Biostratigraphy Of Co-rich Crusts From Western Pacific And Its Growing Response To Cenozoic Climatic And Oceanic Changes

With the potential economic value, comprehensive utilization of Co-rich crusts and earlier exploitation than mentallic nodules, Cobalt-rich crusts become the most important as the "Blue land circling". This work has studied two Co-rich crusts of MHD56 and M17D-1A from Northwest Pacific. For the study on crust MHD56, a large number of calcareous nannofossils have been found and biostratigraphy has been set up. In this work, 41 species of 20 genera have been identified and illustrated. Six calcareous nannofossil zones have been divided, as Neocrepidolithus fossus—Discoaster megastypus; Zygodiscus sigmoides — Cyclicargolithus luminis; Reticulofenestra umbilica — Cyclicargolithus marismontium; Discoaster calculosus — Helicosphaera ampliaperta; Sphenolithus moriformis—S. neoabies; Sphenolithus neoabies—Umbellosphaera tenuis assemblages. The ages of fossil assemblages indicate that the Co-rich crust accumulation is from Paleocene to Pleistocene. The MHD56 crust shows 3 structural layers: compact upper layer, porous middle layer and compact lower layer. Studies show that the age of porous middle layer was formed from late Miocene to late Pliocene (0.5～11.9Ma)and the lower layer was formed from late Paleocene to late Oligocene (56.3～23.2Ma). Furthermore, a sedimentation hiatus has been observed, which was caused by intensive bottom water's corrosion or none deposition which genegated by superficial ocean productive power rapidly falling down. The bottom growing rate of the crust MHD56 was 0.8～3.7mm/Ma, and the middle layer 0.8～3.7mm/Ma. Since few fossils have been found from the crust M17D-1A, the Co-method dating has been used. The crust M17D-1A was also divided into 3 layers: the bottom layer of late Oligocene (24～30Ma), with growing rate of 0.8— 3.7mm/Ma; the middle layer of late Oligocene (24Ma) to Pliocene (1.4Ma), at 2.5～ 5.2mm/Ma rate; and the upper layer of Quaternary (<1.4Ma). Both crusts are of growingrates at 1 ～ 10mm/Ma that is the normal rate in general. The inceptive growing time was accordant with the time of the palaeoceanography events such as Pacific cooling, speeding up of the carbonate solution, strengthening of bottom current and frequent volcano eruptions which inicates a response of the initial growth of Co-rich crust to the paleoceanic and geologic events. The rapid environmental change around the O/M boundary is probably the major cause of the changes of structure, color, luster, and elemental composition between the middle and lower layers. The boundary between the middle and lower layers is of important indication on growing time.