Table T1.
Potential sources of alteration in the 
15N signal.
Cause
|
Process
|
Effect
|
Examples of references
|
|---|---|---|---|
| Denitrification | 14N returned to the atmosphere by denitrifying bacteria. | More 15N enters marine NO3- pool, thereby limiting supply for phytoplankton. | Holmes et al. (1998; 1997; 1996); Naqvi et al. (1998); Pride et al. (1999); Francoise et al. (1993) |
| Nitrogen fixation | Cyanobacteria incorporate "new" nitrogen into system, for example. | More14N enters marine NO3- pool, thereby increasing the amount of utilizable NO3-. | Altabet and Francoise (1994); Peters et al. (1978); Karl et al. (1997); Falkowski (1997) |
| Atmospheric input | Rain incorporates more "new" nitrogen into system. | More 14N enters marine NO3- pool, thereby increasing the amount of utilizable NO3-. | Seitzinger and Sanders (1999); Capone and Carpenter (1982) |
| Terrigeneous input | By rivers, for example. | Marine NO3- pool becomes depleted/enriched depending upon contaminant source. | Holmes et al. (1998; 1997; 1996); Peters et al. (1978); Capone and Carpenter (1982); Falkowski (1997) |
| Grazing and recycling | Enrichment in 15N at each trophic level. | More 15N enters marine NO3- pool, thereby limiting supply for phytoplankton. | Bronk and Ward (1999); Altabet et al. (1991) |
| Diagenetic alteration | May occur both pre- and postburial. Preferential removal of 14N during oxidative degradation of organic matter; however, evidence has been found indicating depletion of 15N. | Will cause enrichment/depletion of 15N in both the NO3- in the nitrate pool and in the sedimentary record. | Holmes et al. (1998; 1996); Altabet and Francoise (1994); Altabet et al. (1991); Sigman et al. (1997); Farrell et al. (1995) |