Palladium on CaCO3 is known as a Lindlar Catalyst. It's widely used for hydrogenation in synthesis of organics such as Vitamin A. In such applications a "catalyst poison" is added to reduce the hydrogenation to a single bond. What I see in my prepared mesh is a very crude form of such a catalyst, where the Calcite is mixed into the deposited Pd by the burnishing process. Details of this structure can be seen in the image below, from my paper from August 2019.