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The water column : Physics , Nutrients , Organic matter

Sediments and interstitial waters : Granulometry, Physical and chemical characteristics of  interstitial waters, Nutrients fluxes, Organic matter mineralization

Mineralization of the organic matter (OM)

L. Charpy

 

Hypothesis 

 

A fundamental hypothesis of diagenetic models such as those used by Berner (1977) or Rabouille and Gaillard  (1990) is the existence of a steady state within the pore water composition. A very active bioturbation is not compatible with this hypothesis, but, the main geochemical processes being now identified, it is possible to quantify the amount of OM mineralized at station 8 through the observed concentration profiles and a simple stoichiometric model.

 

Station 8

 

 

Stoichiometric model

 

The oxidation réactions 

 

At the very top of the sedimentary column (a few millimeters below the interface) dissolved oxygen may diffuse from the water column and part of the Organic Matter (OM) was oxidized according to the following reaction (1) :

"CH2O"* + O2 →  CO2 + H2O         (1)      

The OM can be oxidized by sulfate, in a first approximation, according to the following reaction (3):

"CH2O" + ½ SO4= + H + → ½ H2S + CO2 + H2O      (3)  

*  "CH2O" is OM

 

Reactions (1) and (3) can be rewritten more accurately by taking into account the contributions of nitrogen and phosphorus :

  • In the oxygenated 1 cm top layer :

(CH2O)x (NH3)y H3PO4 + xO2 → xCO2 + yNH3 + H3PO4 + xH2O  (4)  

  • And below :

(CH2O)x (NH3)y H3PO4 + x/2 SO4= + xH+ → x/2 H2S + xCO2 + yNH3 + H3PO4 + xH2O  (5)  

  • For the whole profile : 

CaCO3 + CO2 + H2O ↔ Ca++ + 2HCO3-  (2)  

  • For the total dissolved CO2 

  • Within the oxygenated layer we can write :   

ΔΣCO2 = ΔO2 + ΔCa  (6)

  • And below : 

ΔΣCO2 = ΔH2S + ΔCa

  • For the alcalinity :

  • Within the oxygenated layer we can write : 

ΔAlk = (1-y)/x ΔO2 + 2ΔCa  (7)

In equation (7), the borate contribution is supposed to be constant and the contribution of H3SiO4-  is neglected.

  • And below : 

ΔAlk = 2(1-y-1)/x ΔH2S + 2ΔCa

In order to improve the calculation of Alk, the profile of dissolved sulfide (H2S) has been fitted with a polynomial function. Therefore, it is possible to get a good estimation of H2S for each depth increment z.

 

Theorical profile of alkalinity

The stoichiometric modeling will be applied only to generate a theoretical alkalinity profile. 

 

Station 8

The experimental alkalinity profile is presented on the fig. for station 8 together with the calculated one. The agreement between observed and calculated values is fairly good when the C:N ratio is set to 9.6. This high ratio compared to the Redfield et al. (1963) ratio (6.6), reflects that The OM oxidized within the sediment is a mixture containing phytoplankton and an other type of OM with a lower C:N ratio which could be a contribution of dead benthic material .

 

Diagenic reactions and chemical equilibrium

 

Quantitative estimation of the diagenetic reactions

We can estimate the amount of OM mineralized by the two main oxidation reactions from equation (4) and (5).

  • In the upper centimeter:

ΔOM/ΔO2 = 1/x. If x = 106 and O2 = 187 µM, then OM = 1.8 µM. In the top centimeter of the sediment the porosity = 0.62. Assuming a density of 2.5 g/cm3 for coral sand and 1.025  g/cm3 for sea water, then we find that 2.5 mg of OM is oxidized per kg of total sediment (solid + pore water).

  • Below and down to 34 cm below the interface

ΔOM/ΔO2 = 2/x. Si x = 106 et H2S = 116 µM, then OM = 2.2 µM. With an average porosity value = 0.57 (between 1 and 34 cm below the interface) the same calculation gives 2.7 mg of OM per kg of total sediment.

  • For the whole sediment

The extent of the mineralization processes is restricted to 5.2 mg of organic material per kg of sediment (5.2 ppm). This value seems very low compared to the results obtained by the litterature. Our result emphasizes the oligotrophic status of the global lagoonal ecosystem.

Chemical equilibrium between the aragonitic coral sand and the pore water

To determine if the coupled reaction of dissolution-precipitation of the calcium carbonate occurs within equilibrium conditions, a saturation index (SI) have been calculated with respect to the most probable carbonate phase, the aragonite and alternatively for calcite.

SI = [Ca] [CO3] / Ks

[Ca] and [CO3] represent the total dissolved calcium and carbonate respectively, 

Ks = stoichiometric constant of solubility

  • The values of Ks (Mucci 1983) are 6.34 10-7 and 4.23 10-7 for aragonite and calcite respectively, at T = 301 K and S = 35 psu.

  • [Ca] is directly given by analysis.

  • [CO3] must be calculated from the values of the carbonate alkalinity and the pH:

[CO3] = Alkc / ( 2 + 10 (pK2 – pH) )

pK2 is the second dissociation constant of H2CO3. 

With a value of SI very close to 1, an equilibrium between the pore water and the sediment can be inferred. With SI much greater than 1, the solution is supersaturated with respect to a solid phase which can precipitate. The opposite situation, when SI is smaller than 1, reflects an under-saturation where the solid phase considered can dissolve.

 

The figure shows that the whole pore water profile is supersaturated with respect to both carbonates. The highest supersaturation occurs within the first top centimeter (SI = 5.5 and 8.2 for aragonite and calcite respectively at the SWI). This confirms the hypothesis that the oxidation of organic matter by dissolved oxygen leads to the dissolution of coral sand, while this dissolution does not appear on the calcium profile. Downward, supersaturation decreases but remains close to SI = 2 for aragonite. Then, when oxidation of the OM is controlled by sulfate, the carbonate phase precipitates and refrains any pH increase.

 

Conclusions

 

A great part of the organic matter which reaches the sediment is oxidized in the water column. The organic matter oxidized within the sediment is a mixture of sedimented and benthic material. The amount of  OM mineralized within the sediment is 2.5 mg per kg in the upper centimeter and 5.2 mg per kg below. Interstitial waters are surpersaturated with respect to aragonite and calcite. The kinetics of the carbonate dissolution are faster than the precipitation.

 

 

This page was based on :

Charpy-Roubaud C., Charpy L., Sarazin G. (1996) Diffusional nutrient fluxes at the sediment-water interface and organic matter mineralization in an atoll lagoon (Tikehau, Tuamotu Archipelago, French Polynesia). Mar Ecol. Progr. ser. 132: 181-190

 

References :

Berner RA (1977) Stoichiometric models for nutrient regeneration. Limnol Oceanogr 29: 781-786

Mucci A (1983) The solubility of calcite and aragonite in sea water at various salinities, temperatures and one atmosphere total pressure. Am J Sci 285 : 780-799

Rabouille C, Gaillard JF (1990) The validity of steady-state flux calculations in early diagenesis : a computer simulation of deep-sea silica diagenesis. Deep Sea Res 37 : 625-646

Redfield AC, Ketchum BH, Richards FA (1963) The influence of organisms on the composition of sea water. In : Mac Graw Hill (ed) The Sea, Vol II p 26-77

 

update : 07/10/08

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