Drawdown of Atmospheric pCO₂ Via Variable Particle Flux Stoichiometry in the Ocean Twilight Zone

Abstract

The biological carbon pump is one of the critical mechanisms whereby marine phytoplankton convert inorganic carbon into organic carbon via photosynthesis, and that carbon is subsequently transported into the ocean interior (Sigman & Boyle, 2000). The conventional thinking is that oceanic carbon storage due to the biological carbon pump is proportional to the total inventory of phosphate in the interior ocean that arrived through the biological "regenerated" pathway (Ito & Follows, 2005;. This hypothesis assumes a fixed C:P stoichiometry, where carbon transfer efficiency into the ocean interior is tightly coupled to the transfer of phosphate from surface to depth. Under this framework, an increase in POM transfer efficiency and associated drawdown on atmospheric pCO 2 will rapidly deplete surface nutrients, most notably in the North Atlantic and the Southern Ocean regions, where most deep water is formed (Sarmiento & Toggweiler, 1984).Another school of thought proposes that the biological carbon pump can be strengthened if the C:P ratio of sinking POM increases with depth (Broecker, 1982a(Broecker, , 1982bKnauer et al., 1979;Menzel & Ryther, 1964). This strengthening can happen if the remineralization length scale of particulate organic carbon (POC) is longer than that of particulate organic phosphorus (POP) such that C:P of the sinking POM exceeds C:P of upward inorganic flux (Christian et al., 1997). The seminal study by Menzel and Ryther (1964) demonstrated by sampling POM in the mesopelagic region of the Western North Atlantic that particulate phosphorus is remineralized more quickly than particulate carbon or nitrogen. Subsequent studies based on sediment traps and hydrographic studies have supported this theory (

Integrating Trait‐Based Stoichiometry in a Biogeochemical Inverse Model Reveals Links Between Phytoplankton Physiology and Global Carbon Export

“…Because all optimized models produced faster solubilization of POP relative to POC ( b P > b C ), the ratio of C:P in the particle export flux increases with depth. This is consistent with observational evidence that sinking organic particles become increasingly phosphorus depleted with depth (Tanioka et al., 2021). This mechanism allows for a more efficient biological pump, by allowing the same phosphorus atoms to fuel several cycles of production before being sequestered in the deep ocean.…”

Uncertain response of ocean biological carbon export in a changing world

“…New parameterisations should also be tested in a simplified 1-D framework or semi-empirical model initially, and potentially also in a computationally efficient 3-D framework, such as a transport matrix, e.g. 61,62 . Only if the additional processes are then shown to significantly alter modern-day export flux estimates should they then be implemented in a full climate model to make projections of the future magnitude and efficiency of the biological carbon pump.…”

Knowledge Gaps in Quantifying the Climate Change Response of Biological Storage of Carbon in the Ocean

“…The drawdown of excess carbon relative to nitrogen or phosphate (“carbon over‐consumption”) represents a potential negative feedback mechanism, as it results in additional drawdown of atmospheric CO 2 (Riebesell et al., 2007). Modeling work suggests that C:P or C:N variability in the mesopelagic can alter the strength of carbon sequestration by ∼ 20% (Tanioka et al., 2021; Tian et al., 2004).…”