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Concentration, vertical flux, and remineralization of particulate uranium in seawater
0016-7037/82/071293-07SO3.00/0
Acre Vol. 46, pp. 1293 to 1299 Ceochmrico et Cosmochimica 0 Pergamon Press Ltd. 1982. Printed in U.S.A.
Concentration, vertical flux, and remineralization of particulate uranium in seawater ROBERT F. ANDERSON* Department of Chemistry, Woods (Received
Hole Oceanographic
September
Institution,
Woods
Hole, Massachusetts
02543
3, 1981; accepted in revised form March 4, 1982)
Abstract-Concentrations and fluxes of particulate U were measured throughout the water column at several locations in the Atlantic and Pacific Oceans with in situ filtration systems and sediment traps. The results indicate that dissolved U is fixed to particles in surface seawater. Organic matter appears to be the carrier phase. Formation of particulate authigenic U below the surface waters could not be detected. Authigenic U is remineralized within the bathypelagic layers at the open ocean sites studied. In the Panama Basin, an upwelling area with high biological productivity, remineralization of authigenic U in the deep water column was not observed. The rate of remineralization of authigenic U in the deep sea is insufficient to produce a measurable concentration gradient between surface and deep waters within the mixing time of the oceans. Formation of authigenic U in the water column in areas such as the Panama Basin is not a significant sink for U on an ocean wide basis. INTRODUCIION THE major uranium
features
of the marine
geochemistry
of
are fairly well known.
Uranium is very soluble in oxygenated seawater as a result of the formation of stable uranyl-carbonate complexes (Starik and Kolyadnin, 1957; Langmuir, 1978), with a residence time in the oceans of 2-5 X 10’ years (Turekian and Chan, 1971; Sackett et al., 1973; Ku et al., 1977). Seawater is undersaturated with respect to precipitation of uranium minerals (Naumov et al., 1963; see also Langmuir, 1978). Therefore removal processes other than precipitation of inorganic uranium salts must be responsible for maintaining the concentration of uranium in seawater. Hemipelagic, organic matter-rich sediments contain authigenic uranium derived from seawater (Veeh, 1967; Veeh et al., 1974), and in some cases, uranium concentrations correlate with the organic matter content of the sediments (Baturin, 1969; Baturin et al., 1971; MO et al., 1973; Kolodny and Kaplan, 1973; Sackett et al., 1973). Soluble U(VI) may diffuse from oxygenated bottom waters into sediments where it then may be precipitated by reduction to insoluble U( IV) if the Eh is sufficiently low. Low Eh conditions are often produced by anaerobic decomposition of organic matter just below the sediment-seawater interface in sediments with high organic matter contents. This mechanism was suggested as the cause of enhanced uranium contents of sediments in the Eastern Tropical North Pacific (Bonatti et al., 197 1) and in West African shelf sediments (Veeh et al., 1974). Several investigators prefer an alternative explanation: that the high uranium contents frequently found in organic matter-rich anoxic sediments result
* Present Address: Lamont-Doherty atory, Palisades, New York 10964
Geological
Observ-
from the adsorption or complexation of U(VI) by organic matter (Kochenov et al., 1965; Baturin, 1968, 1969; Baturin et al., 1971; Kolodny and Kaplan, 1973; Degens et al., 1977). Uranium-organic matter associations would then be preserved under reducing conditions either as a result of enhanced preservation of organic matter, which retains complexed U(VI), or as a result of reduction of U(V1) to U(IV) after burial with subsequent adsorption of the reduced uranium to the sediments. Concentrations and vertical fluxes of particulate uranium have been measured at several sites in the oceans with in situ filtration systems and sediment traps to study the formation of particulate authigenic uranium in seawater. Particulate authigenic uranium is formed in surface seawater, apparently associated with organic matter. In the open ocean, most of the particulate authigenic uranium is remineralized within the water column. In contrast, remineralization of particulate authigenic uranium within the water column could not be detected at an ocean margin site. SAMPLE
COLLECTION, INSTRUMENTS, AND METHODS
Sediment trap samples were obtained as part of the PARFLUX Program (Honjo, 1978, 1980; Spencer et al., 1978; Brewer et al., 1980). Descriptions of the sediment traps, the moorings to which they were attached, and sample handling procedures can be found in the above references. Samples of particulate matter were also collected by in situ filtration onto l.O-micron pore size, 293-mm diameter Nuclepore filters with pumping systems described elsewhere (Spencer and Sachs, 1970; Krishnaswami et al., 1976). Sample locations are shown in Fig. 1. Analytical procedures for the radioisotopes are described by Anderson and Fleer (1982). Briefly, the method involved complete dissolution of the samples with HCI-HNO,-HF-HC104 in the presence of isotope yield monitors (*‘% and 229Th). Uranium and thorium were purified by ion exchange (Dowex AG l-X8, Bio Rad Laboratories) and electroplated to provide thin sources suitable for alpha spectrometry. 1293
1294
R. F. ANDERSON
a
\“., i
’ \
’ *.,,HAWAII l
-,.
J; &$., _,’ J
STATION
1
I
180”
150”
’
* BERMUDA
. a- .- \, .
\
- --t+BARBADOS
lSTATION
I
I
120°
9o”
l
I
,
/
-----.I_.
30*
60”
I?0
-I 30” _..._”
FIG. 1. Locations of PARFLUX sediment trap Sites E and P, the Sediment Trap Intercomp,rrisi,r, Experiment (STIE) Site, and station locations occupied for in sifu filtration of large volumes of sea\cnte! (KN73-16). Station I I IO and the STIE site are at the same location.
RESULTS Results of the analysis of sediment trap samples are presented in Table 1. The size fraction greater than I mm is routinely removed for separate studies prior to chemical analysis (Honjo, 1980; Brewer et al., 1980). Only for the shallowest sample at each site did that fraction constitute more than a few per cent of the total sample (Honjo, 1980; Honjo, personal communication). Samples from Site E, for which results are presented in Table 1, were analyzed separately by Brewer et al. (I 980). Their uranium values were consistently about 20 percent higher than those reported here. No reasons can be found for the disagreement. Analysis of standard rock material by the method used to obtain the results in Table 1 produced results equal, within counting errors. to published values (Anderson and Fleer, 1982). The total uranium measured in the samples consists of two parts: uranium present in detrital silicate minerals (U,,) and bio-authigenic uranium (U,,) derived from seawater. Detrital uranium is estimated as Uu = (13’U/Z’zTh),l
x (‘12Th ),,
where (2JaU/*32Th)L, is the detrital U/Th ratio and (z’2Th)M is the measured content of “‘Th in the sediment trap samples The strong correlation of 232Th with other detrital elements in sediment trap samples indicates that it may be considered to be exclusively present in detrital minerals (Brewer ef al., 1980; Anderson, 1981). At Site E, “‘Th/AI and “‘Th/K ratios in surface sediments were close to the ratios in sediment trap samples and to ratios reported for atmospheric dust from a nearby site (Anderson, 1981). Detrital minerals are rapidly transported from the sea surface to the sea floor (Deuser et a/., 1981; Deuser, personal communication), so it is not unexpected that sediment trap samples should have ratios of predominantly detrital elements that are similar to ratios in underlying surface sediments It is assumed then that the (2’*U/232Th)o ratios are also uniform throughout the water column and in the sediments at each site. Nearly all of the biogenic components have been remineralized from the sediments at both sites,
as evidenced by a lack of CO2 evolution upon acidrticatron of the sediments and by the very low concentrations o! organic matter (0.3 percent and 0.2 percent at Sites E and P respectively) and opal (I. I percent and 0.2 percent at Sites E and P respectively) in the sediments (Honjo PI a: in preparation). Therefore, the 13HU/2i2Th ratio in sedi. ments was used as the best available estimate oi (“‘II, 232Th)o at Sites E and P (Table I ). Sediment samples were not available from the STIE Site, so the detrital “‘“U/“‘Th activity ratio was estimated from values tn the lrtcraturt. to be 1.0. Since detrital uranium amounts to oni) IO 30 percent of the total uranium in the STIE samples, an error of as much as 20-30 percent in the detrital *‘slJ!“*Th ratio will result in less than a 10 percent error in the calculated UBA contents. The U,, contents of the samples were then arlcul;iteci as Lt. u,, -= CJi where U7 is the measured total uramum content. i alcuiated UBA contents and fluxes of UB, (equal to the U,,., conten! times the total mass flux from Honjo, 1980: Honio PI ul in preparation) are presented in Table I. Filters were washed with distilled water to I’CIWYLas. much sea salt as possible. Only a negligible amount of the uranium in the samples from the Panama and Guatemala Basins (Table 2) could have been contributed by residual sea salt, since the concentration of uranium in the filtered particles was about an order of magnitude greater than tlx concentration in pure sea salt.
FLUXES
OF URANIUM SEDIMENT
COLLECTED TRAPS
IN THE
Bio-authigenic uranium dominates the total uranium measured in the sediment trap satnpleb. in cvery sample tent
was
except greater
UBA is derived
the deepest than
from
the
at Site
seawater,
E the Cl,,
con-
1). It then its ““1 !“xt J AL:
U, content
(Table
PARTICULATE
tivity ratio should be about 1.14 (Thurber, 1962; Turekian and Chan, 1971; Ku et al., 1977). Detrital uranium would have a corresponding ratio of about 1.0. Counting errors in determining the 234U/238U ratios were fairly large because of the small sample sizes, especially from Sites E and P, and because corrections were required for the 234U present as a contaminant in the 236U yield monitor. The 234U/238U ratios (Table 1) are consistent with a seawater source for most of the uranium, but because of the large counting errors it is not possible to demonstrate a significant correlation between the 234U/238U ratios and the UBA contents. Specific activities of UBa are highest in the shallowest sample at each site and then decrease (Sites E and P) or remain constant (STIE Site) with increasing depth. Most of the formation of UBA must therefore occur in surface waters. Any formation of U,, below the depth of the shallowest trap at each site is counterbalanced by an equal or greater rate of remineralization. If the decrease in U, contents between the shallowest and the deepest sample at each site is used to estimate the proportion of UBA remineralized within the water column, then it is concluded that about ?/3of the UBA at Site E and % of the UBA at Site P are remineralized before the particles reach the sea floor. Krishnaswami ( 1976) estimated an authigenic uranium flux into pelagic
1295
U IN SEAWATER
clays of only 0.03 dpm 238U/cm2-1 O3years, much less than the UBA fluxes collected by even the deepest traps (Table 1). This implies further remineralization at the sediment-water interface. Within the precision of present analytical capabilities, uranium does not show a vertical concentration gradient in the open ocean (Turekian and Chan, 1971; Ku et al., 1977). This means that the flux of remineralized UBA to the deep ocean must be insufficient to measurably change the uranium concentration within the mixing time of the oceans. A simple calculation with the data in Table 1 shows this to be the case. For example, it is assumed that the flux of Usa out of open ocean surface waters is 1.0 dpm 23sU/cm2-103 years, that the surface mixed layer from which the uranium is lost is 100 m thick, and that the entire flux is remineralized within a IOOOm layer of bottom water. Then the 238U concentration would change at a rate of only 0.1 dpm/l- I O3 years in the surface layer and 0.01 dpm/l-IO3 years in the deep layer. Even if a mixing time of the oceans of 1000 years is allowed, the maximum resulting concentration difference between surface and bottom waters (0.11 dpm/l) would be insignificant relative to the uncertainty in the 238U concentration of 2.5 + 0.2 dpm/l (Turekian and Chan, 197 1; Ku er al., 1977). As noted in the introduction, some of the UBA
TABLE 1 Bio-Authigenic Uranium in Sediment Trap Samples.
Sample Site/ Depth (m)
Site E:
232Th
238,, b T
23Bu
' BA
(dpmlg)
234U, 238,, UBA'UT TPercent) (Activity Ratio)
Flux of UsA (dpm,cm2
1O3 yrs,)
13.5'N. 54.O'W - 5288 m
389-Ad 389-Bd 988 3755 5086
0.21+.01a 0.1as.02 0.60~.02 0.99T.03 1.26T.06 _
0.72+.01 0.67T.04 0.76T.03 0.81'.03 o.e3+.03 _
0.61 0.53 0.45 0.42 0.17
85 :9' 52 20
1.15f.04 1.067.08 1.01'.07 0.9t3+.07 1.067.06
O.Oe
w
1.02+.03 _
0.57 0.32 0.25 0.26
92 78 74 59
l.20+.12 1.297.13 1.28'.13 1.107.19 _
1.53L.05
o.oe
w
0.87_'.04
0.79+.05 0.86+.03 0.827.04 0.7e.04 0.8BT.04 o.L79:.03 0.91z.04
0.70 0.70 0.65
89 81 80 74 79 69 71
1.03t.06 1.165.07 ND 1.23+.oe 1.15r.04 1.09T.03 1.167.03
1.15 1.00 0.72 0.82 0.31
CH 75-2, Core 8, 14.1-N. 54.1-W - 5342 m 2-4 cm Site P:
3.25+.0a
1.69z.04
15.3-N, 151.5-W - 5792 m
978 2778 4280 5582
0.07+.01 0.11+.01 O.lZ~.Ol 0.237.02 _
0.62+.04 0.41T.02 0.34T.02 0.445.04
0.15 0.19 0.14 0.09
KKl, Core 1, 14.7'N, 153.1'W - 5962 m o-1 cm STIE:
1.95+.oe 5'21'N, 81'53'W
667 2265RT 3f 2265RT 4f 2265RT 5f 2869 3769 3791 a.
0.088+.007 0.1667.014 0.163T.016 0.189~.014 0.191r.013 0.276+.017 0.265z.017
:.:; 0:61 0.65
2.5 3.0 3.7
Uncertainties include t 1 o counting statistics, for natural and tracer isotopes and the uncertainty in the tracer activity propagated to the final value. Total uranium in sample (measured). Bio-authigenic uranium (calculated - see text). Ouolicate analvses. Laroer errors for second analvsis result from smaller sample size. that all U A has been Pe'lagic clays i& near Sites E and P. It is ?&w&d remineralized and the U/Th ratio accuratelv represents the (U/Th Bn ratio for the corresponding sediment trap samples. f. Rotating sample cup collected sequential samples from the same sediment trap NO = Not determined because of poor resolution in the uranium alpha spectrum. b. c. d. e.
1296
R. F. ANDERSON TARE 2 238 U and "2Th. concentrationsof Particulate 238ba
Station-Location Sample Depth
Cm)
dpmlQC
Liters
-I-_--_--___1
dnm/106 Liters _-
'/oIume Tlltered i.1tersi
Sta. 1110, 05'06'N, rJi"3Z'W. 0.13+.05 0.045.06 0.267.08
1500 2250 3000
i.vi.2
1 .ot3+.20
24+4 0.772.7 15z4
0.7%.Y 4.9T1.6
0.55'.17
l.l%.i 707534 ._
u.63+.19 0.88z.20
< u.7 0.7+0.9 1.5~0.5
0.73+.21 0.895.21 0.91T.26 _
8.8+2.5 9+3 llZ3
0.3+0.9 0.8TO.Y 0.5+1.0 i.PI.0
0.59+.x ;:;7;.~ 0.947'21 _.
4.0'1.7 7.3z2.4 7.452.8 7.871.6
i).i+O.i 0.7TO.6 l.o+l.? 0.17C.l
1.0+.4 0.917.22 0.907.25 0.50~.22 _
8+3 7.371.7 13% 8?3
14.4+1.7 5.010.8 8.9'1.5 ifJ.5+!.2 _
NUe 0.44+.29 0.X37.24 0.67z.26
10,9+2.3 2.F1.5 2.371.7 3.971.5
0.787.22 _
Sta. 1114, 11'09'N, 87'3i'W -3000 4700
O.ll+.oS 36.9-;1.8d
Sta. 1117, 09'36'N. 89*08'E 1500 2250 2900
< 6.06 O.OE+.lO @.12".04
Sta. 1120, 07‘04'N. 91"41_11! ?OOO 1500 2250 3100
0.05+.14 5.087.09 0.06i.12 0.14z.11
Sta. 1122, 04"08'N, 94‘22,'j 1100 % 2600
O.O2+.06 0.09T.07 O.OP.08 O.OP.LVj
Site E, 13‘30'N, 54"00'_~ 800 2200 3600 5000 a. b. :: e.
NO@ u.97*.
16
1.26r.18 1.787.19
For comparjson,dissolved 238~ = 2.5~106 dpmjlO6 Titers. Thorium contents as ppm may be obtained by multiplying dpm/g values by 4.1, m may be obtained by multiplying dpm/g values by 1.34. $nt::s:o,;fS":',r;;hE; 2Th concentration in this sample cannot be explarned at this time, but it is believed to be real (Bacon and Anderson, 1982). Weight of particulatematerial collected could not be determined for th?c sample.
formed in the water column may be preserved in hemipelagic sediments in areas like the Panama Basin. In order for this lJBA flux to represent a significant oceanwide uranium sink, it must be much greater than the average rate of uranium supply by rivers to the oceans (4 dpm 23eU/cm2- IO’ years; Sackett et al., 1973), because ocean margin environments such as the Panama Basin constitute only a small portion of the total ocean area. The maximum Usa flux in the Panama Basin (Table I) is 4 dpm 238U/ cm*-103 years, exactly equal to the average rate of supply to the whole ocean. Therefore, while fixation of uranium to particles in the Panama Basin water column may be locaiiy important as a sink for uranium, it does not represent a major sink for uranium on an oceanwide basis. CARRIER PHASE(S) OF AUTHIGENIC URANIUM It was noted above that most of the UBA formed within open ocean surface water is rapidly reminer-
alized in the deep water. The two most labile major phases.in the trapped particles are CaCO, and or” ganic matter. Pelagic planktonic CaCt$ contains little associated uranium (Ku. 1965; Sackett zr nl.. 1973; Delaney, 1980). The lack of a positive corre-. lation between UBA and CaCU3 contents of the sediment trap samples (Fig. 2) supports the argument (Ku, 1965; Baturin, 1969) that CaC03 is not a major carrier of authigenic uranium. A positive correlation (significant at the YYpercent confidence level) was found between U,, and organic matter contents for the sediment trap samples (Fig. 3). Wakeham ef al. (1980) have shown that certain classes of organic compounds measured in samples from Site E are remineralized much more rapid!) than others. It is impossible at this poinl to determine which class(es) of compounds act as carriers for Un,,. Thus some of the scatter in the data in Fig. 3 may result from a variable ratio of uranium-~omplexiil~ organic matter to total organic matter. .Additiona~ scatter may result from uncertainties in the calcu
PARTICULATE
1297
U IN SEAWATER
I
A
.
.
A STIE . SITE E o SITE P
.
0’
I
’ 25
35
45
55
coca,
65
73
I%/
FIG. 2. Plot of bio-authigenic uranium content against CaCO, content for sediment trap samples. Sample locations are indicated by the key in the figure. CaCO, contents are from Honjo ( 1980 and unpublished results). lation of UBA. Organic matter and CaC03 contents used here are for the
organic matter in the Panama Basin is unknown. However, since U,, seems to be associated with organic matter, the absence of UBA remineralization in the deep Panama Basin is probably related to the enhanced preservation there of total particulate organic matter. There is an intense oxygen minimum in the Panama Basin. The O2 concentration falls below 12 PM/kg between 300-600 m, and increases to -50 PM/kg at 1000 m and -100 FM/kg at 2000 m (P. G. Brewer, unpublished results from Station
A
A
A
4n A 0
A
.
. 0
0 02
I
. SITE E
0
o SITE P A STIE SITE
.
t -0
5
10
ORGAN/C MATTER
15
20
25
I%)
FIG. 3. Plot of bio-authigenic uranium content against organic matter content for sediment trap samples. Sample locations are indicated by the key in the figure. Organic matter contents are from Honjo (I 980 and unpublished results).
129X
R. F. ANDERSON
I1 10, see Table 2). These O2 concentrations below 1000 m are a factor of 2-3 lower than at corresponding depths in the open ocean. Greater preservation of UBA and organic matter at the STIE Site compared to Sites E and P may be related to the lower O2 concentrations in the Panama Basin. If so, further work is required to determine whether it is the intense O2 minimum at 300-600 m or the lower O2 concentration throughout the deep water column that inhibits UBA remineralization. Reduction of U(VI) should not occur in seawater unless
H2S
is present
(Langmuir,
1978).
Therefore.
the redox potential (Eh) is not low enough even in the intense O2 minimum in the Panama Basin to cause the reduction of U(V1) to U(IV) within the water column. Unless uranium is reduced biologically or in reducing microenvironments within the water column (e.g., within fecal pellets), these data are consistent with the conclusion that at least some of the uranium in anoxic sediments is removed from seawater via association of IJ(VI) with organic matter. Fluxes of UBA through the water column in the Panama Basin (Table I ) can be compared to fluxes of authigenic uranium into organic matter-rich shelf and hemipelagic sediments. No data are available for Panama Basin sediments, but authigenic uranium fluxes at other locations (Veeh, 1967; Kolodny and Kaplan, 1973; Veeh et al., 1974) are generally about two orders of magnitude greater than those in the Panama Basin water column. If the UBA fluxes in the Panama Basin are representative of fluxes through the water column at these other locations, then the authigenic uranium in those sediments must be almost entirely fixed by precipitation within the sediments rather than in the water column. Uranium/ organic matter ratios are about 20 times higher in Saanich Inlet sediments (Kolodny and Kaplan, 1973) and range from 12- 100 times higher in West African shelf sediments (Veeh et al., 1974) than in the sediment trap samples. If the organic matter in these sediments has similar uranium binding capabilities to that collected in the sediment traps, then this provides further evidence that most of the authigenic uranium in these sediments was precipitated in situ, supplied by diffusion from the overlying seawater. Thus, these results support to some extent both arguments noted in the introduction: U(VI) does become associated with particulate organic matter in seawater; however, most of the authigenic uranium observed in organic matter-rich sediments results from the precipitation of uranium within the sediments rather than from the complexes formed within the water column. CONCENTRATIONS
OF PARTICULATE
URANIUM
Concentrations of particulate uranium were measured by in situ filtration at Site E and in the Panama and the Guatemala Basins (Table 2). The concen-
tration of particulate uranium in seawiltrr IS very low. Turekian and Chan (I 97 1) and Ku tst ui. ( I977 i showed that filtration of deep ocean water does not measurably reduce the total uranium concentration. Miyake et al. (1973) found less than 0.3 percent 01 the total uranium in Japanese coastal waters to he in particulate form. Hodge er ul. (1979) similarI! found that particulate uranium is only about 0. I percent of the total in California coastal waters. In the deep ocean, where the concentration of particulate material is much less than in coastal waters, and where there is no evidence for the formation of U,,, particulate uranium concentrations (Table 3 1 fall in the range of lo-’ to lO-6 of the total. The 238U//232Th ratios can be used to bhow thai most of the particulate uranium at Site L: ia ~SSO. ciated with detrital minerals, whereas the particulate uranium in the Panama and Guatemala Basins I:, almost entirely authigenic. The detrital “*Uj”‘Th activity ratio is roughly 0.5-- 1 .O. (note the “%,“‘“I% ratios in the sediment samples in Table i ) i’r IS ditficult to determine the 238Uj232Th ratios m samples from the Panama and Guatemala Basins because OI the low 2’ZTh concentrations, but the ratios art clearly much greater than 1.0, and perhaps greater than 10.0 (Table 2). Furthermore, the specitic activity of UBA in filtered particles in the Panama and Guatemala Basins (UBA s UT --- “‘Th) is nearly the same as in the trapped particles at the STIE Site. In contrast, at open ocean Site E, the particulate23”U/232Th activity ratio is less than 1.0 (Table 2). Samples from below 800 m at Site E have ratios in. distinguishable from the 23*U/2’2Th ratio of 0.52 in nearby sediments (CH75-2, Core 8; Table i !. There is an indication of a higher “%J/*‘*Th ratia in the sample from 800 m than in the deeper samples. suggesting that only a small part of the IJs.4 formed a~ the sea surface survives to mesopelagic depths m the fine particle fraction collected by filtration. These results support the conclusion derived from the sediment trap samples that particulate U,, in, rapid11 remineralized in the deep open ocean, where;~s it i> preserved throughout the water column iri octxn margin environments such as the Panama and Gus. temala Basins. The reason for this is not clear; how. ever, as noted in the discussion of the sediment trap results, it may be related to a greater preservation of particulate organic matter in the water column in margin environments compared to the open ocean
Acknowledgments-l am grateful to Drs. Susumu Hon~o. Derek Spencer, and Peter Brewer for providing the sediment trap samples, Peter Sachs and Dr. Michael Bacon assisted with the assembly and deployment of rhe irz sifid pumping systems. Alan Fleer performed the analyses of’ some of the STIE sediment trap samples. Drs Michael Bacon and J. Kirk Cochran provided helpful comments during the preparation of this manuscript. Financial support for this work was provided in part by the National Science Foundation under Grants OCE 7826318, OCE 7825724 and OCE 7727004, the Department of Energy under c<>r.
PARTICULATE tract EY-76-S-02-3566; Education Office.
and a fellowship
from the WHO1
REFERENCES Anderson R. F. ( 198 1) The marine geochemistry of thorium and protactinium. Ph.D. dissertation, Massachusetts Institute of Technology-Woods Hole Oceanographic Institution, WHOI-81-i: Woods Hole, Massachusetts. Anderson R. F. and Fleer A. P. (1982) , Determination of natural actinides and plutonium in marine particulate material. Anal. Chem. (in press). Bacon M. P. and Anderson R. F. (1982) The distribution of thorium isotopes between dissolved and particulate forms in seawater. J. Geophys. Res. 87, 204552056. Baturin G. N. ( 1968) Geochemistry of uranium in the Baltic. Geochem. Internat. 5, 344-348. Baturin G. N. (1969) Uranium in the surface layer of sediments in the northwestern Indian Ocean. Oceanology Academy of Sciences USSR 9, 828-833. Baturin G. N., Kochenov A. V. and Senin Yu. M. (1971) Uranium concentration in recent ocean sediments in zones of rising currents. Geochem. Internat. 8, 281-286. Bonatti E., Fisher D. E., Joensuu 0. and Rydell H. S. ( 197 1) Postdepositional mobility of some transition elements, phosphorous, uranium and thorium in deep sea sediments. Geochim. Cosmochim. Acta 35, 189-201. Brewer P. G., Nozaki Y., Spencer D. W. and Fleer A. P. (1980) A sediment trap experiment in the deep sub-tropical Atlantic: Isotopic and elemental fluxes. J. Mar. Res. 38, 703-728. Degens E. D., Khoo F. and Michaelis W. (1977) Uranium anomaly in Black Sea sediments. Nature 269, 566-569. Delaney M. (1980) U in foraminifera tests. EOS 61, 259. Deuser W. G., Ross E. H. and Anderson R. F. (1981) Seasonality in the supply of sediment to the deep Sargasso Sea, and implications for the rapid transfer of matter to the deep ocean. Deep-Sea Res. 28A, 495-505. Hodge V. F., Koide M. and Goldberg E. D. (1979) Particulate uranium, plutonium and polonium in the biogeochemistries of the coastal zone. Nature 277,206-209. Honjo S. (1978) Sedimentation of material in the Sargasso Sea at a 5367 m deep station. J. Mar. Rex 36, 469-492. Honjo S. ( 1980) Material fluxes and modes of sedimentation in the mesopelagic and bathypelagic zones. J. Mar. Res. 38, 53-97. Honjo S., Manganini S. and Cole J. (1982) Sedimentation of biogenic matter in the deep ocean (in preparation). Kochenov A. V., Baturin G. N., Kovaleva S. A., Yemel’yanov Ye. M. and Shimkus K. M. (1965) Uranium and organic matter in the sediments of the Black and Mediterranean Seas. Geochem. Internat. 2, 212 (Abstract). Kolodny Y. and Kaplan I. R. (1973) Deposition of uranium in the sediment and interstitial water of an anoxic fjord. In Proc. Symposium on Hydrogeochemistry and Biogeochemistry (ed. E. Ingerson), Vol. 1, 418-442. The Clarke Co., Washington, D.C. Krishnaswami S. (1976) Authigenic transition elements in
U IN SEAWATER
1299
Pacific pelagic clays. Geochim. Cosmochim. Acta 40, 425-434. Krishnaswami S., La1 D., Somayajulu B. L. K., Weiss R. F. and Craig H. (1976) Large volume in-situ filtration of deep Pacific waters: mineralogical and radioisotope studies. Earth Planet. Sci. Lett. 32, 420-429. Ku T. L. (1965) An evaluation of the U2’4/U238 method as a tool for dating pelagic sediments. J. Geophys. Res. 70, 3457-3474. Ku T. L., Knauss K. G. and Mathieu G. G. (1977) Uranium in open ocean: concentration and isotopic composition. Deep-Sea Res. 24, 1005-1017. L,angmuir D. (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim. Cosmochim. Acta 42, 5477569. Miyake Y., Saruhashi K. and Sugimura Y. (1973) The isotopic ratio 234U/23*U in seawater and its bearing on the isotopic ratio in river waters. Records of Oceanographic Works in Japan 12, 23-25. MO T., Suttle A. D. and Sackett W. M. (1973) Uranium concentrations in marine sediments. Geochim. Cosmochim. Acta 37, 35-51. Naumov G. B., Kochenov A. V., Gerasimovskii V. I. and Germanov A. I. (1963) Uranium in sedimentary rocks. In Osnovnye Cherty Geokhimii Urana (Basic Features of Uranium Geochemistry) (ed. A. P. Vinogradov) Izd. AN Moskow. Sackett W. M., MO T., Spalding R. F. and Exner M. E. (1973) A revaluation of the marine chemistry of uranium. In Radioactive Contamination in the Marine Environment, 757-769. IAEA-SM-158/51, International Atomic Energy Agency, Vienna. Spencer D. W. and Sachs P. L. (1970) Some aspects of the distribution, chemistry, and mineralogy of suspended matter in the Gulf of Maine. Mar. Geol. 9, I I7- 136. Spencer D. W., Brewer P. G., Fleer A. P., Honjo S., Krishnaswami S. and Nozaki Y. (1978) Chemical fluxes from a sediment trap experiment in the deep Sargasso Sea. J. Mar. Res. 36, 4933523. Starik I. E. and Kolyadnin L. B. (1957) The occurrence of uranium in ocean water. Geochemistry 2, 2455256. Thurber D. (1962) Anomalous U234/U2’B in nature. J. Geophys. Res. 67, 45 18-4520. Turekian K. K. and Chan L. H. (1971) The marine geochemistry of the uranium isotopes, “‘Th and 2”Pa. In Activation Analysis in Geochemistry and Cosmochemistry (eds. A. 0. Brunfelt and E. Steinnes). 31 I-320. Universitetsforlaget: Oslo, Norway. Veeh H. H. (1967) Deposition of uranium from the ocean. Earth Planet. Sri. Lett. 3, 1455150. Veeh H. H., Calvert S. E. and Price N. B. (1974) Accumulation of uranium in sediments and phosphorites on the South West African shelf. Mar. Chem. 2, 189-202. Wakeham S. G., Farrington J. W., Gagosian R. B., Lee C., DeBaar H., Nigrelli G. E., Tripp B. W.. Smith S. 0. and Frew N. M. (1980) Organic matter fluxes from sediment traps in the equatorial Atlantic Ocean. Nature 286, 7988800.
Acre Vol. 46, pp. 1293 to 1299 Ceochmrico et Cosmochimica 0 Pergamon Press Ltd. 1982. Printed in U.S.A.
Concentration, vertical flux, and remineralization of particulate uranium in seawater ROBERT F. ANDERSON* Department of Chemistry, Woods (Received
Hole Oceanographic
September
Institution,
Woods
Hole, Massachusetts
02543
3, 1981; accepted in revised form March 4, 1982)
Abstract-Concentrations and fluxes of particulate U were measured throughout the water column at several locations in the Atlantic and Pacific Oceans with in situ filtration systems and sediment traps. The results indicate that dissolved U is fixed to particles in surface seawater. Organic matter appears to be the carrier phase. Formation of particulate authigenic U below the surface waters could not be detected. Authigenic U is remineralized within the bathypelagic layers at the open ocean sites studied. In the Panama Basin, an upwelling area with high biological productivity, remineralization of authigenic U in the deep water column was not observed. The rate of remineralization of authigenic U in the deep sea is insufficient to produce a measurable concentration gradient between surface and deep waters within the mixing time of the oceans. Formation of authigenic U in the water column in areas such as the Panama Basin is not a significant sink for U on an ocean wide basis. INTRODUCIION THE major uranium
features
of the marine
geochemistry
of
are fairly well known.
Uranium is very soluble in oxygenated seawater as a result of the formation of stable uranyl-carbonate complexes (Starik and Kolyadnin, 1957; Langmuir, 1978), with a residence time in the oceans of 2-5 X 10’ years (Turekian and Chan, 1971; Sackett et al., 1973; Ku et al., 1977). Seawater is undersaturated with respect to precipitation of uranium minerals (Naumov et al., 1963; see also Langmuir, 1978). Therefore removal processes other than precipitation of inorganic uranium salts must be responsible for maintaining the concentration of uranium in seawater. Hemipelagic, organic matter-rich sediments contain authigenic uranium derived from seawater (Veeh, 1967; Veeh et al., 1974), and in some cases, uranium concentrations correlate with the organic matter content of the sediments (Baturin, 1969; Baturin et al., 1971; MO et al., 1973; Kolodny and Kaplan, 1973; Sackett et al., 1973). Soluble U(VI) may diffuse from oxygenated bottom waters into sediments where it then may be precipitated by reduction to insoluble U( IV) if the Eh is sufficiently low. Low Eh conditions are often produced by anaerobic decomposition of organic matter just below the sediment-seawater interface in sediments with high organic matter contents. This mechanism was suggested as the cause of enhanced uranium contents of sediments in the Eastern Tropical North Pacific (Bonatti et al., 197 1) and in West African shelf sediments (Veeh et al., 1974). Several investigators prefer an alternative explanation: that the high uranium contents frequently found in organic matter-rich anoxic sediments result
* Present Address: Lamont-Doherty atory, Palisades, New York 10964
Geological
Observ-
from the adsorption or complexation of U(VI) by organic matter (Kochenov et al., 1965; Baturin, 1968, 1969; Baturin et al., 1971; Kolodny and Kaplan, 1973; Degens et al., 1977). Uranium-organic matter associations would then be preserved under reducing conditions either as a result of enhanced preservation of organic matter, which retains complexed U(VI), or as a result of reduction of U(V1) to U(IV) after burial with subsequent adsorption of the reduced uranium to the sediments. Concentrations and vertical fluxes of particulate uranium have been measured at several sites in the oceans with in situ filtration systems and sediment traps to study the formation of particulate authigenic uranium in seawater. Particulate authigenic uranium is formed in surface seawater, apparently associated with organic matter. In the open ocean, most of the particulate authigenic uranium is remineralized within the water column. In contrast, remineralization of particulate authigenic uranium within the water column could not be detected at an ocean margin site. SAMPLE
COLLECTION, INSTRUMENTS, AND METHODS
Sediment trap samples were obtained as part of the PARFLUX Program (Honjo, 1978, 1980; Spencer et al., 1978; Brewer et al., 1980). Descriptions of the sediment traps, the moorings to which they were attached, and sample handling procedures can be found in the above references. Samples of particulate matter were also collected by in situ filtration onto l.O-micron pore size, 293-mm diameter Nuclepore filters with pumping systems described elsewhere (Spencer and Sachs, 1970; Krishnaswami et al., 1976). Sample locations are shown in Fig. 1. Analytical procedures for the radioisotopes are described by Anderson and Fleer (1982). Briefly, the method involved complete dissolution of the samples with HCI-HNO,-HF-HC104 in the presence of isotope yield monitors (*‘% and 229Th). Uranium and thorium were purified by ion exchange (Dowex AG l-X8, Bio Rad Laboratories) and electroplated to provide thin sources suitable for alpha spectrometry. 1293
1294
R. F. ANDERSON
a
\“., i
’ \
’ *.,,HAWAII l
-,.
J; &$., _,’ J
STATION
1
I
180”
150”
’
* BERMUDA
. a- .- \, .
\
- --t+BARBADOS
lSTATION
I
I
120°
9o”
l
I
,
/
-----.I_.
30*
60”
I?0
-I 30” _..._”
FIG. 1. Locations of PARFLUX sediment trap Sites E and P, the Sediment Trap Intercomp,rrisi,r, Experiment (STIE) Site, and station locations occupied for in sifu filtration of large volumes of sea\cnte! (KN73-16). Station I I IO and the STIE site are at the same location.
RESULTS Results of the analysis of sediment trap samples are presented in Table 1. The size fraction greater than I mm is routinely removed for separate studies prior to chemical analysis (Honjo, 1980; Brewer et al., 1980). Only for the shallowest sample at each site did that fraction constitute more than a few per cent of the total sample (Honjo, 1980; Honjo, personal communication). Samples from Site E, for which results are presented in Table 1, were analyzed separately by Brewer et al. (I 980). Their uranium values were consistently about 20 percent higher than those reported here. No reasons can be found for the disagreement. Analysis of standard rock material by the method used to obtain the results in Table 1 produced results equal, within counting errors. to published values (Anderson and Fleer, 1982). The total uranium measured in the samples consists of two parts: uranium present in detrital silicate minerals (U,,) and bio-authigenic uranium (U,,) derived from seawater. Detrital uranium is estimated as Uu = (13’U/Z’zTh),l
x (‘12Th ),,
where (2JaU/*32Th)L, is the detrital U/Th ratio and (z’2Th)M is the measured content of “‘Th in the sediment trap samples The strong correlation of 232Th with other detrital elements in sediment trap samples indicates that it may be considered to be exclusively present in detrital minerals (Brewer ef al., 1980; Anderson, 1981). At Site E, “‘Th/AI and “‘Th/K ratios in surface sediments were close to the ratios in sediment trap samples and to ratios reported for atmospheric dust from a nearby site (Anderson, 1981). Detrital minerals are rapidly transported from the sea surface to the sea floor (Deuser et a/., 1981; Deuser, personal communication), so it is not unexpected that sediment trap samples should have ratios of predominantly detrital elements that are similar to ratios in underlying surface sediments It is assumed then that the (2’*U/232Th)o ratios are also uniform throughout the water column and in the sediments at each site. Nearly all of the biogenic components have been remineralized from the sediments at both sites,
as evidenced by a lack of CO2 evolution upon acidrticatron of the sediments and by the very low concentrations o! organic matter (0.3 percent and 0.2 percent at Sites E and P respectively) and opal (I. I percent and 0.2 percent at Sites E and P respectively) in the sediments (Honjo PI a: in preparation). Therefore, the 13HU/2i2Th ratio in sedi. ments was used as the best available estimate oi (“‘II, 232Th)o at Sites E and P (Table I ). Sediment samples were not available from the STIE Site, so the detrital “‘“U/“‘Th activity ratio was estimated from values tn the lrtcraturt. to be 1.0. Since detrital uranium amounts to oni) IO 30 percent of the total uranium in the STIE samples, an error of as much as 20-30 percent in the detrital *‘slJ!“*Th ratio will result in less than a 10 percent error in the calculated UBA contents. The U,, contents of the samples were then arlcul;iteci as Lt. u,, -= CJi where U7 is the measured total uramum content. i alcuiated UBA contents and fluxes of UB, (equal to the U,,., conten! times the total mass flux from Honjo, 1980: Honio PI ul in preparation) are presented in Table I. Filters were washed with distilled water to I’CIWYLas. much sea salt as possible. Only a negligible amount of the uranium in the samples from the Panama and Guatemala Basins (Table 2) could have been contributed by residual sea salt, since the concentration of uranium in the filtered particles was about an order of magnitude greater than tlx concentration in pure sea salt.
FLUXES
OF URANIUM SEDIMENT
COLLECTED TRAPS
IN THE
Bio-authigenic uranium dominates the total uranium measured in the sediment trap satnpleb. in cvery sample tent
was
except greater
UBA is derived
the deepest than
from
the
at Site
seawater,
E the Cl,,
con-
1). It then its ““1 !“xt J AL:
U, content
(Table
PARTICULATE
tivity ratio should be about 1.14 (Thurber, 1962; Turekian and Chan, 1971; Ku et al., 1977). Detrital uranium would have a corresponding ratio of about 1.0. Counting errors in determining the 234U/238U ratios were fairly large because of the small sample sizes, especially from Sites E and P, and because corrections were required for the 234U present as a contaminant in the 236U yield monitor. The 234U/238U ratios (Table 1) are consistent with a seawater source for most of the uranium, but because of the large counting errors it is not possible to demonstrate a significant correlation between the 234U/238U ratios and the UBA contents. Specific activities of UBa are highest in the shallowest sample at each site and then decrease (Sites E and P) or remain constant (STIE Site) with increasing depth. Most of the formation of UBA must therefore occur in surface waters. Any formation of U,, below the depth of the shallowest trap at each site is counterbalanced by an equal or greater rate of remineralization. If the decrease in U, contents between the shallowest and the deepest sample at each site is used to estimate the proportion of UBA remineralized within the water column, then it is concluded that about ?/3of the UBA at Site E and % of the UBA at Site P are remineralized before the particles reach the sea floor. Krishnaswami ( 1976) estimated an authigenic uranium flux into pelagic
1295
U IN SEAWATER
clays of only 0.03 dpm 238U/cm2-1 O3years, much less than the UBA fluxes collected by even the deepest traps (Table 1). This implies further remineralization at the sediment-water interface. Within the precision of present analytical capabilities, uranium does not show a vertical concentration gradient in the open ocean (Turekian and Chan, 1971; Ku et al., 1977). This means that the flux of remineralized UBA to the deep ocean must be insufficient to measurably change the uranium concentration within the mixing time of the oceans. A simple calculation with the data in Table 1 shows this to be the case. For example, it is assumed that the flux of Usa out of open ocean surface waters is 1.0 dpm 23sU/cm2-103 years, that the surface mixed layer from which the uranium is lost is 100 m thick, and that the entire flux is remineralized within a IOOOm layer of bottom water. Then the 238U concentration would change at a rate of only 0.1 dpm/l- I O3 years in the surface layer and 0.01 dpm/l-IO3 years in the deep layer. Even if a mixing time of the oceans of 1000 years is allowed, the maximum resulting concentration difference between surface and bottom waters (0.11 dpm/l) would be insignificant relative to the uncertainty in the 238U concentration of 2.5 + 0.2 dpm/l (Turekian and Chan, 197 1; Ku er al., 1977). As noted in the introduction, some of the UBA
TABLE 1 Bio-Authigenic Uranium in Sediment Trap Samples.
Sample Site/ Depth (m)
Site E:
232Th
238,, b T
23Bu
' BA
(dpmlg)
234U, 238,, UBA'UT TPercent) (Activity Ratio)
Flux of UsA (dpm,cm2
1O3 yrs,)
13.5'N. 54.O'W - 5288 m
389-Ad 389-Bd 988 3755 5086
0.21+.01a 0.1as.02 0.60~.02 0.99T.03 1.26T.06 _
0.72+.01 0.67T.04 0.76T.03 0.81'.03 o.e3+.03 _
0.61 0.53 0.45 0.42 0.17
85 :9' 52 20
1.15f.04 1.067.08 1.01'.07 0.9t3+.07 1.067.06
O.Oe
w
1.02+.03 _
0.57 0.32 0.25 0.26
92 78 74 59
l.20+.12 1.297.13 1.28'.13 1.107.19 _
1.53L.05
o.oe
w
0.87_'.04
0.79+.05 0.86+.03 0.827.04 0.7e.04 0.8BT.04 o.L79:.03 0.91z.04
0.70 0.70 0.65
89 81 80 74 79 69 71
1.03t.06 1.165.07 ND 1.23+.oe 1.15r.04 1.09T.03 1.167.03
1.15 1.00 0.72 0.82 0.31
CH 75-2, Core 8, 14.1-N. 54.1-W - 5342 m 2-4 cm Site P:
3.25+.0a
1.69z.04
15.3-N, 151.5-W - 5792 m
978 2778 4280 5582
0.07+.01 0.11+.01 O.lZ~.Ol 0.237.02 _
0.62+.04 0.41T.02 0.34T.02 0.445.04
0.15 0.19 0.14 0.09
KKl, Core 1, 14.7'N, 153.1'W - 5962 m o-1 cm STIE:
1.95+.oe 5'21'N, 81'53'W
667 2265RT 3f 2265RT 4f 2265RT 5f 2869 3769 3791 a.
0.088+.007 0.1667.014 0.163T.016 0.189~.014 0.191r.013 0.276+.017 0.265z.017
:.:; 0:61 0.65
2.5 3.0 3.7
Uncertainties include t 1 o counting statistics, for natural and tracer isotopes and the uncertainty in the tracer activity propagated to the final value. Total uranium in sample (measured). Bio-authigenic uranium (calculated - see text). Ouolicate analvses. Laroer errors for second analvsis result from smaller sample size. that all U A has been Pe'lagic clays i& near Sites E and P. It is ?&w&d remineralized and the U/Th ratio accuratelv represents the (U/Th Bn ratio for the corresponding sediment trap samples. f. Rotating sample cup collected sequential samples from the same sediment trap NO = Not determined because of poor resolution in the uranium alpha spectrum. b. c. d. e.
1296
R. F. ANDERSON TARE 2 238 U and "2Th. concentrationsof Particulate 238ba
Station-Location Sample Depth
Cm)
dpmlQC
Liters
-I-_--_--___1
dnm/106 Liters _-
'/oIume Tlltered i.1tersi
Sta. 1110, 05'06'N, rJi"3Z'W. 0.13+.05 0.045.06 0.267.08
1500 2250 3000
i.vi.2
1 .ot3+.20
24+4 0.772.7 15z4
0.7%.Y 4.9T1.6
0.55'.17
l.l%.i 707534 ._
u.63+.19 0.88z.20
< u.7 0.7+0.9 1.5~0.5
0.73+.21 0.895.21 0.91T.26 _
8.8+2.5 9+3 llZ3
0.3+0.9 0.8TO.Y 0.5+1.0 i.PI.0
0.59+.x ;:;7;.~ 0.947'21 _.
4.0'1.7 7.3z2.4 7.452.8 7.871.6
i).i+O.i 0.7TO.6 l.o+l.? 0.17C.l
1.0+.4 0.917.22 0.907.25 0.50~.22 _
8+3 7.371.7 13% 8?3
14.4+1.7 5.010.8 8.9'1.5 ifJ.5+!.2 _
NUe 0.44+.29 0.X37.24 0.67z.26
10,9+2.3 2.F1.5 2.371.7 3.971.5
0.787.22 _
Sta. 1114, 11'09'N, 87'3i'W -3000 4700
O.ll+.oS 36.9-;1.8d
Sta. 1117, 09'36'N. 89*08'E 1500 2250 2900
< 6.06 O.OE+.lO @.12".04
Sta. 1120, 07‘04'N. 91"41_11! ?OOO 1500 2250 3100
0.05+.14 5.087.09 0.06i.12 0.14z.11
Sta. 1122, 04"08'N, 94‘22,'j 1100 % 2600
O.O2+.06 0.09T.07 O.OP.08 O.OP.LVj
Site E, 13‘30'N, 54"00'_~ 800 2200 3600 5000 a. b. :: e.
NO@ u.97*.
16
1.26r.18 1.787.19
For comparjson,dissolved 238~ = 2.5~106 dpmjlO6 Titers. Thorium contents as ppm may be obtained by multiplying dpm/g values by 4.1, m may be obtained by multiplying dpm/g values by 1.34. $nt::s:o,;fS":',r;;hE; 2Th concentration in this sample cannot be explarned at this time, but it is believed to be real (Bacon and Anderson, 1982). Weight of particulatematerial collected could not be determined for th?c sample.
formed in the water column may be preserved in hemipelagic sediments in areas like the Panama Basin. In order for this lJBA flux to represent a significant oceanwide uranium sink, it must be much greater than the average rate of uranium supply by rivers to the oceans (4 dpm 23eU/cm2- IO’ years; Sackett et al., 1973), because ocean margin environments such as the Panama Basin constitute only a small portion of the total ocean area. The maximum Usa flux in the Panama Basin (Table I) is 4 dpm 238U/ cm*-103 years, exactly equal to the average rate of supply to the whole ocean. Therefore, while fixation of uranium to particles in the Panama Basin water column may be locaiiy important as a sink for uranium, it does not represent a major sink for uranium on an oceanwide basis. CARRIER PHASE(S) OF AUTHIGENIC URANIUM It was noted above that most of the UBA formed within open ocean surface water is rapidly reminer-
alized in the deep water. The two most labile major phases.in the trapped particles are CaCO, and or” ganic matter. Pelagic planktonic CaCt$ contains little associated uranium (Ku. 1965; Sackett zr nl.. 1973; Delaney, 1980). The lack of a positive corre-. lation between UBA and CaCU3 contents of the sediment trap samples (Fig. 2) supports the argument (Ku, 1965; Baturin, 1969) that CaC03 is not a major carrier of authigenic uranium. A positive correlation (significant at the YYpercent confidence level) was found between U,, and organic matter contents for the sediment trap samples (Fig. 3). Wakeham ef al. (1980) have shown that certain classes of organic compounds measured in samples from Site E are remineralized much more rapid!) than others. It is impossible at this poinl to determine which class(es) of compounds act as carriers for Un,,. Thus some of the scatter in the data in Fig. 3 may result from a variable ratio of uranium-~omplexiil~ organic matter to total organic matter. .Additiona~ scatter may result from uncertainties in the calcu
PARTICULATE
1297
U IN SEAWATER
I
A
.
.
A STIE . SITE E o SITE P
.
0’
I
’ 25
35
45
55
coca,
65
73
I%/
FIG. 2. Plot of bio-authigenic uranium content against CaCO, content for sediment trap samples. Sample locations are indicated by the key in the figure. CaCO, contents are from Honjo ( 1980 and unpublished results). lation of UBA. Organic matter and CaC03 contents used here are for the
organic matter in the Panama Basin is unknown. However, since U,, seems to be associated with organic matter, the absence of UBA remineralization in the deep Panama Basin is probably related to the enhanced preservation there of total particulate organic matter. There is an intense oxygen minimum in the Panama Basin. The O2 concentration falls below 12 PM/kg between 300-600 m, and increases to -50 PM/kg at 1000 m and -100 FM/kg at 2000 m (P. G. Brewer, unpublished results from Station
A
A
A
4n A 0
A
.
. 0
0 02
I
. SITE E
0
o SITE P A STIE SITE
.
t -0
5
10
ORGAN/C MATTER
15
20
25
I%)
FIG. 3. Plot of bio-authigenic uranium content against organic matter content for sediment trap samples. Sample locations are indicated by the key in the figure. Organic matter contents are from Honjo (I 980 and unpublished results).
129X
R. F. ANDERSON
I1 10, see Table 2). These O2 concentrations below 1000 m are a factor of 2-3 lower than at corresponding depths in the open ocean. Greater preservation of UBA and organic matter at the STIE Site compared to Sites E and P may be related to the lower O2 concentrations in the Panama Basin. If so, further work is required to determine whether it is the intense O2 minimum at 300-600 m or the lower O2 concentration throughout the deep water column that inhibits UBA remineralization. Reduction of U(VI) should not occur in seawater unless
H2S
is present
(Langmuir,
1978).
Therefore.
the redox potential (Eh) is not low enough even in the intense O2 minimum in the Panama Basin to cause the reduction of U(V1) to U(IV) within the water column. Unless uranium is reduced biologically or in reducing microenvironments within the water column (e.g., within fecal pellets), these data are consistent with the conclusion that at least some of the uranium in anoxic sediments is removed from seawater via association of IJ(VI) with organic matter. Fluxes of UBA through the water column in the Panama Basin (Table I ) can be compared to fluxes of authigenic uranium into organic matter-rich shelf and hemipelagic sediments. No data are available for Panama Basin sediments, but authigenic uranium fluxes at other locations (Veeh, 1967; Kolodny and Kaplan, 1973; Veeh et al., 1974) are generally about two orders of magnitude greater than those in the Panama Basin water column. If the UBA fluxes in the Panama Basin are representative of fluxes through the water column at these other locations, then the authigenic uranium in those sediments must be almost entirely fixed by precipitation within the sediments rather than in the water column. Uranium/ organic matter ratios are about 20 times higher in Saanich Inlet sediments (Kolodny and Kaplan, 1973) and range from 12- 100 times higher in West African shelf sediments (Veeh et al., 1974) than in the sediment trap samples. If the organic matter in these sediments has similar uranium binding capabilities to that collected in the sediment traps, then this provides further evidence that most of the authigenic uranium in these sediments was precipitated in situ, supplied by diffusion from the overlying seawater. Thus, these results support to some extent both arguments noted in the introduction: U(VI) does become associated with particulate organic matter in seawater; however, most of the authigenic uranium observed in organic matter-rich sediments results from the precipitation of uranium within the sediments rather than from the complexes formed within the water column. CONCENTRATIONS
OF PARTICULATE
URANIUM
Concentrations of particulate uranium were measured by in situ filtration at Site E and in the Panama and the Guatemala Basins (Table 2). The concen-
tration of particulate uranium in seawiltrr IS very low. Turekian and Chan (I 97 1) and Ku tst ui. ( I977 i showed that filtration of deep ocean water does not measurably reduce the total uranium concentration. Miyake et al. (1973) found less than 0.3 percent 01 the total uranium in Japanese coastal waters to he in particulate form. Hodge er ul. (1979) similarI! found that particulate uranium is only about 0. I percent of the total in California coastal waters. In the deep ocean, where the concentration of particulate material is much less than in coastal waters, and where there is no evidence for the formation of U,,, particulate uranium concentrations (Table 3 1 fall in the range of lo-’ to lO-6 of the total. The 238U//232Th ratios can be used to bhow thai most of the particulate uranium at Site L: ia ~SSO. ciated with detrital minerals, whereas the particulate uranium in the Panama and Guatemala Basins I:, almost entirely authigenic. The detrital “*Uj”‘Th activity ratio is roughly 0.5-- 1 .O. (note the “%,“‘“I% ratios in the sediment samples in Table i ) i’r IS ditficult to determine the 238Uj232Th ratios m samples from the Panama and Guatemala Basins because OI the low 2’ZTh concentrations, but the ratios art clearly much greater than 1.0, and perhaps greater than 10.0 (Table 2). Furthermore, the specitic activity of UBA in filtered particles in the Panama and Guatemala Basins (UBA s UT --- “‘Th) is nearly the same as in the trapped particles at the STIE Site. In contrast, at open ocean Site E, the particulate23”U/232Th activity ratio is less than 1.0 (Table 2). Samples from below 800 m at Site E have ratios in. distinguishable from the 23*U/2’2Th ratio of 0.52 in nearby sediments (CH75-2, Core 8; Table i !. There is an indication of a higher “%J/*‘*Th ratia in the sample from 800 m than in the deeper samples. suggesting that only a small part of the IJs.4 formed a~ the sea surface survives to mesopelagic depths m the fine particle fraction collected by filtration. These results support the conclusion derived from the sediment trap samples that particulate U,, in, rapid11 remineralized in the deep open ocean, where;~s it i> preserved throughout the water column iri octxn margin environments such as the Panama and Gus. temala Basins. The reason for this is not clear; how. ever, as noted in the discussion of the sediment trap results, it may be related to a greater preservation of particulate organic matter in the water column in margin environments compared to the open ocean
Acknowledgments-l am grateful to Drs. Susumu Hon~o. Derek Spencer, and Peter Brewer for providing the sediment trap samples, Peter Sachs and Dr. Michael Bacon assisted with the assembly and deployment of rhe irz sifid pumping systems. Alan Fleer performed the analyses of’ some of the STIE sediment trap samples. Drs Michael Bacon and J. Kirk Cochran provided helpful comments during the preparation of this manuscript. Financial support for this work was provided in part by the National Science Foundation under Grants OCE 7826318, OCE 7825724 and OCE 7727004, the Department of Energy under c<>r.
PARTICULATE tract EY-76-S-02-3566; Education Office.
and a fellowship
from the WHO1
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