing time of the oceans ( ~ 10 3 years). If there are short rime scale variations in soluble '°Be/9Be in the ocean, part of the variability seen in the sediments may also be due to the fact that the samples do not all correspond to the same time interval of deposition. Whatever the explanation, the observed variability precludes the general and straightforward application of authigenically associated ~°BePBe for dating marine sediments. However, if we restrict ourselves to the open ocean regions of the Pacific, Indian and South Atlantic, we find that all the measured values fall within the range (1.0 ± 0.4). 10-7. This uncertainty in an assumed initial ratio of '°Be/gBe would correspond to a time uncertainty of+7.105 years. An accuracy ofthis magnitude might be quite acceptable for sediments for which there were no alternate dating techniques available. If we restrict ourselves to more limited geographical areas, the spread in ~°Be/gBevalues appears to be even less. For example, the ratios in the Pacific, South Atlantic and Indian ocean samples of figure 1 are (10.2 + 1.8). 10-s, (8.5 -+ 0.7). 10 -s and (6.8 ± 0.7). 10 -s, respectively. For comparison with these results, we show also in Table 7 the '°Be concentrations and total J°BePBe ratios. For the 12 samples taken in the open Pacific, South Atlantic and Indian oceans, for which measurements are available for all these parameters, the average value, and one standard deviation variability, are: t°Be = 3.9.10 9 atoms/g ± 52%, '°Be/ 9Be total = 5.4.10 -8 ± 31% and l°BePBe authigenic: 8.5.10 -8 ± 22%. Thus, the use of authigenically associated '°Be/gBe for dating purposes results in a significant improvement compared to the use of t°Be concentration, and an apparently minor improvement compared to the use of the total I°Be/ 9Be ratio.

Temporal variation of l°BepBe To investigate the constancy of the authigenically associated l°Be/gBe ratio with time at one location, we have mea-

448

D. Bourles, G. M. Raisbeck and F. Yiou

sured it in 32 samples over the length of an independently dated core (RC12-65) from the Central Pacific. In Fig. 2 we show the authigenically associated l°Be and 9Be concentrations, and CaCO3 contents, as a function of depth in core RC12-65. The generally anticorrelated nature of the Be isotopes with CaCO3 demonstrates the diluent effect of the latter for Be, as noted previously by SOUTHON el (/~/. (1987). We believe, however, that the use of 9Be may be preferable to calcium carbonate-free weight as a normalizing parameter for '°Be. Not ouly does the use of 9Be compensate for the possible dilution effect of other components (such as biogenic opal), but it also helps take into account the variable scavenging efficiency of different sedimentary particles. In Fig. 3 we show a semi-logarithmic plot of authigenically associated J°Be/gBe as a function of depth in core RC12-65. While there is no reason that this curve should have a constant slope, it should be monotonically decreasing unless there have been changes in the soluble ~°Bc/gBe ratio in seawater at this location over time. Additional measurements will thus be

10-;

o

E o ¢1 10-;

c~ o

m

.v_

g

J¢ 10-9

1 0 " IO 109,

1()00 Oepth

_o

in

2(~)0 core

(cm)

FIG. 3. Authigenic ~°Be/gBeas a function of depth for RC12-65.

o

.,=

lO~

o 7-- 5 0 0

£ o.

0~

25.

0

16oo Depth in core

2~o (cm)

FIG. 2. '°Be and 9Be extracted with authigenic phases and calcium carbonate as function of depth for sediment core RC12-65. Calcium carbonate date are from HAYSel al. (1969).

interesting to investigate the significance of the apparent deviations from a monotonic decrease at depths of 89 cm and 1565 cm. One can also note a general change in slope at 1000 cm, which corresponds to a change in sedimentation rate at this location, as discussed below. The core RC12-65 has previously been analysed by magnetostratigraphy (FOSTER and OPDYKE, 1970; OPDYKE el al., 1974). In Fig. 4 we show our adopted paleomagnetic age versus depth correlation (BOURLES, 1988) based on the recent work of MILLER et al. 0985) and BERGGREN et al. (1984). In Fig. 5 we show a semi-log plot ofauthigenically associated l°Be/~B¢ as a function of paleomagnetic age, adopting the paleomagnetic correlation scheme of MILLER et al. (1985). In Table 8, we show linear regression fit parameters for the complete set of measurements, and for those corresponding to time periods of 0-5.8 My and 5.8-8.9 My, separately. (The values in Tables 8 and 9 differ from those in BOURLES (1988) which contain a calculational error.) The linear regression of the complete data set implies a decrease in I°Be/ 9Bewith an apparent half-fife of 1.31 ± 0.02 My. This appears inconsistent with the most recent estimate of the '°Be halflife of 1.51 + 0.06 My (HOFMANN et al., 1987). In addition, the standard deviation of the data around the regression line (15.2%) is somewhat larger than the estimated experimental uncertainties in the l°Be/gBe measurements ( ~ 10%). This suggests that there may be other sources of variation. Part of this nonexperimental variation could be due to uncertainties in the paleomagnetic ages. While the correla-

Be isotopes in marine sediments

449

a

10~ 20

E o m o m

A15. E

• lO-e" m

# o

m

.2 ¢ c

.c 10, o.

10-:

a

/

5. @ a

1 0 -'a.

o

~

;

A

Palaomagnetic

8 ( My )

age

O 0

10

5 Paleomagnetic

age

(My)

FIG. 4. Adopted paleomagnetic age v e r s u s depth correlation for RC12-65. Each point represents an observed reversal level (FOSTER and OPDYKE, 1970) and the adopted age (BERC,GREN et al., 1984).

tions and absolute age estimates are believed to be fairly reliable down to 5.4 My, beyond this point the paleomagnetic record of RC 12-65 of FOSTER and OPDYKE (1970) does not correspond uniquely to the paleomagnetic time scale of BERGGREN et al. (1984). We were thus forced to somewhat arbitrarily assume certain correlations (BOURLES, 1988). On the other hand, we note that, assuming the time scale adopted in Fig. 5, there is a significant change in the sedimentation rate ofRC12-65, from ~ 4 m m / k y before 5.8 My ( ~ 1000 cm) to ~ 1.7 m m / k y after 5.8 My. Assuming a bioturbation zone of 10 cm, this implies a time resolution for the samples of ~ 2 5 , 0 0 0 years before 5.8 My and ~ 6 0 , 0 0 0 years since 5.8 My. Thus a part of the larger variability observed in the lower part of the core may also be due to relatively short-term ( ~ 104 year) changes in the ~°Be/gBe ratio in the ocean. For both of the above reasons, we decided to analyse the data in two portions corresponding to 0-5.8 My and 5.8-8.9 My (Table 8). We find from Table 8 that the linear regression from 0 5.8 My corresponds to an effective half-life of '°Be/9Be of 1.26 -+ 0.02 My. The standard deviation of the 18 data points around this regression line is 8.9%, which is comparable to the estimated experimental uncertainty. For the portion of the data corresponding to 5.8-8.9 My, the slope corresponds to an effective ~°Be/gBe half-life of 1.56 _+ 0.19 My. However, the distribution of the data around the regression line has a standard deviation of 18.5%. This is

FIG. 5. Authigenic '°Bc/gB¢ as a function of palcomagnetic age for

RC12-65. The solid line is the best fit regression line, conesponding to an "effective" half-life of 1.31 My. Dashed line indicates expected decay, assuming '°Be half-life of 1.5 My.

significantly higher than the estimated experimental uncertainties, corroborating that there are other sources of variation in this part of the core. In summary then, while the data before 5.8 My appear to have sources of variation other than measurement uncertainties, the data over the whole core suggest a more rapid decrease in '°Be/gBe with time than would be expected from a 1.51 My half-life of ]°Be. What is more surprising is that the deviation from the expected decay appears to be quasilinear with increasing age of Sediment. This can be seen in Table 8. Linear r e g r e s s l o n f i t s

o f t h e data i n F I 8 . 5

to the expression: in(10Be/98e)(authlgenlc) (t

Age

range

- In A - 0.693

t/T

" palemnagnettc age)

Number o f

A

T

(xtO -8 )

(My)

s.d.

data p o £ n t s

(My)

(Z)

.................................................... 0-8.9

31

9.19±0.45

1.31±0.02

15.2

0-8.9

31

6.76

1.5"*

25;9

0-5.8

18

9.56±0.30

1.26±0.02

8.9

0-5.8

18

7.78

1.5**

20.5

5.8-8.9

14

5.30±2.00

1.56±0.19

18.5

1.5"*

18.6

5.8-8.9 14 5.54 .................................................. *

: standard deviation line

**

:

imposed

o f t h e d a t a about r e g r e s s i o n

450

D. Bourles, G. M. Raisbeck and F. Yiou Table 9. Linear regression fits of the data in Flg.7 to the expression

in(10Be/9Be)(total) = in A - 0.693 t/T (t - paleomagnetic

age)

...................................................

Age

Number of

range

A

T

s,d.

data points

(Hy)

(x10 -8)

(Hy)

(z)

................................................... 0-8.9

31

6.36±0.44

1.43±0.03

16.4

0-8.9

3'

5.64

1.5"*

I7.4

0-5.8

12

0-5.8

12

5.98

1.5"*

9.8

5.8-8.9

20

5.77±1.81

1.50±0.27

19.5

5.8-8.9

20

5.41

1.5"*

19.5

6.50~0.27 1.40±0.04

8.0

................................................... *

: standard deviation

of the data about

regression l i n e ** : £mposed

Fig. 6 which shows paleomagnetic and '°Be/9Be ages, with the latter calculated using the zero time intercept (9.19 4- 0.45). 10-s of Fig. 5 as the initial '°Be/gBe value for each of the measured data points.The differencebetween the ,oBe/ 9Be and palcomagnetic ages is approximately 1.5 M y by 9 M y ago. There are severalpossibleexplanations for such behavior. The most obvious of course, is that the '°Be half-lifemight be wrong. Until recently the uncertainty in this parameter would indeed have made such an explanation quite reasonable.However, ifthe estimated uncertaintyin the most recent measurement of this half-lifeis reliable( H O F M A N N el a[., 1987), then thiswould seem to rule out such an explanation. Another possible explanation is that the production rate of ~°Bc has gradually increased by a factorof ~ 2 in the past 9 My, due eitherto an increasedprimary cosmic ray intensity, or to a decreased solar modulation or average geomagnetic field intensity. While there is relatively little direct information about such possibilities, what evidence there is does not favour such dramatic changes over this time period (REEDY eta[., 1983; MCFADDEN and MCELHINNY, 1982). A third possible explanation involves a decreasing soluble 9Be input, either at this particular location, or in the ocean as a whole, over the last 9 My. For example, during the past 9 My, movement of the Pacific plate has resulted in the sediment at the location of RC12-65 moving ~ 7 0 0 km northwest relative to the continents. However, the data for other Pacific surface sediments presented earlier would seem to argue against a change in location of this magnitude as being sufficient to change the '°Be/gBe ratio by a factor of 2. A general decrease in 9Be input to the ocean could arise from decreased erosional input or a decreased hydrothermal contribution. We know of no evidence for or against either of these possibilities. While the 9Be content of hydrothermal solutions can be up to 103 times that of seawater (MEASURES and EDMOND, 1983) the influence of such input on sediment composition does not seem to be simply correlated with distance from the hydrothermal sources (LEINEN, 1987). A final possible explanation is that there has been a gradual diagenesis of the sediment with depth (or time), resulting in a progressive exchange between the authigenitieally extractible Be and phases (IV + V). One argument in favour of this is the decreasing difference between the authigenically asso-

ciated and total ~°Be/9Be ratios with depth (see below). However, an increasing opal content in the lower pan of the core might also account for such a tendency. It is fairly obvious that with the present amount of data it is very difficult to distinguish between the possible explanations mentioned above. Further progress will depend upon making similar measurements in a number of other cores. The origin of the apparent discrepancy between the authigenic ~oBe/gBe and paleomagnetic ages will in turn determine the applicability of using the authigenically associated ,oBe/9Be from sediments for dating purposes. If, for example, the leached ~°Be/gBe is really representative of the soluble ]°Be/ 9Be in that part of the ocean at the time of deposition, then the fact that it does not give a correct absolute date would not preclude it being used for estimating ages. One could readily imagine "calibrating" the '°Be/9Be ratio with independently dated cores (in the same way, for example, that variations in the atmospheric '4C/'2C ratio are corrected for by making measurements in dendrochronologically dated tree tings). The practicality of such a procedure would of course depend on the geographical area over which such a calibration was reliable. If, on the other hand, the problem is due to diagenesis, it is possible that the total '°Be/gBe ratio will be more useful than the leached '°Be/gBe for dating. We can thus try to compare the present results with those for total '°Be/gBe in the same core. In Fig. 7 we show an updated version of some total ~°Be/gBe results presented earlier (RAISBECK, 1985), using the same time scale as Fig. 5. A statistical analysis similar to that of Table 8 is shown in Table 9. Once again there is evidence for '°Be/9Be fluctuations larger than expected for experimental uncertainty alone in the 5.8-8.9 My portion of the core. However, for the 0-5.8 My portion of the core, where the observed data distribution is comparable to the experimental uncertainties, the best fit effective half-life for '°Be/9Be is 1.40 4- 0.04 My. Within uncertainties, this is rea-

/

/

v ~10

J

E

"

.~ ' ~ / ~ / .J"~-/-~'. .

. . . . .

0

,

0

5 (~°Be/9Be)

age

,

,

,

10 (My)

FIG. 6. Calculated ages from authigenic l°B~/9Be,using half-life of 1.5 My, compared to paleomagnetic ages.

Be isotopes in marine sediments

451

residence time of soluble beryllium is lower than, or comparable to, the mixing time of the oceans. The observed spatial variability of authigenically associated ~°Be/9Be in surface sediments precludes its general and straightforward use for dating marine sediments. However, the relatively constant value observed in sediments over large areas of the Pacific, Indian and Southern Atlantic oceans holds promise for approximate dating of open ocean sediments. A profile of the authigenically associated ~°Be/gBe ratio in the Central Pacific core RC12-65 shows an approximately factor of two discrepancy between the observed decrease during the past 9 My and that expected on the basis of :°Be decay. Depending on the origin of this discrepancy, the resolution of which will necessitate similar measurements in other cores, the authigenically associated t°Be/9Be ratio may or may not be a better parameter than total :°Be/9Be for dating marine sediments in the time range 0-15 My.

I0 "7.

A E

2m E o

"~1o-S.

o

10 -q

10-~( 0 Paleomagrtetic

age

(My)

FIG. 7. Total ,oBe/gBeas a function of paleomagnctic age for RC1265. Dashed line indicates expected decay, assuming a l°Be half-life

of 1.5 My and solid line is best fit regression line, corresponding to "effective" half-lifeof 1.43 My.

Acknowledgements--We would like to thank J. Lestringuezand D. Deboflle for help with the chemistry and I°Bemeasurements and D. de Zertucha, A. Castera, J. Dudouit, and M. Arnold for technical assistance with the Tandetron. We also thank J. Lestringuez for assistancewith the statisticalanalysis,and D. Debofllefor artwork. We are very grateful to L. Burkle, J. C. Duplessy, D. Kent, L. Labeyrie, L. Leclaire, and M. Paterne for suggestingand/or providing samples. We are especially indebted to D. Kent for facilitating sample procurement from the Lamont-Doherty Geological Observatory core collection,for providingbibliographicmaterial regardingthese cores, and for advice regardinginterpretation of the imleomagneticstratigraphy of core RC12-65. Core eUration and sampling at LDGO is supported by the NSF (OCE81-00232) and ONR (N00014-84-00132). Tandetron operation is supported by the CNRS, CEA and IN2P3. Editorial handling: J. D. Maedougall

sonably consistent with the estimated ~°Be half-life of 1.51 __.0.06 My. Indeed, with an imposed half-life of 1.5 My, the standard deviation is only marginally larger than the best fit (Table 9), and quite consistent with the experimental uncertainties. If this agreement is not just by chance, it would seem to suggest that, contrary to our initial expectations, and to the surface sediment results, total l°Be/9Be measurements may give a more accurate estimate of sediment age than do the authigenically associated ones. Further testing of this possibility will of course necessitate similar comparisons in a number of other cores. CONCLUSION We have reported here the first results on the distribution of cosmogenic :°Be and its stable isotope 9Be associated with the following phases in marine sediments: exchangeable plus calcium carbonates, iron and manganese oxyhydroxides, organic, opal, detrital.Sequential leaching procedures Ixrmited us to isolate from sediments authigenic phases which contain ~70% of the :°Be but only ~40% of the 9Be. The spatial distribution of the authigenically associated ]°Be/gBe ratio in surface sediments from different locations shows that this parameter is influenced by the proximity to continental input of 9Be to the oceans. This in turn suggests that the oceanic

REFERENCES BERGGREN W . A., KENT D . V., FLYNN J. J. a n d VAN COUVERING

J. A. (1985) Cenozo'fcgeochronology. Geol. Soc. Amer. Bull. 59, 131-145. BOURLESD. (1988) Etude de la gb~chimiede l'isotope cosmo#mique ~°Beet de son isotope stable 9Be en milieu oc~anique.Application la datation des s&timentsmatins. Ph.D. thesis, Univ. Paris XI. BOURLES D., RAISBECK G . M., YIOU F., LOISEAUX J. M., LIEUVIN

M., KLEINJ. and MIDDLETONR. (1984) Investigationof the possibleassociationof*°Beand ~Al with biogenicmatter in the marine environment. Nut. Instr. and Meth. B5, 365-370. BROECKERW. S. and PENGT. H. (1982) Tracers in theSea. LamontDoherty GeologicalObservatory. BROWNT. A., NELSOND. E., SOUTHONJ. R. and VOGELJ. S. (1985) The extraction of ~°Befrom lake sediments leaching versus total dissolution. Chem. Geol. 52, 375-378. CHESTERR. and HUGHESM. J. (1967) A chemical technique for the separation of ferro-manganese minerals, carbonate minerals and adsorbed trace elements from pelagic sediments. Chem. Geol. 2, 249-262. EC~]MANN D. W., MANHE]MF. T. and BETZERP. R. (1980) Dissolution and analysis of amorphous silica in marine sediments. J. Sediment. Petrol. 50, 215-225. FOSTERJ. H. and OPDYKEN. D. (1970) Upper Miocene to Recent magnetic stratigraphy in deep-sea sediments. J. Geophys. Res. 75, 4465-4473. GOELP. S., KHARKARD. P., LALD., NARASAPPAYAN., PETERSB. and YATIRAJAMV. (1957) Beryllium,10 concentration in deepsea sediments. Deep-Sea Res. 4, 202-210.

452

D. Bourles, G. M. Raisbeck and F. Yiou

GUPTA S. K. and CHEN K. Y. (1975) Partitioning of trace metals in selective chemical fractions of near shore sediments. Environ. Lett. 10, 129-158. HAYS J. D., SAITO T., OPDYKE N. D. and BURCKLE L. H. (1969) Pliocene-Pleistocene sediments of the equatorial Pacific. Their paleomagnetic, biostratigraphic and climatic record. Geol. Soc. Amer. Bull. go, 1481-1514. HOFMANN H. J., BEERJ., BONANIG., VON GUNTEN H. R., RAMAN S., SUTER M., WALKER R. L., WOLFLI W. and ZIMMERMANND. (1987) robe: Half-life and A.M.S. standards. NucL Instr. andMeth. B29, 32-36. INOUE T., HUANG g. Y., IMAMURA M., TANAKA S. and USut A. (1983) ~°Beand ~°Be/gBein manganese modules. Geochim. J. 17, 307-312. KRISHNASWAMI S., MANGINI A., THOMASS. H., SHARMAP., COCHRAN J. K., TUREKIAN K. K. and PARKER P. D. (1982) ~°Be and Th isotopes in manganese nodules and adjacent sediments: Nodule growth histories and nuclide behaviour. Earth Planet. Sci. Lett.

59, 217-234. Ku T. L., KUSAKABEM., NELSON D. E., SOUTHONJ. R., KORTELING R. G., VOGEL J. and NOVlKOW I. (1982) Constancy of oceanic deposition of ~°Be as recorded in manganese crusts. Nature 299, 240-242. Ku T. L., SOUTHON J. R., VOGEL J. S., LIANG Z. C., KUSAKABEK. and NELSON D. E. (1985) tuBe distributions in Deep-Sea Drilling Project site 578 sediments studied by accelerator mass spectrometry. Init. Repts. DSDP, Vol. LXXXVI, 539-546. KUSAKABE M. and Ku T. L. (1984) Incorporation of Be isotopes and other trace metals into marine ferromanganese deposits. Geochim. Cosmochim. Acta 48, 2187-2193. KUSAKABE M., KU T. L., VOGEL J. S., SOUTHON J. R., NELSON D. E. and RICHARDS G. (1982). '°Be profiles in sea water. Nature 299, 712-714. KUSAKABEM., KU T. L., SOUTHON J. R., VOGEL J., NELSON D. E., MEASURES C. I. and NOZAKI Y. (1987) Distribution of *°Be and 9Be in the Pacific Ocean. Earth Planet. Sci. Lett. 82, 231-240. LEINEN M. (1987) The origin of paleocbemical signatures in North Pacific pelagic clays: Partitioning experiments. Geochim Cosmochim. Acta 51, 305-319. MACKIN J. E. 0986) Control of dissolved A1 distributions in marine sediments by clay reconstitution reactions: Experimental evidence leading to a unified theory. Geochim. Cosmochim. Acta 50, 207214. MCFADDEN P. L. and MCELHINNY M. W. (1982) Variations in the geomagnetic dipole 2: Statistical analysis of VDMs for the past 5 million years. J. Geomag. Geoelectr. 34, 163-189. MANGINI A., SEGAL M., BONANI G., HOFMANN H. J., MORENZONI E., NESSI M., SUTER M., WOLFL! W. and TUREKIAN K. K. (1984) Mass spectrometric I°Be dating of deep-sea sediments applying the Ziirich tandem accelerator. Nucl. Instr. and Meth. BS, 353-358. MEASURES C. I. and EDMOND J. M. (1982) Beryllium in the water column of the central North Pacific. Nature 297, 51-53. MEASURES C. I. and EDMOND J. M. (1983) The geochemical cycle of 9Be: A reconnaissance. Earth Planet. Sci. Lett. 66, 101-110. MERRILL J. R., LYDEN E. F. X., HONDA M. and ARNOLD J. R. (1960) The sedimentary geochemistry of the beryllium isotopes. Geochim. Cosmochim. Acta 18, 108-129. MILLER K. G., AUBRY M. P., KHAN M. J., MELILO A. J., KENT D. V. and BERGGREN W. A. (1985) Oligocene-Miocene biostratigraphy, magnetostratigraphy and isotopic stratigraphy at the western North Atlantic. Geology 13, 257-27 I.

MOORE R. M. and MILLWARD G. E. (1984) Dissolved-particulate interactions of aluminium in ocean water. Geochim. Cosmochim. Acta 48, 235-241. NISHIIZUMI K., LAL D., KLEIN J., MIDDLETON R. and ARNOLD J. R. (1986) Production of ~0Beand 26A1by cosmic rays in terrestrial quartz in situ and implications for erosion rates. Nature 319, 134136. OPDYKE N. D., BURCKLE L. H. and TODD A. (1974) The extension of the magnetic time scale in sediments of the central Pacific Ocean. Earth Planet. Sci. Lett. 22, 300-306. PFEtFFER G., FORSTNER U. and STOFFERS P. (1982) Speciation of reducible metal compounds in pelagic sediments by chemical extraction. Senckenbergiana Marit 14, 23-28. PICKER]NG W. F. (1981) Selective chemical extraction of soil components and bound metal species. C R . C CriticalRev. Anal. Chem., 233-266. RAMA R. and ZUTSCm P. K. (1958) Annual deposition of cosmic ray produced ~Be at equatorial latitudes. Tellus 10, 99-103. RAISBECK G. M. (1985) Cosmogenic Isotopes. Proc. 19th Internatl. Cosmic Ray Conference, Vol, 9, 73-85. RAISBECK G. M. and YlOU F. (1984) Production of long-lived cosmogenic nuclei and their applications. Nucl. Instr. and Meth. BS, 91-99. RAISBECKG. M., YIOU F., FRUNEAU M., LOISEAUXJ. M., LIEUVlN M., RAVELJ. C., REYSS J. M, and GUICHARD F. (1980) ~°Beconcentration and residence time in the deep ocean. Earth Planet. Sci. Lett. 54, 275-278. RAISBECK G. M., YIOU F., BOURLES D., LESTRINGUEZJ. and DEBOFFLE D. (1984) Measurement of ~°Be with a Tandetron accelerator operating at 2 MV. NucL Instr. andMeth. B$, 175-178. RAISBECK G. M., YIOU F., BOURLES D., LESTRINGUEZJ. and DEBOEFLED. (1987) Measurement of *°Beand 26A1with a Tandctron A.M.S. facility. Nucl. Instr. and Meth. B29, 22-26. REEDY R. C., ARNOLD J. R. and LAL D. (1983) Cosmic ray record in solar system matter. Science 219, 127-135. ROBmNS J. M., LYLE M. and HEATH G. R. (1984) A sequential extraction procedure for partitioning elements among co-existing phases in marine sediments. College of Oceanography, Oregon State University, Report Ref. 84-3. SHARMA P. and SOMAYAJULUB. L. K. (1982) ~°Be dating of large manganese nodules from world oceans. Earth Planet. Sci. Lett. 59, 235-244. SOUTHON J. R., KU T. L., NELSON D. E., REYSS J. L., DUPLESSY J. C. and VOGEL J. S. (1987) J°Be in a deep-sea core: Implications regarding I°Be production changes over the past 420 ka. Earth Planet. Sci. Lett. 85, 356-364. TANAKA S. and INOUE I. (1979) J°Bedating of North Pacific sediment cores up to 2.5 million years B.P. Earth Planet. Sci. Lett. 45, 181187. TANAKA S., 1NOUEI. and IMAMURAM. (1977) The t°Be method of dating marine sediments---Comparison with the paleo-magnetic method. Earth Planet. Sci. Lett. 37, 55-60. TESSmR A., CAMPaELL P. G. C. and BISSON M. (1979) Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51, 844-851. YOKOYAMA Y., GU1CHARDF., REYSS J. L. and NGUYEN HUU VAN (1978) Oceanic residence times of dissolved beryllium and aluminium deduced from cosmogenic tracers ~°Be and 26A1. Science 201, 1016-1017.