C in Marine Organic Matter

An important application of carbon stable isotope ratios lies in the reconstruction of the carbon dioxide concentration in surface waters and subsequently in the atmosphere at the time the organic matter was produced. Experimental and field studies have shown that the fractionation of stable car- 

For algae where CO2 is thought to reach the photosynthetic site only by passive diffusion, p can be calculated by the following equation (Bidigare et al. 1997)  

To obtain C02(aq), Equation 10.9 has to be rearranged, and values for p are determined from the carbon isotopic compositions of the primary photosynthate 5 Cp (%o) and of the ambient dissolved molecular carbon dioxide 5 Cd (%o) (Fig. 10.6)  

Assuming that diagenetic isotopic alterations can be neglected (see below), 8 Cp may be directly substituted by the measured 8 Corg value. 

5 C of calcite tests assuming that the calcite was precipitated in equilibrium with the ECO2, correcting for the temperature-dependent fractionation Sb(a) between C02(aq) and dissolved bicarbonate according to Mook et al. (1974)  

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Hayes J. M., Strauss H., and Kaufman A. J. (1999) The abundance of C in marine organic matter and isotopic fractionation in the global biogeochemical cycle of carbon during the past 800 Ma. Chem. Geol 161, 103-125. 

Wakeham, S.G., C. Lee, J.I. Hedges, P.J. Hernes, and M.L. Peterson. 1997. Molecular indicators of diagenetic status in marine organic matter. Geochimica et Cosmochimica Acta 61 5363-5369. 

In coastal areas, measurements of 5 C in bulk organic matter can help idenhfy the origins of organic material in sediments. In general, material produced using the dominant C3 photos5mthehc pathway has a value of 5 C around -27 %o for terrestrial matter and around -20 %o for marine matter (Deines, 1980). The interpretation of such results is complicated because some plants use the C4 photosynthetic pathway, which... 

Wakeham, S. G., Lee, C., Hedges, J. I., Hernes, P. J., and Peterson, M. (1997). Molecular indicators of diagenetic status in marine organic matter. Geochim. Cosmochim. Acta 61, 5363-5369. 

The other major reactant in Equation 1 is sulfate (SO42 ). Sulfate concentrations are highly variable in lake waters, from 3 x 10 5 mol/L in soft-water lakes in crystalline-rock drainage basins to 1.6 mol/L in hypersaline lakes (2.). In productive, freshwater lakes, sulfate reduction typically goes nearly to completion (5.). As sulfate concentrations increase, amounts of organic matter eventually become insufficient for complete sulfate reduction to occur. This is the case in "normal" marine sediment where a linear relation between total reduced sulfur and organic-carbon concentrations is observed. Sea-water sulfate concentration is 0.028 mol/L and the ratio of total reduced sulfur to organic-carbon concentrations (often referred to as S/C) in marine sediment is 0.33 ( ). ... 

Reaves C.M. (1986) Organic matter metabolizability and calcium carbonate dissolution in nearshore marine muds. J. Sediment. Petrol. 56, 486-494. 

Minor, E. C., and Eglinton, T. I. (1999). Molecular-level variations in particulate organic matter subclasses along the Mid-Atlantic Bight. Marine Chemistry 67(1), 103-122. 

Figure 3 Loss of terrestrial OC in deltaic systems, (a) Organic carbon to mineral surface area ratio (OC SA) plotted against bulk stable carbon isotopic compositions for riverine suspended sediments (closed symbols) and deltaic surface sediments (open symbols). A shift to lower OC SA values indicates net loss of organic matter, and a shift to heavier (i.e., C-enriched) isotopic compositions indicates increasing contributions from marine organic matter, (b) The average ( 1 SD) total amount of terrestrial OC persisting in deltaic sediments, based on the changes in OC SA and composition between river suspended sediments and deltaic sediments for four river systems...

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