Evidence of DOM Removal by Cretaceous CaCO3 Particles During Chalk-Ex 2001

Any process that regulates the distribution of dissolved organic matter (DOM) in seawater assumes considerable significance, given the role of the DOM pool in radiative transfer, remote sensing and carbon sequestration into the oceans. Since particles that constitute the DOM pool are not dense enough for sinking to be the dominant removal process, their transport into the deep ocean interior has often been associated with advection of the source water mass. In recent years, a growing body of evidence linking DOM with inorganic particles has led to the belief that scavenging and ballasting of DOM by inorganic mineral particles could play an important role in DOM removal from the upper ocean. ChalkEx-2001 was a large-scale manipulative experiment, which involved dispersing a known quantity of Cretaceous chalk into seawater and following its fate by shipboard optical surveys, Langrangian drifters and sediment traps (see Balch et al. presentation for experiment overview). These experiments provided us with the opportunity to assess the importance of DOC binding onto CaCO3 particles. Flow Field-Flow Fractograms of sub-micron particles and dissolved organic carbon measurements in samples obtained from discrete depths prior to, and following deployment of the chalk, provided clear evidence of DOM scavenging by CaCO3 particles. Proof of DOM binding onto the chalk particles was also apparent from the continuous absorption measurements at 412nm of 0.2æm pre-filtered seawater, which showed extreme "lows" in the distribution of colored dissolved organic matter coincident with patches of chalk (see also poster by Bowler et al.). The DOM scavenging capacity of the cretaceous coccolith chalk particles was also evident in independent laboratory investigations. The significance of these findings is that CaCO3 particles, by virtue of their ability to scavenge DOM, could potentially accelerate the transport of DOM to the deep sea.