RESULTS

Amount and Composition of the Solvent-Extractable Organic Matter and the Kerogen

The TOC contents of the samples from all three holes increase from 0.3% to 0.5% in the uppermost samples to levels around 1.5% at a depth of about 200 mbsf, and remain constant below (Fig. 1; Table 1). Shipboard and shore-based results correlate despite the fact that quite different methods were used (Shipboard Scientific Party, 1996). The total sulfur contents (TS) are around 1% for all samples. No correlation between the TOC content and the TS nor a depth trend in the TS concentrations was observed. Only samples from within the sulfate-reduction zone appear to be depleted in sulfur.

Between 100 and 1000 ppm soluble organic matter (SOM) could be extracted from the dried sediments. There is a weak but nonlinear correlation to the TOC concentration. Normalized to the TOC content 1%-6% of the organic carbon is soluble with a slight tendency towards an increasing solubility with increasing TOC contents. However, an increase of the SOM/TOC ratio (ratio is not expressed in Table 1) with depth that would indicate a maturation effect is absent.

A large part of the extract consists of asphaltenes (up to 22%) and other polar compounds. The nonaromatic hydrocarbon fraction comprises normal-, iso-, and cyclo-alkanes and alkenes. The n-alkanes contain from 16 to 35 C atoms with a pronounced odd-even-predominance in the high boiling range. The alkane n-C₂₉H₆₀ is typically the most prominent compound in the chromatograms, which indicates a contribution of higher land plants to the organic matter (Eglinton and Hamilton, 1963), although odd-numbered long-chain n-alkanes have been detected also in various species of microalgae (Zegouagh et al., 1998). About 40% of the samples are characterized by n-alkane distribution curves that show a second maximum around n-C₁₇H₃₆, which unequivocally suggests an algal input (Fig. 2A; Blumer et al., 1971). These samples are more concentrated in the Pleistocene-Pliocene part of the profiles. The changes in the relative composition of the organic matter with increasing age of the sediments is expressed by the change in a variety of hydrocarbon ratios (Fig. 3; Table 1).

1. The concentration of the isoprenoids pristane and phytane relative to n-C₁₇H₃₆ and n-C₁₈H₃₈ is constant with depth until about 400 m. At the final depth, pristane/n-C₁₇H₃₆ ratios as high as 2 and phytane/n-C₁₈H₃₈ ratios as high as about 6 were determined, whereas these ratios are below 1 in the near-surface samples (Fig. 3A-3B).
2. The pristane/phytane concentration ratios decrease from levels around 1 at the sediment surface to about 0.3 at the final depth in all three boreholes (Fig. 3C).
3. All samples contain thermally unstable hopenes and ß, ß-hopanes in varying amounts (Fig. 2C-2D; Table 2; Fig. 3D-3F), which indicates the immature stage of the sediments. The concentration of these compounds, relative to that of the stable ,ß-hopanes, increases with depth, which shows that maturation does not play a major role in transforming the organic matter. Otherwise the depth trend of the ratios plotted in Figure 3D-3F should be reversed.
4. The odd-even-predominance (calculated for the n-C₂₇H₅₆) increases steadily from around 1 to 2 at the seabed to ratios of around 3.6 and 3.0 at the final depths of Holes 994C and 995A, whereas it is rather constant at about 2 for the profile of Holes 997A and 997B (Fig. 3E). Again, if maturation would have been the dominating factor, an opposite depth trend for Holes 994C and 995A should occur.

Aside from the features of immaturity present in all samples (odd-even predominance of n-alkanes, unstable hopenes and hopanes), several sediment samples from all three holes show an isomerization pattern of the extended hopanes (22S-compound >22R-compound) that is characteristic of mature organic matter (Fig. 2D; Table 2; compounds 11 and 12, 14 and 15, and 16 and 17). The presence of thermally stable and unstable compounds could be due to a mixture of recycled mature and autochthonous immature organic matter (see chapter discussion). However, because all sediments sampled with the grease-containing core barrel have the most pronounced mature pattern, an artificial contamination by grease cannot be excluded. An analysis of the grease in the same way as the samples has yet to be done.

Organic petrographic investigations provide further insight into the composition of the organic matter. In general, the structured organic matter is evenly distributed and not enriched in distinct layers. The particles are around 10 µm in size, and only pollen may exceed 100 µm. Three groups of particles--inertinite, vitrinite (synonymous with huminite), and liptinite—make up the greater part of the visible organic matter. Bituminite and faunal remains are of minor importance (Table 3). The description of the organic particles visible under the microscope follows the nomenclature of Stach et al., (1982). Submicroscopic organic matter (<1 µm) associated with the inorganic matrix may occur, but it cannot be identified.

1. Inertinite: Highly reflecting inertinite is derived mainly from cell walls of higher plants (mostly trees) and is produced through wildfires. The less-reflecting semi-inertinite is derived from organic matter that has been oxidized in the water column or in the course of early diagenesis. Both constituents are evenly distributed throughout the profiles. Additionally, highly reflective graphite bars, probably derived from eroded crystalline rocks and thin-walled agglomerates, transported as soot, were observed. Altogether they make up about 50% of the particulate organic matter.
2. Vitrinite: Humic material is also derived from terrestrial vegetation that was degraded during transportation, sedimentation, microbial activity, and inorganic oxidation processes. The particles are fine-grained, elongated, thin-walled, and mostly brown in color. Their concentration is below 5% of total particles in the uppermost samples, whereas it is up to about 50% in the deeper sections.
3. Liptinite: Two major subgroups were differentiated: (1) sporinite, predominantly consisting of saccate pollen from conifers, and (2) alginite, chiefly made up of dinoflagellate cysts and solitary, spiny algae of planktonic origin. Because colonial algae of the Botryococcus type (up to 100 µm in size) are rare, fine-grained liptodetrinite, considered to be algal detritus, is dominating.
   These liptinites (pollen and algae) were less degraded than the inertinites and vitrinites. 
   The composition of the liptinites as a multicomponent group changes downhole. The sporinite dominates, or is in equal concentration to the alginite, in samples down to about 160 mbsf, whereas further down alginite is generally the prevailing constituent of the liptinite.
4. Bituminite: Fluorescing lenses, thin streaks, and agglomerates of bituminite are present in all samples, but only in very small amounts. Some samples contain globular or elliptic brown- to yellow-stained bituminite-like particles, about 10 µm in size, which are interpreted as fecal pellets from copepods and similar crustaceans.

In general, the content of vitrinite increases with depth (Fig. 4), whereas the liptinite content decreases (Table 3). Within the liptinite group, marine algae plays a major role below 180 mbsf, whereas above terrigenous pollen dominate. Taking into account these changes in the composition of the particulate organic matter, with broad scattering the content of marine organic matter (algal-derived) amounts to about 30%, whereas that of terrestrial origin is about 70% in all samples (Table 3).

The maturity of the organic matter, expressed as vitrinite reflectance and measured on only 2-14 particles from each of the six selected samples, is around 0.3%.

The varying types of organic matter deduced from chemical and optical investigations cannot be verified by the isotope ratios of the kerogens from the Hole 995A (Table 1). ¹³C values range between -20.4 and -21.9, which is generally believed to indicate marine organic matter (Degens, 1969; Kaplan, 1975). On the other hand, a mixture of type II to type III kerogen (Fig. 5) is indicated by the low hydrogen indices (HI; mean HI around 200mg HC/gC) from the shipboard Rock-Eval data (Shipboard Scientific Party, 1996). However, mixed type II/type III organic matter in marine sediments often arises from partial oxidation of type II algal material. Therefore, bulk analysis have to be interpreted with caution.

Amount and Composition of the Combined Gases

The concentrations of the combined gases from Holes 994C, 995A, 997A, and 997B are plotted vs. depth (mbsf) in Figure 6. The data ranges between 73 and 1554 ppbw, 15 and 107 ppbw, and 6 and 61 ppbw for methane, ethane, and propane, respectively. Ethane and propane are constant with depth with the exception of the uppermost samples where the concentrations are slightly increased. Methane is below 400 ppbw in the depth range of 200-400 mbsf and below 600 mbsf. Slightly higher methane concentrations are found just above 200 mbsf and between ~450 and 550 mbsf. In the uppermost samples methane values are also slightly higher than below.

The carbon isotope ratios of the combined gases from Holes 994C, 995A, 997A, and 997B are plotted vs. depth in Figure 7. The data ranges between -67 and -38, -36 and -23, and -34 and -20 for methane, ethane, and propane, respectively. The isotope data of methane indicates a microbial origin of samples with ¹³C₁ <-55 and a thermal signature for those samples with ¹³C₁ >45. Mixing of methane from different sources or microbial methane oxidation cannot be excluded. Both processes shift the ¹³C₁ pool towards more positive values. No significant trends in the isotope data of methane are observed with depth, which could be related to the depth range of the hydrate stability zone. The more positive levels in the upper samples can be explained by microbial oxidation of the free methane that was found in high concentrations above the hydrate stability zone (Paull, Matsumoto, Wallace, et al., 1996). The small trend of the ethane and propane isotope data towards lighter values from bottom to top is difficult to explain genetically. However, because the samples contained large amounts of microbial methane, especially in the upper part of the holes just below the sulfate-reduction zone (Paull, Matsumoto, Wallace, et al., 1996), small amounts of microbial ethane and propane (with ¹³C₁ values more negative than -50) may also have been formed (Paull et al., Chap. 7, this volume) during methanogenesis. Small amounts of these gases may still be present in the upper samples after ethane- and propane-oxidation and/or after the desorption from the samples ceased. Therefore, the isotope data of ethane and propane may represent a mixture of thermal and, especially in the upper samples, microbial gases.