Geochemistry of Neogene-Quaternary sediments in ODP Site 105-645

Subcontinuously cored early(?)-middle Miocene to recently deposited sediments from ODP Site 645 were studied texturally, mineralogically, and geochemically. The entire sequence contains minerals and associated chemical elements that are chiefly of detrital origin. In particular, the clay minerals, which include smectite, kaolinite, chlorite, and illite, are detrital. No obvious evidence of diagenesis with depth, of burial, of volcanism, or of hydrothermal alteration was observed. The sedimentary textures, clay mineralogy, and <2-µm fraction geochemistry of the early middle Miocene sediments (630 to 1147 mbsf) suggest the pronounced but variable influence of a southward bottom current. Two clay facies are defined. The lower one, Cj (780 to 1147 mbsf), is characterized by the great abundance of discrete smectite (with less than 15% illite interlayers), probably detrital in origin, and reworked older, discrete, smectite-rich sediments. The upper clay facies, C2 (630 to 780 mbsf), shows a net decrease of the fully expandable clay abundances, with a great abundance of mixed-layer, illite-smectite clays (60 to 80% of illite interlayers). Such clay assemblages can be inherited from paleosoils or older sedimentary rocks. An important change occurs at 630 mbsf (clay fraction) or 600 mbsf (sedimentary texture), which may be explained by the beginning of continental glaciation (630 mbsf, ~9 Ma) and the onset of ice rafting in Baffin Bay (600 mbsf, ~8 Ma). Above this level, the characteristics and modifications of the clay assemblages are controlled climatically and can be explained by the fluctuations of (1) ice-rafting, (2) speed of weak bottom currents, and (3) some supply by mud turbiditic currents. Three clay facies (C3, C4, and C5) can be defined by the abrupt increases of the inherited chlorite and illite clays.

Barium in sediments and pore waters of several ODP Leg 170 and Leg 205 sites

Barium concentrations were measured on 17 pore fluid and 13 sediment samples from Sites 1253 and 1254 drilled offshore Costa Rica during Ocean Drilling Program (ODP) Leg 205. An additional 83 pore fluid and 29 sediment samples were analyzed for Ba concentrations from Sites 1039 and 1040 drilled during ODP Leg 170 offshore Costa Rica. Sites 1039/1253 and 1040/1254 are part of a transect across the Middle America Trench offshore Nicoya Peninsula. The entire incoming sediment section is being underthrust beneath the margin, providing an ideal setting to examine Ba cycling in the shallow levels of the subduction zone. Results from these analyses indicate that a significant amount of Ba is liberated from the mineral barite (BaSO4) in the uppermost hemipelagic sediments arcward of the trench. The shallow distillation of Ba may impact the amount of sedimentary Ba reaching the deeper subduction zone.

Composition of drilled sediments and pore waters from the continental slope of the Northern Gulf of Mexico

Pore waters were analyzed from 6 holes drilled from M.V. "Eureka" as a part of the Shell Oil Co. deeper offshore study. The holes were drilled in water depths of 600-3000 ft. (approximately 180-550 m) and penetrated up to 1000 ft. (300 m) of Pliocene-Recent clayey sediments. Salt and anhydrite caprock was encountered in one diapiric structure on the continental slope. Samples from holes drilled near diapiric structures showed systematic increases of pore-water salinity with depth, suggestive of salt diffusion from underlying salt plugs. Anomalous concentrations of K and Br indicate that at least one plug contains late-stage evaporite minerals. Salinities approaching halite saturation were observed. Samples from holes away from diapiric structures showed little change in pore-water chemistry, except for loss of SO4 and other variations attributable to early-stage diagenetic reactions with enclosing sediments. Thus, increased salt concentrations in even shallow sediments from this part of the Gulf appear to provide an indicator of salt masses at depth.

Chemical composition of sediments from DSDP Legs 45 and 71

Results of mineralogical and geochemical investigations of post-Middle Jurassic deposits of the Atlantic Ocean are based on materials of the Deep Sea Drilling Project. Comparative characteristics of primary matter for ''black shales'' are given. Exhalative origin of heavy metal accumulation in near-axial sedimentary deeps of the Mid-Atlantic Ridge (23°N) are shown. History of post-Middle Jurassic sedimentation is considered on the base of clay mineral-, clastic component-, trace and rare- chemical element studies.

Strontium, oxygen and hydrogen isotope ratios of basalts from DSDP Hole 504B

DSDP Hole 504B was drilled into 6 Ma crust, about 200 km south of the Costa Rica Rift, Galapagos Spreading Center, penetrating 1.35 km into a section that can be divided into four zones-Zone I: oxic submarine weathering; Zone II: anoxic alteration; Zones III and IV: hydrothermal alteration to greenschist facies. In Zone III there is intense veining of pillow basalts. Zone IV consists of altered sheeted dikes. Isotopic geochemical signatures in relation to the alteration zones are recorded in Hole 504B, as follows: Zone Depth(m) Average87Sr/86Sr Average delta18O (?) Average deltaD (?) I 275-550 0.7032 7.3 -63 II 550-890 0.7029 6.5 -45 III 890-1050 0.7035 5.6 -31 IV 1050-1350 0.7032 5.5 -36 Alteration temperatures are as low as 10°C in Zones I and II based on oxygen isotope fractionation. Strontium isotopic data indicate that a circulation of seawater is much more restricted in Zone II than in Zone I. Fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 +/- 20°C in Zones III and IV, which is consistent with secondary mineral assemblages. The strontium, oxygen, and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in Zones III and IV are estimated to be 0.7037, 2?, and 3?, respectively. Strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water/rock ratios (in weight) of about 1.6. This value is close to that of the end-member hydrothermal fluids issuing at 21°N EPR. Basement rocks are not completely hydrothermally altered. About 32% of the greenstones in Zones III and IV have escaped alteration. Thus 1 g of fresh basalt including the 32% unaltered portion are required in order to make 1 g of end-member solution from fresh seawater in water-rock reactions.

Pore-water chemistry of ODP sites 111-677 and 111-678

Sites 677 and 678 were drilled on ODP Leg 111 to test hypotheses about the nature and pattern of hydrothermal circulation on a mid-ocean ridge flank. Together with earlier results from DSDP Site 501/504 and several heatflow and piston coring surveys covering a 100-km**2 area surrounding the three drill sites, they confirm that hydrothermal circulation persists in this 5.9-m.y.-old crust, both in basement and through the overlying sediments (Langseth et al., 1988, doi:10.2973/odp.proc.ir.111.102.1988). Profiles of sediment pore-water composition with depth at the three drill sites show both vertical and horizontal gradients. The shapes of the profiles and their variation from one site to another result from a combination of vertical and horizontal diffusion, convection, and reaction in the sediments and basement. Chemical species that are highly reactive in the siliceous-calcareous biogenic sediments include bicarbonate (alkalinity), ammonium, sulfate, manganese, calcium, strontium, lithium, silica, and possibly potassium. Reactions include bacterial sulfate reduction, mobilization of Mn2+, precipitation of CaCO3, and recrystallization of calcareous and siliceous oozes to chalk, limestone, and chert. Species with profiles more affected by reaction in basaltic basement than in the sediments include Mg, Ca, Na, K, and oxygen isotopes. Reaction in basement at 60?C and at higher temperatures has produced a highly altered basement formation water that is uniform in composition over distances of several kilometers. As inferred from the composition of the basal sediment pore water at the three sites, this uniformity extends from up flow zone to downflow zone in basement and the sediments. It exists in spite of large variations in heat flow and depth to basement, apparently as a result of homogenization by hydrothermal circulation in basement. Profiles for chlorinity, Na, Mg, and other species in the sediment pore waters confirm that Site 678, drilled on a localized heatflow high identified by Langseth et al. (1988), is a site of long-lived upwelling of warm water from basement through the sediments at velocities of 1 to 2 mm/yr. The upflow through the anomalously thin sediments is apparently localized above an uplifted fault block in basement. This site and other similar sites in the survey area give rise to lateral diffusion and possibly flow through the sediments, which produces lateral gradients in sediment pore-water composition at sites such as 501/504. The complementary pore-water profiles at the low-heatflow Site 677 2 km to the south indicate that downflow is occurring through the sediments there, at comparable rates of 1 to 2 mm/yr.

Paleoclimate record combinatio of speleothems and of sediment core GeoB3910-2

A substantial strengthening of the South American monsoon system (SAMS) during Heinrich Stadials (HS) points toward decreased cross-equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time-scales. In order to better constrain the exact timing and internal structure of HS1 over tropical South America we assessed two precisely dated speleothem records from central-eastern and northeastern Brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial Atlantic. During HS1 we recognize at least two events of widespread intensification of the SAMS across the entire region influenced by the South Atlantic Convergence Zone (SACZ) at 16.11-14.69 kyr BP and 18.1-16.66 kyr BP (labeled as HS1a and HS1c, respectively), separated by a dry excursion from 16.66-16.11 kyr BP (HS1b). In view of the spatial structure of precipitation anomalies, the widespread increase of monsoon precipitation over the SACZ domain was termed 'Mega-SACZ'.