Macroturbidites of ODP Hole 103-641C

Barremian through uppermost Aptian strata from ODP Hole 641C, located upslope of a tilted fault block on the Galicia margin (northwest Spain), are syn-rift sediments deposited in the bathyal realm and are characterized by rapid sedimentation from turbidity currents and debris flows. Calcarenite and calcirudite turbidites contain shallow-water carbonate, terrigenous, and pelagic debris, in complete or partial Bouma sequences. These deposits contain abraded micritized bioclasts of reefal debris, including rudist fragments. The youngest turbidite containing shallow-water carbonate debris at Site 641 defines the boundary between syn-rift and post-rift sediments; this is also the boundary between Aptian and Albian sediments. Some Aptian turbidites are partially silicified, with pore-filling chalcedony and megaquartz. Adjacent layers of length-fast and -slow chalcedony are succeeded by megaquartz as the final pore-filling stage within carbonate reef debris. Temperatures of formation, calculated from the oxygen isotopic composition of the authigenic quartz, are relatively low for formation of quartz but are relatively warm for shallow burial depths. This quartz cement may be interpreted as a rift-associated precipitate from seawater-derived epithermal fluids that migrated along a fault associated with the tilted block and were injected into the porous turbidite beds. These warm fluids may have cooled rapidly and precipitated silica at the boundaries of the turbidite beds as a result of contact with cooler pore waters. The color pattern in the quartz cement, observed by cathodoluminescence and fluorescence techniques, and changes in the trace lement geochemistry mimic the textural change of the different quartz layers and indicates growth synchronism of the different quartz phases. Fluorescence petrography of neomorphosed low-Mg-calcite bioclasts in the silicified turbidites shows extensive zonation and details of replacive crystal growth in the bioclasts that are not observed by cathodoluminescence. Fluorescence microscopy also reveals a competitive growth history during neomorphism of the adjacent crystals in an altered carbonate bioclast. Barremian-Aptian background pelagic sediments from Hole 641C have characteristics similar to pelagic sediments from the Blake-Bahama Formation described by Jansa et al. (1979) from the western North Atlantic. Sediments at this site differ from the Blake-Bahama Formation type locality in that the Barremian-Aptian pelagic sediments have a higher percentage of dark calcareous claystone and some turbidites are silicified at Site 641. The stable isotopic composition of the pelagic marlstones from Site 641 is similar to those of other Berriasian-Aptian pelagic sediments from the Atlantic.

Pore-water chemistry of ODP Leg 115 samples

Analyses of the Sr2+ concentrations of interstitial fluids obtained from sediments squeezed during Leg 115 were used to estimate the rates and total amount of recrystallization of biogenic carbonates. The total amount of recrystallization calculated using this method varies from less than 1 % in sediments at Site 706 to more than 40% at Site 709 in sediments of 47 Ma. Five of the sites drilled during Leg 115 (Sites 707 through 711) were drilled in a depth transect within a restricted geographic area so that theoretically they received similar amounts of sediment input. Of these, the maximum rate of recrystallization occurred in the upper 50 m of Site 710 (3812 m). The amount of recrystallization decreased with increasing water depth at Sites 708 (4096 m) and 711 (4428 m), presumably as a result of the fact that most of the reactive calcium carbonate was dissolved before burial. We also observed significant alkalinity deficits at many of these sites, a condition which most likely resulted from the precipitation of calcium carbonate either in the sedimentary column, or during retrieval of the core. Precipitation of CaCO3 as a result of pressure changes during core retrieval was confirmed by the comparison of Ca2+ and alkalinity from water samples obtained using the in-situ sampler and squeezed from the sediments. At Sites 707 and 716, the shallowest sites, no calcium or alkalinity deficits were present. In spite of our estimations of as much as 45% recrystallization at Site 709, all the carbonate sites exhibited what would be previously considered conservative Ca2+/Mg2+ profiles, which varied from -1 to -0.5. By virtue of the position of these sites relative to known basaltic basement or through the actual penetration of basalt (i.e., Sites 706, 707 and 712), these sites are all known to be underlain by basalt. Our results suggest, therefore, that more positive Ca2 + /Mg2+ gradients cannot necessarily be used as indicators of the nature of basement material.

Foraminiferal abundance, carbonat concentration and dissolution index of sediment core W9903B-51GC

In order to investigate the paleoceanographic record of dissolution of calcium carbonate (CaCO3) in the central equatorial Pacific Ocean, we have studied the relationship between three indices of foraminiferal dissolution and the concentration and accumulation of CaCO3, opal, and Corg in Core WEC8803B-GC51 (1.3°N, 133.6°W; 4410 m). This core spans the past 413 kyr of deposition and moved in and out of the lysoclinal transition zone during glacial-interglacial cycles of CaCO3 production and dissolution. The record of dissolution intensity provided by foraminiferal fragmentation, the proportion of benthic foraminifera, and the foraminiferal dissolution index consistently indicates that the past corrosion of pelagic CaCO3 in the central equatorial Pacific does not vary with the observed sedimentary concentration of CaCO3. Although there is a weak low-frequency variation (~100 kyr) in dissolution intensity, it is unrelated to sedimentary CaCO3 concentration. There are many shorter-lived episodes where high CaCO3 concentration is coincident with poor foraminiferal preservation, and where, conversely, low CaCO3 concentration is coincident with superb foraminiferal preservation. Spectral analyses indicate that dissolution maxima consistently lagged glacial maxima (manifest by the SPECMAP delta18O stack) in the 100-kyr orbital band. Additionally, there is no relationship between dissolution and the accumulation of biogenic opal or Corg or between dissolution and the burial ratio of Corg/CINorg (calculated from Corg and CaCO3). Because previous studies of this core strongly suggest that surface water productivity varied closely with CaCO3 accumulation, both the mechanistic decoupling of carbonate dissolution from CaCO3 concentration (and from biogenic accumulation) and the substantial phase shift between dissolution and global glacial periodicity effectively obscure any simple link between export production, CaCO3 concentration, and dissolution of sedimentary CaCO3.

Eocene to Quaternary planktonic foraminifers from the Northwest Pacific

Eocene through Quaternary planktonic foraminifers were identified in cores recovered during Leg 126. Turbidites and volcanic ash beds are intercalated with hemipelagic sediments. Preservation of foraminifers is variable, ranging from excellent to poor and appears to have been affected by fluctuations in the carbonate compensation depth (CCD), depth of burial, changes in bottom water temperature, current velocity, sediment accumulation rates and seafloor topography. Preservation of foraminifers in Quaternary sediments is generally good, however, species abundance varies by a factor of I05-106 and reflects dilution by volcanogenic as well as terrigenous constituents and cannot be used for paleoceanographic reconstructions. In pre-Quaternary deposits planktonic foraminiferal tests frequently exhibit dissolution effects; biostratigraphic zonation and placement of zonal boundaries is difficult owing to hiatuses, dissolution facies, extraneously deposited sediments, and discontinuous coring. The Eocene foraminiferal faunas include specimens of the Globorotalia cerroazulensis plexus, markers of Zone P16 as well as Globigerina senni and Globigerinatheka spp., which became extinct before the end of the Eocene. Six hiatuses and/or dissolution periods, probably reflecting global cooling events and/or changes in oceanic circulation patterns were recorded at Site 792. Recrystallized, poorly preserved, possibly reworked Eocene species (Globigerina senni and Globigerapsis sp.) were recorded in sediments at Site 793.

Miocene strontium isotope record of DSDP Hole 30-289

A composite strontium isotopic seawater curve was constructed for the Miocene between 24 and 6 Ma by combining 87Sr/86Sr measurements of planktonic foraminifera from Deep Sea Drilling Project sites 289 and 588. Site 289, with its virtually continuous sedimentary record and high sedimentation rates (26 m/m.y.), was used for constructing the Oligocene to mid-Miocene part of the record, which included the calibration of 63 biostratigraphic datums to the Sr seawater curve using the timescale of Cande and Kent (1992 doi:10.1029/92JB01202). Across the Oligocene/Miocene boundary, a brief plateau occurred in the Sr seawater curve (87Sr/86Sr values averaged 0.70824) which is coincident with a carbon isotopic maximum (CM-O/M) from 24.3 to 22.6 Ma. During the early Miocene, the strontium isotopic curve was marked by a steep rise in 87Sr/86Sr that included a break in slope near 19 Ma. The rate of growth was about 60 ppm/m.y. between 22.5 and 19.0 Ma and increased to over 80 ppm/m.y. between 19.0 and 16 Ma. Beginning at ~16 Ma (between carbon isotopic maxima CM3 and CM4 of Woodruff and Savin (1991 doi:10.1029/91PA02561)), the rate of 87Sr/86Sr growth slowed and 87Sr/86Sr values were near constant from 15 to 13 Ma. After 13 Ma, growth in 87Sr/86Sr resumed and continued until ~9 Ma, when the rate of 87Sr/86Sr growth decreased to zero once again. The entire Miocene seawater curve can be described by a high-order function, and the first derivative (d87Sr/86Sr/dt) of this function reveals two periods of increased slope. The greatest rate of 87Sr/86Sr change occurred during the early Miocene between ~20 and 16 Ma, and a smaller, but distinct, period of increased slope also occurred during the late Miocene between ~12 and 9 Ma. These periods of steepened slope coincide with major phases of uplift and denudation of the Himalayan-Tibetan Plateau region, supporting previous interpretations that the primary control on seawater 87Sr/86Sr during the Miocene was related to the collision of India and Asia. The rapid increase in 87Sr/86Sr values during the early Miocene from 20 to 16 Ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans. In the absence of another source of CO2, these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric CO2 and climatic cooling through a reversed greenhouse effect. The paleoclimatic record, however, indicates a warming trend during the early Miocene, culminating in a climatic optimum between 17 and 14.5 Ma. We suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic CO2 from the eruption of the Columbia River Flood Basalts (CRFB) between 17 and 15 Ma. The decrease in the rate of CRFB eruptions at 15 Ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in Monterey deposits eventually led to cooling and increased glaciation between ~14.5 and 13 Ma. The CRFB hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the Monterey at 17.5 Ma (as marked by increased delta13C values) and the climatic cooling and glaciation during the middle Miocene (as marked by the increase in delta18O values), which did not begin until ~14.5 Ma.

Isotope oxygen record for ODP sediments from the Exmouth Plateau

A new composite d18O record, generated from calcareous fine-fraction and bulk sediments from the Exmouth Plateau, details long-term Cretaceous climatic change at mid-latitudes in the Southern Hemisphere. Assessment of new and previously published d18O data indicates that a mid-Cretaceous global climatic optimum was achieved sometime between the time of the Cenomanian-Turonian boundary and the middle Turonian, when surface-ocean paleotemperatures were the highest of the past 115 m.y. Periods of cooling and warming that reversed the general patterns were superimposed on long-term Aptian-Turonian warming and Turonian-Maastrichtian cooling trends, respectively. Extrapolation of Southern Hemisphere paleotemperature trends to Maastrichtian paleotemperature data from a low-latitude Pacific guyot implies that maximum mid-Cretaceous low-latitude paleotemperatures could have been in excess of 33°C.

(Table 1) Compressional-wave velocities and anisotropies and wet-bulk densities of calcareous rocks and some cherts at DSDP Holes 62-463 and 62-465A

Anisotropy in compressional-wave velocities in sedimentary rocks recovered by DSDP has been recognized by several investigators (Boyce, 1976; Tucholke et al., 1976; Carlson and Christensen, 1977). The anisotropy is also observed at elevated pressures in laboratory experiments, and thus probably persists at depth in some calcareous rocks (Schreiber et al., 1972; Christensen et al., 1973; Carlson and Christensen, 1979). Carlson and Christensen (1979) suggested that the observed velocity anisotropy was produced not by the alignment of cracks but by the alignment of c axes of calcite perpendicular to bedding during compaction, diagenesis, and recrystallization. On DSDP Leg 62, calcareous rocks were recovered from the western Mid-Pacific Mountains (sub-bottom depths of 452-823 m, Site 463) and southern Hess Rise (276-412 m, Site 465). Most of the calcareous rocks are horizontally laminated and color-banded, and ages are early Cenomanian to late Barremian (Site 463 and 465 reports, this volume). The purpose of this study is to confirm the velocity anisotropy in the calcareous rocks and to identify any relationship of anistropy to bulk density, mean velocity, and burial depth.

Occurrence and type of dolomite in ODP Site 124-768 samples (Table 1)

Occurrence of deep-sea dolomites has been reported from numerous settings (for discussion see Lumsden, 1988). Different authors agree that dolomite formation in the pelagic realm is a relatively early diagenetic process (e.g., Jorgensen, 1983; Shimmield and Price, 1984; Kablanow et al., 1984; Kulm et al., 1984). Baker and Burns (1985) suggest that most of the pelagic dolomites formed within a few tens of meters below the seafloor within the zone of microbial sulfate reduction. According to Fuechtbauer and Richter (1988), dolomite can form in the deep-sea at a minimum temperature of 10°C. Other deep-sea dolomites are products of fluids derived from underlying evaporites or submarine weathering of basalts (Garrison, 1981). In some cases (Mullins et al., 1985; Dix and Mullins, 1988; Mullins et al., 1988), the existence of dolomite is linked to disconformities and its formation may have resulted from circulation of seawater through the sediment during prolonged exposure (Dix and Mullins, 1988, p. 287). At Site 768 (Fig. 1), lithified carbonate layers, some containing variable amounts of dolomite, occur below 201 mbsf (Miocene). These beds alternate with unconsolidated or semi-lithified marl layers interbedded in clays and siliciclastic turbidites. The irregular depth distribution of the limestone beds and the variation in preservation and recrystallization of the calcareous microfaunas suggest that lithification of carbonates at Site 768 not only reflects burial diagenesis as described by Garrison (1981) and others, but in part may be a selective, early diagenetic process. The different types and distribution of the dolomite additionally seem to support this assumption. The purpose of this report is to document the occurrence and textural nature of the dolomite at Site 768. Methods used were analyses of stained thin sections (Alizarin S and Ferrocyanide) and studies with the scanning electron microscope. No geochemical analyses (e.g., stable isotopes) were carried out; they will be the subject of further investigations.

Carbon contents of lower Cretaceous sediments from ODP Hole 198-1213B (Table 1)

ODP Leg 198 drilling on Shatsky Rise recovered a lower Aptian porcellanite (~120.5 Ma) deposited during oceanic anoxic event (OAE) 1a that contains C36-C39 alkadienones: C37:2 and C39:2 alkadien-2-ones and C36:2 and C38:2 alkadien-3-ones. This alkenone distribution differs from that typical of contemporary sediments and haptophyte algae, but resembles that of Cretaceous sediments from the Blake-Bahama basin. The discovery of alkenones in the early Aptian extends their sedimentary record by 15 M.y. to 120.5 M.y. and demonstrates the potential for long-term survival of these diagnostic functional lipids under favorable depositional conditions and subsequent shallow burial. It also contributes to the understanding and reconstruction of evolutionary developments in alkenone distributions and biosynthesis over geologic time.

(Appendix) Bulk inorganic chemistry at DSDP Leg 77 Holes

Clay mineralogic and inorganic geochemical investigations of Cretaceous and Cenozoic sediments of the western Gulf of Mexico lead to the following main conclusions. (1) Transition of lowermost Cretaceous continental to marine sedimentation is marked by a clay evaporitic stage, north of the Campeche Escarpment. (2) Existence of combined mineralogic and geochemical stratigraphy allows us to propose correlations between Sites 535 and 540, especially for the Albian. (3) Predominance of detrital clay assemblages is indicative of hot and variably humid continental climate until the early late Cenozoic. (4) Tectonic destabilization of the margins of Gulf of Mexico occurred at different periods, especially until the middle Cretaceous, with a mixed erosion of rocks and soils and temporary oxidized conditions of deposition. (5) Successive developments of confined perimarine basins occurred from the earliest Cretaceous until the Miocene, chiefly in the Florida area. The sources of inorganic materials were chiefly situated on the east of the studied area until the late Tertiary and after that in the Mississippi River basin. (6) Occasionally, volcanic activity influenced the clay mineralogy and mainly the geochemistry, and possibly contributed to the rather strong magnesian character of the deposition until the late Paleogene. (7) The argillaceous diagenesis is weak; variability of the carbonate diagenesis is marked by the relation Sr = f(CaO) and chiefly depends on the depth of burial, the clay content, the porosity, and the geologic age.

Composition of pore water and diagenetic carbonates of ODP Site 123-765

Degradation of organic matter in slightly organic-rich (1 wt% organic carbon) Neogene calcareous turbidites of the Argo Basin at Site 765 by sulfate reduction results in pore-water phosphate, ammonium, manganese, and carbonate alkalinity maxima. Pore-water calcium and magnesium decrease in the uppermost 100 meters below seafloor (mbsf) in response to the precipitation of calcian dolomite with an average composition of Ca1.15Mg0.83Fe0.02(CO3)2. Clear, euhedral dolomite rhombs range from <1 to 40 µm in diameter and occur in trace to minor amounts (<1-2 wt%) in Pleistocene to Pliocene sediment (62-210 mbsf) The abundance of dolomite increases markedly (2-10 wt%) in Miocene sediment (210-440 mbsf). The dolomite is associated with diagenetic sepiolite and palygorskite, as well as redeposited biogenic low-Mg calcite and aragonitic benthic foraminifers. Currently, dolomite is precipitating at depth within the pore spaces of the sediment, largely as a result of aragonite dissolution. The rate of aragonite dissolution, calculated from the pore-water strontium profile, is sufficient to explain the amount of dolomite observed at Site 765. A foraminiferal aragonite precursor is further supported by the carbon and oxygen isotopic compositions of the dolomite, which are fairly close to the range of isotopic compositions observed for Miocene benthic foraminifers. Dolomite precipitation is promoted by the degradation of organic matter by sulfate-reducing bacteria because the lower pore-water sulfate concentration reduces the effect of sulfate inhibition on the dolomite reaction and because the higher carbonate alkalinity increases the degree of saturation of the pore waters with dolomite. Organic matter degradation also results in the precipitation of pyrite and trace amounts of apatite (francolite), and the release of iron and manganese to the pore water by reduction of Fe and Mn oxides. Spherical, silt-sized aggregates of microcrystalline calcian rhodochrosite occur in trace to minor amounts in Lower Cretaceous sediment from 740 to 900 mbsf at Site 765. A negative carbon isotopic composition suggests that the rhodochrosite formed early in the sulfate reduction zone, but a depleted oxygen isotopic composition suggests that the rhodochrosite may have recrystallized at deeper burial depths.

(Table 1) Sound velocity, wet-bulk density, and acoustic impedance of some indurated sediment from DSDP Leg 64 Holes

The Pliocene-Quaternary sediments that we drilled at eight sites in the Gulf of California consist of silty clays to clayey silts, diatomaceous oozes, and mixtures of both types. In this chapter I have summarized various measurements of their physical properties, relating this information to burial depth and effective overburden pressure. Rapid deposition and frequent intercalations of mud turbidites may cause underconsolidation in some cases; overconsolidation probably can be excluded. General lithification begins at depths between 200 and 300 meters sub-bottom, at porosities between 55 and 60% (for silty clays) and as high as 70% (for diatomaceous ooze). Diatom-rich sediments have low strength and very high porosities (70-90%) and can maintain this state to a depth of nearly 400 meters (where the overburden pressure = 1.4 MPa). The field compressibility curves of all sites are compared to data published earlier. Where sediments are affected by basaltic sills, these curves clearly show the effects of additional loading and thermal stress (diagenesis near the contacts). Strength measurements on well-preserved hydraulic piston cores yielded results similar to those obtained on selected samples from standard drilling. Volumetric shrinkage dropped to low values at 100 to 400 meters burial depth (0.3 to 2.0 MPa overburden pressure). Porosity after shrinkage depends on the composition of sediments.

Sedimentary history and chemical characteristics of clay minerals from the Bengal Fan

The purpose of this study is to clarify the sedimentary history and chemical characteristics of clay minerals found in sediments deposited in the distal part of the Bengal Fan since the Himalayas were uplifted 17 m.y. ago. A total of seventy-eight samples were collected from three drilled cores which were to be used for the clay mineral analyses by means of XRD and ATEM. The results obtained from the analyses show that individual clay mineral species in the sediment samples at each site have similar features when the samples are of the same age, whereas these species have different features in samples of differing geological ages. Detrital clay minerals such as illite and chlorite were deposited in greater amounts than kaolinite and smectite during the Early to Middle Miocene. This means that the Himalayan uplift was vigorous at least until the Middle Miocene. In the Pliocene chemical weathering was more prevalent so that instead, in the distal part of the Bengal Fan, kaolinite shows the highest concentrations. This would accord with weaker uplift in the Himalayas. In the Pleistocene period, vigorous Himalayan uplift is characterized by illite-rich sediment in place of kaolinite. In the Holocene, smectite shows the highest concentration in place of the illite and kaolinite which were the predominant clay minerals of the earlier periods. Increasing smectite concentration suggests the Himalayan uplift to have been stable after the Pleistocene period. The smectite analyzed here is found to be dioctahedral Fe-beidellite, and it originated largely from the augite-basalt on the Indian Deccan Traps. The tri-octahedral chlorite is subdivided into three sub-species, an Fe-type, a Mg-type and an intermediate type. The mica clay mineral can be identified as di-octahedral illite which is rich in potassium. The chemical composition and morphology of each clay mineral appears to exhibit no change with burial depth in the sedimentary columns. This implies that there was no systematic transformation of clay minerals with time.