Concentrations of headspace gases were routinely monitored in Hole 1241A sediments according to shipboard safety and pollution prevention considerations. Methane concentrations in headspace samples were low, always <15 ppmv and as low as ambient levels in the shallowest and deepest samples (Fig. F29; Table T13). Methane concentrations were generally >8 ppmv from 62.9 to 287.9 mcd. Low methane concentrations indicate that methanogenesis is limited at this site because of low organic matter concentrations in the sediments (see "Sedimentary Inorganic Carbon, Organic Carbon, and Nitrogen Concentrations"). No higher molecular weight hydrocarbons were observed.
We collected 36 interstitial water samples from Hole 1241A for shipboard analyses. Chemical gradients at this site (Table T14; Fig. F30) reflect the limited influence of organic matter oxidation, the dissolution of biogenic silica driven by the relatively low thermal gradient, and biogenic calcite recrystallization. Many of the profiles are consistent with a flow of relatively unaltered seawater in the underlying oceanic crust, but the lack of major change in composition makes this more difficult to assess than at Site 1240.
Chlorinity averages 559 mM with little variation with depth or lithologic subunit (Fig. F30). Salinity, measured refractively as total dissolved solids, is 35 throughout (Table T14). Sodium concentrations measured by inductively coupled plasma-atomic emission spectrophotometry averaged 2.4% higher than those estimated by charge balance reported here (Table T14). Sodium concentrations parallel the limited variations in chlorinity, with a total range from 477 to 489 mM.
Organic matter diagenesis, driven by microbially mediated oxidation reactions, has a limited influence on the interstitial water chemistry. Sulfate shows depletion relative to seawater values by no more than 4-5 mM, with the smell of hydrogen sulfide noted in some of the interstitial water samples with lower sulfate concentrations. The limited degree of sulfate reduction is a consequence of the relatively organic carbon-poor sediments, and the presence of sulfate limits methanogenesis. Alkalinity increases from 3.6 mM at 3.3 mcd to >5 mM from 62.9 to 95.1 mcd then decreases to <3 mM in Subunit IC.
Dissolved manganese concentrations are >12 µM from 3.3 to 9.3 mcd, decrease to low values throughout Subunit IB, then increase from 0.6 µM at 298.1 mcd to 19.3 µM at 437.5 mcd. The shallow manganese peak represents suboxic diagenesis of organic matter, whereas the deeper one results from basalt alteration. Dissolved iron is present at relatively low concentrations, presumably reflecting a limited supply of reducible iron minerals.
Phosphate concentrations decrease from 4.9 µM at 3.3 mcd to 2.2 µM at 52.5 mcd near the base of Subunit IA then decline further in Subunits IB and IC to below the detection limit (0.2 µM) at 445.4 mcd. Ammonium concentrations increase from 93 µM at 3.3 mcd to >400 µM from 52.5 to 105.7 mcd then decline steadily to 31 µM at 445.4 mcd.
Dissolved silicate concentrations increase from 551 µM at 3.3 mcd to >1000 µM in most of Subunit IC then decline in the deepest two samples. Site 1241 has a significantly lower thermal gradient (~2.7°C/100 m) than Site 1238 (~12.7°C/100 m) or Site 1239 (~9.4°C/100 m) (see "Operations"). The silicate increase with depth for Site 1241 is also much less steep than those observed at Sites 1238 and 1239, and Site 1241 never reaches high enough temperatures to reach the high silicate concentrations (>1800 µM) seen at depth in Sites 1238 and 1239.
Barium concentrations are low (<6 µM) throughout, indicating that sulfate concentrations are sufficient to prevent significant dissolution of barite. Boron concentrations decrease from 502 µM at 3.3 mcd to 349 µM at 445.4 mcd.
Calcium concentrations increase in Subunit IA and the top of Subunit IB from 10.3 mM at 3.3 mcd to >12 mM by 84.5 mcd. Limited organic carbon oxidation drives only a small increase in alkalinity, which is too low to result in authigenic carbonate precipitation sufficient to lower calcium concentrations. Smear slide observations indicated only minor amounts of micrite throughout Site 1241 (see "Lithostratigraphy"). Magnesium concentrations average ~50 mM, with little variation with depth. Magnesium/calcium ratios decrease from 5.0 at 3.3 mcd to <4 by 84.5 mcd and remain low throughout (Fig. F30).
Lithium concentrations decrease from 26 µM at 3.3 mcd to <14 µM from 146.6 to 200.3 mcd then increase to >20 µM from 404.8 mcd and deeper. Strontium concentrations increase from 101 µM at 3.3 mcd to >240 µM from 84.5 to 116.6 mcd and decrease to 85 µM at 445.4 mcd. The middepth maximum is characteristic of the influence of biogenic calcite recrystallization, whereas the decrease with increasing depth must represent either a sink at depth via basalt alteration or a return to seawater values from the influence of fluid flow in the underlying oceanic crust as observed at Site 1240. Potassium concentrations have little variation with depth.
Inorganic carbon (IC), total carbon (TC), and total nitrogen (TN) concentrations were determined on sediment samples from Hole 1241A (Table T15). Organic matter carbon/nitrogen ratios were employed to characterize the organic matter.
Calcium carbonate concentrations range between 12.8 and 89.3 wt% (average = 67.8 wt%) (Table T15; Fig. F31). Concentrations increase gradually from ~63 to generally >80 wt% to 208.8 mcd, with a minimum of 54% at 43.9 mcd. Calcium carbonate concentrations decrease to ~54 wt% at 277.7 mcd then increase to values >80 wt% from 291.6 to 309.5 mcd. Calcium carbonate concentrations then generally decline with depth, with values ranging from 13 to 60 wt% at depths >350 mcd.
Total organic carbon (TOC) concentrations range between 0.05 and 1.4 wt% (average = 0.4 wt%) (Table T15; Fig. F31). In the uppermost ~342 mcd, the TOC concentrations reach their highest values in the depth interval from 345.0 to 446.0 mcd, with peak values as high as 1.4 wt%. The interval of maximum TOC values >350 mcd corresponds to the interval of minimum calcium carbonate concentrations. Calculating TOC on a carbonate-free basis indicate that TOC variations are not caused solely by variable calcium carbonate dilution (Fig. F31).
TN concentrations vary similarly to TOC (Fig. F31). The TOC/TN ratios increase from 5 to 30 in the uppermost ~100 mcd and remain high at greater depth. Low TOC/TN ratios in the uppermost 40 mcd indicate a marine origin of the organic matter (Bordovskiy, 1965; Emerson and Hedges, 1988; Meyers, 1997). Higher TOC/TN ratios can reflect a preferential loss of nitrogen relative to carbon during burial diagenesis of organic matter (Meyers, 1997). In sediments with low TOC concentrations, inorganic nitrogen from ammonium sorbed in the lattice of clay minerals may contribute significantly to the total nitrogen content (Müller, 1977) and TOC/TN ratios may not accurately reflect organic matter composition.