Concentrations of inorganic carbon (IC) and TOC were determined on sediments from Holes 1258A, 1258B, and 1258C. Organic matter atomic carbon/nitrogen (C/N) ratios and Rock-Eval pyrolysis analyses were employed to assess the type of organic matter contained in the sediments. Routine monitoring of interstitial gas contents of the three holes was performed for drilling safety and pollution prevention, and possible microbial activity was investigated from headspace gas contents of Hole 1258A.
Concentrations of inorganic carbon vary from 0.7 to 11.5 wt% at Site 1258 (Table T16). These concentrations are equivalent to 6–96 wt% sedimentary CaCO3, assuming that all of the carbonate is present as calcite or aragonite. All five lithostratigraphic units at this site (see "Lithostratigraphy") contain CaCO3 but show differences in average concentrations related to their facies. Units I and II are strongly carbonate-dominated fine-grained sediments having concentrations in the range of 40–80 wt%. The average carbonate content declines in Unit III to 20–40 wt%. The black shales in Unit IV average ~50 wt% but have a marked layer-by-layer scale variation between 4 and 96 wt%. This variability relates to lithologic changes, mainly the alternating presence of abundant calcite-enriched laminae between more clay-dominated black shale intervals. In Unit V, carbonate content averages 15.5 wt% and ranges between 6 and 24 wt%.
TOC concentrations of the five lithostratigraphic units at Site 1258 have a wide range. The sediments of Units I–III contain <0.7 wt% TOC (average = 0.1 wt% TOC) (Table T16). The sediments of Unit IV have an average TOC concentration of 7.9 wt% but vary widely between 0.1 and 28.3 wt%. In strong contrast to the age-equivalent unit at Site 1257, the calcareous mudstones that compose Unit V at Site 1258 have much higher TOC values, ranging from 2.2 to 5.4 wt% (average = 4.2 wt%). An unusually high TOC of 36.6 wt% was measured in Sample 207-1258C-31R-2, 74–75 cm, in Unit V, which is a piece of charcoal embedded in surrounding shale (see Table T16).
Atomic C/N ratios were employed to help identify the origins of organic matter in sediments at Site 1258. Most of the C/N values in organic-lean Units I–III are low (Table T16) and are below the range typical of fresh algal organic matter (4–10) (Meyers, 1997). These values are probably an artifact of low TOC concentrations combined with the tendency of clay minerals to sorb ammonium ions generated during degradation of organic matter (Müller, 1977). Some values are artificially elevated because the low concentrations of C and N are near the limits of detection of the elemental analyzer.
The C/N ratios of the black shales in Unit IV average 31.2, which is a value typical of land-plant organic matter but is also common to Cretaceous black shales (Meyers, 1997). A van Krevelen–type plot of hydrogen index (HI) and oxygen index (OI) values (Fig. F18) indicates that the black shales in Unit IV contain Type II (algal) organic matter. High HI and low Tmax values (~400°C) like those found in the black shales (Table T17) are characteristic of thermally immature, relatively well preserved marine organic matter (Espitalié et al., 1977; Peters, 1986). Consequently, the elevated C/N values that mimic those of land-derived organic matter are likely to be the result of partial alteration of marine organic matter. A probable scenario is that nitrogen-rich components are preferentially degraded during sinking of organic matter to the seafloor, thereby elevating the C/N ratio of the surviving organic matter (Twichell et al., 2002).
Organic matter in Unit V yields slightly lower HI values than those found in Unit IV, whereas the OI values remain similar. The C/N values of Unit V vary from 18.2 to 30.1 (average = 25.8), which is too high for unaltered marine organic matter. This unit seems to contain mostly marine but also some terrestrial organic matter that represents a mixture of Types II and III. This mixture can be inferred from the lower HI values and their cluster in Figure F18 as well as from the observation of charcoal and wood particles (see "Lithostratigraphy"). The average Tmax value of 418°C for Unit V is higher than that in Unit IV, which can be explained by the mixed organic matter type in Unit V (i.e., namely by the minor contribution of terrestrial organic matter). On the other hand, it cannot be ruled out that the observed difference in Tmax values might also indicate overall poorer conditions of organic matter preservation in Unit V as discussed previously for the age-equivalent sediments of Unit V at Site 1257 (see "Organic Matter Source Characterization" in "Organic Geochemistry" in the "Site 1257" chapter).
Concentrations of interstitial gases in lithostratigraphic Units I–III at Site 1258 were low but increased in Units IV and V. Gas voids were not observed, but some cores from Unit IV showed minor amounts of degassing when brought on deck. Small gas bubbles were visible through the core liner for a short time until the cores adjusted to the surrounding warm temperature and low pressure conditions. A faint odor of hydrogen sulfide was noticeable in cores from organic matter–rich Units IV and V but to a lesser extent than described at Site 1257 (see "Interstitial Gas Contents" in "Organic Geochemistry" in the "Site 1257" chapter).
Headspace gas results from routine safety monitoring and the special microbial gas study are very similar (Fig. F19). Methane was at background levels (<5 ppmv) in the upper part of the sequence and slowly but continuously increased until ~220 mbsf, where concentrations exceeded 1000 ppmv for the first time. Methane concentrations thereafter continued to increase slowly in a more or less linear manner until the top of the black shales of Unit IV was penetrated at ~390 mbsf. Here, the methane yield increased from ~10,000 to 43,300 ppmv and ethane was recorded for the first time in concentrations >100 ppmv (Table T18). Methane concentrations continued to increase with depth and reached a maximum of 65,000 ppmv at 430 mbsf and then slowly declined to ~30,000 ppmv at 482 mbsf in the last core taken. The absolute values of ethane and propane were relatively low compared to the yields of methane, but they also seemed to show a depth-related increase. The natural gas analyzer (NGA) was employed to monitor higher molecular weight volatile compounds like butane, pentane, and hexane, which were present in traces below 425 mbsf (Table T19). The latter compounds are usually not considered to be associated with microbial activity (Claypool and Kvenvolden, 1983). At Site 1257, high methane/ethane ratios and the absence of measurable amounts of higher molecular weight volatile hydrocarbons indicated that the methane was biogenic, rather than thermogenic, in origin. For Site 1258, the situation is more complicated, as it seems that both microbial and thermogenic gases were present. Another difference between the two sites is that the ratio of methane to ethane continued to stay relatively low throughout Unit V at Site 1258. The lower ratio could be explained by the higher amounts of organic carbon in Unit V, which potentially provided a better substrate for microbial activity than in Unit V at Site 1257, or it could indicate migration of gases into this unit from deeper in the section. Because of the possible presence of thermogenic gases, gas monitoring was performed on all three holes drilled at Site 1258.
A point for further consideration would be the potential at Site 1258 for gas hydrate formation or preservation, as it is located at a water depth of 3192 m. The ~450-mbsf drilling depth places the organic-rich Units IV and V in the gas hydrate stability zone (Claypool and Kaplan, 1974) over a rather wide temperature range. Interstitial chlorinity decreases in Units IV and V (see "Inorganic Geochemistry"), which is often an indicator of hydrate decomposition. In contrast, results from downhole logging (see "Downhole Logging") do not provide any supporting evidence for the presence of gas hydrates either in Unit IV or V. Resistivity logs also strongly contradict the observation of lowered interstitial chlorinity for Unit V. Nevertheless, it cannot be completely ruled out that gas hydrates may have been present in Units IV and V during the past and have recently dissociated so that only a relict signal (i.e., in the pore water) has remained. It would be potentially useful to analyze the oxygen isotopic composition of pore water or of newly overgrown carbonate cements in the critical intervals, as gas hydrate formation is known to increase the 18O value of pore water (e.g., Matsumoto and Borowski, 2000).