ORGANIC GEOCHEMISTRY

Shipboard organic geochemical studies of cores from Hole 1166A included monitoring of hydrocarbon gases, carbonate and organic carbon, total sulfur and total nitrogen content, and Rock-Eval pyrolysis characterization of organic matter. Procedures are summarized in "Organic Geochemistry" in the "Explanatory Notes" chapter.

Cores recovered from Hole 1166A were monitored by the headspace method for hydrocarbon gases. All reliable analyses of cores were at background levels (4-10 parts per million by volume [ppmv]) for methane, and no other hydrocarbons were detected.

Thirty-seven sediment samples were analyzed for calcium carbonate (inorganic carbon; IC), and 14 selected (darker colored) samples from Hole 1166A were analyzed for total carbon, OC (by difference), total nitrogen (TN), and total sulfur (TS). The results are reported in Table T5. IC and OC contents are plotted against depth of burial in Figure F31. Carbonate content is generally low (0.01-0.15 wt% IC), with scattered beds having between 0.2 and 0.5 wt% IC and one sample with 1 wt% IC.

OC content varies according to lithostratigraphic unit sampled. The upper diamicton (Unit IB) contains 0.4-1.4 wt% OC. Samples from the massive sand (lithostratigraphic Unit III) contain 0.2-0.5 wt% OC, except for a carbonaceous bed near the base of Unit III, which has 9.2 wt% OC. The carbonaceous claystone (lithostratigraphic Unit IV), from 280 to 315 mbsf, contains 1.5-5.2 wt% OC.

TN content is generally between 0.01 and 0.27 wt%, and carbon/nitrogen (C/N) values range from 7 to 50, except for the sample with 9.2 wt% OC, which has a C/N ratio of 93. Most of the samples are enriched in carbon relative to nitrogen, which suggests input of terrestrial organic matter, especially for samples containing >1% OC.

TS content is similarly scattered, with many analyzed samples showing sulfur below detection limits. When TS is related to OC, most samples are sulfur deficient compared with average marine sediments. The one sample with more sulfur (5.9 wt%) than carbon (4.5 wt%) was from the core catcher, and contamination is suspected.

Ten samples from Hole 1166A were characterized by Rock-Eval pyrolysis (Table T6). Samples with >0.5 wt% OC were selected for analysis. Pyrolyzable hydrocarbons (S₂ yields) range from 0.13 to 4.3 mg hydrocarbon/g of sediment. The pyrolyzable fraction of the OC is low (hydrogen index values 48 mg hydrocarbon/g of carbon), which is consistent with degraded plant material as a possible source of organic matter in the more carbonaceous (>2% OC) samples. The T_max values and the broad S₂ peak shapes in the pyrograms (not shown) indicate that some samples contain mixtures of small amounts of primary organic matter with variable amounts of possibly recycled and degraded thermally mature organic matter. The samples from lithostratigraphic Unit I at 75.4, 124.2, and 130.2 mbsf have lower OC content (0.56-1.39 wt%) and higher T_max values (462°, 477°, and 438°C, respectively), indicating greater proportions of recycled organic matter. The samples from lithostratigraphic Unit IV and the base of Unit III, with OC contents of 2.8 to 9.2 wt%, have T_max values of 420°-425°C. These preglacial shelf sediments appear to contain mostly primary (first cycle) organic matter, whereas younger glacial tills (Unit I) contain ~0.5% recycled higher maturity organic matter, with lesser but variable amounts of primary organic matter. The presence of recycled organic material within Unit I is suggested by Rock-Eval T_max values that approach 480°C as organic carbon decreases toward 0.5 wt%.