Project: US GEOTRACES GP17-OCE: Measurements of the trace gases chlorofluorocarbons and sulfur hexafluoride

NSF Award Abstract:
The goal of the international GEOTRACES program is to understand the distributions in the ocean of trace chemical elements and their isotopes (TEIs), which are different forms of the same element. Many of the TEIs are essential for phytoplankton growth (e.g., iron) and are micronutrients. Yet, other TEIs are pollutants (e.g., lead). The U.S. GEOTRACES GP17-OCE expedition will occur in the South Pacific and Pacific sector of the Southern Ocean. Essential to understanding how TEIs change, e.g., scavenging and regeneration, and for estimating rates of biochemical processes is to identify water masses that are newly ventilated with atmospheric gases. These are waters that have been in contact with the atmosphere in the past few decades. The main objective of this project will be to measure the concentration of three man-made tracers of ocean ventilation that can be used to determine how long since a water mass was at the ocean surface in contact with the atmosphere. The tracers are particularly useful for following water masses along pathways as they age and estimating rates of effective spreading and dilution. An analogy would be adding a colored dye in the surface ocean, with a bit more dye added each year, and then measuring the concentration of this dye as surface water moves into the interior ocean. The three tracers that will be measured are Chlorofluorocarbon-11 (CFC-11), Chlorofluorocarbon-12 (CFC-12) and Sulfur Hexafluoride (SF6). The trace gas concentrations, ventilation ages, and rate data will all be made available to other investigators within six months of collection, forming an important foundation for the broader GEOTRACES program.

The project team will measure CFC-11, CFC-12, and SF6 concentrations at sea aboard the GP17-OCE expedition. In addition to conducting these measurements, the specific objectives are: 1) Calculate tracer ages and identify the relative extent of ventilation and dilution of thermocline, intermediate and bottom waters within which TEIs reside - from the end member sources to downstream locations as waters age, and put the GEOTRACES data in the historical context of changing ventilation due to natural and anthropogenic climate change. The age data will be available for PIs to evaluate how scavenging and regeneration are affected as TEIs are transported downstream. 2) Using tracer ages, calculate rates of biochemically important processes - e.g., apparent oxygen utilization rates, and help to distinguish between contributions from physical and biochemical processes across the different circulation regimes. Tracer ages will be calculated, from the ratio of SF6 to CFCs, and for oldest waters the ratio of CFCs to each other. The most robust tracer combination for ocean processes on time scales of less than 35 years is obtained using the SF6/CFC ratios.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.