Earth and Planetary Science Letters, 113 (1992) 459-472 Elsevier Science Publishers B.V., Amsterdam
The sources and transport of Sr and Nd isotopes in the Baltic Sea Per S. Andersson
and Johan Ingri b
a The Lunatic Asylum of the Charles Arms Laboratory, Dicision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA h Department of Economic Geology, Lule~ Unicersity of Technology, S-951 87 Lule~, Sweden Received April 6, 1992; revision accepted August 21, 1992
ABSTRACT We have determined the concentration and isotopic composition of Sr and Nd in waters from the Baltic Sea. The Baltic Sea is an intracontinental, stratified, brackish water, estuarine-like system, and the rivers emptying into it drain a suite of terranes ranging from Proterozoic-Archean in the north to Phanerozoic in the south. The sampled brackish waters range in salinity from seawater (SW) at 35.289%0 to a minimum of 2.460%~ at the surface in the innermost part of the Gulf of Bothnia. The Sr concentrations show generally conservative behavior, indicating a simple two-component mixing. However, small deviations (3-70 %o) from a perfect mixing line reveal that the imprints from rivers with different Sr concentrations are preserved in the blending. Strontium concentrations from a depth profile across the redoxcline in the Baltic proper indicate that vertical particle transport alters the Sr concentration in the water. Our estimated concentration of Sr in the average freshwater input to the Baltic is ~ 0.03 ppm, which is only about 0.4% of the SW concentration. The Sr isotopic data range from Esr(SW) = 0 in seawater to EsaW(sw) = 7.8 in the least saline Baltic water (BW) sample in the Gulf of Bothnia. The isotopic composition of Sr versus 1 / S t in the Baltic Sea follows an almost perfect mixing line, which shows that seawater Sr is mixed with much more radiogenic components. Calculated end-member values of e~'r(SW) for each sample show that the riverine input into the Gulf of Bothnia has E~'f(SW) = 120-200 and 10-50 • units in the Baltic proper. These values are in general agreement with direct measurements of river waters in each region. However, the calculated values in the Gulf of Bothnia are lower than the measured river water input in this region, which indicates the presence of less radiogenic Sr, presumably originating from the river waters draining the southern part of the basin which are partially transported northward and mixed with Sr from the Gulf of Bothnia rivers. The Nd concentration in the Baltic Sea is not conservative, varying between 5 and 45 ppt, with the highest concentrations in the bottom waters due to vertical particulate transport. A plot of ENd(0) in Baltic water yields a good correlation with the calculated freshwater end member •~'r(SW). The data show that it is possible to unravel the different freshwater sources into the Baltic and to identify the zones of particulate removal of both non-conservative species such as the REE and of quasi-conservative species such as Sr. The use of isotopic tracers in this estuarine environment may provide a much better insight into mixing and element transport, It should also be possible to trace lateral movements of freshwater inputs.
I. Introduction This study reports on the concentration and isotopic composition of strontium and neodymium in brackish water from the Baltic Sea. The 87Sr/ 86Sr a n d 1 4 3 N d / t 4 4 N d r a t i o s v a r y d u e t o l o n g - t e r m
Correspondence to: P.S. Andersson, The Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
radioactive decay and reflect the age of the source t e r r a n e s . T h e v a r i a t i o n in t h e 87Sr//86Sr is c a u s e d b y t h e r a d i o g e n i c d e c a y o f S7Rb t o 87Sr, ( / J - d e c a y , t j / 2 = 4.9 x 101° yrs). T h e r e s i d e n c e t i m e o f S r in t h e o c e a n ( ~ 5 × 10 6 yrs) is l o n g c o m p a r e d t o t h e m i x i n g r a t e o f t h e o c e a n s ( ~ 10 3 y r s ) . O w i n g t o this, s e a w a t e r ( S W ) i n t e g r a t e s t h e flux o f S r from various sources and displays both a rather u n i f o r m S r c o n c e n t r a t i o n ( C s r ~ 7.7 p p m ) a n d a very uniform 87Sr/S6Sr ratio (87Sr/S6Sr = 0.70915) throughout the oceans. However, the concentrat i o n o f S r in t h e o c e a n s c a n b e s o m e w h a t v a r i a b l e
0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
due to the influx of rainwater and to evaporation. By contrast, the average dissolved Sr concentration in major world rivers is about 0.06-0.08 ppm [2,3]. Variation in the za3Nd/144Nd is caused by the radioactive decay of 1475m (a-decay, t~/2 = 1.06 × 10 H yrs) into 143Nd, which increases the abundance of 143Nd through geologic time. A study of Nd concentrations in rivers around the world shows large variations (3-3150 ppt) . The seawater concentration is lower and shows much smaller variations ( ~ 1-5 ppt) . The geochemical behavior of Nd in seawater and in brackish
water is strongly non-conservative  and the oceanic residence time of Nd is much shorter than the mixing time of the oceans. Because of the short residence time of Nd in the oceans, compared to mixing times, the isotopic composition of Nd varies in vertical profiles and in seawater between different ocean basins. These variations reflect the mean age of the terranes that supply the Nd to the oceans and have been used to study both rare earth element geochemistry in seawater and to describe mixing between different water masses in the oceans [5,7,8,9]. • Estuaries and estuarine-like marginal waters
: 5 " " /~." •
' I.~_~ /;
:':" '. : J
[ ] Phonerozoic [ ] Precambrian .--, Watershed boundary • Sampling Station - - - Depth contour, 40 rn ......... Depth contour,lOOm
Fig. 1. Map of the Baltic Sea showing the sampling stations. The inset shows the watershed boundary and the major bedrock units. Note that not all rivers are displayed.
Sr A N D N d I S O T O P E S IN T H E B A L T I C S E A
are the principal places where mixing between seawater and river-introduced freshwater occurs. These areas are important for their role in controlling the input of chemical constituents to the ocean. The general behavior of the elements during seawater-freshwater mixing can be separated into conservative and non-conservative behavior. Salinity is assumed to behave conservatively, which means that it only changes by dilution with freshwater or evaporation and otherwise shows no increase or decrease due to chemical reactions in the estuarine environment. The concentrations of conservative species should show a linear relationship with salinity and depend only on the concentrations in the end members being mixed. The Baltic Sea has been the subject of extensive studies focused on the biogeochemical modeling of nutrients [e.g. 10,11] and the inorganic geochemistry of dissolved and particulate material in the water [e.g. 12-15]. The 878r/S6Sr and 143Nd/144Nd ratios in river water reflect the age and chemistry of the bedrock in the drainage basin from which the river originates, with typically higher 875r/86Sr and lower 143Nd/lanNd ratios in rivers draining older rocks, compared to rivers draining younger rocks [4,16]. Work by o Aberg and Wickman  and I_6fvendahl et al.  has established the Sr concentrations and isotopic compositions of river inputs from the Baltic Sea drainage basin. The data show that rivers draining the Precambrian shield (Fig. 1) have low Sr content (0.02-0.08 ppm) and high 87Sr/S6Sr (0.718-0.745) [17,18]. Rivers draining the Phanerozoic basin in the south and southwest have higher Sr content (0.2-0.5 ppm) and lower 87Sr/86Sr (0.710). However, with the analytical precision available to them, no relationship between 875r/86Sr and strontium concentration was obtained for the Baltic waters that they analyzed, although they did recognize the possibilities of correlated effects. Studies of ferromanganese nodules show significant differences in Nd isotopic composition, with lower values (ENd(0)= --20) in the Gulf of Bothnia compared to the Baltic proper (ENd(0)= --16) . However, no systematic studies of Sr and Nd and their isotopes have previously been undertaken in the Baltic waters themselves. The aim of this study is to investigate the potential of using Sr and Nd isotopes to charac-
terize mixing between river and seawater in a large estuarine area with differing bedrock in the drainage basin. A previous study  in the San Francisco Bay estuary showed a good correlation of concentration and 87Sr/86Sr with salinity, thus demonstrating the generally conservative nature of Sr in the estuarine environment. It is our intent to establish a more detailed study in the Baltic Basin and to relate the Sr and Nd systematics to the different freshwater inputs. 2. Study area The Baltic Sea with its main gulfs, the Gulf of Bothnia and the Gulf of Finland, is a northern European intracontinental sea (Fig. 1) that can be regarded as a large estuarine system. Its surface area is 370,000 km 2, whereas the drainage basin area is about four times larger. Its volume is 21,000 km 3 . The water exchange with the North Sea and the Atlantic Ocean takes place in a transition zone extending from the Kattegatt via the narrow and shallow Belt Sea and the Oresund. The sill depth is 17-18 m in the Belt Sea and 7 - 8 m in 0 r e s u n d . The residence time for water in the Gulf of Bothnia is about 4 years, whereas the total Baltic Sea has a residence time of about 35 years. The salinity distribution is determined by the shape of the connection to the sea and by the net freshwater supply (473 km3/yr). The annual precipitation and evaporation are closely balanced on an annual basis . The freshwater supply from the rivers generates a brackish surface layer of outflowing water separated by a halocline from more saline bottom waters. The depth of the halocline varies from about 20 m in the southern part of the Baltic proper to about 80 m in the northern part. The halocline separates 7-8%o salinity surface water from 10-13%o deep waters. In the Gulf of Bothnia the halocline lies deeper and is not as well developed as in the Baltic proper. The salinities in the Gulf of Bothnia vary between 1 and 7%o. The actual inflow of seawater into the Baltic takes place through a mixing zone in the Kattegatt-Belt S e a - 0 r e s u n d area, where outflowing Baltic surface waters mix with seawater. The salinity difference, i.e. the density difference, generates an average two-layered flow with the saline bottom water flowing in over the sill into
P.S. ANDERSSON ET AL.
the Baltic basin. However, the flow in the mixing zone is often a one-layer flow with the whole water column moving either from or towards the Baltic. For detailed descriptions of the hydrography and water exchange, see Kullenberg [21,23] and Stigebrandt . Due to the combined effect of shallow connections with the sea and stable salinity stratification, the water in the deeper parts of the Baltic proper (below 150 m, station BY-15) is renewed at irregular intervals. Between these renewals, the bottom water becomes anoxic due to the breakdown of organic material [11,25]. The last renewal and complete oxygenation of the deep
water at BY-15 occurred in 1979. Since that time, the deep water has been anoxic. The surface water circulation is weak, with velocities of the order of 1-5 cm/s. The mean circulation in the Baltic proper, Gulf of Bothnia and Gulf of Finland is counterclockwise. The Baltic Sea is almost free of tides and tidal currents. The geology of the Baltic Sea drainage basin is highly variable. The Precambrian crystalline basement of the Baltic Shield is exposed along most of the western, northern and eastern parts of the basin (Fig. 1) and constitutes 45% of the drainage area. About 57% of the water input to the Baltic originates from the Precambrian basement. The
TABLE 1 Salinity, Sr, N d a n d S m c o n c e n t r a t i o n , a n d isotopic d a t a f r o m t h e Baltic Sea
Sample Station location
Baltic Sea brackish water (65°23'5 - N, 23030'0 - E) F-2 A
F-2 D(ut) e
(62035'2"N, 19058'5~E) US-5B A
(57o20'0"N, 20°03'0"E) BY-15 A
- 14.41 +0.37
(57°11'5"N, II*40'O'E) FLADEN A
River water Kemi
0.733035 + 83
336 + 1.1
(57"30'0" N, 6059'0 - F,) H-6 SW
(0o03 ' S, 34049 , IV) CIT SW NBS 987
R e p o r t e d e r r o r s for isotope ratios a r e 20- of t h e m e a n . U n c e r t a i n t i e s for Sr, N d a n d S m c o n c e n t r a t i o n s s p r i n g cruises, 1990 a n d 1991. b p p m ( m g - k g - l ) ; c p p t ( n g - k g - 1 ) ; ~ (f) 0.45 g~m filtered; e (uf) u n f i l t e r e d .
~ 0.1%. a R.V: Argos
Sr AND Nd ISOTOPES
IN T H E B A L T I C S E A
Baltic Shield features ages that become younger from the northeast to the southwest, these ages ranging from Archean > 2.5 A E (1 A E = 1 aeon = 109 yrs) to Proterozoic 1-2.5 A E . In the southern and southeastern part of the basin, the terrain is composed of Phanerozoic sedimentary rocks from diverse provenances .
3. Sampling and analytical methods Samples were collected in May and June of 1990 and 1991 from the R.V. Argos (Fishery Board of Sweden) during one of the vessel's usual monitoring cruise in the Baltic Sea. The sampling stations are shown in Fig. 1 and their coordinates are given in Table 1. Temperature, salinity and nutrients were measured at each site as part of the R.V. Argos research and monitoring program. Samples were taken from surface waters (5 m deep) at four stations. Vertical profiles were taken in the Baltic proper (BY-15), mid-way up the Gulf of Bothnia (US-5B), and in the northernmost part of the gulf (F-2). One sample, H-6, was collected in the North Sea to provide a value for the ocean water that flows into the Baltic. The water was collected in 20 1 and 60 1 GO-FLO bottles internally coated with Teflon and with Teflon valves. During the 1990 cruise the water was immediately transferred to 10 1 polyethylene containers using silicone tubing without filtration. On the 1991 cruise the samples were filtered through 0.45 /zm membrane filters within a few hours of collection by the use of pressurized peristaltic pumps. Samples from the Kalix and Kemi River were collected on 22 May 1991 and 8 June 1991 respectively. The sampled river waters (10 1) were filtered in situ through 0.45 /zm filters using a peristaltic pump. The filtered samples were then acidified with 25 ml of ultrapure HCI. All bottles, tubing, filters and filter holders were acid-cleaned before use. The amount of sample used for Sr concentration and 87Sr/86Sr determination was 1-2 g for brackish water and 40-50 g for river water. Samples were spiked with 84Sr and the Sr concentrations measured by isotope dilution. The Sr in the spiked samples was separated from Rb by a cation-exchange procedure . About 400 ng of Sr were loaded on an oxidized V-shaped Ta-fila-
ment and analyzed on the Lunatic I mass spectrometer  following the procedures described in Papanastassiou and Wasserburg . The data obtained were corrected for mass fractionation in the spectrometer with a discrimination factor calculated using a power law and normalizing to 86Sr/88Sr = 0.1194. Possible isobaric interferences from 87Rb were checked using 85Rb and the S7Rb/86Sr and were found to contribute less than 10 -5 to the measured 87Sr/86Sr ratio. Measurements of the standard SRM 987 (SrCO 3) from the National Institute of Standards are reported in Table 1. Total procedural blanks for Sr were approximately 0.1 ng. The accuracy of the Sr concentration determinations is within 0.1%. The errors quoted for the 87Sr/86Sr ratios are 20- of the mean for 150-200 ratios. The Nd and Sm concentrations and isotopic compositions were determined using ~ 5 1 of water following iron hydroxide precipitation and ion exchange separation techniques. Detailed descriptions of the analytical technique have been given elsewhere [31,32]. Total procedural blanks for the separation of Nd were typically less than 50 pg. The accuracy of the Nd concentrations is within 0.1% and the error in the isotopic composition is 20- of mean for ~ 200 ratios.
4. Data representation Measured 878r/86Sr and 143Nd/144Nd ratios are presented as fractional deviations in parts in 104 (e units) from H-6 (875r/86Sr)sw = 0.709168 and from 143Nd/144Nd in a chondritic uniform reservoir ( C H U R ) having a 143Nd/144Nd of 0.511847 (see  and references therein). All the reported Sr measurements deal with small differences in isotopic composition compared to seawater (10 -4 level). Thus, it is important to have seawater composition well determined under the same measuring conditions as the samples in order to diminish the effects of systematic errors in the method. For comparison, a sample of seawater from the Atlantic Ocean was analyzed (CIT SW). The CIT seawater Sr standard was collected in 1963 and this sample has therefore been used as an internal laboratory isotopic standard for two decades. There is no significant difference between seawater 87Sr/86Sr from station H-6 and the CIT seawater (Table 1). The difference be-
tween the concentrations in H-6 and CIT SW is attributed to evaporative loss over a long period of time. The concentration data for station H-6 are considered to be more precise and reliable. Thus, using the H-6 seawater for normalizing reflects the deviation in the Baltic water (BW) relative to seawater as measured in the North Sea. 5. Results
The results of Sr, Sm and Nd analyses and salinity data are given in Table 1. To evaluate the influence of particles on the Sr analysis, a filtered (0.45 /~m) and an unfiltered aliquot of the same sample were analyzed (F-2 D, Table 1). The Sr concentration was 0.07% higher in the unfiltered portion, which is within error, and there were no significant differences in the 87Sr/86Sr ratios. The particulate load was measured at station BY-15 in June 1991 and varied between 0.1 and 0.2 mgl-1. Particulate load is defined as the weight of inorganic material obtained on the 0.45 /xm filter which was ashed at 550°C; the particulate load in our Baltic Sea samples is too small to have any significant effect on the Sr composition, and we have thus used unfiltered Baltic water for the Sr determinations. We did not filter the samples for the Nd analysis because we wanted to obtain data on the concentration in the Baltic without dealing with contamination problems, which can be introduced during filtering. The salinity in the surface waters (5 m) decreases from 14.277%o at FLADEN to 2.460%0 at station F-2. There is an increase in salinity towards the bottom at both stations BY-15 and F-2, reflecting the mixing between the freshwater layer on top and more saline bottom water below. We determined the Sr concentrations in Kalix and Kemi River and found each to contain 0.011 ppm of Sr (Table 1). Sr concentration data have previously been obtained from weekly sampling of the Kalix River [Pont6r and Ingri, unpublished data]. The results on filtered samples over a one-year period vary from 0.01 ppm in the spring to 0.03 ppm during the winter. Data from a set of rivers that contribute about 80% of the total inflow to the Gulf of Bothnia  give a value of 0.026 ppm Sr for average river input in this area (Table 2). Taking into account all of their mea-
P.S. ANDERSSON ET AL. TABLE 2 C a l c u l a t e d f r e s h w a t e r e n d - m e m b e r v a l u e s C~'r and e~'r(SW) a n d l i t e r a t u r e d a t a on r i v e r w a t e r c o m p o s i t i o n in the Baltic basin Sample
F-2 A F-2 B F-2 C F-2 D F-2 D(0a F-2 D(uf)b
0.0295:0.001 0.0265:0.002 0.0394-0.002 0.0365:0.002 0.0264-0.002 0.0274-0.002
1634-13 1894-19 1224-11 1294-11 2004-19 1955:38
US-5B A US-5B D
BY-15 A BY-15 B BY-15 C BY-15 D
0.085+0.004 (-0.035+0.004 0.3174-0.006 0.163+0.007
53+13 -113+25) ~ 124-7 134-14
Gulf of Bothnia Gulf of Finland Baltic Proper Baltic Sea total
0.015-0.076 n=23 0.029-0.077 n=4 0.040-0.504 n=ll 0.015-0.504 n=38
0.026 0.052 0.367 O.125
f4,"(sw) ~,f,'cs~ 120-507 n=50 287-406 n=4 5-287 n=ll 5-507 n=65
308 90 11 44
a (f) 0.45 ~ m filtered; b (uf) u n f i l t e r e d , c O b s e r v e that C~"r is n e g a t i v e (i.e., no physical m e a n i n g ) , d R a n g e of m e a s u r e d Sr c o n c e n t r a t i o n in r i v e r w a t e r e m p t y i n g into e a c h basin . e M e a n ( m e a s u r e d w e i g h t e d m e a n plus e s t i m a t e d ) riverine Sr c o n c e n t r a t i o n into e a c h b a s i n . f R a n g e of m e a s u r e d S7Sr/86Sr in rivers e m p t y i n g into e a c h basin [17,18]. g M e a n ( m e a s u r e d w e i g h t e d m e a n plus e s t i m a t e d ) riverine 875r/86Sr into e a c h basin . n = n u m b e r of m e a s u r e d rivers.
surements, L6fvendahl et al. sampled about 50% of the total inflow to the Baltic and estimated 0.367 ppm for the average Sr concentration of all rivers emptying into the Baltic proper and 0.125 ppm for the concentration in the average total riverine input to the Baltic Sea. Measured riverwater (RW) ERW(sw) in the Kalix and Kemi River (Table 1) gives eRW(SW) = 316 and 336 respectively, which is in good agreement with the values of 334 and 354 previously reported for these rivers . Literature data [17,18] covering about 80% of the total inflow to the Gulf of Bothnia show that the river waters have eRW(sw) in the range 120-507, with an estimated mean of 308 e units (Table 2). There
are fewer data for the rivers draining the southern Phanerozoic part of the basin, but available measurements point to a much lower eSr value (es~ < 10) for these inflows . The Vistula River, which contributes about 7% of the water and about 25% of the Sr input to the Baltic, has a Sr isotopic composition that appears to be only a few e units above the seawater value [3,18].
6! 4! 2 • ~ " 10 " 1 5
5.1 Strontium concentrations
80 60 40 20
The equation for a two-component mixing between seawater and a riverwater component of any species, C i, is given by: CM = XC sw + ( 1 - X) CRw
where C/M is the concentration of species i in the mixture and X represents the mass fraction of the seawater end member. This linear equation applies to all conservative species in two-component mixing. In our case, if salinity and strontium are used as conservative species, a plot of Cs~ vs. salinity (S) must yield a straight line, if the samples are a result of conservative mixing of two well-defined end members. In Fig. 2a the Sr concentrations from all the Baltic water samples are plotted versus salinity. The graph shows a linear relationship supporting the conservative behavior of Sr. Fitting a straight line through all data points including SW (n = 14) using an isochron program  gives a line with the equation: Csr = (0.222 + 0.006) × S + (0.03 ___0.03)
The intercept at S = 0 suggests a value of = 0.03 p p m for C s~ Rw, but this p a r a m e t e r is not at all well defined by this treatment. In Fig. 2b the deviations for each point from the best fit line are shown. As the analytical uncertainty is less than 0.1%, the plot reveals significant deviations from a perfect mixing line. The three deepest samples from the vertical profile in the Baltic proper, BY-15 B, C and D, show the largest deviations, - 4 3 , 70 and 21%o respectively. Excluding these three points (see later discussion) and calculating a best fit (n = 11) yields the equation: Cs~ = (0.219 + 0.002) × S + (0.032 + 0.009)
which is the same as eq. 2 but with a more precise slope and a better defined intercept at S = 0 of
• BY-15 D • BY-15 A . F-2 | • US-5B D * ~ • US-5B A
-20 -40 -60 -80
• FLADEN A
• BY-15 B
' 10 Salinity
' 15 (o/~)
Fig. 2. (a) Sr (ppm) vs. salinity (%~) in the Baltic water with the best fit line from eq. 2 as described in the text. (b) Deviations (3) in permil of measured C~rw from the best fit line. The analytical error is + 1%o of the measured values.
0.032 ppm. This intercept corresponds effectively to the Sr concentration in the freshwater component. The deviations for each sample from an exact mixing line demonstrate that there are variations of 3-70%0 around the line which are superimposed on the general pattern of two-component mixing. These deviations are probably caused by many rivers with differing Sr concentrations emptying into the Baltic. The intercept, C SRw r , is distinct from zero as seen in eq. 2 and 3, with a value, however, of only sw . Using eq. 1 for both strontium 0.4% of Csr concentration and salinity and assuming, S RW = 0, ( X = s B W / s s w ) eq. 1 for Sr then becomes:
Ic w+ (,sw
If we consider each sample as a mixture of seawater and a single freshwater component, we may calculate a value of Csr for the freshwater concentration by forcing a straight line through the seawater value and each data point. We define the value for the Sr concentration calculated in
P.S. ANDERSSON ET AL.
0.35 0.30 0.25 0.20 o, 0.15 ° ~ 0.10 0.05 0.00 -0.05
Nd, Sm (ppt) 10 20
-0.05 0 F .
0.10 C~ 0.25 . . . .
30 " • F-2 A *
US-mSBA • US-SB D
1'2 1'4 ' 16 8 10 Salinity (%o) Fig. 3. Calculated Sr concentration in the freshwater end member, C*sr, for each sample vs. salinity (%0). C~r is the calculated Sr concentration of the freshwater component assuming the sample is a mixture of seawater and freshwater with zero salinity. Note that sample BY-15 B shows a negative C~. Error bars from Table 2. 4
this way for an end member as: -80
c wssw_ csws w c¢, =
ss w _ sB w
The results are not significantly dependent on the assumption that the salinity of the freshwater component is zero. We note that if (as is certainly the case) the freshwater component is a blend of different waters, C~'r is the effective average concentration in that mixture Carw. In any case, for conservative systems the values of Cs* will always be physically reasonable. For multicomponent freshwater sources, changes in C~ w would give deviations from a pure two-component mixing curve. The calculated values, C*st, are given in Table 2 and are shown in Fig. 3. As we will argue later, most samples give values of C~'r which do appear to be physically sensible. The samples from the depth profile in the Baltic proper (BY15) show both large deviations from the mixing line and large variations in C~. However, one of the samples (BY-15 B) displays a negative C*Sr, which is a physically meaningless value, that indicates removal of Sr. Plotting C~'r for the samples from the depth profile and the deviation (/~) from the conservative mixing line vs. depth (Fig. 4) shows that the maximum Cs* and 6 coincide with the redox interface. In Fig. 5 a schematic picture of conservative mixing between SW and RW is drawn. The arrows show the consequences of deviation from a mixing line on the calculated end member concentration ( C*Sr)" The calculated end member concentration will be too high (C~'r+)
Fig. 4. A vertical profile at station BY-15. C~r (©) and 6 (%0) deviations from the best fit line (see Fig. 2) (o). Nd ( [ ] ) and Sm (11) in ppt. The water depth at BY-15 is 240 m. The redoxcline at 150 m coincides with the maximum for Cs*r and 6. An increase in Nd and Sm concentration is observed between 75 m and 150 m, which is coincident with the redoxcline and anoxic water.
if salinity is constant and Sr is added or if the Sr concentration is constant and salinity decreases. The calculated end-member concentration will give too low a value of C~'r (C~'r-) at constant
r,.) .... ..... ....
c•t c~' c;; Salinity --~ Fig. 5. Schematic diagram of Sr concentration, Csr, vs. salinity, showing the result of a non-conservative behavior. The solid line shows the mixing between SW and RW. The arrows denote the results on the calculated end member, C~r, when measured Csr deviates from a mixing line. The calculated end-member concentration will be too high ( + ) (point P ) when salinity is constant and Sr is added; it will be too low ( - ) (point Q), when Sr is removed at constant salinity.
Sr AND Nd ISOTOPES IN THE BALTIC SEA
0.6 0 30
U (ppb) 0.8
Mn dissolved (ppb) 100 200 300 400
g 9 10 Salinity (%0) L . , . , . i . . . 0 20 40 60 80 Oxygen saturation
. . . 100 120 (%)
1 2 3 4 5 Mn particulates (%)
Fig. 6. (a) Vertical profile of dissolved ( < 0.45/zm) U concentration (ppb, ~g-kg -t) (o), oxygen saturation (%, dashed line), and salinity (solid line) from BY-15, 11 June 1985, L6fvendahl . The redoxcline coincides with the U maximum. (b) A schematic vertical profile for dissolved Mn (ppb, solid line) and particulate Mn (%, dashed line). Mn particles expressed as percentage of total inorganic particles. Data taken from Bostr6m et al. , who measured dissolved and particulate Mn across the redoxcline at the Landsort Deep, NW Baltic Sea, in August 1982. The arrows show the migration of dissolved (aq) Mn(lI) from the anoxic water into the oxic water, where it oxidizes to insoluble (s) Mn(IV). These oxyhydroxide particles fall in the water and redissolve in the anoxic water below the redoxcline.
salinity when Sr is being removed or when salinity increases at constant Sr. As the salinity is conservative, the BY-15 depth profile shows that the Sr in the water column is depleted above the redoxcline and is enriched at the redoxcline. Thus, it is clear that Sr does not behave conservatively in the depth profile. The cause for the observed non-conservative behavior of Sr at BY-15 could be redistribution by the trapping of Sr in sinking particles. Earlier m e a s u r e m e n t s of dissolved uranium at BY-15  show the same patterns as C*S t , with a maximum in U concentration near the redoxcline (Fig. 6a). The similarities between the measured U concentration and our C~r points to similarities in the removal mechanisms. The region above the redoxcline is associated with a high content of authigenic Fe-Mn oxyhydroxides [14,15,36]. These particles originate from anoxic
waters with high concentrations of dissolved Mn(II), which migrates upwards into the oxic waters where it can precipitate as insoluble oxyhydroxides (Mn(IV)) [14,15,36]. In Fig. 6b a schematic Mn profile across the redoxcline at BY-15 is plotted using data from the Landsort Deep, NW Baltic Sea . The profile demonstrates the migration of dissolved Mn(II) from the anoxic water into the oxic zone below the halocline, where it oxidizes to insoluble Mn(IV). As the insoluble Mn(IV) forms it can scavenge Sr. T h e r e is evidence of Sr enrichment in Fe-Mn hydroxides . The S r / A l ratio is enhanced in Fe-Mn concretions at the sediment surface in the Baltic compared to average crust  which indicates that Sr is associated with Fe and Mn. It would appear that particulate formation, settling and redissolution may affect the Sr concentration in the anoxic zones. The presence of suspended BaSO 4 (barite) particles in the Baltic proper have also been reported [14,15]. The abundance of barite is about 5% of the suspended particle content in the Baltic Sea, but it could reach 44% . It has been shown that barite may contain 3% Sr  and that Mn and Ba are correlated both in Fe-Mn particles [15,36] and in Fe-Mn concretions in the Baltic . Thus, barite formation and settling could also play a role in Sr redistribution. The calculated values of C~'r for the freshwater end m e m b e r s for samples from the Gulf of Bothnia (F-2 and US-5B) yield low values ( --- 0.03 ppm), whereas in the Baltic proper (BY-15) they yield higher values ( = 0.1-0.3 ppm). The value of C ~ r ~ 0.06 p p m at FLADEN is about twice the value in the Gulf of Bothnia. The variable results for C~r show that the lowest values occur at the northernmost stations and high values occur in the Baltic proper. These values are in general agreement with literature data for riverwater input (Table 2), with lower Sr concentrations for rivers draining the Precambrian and higher concentrations for rivers draining the Phanerozoic. From our calculated values of C~r and the good correlation with the published river data, we infer that the C~'~ values indicate distinct variabilities in the river water Sr concentrations from different areas and that the different sources contributing to the Sr in Baltic waters can be seen through the various blends of freshwaters and seawater.
P.S. ANDERSSON ET AL.
5.2 Strontium isotopes
The general equation for isotopic composition in a two-component mixing between SW and RW has the form:
r(SW) XCsSW SW( s w ) + (1 -x)c
XCssw + (1 - x)c
where X is the mass fraction of SW in the mixture. By choosing seawater as a standard, esSW(sw) = 0 and the SW term is eliminated in the numerator but not in the denominator. The conservative behavior of salinity can be used to obtain the fraction of SW in the mixture. Assuming salinity in the riverwater to be zero, and the mass fraction of SW to be given by X = SBw/ssw, the isotopic composition in the Baltic Sea can be rewritten in terms of salinity using eq. 1 and 6 as: SSW ~-sBT(SW) =
RW (SW) Csr RW )es~
Note that the parameters representing the isotopic composition of the freshwater component occur as the product eSr RW(SW)Cs~ RW. Further, the contribution of C Rw in the denominator (eq. 6) is small for the range of salinity values measured. The measured values of eBW(sw) in the Baltic water samples were found to increase with decreasing salinity, displaying the highest values in the surface at the northernmost station (F-2, Table 1). Equation 7 is a hyperbola and it can be rewritten as:
RW RW ~Sr ( S W ) C s r
Cs~w RW RW -- ESr ( S W ) f s r
This yields a straight line if plotted vs. ( 1 ) / ( C ~ w ) (Fig. 7). A mixing line is calculated by using seawater as an end member with ESW(sw)---0 and Cssw -- 7.73 ppm. The data yield a good fit to a mixing line with ESr RW(SW)Cs~ RW = 4.8 _+ 0.2 and an intercept of - 0.62 _+ 0.03. Assuming C Rw 0.03 ppm in the freshwater end member, as calculated from eq. 3, yields Esr(SW) = 149 _+ 43 in the
110 */C~w (ppm")
Fig. 7. E~W(sw) vs. 1/Csr (ppm -1) showing that the data follow eq. 8 for mixing between seawater and freshwater.
freshwater component. As the mixing curve is governed by the product, eSr Rw(SW)CsrRW, we will obtain the same shape either with high esr and low Csr or vice versa. We note that there are undoubtedly deviations from a single mixing line which may be resolved with the ultrahigh precision measurements that are now becoming available. The freshwater end member component calculated from eq. 7 and 8 falls within the reported data for Baltic river input (Table 2). However, as the variations in concentrations and isotopic composition between different rivers are large, a more complicated case than a simple two-component mixing is at hand. In the general case of many freshwater sources contributing to a given sample, an equation analogous to eq. 7 applies but with ~'Sr RW( S W ) C s RW r replaced by ~sR~W(sW)Csarw, where the bar represents the average values of the mixture. If, for example, the average concentration stays fixed and the gRW(sw) shifts, the simple mixing rule is no longer linear. Using the calculated end-member concentration, C*sr, for each sample and using the mixing relationship in eq. 7 and 8, we may calculate the effective isotopic composition of Sr in the endmember freshwater component for each sample (Table 2). The values for the Gulf of Bothnia vary between 120 and 200 E units. In the Baltic proper there is a substantially lower e~'~(SW), between 10 and 50 e units. Comparing these results with the direct measurements for Kalix and Kemi River (Table 1) and with the measured and estimated input to the Gulf of Bothnia shows that our calculated e~'r(SW) is a factor of ~ 0.5 lower.
This implies that a substantial portion ( ~ 40%) of the Sr in the Gulf of Bothnia must have a less radiogenic source than the rivers draining the Precambrian basement. Thus, it is plausible that Sr from the southern drainage basin is transported to the Gulf of Bothnia. This may be due to movements of waters above the halocline and possibly upward transport across the halocline. Riverwaters from the southeastern part of the Baltic basin will go northward due to the counterclockwise circulation in the Baltic Sea.
5.3 Neodymium There is no correlation between Nd and Sm concentrations and salinity in Baltic waters (Fig. 8), clearly showing that Nd and Sm are non-conservative in their behavior. There is a general increase in both Sm and Nd concentrations as a function of depth (Table 1), which either indicates resuspension of bottom sediments or scavenging by sinking particles in the water. This increase in concentration with depth is similar to that found in the oceans , but the increase in the Baltic is more extreme. At station BY-15 the Nd concentration is almost 5 times higher at the bottom compared to the surface, whereas eNd(0) is constant ( ~ --14.7) throughout the water column. This implies that there is vertical downward transport of Nd with a uniform Nd isotopic composition in the profile. This point indicates that surface sources (e.g. freshwater sources) will dominate the particulate removal. Compared to Cs* there is no maximum at the redox interface for Nd and Sm concentrations
z 20 I
~0 8 " 1'0 " 1'2 " 1 4 " 16 Salinity (%o) Fig. 8. Concentration of Nd (ppt, o) and Sm (ppt, ©) vs. salinity (%o) in the Baltic water. oq
-10 -12 -14 "~-16
8 10 Salinity (%o)
Fig. 9. ~Nd(0) VS. salinity (%0) in the Baltic water. A general trend is evident but there does not appear to be any quantitative relationship.
(Fig. 4), which indicates different mechanisms for the downward particulate transport between Sr and Nd and Sm. There is no clear relation between eNd(0) and salinity (Fig. 9). However, there is a general trend with lower eNd(0) values at lower salinities, which shows that the Nd in the riverwater entering the Gulf of Bothnia reflects the age of the source terrain. From the relationship eNd(0)= Ofsm/NdT  we can calculate a model age, T, in aeons. Here, fSm/Nd is the enrichment factor giving the Sm-Nd fractionation in the crust related to the bulk earth by: fSm/Nd = [(147Sm/144Nd)m/(147Sm/t44Nd)cHUR
-1] where m is the measured value and ( 1 4 7 5 m / 144Nd)cHUR = 0.1967; Q is a constant ( = 25.13 A E - t ). Assuming an average fSm/Nd of ------0.4 in crustal samples, we obtain a model age of = 2 A E for the age of the Nd source rocks in the basin draining into the Gulf of Bothnia. This is in accordance with the Precambrian age of the rocks of this region . eNd(0) shows a general correlation with e~W(SW) (Fig. 10a), with low eNd(0) at high eBW(sw). Due to the non-conservative behavior RW but it should be of Nd we cannot calculate CNd, at least a factor of 10 higher than the seawater concentration. Therefore it is not possible to calculate a meaningful mixing curve for ENdBW--EsrBW. Due to the very low Csr in the riverwater compared to the seawater concentration, the isotopic signal of Sr from the freshwater is largely over-
P.S. ANDERSSON ET AL. -8 "IOf~A -12[
Bvqse Bv-~s^ 1. " -" :I , ) ' 1 6 l B:,SD'" B Y - I , B S-14
-22t _241 0
[ , US-SBD
~ xx : i , i 5 6
i , i 3 e~Wsw)4
~ ' ,
-12 ~ Y - 1 5
y=-12.3 - 0.04x
5'0 7'5 '1
E ~, (Sw)
Fig. 10. (a) eNd(0) VS. eBrW(SW) in the Baltic water. T h e r e is a general relationship but no clear correlation between these parameters. Note that samples BY-15 B, C and D are from the vertical profile crossing the oxic/anoxic interface. These samples show evidence of non-conservative behavior of Sr (see Fig. 4). (b) ENd(0) VS. ~ r ( S W ) in the Baltic water. Solid line r e p r e s e n t s the best fit for BY-15 A, F-2 C and F-2 B.
10b.) This could be due to the fact that Sr in the BY-15 profile shows evidence of non-conservative behavior. Non-conservative behavior of Sr yields values of Cs*r that are erroneous and thus lead to erroneous calculated values of e~r. The difference between the surface and deeper water could also be due to different sources for the Sr. Strontium in the deeper parts of the Gulf of Bothnia could be influenced by less radiogenic Sr transported from the southern part of the Baltic Sea. The intercept calculated from the line in Fig. 10b at e~r(SW)= 0 points to a seawater ENd(0) value of = - 12. This value is roughly 2 - 4 e units lower than values reported from the Norwegian Sea (ENd(0)of between -- 10.3 and - 7.7 ). This discrepancy could indicate that we have an incorrect estimate of ENd(0)in the part of the Norwegian Sea that contributes Nd to the Baltic Sea. Another explanation could be that our Nd samples were unfiltered and that particulates contribute to the net Nd. This would imply that the suspended load has a lower ENd(0) than the dissolved Nd . Thus, our ENd(0) could be systematically 1 or 2 units too low. Further experiments need to be carried out to evaluate the significance of these factors. 6. Conclusions
printed by seawater. For Nd we have the opposite relationship, with more Nd in freshwater compared to seawater. This means that the Nd isotopic signal in the Baltic mainly reflects the freshwater input. Labelling the freshwater with regard to Sr and Nd, we use the calculated end-member Sr isotopic composition, e~'r(SW) vs. ENd(0) and exclude station FLADEN, which obviously is very close to SW in Sr isotopic composition (Fig. 10b). The E N d ( 0 ) values show better correlation with e~'r(SW) than with esr(SW). This relationship is not due to mixing; instead it displays the correlation between Nd and Sr isotopic compositions in the freshwater component. Sample BY-15 B is not plotted in Fig. 10b because of its negative C*Sr, which gives an erroneous e~r(SW). The three samples, F-2 B, F-2 C and BY-15 A, fall on a perfect line (Fig. 10b). The deep-water samples, below the halocline, from the Baltic proper (BY15 C and D) and from the Gulf of Bothnia (US-SB D and F-2 D), deviate from the line (Fig.
The Sr concentration in the Baltic Sea shows a general two-component conservative mixing between sea- and riverwater. However, using all the data points we observe small deviations from a perfect mixing line. We attribute this to the fact that there are many different inflows into the Baltic draining different terrains with correspondingly different concentrations (C Rw) of Sr. If we assume that each Baltic water sample corresponds to a mixture of a single effective riverwater source and seawater, we can calculate the concentration of Sr in that source (C~r). The calculated values for mean surface waters show higher concentration in the Baltic proper compared to the Gulf of Bothnia. This is in good agreement with observed Sr concentrations in rivers from different regions of the Baltic, the Precambrian in the north having low Sr in rivers, while the rivers draining the Phanerozoic sedimentary rocks in the south have a higher Sr content.
Sr AND Nd ISOTOPES
IN THE BALTIC SEA
The data from the depth profile in the Baltic proper, which comprise samples across a redoxcline, show that there are small but substantial variations in the C~r that are not simply related to different freshwater sources. This suggests that Sr is subject to redistribution by particulates in this zone and thus that Sr is not strictly conservative. The strontium isotopic composition follows an almost perfect mixing line, with higher e~W(sw) at lower salinities. Calculated values of the Sr isotopic composition in the freshwater end member for each sample show the most radiogenic Sr (120-200 e units) for the Gulf of Bothnia and least radiogenic Sr for the Baltic proper (10-50 e units). The values of the calculated concentrations and isotopic compositions of the freshwater end members in all surface waters are in general agreement with direct measurements of the river inflows from the different subregions of the Baltic basin. However, the calculated data from the Gulf of Bothnia indicate the presence of a less radiogenic freshwater component than in the rivers entering this region. These results suggest that it may be possible to trace the lateral transport of freshwaters across the Baltic. The Nd and Sm concentrations in the Baltic Sea do not show conservative behavior. The concentrations increase with depth, as in the ocean, but with larger increases in the Baltic. This is attributed to particulate scavenging and downward transport. The eya(0) values show a general trend of increasing with salinity but do not fall on a mixing line. The lowest ENd(0) value (--22) is found in the innermost part of the Gulf of Bothnia, which is in general agreement with the ages of the Baltic Shield in that region. The measured Nd isotopic compositions correlate with the calculated isotopic compositions of the river input E~'r(SW) for the same water and we assume that ENd(0) in the Baltic mainly reflects the freshwater input. The correlation between ENd(0) and the calculated E~'r(SW) of the freshwater end members shows that the isotopic compositions of the end members are reasonably well defined. As the Baltic Sea is relatively stable compared to typical estuaries, it appears to be an excellent area for studying both the geochemical removal processes in estuaries and the mixing of different waters draining terrains of different ages. The
use of coupled isotopic tracers shows well-defined relationships that may serve as a key to the chemistry of almost conservative and non-conservative species in estuarine environments.
Acknowledgements We acknowledge the advice and aid of D.A. Papanastassiou. We also thank H. Ngo for analytical assistance and advice and Kurt Bostr6m for his generous support during this study. In addition, we thank the SMHI (Swedish Meteorological Hydrological Institute) for allowing us to participate in the R.V. Argos cruises; the assistance and hospitality of both the SMHI scientific party and the crew on the Argos are greatly appreciated. We are also grateful for the comments of J.F. Minster and an anonymous reviewer, who contributed to substantial improvement of the manuscript. This work was supported by a grant from the Department of Energy (DEFG0388ER13851 and the National Science Foundation (NSF O C E 9018534). Per S. Andersson was supported by a post-doctoral fellowship from the Swedish Natural Science Research Council (NFR G-PD 6331-300 and G - G U 6331-301). This is Division Contribution 5131(770).
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