The following article assesses the potential of ammonium as a biomarker in
Precambrian metasediments. A modern marine environment is considered to hig
hlight the processes that contribute to the isotopic composition of organic
nitrogen, notably the roles played by nitrogen-fixing organisms (cyanobact
eria) and denitrifying bacteria. The carbon and nitrogen biogeochemical cyc
les are compared and contrasted. Unlike carbon. the major nitrogen isotope
variations may be controlled by local environmental conditions, such as sea
son of the year and water depth. However. such local environmental factors
may have been superimposed on more gradual changes related to the long-term
chemical evolution of the exterior environments of the Earth. The great po
tential of nitrogen as a biomarker is that the breakdown product of organic
nitrogen (ammonium) behaves like a trace element, and is stable in high te
mperature crustal processes. Thus, ammonium may survive as a biomarker when
all other evidence for former life has been destroyed, and ammonium studie
s may be able to identify sedimentary (pelitic) components in high-grade Pr
ecambrian terrains. Previous results from Precambrian rocks are reviewed, t
o highlight that very little work has been completed to date, and that only
very general conclusions can be drawn. Much work needs to done in order to
understand the basic biogeochemistry and geochemistry of crustal nitrogen,
and it is suggested that the best way to proceed is by numerous detailed c
ase studies. The interpretation of isotope ratios in ancient sediments usua
lly assumes uniformitarianism, and the justification of this approach is co
nsidered briefly. Overall, this article highlights the under-developed pote
ntial of Precambrian ammonium biogeochemistry, and suggests area for future
research. Not only does the development of the subject have implications f
or the search for former life on Mars, but it may also open up new areas of
Precambrian research. (C) 2001 Elsevier Science B.V. All rights reserved.