GEOCHEMISTRY

Concentrations of headspace gases were routinely monitored in Hole 1236A sediments according to shipboard safety and pollution prevention considerations. Headspace methane concentrations were below 2 ppmv throughout the entire hole, equivalent to blank (i.e., laboratory air) concentrations. This indicates that no methanogenesis is now occurring at this site, consistent with the low organic matter contents in sediments (see "Sedimentary Inorganic Carbon Concentrations").

We collected 22 interstitial water samples from Hole 1236A. Chemical gradients at this site (Table T12; Fig. F27) reflect a minor influence of organic matter diagenesis by microbially mediated oxidation reactions, a limited degree of biogenic opal dissolution along with other controls on dissolved silicate, and a minor signature of calcite diagenesis. In general, chemical variations with depth at Site 1236 are minor compared to those at the Chile margin sites (1233-1235). Interstitial water geochemistry varies little with lithologic unit, except in the deepest sedimentary unit overlying presumed basement (Fig. F27).

Chlorinity increases from 553 mM at 2.9 mcd to 570 mM at 43.6 mcd then decreases to values ~560-565 mM from 54.2 to 147.1 mcd (Fig. F27). Chlorinity is 573 mM at 170.4 mcd in a sample taken from an ash layer. Salinity, measured refractively as total dissolved solids, ranges from 34 to 36 (Table T12). Sodium concentrations measured by inductively coupled plasma-atomic emission spectrophotometry averaged 3% lower than those estimated by charge balance (Table T12). Sodium concentrations parallel chlorinity, with a total range from 473 to 492 mM.

Organic matter diagenesis, driven by microbial oxidation reactions, has a relatively minor influence on interstitial water composition. Sulfate undergoes a limited degree of reduction, by no more than 6 mM from typical seawater values of ~29 mM, to the lowest values in lithologic Unit III, with a value of 23.6 mM at 191.1 mcd coincident with a small local maximum in alkalinity. Alkalinity averages ~2.2 mM and varies little with depth except for a decrease to 1.1 mM observed in the deepest three samples from 202.3 to 220.2 mcd.

Dissolved manganese concentrations initially decrease from 2.3 µM at 2.9 mcd to below the detection limit (<0.1 µM) at 20.3 mcd then increase to 3.8 µM at 77.3 mcd before decreasing again to low values (generally below the detection limit) throughout lithologic Unit II (see "Lithostratigraphy"). Manganese increases in Unit III to 8.9 µM at 212.0 mcd then decreases to 6.5 µM in the deepest sample at 220.2 mcd. The magnitude of the middepth peak is equivalent to those at Chile margin Sites 1233 and 1234, with the peak near the contact with presumed basement larger than any values at Sites 1233-1235. Dissolved iron is below the detection limit (<0.3 µM) in all samples, reflecting the low content of terrigenous material in this site relative to Chile margin Sites 1233-1235.

Phosphate concentrations decrease from 2 µM at 2.9 mcd to generally low values no more than a factor of 3-4 times the detection limit (0.4 µM) throughout. Ammonium concentrations are generally below the detection limit (2.2 µM), and the concentrations reported reflect a substantial blank correction relative to the measured signal. Phosphate and ammonium concentrations at Site 1236 are an order of magnitude lower than those at Chile margin Sites 1233-1235, as is alkalinity.

Dissolved silicate concentrations increase from 210 µM at 2.9 mcd to 492 µM at 86.1 mcd, near the only biostratigraphy sample observed to have abundant diatoms (see "Biostratigraphy"). Silicate concentrations drop sharply to values averaging 210 µM from 127.9 to 179.9 mcd in lithologic Unit II. They then increase to 645 µM at 202.3 mcd, the highest value seen at this site. This sample was taken in an ash layer from the same core where one sample contained diatoms (rare), which were otherwise nearly absent throughout the drilled sequence (see "Diatoms" in "Biostratigraphy"). The interstitial waters are below saturation with respect to biogenic opal, and the profile indicates multiple influences with depth.

Barium concentrations are much lower than those in the reducing Chile margin Sites 1233-1235. The blank-corrected concentrations reported here are of similar magnitude to the analytical blank and not significantly greater than the detection limit, and show no significant variation with depth. The lack of a barium increase here, where sulfate values remain high throughout, is consistent with barite dissolution driven by solubility controlling site-to-site and depth variations in dissolved barium.

Boron concentrations decrease slightly with increasing depth, from 433 µM at 2.9 mcd to 385 µM at 196.9 mcd, with a steeper decrease to 284 µM in the deepest sample at 220.2 mcd. Boron concentrations at Site 1236 are lower than those at the Chile margin Sites 1233-1235, and the profile lacks the middepth maximum seen at those sites.

Calcium concentrations increase with depth, from 11.0 mM at 2.9 mcd to 12.9 mM at 32.2 mcd, then decline to 11.4 mM at 170.4 mcd before increasing to 14 mM from 191.1 to 220.2 mcd. Magnesium concentrations have minor variations with depth. Magnesium/calcium ratios decrease with depth from 4.7 at 2.9 mcd and to <4.2 from 10.9 to 43.6 mcd, increase slightly with depth to 4.5 from 170.4 to 180.0 mcd, and then decrease in the deepest four samples to 3.4-3.7 (Fig. F27). The limited range of variability in calcium and magnesium and therefore magnesium/calcium ratios at Site 1236 contrasts strongly with the nonconservative profiles of Chile margin Sites 1233-1235, which were dominated by the influence of authigenic mineralization reactions. Lithium concentrations increase slightly with depth, from 25 to 28 µM. Strontium concentrations are >100 µM from 2.9 to 54.2 mcd, generally decrease to 89 µM near the base of lithologic Unit II, and then increase to 100 µM from 202.3 to 220.2 mcd. Potassium concentrations range from 9.8 to 11.0 mM (Fig. F27).