Good question! One way to look at changes in salinity in the geologic past is to look at oxygen isotopes in shells and skeletons of marine fossils, including in bivalves, corals, and foraminifera (which are tiny marine organisms) that have been preserved in the geologic record somehow (either in a rock formation, in coral reefs, in ocean sediment etc.). An isotope is an atom with the same number of protons but different number of neutrons as a different atom of the same element. For example, an atom of oxygen has eight protons (that’s what makes it oxygen) but can have 8, 9, or 10 neutrons. Because of the different number of neutrons, different isotopes have different masses. The ratio of heavy (i.e., oxygen atoms with 10 neutrons) to light (i.e., oxygen atoms with 8 neutrons) in a marine fossil shell/skeleton is dependent on both the oxygen isotopes and the temperature of the water during the time that the shell/skeleton of the fossil formed (i.e., when the organisms was living in the geologic past). The oxygen isotopes of ocean water have a very close relationship with salinity because both are influenced by the amount of precipitation and evaporation occurring in the ocean: if there is increased evaporation, the ocean water gets saltier because the water evaporates but the salt does not. Similarly, with increased evaporation, oxygen isotopes of the ocean water get heavier because the lighter isotopes evaporate to a greater extent than the heavy isotopes do.
So, if we can figure out what the temperature of the water was at the time the fossil formed, which we can do by looking at other chemical properties of the shell/skeleton such as the ratio of the elements strontium to magnesium in coral fossils, we can then essentially factor out the influence of temperature on the oxygen isotopes in that coral fossil and we know that the remaining changes in the oxygen isotopes of that coral are related to salinity. We can therefore figure out what the salinity in that part of the ocean (where the fossil formed) was in the past and how it changed through time.
Good question! One way to look at changes in salinity in the geologic past is to look at oxygen isotopes in shells and skeletons of marine fossils, including in bivalves, corals, and foraminifera (which are tiny marine organisms) that have been preserved in the geologic record somehow (either in a rock formation, in coral reefs, in ocean sediment etc.). An isotope is an atom with the same number of protons but different number of neutrons as a different atom of the same element. For example, an atom of oxygen has eight protons (that’s what makes it oxygen) but can have 8, 9, or 10 neutrons. Because of the different number of neutrons, different isotopes have different masses. The ratio of heavy (i.e., oxygen atoms with 10 neutrons) to light (i.e., oxygen atoms with 8 neutrons) in a marine fossil shell/skeleton is dependent on both the oxygen isotopes and the temperature of the water during the time that the shell/skeleton of the fossil formed (i.e., when the organisms was living in the geologic past). The oxygen isotopes of ocean water have a very close relationship with salinity because both are influenced by the amount of precipitation and evaporation occurring in the ocean: if there is increased evaporation, the ocean water gets saltier because the water evaporates but the salt does not. Similarly, with increased evaporation, oxygen isotopes of the ocean water get heavier because the lighter isotopes evaporate to a greater extent than the heavy isotopes do.
So, if we can figure out what the temperature of the water was at the time the fossil formed, which we can do by looking at other chemical properties of the shell/skeleton such as the ratio of the elements strontium to magnesium in coral fossils, we can then essentially factor out the influence of temperature on the oxygen isotopes in that coral fossil and we know that the remaining changes in the oxygen isotopes of that coral are related to salinity. We can therefore figure out what the salinity in that part of the ocean (where the fossil formed) was in the past and how it changed through time.