3.2 Dissolved Gases
Dissolved gases in seawater are essential for marine life and play crucial roles in global biogeochemical cycles. Oxygen, carbon dioxide, and nitrogen are the most important gases in ocean chemistry.
Gas Solubility
\( C_{eq} = K_H \cdot p_{gas} \)
Henry's Law: gas concentration proportional to partial pressure
Temperature Effect
Cold water holds more dissolved gas - polar waters are oxygen-rich
Salinity Effect
Higher salinity reduces gas solubility - "salting out" effect
Oxygen in the Ocean
~5-8 mL/L
Surface concentration
OMZ
Oxygen Minimum Zone (200-1000m)
~4-6 mL/L
Deep water (cold source)
Sources: photosynthesis, air-sea exchange. Sinks: respiration, decomposition.
Carbon Dioxide
The ocean absorbs ~30% of anthropogenic CO₂. Forms carbonic acid, affecting pH (ocean acidification).
Physical Pump
Cold water absorbs CO₂, sinks at poles, carries carbon deep
Biological Pump
Phytoplankton fix CO₂, organic matter sinks, sequesters carbon
Python: O₂ Saturation
#!/usr/bin/env python3
"""dissolved_gases.py - Oxygen saturation calculation"""
import numpy as np
def oxygen_saturation(T, S):
"""
Calculate O2 saturation (mL/L) using Garcia & Gordon (1992)
T: temperature (°C), S: salinity (PSU)
"""
Ts = np.log((298.15 - T) / (273.15 + T))
A0, A1, A2 = 2.00907, 3.22014, 4.05010
A3, A4, A5 = 4.94457, -0.256847, 3.88767
B0, B1, B2, B3 = -0.00624523, -0.00737614, -0.0103410, -0.00817083
lnC = A0 + A1*Ts + A2*Ts**2 + A3*Ts**3 + A4*Ts**4 + A5*Ts**5
lnC += S * (B0 + B1*Ts + B2*Ts**2 + B3*Ts**3)
return np.exp(lnC)
# Example
for T in [5, 15, 25]:
print(f"T={T}°C, S=35: O2_sat = {oxygen_saturation(T, 35):.2f} mL/L")