Question

The concentrations of Mg2+ and Cl- in seawater are 52.8 mmol kg-1 and 545.9 mmol kg-1 , respectively. Globally, the average concentration of Mg2+ in river water is 0.128 mmol kg-1 , while that of Cl- is 0.220 mmol kg-1 . Given a global ocean mass of 1.38 x 1021 kg and an annual river flow rate of 3.5 x 1016 kg y-1 , how much longer is the residence time of Cl- in the ocean to that of Mg2+?

Answer 1

Element	Conc in River (ppm)	Annual river Q (kg/yr)	Conc in ocean (ppm)	Ocean mass (kg)	RTD (yr)
Mg2+	13.27 (545.9*24.31 (mol.wt)/1000)	3.5*1016	1.28	1.38*1021	3803
Cl-	0.0078	3.5*1016	0.0045	1.38*1021	22747

River input of element ------> A

Amount of element in ocean ------>B

RTD----->C

A = conc.in river * Amt.of water flow from river to ocean

B = conc.in ocean * Amt. of water in ocean

C = B/A

for Mg²⁺,

A = (13.27*3.5*10¹⁶) = 4.6445 E17

B= (1.28*1.38*10²¹) = 0.38E21

RTD of Mg²⁺ is C= B/A = 3803

Similarly, RTD of Cl^- is 22747