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ABSTRACT: Paleoceanographic Implications from High-Frequency Dark/Light Rhythms in the Sea of Japan (ODP Legs 127 and 128)

Karl B. Follmi

One of the most remarkable results of companion legs 127 and 128 is the detection of prominent high-frequency dark/light alternations throughout Pleistocene sediments in the Japan Sea. The distinct rhythms include a dark-colored, massive or laminated unit rich in diatoms and foraminifera, as well as in TOC (max 5.6 wt %), and a light-colored massive or bioturbated unit rich in terrigenous silts and clays. The dark intervals are frequently built up by a stack of laminated or thinly bedded couplets. Each couplet consists of a dark organic-rich subunit and a light-colored clay and/or silt rich subunit, and mirrors the general sequence in a self-repetitive pattern. The dark/light rhythms display a variability in thickness (10-200 cm) and bulk composition (nannofossils versus iatoms). The internal configuration of the rhythms, however, is generally consistent: The basal contact is commonly sharp and well-defined, the dark unit is either laminated of massive, the transition into the superjacent light-colored unit is rapid but may be obscured by dense burrowing, and the light-colored unit is either massive or bioturbated.

Although dark/light rhythms are present in sediments of upper Miocene to lower Pliocene age, they are most prominent and consistently developed in sediments of Pleistocene age. The rhythms reflect high-frequency paleoceanographic changes in the Japan Sea including variations in the input of terrigenous material, preservation of organic matter, as well as changes in bottom water oxygenation, probably in concordance with tectonic and climatic changes. The well-defined and generally sharp boundaries between the dark/light rhythms may indicate that transitions between the different oceanographic operational modes of the Japan Sea were rapid.

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