Optical Results From the November '01 "Chalk-Ex" Ocean Optics Manipulation Experiment

Suspended calcium carbonate particles in the sea create a significant amount of backscattered light and are highly relevant to a wide variety of processes ranging from radiative transfer to biogeochemical cycles. A large-scale ocean optics manipulation experiment (dubbed "Chalk-Ex") was performed in November of 2001 which involved adding Cretaceous coccolith chalk to seawater and following its fate over time (see overview abstract by Balch et al.). The chalk had extremely well-defined optical backscattering properties, which allowed us to precisely map its distribution using several approaches: a towed, undulating, Scan-Fish equipped with a backscattering sensor, shipboard measurements of inherent optical properties, plus above-water radiometry measurements. We will present vertical sections and aerially-integrated estimates of the chalk during patch evolution. High-altitude images of the patch provided synoptic estimates of the patch's shape and size. Moreover, the images illustrated the importance of wind-induced surface shear and confirm shipboard optical results. Backscattering of the chalk spread from its "injection isopycnal" and was gradually sheared into other density horizons, spreading in various horizontal directions. There was evidence of dissolved organic matter binding to the chalk, which, in a few cases produced significant "holes" in the spatial distribution of colored dissolved organic matter (as evidenced by results of the 412nm absorption for the <0.2æm filtered size fraction). The patch was observed by the MODIS sensor aboard NASA's Terra spacecraft and the satellite estimates of calcite were within a factor of 2 of the values measured aboard ship. In terms of the mass conservation of chalk, it decreased exponentially through time (both on a mass-specific and area-specific basis). This was likely due to the combination of our ability to find the ever-expanding chalk patch as well as the possibility that microzooplankton were grazing on the chalk (see McManus et al. poster). Nonetheless, in a few specific regions, the integrated chalk per m2 remained relatively constant over the first 32h (only decreasing about 25 percent). This suggested that the various loss processes (i.e., dissolution, grazing, aggregation and sinking) were negligible in these select areas over that time period.