Interstitial water chemistry at DSDP Leg 67 Holes

Interstitial water chemistry has proved to be a sensitive indicator for early diagenetic reactions, particularly those related to organic matter oxidation. Downhole chemical variations in the pore waters from Deep Sea Drilling Project Holes 496 and 497 on the Middle America Trench slope off Guatemala are anomalous because both salinity and chlorinity show strong decreases to half the values of seawater, and d18O values become positive (maximum of about +2.5% at the bottom of the holes). These observations are explained in terms of dilution of pore waters after retrieval as a result of decomposition of the gas hydrates before removal of pore waters by shipboard squeezing techniques. In all holes, except Hole 495 (drilled in pelagic sediments), decomposition of organic matter leads to rapid sulfate depletion and subsequent methane generation. Associated with methane generation are large increases in alkalinity and dissolved ammonia. The latter component causes ion exchange reactions with clay minerals, which results in maxima in magnesium and perhaps potassium. At greater depths, as yet unidentified reactions cause the removal of magnesium. Especially in the deeper Trench Sites 499 and 500, rapid variations in calcium, magnesium, and alkalinity occur in turbidite sequences.

(Table 1) Heavy minerals in silt and sand samples of IODP Site 301-U1301

The northwestern Cascadia Basin of western North America accumulated high-sedimentation-rate sequences during the Pleistocene sea-level low-stands. The continental shelf was largely exposed at that time, and rivers and estuaries delivered large sediment fluxes directly to the deep ocean. The IODP EXP1301 core, which was taken from the middle portion of the Cascadia Basin, is well preserved and exhibits the deeper and - more distal sedimentary facies. The lithology in this location is composed of two units, 1) hemipelagic mud with a thin sand layer and 2) thick, coarsening upward silt-sand turbidites with a small proportion of granules at the top. We will focus on the detailed sand-grain proportions in order to understand the origin of these sediments. We determined the modal proportions of the heavy minerals, and the chemical composition of olivine and orthopyroxene in fourteen samples. These are characterized by an abundance of amphibole, pyroxenes and epidote, and the presence of minerals derived from peridotite. There is no drastic change in the modal and mineral compositions of the sands and silts between the turbidite and hemipelagic sequences. There were two probable drainage systems on the continent, the Frazer and Columbia rivers, which shed turbidites into the Cascadia Basin after 1.6 Ma, especially at 0.46-0.76 Ma. Based on a comparison of the modal and mineral compositions, the Northern Cascadia Basin has been supplied with sediments, mainly from the Frazer River, through the Straits of Juan de Fuca, by Pleistocene to Holocene turbidites.

Organic carbon chemistry of late Tertiary to Quaternary sediments ODP Leg 107

We analyzed samples from ODP Holes 652A and 654A (Leg 107, Tyrrhenian Sea) for the amount, type, and thermal maturity of organic matter. The sediments encompass clastic and biogenic lithologies, which were deposited on the passive margin east of Sardinia since the late Miocene to the Pleistocene. Marine, hypersaline/evaporitic, lacustrine/riverine, and finally hemipelagic marine conditions with occasional anoxic(?) interludes gave rise to very diverse sedimentary facies. The majority of samples is lean in organic matter (<0.2% TOC). Notable exceptions are Tortonian sediments (TOC average 0.3%), Messinian oil shales from Core 107-652A-64R (up to 11% TOC), Messinian lacustrine/fluvial sediments from Hole 652A (TOC average 0.42%,), and Pleistocene sapropel samples (>2% TOC). The Messinian oil shale in Hole 652A appears to be the only mature hydrocarbon source rock. In general, Pliocene sediments are the leanest and least mature samples. Pleistocene and Pliocene samples derive organic matter from a marine source. In spite of obvious facies differences in the Messinian between the two sites, pyrolysis results are not conclusive in separating hypersaline facies of Site 654 from the fresh water facies of Site 652, because both appear to have received terrestrial organic tissue as the main component of TOC. It is apparent from the distribution of maximum pyrolysis temperatures that heat flow must have been considerably higher at Site 652 on the lower margin in the Messinian. Molecular maturity indices in lipid extracts substantiate the finding that the organic matter in Tortonian and Messinian samples from Hole 654A is immature, while thermal maturation is more advanced in coeval samples from Hole 652A. Analyses of lipid biomarkers showed that original odd-even predominance was preserved in alkanes and alkylcyclohexanes from Messinian samples in Hole 654A, while thermal maturation had removed any odd-even predominance in Hole 652A. Isomerization data of hopanes and steranes support these differences in thermal history for the two sites. Hopanoid distribution further suggests that petroleum impregnation from a deeper, more mature source resulted in the co-occurrence of immature and mature groups of pentacyclic biomarkers. Even though the presence of 4-methylsteranes may imply that dinoflagellates were a major source for organic matter in the oil shale interval of Hole 652, we did not find intact dinoflagellates or related nonskeletal algae during microscopic investigation of the organic matter in the fine laminations. Morphologically, the laminations resemble bacterial mats.

Magnetic susceptibility and documentation of sediment cores from the Antarctic Peninsula Pacific margin

The effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the Antarctic Peninsula Pacific margin. Nineteen sediment cores were visually described, logged for magnetic susceptibility, and X-radiographed. About 1000 analyses were performed for grain size, clay minerals and biostratigraphy (foraminifera, nannofossils and diatoms). Four sediment types associated with distinct sedimentary processes are recognised based on textural/compositional analysis. (1) Hemipelagic mud forms the bulk of the interglacial sediment, and accumulated from the pelagic settling of bioclasts and ice-rafted/windtransported detritus. (2) Terrigenous mud forms the bulk of the glacial sediment, and accumulated from a combination of sedimentary processes including turbidity currents, turbid plumes, and bottom current reworking of nepheloid layers. (3) Silty deposits occurring as laminated layers and lenses, represent the lateral spillout of lowdensity turbidity currents. (4) Lastly, glacial/interglacial gravelly mud layers derive from settling of ice-rafted detritus. Five depositional settings are interpreted within sediment Drift 7, each characterised by the dominance/interaction of one or several depositional processes. The repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages (glaciation, glacial, deglaciation, and interglacial), each one characterised by a distinctive clay mineral assemblage and bioclastic content. Variations in clay mineral assemblage within interglacial stage 5 (core SED-06) suggest minor colder climatic fluctuations, possibly correlatable with substages 5a to 5e.

Geochemistry, mineralogy and isotopes at DSDP Hole 62-464

The relationships between mineralogical and geochemical data on the three successive sedimentary facies at Deep Sea Drilling Project Site 464 are studied. The evolution of siliceous biogenic sediments is derived from the analyses of one Fe-Ti smectite concretion, and of siliceous aggregates occurring in the pelagic "brown clays." Along the sedimentary section, the trace elements enriching the authigenic silicates and the Fe-Mn oxyhydroxides vary, depending on the marine environment. The proportion of clays and carbonates into the siliceous deposits controls the diagenetic evolution of silica making up the quartz aggregates from the "brown clay" or the cristobalite cherts.

Geochemistry of Neogene-Quaternary sediments in ODP Site 105-645

Subcontinuously cored early(?)-middle Miocene to recently deposited sediments from ODP Site 645 were studied texturally, mineralogically, and geochemically. The entire sequence contains minerals and associated chemical elements that are chiefly of detrital origin. In particular, the clay minerals, which include smectite, kaolinite, chlorite, and illite, are detrital. No obvious evidence of diagenesis with depth, of burial, of volcanism, or of hydrothermal alteration was observed. The sedimentary textures, clay mineralogy, and <2-µm fraction geochemistry of the early middle Miocene sediments (630 to 1147 mbsf) suggest the pronounced but variable influence of a southward bottom current. Two clay facies are defined. The lower one, Cj (780 to 1147 mbsf), is characterized by the great abundance of discrete smectite (with less than 15% illite interlayers), probably detrital in origin, and reworked older, discrete, smectite-rich sediments. The upper clay facies, C2 (630 to 780 mbsf), shows a net decrease of the fully expandable clay abundances, with a great abundance of mixed-layer, illite-smectite clays (60 to 80% of illite interlayers). Such clay assemblages can be inherited from paleosoils or older sedimentary rocks. An important change occurs at 630 mbsf (clay fraction) or 600 mbsf (sedimentary texture), which may be explained by the beginning of continental glaciation (630 mbsf, ~9 Ma) and the onset of ice rafting in Baffin Bay (600 mbsf, ~8 Ma). Above this level, the characteristics and modifications of the clay assemblages are controlled climatically and can be explained by the fluctuations of (1) ice-rafting, (2) speed of weak bottom currents, and (3) some supply by mud turbiditic currents. Three clay facies (C3, C4, and C5) can be defined by the abrupt increases of the inherited chlorite and illite clays.