Persistent organic matter and carbon cycle

　Considering the main location of non-living organic matter on Earth, in terrestrial ecosystems soil organic matter is mentioned. You probably know that the dead bodies and excretions of plants, microorganisms, and animals make up soil organic matter. On the other hand, one of the locations of non-living organisms in the ocean is in marine sediments, and the other is in seawater. Abiotic organic matter in seawater is mostly present in the dissolved fraction (in the open ocean, more than 90% of abiotic matter in seawater is dissolved organic matter). Although it is not well known, the total amount of dissolved organic matter in the ocean is more than 200 times the amount of marine biomass, and most of the organic matter in the ocean is abiotic organic matter (figure below).

Major forms, locations and amounts of carbon present on the Earth's surface

　In addition, the total amount of carbon in dissolved organic matter in the ocean is almost the same as the total amount of carbon dioxide in the atmosphere, and it is attracting attention as an important piece in the carbon cycle of the earth's surface. Taking an extreme example, if 10% of the dissolved organic matter in the ocean were decomposed and all the resulting inorganic carbon was released into the atmosphere, the concentration of carbon dioxide in the atmosphere would increase by about 10%. However, most of the dissolved organic matter in the ocean (more than 95%) is a persistent component with a lifetime of several thousand years or more, so it is not easily decomposed.It is extremely important in considering the carbon cycle to know whether or not the concentration of persistent components fluctuates due to their abundance. If it is increasing, it is possible that part of the anthropogenic carbon dioxide is transformed into persistent components and sequestered in the ocean.
Even though there is a large amount of dissolved organic matter in the ocean, the concentration in seawater is low (1L of seawater contains about 35g of salt, but only about 1mg of dissolved organic matter). Its analysis is difficult. Determining the proportion of persistent components in dissolved organic matter is even more difficult.Furthermore, we do not know at all whether the persistent components are increasing, decreasing, or whether the concentration remains constant from the past to the future.Furthermore, there are many mysteries regarding the origin of the persistent components and the mechanism of their formation, maintenance, and decomposition.

　In the first place, how did you know that persistent organic matter exists in the sea? The most direct evidence comes from the radiocarbon isotope (14C) measurement of carbon dioxide obtained by oxidative decomposition of dissolved organic matter in 5 liters of seawater. By the way, the 14C dating method estimates how much time has passed since organisms took in 14C from the atmosphere from the rate of radioactive decay. With advances in technology, it may be possible to measure 14C dissolved organic matter in even 1 liter of seawater. The difference between 1 liter and 5 liters is big, and the trouble of ocean observation will be greatly reduced. We expect to see progress in this area in the near future.

（Faculty of Environmental Earth Science：Yohei Yamashita）