Clay minerals and their assemblages at DSDP Leg 63 Holes

This paper discusses the distribution of clay minerals and identification of their assemblages in relation to sedimentary facies encountered during DSDP Leg 63 drilling off southern California and Baja California. We also consider how these assemblages are determined by source areas and changes in general paleogeographic environments during different periods of sedimentation.

Geochemistry of minerals in medium-grained diabase from ODP Holes 137-504B and 140-504B

Samples recovered from Hole 504B during Leg 140 include a number of medium-grained, holocrystalline diabases that appear to represent the cores of thick dikes. The plagioclase and pyroxene in these samples occur in a variety of crystal morphologies. Plagioclase occurs as phenocrysts, microphenocrysts, elongate crystals, skeletal crystals, and branching radial clusters. Pyroxene occurs as phenocrysts, microphenocrysts, ophitic crystals, and poikilitic crystals. Plagioclase compositions became progressively poorer in anorthite and MgO and progressively richer in FeO as crystallization proceeded, while the average grain volume decreased and the aspect ratio of individual grains increased. Pyroxene compositions are largely independent of crystal morphology. The diabase dikes recovered from Hole 504B during Leg 140 appear to have crystallized in situ. Crystal compositions and morphologies are consistent with a rapid cooling rate and solidification times for individual dikes on the order of hours or days. The crystallization rate and nucleation rate of plagioclase lagged behind the cooling rate so that the degree of undercooling progressively increased as crystallization proceeded. Plagioclase crystal morphologies indicate much greater degrees of supersaturation than do pyroxene or olivine crystal morphologies. The 504B diabase magmas appear to have been emplaced with abundant preexisting pyroxene and olivine nuclei, but with few preexisting plagioclase nuclei. The suppression of plagioclase nucleation and crystallization relative to that of pyroxene and olivine could provide a mechanism by which the actual fractionation assemblage is more pyroxene-rich and plagioclase-poor than that predicted from thermodynamic models, or that observed in isothermal crystallization experiments.

40Ar-39Ar geochronological studies on rocks at DSDP Leg 58 Holes

The technique of 40Ar-39Ar step-heating dating was applied to three rock samples from core of DSDP Site 443, one sample from Site 445, and four samples at Site 446. All sites were drilled during DSDP Leg 58. At Site 443 (Shikoku Basin), about 116 meters of basalt basement was drilled. Three samples were chosen for dating from different levels in the basalt; two samples are aphyric basalt, and the other is subophitic dolerite. At Site 445 (Daito Ridge), no basement rock was drilled; however, conglomeratic sandstone was cored in the lower part of the hole. 40Ar-39Ar dating was applied to a basalt pebble in the conglomerate. At Site 446 (Daito Basin), the lower cored sequence is clay stone interlayered with 16 basalt sills. Four samples were chosen from sills at different levels.

Geochemistry of Neogene volcanic ashs in ODP Leg 124 holes

Mineralogical and geochemical analyses were performed on 40 ash layers of Pleistocene to late Miocene age, recovered during Leg 124 in the Celebes and Sulu Seas (Sites 767, 768, and 769). They provide information on alteration processes related to burial diagenesis. The zonal distribution of secondary volcanic products emphasizes a major diagenetic change, characterized by the complete replacement of volcanic glass by an authigenic smectite-phillipsite assemblage, in tephra layers dated at 3.5-4 Ma. This diagenetic "event" occurs simultaneously in the two basins, and, on the basis of isotopic data, under low-temperature conditions. It is independent of distinct sedimentation rates and related to a relative quiescence of on-land volcanic activity. This period suggests a more uniform paleooceanographic situation having tectonic significance, and probably reflects a kinetic and environmental control of diagenetic reactions.

Carbonate, organic carbon and nitrogen, bitumen and rock-eval pyrolysis at DSDP Leg 79 Holes

The lipids and kerogens of 15 sediment samples from Site 547 (ranging from Pleistocene to Early Jurassic/Triassic) and 4 from Site 545 (Cretaceous) have been analyzed. A strong terrestrial contribution of organic matter was found, and significant autochthonous inputs were also present, especially at Site 545. Both strongly reduced and highly oxidized sediments have been found in the Cenozoic and Jurassic samples of Site 547. On the contrary, all the Cretaceous sections of Sites 547 and 545 are anoxic. Sediments from anoxic paleoenvironments are immature and have a high content of sterenes, diasterenes, steradienes, hopenes, and ßß hopanes. Samples from oxic paleoenvironments are mainly mature and their content of hopenes and steriod structures is below the detection level. Nevertheless, their hopane distributions have the immature ßß homologs as the predominant molecular markers. For Site 545 the most abundant molecular markers are ring A monoaromatic steranes, and their presence is attributed to microbial and chemical transformations during early diagenesis.

Lower Jaramillo polarity transition records from ODP Holes 108-665A and 121-758B

Two records of the geomagnetic polarity transition at the beginning of the Jaramillo Subchron (0.97 Ma) have been obtained from sediments in the equatorial Atlantic (Leg 108, Site 665; 2.95°N, 340.33°E) and Indian (Leg 121, Site 758; 5.38°N, 90.35°E) oceans. Both cores yielded high-quality magnetostratigraphic results; however, the relatively low sedimentation rates, the weak magnetizations, and complex demagnetization behavior of some transitional samples suggest that the record of the transitional field behavior may be less reliable. In addition, variations in grain size preclude reliable paleointensity determinations although the remanence in both cores is apparently dominated by magnetite. Despite these possible complications, the two cores yield transitional paths that are neither far-sided nor near-sided. Together with published records that meet minimum reliability standards, the two equatorial records presented here suggest that the lower Jaramillo transitional field morphology was significantly nonaxisymmetric. The mean normal and reversed inclinations from both cores deviate from the inclination expected from a geocentric axial dipole, as noted in virtually all marine sediment cores. The observed inclinations provide further support for a polarity-dependent nondipole contribution to the time-averaged field.

Biogenic and terrigeneous components in ODP Leg 108 holes

Five Ocean Drilling Program sites (657-661), which form a north-south transect off the western periphery of the Sahara, were selected to measure the long-term history of Saharan/Sahelian dust flux and fluvial sediment discharge and the fluxes of marine CaCO3 and opal over the last 8 m.y. Sites 658 and 659 served for high-resolution studies, and Sites 657, 660, and 661 for insights into the spatial patterns of dust flux. The nearshore mean flux of opal off Cap Blanc (21 °N) showed an abrupt increase about 3 Ma that appears to reflect the main onset of coastal upwelling fertility and enhanced trade winds. At the same time, the input of river-borne clay strongly decreased, suggesting a dry up of the central Saharan rivers. Later, marked short-lived spikes of clay and opal may indicate ongoing ephemeral pulses of fluvial runoff linked to peak interglacial stages. Given the zonal dust discharge centered near 18 °N at Site 659, the aridification of the south Sahara and Sahel increased in several steps: at 4.6, 4.3, and especially at 4.0, 3.6, and 2.1 Ma, and again, at 0.8 Ma. The late Miocene and earliest Pliocene were humid. Although the central and north Saharan climate appears to be linked to the glaciation history of the Northern Hemisphere, the long-term aridification further south followed a different schedule. The spatial distribution of quartz accumulation suggests that the dust outbreaks linked to the Intertropical Convergence Zone during summer did not shift in latitude back to 4.0 Ma, at least. The short-term variations of dust output over the last 0.5 m.y. followed orbital scale pulses with a strong precessional signal, showing a link of Sahelian humidity changes to the variation of sea-surface temperature and evaporation in the tropical Atlantic.

Ostracoda communities from DSDP Holes 95-612 and 95-613

Ninety-three samples from DSDP Leg 95, Sites 612 and 613, were examined for ostracodes to aid in the study of paleoceanography and paleodepth. In total, more than 25 genera were recovered. The most abundant and diverse ostracode assemblages were from the middle Eocene at both sites; lower and upper Eocene and Pliocene-Pleistocene assemblages were less abundant and were dominated by only three or four species. The middle Eocene assemblages were the most diagnostic of paleoenvironment and suggest water depths of 1000 to 2000 m. These assemblages are similar to other middle Eocene assemblages known from the Caribbean and North Atlantic, and signify a relatively cosmopolitan fauna that inhabited moderately deep but relatively warm bottom waters.

Calcareous nannofossil range-distribution and zonation of sediments from three ODP Leg 205 holes

Ocean Drilling Program Leg 205 of the research vessel JOIDES Resolution was a return expedition to the Leg 170 sites located on the Costa Rica subduction zone. Here the entire sediment cover on the incoming Cocos plate, including significantly large sections of calcareous nannofossil ooze and chalk, is underthrust beneath the overriding Caribbean plate. The large amount of subducted carbonate produces characteristic styles of volcanic and seismic activity that differ from those found farther along strike in Nicaragua and elsewhere. An understanding of the fate of subducted carbonate sediment sections is an essential component to our understanding of the global biogeochemical cycling of carbon dioxide. Because Leg 205 drilling operations were performed within meters of the Leg 170 drill sites occupied during October-December 1996, minimal coring was done during Leg 205. Although the biostratigraphy of the Leg 170 sites has since been documented in detail, questions remained regarding the age and nature of a gabbro sill that was only partially penetrated by coring during Leg 170. Coring operations during Leg 205 fully penetrated the gabbro sill, followed by an additional 12 m of sediments below the sill, and then ~160 m of gabbro. Coring halted at 600 meters below seafloor (mbsf). Calcareous nannofossil age dating of the sediments immediately above the igneous sill, as well as the sediment between the sill and the lower igneous unit, indicates a minimum age of 15.6 Ma and a maximum age of 18.2 Ma for the sediments. This implies that the sill was emplaced more recently than 18.2 Ma. The calcareous nannofossil assemblage in baked sediments in contact with the top of the lower igneous unit also suggests that the maximum age for emplacement is 18.2 Ma. At Site 1254, coring was accomplished between 150 and 230 mbsf (prism section), and from 300 to 367.5 mbsf (prism and through the décollement into the underthrust section). In the interval from 150 to 322 mbsf, the biostratigraphic analysis of calcareous nannofossils suggests that the sediments are early Pleistocene age between 150 and 161 mbsf, late Pliocene age from 161 to 219 mbsf, and early Pliocene age from 219 to 222 mbsf (no younger than 3.75 Ma). The lack of marker fossils in the interval of sediments cored from 300 to 350.6 mbsf does not allow for any age determinations; however, sediments from 351.6 to 359.81 mbsf could be age dated and are also early Pliocene age, but no younger than 3.75 Ma.

Grain-size, carbona and calcium carbonate at DSDP Leg 58 Holes

Sand-silt-clay distribution was determined on 10-cm**3 sediment samples collected at the time the cores were split and described. The sediment classification used here is that of Shepard (1954), with the sand, silt, and clay boundaries based on the Wentworth (1922) scale. Thus the sand, silt, and clay fractions are composed of particles whose diameters are 2000 to 62.5 µm, 62.5 to 3.91 µm, and less than 3.91 µm, respectively. This classification is applied without regard to sediment type and origin; therefore, the sediment names used in this table may differ from those used elsewhere in this volume; e.g., a silt composed of nannofossils may be called a nannofossil ooze in a site chapter.

Bulk and clay mineralogy of ODP Leg 110 holes

The mineralogy of both bulk- and clay-sized fractions of samples from Sites 671, 672, and 674 of ODP Leg 110 was determined by X-ray diffraction. The major minerals include quartz, calcite, plagioclase feldspar, and the clay minerals smectite, illite, and kaolinite. The smectite is a dioctahedral montmorillonite and is derived primarily from degradation of volcanic ash. Percentage of smectite varies with sediment age; Miocene and Eocene sediments are the most smectite-rich. High smectite content tends to correlate with elevated porosity, presumably because of the ability of smectite clays to absorb significant amounts of interlayer water. Because of a change in physical properties, the decollement zone at Site 671 formed in sediments immediately subjacent to a section of smectite-rich, high-porosity, Miocene-age sediments. Sediments above the decollement at Site 671, as well as all sediments analyzed from Sites 672 and 674, contain nearly pure smectite characteristic of the alteration of volcanic ash. Within the decollement zone and underthrust sequence, however, the smectite contains up to 65% illite interlayers. Although the illite/smectite could be interpreted as detrital clay derived from South America, its absence in the sediments stratigraphically equivalent to the decollement and underthrust sequences at Sites 672 and 674 favors the interpretation that it originated by diagenetic alteration of pre-existing smectite similar to that in the overlying sediments. A significant percentage of the freshening of the pore waters observed in these zones could be due to the water released during smectite dehydration.

Alkali contents, Sr isotope and U-Th-Pb data for ocean crust vein minerals from DSDP Holes 27-261, 61-462A, and 72-516F

Cation exchange experiments (ammonium acetate and cation resin) on celadonite-smectite vein minerals from three DSDP holes demonstrate selective removal of common Sr relative to Rb and radiogenic Sr. This technique increases the Rb/Sr ratio by factors of 2.3 to 22 without significantly altering the age of the minerals, allowing easier and more precise dating of such vein minerals. The ages determined by this technique (Site 261 - 121.4+/-1.6 m.y.; Site 462A - 105.1+/-2.8 m.y.; Site 516F - 69.9+/-2.4 m.y.) are 34, 54 and 18 m.y. younger, respectively, than the age of crust formation at the site; in the case of site 462A, the young age is clearly related to off-ridge emplacement of a massive sill/flow complex. At the other sites, either the hydrothermal circulation systems persisted longer than for normal crust (10-15 m.y.), or were reactivated by off-ridge igneous activity. Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Th contents are significantly lower. Celadonites thus have unusually high alkali/U,Th ratios and low Th/U ratios. If this celadonite alteration signature is significantly imprinted on oceanic crust as a whole, it will lead to very distinctive Pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust. Initial Sr isotope ratios of ocean crust vein minerals (smectite, celadonite, zeolite, calcite) are intermediate between primary basalt values and contemporary sea water values and indicate formation under seawaterdominated systems with effective water/rock ratios of 20-200.