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doi:10.2204/iodp.proc.344.203.2016

Results

Headspace gas

The HS analysis results show that methane is a major component and ethane a minor component at all sites (see the “Expedition 344 summary” chapter [Harris et al., 2013a]) (Table T1; Fig. F1). Methane concentration is relatively constant with depth at Sites U1380, U1412, and U1413. Ethane concentration does not show vertical variation with depth at Site U1380, whereas it increases with depth at Sites U1412 and U1413. In general, C₁/C₂₊ ratios are >1000 in biogenic gases and <300 in thermogenic gases (Claypool and Kvenvolden, 1983; Whiticar et al., 1986; Whiticar, 1999; Milkov et al., 2005; Pape et al., 2010; Kim et al., 2011, 2012). In this study, the C₁/C₂₊ ratio is lowest (<300) in lithostratigraphic Unit I at Site U1380 and Unit III at Site U1413.

The δ¹³C_CH4 and δD_CH4 values of biogenic methane typically range from –110‰ to –50‰ and –400‰ to –100‰, respectively, whereas those of thermogenic methane range from –50‰ to –20‰ and –275‰ to –50‰, respectively (Whiticar, 1999). In this study, δ¹³C_CH4 and δD_CH4 values range from –64.5‰ to –57.7‰ and –210‰ to –187‰, respectively, at Site U1380; from –81.3‰ to –57.0‰ and –194‰ to –185‰, respectively, at Site U1412; and from –71.7‰ to –58.2‰ and –209‰ to –181‰, respectively, at Site U1413. The lowest values of δ¹³C_CH4 (approximately –80‰) are found in the upper part of Site U1412 (Fig. F1; Table T1). This interval contains a sulfate methane transition zone (see the “Expedition 344 summary” chapter [Harris et al., 2013a]). The δ¹³C_CH4 values are constant with depth except in the upper part at Site U1412. Crossplots of δ¹³C_CH4 versus C₁/C₂₊ and δ¹³C_CH4 versus δD_CH4 illustrate that most analyzed data are located in the field of biogenic methane generated by CO₂ reduction (Fig. F2).

Void gas

The VG analysis results also show that methane is a major component and ethane is a minor component at all sites (see the “Expedition 344 summary” chapter [Harris et al., 2013a]) (Table T2; Fig. F3). Methane concentration is relatively constant with depth at Sites U1412 and U1413. Ethane concentrations increase with depth at Sites U1412 and U1413. Ethane concentrations at Site U1412 (average = 41 ppmv) are greater than those at Site U1413 (average = 6 ppmv).

The δ¹³C_CH4 and δD_CH4 values vary from –74.9‰ to –64.3‰ and –193‰ to –183‰, respectively, at Site U1412 and from –73.0‰ to –71.3‰ and –187‰ to –183‰, respectively, at Site U1413. The δ¹³C_CO2 values range from –10.8‰ to 1.4‰ at Site U1412 and from –3.8‰ to 0.5‰ at Site U1413, indicating a decrease with depth (Fig. F3; Table T2). Crossplots of δ¹³C_CH4 versus C₁/C₂₊, δ¹³C_CH4 versus δD_CH4, and δ¹³C_CH4 versus δ¹³C_CO2 indicate that the gas is generated by CO₂ reduction (Fig. F4).

Conversion ratio

We calculate how much organic matter should convert to hydrocarbon gas in situ using the Rayleigh ratio based on the carbon stable isotopic data for organic matter and VG (modified from Pohlman et al., 2009) (Table T2; Fig. F3). Carbon stable isotopes of organic matter are between –25.0‰ and –23.5‰ at Site U1412 and between –26.0‰ and –23.6‰ at Site U1413 (see the “Expedition 344 summary” chapter [Harris et al., 2013a]). Estimated conversion ratios vary between 21.8% and 30.7% at Site U1412 and between 26.4% and 30.2% at Site U1413, slightly decreasing with depth (Fig. F3).

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