Ocean Worlds The Story of Seas on Earth and other Planets

Life most likely originated in the oceans, and it is to oceans that astronomers are looking to find life elsewhere in the universe. With the publication last year of Kevin Peter Hand's Alien Oceans, I decided this was the right time to finally review Ocean Worlds, a book that I have been very keen to read ever since buying it some years ago. This, then, is the first of a two-part dive into the story of oceans on Earth and other planets.

Palaeobiologists Jan Zalasiewicz and Mark Williams here provide a deep history of our oceans. As soon as I tucked in, it became clear that they go deeper than Eelco Rohling did in the previously reviewed The Oceans: A Deep History, a book that focused heavily on palaeoclimatology. Even though most of the action in Ocean Worlds takes place on Earth, and the wider universe is only considered in the opening and closing two chapters, the book is characterised by an almost cosmic perspective on the subject. The writing of Zalasiewicz and Williams is such that I felt as if was surveying major developments in the history of our universe from an elevated, slightly detached, almost omniscient position. The result is thrilling and at times awe-inspiring. What follows are some of the big questions and outrageously fascinating topics they consider.

To have an ocean we first need water. Hydrogen was an immediate byproduct of the Big Bang. Oxygen, however, did not appear until after the universe had gone through its first cycle of stars being born and dying, as its creation required nuclear fusion. Likely, the formation of water had to wait for a few hundred million years, though some have argued it could have started much sooner. As is usual when dealing with processes that took place in such a distant past, opinions are divided and there are several reasonable scenarios.

With water present in the universe, how did Earth acquire its oceans? After all, "There is a wild card here, which surely had an impact" (p. 18). We have good evidence that our proto-Earth, called Tellus by some, was hit by a small planetoid, Theia, with the resulting debris forming our current Earth-Moon system. This event would likely have obliterated what early oceans we had, if any. Various authors have proposed that certain meteorites (carbonaceous chondrites) or comets might have subsequently been water's cosmic delivery vehicle.

However it got here, the first major effect it had was kick-starting plate tectonics. The early Earth was hot, but without the lubrication provided by water, the heat-venting mechanism of plate tectonics was not in place. How did molten rock make its way to the surface? Some scientists argue that it was through simple vertical conduits, so-called heat pipes, which would have made for a radically different surface topography: "the fundamental proportions of land area and ocean area [...] would have been utterly different to today's familiar patterns" (p. 34). Though, again, this idea is contested by others. The puzzle of when plate tectonics started, possibly 3 billion years ago, relies on truly ancient rocks, 3.5 to 3.8 billion years old, of which we have precious few remaining in places such as Australia and Greenland.

Beyond those earliest days, Ocean Worlds has much interesting material about later episodes. Life likely started in the oceans, this much I knew, but these were iron seas. Water without oxygen can hold large amounts of dissolved iron, and early organisms used this in their biochemistry to generate energy. This was the realm of the Archaea: the salt-tolerant, heat-loving, chemoautotrophic microbes for whom oxygen was poison and the Great Oxygenation Event murder. It was also a time when banded iron formations (BIFs) were built up, relevant to us today as they formed the ore deposits providing most of our iron and steel. Though, as clarified here, their formation was anything but straightforward. Other fascinating episodes are the Messinian Salinity Crisis, some 5.6 million years ago, when the Mediterranean repeatedly dried up, leaving behind kilometre-thick salt layers that reduced global ocean salinity.

Of course, a book about oceans has to consider current human impacts. With due diligence, the authors tackle the problems of overfishing, shifting baselines, trawling, litter, ocean warming, oxygen loss, and acidification. If that was not sobering enough, what really made me feel small was when they pulled back from our timescale and the current "brief ecological wrecking spree" (p. 195), to the long-term future. Our oceans are not forever. As the Sun grows hotter they will evaporate, though the "end of the oceans is not likely to be simple" (p. 207). Whether through a moist greenhouse phase where water is gently siphoned off into space by solar winds, or a runaway greenhouse hot enough to melt rock, a dry future awaits, and plate tectonics will once again grind to a halt. As this process "is unlikely to simply just stop, smoothly and without fuss" (p. 211), expect some extraordinary landscapes.

Amidst these grand, cosmic scenes, the authors highlight the human stories behind this research. Such as the pioneering contributions to oceanography by the people on board the HMS Challenger expedition, the mapping of the seafloor by Marie Tharp, or the work of Wally Broecker who established a link between ocean currents and rapid climatic changes. And while Svante Arrhenius is better remembered for linking historical changes in carbon dioxide concentrations to past ice ages, both he and Fritz Haber tried to extract gold from sea water. Unsuccessfully, I might add.

In the last two chapters, the authors turn their gaze to the skies once more, discussing past and present oceans inside and outside of our solar system. With the many exoplanets discovered by the Kepler space telescope, "We are on the verge of not just a new chapter in oceanography – or exo-oceanography, if you like – but of setting up an entirely new library of oceans, for the diversity and complexity of cosmic oceans will be beyond anything that we can dream of" (p. 264).

I will explore this topic more in-depth in my review of Alien Oceans. But, as a warming-up exercise and a proper deep history of oceans, Ocean Worlds is a fantastic book that strikes the right balance. Zalasiewicz and Williams present fascinating science with enviable ease, without smoothing over the fact that science is rarely a straightforward affair, proceeding by means of conflicting scenarios and competing hypotheses. The deep-time perspective and big questions asked make this one awe-inspiring book.