Paleomagnetic and rock magnetic study of North Pacific Ocean and South China Sea sediments

Abstract

Magnetism plays an important role in marine science. On the one hand, paleomagnetism helps us to understand ancient geomangetic field behaviour that can also facilitate geochronological studies. On the other hand, rock magnetism can provide valuable information concerning paleoenvironmental and paleoclimatic processes. However, magnetic particles in marine sediments are influenced by multiple geological and environmental processes, and commonly occur in mixed magnetic mineral assemblages, which complicates magnetic parameter interpretation. Therefore, to apply environmental magnetism in marine science, it is crucial to be able to discriminate magnetic signals in marine sediments and to relate these signals to relevant processes. In this study, detailed paleomagnetic and rock magnetic studies have been performed on sediments from the North Pacific Ocean (NPO) and the South China Sea (SCS). The main conclusions of this work are as follows. Magnetite is the dominant magnetic mineral throughout SCS Hole U1431D, and its remanence is used to reconstruct a magnetostratigraphic time framework for the past ~6.5 Ma. A single authigenic greigite-bearing interval is identified at 130.5-132.0 mbsf, whose age is estimated at ~2.53-2.55 Ma is associated with oceanographic changes that coincided with intensification of northern hemisphere glaciation. Sedimentary magnetic parameters are affected mainly by the East Asian monsoon and are used to indicate monsoon evolution over the past 6.5 Ma. The summer and winter monsoon were stable before 5 Ma. The summer monsoon intensified from 5 Ma to 3.8 Ma, when it started to weaken. The winter monsoon weakened at 5 Ma, and then intensified after 3.8 Ma, finally stabilizing at 0.6 Ma. Spectral analyses indicate that there was a direct response of the summer monsoon to low-latitude insolation between 6.5 and 3.2 Ma. Since 3.2 Ma, both low-latitude insolation and high-latitude global ice volume have influenced summer monsoon evolution. In NPO core NP02, three visible tephra layers are identified, and can be correlated to the To-Of, Spfa-1, and Kt-3 tephra layers. Together with a radiocarbon date, an initial age model was reconstructed over the ~22-57 ka interval for core NP02. Magnetic analyses suggest that the remanence is carried by detrital vortex state and biogenic single domain magnetite. These two components record similar relative paleointensity (RPI) patterns, and the fidelity of the core NP02 RPI record is further verified by comparison with other RPI stacks and records. Therefore, the RPI-assisted chronology is used to date the studied NPO sediments by tuning the initial core NP02 age model using RPI variations. Hard isothermal remanent magnetization (HIRM) is dominated by the hematite concentration and is used as a proxy for Asian-sourced dust content. Low dust contents are identified at ~25, 39, 48, and 55 ka, which can be linked to Heinrich events 2, 4, 5, and 5a, and are likely caused by southward shifts of the westerlies associated with Atlantic meridional overturning circulation (AMOC) slowdowns. AMOC slowdowns increase the meridional temperature gradient and, therefore, lead to intensified and southward-shifted westerlies. In this situation, the main westerly axis shifts away from the NP02 core site and, therefore, low dust contents are recorded at this site.