Interstitial water chemistry of DSDP Leg 85 sites

The concentration changes in pore waters of dissolved calcium, magnesium, sulfate, strontium, and silica and of alkalinity are controlled by diagenetic reactions occurring within the biogenic sediments of DSDP Sites 572, 573, and 574. Downcore increases in dissolved Sr2 + indicate recrystallization of calcite, and increases in dissolved SiO2 reflect dissolution of amorphous silica. Minor gradients in dissolved Ca(2+) and Mg(2+) suggest little if any influence from reactions involving volcanic sediments or basalt. Differences in interstitial water profiles showing the downhole trends of these chemical species mark variations in carbonate and silica diagenesis, sediment compositions, and sedimentation rate histories among the sites. The location and extent of carbonate diagenesis in these sediments are determined from Sr/Ca distributions between the interstitial waters and the bulk carbonate samples. Pore water strontium increases in the upper 100 to 250 m of sediment are assumed to reflect diffusion from underlying zones where calcite recrystallization has occurred. On the basis of calculations of dissolved strontium production and comparisons between observed and calculated "equilibrium" Sr/Ca ratios of the solids, approximately 30 to 50% of the carbonate has recrystallized in these deeper intervals. These estimates agree with the observed amounts of chalk at these sites. Variations in Sr/Ca ratios of these carbonates reflect differences in calcareous microfossil content, in diagenetic history, and, possibly, in changes in seawater Sr/Ca with time. Samples of porcelanite recovered below 300 m at Site 572 suggest formation at temperatures 20 to 30° C greater than ones estimated assuming oceanic geothermal gradients from sedimentary sections similar to those recovered on Leg 85. The higher temperatures may partially account for higher Sr/Ca ratios determined for recrystallized carbonates from this site.

(Table 1) Physical properties at DSDP Hole 72-516F

Forty indurated sediment samples from Site 516 were studied to determine the cause of acoustic anisotropy in carbonate- bearing deep-sea sediments. Recovered from sub-bottom depths between 388 and 1222 m, the samples have properties exhibiting the following ranges: wet-bulk density, 1.90-2.49 g/cm3; fractional porosity, 0.45-0.14; carbonate content, 33-88%; compressional-wave velocity (at 0.1 kbar pressure), 1.87-4.87 km/s; and anisotropy, 1-13%. Velocities were measured in three mutually perpendicular directions through the same specimen in 29 of the 40 samples studied. Calcite fabric has been estimated by X-ray pole figure goniometry. The major findings of this study are: 1) Carbonate-bearing deep-sea sediments may be regarded as transversely isotropic media with symmetry axes normal to bedding. 2) Calcite c-axes are weakly concentrated in a direction perpendicular to bedding, but the preferred orientation of calcite does not contribute significantly to velocity anisotropy. 3) The properties of bedded and unbedded samples are distinctly different. Unbedded sediments exhibit low degrees of acoustic anisotropy (1-5%). By contrast, bedded samples show higher degrees of anisotropy (to 13%), and anisotropy increases markedly with depth of burial. Thus, bedding must be regarded as the principal cause of acoustic anisotropy in calcareous, deep-sea sediments.

Eolian mass-accumulation rates and grain size at DSDP Sites 86-576 and 86-578

The mass-accumulation rate and grain size of the total eolian component of North Pacific pelagic clays at Deep Sea Drilling Project Sites 576 and 578 have been used to evaluate changes in eolian sedimentation and the intensity of atmospheric circulation that have occurred during the past 70 m.y. Eolian deposition, an indicator of source area aridity, was low in the Paleocene, Eocene, and Oligocene, apparently reflecting the humid environments of that time as well as the lack of glacial erosion products. A general increase in eoiian accumulation in the Miocene apparently reflects the relative increase in global aridity during the latter part of the Cenozoic. A dramatic increase in eolian accumulation rates in the Pliocene reflects the increased aridity and availability of glacial erosion products associated with Northern Hemisphere glaciation 2.5 m.y. ago. Eolian grain size, an indicator of wind intensity, suggests that Late Cretaceous wind strength was comparable to present-day wind strength. A sharp decrease in eolian grain size across the Paleocene/Eocene boundary is not readily interpreted, but may indicate a significant reduction in the intensity of atmospheric circulation at that time. Fine eolian grain size and low accumulation rates in the Eocene and early Oligocene are in agreement with low early Tertiary thermal gradients and less vigorous atmospheric circulation. Large increases in grain size during the Oligocene, mid-to-late Miocene, and Pliocene appear to be a response to steepening thermal gradients resulting from increasing polar isolation.

Isotopic geochemistry of lavas at DSDP Holes 81-553 and 81-555

It is demonstrated by K-Ar analyses that the age of reversely magnetized basalts, which immediately predate magnetic Anomaly 24B, is 53.5 ± 1.9 m.y. Samples from deep levels appear to be grossly contaminated by an extraneous argon component with a uniform argon-40/argon-36 ratio 440. This component is thought to have been derived from fluids circulating in the lava pile during burial. The age result corroborates the assignment previously made to Anomaly 24B by Hailwood et al. (1979) and Lowrie and Alvarez (1981). It additionally suggests that lava extrusion formed part of a much larger magmatic event, which affected wide areas of the North Atlantic margins around the Paleocene/Eocene boundary, and can therefore probably be considered a good estimate of the age of this boundary. Initial 143Nd/144Nd ratios lie in the very restricted range 0.512920 ± 19 to 0.513026 ± 24 and initial 8 7Sr/86Sr ratios from ca. 0.703 to ca. 0.705. Acid leaching reduces the latter range to 0.70264 ± 4 to 0.70384 ± 4, suggesting that the higher 87Sr/86Sr ratios resulted from interaction with seawater. The array of data for treated samples is closely conformable on a 143Nd/144Nd-87Sr/86Sr diagram with the main oceanic mantle array and with previously published fields for Atlantic Ocean basalts. No evidence for any continental crustal contamination has been found. This suggests, but does not prove, that continental crust played no part in the genesis of these rocks.

Accumulation rates, coarse fraction and carbonate abundances at DSDP Hole 82-558A

Carbonate and coarse fraction data from Hole 558A record variations in the carbonate system for the Pleistocene. The late Pleistocene carbonate record from Hole 558A can be correlated to Atlantic carbonate records with similar accumulation rates. The correlation between carbonate and coarse fraction record in Hole 558A does not remain constant. The early Pleistocene carbonate-coarse fraction correlation is opposite of the typical North Atlantic pattern, whereas late Pleistocene carbonate-coarse fraction correlation does not show consistent pattern. The different relationships between the carbonate and coarse fraction records suggests that the carbonate system in the North Atlantic changed between the early and late Pleistocene.

(Table 1) Carbon and carbonate at DSDP Leg 71 Holes

Leg 71 sediments were analyzed for total carbon and acid-insoluble (organic) carbon, using the Leco WR-12 analyzer, according to the standard technique outlined below. The 3 cm**3 sediment samples were first dried and ground into a homogeneous powder. The ground sediment was redried at 105 to 110°C, and two samples, a 0.1 g and a 0.5 g sample, were weighed into Leco clay crucibles. The 0.5 g sample was acidified with 10% hydrochloric acid and washed with distilled water. The sample was then dried and analyzed for acid-insoluble carbon. The 0.1 g sample was analyzed for total carbon without further treatment. If the sample contained less that 10% CaCO3, an additional 0.5 g sample was analyzed for greater accuracy. The calcium carbonate percentages were calculated as follows: (% total C - % organic C) x 8.33 = % CaCO3. Although other carbonates may be present, all acid-soluble carbon was calculated as calcium carbonate.

Geochemistry, minerals, color, rare earth elements and d18O at DSDP Hole 92-504B

Investigations of borehole waters sampled in Hole 504B during Leg 92 revealed changes in major-ion composition similar to changes observed previously (during Leg 83). The uniformity of chloride concentrations with increasing depth suggests efficient downhole mixing processes along density gradients caused by large temperature gradients. Chemical and mineralogical studies of suspended drilling mud (bentonite) suggest that this material has undergone substantial alteration and that CaSO4 (anhydrite/gypsum) has precipitated in the deeper parts of the hole. Rare earth element studies suggest contributions of both the bentonites and the basalts to the REE distributions. Studies of the isotopic composition (87Sr/86Sr) of dissolved strontium indicate a strong contribution of basaltic nonradiogenic strontium, although differences between the Leg 83 and Leg 92 data indicate an influence of bentonite during Leg 92. The oxygen isotope composition of the water does not change appreciably downhole. This uniformity can be understood in terms of high water-rock ratios and suggests that the chemical changes observed are due either to alteration processes involving bentonites and basaltic material from the walls of the hole or to exchange with formation fluids from the surrounding basement, which may have altered in composition at relatively high water-rock ratios.

Sample descriptions, d-spacing and geochemistry at DSDP Holes 68-501 and 69-504B

Mössbauer analyses were conducted on a sample of saponite selected from DSDP Leg 69 basalt core. The sample was initially placed within a nitrogen-purged container on-board Glomar Challenger approximately three hours after recovery, where it remained until analysis. The Mössbauer data revealed an original, in situ Fe2O3/FeO ratio of 0.46, with both Fe**2+ and Fe**3+ in octahedral coordination. With controlled exposure to air under ambient laboratory storage conditions, the proportion of Fe**3+ increased from an original 30% to 51% over a period of about 11.5 months. The Fe**3+ thus produced remained in octahedral coordination, and no observable changes occurred in the physical appearance of the sample.