(a) if the seawater carbonate ion concentration is 270 ? mol/kg, what is the approximate rate of calcification, and approximately how many days would it take 1 square meter of reef to accumulate 30 mmol of calcium carbonate (caco3)?

Using the graph provided with the question, we are able to find that that for a concentration of 270 micromol per kilogram, the calcification rate is 20 mmol calcium carbonate per meter squared per day.

Using this calcification rate,
30 = 20 * 1 * days

days = 1.5

The deposition will require 1.5 days.

Abay where the river empties into the sea