Chalk-Ex: An Ocean Optics Manipulation Experiment on the Fate of Calcite Particles

Large-scale manipulation experiments such as Iron-Ex have allowed the testing of fundamental hypotheses about the ocean, not easily approachable with typical bench-style experimentation. Manipulation experiments provide tremendous insight by integrating the entire biogeochemical system into the results; they arguably give some of the most valuable tests for today's complex numerical models, at relatively fine spatial scales. One poorly understood biogeochemical cycle is that of CaCO3. About one quarter of the earth's marine sediment is CaCO3, much of which is composed of small coccoliths. How these particles are transported to the sea floor is still an open question. In this talk, we will present an overview of experiments from November 2001 from Continental Shelf and Slope waters off the NE U.S., in which Cretaceous coccolith chalk was dispersed into a patch of about 1.5 km2 (dubbed "Chalk-Ex"). The chalk's extremely well-defined light scattering and stable isotope properties made it possible to monitor the patch evolution and examine the importance of physical processes (horizontal shear, vertical mixing), grazing (macro- and micro-zooplankton), aggregation, interactions with dissolved organic carbon and sinking (estimated with drifting sediment traps). Lagrangian drifters were used to follow the patch and an instrumented drifter was used to characterize T-S properties over several days. From a particle perspective, the power of Chalk-Ex was that it simplified particle turn-over calculations. That is, while most particle experiments must simultaneously quantify both the production and loss terms of the particles in question (often with limited statistical precision), in Chalk-Ex, the production term was known almost exactly, so that we could focus on subsequent loss terms. The process that appeared to be most important to the patch evolution was horizontal dispersion at the "injection density", driven by shear between the surface and the maximum injection depth of about 25 m. We observed a shift in the size spectrum of sub-micron particles in the patch as dissolved organic matter appeared to bind to the chalk. Aggregation of chalk particles was observed, but did not result in particles large enough to sink out of the mixed layer during the 48h trap deployment. Mesozooplankton did not appreciably consume chalk based on laboratory feeding experiments, but shipboard bottle experiments suggested that microzooplankton grazing may have been an important loss term for the chalk particles from the surface waters. Nonetheless, isotope analyses revealed that negligible Cretaceous chalk was collected by the drifting traps below the mixed layer. This indicated that the chalk was never concentrated into large enough particles (either biologically or physically) to rapidly sink and be intercepted by the traps. If it was concentrated into large particles, then an alternative hypothesis is that the chalk dissolved above the traps (associated with microbial or grazing activity). The patch was eventually "lost" between two horizontal intrusions. This talk will serve as an introduction to a series of posters to be presented in a parallel session.