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doi:10.2204/iodp.proc.335.104.2012

Summary of scientific ocean drilling of the ocean basement, “Project Mohole” to IODP Expedition 335

In March–April 1961, the drilling barge CUSS1 undertook the first scientific ocean drilling operation off Guadalupe Island, ~240 km west of Baja California (Mexico). This expedition, beautifully reported in LIFE magazine by the novelist John Steinbeck and the renowned science photographer Fritz Goro, was the first (and eventually only) concrete manifestation of Project Mohole. This project was a very ambitious endeavor proposed in the late 1950s by the American Miscellaneous Society (AMSOC), an informal group of notable US scientists, mostly geophysicists and oceanographers associated with the Office of Naval Research, including Harry Hess and Walter Munk. The principal aim was to drill through the oceanic crust, through the Mohorovicic discontinuity, and to retrieve samples from Earth’s mantle. In his book A Hole in the Bottom of the Sea, Willard Bascom, Director of Project Mohole, records that the AMSOC elaborated on and initiated the project over a wine breakfast at Munk’s La Jolla home in April 1957, following on from original ideas discussed by Walter Munk and Harry Hess (Bascom, 1961). Bascom also notes that probably the first written suggestion for a deep penetration down into the mantle was given by Frank Estabrook, an astrophysicist from the Basic Research branch of the US Army in Pasadena (California, USA) in a letter “Geophysical Research Shaft” published in Science in 1956 (Estabrook, 1956).

IODP Expedition 335, the fourth expedition of the “Superfast” campaign to core an intact section of ultrafast-spread oceanic crust, coincided with the fiftieth anniversary of the drilling expedition in 1961 (Teagle and Ildefonse, 2011). The US National Academy of Science has launched a web page to commemorate the innovative accomplishments of Project Mohole (www.nationalacademies.org/​mohole.html). These accomplishments include the invention of dynamic positioning, the drilling guide horn, and deepwater drill hole reentry—all conceived and accomplished years before the offshore petroleum industry ventured into the open ocean. The drilling expedition in 1961 cored for the first time seismic Layer 2 and demonstrated with core that the uppermost ocean crust was made up of basaltic lavas. This achievement received a personal letter of congratulations from President Kennedy. Unfortunately, following divorce from the original scientific architects and vast cost overruns, Project Mohole progressively lost momentum with no further drilling accomplished, resulting in the ignominious termination of the project by the US Congress in 1965 (Shor, 1985; Greenberg, 1974). Despite often being recounted as a major geopolitical fiasco, this project has had an enduring impact on the Earth sciences by demonstrating that drilling in the deep ocean was technically feasible. This coincided with the formulation and growing acceptance of plate tectonic theory and recognition of the high-resolution geological records and key roles played by the oceanic crust and overlying sediments in major Earth cycles. Project Mohole’s direct offspring was the pioneering Deep Sea Drilling Project (DSDP) that initiated more than 40 years of international collaboration for scientific ocean drilling.

Since the start of DSDP in 1968, oceanic basement has been drilled in a range of geodynamic settings, and a compilation of holes into the ocean crust cored by scientific ocean drilling since the beginning of DSDP is presented in Table T1 and Figures F5 and F6. This compilation does not include other “hard rock” drill holes in oceanic plateaus, arc basement, hydrothermal mounds, or passive margins. Only 34 holes deeper than 100 m have been cored in oceanic crust since DSDP Leg 37 in 1974 (see Fig. F6). The recovered material represents <2% of the ~330 km of cores recovered to date by DSDP, ODP, and IODP. In spite of this relatively cursory sampling, scientific drilling has contributed significantly to advance knowledge of ocean crust architecture and mid-ocean-ridge accretion and hydrothermal processes (e.g., Alt et al., 1996; Teagle et al., 1998b; Dick et al., 2000, 2006; Ildefonse et al., 2007a, 2007c; Wilson et al., 2006; Blackman et al., 2011). Hole 504B, located on 6.9 Ma crust formed at an intermediate rate at the Costa Rica Rift (Fig. F5), remains the deepest hole (2111 mbsf) in all of scientific ocean drilling (Alt et al., 1996). This site was host to drilling and other experiments over eight DSDP and ODP legs and was the first hole to penetrate completely through the volcanic lava sequences and ~1 km into sheeted dikes. It remains a reference hole for hydrothermal alteration of the ocean crust (e.g., Alt et al., 1986a, 1986b) and the geological structure of seismic Layers 2A, 2B, and 2C (e.g., Carlson, 2011). Hole 504B is the only location where the seismic Layer 2/3 boundary has been sampled in situ (Detrick et al., 1994; Carlson, 2010). However, a complete, continuous section of intact, homogeneous fast-spread crust down to the cumulate gabbro layers has yet to be drilled and remains a first-order scientific target for ocean drilling for the ocean crust research community (e.g., Dick and Mével, 1996; Murray et al., 2002; Teagle et al., 2004, 2009; Ildefonse et al., 2007b, 2010a, 2010b; Ravelo et al., 2010; IODP Science Plan 2013–2023 [campanian.iodp.org/​NSP/​iodp_sci_plan_broch.pdf]). Recently, IODP Expedition 312 penetrated to the base of the sheeted dike complex and the uppermost gabbro in Hole 1256D, which was the first sampling of the transition to plutonic rocks in intact ocean crust (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the 309/312 Scientists, 2006; Wilson et al., 2006). Further deepening of Hole 1256D into cumulate gabbros was the primary sampling objective of Expedition 335.

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