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ABSTRACT: Downhole Measurement Data as Paleoclimate Indicators: a Plio-Pleistocene Record of Asian Eolian Dust Deposition to the Sea of Japan

Peter Demenocal, James F. Bristow

Ocean Drilling Program Site 798 (Leg 128) presents a unique opportunity to monitor the late Neogene paleoclimatic evolution of the SE Sea of Japan region because it is positioned on an elevated ridge (the Oki Ridge, 911 mbsl) above the present lysocline and sedimentation rates here are unusually high (^sim120 m/Ma). Since the site is located downwind of the westerly Asian dust trajectory, it was anticipated that these diatomaceous sediments would yield a detailed late Neogene record of eolian dust deposition. We have collected geophysical and geochemical logs as part of the downhole measurements program at Hole 798B; these data represent continuous in situ measurements of bulk density, porosity, sonic velocity, natural gamma ray activity, and a suite of elemental abundanc logs (K, U, Th, Al, Si, Ti, Ca, Gd, Fe, S, H, Cl, and Mg+Na).

Based on a paleomagnetic timescale, the natural gamma ray log (which largely reflects variations in gamma-emitting K and Th bound in clay minerals) is dominated by cyclic 41 kyr variations over the interval corresponding to 2.8-1.0 Ma; these cyclic variations are also apparent in other log responses. We interpret these periodic increases in terrigenous sediment content to reflect variations in eolian dust input from Asian source areas. Studies of Chinese loess deposits have demonstrated that Asian continental aridity has been strongly modulated by variations in Northern Hemisphere glacial ice volume which varied predominantly at the 41 kyr orbital obliquity periodicity during the Matuyama Chron (2.47-0.73 Ma). Cross-spectral analysis of the Hole 798B gamma ray record and the marine ox gen isotopic record over

this interval supports this linkage. Work in progress will define more accurately the sedimentologic origin of the various log responses, and hence their fidelity as paleoclimate proxy indicators.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990