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Abstract. Export of carbon from the euphotic zone to intermediate and deep water plays a critical role in the ocean's feedback response to a warming climate. However, as water temperature increases so does the rate of bacterial respiration at the base of the biological pump, resulting in more efficient recycling of carbon in the upper ocean, less efficient export of carbon to depth, and a diminished net negative feedback on climate. Therefore, to better predict climate response associated with changes in ocean carbon storage in warming scenarios, it is imperative to incorporate temperature-sensitive mechanisms, such as bacterial respiration (remineralization), into Earth system models. Here, we employ a new temperature-dependent parameterization for remineralization (Tdep) in the Community Earth System Model version 1 (CESM1) applied to gravitationally sinking particulate organic carbon (POC) in a preindustrial control simulation. We find that the inclusion of Tdep in both low and high-resolution model configurations more accurately captures regional heterogeneity in POC transfer efficiency while preserving the overall trends in nutrient distribution and attenuation of sinking particulate matter when compared with modern empirical data. Inclusion of this parametrization will allow for improved predictions of temperature-sensitive mechanisms impacting carbon storage in the warming ocean.
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What Controls the Large‐Scale Efficiency of Carbon Transfer Through the Ocean's Mesopelagic Zone? Insights From a New, Mechanistic Model (MSPACMAM)
Abstract A key challenge for current‐generation Earth system models (ESMs) is the simulation of the penetration of sinking particulate organic carbon (POC) into the ocean interior, which has implications for projections of future oceanic carbon sequestration in a warming climate. This paper presents a new, cost‐efficient, mechanistic 1D model that prognostically calculates POC fluxes by carrying four component particles in two different size classes. Gravitational settling and removal/transformation processes are represented explicitly through parameterizations that incorporate the effects of particle size and density, dissolved oxygen, calcite and aragonite saturation states, and seawater temperature, density, and viscosity. The model reproduces the observed POC flux attenuation at 22 locations in the North Atlantic and North Pacific. The model is applied over a global ocean domain with seawater properties prescribed from observation‐based climatologies in order to address an important scientific question: What controls the spatial pattern of mesopelagic POC transfer efficiency? The simulated vertical POC transfer is more efficient at high latitudes than at low latitudes with the exception of oxygen minimum zones, which is consistent with recent inverse modeling and neutrally buoyant sediment trap studies. Here, model experiments show that the relative abundance of large‐sized, rapidly sinking particles and the slower rate of remineralization at high latitudes compensate for the region's lack of calcium carbonate ballast and the cold‐water viscous resistance, leading to higher transfer efficiencies compared to low‐latitude regions. The model could be deployed in ESMs in order to diagnose the impacts of climate change on oceanic carbon sequestration and vice versa.
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- Award ID(s):
- 2019589
- PAR ID:
- 10559555
- Publisher / Repository:
- AGU Publications
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 36
- Issue:
- 10
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Understanding particle cycling processes in the ocean is critical for predicting the response of the biological carbon pump to external perturbations. Here, measurements of particulate organic carbon (POC) concentration in two size fractions (1–51 and >51 μm) from GEOTRACES Pacific meridional transect GP15 are combined with a POC cycling model to estimate rates of POC production, (dis)aggregation, sinking, remineralization, and vertical transport mediated by migrating zooplankton, in the euphotic zone (EZ) and upper mesopelagic zone (UMZ) of distinct environments. We find coherent variations in POC cycling parameters and fluxes throughout the transect. Thus, the settling speed of POC in the >51 μm fraction increased with depth in the UMZ, presumably due to higher particle densities at depth. The settling flux of total POC (>1 μm) out of the EZ was positively correlated with primary production integrated over the EZ; the highest export occurred in the subarctic gyre while the lowest occurred in the subtropical gyres. The ratio of POC settling flux to integrated primary production was low (<5%) along GP15, which suggests an efficient recycling of POC in the EZ in all trophic regimes. Specific rates of POC remineralization did not show clear variations with temperature or dissolved oxygen concentration, that is, POC recycling was apparently controlled by other factors such as microbial colonization and substrate lability. Particle cohesiveness, as approximated by the second‐order rate constant for particle aggregation, was negatively correlated with trophic regime: particles appeared more cohesive in low‐productivity regions than in high‐productivity regions.more » « less
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Abstract The ocean contributes to regulating atmospheric CO2levels via the biological carbon pump (BCP). One critical aspect of the BCP is the depth at which sinking particulate organic carbon (POC) remineralizes in the mesopelagic zone (200–1,000 m). In the Southern Ocean, the circulation is such that the products generated from POC remineralization may have drastically different fates depending on (a) the latitude at which sinking particulate material is produced and (b) the depth at which its remineralization occurs. Here, we assess latitudinal and depth variations of POC export marine aggregate abundance and composition in the Southeast Pacific sector of the Southern Ocean. We show changes in flux attenuation depth horizons in the upper mesopelagic in the subantarctic zone. These correspond to rapid particle accumulation below the depth of the euphotic zone followed by abrupt export. We believe that such rapid changes may be linked to diatom life cycles, including resting cell and spore formation and resulting changes in particle sinking velocities rather than attenuation due to heterotrophic degradation or solubilization in the upper mesopelagic zone. We further discuss the occurrence of such features in the Southern Ocean and at the global scale. Our results highlight the importance of alternative flux attenuation processes, such as sudden changes in particles sinking velocities, in explaining variability in organic carbon sequestration by the ocean's BCP.more » « less
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Not AvailThe oceanic biological carbon pump (BCP) is thought to be driven by the vertical export of particulate organic carbon (POC) derived from fresh photosynthetically-fixed carbon in the surface waters to depth. Yet the chemical and isotopic composition of deep ocean POC is often inconsistent with this one-dimensional perspective of the BCP. Here, we investigate the sources and temporal dynamics of POC in the deep-sea using the concentration and carbon isotopic composition (Δ14C and δ13C) of POC intercepted by time-series sediment traps deployed in the Sargasso Sea near Bermuda over a 43-month time period. Seasonal variability in POC collected at 3200 m between 2012 and 2015 reveals a recurring springtime increase in Δ14C and δ13C, indicating rapid transfer of POC produced in the surface ocean to depth. In contrast, summer and autumn periods are characterized by lower Δ14C and δ13C values, reflecting higher contributions of millennial to centennial aged carbon sources. Isotopic mass balance calculations indicate that, on average, 63 ± 14 % of sinking POC originates from freshly produced surface-derived OC and 24 ± 15 % derives from mineral-free suspended pre-aged POC (partly surface-derived), while mineral-associated OC (8 ± 6 %), and DOC (5 ± 4 %) further contribute to the aged carbon signature. The contribution of surface-derived OC is highest during spring, while the fraction of aged mineral-bound OC, likely linked to Gulf Stream-driven lateral sediment transport from the Northwest Atlantic continental margin, is highest in autumn. Our findings highlight the complexity of the BCP in the open ocean, where pre-aged OC contributes significantly to deep-sea carbon sequestration, challenging traditional one-dimensional vertical perspectives of carbon export.ablemore » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2021GB007131
