GEOCHEMISTRY

Concentrations of headspace and vacutainer gases were routinely monitored in Hole 1235A sediments according to shipboard safety and pollution prevention considerations (Fig. F25; Table T11). The high gas pressures in the cores required perforating the core liners to prevent excessive core expansion. Methane concentrations increased rapidly to 55,648 ppmv at 19.6 mcd, and vacutainer samples had high methane concentrations (>95% by volume) at all depths sampled (43.2 to 208.6 mcd). Low ethane (C₂) concentrations were detected in the headspace and vacutainer samples. Ethane values gradually increase with depth and reach 105 ppmv at 208.6 mcd. No significant amounts of higher molecular weight hydrocarbons were observed.

High methane concentrations, low ethane concentrations, and the resulting high C₁/C₂ ratios (Fig. F25) indicate that methane originates from in situ formation (methanogenesis) of sedimentary organic matter (Claypool and Kvenvolden, 1983). A biogenic origin for the methane is supported by the disappearance of dissolved sulfate by 19.6 mcd, coincident with the increase in methane.

We collected 20 interstitial water samples from Hole 1235A. Chemical gradients at this site (Table T12; Fig. F26) reflect the likely presence of gas hydrates, the influence of organic matter diagenesis by microbially mediated oxidation reactions, a limited degree of biogenic opal dissolution, and the effects of authigenic mineralization reactions on fluid composition.

Chlorinity decreases by more than 12%, from 553 mM at 1.5 mcd to 483 mM by 210.0 mcd (Fig. F26). The chlorinity gradient with depth is not as steep as that observed at Site 1233, although both sites have similar chlorinity values from ~75 to 100 mcd. Salinity, measured refractively as total dissolved solids, ranges from 35 to 28, decreasing by ~20% with increasing depth (Table T12). Sodium concentrations measured by inductively coupled plasma-atomic emission spectrophotometry were typically within <1% of those estimated by charge balance reported here (Table T12). Sodium concentrations decrease by ~20% with depth from a high of 483 mM at 41.7 mcd to 381 mM at 210.0 mcd. Site 1235 was drilled to a greater depth than Site 1233, and the decreases in chlorinity, salinity, and sodium concentrations previously observed at Site 1233 continue through the total depth drilled at Site 1235.

The decreasing chlorinity gradient at Site 1235, like that at Site 1233, is significantly larger than those observed at the deeper-water midslope Peru basin sites drilled during Leg 112 (Sites 682, 683, 685, and 688; Suess, von Huene, et al., 1988). Decomposition of gas hydrates in Site 1235 sediments could explain the observed chloride gradient in interstitial water as a result of dilution, either through in situ decomposition at depth in the sediment column or during sediment recovery. Other possible explanations for the chloride gradient include mineral dehydration reactions at depth, clay membrane ion filtration reactions at depth, and advection of fresher fluid at depth (Kastner et al., 1990).

Organic matter diagenesis, driven by microbial oxidation reactions, dominates many of the interstitial water profiles. The relatively low organic carbon contents are apparently counterbalanced by high total sedimentation rates, as at Site 1234, and result in pronounced depth variations in interstitial water chemistry as typically observed in more organic carbon-rich continental margin settings. Sulfate concentration at 1.5 mcd is 25.5 mM, only slightly reduced from typical seawater values of 29 mM, and sulfate concentrations are below the detection limit (~0.6 mM) by 19.6 mcd.

Organic matter decomposition by sulfate reduction and methanogenesis drives large increases in alkalinity, which increases to peak values of >60 mM from 31.1 to 41.7 mcd, then decreases to a minimum of 25 mM at 96.5 mcd. After a slight increase, alkalinity declines to 6 mM at 210.0 mcd. The alkalinity maximum at Site 1235 has peak values ~10% lower than those at Site 1234, and the alkalinity maximum at Site 1235 is deeper and narrower than that at Site 1234. These differences are consistent with less rapid sulfate depletion with depth at Site 1235. Alkalinity declines with increasing depth below the maxima at Sites 1233, 1234, and 1235, with the lowest values seen in the deepest section of Site 1235 (>166 mcd), consistent with the greater degree of authigenic carbonate mineralization at Site 1235 (see "Lithostratigraphy").

The reduced forms of the secondary oxidants manganese and iron are not simply related to that of sulfate. Dissolved manganese averages 2 µM, and concentrations at Site 1235 are generally greater than those at Site 1234. The profile has a subsurface peak from 19.6 to 41.7 mcd, reaches minimum values around 100 mcd, and is followed by a secondary maximum at greater depth (Fig. F26), although concentrations throughout are not large relative to the blank levels for which they have been corrected. Dissolved iron concentrations reach a peak of 10.7 µM from 41.7 mcd, followed by a steep then more gradual decline to values generally <2 µM deeper than 77 mcd. Iron concentrations are generally higher at Site 1235 than those at Site 1234. Manganese and iron profiles do not show a clear succession of redox zonation with sulfate profiles.

Organic matter decomposition generates increases in phosphate and ammonium in interstitial water. Phosphate concentrations are >200 µM from 19.6 to 41.7 mcd then decline sharply to 62 µM at 108.2 mcd, then to 4 µM at 210.0 mcd. Maximum phosphate values at Site 1235 are higher than those at Site 1234, with a steeper decline with depth, indicating greater uptake in authigenic mineralization reactions.

Ammonium concentrations increase from below the detection limit (0.3 mM) at 1.5 mcd to values >8 mM from 31.1 to 41.7 mcd, decrease to a low of 4.6 mM at 96.5 mcd, and persist at concentrations of 5-7 mM throughout. The ammonium peak at Site 1235 is narrower in depth and lower in peak amplitude than that at Site 1234, consistent with the alkalinity differences between the two sites.

Dissolved silicate concentrations average ~720 µM, with little depth variation, (Fig. F26), similar to the profile at Site 1234. The interstitial waters are under saturation with respect to biogenic opal (saturation value >1000 µM). This may reflect the limited amount of biogenic opal available for dissolution or other controls on opal solubility in these sediments. Diatoms are less well preserved at Site 1235 than at Site 1234 (see "Biostratigraphy"), despite similar silicate concentrations in interstitial waters. Barium concentrations are significantly lower than those at Sites 1233 (peak value = >35 µM) and 1234 (peak value = up to 25 µM), with barium always <4 µM at Site 1235. All three sites experience total sulfate reduction at shallow depth, which should drive barite dissolution and produce dissolved barium in proportion to the sulfate decline. The boron profile in the upper 150 mcd is similar in character to those at Sites 1233 and 1234, with an increase to values >800 µM from 31.1 to 41.7 mcd. The boron peak is lower in concentration and reached deeper at Site 1235 than that at Site 1234. Boron reaches a low of 637 µM at 142.2 mcd then increases steeply to >1200 µM at 210.0 mcd, indicating a source of boron at depth. The contrasts in alkalinity between Sites 1233, 1234, and 1235 profiles appears to be reflected in the boron profiles.

Calcium concentrations decrease sharply from 10.3 mM at 1.5 mcd to 1.7 mM at 19.6 mcd, persist at low values to a minimum of 1.1 mM at 96.5 mcd, and then increase sharply with depth to 14.6 mM (greater than seawater calcium) at 210.0 mcd. Sites 1233 and 1235, both characterized by strong chlorinity decreases with depth, have strong minima in calcium at ~100 mcd. Magnesium concentrations increase to >54 mM from 31.1 to 41.7 mcd, decrease to a minimum of 41.2 mM at 96.5 mcd, increase again, then decrease to 34.3 mM at 210.3 mcd. The very strong decrease in calcium by 19.6 mcd is consistent with authigenic mineralization reactions driven by the alkalinity increase. Increasing magnesium/calcium ratios with the calcium decrease indicate that calcite precipitation is the most likely reaction taking place in shallow sediments. Magnesium/calcium ratios increase to a maximum of 41 at 53.2 mcd, with a secondary maximum of 38 at 96.5 mcd driven by the strong calcium minimum at that depth. This peak in Mg/Ca is not observed at Site 1234. A strong decline in Mg/Ca below this depth, to 2.4 at 210.3 mcd, compares with higher values at the comparable depth range of Site 1234. This may reflect more significant uptake of Mg in authigenic dolomite at this site, where dolomite nodules were found (see "Lithostratigraphy").

Lithium profiles are similar at Sites 1233, 1234, and 1235, with a sharp initial decrease followed by a general increase with increasing depth before a decrease in the deepest samples (Fig. F26). Strontium concentrations decrease quickly with increasing depth at all three sites, reaching a broad minimum <55 µM at Site 1235 from 19.6 to 119.8 mcd. Strontium concentrations at Site 1235 increase sharply below ~150 mcd, to 420 µM at 210.0 mcd, a feature not seen at Site 1234. The strontium increase in the deepest part of Site 1235, like the calcium and boron increases over a similar depth range, requires a source at depth for these elements, presumably from the diffusive influence of basement alteration reactions. Potassium concentrations generally decrease from 14.6 mM at 1.5 mcd to 10.1 mM at 210.0 mcd (Fig. F26).

Inorganic carbon (IC), total carbon (TC), total nitrogen (TN), and total sulfur (TS) concentrations were determined on sediment samples from Holes 1235A and 1235B (Table T13). Organic matter carbon/nitrogen ratios and Rock-Eval pyrolysis were employed to characterize the organic matter.

Calcium carbonate concentrations are low, ranging between 0.3 and 15.5 wt% (average = 2.4 wt%) (Fig. F27). In the uppermost 70 mcd, the calcium carbonate concentrations are small. Below this depth, calcium carbonate concentrations are somewhat higher and show larger amplitude variations of 2-5 wt%. The highest calcium carbonate concentration of 15.5 wt% is present between 206.4 and 213.6 mcd. Calcium carbonate originates mainly from calcareous plankton, although diagenetic inorganic carbon is visible in some intervals (see "Lithostratigraphy"). The low calcium carbonate concentrations and amplitude fluctuations result from high fluvial supply of siliciclastics. The control of calcium carbonate at Site 1235 by dilution rather than productivity and dissolution, is consistent with well-preserved microfossils observed in sediments. Calcium carbonate concentrations at Site 1235 are lower than those at Site 1234, suggesting an even stronger dilution effect at the shallower Site 1235 than at the deeper Site 1234.

TOC concentrations range between 0.4 and 1.5 wt% (average = 0.6 wt%) (Fig. F27). The shallowest sample at 0.7 mcd has the highest concentration measured at the site. Below this surface sample, TOC contents remain and small variations throughout the sedimentary record. Two intervals have slightly higher TOC concentrations, between 27.4 and 65.1 mcd and between 78.9 and 84.3 mcd. The TN record contains very similar trends. We infer that the TOC variations are driven by the interplay between siliciclastic dilution and export productivity from the overlying waters. As it was for calcium carbonate, the dilution effect on TOC is more pronounced at the shallower Site 1235 than at the deeper Site 1234. Such a large supply of siliciclastics is not typical for upwelling regions. For example, the coastal upwelling areas of the Namibia and California margins are characterized by upper Pleistocene TOC contents as high as 17 and 7 wt%, respectively (Lyle, Koizumi, Richter, et al., 1997; Berger et al., 1998).

TS concentrations are high throughout the record, varying between 0.3 and 2.1 wt% (Table T13), with a long-term tendency for higher contents toward the bottom of the sedimentary record.

TOC/TN ratios typically range between 5 and 10, which indicates a predominantly marine origin of the organic material (Fig. F28) (Bordovskiy, 1965; Emerson and Hedges, 1988; Meyers, 1997). Lower TOC/TN ratio are associated with lower TOC concentrations (Fig. F28) indicating an increased supply of terrigenous organic matter and/or differential preservation during diagenesis.

Five samples were selected for Rock-Eval measurements. Low T_max values indicate that the organic matter is thermally immature (Table T14). The relationship between S₂ and TOC concentrations shows that the organic matter is dominantly Type II (Fig. F29) (i.e., marine algal organic matter) (Tissot and Welte, 1984; Langford and Blanc-Valleron, 1990), which is consistent with the low TOC/TN ratios at Site 1235.

Fresh marine plankton has a relatively high lipid content and thus high H/C ratios. Therefore, well-preserved organic matter of marine algal origin yields high hydrogen index (HI) values when subjected to pyrolysis. The HIs measured in these samples are low, ranging from 220 to 289 (Table T14). This indicates a significant degradation of organic matter. The correspondence between decreases in both TOC concentrations and HI values (Fig. F29) could indicate that preservation of marine organic matter during diagenesis is important in controlling the organic carbon concentrations on the Chile margin.