Concentrations of inorganic carbon (IC) and TOC were determined on sediments from Holes 1257A, 1257B, and 1257C. 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 1257A was performed for drilling safety, and possible microbial activity was determined from headspace gas contents of this hole.
Concentrations of IC vary from 1 to 11.9 wt% at Site 1257 (Table T16). These concentrations are equivalent to 9–99 wt% sedimentary CaCO3, assuming that all of the carbonate is present as calcite or aragonite. Carbonate concentrations of the five lithostratigraphic units (see "Lithostratigraphy") generally decrease 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.
TOC concentrations of the five lithostratigraphic units have a wide range. The sediments of Units I, II, and III contain <0.1 wt% TOC (Table T16). In marked contrast, the black shales in Unit IV have TOC concentrations between 1 and 15.8 wt%. The calcareous siltstones that compose Unit V have TOC values that cluster closely around 0.6 wt%, which is twice that of the average deep-sea value of 0.3 wt% compiled by McIver (1975) from DSDP Legs 1–33.
Atomic C/N ratios were employed to help identify the origins of organic matter in Site 1257 sediments. C/N values in organic-lean Units I–III are low, commonly 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 sorb ammonium ions generated during degradation of organic matter (Müller, 1977).
The C/N ratios of the black shales in Unit IV average 27, 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. F14) 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). 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 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).
Based on its low HI values, elevated OI values, and marine C/N values, organic matter in the calcareous siltstones of Unit V appears to be composed of Type IV kerogen. This type of kerogen represents marine organic matter that has experienced substantial degradation and oxidation. The presence of relatively uniform and elevated concentrations (TOC = 0.6 wt%) of detrital organic matter in these middle–upper Albian siltstones represents unusual conditions of deposition.
Sediments at Site 1257 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 shale in lithostratigraphic Unit IV (174–227 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 very similar (Fig. F15). Methane first appears at 107 mbsf. Concentrations rapidly increase to reach a broad maximum between 180 and 218 mbsf before decreasing with greater depth in the core. High C1/C2 ratios and the absence of measurable amounts of higher molecular weight volatile hydrocarbons 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).