Award: OCE-1829614

The biological carbon pump is one of the ways that the ocean takes up carbon dioxide from the atmosphere and transfers it to the deep ocean, where it can be stored for decades to centuries. This occurs when phytoplankton that live in the sunlit surface ocean photosynthesize, turning carbon dioxide into organic carbon. Phytoplankton can then have several fates: they can be respired back into carbon dioxide, returning the carbon to the atmosphere, they can be consumed by zooplankton, or they can aggregate into larger particles that can sink (gravitationally) into the deep ocean. Zooplankton can also migrate to depth and excrete a portion of what they eat, also transferring organic carbon into the deep ocean. Large particles can break up and disaggregate, which essentially slows down the descent of particles and hastens respiration back to carbon dioxide. The rates at which these various processes occur determine how effective the biological pump is at transferring carbon from the atmosphere to the deep ocean. Processes that favor aggregation and sinking increase its effectiveness; processes that favor respiration and disaggregation decrease its effectiveness. Despite their importance, these particle cycling rates are notoriously difficult to measure directly. This projects contribution was to develop and apply inverse methods that can quantify these particle cycling rates using a simple particle cycling model combined with field observations of particulate organic carbon in two size classes: a small, mainly suspended size class of particle, and a large, mainly sinking size class of particle. We applied this method to data collected from three major oceanographic expeditions funded by NASA and the NSF: 1) the EXPORTS North Pacific cruise, 2) the GEOTRACES Pacific Meridional Cruise, and 3) the EXPORTS North Atlantic cruise. These oceanographic expeditions spanned many ocean biomes, allowing us to contrast particle cycling rates from low and high productivity areas on a long transect from Alaska to Tahiti (GEOTRACES), as well as areas that are relatively invariant (EXPORTS North Pacific) and those that have dramatic phytoplankton blooms and declines (EXPORTS North Atlantic). During the EXPORTS North Pacific cruise, which was a low productivity site dominated by small phytoplankton, we found that small particles contributed a significant fraction of the total gravitational sinking fluxa surprising result since small particles were not thought to sink on their own. On the Pacific Meridional Transect, we found that higher productivity areas had larger sinking fluxes, but we were surprised to find that the fraction of biological production that sank out of the surface was low relatively constant throughout the transect, regardless of productivity. This may have been because we sampled this transect in the autumn season, whereas most oceanographic expeditions from which we build out intuition are conducted in the spring and summer seasons. This demonstrated the value in sampling at different times in the seasonal cycle. The long transect down the Pacific originating in the cold, low oxygen waters of Alaska and ending in the tropical waters of Tahiti allowed us to examine the effect of environmental variables on particle cycling rates. Surprisingly, we found no evidence for a strong dependence of respiration rate on oxygen and temperature, a finding with interesting implications for modelling the response of the biological pump to global warming. Our results suggest that we cannot ignore the active transport of carbon from the surface to the deep by zooplankton that migrate hundreds of meters daily, as this flux can be comparable to the flux from gravitational settling of large particles. Our results are some of the most detailed examination of the rates of these particle cycling processes in a wide range of locations, and helps us better understand and predict the biological carbon pump and the role that the ocean plays in the carbon cycle. Our results have been published in open access journals that are freely available to the public. Last Modified: 12/08/2024 Submitted by: PhoebeJLam