Only trace amounts of methane (<16 ppmv) and no other hydrocarbon gases were detected in sediments at Site 1145. Carbonate concentrations were mostly low, between 3 and 19 wt%, in the upper 190 mcd. There is a marked increase at 170 mcd, and in the lower 170-210 mcd of the sediment section, carbonate represents as much as 30 wt%. Total organic carbon (TOC) obtained by difference calculation (total carbon [TC] - inorganic carbon [IC]) declined steadily from >1 wt% at the top of Hole 1145A to much lower concentrations (~0.2 wt%) at the base, consisting of organic matter (OM) with a lower organic C/N ratio (probably marine OM).
Three samples per core were analyzed for inorganic carbon content, and one sample per core for TC, total nitrogen (TN), and total sulfur (TS) content from Hole 1145A (Table T12). Carbonate varies from 3.2 to 30.5 wt%, with two distinct intervals apparent (Fig. F15A). The upper interval (0-170 mcd) contains sediments with lower carbonate content (average [AV] = 10.65 wt%; standard deviation [SD] = 3.40), with some notable excursions (e.g., Sections 184-1145A-5H-5, 17.5 wt%; 12H-3, 3.2 wt%). However, below 170 mcd, carbonate content and variability increase significantly (AV = 18.82 wt%; SD = 7.07), reaching a maximum in Core 184-1145A-31X, then declining to the base of Hole 1145A.
The TOC concentration by difference (TC - IC) was determined for one sample per core for Hole 1145A (Table T12). It decreases from 1 wt% at the top of the hole to ~0.25 wt% from 0 to 130 mcd; below this level, values remain near 0.25 wt% to the bottom of the hole. Little correlation is observed between organic and inorganic carbon, and the decrease in TOC in the upper interval is independent of carbonate abundance (Fig. F15B).
Sulfur values vary between <0.02 and 0.36 wt%, with higher values paired with higher TOC values, mostly from the top of Hole 1145A down to Core 184-1145A-12H (Table T12). The very low values (<0.02 wt%) probably result from the small physical size of the carbonate samples (as opposed to a channel or composite sample representing the full section length) and from the localization of S in pyrite-filled burrows. Such burrows are reported (see "Lithostratigraphy") in the lower cores of Hole 1145A, where these values are observed. The S analyses with very low values (<0.02 wt%) were made with a new reaction column, which produced a small S peak that apparently was below the integration limit of the instrument software. A blank (tin crucible with V2O5) run in the same set showed no S peak.
Geochemical analysis of the sediments from Hole 1145A allows some characterization of the organic matter. As at previous sites on this leg (see "Organic Geochemistry" in the "Site 1143" chapter and "Organic Geochemistry" in the "Site 1144" chapter) and during previous Ocean Drilling Program (ODP) legs (cf. Carter, McCave, Richter, Carter, et al., 1999), the C/N ratio data include some unreasonably low values, even for fresh marine OM. These data suggest a systematic error in either the TOC by difference calculation or N determination or, alternatively, a significant contribution from inorganic N (Table T12). However, the range of C/N values is informative and indicates that the residual OM in Hole 1145A is of a degraded, predominantly marine origin. A significant shift to lower C/N values occurs at 100 mcd, well below the depth of most of the TOC reduction as indicated by the profile of dissolved sulfate (Figs. F15C, F16C).
Average marine organic matter is characterized by the "Redfield" ratio of carbon to nitrogen (106:16), a requirement of phytoplankton cells (Redfield et al., 1982; Riebesell et al., 1993; Broecker and Peng, 1982), yielding an expected average C/N value of ~7. However, research suggests that carbon consumption, relative to nitrogen, may occur in coastal and deep-sea waters both greater than and less than that predicted. In the deep ocean, most nitrogen is cycled within the upper layers; production is driven by ammonia released from degrading plankton. Nitrogen in plankton is present in an average weight ratio of 100:20 (Romankevich, 1984). For certain plankton the C/N ratio decreases to near 4 (e.g., diatomaceous suspensions). During complete OM mineralization, N is liberated as ammonium salts, which may undergo further oxidation to free nitrogen, nitrite, and nitrate ions as a result of bacterial activity. In sediments containing predominantly clay, nitrogenous compounds may be preferentially preserved from bacterial metabolism as the major proportion is tightly sorbed by the mineral base of the sediment (Suess and Müller, 1980). The impact of differences in composition of clay minerals, and benthic enrichment of the OM by clays with nitrogenous compounds, appears small (Romankevich, 1984).
In summary, the low C/N values recorded at this site may be partly explained by the origin of the OM. However, the declining TOC values with depth may suggest a possible increased ammonium content during OM destruction; such an increase in inorganic nitrogen may well explain the lower C/N values observed.
Seven samples from above 100 mbsf containing >0.5 wt% TOC (by difference) were analyzed by Rock-Eval pyrolysis (Table T13). All samples yielded calculated TOC values ~30% lower than calculated by difference (TC - IC). All samples yielded Tmax values <400°C, indicating immature organic matter as expected from the shallow burial depth and normal geothermal gradient (see "Physical Properties"). The exception is Sample 184-1145A-12H-3, 107-108 cm, with a higher Tmax. However, the inflection point of the S2 peak from which the Tmax is determined is indistinct, and the peak temperature assignation is therefore not precise. The S2 peak for Sample 184-1145A-12H-3, 107-108 cm, appears to be the same as the other samples from Hole 1145A. The production index appears anomalously high for all samples relative to the Tmax. This suggests that S1 values are also anomalously high and would be expected to increase—not decrease—with depth if there was thermogenic generation of hydrocarbons. Both hydrogen index (HI) and oxygen index (OI) values (see "Organic Geochemistry" in the "Explanatory Notes" chapter) are less reliable for young, immature OM (Katz, 1983) but appear to be indicative of type III kerogen, lignin-rich of terrestrial origin or more oxidized marine OM. Nevertheless, the suggestion of highly oxidized residual organic material may well be applicable to such low HI and high OI values at this deep-water site, where the OM has sufficient time to be oxidized during settling through the water column to the sediment surface.