Characterization of sorbed volatile hydrocarbons from the Peru margin

Bacterial and thermogenic hydrocarbons are present in the sorbed-gas fraction of Peru margin sediments. At Ocean Drilling Program (ODP) Sites 681, 682, 684, and 686, bacterial gases are restricted to the early diagenetic zones, where dissolved sulfate has been exhausted and methanogenesis occurs. Methane migrating into the sulfate zone at Sites 681, 684, 686, and possibly 682, has been consumed anaerobically by methanotrophs, maintaining the low concentrations and causing an isotope shift in d13C(CH4) to more positive values. Significant amounts of C2+ hydrocarbons occur at the shelf Sites 680/681, 684, and 686/687, where these hydrocarbons may be associated with hypersaline fluids. There is evidence at Site 679 that sorbed C2+ hydrocarbons may also have been transported by hypersaline fluids. This characteristic C2+ hydrocarbon signature in the sorbed-gas fractions of sediments at Site 679 is not reflected in data obtained using the conventional "free-," "canned-," or "headspace-gas" procedures. The molecular and isotope compositions of the sorbed-gas fraction indicate that this gas may have a thermogenic source and may have spilled over with the hypersaline fluids from the Salaverry Basin into the Lima Basin. These traces of thermogenic hydrocarbon gases are over-mature (about 1.5% Ro) and are discordant with the less-mature sediments in which they are found. This observation supports the migration of these hydrocarbons, possibly from continental sources. Sorbed-gas analyses may provide important geochemical information, in addition to that of the free-gases. Sorbed-gases are less sensitive to activities in the interstitial fluids, such as methanogenesis and methanotrophy, and may faithfully record the migration of hydrocarbons associated with hypersaline fluids.

Lithofacies, chronostratigraphy, minerals and diagenetic silica facies at DSDP Leg 80 Holes

X-ray powder diffraction and optical and scanning-electron microscope analyses of sediment samples taken from four sites drilled in the Goban Spur area of the northeast Atlantic show variable diagenetic silicification of sediments at several stratigraphic horizons. The results are as follows: 1. The silicified sediments are middle Eocene at Site 548, Paleocene to lower Albian at Site 549, upper to lower Paleocene at Site 550, and lower Turanian at Site 551. 2. There are three types of these silicified sediments: nodular type in carbonate-rich host sediments, bedded type in clayey host sediments, and a type transitional between the other two. 3. Silica diagenesis is considered to progress as follows: dissolution of siliceous fossils; precipitation of opal CT in pore spaces and transformation of biogenic silica (opal A) to opal CT, development of opal CT cement; chalcedonic quartz precipitation in pore spaces and replacement of foraminiferal tests by chalcedonic quartz; and finally, transformation of opal CT to quartz, and cementation. But the strong influence of host-sediment types on diagenetic silica fades is recognized. Bedded-type silicified sediments in a clayey environment indicate a lower grade of silica diagenesis. Only very weak chalcedonic quartz formation is recognized, and there is no opal CT cementation, even in Lower Cretaceous bedded-type clayey silicified sediments. 4. The rf(101) spacing of opal CT shows two distinct trends of ordering or decrease with burial depth; one is a rapid change, in the case of nodular silicified sediments, and the other is a more gentle shift, found in bedded silicified sediments. 5. Diagenetic silica facies of the nodular type develop as irregular concentric zones around some nodule nuclei. Also, quartz-chert nodule formation occurs at rather shallower horizons, and is discordant with the trend of decreasing d(101) spacing in opal CT. 6. Silicified sediments at Site 551 are shallower than at the other sites. The diagenetic silica facies suggest the probable erosion of 300 m or more of sediment at this site. 7. The zeolites clinoptilolite and phillipsite were found in the sediment samples recovered on Leg 80. Clinoptilolite occurs from the shallower levels to the deepest horizons of diagenetically silicified zones, suggesting that clinoptilolite formation is related to diagenesis of biogenic silica. Phillipsite at Site 551 (Section 551-5-2) may originate from volcanogenie material.

40Ar/39Ar analysis for incremental heating experiments at DSDP Holes 77-537 and 77-538A

Drilling penetrated pre-Mesozoic crystalline basement beneath abbreviated sedimentary sequences overlying fault blocks in the southeastern Gulf of Mexico. At Hole 538A, located on Catoche Knoll, a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite is intruded by post-tectonic diabase dikes. Hornblende from the amphibolite displays internally discordant 40Ar/39Ar age spectra, suggesting initial post-metamorphic cooling at about 500 Ma followed by a mild thermal disturbance at about 200 Ma. Biotite from the gneiss yields a plateau age of 348 Ma, which is interpreted to result from incorporation of extraneous argon components when the biotite system was opened during the about 200 Ma thermal overprint. A whole-rich diabase sample from Hole 538A records a crystallization age of 190.4 ± 3.4 Ma. A lower grade phyllitic metasedimentary sequence was penetrated at Hole 537, drilled about 30 km northwest of Catoche Knoll. Whole-rock phyllite samples display internally discordant 40Ar/39Ar age spectra, but plateau segments clearly document an early Paleozoic metamorphism at about 500 Ma. The age and lithologic character of the basement terrane penetrated at Holes 537 and 538A suggest that the drilled fault blocks are underlain by attenuated fragments of continental crust of "Pan-African" affinity. This supports pre-Mesozoic tectonic reconstructions that locate Yucatan in the present Gulf recess during the amalgamation of Pangea.

Age determination of foraminifera shells

The causes for discordant radiocarbon results on multiple species of planktonic foraminifera from high-sedimentation-rate marine sediments are investigated. We have documented two causes for these anomalous results. One is the addition of secondary radiocarbon for which we have, to date, only one firm example. It involves an opal-rich sediment. The other is the incorporation of reworked material. Again, we have, to date, only one firm example. It involves a rapidly deposited ocean margin sediment. However, we have three other examples where reworking is the most likely explanation. On the basis of this study it is our conclusion that, where precise radiocarbon dating of high-deposition-rate marine sediment is required, a prerequisite is to demonstrate that concordant ages can be obtained on pairs of fragile and robust planktic shells. For sediment rich in opal, it is advisable to check for secondary calcite by comparing ages obtained on acid-leached samples with those on unleached samples.

Peak areas and heights for minerals of ODP Leg 113-696B (Table 1)

Seismic data acquired over the eastern shelf and margin of the South Orkney microcontinent, Antarctica, have shown a high-amplitude reflection lying at a sub-bottom two-way traveltime (TWT) of 0.5-0.8 s. There appear to be two causes for the reflection which apply in different parts of the shelf. The more widespread cause of the reflection is a break-up unconformity associated with the opening of Jane Basin to the east. This is clearly seen where reflections in the underlying sequence are discordant. In contrast, in Eotvos Basin and the southeastern part of Bouguer Basin, the high-amplitude reflection in places cuts across bedding and is interpreted to be caused by silica diagenesis. A post-cruise analysis of core samples from Site 696 in Eotvos Basin by X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed the presence of a silica diagenetic front at 520-530 mbsf. The position of the unconformity at this site is uncertain, but probably coincides with a change of detrital input near 548 mbsf. Fluctuations of physical properties related to the depth of the diagenetic front are difficult to separate from those related to the variation of detrital composition over the same depth interval. Correlation of the drilling record with the seismic record is difficult but with a synthetic seismogram it is demonstrated that diagenesis is the probable cause of the high-amplitude reflection. In Bouguer Basin at Site 695 the depth of the high-amplitude reflection was not reached by drilling; however, the reflection is probably also caused by silica diagenesis because of the biogenic silica-rich composition of the sediments cored. The estimated temperatures and ages of the sediments at the depths of the high-amplitude reflections at Sites 695 and 696 compare favorably with similar data from other diagenetic fronts of the world. The high-amplitude reflection in Bouguer Basin is commonly of inverse polarity, possibly caused either by interference between reflections from several closely-spaced reflecting layers, such as chert horizons, or by free gas trapped near the diagenetic front.

Whole rock and elements concentrations from IODP Hole 343-C0019E

The microstructures, mineralogy and chemistry of four representative samples collected from cores extracted from the Japan Trench during Integrated Ocean Drilling Project Expedition 343, the Japan Trench Fast Drilling Project (JFAST) have been studied using optical microscopy, TEM, SEM, XRF, XRD and microprobe analyses. The samples provide a transect from relatively undeformed marine sediments in the hanging wall, to the undeformed footwall material, crossing the thrust interface between the Pacific and North American plate, where the fault slipped during the March 2011 Tohoku-Oki earthquake. Our preliminary results suggest that the low strength of JFAST fault gouge material is caused by the high amount of clay minerals (~ 60% smectite, ~ 14 illite). Clay minerals in the décollement (gouge) sample are partly replaced by newly formed manganese oxide, which precipitated from hydrothermal fluids. Dauphine twins were found in quartz grains of the décollement sample suggesting local high stress possible during seismic loading. Other microstructures cannot be assigned unambiguously to co-seismic or a-seismic faulting processes. The observed scaly clay fabric is consistent with observations in many other plate-boundary fault zones. Significant grain size reduction was found in the fault (decollement) zone sample. But a change in lithology of the fault material cannot be ruled out. Microstructures typical for a-seismic deformation like dissolution-precipitation features (e.g. dissolved grain boundaries, mineral alteration) occur in all JFAST core samples, but more frequently in the décollement sample.

Chemical compositions of monomineral fractions from Fe-Mn crusts and of their host rocks, seamounts of the Sea of Japan

It has been established (with use of analytical electron microscopy) that Fe-Mn crusts from two seamounts of the Sea of Japan consist predominantly of an association of birnessite minerals including Ca-birnessite and clinobirnessite. The latter is represented by twins, which form regular integrowths with 14 Å Ca-birnessite. Ore material from the third seamount (Tarasov submarine volcano) consists only of fibrous todorokite, which forms spherulitic nodules, blocks, and laminar formations. Its chemical composition is close to that of todorokite from the Galapagos rift zone.