The shipboard geochemistry program for Leg 210 included (1) real-time monitoring of volatile hydrocarbons in cores as required by ODP safety regulations; (2) measurement of sediment carbonate content; (3) elemental analysis of total carbon (TC), nitrogen, and hydrogen contents of sediment; (4) characterization of bulk organic matter by Rock-Eval pyrolysis; and (5) elemental analysis of major and trace elements by ICP-AES.
Laboratory procedures and instruments employed during Leg 210 are described in Pimmel and Claypool (2001) and Murray et al. (2000). The procedures and instruments are primarily those used during the most recent ODP legs.
Compositions and concentrations of gases in sediment were monitored for safety and pollution prevention with an average sample interval of two per core. Headspace (HS) gas samples were collected immediately after core retrieval. On deck, 5-cm3 sediment samples were obtained using a borer tool, placed in a glass serum vial, and sealed with a septum and metal crimp cap. For consolidated or lithified sediment, chips of material were sampled. Each vial was then heated at 60°C for 30 min, and a 5-cm3 subsample of the HS gas was extracted using a glass syringe.
Gas obtained by HS sampling was analyzed using a Hewlett-Packard (HP) 6890 gas chromatograph (GC) equipped with a flame ionization detector (FID). Chromatographic responses were calibrated using Scotty IV gas mixtures, and the results were reported as component parts per million by volume (ppmv) relative to the analyzed gas. This system determines concentrations of C1–C3 (methane, ethane, ethene, propane, and propene) hydrocarbons.
Gas samples were also analyzed with a natural gas analyzer (NGA) when high concentrations of C2+ hydrocarbons or nonhydrocarbon gases such as H2S or CO2 were anticipated. The NGA system consists of an HP 6890 GC equipped with both FID and thermal conductivity detectors. Detectable gases are N2, O2, CO2, H2S, and hydrocarbons in the range of C1–C6.
To the extent that sampling procedures are uniform, the differences in the HS results reflect differences in the amounts of gas remaining in the cores.
Inorganic carbon (IC) and carbonate contents were determined by a titration method. Amounts of CO2 evolved from sediment samples were measured by a spectrophotometer through a colorimetric reaction, using a Coulometrics 5011 CO2 coulometer equipped with a System 140 carbonate analyzer. Aliquots of 10 mg of freeze-dried, ground sediment were reacted with 2-N HCl to liberate CO2. Carbonate content (in weight percent) was then calculated from the IC content using the following equation:
This method assumes that the CO2 evolved from the sediment was derived solely from dissolution of calcium carbonate. No corrections were made to account for the possible presence of other carbonate minerals.
TC, nitrogen, and hydrogen were determined using a Carlo Erba NA 1500 CHNS analyzer. The analytical procedure employs a 10-mg subsample of freeze-dried, ground sediment. The amount of total organic carbon (TOC) was calculated as the difference between TC and IC (determined by coulometry on the same sample),
The type of organic matter was characterized by programmed pyrolysis using a Delsi Nermag Rock-Eval II system. Aliquots of 100 mg of freeze-dried, ground sediment are employed in this technique. The amount of volatile hydrocarbons released by heating organic matter at 300°C for 3 min is S1. The quantity of hydrocarbons subsequently released by thermal cracking of kerogen as the temperature is increased to 550°C at a rate of 25°C/min is S2. The nominal temperature at which the kerogen yields the maximum amount of hydrocarbons (top of the S2 peak) is Tmax, which is used to assess the thermal maturity of the organic matter. The amount of CO2 released from thermal degradation of organic matter (between 300° and 390°C) is S3. S1, S2, and S3 are reported in milligrams per gram of dry sediment.
Rock-Eval analysis provides the following calculated parameters: S2/S3 ratio and hydrogen, oxygen, and production indexes (HI, OI, and PI, respectively). These parameters are calculated using the following equations:
Samples with <0.5 wt% TOC may not give reliable results because of the small size of the S1, S2, and S3 signals.
Mainly fine grained sediments were routinely analyzed for elemental composition using ICP-AES. The main objective was to highlight downhole trends in bulk geochemistry that could help indicate changes in sediment provenance, diagenesis, and organic productivity. Additional samples were taken as appropriate to highlight finer-scale variations and to help with lithostratigraphic identifications. Samples from Cretaceous black shales were analyzed to help characterize paleoceanographic conditions during oceanic anoxic events.
Elemental composition of bulk sediment was determined using a Jobin-Yvon 2000 Ultrace ICP-AES using a type-C Meinhard concentric nebulizer. Our analytical approach followed the general procedure outlined by Murray et al. (2000) and the constraints indicated by Quintin et al. (2002). Analytical blanks were prepared using 400 mg of lithium metaborate (LiBO2) flux to ensure matrix matching. Samples analyzed by ICP-AES were ignited before dissolution by heating 1 g of freeze-dried, ground sediment at 900°C for 4 hr to determine weight loss on ignition (LOI), to release volatile phases (H2O, CO2, and S), and to fully oxidize all iron to ferric iron.
Aliquots of 100 mg of ignited sediment and standards were mixed with 400 mg of LiBO2 flux. Subsequently, 10 µL of a wetting agent, 0.172-mM lithium bromide (LiBr), was added to the samples, standards, and blanks. This mixture was fused at 1050°C for 3 min in a NT-2100 Bead Sampler prior to dissolution in 50 mL of 10% HNO3. For complete dissolution, 1 hr of shaking with a Burrell wrist-action shaker was required. Aliquots of 5 mL of the resulting solutions were filtered (0.45 µm) and diluted with 35 mL of 10% HNO3, resulting in a 4000x dilution of the original sediment.
A range of standards was initially selected to cover the entire range of expected sediment compositions, and their suitability was monitored during the leg. These standards were as follows: Cody shale (SCO-1), pahoehoe basalt lava flow (BHVO-2), marine mud (MAG-1), Green River shale (SGR-1), Glass Mountain rhyolite (RGM), dolomitic limestone (NBS-88), and argillaceous limestone (NBS-1c). The Cody shale (SCO-1) was also selected as both the drift and the consistency standard.
We decided to analyze a range of major and trace elements that would be useful for interpretation. The major elements are Si, Al, Fe, MgO, CaO, Na, K, Ti, Mn, and P, and the trace elements are Ba, V, La, Ce, Nd, Cr, Cd, Ni, Cu, Zn, Pb, Rb, Sr, Y, Zr, Nb, and Mo. Major elements were expressed as weight percent oxide and trace elements as parts per million. LOI values were determined routinely. Samples were analyzed in duplicate. There was insufficient time at the end of the cruise for the analysis of some chemical constituents, and thus some values are not included in the database.
Selected representative samples were cut with a diamond-impregnated saw blade and were wet ground on a diamond abrasive wheel to remove surface contamination. Samples were washed in an ultrasonic bath with methanol for 10 min, followed by three consecutive 10-min washes in an ultrasonic bath containing nanopure deionized water. Samples were then dried for 12 hr in an oven at 110°C. They were subsequently reduced to fragments <1 cm in diameter by crushing between two disks of Delrin plastic in a hydraulic press, followed by grinding to a fine powder for 5 min in a Spex 8510 shatterbox with a tungsten carbide barrel. Approximately 1 g of sample powder was ignited at 1025°C for 4 hr.
Sample dissolution and analyses followed procedures described for sediment samples (see "Sediments and Sedimentary Rocks").
Interstitial water analyses were performed on 10-cm whole-round sections cut on deck immediately after core retrieval. After extrusion from the core liner, the surface of each whole-round sample was scraped with a spatula to remove potential contamination. Interstitial waters were collected using a titanium squeezer, modified after Manheim and Sayles (1974). Pressure up to 45,000 lb was applied using a hydraulic press. Pore waters were double filtered through prewashed 0.45-µm Acrodisk filters fitted above a titanium screen and subsequently extruded through Whatman number 1 filters into a plastic syringe attached to the bottom of the squeezer assembly.
Interstitial water samples were analyzed for salinity as total dissolved solids with a Goldberg optical handheld refractometer. Sulfate (SO42–) was analyzed by nephelometry using a spectrophotometer (Gieskes et al., 1991). Results are reported in molar units. International Association of Physical Sciences of the Oceans seawater was used as standard.