Evolution of stratification and shear during ChalkEx-2001

ChalkEx-2001 was a manipulative experiment wherein a known quantity of CaC03 (chalk) was injected into the surface mixed layer from a ship and tracked with the aid of Lagrangian drifters. After injection, the "chalk patch" was identified from spatial surveys using optical backscattering sensors on the ship and on a towed, undulating vehicle. The goal of the experiment was to identify and quantify the relevant physical and biological processes that remove CaC03 particles from the mixed layer. Observations of temperature, conductivity and horizontal velocity in the upper 100 m of the water column were used to characterize the evolution of stratification and shear during the experiment. The mixed layer depth and density at the time of chalk injection, and the subsequent development of stratification and shear were used to diagnose the development of the chalk patch due to vertical mixing and horizontal advection. The expected patch development was compared to the observed patch distribution as determined from spatial surveys. Although vertical mixing dictated the initial penetration depth of the chalk, patch development during the first 48 hours was controled primarily by horizontal advection during a period of restratification. The horizontal displacement estimated from the shear between the surface and the maximum injection depth was consistent with the observed size and orientation of the patch.