Concentrations of inorganic carbon and TOC were determined on sediments from Holes 1259A, 1259B, and 1259C. 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 Hole 1259A was performed for drilling safety, and the possiblity of microbial activity was evaluated from headspace gas contents of this hole.
Concentrations of inorganic carbon vary from 0.02 to 11.3 wt% at Site 1259 (Table T16). These concentrations are equivalent to 0.2–93.9 wt% sedimentary CaCO3, assuming that all of the carbonate is calcite or aragonite. Carbonate concentrations in the five lithostratigraphic units (see "Lithostratigraphy") generally become less with greater depth. However, the black shales that compose most of Unit IV still contain ~50 wt% carbonate, largely because calcite laminae were interspersed among the black shale laminae.
Site 1259 sediments have a wide range of TOC concentrations. The sediments of Units I–III contain <0.8 wt% TOC (Table T16). In marked contrast, the black shales in Unit IV have TOC concentrations between 1.3 and an exceptionally high 29 wt% (Sample 207-1259B- 24R-3, 141–142 cm; C/N ratio = 41). The average TOC content for Unit IV is 9.3 wt%. One sample in Unit V (Sample 207-1259C-19R-3, 83–84 cm) has a TOC of 6 wt% and probably contains woody material; its high C/N ratio of 61 indicates its terrestrial origin. Otherwise, Unit V has low TOC values of <0.5 wt%.
Atomic Corganic/Ntotal ratios were employed to help identify the origins of organic matter in Site 1259 sediments. C/N values in organic-lean Units I, II, and III are low, commonly below the range typical of fresh algal organic matter (4–8) (Meyers, 1997). These values are probably an artifact of the low TOC concentrations combined with the tendency of clay minerals to absorb ammonium ions generated during degradation of organic matter (Müller, 1977).
The C/N ratios of the black shales in Unit IV average 29, which is a value typical of land-plant organic matter but also common to Cretaceous black shales (Meyers, 1997). A van Krevelen–type plot of hydrogen index (HI) and oxygen index (OI) values (Fig. F21) indicates that the black shales in Unit IV contain Type II (algal) organic matter. High HI and low Tmax values 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). There are four data points in the van Krevelen plot that appear to be a mixture of Types II and III organic matter; Tmax values >400°C for three out of four of these samples confirm greater maturity that is most probably due to incorporation of some organic detrital matter in the black shales. 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).
Sediments at Site 1259 have low interstitial gas concentrations. Neither gas voids nor other evidence of gas release from cores was observed. A significant odor of hydrogen sulfide was noticeable in cores from the black shales (Unit IV; 493–563 mbsf), but the natural gas analyzer, which has a sensitivity of ~1 ppmv, did not detect this gas in headspace samples.
Headspace gas results from routine safety monitoring and the special microbial gas study are shown in Figure F22. The routine safety monitoring analyses show methane first appearing at 350 mbsf (bottom of Subunit IIC; foraminifer-nannofossil chalk) and concentrations steadily increasing to a maximum 76,400 ppmv at 545 mbsf (bottom of Unit IV; black shale) before falling to 2,300 ppmv in the last core (Unit V; quartz sandstone). Methane determined as part of the microbial gas study shows higher, though still small (3–50 ppmv), concentrations between 0 and 350 mbsf, then a rapid increase to 56,400 ppmv at the bottom of the black shales in Unit IV. The last core was not measured in the microbial gas study. The differences observed between the two experiments most likely reflects the different treatment of samples prior to analysis (see "Organic Geochemistry" in the "Explanatory Notes" chapter). In both cases, however, high methane/ethane (C1/C2) ratios and the absence of significant amounts of C2 and C3 indicate that the methane is biogenic, rather than thermogenic, in origin (Table T18). A biogenic origin is also supported by the disappearance of interstitial sulfate at the same subbottom depth where methane concentrations begin to rise (see "Inorganic Geochemistry"); interstitial sulfate generally inhibits microbial methanogenesis (Claypool and Kvenvolden, 1983).
Interstitial gas concentrations and compositions were found to relate to lithology; concentrations increased abruptly and peaked in the black shales. The possible relation between sediment organic matter contents and gas concentrations was investigated by measuring the TOC concentrations of the headspace sediment samples from Hole 1259A. A rough correspondence exists between higher TOC and larger gas concentrations (Fig. F23). Excursions from a simple linear relation suggest that organic matter quality, and not simply quantity, affect gas generation from the black shales. Moreover, dramatic changes in methane concentrations at lithologic boundaries (Fig. F22) suggest either that gas does not freely migrate from its origin in the black shales or that it migrates and is consumed/oxidized at the base of the sulfate reduction zone (see "Inorganic Geochemistry").