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doi:10.2204/iodp.proc.336.106.2012

Organic geochemistry

Determination of the concentrations of organic carbon compounds in the subsurface is important to understand the extent of biological processes within the biome. Such measurements address questions central to the goals of this expedition. Organic compounds provide both an electron donor source for heterotrophic organisms and an indication of potential metabolic activity. The main focus of study for organic geochemistry was within sediment core samples acquired from Holes U1382B, U1383D, U1383E, and U1384A. Shipboard analysis was limited to volatile hydrocarbons, with total organic carbon concentrations and acetate concentrations measured postexpedition.

Volatile hydrocarbons

Sample collection and processing

Sediment samples for volatile hydrocarbon analysis were collected (up to three samples per core). A volume of 2 cm³ of sediment was collected from the center of the core using a cut-end syringe and transferred to a 20 mL glass serum vial with a crimp cap for volatile hydrocarbon concentration analysis on board, in accordance with standard shipboard protocol (i.e., heating at 60°C for 30 min; see “Organic geochemistry” in the “Methods” chapter [Expedition 336 Scientists, 2012a]). Samples were then analyzed on a gas chromatograph to determine concentrations of C₁–C₃ compounds.

Volatile hydrocarbon concentrations were determined in a total of 25 (Hole U1382B), 12 (Hole U1383D), 15 (Hole U1383E), and 28 (Hole U1384A) headspace samples. All measured concentrations are considered to be at the detection limit of the analysis.

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