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ar_track_alvar publishes by default the detected markers to the topic : ar_pose_marker

You can see in this topic that the pose contains the distance from the camera to the marker in the variable pose.position.z

Alternatively, since you are using a kinect, you could recalculate the pixel at which the marker is centered and get its corresponding depth from the kinect.

In both cases you will get a distance from the marker to the camera and you need to use tf to transform it such that you get the distance from the marker to the create base.