Carbon Sequestration

By far the largest reservoir of biospheric Carbon is in the ocean as dissolved CO2 in balance with atmospheric CO2.

Replenishment by rivers and upwellings of nutrient in the euphotic zone, the sunlit surface layers of the ocean, enhances marine ecosystems, starting with phytoplankton blooms feeding on both nutrient and dissolved CO2.

Dissolved Carbon as CO2 is thus being continually sequestered as living biomass, corpses, "whale fall", faecal pellets and other detritus. These corpses and detritus feed deeper biomass, and much of it ultimately becomes deep sediment "exported" from the atmosphere.

Such sequestration and export of carbon from the marine ecosystem is proportional to it. And one way of enhancing the marine ecosystem is via our Nutrient Megapump, as explained under Fisheries.

So our Nutrient Megapump acts also as a Carbon sequestrator and exporter. Hence explanation in other sections about both Fisheries and Carbon Credit values of our Megapump and its artificial upwellings.


Will a Plankton Bloom cause Sequestration?

A good first guess might be that Carbon, Nitrogen and Phosphorus will be consumed according to the Redfield ratio of 108:16:1. However this does not take into account the fact that throughout most of the ocean these three elements are not found in the Redfield ratio. Carbon is invariably greatly in excess of this ratio which reflects the fact that carbon can be dissolved in the ocean by purely physical processes. Thus when a parcel of water is raised to the surface it will contain excess carbon which may be released into the atmosphere over and above that carbon which is taken up by the biota.

The situation is made even more complicated by the complex physical chemistry of CO2 in water. It reacts with water to form carbonate and bicarbonate ions. Furthermore the reaction rates of the various chemical reactions involved are strong functions of temperature. An excellent description is given by Broecker and Peng, 1982, and will not be repeated here. This complex physical chemistry is encapsulated in the widely-used computer program called CO2SYS of Lewis and Wallace, which allows concentrations and physical parameters to be read in and predictions made of unknown concentrations according to state-of-the-art physical chemistry formulae.

Oceanographic data from WOCE section P18 was input into the CO2SYS program to determine the rate at which carbon would be either sequestered or released when deep nutrients are brought to the surface by means of a hydrothermal heat engine.


Setting the Parameters

Hydrothermal vents occur on mid-oceanic ridges. Their abundance and power are related to the rate of spreading of the plates that define the ridge. For the purposes of this study we take the ridge known as the East Pacific Rise (EPR) as a suitable candidate because it has the fastest spreading rate of any large ocean ridge. Fortuitously, good oceanographic data are available from WOCE (World Ocean Circulation Experiment) section P18 which lies close to the EPR on longitude 105o W and extends from 67 deg S to 23 deg N We use this section in this study. The coloured image at the end of this report shows the distribution of nitrate along this section.

The nitrate density is important because it defines the location and parameters of the hydrothermal pump. The CO2SYS program determines what happens to a parcel of water when it is moved from an input location to an output location. For most of the calculations a depth of 800m at 15 deg S where nitrate concentration is a maximum was chosen as the input location of the pump and the surface at 15 deg S was chosen as the output location of the pump.

The various properties of the water parcel were input to the program. These included the salinity, the concentrations of phosphate and silicate, the input temperature and pressure, the output temperature and pressure, the output total alkalinity, TA, and output total carbon, TC. The program then computed the output partial pressure of CO2, pCO2. The salinity, the concentrations of P and Si and the input temperature were determined by inspection of the relevant WOCE sections.

Input TA and TC and nitrate concentrations were also determined from these sections and the output TA and TC determined from them by assuming that photosynthesis takes place and that this removes carbon and increases alkalinity. The total carbon was assumed to be removed at 6.8 times the nitrate concentration (i.e. according to the Redfield Ratio). Removal of nitrate was assumed to increase the alkalinity by an amount numerically equal to the original nitrate concentration. The computed output TA and TC were also fed to the CO2SYS program to compute the output partial pressure, pCO2.


CO2SYS Predictions

The values discussed above and the computed values of pCO2 are listed in the table below for a range of output temperatures. The saturation partial pressure of CO2 can be taken as 380 micro-atmospheres corresponding to a concentration in the atmosphere of 380 parts per million by volume as observed by various baseline monitoring stations such as that at Mauna Loa, Hawaii during 2007. The pCO2 values shown in the second last column of Table 6.1 are less than this value for all output temperatures below 27 degC, indicating that, in general, carbon will be removed from the atmosphere by the nutrient upwelling, even in tropical latitudes.

In order to arrive at a quantitative estimate of the extent of this carbon removal we need to know the total inorganic carbon concentration which would be achieved if the surface waters were saturated at the specified temperature. This too can be calculated using the CO2SYS program. The values are shown in the third last column of the table.

The amount of carbon absorbed from the atmosphere can then be calculated by taking the difference between the output total carbon and the saturation value. These difference values, dC, are shown in the final column of the table. Evidently they are strongly dependent on temperature.


The Effect of Surface Temperature

The effect of variables other than temperature was assessed using the CO2SYS program. Firstly the effect of output depth was tested by taking an output depth value of 50m. The results are shown in the second last row, (b), of the table. The output values of pCO2 and dC are almost indistinguishable from the previously calculated values for a surface output at the same temperature of 25 degC.

The values of pCO2 and dC were calculated for a temperate latitude (40 deg S) with different P, Si and nitrate values and lower surface temperature (15 degC). The results are shown in the bottom row, (c), of the table. It can be seen that the computed values (241 micro-atmospheres and 92.5 micromole per kg) are little different from the values at 15 degS for the same temperature (233 and 102.7).

InInOutOut InInInOutOutp=380
SalPSiTPrTPr TATCNO3TATCTCpCO2dC
psuÁmol/kgÁmol/kgdegCdbardegCdbar Ámol/kgÁmol/kgÁmol/kgÁmol/kgÁmol/kgÁmol/kgÁatmÁmol/kg
(a)34.536058002702320 230043236320082004386-3.8
34.536058002602320 2300432363200820133715.2
34.536058002502320 23004323632008202235714.1
34.536058002402320 23004323632008203134323.0
34.536058002302320 23004323632008203932931.9
34.536058002202320 23004323632008204831640.9
34.536058002102320 23004323632008205730349.8
34.536058002002320 23004323632008206629058.7
34.536058001502320 230043236320082110233102.7
34.536058001002320 230043236320082153184145.7
(b)34.5360580025502320 23004323632008202235714.2
(c)34.32.2304.110001502295 22003123631989208224192.5

Data values input to the CO2SYS program. Output values are shown in the last two columns. (a) 800m depth to surface at 15 deg S. (b) 800m depth to 50m depth at 15 deg S. (c)1000m to surface at 40 deg S.

Obviously the major factor controlling the degree of carbon uptake is the final temperature. Water brought up from depth by the hydrothermal pump will always have a much lower temperature than the surface water with which it is mixed and this will bring about some cooling of the final mixture. How much cooling is problematic because it depends on the degree of mixing and the subsequent heating effect due to solar insolation. The modeling of the output plume from the hydrothermal pump and of the nearby mixed layer promises to be difficult to impossible. All that can be said at this stage is that the carbon sequestration figures listed in Table 6.1 are pessimistic. Because of the cooling effect of the deeper water, more atmospheric carbon will be sequestered than has been calculated here.


Dollar Yield

The dC values listed in the table enable an estimate to be made of the total rate of carbon sequestered by a single, typical hydrothermal pump.

A spread sheet showing the dollar yield derived from three dC values corresponding to three output temperatures is shown in the table below. The carbon credit trading price of $20 per tonne has not been fixed at the time of writing this report. The figure of $20 is little more than a guess.

25 degC20 degC15 degCunit
HTBP Yield220000220000220000kg/s
C rate14.158.7102.7umol/kg
mol/umol100000010000001000000
C Yield3.10212.91422.594mol/s
C Atomic Weight121212
C Mass Yield37.224154.968271.128g/s
C Mass Yield37.224154.968271.128g/s
s/day864008640086400
C Mass Yield0.0372240.1549680.271128kg/day
day/year365365365
C Mass Yield117389648870718550293kg/year
tonne/kg0.0010.0010.001
C Mass Yield117448878550tonne/year
C Credit Price202020$A/tonne
Dollar Yield$23478$97741$171006$A/year

Spreadsheet showing carbon credit dollar yields using 3 values of dC from the previous table.


Conclusion

The phytoplankton bloom generated by a hydrothermal pump will bring about the sequestration of atmospheric carbon. The rate of sequestration is a strong function of temperature but significant income from carbon trading is possible even in tropical locations.


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