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doi:10.2204/iodp.proc.333.102.2012

Organic geochemistry

The shipboard organic geochemistry program during Expedition 333 includes analyses of volatile hydrocarbon contents (C1–C4), elemental analyses of total carbon, nitrogen, and sulfur, as well as measurement of inorganic carbon content in sediments. The type and maturity of organic matter was characterized using Rock-Eval pyrolysis.

Hydrocarbon gas

For safety monitoring, concentrations and distributions of light hydrocarbon gases (C1–C4) were measured for each core following standard headspace sampling protocols. A 5 cm³ sediment sample was collected with a cut-off plastic syringe from the freshly exposed end of the first section that was cut open in each core. When the sediments were too lithified, a cork borer was used for sampling. The sample was extruded into a 20 mL glass vial, immediately sealed with a PTFE coated septum, and then placed in a headspace sampler (Agilent Technologies G1888 network headspace sampler). The sample was heated at 70°C for 30 min before headspace gas was automatically injected into an Agilent 6890N gas chromatograph (GC) equipped with a packed column (GL HayeSep R) and flame ionization detector (FID). The carrier gas was helium at a constant flow rate of 32.8 mL/min. In the temperature program of the GC, the initial temperature of 100°C was held for 5.5 min before the temperature was ramped up at a rate of 5°C/min to 140°C and held isothermal for 4 min. Chromatographic response of the GC was calibrated against five different standards with variable quantities of low molecular weight hydrocarbons and checked on a daily basis.

Total carbon, nitrogen, and sulfur contents

Sediments for bulk geochemistry analyses were mostly taken from cluster samples, though some samples were collected from working cores according to the requests of shipboard sedimentologists and organic geochemists. Sediments were freeze-dried and then ground using a planetary mill (Fristch). Total carbon (TC), total nitrogen (TN), and total sulfur (TS) contents were determined using a Thermo Finnigan Flash EA 1112 elemental analyzer. Calibration was based on the synthetic standard sulfanilamide, which contains 41.81 wt% C, 16.27 wt% N, and 18.62 wt% S.

About 40 mg of sediment was weighed and placed in a tin container for carbon and nitrogen analyses. For sulfur analysis, approximately 20 mg of sediment was weighed and put in a tin container with the V₂O₅ catalyst that was slightly >20 mg. Sediment samples were combusted in a stream of oxygen at 900°C for TC and TN and at 1000°C for TS. Nitrogen oxides were then reduced to N₂, and the mixture of CO₂, N₂, and SO₂ was separated by gas chromatography and detected by a thermal conductivity detector (TCD). The standard deviation of TC, TN, and TS concentrations determined from triplicate measurements of selected samples was within 0.01 wt%. Accuracy was confirmed using soil NCS reference material and a GSJ reference sample, with the measured value within 0.05, 0.04, and 0.1 wt% of the certified values for TC, TN, and TS, respectively.

Inorganic and organic carbon contents

In the same set of samples that was used for elemental analyses, inorganic carbon concentration was determined using a Coulometrics 5012 CO₂ coulometer. About 15–25 mg of freeze-dried ground sediment was weighed and reacted with 2M HCl. The amounts of liberated CO₂ were determined by trapping the CO₂ with ethanolamine and titrating coulometrically the hydroxyethylcarbamic acid that is formed. The weight percentage of calcium carbonate was calculated from the inorganic carbon content, assuming that all the measured CO₂ was derived from dissolution of calcium carbonate, by the following equation:

No correction was made for the presence of other carbonate minerals. National Institute of Standards and Technology–Standard Reference Material 88b was used to confirm accuracy, which was ±0.2 wt% from the certified value of inorganic carbon content (12.65 wt%). The standard deviation of inorganic carbon measurements was <0.02 wt%, based on triplicate analyses of selected samples. Total organic carbon (TOC) contents were calculated by subtracting inorganic carbon from the TC contents determined in elemental analyses.

Rock-Eval pyrolysis

Rock-Eval pyrolysis was used to characterize the type and maturity of the sedimentary organic matter. In principle, Rock-Eval pyrolysis consists of sequentially heating a sample in an inert atmosphere (nitrogen gas) within a pyrolysis oven. It allows us to quantitatively and selectively determine both the quantity of free hydrocarbons present in samples (S1) and the amounts of hydrocarbons (S2) and oxygen-containing compounds (S3) that are volatilized during thermal cracking of unextractable organic matter (kerogen).

During Expedition 333, Rock-Eval pyrolysis was performed using a Rock-Eval 6 analyzer (Vinci Technologies). S3 was not measured because the infrared cell was not functioning properly. S1, S2, and T_max were measured; hydrogen index (HI) and production index (PI) were calculated. S1 and S2 are expressed in milligrams of hydrocarbons per gram of sediment. S2 is an indicator of the quantity of hydrocarbons that the sediment can potentially produce should burial and maturation continue. T_max is the temperature at which the maximum release of hydrocarbons occurs from cracking of kerogen during pyrolysis. T_max is an indicator of organic matter maturity. HI (HI = [100 × S2]/TOC; in milligrams of hydrocarbon per gram of TOC) was calculated using TOC values yielded from the elemental analyzer. HI correlates with the ratio of H to C, which is higher for lipid- and protein-rich organic matter derived from marine algae than that from terrestrial plants. PI (PI = S1/[S1 + S2]) can indicate the evolution level of organic matter.

Rock-Eval analysis was conducted on samples that were selected according to their TOC and hydrocarbon gas contents. About 60–80 mg of sediment samples were obtained from the same freeze-dried and homogenized bulk samples that had been used for elemental analyses. The pyrolysis was kept at 300°C for 3 min, and then volatilized free hydrocarbons were measured by FID as the S1 peak. Subsequently, the temperature was raised from 300° to 550°C at 25°C/min. The hydrocarbons released from this thermal cracking were detected as the S2 peak by FID. The temperature of maximum detection of S2 was recorded as T_max. The values of all parameters were calibrated against IFPI 60000 reference materials (Vinci Technologies).

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