Seawater is a remarkably consistent chemical solution despite the ocean's vast, turbulent complexity.

Nature · Published 30 September 2026 Swipe more like this →

In short: Covering over 70 percent of Earth, seawater is far more than just salty water. It is a precise, complex mixture of dissolved ions and gases that remains surprisingly uniform across the globe. While local conditions shift, the fundamental chemistry of our oceans is governed by a delicate, invisible balance.

Seawater is composed of 96.5 percent water and 2.5 percent salts, with the remainder consisting of dissolved gases, organic materials, and particulates. Six ions—chloride, sodium, sulfate, magnesium, calcium, and potassium—account for about 99 percent of all sea salts. Remarkably, the relative concentrations of these principal constituents remain nearly constant throughout the world's oceans. This consistency exists because the time required for oceanic mixing is significantly shorter than the time needed for input or removal processes to alter these concentrations.

Salinity, typically ranging from 34 to 37 parts per thousand in the open ocean, is primarily influenced by the local exchange of water through evaporation and precipitation. While physical mixing tends to homogenize the ocean, biogeochemical processes create spatial variations. For instance, organisms in the upper 100 metres transform dissolved inorganic carbon into organic matter, which then settles into the deep ocean. Hydrothermal solutions and river runoff further contribute to the complex chemical distribution found at different depths.

Beyond its chemical makeup, seawater supports a staggering diversity of life. Modern DNA-based identification techniques have revealed that a single litre of seawater may contain over 20,000 species of microbes. Researchers suggest the total number of bacterial types in the oceans could reach between five and 10 million. This biological richness, combined with the ocean's physical properties—such as higher viscosity and density compared to fresh water—makes seawater a unique medium that sustains the planet's climate and its vast, hidden ecosystems.

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