Calcium carbonate, isotopes and Rock-Eval pyrolysis at DSDP Holes 77-535 and 77-540

At DSDP Sites 534 (Central Atlantic) and 535 and 540 (Gulf of Mexico), and in the Vocontian Basin (France), Lower Cretaceous deposits show a very pronounced alternation of limestone and marl. This rhythm characterizes the pelagic background sedimentation and is independent of detritic intercalations related to contour and turbidity currents. Bed-scale cycles, estimated to be 6000-26,000 yr. long, comprise major and minor units. Their biological and mineralogic components, burrowing, heavy isotopes C and O, and some geochemical indicators, vary in close correlation with CaCO3 content. Vertical changes of frequency and asymmetry of the cycles are connected with fluctuations of the sedimentation rate. Plots of cycle thickness ("cyclograms") permit detailed correlations of the three areas and improve the stratigraphic subdivision of Neocomian deposits at the DSDP sites. Small-scale alternations, only observed in DSDP cores, comprise centimetric to millimetric banding and millimetric to micrometric lamination, here interpreted as varvelike alternations between laminae that are rich in calcareous plankton and others rich in clay. The laminations are estimated to correspond to cycles approximately 1,3, and 13 yr. in duration. The cyclic patterns appear to be governed by an interplay of continental and oceanic processes. Oceanic controls express themselves in variations of the biogenic carbonate flux, which depends on variations of such elements as temperature, oxygenation, salinity, and nutrient content. Continental controls modulate the influxes of terrigenous material, organic matter, and nutrients derived from cyclic erosion on land. Among the possible causes of cyclic sedimentation, episodic carbonate dissolution has been ruled out in favor of climatic fluctuations with a large range of periods. Such fluctuations are consistent with the great geographic extension shown by alternation controls and with the continuous spectrum of scales that characterizes limestone-marl cycles. The climatic variations induced by the Earth's orbital parameters (Milankovitch cycles) could be connected to bed-interbed alternations.

Isotopic ratios of methane, ethane, carbon dioxide, inorganic carbon and organic matter from ODP Leg 164 sites

The isotopic characteristics of CH4 (d13C values range from -101.3 per mil to -61.1 per mil PDB, and dD values range from -256 per mil to -136 per mil SMOW) collected during Ocean Drilling Program (ODP) Leg 164 indicate that the CH4 was produced by microbial CO2 reduction and that there is not a significant contribution of thermogenic CH4 to the sampled sediment gas from the Blake Ridge. The isotopic values of CO2 (d13C range -20.6 per mil to +1.24 per mil PDB) and dissolved inorganic carbon (DIC; d13C range -37.7 per mil to +10.8 per mil PDB) have parallel profiles with depth, but with an offset of 12.5 per mil. Distinct downhole variations in the carbon isotopic composition of CH4 and CO2 cannot be explained by closed-system fractionation where the CO2 is solely derived from the locally available sedimentary organic matter (d13C -2.0 per mil ± 1.4 per mil PDB) and the CH4 is derived from CO2 reduction. The observed isotopic profiles reflect the combined effects of upwards gas migration and decreased microbial activity with depth.

Geochemistry at DSDP Hole 57-439

Acidic to intermediate volcanic rocks were obtained as boulders, pebbles, and clasts with intercalated matrix sediments near the Japan Trench. A 47.5-meter conglomerate bed unconformably overlies acoustic basement consisting of Upper Cretaceous siltstone and is overlain in turn by massive coarse-sandstone and siltstone beds with many fossil mollusks. The volcanic cobbles and boulders in the conglomerate show pronounced porphyritic texture. Their phenocrysts are plagioclase, hornblende, and biotite; the groundmass consists of plagioclase, K-feldspar, quartz, iron oxide, and altered interstitial glass. The Plagioclase content of these volcanic rocks is very high, whereas iron oxide minerals are rare. The chemical composition of these volcanic rocks was analyzed to determine the rock series. Matrix sediments were also analyzed chemically, and their chemical composition was found to be similar to that of volcanic rocks, except for a lower CaO content. SiO2 content of the volcanic rocks ranges from 60.23 to 73.90, corresponding to that of andesite to rhyolite. All the samples show extremely high Al2O3 content, which reflects the high amounts of modal plagioclase. These volcanic rocks belong to both the calc-alkalic and tholeiitic rock series, and the differentiation trend is controlled by fractional crystallization, mainly of plagioclase, K-feldspar, and hornblende. The assemblage of calc-alkalic and tholeiitic rock series is frequently observed in island arcs and active continental margins. These volcanic rocks are derived from the Oyashio ancient landmass, which is a slightly matured island arc.

Isotopic composition and Strontium/Calcium ratios of foraminifera of ODP Holes 113-689B and 113-690C

Oxygen and carbon isotopic ratios were measured from Maestrichtian benthic and planktonic foraminifer species and bulk carbonate samples from ODP Sites 689 and 690, drilled on the Maud Rise during Leg 113. Careful scanning electron microscope observations reveal that test calcite in some intervals was diagenetically altered, although Sr/Ca and isotopic ratios of these tests do not appear to have been modified significantly. Foraminifer d18O values at both sites document a cooling trend during early Maestrichtian time, a rapid drop in water temperatures at the time of the first appearance of Abathomphalus mayaroensis in the high southern latitude regions (about 69.9 Ma), and lower water temperatures during late Maestrichtian time. d13C values record a depletion in 13C in the latest early Maestrichtian time beginning at about 72.2 Ma, just prior to the sharp late Maestrichtian increase in d18O values. These trends are similar to those previously reported for well-preserved benthic foraminifer species from Seymour Island, in the Antarctic Peninsula. Paleotemperature estimates are also comparable to those at Seymour Island and suggest temperate climatic conditions in Antarctica and that bottom waters in the southern South Atlantic region were of Antarctic origin. Benthic and planktonic foraminifer 613C values fluctuate sympathetically and are higher in upper Maestrichtian sediments than in the lower Maestrichtian sequence.

Mineralogy and diagenesis: Their effect on acoustic and electrical properties of pelagic clays at DSDP Leg 86 Holes

Analysis of pelagic clay samples from Sites 576, 578, and 581 shows that physical, acoustic, and electrical trends with increasing burial depth are related to mineralogical and diagenetic changes. The properties of interest are bulk density (roo), porosity (phi), compressional-wave velocity (Vp) and velocity anisotropy (Ap), and electrical resistivity (Ro) and resistivity anisotropy (Ar). In general, as demonstrated in particular for the brown pelagic clay, the increase in roo, Vp, Ro, and to a lesser extent Ap and Ar with increasing depth is primarily caused by decreasing phi (and water content) as a result of compaction. The mineralogy and chemistry of the pelagic clays vary as a function of burial depth at all three sites. These variations are interpreted to reflect changes in the relative importance of detrital and diagenetic components. Mineralogical and chemical variations, however, play minor roles in determining variations in acoustic and electrical properties of the clays with increasing burial depth.

High resolution paleoceanography of the Guaymas Basin, Gulf of California, during the past 15000 yeras

Deep Sea Drilling Project Site 480 (27°54.10'N, 111°39.34'W; 655 m water depth) contains a high resolution record of paleoceanographic change of the past 15000 years for the Guaymas Basin, a region of very high diatom productivity within the central Gulf of California. Analyses of diatoms and silicoflagellates were completed on samples spaced every 40-50 yr, whereas ICP-AES geochemical analyses were completed on alternate samples (sample spacing 80-100 yr). The Bolling-Allerod interval (14.6-12.9 ka) (note, ka refers to 1000 calendar years BP throughout this report) is characterized by an increase in biogenic silica and a decline in calcium carbonate relative to surrounding intervals, suggesting conditions somewhat similar to those of today. The Younger Dryas event (12.9-11.6 ka) is marked by a major drop in biogenic silica and an increase in calcium carbonate. Increasing relative percentage contributions of Azpeitia nodulifera and Dictyocha perlaevis (a tropical diatom and silicoflagellate, respectively) and reduced numbers of the silicoflagellate Octactis pulchra are supportive of reduced upwelling of nutrient-rich waters. Between 10.6 and 10.0 ka, calcium carbonate and A. nodulifera abruptly decline at DSDP 480, while Roperia tesselata, a diatom indicative of winter upwelling in the modern-day Gulf, increases sharply in numbers. A nearly coincident increase in the silicoflagellate Dictyocha stapedia suggests that waters above DSDP 480 were more similar to the cooler and slightly more saline waters of the northern Gulf during much of the early and middle parts of the Holocene (~10 to 3.2 ka). At about 6.2 ka a stepwise increase in biogenic silica and the reappearance of the tropical diatom A. nodulifera marks a major change in oceanographic conditions in the Gulf. A winter shift to more northwesterly winds may have occurred at this time along with the onset of periodic northward excursions (El Nino-driven?) of the North Equatorial Countercurrent during the summer. Beginning between 2.8 and 2.4 ka, the amplitude of biogenic silica and wt% Fe, Al, and Ti (proxies of terrigenous input) increase, possibly reflecting intensification of ENSO cycles and the establishment of modern oceanographic conditions in the Gulf. Increased numbers of O. pulchra after 2.8 ka suggest enhanced spring upwelling.

Chemical composition of phillipsites from Core AKO26-35

Behavior of rare earth elements (REE) was examined in oceanic phillipsites collected from four horizons of eupelagic clay in the Southwest Basin of the Pacific Ocean. REE concentrations were determined in >50 ?m size fraction phillipsite samples by the ICP-MS method. Composition of separate phillipsite aggregates was studied by electron microprobe and secondary ion mass-spectrometry. Rare earth elements in phillipsite samples are related to admixture of ferrocalcium hydroxophosphates. Analysis of separate phillipsite aggregates reveals low (<0.1-18.1 ppm) REE(III) concentrations. Ce concentration varies between 2.7 and 140 ppm. The correlation analysis shows that REE(III) present in admixture of iron oxyhydroxides in separate phillipsite aggregates. Based on the REE(III) concentration in iron oxyhydroxides we can identify two generations of phillipsite aggregates. Massive rounded aggregates (phillipsite I) are depleted in REE, while pseudorhombic (phillipsite II) aggregates are enriched in REE and marked by a positive Ce anomaly. Oceanic phillipsites do not accumulate REE or inherit the REE signature of volcaniclastic material and oceanic deep water. Hence, REE distribution in phillipsites does not depend on sedimentation rate and composition of host sediments.

Sedimentology and origin of lower Cretaceous pelagic carbonates and redeposited clastics at DSDP Hole 76-534A

Drilling at Site 534 in the Blake-Bahama Basin recovered 268 m of Lower Cretaceous, Berriasian to Hauterivian, pelagic carbonates, together with volumetrically minor intercalations of claystone, black shales, and terrigenous and calcareous elastics. Radiolarian nannofossil pelagic carbonates accumulated in water depths of about 3300 to 3650 m, below the ACD (aragonite compensation depth) but close to the CCD (calcite compensation depth). Radiolarian abundance points to a relatively fertile ocean. In the Hauterivian and Barremian, during times of warm, humid climate and rising sea level, turbiditic influxes of both terrigenous and calcareous sediments, and minor debris flows were derived from the adjacent Blake Plateau. The claystones and black shales accumulated on the continental rise, then were redeposited onto the abyssal plain by turbidity currents. Dark organic-rich and pale organic-poor couplets are attributed to climatic variations on land, which controlled the input of terrigenous organic matter. Highly persistent, fine, parallel lamination in the pelagic chalks is explained by repeated algal "blooms." During early diagenesis, organic-poor carbonates remained oxygenated and were cemented early, whereas organic-rich intervals, devoid of burrowing organisms, continued to compact later in diagenesis. Interstitial dissolved-oxygen levels fluctuated repeatedly, but bottom waters were never static nor anoxic. The central western Atlantic in the Lower Cretaceous was thus a relatively fertile and wellmixed ocean basin.

Geochemistry of volcanic glass from ODP holes 129-801C and 185-801C in the western Pacific

Major element chemistry of basalt from the southern East Pacific Rise (EPR) is different from that of the EPR at the time of the formation of the Pacific Plate at 170 Ma.Glass recovered from Jurassic age (170 Ma) Pacific ocean crust (Bartolini and Larson, 2001, doi:10.1130/0091-7613(2001)029<0735:PMATPS>2.0.CO;2) at Ocean Drilling Program Hole 801C records higher Fe8 (10.77 wt%) and marginally lower Na8 (2.21 wt%) compared to the modern EPR, suggesting deeper melting and a temperature of initial melting that was 60°C hotter than today.Trace element ratios such as La/Sm and Zr/Y, on the other hand, show remarkable similarities to the modern southern EPR, indicating that Site 801 was not generated on a hotspot-influenced ridge and that mantle composition has changed little in the Pacific over the past 170 Ma. Our results are consistent with the observation that mid-ocean ridge basalts (MORBs) older than 80 Ma were derived by higher temperature melting than are modern MORBs (Humler et al., 1999, doi:10.1016/S0012-821X(99)00218-6), which may have been a consequence of the Cretaceous superplume event in the Pacific.Site 801 predates the formation of Pacific oceanic plateaus and 801C basalt chemistry indicates that higher temperatures of mantle melting beneath Pacific ridges preceded the initiation of the superplume.

Platinum group elements and geochemistry of altered oceanic crust of ODP Hole 140-504B

Primary chemical heterogeneity in the sheeted dike complex in Deep Sea Drilling Project Hole 504B makes these rocks unsuitable for conventional mass balance calculations in determining element mobility associated with hydrothermal alteration. Due to the original heterogeneity and variable degrees of fractionation in the dikes, an appropriate reference sample on which calculations can be based is difficult to find. Therefore, the use of incompatible element ratios is developed to evaluate geochemical changes during alteration(s). For example, on a Zr/Yb-La/Yb plot, scatter along a straight line suggests tapping of a variably depleted mantle source and deviation from the line suggests element mobility (gain or loss). Using this method, our data indicates that the hydrothermal evolution of the sheeted dike complex was accompanied by significant loss of Cu, Zn, and Ti and some loss of La. The sheeted dike complex has low platinum group element (PGE) concentrations and steep PGE patterns, typical of mid-ocean ridge basalts (MORBs) on the global scale. We propose that the unusual PGE patterns of MORBs cannot be entirely generated by a partial melting and sulfide segregation model; instead, these patterns in part must have been inherited from their mantle source. The Au data show no evidence for mobilization during hydrothermal alteration of the dikes.

Chemical composition and elemental oxide ratios to AI2O3 of bulk solids at DSDP Leg 64 Holes

Analyses of sediments from Leg 64 sites reveal a diverse and in one case unique geochemistry. Sites are characterized by high heat flow along an active, divergent plate boundary, or rapid accumulation of diatom muds, or both. The geochemical trends of Sites 474-476 at the tip of Baja California reflect changes4n the percentages of sedimentary components - particularly biogenous matter and mineralogy - that support interpretations of sedimentary environments inferred to be present since the commencement of subsidence along this young, passive continental margin. The sediments below dolerite sills in Holes 477, 477A, 478, and 481 show major mineralogic and chemical deviations from "average" hemipelagic sediments. The sills appear to have two functions: (1) they allow hydrothermal circulation and metamorphism in a partially closed system by trapping heat and fluids emanating from below, and (2) they expel heated interstitial fluids at the moment of intrusion and mobilize elements, most likely leading to the formation of metalliferous deposits along the surface traces of normal faults in the basin. The hydrothermal system as a whole appears to be localized and ephemeral, as is indicated by the lack of similar geochemical trends and high heat flow at Sites 478 and 481. Site 479 illustrates sedimentation in an oxygen-minimum zone with anoxic sediments and concomitant geochemical trends, especially for MnO. With few exceptions, geochemical trends are remarkably constant with depth, suggesting that Site 479 can serve as an "internal" standard or average sediment against which the magnitude of hydrothermal alteration at the basinal Sites 477, 478, and 481 can be measured.

Composition of volcanic rocks from the Alchan Basin, Northwestern Primorie

Geological, petrochemical, and geochemical data are reported for volcanic rocks of a Cretaceous pull-apart basin in the Tan Lu strike-slip system, Asian continental margin. A comparison of these volcanic rocks with magmatic rocks from typical Cenozoic transform margins in the western North America and rift zones of Korea made it possible to distinguish some indicator features of transform-margin volcanic rocks. Magmatic rocks from strike-slip extension zones bear island-arc, intraplate, and occasionally depleted MORB geochemical signatures. In addition to calc-alkaline rocks there are bimodal volcanic series. The rocks are characterized by high K2O, MgO, and TiO2 contents. They show variable enrichment in LILE relative to HFSE, which is typical of island-arc magmas. At the same time they are rich in compatible transition elements, which is a characteristic of intraplate magmas. Trace element distribution patterns normalized to MORB or primitive mantle usually show a negative Ta-Nb anomaly typical of suprasubduction settings. Their Ta/Nb ratio is lower, whereas Ba/Nb, Ba/La, and La/Yb ratios are higher than those of some MORB and OIB. In terms of trace element systematics, for example, Ta-Th-Hf, Ba/La-(Ba/La)_n, (La/Sm)_n-La/Hf, and others, they fall within the area of mixing of magmas from several sources (island arc, intraplate, and depleted reservoirs). Magmatic rocks of transform settings show a sigmoidal chondrite-normalized REE distribution pattern with a negative slope of LREE, depletion in MREE, and an enriched or flat HREE pattern. Magmas with mixed geochemical characteristics presumably originated in a transform margin setting in local extension zones under influence of mantle diapirs, which caused metasomatism and melting of the lithosphere at different levels, and mixing of melts from different sources in variable proportions.

Chemical composition of phosphorites from the Peru and Chile shelves

Recent and Late Quaternary shelf phosphorites have low Fe, Ti and Al contents. These elements enter the phosphorites with terrigenous impurities and organic detritus. Ti, Al, and some Fe are removed when the phosphorites are lithified, whereas remaining iron settles in the phosphorites as sulfides. Ti/Fe, Al/Fe and Ti/Al ratios are used as examples of difference between behavior of Fe and that of Ti and Al.

Chemical and isotopic compositions of basalts and glasses from lavas of the Sierra Leone fault site

According to detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial rift zone of the MAR between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts, but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in basalts and their glasses. It was shown that some samples have significant nonsystematic differences in the 87Sr/86Sr ratio between basalts and their chilled glasses and less significant difference in e-Nd; higher Sr ratios can be observed both in glasses and basalts of the same lava fragments. No significant correlation is observed between isotope characteristics of samples and their geochemistry; it was also shown that seawater did not affect Sr and Nd isotope compositions of the chilled glasses from the studied pillow lavas. It is suggested that such differences in isotope ratios are related to small-scale heterogeneity of melts owing to incomplete homogenization during their rapid ascent to the surface. Heterogeneity of basaltic melts is explained by their partial contamination by older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the surface of the ocean floor. The wider scatter of the Sr isotopic ratios relative to Nd ones is related to presence of xenocrysts of calcic plagioclase; correspondingly, absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. Causes of this phenomenon remain unclear.

Petrology and chemistry of ODP Site 134-828 magmatic rocks

Petrography, major and trace elements, mineral chemistry, and Sr, Nd, and Pb isotopic ratios are reported for igneous rocks drilled on the northern flank of the North d'Entrecasteaux Ridge (NDR) during Ocean Drilling Program (ODP) Leg 134 Site 828. These rocks comprise a breccia unit beneath a middle Eocene foraminiferal ooze. Both geophysical characteristics and the variety of volcanic rocks found at the bottom of Holes 828A and 828B indicate that a very immature breccia or scree deposit was sampled. Basalts are moderately to highly altered, but primary textures are well preserved. Two groups with different magmatic affinities, unrelated to the stratigraphic height, have been distinguished. One group consists of aphyric to sparsely plagioclase + clinopyroxene-phyric basalts, characterized by high TiO2 (~2 wt%) and low Al2O3 (less than 15 wt%) contents, with flat MORB-normalized incompatible element patterns and LREE-depleted chondrite-normalized REE patterns. This group resembles N-MORB. The other group comprises moderately to highly olivine + plagioclase-phyric basalts with low TiO2 (<1 wt%) and high Al2O3 (usually >15 wt%) contents, and marked HFSE depletion and LFSE enrichment. Some lavas in this group are picritic, with relatively high modal olivine abundances, and MgO contents up to 15 wt%. Both the basalts and picritic basalts of this group reflect an influence by subduction-related processes, and have compositions transitional between MORB and IAT. Lavas with similar geochemical features have been reported from small back-arc basins such as the Mariana Trough, Lau Basin, Sulu Sea, and the North Fiji Basin and are referred to as back-arc basin basalts. However, regional tectonic considerations suggest that the spreading that produced these backarc basin basalts may have occurred in the forearc region of the southwest-facing island arc that existed in this region in the Eocene.