Previous   |    Next

doi:10.2204/iodp.proc.318.203.2015

Results and discussion

At Site U1356, 25 samples ranging in age from early Eocene to late Miocene were extracted for lipids, separated into chemical fractions, and analyzed by GC-MS onboard the R/V JOIDES Resolution. The compound classes reported include aliphatic hydrocarbons, hopanes, unresolved complex mixtures of branched alkanes, ketones, n-alkanols, sterols, diols, and n-alkanoic acid methyl esters (FAMES). Relative to the Oligocene and Miocene, molecular concentrations generally increase by several orders of magnitude in the early Eocene sediments. The N1, N3, N4, and FAMES fractions were analyzed by GC-MS, and the results from each fraction are summarized in Tables T10, T11, T12, and T13 in the “Site U1356” chapter [Expedition 318 Scientists, 2011b]), respectively (the N2 and N5 fractions were archived). Examples of GC-MS total- and single-ion chromatograms (of the four analyzed fractions) from four samples are presented in Figures F23 (middle Miocene), F24 (late Oligocene), F25, and F26 (both early Eocene) in the “Site U1356” chapter [Expedition 318 Scientists, 2011b]).

Shipboard analysis revealed a grouping of three to four peaks eluting late in the ketone fraction (N3) in >50% of the samples. No alkenone standards were available on ship for comparison. However, based on the general retention time and distribution, these peaks superficially suggested a distribution similar to that expected for the C_37:2, C_38:2, and C_39:2 alkenones. The detector response of these late-eluting peaks was weak when analyzed by the shipboard GC-MS in “scanning mode,” and the resultant mass spectra were inconclusive. However, the late-eluting peaks appeared more prominent when analyzed in “single ion monitoring” (SIM) mode for a number of individual, characteristic mass fragments associated with alkenones (e.g., m/z 81, 109). Thus, the initial shipboard assignation of alkenones was made (see Figs. F25 and F26 and Table T11 in the “Site U1356” chapter [Expedition 318 Scientists, 2011b]).

The additional 50 ketone fraction samples taken from throughout the lowermost 110 m (Units X and XI) of Site U1356 were analyzed on shore to assay for alkenone compounds and confirm the shipboard analyses. As with the shipboard analyses, a grouping of three to four peaks eluted late in the ketone fraction in the majority of the samples. However, the mass spectra of the late-eluting analytes, as produced by shore-based GC-MS, do not show any diagnostic similarity with known alkenone compounds. Shore-based GC-MS results from a representative sample from Site U1356 are given in Figure F1A. For comparison, an alkenone-yielding, Quaternary age sample from the Japan Sea was analyzed in the same laboratory, using the same GC-MS, under the same temperature program and conditions. Figure F1B confirms that positive alkenone identification using the shore-based GC-MS and conditions was feasible, with mass spectra exhibiting characteristic alkenone fragmentation patterns. Moreover, comparison of Figures F1A and F1B unambiguously indicates that the late-eluting peaks in the Site U1356 samples are not alkenones on the basis of both peak retention time and mass spectra. This negative result supersedes the initial report of alkenones in the Eocene sediments of Site U1356 and indicates that shipboard assignation of alkenones was erroneous.

Top of page   |    Previous   |    Next