MOLECULAR FOSSIL AND NANNOFOSSIL RECORDS IN A Co-RICH CRUST OF WEST PACIFIC SEAMOUNTS: IMPLICATION FOR STRATIGRAPHIC DIVISION AND PALEOECOLOGY AND PALEOENVIRONMENT

Calcareous nannofossils in CM1D03 Co-rich crust of west Pacific Seamounts were analyzed to esimate the stratigraphic ages. The results showed that the lower layer was formed in the late Paleocene-early Eocene (54~51 Ma), whereas the porous middle layer was formed in the middle Eocene (45~40 Ma) and the upper layer was in the Miocene-Pliocene (22~2.4 Ma) and the Pliocene-Pleistocene (3.6~1.2 Ma). The molecular fossils, including chloroform bitumen "A", n-alkanes, isoprenoids, steranes, in the Co-rich crust were measured using gas chromatography and gas chromatography-mass spectrum. The source composition, depositional environment and palaeoecological community succession in the Co-rich crust during its growth have been discussed by analyzing the characteristics of these fine molecules (C₂₇, C₂₈, and C₂₉ steranes) and their molecular indices (ΣC₂₃^-/ΣC₂₄⁺, CPI and Pr/Ph) with consideration of the variation in organic carbon (TOC) content and its stable isotope compositions (δ¹³C) records. The results showed that chloroform bitumen "A"/TOC ("A"/C) ratio was 10.51%~21.74%, showing significant hydrocarbon transport pattern. The ratio of ΣC₂₃^-/ΣC⁺₂₄ for n-alkanes was 0.74~1.47,the CPI was 0.80~1.45, and the value of δ¹³C was -24.00‰~-25.48‰, indicating that organic matter in the Co-rich crust mainly origined from phytoplankton. The results also indicated that the source of organic matter, C₂₇, C₂₈, and C₂₉ steranes distribution variation, TOC preservation and δ¹³C were related to changes in marine environment, global climate and Antarctic Bottom Water during the formation of Co-rich crust.