Every one of these elements is present at some concentration in seawater. As you have seen, some elements have high concentrations (e.g. Na, Cl) whereas others (e.g. Au or Fe, etc.) have very low concentrations. Very few elements are near saturation (the maximum amount that could be held in seawater of a certain salinity, temperature, and pressure). The chemistry and behavior of elements differ among the various groups (for example redox-sensitive metals vs. alkaline earths).
Residence time is the average time that a substance remains in solution in seawater. It can be calculated for any element by a standard equation. Note that this is cast in terms of the riverine input only (Activity 2 will ask you why this could be incorrect):
Residence Time (yrs.) = Total amount ion in seawater (kg) / Input rate (kg/yr)
where Input rate = Avg. ion conc. in rivers (kg/km3) x River discharge (km3/yr)
What is the residence time of all of the salt in seawater? This is an interesting consideration because, in the past, this question was used to argue something about the age of the Earth (How long would it take rivers to deliver all the salt in seawater today?). There are about 5 x 1022 g of dissolved solids in oceans, and rivers bring in about 2.5 x 1015 g of dissolved solids per year. Think about it. It should only take about 2 x 107 years (20 million years) to bring the oceans to their present salinity, but we know that the oceans are 3.8 billion years old, and if rivers have been providing approximately the same input through time, and if the oceans have maintained approximately the same composition through time, there has to be an output of material that balances the inputs; otherwise, we are wrong about the age of the Earth and its oceans, and that, for various reasons, seems unlikely. This question is still worth exploring with your students because it gets them to think about the dynamic Earth. Interestingly, scientist John Joly (Irish), first tried this calculation around 1901 and obtained an age for the Earth of 90-100 million years. This was too long to suit Irish Archbishop Usher's (1654) supporters who, based on biblical genealogy, believed that the Earth was created in 4004 BC.
You will calculate the residence time for several elements to gain insights into their rate of cycling through the ocean system. Think about what it means to have a long residence time vs. a short residence time. For example, we like to think of Penn State as a system. Students come in; students go out. If we simply assume that all students graduate and that the total number of students allowed at the University Park campus does not change, we can calculate the average residence time of a student at the main campus. There are about 42 thousand undergraduate students, with just over 8 thousand students admitted per year. Residence time? Just over 5 years (ouch!). Of course, we have glossed over the details, right? How many students simply left without their diplomas? You get it--it's the same for geochemical cycle considerations of residence times. We tend to simplify, thereby missing some of the important stories.