When rain falls on land it contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid (which forms from carbon dioxide and water). The rain erodes the rock and the acid breaks down the rocks and carries it along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers to the seas and oceans. Some of the dissolved ions are are removed from the water by organisms in the ocean. Others are not used up and are left for long periods of time where their concentrations increase over time. 90% of all dissolved ions present in seawater are either chloride and sodium which as we all know combine to make SALT