Concentrations of inorganic carbon and TOC were determined on sediments from Holes 1261A and 1261B. 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 two holes was performed for drilling safety and pollution prevention, and the possibility of microbial activity was evaluated from headspace gas contents of Hole 1261B.
Concentrations of inorganic carbon vary from 0.1 to 11.7 wt% at Site 1261 (Table T14). These concentrations are equivalent to 0.9–93 wt% CaCO3, assuming that all of the carbonate is calcite or aragonite. All five lithostratigraphic units at this site (see "Lithostratigraphy") contain carbonate but show differences in average concentrations related to their facies. In Subunits IA and IB, carbonate concentrations range between 15 and 30 wt%, whereas Subunit IC shows a wider scatter between 25 and 78 wt%. Unit II is composed of calcareous chalk, representing the most carbonate-rich unit at Site 1261, with most samples clustering close to the average concentration of 75 wt% carbonate. The clayey calcareous chalks of Unit III show a wider scatter of carbonate contents, between 17 and 77 wt% (average = 47 wt%). The black shales in Unit IV average ~43 wt% carbonate but have a marked layer-by-layer scale variation between 1 and 97 wt%. This variability is associated with lithologic changes, mainly the alternating abundant calcite-enriched laminae between clay-dominated black shale intervals. From Unit V, only one sample (Sample 207-1261B-16R-1, 99–100 cm), from a short, dark, clay-rich interval intercalated with the dominating sandy lithofacies, was analyzed, yielding 3.8 wt% carbonate.
There is a wide range of TOC concentrations among the five lithostratigraphic units at Site 1261. The sediments of Units I–III contain <0.5 wt% TOC (average = 0.1 wt% TOC) (Table T14). The sediments of Unit IV have an average TOC concentration of 9.3 wt% but vary widely (0.4–16.1 wt%). The TOC of Sample 207-1261B-16R-1, 99–100 cm, from Unit V, is 1.0 wt%. However, this clay-rich sample is not representative of this sandstone unit.
Atomic Corganic/ Ntotal ratios were employed to help identify the origin of organic matter in sediments at Site 1261. Most of the C/N values in organic-lean Units I–III are low (Table T14) 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). Some values are artificially elevated because the low amounts 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 34.3, 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. F13) indicates that the black shales in Unit IV contain Type II (algal) organic matter. High HI and low Tmax values (average = 397°C) like those found in the black shales (Table T15) are characteristic of thermally immature, relatively well preserved marine organic matter (Espitalié et al., 1977; Peters, 1986). Consequently, the elevated C/N values, which mimic those of land-derived organic matter, are probably the result of partial alteration of marine organic matter. A likely 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 is represented by only one value of 1.0 wt% TOC (Table T14). Considering this low TOC yield in comparison to the black shales in Unit IV, the C/N ratio of 38.3 could indicate land-plant derived organic matter. Rock-Eval HI and OI values (Fig. F13) place this sample into the transitional field between Types II and III kerogen, indicating a mixture of predominately algal and some terrigenous organic matter. The Tmax of this sample is 412°C and is therefore slightly higher than the average Tmax of the samples from Unit IV. This difference can be explained by either the mixed organic matter type or overall poorer conditions of organic matter preservation in Unit V.
Concentrations of interstitial gases in lithostratigraphic Units I–III at Site 1261 were generally low but slightly higher in comparison to other Leg 207 sites. The gas concentrations increase dramatically in Units IV and V. In these units, gas voids were not observed, but most cores from Unit IV showed active degassing when brought on deck. Small gas bubbles were visible through the core liner for several minutes until the cores adjusted to surrounding warm temperature and low pressure conditions. When the cores were cut after several hours, some still yielded a large amount of gas that was liberated during the cutting process.
The results of the headspace and microbial gas analysis are very similar (Fig. F14). Methane (C1) concentrations rose above background values (>3 ppmv) in lithostratigraphic Unit I. After a concentration maximum (10 ppmv) was reached at 251 mbsf, methane yields gradually returned to background values until 394 mbsf. Although the C1 concentrations in this specific interval are very low, the presence of some methane is consistent with a decrease in interstitial sulfate concentrations (refer to "Inorganic Geochemistry"). Interstitial sulfate is known to generally inhibit microbial methanogenesis (Claypool and Kvenvolden, 1983), and sulfate disappeared completely from the pore water at ~250 mbsf in Hole 1261A.
Below 400 mbsf, the methane concentrations continuously increased to reach 19,300 ppmv at the base of Unit III (562 mbsf). When the top of the black shales in Unit IV was penetrated at ~564 mbsf, methane increased suddenly to 62,000 ppmv and ethane (C2) was recorded for the first time in concentrations >100 ppmv (Table T16). The methane concentrations stayed elevated (average = 50,000 ppmv) through the black shale unit, reaching a prominent maximum of 111,000 ppmv at 586 mbsf. The last sample from Unit IV at 653 msbf contained 53,800 ppmv methane. The two cores from Unit V were not suitable for gas monitoring because of their quartz sandstone lithology.
The natural gas analyzer was employed to monitor the higher molecular weight volatile compounds butane (C4), pentane (C5), and hexane (C6) in the black shale unit and to augment the routine monitoring. These three compounds are usually not considered to result from microbial activity (Claypool and Kvenvolden, 1983), and their presence might therefore indicate thermogenic gas. They were found only in traces (<5 ppmv) at Site 1261. The methane/ethane (C1/C2) ratio dropped from >1000 ppmv (Table T16) above Unit IV to somewhat lower values in the black shales. The ratio stayed at relatively high levels (~500 ppmv) throughout the middle of the unit and increased at the base. The overall depth trend of the C1/C2 ratio in Unit IV and the complete disappearance of interstitial sulfate just above the unit (see "Inorganic Geochemistry") strongly suggest an exclusively microbial origin for the gas at this site. As neither the organic matter concentration nor its quality (the lipid richness of organic matter expressed by its average HI values) are elevated with respect to other Leg 207 sites, the relatively high methane concentrations observed at Site 1261 can be most likely explained by higher lithostatic pressure.