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doi:10.2204/iodp.proc.307.205.2009

Results

Site U1316, downslope site

TOC content in the sediments of Site U1316 varies between 0.24% and 0.97% (Fig. F1A; Table T1) and seems to be more variable above (Hole U1316B) the coral intersection (flank of Challenger Mound, lithostratigraphic Unit 2) than below (Hole U1316C). The δ¹³C signal of the TOC above the coral intersection has values between–24.9‰ and –26.0‰, on average ~2.1‰ lighter than values below the coral layer, which vary between –22.3‰ and –23.8‰ (Fig. F1A; Table T1). This shift in the bulk carbon isotope data of the organic matter coincides with the n-alkane distribution in Figure F1B, indicating a higher supply of long-chain cuticular wax n-alkanes from higher land plants (Eglinton and Hamilton, 1967) in the sediments above the coral intersection (Hole U1316B).

Phospholipids were not detected in the sediments of Site U1316. In contrast, a series of hopanoids (17β(H),21β(H)-hopanes, 17α(H),21β(H)-hopanes, and hopenes) were detected in all samples investigated (Fig. F2; Table T1). With one exception (Sample 307-U1316C-8R-2, 28–40 cm) showing a relatively high hopanoid content, the hopanoid distribution related to the TOC content is relatively similar, ranging ~43.1 µg/g TOC. The conformity in the distributions of the hopanoids with the β,β-configuration, being characteristic for immature microbial biomass, and those with the α,β-configuration indicates that the α,β-hopanes are also from an immature microbial source rather than representing oil-derived mature hydrocarbons (Fig. F2B; F2C). This is corroborated by the lack of the typical α,β-hopane distribution with S- and R-epimer pairs in the carbon-number range between C₃₁ and C₃₅, as usually observed in oils. Thus, oil-derived hydrocarbons were not detected in sediments of Site U1316.

Site U1317, mound site

Although there are some samples with a relatively high TOC content in the sediments of the carbonate mound, the TOC content ~0.46%, in general, is higher in the Miocene sediments below the mound base (Fig. F3A; Table T2). The δ¹³C data of the total organic matter in the samples from below the mound base range from –23.6‰ to –22.4‰ (Fig. F3A; Table T2) and are comparable to those of the sediments below the coral intersection at Site U1316. With exception of two samples (307-U1317A-2H-3, 58–70 cm, and 12H-3, 68–80 cm) the bulk carbon isotope signal shows a gradual increase from the mound base to the top to a value of –20.6‰. This might resemble a slightly increasing proportion of marine organic matter in the mound section.

Related to the sediment extracted, a higher content of terrestrial organic matter is indicated below the mound base by the amount of long-chain n-alkanes (Fig. F3B) being somewhat higher than in the mound section. In general, the n-alkane concentrations from below the mound base correspond to the n-alkane concentrations below the coral intersection at Site U1316. However, because of the higher TOC contents below the mound section, the higher amounts of terrestrial organic matter are not recognizable if related to the TOC contents. Thus, there seems to be not much difference in the n-alkane proportions related to TOC between the mound section and the sediments below, which might explain the small difference in the carbon isotope signal of the organic matter above and below the carbonate mound base. The exceptionally low δ¹³C signal of Sample 307-U1317A-12H-3, 68–80 cm (–26.7‰), from within the mound section, might be explained by a higher proportion of terrestrial organic matter, as indicated by a very high n-alkane concentration (also related to the TOC content) in this sample (Fig. F3B; Table T2).

Traces of phospholipids, just above the detection limit of the applied method, were detected in only one sample ~10 m below the mound base (Sample 307-U1317D-6R-3, 5–17 cm). Surprisingly, microbial biomarker proportions such as the hopanoids (Fig. F4; Table T2) are distinctly lower in the mound section than below the mound base. The lower hopanoid concentrations in the mound section related to the amount of sediment extracted does not derive from a dilution effect by the coral carbonate in the mound sequence, because almost the same distribution can be observed when the sediments are corrected for carbonate contents (carbonate-free basis). Again, there is no indication for migrated oil-derived organic compounds.

Headspace gas analysis reveals low methane concentrations above the mound base (Fig. F5A; Table T3). However, below the mound base (130.1 meters below seafloor [mbsf] in Hole U1317A and 146.1 mbsf in Hole U1317D) the methane concentrations increase with increasing depth (Table T3). The same can be observed for the ethane and propane concentrations. The gas wetness data [C₁/(C₂ + C₃)] plot in the mixed gas zone between thermogenic and biogenic gas with a higher tendency to a biogenic gas source. The same trend can be observed when considering the carbon isotope data of the indigenous methane showing values around –64.3‰ (Fig. F5B; Table T3).

Site U1318, upslope site

The highest TOC values (average = 1.06%) are observed in the sediments of the lithostratigraphic Subunits 1B and 1C (Fig. F6A; Table T4). Above and below these lithologic units the TOC values range between 0.25% and 0.36%. TOC values of sediments from the lower part of this site (Hole U1318B) show slightly higher values ranging from 0.50% to 0.70%. The δ¹³C data of the sediments below Subunits 1B and 1C are characterized by values of –23.2‰ to –22.3‰ (Fig. F6A; Table T4). In Subunits 1B and 1C there is a clear shift (average = 2.6‰) to lighter carbon isotope data ranging from –25.8‰ to –24.8‰. Above these lithologic units the values are again slightly heavier (–24.4‰).

A higher supply of terrestrial organic matter during sedimentation of Subunits 1B and 1C is indicated by higher proportions of long-chain n-alkanes (Fig. F6B; Table T4), which coincides with the lighter δ¹³C data for this section. However, the high concentrations of n-alkanes in the uppermost samples are not recognizable in the carbon isotope data.

Hopanoid biomarkers are highest in Subunits 1B and 1C and in the sample nearest to the seafloor (Fig. F7; Table T4). Again, there is no indication of oil-derived hydrocarbons. In contrast to the other sites, in the samples of Hole U1318A microbial phospholipids were detected in significant amounts (phosphatidylglycerol and phosphatidylethanolamine esters) (Fig. F8; Table T5). In general, there is a decrease of the phospholipid concentrations from the top to the bottom of Hole U1318A. However, there is a small increase of these microbial life markers in the section of Subunits 1B and 1C, which corresponds to the higher amount of hopanoid biomarkers and higher TOC contents. In Hole U1318B, phospholipids were not detected.

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