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December 2, 2021
JAMSTEC

As a part of International Ocean Discovery Program (IODP, *1), a research cruise of the JOIDES Resolution (*2) is set to begin on December 6, 2021 as IODP Expedition 391 program (see the attached schedule). The purpose of this cruise is to understand the formation mechanism of the Walvis Ridge Hotspot. Proposed coring at six locations on the South Atlantic Ocean Ridge along the older basaltic lava flows are ranging in age from ~59 to 104 Ma. Samples will reveal the trace of the evolution of geochemical and isotopic signatures as the hotspot track became zoned, offering vital clues about compositional changes of the plume source and important implications for understanding the origin of hotspot zonation.

A total of 26 researchers from Austria, Canada, China, France, Germany, India, Italy, Japan, Korea, Sweden, the United States, and United Kingdom are scheduled to join the research cruise. One of them from Japan is scheduled to participate on board.

*1　International Ocean Discovery Program (IODP)
This international scientific research cooperative project started in October 2013. By drilling deep below the ocean floors using drilling vessels provided by Japan (JAMSTEC D/V Chikyu), the United States (JOIDES Resolution), and Europe (Mission-Specific Platforms), IODP promotes research aimed at elucidating the internal structure and the biosphere within and below the Earth’s crust.

*2　JOIDES Resolution
The JOIDES Resolution Science Operator (JRSO) manages and operates the riserless drillship, JOIDES Resolution, for the International Ocean Discovery Program (IODP). The JRSO is based in the College of Geosciences of Texas A&M University.

Addendum

Walvis Ridge Hotspot

1. Schedule (local time)

Expedition 391

December 6, 2021

Research cruise begins
(Principal Investigator (PI) boards at Cape Town, South Africa)

February 5, 2022

Return to Cape Town, South Africa

※ The schedule may change depending upon the condition/inflences of COVID-19, the of progress in sailing preparations, weather conditions, and/or research activities.

2. Scientific Participants from Japan

Name	Affiliation / position	Specialized field
Yusuke Kubota	Department of Earth and Planetary Sciences, Tokyo Institute of Technology/Ph.D Student	Igneous Geochemist

3. Research Purposes and Objectives

Walvis Ridge (WR) is a long-lived hotspot track that began with a continental flood basalt event at ~132 Ma during the initial opening of the South Atlantic Ocean. WR stretches ~3300 km to the active volcanic islands of Tristan da Cunha and Gough, and it was originally paired with Rio Grande Rise (RGR) oceanic plateau. Because of the duration of its volcanism and the length of its track, the Tristan-Gough hotspot forms the most pronounced bathymetric anomaly of all Atlantic hotspots. Its age progression, chemistry, and connection to flood basalts point to a lower mantle plume source, projected to be the hypothesized plume generation zone at the margin of the African large low shear-wave velocity province. The hotspot interacted with the Mid-Atlantic Ridge (MAR) during its early history, producing WR and RGR through plume-ridge interaction. Valdivia Bank, a WR plateau paired with the main part of RGR, represents heightened hotspot output and may have formed with RGR around a microplate, disrupting the expected hotspot age progression. After producing a relatively uniform composition from ~120 to ~70 Ma, WR split into three seamount chains with distinct isotopic compositions at about the time that the plume and MAR separated. With ~70 My spatial zonation, the hotspot displays the longest-lived geochemical zonation known. Currently at ~400 km width with young volcanic islands at both ends, the hotspot track is far wider than other major hotspot tracks. Thus, WR displays global extremes with respect to (1) width of its hotspot track, (2) longevity of zonation, (3) division into separate chains, and (4) plume-ridge interaction involving a microplate, raising questions about the geodynamic evolution of this hotspot track. Understanding WR is critical for knowledge of the global spectrum of plume systems. To test hypotheses about mantle plume zonation, plume activity around a microplate, and hotspot drift, we propose coring at six locations along the older ridge to recover successions of basaltic lava flows ranging in age from ~59 to 104 Ma. Samples will help us trace the evolution of geochemical and isotopic signatures as the hotspot track became zoned, offering vital clues about compositional changes of the plume source and important implications for understanding the origin of hotspot zonation. Dating will show the age progression of volcanism both at individual sites and along the ridge, testing whether WR formed as a strictly age-progressive hotspot track and whether Valdivia Bank formed as a plume pulse, extended volcanism around a microplate, or possibly even a continental fragment. Paleomagnetic data will track paleolatitude changes of the hotspot, testing whether hotspot drift or true polar wander, or both, explain changes in paleolatitude.

¹Sager, W., Hoernle, K., and Petronotis, K., 2020. Expedition 391 Scientific Prospectus: Walvis Ridge Hotspot. International Ocean Discovery Program. https://doi.org/10.14379/iodp.sp.391.2020

Figure  Locations of drilling sites. The colors represent the primary target sites (red circles) and alternate sites (yellow circles).

*Figure is cited from the IODP website with partial modification.
IODP JRSO・Expeditions・Walvis Ridge Hotspot
http://iodp.tamu.edu/scienceops/expeditions/walvis_ridge_hotspot.html
http://publications.iodp.org/scientific_prospectus/391/391SPADD/index.html
http://publications.iodp.org/scientific_prospectus/391/391SP.PDF
http://iodp.tamu.edu/about/copyright.html