Post-mortem pteropod degradation in the Southern Atlantic twilight zone

Part of the carbon taken up by the ocean is transformed into biogenic particulate matter, that eventually leaves the surface ocean, settling toward the seafloor. Planktonic organisms secreting a calcium carbonate (CaCO₃) shell occupy a key, but ambivalent role in this scheme. First, the precipitation of their shell generates CO₂, thereby reducing the ocean CO₂ sink, while the sinking of their shell constitutes a direct export of carbon to the deep ocean. Meanwhile, the dissolution of their shells generates alkalinity, which in turn boosts the capacity of seawater to take up more CO₂ from the atmosphere.

 

CaCO₃ is present in the ocean under two main mineral forms: calcite (relatively stable) and aragonite (relatively soluble). Aragonite, produced in today’s oceans mostly by pteropods, a group of pelagic snails, has a very poorly understood role in the marine carbon cycle, and key questions remain unanswered: what controls their soft parts degradation and shell dissolution in the upper kilometer of the water column? How do both processes interact?

 

During the BEYΩND expedition that took place in March 2023 across the Southern Atlantic, we sampled pteropods using a multinet at 5 different depth ranges in the upper kilometer. Retrieved pteropods were representative of different life stages (adults, juveniles), some still well preserved, some dead with various stages of decomposition. Individuals were then preserved into ethanol, and later scanned with a micrometric resolution using microtomography. From the scans, the post-mortem degradation of the different body parts can be appreciated, shell micro-ornamentations can be seen, and possibly gut contents, which may influence dissolution and degradation processes. From the collected scans, a reactive transport model is then applied to predict in 3D the rates at which both organic matter degradation and aragonite dissolution occur, as well as how they interact.