GEOCHEMISTRY of URANIUM in the CARIACO Trencht

GEOCHEMISTRY OF URANIUM IN THE CARIACO TRENCHt

CLARA C. DORTA AND ELIZABETH RONA University of Miami, Rosenstiel School of Marine and Atmospheric Science

ABSTRACT During a cruise (P6603) of the R/V PILLSBURY of the University of Miami, several cores were taken from the Cariaco Trench. The concentrations of thorium, protactinium, and uranium have been studied. A layer of oxidized clay is present below a few meters of reduced sediments. Anal- yses show high concentration of uranium to be associated with the organic- rich reducing sediments. In one core, a fivefolddecrease in uranium content is found in the oxidized layer, whereas the thorium and protactinium re- main the same. Aeration of the sediment at the end of the last glaciation dissolved uranium in the oxidized layer. In the reducing layers, uranium concentrations are higher than in sediments of the open sea.

INTRODUCTION The Cariaco Trench is a depression about 1390 meters deep on the Con- tinental Shelf off the Venezuelan coast, 100 miles long in the east-west direction, and 40 miles wide in the north-south direction. It is separated from the Caribbean Sea by a ridge which includes a line of islands. The sill is nowhere deeper than 150 meters, thus shutting off the deeper water from that of the Caribbean. Above sill depth, the waters of the trench are in free communication with the Caribbean Sea, while below, an isolated marine environment undisturbed by mixing exists. The Cariaco Trench offers an opportunity to study the geochemistry of uranium in an anaerobic environment, and the stratigraphy of Recent marine deposits. The chemistry of the waters of the Cariaco Trench has been studied extensively by Richards & Vaccaro (1956). The oxygen content of the water below sill depth decreases to zero at about 475 meters (Fig. 1); here hydrogen sulfide can first be detected. Below this depth the concentration of hydrogen sulfide increases. At 700 meters, nitrogen as nitrate disappears, and the only inorganic nitrogen is ammonia. This situation is similar to that of other euxinic basins, such as the Norwegian Fjords and the Black Sea. The sediments are rich in "black mud" of relatively high organic content. The sediments from the Cariaco Trench have been investigated by Heezen et at. (1959), Broecker et at. (1960), and Lidz et at. (1969). The upper 5 or 10 meters of the cores are composed of dark greenish gray laminated

1 Contribution No. 1400 from the University of Miami, Rosenstiel School of Marine and Atmos- pheric Science. 1971] Dorta & Rona: Geochemistry of Uranium, Cariaco Trench 755

DISSOLVED OXYGEN AND TOTAL REDOX REACTANTS MIL L I REACTANTS OR mg AIL 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-tt, ":t!..' I , I I I , , I I I ~ 100 .••...••..•. r,1) __ .•.,.," f \ r \ i + ~ ~\.\ \ . en v VII a: + , w 600 t I- W v~ ++ ~ IV . \ r I ffl I- \ a.. 800 w o Jz 1++ 'v\ CARIACO TRENCH ~ T • DISSOLVED OXYGEN 1 1000 I v SULFIDE CONCENTRATION + vf + TOTAL REDOX REACTANTS I v I, CARIBBEAN , 1200 o DISSOLVED OXYGEN + ,, v A

1400 0.00 0.05 5 CONC., mg AIL

FIGURE 1. Dissolved oxygen in the Cariaco Trench and the nearby Caribbean; sulphide concentrations and total oxygen and sulphate consumed in the Cariaco Trench. (After Richard & Vaccaro, 1956.) lutite, deposited under reducing conditions. A layer of oxidized clay is present beneath this organic layer (an indication that aeration occurred sometime in the past) and is followed below by another laminated layer (cf., Fig. 3). A radiocarbon date on organic material from the base of the 756 Bulletin of Marine Science [21 (3)

61° 66° 65°

VENEZUELA

66" . Lo"t;on of C.,;a ". ,~ FIGURE 2 Navy H. O. Cha" ~ 0.2319,TeenehApriland hidrographic statio s .~ upper anacrob' I 917. 13th Ed., 195 0iMler u.s.

11,000 years ~ :yer indicates that sta . . belowd' must have. .bc(Heezend el a.,l 1959)gnahon of the trench b an .m'eropaleonlo eo eposited dnrin . Therefore th eg~n.abruptly P,gure 2 show log,eal analyses by Lgdthe last glacia;;o e ox.,d,zctl day 'he cores were ta~:n map of lhe Cariae~ ~ et al. (1969) s~' M,""~logieal Melh d . Table 1 gives 1 . rench and the I pp.ort this view o s.-P oea',o oeat.o . 2-5) have b our cores from th . n, water dep'h, and ns at which een analyzed f ~ Canaco Tre h core length. or maJor', mmor, andnctrace(seeel F Ig. 2 and Tables ements , usmg. effilS-.

DATA FOR C TABLE 1 ORES FROM THE CARIACO T

Latitude RENCH (N) Longitude Water depth Core length Core No. (W) (m) (em)

P6603-1 10°28' 64°39' 1346 790 P6603-2 10°35' 64°38' 940 820 P6603-4 10°48.6' 65°21' 1124 210 P6603-7 10°42.5' 65°50' 1339 504 1971 ] Dorta & Rona: Geochemistry of Uranium, Cariaco Trench 757

TABLE 2 ELEMENTALANALYSESOF CORE P6603-2

-_ .. ~----- ,----- ~-~--- Depth (em) 4-8 254-259 354-359 504-509 ------~- p~O.; (%) 0.163 0.231 0.189 0.121 SiO~ ('70) 31.53 50.76 42.68 33.33 AI ('70) 5.93 2.80 6.24 5.56 Fe ('70) 2.9 1.5 3.1 2.9 Na ('7~) 0.519 1.63 2.22 1.63 Mn (%) 0.019 0.009 0.029 0.032 Ca ('70) 4.07 1.64 4.15 5.07 Mg (%) 0.84 0.72 0.78 1.20 K ('70) 1.16 0.664 1.16 1.08

TABLE 3 ELEMENTALANALYSESOF CORE P6603-1

Depth (em) 0-7 50-54 103-109 357

TiO~ ('70) 0.70 0.80 0.70 0.75 Fe ('70) 3.75 3.72 3.60 3.90 AI~O:l ('70) 13.4 13.2 13.0 13.6 MnO ('70) 0.028 0.018 0.030 0.033 P~O~ ('70) 0.18 0.20 0.25 0.25 MgO ('/'0) 2.05 1.95 1.85 1.90 CaO ('70) 8.60 9.25 9.15 9.70 Na~O ('70) 2.75 2.75 2.60 2.45 K~O (70) 1.80 1.80 1.72 1.62 B (ppm) 100 130 115 110 Ba (ppm) 400 350 400 350 Co (ppm) 12 5 5 10 Cr (ppm) 95 85 95 100 Cu (ppm) 25 30 25 30 La (ppm) 25 30 35 40 Ni (ppm) 80 85 70 80 Sc (ppm) 12 14 13 13 V (ppm) 200 250 190 200 Y (ppm) 20 25 25 25 Zr (ppm) 90 95 105 105 Mo (ppm) 75 90 85 85 --- 758 Bulletin at Marine Science [21(3) TABLE 4 ELEMENTAL ANALYSES OF CORE P6603-4

Depth (em) 6 53 74

Ti02 (%) 0.40 0.40 0.45 Fe (%) 2.45 2.70 2.80 AI203 (%) 9.0 10.0 10.0 MnO (%) 0.025 0.035 0.022 P205 (%) 0.17 0.20 0.30 MgO (%) 1.85 1.95 1.80 CaO (%) 21.2 19.8 18.0 Na20 (0/0) 2.65 2.85 3.00 K20 (%) 1.80 1.25 1.45 B (ppm) 95 110 120 Ba (ppm) 250 400 450 Co (ppm) 5 10 5 Cr (ppm) 80 90 80 Cu (ppm) 20 18 13 La (ppm) 20 20 16 Ni (ppm) 70 80 75 Sc (ppm) 9 10 10 V (ppm) 180 145 190 Y (ppm) 16 18 13 Zr (ppm) 40 60 50 Mo (ppm) 110 60 40 sion spectroscopy and atomic absorption. Uranium, thorium isotopes, and protactinium were separated by ion exchange and solvent extraction tech- niques. The concentrations of the radio elements were determined by alpha particle spectrometry (Rona, 1969) (Tables 6-9). Uranium concentrations are high in all cores, compared to the Caribbean cores (cf., Rona & Emiliani, 1969; Rona, Emiliani & Dorta, 1970)-up to 30 p.g/g sediment in trench cores compared to 1 to 2 p.g/g in the Carib- bean cores. Anoxic environments are rich in organic material which decom- poses slowly, utilizing all the oxygen available. Uranium forms complexes with organic matter and is released after decomposition. In the reducing environment of the sediment interface, uranium is immobilized as insoluble sulfide or hydroxide. In all cores, high concentrations of uranium show correlation with organic matter and with molybdenum and vanadium. These elements are also scavenged by organic matter. The presence of Ti02 indicates the intrusion of detrital material in some sediment profiles. 1971] Dorta & Rona: Geochemistry of Uranium, Cariaco Trench 759

TABLE 5 ELEMENTAL ANALYSES OF CORE P6603-7

Depth (em) 2-6 50-54 100-104 320

Ti02 ('fa) 0.70 0.65 0.85 0.55 Fe ('70) 4.00 3.55 4.20 3.15 AI203 ('fa) 13.8 13.0 13.6 13.2 MnO ('fa) 0.054 0.038 0.069 0.024

P205 ('fa) 0.20 0.14 0.30 0.14 MgO ('fa) 2.33 1.85 2.20 4.20 CaO ('fa) 10.5 15.0 8.3 13.5 Na20 ('fa) 3.40 2.00 2.55 1.70 K20 ('fa) 1.50 2.00 1.82 1.90 B (ppm) 105 100 60 120 Ba (ppm) 200 300 300 300 Co (ppm) 12 11 16 5 Cr (ppm) 90 80 90 70 Cu (ppm) 20 25 25 20 La (ppm) 30 30 35 30 Ni (ppm) 70 70 75 50 Sc (ppm) 14 13 16 12 V (ppm) 185 180 135 140 Y (ppm) 20 25 30 20 Zr (ppm) 95 95 55 80 Mo (ppm) 50 30 19 60

TABLE 6 ISOTOPIC ANALYSES OF URANIUM, THORIUM, AND PROTACTINIUM OF CORE P6603-2

U-23S* Depth U-234" U-234 Th-232 Th-230"t Pa-231*t Th-230 CaCO. (em) (d/h/g) (p.g/g) (d!h/g) U-238 (d/h/g) (d/h/g) (d/h/g) Pa-231 (%) 4-8 574.0 13.0 605.0 1.05 92.0 110.0 13.1 8.4 21.4 254-259 729.0 16.6 828.0 1.10 42.0 128.5 13.1 9.8 9.1 354-359 206.0 4.7 255.0 1.20 76.0 156.0 17.0 9.2 20.7 504-509 817.0 18.5 795.0 0.97 150.0 243.0 28.2 • Carbonate-free basis. t Not corrected for uranium. 760 Bulletin of Marine Science [21(3)

P6603-2

LAMINATIONS 100 SHL HOMOGENEOUS LUTITE/SLT

200WOOD FRAG AT 205 CM

30_0 .•.~ 237-389. LUTITE BANDING ALTERNATING W/MICRO-BANDING STARTING 356 @ 237CM 400

CARIACO TR 500

SHLS 600

700

800

FIGURE 3. Stratigraphy of core P6603-2.

Core P6603-2.-This core, from a depth of 940 meters, spans the late Pleistocene. There is a sharp break at about 350 em between the laminated sediments above and homogeneous clay below (Fig. 3). Similar boundaries were observed by Heezen et at. (1959) and Athearn (1965) in cores from nearby areas. Heezen dated the homogeneous clay by the C-14 method and established that it represents the last glaciation. At about 350 em, the location of the break, the uranium concentration (Table 6) drops dra- matically to about one-fifth of the value found above or below this narrow band; at this depth the nitrogen and organic carbon are at minimum (Fig. 4) 1971] Dorta & Rona: Geochemistry of Uranium, Cariaco Trench 761

P6603-2 PER CENT NITROGEN PERCENT ORGANIC CARBON PERCENT c,j:.0, 6 COARSE FRACTION o .10 .20 .~o ~o .50 o 1.0 2.0 3.0 4.0 50 6.0 0 10 20 30 40 50 o i , , 100

, ,. , ------CaCO. 200 -- COARSE FRACTION ,. ( " t. ~OO ('.I" t•. ...._~.•.

E u

500

600

700

800

FIGURE 4. Percentages of nitrogen, organic carbon, calcium carbonate, and coarse fraction (> 62p.) in core P6603-2, from the Cariaco Basin. (From Lidz et al., 1969). 762 Bulletin of Marine Science [21(3)

P6603-1 PER CENT NITROGEN PERCENT ORGANIC C'R80N PERCENT CoCO, II COARSE FRACTION o .10 .20 .30 .40 .50 o 1.02.0 3.0 4.0 5.0 6.0 0 10 20 30 40 50 o

100

200

300 ., ,, I

400 ~ I I

500

600

700

800

FIGURE 5. Percentages of nitrogen, organic carbon, calcium carbonate, and coarse fraction (> 62p.) in core P6603-1, from the Cariaco Basin. (From Lidz et at., 1969). 1971 ] Dorta & Rona: Geochemistry of Uranium, Cariaco Trench 763

TABLE 7 ISOTOPIC ANALYSES OF URANIUM OF CORE P6603-1

U-238* Depth U-234* U-234 CaC03 (em) (djhjg) (/Lgjg) (djhjg) U-238 (%) 0-7 465.03 10.57 515.24 1.11 16.35 50 404.67 9.20 437.20 1.08 11.78 103-109 377.03 8.57 437.73 1.16 14.33 357 402.23 9.14 437.63 1.09 17.51 • Carbonate-free basis.

TABLE 8 ISOTOPIC ANALYSES OF URANIUM OF CORE P6603-4

U-238* Depth U-234* U-234 CaC03 (em) (djhjg) (/Lgjg) (djhjg) U-238 (%) 0-6 930.87 21.16 1016.44 1.09 40.82 53 826.52 18.78 971.37 1.17 41.32 74 1120.08 25.46 1243.05 1.11 34.21 • Carbonate-free basis.

TABLE 9 ISOTOPIC ANALYSES OF URANIUM OF CORE P6603-7

U-238* Depth U-234* U-234 CaCOo (em) (d/h/g) (,ug/g) (d/h/g) U-238 (%) 2-6 479.85 10.90 501.83 1.04 18.10 50-54 894.25 20.32 922.58 1.03 25.86 100-105 267.71 6.08 307.29 1.15 14.31 350 458.75 10.43 530.17 1.16 23.27 • Carbonate-free basis. whereas the concentrations of the thorium isotopes and that of protactinium do not change, nor do those of the other elements which have been analyzed. At 350 to 359 em, a change in color is observed and an increase of benthonic foraminifers has been detected (Lidz, 1966). A large dis- 764 Bulletin of Marine Science [21(3) charge of river water may have happened at this time, with a sudden increase of oxygen leading to a solubilization of uranium and change from an as- sumed +4 to a +6 valence. This would explain why the concentration of thorium and protactinium did not change, both elements having only one valence state. Core P6603-1.-This core contains 748 cm of undisturbed sediments. Con- centration of CaC03 is low and fairly uniform (Fig. 5), and the uranium concentration (Table 7) is also uniform. Core P6603-4.-Taken from the central part of the Cariaco Trench, south of Tortuga, is similar (Table 8) to the upper sections of cores P6603-1 and P6603-2. Core P6603-7.-Taken from the western part of Cariaco Trench, differs (Table 9) from the other three cores discussed above. At the depth of 100 em a band of coarse, lighter colored bioclastic material is interposed into the clay. This material comes from adjacent extended shoals and shoreline. The uranium concentration in this band is low. Molybdenum is likewise low, while TiO~, a fair indicator for detrital material, is high. Acknowledgments.-We thank Mrs. Sylvia Valdes and Dr. Oiva Joensuu, who performed many of the chemical analyses. Research was supported by AEC Contract AT (40-1 )-3622.

SUMARIO

GEOQUIMICA DEL URANIO EN LA FOSA DE CARIACO Muestras tomadas en la parte este, oeste y central de la Fosa de Cariaco fueron analizadas para determinar uranio; en una de ellas, P6603-2, tambien se determin6 thorio y protactinio. Elementos mayores y menores y trazas de elementos fueron determinados. La correlaci6n de urania can molibdeno y vanadio indica la presencia de materia organica en gran concentraci6n, as! como Ti02 indica la intrusi6n de material detritico en la formaci6n de algunos sedimentos. Todas las muestras estudiadas presentan una alta con- centraci6n de urania y una estrecha correlaci6n can la materia organica.

REFERENCES ATHEARN, W. D. ]965. Sediment cores from the Cariaco Trench. Tech. Rep. Woods Hole Oceanogr. Inst., Ref. 65-37,31 pp. (Mimeographed.) BROECKER, W. S., M. EWING, AND B. C. HEEZEN 1960. Evidence for an abrupt change in climate close to 11,000 years ago. Am. J. Sci., 258: 429-448. HEEZEN, B. c., W. S. MENZIES, W. S. BROECKER, AND M. EWING 1971] Dorta & Rona: Geochemistry of Uranium, Cariaco Trench 765 1959. Date of stagnation of the Cariaco Trench, southeast Caribbean. Bull. geoI. Soc. Am., 60: 1579. (Abstract.) LIDZ, L. 1966. Deep-sea Pleistocene stratigraphy. Science, 154: 1448-1452. LIDZ, L., W. B. CHARM, M. M. BALL, AND S. VALDES 1969. Marine basins off the coast of Venezuela. Bull. Mar. Sci., 19: 1-17. RICHARDS,F. A. ANDR. F. VACCARO ] 956. The Cariaco Trench and the Gulf of Cariaco. Deep-Sea Res., 7: ] 63- 182. RONA, E. 1969. Quantitative radiochemical methods for the determination of uranium, thorium, isotopes and protactinium in sea water and marine sediments. Tech. Rep. Inst. Mar. Atmos. Sci. Univ. Miami, ML 69070, 8 pp. (Mimeographed. ) RONA, E. ANDC. EMILIANI 1969. Absolute dating of Caribbean cores P6304-8 and P6304-9. Science, 163: 66-68. RONA, E., C. EMILIANI, ANDC. C. DORTA 1970. Paleotemperature analysis and absolute dating of Caribbean deep-sea core P6304-1. 10 pp. in Annual Progress Report, AEC Contract AT- (40-1 )-3622, Rosenstiel School Mar. Atmos. Sci. Univ. Miami, ML 70080, 46 pp.