A report to be presented at the December AGU conference will discuss highly variable isotope fractionations observed in ALH84001:

AN: P71C-0470
TI: Variable Carbon Isotopes in ALH84001 Carbonates.

The report does suggest some possible abiotic causes but it is notable that variable isotope fractionation is a hallmark of biologically mediated fractionation. Indeed, when seen *in nature* it is usually regarded as a signature of biological activity, as described here:

SES302 Lecture 12 Stable Isotopes as Paleoclimatic and Paleontologic tools.
"Because bonds involving light isotopes are more easily broken,the lighter isotope will usually be preferentially concentrated in the reaction products when reactions do notachieve equilibrium. Large kinetic effects are associated with biologically mediated reactions (e.g., photosynthesis, bacterial reduction), because such reactions generally do notachieve equilibrium and do not go to completion (e.g., plants don't convert all CO2to or-ganic carbon)."
p.5
http://www.geo.cornell.edu/geology/classes/SES302/00notes/00SES302Lect12.pdf

Here's a report that discusses variation in carbon isotope fractionation that occurs diurnally:

Carbon isotope evidence for diurnal variations in methanogenesis in freshwater lake sediments.
Geochimica et Cosmochimica Acta, Vol. 59 (3) (1995) pp. 557-561
http://www.elsevier.com/gej-ng/10/20/36/36/38/30/abstract.html#cor1

Other biologically mediated isotopic fractionations are observed in nitrogen and hydrogen:

Stable Hydrogen and Carbon Isotope Fractionation during Microbial Toluene Degradation: Mechanistic and
Environmental Aspects.
"Biological isotope fractionation leads to unequal distribution of heavier and lighter isotopes between the educts and
products of a reaction. In most cases, the lighter isotopes are
preferentially used and the heavier isotopes are enriched in the
residual substrate fraction. This is a well-known phenomenon
for CO2 fixation during photosynthesis (25) and methanogen-esis
(11, 17) and for methane oxidation (5, 8, 36). In recent years, isotope fractionation has been described for bacterial degradation of environmental contaminants, such as chlorinated hydrocarbons (6, 10, 16) and aromatic compounds (1, 18, 21, 22, 32, 38). In addition, 13 C/12 C isotope fractionation of chlorinated and aromatic hydrocarbons was demonstrated in
contaminated aquifers (9, 16, 29), and this finding was taken as an indication of microbial degradation."
APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Oct. 2001, p. 4842-4849
http://www.uni-tuebingen.de/geo/zag/members/meckenstock/bilder/pdf/Morasch01.pdf

Both hydrogen and nitrogen have been observed in the carbonates in ALH84001:

Nitrogen and Noble Gas Isotopic Signatures in Bulk ALH84001 with Carbonates.
http://www.lpi.usra.edu/meetings/LPSC98/pdf/1825.pdf

HYDROGEN ISOTOPE STUDIES OF CARBONATE AND PHOSPHATE IN MARTIAN METEORITE ALH84001.
http://www.lpi.usra.edu/meetings/metsoc98/pdf/5275.pdf

It would be interesting to find out if there are variable hydrogen and nitrogen isotope fractionations observed in the ALH84001 carbonates as well.

Bob Clark