Award: OCE-1406552

Carbon-containing particles are produced from CO2 by tiny organisms in the surface ocean. A small fraction of these particles sink into the deep ocean, ultimately removing CO2 from the atmosphere in a process termed "the biological pump". A shifting balance among physical motions of the water, chemical reactions, and the activities of ocean life determines whether and how much of the sinking carbon is ultimately sequestered in the deep ocean. These processes affecting sinking carbon are most variable and hardest to measure at depths between roughly 100 and 1000 meters, sometimes called "the twilight zone". Our challenge is to expand the number of particle flux observations in the twilight zone, something that has proven elusive with ship-based "snapshots" that have lengths of, at most, a few weeks. In this project we employed a type of optical sensor to measure the flux of sinking particles using self-sufficient, robotic profiling floats. New developments in data interpretation, sensor operation, and platform control allow us to make measurements at fast time resolution, similar to the rapid changes that can occur in the natural processes that affect sinking carbon in the ocean. The sensors and floats that we have used are simple, robust, and commercially-available, which allows these tools to be adopted by other researchers in many areas of the ocean. The project has two main goals: First, we have quantified the flux of sinking carbon (i.e., the amount of carbon sinking through an area of the ocean during a given time period) by comparing the optical sensor observations to fluxes measured simultaneously with the established, but more labor-intensive method of using rain gauge-like sediment traps. Second, we have tested how much rapid export "events" contribute to total sinking carbon fluxes in the open ocean by using profiling floats to measure the flux of sinking carbon as well as measurements of dissolved oxygen, temperature, salinity, particles, and photosynthesis pigments. During this project, we conducted experiments in the laboratory to determine the detection limit of the optical flux sensor, and to determine whether many small particles give the same sensor response as fewer, larger particles of the same mass. We also performed a series of five comparisons of sediment traps to optical flux sensors in conjunction with monthly Bermuda Atlantic Time-series Study (BATS) cruises, taking advantage of the timeseries measurements and the context provided by the 25-year record of sinking carbon flux at that site. Finally, we used measurements, transmitted by two profiling floats allowed to operate on their own in the northwest Atlantic Ocean, to interpret sinking carbon fluxes over a year-long period in the context of physical and biological events occurring in the ocean during the deployments. Broader impacts of this project include the close involvement of two early-career researchers, and the training of one undergraduate student to work with profiling float data and conduct original research. The project has enhanced research infrastructure by utilizing profiling floats developed through an academic-private sector collaboration. These floats are now being used in a variety of other projects around the world, contributing both to our understanding of the ocean and to the competiveness of U.S.-produced oceanographic equipment. The use of commercially-available, off-the-shelf technology for field observations has ensured that other researchers can independently move forward with similar observations in the future. Finally, the proposed project benefits society by addressing gaps in our understanding of the marine carbon cycle that limit our ability to assess and adapt to the consequences of climate change. Last Modified: 06/29/2016 Submitted by: Margaret Estapa