Late Eocene to Early Oligocene (36–26 Ma) Rock Magnetism and Magnetostratigraphy of IODP Site U1553, Southwestern Pacific: Defying the Challenges of Carbonate Dilution and Reductive Diagenesis

Abstract The late Eocene to early Oligocene is an intriguing Paleogene period in which the Earth climate state changed from Warmhouse to Coolhouse. Accurate age models are essential for investigating these climate transitions. The southwestern Pacific International Ocean Discovery Program (IODP) Site U1553, with its almost complete succession of the latest Eocene to early Oligocene, provides a rare opportunity to explore the Paleogene climate shifts at high southern latitude. Here we report on paleomagnetic results from ∼600 paleomagnetic samples collected throughout the manganous, ferruginous and sulfidic zones at 5–260 m sediment depth. We identify fine‐grained titanomagnetite as the primary remanence carrier through rock magnetic analyses, including electron microscopy, isothermal remanent magnetization unmixing and endmember modeling, low‐temperature cycling, and first‐order reversal curves. We find that high carbonate dilution rather than the previously proposed reductive dissolution is the main cause of low ferrimagnetic mineral content and thus very weak natural magnetization. Reductive diagenesis and precipitation of secondary iron sulfide minerals also perceivably affect paleomagnetic recording quality. Thanks to an extensive sample set, sensitive SQUID magnetometry, and rigorous data evaluation, we were nonetheless able to identify 15 geomagnetic reversals. Supported by micropaleontology data, these could be confidently correlated with Chrons C16–C8r of the geomagnetic polarity time scale. Our Paleogene magnetostratigraphy can be used as a chronological anchor for future climate studies. We also discussed the long‐term paleomagnetic consequences of mild reductive diagenesis in the suboxic and sulfidic zones and identified detrital titanomagnetite as the most alteration‐resistant magnetic carrier mineral.