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Uranium and thorium concentrations in wind-borne Saharan dust over the Western Equatorial North Atlantic Ocean
EARTH AND PLANETARY SCIENCE LETTERS 14 (1972) 397-402. NORTH-HOLLAND PUBLISHING COMPANY
URANIUM
AND THORIUM
CONCENTRATIONS
DUST OVER THE WESTERN
EQUATORIAL
IN WIND-BORNE NORTH
ATLANTIC
SAHARAN OCEAN
Harold S. RYDELL and Joseph M. PROSPERO University of Miami, Rosenstiel School o f Marine and Atmospheric Science, 10 Rickenbacker Causeway, Miami, Florida 33149, USA Received 13 December 1971 Revised version received 3 February 1972 The average uranium and thorium concentration in 15 samples of wind-borne Saharan dust collected at Barbados, West Indies, is 3.6 and 12.4 ppm, respectively; these values are approximately one-third greater than that of average crustal material. The thorium-uranium weight ratio of the dust is 3.5, about the same as that of the crust; the 23"U/ 238 U activity ratio is 1.08 and the 2 3 0 Th/ 2 3 U activity ratio, 0.97. We conclude that the presence of large amounts of African dust in Atlantic sediments would not significantly affect the validity of the assumptions inherent in the 231pa/23°Th and 23°Th/232Th dating methods.
1. Introduction Marine sediment is a composite o f many genetically different constituents. Once these diverse materials have become incorporated in marine sediment, it is very difficult or impossible to identify the component fractions, with the exception of biological material, and to separate them for analysis. Traditionally it has been assumed that essentially all o f the detrital fractions of pelagic sediments were originally transported to the oceans by rivers and thence by ocean currents. However, there is an increasingly large b o d y o f evidence which suggests that a significant fraction o f the detrital component o f sediments in some oceanic regions consists o f windtransported material. For the most part, this evidence is inferred from the configuration o f the concentration gradients o f certain mineral species in the sediments when viewed with respect to the disposition o f land masses and the major wind systems [ 1 - 3 ] . Qualitative support for the eolian hypothesis was obtained from a number o f studies of the concentration o f air-borne mineral dust collected from aboard ships on the Atlantic, Pacific, and Indian Oceans [ 4 - 8 ] . However, because these studies were o f a short duration and
were all performed at sea level, and because of a lack o f knowledge o f removal rates from the atmosphere, it is not possible to make a quantitative estimate of the mass of material transported from the continents to the seas by winds. Recently, Prospero and Carlson [9] established that the quantity o f dust transported annually from North Africa to the Caribbean area by the trade winds between 10°N and 25°N was in excess of 50 megatons; this calculation was based on five years o f essentially continuous sea level aerosol studies at Barbados, West Indies, [ 9 - 1 2 ] and on extensive aircraft measurements made during the Barbados Oceanographic and Meteorological Experiment [13]. To visualize this mass in terms o f sedimentation rates, assume that an equal quantity of material was deposited en route; then the effective sedimentation rate in the 15 ° latitude band between Africa and Barbados would be approximately 0.5 g o f dry sediment per thousand years, a rate commensurate with the measured carbonate-free sedimentation rates in this region [ 14]. Thus, analyses o f wind-borne dust can provide the foundation for some unique geochemical measurements o f the interaction o f the atmosphere and hydrosphere and should establish elemental and isotopic parameters
398
H.S. Rydell, J.M. Prospero, Uranium and thorium concentrations
for this material prior to permanent inclusion and alteration in the water-sediment column. In this paper we report on the analyses of dust for uranium (238U) and thorium (232Th) and two of their daughter products, 234U and 230Th. These nuclides are important because two o f the commonly used methods for the absolute dathag o~foceanic sediments are based on the relative concentrations of these nuclides. In the 230Th/ 231pa and 230Th/232Th dating methods [15, 16], a uranium correction is made to distinguish between the unsupported authigenic 23°Th and 231pa (the datable part) and the allogenic (old terrestrial part). The aUogenic 23°Th and 231pa is presumed to be contained in detrital particles and to be in radioactive equilibrium with the parent uranium nuclides, 238U and 235U, respectively. If wind-borne dust does constitute a major portion of marine sediment in some areas and if considerable radioactive disequilibrium exists in the dust, then the dates obtained by these methods would be rendered inaccurate.
2. Procedures The air-borne dust collections were made at the University of Miami Marine Aerosol Studies Station, Barbados, West Indies (13°N, 59°W), where essentially continuous aerosol sampling has been carried out since August 1965 [10-13]. It has been shown conclusively that this material is derived from the arid regions of West Africa, over 4500 km to the east [13]. Aerosols were collected using a mesh technique; a detailed de, scription of the aerosol facility and the procedures employed there is presented by Delany et al. [10]. In addition to the Barbados dusts, we analyzed several samples of air-borne dust collected at a coastal site in the Miami area during westerly (offshore) winds. Isotopic uranium and thorium analyses were done by alpha particle spectrometry using a solid state detector and a multichannel analyzer. After total dissolution of the samples, uranium and thorium were concentrated and separated by a procedure employing coprecipitation, solvent extraction, ion exchange, and electrodeposition. 232U and 234Th tracers were used to correct for analytical losses. These are procedures similar to those used by other workers [17-19].
3. Results and discussion
Before discussing our results we will describe briefly the mineralogical and chemical composition of the dust. X-ray diffraction and optical studies of Barbados dusts show the dominant mineral to be quartz which comprises about one-third of the sample on a bulk weight basis; many of the quartz grains are coated with desert varnish (ferruginous oxides) thereby giving the dust a characteristic deep red-brown coloration [ 12]. Layered silicates (kaolinite, chlorite, muscovite) also are present in abundance. All samples contain sufficient quantities of amorphous material [12] to produce a pronounced diffuse X-ray scatter background. Thirty representative dust samples analyzed for 26 elements (Prospero, unpublished data) were found to have a composition essentially identical to that of average crustal material [20]. Miami dust collected during westerly (offshore) winds is largely calcite with lesser amounts of aragonite and minor amounts of feldspar and quartz [21].* In addition, the Miami dusts contain large quantities of organic matter and unidentified amorphous material. Our uranium and thorium results are presented in table 1. For Barbados dust, the average U and Th content is 3.6 and 12.4 ppm, respectively, and the Th/U ratio is 3.5. Miami westerly dust has 5.5 ppm U and 1.65 ppm Th, yielding a Th/U ratio of 0.3. Krauskopf [20] gives the average U and Th contents (in ppm) for the earth's crust as 2.7 and 9.6, and for shale as 3.2 and t 1, respectively; therefore, the ratio of Th/U for crust would be 3.6 and for shale 3.4. Thus, the Barbados samples show a small enrichment, approximately 30%, of U and Th compared to average crust, but the Th/U ratio is about the same. We considered the possibility that a bias could be introduced in our results because of the low capture efficiency of our collectors for particles below several microns diameter; consequently, our dust samples are not truly representative of the actual airborne material since it is known that some compositional characteristics, such as the mineralogy [12], are size dependent. In order to ascertain whether a significant size dependence existed for uranium and thorium, we removed an * During the summer easterlies, the composition of Miami dust is identical to that of Barbados dust, indicating again the scale of the African dust transport phenomenon [21].
24-26/6 24-26/6 14-16/7 5/6 7-1218 12/8 17-22/8 3-10/9 24-10/9 1-8~10 10/28 5/19 6/22 7/5 7/26 8/9 9/6 5/5 9/9 11/5
B (bulk sample) B (< 2t~m) * B
0.12 0.16 0.12 0.12 0.24 0.13 0.11 0.12 0.13 0.15 0.13 0.43 0.11 0.24
0.11
0.17 0.16
Error ±
Error l o radiometric count. B - Barbados dust. M - Miami dust. * Analyses by WiUard S. Moore. ** Weight ratio. ~" Based on total dissolved and suspended solids (6.81g). t t Activity ratio.
5.49
Mean M
2.95 2.93 3.1 3.4 3.96 0.05 3.48 3.79 3.81 3.35 4.70 3.36 3.31 3.72 3.39 4.10 3.93 5.33 4.70 6.44
U (ppm)
3.58
/67 /67 /67 /68 /68 /68 /68 /68 /68 /68 /68 /69 /69 /69 /69 /69 /69 /70 /70 /70
Year
Mean B
B B B B B B B B B B B RSMAS M # 40 RSMAS M # 91 RSMAS M #107
* D (aqueous)~
B
* B
Date
Location
1,65
12,41
11.44 14.14 8-5 10. 13.26 < 0.01 14.21 12.42 12.96 10.98 13.16 13.35 13.60 14.51 12.33 11.25 12.39 1.86 0.68 2.40
Th (ppm)
0.55 0.39 0.34 0.51 0.54 0.54 0.52 0.64 0.51 0.47 0.45 0.41 0.13 0.26
0.45
0,84
1.23
Error +
Table 1
1.07
1.08
1.01
1.08
1.9 1.11 1.16 1.01 1.22 1.14 1.05 1.05 1.01 1.03 1.13 1.03 1.11
1.00
1.13
1.05
1.06 1,06
U-234~t U-238 0.08 0.09 0.06 0.05 0.04 0.5 0.06 0.06 0.04 0.06 0.08 0.06 0.05 0.05 0.06 0.06 0.05 0.12 0.04 0.05
+
Error
0.29 0.85
4.09 3.28 3.40 3.27 2.80 3.97 4.11 3.90 4.06 2.74 3.15 0.35 0.15 0.37 3.52
0.06 0.04 0.04 0.05 0.05 0.07 0.06 0.07 0.06 0.04 0.05 0.07 0.04 0.05
0.05
3.87 4.82 2.7 2.9 3.35
U
+
0.11 0.10
Th**
Error
0.97
0.95 0.84 0.71 1.01
0.83
1.10 ,1.19 0.73 0.66 1.10 <0.01 0.97 0.93 0.83 0.86 0.73 1.13 1.22 1.14 1.10
Th-2301t U-234
0.22 0.17 0.14 0.19 0.18 0.22 0.21 0.21 0.23 0.15 0.15 0.08 0.03 0.04
0.15
0.47 0,42
_+
Error
~D
400
H.S. Rydell, J.M. Prospero, Uranium and thorium concentrations
aliquot of sample B-6/24-26/67 and, using an aqueous sedimentation technique, separated the less than 2/am diameter size fraction which we then analyzed.* The data in table 1 show that the composition of the small size fraction is not significantly different from that of the bulk sample or of the other B samples analyzed. We also established that no error was introduced in neglecting to analyze the supernatant liquid from the mesh wash water which normally is discarded after the dust is recovered. This phase contains dissolved salts, mostly sea salt, and the extremely fine particulates still in suspension after the one-day settling period. We analyzed the supernatant liquid from sample D (aqueous) 8/12/68 which was collected aboard OSS DISCOVERER, on station 100 km east of Barbados, concurrently with Barbados sample B 8/7-12/68. The mineralogical composition of these two dust samples is identical. The salt content of the D sample was equivalent to that of about 200 ml of sea water, while the uranium content (0.36/~g) was approximately equal to that of 100 ml of sea water. [The average uranium concentration of sea water is 3.3/ag 1-1 [22, 23].] Because of the low uranium concentration, the counting statistics are too poor for the measured 234U/238U ratio of 1.9 to be meaningful. Thus we conclude that in disregarding the aqueous phase we are not introducing any significant bias into our results. The uranium concentration in Miami westerly dust is higher than the 1 4 ppm that would be expected for Pleistocene to Recent marine carbonates [19]. This enrichment may be due to the presence of non-carbonate material such as organic matter, mineral dust, and atmospheric pollutants. The high uranium concentration may also be attributed to the use of phosphate fertilizers which typically have a high uranium content. These possibilities seem plausible as the samples were collected when the wind was from the mainland and probably bore dust derived from the agricultural and urban areas of Dade County. In some respects, the Miami westerly dust may be considered to be a "contaminated" sample of local *
Air-borne particulates frequently exist as loosely-bound agglomerates; consequently, after dispersing our samples in water, we find that 40% to 50% of the dust weight consists of particles in the less than 2ttm diameter size range [lZl.
marine carbonate. Pleistocene and Recent corals and oolites from the Miami area have been studied by Osmond et al. [19], for the purpose of dating the Pleistocene Key Largo and Miami Oolite formations by the 230Th/234U method. The age determined by Osmond for these formations is about 130 000 yr bp. Our sample RSMAS M#91, which, on the basis o f having the lowest U and Th content, would seem to be the least "contaminated" example of dust from the local lithology, is strikingly similar to the data of Osmond et al. [19], and yields a 230Th/234U age of approximately 135 000 yr.** Despite the good agreement in ages, we would not advocate dust analysis as a means of dating Pleistocene marine carbonate formations! However, the isotropic composition of dusts may be useful as an indicator of provenance. As mentioned above, the inclusion of significant amounts of terrestrial dust in marine sediments could conceivably cause problems in absolute age dating by the 230Th/231pa and 23°Th/232Th methods if these dusts exhibited large departures from equilibrium. In closed geological systems older than about 106 yr, 230Th and 234U will have achieved radioactive equilibrium with their parent, 238U; that is, their alpha activity ratio would be 1.00. However, in open systems exposed to weathering and the circulation of groundwater, separation of these nuclides can occur, giving rise to a state of radioactive disequilibrium [24-27[..Consequently, the activity ratio of 234U to 238U in many natural waters, including most rivers, is greater than 1.00, that of the oceans being 1.15 [26, 28, 29]. Our analyses show a considerable variation in the degree of disequilibrium between 234U/238Uand 230Th/234U. However, the average for the activity ratio 234U/238U is 1.08 and for 230Th/234U is 0.97 for Barbados dust. Since a 230Th/234U ratio of 1.0 is assumed in correcting for U-supported 230Th in dating by the 230Th/231pa and 230Th/232Th methods, we would conclude that the influx of dust from Africa into the equatorial Atlantic and Caribbean would not, in itself, significantly affect the dates obtained for marine sediments. The average 8% excess of 234U is probably the result of weathering processes occurring near the source area in Africa. The excess 234U is possibly associated with the desert varnish coating depos** Ideally, marine carbonates intended for datine should contain only radiogenic 23°Th and no 232Th.
H.S. Rydell, J.M. Prospero, Uranium and thorium concentrations
ited by secondary processes; it is unlikely that the excess 234U is derived from sea-spray salts as the samples are washed prior to analysis. Because of the small sample sizes ( ~ 2g), 231pa could not be measured, but a rough estimate can be made. On the basis of the average 230Th/234U activity ratio of 0.97 and the 234U/238U ratio of 1.08, the net disequilibria between parent 238U and its eventual daughter 230Th ia about 5%. Since the activity ratio 238U/235U (21.8) is constant in nature, it can be assumed that if 231pa is in equilibrium with its parent, 235U, then there would be a 5% deficiency of 231pa relative to 230Th in African dust. Again a discrepancy of this magnitude, if existent, would not greatly influence the correction for the allogenic component of radio-elements in dating of marine sediments. Probably a greater source of potential dating errors than the disequilibrium in dust is (a) the postdepositional mobility of uranium [18, 30, 31] and (b) the separation of 230Th and 231pa in oceanic regions where manganese nodules and encrustations are common [32].
Acknowledgements We thank W.S. Moore for permission to use his data in this report, R. Nees and C. Dorta for their technical assistance, and Judge G.L. Taylor, Barbados, for the continued use of his coastal land site for our field operations. A portion of this research was supported by the Office of Naval Research Contract N00014-67-A0201-0013, Atomic Energy Commission Contract AT-(40-1)-3622, and National Science Foundation Grants GA-25916, GA-20112 and GA-22033. Contribution no. 1465 from the Rosenstiel School of MarLle and Atmospheric Science.
References [1] R.W. Rex and E.D. Goldberg, Quartz contents of pelagic sediments of the Pacific Ocean, Tellus 10 (1958) 153. [2] P.E. Biscaye, Mineralogy and sedimentation of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans, Geol. Soc. Am, Bull. 76 (1965) 803.
401
[3] J.J. Griffin, H. Windom and E.D. Goldberg, The distribution of clay minerals in the world ocean, Deep Sea Res. 15 (1968) 433. [4] J.M. Prospero and E. Bonatti, Continental dust in the atmosphere of the eastern equatorial Pacific, J. Geophys. Res. 74 (1969) 3362. [5] D.W. Parkin, D.R. Phillips and R.A.L. Sullivan, Airborne dust collections over the North Atlantic, J. Geophys. Res. 75 (1970) 1782. [6] D.W. Folger, Wind transport of land-derived mineral, biogenic and industrial matter over the North Atlantic, Deep Sea Res. 17 (1970) 337. [7] E.D. Goldberg and J.J. Griffin, The sediments of the northern Indian Ocean, Deep Sea Res. 17 (1970) 513. [8] R. Chester, H. Elderfield and J.J. Griffin, Dust transported in the North-east and South-east Trade Winds in the Atlantic Ocean, Nature 233 (1971) 474. [9] J.M. Prospero and T.N. Carlson, Saharan dust in the atmosphere of the northern equatorial Atlantic Ocean: A major constituent of the marine aerosol, Bull. Am. Meteorol. Soc. 53 (1972) in press. (Abstract: Working Symposium on Sea-Air Chemistry, Ft. Lauderdale, Fla., January; proceedings to be published in J. Geophys. Res.) [10] A.C. Delany, D.W.. Parkin, J.J. Griffin, E.D. Goldberg and B.E.F. Reimann, Airborne dust collected at Barbados, Geochim. Cosmochim. Acta 31 (1967) 885. [11] J.M. Prospero, Atmospheric dust studies on Barbados, Bull. Am. Meteor. Soc. 49 (1968) 645. [12] J.M. Prospero, E. Bonatti, C. Schubert and T.N. Carlson, Dust in the Caribbean atmosphere traced to an African dust storm, Earth Planet. Sci. Letters 9 (1970) 287. [13] T.N. Carlson and J.M. Prospero, The large-scale movement of Saharan air outbreaks over the equatorial North Atlantic, J. Applied Meteorology, (1972) in press. [14] T.L. Ku, W.S. Broecker and N. Opdyke, Comparison of sedimentation rates measured by paleomagnetic and the ionium methods of age determination, Earth Planet. Sci. Letters 4 (1968) 1. [15] J.N. Rosholt, C. Emiliani, J. Geiss, F.F. Koczy and P.J. Wangersky, Absolute dating of deep-sea cores by the pa231]Th23° method, J. Geol. 69 (1961) 162. [16] E.D. Goldberg and M. Koide, Geochronological studies of deep sea sediments by the ionium/thorium method, Geochim. Cosmochim. Acta 26 (1962) 417. [17] M. Bernat, R.H. Bieri, M. Koide, J.J. Griffin and E.D. Goldberg, Uranium, thorium, potassium and argon in marine phillipsites, Geochim. Cosmochim. Acta 34 (1970) 1053. [18] T.L. Ku, An evaluation of the U234/U238 method as a tool for dating pelagic sediments, J. Geophys. Res. 70 (1965) 3457. [19] J.K. Osmond, J.R. Carpenter and H.L. Windom, Th23°/ U234 age of the Pleistocene corals and oolites of Florida, J. Geophys. Res. 70 (1965) 1843. [20] K.B. Krauskopf, Introduction to Geochemistry (McGrawHill, New York, 1967) 639.
402
H.S. Rydell, J.M.Prospero, Uranium and thorium concentrations
[21] J.M. Prospero and T.N. Carlson, Mineralogy of aerosols collected at Miami, Florida: Evidence for the frequent presence of African dust, EOS 52 (1971) 370. [22] E. Rona, L.O. Gilpatrick and L.M. Jeffrey, Uranium determination in sea water, Trans. Am. Geophys. Union 37 (1956) 697. [23] D.W. Spencer, D.E. Robertson, K.K. Turekian and T.R. Folsom, Trace element calibrations and profiles at the GEOSECS Test Station in the Northeast Pacific Ocean, J. Geophys. Res. 75 (1970) 7688. [24] P.I. Chalov, The U234/U 238 ratio in some secondary minerals, Geochem. 1 (1959) 203. [25] E.A. Isabaev, E.P. Usatov and V.V. Cherdyntsev, Isotopic composition of uranium in natural samples, Radiochem. 2 (1960) 97. [26] D.L. Thurber, Anomalous U234/U 238 in nature, J. Geophys. Res. 67 (1962) 4518. [27] J.R. Dooley, H.C. Granger and J.N. Rosholt, Uranium234 fractionation in the sandstone-type uranium deposits of the Ambrosia Lake district, New Mexico, Econ. Geol. 61 (1966) 1362.
[28] M. Koide and E.D. Goldberg, Uranium-234/uranium238 ratios in sea water, in: Progress in Oceanography, Vol. 3, ed. M. Sears (Pergamon Press, London, 1965) 173. [29] R.L. Blanchard, U234/U 238 ratios in coastal marine waters and calcium carbonates, J. C-eophys. Res. 70 (1965) 4055. [30] Y. Kolodny and I.R. Kaplan, Uranium isotopes in sea floor phosphorites, Geochim. Cosmochim. Acta 34 (1970) 3. [31] E. Bonatti, D.E. Fisher, O. Joensuu and H.S. Rydell, Postdepositional mobility of some transition elements, phosphorus, uranium and thorium in deep sea sediments, Geochim. Cosmochim. Acta 35 (1971) 184. [32] K.K. Turekian and L.H. Chan, The marine geochemistry of the uranium isotopes, 23oTh and 231pa, in: Activation Analysis in Geochemistry and Cosmochemistry, A.O. Brunfelt and E. Steinnes, eds. (Universitetsforlaget, Oslo, 1971) 311.
URANIUM
AND THORIUM
CONCENTRATIONS
DUST OVER THE WESTERN
EQUATORIAL
IN WIND-BORNE NORTH
ATLANTIC
SAHARAN OCEAN
Harold S. RYDELL and Joseph M. PROSPERO University of Miami, Rosenstiel School o f Marine and Atmospheric Science, 10 Rickenbacker Causeway, Miami, Florida 33149, USA Received 13 December 1971 Revised version received 3 February 1972 The average uranium and thorium concentration in 15 samples of wind-borne Saharan dust collected at Barbados, West Indies, is 3.6 and 12.4 ppm, respectively; these values are approximately one-third greater than that of average crustal material. The thorium-uranium weight ratio of the dust is 3.5, about the same as that of the crust; the 23"U/ 238 U activity ratio is 1.08 and the 2 3 0 Th/ 2 3 U activity ratio, 0.97. We conclude that the presence of large amounts of African dust in Atlantic sediments would not significantly affect the validity of the assumptions inherent in the 231pa/23°Th and 23°Th/232Th dating methods.
1. Introduction Marine sediment is a composite o f many genetically different constituents. Once these diverse materials have become incorporated in marine sediment, it is very difficult or impossible to identify the component fractions, with the exception of biological material, and to separate them for analysis. Traditionally it has been assumed that essentially all o f the detrital fractions of pelagic sediments were originally transported to the oceans by rivers and thence by ocean currents. However, there is an increasingly large b o d y o f evidence which suggests that a significant fraction o f the detrital component o f sediments in some oceanic regions consists o f windtransported material. For the most part, this evidence is inferred from the configuration o f the concentration gradients o f certain mineral species in the sediments when viewed with respect to the disposition o f land masses and the major wind systems [ 1 - 3 ] . Qualitative support for the eolian hypothesis was obtained from a number o f studies of the concentration o f air-borne mineral dust collected from aboard ships on the Atlantic, Pacific, and Indian Oceans [ 4 - 8 ] . However, because these studies were o f a short duration and
were all performed at sea level, and because of a lack o f knowledge o f removal rates from the atmosphere, it is not possible to make a quantitative estimate of the mass of material transported from the continents to the seas by winds. Recently, Prospero and Carlson [9] established that the quantity o f dust transported annually from North Africa to the Caribbean area by the trade winds between 10°N and 25°N was in excess of 50 megatons; this calculation was based on five years o f essentially continuous sea level aerosol studies at Barbados, West Indies, [ 9 - 1 2 ] and on extensive aircraft measurements made during the Barbados Oceanographic and Meteorological Experiment [13]. To visualize this mass in terms o f sedimentation rates, assume that an equal quantity of material was deposited en route; then the effective sedimentation rate in the 15 ° latitude band between Africa and Barbados would be approximately 0.5 g o f dry sediment per thousand years, a rate commensurate with the measured carbonate-free sedimentation rates in this region [ 14]. Thus, analyses o f wind-borne dust can provide the foundation for some unique geochemical measurements o f the interaction o f the atmosphere and hydrosphere and should establish elemental and isotopic parameters
398
H.S. Rydell, J.M. Prospero, Uranium and thorium concentrations
for this material prior to permanent inclusion and alteration in the water-sediment column. In this paper we report on the analyses of dust for uranium (238U) and thorium (232Th) and two of their daughter products, 234U and 230Th. These nuclides are important because two o f the commonly used methods for the absolute dathag o~foceanic sediments are based on the relative concentrations of these nuclides. In the 230Th/ 231pa and 230Th/232Th dating methods [15, 16], a uranium correction is made to distinguish between the unsupported authigenic 23°Th and 231pa (the datable part) and the allogenic (old terrestrial part). The aUogenic 23°Th and 231pa is presumed to be contained in detrital particles and to be in radioactive equilibrium with the parent uranium nuclides, 238U and 235U, respectively. If wind-borne dust does constitute a major portion of marine sediment in some areas and if considerable radioactive disequilibrium exists in the dust, then the dates obtained by these methods would be rendered inaccurate.
2. Procedures The air-borne dust collections were made at the University of Miami Marine Aerosol Studies Station, Barbados, West Indies (13°N, 59°W), where essentially continuous aerosol sampling has been carried out since August 1965 [10-13]. It has been shown conclusively that this material is derived from the arid regions of West Africa, over 4500 km to the east [13]. Aerosols were collected using a mesh technique; a detailed de, scription of the aerosol facility and the procedures employed there is presented by Delany et al. [10]. In addition to the Barbados dusts, we analyzed several samples of air-borne dust collected at a coastal site in the Miami area during westerly (offshore) winds. Isotopic uranium and thorium analyses were done by alpha particle spectrometry using a solid state detector and a multichannel analyzer. After total dissolution of the samples, uranium and thorium were concentrated and separated by a procedure employing coprecipitation, solvent extraction, ion exchange, and electrodeposition. 232U and 234Th tracers were used to correct for analytical losses. These are procedures similar to those used by other workers [17-19].
3. Results and discussion
Before discussing our results we will describe briefly the mineralogical and chemical composition of the dust. X-ray diffraction and optical studies of Barbados dusts show the dominant mineral to be quartz which comprises about one-third of the sample on a bulk weight basis; many of the quartz grains are coated with desert varnish (ferruginous oxides) thereby giving the dust a characteristic deep red-brown coloration [ 12]. Layered silicates (kaolinite, chlorite, muscovite) also are present in abundance. All samples contain sufficient quantities of amorphous material [12] to produce a pronounced diffuse X-ray scatter background. Thirty representative dust samples analyzed for 26 elements (Prospero, unpublished data) were found to have a composition essentially identical to that of average crustal material [20]. Miami dust collected during westerly (offshore) winds is largely calcite with lesser amounts of aragonite and minor amounts of feldspar and quartz [21].* In addition, the Miami dusts contain large quantities of organic matter and unidentified amorphous material. Our uranium and thorium results are presented in table 1. For Barbados dust, the average U and Th content is 3.6 and 12.4 ppm, respectively, and the Th/U ratio is 3.5. Miami westerly dust has 5.5 ppm U and 1.65 ppm Th, yielding a Th/U ratio of 0.3. Krauskopf [20] gives the average U and Th contents (in ppm) for the earth's crust as 2.7 and 9.6, and for shale as 3.2 and t 1, respectively; therefore, the ratio of Th/U for crust would be 3.6 and for shale 3.4. Thus, the Barbados samples show a small enrichment, approximately 30%, of U and Th compared to average crust, but the Th/U ratio is about the same. We considered the possibility that a bias could be introduced in our results because of the low capture efficiency of our collectors for particles below several microns diameter; consequently, our dust samples are not truly representative of the actual airborne material since it is known that some compositional characteristics, such as the mineralogy [12], are size dependent. In order to ascertain whether a significant size dependence existed for uranium and thorium, we removed an * During the summer easterlies, the composition of Miami dust is identical to that of Barbados dust, indicating again the scale of the African dust transport phenomenon [21].
24-26/6 24-26/6 14-16/7 5/6 7-1218 12/8 17-22/8 3-10/9 24-10/9 1-8~10 10/28 5/19 6/22 7/5 7/26 8/9 9/6 5/5 9/9 11/5
B (bulk sample) B (< 2t~m) * B
0.12 0.16 0.12 0.12 0.24 0.13 0.11 0.12 0.13 0.15 0.13 0.43 0.11 0.24
0.11
0.17 0.16
Error ±
Error l o radiometric count. B - Barbados dust. M - Miami dust. * Analyses by WiUard S. Moore. ** Weight ratio. ~" Based on total dissolved and suspended solids (6.81g). t t Activity ratio.
5.49
Mean M
2.95 2.93 3.1 3.4 3.96 0.05 3.48 3.79 3.81 3.35 4.70 3.36 3.31 3.72 3.39 4.10 3.93 5.33 4.70 6.44
U (ppm)
3.58
/67 /67 /67 /68 /68 /68 /68 /68 /68 /68 /68 /69 /69 /69 /69 /69 /69 /70 /70 /70
Year
Mean B
B B B B B B B B B B B RSMAS M # 40 RSMAS M # 91 RSMAS M #107
* D (aqueous)~
B
* B
Date
Location
1,65
12,41
11.44 14.14 8-5 10. 13.26 < 0.01 14.21 12.42 12.96 10.98 13.16 13.35 13.60 14.51 12.33 11.25 12.39 1.86 0.68 2.40
Th (ppm)
0.55 0.39 0.34 0.51 0.54 0.54 0.52 0.64 0.51 0.47 0.45 0.41 0.13 0.26
0.45
0,84
1.23
Error +
Table 1
1.07
1.08
1.01
1.08
1.9 1.11 1.16 1.01 1.22 1.14 1.05 1.05 1.01 1.03 1.13 1.03 1.11
1.00
1.13
1.05
1.06 1,06
U-234~t U-238 0.08 0.09 0.06 0.05 0.04 0.5 0.06 0.06 0.04 0.06 0.08 0.06 0.05 0.05 0.06 0.06 0.05 0.12 0.04 0.05
+
Error
0.29 0.85
4.09 3.28 3.40 3.27 2.80 3.97 4.11 3.90 4.06 2.74 3.15 0.35 0.15 0.37 3.52
0.06 0.04 0.04 0.05 0.05 0.07 0.06 0.07 0.06 0.04 0.05 0.07 0.04 0.05
0.05
3.87 4.82 2.7 2.9 3.35
U
+
0.11 0.10
Th**
Error
0.97
0.95 0.84 0.71 1.01
0.83
1.10 ,1.19 0.73 0.66 1.10 <0.01 0.97 0.93 0.83 0.86 0.73 1.13 1.22 1.14 1.10
Th-2301t U-234
0.22 0.17 0.14 0.19 0.18 0.22 0.21 0.21 0.23 0.15 0.15 0.08 0.03 0.04
0.15
0.47 0,42
_+
Error
~D
400
H.S. Rydell, J.M. Prospero, Uranium and thorium concentrations
aliquot of sample B-6/24-26/67 and, using an aqueous sedimentation technique, separated the less than 2/am diameter size fraction which we then analyzed.* The data in table 1 show that the composition of the small size fraction is not significantly different from that of the bulk sample or of the other B samples analyzed. We also established that no error was introduced in neglecting to analyze the supernatant liquid from the mesh wash water which normally is discarded after the dust is recovered. This phase contains dissolved salts, mostly sea salt, and the extremely fine particulates still in suspension after the one-day settling period. We analyzed the supernatant liquid from sample D (aqueous) 8/12/68 which was collected aboard OSS DISCOVERER, on station 100 km east of Barbados, concurrently with Barbados sample B 8/7-12/68. The mineralogical composition of these two dust samples is identical. The salt content of the D sample was equivalent to that of about 200 ml of sea water, while the uranium content (0.36/~g) was approximately equal to that of 100 ml of sea water. [The average uranium concentration of sea water is 3.3/ag 1-1 [22, 23].] Because of the low uranium concentration, the counting statistics are too poor for the measured 234U/238U ratio of 1.9 to be meaningful. Thus we conclude that in disregarding the aqueous phase we are not introducing any significant bias into our results. The uranium concentration in Miami westerly dust is higher than the 1 4 ppm that would be expected for Pleistocene to Recent marine carbonates [19]. This enrichment may be due to the presence of non-carbonate material such as organic matter, mineral dust, and atmospheric pollutants. The high uranium concentration may also be attributed to the use of phosphate fertilizers which typically have a high uranium content. These possibilities seem plausible as the samples were collected when the wind was from the mainland and probably bore dust derived from the agricultural and urban areas of Dade County. In some respects, the Miami westerly dust may be considered to be a "contaminated" sample of local *
Air-borne particulates frequently exist as loosely-bound agglomerates; consequently, after dispersing our samples in water, we find that 40% to 50% of the dust weight consists of particles in the less than 2ttm diameter size range [lZl.
marine carbonate. Pleistocene and Recent corals and oolites from the Miami area have been studied by Osmond et al. [19], for the purpose of dating the Pleistocene Key Largo and Miami Oolite formations by the 230Th/234U method. The age determined by Osmond for these formations is about 130 000 yr bp. Our sample RSMAS M#91, which, on the basis o f having the lowest U and Th content, would seem to be the least "contaminated" example of dust from the local lithology, is strikingly similar to the data of Osmond et al. [19], and yields a 230Th/234U age of approximately 135 000 yr.** Despite the good agreement in ages, we would not advocate dust analysis as a means of dating Pleistocene marine carbonate formations! However, the isotropic composition of dusts may be useful as an indicator of provenance. As mentioned above, the inclusion of significant amounts of terrestrial dust in marine sediments could conceivably cause problems in absolute age dating by the 230Th/231pa and 23°Th/232Th methods if these dusts exhibited large departures from equilibrium. In closed geological systems older than about 106 yr, 230Th and 234U will have achieved radioactive equilibrium with their parent, 238U; that is, their alpha activity ratio would be 1.00. However, in open systems exposed to weathering and the circulation of groundwater, separation of these nuclides can occur, giving rise to a state of radioactive disequilibrium [24-27[..Consequently, the activity ratio of 234U to 238U in many natural waters, including most rivers, is greater than 1.00, that of the oceans being 1.15 [26, 28, 29]. Our analyses show a considerable variation in the degree of disequilibrium between 234U/238Uand 230Th/234U. However, the average for the activity ratio 234U/238U is 1.08 and for 230Th/234U is 0.97 for Barbados dust. Since a 230Th/234U ratio of 1.0 is assumed in correcting for U-supported 230Th in dating by the 230Th/231pa and 230Th/232Th methods, we would conclude that the influx of dust from Africa into the equatorial Atlantic and Caribbean would not, in itself, significantly affect the dates obtained for marine sediments. The average 8% excess of 234U is probably the result of weathering processes occurring near the source area in Africa. The excess 234U is possibly associated with the desert varnish coating depos** Ideally, marine carbonates intended for datine should contain only radiogenic 23°Th and no 232Th.
H.S. Rydell, J.M. Prospero, Uranium and thorium concentrations
ited by secondary processes; it is unlikely that the excess 234U is derived from sea-spray salts as the samples are washed prior to analysis. Because of the small sample sizes ( ~ 2g), 231pa could not be measured, but a rough estimate can be made. On the basis of the average 230Th/234U activity ratio of 0.97 and the 234U/238U ratio of 1.08, the net disequilibria between parent 238U and its eventual daughter 230Th ia about 5%. Since the activity ratio 238U/235U (21.8) is constant in nature, it can be assumed that if 231pa is in equilibrium with its parent, 235U, then there would be a 5% deficiency of 231pa relative to 230Th in African dust. Again a discrepancy of this magnitude, if existent, would not greatly influence the correction for the allogenic component of radio-elements in dating of marine sediments. Probably a greater source of potential dating errors than the disequilibrium in dust is (a) the postdepositional mobility of uranium [18, 30, 31] and (b) the separation of 230Th and 231pa in oceanic regions where manganese nodules and encrustations are common [32].
Acknowledgements We thank W.S. Moore for permission to use his data in this report, R. Nees and C. Dorta for their technical assistance, and Judge G.L. Taylor, Barbados, for the continued use of his coastal land site for our field operations. A portion of this research was supported by the Office of Naval Research Contract N00014-67-A0201-0013, Atomic Energy Commission Contract AT-(40-1)-3622, and National Science Foundation Grants GA-25916, GA-20112 and GA-22033. Contribution no. 1465 from the Rosenstiel School of MarLle and Atmospheric Science.
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401
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