1. Water and the Oceans
What are the distribution and flows of water through the Earth system?
What factors control these flows and what conditions do they influence?
What chemical processes occur as a result of water cycling and how do these processes shape the composition of oceans and other bodies of water?
How is ocean composition changing as a result of anthropogenic activities? What are the impacts of these changes on ocean life and ecosystems?

2. Important water properties
Extremely strong intermolecular interactions (hydrogen bonding)
Liquid is more dense than solid  volume minimum at 4 C
High heat capacity  stabilizes temperature swings
High melting and boiling points  water stays liquid over wide T range
Extremely good solvent of ionic and polar compounds

3. Hydrological Cycle
Estimates of the main water reservoirs in plain font (e.g. Soil moisture) are given in 103 km3 and estimates of the flows between the reservoirs in italic (e.g. Surface flow) are given in 103 km3/year. Figure from Trenberth et al. (2007) American Meteorological Society.

4. Water Distribution

5. Water availability
Available renewable fresh water in 1000 m3 per capita per year.

6. Water density Temperature Density kg/m^3 999.868 4 1000.000 10 999.728 4 10 20
Ice at 0
917

7. So What? Ice Floats Seasonal overturn of lakes
Consider a typical lake in the Summer
This is a lake
---- T, r ---->
Epilimnion (25 oC)
Thermocline
Depth
Hypolimnion (15 oC)

8. Lake in the Fall
---- T, r---->
(15 oC)
Mixing
(15 oC)

9. Winter ---- T, r ----> Epilimnion (<4 oC) Thermocline Depth
Hypolimnion (4 oC)

10. Lake in the Spring
---- T, r ---->
(4 oC)
Mixing
(4 oC)

11. What about the ocean
Due to the salts dissolved in seawater, seawater does not have a density minimum.
Cold water is more dense
Saltier water is more dense
Cold, salty water sinks

12. Thermohaline circulation

13. Ocean water composition

14. Notes on units 1 M (molar) = 1 mol L-1
ppm, ppb etc. are on a mass basis (ppmm)
1 ppm = 1 mg/kg = 1 mg/L
Assumes water density of 1 kg/L
Concentrations are sometimes expressed as concentration of an element rather than the compound
E.g., 7 ppm N as NO3-

15. Ocean water composition

16. Water exploitation index (WEI) = total freshwater abstraction / total renewable resource. >20% is “under pressure”

17. Carbonate-silicate cycle
Timescale = 105 – 107 yrs
J. F. Kasting, Science Spectra, 1995

18. Water and the Oceans
What are the distribution and flows of water through the Earth system?
What factors control these flows and what conditions do they influence?
What chemical processes occur as a result of water cycling and how do these processes shape the composition of oceans and other bodies of water?
How is ocean composition changing as a result of anthropogenic activities? What are the impacts of these changes on ocean life and ecosystems?
Specifically, how do increases in atmospheric CO2 affect the chemistry and biology of aquatic systems?

19. Forms of CO2 in water
H2CO3 (aq) ↔ H+ + HCO3-
HCO3- ↔ H+ + CO3-

20. Factors affecting CaCO3 solubility
Solubility increases as…
PCO2 increases
Salinity increases
Temperature decreases

21. Variation in CO3 species with depth
PCO2 initially increases with depth as T decreases and respiration increases
Hence, pH decreases and CaCO3 solubility increases

22. Patterns in PCO2 and CaCO3 solubility
CaCO3 is most supersaturated in surface waters
Dissolution of CaCO3 occurs at depth

23. Trends in CO2 and ocean pH

24. Forms of CO2 in water
H2CO3 (aq) ↔ H+ + HCO3-
HCO3- ↔ H+ + CO3-

25. CO2 Uptake and Ocean pH
"Since preindustrial times, the average ocean surface water pH has fallen by approximately 0.1 units, from approximately 8.21 to 8.10 (Royal Society 2005), and is expected to decrease a further 0.3–0.4 pH units (Orr et al. 2005) if atmospheric CO2 concentrations reach 800 ppmv [the projected end-of-century concentration according to the Intergovernmental Panel on Climate Change (IPCC) business-as-usual emission scenario]."
From Doney et al., 2009.
A decrease of 0.1 pH units represents a 30% increase in H+

26. CaCO3 saturation
Vertical distributions of anthropogenic CO2 concentrations in μmol kg−1 and the saturation state Ω = 1.0 horizons for aragonite (red) and calcite (white) for present (solid line) and preindustrial (dashed line) conditions along north-south transects in the (a) Atlantic, (b) Pacific, and (c) Indian Oceans as in Feely et al. (2004). Adapted with permission from AAAS.

27. https://climateinterpreter

28. Ocean acidification effects
Coral reefs are threatened by both warming temperatures and acidification, which reduces CaCO3 saturation.
Reefs grow very slowly, with the rate of calcification only slightly greater than the rate of loss. A small shift could tip the balance toward net loss of CaCO3.
In a lab experiment, a sea butterfly (pteropod) shell placed in seawater with increased acidity slowly dissolves over 45 days.

29. The effect of increasing CO2 on marine biota
"Although researchers recognized that the concentration of carbon dioxide in the surface ocean was more or less in equilibrium with overlying atmosphere CO2, they largely dismissed the potential impact [of increasing atmospheric CO2] on the ocean biota because calcite (the assumed CaCO3 mineralogy of most calcifying organisms) would remain supersaturated in the surface ocean.
Since then, multiple studies revealed several issues that elevate ocean acidification as a threat to marine biota: (a) the calcification rates of many shell-forming organisms respond to the degree of supersaturation (e.g., Smith & Buddemeier 1992, Kleypas et al. 1999); (b) aragonite, a more soluble CaCO3 mineral equally important in calcifying organisms, may become undersaturated in the surface ocean within the early 21st century (Feely & Chen 1982, Feely et al. 1988, Orr et al. 2005); and (c) the biological effects of decreasing ocean pH reach far beyond limiting calcification."
From Doney et al.

30. Effect on food webs

31. “Historically, about half of the pollution from human sources has been absorbed by the oceans and by terrestrial plants, preventing temperatures from rising as quickly as they otherwise would, scientists say…. “If the oceans and the biosphere cannot absorb as much carbon, the effect on the atmosphere could be much worse,” said Oksana Tarasova, a scientist and chief of the WMO’s Global Atmospheric Watch program, which collects data from 125 monitoring stations worldwide.”

32. Rate of CO2 increase
“The new figures for carbon dioxide were particularly surprising, showing the biggest year-over-year increase since detailed records were first compiled in the 1980s, Tarasova said in an interview. The jump of nearly three parts per million over 2012 levels was twice as large as the average increase in carbon levels in recent decades, she said.”

34. Carbonate-silicate cycle
Timescale = 105 – 107 yrs
J. F. Kasting, Science Spectra, 1995

35. Feedback on CO2
Figure 09-11
How to Build a Habitable Planet Copyright © Princeton University Press 2012