The Biological Carbon Pump
The biological carbon pump (BCP): the ocean’s role in the global carbon cycle and climate regulation
The ocean is a central cog in the global carbon cycle and one of the largest active carbon reservoirs; storing ~50 times the amount of carbon held in the atmosphere and driving 30% of the annual net removal of anthropogenic carbon dioxide (CO₂) (Sarmiento and Gruber, 2006; DeVries, 2022; Iversen, 2023). This net uptake of atmospheric CO₂ by the global ocean actively lowers atmospheric CO₂ concentrations and plays a major role in regulating the Earth’s climate, the extent to which has been hypothesized to be associated with glacial and interglacial climate variability (Broecker, 1982; Sigman and Boyle, 2000). The magnitude of this inorganic carbon storage gradient from the atmosphere to the deep ocean is governed by two mechanisms; the solubility and the biological carbon pumps (BCP) (Volk and Hoffert, 1985; McKinley et al., 2016). The former, driven by temperature, underpins the bulk of oceanic CO₂ uptake at the atmosphere-ocean surface interface while the latter, the BCP, accounts for ~90% of the contemporary storage gradient observed today (Sarmiento and Gruber, 2006). Believed to be the dominant component of the BCP, the gravitational sinking of newly formed particulate organic matter (POM) by phytoplankton photosynthesis (the fixing of inorganic CO₂ into organic carbon by ubiquitous single-celled marine algae) connects surface and deep ocean processes on shorter time scales (days to weeks) than physical advection (decades to centuries) (Boyd and Trull, 2007; Buesseler et al., 2007) and drives long-term storage in the deep ocean on centennial timescales (Volk and Hoffert, 1985; Sigman and Boyle, 2000; Ducklow, Steinberg and Buesseler, 2001). This sinking POM is largely composed of detrital material (i.e., dead phytoplankton cells and fecal pellets produced from zooplankton – heterotrophic consumers), aggregates and living microbial cells (Turner, 2015; Boyd et al., 2019). While the vast majority of flux attenuation (70 – 85%) occurs within the mesopelagic (~200 – 1000 m) primarily due to microbe-mediated diagenetic transformations (Martin et al., 1987; Karl et al., 1988; Buesseler and Boyd, 2009; Guidi et al., 2015), the strength (i.e., the rate of organic carbon export) and efficiency (i.e., the extent to which newly supplied nutrients are consumed and thus support export production) of the BCP is largely governed by euphotic zone (sunlit upper layer of the ocean) food-web structures and system productivity, and thus is not uniform across the global ocean (Boyd et al., 2024; Stukel et al., 2024).
70 - 85% particle organic matter flux attenuation
Zooplankton consumption/ ‘sloppy feeding’
Microbial diagenesis
Particle fragmentation
Drivers of the BCP: nutrient availability and food-web structure
Marine macro- and micronutrient landscapes govern productivity regimes, shaping subsequent plankton community dynamics and export fluxes (Dugdale and Goering, 1967; Moore et al., 2013; Stukel and Barbeau, 2020). Essential macronutrients including nitrate, ammonium, phosphate and at times silicic acid, as well as micronutrients including iron are required by and often (co-)limiting for all phytoplankton taxa (Dugdale and Goering, 1967; de Baar et al., 1995; Browning and Moore, 2023). The availability (or limitation) of these nutrients broadly defines important marine biogeochemical regions and exerts a strong control on phytoplankton community composition (Deutsch and Weber, 2012; Moore et al., 2013). Productive upwelling and mixing regions are generally observed to have cooler, more active (with regards to surface water mass overturning) euphotic zones that are nutrient replete, supporting rapid growth of large phytoplankton taxa (e.g., diatoms) and successive mesozooplankton communities (Stukel et al., 2011, 2013, 2023; Valencia et al., 2022; Browning and Moore, 2023). Conversely, oligotrophic regions are largely defined by warmer, stratified euphotic zones that are nitrate (and/or phosphate and iron) (co-)limited, supporting small phytoplankton taxa (e.g., cyanobacteria and picoplankton) and microzooplankton communities (Stukel et al., 2011; Selph et al., 2022; Valencia et al., 2022; Browning and Moore, 2023). Although biogeochemical regions supporting food-webs of larger phytoplankton and mesozooplankton are generally observed to drive higher export fluxes (Krause et al., 2015; Stukel et al., 2017; Buesseler et al., 2008b), an increasing body of literature describes elevated export efficiencies in more oligotrophic regions (Puigcorbé et al., 2015; Mouw et al., 2016; Stukel and Barbeau, 2020) .
Nitrogen (N) is arguably the most influential growth-limiting element across the global ocean (Moore et al., 2013; Browning and Moore, 2023). While bioavailable in numerous forms including nitrate, nitrite and ammonium, with the former constituting the largest marine N pool, the marine N inventory is unique to other elemental cycles (e.g., carbon and phosphorus) since the sources/losses are largely controlled by biological processes. The biological fixation of dinitrogen (N₂) gas, mediated primarily by marine prokaryotes (“diazotrophs”), is the largest source of newly fixed N to the global ocean, while denitrification and anammox control the loss terms of the marine N budget (Gruber, 2004; Zehr and Kudela, 2011; Casciotti, 2016; Landolfi et al., 2018). Although the global rate and distribution of marine N₂ fixation remains uncertain and is co-limited by diazotrophic iron and phosphorus requirements (Moore et al., 2009; Monteiro et al., 2011; Dutkiewicz et al., 2012; Weber and Deutsch, 2014), this process is observed to predominantly occur in oligotrophic surface waters and has large ecological impacts on marine food-web structures and subsequent biogeochemical impacts on carbon export potential (Dugdale and Goering, 1967; Karl et al., 1997; Capone, 2001; Capone et al., 2005). Although export in oligotrophic regions is observed to be very low, areas of high euphotic zone N₂ fixation in the western tropical South Pacific (WTSP) have recently been associated with elevated export fluxes (Bonnet et al., 2023b, 2023a), suggesting a uniquely amplified BCP in an otherwise oligotrophic region. Conversely, regions with nitrate-replete euphotic zones including upwelling regions and the high-nutrient, low-chlorophyll Southern Ocean are observed to promote stronger BCPs with variable efficiencies due to micronutrient (largely iron) availability (Stukel and Ducklow, 2017; Stukel et al., 2017, 2023; Décima et al., 2023).
Current approaches constraining BCP variability
Considering the vast implications the BCP has for climate regulation and potential mitigative impacts on anthropogenic-driven climate change, the spatial and temporal patterns of the strength and efficiency of the BCP have been a key research focus for close to three decades. As a result, a variety of approaches have been developed to investigate and constrain the observed variability and magnitude of the BCP. Direct in situ field observations and sample collection often include the deployment of sediment traps to capture sinking POM throughout the mesopelagic and deep ocean, followed up with an array of chemical analyses of the particulate material (Volk and Hoffert, 1985; Martin et al., 1987; Buesseler et al., 2008b). Export estimates can be measured using radionuclide disequilibrium approaches (between 238U and 234Th, the latter adhering to POM; Henson et al., 2011; Owens et al., 2011; Stukel et al., 2019), and nutrient mass balance proxies and budgets, particularly relating to N, and largely focusing on productivity metrics within the euphotic zone (Fawcett et al., 2011; Yingling et al., 2022; Stirnimann et al., 2024). Recent research has focused on different pathways driving this export such as phytoplankton size and system productivity (Stukel et al., 2024), phytoplankton community contributions (e.g., diatoms; Krause et al., 2015), and zooplankton community contributions (e.g., salps; Décima et al., 2023). Additional advancements in genetic sequencing through metabarcoding, has aimed to constrain the microbial communities that drive remineralization processes, while further enumerating phytoplankton taxa contribution to sinking fluxes (Amacher et al., 2013; Guidi et al., 2016; Valencia et al., 2021, 2021). Together, the methods above describe various frameworks to assess the strength, efficiency and variability of the BCP.
These approaches along with satellite data often form the foundation for data-assimilative modeling approaches used to quantify the BCP on a global scale, including food-web models and biogeochemical inverse models (Siegel et al., 2014; DeVries and Weber, 2017; Nowicki et al., 2022). Despite extensive work, large discrepancies remain across measurements and models, describing poorly constrained BCP estimates with large uncertainties (varying from ~5 – 12 Pg carbon per year; Laws et al., 2000, 2011; Henson et al., 2011; Siegel et al., 2014; DeVries and Weber, 2017; Nowicki et al., 2022). This indicates continued critical knowledge gaps in BCP variability that are manifested in the general spatial and temporal inconsistency of in situ measurements as well as our insufficient understanding of the various BCP mechanisms, specifically 1) drivers of export flux in N-limited regions, accounting for much of the global ocean, and 2) the molecular-level composition (presence of individual molecules) of sinking POM and how this changes over trophic gradients with varying food-web structures. Greater insights into these mechanisms, such as the molecular-level composition of sinking POM (largely reflecting the major organic compounds of life such as carbohydrates, proteins, lipids, and amino acids), will allow for a greater understanding of BCP variability while having large implications for nutrient cycling and ecosystem functioning today and in a warming world.
Examples of in situ observations of the BCP magnitude
Niskin rosette collects water column samples
Sediment trap deployment
Satellite/remote observations
Global BCP estimates
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All information above taken from:
Forrer, H. J. (2024). Investigating Biological Carbon Pump Variability and Functionality: The Application of Old and New Conceptual Frameworks across Spatial and Temporal Scales. Doctoral dissertation, Florida State University, USA.
All graphics, unless credited, were created by Heather Forrer and use is encouraged. If you choose to, you can credit them as Heather Forrer.