Sulfur speciation in bottom sediments from the Peru upwelling area

Sulfur speciation in bottom sediments from the area of the Peru upwelling has been studied. Data on sulfur contents in different compounds (sulfide, elemental, sulfate, pyritic and organic), water content, Eh, and organic carbon content in the bottom sediments have been obtained. The bottom sediments from the area are characterized by high content of organic carbon and low contents of total and reactive iron; this is typical for bottom sediments from ocean upwelling areas. Intense process of sulfate reduction occurs in the bottom sediments of the area, and accumulation of reduced sulfur compounds derivated from bacterial hydrogen sulfide does not exceed previously known values for other regions of the ocean.

Petrology at DSDP Leg 66 Holes

The sandstone petrology of Leg 66 samples provides insights into changes through time in the geology of the source regions along the Guerrero portion of the Middle America continental margin. This in turn constrains possible models of the evolution of the Middle America Trench (e.g., de Czerna, 1971; Malfait and Dinkleman, 1972; Karig, 1974). Primarily medium-grained sands and sandstones, representing the widest variety available of trench/trench slope settings and ages, were analyzed in both light and heavy mineral studies. Standard techniques were used as much as possible in order to compare results from other margins and from ancient rocks.

Stable isotope analyses of Cretaceous sediment samples

We report the sulfur and oxygen isotope composition of sulfate (d34SSO4 and d18OSO4, respectively) in coexisting barite and carbonate-associated sulfate (CAS), which we use to explore temporal variability in the marine sulfur cycle through the middle Cretaceous. The d34SSO4 of marine barite tracks previously reported sulfur isotope data from the tropical Pacific. The d18OSO4 of marine barite exhibits more rapid and larger isotopic excursions than the d34SSO4 of marine barite; these excursions temporally coincide with Ocean Anoxic Events (OAEs). Neither the d34SSO4 nor the d18OSO4 measured in marine barite resembles the d34SSO4 or the d18OSO4 measured in coexisting CAS. Culling our data set for elemental parameters suggestive of carbonate recrystallization (low [Sr] and high Mn/Sr) improves our record of d18OSO4 in CAS in the Cretaceous. This suggests that the CAS proxy can be impacted by carbonate recrystallization in some marine sediments. A box model is used to explore the response of the d34SSO4 and d18OSO4 to different perturbations in the marine biogeochemical sulfur cycle. We conclude that the d34SSO4 in the middle Cretaceous is likely responding to a change in the isotopic composition of pyrite being buried, coupled possibly with a change in riverine input. On the other hand, the d18OSO4 is likely responding to rapid changes in the reoxidation pathway of sulfide, which we suggest may be due to anoxic versus euxinic conditions during different OAEs.

Mineralogy and major-element chemistry of volcanic ash of ODP Site 131-808

Mineralogical and major-element compositions of 72 samples of volcanic ash, recovered from Site 808 at Nankai Trough during Leg 131, were analyzed in relation to the early diagenetic alteration. Alteration products are first observed at the following depths: smectite, 200 mbsf; clinoptilolite, 646 mbsf; and analcite, 810 mbsf. Glass decomposition dominates over authigenic mineral formation between 200 and 550 mbsf in the sediment column, whereas mineral formation becomes dominant below 550 mbsf. Based on the X-ray diffraction patterns, a broad and asymmetric peak of 15A suggests a presence of illite/smectite (I/S) mixed-layered minerals in a sample from 646 mbsf. I/S mixed-layered mineral formation, however, rarely occurs even at the bottom of the sediment column (1290 mbsf) at 120° C. This is possibly because zeolite (especially clinoptilolite) formed in the ash interferes with illite formation in the smectite. The formation of alteration minerals affects the major-element chemistry of the ash and the interstitial waters. H4SiO4 concentrations in interstitial waters increase during glass decomposition and decrease with smectite and clinoptilolite formation. K is removed from interstitial water into smectite and/or clinoptilolite. Mg is fixed into smectite (and/or chlorite).

Geochemical characterizations of the Cretaceous formations at DSDP Holes 93-603B and 11-105

Geochemical characterizations of the Cretaceous formations at Site 603 are quite comparable with those at Site 105. In the Blake-Bahama and the Hatteras formations, the petroleum potential is medium (<5 kg HC/t of rock) to very low (<0.5 kg HC/t of rock), and the organic matter is mainly of type III origin, that is, terrestrial. At the top of the Hatteras Formation, there is a condensed series, which chiefly contains organic matter of type II origin, with up to 20 wt.% total organic carbon content in Core 603B-34 and 25 wt.% in Core 105-9. This accumulation corresponds to the Cenomanian/Turonian boundary event. An examination of dinoflagellates in the kerogen concentration assigns dates to the samples studied by organic geochemistry. The Cenomanian and Turonian age of the organic-matter-rich black claystones indicates a low rate of sedimentation, about 1 m/Ma. Furthermore, the occurrence of type II organic matter indicates an anoxic environment with insufficient oxygen renewal to oxidize the sinking hemipelagic organic matter. This organic enrichment is not related to local phenomena but to sedimentation over an extended area, because deposits are well known in various areas with different paleodepths in the North Atlantic.

Iron and carbon geochemistry of sediment core GeoB1401-4

Iron speciation was determined in hemiplegic sediments from a high productivity area to investigate systematically the early diagenetic reactivity of Fe. A combination of various leaching agents (1 M HCI, dithionite buffered in citrate/acetic acid, HF/H2SO4, acetic Cr(II)) was applied to sediment and extracted more than 80% of total Fe. Subsequent Fe species determination defined specific mineral fractions that are available for Fe reduction and fractions formed as products of Fe diagenesis. To determine the Fe speciation of (sheet) silicates we explored an extraction procedure (HF/H2SO4) and verified the procedure by application to standard rocks. Variations of Fe speciation of (sheet) silicates reflect the possible formation of Fe-bearing silicates in near surface sediments. The same fraction indicates a change in the primary input at greater depth, which is supported by other parameters. The Fe(II)/ Fe(III) -ratio of total sediment determined by extractions was compared with Mössbauer-spectroscopy ] at room temperature and showed agreement within 10%. M6ssbauer-spectroscopy indicates the occurrence of siderite in the presence of free sulfide and pyrite, supporting the importance of microenvironments during mineral formation. The occurrence of other Fe(II) bearing minerals such as ankerite (Ca-, Fe-, Mg-carbonate) can be presumed but remains speculative.

Mineralogical, sedimentological and paleontological analyses of ODP Hole 127-797B upper Quarternary sediments

Nearly continuous cores of Quaternary fine-grained sediments with distinct dark-light colored cycles were recovered from Sites 794, 795, and 797 in the basinal parts of the Japan Sea during Leg 127. A comparison of gray value (darkness) profiles supplemented by visual inspection of core photographs between sites indicated that most of the dark and light layers were correlatable between sites, and that two of the dark layers lie close to adjacent marker ash layers. These observations indicate that deposition of dark and light layers resulted from basin-wide synchronous events. In order to understand the origin of these dark-light cycles, petrographical, mineralogical, compositional, and paleontological studies were carried out on closely spaced samples from the upper Quaternary sediments recovered from Site 797. Age model was constructed based on comparison between variation in diatom abundance and the standard oxygen isotope curve of Imbrie et al. (1984), the latter was interpolated between the five age controlled levels established at Site 797. The two curves show similar patterns which enabled us to "tune" the sediment ages to the oxygen isotope stages. We have to use variation in diatom abundance as a substitute for oxygen isotope curve since oxygen isotopic data are not available at the studied sites. Bottom water oxygenation conditions were estimated based on two criteria: (1) the degree of lamina preservation and (2) the ratio of Corg to Stot. The surface water productivity was deduced from the Corg and biogenic silica content. Results suggest that the bottom water oxygenation level and the surface water productivity varied significantly in response to the glacial-interglacial cycles. Glacio-eustatic sea-level changes and subsequent changes in water circulation in the Japan Sea appear to have been responsible for these variations and consequent changes in sediment composition throughout the Quaternary.

Geochemical analysis of sediments from three coring sites in the Mediterranean

In a gravity core from the eastern Mediterranean Sea, a chemically and mineralogically distinct, 5.5-cm-thick layer is present above sapropel S-1 and overlain by hemipelagic marls. Calcite is completely absent in this exotic layer, dolomite is present only in small amounts, and the Cr concentrations are significantly enhanced. The layer was deposited primarily under reducing conditions, but the distributions of redox-sensitive elements show that a large part of the exotic layer is now oxidised by a downward-progressing oxidation front. Sediments from within the nearby anoxic, hypersaline Urania Basin are similar to those from the exotic layer, in particular in S-, C-, and O-isotope distributions of pyrite and dolomite, as well as increased Cr concentrations. Mud expulsion due to expansion of gas-rich mud is proposed to explain the presence of the exotic layer outside the Urania Basin. The deposition of an anoxic layer above S-1 shielded the sapropel from oxidation which resulted in the rare occurrence of a complete preservation of S-1 and provides the first minimum age for the start of anoxic mud accumulation in the Urania Basin.

Pore water (SO4, CH4, alkalinity, HS-, Ca, Sr, Mg, Ba) and solid phase (Ca, Sr, Mg, Ba) data measured in pockmark sediments of the Northern Congo Fan

This study focuses on the vertical distribution of authigenic carbonates (aragonite and high Mg-calcite) in the form of finely disseminated precipitates as well as massive carbonate concretions present in and above gas hydrate bearing sediments of the Northern Congo Fan. Analyses of Ca, Mg, Sr and Ba in pore water, bulk sediments and authigenic carbonates were carried out on gravity cores taken from three pockmark structures (Hydrate Hole, Black Hole and Worm Hole). In addition, a background core was retrieved from an area not influenced by fluid seepage. Pore water Sr/Ca and Mg/Ca ratios are used to reveal the current depths of carbonate formation as well as the mineralogy of the authigenic precipitates. The Sr/Ca and Mg/Ca ratios of bulk sediments and massive carbonate concretions were applied to infer the presence and depth distribution of authigenic aragonite and high Mg-calcite, based on the approach presented by Bayon et al. [Bayon et al. (2007). Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241(1-4), 93-109, doi:10.1016/j.margeo.2007.03.007]. We show that the approach developed by Bayon et al. (2007) for sediments of cold seeps of the Niger Delta is also suitable to identify the mineralogy of authigenic carbonates in pockmark sediments of the Congo Deep-Sea Fan. We expand this approach by combining interstitial with solid phase Sr/Ca and Mg/Ca ratios, which demonstrate that high Mg-calcite is the predominant authigenic carbonate that currently forms at the sulfate/methane reaction zone (SMRZ). This is the first study which investigates both solid phase and pore water signatures typical for either aragonite or high Mg-calcite precipitation for the same sediment cores and thus is able to identify active and fossil carbonate precipitation events. At all investigated pockmark sites fossil horizons of the SMRZ were deduced from high Mg-calcite located above and below the current depths of the SMRZ. Additionally, aragonite enrichments typical for high seepage rates were detected close to the sediment surface at these sites. However, active precipitation of aragonite as indicated by pore water characteristics only occurs at the Black Hole site. Dissolved and solid phase Ba concentrations were used to estimate the time the SMRZ was fixed at the current depths of the diagenetic barite fronts. The combined pore water and solid phase elemental ratios (Mg/Ca, Sr/Ca) and Ba concentrations allow the reconstruction of past changes in methane seepage at the investigated pockmark sites. At the Hydrate Hole and Worm Hole sites the time of high methane seepage was estimated to have ceased at least 600 yr BP. In contrast, a more recent change from a high flux to a more dormant stage must have occurred at the Black Hole site as evidenced by active aragonite precipitation at the sediment surface and a lack of diagenetic Ba enrichments.

Composition of sulfide minerals of ODP Hole 209-1268A

This paper presents sulfide mineral occurrence, abundance, and composition in samples from hydrothermally altered peridotite and gabbro recovered during Ocean Drilling Program (ODP) Leg 209 from south of the 15°20'N Fracture Zone on the Mid-Atlantic Ridge at Site 1268. Most of the sulfide minerals occur in veins and halos around veins in serpentinized peridotite. The only sulfide phases reported that occur in proximity to gabbro are those associated with a mafic intrusion into serpentinized peridotite. Sulfide mineral species change predictably downsection but are perturbed coincident with a breccia interpreted to be generated by intrusion of a gabbroic magma. The general downhole trend suggests sulfide mineral precipitation in conditions with decreasing sulfur and oxygen fugacity. Sulfide minerals that indicate precipitation at relatively higher sulfur and oxygen fugacity occur in the central core of the intrusion breccia. Sphalerite makes a fleeting appearance in the sulfide mineral assemblage in samples from the lower part of the intrusion breccia. Strongly contrasting pyrite compositions suggest at least two episodes of pyrite precipitation, but there is no clear morphological distinction between phases. Heazelwoodite, tentatively identified in shipboard examinations, could not be confirmed in this study.