Upper Ocean Mixing and Oxygen Utilization Rates in the Changing Sargasso Sea with Development of an Improved Beryllium-7 Sampling System

Oceanographers often use radioactive elements coming from the atmosphere to estimate how quickly the ocean circulates. For example, radiocarbon, which decays over thousands of years, can be used to determine that ocean currents travel from the surface to the deep sea in about 300 to 1000-years, depending on where you are. Radiocarbon decay is too slow for this technique to be used in the upper ocean.

Beryllium-7 is another radioactive element generated in the atmosphere. Beryllium-7 decays in months so it is well suited to the faster circulation of the upper ocean. Unfortunately, beryllium-7 is very rare, so measurements are expensive.

In this project an affordable and efficient system will be designed and built to sample beryllium-7 in seawater. The system will be tested near Florida and then used near Bermuda to estimate how quickly the upper ocean circulates. The circulation timescales will be used with oxygen measurements to estimate the rate of biological activity in this part of the Atlantic Ocean.

The project will develop an in situ pumping system to more easily collect beryllium-7 from seawater. The prototype will be tested on a brief cruise near Florida and then deployed as part of Bermuda Atlantic Time Series (BATS) cruises over four seasons and two years. To estimate lateral trends in preformed oxygen and in beryllium-7, additional sampling near the BATS site is planned on all cruises.

The beryllium-7 data collected at BATS will be applied in an established vertical mixing model to estimate vertical diffusivities (Kz) and Oxygen Utilization Rates (OURs) over a full seasonal cycle. The lateral beryllium-7 and oxygen trends measured will be used to calculate the impacts of horizontal transport on the vertical model. The Kz and OUR estimates will be compared with those determined at BATS in 2007/8 using the older, large volume sampling technique for beryllium-7.

The sampling system has promising applications to other trace elements and isotopes and will provide for more widespread applications of beryllium-7. The project supports instrument development and undergraduates from Florida International University will gain hands-on research and engineering skills.

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.