Organic delta/sup 13/C varies slightly with salinity in microbial mats at Guerrero Negro, Baja California, Mexico: Implications for stromatolite /sup 13/C values

Organic carbon preserved in Precambrian stromatolites has delta/sup 13/C values typically in the range -19 to -35, whereas microbial mats growing today in marine hypersaline environments have delta/sup 13/C values in the range -6 to -14. This /sup 13/C shift has been attributed to selective preservation of isotopically light organics, a decline in atmospheric CO/sub 2/ levels, or the restriction of microbial stromatolitic communities to hypersaline environments in the Phanerozoic due to grazing pressures in lower salinity environments. The salinity argument requires that the organic delta/sup 13/C of microbial mats increases with salinity, perhaps ranging from -20 to -35 in less saline water to -6 to -14 in hypersaline waters. Microbial mats inhabit those Guerrero Negro salt works ponds with salinities between 5.0 and 13.0%. Carbonate alkalinities in the ponds range from 2.7x10/sup -3/ moles/L (4%) to 3.6x10/sup -3/ moles/L (13%). Microcoleus mats' organic delta/sup 13/C vary from -12.4 to -14.2 with no clear dependence upon salinity. Aphanothece mats appear to be 1 to 2 permil enriched, relative to Microcoleus mats. The data suggest that salinity effects are insufficient by themselves to cause the /sup 13/C differences observed between Precambrian stromatolites and modern microbial mats.