ORGANIC GEOCHEMISTRY

At Site 1143, the shipboard organic geochemistry analysis consisted of determinations of total organic carbon (TOC), total carbon (TC), total nitrogen (TN), total sulfur (TS), and carbonate as well as Rock-Eval pyrolysis and hydrocarbon (HC) monitoring. The analytical procedures are outlined in "Organic Geochemistry" in the "Explanatory Notes" chapter.

As part of the shipboard safety and pollution-prevention monitoring program, hydrocarbon gases were analyzed in every core of Hole 1143A and in the lower 100 m of Hole 1143C (Cores 184-1143C-44X through 54X) by the headspace (HS) technique. No gas expansion voids or hydrates were observed. Only trace amounts of methane (10 ppmv) were detected, and no ethane was found in the headspace gas. Higher methane values were in the top 120 m; below this level, concentrations decreased, slowly increasing again in the bottom 190 m of the hole (especially Cores 184-1143C-49X through 54X), partly because of increased sample size. Because the values were so low, a headspace vial and cap without sediment were similarly analyzed with a result of 1.9 ppmv methane, approximately the minimum value recorded downcore. This corresponds to the ambient atmospheric methane concentration in the laboratory. Low methane concentrations at Site 1143 may have resulted either from very low levels of TOC or because sulfate reduction was not complete (see "Inorganic Geochemistry"). Therefore, methanogenic bacteria were not able to successfully compete for organic matter.

Analytical methods for inorganic carbon (IC), TC, and TOC determination are discussed in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Sampling was conducted in three sections per core for carbonate and at least one sample per core for carbon-nitrogen-sulfur (CNS) analysis. Carbonate varies generally from 10 to 60 wt%, with only four values above and four below this range. The carbonate plot (Fig. F18) tends to mirror that of TOC (Fig. F19) in the upper 200 mcd (Cores 184-1143A-1H through 21H), suggesting that the carbonate concentrations reflect varying amounts of noncarbonate sediment input (e.g., clastics and admixed volcanic ash). Below this depth, carbonate values rise markedly from ~20 wt% in the upper 150 mcd to an average of 45 wt% below that depth. Below 300 mcd, carbonate increases to ~50 wt% to the bottom of the hole. The higher carbonate content is also reflected in the interstitial water Ca²⁺ and alkalinity data (see "Inorganic Geochemistry"). Two exceptionally high carbonate samples at 413 and 433 mcd were remeasured and confirmed as calcite rich and are coincident with foraminifer sand layers.

The TOC values at Site 1143 are consistently low (mostly between 0.1 and 0.4 wt%) except in the top 70 m of the core, where half of the data are between 0.4 and 0.6 wt% (Table T12). The percentage of TOC declines rapidly from >0.3 wt% in the upper 70 m (Cores 184-1143A-1H through 8H) to <0.3 wt% throughout the remainder of the hole, except Sections 184-1143A-30X-4 and 33X-3. These latter samples were characterized by higher TOC (>0.8 wt%) and correspond to two discrete samples with lower carbonate concentrations.

Calculated C/N ratios are remarkably low (Table T12). A value of 5 to 6 indicates that most of the organic matter is derived from fresh marine phytoplankton. However, the low values (<5) measured on samples from Site 1143 are likely an artifact caused by a combination of very low organic carbon concentrations and the absorption of ammonium-nitrogen by abundant clay minerals (see "Organic Geochemistry" in the "Site 1145" chapter), (Müller, 1977).

One hundred samples were analyzed for CNS content, but many were found to give low or zero values (there is no SO₂ peak). The problem is sorption or reaction of SO_x on the CaO that remains in the furnace. Total sulfur varies from 0.1 to 0.8 wt%, with most values ranging from 0.1 to 0.4 wt% (Table T12). We assume that this is all pyrite-S because pyrite is observed in many cores as thin burrow fillings and fine-grained black concentrations.

Rock-Eval pyrolysis analysis was made on seven samples with higher TOC (by difference) values. The results are presented in Table T12. The sample from Section 184-1143A-30X-4 was run in duplicate with two different masses. The reproducibility for the replicate analysis is quite good, with precision of better than 10 wt% for all parameters. Rock-Eval TOC and TOC by difference (TC - IC) do not agree. On only one sample do the results by both methods agree; on the other samples, Rock-Eval shows TOC values from 15 wt% to a factor of 4 lower than TOC by difference. The two samples that show the best agreement in TOC are also those with the highest TOC values. It is unclear whether the TOC values obtained by subtracting a carbonate value >96 wt% of the TC value, or by Rock-Eval analysis, are more reliable at such low concentrations. However, a possible systematic error in the by-difference method does not invalidate the discussion of relative TOC variations with carbonate. Because of the low TOC, none of the other Rock-Eval parameters is considered reliable (Tissot and Welte, 1984) and cannot be used to examine the origin of the organic matter at this site.

All routine headspace samples were taken from sediment exposed during division of the core on the catwalk, usually from the upper 5 cm of Section 4. Sediment disturbance during extended core barrel usage, especially below 250 meters below seafloor, may have resulted in some of HS samples containing mainly drilling-disturbed sediment. For this reason, an aliquot of interval 184-1143A-29X-1, 145-150 cm, was also analyzed on the natural gas analyzer (see "Organic Geochemistry" in the "Explanatory Notes" chapter). A very marked C₅ peak, with a concentration of 3376 ppmv, was noted. Upon further investigation, a similar result was obtained from Section 184-1143A-27X-CC, and the source was identified as acetone contamination (liberally used to seal section end-caps). Such an isolated peak without HC homologues should be immediately regarded as contamination, a valuable indicator for future organic geochemical analyses.