3.1 Seawater Chemistry
Understanding ocean chemistry is fundamental to oceanography. Seawater is a complex solution containing dissolved salts, gases, nutrients, and organic matter that drive marine biogeochemical cycles.
Salinity
\( S = \frac{\text{mass of dissolved salts}}{\text{mass of seawater}} \times 1000 \)
Average ocean salinity: 35 PSU (practical salinity units)
High Salinity
Red Sea (~40 PSU), Mediterranean (~38 PSU) - high evaporation
Low Salinity
Baltic Sea (~7 PSU), near river mouths - freshwater input
pH and the Carbonate System
Ocean pH averages 8.1 (slightly alkaline). The carbonate system buffers pH:
\( \text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^- \rightleftharpoons 2\text{H}^+ + \text{CO}_3^{2-} \)
Python: Salinity Calculator
#!/usr/bin/env python3
"""seawater_chemistry.py - Ocean chemistry calculations"""
import numpy as np
def density_seawater(T, S, P=0):
"""
Calculate seawater density (kg/m³)
T: temperature (°C), S: salinity (PSU), P: pressure (dbar)
"""
# UNESCO 1983 equation of state (simplified)
rho_0 = 999.842594 + 6.793952e-2*T - 9.095290e-3*T**2
rho_0 += 1.001685e-4*T**3 - 1.120083e-6*T**4 + 6.536332e-9*T**5
A = 8.24493e-1 - 4.0899e-3*T + 7.6438e-5*T**2
A += -8.2467e-7*T**3 + 5.3875e-9*T**4
B = -5.72466e-3 + 1.0227e-4*T - 1.6546e-6*T**2
C = 4.8314e-4
rho = rho_0 + A*S + B*S**1.5 + C*S**2
return rho
# Example
T, S = 20, 35 # 20°C, 35 PSU
print(f"Density at T={T}°C, S={S}: {density_seawater(T,S):.2f} kg/m³")