Ground truth · a balance in blue water
The Water Leaves, the Salt Stays
Rivers carry dissolved rock into the sea. That part of the childhood answer is true. The missing half is the door out: particles, shells, altered seafloor and evaporite rock remove ions too. The present ocean is roughly balanced, and each ion keeps its own clock.
Ask how long it stays
Choose one chemical species. The bench divides its estimated ocean inventory by its effective river input flux. Exactly the same arithmetic turns aluminium into a visitor and chloride into a geological resident.
14,850,000 Pg / 0.270 Pg per year = 55,000,000 years
River water is not young seawater
These rounded shares include an explicit other category. Sodium and chloride together move from 16% of river dissolved solids to 85.7% in seawater. Bicarbonate moves from 48% to 0.14%, because carbonate chemistry, organisms and minerals remove it.
At 1x, Na + Cl is still 16.0% of dissolved solids. Multiplying everything cannot make 85.7%.
The check
The relation is residence time = reservoir amount / flux. Sodium: 14,850,000 Pg / 0.270 Pg per year = 55 million years. Chloride: 26,535,000 Pg / 0.305 Pg per year = 87 million years. Iron: 0.0048 Pg / 0.0000137142857 Pg per year = 350 years. Aluminium: 2.74 Pg / 0.0274 Pg per year = 100 years. Published estimates differ: sodium is reported near 55, 68 or 80 million years; chloride near 87 or 100 million; iron near 200 to 500 years. The bench uses named anchors, not unique true values.
NOAA estimates that rivers carry 4 billion tons of dissolved salts to the ocean each year and says roughly comparable removal probably balances it. Typical open-ocean salinity is about 35 g/kg. River flux includes recycled sea spray, while hydrothermal exchange and many sinks complicate attribution. Much chloride is understood as tracing interior degassing rather than rock weathering, but this instrument does not fix the split.
Apparatus, sources and what is not proven
The residence-time anchors and formula come from Chamberlin, Shaw and Rich, Our World Ocean, section 8.9 (2023), corroborated by the UCLA EPSS 15 teaching table. Composition anchors come from Encyclopaedia Britannica, "Seawater". The balance and 4-billion-tonne estimate come from NOAA National Ocean Service. The Messinian dates and 7 to 10% salt-giant extraction range come from Krijgsman et al., Nature Reviews Earth & Environment (2024). Dittmar's 1884 Challenger analysis established near-constant major-salt proportions in open-ocean water; it does not mean every estuary matches. Rubey (1951) is the historical source for the excess-volatiles argument.
The inventory and flux pairs are a computational representation of published residence-time anchors, not a newly reconciled modern flux dataset. The evaporation slider holds chemistry fixed and therefore proves only that scalar concentration cannot change composition shares. It does not model precipitation, biological uptake, carbonate equilibria or hydrothermal reactions. Early ocean salinity changed, present salinity varies in space and time, and modern regional trends are a separate water-cycle question.