Quantifying biological carbon pump pathways with a data-constrained mechanistic model ensemble approach

The ability to constrain the mechanisms that transport organic carbon into the deep ocean is complicated by the multiple physical, chemical, and ecological processes that intersect to create, transform, and transport particles in the ocean. In this manuscript we develop and parameterize a data-assimilative model of the multiple pathways of the biological carbon pump (NEMUROBCP). The mechanistic model is designed to represent sinking particle flux, active transport by vertically migrating zooplankton, and passive transport by subduction and vertical mixing, while also explicitly representing multiple biological and chemical properties measured directly in the field (including nutrients, phytoplankton and zooplankton taxa, carbon dioxide and oxygen, nitrogen isotopes, and 234Thorium). Using 30 different data types (including standing stock and rate measurements related to nutrients, phytoplankton, zooplankton, and non-living organic matter) from Lagrangian experiments conducted on 11 cruises from four ocean regions, we conduct an objective statistical parameterization of the model and generate one million different potential parameter sets that are used for ensemble model simulations. The model simulates in situ parameters that were assimilated (net primary production and gravitational particle flux) and parameters that were withheld (234Thorium and nitrogen isotopes) with reasonable accuracy. Model results show that gravitational flux of sinking particles and vertical mixing of organic matter from the surface ocean are more important biological pump pathways than active transport by vertically-migrating zooplankton. However, these processes are regionally variable, with sinking particles most important in oligotrophic areas of the Gulf of Mexico and California, sinking particles and vertical mixing roughly equivalent in productive regions of the CCE and the subtropical front in the Southern Ocean, and active transport an important contributor in the Eastern Tropical Pacific. We further find that mortality at depth is an important component of active transport when mesozooplankton biomasses are high, but that it is negligible in regions with low mesozooplankton biomass. Our results also highlight the high degree of uncertainty, particularly amongst mesozooplankton functional groups, that is derived from uncertainty in model parameters, with important implications from results that rely on non-ensemble model outputs. We also discuss the implications of our results for other data assimilation approaches.

210Po-210Pb Disequilibrium in the Western North Pacific Ocean: Particle Cycling and POC Export

Estimating the particulate organic carbon (POC) export flux from the upper ocean is fundamental for understanding the efficiency of the biological carbon pump driven by sinking particles in the oceans. The downward POC flux from the surface ocean based on 210Po-210Pb disequilibria in seawater samples from the western North Pacific Ocean (w-NPO) was measured in the early summer (May-June) of 2018. All the profiles showed a large 210Po deficiency relative to 210Pb in the euphotic zone (0–150 m), while this 210Po deficiency vanished below ∼500 m (with 210Po/210Pb ∼1 or > 1). A one-dimensional steady-state irreversible scavenging model was used to quantify the scavenging and removal fluxes of 210Po and 210Pb in the euphotic zone of the w-NPO. In the upper ocean (0–150 m), dissolved 210Po (D-Po) was scavenged into particles with a residence time of 0.6–5.5 year, and the 210Po export flux out of the euphotic zone was estimated as (0.33–3.49) × 104 dpm/m2/year, resulting in a wide range of particulate 210Po (P-Po) residence times (83–921 days). However, in the deep ocean (150–1,000 m), 210Po was transferred from the particulate phase to the dissolved phase. Using an integrated POC inventory and the P-Po residence times (Eppley model) in the w-NPO euphotic zone, the POC export fluxes (mmol C/m2/d) varied from 0.6 ± 0.2 to 8.8 ± 0.4. In comparison, applying the POC/210Po ratio of all (>0.45 μm) particles to 210Po export flux (Buesseler model), the obtained POC export fluxes (mmol C/m2/d) ranged from 0.7 ± 0.1 to 8.6 ± 0.8. Both Buesseler and Eppley methods showed enhanced POC export fluxes at stations near the continental shelf (i.e., Luzon Strait and the Oyashio-Kuroshio mixing region). The Eppley model-based 210Po-derived POC fluxes agreed well with the Buesseler model-based fluxes, indicating that both models are suitable for assessing POC fluxes in the w-NPO. The POC export efficiency was < 15%, suggesting a moderate biological carbon pump efficiency in the w-NPO. These low export efficiencies may be associated with the dominance of smaller particles and the processes of degradation and subsequent remineralization of these small particles in the euphotic zone of oligotrophic regions in the w-NPO.

Sea-ice derived meltwater stratification slows the biological carbon pump: results from continuous observations

The ocean moderates the world’s climate through absorption of heat and carbon, but how much carbon the ocean will continue to absorb remains unknown. The North Atlantic Ocean west (Baffin Bay/Labrador Sea) and east (Fram Strait/Greenland Sea) of Greenland features the most intense absorption of anthropogenic carbon globally; the biological carbon pump (BCP) contributes substantially. As Arctic sea-ice melts, the BCP changes, impacting global climate and other critical ocean attributes (e.g. biodiversity). Full understanding requires year-round observations across a range of ice conditions. Here we present such observations: autonomously collected Eulerian continuous 24-month time-series in Fram Strait. We show that, compared to ice-unaffected conditions, sea-ice derived meltwater stratification slows the BCP by 4 months, a shift from an export to a retention system, with measurable impacts on benthic communities. This has implications for ecosystem dynamics in the future warmer Arctic where the seasonal ice zone is expected to expand.

Multi-year robotic observations reveal the seasonality of downward carbon export pathways in the Southern Ocean

At high latitudes, the export of organic matter from the surface to the ocean interior, the biological carbon pump, has conventionally been attributed to the gravitational sinking of particulate organic carbon (POC). Conspicuous deficits in ocean carbon budgets have recently challenged this long-lived paradigm of a sole pathway. Multiple strands of evidence have demonstrated the importance of additional export pathways, including the particle injection pumps (PIPs). Recent model estimates revealed that PIPs have a comparable downward POC flux to the biological gravitational pump (BGP), but with potentially different seasonal signatures. To date, logistical constraints have prevented concomitant and extensive observations of these pumps, and little is known about the seasonality of their fluxes. Here, using year-round robotic observations and recent advances in optical signal analysis, we concurrently investigated the functioning of two PIPs - the mixed layer and eddy subduction pumps - and the BGP in Southern Ocean waters. By comparing three phytoplankton bloom cycles in contrasting environments, we show how physical forcing and phytoplankton phenology influence the magnitude and seasonality of these pumps, with implications for carbon sequestration efficiency.

The Seasonal Flux and Fate of Dissolved Organic Carbon Through Bacterioplankton in the Western North Atlantic

The oceans teem with heterotrophic bacterioplankton that play an appreciable role in the uptake of dissolved organic carbon (DOC) derived from phytoplankton net primary production (NPP). As such, bacterioplankton carbon demand (BCD), or gross heterotrophic production, represents a major carbon pathway that influences the seasonal accumulation of DOC in the surface ocean and, subsequently, the potential vertical or horizontal export of seasonally accumulated DOC. Here, we examine the contributions of bacterioplankton and DOM to ecological and biogeochemical carbon flow pathways, including those of the microbial loop and the biological carbon pump, in the Western North Atlantic Ocean (∼39–54°N along ∼40°W) over a composite annual phytoplankton bloom cycle. Combining field observations with data collected from corresponding DOC remineralization experiments, we estimate the efficiency at which bacterioplankton utilize DOC, demonstrate seasonality in the fraction of NPP that supports BCD, and provide evidence for shifts in the bioavailability and persistence of the seasonally accumulated DOC. Our results indicate that while the portion of DOC flux through bacterioplankton relative to NPP increased as seasons transitioned from high to low productivity, there was a fraction of the DOM production that accumulated and persisted. This persistent DOM is potentially an important pool of organic carbon available for export to the deep ocean via convective mixing, thus representing an important export term of the biological carbon pump.

Biological carbon pump affected by CO2 uptake in the Benguela Upwelling System

Eastern Boundary Upwelling Systems (EBUS) are well-known for their high productivity and fishery yields. However, being scarcely sampled and poorly represented in global models, their role as CO2 sources and sinks to the atmosphere remains elusive. Here, we present a compilation of shipboard measurements over the past two decades, showing how the Benguela Upwelling System (BUS) in the southeast Atlantic Ocean acts as a CO2 source in the north and CO2 sink in the south. Surface warming of upwelled waters increases the partial pressure of CO2 (pCO2) and outgassing in both regions, but in the south, the biologically-mediated drawdown of CO2 exceeds this warming effect. Here, the biological carbon pump owes its stronger impact on pCO2 to higher shares of upwelling source waters carrying preformed nutrients supplied from the Southern Ocean. Their formation increases pCO2 in surface waters and counteracts human-induced invasion of CO2 in the Southern Ocean. However, their utilization in the BUS compensates for over 20% of the CO2 loss occurring in the Atlantic sector of the Southern Ocean. This emphasizes the role of the BUS as key to improve our understanding of the ocean’s response to climate change and the future evolution of CO2 in the atmosphere.