The Mid-Pleistocene Transition and the Resulting Paleoproductivity Cycles Within the Japan Sea.

Japan Sea sediments provide a geochemical history of past oceanographic conditions in the shallow, marginal sea during the Pleistocene. Carbon and nitrogen stable isotope analyses of sediment allow for the ability to better understand the temporal variations in surface productivity levels during glacial/interglacial cycles. The timing and intensity of glacial/interglacial cycles have previously been shown to have shifted significantly during the Mid-Pleistocene Transition (MPT). Before the MPT, the low amplitude 41ky obliquity cycle dominated the orbital forcing records, but after the MPT a low frequency, high amplitude 100 ky glacial/interglacial cycle emerged, which was dominated by the 21ky precession cycle. This study uses the isotope geochemistry (%TOC, %N, %CaCO₃, δ¹³C, and δ¹⁵N) of sediment cores collected during IODP Expedition 346 in the Japan Sea as well as continuous wavelet analysis and multi-taper method (MTM) of spectral analysis to better understand the variation of surface paleoproductivity during glacial/interglacial cycles before and after the MPT. Increased productivity occurs in the interglacial periods when there is a larger flux of water into the Japan Sea than compared to the partially isolated conditions and lower productivity levels during glacial periods. Continuous wavelet analysis shows an apparent shift in periodicity of paleoproductivity levels at 773 ka during the Matuyama-Brunhes paleomagnetic boundary, which served as the MPT boundary in this study. MTM analysis of the geochemical data indicates the apparent shift in periodicity in paleoproductivity levels in the Japan Sea from 41ky cycles to 21ky/100ky cycles during the MPT.