Ocean Biogeochemical Cycles

Carbon, nitrogen, phosphorus, and other elements cycle through the ocean in tightly coupled loops driven by biology, chemistry, and circulation, making the sea a vast regulator of Earth's climate and chemistry.

Definition

Ocean biogeochemical cycles are the coupled pathways by which biologically and geochemically important elements move among dissolved, particulate, living, and sedimentary reservoirs in the sea and exchange with the atmosphere.

Scope

This topic covers the marine carbon cycle and the solubility and biological carbon pumps, the nitrogen cycle including fixation, nitrification, and denitrification, the cycling of phosphorus, silicon, and iron, the exchange of gases with the atmosphere, and the coupling of these cycles to ocean circulation and sediments.

Core questions

• How do the solubility and biological pumps move carbon from the surface into the deep ocean?
• What microbial transformations drive the marine nitrogen cycle, and how are nitrogen inputs and losses balanced?
• How are the cycles of different elements linked through organic matter and circulation?
• How do these cycles regulate atmospheric carbon dioxide over short and geological timescales?

Key theories

Solubility and biological carbon pumps

Carbon is drawn into the deep ocean both physically, as cold surface water sinks carrying dissolved carbon dioxide, and biologically, as sinking organic particles export carbon away from the surface.

Marine nitrogen cycle balance

Biological nitrogen fixation adds fixed nitrogen while denitrification and anammox remove it, and the long-term balance of these microbial processes regulates the ocean's inventory of this often-limiting nutrient.

Mechanisms

Photosynthesis fixes carbon and nutrients into organic matter that sinks and is respired at depth, while gas exchange and the sinking of cold dense water transfer carbon dioxide downward; microbial communities transform nitrogen among its oxidation states, and slow burial in sediments closes the cycles over geological time. Circulation ties surface and deep reservoirs together, setting the timescales of exchange.

Clinical relevance

These cycles control how much carbon dioxide the ocean removes from the atmosphere and thus its role in moderating climate, while shifts in nutrient and oxygen cycling driven by warming and pollution alter productivity, dead zones, and the emission of greenhouse gases such as nitrous oxide.

History

Building on Redfield's elemental ratios, the modern synthesis of marine biogeochemistry emerged from global survey programs (GEOSECS, JGOFS, WOCE) in the late twentieth century and from Earth-system thinking, which framed the ocean's cycles as central components of the planetary carbon and nitrogen budgets.

Key figures

• Alfred Redfield
• Paul Falkowski
• Nicolas Gruber

Seminal works

• sarmientoGruber2006
• falkowski2000

Frequently asked questions

What is the biological pump?

It is the suite of processes by which marine life converts dissolved carbon dioxide into organic matter that sinks into the deep ocean, transferring carbon away from contact with the atmosphere.

How does the ocean affect atmospheric carbon dioxide?

Through gas exchange and its solubility and biological pumps, the ocean absorbs a large fraction of human carbon dioxide emissions and, over longer times, regulates the natural level of the gas in the atmosphere.