8.5: Salinity over Long Timescales

Why don’t salts build up in the ocean? Why isn’t the ocean becoming saltier? Just how has ocean salinity changed over long timescales—centuries to millennia?

To understand the answers to these questions, we must go back in time. We begin in a period of geologic time known as the Archean, an eon that stretched from 4 to 2.5 billion years ago. Earth’s first ocean—presumably freshwater—formed from outgassing of water vapor from the interior. Subsequent cooling of that water vapor brought torrential rains that covered Earth’s basaltic crust. Higher elevations—probably volcanoes—stuck up like islands, but there were no continents. At some point, Earth’s crust began to cool, and an early form of plate tectonics took hold. The sinking, melting, and reworking of Earth’s oceanic crust gave rise to a smallish amount of continental crust. The first continent was born—Ur, named by geologist John Rogers in 1996. At a billion and a half years old, the relatively new Earth had one enormous ocean and one small continent (Rogers 1996).

While this was happening, water began to dissolve the soluble components of Earth’s crust. Eventually, whatever was soluble became dissolved in the ocean, and the ocean became salty. Hydrothermal vents—known to be present at least 3.2 billion years ago—added and subtracted their share of minerals, mostly NaCl. Some research suggests that the early ocean may have been twice as salty as the current ocean (e.g., Knauth 2005). Other evidence points to an Archean ocean remarkably similar to our modern one (e.g., Marty et al. 2018). We’ll have to wait for further studies to get a definitive answer.

Over time, through plate tectonic conversion of oceanic crust, other continents appeared: Arctica, Baltica, and Atlantica. About a billion years ago, they collided and formed a supercontinent called Rodinia. While this supercontinent-building was going on, parts of the coastal ocean were cut off. Seawater in the resultant isolated basins was trapped. The trapped water evaporated, the salts were left behind, and vast salt deposits called evaporites were formed.

Through tectonic uplift of the land, the evaporite deposits became permanent parts of the continents. At certain periods during Earth’s history, tectonic and climatic conditions were such that huge deposits of evaporites were formed, called mega-evaporites. You can find these vast deposits of salt in places like Australia, Oman, Iran, Pakistan, and beneath the Gulf of Mexico. During periods of mega-evaporite formation, the oceans got less salty.

The latest episodes of salinity reduction appear to have occurred in the Mesozoic during the breakup of the supercontinent Pangea (~250 million years ago) and later when the ice caps formed (~35 million years ago). Prior to the Mesozoic, salinity values may have ranged as high as 50 parts per thousand (ppt). By the early Miocene (~23 million years ago), salinity values had fallen to 37 to 39 ppt (e.g., Hay et al. 2006). Eventually, the sources of salt into the ocean balanced out the sinks. The salinity of the world ocean was established at the value that it is today, about 35 ppt. Though the salinity of the world ocean continues to change (the plates haven’t stopped moving, and sea level is rising due to melting of glaciers and ice caps), it’s reasonable to assume that over time periods of a few million years, the salinity of the ocean is constant.