Cyclostratigraphy of the Late Miocene to Pliocene sediments at IODP sites U1425 and U1430 in the Japan Sea and paleoceanographic implications

摘要

High-resolution studies of sedimentary sequences have reconstructed paleoceanographic changes in the Atlantic and southern and equatorial Pacific Oceans during the Late Miocene and Pliocene, but comparable analyses are lacking for the North Pacific Ocean. However, continuous samples of hemipelagic sequences covering this time interval were obtained at Integrated Ocean Drilling Program sites U1425 and U1430 in the Japan Sea during expedition 346. Because the paleoceanography of the Japan Sea was sensitive to glacio-eustatic sea-level changes, changes in the sediment record are manifested as cyclic lithological changes. By using a gamma ray attenuation (GRA) density, which reflects biogenic silica content, as an indicator of sea-level changes, we developed an independent orbitally tuned chronology at each site to construct high-resolution, high-precision age models for the time span of 11.8 to 1.45?Ma. First, the 405-kyr-filtered GRA profile at each site was tuned to 405-kyr-filtered orbital eccentricity. Then, using the 405-kyr-tuned age model, the 100-kyr-filtered GRA profile was tuned to short-eccentricity cycles to adjust the 405-kyr-tuned age models. We used the resulting age models to revise the time intervals of previously published lithological units and paleoceanographic stages for the Japan Sea. Our profiles based on physical properties are a good match to the oxygen isotope record, except for 6.6–3.6?Ma, a time interval with small-amplitude fluctuations in the oxygen isotope record.