HR: 0830h
AN: PP41B-0839    [PDF]
TI: Negative spikes of isotopic composition of organic matter in Cretaceous OAE-2 black shales
AU: * Kuroda, J
EM: kuroda@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Tokuyama, H
EM: tokuyama@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Ogawa, N O
EM: nanaogawa@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Matsumoto, K
EM: kohei@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Suga, H
EM: suga@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Taira, A
EM: ataira@jamstec.go.jp
AF: Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Ohkouchi, N
EM: nohkouchi@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AB: To demonstrate paleoecological changes during the mid-Cretaceous Oceanic Anoxic Event-2 (OAE-2, 93 Ma), we determined isotopic ratios of total organic carbon ($\delta$$^{13}$C$_{org}$) for samples collected across the "Livello Bonarelli" black shale enriched in organic carbon (up to 25$%$). About 500 samples were measured in a 1 m sequence of the Bonarelli black shale to reconstruct ultra-high-resolution $\delta$$^{13}$C$_{org}$ record during the OAE-2. The $\delta$$^{13}$C$_{org}$ profile shows fine-scale fluctuations as well as an upward increasing trend (up to -22$\permil$) due to an enhanced organic carbon burial during the OAE-2. We found several narrow $^{13}$C-depleted intervals down to -35$\permil$. This value is substantially lighter than the adjacent samples (-22 to -27$\permil$) as well as those previously reported both from the Bonarelli and other Cretaceous black shales. Such large, rapid negative spikes of $\delta$$^{13}$C$_{org}$ could not simply be ascribed to the variability of isotopic fractionations by primary producers during the event, but it may reflect the contribution of organisms metabolizing isotopically light carbon like methane. In order to examine this, we measured carbon isotope ratios of lipid biomarkers extracted from several intervals including the isotopically light intervals. Unexpectedly, the results show that the extractable lipid compounds such as alkanes, hopanoids, isoprenoids, alkanols and fatty acids indicate relatively constant $\delta$$^{13}$C$_{org}$ values mostly ranging from -25 to -30$\permil$, even in the $^{13}$C-depleted intervals. It suggests that the $^{13}$C-depleted compounds exist as non-extractable form of organic matter (kerogen) in the black shale. In the presentation, we will report carbon isotopic compositions of kerogen-bound compounds in the $^{13}$C-depleted intervals.
DE: 1055 Organic geochemistry
DE: 4802 Anoxic environments
DE: 4870 Stable isotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting