Why is the ocean salty? // Brains On! Science Podcast For Kids

Episode Overview

• This episode explores the scientific reasons behind the saltiness of oceans compared to the apparent freshness of rivers and lakes.
• It defines salinity and explains how minerals from eroded rocks on land are carried by rivers into the ocean.
• The episode details how evaporation concentrates these salts in the ocean, which acts as a basin with no outlet.
• It also touches upon the complex cycle of mineral precipitation that helps maintain a stable level of salinity in the oceans over millions of years.

Key Concepts

• Salinity: The measure of the amount of dissolved salts in water. Ocean salinity is typically around 35 parts per thousand, meaning for every kilogram of seawater, there are 35 grams of dissolved salts (mostly sodium chloride).
• Erosion and River Transport: The primary source of salt in the ocean is from the erosion of rocks and soil on land. Rainwater, which is slightly acidic, dissolves minerals from rocks, which are then carried by rivers and streams into the ocean.
• Evaporation and Concentration: The ocean acts as a giant basin with no outlets. While rivers continuously bring in water and dissolved minerals, water escapes through evaporation, leaving the salts and minerals behind. This process concentrates the salinity over time.
• Freshwater vs. Saltwater: Rivers and lakes do contain salts, but in much lower concentrations because they have outlets that allow the water (and some salts) to flow through them, preventing accumulation. Lakes without outlets, like the Great Salt Lake, are also very salty.
• Ocean Salinity Balance: The ocean's saltiness remains relatively stable because minerals are also removed from the water through processes like mineral precipitation, where dissolved minerals form solids and settle on the ocean floor, eventually becoming sedimentary rock.

Quotes

• At 00:25 - "Our question is, why are oceans salty while lakes and rivers are not?" - This is the central question of the episode, posed by young listeners Corinne and Sophie.
• At 00:53 - "The way they defined it originally was they would take a kilogram of seawater and then they would evaporate it, and they would measure what was left. And you'd get some like a pile of white powder at the end, and you weigh it." - Chemical oceanographer Phoebe Lam explains the fundamental method used to measure salinity.
• At 02:15 - "It comes from the rivers. So ultimately, all of the saltiness in the ocean got there from rivers." - Phoebe Lam provides the surprising core answer to the question, explaining that the source of the ocean's salt is freshwater rivers.

Takeaways

• Understand that the ocean's saltiness is a result of it being the final destination for water and minerals flowing from the land, with evaporation leaving the minerals behind.
• Recognize that "freshwater" is not entirely salt-free; it contains low concentrations of dissolved minerals that accumulate in terminal basins like oceans or salt lakes.
• Appreciate that the Earth's systems are in a dynamic balance; the ocean's salinity has been stable for millions of years because salts are removed at roughly the same rate as they are added.