Nuclear Instruments and Methods in Physics Research B29 (1987) 332-334 North-Holland, Amsterdam
332
“Be IN THE ATLANTIC OCEAN, A TRANSECT AT 25 ’ N M. SEGL I)*, A. MANGINI and C. MEASURES
‘), J. BEER 3), G. BONANI 3), M. SUTER 3), W. WijLFLI
3,
4,
‘) Institut fti Umweltphysik, Universittit Heidelberg Im Neuenheimer Feld 366, 6900 Heidelberg, FRG ‘) Heidelberg Academy of Sciences, Paleoclimate and Dating Project c/o Instiiut ftir Umweltphysik, Universitiit Heidelberg, Im Neuenheimer Feld 366, 6900 Heidelberg, FRG 3, Institut ftir Mittelenergiephysik, ETH Zurich, Hiinggerberg, CH-8093 Zurich, Switzerland 4, Department of Earth and Planetary Sciences, Massachusetts Institute of Technology Cambridge, MA 02139, USA
The “Be concentration of the water column was measured on 5 profiles on a transect across the Atlantic at = 25 o N. In the deep water the profiles display a “Be concentration of 1920 atoms/g near the Mid-Atlantic Ridge, 1300 atoms/g in the Western Basin, and 1300-1670 atoms/g in the Eastern Basin, From this, the residence time of “Be in the deep water can be calculated to 380-560 Ma. This agrees well with earlier estimates from the flux into sediments from the Eastern Basin. For the surface water, the residence time yields to lo-13 a.
1. Introduction Cosmogenic “Be
is used for dating
deep-sea-sedi-
BP [8,4]. At present only few “Be measurements in ocean water are available so that its behavior in the water column is poorly understood. Values for the residence time for “Be in the water column range between a few thousand years for the Central Pacific Ocean from one single measurement of Krishnaswami et al. [3] and = 630 a for the Atlantic as given by Raisbeck et al. [7] assuming a “Be production of 1.3 X lo6 at./cm2 a. Studies of the “Be flux to sediments from an upwelling area off West-Africa and from the Antarctic Convergence Zone suggest a residence time in the Atlantic Ocean of about 400 a [4], a value comparable to Raisbeck’s result. This residence time was estimated by comparison of the “Be and the 230Th fluxes into the sediments, applying only the residence time of 230Th in the water column of 26 a [l]. We present “Be data from a transect across the Atlantic Ocean at 25 “-28 o N. ments
and
Mn-nodules
back
to 14 Ma
56 in 1980. The depth-resolution is 8-10 depths/profile. 30 liter samples for “Be were taken with 101 Niskin bottles on a rosette. On board the water was emptied into 301 containers, acidified with 8N HCl to pH l-2 and brought to the laboratory for further treatment. The “Be samples were spiked with 1.35 mg of 9Be carrier. Then the pH was brought to 8 adding NH,OH and samples were stored over night. The precipitate, consisting of Be(OH), together with Mg(OH), and other hydroxides was filtered. The precipitate was dissolved again in 8N HCl and the Be was purified using wet chromatography techniques described elsewere [8]. The “Be/‘Be ratio was measured at the AMS facility of the ETH at Ztirich. At each depth where “Be was sampled, 200 ml of water was filled in acid-cleaned PE bottles and acidified with destilled 6N HCl for 9Be analysis. However, the sampling conditions on board were unfavourable for 9Be measurements because of a weight of 10 ton of lead attached directly over the rosette. As the Niskin bottles were closed during upward motion, some contamination was expected. 9Be was measured by Measures and Edmond. MIT.
2. Sampling and preparation techniques The samples were taken at = 25 “-27 o N, and 60 ’ W, 50° W, 45O W, and 35” W during cruise SONNE 41 in 1986. This data was completed by a profile measured on samples taken at-28 o N, 25 o W during cruise METEOR * Present address: Fachbereich Geowissenschaften, Universitat Bremen, Bibliothekstrasse, 2800 Bremen, FRG. 0168-583X/87/$03.50 (North-Holland
0 Elsevier Science Publishers Physics Publishing
Division)
B.V.
3. Results The “Be data are shown on fig. 1 and table 1. On all five profiles the “Be concentration decreases from = 1500 atoms/g at the surface water to about 1000 atoms/g at 200-300 m depth. In the deep water the “Be concentrations are slightly higher near the Mid-
M. Segl et al. / “Be in the Atlantic Ocean “Be concentration Water ,~~2~i~~~?~i~?~ depthkr
lxlOOOatoms/c
11
I23
1000
2000
Table 1 “Be concentrations of the 5 profiles together with ‘Be concentration at the stations 3MS and 9MS. For “Be measurement on station ME 56, two samples from different depths were combined to get enough water. Depth (m)
“Be (at./&
3MS
10 100 200 500 1000 1500 2000 3000 4500
1407+ 10% 1318& 13% 1127*12% 1013 zt 13% 1418&10% 1128k 13% 1314 * 14% 1184k 16% 1527& 12%
9MS
10 100 200 500 800 1200 2000 3000 4500
1671 f 12% 1479+ 11% 1025 + 12% 1459 * 13% 1409+12% 2432+11% 2272+ 12% 1561 f 10%
100 200 500 800 1200 1500 2500 3319
1310+ 980 + 1153+ 1871+ 1905 + 2189+ 1595 f 1495*
18MS
10 100 200 500 800 1200 2000 3000 4500
1416+ 8% 1091 f 13% 820+ 13% 1043*16% 1505 + 13% 1310*125g 1226+_13% 1361_+17% 1361+ 10%
ME 56
10+ 120 5Oc+ 350 1200 + 1500 2100+2340 2900+3150 3850+4090 5050+ 5140
1293 1006 1676 1694 1551 1685 1730
3000
4000
5000
;I r
HE.56 2'BON, 25%
Fig. 1. “Be
depth profiles in the water column on the 5 profiles across the Atlantic Ocean.
Atlantic Ridge than in the Sargasso Sea and in the Canary Basin. The profile in the Sargasso Sea at 60 o W displays a constant concentration of 1300 + 160 atoms/g below 1000 m. On the east-side of the Midin the deep water Atlantic Ridge the “Be concentration is 1300 _C70 atoms/g at 35 o W and 1670 f 70 atoms/g at 25OW. The profiles at 50 o W and 45 o W display mean lo Be concentrations of 1920 f 600 and 1830 + 300 atoms/g, respectively. From these profiles the mean residence time ‘of “Be for the deep-water can be calculated to 380-560 a as:
12MS
15MS
7 = (hC)/P, with h = 3600 m, the mean depth of the Atlantic Ocean and a production rate P of 1.21 X lo6 at/cm* a [5]. This agrees very well with the residence time calculated from the 230Th and “Be fluxes to sediments from the high productivity areas. The residence time in the surface layer, where “Be shows a nutrient-type behaviour, can be calculated to 13-15 a as:
were C, and C, are concentrations in the surface layer and in the deep-water, respectively, h is the surface layer depth, here assumed to be 200 m, P the “Be production rate, and w the upwelling rate, here assumed to be 4 m/a, the global upwelling rate [2]. With a mean surface concentration of 1000 atoms/g and a deep water concentration of 1300-1900 atoms/g, the residence of “Be in the surface layer is lo-13 a. Assuming a negligible upwelling rate the maximum residence time would be 15 a. This is in agreement with Raisbeck’s
333
9Be (at/g) X 10" 2.3 2.5 2.4 2.2 (3.7) a) 2.0 2.1 2.1 2.7 2.5 2.5 2.4 2.5 1.9 1.9 2.0 2.3
16% 12% 12% 14% 12% 10% 13% 9%
‘) Contaminated.
estimate of 16 a derived from a single measurement on water from the Pacific Ocean [6]. Recalculation of his data point with a production rate of 1.21 x lo6 at/cm2 a instead of 0.47 X lo6 at./cm’ a, however, gives a residence time of 6.2 a. III(d). OCEAN/ATMOSPHERIC
SCIENCES
334
M. Segl et al. / “Be in the Atlantic Ocean
The 9Be concentrations are listed in table 1. Unfortunately, the samples were contaminated. An estimate of the degree of contamination is possible by comparing our values with those sampled by Measures at another station at 34ON, 63“W which display contents of 9Be about 9 pM lower than profiles at stations 3MS and 9 MS. The corrected 9Be values in deep water should be about 1.8 f 0.3 x 10” atoms/g and the ratio of “Be/‘Be approximately 8 X lo-*.
References [l) M.P. Bacon and R.F. Anderson, J. Geophys. Res. 87 (1982) 2045. [2] W.S. Broecker and T.H. Peng, Tracers in the sea, Lamont-
Geological Observatory, Columbia University, New York (1982). 131 S. Krishnaswami, A. Mangini, J.H. Thomas, P. Sharma, J.K. Co&ran, K.K. Turekian and P.D. Parker, Earth Planet. Sci. Lett. 59 (1982) 217. I41 A. Mangini, M. Segl, G. Bonani, H.J. Hofmann, E. Morenzoni, M. Nessi, M. Suter, W. Wiilfli and K.K. Turekian, Nucl. Instr. and Meth. B5 (1984) 353. I51 M.C. Monaghan, S. Krishnaswami and K.K. Turekian, Earth Planet. Sci. Lett. 76 (1985/86) 279. (61 G.M. Raisbeck, F. Yiou, M. Fruneau, J.M. Loiseaux and Dahorty
M. Lieuvin, Earth Planet. Sci. Lett. 43 (1979) 237. [71G.M. Raisbeck, F. Yiou, M. Fruneau, J.M. Loiseaux, M. Lieuvin, J.C. Ravel, J.M. Reyss and F. Guichard, Earth Planet. Sci. Lett. 51 (1980) 275. G. Bonani, H.J. Hofmann, E. PI M. Se& A. Mangini, Morenzoni, M. Nessi, M. Suter and W. WBlfli, Nucl. Instr. and Meth. B5 (1984) 359.