Rare earth element composition and mass accumulation rates at DSDP Hole 92-598

Site 598 sediments were analyzed to determine the factors controlling the rare earth element (REE) geochemistry of the hydrothermal component. Site 598 provides an ideal sample suite for this purpose. Samples are lithologically "simple," primarily consisting of a hydrothermal component and biogenous carbonates. Also, the composition of the hydrothermal component appears unchanged through time or space, and the site appears to have undergone minimal diagenetic alteration. The shale-normalized REE patterns are similar to the pattern of seawater, varying only in absolute REE content. The REE content increases with distance from the paleorise crest and exhibits a pronounced increase in sediments deposited below the paleolysocline. Results presented are consistent with the following model: the source mechanism for the REE content of hydrothermal sediments is scavenging by Fe oxyhydroxides from seawater. With prolonged exposure to seawater resulting from transport far from the injection point and/or long residence at the seawatersediment interface, the absolute REE content of hydrothermal sediments increases and becomes more like seawater.

Geochemistry at DSDP Holes 80-550 amd 80-548

At Sites 548 and 550 of DSDP Leg 80 several condensed sedimentary sections contain various types of polymetallic crusts. The relationships between mineralogic and geochemical data in the sections have been studied in the context of the biostratigraphic and sedimentologic results. The diagenetic evolution during periods of low accumulation rate varies according to depth and sedimentary environment. At Site 548 on the continental margin, the phosphatic and manganiferous crusts are similar to those related to upwelling influences before Late Cretaceous deposition. At Site 550 the upper Paleocene cherts, deposited directly on oceanic crust, are overlain by pelagic brown clays containing diagenetic manganiferous concretions characterized by very high Sr and Ba contents. The origin of these small nodules is probably related to the authigenesis of fecal pellets. The upper Eocene indurated section is made up of authigenic zeolites, clays, and Fe-Mn phases and is similar to the volcanic-sedimentary deposits described in deep basins and seamounts of the Pacific. These crusts and a polynucleated nodule within the overlying sediments have geochemical characteristics (high Ni, Co, and Cu contents) similar to those formed in the deep ocean under volcanic influences during periods of low sedimentation rates or sedimentary hiatuses. Volcaniclastic material is ubiquitous and peculiarly abundant in Eocene sections and can be related to the volcanic formation of Iceland in the North Atlantic.

Lead and strontium isotope composition, mass accumulation rates and rare earth elements of selected samples from DSDP Sites 92-597 to 92-601

Most of the Pb isotope data for the Leg 92 metalliferous sediments (carbonate-free fraction) form approximately linear arrays in the conventional isotopic plots, extending from the middle of the field for mid-ocean ridge basalts (MORB) toward the field for Mn nodules. These arrays are directed closely to the average values of Mn nodules, the composition of which reflects the Pb isotope composition of seawater (Reynolds and Dasch, 1971). Since the Leg 92 samples are almost devoid of continentally derived detritus, it can be inferred that the more radiogenic end-member is seawater. The less radiogenic end-member lies in the very middle of the MORB field, and hence can be considered to reflect the Pb isotope composition of typical ocean-ridge basalt. The array of data lying between these two end-members is most readily interpreted in terms of simple linear mixing of Pb from the two different end-member sources. According to this model, eight samples from Sites 599 to 601 contain 50 to 100% basaltic Pb. Five of these samples have compositions that are identical within the uncertainty of the analyses. We use the average of these five values to define our unradiogenic end-member in the linear mixing model. The ratios used for this average are 206Pb/204Pb = 18.425 ± 0.010; 207Pb/204Pb = 15.495 ± 0.018; 208Pb/204Pb = 37.879 ± 0.068. These values should approximate the average Pb isotope composition of discharging hydrothermal solutions, and therefore also that of the basaltic crust, over the period of time represented by these samples ( 4 m.y., from 4 to 8 Ma). Sr isotope ratios show a significant range of values, from 0.7082 to 0.7091. The lower ratios are well outside the value of 0.70910 ± 6 for modern-day seawater (Burke et al., 1982). However, most values correspond very closely to the curve of 87Sr/86Sr versus age for seawater, with older samples having progressively lower 87Sr/86Sr ratios. The simplest explanation for this progressive reduction is that recrystallization of the abundant biogenic carbonate in the sediments released older seawater Sr which was incorporated into ferromanganiferous phases during diagenesis. Leg 92 metalliferous sediments have total rare earth element (REE) contents that range on a carbonate-free basis from 131 to 301 ppm, with a clustering between 167 and 222 ppm. The patterns have strong negative Ce anomalies. Samples from Sites 599 to 601 display a slight but distinct enrichment in the heavy REE relative to the light REE, whereas those from Sites 597 to 598 show almost no heavy REE enrichment. The former patterns (those for Sites 599 to 601) are interpreted as indicating moderate diagenetic alteration of metalliferous sediments originating at the EPR axis; the latter reflect more complete diagenetic modification.

Distribution of late Quaternary calcareous nannofossils in DSDP Leg 96 sites

Sediments from holes drilled at 11 sites in the northern Gulf of Mexico during Deep Sea Drilling Project Leg 96 were analyzed for calcareous nannofossil content. All sediments recovered are Holocene and late Pleistocene in age and are within the Emiliania huxleyi Zone. The datum level represented by the lowest stratigraphic occurrence of dominant E. huxleyi occurs at two sites (Sites 615 and 619) and can be dated at approximately 84,000 yr. ago at Site 619. Reworked Cretaceous nannofossils are generally common or abundant and dominate the floral assemblages of the late Wisconsin glacial sediments. When present, indigenous late Quaternary species are rare or few in abundance. Slight increases in the contemporaneous Quaternary component of the floral assemblages can be documented by the use of a calculated in s/fu/reworked ratio. This ratio, based on the relative abundances of the indigenous Quaternary taxa and reworked taxa, shows potential both for local correlations between drill sites and for correlation with glacio-eustatic fluctuations during the late Pleistocene.

Isotopic analysis of core gases at DSDP Leg 84 Holes

Methane carbon-isotopic compositions (d13C values relative to the PDB standard) at Sites 565, 566, 567, and 569 were lighter (enriched in 12C) than -60 per mil, indicating a biogenic origin. In the deeper sections at Sites 568 and 570, d13C values were heavier, approaching -40 per mil, and therefore suggest a thermogenic source. A significant thermogenic source was discounted, however, because the carbon dioxide d13C values in these sections were also anomalously heavy, suggesting that the methane may have formed biogenically by reduction of the heavy carbon dioxide. d13C values of ethane and higher hydrocarbons were measured in several sections from Sites 566 and 570 that contained sufficient C2-C4 hydrocarbon concentrations. Ethane values in six sections (245-395 m sub-bottom) from Site 570 were fairly uniform, ranging from -24 to -26 per mil. These values are among the heaviest ethane values reported for natural gases. The isobutane/ n-butane and isopentane/n-pentane ratios of the core gases suggested that the C2-C5 hydrocarbons are thermally produced by low-temperature chemical diagenesis of indigenous organic matter. This process apparently generates isotopically heavy C2-C5 hydrocarbons. High gas concentrations in the serpentinite basement rocks at Sites 566 and 570 appear to have resulted from migrated biogenic methane gas containing small amounts of immature C2-C5 hydrocarbons.

Sediment description, CaCO3, density and porosity at DSDP Site 89-585

Siliceous sediments and sedimentary rocks occur as chert and silicified chalk, limestone, and claystone in Site 585 lower Miocene to Campanian sediments, with one older occurrence of chert near the Cenomanian/Turonian boundary. The recovered drill breccia in the Miocene to middle Eocene interval is dominated by bright red, orange, yellow, and brown chips and fragments of chert. In early Eocene and older sediments gray silicified limestone and yellowish brown chert fragments predominate. Recovery is poor in cores with chert because chert tends to fracture into smaller pieces that escape the drill and because the hard chert fragments grind away other sediments during rotary drilling. Thin-section and hand-sample studies show complex diagenetic histories of silicification (silica pore infill) and chertification (silica replacement of host rock). Multiple events of silicification can occur in the same rocks, producing chert from silicified limestone. Despite some prior silicification, silicified limestone is porous enough to provide conduits for dissolved silica-charged pore waters. Silicification and chert are more abundant in the coarser parts of the sedimentary section. These factors reflect the importance of porosity and permeability as well as chemical and lithologic controls in the process of silica diagenesis.

Facies and diagenesis of organic matter in sediments at DSDP Leg 87

A series of upper Pliocene to Pleistocene sediment samples from DSDP Sites 582 and 583 (Nankai Trough, active margin off Japan) were investigated by organic geochemical methods including organic carbon determination, Rock- Eval pyrolysis, gas chromatography of extractable hydrocarbons, and kerogen microscopy. The organic carbon content is fairly uniform and moderately low (0.35 to 0.77%) at both sites, although accompanied by high sedimentation rates. The low organic matter concentrations are the result of the combined effect of several factors: low bioproductivity, oxic depositional environment, and dilution with lithogenic material. Organic petrography revealed a mixture of three maceral types: (1) fresh, green fluorescent alginites of aquatic origin probably transported by turbidites from the shelf edge, (2) gelified huminites and paniculate liptinites derived from the erosion of unconsolidated peat, and (3) highly reflecting inertinites derived from continental erosion. By a combination of organic petrography and Rock-Eval pyrolysis results, the organic matter is characterized as mainly type III kerogen with a slight tendency to a mixed type II-III. During Rock-Eval pyrolysis, a mineral matrix effect on the generated hydrocarbons was observed. The organic matter in all sediments has a low level of maturity (below 0.45% Rm) and has not yet reached the onset of thermal hydrocarbon generation according to several geochemical maturation parameters. This low maturity is in contrast to anomalously high extract yields at both sites and large hydrocarbon proportions in the extracts at Site 583. This contrast may be due to early generation of polar compounds and perhaps redistribution of hydrocarbons caused by subduction tectonics. Carbon isotope data of the interstitial hydrocarbon gases indicate their origin from bacterial degradation of organic matter, although only very few bacterially degraded maceral components were detected.

Inclination of the two main components of magnetization and 87Sr/86Sr ratios for dolomites at DSDP Hole 77-536

Site 536 terminated in a shallow-water dolomite of unknown age. Paleomagnetic measurements combined with strontium isotope analyses suggest that the dolomite was deposited in the Middle Jurassic to Early Cretaceous time interval. However, the assumptions required to reach this determination make these results less than conclusive.

Carbonate fraction, magnesium and strontium concentration, oxygen and carbon isotopes at DSDP Hole 92-600C

Strontium, magnesium, oxygen, and carbon isotope profiles of the carbonate fraction of Hole 600C sediments support the lithologic and petrographic observations of extensive CaCO3 dissolution and recrystallization in the Pliocene basal section. Convective fluid flow through the sediments during the first 1 to 1.5 m.y. of the sedimentary history of these sediments may explain these observations.

87Sr/86Sr ratios of planktonic foraminifers and interstitial waters at DSDP Hole 90-593

Detailed profiles of the Sr isotopic compositions of fossil planktonic foraminifers and interstitial waters have been measured from DSDP Site 593 to determine the Sr isotopic composition of seawater during the last 40 m.y. Foraminiferal recrystallization was assessed through scanning electron microscopy (SEM) and Sr/Ca ratios. Foraminifers were shown to be well preserved. Results document that the seawater 87Sr/86Sr has increased continuously but not uniformly, since the latest Eocene.

Accumulation rates, grain-size fractions and calcium carbonate at DSDP Sites 89-586 and 90-591

Carbonate oozes recovered by hydraulic piston coring at DSDP Site 586 on Ontong-Java Plateau and Site 591 on Lord Howe Rise have carbonate contents that are consistently higher than 90% with only minor variations. Consequently, paleoceanographic signals were not recorded in detail in the carbonate contents. However, mass accumulation rates of carbonate increased in the late Miocene to mid-Pliocene, reflecting an increase in productivity, then abruptly decreased from mid-Pliocene to the present. Variations in relative abundances of coarse material (foraminifers) and fine material (mostly calcareous nannofossils) do reflect histories of current winnowing and biogenic productivity at the two sites. The late Miocene from 10.5 to 6.5 m.y. ago was a time of relatively constant, quiet, pelagic sedimentation with typical southwest Pacific sedimentation rates of 20-25 m/m.y. The average coarse-fraction abundances are always higher at Site 586 than at Site 591, which reflects winnowing at Site 586. These conditions were interrupted between 6.5 to 4.0 m.y. ago when increased upwelling at the Subtropical Divergence and the Equatorial Divergence produced greater productivity of calcareous planktonic organisms. The increased productivity is suggested by large increases in both fineand coarse-fraction material and constant ratios of foraminifers to nannofossils. The maximum of productivity was about 4.0 m.y. ago. This period of increased upwelling is coincident with the inferred development of the West Antarctic ice sheet. The high productivity was followed by an abrupt increase in winnowing about 2.5 m.y. ago at Site 591, but not until about 2.0 m.y. ago at Site 586. By 2.0 m.y. ago in the late Pliocene, quiet, pelagic sedimentation conditions prevailed, similar to those of the late Miocene. The last 0.7 m.y. has been a period of relatively intense winnowing on Lord Howe Rise but not on Ontong-Java Plateau. The coarse-fraction data have both long- and short-period fluctuations. Long-period fluctuations at Site 591 average about 850 *10**3 yr./cycle and those at Site 586 average 430*10**3 yr./cycle. The highest amplitudes are found in the Pliocene and Quaternary sections. The short-period fluctuations range from 100 to 48*10**3 yr./cycle at Site 586 and from 250 to 33 *10**3 yr./cycle at Site 591. The effects of local fluctuations of productivity and winnowing have modified the primary orbital forcing signals at these two sites to yield complex paleoceanographic records.

Age profiles and isotopes d18O and d13C in Globigerina angiporoides and Oridorsalis umbonatus at DSDP Site 90-593 and Holes 12-111, 29-277 and 90-592

The late Eocene through earliest Miocene stable-isotope composition of southwest Pacific microfossils has been examined in a traverse of high-quality sedimentary sequences ranging from subantarctic (DSDP Site 277) through temperate regions (DSDP Sites 592 and 593). Changes in oxygen-isotope values, measured in benthic and planktonic foraminifers, document the Oligocene development and strengthening of latitudinal thermal zonation from water masses with broad temperature gradients during the Eocene to the steeper gradients and more distinct latitudinally distributed surface water-mass belts of the Neogene. The oxygen-isotope records can be divided into three intervals: late Eocene, early Oligocene, and middle to late Oligocene. Each interval represents a successive stage in the evolution of latitudinal thermal gradients between subantarctic and temperate regions in the Southern Hemisphere. During the late Eocene, oxygen-isotope values at subantarctic Site 277 were similar to those at temperate Sites 592 and 593. The isotope values suggest that, although the inferred paleotemperatures at Site 277 are slightly cooler on average than those at the temperate sites, there is no evidence for a major thermal boundary between the regions at this time. All three sites record the well-known oxygen-isotope enrichment of about 1 per mil in both planktonic and benthic foraminifers in close association with the Eocene/Oligocene boundary. In contrast to the earliest Oligocene enrichments in the planktonic and benthic oxygen-isotope composition at Site 277, more northern Sites 592 and 593 exhibit a depletion through the early-middle Oligocene. This documents the beginning of thermal segregation as subantarctic waters cooled relative to those at temperate latitudes. During the Oligocene, this surface-water differentiation continued, as measured by planktonic d18O values. The oxygen-isotope records of the benthic foraminifers also began to diverge in the earliest Oligocene. The most enriched oxygen-isotope values in all records cluster in the middle Oligocene, marked by oscillating episodes of enrichments >0.5 per mil occurring most prominently in the subantarctic record of Site 277. These values can be interpreted as recording either the coldest oceanic temperatures of the Paleogene and/or accumulations of Antarctic ice. After this interval, latitudinal thermal differentiation developed rapidly during the middle Oligocene, especially in the surface waters which actually warmed in temperate areas. If the enriched Oligocene oxygen-isotope values indicate that ice had accumulated, this ice must have disappeared by the early Miocene, when depleted oxygen-isotope values suggest very warm conditions. The data presented in this chapter document the progressive increase of latitudinal temperature gradients from the late Eocene through the late Oligocene. This pattern of increasing isotopic offset between latitudinally distributed southwest Pacific sites is linked to the establishment and strengthening of the Circum-Antarctic Current, previously considered to have developed during the middle to late Oligocene. The intensification of this current system progressively decoupled the warm subtropical gyres from cool polar circulation, in turn leading to increased Antarctic glaciation.

Porphyrin geochemistry of DSDP Hole 71-511

Late Jurassic-early Cretaceous black shales and an overlying sequence of Albian-Campanian zeolitic claystones from the Falkland Plateau (DSDP/IPOD Leg 71, Site 511) were analyzed for tetrapyrrole pigment type and abundance. The "black shale" sequence was found to be rich in DPEP-series dominated free-base, nickel (Ni) and, to a lesser extent, vanadyl (V = 0) porphyrins. A low level of organic maturity (i.e. precatagenesis) is indicated for these strata as nickel chelation by free-base porphyrins is only 50-75% complete, proceeding down-hole to 627 meters sub-bottom. Electronic and mass spectral data reveal that the proposed benzo-DPEP (BD) and tetrahydrobenzo-DPEP (THBD) series are present in the free-base and Ni species, as well as the more usual occurrence in V = 0 porphyrin arrays. Highly reducing conditions are suggested by an abundance of the PAH perylene, substantial amounts of the THBD/BD series and a redox equilibrium between free-base DPEP and 7,8-dihydro-DPEP series, which exist in a 7:1 molar ratio. The Albian-Campanian claystone strata were found to be tetrapyrrolepoor, and those pigments present were typed as Cu/Ni highly dealkylated (C26 max.) etioporphyrins, thought to be derived via redeposition and oxidation of terrestrial organic matter (OM). Results from the present study are correlated to our past analyses of Jurassic-Cretaceous sediments from Atlantic margins in an effort to relate tetrapyrrole quality and quantity to basin evolution and OM sources in the proto-Atlantic.