Francisco de Melo Virissimo (National Oceanography Centre, UK)
Abstract: The ocean is a massive carbon reservoir. It is estimated that it stores about 18 times more carbon than the terrestrial biosphere, and over 50 times more carbon than the atmosphere. It is also constantly exchanging carbon with the latter, but luckily in a way that it actually takes up and buffers some of the extra CO2 that has been released by the humankind since the industrial revolution times – thanks to this uptake, the atmospheric levels are actually lower than what they would be otherwise. An important pathway that allows for this buffering of carbon is the so-called biological carbon pump (BCP). The BCP can be roughly described as a series of biota-driven processes that effectively “pumps” organic carbon from the surface ocean into the deep ocean, where it can remain for 100s to 1,000s of years before returning to the surface ocean as inorganic carbon.
Over the past 40 years, our knowledge of the BCP has grown tremendously, particularly thanks to the fruitful combination of many dedicated expeditions that have been carried since the 1980s, and to the advent of marine biogeochemical models in the late 1980s and 1990s – which are now integral part of the most advanced CMIP-like Earth System models used for climate modelling. Despite that, we are relatively far from understanding and quantifying the BCP: for instance, the carbon fluxes from the ocean surface to deep ocean are only poorly quantified, and we also lack a consensual understanding of what processes might influence these fluxes – particularly in the mesopelagic ocean.Much of our understanding of the BCP comes from studies that are based on annual averages and assumes constancy in key parameters (e.g. the sinking speed and the remineralisation rate). But in reality, the ocean detrital pool is a beast in constant change – seasonal at first order. This consideration prompts a very simple question: are such temporal variations important?
In this talk I will review these topics and argue why do I consider seasonal variability to be of relevance for a proper account of the BCP – the figures above are a hint. I will then present some preliminary theoretical results about the effect of an imposed seasonal variation in the flux attenuation and detritus sinking speed has on the global nutrient distributions and particulate organic carbon (POC) transfer to depth. Particular attention is given to the effects of the strength of seasonality and its relative phase to other fields such as the growth rate of phytoplankton and the incident shortwave solar radiation.I will conclude by discussing some of the difficulties I have found in advancing in this area, as well as some ideas on how to overcome these difficulties.
This work highlights the importance of temporal variability in the BCP and POC transfer efficiency in the global ocean, as well as the effects of shifts between slow sinking and fast sinking detritus in the mesopelagic ocean.I aim to tailor this talk to a non-expert audience so that it should be accessible to anyone with a scientific background – no previous knowledge of marine biogeochemistry should be required.