History of Deep-sea Ocean Basement Drilling Programs and Contributions to the Earth Sciences

Abstract

It is now more than half a century since the Deep Sea Drilling Project (DSDP 19681983) was launched in the United States. The DSDP was followed by the Ocean Drilling Program (ODP 19832003) and subsequently by the Integrated Ocean Drilling Program (IODP 20032013) . The program has been ongoing since 2013 as the International Ocean Discovery Program (IODP 20132023) . In October 2020, the long-term scientific outlook to 2050 (Science Framework 2050) for a new deep sea drilling program was published electronically in a follow-up to the second phase of the IODP that will be completed in 2023. Deep-sea drilling science must be one of the most successful international collaborations in the Earth sciences, with more than 1,000 deepsea scientific drilling sites operated to date, making it an essential approach for the direct exploration of the Earth's interior. The research products have promoted a scientific understanding of the ocean floor in a wide range of fields, including verification of the seafloor spreading theory of plate tectonics in the early days, elucidation of paleoenvironmental changes, and investigations of the subsurface biosphere. However, the so-called "basement drilling" of rocks (mainly basaltic igneous rocks) , which are the main material of the oceanic crust covered by deep-sea sediments, has not been developed smoothly. In more than half a century of deep-sea drilling programs, only 38 holes more than 100 m have been drilled through the basement rock, and only 20 holes deeper than 200 m have been drilled. In addition, the total amount of marine crustal material recovered by basement drilling is less than 2％ of the total depth of drilling. However, the structure and petrological properties of the oceanic crust have been gradually elucidated from available basement materials, although it is not easy to drill deep seafloor boreholes and to recover drilled rocks. An overview is presented of several important studies on deep-sea basement drilling to date in as much chronological order as possible, including a brief history of scientific drilling prior to the Deep Sea Drilling Program. Moreover, some plans are briefly introduced for ultra-deep basement rock drilling to the mantle in the future.