Particulate inorganic carbon

Satellite imagery of particulate inorganic carbon (PIC) – NASA 2014^[1][2][3]

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.

Most PIC is calcium carbonate, CaCO₃, particularly in the form of calcite, but also in the form of aragonite. Calcium carbonate makes up the shells of many marine organisms. It also forms during whiting events and is excreted by marine fish during osmoregulation.

Overview

Carbon compounds can be distinguished as either organic or inorganic, and dissolved or particulate, depending on their composition. Organic carbon forms the backbone of key component of organic compounds such as – proteins, lipids, carbohydrates, and nucleic acids. Inorganic carbon is found primarily in simple compounds such as carbon dioxide, carbonic acid, bicarbonate, and carbonate (CO₂, H₂CO₃, HCO₃⁻, CO₃²⁻ respectively).

Marine carbon is further separated into particulate and dissolved phases. These pools are operationally defined by physical separation – dissolved carbon passes through a 0.2 μm filter, and particulate carbon does not.

There are two main types of inorganic carbon that are found in the oceans. Dissolved inorganic carbon (DIC) is made up of bicarbonate (HCO₃⁻), carbonate (CO₃²⁻) and carbon dioxide (including both dissolved CO₂ and carbonic acid H₂CO₃). DIC can be converted to particulate inorganic carbon (PIC) through precipitation of CaCO₃ (biologically or abiotically). DIC can also be converted to particulate organic carbon (POC) through photosynthesis and chemoautotrophy (i.e. primary production). DIC increases with depth as organic carbon particles sink and are respired. Free oxygen decreases as DIC increases because oxygen is consumed during aerobic respiration.

Particulate inorganic carbon (PIC) is the other form of inorganic carbon found in the ocean. Most PIC is the CaCO₃ that makes up shells of various marine organisms, but can also form in whiting events. Marine fish also excrete calcium carbonate during osmoregulation.^[4]

Some of the inorganic carbon species in the ocean, such as bicarbonate and carbonate, are major contributors to alkalinity, a natural ocean buffer that prevents drastic changes in acidity (or pH). The marine carbon cycle also affects the reaction and dissolution rates of some chemical compounds, regulates the amount of carbon dioxide in the atmosphere and Earth's temperature.^[5]

Carbon is separated into four distinct pools based on whether it is organic/inorganic and whether it is dissolved/particulate. The processes associated with each arrow describe the transformation associated with the transfer of carbon from one reservoir to another.

Natural particle size distributions in the ocean

Natural particle size distributions in the ocean broadly follow a power law over many orders of magnitude, from viruses and bacteria to fish and whales. Non-living material contained in the particle size distribution may also include marine snow, detritus, sediment and microplastic. The power law particle size distribution is the sum of log-normal distributions for each sub-population, four examples of which are illustrated in this figure. N is the number of particles of diameter, D; K is the number of 1 μm particles per volume; J is the slope of the power-law distribution.^[6]

Particulate inorganic carbon budget for Hudson Bay

Black arrows represent DIC produced by PIC dissolution. Grey lines represent terrestrial PIC.^[7]                      Units are Tg C y⁻¹

Calcium carbonate

Particulate inorganic carbon (PIC) usually takes the form of calcium carbonate (CaCO₃), and plays a key part in the ocean carbon cycle.^[8] This biologically fixed carbon is used as a protective coating for many planktonic species (coccolithophores, foraminifera) as well as larger marine organisms (mollusk shells). Calcium carbonate is also excreted at high rates during osmoregulation by fish, and can form in whiting events.^[9] While this form of carbon is not directly taken from the atmospheric budget, it is formed from dissolved forms of carbonate which are in equilibrium with CO₂ and then responsible for removing this carbon via sequestration.^[10]

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻

Ca²⁺ + 2HCO₃⁻ → CaCO₃ + CO₂ + H₂O

While this process does manage to fix a large amount of carbon, two units of alkalinity are sequestered for every unit of sequestered carbon.^[11][12] The formation and sinking of CaCO₃ therefore drives a surface to deep alkalinity gradient which serves to raise the pH of surface waters, shifting the speciation of dissolved carbon to raise the partial pressure of dissolved CO₂ in surface waters, which actually raises atmospheric levels. In addition, the burial of CaCO₃ in sediments serves to lower overall oceanic alkalinity, tending to raise pH and thereby atmospheric CO₂ levels if not counterbalanced by the new input of alkalinity from weathering.^[13] The portion of carbon that is permanently buried at the sea floor becomes part of the geologic record. Calcium carbonate often forms remarkable deposits that can then be raised onto land through tectonic motion as in the case with the White Cliffs of Dover in Southern England. These cliffs are made almost entirely of the plates of buried coccolithophores.^[14]

Carbonate pump

Sea surface dissolved inorganic carbon

The carbonate pump, sometimes called the carbonate counter pump, starts with marine organisms at the ocean's surface producing particulate inorganic carbon (PIC) in the form of calcium carbonate (calcite or aragonite, CaCO₃). This CaCO₃ is what forms hard body parts like shells.^[5] The formation of these shells increases atmospheric CO₂ due to the production of CaCO₃^[15] in the following reaction with simplified stoichiometry:^[16]