Developing paleomagnetic chronologies of, and paleogeomagnetic understanding from, IODP sites U1486 and U1489 in the western Pacific warm pool.

点击翻译按钮获取中文摘要

Observations of Earth's magnetic field over timescales from thousands to millions of years are important for understanding how the geodynamo operates and shields the Earth from harmful incoming solar radiation. As ocean sediments accumulate on the seafloor they record the ambient magnetic field and thereby provide an important record of past geomagnetic behavior. Many of the existing high quality records that span the last 2-3 million years are from North Atlantic Ocean sediments, which represents a narrow, and maybe even unique, magneto-geographical range. To properly represent the global field and its variability requires records from other regions of the globe for the same time period. Integrated Ocean Drilling Program (IODP) Expedition 363 to the West Pacific Warm Pool drilled nine equatorial sites in the Indian and Pacific Oceans and recovered a total of 7km of ocean sediment core. Preliminary measurements suggest two of these sites, U1486 and U1489, contain high quality magnetic records that can be used to investigate and understand the behavior of Earth's magnetic field over the last 1.25 (U1486) and 5 (U1489) million years. This project seeks to measure the magnetism of these sediments to develop and refine high resolution records of Earth's recent magnetic field behavior and compare them to North Atlantic records. The project will also develop the longest continuous relative paleomagnetic intensity time-series to date back to the earliest Pliocene (~5 million years ago). In addition, these paleomagnetic records will provide a chronological framework for related detailed isotope and climate studies for the region. In terms of broader impacts and outreach, this project will directly support the training and development of a post-doc as the principal investigator and will expose undergraduate students from Oregon State University to basic research methods and concepts. There will be special attention placed on undergraduates being involved in original research, the development of project "ownership" for undergraduate students, and in recruiting a diverse team of researchers.Paleomagnetic records derived from relatively high sedimentation rate marine cores in the North Atlantic strongly inform and define our current understanding of paleomagnetic secular variation and relative paleointensity (RPI). Replication of records from within this basin argue for RPI variations that have a global expression and reflect stochastic variations in the intensity of the dipolar field. This assumption is likely true, however, it has never explicitly been tested, in part due to a lack of records from different regions of the globe that have a similar resolving ability (i.e. sedimentation rate) to those in the North Atlantic. We will take u-channel samples from the archives halves of U1486 and U1489 and measure the magnetic susceptibility and demagnetization behaviors of the natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) at centimeter scale. We will use these data to generate characteristic remanent directions and generate estimates of RPI over long Pleistocene and Pliocene timeframes. The 1.25 Myr equatorial Pacific marine sediment record from Site U1486 can be queried for evidence of non-axial-dipole structure and regional heterogeneities at a resolution several times higher than existing records and stacks and is more similar to high quality North Atlantic records. Site U1489 contains a 5 Myr record of sedimentation that will be developed into the longest continuous RPI record ever produced. Preliminary data suggest it is of remarkable fidelity and, when paired with the d18O benthic foraminifer stratigraphy being developed concurrently for this site, can potentially become the type record for Pliocene geomagnetic field behavior. Not only will this record be a magnetostratigraphic tuning target for the Pliocene, it will also inform inquiries into geomagnetic field evolution and organization over the scale of (sub-)orbital variations to full chrons and epochs. RPI-assisted chronologies will be developed for the mid-late Pleistocene records of both U1486 and U1489 (through correlation to existing global stacks) that can then be tandemly tuned with d18O records to develop robust chronologies and the establishment of a strong chronostratigraphic framework to directly facilitate the science objectives of IODP Expedition 363.