MESMO 3: Flexible phytoplankton stoichiometry and refractory DOM

Global Biogeochemical Cycles

Tanioka

Gilchrist

2022

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Most of the organic matter produced in the surface ocean by photoautotrophs is grazed by zooplankton and recycled quickly within the upper ocean by heterotrophic microbes. The small fraction of organic matter that escapes recycling accumulates in the ocean in the dissolved phase. In terms of carbon, the oceanic inventory of the dissolved organic carbon (DOC) has been previously estimated to be 662-700 PgC (Hansell et al., 2009;Hedges, 1992). Previous estimates of the global ocean DOC export production have employed ocean models and range from roughly 20% (Hansell et al., 2009) to 27% (Yamanaka & Tajika, 1997 of the global ocean carbon export production. The potential importance of marine DOC to global climate is indicated by the fact that the marine DOC inventory rivals the atmospheric carbon inventory. Also, carbon export is a key driver of atmospheric pCO 2 . Indeed, some paleoclimate events have been attributed to changes in the marine DOC cycle (Sexton et al., 2011).The marine DOC cycle has been studied for decades, and as yet, our understanding of DOC dynamics is incomplete. DOC is a complex mixture of thousands of compounds that are chemically and structurally diverse and thus poorly characterized (e.g., Hedges et al., 2000;Hertkorn et al., 2006). Radiocarbon ( 14 C) measurements of deep ocean DOC samples indicate ages as old as 6,000 years (Williams & Druffel, 1987). Thus, accumulated DOC has somehow survived global ocean overturning multiple times.There are also aspects of the global distribution of DOC that remain unexplained. Measurements indicate that DOC in the open ocean exists at very low concentrations, typically in the range of 35-80 μmol kg −1 (Figure 1). The concentration is relatively high and variable near the surface and lower and more uniform at depth. It is typically 65-80 μmol kg −1 in the stratified, warm waters of the tropical and subtropical regions. In the subpolar surface waters, where vertical mixing is strong, the DOC concentration is lower and approaches the deep ocean values of 35∼40 μmol kg −1 . The deep ocean DOC concentration was once thought to be uniform (Martin & Fitzwater, 1992), but as Hansell and Carlson (1998) first reported, there is a modest concentration gradient representing a decrease along the path of the meridional overturning circulation from the North Atlantic to the North Pacific by approximately 14 μmol kg −1 . The surface distribution as well as the deep ocean gradient are not well explained in terms of sources and sinks of DOC. In particular, the DOC sinks (e.g., photodegradation, hydrothermal vents) and their rates are poorly understood (Lang et al., 2006;Mopper et al., 1991). In one case, a DOC

supporting

confidence: 68%

Section: Mesmomentioning

confidence: 99%

Section: Prescribing Preformed Doc R In Mesmomentioning

confidence: 99%

Section: Mesmomentioning

confidence: 99%

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Sensitivity of Steady State, Deep Ocean Dissolved Organic Carbon to Surface Boundary Conditions

Global Biogeochemical Cycles

Tanioka

Gilchrist

2022

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“…b The calculation of the PFT abundance requires NPP in terms of P. NPP was unavailable as a model output for MESMO 2, so PFT % was estimated from POC export. References for independent constraints: (1) global NPP (Carr et al, 2006); (2) global POC export (DeVries and Weber, 2017); (3) global DOC export assumed to be 20% of total carbon export (Hansell et al, 2009;Roshan and DeVries, 2017); (4) global opal ; (5) global CaCO3 export (Berelson et al, 2007); (6) global N fixation and denitrification rates (Landolfi et al, 2018); (7) uptake C:N:P ratio is based on POM measurements (Martiny et al, 2013); (8) export C:N:P ratio is assumed to equal the subsurface remineralization ratio (Anderson and Sarmiento, 1994); (9) Deep O2 from 13 WOA13 below 100 m (Garcia et al, 2013…”

Section: Large-scale Patterns Of N2 Fixation and Denitrificationmentioning

confidence: 99%

Author response to two reviewers on gmd-2020-408

2021

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We describe the third version of Minnesota Earth System Model for Ocean biogeochemistry (MESMO 3), an earth system model of intermediate complexity, with a dynamical ocean, a dynamic-thermodynamic sea ice, and an energy moisture balanced atmosphere. A major feature of Version 3 is the flexible C:N:P ratio for the three phytoplankton functional types represented in the model. The flexible stoichiometry is based on the power law formulation with environmental dependence on phosphate, nitrate, temperature, and light. Other new features include nitrogen fixation, water column denitrification, oxygen and temperaturedependent organic matter remineralization, and CaCO3 production based on the concept of the residual nitrate potential growth. Also, we describe the semi-labile and refractory dissolved organic pools of C, N, P, and Fe that can be enabled in MEMSO 3 as an optional feature. The refractory dissolved organic matter can be degraded by photodegradation at the surface and hydrothermal vent degradation at the bottom. These improvements provide a basis for using MESMO 3 in further investigations of the global marine carbon cycle to changes in the environmental conditions of the past, present, and future.

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

Tanioka

Geophysical Research Letters

Lomas

2021

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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 (