David, haven't looked at tracer code in some time, but at least in earlier versions (and this is also true for temp and salt) array gTracer is used as temporary storage for tracer, later cycle by TRACERS_CORRECTION_STEP > gTracer(i,j,k,bi,bj)=tracer(i,j,k,bi,bj) > & +dTtracerLev(k)*gTracer(i,j,k,bi,bj)