Geochemical composition and granulometry of ODP Hole 167-1017E sediments

Intensification of North Pacific Intermediate Water during the Younger Dryas and stadials of the last glacial episode has been advocated by Kennett and his colleagues based on studies of ventilation history in Santa Barbara Basin. Because Santa Barbara Basin is a semi-isolated marginal basin, this hypothesis requires testing in sequences on the upper continental margin facing the open-ocean of the Pacific. Ocean Drilling Program Site 1017 is located on the upper slope of southern California off Point Conception close to the entrance of Santa Barbara Basin, an ideal location to test the hypothesis of late Quaternary switching in intermediate waters. We examined chemical and mineral composition, sedimentary structures, and grain size of hemipelagic sediments representing the last 80 k.y. at this site to detect changes in behavior of intermediate waters. We describe distinct compositional and textual variations that appear to reflect changes in grain size in response to flow velocity fluctuations of bottom waters. Qualitative estimates of changes in degree of pyritization indicate better ventilation of bottom water during intervals of stronger bottom-water flow. Comparison between variations in the sediment parameters and the planktonic d18O record indicates intensified bottom-current activity during the Younger Dryas and stadials of marine isotope Stage 3. This result strongly supports the hypothesis of Kennett and his colleagues. Our investigation also suggests strong grain-size control on organic carbon content (and to less extent carbonate carbon content). This, in turn, suggests the possibility that organic carbon content of sediments, which is commonly used as an indicator of surface productivity, can be influenced by bottom currents.

Organic geochemical character of selected cores at DSDP Hole 75-530A

The Albian/Cenomanian strata in Hole 530A are organically richer than are the post-Cenomanian strata. Organic matter is thermally immature and appears to be of dominantly marine origin with either variable levels of oxidation or variable amounts of terrestrial input. Geochemical data alone cannot establish whether the black shales present in Hole 530A represent deposition within a stagnant basin or within an expanded oxygen-minimum layer

Geochemical composition of sediments from ODP Site 164-997, Blake Ridge

Twenty-two trace elements in 355 sediment samples from Site 997 on the Blake Ridge were examined by inductively coupled plasma-optical emission spectrometry and inductively coupled plasma-mass spectrometry, for respective fractions of acid-soluble and insoluble compositions. Downhole profiles of these elements exhibit complicated fluctuations throughout late Miocene to Pleistocene, principally due to the variations in the acid-soluble fraction. Noncarbonate composition is given from the acid-insoluble residues, which permits us to recognize secular feature of selected element variance for four intervals. These intervals (I: 0-183 mbsf; II: 183- 440 mbsf; III: 440-618 mbsf; and IV: 618-750 mbsf) are interpreted to have originated from changes in the suite of sediments of particular sources and chemical composition, sedimentation rate, dilution of biogenic carbonate abundance, and possibly the current system that controlled deposition and reworking of the terrigenous materials.

Geochemical characterization of black shales from the Tarfaya Basin

Organic geochemical and petrological investigations were carried out on Cenomanian/Turonian black shales from three sample sites in the Tarfaya Basin (SW Morocco) to characterize the sedimentary organic matter. These black shales have a variable bulk and molecular geochemical composition reflecting changes in the quantity and quality of the organic matter. High TOC contents (up to 18wt%) and hydrogen indices between 400 and 800 (mgHC/gTOC) indicate hydrogen-rich organic matter (Type I-II kerogen) which qualifies these laminated black shale sequences as excellent oil-prone source rocks. Low Tmax values obtained from Rock-Eval pyrolysis (404-425 MC) confirm an immature to early mature level of thermal maturation. Organic petrological studies indicate that the kerogen is almost entirely composed of bituminite particles. These unstructured organic aggregates were most probably formed by intensive restructuring of labile biopolymers (lipids and/or carbohydrates), with the incorporation of sulphur into the kerogen during early diagenesis. Total lipid analyses performed after desulphurization of the total extract shows that the biomarkers mostly comprise short-chain n-alkanes (C16-C22) and long-chain (C25-C35) n-alkanes with no obvious odd-over-even predominance, together with steranes, hopanoids and acyclic isoprenoids. The presence of isorenieratane derivatives originating from green sulphur bacteria indicates that dissolved sulphide had reached the photic zone at shallow water depths (~100m) during times of deposition. These conditions probably favoured intensive sulphurization of the organic matter. Flash pyrolysis GC-MS analysis of the kerogen indicates the aliphatic nature of the bulk organic carbon. The vast majority of pyrolysis products are sulphur-containing components such as alkylthiophenes, alkenylthiophenes and alkybenzothiophenes. Abundant sulphurization of the Tarfaya Basin kerogen resulted from excess sulphide and metabolizable organic matter combined with a limited availability of iron during early diagenesis. The observed variability in the intensity of OM sulphurization may be attributed to sea level-driven fluctuations in the palaeoenvironment during sedimentation.

Geochemical investigation of ODP Leg 113 sites

Dissolved organic carbon (DOC) was determined in pore water extracted from pelagic and hemipelagic sediments recovered during Leg 113. DOC concentration varied between 1.82 and 13.6 mg C/L which is one to two orders of magnitude less than previously reported for hemipelagic sediments. It is argued that this difference is related to differences in the intensity of degradation of organic matter. As a first approximation it is found that in reducing sediments, the level of DOC is proportional to the intensity of sulfate reduction. It is suggested that DOC is formed by different mechanisms in oxic and reducing environments.

Organic geochemical character and hydrocarbon source potential of selected black shales at DSDP Hole 93-603B

LECO analysis, pyrolysis assay, and bitumen and elemental analysis were used to characterize the organic matter of 23 black shale samples from Deep Sea Drilling Project Leg 93, Hole 603B, located in the western North Atlantic. The organic matter is dominantly gas-prone and/or refractory. Two cores within the Turonian and Cenomanian, however, contained significant quantities of well-preserved, hydrogen-enriched, organic matter. This material is thermally immature and represents a potential oil-prone source rock. These sediments do not appear to have been deposited within a stagnant, euxinic ocean as would be consistent with an "oceanic anoxic event." Their organic geochemical and sedimentary character is more consistent with deposition by turbidity currents originating on the continental shelf and slope.

Geochemical parameters of bottom sediments from the Stromboli Canyon

In recent sediments of the Stromboli Canyon sides features of mineral and grain size compositions, redox conditions, behavior of Fe, Mn, organic carbon, Mo, and W in an environment of active input of pyroclastic material are considered. Differences in conditions of sedimentation and early diagenesis in the east and west sides of the canyon depending on position of the prevailing direction of drift and steepness of the slopes, as well as types of differentiation of detrital material in sediments under conditions of permanent vibrations are specified.

Geochemical history at DSDP Leg 56 Holes

A geological model of subduction postulated by Karig, Ingle, et al. (1975) and Karig and Sharman (1975) proposes that the sedimentary prism at the foot of the landward wall is being actively built as sediment is scraped off the subducting oceanic and plastered onto the base of the wedge, forming an accretionary wedge containing overthrust sedimentary layers or intense sedimentary folding. Because overlying layers must continually be uplifted and compressed to accommodate new matter at the base, the accreting wedge will provide a geochemical record of this process at or near the Japan Trench. Several recent papers have discussed the metalliferous sediments on the active oceanic ridges. The geochemistry of such sediments is now reasonably well known: generally these deposits are considered products of volcanic processes (Boström and Peterson, 1969; Böstrom et al., 1969; Horowitz, 1970, 1974; Cronan et al., 1972; Cronan and Garrett, 1973). The geochemistry of subduction zone sediments, however, is less well known, and the need for studies of these sediments is particularly urgent if such sediments provide a record of the effects of subduction of oceanic plates under continental crust. Because the Japan Trench contains welldeveloped subduction zone deposits, Leg 56 sampling was of utmost importance to the discovery of how they originate.

Geochemical composition of sediment samples from Krasnov field in the Mid Atlantic Ridge

This paper is dedicated to the geochemical studies of two bottom sediment cores that were taken during cruise 28 of the R/V Professor Logachev in the Mid-Atlantic Ridge (MAR) 16°38'N area in 2006. The chemical compositions of background metalliferous and ore (ore-bearing) carbonate sediments are presented and inter-element correlations are examined. Individual episodes are distinguished in the accumulation history of the ore-bearing and metalliferous sediments on the basis of element factor analysis.

Organic-geochemical and petrographical composition of sediments from ODP Leg 164 sites

Sediments from Holes 994C, 995A, 997A, and 997B have been investigated for "combined" gases (adsorbed gas and that portion of free gas that has not escaped from the pore volume during core recovery and sample collection and storage), solvent-extractable organic compounds, and microscopically identifiable organic matter. The soluble materials mainly consist of polar compounds. The saturated hydrocarbons are dominated by n-alkanes with a pronounced odd-even predominance pattern that is derived from higher plant remains. Unsaturated triterpenoids and 17ß, 21ß-pentacyclic triterpenoids are characteristic for a low maturity stage of the organic matter. The low maturity is confirmed by vitrinite reflectance values of 0.3%. The proportion of terrestrial remains (vitrinite) increases with sub-bottom depth. Within the liptinite fraction, marine algae plays a major role in the sections below 180 mbsf, whereas above this depth sporinites and pollen from conifers are dominant. These facies changes are confirmed by the downhole variations of isoprenoid and triterpenoid ratios in the soluble organic matter. The combined gases contain methane, ethane, and propane, which is a mixture of microbial methane and thermal hydrocarbon gases. The variations in the gas ratios C1/(C2+C3) reflect the depth range of the hydrate stability zone. The carbon isotopic contents of ethane and propane indicate an origin from marine organic matter that is in the maturity stage of the oil window.

Geochemical analyses at DSDP Holes 72-515B and 72-516F

The petroleum-generating potential of five samples from Hole 515B, Vema Channel, and of 23 samples from Hole 516F, Rio Grande Rise, was analyzed. Organic carbon and pyrolysis data indicated that source rocks of good quality are not present. Microscopic examination showed predominance of woody organic matter, which is more favorable for the generation of gas in a mature stage; all samples, however, are still thermally immature.

Organic geochemical characteristics at DSDP Holes 76-534A and 44-391C

Pyrolysis assay, bitumen analysis, and elemental analysis of kerogen were used to characterize the organic matter of selected core samples from Hole 534A (Leg 76) and Hole 391C (Leg 44) on the Blake-Bahama Plateau. The organic matter throughout the stratigraphic section appears to be principally of a terrestrial origin. The data from several isolated horizons in the Hatteras and Blake-Bahama Formations imply the presence of significant quantities of autochthonous marine organic matter. However, these horizons appear so limited that they cannot be considered potential liquid hydrocarbon source rocks. All the analyzed samples are immature and have not evolved sufficiently to enter into the main stage of hydrocarbon generation. The temporal and spatial restrictions of strata rich in marine organic matter suggest that they do not represent major expansions and contractions of anoxic bottom-water masses, but represent limited occurrences of anoxic conditions.

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.

Geochemical processes and rare earth element analysis in cold-seep pore waters of Hydrate Ridge, northeast Pacific Ocean

The concentrations of rare earth elements (REEs), sulphate, hydrogen sulphide, total alkalinity, calcium, magnesium and phosphate were measured in shallow (<12 cm below seafloor) pore waters from cold-seep sediments on the northern and southern summits of Hydrate Ridge, offshore Oregon. Downward-decreasing sulphate and coevally increasing sulphide concentrations reveal sulphate reductionas dominant early diagenetic process from ~2 cm depth downwards. A strong increase of total dissolved REE concentrations is evident immediately below the sediment-water interface, which can be related to early diagenetic release of REEs into pore water resulting from the remineralization of particulate organic matter. The highest pore water REE concentrations were measured close to the sediment-water interface at ~2 cm depth. Distinct shale normalized REE patterns point to particulate organic matter and iron oxides as main REE sources in the upper ~2-cm depth interval. In general, the pore waters have shalenormalized patterns reflecting heavy REE (HREE) enrichment, which suggests preferential complexation of HREEs with carbonate ions. Below ~2 cm depth, a downward decrease in REE correlates with a decrease in pore water calcium concentrations. At this depth, the anaerobic oxidation of methane (AOM) coupled to sulphate reduction increases carbonate alkalinity through the production of bicarbonate, which results in the precipitation of carbonate minerals. It seems therefore likely that the REEs and calcium are consumed during vast AOM-induced precipitation of carbonate in shallow Hydrate Ridge sediments. The analysis of pore waters from Hydrate Ridge shed new light on early diagenetic processes at cold seeps, corroborating the great potential of REEs to identify geochemical processes and to constrain environmental conditions.