The amounts and compositions of headspace gases exhibit similar patterns that are related to the lithostratigraphic units and carbon concentrations at the five Leg 207 sites. The general pattern is that interstitial methane concentrations start to increase near the top of the nannofossil chalks and clays of Unit III, reach maximum values in the black shales of Unit IV, and decrease near the base of Unit V. In addition, a weak odor of hydrogen sulfide was noted in the black shale sequences at all the sites, although detectable concentrations (>1 ppmv with the NGA) of this gas were not found. A petroliferous odor present in parts of some of the black shale units may indicate the presence of organosulfur compounds. Concentration patterns of CaCO3, total organic carbon (TOC), and headspace methane at each site are summarized in the following sections.
Concentrations of CaCO3 in the five lithostratigraphic units generally diminish with depth (Fig. F1). However, Unit IV still contains relatively high CaCO3 (average = 50 wt%) but the concentrations of individual sediment samples vary greatly, largely because of the calcite laminae in the black shales of this unit and because the claystones are interbedded with limestones. The sediments of Units I, II, and III contain <0.1 wt% TOC. In marked contrast, the black shales have TOC concentrations of 1–16 wt%. The calcareous siltstones that compose Unit V have TOC values of ~0.6 wt%, which is double the average DSDP value (TOC = 0.3 wt%) compiled by McIver (1975). The Unit V siltstones are hence relatively rich in organic matter.
Concentrations of headspace methane remain at background levels (<3 ppmv) until Unit III, where they start to increase in conjunction with the depletion of concentrations of interstitial sulfate (see "Interstitial Gas Contents" in "Organic Geochemistry" in the "Site 1257" chapter). Maximum headspace methane concentrations of ~25,000–30,000 ppmv are reached in black shale Unit IV. These values are not particularly large. The maximum concentrations are accompanied by sharp decreases in C1/C2 ratios from 2300–1900 ppmv in Unit III to 1300–600 ppmv in the black shales (Fig. F1). Propane also appears at minor but significant concentrations (14–48 ppmv). Methane concentrations decrease below the black shale unit, and C1/C2 ratios are larger than values in Unit IV.
Concentrations of CaCO3 vary between 5 and 95 wt% in the sediments and rocks of Site 1258 (Fig. F2). Sediments of Units I, II, and III average 0.1 wt% TOC, whereas the black shales of Unit IV have an average of 7.9 wt% TOC. The calcareous mudstones that compose Unit V have an average TOC value of 4.2 wt%, which is much higher than in Unit V at the other four sites.
Concentrations of interstitial gases in lithostratigraphic Units I, II, and III at Site 1258 are low, but they increase in Units IV and V (Fig. F2). Methane concentrations first exceed 1000 ppmv at 220 meters below seafloor (mbsf) and thereafter increase slowly to the top of the black shale Unit IV at ~390 mbsf. Here, methane increases abruptly from ~10,000 to 43,300 ppmv and ethane concentrations exceed 100 ppmv for the first time at this site (see "Interstitial Gas Contents" in "Organic Geochemistry" in the "Site 1258" chapter). The methane concentrations reach a maximum of 65,000 ppmv at 430 mbsf and then slowly decrease to ~30,000 ppmv at the bottom of the hole. The concentrations of ethane and propane are relatively low compared to the yields in methane, yet they also seem to show a depth-related increase. The C1/C2 ratio decreases to values between 500 and 100 in the black shales and remains relatively low throughout Unit V.
Because gas concentrations are higher at Site 1258 than those at Site 1257, the NGA was employed to monitor the higher molecular weight volatile hydrocarbons butane (C4), pentane (C5), and hexane (C6) that occur in trace amounts below 425 mbsf (see "Interstitial Gas Contents" in "Organic Geochemistry" in the "Site 1258" chapter). These hydrocarbons are usually not considered to be microbially generated (Claypool and Kvenvolden, 1983). Because the organic matter in these sediments is thermally immature (see "Organic Matter Source Characterization" in "Organic Geochemistry" in the "Site 1258" chapter), the presence of the C4–C6 hydrocarbons suggests that some gases have migrated into Unit IV from an unidentified source deeper in the sedimentary section.
CaCO3 concentrations of the five lithostratigraphic units generally decrease with greater depth. However, the black shales that compose most of Unit IV still contain ~50 wt% CaCO3, largely because calcite laminae are interspersed among the black shale laminae. Sediments in Units I, II, and III contain <0.8 wt% TOC. The average TOC content for the black shale unit is 9.3 wt%. Unit V has TOC values that are <0.5 wt%.
Concentrations of headspace methane remain at background levels (<3000 ppmv) until near the base of Unit II, when they start to rise as concentrations of interstitial sulfate decrease (see "Interstitial Gas Contents" in "Organic Geochemistry" in the "Site 1259" chapter). Concentrations remain below 5000 ppmv until the black shale in Unit IV (Fig. F3). Maximum concentrations on the order of 62,000–76,000 ppmv are reached near the base of this unit. The maximum concentrations are accompanied by sharp decreases in C1/C2 ratios from 1600–1200 in Unit III to 800–500 in the black shales (Fig. F3). Propane also appears at minor concentrations near the base of Unit IV. The methane concentration decreases abruptly to 2300 ppmv in the sandstone unit underlying the black shale unit.
Concentrations of CaCO3 generally decrease with depth in the sedimentary succession of Site 1260. The black shale Unit IV averages ~50 wt% CaCO3 because calcite laminae are interspersed in this unit. The quartz sandstones of Unit V are low in calcite (Fig. F4). Most of the sediments of lithostratigraphic Units I, II, and III contain <0.1 wt% TOC. The black shales of Unit IV have TOC concentrations that average 7.2 wt% and reach as high as 13.9 wt%.
Concentrations of headspace methane first increase above background levels at the top of Unit III. Concentrations remain <4,000 ppmv until the top of black shale unit, where they abruptly increase to 17,000 ppmv (Fig. F4). A maximum concentration of 68,000 ppmv is attained near the middle of this unit. C1/C2 ratios sharply decrease from 1500 to 1000 ppmv in Unit III and from 900 to 300 ppmv in the black shales. Propane also appears at concentrations of 30–5 ppmv near the middle of Unit IV. Methane concentrations decrease moderately to 7000–2000 ppmv in the siltstone unit underlying the black shale unit, and C1/C2 ratios increase slightly (Fig. F4).
The sedimentary succession at Site 1261 differs from the other four Demerara Rise sites by its 350-m-thick clay-rich Unit I. Concentrations of CaCO3 are between 15 and 30 wt% through most of this unit and reach ~80 wt% only in the debris flow layer that is its base (Fig. F5). Unit II is composed of calcareous chalk and averages 75 wt% CaCO3, whereas the clayey calcareous chalks of Unit III average 47 wt%. The black shale unit is again quite variable in carbonate content, and the underlying sandstone is carbonate poor. Sediments of Units I, II, and III contain <0.5 wt% TOC, and the black shales of Unit IV have an average TOC concentration of 9.3 wt%,
Methane concentrations increase above background values within Unit I to reach 10 ppmv at 250 mbsf before decreasing to <3 ppmv. The appearance of methane coincides with the disappearance of interstitial sulfate at this depth (see "Interstitial Gas Contents" in "Organic Geochemistry" in the "Site 1261" chapter). Methane concentrations increase again above baseline levels starting at ~400 mbsf in Unit II to reach 19,300 ppmv at the base of Unit III (Fig. F5). The amount of methane increases sharply to 62,000 ppmv at the top of the black shale Unit IV, and ethane appears at concentrations above 100 ppmv (see "Interstitial Gas Contents" in "Organic Geochemistry" in the "Site 1261" chapter). Methane concentrations remain high through the black shale unit, reaching a maximum of 111,000 at 586 mbsf. The C1/C2 ratio decreases from >1000 in Unit III to values ~500 in the black shales.