HCO3 marine carbonate

Figure 10.12. A. Carbon isotopic composition of major groups of higher plants and autotrophic microorganisms compared with oxidized carbon (CO2, HCO3, CC>32 )- The triangles are mean values. (After Holser et al 1988 Schidlowski, 1988). B. Sulfur isotopic composition of bacteriogenic sulfide in modern marine anaerobic sediments (open bars, 1-6) and in the Permian Kupferschiefer (black bar, 7) compared with oxidized sulfate of modem (1-6) and Permian seawater (+11 %o). The black triangles are mean values. (After Holser et al., 1988.)...

The influence of continental carbon weathering on marine HCO3 5 C values can be evaluated using Equations (3) and (8). The overall carbon isotopic value of weathering products can be estimated using the relationship ... 

A brief summary of the effect of several carbon weathering-related factors on the 5 C value of the marine HCO3 reservoir is given in Figure 5. 

The impact of methane clathrate formation or release on the 5 C value of the marine HCO3 can be evaluated using a modification of Equation (8). Biologically-produced methane typically has values between -110 to -50%o vs PDB (Schoell 1988), with a typical value of —65%o (Cicerone and Oremland 1988). This is substantially lower than the average 5 C value of organic carbon produced by photosynthesis, which is —25%o. Incorporating methane burial into Equation (8) therefore requires modification of the term Ab. This can be done through the simple expression ... 

Oceans play a significant role in the exchange of CO2 between the atmosphere and biosphere. Carbon isotope exchange between CO2 in the air and HCO3 in the oceans results in C enrichment of the latter. CO2 in air is well mixed globally and currently has a <5 C value of - 84 V-PDB compared to marine limestones at 04 V-PDB. Freshwater carbonates have much more variable and lower values due to the oxidation of C depleted organic matter and subsequent formation of HCOj. 

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