SGER: Determining Equatorial Upwelling Rates with a Novel Geochemical Tracer Technique

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In the exploratory project, the investigators propose to develop a novel isotopic technique, based on the isotope Beryllium 7, to infer rates of upwelling along the equator. Beryllium 7 samples will be collected on an NSF-funded research cruise to the equatorial eastern Atlantic in April, 2009. The cruise track is ideal for the proposed study. Several stations away from the equator will be occupied to make numerous mixed layer measurements and several profiles of Beryllium 7 within the areas of upwelling in the cold tongue region, focusing primarily on the region between 5°E and 10°W. The Beryllium 7 profiles will be collected simultaneously with the CTD stations using well established techniques developed by the investigators. The key hypothesis to be tested is the idea that the observed mixed layer Beryllium 7 concentration is an extremely sensitive indicator of the upwelling rate. Two properties of Beryllium 7 make it an ideal tracer for investigation of upwelling. First, its source function and water column distribution (high contrast between mixed layer and sub-mixed layer depth) allow it to be a sensitive indicator of upwelling. Secondly, its characteristic time scale is long enough to integrate upwelling over periods of weeks but still be sensitive to variations on seasonal timescales, unlike tracers with significantly longer half-lives.Broader Impacts. Upwelling is an important physical process affecting climate dynamics and biogeochemical cycling within the global ocean. Direct measurements of upwelling are difficult because of the relatively small velocities involved, and it must therefore be inferred by indirect methods such as those provided by tracer observations. If successful, this method will provide an important tracer tool to the oceanographic community. The findings will be incorporated into undergraduate and graduate courses that the co-investigator teaches.