The silicon cycle describes the movement of silicon through Earth’s various systems, from its release from rocks to its journey through water bodies and eventual return to the crust. This biogeochemical cycle influences nutrient levels and helps regulate carbon in the atmosphere and oceans. Silicon, the second most abundant element in the Earth’s crust, is a foundational element for many marine organisms. The cycle involves processes like rock weathering, which releases silicon into the environment.
The Journey of Silicon Through Earth
The journey of silicon begins with the weathering of silicate rocks, which constitute about 90% of all minerals in the Earth’s crust. As rainwater, often enriched with carbon dioxide to form carbonic acid, interacts with these rocks, it chemically breaks them down. This process releases dissolved silicon, primarily as silicic acid, into soils and freshwater systems.
From terrestrial environments, dissolved silicon is transported by rivers to the oceans, representing the largest source of silicon to the marine environment, contributing up to 90% of all silicon delivered to the ocean. Once in the ocean, silicon is taken up by various marine organisms, particularly diatoms, which are microscopic algae. These organisms use the dissolved silicon to construct their intricate cell walls, known as frustules, made of biogenic silica.
After diatoms and other siliceous organisms die, their biogenic silica shells sink through the water column. A significant portion of this biogenic silica dissolves back into silicic acid as it sinks, replenishing the dissolved silicon pool in the deeper ocean. However, some of these shells reach the seafloor, forming siliceous sediments.
Over geological timescales, these buried siliceous sediments can undergo processes like subduction and metamorphosis, eventually returning silicon to the Earth’s crust. This long-term geological recycling, which can take tens to hundreds of millions of years, makes silicon available for future weathering events.
How Silicon Shapes Life and Climate
The silicon cycle profoundly shapes marine life, particularly through its interaction with diatoms, which are single-celled algae. Diatoms require dissolved silicon to construct their rigid, glass-like cell walls, or frustules. This structural requirement makes silicon availability a primary factor regulating diatom growth in marine environments.
Diatoms are fundamental to marine food webs, contributing up to 45% of the total primary production in the ocean. Their abundance supports a wide range of marine organisms at higher trophic levels, acting as a significant link in the transfer of energy within the food chain. Without sufficient silicon, diatom populations would decline, impacting the entire marine ecosystem.
Beyond their role in food webs, diatoms play a substantial part in the global carbon cycle. As they photosynthesize, diatoms absorb carbon dioxide from the atmosphere and convert it into organic carbon. When diatoms die and sink, a portion of this organic carbon, along with their silica frustules, is transported to deeper ocean layers and sequestered in seafloor sediments.
This process, often referred to as the biological pump, effectively removes carbon dioxide from the atmosphere, influencing global climate regulation over geological timescales. The silicon cycle thus directly connects to the carbon cycle, demonstrating silicon’s role in balancing atmospheric and oceanic carbon levels.
Human Influence on the Silicon Cycle
Human activities have significantly altered the natural flow and balance of the silicon cycle, particularly impacting the transport of silicon from land to oceans. River damming, for instance, is a major human intervention that disrupts silicon delivery.
Dams slow down water flow, causing sediment, including silicon-rich material, to settle and accumulate in reservoirs upstream. This increased residence time of water in dammed reservoirs promotes the growth of diatoms and other algae, which consume dissolved silicon before it can reach downstream ecosystems and the ocean. Consequently, damming can reduce the amount of dissolved silicon reaching coastal waters, with global estimates suggesting a nearly 30% decrease in total global dissolved silicon movement to the ocean.
Agricultural practices also exert considerable influence on the silicon cycle. Intensive farming can lead to changes in soil silicon content, as crops accumulate silicon in their tissues and its removal during harvesting prevents its return to the soil through natural decomposition. This results in substantial silicon losses from agricultural plant-soil systems, altering the natural cycling of biogenic silica in terrestrial environments.
Furthermore, the application of fertilizers and other pollutants in agriculture can introduce large amounts of non-silicon nutrients into rivers, lakes, and reservoirs. These excess nutrients can trigger algal blooms, which rapidly consume available silicon, further preventing its transport to the oceans. These human-induced alterations have far-reaching consequences for marine ecosystems and the carbon cycle.