Mysteries of the Deep How Seafloor Drilling Expeditions Revolutionized Our Understanding of Earth History

Many advances in the earth sciences have come from one particular feat of technology and engineering: deep-sea drilling. Or to be more precise, it is the sediment cores thus extracted from the seafloor that have offered a wealth of information. In Mysteries of the Deep, retired geologist James Lawrence Powell gives a very readable whistlestop tour of the many remarkable insights these drilling expeditions have given us. In the process, he provides a microcosm of how science advances and how scientists change their minds, or sometimes fail to, in the face of new evidence.

As with other things oceanographic, we can trace the origins of deep-sea drilling to the Challenger expedition. Next to many extraordinary animals, it collected sediment samples using a hollow tube that would plunge into the seabed. In various chapters, Powell traces how successive expeditions improved on this technique and retrieved longer sediment cores. A first attempt in 1961 at drilling through sediment to reach the underlying mantle rock, project Mohole, succeeded technically but subsequently floundered bureaucratically. It did lead to the collaborative partnership JOIDES (Joint Oceanographic Institutions for Deep Earth Sampling) in 1964 in which several universities teamed up and successfully obtained funding from the National Science Foundation. This was the start of a series of research projects that have been ongoing for the last six decades. Powell provides a fair amount of detail on the first project and the engineering challenges and solutions but has relegated information on later vessels and programmes to a brief 2-page appendix. There are no doubt stories left untold here. Instead, the focus is squarely on the fruits of all this labour. What has deep-sea drilling ever done for us? Actually, a surprising amount!

Anyone familiar with the history of geology will not be surprised that several chapters deal with continental drift and plate tectonics. This decades-long saga saw independent lines of evidence pile up, even as the idea continued to be staunchly resisted. Powell gives a concise overview and highlights the contributions made by deep-sea drilling. For instance, drilling at increasing distance from the Mid-Atlantic Ridge revealed progressively older rocks, consistent with the idea of seafloor spreading. It also vindicated Marie Tharp's hunch, based on her and Bruce Heezen's meticulous mapping of the Atlantic seafloor, that the ridge was a rift valley where two plates were separating.

Beyond the obvious, deep-sea drilling furthermore contributed to other, less expected topics. For example, in recent geologic history, the Mediterranean Sea became cut off from the Atlantic and dried up, an episode known as the Messinian Salinity Crisis. Deep-sea drilling found large deposits of evaporite salt that were left behind, as well as very deeply eroded river mouths that were later backfilled with newer marine sediment. Other drilling expeditions have found evidence of bacterial life deep beneath the Earth's crust. Drilling has delivered insights on palaeoclimatology, with one of the nine holes drilled in 1987 during Leg 113 of the Ocean Drilling Program becoming "a shining example of the value of scientific ocean drilling" (p. 210), spawning hundreds of scientific papers. It provided crucial evidence for a period of intense runaway climate change known as the Paleocene-Eocene Thermal Maximum 56 million years ago where average global temperatures soared by approximately 7°C. Deep-sea drilling has even shed light on human evolution. Cores extracted off the coast of East and West Africa show that in the last 3 million years there were periods where the climate rapidly alternated between wet and dry conditions. These periods coincided with new hominin species appearing in the fossil record.

A noticeable second component in this book is Powell's interest in science history and the progression of ideas. His previously reviewed Four Revolutions in the Earth Sciences explicitly examined four such episodes and it makes a welcome return here. Next to the abovementioned history of plate tectonics, Powell gratefully draws on the 1986 book by the Imbries to recount the discovery of the Pleistocene Ice Ages. Though the name of Louis Agassiz is usually attached to this, it was more of a relay race. In the face of almost wholesale rejection, you can trace a concatenation of scholars (there is a new collective noun for you) who defended the idea before it reached Agassiz in 1834. Powell also highlights examples of scientists changing their minds. Darwin himself did not notice anything out of the ordinary when visiting glacial deposits in Wales in 1831, but upon a second visit in 1842 thought the evidence could not be clearer. "Even the greatest minds cannot think the unthinkable" (p. 131). Similarly, geologist J. Tuzo Wilson went from opposing continental drift in 1960 to supporting it in 1961 after reading about a novel hypothesis for the formation of the chain of volcanic islands of Hawai'i. Rather than a large volcanic fissure moving along a static seafloor and periodically extruding islands (for which no mechanism was proposed), it was actually the seafloor that moved over a volcanic hotspot.

What makes this book a pleasure to read is the conciseness of the chapters, which never exceed 20 pages; no topic overstays its welcome. Additionally, Powell makes good use of (historical) illustrations to visually explain concepts, as well as show what scientists thought at certain points in time. There are one or two chapters that can get quite technical. Although I am familiar with Milankovitch cycles (another topic where Powell adds a nice science history angle), the chapter discussing the subsequent development of astrochronology and cyclostratigraphy was a challenging read. Overall, at least some familiarity with earth sciences will be beneficial to the reader.

Whereas the related topic of ice core drilling has been the subject of at least two popular books in the last ten years, there has to my knowledge not been a popular book on deep-sea drilling since Kenneth Hsü's 1992 book Challenger at Sea. With Mysteries of the Deep, Powell delivers that book: a fascinating and accessible whistlestop tour of what international collaborative science can achieve. As mentioned in his final chapter, though the current International Ocean Discovery Program is scheduled to conclude next month in September 2024, the earth science community is busy planning its follow-up. There are many more questions that we did not even know we had before deep-sea drilling.