Question

What is the second major source of elements in seawater?

Answer 1

 After doing some research........(from 
http://oceanplasma.org/documents/chemistry.html)(works cited so im not plagiarizing)

99% of the total elements occur naturally. They are sodium ion, chloride,sulfate, magnesium ion , calcium ion , and potassium ion. other wise Every naturally-occurring element has been found in seawater.
here is a link for a picture that might help:
https://www.google.com/search?q=chemistry+of+seawater&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiZ756ZivX....:

hope this helps!

Answer 2

Answer: The second major source of elements in seawater is the interior of the earth.The elements such as calcium, phosphorous and other minerals are found deep inside the earth's interior.

The interior of earth is full of elements which comes out of the earth by the volcanoes and earthquakes.

These two phenomenon brings the nutrients into the seawater.