Midterm study guide Flashcards
Why are mmol/kg used as units for marine chemistry, instead of mmol/L?
The ocean is deep, and water is slightly compressible. So 1.0 L of water at the surface (sea level) would be slightly smaller in volume at depth, where pressure is higher. So we use mass of seawater, rather than volume, as units for the concentration of solutes in seawater
How do we know that the salt composition of the ocean is similar today to that found
900,000,000 years ago?
The composition, and layering, of evaporites made from ancient oceans have the same sequence of
salts we would expect from our present ocean. In other words, calcium carbonate (CaCO3)
precipitates out first, then CaSO4, then NaCl, then MgSO4, and so on.
If CaCO3 was not found at the bottom of evaporite deposits, then the ocean chemistry would not have
been the same 900,000,000 years ago as it is today. For example, if you found the very insoluble AgCl at
the bottom, then you would know that there was a whole lot more Ag+ in ancient seawater than there is
today (practically none)
