GEOCHEMISTRY

Concentrations of headspace gases were routinely monitored in sediments from Hole 1232A according to shipboard safety and pollution prevention considerations. Methane concentrations above ambient background first appeared in a headspace gas sample at 27.3 mcd (Table T11; Fig. F18). Methane concentrations increased to ~9000 ppmv by 64.2 mcd, then remained typically above 10,000 ppm. High methane concentrations and the absence of higher molecular weight hydrocarbon gases (Table T11) indicate that the methane is of biogenic origin, probably originating from in situ degradation via microbial diagenesis (methanogenesis) of sedimentary organic matter. A biogenic origin for the methane is supported by the low values of sulfate in interstitial water at depths below ~20 mcd (Fig. F19). The presence of interstitial sulfate inhibits methanogenesis in marine sediments (Claypool and Kvenvolden, 1983).

We collected 30 interstitial water samples from Site 1232, 20 from Hole 1232A and 10 from Hole 1232B. These are treated as constituting a single mcd profile. Chemical gradients at this site (Table T12) reflect the influence of organic matter diagenesis, some degree of biogenic opal dissolution, and other processes.

Chlorinity ranges from 559 to 576 mM, with low values near the sediment/water interface and from 159.2 to 277.9 mcd (Fig. F19). Salinity, measured refractively as total dissolved solids, ranges from 33 to 35. Sodium concentrations measured by ion chromatography were, on average, within <2% of those estimated by charge balance reported here (Table T12).

Alkalinity increases to peak values >30 mM from 26.3 to 159.2 mcd then decreases to <20 mM at 248.5 mcd, with values in deeper samples >25 mM. Sulfate concentrations decrease to values <2 mM by 26.3 mcd, remaining low with increasing depth except for a small maximum of ~5 mM at 248.5 mcd. Dissolved manganese concentrations generally decrease with increasing depth from 37 µM at 6.7 mcd to 7 µM from 56.6 to 66.3 mcd. Manganese concentrations reach a secondary maximum, with values increasing from 12 µM at 75.5 mcd to 22 µM at 85.7 mcd; farther downhole, values are generally <10 µM. Dissolved iron concentrations increase with increasing depth, with values generally >20 µM from 36.9 to 124.2 mcd and lower values in deeper samples. The iron data show considerable scatter, and many samples have concentrations close to the detection limit (~6 µM). Phosphate concentrations are >50 µM throughout. Ammonium concentrations increase from 0.9 mM at 6.7 mcd to values generally >2 mM by 36.9 mcd. Slightly lower ammonium concentrations are found from 93.4 to 124.2 mcd.

Dissolved silicate concentrations are always <600 µM, indicating that the interstitial waters are not at saturation with respect to biogenic opal. Barium concentrations are generally <20 µM, with a pronounced barium peak up to 34 µM from 83.6 to 95.4 mcd and with values typically >20 µM from 219.7 to 366.2 mcd. Boron concentrations generally decrease with increasing depth, from 472 µM at 6.7 mcd to 325 µM at 366.2 mcd.

Calcium concentrations decrease sharply to values <6 mM by 27.2 mcd and increase to values >6 mM from 64.1 to 75.0 mcd, with a sharp minimum <4 mM from 93.4 to 104.9 mcd. Magnesium concentrations range from 45 to 55 mM, with no pronounced variations with depth. Magnesium/calcium ratios increase from 5.6 at 6.7 mcd to ~8-9 from 26.3 to 76.0 mcd, reach a sharp maximum >14 from 93.4 to 104.9 mcd, decline to <10 from 124.2 to 134.1 mcd, and then remain >11 until the deepest sample. Strontium and potassium concentrations generally decrease slightly with increasing depth (Fig. F19).

Inorganic carbon (IC), total carbon (TC), and total nitrogen (TN) concentrations were determined on sediment samples from Holes 1232A and 1232B (Table T13). Organic matter carbon/nitrogen ratios were used to characterize the organic matter.

Total organic carbon (TOC) concentrations are very low throughout the sediment column, with a range between 0.05 and 0.35 wt% (average = 0.17 wt%). Most of the sediment samples contain <0.25 wt% TOC, which is typical of open-ocean sediments. TN values at Site 1232 are very low and vary between 0.1 and 0.8 wt% (average = 0.02 wt%) (Table T13). In some sediment samples, nitrogen contents are too low to be measured accurately. Calcium carbonate concentrations range between 0.3 and 27 wt%, and most sediment samples contain <2 wt% calcium carbonate (Table T13). These generally low concentrations agree with the paucity of microfossils (coccoliths and foraminifers) and the high abundance of continental clastic sediments at this site.

Most TOC/TN ratios are below 12 (average = 8) (Table T13), indicating a predominance of marine organic matter throughout the record (Emerson and Hedges, 1988; Meyers, 1997). Most of the samples in which turbidites are observed have very low TOC contents (e.g., Samples 202-1232A-3H-3, 72-73 cm; 8H-6, 75-75 cm; 10H-3, 74-75 cm; and 11H-6, 74-75 cm) (Table T13) and nitrogen contents too low to be accurately measured.