GEOCHEMISTRY

Concentrations of headspace gases were routinely monitored in Hole 1242A sediments according to shipboard safety and pollution prevention considerations. Methane increased from ambient concentrations at 1.5 mcd to >20,000 ppmv at 23.4 mcd (Table T14; Fig. F26). Methane concentrations were high at greater depth (average = 40,000 ppmv). Small amounts of ethane were detected, varying between 1.4 and 14.8 ppmv. High methane/ethane ratios, representative of microbial gases, indicate that the gases are of biogenic origin. This is supported by the disappearance of interstitial sulfate at approximately the same depth where methane contents begin to increase. The presence of interstitial sulfate inhibits methanogenesis in marine sediments (Claypool and Kvenvolden, 1983).

We collected 19 interstitial water samples from Hole 1242A for shipboard analyses. All of these samples were taken from Unit I (see "Lithostratigraphy"). Chemical gradients at this site (Table T15; Fig. F27) reflect the influence of organic matter oxidation by sulfate reduction, authigenic mineralization, the dissolution of biogenic silica, and the diffusive influence of basalt alteration reactions at greater depth.

Chlorinity generally increases with depth, from 552 mM at 1.5 mcd to 564 mM at 275.5 mcd (Fig. F27). Salinity, measured refractively as total dissolved solids, ranges between 32-35 (Table T15). Sodium concentrations measured by inductively coupled plasma-atomic emission spectrophotometry averaged 2.0% lower than those estimated by charge balance reported here (Table T15). Sodium concentrations show a small increase from 478 mM at 1.5 mcd to >490 mM from 34.3 to 74.6 mcd then decrease substantially to 363 mM at 275.5 mcd. The lack of decrease in salinity or chlorinity is because the decrease in sodium is counterbalanced by a substantial increase in calcium.

Dissolved manganese concentrations have a near-surface maximum of 5.8 µM at 1.5 mcd, decrease to low values from 23.4 to 169.5 mcd, then increase to >10 µM from 232.3 to 275.5 mcd. Dissolved iron has a small maximum of >15 µM from 83.5 to 104.3 mcd. At greater depth, the iron profile resembles that of manganese, with a significant increase from 12.4 µM at 169.5 mcd to >110 µM from 210.1 to 255.1 mcd. These deep maxima of iron and manganese may indicate the persistence of suboxic diagenesis of organic carbon at depths greater than ~180 mcd, consistent with the presence of measurable sulfate at depth. Organic matter diagenesis, driven by microbially mediated oxidation reactions, significantly influences the interstitial water chemistry. Sulfate decreases to nearly nondetectable levels by 23.4 mcd and remains at that level to 191.5 mcd. The disappearance of sulfate coincides with the increase in methane. Interstitial water samples from 210.1 to 275.5 mcd, taken from XCB cores, have detectable sulfate concentrations (1.4-3.4 mM). This is often interpreted as reflecting seawater contamination in the XCB whole rounds, but the evidence from barium concentrations indicates that these may in fact represent in situ sulfate concentrations. Alkalinity rapidly increases from 3.9 mM at 1.5 mcd to >25 mM from 23.4 to 93.6 mcd then decreases to 3.3 mM at 275.5 mcd.

Phosphate concentrations increase from 10 µM at 1.5 mcd to 60 µM at 34.4 mcd, decline steeply to 6 µM at 146.2 mcd, then decrease more slowly with depth to values below the detection limit (1 µM). Ammonium concentrations increase from below the detection limit (0.4 mM) at 1.5 mcd to >7.5 mM from 93.6 to 169.5 mcd then decline steadily to 3.7 mM at 275.5 mcd.

Dissolved silicate increases from 468 µM at 1.5 mcd to >1400 µM at 275.5 mcd, indicating that interstitial waters could be at equilibrium with biogenic opal solubility. Despite the high thermal gradient at this site, dissolved silicate concentrations do not reach the extremely high values (>1800 µM) seen at Sites 1238 and 1239.

Barium concentrations increase significantly with the disappearance of sulfate to values of >200 µM from 44.0 to 124.9 mcd then increase sharply to 872 µM at 191.5 mcd. The increase in barium with the disappearance of dissolved sulfate is consistent with the dissolution of barite from sediments with sulfate reduction. In the deeper samples with small but measurable sulfate concentrations, barium concentrations return to low values, 150 µM from 210.1 to 275.5 mcd. Boron concentrations decrease overall from 529 µM at 1.5 mcd to 351 µM at 275.5 mcd.

Calcium decreases from 9.9 mM at 1.5 mcd to 2.5 mM at 34.4 mcd then increases to 5.5 mM at 104.3 mcd and to 76.6 mM at 275.5 mcd. The deeper calcium gradient is equivalent to a 48 mM/100 m change in calcium concentration, the largest observed during this leg and a factor of 10 larger than the average oceanic gradient. Magnesium concentrations decrease overall, with the decrease in the deeper section linearly correlated to the calcium increase, indicating that these profiles are diffusionally controlled from a reaction source/sink at depth such as basalt alteration reactions. Magnesium/calcium ratios increase from 5.1 at 1.5 mcd to >10 from 23.4 to 54.0 mcd (Fig. F27). The decrease in calcium can result from authigenic calcite precipitation and drive magnesium/calcium ratios higher. High magnesium/calcium ratios, along with the disappearance of sulfate, can subsequently result in dolomite formation, and observations indicate the presence of dolomite in this site (see "Lithostratigraphy").

Lithium concentrations increase from 23 µM at 1.5 mcd to 310 µM from 191.5 to 210.1 mcd then decrease to 191 µM at 275.5 mcd. Strontium concentrations increase with depth from 85 µM at 1.5 mcd to >450 µM from 255.1 to 275.5 mcd. Potassium concentrations increase from 12 mM at 1.5 mcd to >15 mM from 93.6 to 210.1 mcd then decrease to 9.6 mM at 275.5 mcd.

Inorganic carbon (IC), total carbon (TC), and total nitrogen (TN) concentrations were determined on sediment samples from Hole 1242A (Table T16). Organic matter carbon/nitrogen ratios were employed to characterize the organic matter contained within the sediments.

Calcium carbonate concentrations range between 5.7 and 51.2 wt% (average = 24.4 wt%) (Table T16; Fig. F28). Calcium carbonate concentrations generally increase with increasing depth, with fluctuations of ~10 to ~20 wt% around this trend.

Total organic carbon (TOC) concentrations range between 0.9 and 3.5 wt% from 2.2 to 238.8 mcd (average = 1.6 wt%) (Table T16; Fig. F28). TOC concentrations decrease from 3.5 wt% at 2.2 mcd to typically <1.5 wt% at depths >~90 mcd.

TOC/TN ratios range between 4.9 and 8.8 (average = 7.2), indicating that the sedimentary organic matter is dominantly of marine origin (Bordovskiy, 1965; Emerson and Hedges, 1988; Meyers, 1997). In the uppermost ~50 mcd, TOC/TN ratios average 8.2 (Fig. F28). TOC/TN ratios decrease to a minimum of 4.9 at 122.7 mcd.