IODP ³ Expedition 504S “Volcanic ash on the Ontong Java Plateau: testing models of subduction reversal and wind transport in the western equatorial Pacific” Scientific Prospectus

IODP3 Expedition 504S will re-examine legacy cores of the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drilled on the Ontong Java Plateau (OJP). Positioned in the western equatorial Pacific, the OJP is the world's largest oceanic plateau and is flanked by island arc and hotspot volcanoes. Since at least the Eocene, eruptions have deposited volcanic ash throughout the marine sediment, accumulating on the OJP. This expedition will focus on the provenance, history, and evolution of the volcanic ash record to achieve two principal objectives: (1) pinpoint the timing and mechanism of the subduction reversal resulting from collision of the OJP with the Solomon Islands arc and (2) reconstruct wind patterns and the deposition of dust and volcanic ash in the western equatorial Pacific since the Eocene. Collision of the OJP with the arc is thought to have congested the subduction zone and caused subduction reversal. Despite being regarded as the prototypical example of this process, there are three incompatible hypotheses for the timing and mechanism of the collision and subduction reversal. One widely accepted hypothesis involves “soft docking”, a concept involving an initial collision that halts subduction but is not marked by local deformation. Soft docking has been invoked in other collisional settings and locally as the cause of a change in Australian plate motion. The determination of whether the OJP ashes are arc- or hotspot derived will differentiate between the tectonic hypotheses, testing the validity of soft docking. The expedition will fingerprint the origin of volcanic ashes and establish a chronology of eruption history. We will characterise both ash deposited in layers and ash dispersed throughout the bulk sediment separately from aeolian dust derived from continental crust for a comprehensive perspective on ash and aeolian dust deposited on the OJP. Legacy cores from the OJP have marine sediment archives that cover the late Eocene to the present, allowing for the reconstruction of aeolian deposition over million-year time scales. The expedition will test hypotheses of dust fluxes increasing from the strengthening of the monsoon systems and the migration of the inter-tropical convergence zone since the Eocene. Expedition 504S addresses strategic objectives of the 2050 Science Framework, including the oceanic life cycle of tectonic plates, natural hazards impacting society, feedbacks in the Earth system, and global cycles of energy and matter. Expedition 504S will operate in parallel with a shore-based study of volcaniclastic rocks from the Malaita Terrane, an accreted fragment of OJP-related crust.