3) Introduction: Geochemical tracers as fingerprints

Imagine you have a cup of soda containing Coca-Cola, Pepsi, Dr. Pepper, and Mug Root Beer.  Can you tell how much of each brand was added to your cup by taste?  Probably not.

Like the mix of different sodas, Arctic seawater can contain Pacific water, Atlantic water, Siberian rivers, North American rivers, and sea-ice melt.  We call these water types.

Also like the soda, Arctic water types have different “flavors”.  Instead of tasting them, we use chemistry to tell the water types apart.  By measuring many types of “flavors” (different minerals dissolved in the water) we can identify each water type.

We will use salinity, stable isotopes of oxygen (d18O), total alkalinity, barium, and nutrients (e.g., nitrate, phosphate, silicate) to distinguish the different water types.  Individually, these chemical tracers cannot be used to tell all the different water types apart.  But together they form a type of chemical fingerprint specific to each water type.  For example, both Siberian and North American rivers are characterized by very similar salinities and d18O but they have different total alkalinities and barium concentrations (see Figure 3).  So, while salinity or d18O can be used to identify these river waters as freshwater sources, total alkalinity and barium are needed if the specific source (North America or Siberia) is to be identified.

Figure 3. Bar graphs comparing the magnitudes of salinity, oxygen-18 isotopes, barium, and total alkalinity for different sources of freshwater to the Arctic Ocean, including Siberian rivers, North American rivers, Pacific water, and sea ice melt.  From this graph, one can see that the lowest values of all four tracers are found within sea ice melt.  Siberian and North American rivers are associated with highly negative oxygen-18 but differ somewhat in barium and total alkalinity.  Pacific water has the highest salinity and total alkalinity, but is relatively low in barium and is associated with near-zero values for oxygen-18 (similar to sea ice melt).

Figure 3. Bar graphs comparing the magnitudes of salinity, oxygen-18 isotopes, barium, and total alkalinity for different sources of freshwater to the Arctic Ocean, including Siberian rivers, North American rivers, Pacific water, and sea ice melt. From this graph, one can see that the lowest values of all four tracers are found within sea ice melt. Siberian and North American rivers are associated with highly negative oxygen-18 but differ somewhat in barium and total alkalinity. Pacific water has the highest salinity and total alkalinity, but is relatively low in barium and is associated with near-zero values for oxygen-18 (similar to sea ice melt).