How salty has the sea been over the past 541 million years?

Take a bottle of water from the sea and try to drink it. You gag and your lips pucker. After all, dissolved in that liter of the ocean are around 35 grams of salts (mostly sodium chloride). Now, imagine you tried to do this same thing 1 million years ago, 10 million years ago, 100 million years ago, even 500 million years ago (that is, throughout the Phanerozoic eon). Would you ever be able to drink the water? Alternatively, would the sea ever have been so salty that today’s ocean creatures would not have survived? A 2006 article by Hay et al. helps answer precisely these questions. The authors tracked variable chloride levels to demonstrate how salinity has changed throughout the Phanerozoic, noting a significant overall decline. These changes have had important effects on ocean circulation and on plankton levels — and possibly contributed to the explosion of complex life in the Cambrian, 541–520 million years ago.

From the late nineteenth century to at least 1925, some scientists tried to use salinity levels to calculate the age of the earth — leading to grossly low estimates of only 100 million years or so. After geologists realized that salt was being removed as well as added to the oceans, some switched to the assumption that salinity has remained relatively constant at 34–35‰ (i.e. 34–35 grams of salt per liter of seawater). What Hay et al. present against these assumptions is evidence from evaporites: large amounts of solids that represent the extraction of salt from the oceans. For instance, drilling in the Gulf of Mexico in 1967 revealed a vast layer of salt from the Jurassic beneath the oceanic sediments. The total global amount of halite (salt deposits) discovered was astonishing: the minimum estimate is 19.6 quintillion kilograms (19.6 × 1018 kg), or more than half of the total salt in the oceans today — the maximum estimate is that an unbelievable 95% of the present total is trapped in halite deposits. If all of this had been dissolved at one time, the salinity of seawater would have been much higher: 57‰ to 73‰ instead of today’s 34‰ to 35‰. This extreme scenario is unlikely, but one thing is clear: the salinity of the oceans has varied significantly over geologic time. The problem is how to quantify this change.

One issue faced by the authors is that the evaporite inventory is incomplete. For instance, data from Antarctica are hard to come by — expeditions to drill through hundreds of meters of ice and then kilometers of sediments give nightmares to engineers (and logisticians). New discoveries could easily change the tables of evaporite volumes from different periods that Hay et al. compile. There are other issues, too, that need to be addressed to make estimates as precise as possible. For instance, what exactly are the authors measuring to tell us paleosalinity (how salty the earth was at a given time)? The option Hay et al. go with is to use the concentration of chloride ions (Cl) as a proxy for salinity. Four anions and four cations (Cl, SO42−, HCO3, Br, and Na+, Mg2+, Ca2+, K+) make up 99.8% by weight of the solids dissolved in seawater. To precisely measure salinity one must find the total mass of solids dissolved in a volume of water, which is usually expressed as grams per liter or (equivalently) parts per thousand (‰). In seawater, these solids are mostly Cl and Na+, the constituent ions of table salt (sodium chloride). Chloride alone makes up 55% of the dissolved solids in seawater. It is therefore a good measure of salinity. One issue, though, is that the relative proportion of ions in seawater has changed. In other words, an increase in the concentration of chloride ions could just show that chloride dominated more than other ions — not that the overall salinity itself increased. Hay et al. ultimately settle on chloride as a proxy because it, unlike other ions, does not easily enter into minerals. Chloride instead resides almost entirely in seawater, ocean sediments, and evaporites.  Bearing in mind these issues, Hay et al. turn to measuring the mass of solids and volume of water throughout the earth’s history to determine paleosalinity.

Chloride comes mostly from physical weathering, but also partly through rainfall that has picked up chloride from sea spray. Either way, most chloride ions are then delivered to the ocean by rivers. Some chloride also comes from volcanic emissions, and more and more now comes from human activities. The authors thus summarize the changes in chloride, which is a proxy for the total mass of dissolved solids; to determine paleosalinity, they must also track the volume of water. The earth’s volume of water can increase because of comets or outgassing from the earth’s interior — though outgassing often just recycles seawater instead of actually increasing the volume of water. Can the volume of water decrease? Indeed, some is lost to space — though this has become negligible as the amount of oxygen (from life) has increased. But the problem, Hay et al. note, is that these processes are too uncertain to effectively determine how the volume of water has changed over the earth’s history. They therefore resort to using two different models for the history of water volume on the earth: the first assumes that the volume has remained constant in the Phanerozoic; the second assumes a steady decline of 0.256 × 1012 kg/yr, based on observations of continental flooding and emergence (that is, paleogeography). On the other hand, it is easier to determine how the volume of ocean water has changed. When the climate was cold, more water was locked up in ice sheets and glaciers — which caused an oscillation of ocean salinity from 34.7‰ to 36‰. Hay et al. have at this point dealt with starting assumptions and problems and shown us why chloride and water levels have changed in the earth’s history. They use calculations from the evaporite inventory and the two different models of ocean history to compile the following graph of paleosalinity:

What have been the effects of this decline from 50‰ to 35‰ over the past 500 million years? An increase in salinity results in greater density, lower freezing temperature, and higher osmotic pressure. These effects change patterns of ocean circulation. At higher salinities, much less energy is needed to make water more dense — and it is density that drives convection and hence ocean circulation. Since ocean convection requires less energy at higher salinity, ocean water mixed more actively in the past. What does higher salinity mean for life? This is an easier question to answer, since there are plenty of small habitats with locally high salinity. For example, corals in the northern Red Sea thrive in salinities ranging between 41‰ and 43‰ — conditions that were even more extreme during the last ice age. Hay et al. therefore suggest that “at present many marine animals and plants are living nearer their low salinity tolerance limit rather than their high limit.” Unfortunately, it is difficult to test this hypothesis with fossils of marine creatures from the open ocean because most ocean crust is geologically young — that is, less than 200 million years old. (Marginal seas — that is, flooded continents — do have a useful fossil record, but their salinity is strongly affected by local differences in fresh water supply rather than broader trends.) Hay et al. also observe that plankton may have bloomed as salinity declined, which might explain why deposits of the Late Jurassic and Early Cretaceous are such good producers of petroleum. The other tempting correlation Hay et al. point to is between the sharp decline in salinity and explosion of complex life at the beginning of the Cambrian, 541 million years ago. They hesitate, though, to assign a specific date to the decline in salinity and even more so decline to hypothesize how this might be linked to the Cambrian explosion.

In their paper, Hay et al. present compelling evidence for changes in salinity. They account for issues of definition; different measurements of solids; and the sources and sinks of chloride and water. By combining this discussion with a comprehensive inventory of evaporites in previously published literature, they arrive at detailed estimates of paleosalinity from the past 540 million years. This work lays a strong foundation for tantalizing future work on the consequences of paleosalinity variation for life. Later articles have expanded the scope of the paper to other time periods; focused on the consequences of salinity for specific organisms; refined modeling of ocean circulation; incorporated the estimates of paleosalinity in broader histories of the earth’s climate and carbon cycles; and even considered how salinity might make oceans different on planets outside our solar system.

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