Experimental and field evidence support the assumption that global warming and ocean acidification is decreasing calcification rates in the oceans. Measured differences in conservation of total-alkalinity with salinity of the Red Sea surface water indicate a 26±16% decline in CaCO3 precipitation rates from 1998 to 2015 and stability during 2015-2018. Strontium-to-alkalinity ratios reveal that the net contribution of hermatypic corals to the CaCO3 budget of the southern and central Red Sea declined from ~20% to nearly 0, while pelagic calcification rates decreased by 7.5±7.5%. The decline in coral calcification rates is much larger than expected based on warming and acidification. Part of this change is possibly associated with eutrophication which can promote ecological change and enhance dissolution of calcium carbonate. Commercial fish-farming in the oligotrophic northern Gulf of Aqaba between 1997 and 2008 provided an opportunity to test the effect of long-term fertilization supplying nitrogen and phosphate on CO2 sequestration by the sea through changes in the CaCO3 cycle which I will explore using sedimentary and ecological observations.