Parameters affecting the stable carbon isotope ratio in CaCO3		  
 
The dynamics of the stable carbon isotope ratio 13C in calcium carbonate CaCO3 are less understood and more complex than those of 18O. 13C (CaCO3) is less directly determined by physical factors such as temperature but seems to be affected primarily by

- taxon and age specific metabolic activity, and

- primary production

  
 
Metabolic activity

Metabolism and changes in metabolism during life, such as onset of maturity, seem to play a significant role (Krantz et al. 1987) and may obscure seasonal signals. Very often we find ontogenetic trends towards lighter 13C (examples in Krantz et al. 1987) but sometime the opposite, too (Brey & Mackensen 1997).

  
 
Primary production

Primary production requires carbon dioxide CO2, which is taken from the water column. As phyto plankton selects towards the lighter 12C carbon isotope, primary production causes the 13C (CO2) of the remaining water carbonate pool to become heavier.

Hence, if primary production is seasonal as in temperate regions, we have a seasonal signal in 13C (CO2) of the water carbonate pool which should be reflected in 13C (CaCO3) of calcium carbonate skeletons produced in this water. The annual primary production oscillation is reflected in a corresponding 13C (CaCO3) oscillation in the skeleton.

  
 
Usually the water column is stratified during the productive spring/summer period and primary production is restricted to the upper euphotic layer. Only in this layer we find heavier 13C (CO2) in spring/summer, and only here calcium carbonate skeletons will reflect this pattern, i.e. heavy 13C (CaCO3) during spring/summer and lighter 13C (CaCO3) during autumn/winter (see example A).


 

 
 
In deeper water layers below the euphotic zone things are different. There is no primary production, but the opposite process may take place. Phytoplankton sedimentates from the euphotic zone and is remineralised by the food web. The degradation of the phytoplankton organic matter produces CO2 which enters the water carbonate pool at depth or even in bottom sediments. As this CO2 contains comparatively light carbon previously enriched in the plankton cells during primary production, 13C (CO2) of the water carbonate pool at depth becomes lighter.

Hence, if sedimentation is a seasonal event, we have a seasonal signal in 13C (CO2) of the water carbonate pool at depth which should be reflected in 13C (CaCO3) of calcium carbonate skeletons produced in this water. Light 13C (CaCO3) indicates strong primary production & sedimentation (see example B).








 
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Introduction
Chemistry
18O
 13C Methods