Interstitial Water Studies, Deep Sea Drilling Project, Leg 75

28. INTERSTITIAL WATER STUDIES, DEEP SEA DRILLING PROJECT, LEG 751 Joris M. Gieskes, Kirk Johnston, and Marcus Boehm, Scripps Institution of Oceanography, La Jolla, California and Masato Nohara, Geological Survey of Japan Ibarak, 305, Japan ABSTRACT Interstitial water profiles obtained at Sites 530 and 532 of DSDP Leg 75 indicate complex concentration depth profiles resulting from diagenetic reactions taking place in the sediments. At both sites, large depletions in dissolved sulfate, resulting from bacterial sulfate reduction reactions, are accompanied by increased alkalinity values and also by increased dissolved ammonia concentrations. At Site 530, high sedimentation rates in the upper 200 m of the sediment column have led to a minimum in dissolved sulfate. Deep-seated reactions in basal sediments and/or basalts at this site cause downhole increases in dissolved calcium and decreases in dissolved magnesium. At Site 532, phosphate liberated by sulfate reduction has led to reaction with calcium ions to form authigenic Ca-phosphate minerals. INTRODUCTION During Leg 75 of the Deep Sea Drilling Project (DSDP) two sites were drilled—Sites 530 and 532— which were sampled in great detail for interstitial waters. The drill sites are closely related to two sites occupied during Leg 40: Site 530 is 55 km NE of Site 363, at a water depth of 4629 m in the Angola Basin, and Site 532 is essentially a reoccupation of Site 362 of DSDP Leg 40. Site 530 was piston cored to a depth of 180 m (Hole 53OB) and rotary drilled to basement at 1103 m sub-bottom depth. Site 532 was piston cored to a depth of 300 m (Hole 532B); at this site a very detailed sampling program for interstitial waters was undertaken. Both Sites 530 and 532 are characterized by very high sedimentation rates, especially in the younger sections, and this leads to complications in the interstitial water profiles as will be discussed. RESULTS The shipboard data (pH, alkalinity, salinity, chloride, calcium, and magnesium) and the data obtained in our laboratory are presented in Table 1 and Figures 1, 2, and 3. Methods used were those described by Gieskes (1974), Gieskes and Lawrence (1976), and Gieskes and Johnson (1981). DISCUSSION Site 530 The upper 100 m (Units la, lb) were deposited at rates in excess of 65 m/m.y. and consist of diatom-nannofossil marls and debris-flow deposits. Sedimentation rates in the lower lying lithologic units were much less. Hole 53OB (Fig. 1) indicates that sulfate reduction is an important process in the upper section of the sedi- Hay, W. W., Sibuet, J . - C , et al., Init. Repts. DSDP, 75: Washington (U.S. Govt. Printing Office). ment column. This is evident from the rapid decrease in dissolved sulfate, the increase in dissolved ammonia (maximum of 3.25 mM at 80 m), and the increase in alkalinity (maximum of 25 meq/dm3 at 80 m). Typically, the production of bicarbonate has led to the precipitation of calcium carbonate, thus causing the low concentrations of dissolved calcium in the upper 200 m. A rapid decrease in magnesium occurs which is not readily explained, but may be the result of processes involving the diagenesis of opaline silica (Kastner et al., 1977) or the formation of dolomite. Strontium concentrations increase rapidly below 40 m, probably as a result of carbonate diagenesis (Baker et al., 1982). Dissolved lithium appears to have a source in the lower lying sediments, i.e., in Unit 3 (Fig. 2). Data on dissolved silica indicate high concentrations, representative of those often found in siliceous sediments (Gieskes, 1981). Holes 530 and 53OA sampled the deeper section of Site 530, and the data indicate a well-established minimum in dissolved sulfate, located at about 200 m subbottom depth. This can be understood in terms of the higher sedimentation rates in the upper 200 m of the sediments, usually associated with increased levels of reactive organic carbon. With sedimentation rates of ~ 50 m/m.y. in the upper 200 m of the sediment column the communication length for diffusion is between 100-150 m, and thus nonsteady-state dissolved sulfate profiles, especially as a result of higher sulfate reduction rates in the upper sediment column, are to be expected (Gieskes et al., 1978; Gieskes, 1981). The sulfate minimum is accompanied by an ammonia maximum as well as an alkalinity maximum. Unit 3 (250-450 m) is characterized by red and green muds, with appreciable volcanic contributions. Dissolved silica values are low, indicating little contribution of biogenic silica. The profiles of dissolved lithium and potassium indicate a source for lithium, leading to a maximum in this zone, and a sink for potassium, perhaps as a result of uptake in clay minerals. No sink for magnesium is indicated by the dissolved magnesium profile. 959 J. M. GIESKES, K. JOHNSTON, M. BOEHM, M. NOHARA Table 1. Interstitial water analyses, Leg 75. Sample (interval in cm) pH Alk. (meq/dirr) S (g/kg) Ca (mM) Mg (mM) Cl (g/kg) Sr (µM) Li (µM) K (mM) So 4 (mM) 124 7.54 16.32 33.0 5.50 39.7 23.49 166 88 10.7 5.0 1407 172 219 265 313 362 406 416 500 594 687 965 1050 7.77 7.44 7.52 6.81 7.42 6.81 7.06 7.58 7.64 7.25 12.40 6.79 2.19 3.26 2.32 1.96 1.50 1.46 0.92 0.22 - 7.39 8.09 8.13 13.41 17.91 23.61 24.68 21.77 24.42 32.89 87.70 43.53 38.3 38.3 37.6 37.7 36.5 29.8 31.4 36.2 29.1 16.5 15.8 13.1 19.55 19.55 19.31 19.41 18.67 19.11 18.87 19.78 19.75 19.61 18.93 18.33 254 197 167 234 275 320 387 387 593 725 735 630 161 158 136 136 145 160 197 133 97 90 — 182 10.0 6.95 6.65 6.60 3.00 2.90 2.50 4.20 2.86 2.55 1.39 1.39 4.5 — — 8.9 - 33.0 32.2 32.4 33.0 31.9 33.0 32.4 34.1 34.1 34.1 31.9 31.4 17.1 — 20.4 — 18.5 — — 1425 1113 871.5 582 378 370 274 252 340 250 — — 1227 495 118 82 167 125 107 781 375 122 — — 14 35 57 83 106 124 149 172 7.65 7.47 7.57 7.35 7.41 7.64 7.48 7.47 12.53 19.95 23.38 24.02 20.82 14.81 10.56 10.66 35.2 34.4 33.3 33.6 32.7 32.4 32.2 32.2 6.93 6.74 5.63 4.88 5.69 5.12 5.33 6.35 52.4 46.6 41.2 40.3 38.5 37.1 31.3 35.6 19.46 19.46 19.46 19.69 19.39 19.63 19.29 19.49 96 81 124 178 186 177 170 170 48 53 57 69 69 98 120 142 10.72 8.46 11.34 9.07 9.70 8.20 10.0 7.41 20.9 8.4 7.4 — 3.8 3.5 3.6 4.5 1550 2518 3041 3252 3238 2934 2559 2209 759 787 944 916 974 970 875 787 8 51 90 130 164 211 7.01 7.52 7.20 7.27 6.93 7.92 2.23 13.22 19.35 19.88 19.81 19.47 35.5 34.4 32.7 32.2 32.4 33.0 10.73 9.00 7.06 8.31 6.65 9.57 49.8 41.1 36.9 27.8 27.9 24.7 — — 104 134 195 234 298 351 47 70 123 163 208 267 6.90 7.17 6.83 10.29 8.87 10.60 30.0 15.0 7.2 2.9 3.9 3.9 40 2198 3784 4421 5242 5505 44 782 893 866 840 657 8 17 25 34 43 52 61 69 78 87 96 105 113 121 129 138 146 154 162 170 177 184 1% 208 216 224 232 243 251 258 264 271 279 285 290 7.61 7.69 7.46 7.53 7.49 7.34 7.52 7.48 7.49 7.52 7.51 7.38 7.54 7.52 7.39 7.45 7.63 7.67 7.7 7.67 7.31 7.54 7.68 7.48 7.74 7.67 7.82 7.65 7.48 7.55 7.69 7.66 7.59 7.57 7.84 3.76 4.20 4.96 7.49 9.96 12.34 13.55 14.68 16.34 18.98 18.61 17.93 15.20 18.64 18.20 20.46 18.98 18.83 17.80 18.13 11.06 18.65 17.42 13.84 17.30 17.86 16.01 15.51 12.24 12.39 13.70 11.98 9.70 10.41 12.56 34.9 35.2 35.2 35.2 35.2 35.2 34.3 33.8 34.1 33.3 33.0 33.0 32.4 32.7 33.0 33.0 33.0 32.4 32.2 31.9 31.9 32.2 32.4 32.2 32.2 32.4 32.7 32.2 32.2 32.2 32.4 32.2 32.2 31.9 34.1 10.82 10.71 10.22 9.35 8.08 7.28 7.06 5.02 4.89 5.00 4.94 5.04 5.20 5.24 4.98 6.28 5.12 6.00 5.77 6.24 5.41 6.31 6.94 7.18 7.20 7.59 7.43 7.18 5.57 6.98 7.71 5.63 4.94 5.57 7.28 54.2 53.8 53.7 54.2 52.2 50.8 46.9 44.0 43.8 40.8 36.7 35.1 32.3 32.2 31.7 30.9 31.8 30.9 33.7 29.1 24.6 30.9 28.3 24.4 27.8 28.4 27.7 27.0 26.0 26.4 26.8 26.2 26.2 25.9 28.4 18.83 18.77 19.92 19.68 19.41 19.78 19.98 19.88 19.41 19.34 19.14 19.41 19.21 18.97 19.17 19.17 19.34 19.14 20.12 19.54 19.51 19.34 19.34 19.44 19.21 19.37 19.68 19.51 19.41 19.17 19.61 19.44 19.88 19.14 19.27 104 122 124 126 129 130 134 147 151 173 184 184 195 198 196 235 237 274 272 284 219 279 302 274 320 366 366 375 325 365 377 376 364 316 304 50 50 51 58 59 59 79 81 89 82 118 124 132 139 138 129 177 188 196 216 234 203 265 279 288 274 274 292 294 297 295 312 291 285 254 11.24 9.28 8.27 11.44 10.61 10.64 9.84 12.62 10.97 11.15 9.07 11.04 11.75 11.13 10.0 10.02 11.81 10.87 10.86 11.47 15.17 11.42 8.49 12.14 10.63 10.40 10.82 10.86 8.05 9.45 8.24 8.95 — 10.55 8.90 31.0 28.4 31.0 _ 24.3 23.3 — 13.1 — 8.3 — 4.9 — 7.3 9.4 — 7.3 328 398 510 958 1322 1719 2153 2584 2757 3043 3241 2889 4126 4130 4955 4851 4974 5264 5609 5358 5061 5649 6873 5716 6106 5061 6458 6209 6858 6230 6011 6272 6591 5500 3500 512 536 565 687 700 832 761 883 788 839 840 829 786 831 812 800 801 804 753 757 349 831 673 372 653 725 764 726 452 527 678 457 363 363 883 Sub-bottom depth (m) NH4 (MM) Si 0*M) Hole 530 2-6, 140-150 895 Hole 530A 5-6, 140-150 10-6, 140-150 15-5, 140-150 20-5, 140-150 25-6, 140-150 30-4, 140-150 35-4, 140-150 40-3, 140-150 50-3, 110-120 60-1, 140-150 89-5, 140-150 99-4, 140-150 Hole 530B 4-2, 143-150 9-2, 143-150 14-2, 140-150 20-2, 140-150 26-2, 140-150 32-1, 140-150 38-2, 140-150 46-1, 140-150 Hole 532 3-4,0-11 12-2, 140-150 21-2, 140-150 31-2, 140-150 40-2, 143-150 51-2, 140-150 — Hole 532B 2-2, 140-150 4-2, 140-150 6-2, 140-150 8-2, 140-150 10-2, 140-150 12-1, 140-150 14-2, 140-150 16-2, 140-150 18-1, 140-150 20-1, 140-150 22-1, 140-150 24-2, 140-150 26-3, 140-150 28-2, 140-150 30-2, 143-150 32-2, 140-150 34-2, 140-150 36-2, 143-150 38-3, 0-006 40-2, 140-150 42-2, 140-150 44-2, 140-150 47-2, 140-150 50-2, 140-150 52-2, 140-150 54-2, 140-150 56-2, 140-150 59-2, 140-150 61-2, 140-150 63-2, 140-150 65-2, 140-150 67-2, 140-150 69-2, 140-150 71-1, 140-150 73-1, 140-150 9.4 _. 6.7 — 9.3 9.3 _ 9.2 — 8.9 — 8.9 — 6.7 _ 7.8 Note: Dash = data not available. Below Unit 3 the profile of dissolved calcium indicates a source of calcium in the deeper section of the sediment column, perhaps in the carbonate layers or in the underlying basement. For dissolved magnesium the situation is less clear, with possible uptake both in the sediments (dolomitization?) and in the underlying basalts. Dissolved strontium has a significant source in 960 Unit 5, which is characterized by calcareous sediments and limestones. At great depth dissolved strontium values again decrease. In general the dissolved constituents of the interstitial waters recovered from Site 530 sediments indicate a complex set of reactions reflecting the variable lithological features of the sediments. Biogenic sulfate reduc- INTERSTITIAL WATER STUDIES Sulfate (mM) and ammonia ( µM) 0 1000 2000 3000 N H 4 10 20 30 SO. pH Alkalinity (meq/dm3) 6 8 10 20 Calcium and Magnesium (mM) 0 20 40 60 \T 1a 40 - ; i 80 " 1 b .J - • 120 " 2 160 - ,0 - \ / • . Silica (µM) 400 800 V J \ ; Lithium (µM) 50 100 Strontium (µM) 0 100 200 1 40 Mg _ u 80 -lea 1 150 0 Potassium (mM) 5 10 V ] 120 160 j Figure 1. Interstitial water data, Hole 530B. Unit la: Diatom-nannofossil marl and ooze; debris-flow deposits 65 m/m.y. Unit lb: Diatom ooze and debris-flow deposits 65 m/m.y. Unit 2: Nannofossil clay, marl, and ooze; debris-flow deposits -20 m/m.y. tion processes in the upper sediment column cause complex nonsteady state profiles in dissolved sulfate, ammonia, alkalinity, and calcium. Site 532 Site 532 was essentially a reoccupation of Site 362 on the Walvis Ridge. Only the upper 300 m of this site were sampled, but very detailed sampling allowed a detailed determination of the interstitial water profiles. Sedimentation rates have varied between 40 and 50 m/m.y., and the sediments consist mostly of nannofossil marls (Fig. 3). The alkalinity profile shows a broad maximum of - 2 0 meq/dm3 between 80-200 m. Dissolved sulfate shows a minimum located at the base of Unit lb (diatom-nannofossil marl). This minimum is not reflected in the dissolved ammonia profile. Methane gas levels (not reported here) only become significant below 120 m, i.e., below the sulfate minimum. Dissolved ammonia has its main source at ~ 250 m, with high production in the methane zones (120-250 m). Alkalinity increases, causing authigenic apatite (cf., site summary) and calcium carbonate precipitation and consequently a decrease in dissolved calcium. Dissolved magnesium appears to have a sink in Unit lb. Perhaps the decrease in magnesium is the result of partial dolomitization of carbonates in the low sulfate zone at -100 m. Both dissolved lithium and dissolved strontium have sources at -280 m. However, the nature of these sources remains unclear, though carbonate recrystallization reactions are the most likely source of dissolved strontium. The data for dissolved potassium show little trend and are too scattered to suggest any possible significant variability downhole. Data on dissolved chloride are not precise enough to confirm the gradual downhole increase in dissolved chloride that characterized Site 362, particularly below a depth of -300 m (Sotelo and Gieskes, 1978). Agreement between alkalinity, dissolved calcium, and dissolved magnesium profiles obtained in Site 362 (Sotelo and Gieskes, 1978) and in Site 532 is very good. ACKNOWLEDGMENTS We appreciate the efforts by the shipboard chemist, Mr. Ken Thompson. The manuscript has been reviewed by Drs. R. E. McDuff and G. Klinkhammer, whose criticisms are appreciated. This work was supported by NSF Grant OCE-8023966 to JMG. REFERENCES Baker, P. A., Gieskes, J. M., and Elderfield, H., 1982. Diagenesis of carbonates in deep sea sediments—Evidence from Sr/Ca ratios and interstitial dissolved Sr2* data. J. Sed. Petrol., 52:71-82. Gieskes, J. M., 1974. Interstitial water studies, Leg 25. In Simpson, E. S. W., Schlich, R., et al., Init. Repts. DSDP, 25: Washington (U.S. Govt. Printing Office), 361-394. , 1975. Chemistry of interstitial waters of marine sediments. Ann. Rev. Earth Planet. Sci., 3:433-453. _, 1981. Deep-sea drilling interstitial water studies: Implications for chemical alteration of the oceanic crust, Layers I and II. Soc. Econ. Paleontol. Mineral., Spec. Pub!., 32:149-167. Gieskes, J. M., and Johnson, J., 1981. Interstitial water studies, Leg59. In Kroenke, L., Scott, R., et al. Init. Repts. DSDP, 59: Wasfiington (U.S. Govt. Printing Office), 627-630. Gieskes, J. M., and Lawrence, J. R., 1976. Interstitial water studies, Leg 35. In Hollister, C. D., Craddock, C , et al., Init. Repts. DSDP, 35: Washington (U.S. Govt. Printing Office), 407-423. 961 J. M. GIESKES, K. JOHNSTON, M. BOEHM, M. NOHARA Gieskes, J. M., Lawrence, J. R., and Galleisky, G., 1978. Interstitial water studies, Leg 38. In Bolli, H. M., Ryan, W. B. F., et al., Init. Repts. DSDP, Suppl. to Vols. 38, 39, 40, and 41: Washington (U.S. Govt. Printing Office), 121-133. Kastner, M., Keene, J. B., and Gieskes, J. M., 1977. Diagenesis of siliceous oozes. I. Chemical controls on the rate of opal-A to opalCT transformation—An experimental study. Geochim. Cosmochim. Acta, 41:1041-1059. pH Alkalinity (meq/dm3) 10 20 6 8 1 2 400 -. Sulfate (mM) and ammonia (µM) NH4 400 800 12O o 0 10 20 30 SO. 0 f \ '"" «6 800 — • 8 - 1200 -- 9 - 0 Date of Initial Receipt: October 6,1982 Calcium and Magnesium (mM) 0 20 40 60 • ×* 3 4 7 Sotelo, V., and Gieskes, J. M., 1978. Interstitial water studies, Leg 40: Shipboard studies. In Bolli, H. M., Ryan, W. B. F., et al., Init. Repts. DSDP, 40: Washington (U.S. Govt. Printing Office), 549-554. - Mg - Silica (µM) 400 800 1200 0 Strontium (µM) 200 400 600 0 Lithium (µM) 100 200 0 Potassium (mM) 5 10 400 800 1200 Figure 2. Interstitial water data, Hole 53OA. Unit 1: Diatom-nannofossil oozes, marls, and debris-flow deposits 65 m/m.y. Unit 2: Nannofossil clay, marl, and ooze; debris-flow deposits 20 m/m.y. Unit 3: Red and green mud 9 m/m.y.; Unit 4: Multicolored mudstone, marlstone, chalk, and clas5 m/m.y. Unit 5a: Mudstone, marlstone, limestone 15 m/m.y. Unit 5b: Mudtic limestone stone, marlstone, limestone, and siliciclastic sandstone 38 m/m.y. Unit 5c: Mudstone, marlstone, 11 m/m.y. Unit 6: Glauconitic sandstone 20 m/m.y. Unit 7: calcareous siliclastic sandstone Claystone, siltstone, sandstone 31 m/m.y. Unit 8: Claystone, marlstone, black shales 9 m/m.y. Unit 9: Basalt. 962 INTERSTITIAL WATER STUDIES pH Alkalinity (meq/dm3) 6 8 10 20 Sulfate (mM) and ammonia (µM) 0 2000 4000 6000 NH 0 10 20 30 SO. Calcium and Magnesium (mM) 0 20 40 60 la « 80 - 1b 160 - 1c 240 - < j J J T 1 ; > SO4 St + +++ NH4 \ \ 320 -- .0 80 160 Silica (µM) 400 800 0 Strontium (µM) 200 400 Ca > 0 Lithium (µM) 100 200 300 0 Potassium (mM) 5 10 V • 240 •/• 320 Figure 3. Interstitial water chemistry, Site 532. Unit la: Foram-nannofossil marl and ooze 41 m/m.y. Unit lb: Diatom-nannofossil marl 52 m/m.y. Unit lc: Nannofossil marl 40 m/m.y. 963