WHY IS IT SO?

The wa­ter on the Earth’s sur­face was once fresh. JAKE PORT ex­plains how that changed.

Cosmos - - Contents - JAKE PORT, a Mel­bourne-based writer, con­trib­utes to the ex­plainer se­ries on cos­mos­magazine.com

IT IS ES­TI­MATED there is enough salt in the world’s oceans to cover all the planet’s land sur­faces with a layer about 40 sto­ries thick. But sea­wa­ter wasn’t al­ways so salty; when the Earth’s oceans first formed about 3.8 bil­lion years ago, as the sur­face of the planet cooled enough to al­low wa­ter vapour to liquify, the oceans were mostly fresh wa­ter. So where did all the salt come from?

It came from rock, laden with el­e­men­tal salts in­clud­ing sodium, chlo­rine and potas­sium, that was spewed forth as mag­matic ma­te­rial by mas­sive vol­canos from the depths of the planet.

En­ter ero­sion, the process lib­er­at­ing th­ese salts from their rocky prison, thanks to an at­mos­phere dom­i­nated by gases in­clud­ing ni­tro­gen and, im­por­tantly, car­bon diox­ide.

When mixed with wa­ter (H O), car­bon diox­ide 2 (CO ) can form car­bonic acid (H CO ), a weak but 2 2 3 cor­ro­sive acid. This car­bonic acid rained down on salt-rich rock, slowly break­ing through and re­leas­ing the trapped salt into rain­wa­ter. The runoff slowly car­ried the salt to nearby lakes and rivers, which in turn car­ried it to the seas. Al­though the amount de­posited by any one out­let was small, the con­tri­bu­tion of mil­lions of out­lets over mil­lions of years grad­u­ally ri­ased the salin­ity of the oceans. The process con­tin­ues.

Along the way from rock to sea, a fair pro­por­tion of the salt re­leased from rock is used by liv­ing things. Salt is cru­cial to both plant and an­i­mal life, reg­u­lat­ing the amount of fluid in cells and neu­ron func­tion. When an or­gan­ism dies and de­com­poses, the salt is freed to con­tinue its sea­ward jour­ney.

Acid rain isn’t the only way the seas are fed with salt. On­go­ing vol­can­ism still has an im­por­tant role to play. Hy­dro­ther­mal vents al­low sea­wa­ter that has seeped though the rock of the oceanic crust to re­turn to the sur­face. The wa­ter is su­per­heated from magma be­low, and as it trav­els up it dis­solves min­er­als locked in the crust, erupt­ing as min­eral-rich steam.

A sim­i­lar process in­volves the in­ter­ac­tion of sub­ma­rine vol­ca­noes with sur­round­ing sea­wa­ter. Sub­ma­rine vol­ca­noes are com­pa­ra­ble to their above-ground rel­a­tives ex­cept that their lava cools much more rapidly, al­low­ing for speedy growth. Magma erupt­ing through sub­ma­rine fis­sures boils the sur­round­ing wa­ter, which then dis­solves salts in the cool­ing rock to es­cape in a man­ner sim­i­lar to hy­dro­ther­mal vents. Many of the worlds is­lands were formed by this process, re­leas­ing thou­sands of tons of salt in the process.

While sea­wa­ter con­tains, on av­er­age, about 35 grams of salt per litre, the oceans and seas are not uni­formly salty; gen­er­ally the closer you get to the poles the less saline the wa­ter be­comes, as fresh wa­ter re­leased from the ice of the frozen poles di­lutes the con­cen­tra­tion of the salt.

There is still one ques­tion left: if most of the salt in the sea comes via rivers and streams, why are they not also salty? The sim­ple ex­pla­na­tion is that they do con­tain salt, but the con­cen­tra­tion is much lower, and the salt flows rather than ac­cu­mu­lates. It is es­ti­mated that each year four bil­lion tons of dis­solved salts are car­ried to the sea by rivers.

So is the ocean get­ting saltier? The an­swer right now is prob­a­bly not. The in­put of salts is bal­anced by salts be­ing buried un­der­ground by the move­ment of tec­tonic plates, the flow of fresh­wa­ter and a host of other pro­cesses.

IM­AGE Tommy Clarke / Getty Im­ages IL­LUS­TRA­TIONS Jef­frey Phillips

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