1. Is gas transfer at the ocean’s air-water interface a good place to focus my energy? Or are interfaces at lakes and/or rivers a better focus? [this question will not be answered here]

2. Is the rate of exchange of CO2 across the ocean’s interface a limiting process in the global CO2 cycle on long time scales? [this is one of the core questions that you had with Melack]

3. When, where, and how is my research on gas transfer able to help the environmental sciences?

Guiding Questions

Evidence supporting the idea that mixing of intermediate/deep waters with the surface layer of the ocean is the process controlling the rate of carbon sequestration into the ocean:

by citation

(Schlesinger, 1997, page 361) arguments

(Schlesinger, 1997, page 316) arguments

(Keeling CD, 1983) arguments

(Broecker, page 150) arguments

(Broecker, page 525-530) arguments

Let’s analyze question 2 [but first define “long” time scales to be those related to humankind induced climate changes, so 10 to 10,000 years]

Evidence supporting the idea that the interfacial gas exchange rate limits the rate of carbon sequestration in the oceans:

by citation

(Java Climate Model, http://www.climate.unibe.ch/jcm/) arguments

(Takahashi et al, 2002 and Oceanography Mag. JGOFS, 2001) arguments

Let’s analyze question 2

1) Knowing kl allows calculation of F, if delta-pCO2 is known. It may be possible to measure kl from satellite radar. This may someday allow routine measurements of F that are more spatially and temporally frequent. “Current global carbon models generally agree with the total inventory estimates, but discrepancies still exist in the regional distribution of the anthropogenic inventories” (Broecker,1982)

2) The knowledge of F on global oceans is not only important to oceanic carbon sequestration, but also to (Battle et al 2000 SCIENCE state the importance of uncertainty in air sea gas exchange in the middle of the third column.) (Gruber, 2001 need the exchange coefficient to use d13C as a method of calculation of anthropogenic C. This state of the art article uses a parameterization that is only a function of wind speed, obviously in error.)

3) There are needs to measure gas transfer out of inland water bodies for the purpose of global carbon cycle. For example, Richey 2002,

4) Reaeration of rivers with respect to O2 in polluted settings to avoid fish kills.

5) Transfer of pollutants such as mercury PCB PAHs, etc.. Lots of examples.

6) Transfer of heat (heat is often limited by transfer on the air side, but in very windy situations this may not be the case)

Let’s analyze question 3

1) Since there is no “proven” theory of gas transfer into liquids, there exists much diversity of opinion among people who measure gas transfer. This scatters peoples efforts. There is also no good method for measuring gas transfer in large water bodies. A solution to either of these problems would be very useful to the environmental sciences.

2) There is growing interest in using diel dissolved oxygen changes as a measurement of whole ecosystem metabolism. (Kalfs book page 227)

3) DO in lakes also affects the redox potential, in turn affecting the fate and transport of phosphorus, other nutrients, nutrient metals, toxic metals.

4) CO2 loss in lakes causes CaCO3 precipitation and “whiting” events. (Kalfs book)

Let’s analyze question 3 more

(Schlesinger, 1997, page 361) For his argument, Schlesinger calculates a residence time for carbon in the surface layer of the ocean to be 11 years, due to exchange with the atmosphere. The same calculation for the deep water of the ocean gives 350 years, due to exchange with the surface layer. “Thus, the uptake of CO2 by the ocean is constrained by mixing of surface and deep waters – not by the rate of dissolution of CO2 across the surface”. This argument assumes the surface layer and the deep ocean are, individually, well mixed reservoirs with homogenous chemical/physical/biological properties. This is not true, in fact the two bodies of water have complicated and interacting dynamics, e.g. the thermohaline circulation.

(Schlesinger, 1997, page 316) “[The] surface ocean provides only a limited volume for CO2 uptake, and the atmosphere is not in immediate contact with the much larger volume of the deep ocean.” In some regions of the ocean the coupling between the deep ocean and the atmosphere is strong, e.g. North Atlantic or Southern Oceans. In regions such as these the surface-deep water mixing is so strong as to cause limitation by gas exchange across the air-water interface.

Arguments

(Keeling CD, 1983) Keeling calculates the gas transfer across the global oceans, if the ocean is entirely well mixed, to be 6 Pg C / yr. The point is that this number is larger than the nominal 2 Pg C / yr. This is interpreted (by Schlesinger, page 316) as proof that gas exchange across the interface is not the limiting process. Even if interfacial gas transfer is not the only limiting process (where we assume the 6 Pg C / yr calculation is accurate), it still could be a limiting process in key regions.

(Broecker, page 150) Equilibrium time of CO2 in surface layer is about a year. The surface layer “never quite catches up with the processes tending to disrupt this equilibrium.”

Arguments

(Broecker, page 525-530) Broecker uses the “Oeschger, H.” model to show numerically that the ocean surface layer has pCO2 at “only 0.86 the equilibrium amount of fossil fuel CO2.” But the numerical model is very simple, and calculates a carbon flux into the ocean that is low. Broecker says this could be due to the simplistic one dimensional Oeschger model which doesn’t account for regions of outcropping isopycnals, where down-welling occurs.

(Java Climate Model, http://www.climate.unibe.ch/jcm/) This model uses the Bern Model, a more complicated Oeschger model which uses a down-welling region in the ocean, the same model used by the IPCC 2001 report. It includes average air-sea gas exchange rate as a parameter. One can vary this parameter and see that the air-sea gas exchange rate has a small, ~1 to 10%, effect on global carbon cycle evolution over 10s to 100s of years.

Arguments

(Takahashi et al, 2002 and Oceanography Mag. 2001) The ocean (theoretically) will take up C in a “permanent” sense in waters that are freshly brought up to the surface from the deep, and in waters about to be plunged to the bottom. Waters freshly brought to the surface are depleted in C, since they formed in an atmosphere that is thousands of years old, with lower C concentrations. These waters will stay at the surface for long enough for wind mixing to not make a difference in the magnitude of the C they absorb. Waters about to be plunged to the bottom, on the other hand, will carry C down with them. The magnitude of this sink of C depends on the [C] at the time of sinking. This [C] will probably depend on photosynthesis and wind forcing at the surface. The distance of [C] from equilibrium of North Atlantic Waters can be up to 75% out of equilibrium with the atmosphere. However, most anthropogenic C is taken up homogenously in the open ocean. This is because the atmospheric pCO2 is ramping GLOBALY. Thus, open ocean waters should continue to take up most of the anthropogenic C, as long at atmospheric pCO2 is ramping. This open ocean water is strongly stratified. Now the question is, is this open ocean water in equilibrium with the atmosphere. The answer is NO, the delta pCO2 can be as large as 60% of atmospheric pCO2, with variability and sign driven mainly by temperature changes. The fact of the matter is, wind forcing does limit the response of the ocean to ramping atmospheric pCO2. Think of extreme examples. If the exchange of CO2 between the atmosphere and surface waters was extremely strong, then surface waters would always be in equilibrium with the atmosphere….

Stratified surface layer open ocean waters is where all the anthropogenic C is going. These same waters are up to 60% out of equilibrium with the atmosphere at the surface, sometimes overpressured and underpressured, due to temperature changes. Thus, the wind speed forcing is the factor that modulates how much C gets into these perturbed systems.

Arguments

• “the equatorial Pacific is a strong … of CO2 to the atmosphere throughout the year as a result of the upwelling and vertical mixing of deep waters in the central and eastern regions of the equatorial zone….Strong convective mixing that brings up deep waters rich in CO2 produces the net release of CO2 in the subarctic Pacific. The effect of increased DIC concentration surpasses the [effect of the cooling of the water] during winter……and on and on… These observations point out that the deltapCO2 in high-latitude oceans is governed primarily by deep-water upwelling in winter and biological uptake in spring and summer, whereas in the temperate and subtropical oceans, the deltapCO2 is governed primarily by water temperature….” [Oceanography JGOFS, p. 23]

• “Since effects other than wind speed have not been well quantified, the processes controlling gas transfer have been parameterized solely with wind speed, in large part because k is strongly dependent on wind, and global and regional wind-speed data are readily available.” [Oceanography JGOFS, p.23]

• The quadratic relationship of Wanninkhof (1992) was constructed to follow the general shape of curves derived in wind-wave tanks but adjusted so that the global mean transfer velocity corresponds with the long-term global average gas transfer velocity deter-mined from the invasion of bomb14C into the ocean.Because the bomb14C is also used as a diagnostic or tuning parameter in global ocean biogeochemical circulation models, this parameterization yields internally consistent results when used with these models, making it one of the more favored parameterizations.” [Oceanography JGOFS, p.23]

• “The global oceanic CO2 uptake using different wind speed/gas transfer velocity parameterizations differs by a factor of three (Table1). The wide range of global CO2 fluxes for the different relationships illustrates the large range of results and assumptions that are used to produce these relationships. Aside from differences in global oceanic CO2 uptake, there are also significant regional differences. Figure 5 shows that the relation-ship of W&M-99 yields systematically lower evasionrates in the equatorial region and higher uptake rates at high latitudes compared with W-92, leading to signifi-cantly larger global CO2 uptake estimates.”[Donelan] But I thought the k models were normalized to the same global bomb 14C data, so they shouldn’t differ at all when integrated globally?

Notes:

•“Our present understanding of the temporal and spatial distribution of net CO2 flux into or out of the ocean is derived from a combination of field data, shich is limited by sparse temporal and spatial coverage, and model results, which are validated by comparisons with the observed disributions of tracers, …natural 14C … 3He … chlorofluorocarbons… and bomb 14C.” [Oceanography JGOFS, p. 18]

•“the pCO2 in surface swawater isknown to vary geographically and seasonally over a range .. 150 uatm and 750 uatm … 60% below and 100% above current atmospheric pCO2 of 370 uatm…….The average pCO2 of the global ocean is about 7 uatm lower than the atmosphere,…In a parcel of seawater with constant chemical composition, pCO2 would increase by a factor of 4 when the water is warmed from polar temperatures of about –1.9 oC to equatorial temperatures of about 30 oC..[However] the DIC ….2150 umol kg-1 in polar regions to 1850 umol kg-1 in the tropics….should reduce pCO2 by a factor of 4…. The two effects are often compensating… balancing…” [Oceanography JGOFS, p. 22]

•“waters in subtropical gyres mix vertically at [relatively] slow rates with [deep water]… As a result, they are in contact with the atmosphere and can exchange CO2 for a long time. Consequently, the pCO2 in these warm waters follows the increasing trend of atmospheric pCO2 conc… Accordingly, the pCO2… in a given year is corrected to the same month in the reference year 1995 using … In contrast to the waters of the subtropical gyres, surface waters in high-latitude regions are mixed convectively [relatively quickly] with deep waters following fall and winter, and their CO2 properties tend to remain unchanged from year to year…. Thus no correction is needed from year to year…”

•http://cdiac.esd.ornl.gov/oceans/ndp_076/intro076.html and http://cdiac.esd.ornl.gov/oceans/ndp_076/result076.html give the TIC below the mixed layer. The authors state that TIC above the mixed layer has too much variability to report mean values for.

•http://cdiac.esd.ornl.gov/oceans/ndp_076/ndp076fig1.htmlMAXIMUM DEPTH OF MIXED LAYER

•http://cdiac.esd.ornl.gov/oceans/ndp_076/ndp076fig2.html TIC BELOW MIXED LAYER

•http://cdiac.esd.ornl.gov/oceans/ndp_076/ndp076fig3.html Total Alkalinity below MIXED LAYER

Data from JGOFS WOCE http://cdiac.ornl.gov/oceans/home.html

Data from JGOFS WOCE http://cdiac.ornl.gov/oceans/home.html

At a PH of 8 and temp near 10 C, water in equilibrium with an atmosphere of 370 microatm has a total DIC concentration of ~

Refs• Broecker WS, Tracers in the Sea, 1982

• Broecker WS, The distribution of bomb radiocarbon in the ocean,..

• Millero F and Sohn M, Chemical Oceanography, 1991

• Takahashi 2002

• Keeling, 1983. The global carbon cycle: What we know and could know from atmospheric, biospheric, and oceanic observations. Pp. II.3-62 In Proceedings: Carbon dioxide research conference: carbon dioxide, science, and consensus. CONF 820970 US DOE Washington DC

• Keeling, 1996

• Battle, 2000

• [1] Oceanography, JGOFS, 2001

• Schlesinger W.H., Biogeochemistry: An analysis of Global Change. London, 1997

• Donelan, M. A. and R. H. Wanninkhof (2002). Gas Transfer at Water Surfaces - Concepts and Issues. Gas Transfer At Water Surfaces. M. A. Donelan, W. M. Drennan, E. S. Saltzman and R. H. Wanninkhof. Washington D.C., American Geophysical Union: 1.

•

•Ar

•CO2

•CH4

•Dimethyl Sulfide:

•H20

•N20

•N2

•O2

•O3

•PCBs and other phenolic pollutants

The conclusions we are coming to are applicable to ANY sparingly soluble gas that does not undergo a reaction in the surface film layer.