Reading Seawater
“How inappropriate,” Arthur C. Clarke once observed, “to call this planet Earth when it is so clearly Ocean.”1 Most of our planet is covered with water, almost all of it salty. “Water, water everywhere, nor any drop to drink,” the Ancient Mariner complained. Yet it remains surprisingly difficult to explain why the sea is salty.
Most coastal cultures have a story that explains the origin of sea salt. These accounts typically suggest that the waters were fresh to begin with and salt was added later. In Norse mythology, the sea was made unpotable through an act of revenge by two enslaved giantesses, Fenja and Menja, who tended a magic mill that they alone were strong enough to turn. Held captive on a ship and forced to grind salt day and night by the sea-king Mýsingr, the giantesses kept on grinding until they produced enough to sink the vessel. The still-churning millstone fell into the abyss, creating a whirling maelstrom and forever mixing salt into the sea.
For Pythagoras, the sea represented the tears of Kronos, a Titan born of Gaia and Uranus. Empedocles suggested that seawater was the sweat of the earth. Aristotle wrote at length on the composition of seawater in his treatise Meteorology, sweeping aside earlier theories with scorn. “Metaphors are poetical,” Aristotle sniffed, “and so that expression of [Empedocles] may satisfy the requirements of a poem, but as a scientific theory it is unsatisfactory.”2 Aristotle observed that seawater is not merely salty but also bitter, concluding that it must therefore be an admixture of water and various earthy residues borne by rivers. He recognized that this idea posed a problem. “If it is maintained that an admixture of earth makes the sea salt,” Aristotle wrote, “it is strange that rivers should not be salt too.” He suggested that the action of the sun’s heat on seawater played a transformative role, and offered a comparison with the production of cinders and bodily waste:
What heat fails to assimilate becomes the excrementary residue in animal bodies, and, in things burnt, ashes. That is why some people say that it was burnt earth that made the sea salt. To say that it was burnt earth is absurd; but to say that it was something like burnt earth is true. … [E]verything that grows … always leaves an undigested residue, like that of things burnt.3Aristotle further argued that the accumulation of salt in the sea was counterbalanced by rainfall, thereby keeping its composition in a steady state, an early articulation of a key concept in chemical oceanography.
More than three centuries later, Pliny the Elder, in descriptions of salt extraction methods around the Roman Empire, still espoused Aristotle’s views, although he downplayed the excretory analogies. Pliny added his own elaboration, suggesting that because salt precipitated in pools and pans at night, the action of the moon must also be significant.4
The continuing influence of Aristotle and Pliny into the Middle Ages is reflected in the writings of the tenth-century Arabic philosopher and historian al-Mas’udi.5 In his Muruj adh-dhahab wa ma’adin al-jawahir (Meadows of Gold and Mines of Gems), al-Mas’udi described the continuous chemical exchange between earth and ocean. Water flowing into the sea carries salt absorbed from the earth; heat from the sun and moon evaporate the “sweet portions of the water,” which later fall as rain. “This process,” he remarked, “is constantly repeated.”6
After two millennia, Aristotelian doctrine was finally overturned by Robert Boyle. In the opening paragraph of his Experiments and Observations upon the Saltness of the Sea, published in 1675, Boyle scolded Aristotle:[H]is Authority, perhaps much more than his Reasons, did for divers Ages make the Schools and the generality of Naturalists of his Opinion, till towards the end of the last Century and the beginning of ours, some Learned Men took the boldness to question the common Opinion…7Boyle’s empirical approach laid the groundwork for the practice of modern chemistry. He systematically analyzed seawater through hydrometer measurements, as well as evaporation, precipitation, and primitive titration experiments. Boyle disproved Aristotle’s hypothesis that some unspecified action of the sun’s rays on water left behind a residue of concentrated salts, suggesting instead that evaporation of riverine waters was sufficient to explain seawater chemistry.
By the nineteenth century, geology was emerging as a discipline in its own right, revealing increasing evidence that the earth had a far longer history than previously imagined. In 1899, John Joly suggested that the earth’s age could be estimated from the salinity of seawater. He assumed that the sea had started as fresh water and had grown more saline over geologic time. Joly gathered information about the composition of river water from around the British Empire to determine the average annual flux of sodium into the sea. He divided the total inferred volume of sodium in ocean water by this number and estimated that the age of earth, or at least its oceans, was between 90 and 100 million years.8
Joly’s work was criticized by the geological community, not least for his assumption that the salt content of the oceans had increased continuously over geological time. This seemed inconceivable to geologists steeped in the uniformitarian doctrine popularized by Charles Lyell in his Principles of Geology, which stressed that the earth’s processes and natural laws have remained constant.9 Moreover, geologists had also documented thick evaporite, or rock salt, deposits left by ancient marine waters in sedimentary sequences around the world. These clearly indicated that sodium and other elements did not accumulate continuously, but could also exit the sea in large volumes. In arguing for a steady state in seawater chemistry, both Aristotle and al-Mas’udi had been correct in a way that Joly was not.
As it turned out, Joly’s efforts to date the earth were eclipsed in the following decade by the development of radiometric dating techniques. But his result was not entirely without significance. Joly’s estimate of approximately 100 million years is close to the residence time of sodium in seawater: the average time a sodium ion stays in the sea before leaving via mineral precipitation, salt spray, or other sinks. Sodium is, in fact, part of a geochemical cycle—atoms do not simply take one-way trips into the ocean.
The emergence of a more nuanced understanding of ocean chemistry can be seen in a paper published by William Rubey in 1951.10 Rubey amassed data about the chemical composition of present-day river and ocean waters, ancient marine deposits, and igneous rocks to ascertain whether ocean chemistry could be explained solely by rivers conveying the dissolved products of rock weathering.In what can be recognized as an early effort to model global biogeochemical cycles, Rubey attempted to balance the budgets of the various constituents of seawater. Like Aristotle, he assumed that the composition of seawater had remained constant over geologic timescales. In his mass balance calculations, Rubey was unable to account for the profusion of volatile elements and compounds found in seawater, including chlorine, its most abundant ion. He concluded that there must be another source for some of seawater’s major constituents and that source might be deep-sea “volcanoes, fumaroles, and hot springs.”11 He was right.
