Ketones in hydrothermal petroleums and sediments from Guaymas Basin

The polar compound (NSO) fractions of seabed petroleums and sediment extracts from the Guaymas Basin hydrothermal system have been analyzed by gas chromatography and gas chromatography-mass spectrometry. The oils were collected from the interiors and exteriors of high temperature hydrothermal vents and represent hydrothermal pyrolysates that have migrated to the seafloor by hydrothermal fluid circulation. The downcore samples are representative of both thermally unaltered and thermally altered sediments. The survey has revealed the presence of oxygenated compounds correlated with samples exhibiting a high degree of thermal maturity. Several homologous series of related ketone isomers are enriched in the interiors of the hydrothermal vent samples or in hydrothermally-altered sequences of the downcore sediments (DSDP Holes 477 and 481A). The n-alkanones range in carbon number from C11 to C33 with a Cmax from 14 to 23, distributions that are similar to those of the n-alkanes. The alkan-2-ones are usually in highest concentrations, with lower amounts of 3-, 4-, 5-, 6-, 7- (and higher) alkanones, and they exhibit no carbon number preference (there is an odd carbon number preference of alkanones observed for downcore samples). The alkanones are enriched in the interiors of the hydrothermal vent spires or in downcore hydrothermally-altered sediments, indicating an origin at depth or in the hydrothermal fluids and not from an external biogenic deposition. Minor amounts of C13 and C18 isoprenoid ketones are also present. Simulation of the natural hydrothermal alternation process by laboratory hydrous pyrolysis techniques provided information regarding the mode of alkanone formation. Hydrous pyrolysis of n-C32H66 at 350°C for 72 h with water only or water with inorganic additives has been studied using a stainless steel reaction vessel. In each experiment oxygenated hydrocarbons, including alkanones, were formed from the n-alkane. The product distributions indicate a reaction pathway consisting of n-alkanes and a-olefins as primary cracking products with internal olefins and alkanones as secondary reaction products. Hydrous pyrolyses of Messel shale spiked with molecular probes have been performed under similar time and temperature constraints to produce alkanone distributions like those found in the hydrothermal vent petroleums.

Pleistocene and late Miocene Calcareous nannofossil biostratigraphy of ODP Hole 150-905A

Continental rise Site 905 yielded upper Miocene and Pliocene uniform hemipelagic mud (a contourite) from approximately 215 to 540 meters below seafloor. The nannofossil biostratigraphy of this interval was reexamined using closely spaced samples from core interiors. Additionally, total nannofossil abundances and dominant species and species group abundances were determined to evaluate the potential of this section for extracting sequence stratigraphic information. The data indicate that the putative hiatuses at the end of the late Pliocene (Zones NN17 and NN18) and in the early Pliocene (Zones NN13 and NN14) probably are condensed intervals, but the base of the late Miocene is almost certainly marked by an unconformity. Judging from carbonate content and sedimentation rate both, nannofossil abundance may be governed by carbonate dissolution or by siliciclastic dilution. Consequently, condensed sections cannot be identified by the abundance of pelagic component in the sediment alone, as is possible in equivalent age Gulf of Mexico sediments. Where nannofossil preservation is adequate in consecutive samples, as in the early Pliocene and latest late Miocene, total nannofossil abundance fluctuates regularly and with a periodicity of less than 105 yr, which suggests that dilution of the pelagic component occurred with a frequency probably related to astronomical forcing.

Geochemistry of minerals at DSDP Holes 68-501 and 69-504B

The interaction of seawater with basalts in DSDP Hole 501 and the upper part of Hole 504B (Costa Rica Rift) produced oxidative alteration and a zonation of clay minerals along cracks. From rock edges to interiors in many cracks the following succession occurs, based on microscopic observations and microprobe analysis: iron hydroxides (red), "protoceladonite" (green), iddingsite (orange), and saponite (yellow). Clay minerals replace olivines and fill vesicles and cracks. Other secondary minerals are phillipsite, aragonite, and unidentified carbonates. Some glass is transformed to Mg-rich palagonite. Bulk rock chemistry is related to the composition of the secondary minerals. The zonation can be interpreted as a succession of postburial nonoxidative and oxidative diagenesis similar to that described in the Leg 34 basalts.

Crystal morphologies and pyroxene compositions in boninites and tholeiitic basalts at DSDP Holes 60-458 and 60-459B

Not all boninites are glassy lavas. Those of Hole 458 in the Mariana fore-arc region are submarine pillow lavas and more massive flows in which glass occurs only in quenched margins. Pillow and flow interiors have abundant Plagioclase spherulites, microlites, or even larger crystals but can be recognized as boninites by (1) occurrence of bronzite, (2) presence of augite-bronzite microphenocryst intergrowths, and (3) reversal of the usual basaltic groundmass crystallization sequence of plagioclase-augite to augite-plagioclase. The latter is accentuated by sharply contrasting augite and Plagioclase crystal morphologies near pillow margins, a consequence of rapid cooling rates. This crystallization sequence appears to be a consequence of boninites having higher SiO2 and Mg/Mg + Fe than basalts but lower CaO/Al2O3. Microprobe data are used to illustrate the effects of rapid cooling on the compositions of pyroxene and microphenocrysts in a glassy boninite sample and to estimate temperatures of crystallization of coexisting bronzite and augite. A range from 1320°C to 1200°C is calculated with an average of 1250°C. This is higher by 120°-230° than the known range for western Pacific arc tholeiites and by over 300° than for calc-alkalic andesites. Boninites of Hole 458 lack olivine and clinoenstatite but are otherwise chemically and petrographically similar to boninites that have these minerals. In order to distinguish the two types, the Hole 458 lavas are here termed boninites and the others are termed olivine boninites. Arc tholeiite pillow lavas from Holes 458 and 459B are briefly described and their textures compared to fractionated, moderately iron-enriched, abyssal tholeiites. Massive tholeiite flows contain striking quartz-alkali feldspar micrographic intergrowths with coarsely spherulitic textures resulting from in situ magmatic differentiation. Such intergrowths are rare in massive abyssal tholeiites cored by DSDP and probably occur here because arc tholeiites have higher normative quartz at comparable degrees of iron enrichment - a result of higher oxygen fugacities and earlier separation of titanomagnetite - than abyssal tholeiites.

Low-temperature magnetic remanence and hysteresis measurements on sediment core GeoB4901-8 from the Niger deep-sea fan

Low-temperature (LT) magnetic remanence and hysteresis measurements, in the range 300-5 K, were combined with energy dispersive spectroscopy (EDS) in order to characterize the magnetic inventory of strongly diagenetically altered sediments originating from the Niger deep-sea fan. We demonstrate the possibility of distinguishing between different compositions of members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series on a set of five representative samples, two from the upper suboxic and three from the lower sulfidic anoxic zone of gravity core GeoB 4901. Highly sensitive LT magnetic measurements were performed on magnetic extracts resulting in large differences in the magnetic behavior between samples from the different layers. This emphasizes that both Fe-Ti oxide phases occur in different proportions in the two geochemical environments. Most prominent are variations in the coercivity sensitive parameter coercive field (BC). At room-temperature (RT) hysteresis loops for all extracts are narrow and yield low coercivities (6-13 mT). With decreasing temperature the loops become more pronounced and wider. At 5 K an approximately 5-fold increase in BC for the suboxic samples contrasts a 20-25-fold increase for the samples from the anoxic zone. We demonstrate that this distinct increase in BC at LT corresponds to the increasing proportion of the Ti-rich hemoilmenite phase, while Fe-rich (titano-)magnetite dominates the magnetic signal at RT. This trend is also seen in the room-temperature saturation isothermal remanent magnetization (RT-SIRM) cycles: suboxic samples show remanence curves dominated by Fe-rich mineral phases while anoxic samples display curves clearly dominated by Ti-rich particles. We show that the EDS intensity ratios of the characteristic Fe Kalpha and Ti Kalpha lines of the Fe-Ti oxides may be used to differentiate between members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series. Furthermore it is possible to calculate an approximate composition for each grain if the intensity ratios of natural particles are linked to well-known standards. Thus, element spectra with high Fe/Ti intensity ratios were found to be rather typical of titanomagnetite while low Fe/Ti ratios are indicative of hemoilmenite. The EDS analyses confirm the LT magnetic results, Fe-rich magnetic phases dominate in the upper suboxic environment whereas Ti-rich magnetic phases comprise the majority of particles in the lower anoxic domain: The mineral assemblage of the upper suboxic environments is composed of magnetite (~19%), titanomagnetite (~62%), hemoilmenite (~17%) and ~2% other particles. In the lower anoxic sediments, reductive diagenetic alteration has resulted in more extensive depletion of the (titano-)magnetite phase, resulting in a relative enrichment of the hemoilmenite phase (~66%). In these strongly anoxic sediments stoichiometric magnetite is barely preserved and only ~5% titanomagnetite was detected. The remaining ~28% comprises Ti-rich particles such as pseudobrookite or rutile.

Geochemical composition and atomic formulas of saponites, celadonites and clays from ODP Leg 168 sites

A drilling transect across the sedimented eastern flank of the Juan de Fuca Ridge, conducted during Leg 168 of the Ocean Drilling Program, resulted in the recovery of samples of volcanic basement rocks (pillow basalts, massive basalts, and volcanic glass breccias) that exhibit the effects of low-temperature hydrothermal alteration. Secondary clays are ubiquitous, with Mg-rich and Fe-rich saponite and celadonitic clays commonly accounting for several percent, and up to 10%-20% by volume. Present-day temperatures of the basement sites vary from 15° to 64°C, with the coolest site being about 0.8 Ma, and the warmest site being about 3.5 Ma. Whereas clays are abundant at sites that have been heated to present temperatures of 23°C and higher, the youngest site at 15°C has only a small trace of secondary clay alteration. Alteration increases as temperatures increase and as the volcanic basement ages. The chemical compositions of secondary clays were determined by electron microprobe, and additional trace element data were determined by both conventional nebulization inductively coupled plasma-mass spectroscopy (ICP-MS) and laser-ablation ICP-MS. Trioctahedral saponite and pyrite are characteristic of the interior of altered rock pieces, forming under conditions of low-oxygen fugacity. Dioctahedral celadonite-like clays along with iron oxyhydroxide and Mg-saponite are characteristic of oxidized haloes surrounding the nonoxidized rock interiors. Chemical compositions of the clays are very similar to those determined from other deep-sea basalts altered at low temperature. The variable Mg:Fe of saponite appears to be a systematic function both of the Mg:Fe of the host rock and the oxidation state during water-rock interaction.

Mineral and chemical composition of hydrothermally altered rocks and sediments from the Middle Valley, ODP Leg 139 data

The Middle Valley segment at the northern end of the Juan de Fuca Ridge is a deep extensional rift blanketed with 200-500 m of Pleistocene turbiditic sediment. Sites 857 and 858 were drilled during Ocean Drilling Program Leg 139 to determine whether these two sites were hydrologically linked end members of an active hydrothermal circulation system. Site 858 was placed in an area of active hydrothermal discharge with fluids up to 270°C venting through anhydrite-bearing mounds on top of altered sediment. The shallow basement of fine-grained basalt that underlies the vents at Site 858 is interpreted as a seamount that was subsequently buried by turbidites. Site 857 was placed 1.6 km south of the Site 858 vents in a zone of high heat flow and numerous seismically imaged ridge-parallel faults. Drilling at Site 857 encountered sediments that are increasingly altered with depth and that overlie a series of mafic sills at depths of 460-940 m below sea floor. Sill margins and adjacent baked sediment are highly altered to magnesian chlorite and crosscut with veins filled with quartz, chlorite, sulfides, epidote, and wairakite. The sill interiors vary from slightly altered, with unaltered plagioclase and clinopyroxene in a mesostasis replaced by chlorite, to local zones of intense alteration and brecciation. In these latter zones, the sill interiors are pervasively replaced by chlorite, epidote, quartz, pyrite, titanite, and rare actinolite. The most complete replacement is associated with brecciated horizons with low recovery and slickensides on fracture surfaces, which we interpret as intersections between faults and the sills. Geochemically, the alteration of the sill complex is reflected in significant whole-rock depletions in Ca, Sr, and Na with corresponding enrichments in Mg, Al, and most metals. The latter results from the formation of conspicuous sulfide poikiloblasts. In contrast, metamorphism of the Site 858 seamount includes incomplete albitization of plagioclase phenocrysts and replacement of sparse mafic phenocrysts. Much of the basement alteration at Site 858 is confined to crosscutting veins except for a highly altered and veined horizon at the contact between basaltic basement and the overlying sediment. The sill complex at Site 857 is more highly depleted in 18O (d18O = 2.4 per mil - 4.7 per mil) and more pervasively replaced by secondary minerals relative to the extrusives at Site 858 (d18O = 4.5 per mil - 5.5 per mil). There is no evidence of significant albitization of the plagioclase at Site 857, suggesting high Ca/Na in the pore fluids. Fluid-inclusion data from hydrothermal minerals in altered mafic rocks and veins at Sites 857 and 858 show a consistency of homogenization temperatures, varying from 245 to 270°C, which is within the range of temperatures observed for the fluids venting at Site 858. The consistency of the fluid inclusion temperatures, the lack of albitization within the Site 857 sills, and the apparently low water/rock ratio collectively suggest that the sill complex at Site 857 is in thermal equilibrium and being altered by a highly evolved Ca-rich fluid similar to the fluids now venting at Site 858. The alteration evident in these two deep crustal drillsites is a result of the ongoing hydrothermal circulation and is consistent with downhole logging results, instrumented borehole results, and hydrothermal fluid chemistry. The pervasive alteration of the laterally extensive sill-sediment complex at Site 857 determines the chemistry of the fluids that are venting at Site 858. The limited alteration of the Site 858 lavas suggests that this basement edifice acts as a penetrator or ventilator for the regional hydrothermal reservoir with much of the flow focussed at the highly altered and veined sediment-basalt contact.

Atlantic 231Pa/230Th and biogenic opal compilation of the Holocene and the LGM

The strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial-interglacial and millennial timescales, but has proved difficult to reconstruct, particularly for the Last Glacial Maximum. Today, the return flow from the northern North Atlantic to lower latitudes associated with the Atlantic meridional overturning circulation reaches down to approximately 4,000 m. In contrast, during the Last Glacial Maximum this return flow is thought to have occurred primarily at shallower depths. Measurements of sedimentary 231Pa/230Th have been used to reconstruct the strength of circulation in the North Atlantic Ocean, but the effects of biogenic silica on 231Pa/230Th-based estimates remain controversial. Here we use measurements of 231Pa/230Th ratios and biogenic silica in Holocene-aged Atlantic sediments and simulations with a two-dimensional scavenging model to demonstrate that the geometry and strength of the Atlantic meridional overturning circulation are the primary controls of 231Pa/230Th ratios in modern Atlantic sediments. For the glacial maximum, a simulation of Atlantic overturning with a shallow, but vigorous circulation and bulk water transport at around 2,000 m depth best matched observed glacial Atlantic 231Pa/230Th values. We estimate that the transport of intermediate water during the Last Glacial Maximum was at least as strong as deep water transport today.

(Table 1) Age markers and errors for the last 1215 ka in IODP Exp302

Despite its importance in the global climate system, age-calibrated marine geologic records reflecting the evolution of glacial cycles through the Pleistocene are largely absent from the central Arctic Ocean. This is especially true for sediments older than 200 ka. Three sites cored during the Integrated Ocean Drilling Program's Expedition 302, the Arctic Coring Expedition (ACEX), provide a 27 m continuous sedimentary section from the Lomonosov Ridge in the central Arctic Ocean. Two key biostratigraphic datums and constraints from the magnetic inclination data are used to anchor the chronology of these sediments back to the base of the Cobb Mountain subchron (1215 ka). Beyond 1215 ka, two best fitting geomagnetic models are used to investigate the nature of cyclostratigraphic change. Within this chronology we show that bulk and mineral magnetic properties of the sediments vary on predicted Milankovitch frequencies. These cyclic variations record ''glacial'' and ''interglacial'' modes of sediment deposition on the Lomonosov Ridge as evident in studies of ice-rafted debris and stable isotopic and faunal assemblages for the last two glacial cycles and were used to tune the age model. Potential errors, which largely arise from uncertainties in the nature of downhole paleomagnetic variability, and the choice of a tuning target are handled by defining an error envelope that is based on the best fitting cyclostratigraphic and geomagnetic solutions.