BIOTURBATION AND POROSITY GRADIENTS

Ubiquitous porosity gradients have a potentially important effect on t he mixing of particle-bound tracers, such as Pb-210. Mass-depth coordi nates cannot be used to deal with these effects if values of the tradi tional mixing coefficient, D-B, are required. This paper compares and evaluates three different means of dealing directly with porosity grad ients while modeling bioturbation, i.e. mean constant porosity, interp hase mixing (porosity mixed), and intraphase mixing (porosity not mixe d). We apply these models to 11 different Pb-210 profiles collected at various depths and times on the eastern Canadian Margin. A statistica l 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 th e existence of well-mixed near-surface zones on the time scale of Pb-2 10 decay, a phenomenon not predicted by the other models. Unfortunatel y, our tracer dataset by itself cannot be used to establish which mixi ng mode is actually operative at each station.