They are both found in the waters of the Arctic Ocean.  When studying the ocean, one of the things we are looking at is a surface layer that contains fresher water, which could have its origin in continental river water runoff, or melting sea ice.  Both river water and sea ice have low salinity, so oceanographers use a trick to distinguish between them. 
 

O18

  There are two common forms (isotopes) of the atom oxygen.  O₁₆ is a more common isotope than O₁₈.  O₁₈ weighs more than O₁₆ and has two more neutrons.  The chemical formula for water is H₂O.  The oxygen in water can be either O₁₆ or O₁₈.  When evaporation from the sea surface occurs, it is easier to lift the lighter O₁₆ than the heavier O₁₈.  Thus precipitation contains more O₁₆ than O₁₈.  When the precipitation containing O₁₆ is returned to the ground, it flows as river runoff.  By determining whether your seawater sample contains more O₁₆ or O₁₈, you can determine whether it is from land or ocean.
 

  In simple terms, when we find an input of fresh water, "O₁₈" tells us if we are looking at sea ice melting or river water. 

  Scientists have been studying Barium since the early 90's.    It is known that sediments contain Barium. 

  The North American continent contains higher levels of Barium than the lands of Russia.  By looking at the levels of Barium in the seawater along with the salinity and the O₁₈, you can figure out which continent the river water came from. 

  Barium is not a simple tracer element, because it can be dissolved, consumed by critters, or changed into something else which then sinks to the bottom of the ocean.  Barium travels to the ocean from rivers by attaching itself to clay matter.  Once this clay enters the ocean, the Barium is released.  Barium can then be traced back to a particular river.  If you know which river it came from, the Barium tells you something about the circulation of the Arctic Waters.

  In the pictures you can see two sample bottles.  The glass container is for O₁₈ and the plastic is for Barium.  You can see the cast number identifier, station number identifier, and which niskin bottle it was taken from.  This is how the data are logged for reference and storage.