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doi:10.2204/iodp.proc.318.109.2011

Geochemistry

Organic geochemistry

Gas analysis was performed typically once per core from Hole U1361A using the methods described in “Geochemistry and microbiology” in the “Methods” chapter.

Methane concentrations vary by four orders of magnitude downhole, from 2 to 15,363 ppmv (Fig. F14A). Concentrations are 2–3 ppmv in the uppermost 163 mbsf and increase to 7,248 ppmv at 240 mbsf. A maximum value of 15,363 ppmv is reached at 355 mbsf. Methane levels from 364 to 381 mbsf are lower, with an average concentration of ~8,500 ppmv. Ethane was detected below 216 mbsf, and C₁/C₂ levels are between 4,455 and 8,816 (Fig. F14B).

Inorganic geochemistry

Forty samples from Hole U1361A were taken for analyses of weight percent carbonate, carbon, nitrogen, and sulfur content, as well as major and trace element analyses. Samples were collected in close collaboration with the sedimentology group to sample the main lithologies represented. Because of technical problems with the ICP-AES, no major and trace element analyses could be obtained.

CaCO₃ (in weight percent) was determined on all sediment samples by coulometric methods (see “Geochemistry and microbiology” in the “Methods” chapter). CaCO₃ contents for most samples are well below detection limit (<1 wt%) (Fig. F15). Between 313.96 and 342.04 mbsf, however, carbonate contents increase to 12.1–24.8 wt%. This is in agreement with the description of nannofossil-bearing clays constituting one of three major facies below 313.2 mbsf (lithostratigraphic Subunit IIb; see “Lithostratigraphy”).

Carbon, nitrogen, and sulfur contents were measured on 15 selected samples covering the full range of CaCO₃ contents (0–24.8 wt%). All concentration levels are very low (i.e., C < 0.5 wt%, N < 0.03 wt%, and S < 0.02 wt%), except for the four samples with high calcium carbonate contents (Fig. F16). Taken together with the CaCO₃ measurements, these samples yield total organic carbon concentrations below 0.3 wt%, which is within the error or the respective measurements.

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