Low-molecular-weight hydrocarbon concentrations, organic carbon contents, and Rock-Eval pyrolysis hydrogen indices at DSDP Holes 75-530A and 75-532

A series of C2-C8 hydrocarbons (including saturated, aromatic, and olefinic compounds) from deep-frozen core samples taken during DSDP Leg 75 (Holes 530A and 532) were analyzed by a combined hydrogen-stripping/thermovaporization method. Concentrations representing both hydrocarbons dissolved in the pore water and adsorbed on the mineral surfaces vary in Hole 530A from about 10 to 15,000 ng/g of dry sediment weight depending on the lithology (organic-carbon-lean calcareous oozes versus "black shales"). Likewise, the organic-carbon-normalized C2-C8 hydrocarbon concentrations vary from 3,500 to 93,100 ng/g Corg, reflecting drastic differences in the hydrogen contents and hence the hydrocarbon potential of the kerogens. The highest concentrations measured of nearly 10**5 ng/g Corg are about two orders of magnitude below those usually encountered in Type-II kerogen-bearing source beds in the main phase of petroleum generation. Therefore, it was concluded that Hole 530A sediments, even at 1100 m depth, are in an early stage of evolution. The corresponding data from Hole 532 indicated lower amounts (3,000-9,000 ng/g Corg), which is in accordance with the shallow burial depth and immaturity of these Pliocene/late Miocene sediments. Significant changes in the light hydrocarbon composition with depth were attributed either to changes in kerogen type or to maturity related effects. Redistribution pheonomena, possibly the result of diffusion, were recognized only sporadically in Hole 530A, where several organic-carbon lean samples were enriched by migrated gaseous hydrocarbons. The core samples from Hole 530A were found to be severely contaminated by large quantities of acetone, which is routinely used as a solvent during sampling procedures on board Glomar Challenger.

Interstitial water d13C and sediment geochemistry of ODP Leg 166 sites

A review of interstitial water samples collected from Sites 1003-1007 of the Bahamas Transect along with a shore-based analysis of oxygen and carbon isotopes, minor and trace elements, and sediment chemistry are presented. Results indicate that the pore-fluid profiles in the upper 100 meters below seafloor (mbsf) are marked by shifts between 20 and 40 mbsf that are thought to be caused by changes in sediment reactivity, sedimentation rates, and the influence of strong bottom currents that have been active since the late Pliocene. Pore-fluid profiles in the lower Pliocene-Miocene sequences are dominated by diffusion and do not show significant evidence of subsurface advective flow. Deeper interstitial waters are believed to be the in situ fluids that have evolved through interaction with sediments and diffusion. Pore-fluid chemistry is strongly influenced by carbonate recrystallization processes. Increases in pore-fluid Cl- and Na+ with depth are interpreted to result mainly from carbonate remineralization reactions that are most active near the platform margin. A lateral gradient in detrital clay content observed along the transect, leads to an overall lower carbonate reactivity, and enhances preservation of metastable aragonite further away from the platform margin. Later stage burial diagenesis occurs at slow rates and is limited by the supply of reactive elements through diffusion.

Neogene ice-rafted debris record of ODP Hole 120-751A

One of the primary objectives of Leg 120 was to obtain a high-resolution Neogene stratigraphic section from the Kerguelen Plateau. Site 751, located in the central part of the Raggatt Basin on the Southern Kerguelen Plateau in 1633.8 m of water (57°43.56'S; 79°48.89'E), was selected as the dedicated Neogene site for this objective. High-resolution sampling at Site 751 was used to delineate in detail the Neogene ice-rafted debris (IRD) occurrences on the Kerguelen Plateau. The oldest IRD found at Site 751 was approximately 9.9 Ma, and it was not until approximately 8.5 Ma that significant concentrations of IRD were detected. The first major IRD event at this site occurred in the uppermost Miocene between 6.0 and 5.5 Ma. During this time period, a general climatic cooling and glacial expansion occurred on Antarctica. The late Miocene IRD event was followed by a continuous episode of elevated IRD deposition in the lowermost Pliocene between 4.5 and 4.1 Ma. The 0.4-m.y. duration and the timing of the early Pliocene IRD event on the Kerguelen Plateau corresponds with IRD fluxes observed on the Falkland Plateau and in the Weddell Abyssal Plain. This correspondence of data indicates that a major global climatic event occurred during the early Pliocene. The East Antarctic Ice Sheet may have experienced deglaciation between 4.5 and 4.1 Ma and, as a result, released large volumes of sediment-laden ice into the Southern Ocean.