 Zivori et al. find calcifying phytoplankton dominate pelagic CaCO3 production, but a large portion of this CaCO3 dissolved in the photic zone, they suggest the processes driving shallow CaCO3 dissolution are key to understanding the role of planktonic calcifiers in regulating atmospheric CO2. This article was authored by Patrizia Zivori, William Robert Gray, Griselder and Glader or Thies, and others.