Intellectual merit: Summary: Many processes in the ocean cannot be directly observed and as such, tracers are used to provide important constraints on their rates and pathways. Be-7 is a tracer that, because of its half-life (53.3d), allows the study of processes which occur over time (seasonal) and spatial (shallow thermocline) scales that are otherwise difficult to obtain but which are critically important to studies of biological production, nutrient regeneration, and atmospheric deposition, to name a few. However, it has been one that is difficult to approach due to the lack of oceanographic tracers suitable for integrating processes over this temporal range. Advances in sampling and analytical techniques, coupled with a better understanding of the behavior of 7Be in ocean biogeochemical cycles, present an opportunity to fully utilize this tracer and address key tasks formulated within the GEOTRACES science plan. In this work we provided realistic estimates of the underlying transport processes influencing measured trace element distributions, and provided estimates of the atmospheric deposition of trace elements. Outcomes: We participated in the 55 day Peru-Tahiti US GEOTRACES leg in November-December, 2013. All of our sampling objectives were met. This included collection of 13 water column profiles for 7Be and 17 high volume aerosol samples. All of these samples have been analyzed. In addition, we measured high volume size-distributed aerosol samples. Our hypothesis that the 7Be inventories in the eastern section would reflect upwelling was proven. That is, the 7Be concentration in the usually 7Be-rich surface mixed layer was diluted from penetration of 7Be "dead" water upwelled from below. An obvious correlation between 7Be inventory and water temperature was observed; cold, upwelled water had a deficit of 7Be. This dilution of surface 7Be with upwelled water provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, 7Be profiles were used to constrain vertical diffusivity within the upper thermocline. The PI has presented these results at the AGU meeting in December, 2014. In addition, the results were published in a peer-reviewed journal, Global Biogeochemical Cycles. In this work, the transport parameters derived from 7Be were used with nitrate profiles to calculate net community production. Finally, the 7Be aerosol concentrations are being applied to trace element aerosol concentrations to derive the flux of trace elements to the ocean of this study region. The derived transport parameters have been used by other PIs ion the program who have thus far used the information to interpret Hg, 234Th, POC flux, redox behavior, and nitrate flux along that expedition transect. Broader Impacts: The derivation of upwelling rates with vertical diffusion parameters enables calculation of new primary production. This could be important in economic fisheries, and social disciplines. The use of 7Be to derive the atmospheric input of trace elements(TE) offers robust estimates of TE input to the surface ocean averaged over several weeks and over a sub-regional to regional spatial scale. The method is particularly useful for remote areas where fixed sampling stations do not exist; that is, the majority of the global ocean. The study of marine trace element geochemistry in general and the study of micronutrient (e.g., Fe, Mn, Zn) biogeochemistry in marine ecology will benefit greatly from this method. All data has been reported to, and is accessible in, the Biological and Chemical Oceanography Data Management Office (BCO-DMO). Last Modified: 10/02/2017 Submitted by: David C Kadko
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