The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is considered a bioessential element and is one of the most abundant elements on Earth. [2] [3] The silica cycle has significant overlap with the carbon cycle (see carbonate–silicate cycle) and plays an important role in the sequestration of carbon through continental weathering, biogenic export and burial as oozes on geologic timescales. [4]
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Silicon is the eighth most abundant element in the universe and the second most abundant element in the Earth's crust (the most abundant is oxygen). The weathering of the Earth's crust by rainwater rich in carbon dioxide is a key process in the control of atmospheric carbon dioxide. [5] [6] It results in the generation of silicic acid in aqueous environments. Silicic acid, Si(OH)4, is a hydrated form of silica found only as an unstable solution in water, yet it plays a central role in the silica cycle. [1]
Silicifiers are organisms that use silicic acid to precipitate biogenic silica, SiO2. Biogenic silica, also referred to as opal, is precipitated by silicifiers as internal structures [7] and/or external structures. [8] Silicifiers are among the most important aquatic organisms. They include micro-organisms such as diatoms, rhizarians, silicoflagellates and several species of choanoflagellates, as well as macro-organisms such as siliceous sponges. Phototrophic silicifiers, such as diatoms, globally consume vast amounts of silicon along with nitrogen (N), phosphorus (P), and inorganic carbon (C), connecting the biogeochemistry of these elements and contributing to the sequestration of atmospheric carbon dioxide in the ocean. [9] Heterotrophic organisms like rhizarians, choanoflagellates, and sponges produce biogenic silica independently of the photoautotrophic processing of C and N. [10] [8] [11] [1]
The diatoms dominate the fixation and export of particulate matter in the contemporary marine silica cycle. This includes the export of organic carbon from the euphotic zone to the deep ocean via the biological carbon pump. As a result, diatoms, and other silica-secreting organisms play crucial roles in the global carbon cycle by sequestering carbon in the ocean. The connection between biogenic silica and organic carbon, together with the significantly higher preservation potential of biogenic siliceous compounds compared to organic carbon makes opal accumulation records of interest in paleoceanography and paleoclimatology.
Understanding the silica cycle is important for understanding the functioning of marine food webs, biogeochemical cycles, and the biological pump. Silicic acid is delivered to the ocean through six pathways as illustrated in the diagram above, which all ultimately derive from the weathering of the Earth's crust. [12] [1]
Silica is an important nutrient utilized by plants, trees, and grasses in the terrestrial biosphere. Silicate is transported by rivers and can be deposited in soils in the form of various siliceous polymorphs. Plants can readily uptake silicate in the form of H4SiO4 for the formation of phytoliths. Phytoliths are tiny rigid structures found within plant cells that aid in the structural integrity of the plant. [2] Phytoliths also serve to protect the plants from consumption by herbivores who are unable to consume and digest silica-rich plants efficiently. [2] Silica release from phytolith degradation or dissolution is estimated to occur at a rate double that of global silicate mineral weathering. [3] Considering biogeochemical cycling within ecosystems, the import and export of silica to and from terrestrial ecosystems is small.
Silicate minerals are abundant in rock formations all over the planet, comprising approximately 90% of the Earth's crust. [4] The primary source of silicate to the terrestrial biosphere is weathering. The process and rate of weathering is variable, depending on rainfall, runoff, vegetation, lithology, and topography.
Given sufficient time, rainwater can dissolve even a highly resistant silicate-based mineral such as quartz. [13] Water breaks the bonds between atoms in the crystal: [14]
The overall reaction for the dissolution of quartz results in silicic acid
Another example of a silicate-based mineral is enstatite (MgSiO3). Rainwater weathers this to silicic acid as follows: [15]
In recent years, the effect of reverse weathering on biogenic silica has been of interest in quantifying the silica cycle. During weathering, dissolved silica is delivered to oceans through glacial runoff and riverine inputs. [16] This dissolved silica is taken up by a multitude of marine organisms, such as diatoms, and is used to create protective shells. [16] When these organisms die, they sink through the water column. [16] Without active production of biogenic SiO2, the mineral begins diagenesis. [16] Conversion of this dissolved silica into authigenic silicate clays through the process of reverse weathering constitutes a removal of 20-25% of silicon input. [17]
Reverse weathering is often found in river deltas as these systems have high sediment accumulation rates and are observed to undergo rapid diagenesis. [18] The formation of silicate clays removes reactive silica from the pore waters of sediment, increasing the concentration of silica found in the rocks that form in these locations. [18]
Silicate weathering also appears to be a dominant process in deeper methanogenic sediments, whereas reverse weathering is more common in surface sediments, but still occurs at a lower rate. [19]
The major sink of the terrestrial silica cycle is export to the ocean by rivers. Silica that is stored in plant matter or dissolved can be exported to the ocean by rivers. The rate of this transport is approximately 6 Tmol Si yr−1. [20] [3] This is the major sink of the terrestrial silica cycle, as well as the largest source of the marine silica cycle. [20] A minor sink for terrestrial silica is silicate that is deposited in terrestrial sediments and eventually exported to the Earth's crust.
As of 2021, the best estimate of the total riverine input of silicic acid is 6.2 (±1.8) Tmol Si yr−1. [12] This is based on data representing 60% of the world river discharge and a discharge-weighted average silicic acid riverine concentration of 158 μM−Si. [22] [12] However, silicic acid is not the only way silicon can be transferred from terrestrial to riverine systems, since particulate silicon can also be mobilised in crystallised or amorphous forms. [22] According to Saccone and others in 2007, [23] the term "amorphous silica" includes biogenic silica (from phytoliths, freshwater diatoms, sponge spicules), altered biogenic silica, and pedogenic silicates, the three of which can have similar high solubilities and reactivities. Delivery of amorphous silica to the fluvial system has been reviewed by Frings and others in 2016, [24] who suggested a value of 1.9(±1.0) Tmol Si yr−1. Therefore, the total riverine input is 8.1(±2.0) Tmol Si yr−1. [1]
No progress has been made regarding aeolian dust deposition into the ocean [25] and subsequent release of silicic acid via dust dissolution in seawater since 2013, when Tréguer and De La Rocha summed the flux of particulate dissolvable silica and wet deposition of silicic acid through precipitation. [12] Thus, the best estimate for the aeolian flux of silicic acid, FA, remains 0.5(±0.5) Tmol Si yr−1. [1]
A 2019 study has proposed that, in the surf zone of beaches, wave action disturbed abiotic sand grains and dissolved them over time. [26] To test this, the researchers placed sand samples in closed containers with different kinds of water and rotated the containers to simulate wave action. They discovered that the higher the rock/water ratio within the container, and the faster the container spun, the more silica dissolved into solution. After analyzing and upscaling their results, they estimated that anywhere from 3.2 ± 1.0 – 5.0 ± 2.0 Tmol Si yr−1 of lithogenic DSi could enter the ocean from sandy beaches, a massive increase from a previous estimate of 0.3 Tmol Si yr−1. [27] If confirmed, this represents a significant input of dissolved LSi that was previously ignored. [26] [1]
Siliceous organisms in the ocean, such as diatoms and radiolaria, are the primary sink of dissolved silicic acid into opal silica. [32] Only 3% of the Si molecules dissolved in the ocean are exported and permanently deposited in marine sediments on the seafloor each year, demonstrating that silicon recycling is a dominant process in the oceans. [3] This rapid recycling is dependent on the dissolution of silica in organic matter in the water column, followed by biological uptake in the photic zone. The estimated residence time of the silica biological reservoir is about 400 years. [3] Opal silica is predominately undersaturated in the world's oceans. This undersaturation promotes rapid dissolution as a result of constant recycling and long residence times. The estimated turnover time of Si is 1.5x104 years. [20] The total net inputs and outputs of silica in the ocean are 9.4 ± 4.7 Tmol Si yr−1 and 9.9 ± 7.3 Tmol Si yr−1, respectively. [20]
Biogenic silica production in the photic zone is estimated to be 240 ± 40 Tmol Si year −1. [20] Dissolution in the surface removes roughly 135 Tmol Si year−1, while the remaining Si is exported to the deep ocean within sinking particles. [3] In the deep ocean, another 26.2 Tmol Si Year−1 is dissolved before being deposited to the sediments as opal rain. [3] Over 90% of the silica here is dissolved, recycled and eventually upwelled for use again in the euphotic zone. [3]
The major sources of marine silica include rivers, groundwater flux, seafloor weathering inputs, hydrothermal vents, and atmospheric deposition (aeolian flux). [15] Rivers are by far the largest source of silica to the marine environment, accounting for up to 90% of all the silica delivered to the ocean. [15] [20] [35] A source of silica to the marine biological silica cycle is silica that has been recycled by upwelling from the deep ocean and seafloor.
The diagram on low-temperature processes shows how these can control the dissolution of (either amorphous or crystallized) siliceous minerals in seawater in and to the coastal zone and in the deep ocean, feeding submarine groundwater (FGW) and dissolved silicon in seawater and sediments (FW). [1] These processes correspond to both low and medium energy flux dissipated per volume of a given siliceous particle in the coastal zone, in the continental margins, and in the abysses and to high-energy flux dissipated in the surf zone. [1]
Rapid dissolution in the surface removes roughly 135 Tmol opal Si year−1, converting it back to soluble silicic acid that can be used again for biomineralization. [20] The remaining opal silica is exported to the deep ocean in sinking particles. [20] In the deep ocean, another 26.2 Tmol Si Year−1 is dissolved before being deposited to the sediments as opal silica. [20] At the sediment water interface, over 90% of the silica is recycled and upwelled for use again in the photic zone. [20] Biogenic silica production in the photic zone is estimated to be 240 ± 40 Tmol si year −1. [36] The residence time on a biological timescale is estimated to be about 400 years, with each molecule of silica recycled 25 times before sediment burial. [20]
Deep seafloor deposition is the largest long-term sink of the marine silica cycle (6.3 ± 3.6 Tmol Si year−1), and is roughly balanced by the sources of silica to the ocean. [15] The silica deposited in the deep ocean is primarily in the form of siliceous ooze. When opal silica accumulates faster than it dissolves, it is buried and can provide a diagenetic environment for marine chert formation. [37] The processes leading to chert formation have been observed in the Southern Ocean, where siliceous ooze accumulation is the fastest. [37] Chert formation however can take tens of millions of years. [38] Skeleton fragments from siliceous organisms are subject to recrystallization and cementation. [37] Chert is the main fate of buried siliceous ooze and permanently removes silica from the oceanic silica cycle.
The siliceous ooze is eventually subducted under the crust and metamorphosed in the upper mantle. [39] Under the mantle, silicate minerals are formed in oozes and eventually uplifted to the surface. At the surface, silica can enter the cycle again through weathering. [39] This process can take tens of millions of years. [39] The only other major sink of silica in the ocean is burial along continental margins (3.6 ± 3.7 Tmol Si year −1), primarily in the form of siliceous sponges. [15] Due to the high degrees of uncertainty in source and sink estimations, it's difficult to conclude if the marine silica cycle is in equilibrium. The residence time of silica in the oceans is estimated to be about 10,000 years. [15] Silica can also be removed from the cycle by becoming chert and being permanently buried.
The rise in agriculture of the past 400 years has increased the exposure rocks and soils, which has resulted in increased rates of silicate weathering. In turn, the leaching of amorphous silica stocks from soils has also increased, delivering higher concentrations of dissolved silica in rivers. [15] Conversely, increased damming has led to a reduction in silica supply to the ocean due to uptake by freshwater diatoms behind dams. The dominance of non-siliceous phytoplankton due to anthropogenic nitrogen and phosphorus loading and enhanced silica dissolution in warmer waters has the potential to limit silicon ocean sediment export in the future. [15]
In 2019 a group of scientists suggested acidification is reducing diatom silica production in the Southern Ocean. [40] [41]
The silica cycle plays an important role in long term global climate regulation. The global silica cycle also has large effects on the global carbon cycle through the carbonate-silicate cycle. [43] The process of silicate mineral weathering transfers atmospheric CO2 to the hydrologic cycle through the chemical reaction displayed above. [4] Over geologic timescales, the rates of weathering change due to tectonic activity. During a time of high uplift rate, silicate weathering increases which results in high CO2 uptake rates, offsetting increased volcanic CO2 emissions associated with the geologic activity. This balance of weathering and volcanoes is part of what controls the greenhouse effect and ocean pH over geologic time scales.
Biogenic silica accumulation on the sea floor contains lot of information about where in the ocean export production has occurred on time scales ranging from hundreds to millions of years. For this reason, opal deposition records provide valuable information regarding large-scale oceanographic reorganizations in the geological past, as well as paleoproductivity. The mean oceanic residence time for silicate is approximately 10,000–15,000 yr. This relative short residence time, makes oceanic silicate concentrations and fluxes sensitive to glacial/interglacial perturbations, and thus an excellent proxy for evaluating climate changes. [44] [45]
Isotope ratios of oxygen (O18:O16) and silicon (Si30:Si28) are analysed from biogenic silica preserved in lake and marine sediments to derive records of past climate change and nutrient cycling (De La Rocha, 2006; Leng and Barker, 2006). This is a particularly valuable approach considering the role of diatoms in global carbon cycling. In addition, isotope analyses from BSi are useful for tracing past climate changes in regions such as in the Southern Ocean, where few biogenic carbonates are preserved.
The silicon isotope compositions in fossil sponge spicules (δ30Si) are being increasingly often used to estimate the level of silicic acid in marine settings throughout the geological history, which enables the reconstruction of past silica cycles. [46]
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic table: carbon is above it; and germanium, tin, lead, and flerovium are below it. It is relatively unreactive. Silicon is a significant element that is essential for several physiological and metabolic processes in plants. Silicon is widely regarded as the predominant semiconductor material due to its versatile applications in various electrical devices such as transistors, solar cells, integrated circuits, and others. These may be due to its significant band gap, expansive optical transmission range, extensive absorption spectrum, surface roughening, and effective anti-reflection coating.
A diatom is any member of a large group comprising several genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of the Earth's biomass: they generate about 20 to 50 percent of the oxygen produced on the planet each year, take in over 6.7 billion tonnes of silicon each year from the waters in which they live, and constitute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half-mile deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by transatlantic winds from the African Sahara, much of it from the Bodélé Depression, which was once made up of a system of fresh-water lakes.
Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.
The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments. In other words, it is a biologically mediated process which results in the sequestering of carbon in the deep ocean away from the atmosphere and the land. The biological pump is the biological component of the "marine carbon pump" which contains both a physical and biological component. It is the part of the broader oceanic carbon cycle responsible for the cycling of organic matter formed mainly by phytoplankton during photosynthesis (soft-tissue pump), as well as the cycling of calcium carbonate (CaCO3) formed into shells by certain organisms such as plankton and mollusks (carbonate pump).
High-nutrient, low-chlorophyll (HNLC) regions are regions of the ocean where the abundance of phytoplankton is low and fairly constant despite the availability of macronutrients. Phytoplankton rely on a suite of nutrients for cellular function. Macronutrients are generally available in higher quantities in surface ocean waters, and are the typical components of common garden fertilizers. Micronutrients are generally available in lower quantities and include trace metals. Macronutrients are typically available in millimolar concentrations, while micronutrients are generally available in micro- to nanomolar concentrations. In general, nitrogen tends to be a limiting ocean nutrient, but in HNLC regions it is never significantly depleted. Instead, these regions tend to be limited by low concentrations of metabolizable iron. Iron is a critical phytoplankton micronutrient necessary for enzyme catalysis and electron transport.
Orthosilicic acid is an inorganic compound with the formula Si(OH)4. Although rarely observed, it is the key compound of silica and silicates and the precursor to other silicic acids [H2xSiOx+2]n. Silicic acids play important roles in biomineralization and technology. It is the parent acid of the orthosilicate anion SiO4−
4.
Lithogenic silica (LSi) is silica (SiO2) derived from terrigenous rock (Igneous, metamorphic, and sedimentary), lithogenic sediments composed of the detritus of pre-existing rock, volcanic ejecta, extraterrestrial material, and minerals such silicate. Silica is the most abundant compound in the Earth's crust (59%) and the main component of almost every rock (>95%).
Biogenic silica (bSi), also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. For example, microscopic particles of silica called phytoliths can be found in grasses and other plants.
Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles either have their origins in soil and rocks and have been transported from the land to the sea, mainly by rivers but also by dust carried by wind and by the flow of glaciers into the sea, or they are biogenic deposits from marine organisms or from chemical precipitation in seawater, as well as from underwater volcanoes and meteorite debris.
Siliceous ooze is a type of biogenic pelagic sediment located on the deep ocean floor. Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor. Oozes are defined as sediments which contain at least 30% skeletal remains of pelagic microorganisms. Siliceous oozes are largely composed of the silica based skeletons of microscopic marine organisms such as diatoms and radiolarians. Other components of siliceous oozes near continental margins may include terrestrially derived silica particles and sponge spicules. Siliceous oozes are composed of skeletons made from opal silica SiO2·nH2O, as opposed to calcareous oozes, which are made from skeletons of calcium carbonate (CaCO3·nH2O) organisms (i.e. coccolithophores). Silica (Si) is a bioessential element and is efficiently recycled in the marine environment through the silica cycle. Distance from land masses, water depth and ocean fertility are all factors that affect the opal silica content in seawater and the presence of siliceous oozes.
The carbonate–silicate geochemical cycle, also known as the inorganic carbon cycle, describes the long-term transformation of silicate rocks to carbonate rocks by weathering and sedimentation, and the transformation of carbonate rocks back into silicate rocks by metamorphism and volcanism. Carbon dioxide is removed from the atmosphere during burial of weathered minerals and returned to the atmosphere through volcanism. On million-year time scales, the carbonate-silicate cycle is a key factor in controlling Earth's climate because it regulates carbon dioxide levels and therefore global temperature.
Fragilariopsis kerguelensis, is a pennate diatom native to the Southern Ocean. It has been characterized as "the most abundant diatom in the Antarctic Seas".
Reverse weathering generally refers to a process of clay neoformation consuming cations and alkalinity in a way unrelated to the weathering of silicates. More specifically reverse weathering refers to the formation of authigenic clay minerals from the reaction of 1) biogenic silica with aqueous cations or cation-bearing oxides or 2) cation poor precursor clays with dissolved cations or cation-bearing oxides.
Marine biogeochemical cycles are biogeochemical cycles that occur within marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. These biogeochemical cycles are the pathways chemical substances and elements move through within the marine environment. In addition, substances and elements can be imported into or exported from the marine environment. These imports and exports can occur as exchanges with the atmosphere above, the ocean floor below, or as runoff from the land.
Aluminum is the third most abundant element in the lithosphere at 82,000 ppm. It occurs in low levels, 0.9 ppm, in humans. Aluminum is known to be an ecotoxicant and expected to be a health risk to people. Global primary production (GPP) of aluminum was about 52 million tons in 2013 and remains one of the world's most important metals. It is used for infrastructure, vehicles, aviation, energy and more due to its lightweight, ductility, and cheap cost. Aluminum is harvested from gibbsite, boehmite, and diaspore which make up bauxite. The aluminum cycle is the biogeochemical cycle by which aluminum is moved through the environment by natural and anthropogenic processes. The biogeochemical cycle of aluminum is integral with silicon and phosphorus. For example, phosphates store aluminum that has been sedimented and aluminum is found in diatoms. Aluminum has been found to prevent growth in organisms by making phosphates less available. The humans/lithosphere ratio (B/L) is very low at 0.000011. This level shows that aluminum is more essential in the lithospheric cycle than in the biotic cycle.
Many protists have protective shells or tests, usually made from silica (glass) or calcium carbonate (chalk). Protists are a diverse group of eukaryote organisms that are not plants, animals, or fungi. They are typically microscopic unicellular organisms that live in water or moist environments.
Particulate inorganic carbon (PIC) can be contrasted with dissolved inorganic carbon (DIC), the other form of inorganic carbon found in the ocean. These distinctions are important in chemical oceanography. Particulate inorganic carbon is sometimes called suspended inorganic carbon. In operational terms, it is defined as the inorganic carbon in particulate form that is too large to pass through the filter used to separate dissolved inorganic carbon.
The Great Calcite Belt (GCB) refers to a region of the ocean where there are high concentrations of calcite, a mineral form of calcium carbonate. The belt extends over a large area of the Southern Ocean surrounding Antarctica. The calcite in the Great Calcite Belt is formed by tiny marine organisms called coccolithophores, which build their shells out of calcium carbonate. When these organisms die, their shells sink to the bottom of the ocean, and over time, they accumulate to form a thick layer of calcite sediment.
In geology, silicification is a petrification process in which silica-rich fluids seep into the voids of Earth materials, e.g., rocks, wood, bones, shells, and replace the original materials with silica (SiO2). Silica is a naturally existing and abundant compound found in organic and inorganic materials, including Earth's crust and mantle. There are a variety of silicification mechanisms. In silicification of wood, silica permeates into and occupies cracks and voids in wood such as vessels and cell walls. The original organic matter is retained throughout the process and will gradually decay through time. In the silicification of carbonates, silica replaces carbonates by the same volume. Replacement is accomplished through the dissolution of original rock minerals and the precipitation of silica. This leads to a removal of original materials out of the system. Depending on the structures and composition of the original rock, silica might replace only specific mineral components of the rock. Silicic acid (H4SiO4) in the silica-enriched fluids forms lenticular, nodular, fibrous, or aggregated quartz, opal, or chalcedony that grows within the rock. Silicification happens when rocks or organic materials are in contact with silica-rich surface water, buried under sediments and susceptible to groundwater flow, or buried under volcanic ashes. Silicification is often associated with hydrothermal processes. Temperature for silicification ranges in various conditions: in burial or surface water conditions, temperature for silicification can be around 25°−50°; whereas temperatures for siliceous fluid inclusions can be up to 150°−190°. Silicification could occur during a syn-depositional or a post-depositional stage, commonly along layers marking changes in sedimentation such as unconformities or bedding planes.
Silicon isotope biogeochemistry is the study of environmental processes using the relative abundance of Si isotopes. As the relative abundance of Si stable isotopes varies among different natural materials, the differences in abundance can be used to trace the source of Si, and to study biological, geological, and chemical processes. The study of stable isotope biogeochemistry of Si aims to quantify the different Si fluxes in the global biogeochemical silicon cycle, to understand the role of biogenic silica within the global Si cycle, and to investigate the applications and limitations of the sedimentary Si record as an environmental and palaeoceanographic proxy.
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