[M-IS16] 古気候・古海洋変動

[MIS16-18] Increased eolian dust in the Pacific since around the Eocene-Oligocene boundary estimated from a rock-magnetic study of red clay

*山崎 俊嗣¹、Fu Wei¹、下野 貴也²、臼井 洋一³ (1.東京大学大気海洋研究所、2.産業技術総合研究所地質調査総合センター、3.海洋研究開発機構)

キーワード：風成塵、生物源マグネタイト、赤色粘土、始新世ー漸新世境界

Red clay widely occupies the seafloor of pelagic environments in middle latitudes, and potentially preserves long paleoceanographic records. We conducted a rock-magnetic study of Pacific Ocean red clay to elucidate paleoenvironmental changes. Three piston cores from the western North Pacific Ocean and IODP Hole U1365A cores in the South Pacific Ocean were studied here. Principal component analyses applied to first-order reversal curve diagrams (FORC-PCA) reveal three magnetic components (endmembers EM1 through EM3) in a core of the western North Pacific. EM1, which represents the features of interacting single-domain (SD) and vortex states, is interpreted to be of terrigenous origin. EM2 and EM3 are carried by non-interacting SD grains with different coercivity distributions, which are interpreted to be of biogenic origin. The EM1 contribution suddenly increases upcore at a depth of ~2.7 m, which indicates increased eolian dust input. The age of this event is estimated to be around the Eocene-Oligocene (E/O) boundary. Transmission electron microscopy reveals that EM2 is dominated by magnetofossils with equant octahedral morphology, while EM3 has a higher proportion of bullet-shaped magnetofossils. An increased EM3 contribution from ~6.7 to 8.2 m suggests that the sediments were in the oxic-anoxic transition zone (OATZ), although the core is oxidized in its entire depths now. The chemical conditions of OATZ may have been caused by higher biogenic productivity near the equator. FORC-PCA of Hole U1365A cores identified two EMs, terrigenous (EM1) and biogenic (EM2). The coercivity distribution of the biogenic component at Hole U1365A is similar to that of the lower coercivity biogenic component in the western North Pacific. A sudden upcore terrigenous-component increase is also evident at Hole U1365A with estimated age around the E/O boundary. The eolian increase may have been coeval in both hemispheres and associated with the global cooling at the E/O boundary.