As part of the shipboard safety and pollution-prevention monitoring program, hydrocarbon gases were analyzed in each core of Hole 1123A, in each core below 150 mbsf of Hole 1123B, and in each core below 490 mbsf of Hole 1123C by the headspace technique. Gas pockets were not encountered. The headspace methane concentrations are very low (<10 ppm). Despite the relatively high organic-carbon concentrations (see "Carbonate and Organic Carbon"), this finding indicates low amounts of metabolizable organic matter and is corroborated by interstitial sulfate concentrations above 10 mM (see "Inorganic Geochemistry").
The abundance of total, inorganic, and organic carbon and of calcium carbonate in sediments from Holes 1123A, 1123B, and 1123C is summarized in Table T18 (also in ASCII format). Random sampling of all lithologies was performed for carbonate analysis, and one sample per core was analyzed for organic carbon.
Carbonate contents vary between 10.3 and 84.3 wt% with an average of 57.5 wt% (Fig. F35). Variations are higher in the upper 180 mbsf (lithostratigraphic Subunit IA; see "Lithostratigraphy") and below 470 mbsf (Units III and IV) than in the section in between (Units IB and II). The upper boundary may reflect the Miocene/Pliocene boundary, whereas the lower one does not correspond to a chronostratigraphic transition. The sample with the value of 84.3 wt% (Sample 181-1123C-25X-2, 38-39 cm; 548.08 mbsf) is from the debris flow within the sequence.
Sediments at Site 1123 average 0.59 wt% organic carbon (Fig. F36), which is twice the average for deep-sea sediments of 0.3% compiled by McIver (1975) for DSDP Legs 1 through 33. The highest values were found in the uppermost samples and concentrations decrease slightly with increasing depth. The variations are high and without any apparent correlation with lithology. Despite the relatively high concentrations of organic carbon, there is no indication for present-day bacterial degradation of organic matter as indicated by low methane concentrations. Thus, the organic matter may be already reworked by bacteria, or large amounts are derived from terrigenous sources and are thus more refractory.
The source of organic matter in Site 1123 samples was determined by either atomic organic carbon/nitrogen values or by Rock-Eval pyrolysis. Algal organic matter generally has C/N values of between 5 and 10, whereas organic matter derived from land plants has values of between 20 and 100 (e.g., Premuzic et al., 1982; Meyers, 1994).
Atomic organic carbon/nitrogen values were calculated for Site 1123 samples using total organic carbon and total nitrogen concentrations. The C/N ratios vary from 3.2 to 25.9 with an average of 10.9 (Table T18), without apparent correlation with depth, age, or lithology. These values suggest a predominantly marine source with varying amounts of terrestrial organic matter. Alternatively, the initial predominantly terrigenous organic material was affected by postdepositional diagenetic processes, and, thus, C/N values were altered. Degradation of organic matter in organic carbon-poor marine sediments tends to lower C/N values as nitrogenous compounds break down to produce ammonia, which is retained by clay minerals (Müller, 1977), and the produced CO2 diffuses out of the sediment.
Rock-Eval analyses were conducted for 10 selected samples with organic carbon contents higher than 0.5% (Table T19). In general, volatile hydrocarbons (S1), and hydrocarbons (S2) generated by thermal cracking of kerogen can only be detected in very low concentrations, and, thus, the results must be interpreted carefully to eliminate a possible artifact caused by measurement error. Plotted in a van Krevelen-type diagram as hydrogen index (HI) vs. oxygen index (OI) values, the data suggest that the sediments contain Type III (terrigenous) organic matter (Fig. F37). This source assignment conflicts with the intermediate C/N values for these samples. The contradiction between the Rock-Eval source characterization and the elemental source characterization indicates that the organic matter has been heavily oxidized, probably by microbial reworking.
In summary, an unambiguous source assignment by determination of C/N values and Rock-Eval parameters is not possible because of strong organic matter degradation.