Bioturbation and porosity gradients

Abstract

Ubiquitous porosity gradients have a potentially important effect on the mixing of particle-bound tracers, such as Pb-210. Mass-depth coordinates cannot be used to deal with these effects if values of the traditional mixing coefficient, D-B, are required. This paper compares and evaluates three different means of dealing directly with porosity gradients while modeling bioturbation, i.e. mean constant porosity, interphase mixing (porosity mixed), and intraphase mixing (porosity not mixed). We apply these models to 11 different Pb-210 profiles collected at various depths and times on the eastern Canadian Margin. A statistical analysis of the resulting best fits shows that these models produce equivalent mixing coefficient values for 55% of the profiles. For the remaining 45% of the profiles, the interphase mixing model predicts the existence of well-mixed near-surface zones on the time scale of Pb-210 decay, a phenomenon not predicted by the other models. Unfortunately, our tracer dataset by itself cannot be used to establish which mixing mode is actually operative at each station.