Siegel David A., et al. (2021) Assessing The Sequestration Time Scales Of Some Ocean-Based Carbon Dioxide Reduction Strategies. 2021 Environ. Res. Lett. (In Press)

Summary of the study: Carbon dioxide (CO2) removal (CDR) strategies that use the ocean are an important part of the set of steps needed to get negative greenhouse gas emissions. Many ocean-based CDR strategies depend on putting CO2 or organic carbon (which will eventually turn into CO2) into the deep ocean or making the biological pump of the ocean work better. These methods won’t lead to permanent sequestration because ocean currents will eventually bring the CO2 back to the surface, bringing it back into balance with the atmosphere. Here, a model of steady state global ocean circulation and mixing is used to figure out how long CO2 that has been put into the deep ocean stays out of the air. Because there are an infinite number of ways to get from a place at depth to the surface of the water, there will be a range of sequestration times for any single discharge location. The result is that the probability distribution is very skewed, with a long tail of very long transit times. This makes the mean sequestration times much longer than normal time scales. Deeper discharge sites will store CO2 for a much longer time than shallower ones. The average time to store CO2 is between a few decades and a few centuries, but in the deep North Pacific, it takes close to a thousand years. There are big differences in how long it takes for carbon to be stored within and between the major ocean basins. In general, the Pacific and Indian Oceans take longer to store carbon than the Atlantic and Southern Oceans. Assessments done over a 50-year period show that most of the carbon injected at depths greater than 1000 m will stay there, with a few places like the Western North Atlantic being an exception. Ocean CDR strategies that aim to increase the productivity of upper ocean ecosystems so that more carbon can be exported to depth will have a short-term effect on CO2 levels in the air because about 70 percent of the carbon will be brought back to the surface ocean within 50 years. The results shown here will help plan ocean CDR strategies that can help limit the damage to the climate caused by CO2 emissions from fossil fuels.

Dittmar Thorsten, et al. (2021) Enigmatic Persistence Of Dissolved Organic Matter In The Ocean. June 2021 Nature Reviews Earth & Environment.

Summary: Marine dissolved organic matter (DOM) has more carbon than all the Earth’s living things put together. Organisms in the ocean constantly release a wide variety of molecules that become food for microheterotrophs. However, for reasons that aren’t clear, some of these molecules stay in the ocean as DOM for thousands of years. In this Perspective, we look at two ideas that could explain this persistence and compare them. The “intrinsic recalcitrance” paradigm, which has been around for a long time, says that the stability of DOM comes from the way molecules are made. In the “emergent recalcitrance” theory, marine microheterotrophs change DOM all the time, and recalcitrance develops on an ecosystem level. Both ideas make sense based on what we know about the ocean today, but they predict very different responses of the DOM pool to changes in the climate. To learn more about how DOM stays around, we propose a new research strategy called “the ecology of molecules.” This strategy combines the ideas of “intrinsic” and “emergent” recalcitrance with the ecological and environmental context.

Baklouti M., Pagès R., Alekseenko E., Guyennon A., Grégori G. (2021). On the Benefits of Using Cell Quotas in Addition to Intracellular Elemental Ratios in Flexible-Stoichiometry Plankton Functional Type Models. Application to the Mediterranean Sea. Prog. Oceanogr. 197, 102634.

Summary: In the last few decades, there have been a growing number of marine Plankton Functional Type (PFT) models. Since the NPZD models that came before them, the general trend has been toward making models that are more and more complicated, either in terms of the number of variables they include or the level of detail they consider. In this way, flexible-stoichiometry models have been an important step in the history of this type of models. Since then, new developments have been written about in the literature, and this paper focuses on one of them: the addition of abundance to biomass to describe PFTs. With the growing number of studies and data sets from flow cytometry, this new feature makes it possible to test the model. It also makes it possible to get cell quotas (expressed as the amount of a given biogenic element per unit cell) for each PFT of the model. Cell quotas and intracellular ratios are used to control the speed of the different biogeochemical processes in the model. It is suggested here that their role may be crucial for accurately representing some processes, such as mineralization by heterotrophic bacteria. In this paper, we talk about some of the other benefits of cell quotas, such as how they help us understand how the trophic web works and how well organisms are eating. Lastly, the seasonal changes in the number of cells in the PFT that represent large phytoplankton can be used to explain why the NW Mediterranean spring bloom happens from an internal status point of view.