Studies on Middle Eocene Stratigraphy and Paleoceanography in the Atlantic and Pacific

概要

This PhD thesis focuses on integrated bio- and chemostratigraphy and paleoceanographic reconstruction of the middle Eocene interval (Lutetian–Bartonian Stages/Ages) in the Northwest Alantic and Northwest Pacific Oceans. Studying middle Eocene geological records are essential to understand the long-term history of environmental changes and biotic evolution during the transitional period, from the maximum warmth in the early Eocene greenhouse world to the late Eocene as the prelude to the Oligocene-to-modern icehouse world.

From this perspective, the expanded and well-preserved sedimentary records at IODP Site U1408 (Expedition 342; Southwest Newfoundland Ridge) provides both the new reference for the regional and global stratigraphic scheme and information on previously not well-known upper ocean hydrographic and productivity responses to the environmental perturbation across the MECO (~40.5–40.0 Ma) (Chapter 2). Newly generated bulk carbonate carbon and oxygen isotope (δ13C and δ18O) stratigraphy in the U1408 succession, spanning 4.5-million-year-long interval, is firmly correlative with those in the circum-North Atlantic reference sections, combined with the existing orbitally-tuned magnetobiochronology. Well-preserved planktic foraminiferal multi-species δ18O data reveal the upper ocean warming of ~4 °C and complex response of thermocline during the MECO, associated with declined foraminiferal productivity.

On the other hand, the integrated stratigraphy of the middle Eocene hemipelagic succession, the Sakasegawa Group, exposed in Amakusa area (Southwest Japan) was investigated to refine the chronostratigraphy and provide the basis for the future studies to reveal paleoenvironmental history at the previously under-studied Northwest Pacific continental margin (Chapter 3). Revised δ13C stratigraphy derived from sedimentary total organic carbon (TOC) in the Amakusa composite section contains several globally traceable marker horizons, which is calibrated with planktic foraminiferal and calcareous nannofossil biozones. The integrated stratigraphic correlation with the reference strata in the Pacific and circum-Atlantic regions has revealed that the Sakasegawa Group is correlative with the stratigraphic interval from the middle Lutetian to the upper Bartonian, spanning ~8 million years at maximum estimation according to the latest age model. Finally, the MECO horizon would be constrained in the middle part of the Sakasegawa Group.