Palaeomagnetism of marine sediments from the Japan Sea: insight into the Plio-Pleistocene East Asian Monsoon

Abstract

The East Asian monsoon (EAM) system stands as a pivotal component of the global climate, significantly impacting weather patterns and seasonal precipitation across East Asia inhabited by a substantial portion of the world’s population. Despite its importance, the understandings of EAM variability remains incomplete, hindered by gaps in knowledge regarding its interactions with global climate drivers and regional dynamics. Moreover, existing studies often face limitations in explaining long-term EAM trends, particularly at orbital and tectonic scales. The Japan Sea, with its unique geographic settings and sensitivity to environmental changes, offers promising opportunities for enhancing current understanding of EAM variability and improving future climate projections under global warming.
Here I present long, well-dated, high-resolution and continuous environmental magnetic records that are indicative of East Asian monsoon variabilities on various time scales using sediments from the Japan Sea recovered during the Integrated Ocean Drilling Program (IODP) Expedition 346. Chapter 2 conducts detailed rock magnetic analyses on Quaternary dark and light-layered Japan Sea sediments, linking sediment magnetic properties to millennial- and orbital-scale climatic and oceanographic changes. Results from the chapter suggest that the dark and light layered sediments contain a varying amount of detrital and biogenic magnetite, iron sulfides, and hematite. Samples from thick dark layers deposited during glacial maxima have lower concentrations and coarser magnetic grains (due to enhanced reductive diagenesis) and contain significantly more hematite. Samples from thick light layers deposited during interglacial peaks contain mainly a mixture of detrital and biogenic magnetite. Samples from suborbital-scale alterations of dark/light layers have similar magnetic mineral compositions with dark layers containing less fine-grained magnetic minerals (e.g., greigite and biogenic magnetite) due to enhanced reductive diagenesis. The magnetic grain size of the sediments appears to co-vary with the proportion of coarser (>14 micron) grains in the sediments that are dominated by aoelian dust, and can be used as a proxy for dust transportation to the Japan Sea.
Chapter 3 builds upon the findings of Chapter 2, creating high-resolution environmental magnetic records spanning the past 500,000 years from sediments at Site U1424. The magnetic grain size parameter (κARM/κ) in these sediments strongly correlates with the abundance of coarse particles (> ~14 μm), primarily comprising eolian dust. Changes in the magnetic grain size proxy are utilized to investigate dust transportation to the Japan Sea, influenced by the East Asian winter monsoon (EAWM) and/or the Westerly Jet (WJ). The shifting main axis of the WJ significantly impacts the interaction between the WJ and EAWM, affecting dust transports to the Japan Sea. During interglacials and glacial inceptions, when the WJ's main axis locates near the Northern China dust source region of the EAWM, it facilitates long-distance transportation of coarse particles to the Japan Sea. Conversely, during glacial maxima, when the WJ's main axis shifts away from the EAWM source regions, the interaction between the WJ and EAWM largely reduces, restricting the transport of very coarse particles over long distance. The proposed conceptual model illustrates the effects of these changes on dust transportation along the Asian dust pathway during glacial and interglacial cycles. The high-resolution magnetic records from Site U1424 offer valuable insights into the evolution of the EAWM and its impact on eolian inputs in the Japan Sea.
Chapter 4 explores the evolution of the East Asian summer monsoon (EASM) during the early Pliocene, using high-resolution environmental magnetic records from Site U1424. The early Pliocene Japan Sea sediments are dominated by magnetite and titanomagnetite. The soft magnetic flux was calculated using SIRM and IRM950 after 60mT demagnetization. Increased soft magnetic flux indicates increased riverine inputs due to intensified rainfall under a strengthened EASM. This new magnetic proxy for EASM correlates well with published records from Chinese Loess and the South China Sea. The magnetic soft flux record suggests EASM strengthened slightly from 4.5 to 4.2 Ma, and then intensified significantly at ~4.0 Ma until about 3.6 Ma. These changes are likely due to Tibetan Plateau uplift and Panama Seaway closure, enhancing land-sea temperature gradients and moisture transport to East Asia. The record also reflects responses to both precession modulation and obliquity amplification on the orbital scale like other EASM records. The high-resolution EASM record improves the understanding of warm-Pliocene EASM evolution and future precipitation predictions in East Asia.

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Identifiers

ORCID for Jiachun Wang: orcid.org/0009-0000-6818-1002

ORCID for Chuang Xuan: orcid.org/0000-0003-4043-3073

ORCID for Paul Wilson: orcid.org/0000-0001-6425-8906

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