GEOCHEMISTRY

Headspace gas analysis was conducted as part of the standard protocol required for shipboard safety and pollution prevention monitoring. On average, two samples per core were analyzed (Table T15). Methane (C₁) concentrations remain at low background levels (7 ppmv; average = 3 ppmv) to a depth of 1140 mbsf in lithologic Subunit 5B (Fig. F152). From 1140 to 1610.58 mbsf, just above the upper diabase sill (Subunit 5C1), C₁ concentrations generally increase downhole with a range of values between 1 and 1353 ppmv. Small amounts (1 ppmv) of ethane (C₂) were detected in only five samples above 1176 mbsf (Table T15; Fig. F152). Below this level, down to 1610.58 mbsf, ethane was detected in almost every sample, and its concentrations increase downhole (up to 74 ppmv).

The C₁ concentrations increase dramatically to 5854 ppmv in the sediments just below Subunit 5C1 (diabase sill) in Core 210-1276A-88R (1624.01 mbsf), and high concentrations persist in deeper cores (Table T15; Fig. F152). Layers of anomalously unconsolidated high-porosity clay-rich sediments are present in Cores 210-1276A-96R and 97R (1692.33-1703.5 mbsf) (see "Undercompacted Systems: High-Porosity and Low-Velocity Mudstones" in "Density and Porosity" in "Physical Properties"). In these layers, C₁ levels rise to 18,669 ppmv. At 1713.01 mbsf (Core 210-1276A-98R), C₁ concentrations drop to 556 ppmv in strongly hydrothermally altered sediments. Between 1624.01 and 1703.5 mbsf, the high methane concentrations may be caused by the sill acting as a seal, preventing gas and interstitial water from escaping. In this same interval, C₂ concentrations (95-7501 ppmv) increase with depth, following the C₁ profile (Fig. F152).

Propane (C₃) (up to 1417 ppmv) and traces of branched and straight-chain C₄ (up to 102 ppmv) and C₅ (up to 6 ppmv) components were commonly detected below 1470 mbsf and always detected below the upper sill (>1624.01 mbsf). C₆ was detected in only two samples (210-1276A-94R-4, 0-1 cm, and 97R-3, 0-1 cm), both below the upper sill (Table T15).

C₁/C₂ ratios are useful to distinguish between biogenic and thermogenic gaseous hydrocarbons. Biogenic gases generally have ratios >1000, whereas ratios <200 may indicate gas generation related to increasing depth and temperature (Claypool and Kvenvolden, 1983; Stein et al., 1995; Whiticar, 1999). C₁/C₂ ratios in lithologic Subunits 5B and below show a trend of decreasing values with depth (Fig. F152), from values of ~100 at 1200 mbsf to ~10 at 1610.58 mbsf (Table T15). Below the upper sill (1624.01 mbsf), C₁/C₂ ratios stabilize at low values, ranging between 2 and 6. Overall, low C₁/C₂ ratios and the presence of longer-chain volatile hydrocarbons indicate that along the cored sequence, thermogenically derived gas is present, either generated in situ or migrating from a source. A weak correspondence exists between higher organic carbon percentages and higher methane concentrations between 1140 and 1684 mbsf (Fig. F153). This suggests that the amount of organic matter present may play a role in the generation of methane.

Carbonate determinations were made for a total of 758 samples from Hole 1276A (Table T16). Samples were selected to provide a measure of the carbonate content in different lithologic units and to assess the influence of carbonate content on seismic velocity (see "Velocity Structure of Turbidite Sequences" in "Factors Affecting Sediment Velocity" in "Physical Properties"). Carbonate contents range between 0.1 and 92.9 wt% (Fig. F154). Although the carbonate content profile shows widely varying values downhole, there is a general correlation with the lithologic units. Units 1 (mudstone) and 4 (burrowed mudstones and sandstones) show lower carbonate values (averages = 6.7 and 8 wt%, respectively), whereas Unit 2 (grainstones and marlstones) shows the highest carbonate contents (average = 30.6 wt%). In Unit 5, average carbonate content tends to decrease downhole (average = 22 wt% in Subunit 5A, 14.1 wt% in Subunit 5B, and 9.5 wt% in Subunit 5C) in association with an increase of the mud/clay fraction and a general downhole increase in Al (Fig. F155).

Elemental concentrations of C, N, and H were measured for a total of 443 samples (Table T17; Fig. F154). TOC contents vary between 0 and 9.4 wt%. Unit 4 has uniformly low TOC percentages (average = 0.1 wt%), whereas Unit 5 (mainly dark claystones) shows intermittently higher values but a somewhat low average of 1 wt% (Fig. F154).

C/N ratios range between 0 and 136.8 (Table T17; Fig. F154). C/N ratios are often used as indicators of the origin of organic matter, with lower values (4-10) typical of marine algae and higher values (>20) considered to be typical of land-derived material (Meyers, 1994). In Unit 4, C/N ratios (<9.6) are characteristic of algal material, which could indicate a pelagic origin for the organic fraction. C/N ratios show scattered high and low values in the other lithologic units, although averages of Units 1 and 3 are still typical of marine-derived organic matter (12.2 and 12.7, respectively). These units also show scattered higher values (>25-30), more typical of land-plant material, that could be explained by strong terrigenous turbidite influence. Subunits 5A, 5B, and 5C show somewhat higher C/N values (averages = 21, 16.6, and 18.6, respectively) together with higher organic carbon contents.

A general correlation exists between higher C/N ratios and higher TOC contents in Hole 1276A (Fig. F154), suggesting the possible importance of a terrigenous contribution to the sedimentary organic matter. Smear slide observations (see "Unit 5" in "Lithostratigraphy" and "Site 1276 Smear Slides") confirm an abundance of terrestrial organic matter over the whole cored sequence, with algal material mainly localized in higher-TOC intervals. However, many Cretaceous black shales (Meyers et al., 1984; Meyers, 1987) and Mediterranean sapropels (Bouloubassi et al., 1999; Meyers and Doose, 1999; Nijenhuis and de Lange, 2000) have high C/N ratios even though the organic matter appears to be marine derived. Correspondence of high C/N ratios with high TOC percentages has been postulated to result from a coupling between higher fluxes of organic matter and improved preservation of carbon content (Twichell et al., 2002). This process is promoted by suboxic conditions and appears to be driven by denitrification (Van Mooy et al., 2002).

From the above considerations, C/N values in Unit 5 may indicate partial input of terrestrial material and/or differential nitrogen cycling in this black shale-dominated sequence.

Rock-Eval investigations were conducted for purposes of safety and pollution prevention, with a resolution of two samples per core. To minimize mineral matrix effects, only results from samples with TOC 0.5 wt% (Peters, 1986; Bordenave et al., 1993) and CaCO₃ 50 wt% (G.E. Claypool, pers. comm., 2003) were considered. Samples with <0.5 wt% TOC and >50 wt% CaCO₃ are included in the data table (Table T18) but are not included in the interpretative plots (Fig. F156). During Leg 210, because of technical problems with the Delsi Nermag Rock-Eval II, S₃ and TOC data were not acquired. In order to calculate the HI, TOC results from the CHNS analyzer were employed. Rock-Eval parameters are useful in constraining the origin of organic matter and its thermal maturity.