The largest reservoir of carbon in the Earth system is the lithosphere, where carbon is stored in sedimentary rocks and kerogen on timescales of millions of years. The ocean is the second-largest reservoir, storing carbon as dissolved inorganic carbon and in marine sediments. Soils store large amounts of organic carbon, some of which is stabilised and can remain in the soil for centuries to millennia. Fossil fuels represent a geologic carbon reservoir formed over millions of years from buried organic matter.

The bulk of Earth’s carbon is stored in the lithosphere in the form of marine and terrestrial sediments and rocks, which exchange carbon with the atmosphere and ocean only on geological time scales (thousands to millions of years). The ocean is the largest active reservoir of carbon on the planet, containing about 38,000 GtC, most of it in the deep ocean. Soils are also a major carbon reservoir, storing on the order of 1500–2400 GtC in organic and inorganic forms, some of which can persist for centuries to millennia.

Carbon can be "stored" in natural systems over long time scales, called carbon sinks. Underground oil and gas reserves, carbon-rich ecosystems, and deep oceans are some of the largest carbon sinks. The page also notes that carbon dissolves into porous rocks and warm ocean water.

NASA states: "Most of Earth's carbon—about 65,500 billion metric tons—is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels." It explains that carbon moves between these reservoirs through processes like photosynthesis, respiration, weathering, and sedimentation, but large fractions remain stored in them for very long periods.

Most carbon is stored in reservoirs, or sinks, such as rocks and sediments, while the rest is stored in the atmosphere, oceans, and living organisms. It also says that organic carbon is found in fossil fuels and small deposits in rocks, and that more than 99 per cent of the carbon in the carbon cycle is found in the Earth's crust.

The IPCC explains that over millions of years, "CO2 is removed from the atmosphere through weathering by silicate rocks and through burial in marine sediments of carbon fixed by marine plants." It further notes that burning fossil fuels "returns carbon captured by plants in Earth’s geological history to the atmosphere," indicating that fossil fuels are a long‑term geological carbon reservoir formed from buried organic matter in sediments and rocks.

Biological carbon sequestration is the natural ability of life and ecosystems to store carbon. Forests, peat marshes, and coastal wetlands are particularly good at storing carbon. The page also explains that geologic carbon sequestration is the injection and storage of carbon dioxide into deep subsurface rocks.

Most of Earth’s carbon is stored in rocks and sediments. The rest is in the ocean, atmosphere, and in living organisms. Over long timescales, carbon moves between rocks, ocean, atmosphere, and living things through processes such as weathering, sedimentation, and burial, which can lock carbon away in sediments and rocks for millions of years.

Most of Earth’s carbon is stored in the deep ocean, rocks, and sediments, where it can stay locked away for thousands to millions of years. Much smaller amounts reside in the atmosphere, plants and soils, and the surface ocean, which exchange carbon with the atmosphere on shorter time scales. Fossil fuels such as coal, oil, and natural gas are ancient carbon-rich deposits formed from buried organic matter and represent another long-term carbon reservoir until they are extracted and burned.

CRS describes the main carbon reservoirs: "The atmosphere, oceans, vegetation, and soils on the land surface all store carbon." It adds: "Geological reservoirs also store carbon in a variety of forms, including fossil fuels, such as oil, gas, and coal." It notes that dissolved inorganic carbon in the ocean is the largest of these exogenic reservoirs, followed by fossil carbon in geological reservoirs, and then by the total amount of carbon contained in soils.

This review states: "The ocean contains about 50 times as much carbon as the present-day atmosphere, which makes it an important reservoir for exchange with atmospheric CO2." It further notes that "On the Earth surface, the sediments are by far the largest carbon reservoir containing carbonate sediments and rocks, mostly calcite and dolomite, and organic matter," indicating that sediments and sedimentary rocks hold very large, long‑term carbon stores.

The chapter describes long‑term geological processes: it shows CO2 being buried in deep‑sea sediments, "which are the precursor of carbonate rocks." It also mentions "burial of organic matter as fossil organic carbon (including fossil fuels), and outgassing of CO2 through tectonic processes (vulcanism)," indicating that sediments, rocks, and fossil fuels are long‑term reservoirs where carbon is stored over geological timescales.

The global carbon cycle is a whole system of processes that transfers carbon in various forms through the Earth’s different parts. First, let’s consider the main reservoirs of carbon. These can be seen in the diagram below, where each box represents a different reservoir... Soil and Deep Oceans are larger, reflecting their significant carbon storage; Sedimentary Rocks is the largest box, emphasizing its massive carbon reservoir. Among the other reservoirs, you can see that there is a huge range in the sizes. The ocean biota contain a very small amount of carbon relatively speaking, while sedimentary rocks contain a vast quantity (in the form of calcite — CaCO3 — that forms limestones, and coal, petroleum, etc.).

The document defines the global carbon cycle as reservoirs linked by exchange fluxes and notes that reservoir turnover times range "from a few years for the atmosphere to decades to millennia for the major carbon reservoirs of the land vegetation and soil and the various domains in the ocean." It describes a "slow domain" consisting of "the huge carbon stores in rocks and sediments" with reservoir turnover times of 10,000 years or longer, showing that rocks and sediments act as long‑term carbon stores.

On a global level, the total carbon cycle is more complex, and involves carbon stored in fossil fuels, soils, oceans, and rocks. We can organize all the carbon on earth into five main pools, listed in order of the size of the pool: 1. Lithosphere (Earth's crust). This consists of fossil fuels and sedimentary rock deposits, such as limestone, dolomite, and chalk. This is far and away the largest carbon pool on earth.

Carbon sequestration describes the processes that capture carbon long term in the soil, rocks or aquatic systems. Atmosphere, vegetation, soils, and the ocean are primary reservoirs of carbon. Plants and soil sequester carbon from the atmosphere. Ocean sediment can sequester carbon as well.

Biologic carbon sequestration involves storing CO2 in places where it is stored naturally as part of the carbon cycle. Some carbon is stored in plants—especially woody plants and grasslands—as a result of the biological process of photosynthesis. Plants also move carbon into soil, producing something called soil organic carbon. Soil contains massive amounts of organic carbon. Some of this carbon is eventually broken down by natural processes and returns to the atmosphere as CO2, but some becomes stable and can remain locked in the soil for long periods of time. In the oceans, CO2 is stored as dissolved gas in the water and carbonate sediments on the seafloor.

Carbon capture and storage involves capturing carbon dioxide at emission sources, such as power stations, then transporting and storing it underground. In suitable geological formations, carbon dioxide can be stored for thousands to millions of years. These formations include deep saline aquifers, depleted oil and gas fields and unmineable coal seams, where the CO2 is trapped within the pore spaces of rocks or dissolved and eventually mineralised.

The teaching material explains that "Before farming and fossil fuels, the major pools of carbon were largely stable. The land lost a small amount of carbon to the sea each year; some of this remained in the sea, and some was deposited in sediments on the sea bottom." It also notes that "Most of the carbon is recycled by mixing, but a tiny amount settles to the ocean bottom and accumulates as sediment," and separately that there are large quantities of carbon in fossil fuel reserves such as coal, oil, and natural gas.

Carbon may be stored for long periods of time in the atmosphere, bodies of liquid water—mostly oceans—ocean sediment, soil, rocks, fossil fuels, and Earth's interior. The article distinguishes between rapid biological cycling and slow geological cycling over long timescales.

The largest long-term carbon reservoir is sedimentary rock, not the atmosphere. Photosynthesis removes CO₂ from the atmosphere, while respiration returns it. The largest long-term carbon reservoir is sedimentary rock, not the atmosphere.

Sedimentation and burial move carbon into long-term storage in sediments, fossil fuels, and rock. Combustion transfers carbon rapidly from fossil fuels to the atmosphere. The slow part of the carbon cycle involves long‑term storage of carbon in rocks, deep ocean sediments, and fossil fuels, which can last millions of years.

Carbon is stored for long periods in carbon reservoirs, which include the atmosphere, bodies of liquid water, ocean sediment, soil, rocks, and fossil fuels. The page uses this reservoir concept to explain long-term carbon storage.

Carbon is stored in reservoirs like forests, soils, oceans, and fossil fuels. It flows through processes like photosynthesis, respiration, decomposition, and combustion, cycling between the atmosphere, biosphere, hydrosphere, and lithosphere.

Coal, oil, and natural gas are fossil fuels formed from ancient organic matter that was buried and subjected to heat and pressure over millions of years. Because the carbon in fossil fuels has been isolated from the active atmosphere–biosphere carbon cycle on geologic timescales, these fuels constitute a long-term carbon reservoir until they are extracted and burned.