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Organic geochemistry

Hydrocarbon gas

At Site C0012, methane and ethane are either below detection or present at low concentrations. No heavier hydrocarbon gases (C3 and C4) were found (Table T12; Fig. F49). Hydrocarbon gases are absent in the upper 177 mbsf (Holes C0012C and C0012D), with one exception at 174.1 mbsf, where methane appears at 2.5 parts per million by volume (ppmv). In the sediments between 501.4 and 520.4 mbsf (Hole C0012E), methane occurs at low concentrations, varying from 23.3 to 119.7 ppmv, and ethane was only sporadically detected, ranging between 0 and 1.9 ppmv. In the two horizons where both methane and ethane were detected, C1/C2 ratios are <100, possibly indicating the organic matter is mature and the hydrocarbon gases are thermogenic.

Sediment carbon, nitrogen, and sulfur composition

Calcium carbonate (CaCO3) concentration (Table T13; Fig. F50) ranges between 0.21 and 45.06 wt% with an average of 6.08 wt%. The variations of CaCO3 content defines the following five stages:

  1. Upper ~11 mbsf: the values are relatively high, varying between 6.58 and 16.45 wt%.

  2. Approximately 11–52 mbsf: the concentration remains low and displays little variation, ranging between 0.29 and 4.52 wt%.

  3. Approximately 61–137 mbsf: CaCO3 content is scattered, fluctuating in a wide range of 0.55–18.20 wt%, but exhibits an overall positive excursion.

  4. Approximately 137–180 mbsf: the amounts of CaCO3 are low and uniform (mostly <2.5 wt%) in most cores and show several elevated values at the bottom of the Hole C0012D.

  5. Approximately 500–525 mbsf: high CaCO3 concentration varies between 7.02 and 45.06 wt%.

Notably, the Stage 1/2 boundary is roughly consistent with the top of a slump (see “Structural geology”), where the stratigraphic record also presents a hiatus (see “Paleomagnetism” and “Biostratigraphy”). The high CaCO3 values in Stages 1 and 3 are consistent with the abundance of nannofossils observed by smear slides in the sediments in the upper ~8 mbsf, between ~68 and ~79 mbsf, and at ~108 mbsf (see Site C0012 smear slides in “Core descriptions”).

Total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) concentrations are low at Site C0012, ranging between 0.03 and 0.52 wt%, 0.02 and 0.07 wt%, and 0 and 0.49 wt%, respectively (Table T13; Fig. F50). The atomic ratios of TOC to TN (TOC/TNat) fall in the range of 1.3–10.6, suggesting a marine origin of the organic matter. The variations of these four parameters share an overall similar pattern, including scattered but progressively increasing values in the upper ~52 mbsf, relatively uniform values between ~52 and ~180 mbsf, and relatively low values between ~500 and 525 mbsf. A distinct increase in TS content is also observed at ~10 mbsf, which corresponds to the time gap observed in the paleomagnetic analyses (see “Paleomagnetism”).

Rock-Eval pyrolysis

We selected 23 samples for Rock-Eval analyses (Table T14; Fig. F51). The amounts of hydrocarbon already present in the samples (S1) and hydrocarbon generated by pyrolytic degradation (S2) are low, with S1 ranging between 0 and 0.02 mg hydrocarbon/g sediment (mg HC/g sediment) and S2 between 0.04 and 0.35 mg HC/g sediment. Samples in the upper ~10 mbsf and below ~80 mbsf have Tmax values <430°C, suggesting the organic matter is thermally immature. High Tmax values (604°–607°C) of the sediments between ~20 and ~80 mbsf are indicative of thermally overmature organic matter, perhaps suggesting the organic matter at this depth interval was transported from a region where thermal gradients have been sufficiently high to mature the organic matter. The low production index values in all the cores (ranging from 0.01 to 0.18) reflect either immaturity of the organic matter for the sediments in the upper 10 mbsf and below 80 mbsf or overmaturity of the organic matter for the sediments between ~20 and 80 mbsf. Hydrogen index values vary between 15 and 86 mg HC/g TOC and hence fall in the range for terrigenous organic matter (Tissot and Welte, 1984). This source characterization is inconsistent with the low TOC/TNat ratios, which suggests that organic matter is mainly derived from marine sources. It is important to note that TOC concentration at Site C0012 is <0.5 wt%, which could affect the reliability of the Rock-Eval parameters (Espitalié et al., 1984). The interpretations herein about the thermal maturity and sources of the organic matter remain to be verified by further shore-based analyses.