The ocean contains a massive, globally distributed reservoir of magnesium (Mg), which plays a fundamental role in the chemistry and biology of seawater. Magnesium is one of the most plentiful dissolved ions found in the global ocean, ranking as the second most abundant cation after sodium. It functions as an essential element, providing the chemical foundation for marine life and influencing global geological cycles. The vast quantity of dissolved magnesium has maintained a remarkably stable concentration for millions of years, reflecting a balance between the processes that add and remove it.
The Chemistry of Magnesium in Seawater
Magnesium is present in seawater at an average concentration of approximately 1,350 milligrams per liter, or about 55 millimoles per kilogram of water. This high concentration makes it a major component of sea salt, maintaining a consistent proportion relative to the total salinity. Magnesium exists almost entirely as the dissolved divalent cation, Mg2+, a stable form that interacts readily with other ions.
The constancy of this concentration results from magnesium’s exceptionally long residence time, estimated to be between 10 and 13 million years. This timescale ensures the element is mixed throughout the global ocean before removal, leading to a uniform distribution. Compared to calcium, magnesium is significantly less reactive in surface waters. Magnesium ions also help regulate the precipitation of calcium carbonate, the mineral used by many organisms for shells and skeletons. Dissolved magnesium helps keep calcium in solution, preventing the spontaneous formation of calcium carbonate crystals.
Input Pathways: Sources of Oceanic Magnesium
The primary mechanism introducing magnesium into the ocean is continental weathering, the constant breakdown of continental rock surfaces. This process involves rainwater and groundwater chemically reacting with rocks like silicates and carbonates, releasing magnesium ions into runoff. Rivers act as the main delivery system, transporting this dissolved magnesium from terrestrial environments into the sea. The riverine flux contributes a substantial input to the ocean’s inventory annually.
Other minor pathways also contribute to the overall supply:
- Submarine groundwater discharge, where mineral-rich water seeps from coastal aquifers.
- Wind-blown dust and volcanic emissions, which carry magnesium-containing particles that settle on the sea surface.
These external processes sustain the ocean’s magnesium reservoir.
Biological Functions in Marine Life
Magnesium performs fundamental tasks across all levels of marine biology. In phytoplankton, the microscopic plants that drive ocean productivity, magnesium is an indispensable component of the chlorophyll molecule. A single magnesium atom sits at the center of the chlorophyll structure, enabling the capture of sunlight necessary for photosynthesis.
Beyond primary producers, magnesium acts as a co-factor for hundreds of enzymes in every marine organism. These enzymes regulate metabolic functions, including the transfer of energy via adenosine triphosphate (ATP) and the synthesis of DNA. Without sufficient magnesium, these biochemical reactions cannot proceed efficiently. Magnesium is also incorporated into the biomineral structures of certain marine species, even though calcium is the main component of shells and skeletons. Coralline algae, sea urchins, and some foraminifera build their hard parts using magnesium-calcite, a form of calcium carbonate where magnesium is structurally integrated.
Geochemical Removal and Long-Term Stability
The long-term stability of the ocean’s magnesium concentration depends on powerful removal mechanisms that balance the constant input. The largest sink for magnesium is high-temperature hydrothermal circulation occurring at mid-ocean ridges. Seawater penetrates the oceanic crust through fractures and is heated by magma. This hot, chemically aggressive water reacts with basaltic rock, stripping magnesium from the seawater and incorporating it into new minerals in the crust. This process effectively removes a massive quantity of magnesium from the water column. The rate of this hydrothermal removal is a primary control on the global magnesium cycle.
Another significant sink is reverse weathering, which involves the formation of new clay minerals in marine sediments. Dissolved magnesium ions react with elements like silicon and aluminum to form authigenic silicates on the seafloor, sequestering magnesium into the sedimentary layer. Finally, the formation of dolomite, a calcium-magnesium carbonate mineral, also removes magnesium from seawater over geological timescales.