4.1 Chemical composition of seawater

Major Ions and Chemical Composition of Seawater

Seawater's chemical composition is dominated by major ions like chloride and sodium. These ions maintain constant ratios throughout the ocean, which simplifies analysis and enables the study of ocean circulation patterns. This consistency is known as Marcet's Principle.

The ocean's dissolved ions come from various sources, including rock weathering, volcanic activity, and biological processes. They're removed through biological uptake, chemical precipitation, and other mechanisms. Understanding how long each ion stays in the ocean (its residence time) helps explain oceanic chemical cycles and predict environmental impacts.

Major dissolved ions in seawater

Two ions account for the vast majority of what's dissolved in seawater:

• Chloride ($Cl^-$) is the most abundant ion, making up about 55% of total dissolved solids.
• Sodium ($Na^+$) is the second most abundant at roughly 30% of total dissolved solids.

Together, those two alone represent 85% of the dissolved material. The remaining major ions round out the picture:

• Sulfate ($SO_4^{2-}$) comprises about 8% and plays a key role in marine biogeochemical cycles.
• Magnesium ($Mg^{2+}$), calcium ($Ca^{2+}$), and potassium ($K^+$) collectively contribute the remaining ~7%. All three are essential for marine life processes like shell-building and cellular function.

Beyond the major ions, several minor ions matter for specific reasons:

• Bicarbonate ($HCO_3^-$) helps regulate ocean pH, acting as part of the ocean's buffering system.
• Bromide ($Br^-$) is involved in atmospheric chemistry.
• Strontium ($Sr^{2+}$) gets incorporated into carbonate shells and is widely used in paleoceanography to reconstruct past ocean conditions.
• Boron ($B$) influences marine productivity.

Trace elements like iron ($Fe$), zinc ($Zn$), and copper ($Cu$) are present in extremely small quantities but are vital as enzyme cofactors. Iron, for example, often limits phytoplankton growth in large regions of the open ocean.

Concept of constant proportions

Even though total salinity varies from place to place, the relative ratios of the major ions stay remarkably consistent across the global ocean. This is Marcet's Principle (also called the principle of constant proportions), and it's one of the most useful facts in chemical oceanography.

Why does it matter so much? Because if the ratios are constant, you only need to measure one ion to figure out the total salt content. Historically, oceanographers measured chlorinity (the concentration of chloride) and used it as a proxy for total salinity. This dramatically simplified fieldwork.

Constant proportions also make it possible to trace ocean circulation and mixing patterns. If you see a deviation from the expected ratios, that tells you something unusual is happening at that location.

Exceptions to Marcet's Principle occur in a few specific environments:

• Estuaries and coastal areas where significant freshwater input alters the ratios
• Deep-sea hydrothermal vents where superheated water exchanges ions with the oceanic crust
• Isolated basins (like the Dead Sea) where extreme evaporation or limited mixing skews the chemistry

Sources and sinks of dissolved ions

The ocean's chemistry stays roughly in a steady state because ions are continuously added and removed. Think of it like a bathtub with the faucet on and the drain open: the water level stays constant as long as input matches output.

Sources (how ions enter the ocean):

1. Weathering of terrestrial rocks is the largest source. Rain and rivers dissolve minerals from rocks on land and carry the ions to the sea.
2. Volcanic activity introduces ions through submarine eruptions and hydrothermal vents on the seafloor.
3. Atmospheric deposition contributes ions via wind-blown dust and recycled sea spray.
4. Biological decomposition releases ions back into the water when organic matter breaks down or shells dissolve.

Sinks (how ions leave the ocean):

1. Biological uptake removes ions as organisms build shells, skeletons, and cell structures (calcium and silica are big ones here).
2. Chemical precipitation forms mineral deposits, such as evaporites in shallow basins where seawater evaporates.
3. Adsorption onto particles like clay minerals and organic matter pulls ions out of solution as those particles settle to the seafloor.
4. Hydrothermal alteration exchanges ions between seawater and hot oceanic crust at mid-ocean ridges, removing some ions while adding others.

Residence time of seawater ions

Residence time is the average length of time an ion remains dissolved in the ocean before being removed. It tells you how reactive or stable that ion is in seawater.

The formula is straightforward:

$\text{Residence time} = \frac{\text{Total amount of ion in the ocean}}{\text{Rate of input (or removal)}}$

A few patterns to know:

• Longer residence times generally correlate with higher concentrations. Sodium, for instance, has a residence time of about 260 million years. It's abundant and not very reactive, so it accumulates.
• Shorter residence times indicate more active cycling. Iron has a residence time of only a few hundred years because organisms consume it quickly and it readily binds to particles.

Residence time is also useful for predicting how the ocean will respond to human inputs. An ion with a short residence time will cycle through and be removed relatively quickly. An ion with a very long residence time will accumulate if we increase its input, because the ocean's removal mechanisms can't keep up on human timescales. This makes residence time a valuable concept for understanding how pollution and climate change affect ocean chemistry.