Concentrations of calcium carbonate and TOC were determined on sediments from Holes 1260A and 1260B. 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 was performed for drilling safety, and possible microbial activity was assessed from headspace gas contents of Hole 1260A.
Concentrations of inorganic carbon vary from virtually 0 to 11.6 wt% in sediments at Site 1260 (Table T16). These concentrations are equivalent to 0.1 to 96 wt% CaCO3, assuming that all of the carbonate is calcite or aragonite. Carbonate concentrations of the five lithostratigraphic units (see "Lithostratigraphy") generally decrease with greater depth. However, black shales in Unit IV still contain ~50 wt% carbonate, largely because calcite laminae are interspersed in this laminated unit.
TOC concentrations of sediments at Site 1260 have a wide range. Most of the sediments of lithostratigraphic Units I–III contain <0.1 wt% TOC (Table T16). In marked contrast, the black shales in Unit IV have TOC concentrations that average 7.2 wt% and reach as high as 13.9 wt%. A piece of carbonized wood was found in this unit; its TOC value is 55.9 wt% (Sample 207-1260B-39R-7, 29–30 cm). Unit V (calcareous siltstones) has TOC values that cluster at ~0.5 wt%, which is nearly twice that of the average deep-sea value of 0.3 wt% compiled by McIver (1975) from Deep Sea Drilling Project Legs 1–33, and this unit is therefore relatively rich in organic matter.
Atomic Corganic/ Ntotal ratios were employed to help identify the origins of the organic matter of Site 1260 sediments. Most of the C/N values in organic carbon–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 the low TOC concentrations combined with the tendency of clay minerals to absorb ammonium ions generated during degradation of organic matter (Müller, 1977). Values of other samples are artificially elevated because their low amounts of C and N are near the limits of detection of the shipboard elemental analyzer.
The C/N ratios of the black shales in Unit IV average 33, which is a value usually considered typical of land-plant organic matter but that is also common to Cretaceous black shales (Meyers, 1997). A 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 that is thermally immature and relatively well preserved (Espitalié et al., 1977; Peters, 1986). The generally low Tmax values of this unit (Table T17) confirm the thermal immaturity. 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).
The C/N value of the carbonized wood in Sample 207-1260B-39R-7, 29–30 cm, is 81, which clearly identifies the organic matter of this sample as land-plant woody tissue (Meyers et al., 1995). A number of C/N values between 40 and 60 in the lower half of Units IV and V (Table T16) suggests some fraction of land-plant debris in these sequences.
Based on its low HI values (Table T17), organic matter in the lower Albian siltstones of Unit V is composed of Type III kerogen (Fig. F18). This type of kerogen appears to represent a mixture of degraded marine organic matter and detrital land-derived organic matter at Site 1260.
Sediments at Site 1260 have fairly low interstitial gas concentrations. Neither gas voids nor other evidence of gas release from cores was observed. A faint odor of hydrogen sulfide was noticeable in cores from the black shales in lithostratigraphic Unit IV (391–484 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 similar (Fig. F19). Methane (C1) first appears in the routine monitoring results at 273 mbsf. Concentrations increase to reach a broad maximum between 393 and 474 mbsf before decreasing with greater depth in the core. High methane/ethane (C1/C2) ratios and the absence of measurable amounts of higher molecular weight volatile hydrocarbons indicate that the origin of the methane is biogenic and not thermogenic (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 consistently increase and abruptly peak 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 Holes 1259A, 1260A, and 1260B. A rough correspondence exists between higher TOC and larger gas concentrations (Fig. F20). Marked 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. F19) suggest either that gas does not freely migrate from its origin in the black shales or that it migrates and is being quickly generated from the organic matter in this unit so that elevated concentrations are maintained.