  We place a 2.5% limit on the anthropogenic contribution to the modern
abundance of 81Kr/Kr in the atmosphere at the 90% confidence level. Due to its
simple production and transport in the terrestrial environment, 81Kr (halflife
= 230,000 yr) is an ideal tracer for old water and ice with mean residence
times in the range of 10^5-10^6 years. In recent years, 81Kr-dating has been
made available to the earth science community thanks to the development of Atom
Trap Trace Analysis (ATTA), a laser-based atom counting technique. Further
upgrades and improvements to the ATTA technique now allow us to demonstrate
81Kr/Kr measurements with relative uncertainties of 1% and place this new limit
on anthropogenic 81Kr. As a result of this limit, we have removed a potential
systematic constraint for 81Kr-dating.
