Delayed settling of marine snow at sharp density transitions driven by fluid entrainment and diffusion-limited retention

Abstract

Marine snow is central to the marine carbon cycle, and quantifying its small-scale settling dynamics in different physical environments is essential to understanding its role in biogeochemical cycles. Previous field observations of marine aggregate thin layers associated with sharp density gradients have led to the hypothesis that these layers may be caused by a decrease in aggregate settling speed at density interfaces. Here, we present experimental data on aggregate settling behavior, showing that these particles can dramatically decrease their settling velocity when passing through sharp density transitions. This delayed settling can be caused by 2 potential mechanisms: (1) entrainment of lighter fluid from above as the particle passes through the density gradient, and (2) retention at the transition driven by changes in the density of the particle due to its porosity. The aggregates observed in this study exhibited 2 distinct settling behaviors when passing through the density transition. Quantitatively comparing these different behaviors with predictions from 2 models allow us to infer that the delayed settling of the first group of aggregates was primarily driven by diffusion-limited retention, whereas entrainment of lighter fluid was the dominant mechanism for the second group. Coupled with theory, our experimental results demonstrate that both entrainment and diffusion-limited retention can play an important role in determining particle settling dynamics through density transitions. This study thus provides insight into ways that delayed settling can lead to the formation of aggregate thin layers, important biological hotspots that affect trophic dynamics, and biogeochemical cycling in the ocean. © Inter-Research 2013.