14:00 〜 14:15
[BPT02-02] Organic geochemical analysis of marine mammal bone fossils from the Oligocene sediments of Hokkaido, Japan
キーワード：Biomarker , Oligocene, Desmostylian, PAH
Preservation and diagenesis of lipids from plant, algal and bacterial sources have been extensively studied, but little research has been performed towards the elucidation of the diagenetic mechanisms undergone by the lipids that constitute animal bone tissue. In this study, we performed biomarker analysis on three samples of fossil bones – a whale bone, and two desmostylian bones (Ashoroa laticosta and Behemotops katsuiei) – from the Oligocene Morawan Formation near Ashoro, Hokkaido, and their associated sediment matrices in an attempt to characterize their diagenetic products and taphonomic scenario. The bone samples were first demineralized with HCl. Subsequently, they were crushed to fine power and extracted using suitable organic solvents, followed by silica gel column chromatography. Multiple fractions were thus separated representing saturated, aromatic, ketone, and polar fractions. The fractions were then analyzed using gas chromatography-mass spectrometry (GC-MS). The surrounding matrices were also anlayzed.
Short-mid chained n-alkanes predominated the three samples, with whale bone displaying less abundance and possibly branched alkanes. C28-C30 hopanoids were detected in all the samples while the desmostylians additionally displayed C31-C35 hopanes. Homohopane indices for the desmostylians were 0.11 (A. laticosta) and 0.09 (B. katsuiei) respectively, indicating a slightly reducing depositional environment. The abundance and redox levels indicated by the homohopanes suggest post-mortem bacterial activity on the desmostylians, and their lack could suggest that bacterial activity on the whale bone was less. C27-C29 steranes and diasteranes along with C21 sterane (possibly a direct diagenetic product) were detected at in case of the desomstylians, while only cholestane and its isomer coprostane (both C27) were detected in the whale bone. Cholestanone and cholesterol were also detected in the whale bone. Cholestanone was detected, but less abundant in the desmostylians and cholesterol was not detected in B. katsuiei. The whale bone also contains possible aromatized derivative of cholesterol. The most prominent polyaromatic hydrocarbons (PAHs) in the fossils were likely to be isomers of n-methylalkenyl napthalenes. As these are also the most prominent PAHs in the surrounding matrices, it is possible that migration has occurred, either from bone to matrix, or matrix to bone. Other PAHs in the desmostylian bones were phenanthrene, indene, pentene, biphenyl and their methylated homologues, while there were no other notable PAHs in the whale bone. Methylphenanthrene indices (MPI-1) were calculated to determine maturity in the desmostylians. The MPI-1 values were 0.21 for A. laticosta and 0.19 for B. katsuiei. Phenanthrene was not detected in the whale bone. From the data, we interpret that the all three samples are fairly immature, with whale bone being less mature and less diagenetically altered than desmostylians. Due to their high abundance in the whale bone, and lack of presence of their methylated homologues, cholestane and its derivatives are interpreted as indigenous. and were probably derived from bone cholesterol. There is a possibility, however, that the same compounds could be derived from post-mortem microbial action in the desmostylians. Due to its relatively lower maturity, it is possible that the aromatized cholestane derivative from the whale bone represents a potential intermediary step in the aromatization of animal-derived cholesterol to PAHs. We also conclude that biomarker analysis of animal bones has the potential to reveal valuable information which can be used to evaluate endogeneity and diagenetic pathways of the constituent lipids, and recommend further such analyses.
Short-mid chained n-alkanes predominated the three samples, with whale bone displaying less abundance and possibly branched alkanes. C28-C30 hopanoids were detected in all the samples while the desmostylians additionally displayed C31-C35 hopanes. Homohopane indices for the desmostylians were 0.11 (A. laticosta) and 0.09 (B. katsuiei) respectively, indicating a slightly reducing depositional environment. The abundance and redox levels indicated by the homohopanes suggest post-mortem bacterial activity on the desmostylians, and their lack could suggest that bacterial activity on the whale bone was less. C27-C29 steranes and diasteranes along with C21 sterane (possibly a direct diagenetic product) were detected at in case of the desomstylians, while only cholestane and its isomer coprostane (both C27) were detected in the whale bone. Cholestanone and cholesterol were also detected in the whale bone. Cholestanone was detected, but less abundant in the desmostylians and cholesterol was not detected in B. katsuiei. The whale bone also contains possible aromatized derivative of cholesterol. The most prominent polyaromatic hydrocarbons (PAHs) in the fossils were likely to be isomers of n-methylalkenyl napthalenes. As these are also the most prominent PAHs in the surrounding matrices, it is possible that migration has occurred, either from bone to matrix, or matrix to bone. Other PAHs in the desmostylian bones were phenanthrene, indene, pentene, biphenyl and their methylated homologues, while there were no other notable PAHs in the whale bone. Methylphenanthrene indices (MPI-1) were calculated to determine maturity in the desmostylians. The MPI-1 values were 0.21 for A. laticosta and 0.19 for B. katsuiei. Phenanthrene was not detected in the whale bone. From the data, we interpret that the all three samples are fairly immature, with whale bone being less mature and less diagenetically altered than desmostylians. Due to their high abundance in the whale bone, and lack of presence of their methylated homologues, cholestane and its derivatives are interpreted as indigenous. and were probably derived from bone cholesterol. There is a possibility, however, that the same compounds could be derived from post-mortem microbial action in the desmostylians. Due to its relatively lower maturity, it is possible that the aromatized cholestane derivative from the whale bone represents a potential intermediary step in the aromatization of animal-derived cholesterol to PAHs. We also conclude that biomarker analysis of animal bones has the potential to reveal valuable information which can be used to evaluate endogeneity and diagenetic pathways of the constituent lipids, and recommend further such analyses.