ORGANIC GEOCHEMISTRY

Carbonate determinations by coulometry were made for a total of six samples from Hole 1214A (Table T6). Samples were chosen to provide a measure of the carbonate content in Albian limestones from Subunit IIB. Elemental concentrations of C, H, N, and S were also measured (Table T6) for three samples from the lower Aptian (Subunit IID) to assess the enrichment of organic matter in discrete intervals. Two samples (198-1214A-23R-1, 20-22 cm, and 87-89 cm) taken from darker horizons interbedded with green claystones proved lean in organic matter (0-0.1 wt%). The contents of organic carbon are higher (1.4 wt%) in a clayey porcellanite (Sample 198-1214A-23R-1, 5-7 cm), though lower than the enriched values found in the lower Aptian at Sites 1207 and 1213 (Tables T12 in the "Site 1207" chapter and T11 in the "Site 1213" chapter).

Hydrocarbon compositions were examined in the lower Aptian sample (198-1214A-23R-1, 5-7 cm) containing elevated organic carbon contents (>1 wt%) (Table T6) to characterize the biological sources and thermal maturity of its organic matter. In addition, comparison of biomarker distributions with other lower Aptian C_org-rich intervals can help assess the depositional conditions that led to enhanced sequestration of organic matter. The procedure followed the methodology described in "Organic Geochemistry" in the "Explanatory Notes" chapter. In addition, gas chromatography-mass spectrometry (GC-MS) was used to examine the composition of CH₂Cl₂ eluates from the silica separation step of the samples consisting primarily of ketones.

Examination of the lower molecular weight hydrocarbons was precluded by the presence of significant contaminant hydrocarbons from petroleum products, but the polycyclic steroid and hopanoid hydrocarbons could be evaluated. Components were identified from responses at specific target intervals in individual ion chromatograms and from their mass spectra. All of the hydrocarbons are well documented as components of mid-Cretaceous black shales (e.g., Barnes et al., 1979; Rullkötter et al., 1984, 1987; Farrimond et al., 1986b, 1990; Simoneit, 1986; Simoneit and Stuermer, 1982). There are fewer published identifications of ketones in sediments of this age (Comet et al., 1981; Farrimond et al., 1986a).

The patterns of distributions of the hydrocarbons and ketones (Table T7; Fig. F7) are broadly similar to those observed at Sites 1207 and 1213. Among the hydrocarbons, the prominence of 4- and 5-sterenes derived from eukaryotic sterols is more reminiscent of Site 1207, although none of the bacterial methylhopanoids observed in the C_org-rich intervals at that site are present. The distribution pattern of steroidal ketones closely resembles that found in Hole 1213B, but the distinctive components derived from various bacterial sources (cf. Table T14 in the "Site 1213" chapter) are absent. Thus, the uniformity in characteristics attributable to algal primary producers over the region of Shatsky Rise provides a stark contrast to the restricted occurrence of critical bacterial markers, notably methylhopanoids. However, it is also important to emphasize that the limited recovery means that Sample 198-1214A-23R-1, 5-7 cm, is probably not representative of the episode of maximum sequestration of organic matter during OAE1a at Site 1214. The close resemblance of its polycyclic hydrocarbon distribution to the leanest interval at Site 1207 (Sample 198-1207B-44R-1, 103-104 cm) supports this assessment. These considerations reinforce the interpretation that the depositional conditions that led to accumulation of horizons containing >10% of C_org were clearly exceptional and suggest that they promoted bacterial processes that otherwise were of minor significance.