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Total trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn), Al, and pyrite- and reactive-associated metals were measured for the first time in a microbial mat and its underlying anoxic-sulfidic sediment collected in the saltern of Guerrero Negro (GN), Baja California Sur, Mexico. It is postulated that the formation of acid volatile sulfide (AVS) and pyrite in the area of GN could be limited by the availability of reactive Fe, as suggested by its limited abundance (mat and sediment combined average value of only 19 +/- A 10 mu mol g(-1); n = 22) as well as the low pyrite (0.89-7.9 mu mol g(-1)) and AVS (0.19-21 mu mol g(-1)) concentrations (for anoxic-sulfidic sediments), intermediate degrees of pyritization (12-50%), high degrees of sulfidization (14-100%), generally low degrees of trace metal pyritization, and slight impoverishment in total Fe. This is a surprising result considering the large potential reservoir of available Fe in the surrounding desert. Our findings suggest that pyrite formation in the cycling of trace metals in the saltern of GN is not very important and that other sedimentary phases (e.g., organic matter, carbonates) may be more important reservoirs of trace elements. Enrichment factors [EFMe = (Me/Al)(sample)/(Me/Al)(background)] of Co, Pb, and Cd were high in the mat (EFMe = 2.2 +/- A 0.4, 2.8 +/- A 1.6 and 34.5 +/- A 9.8, respectively) and even higher in the underlying sediment (EFMe = 4.7 +/- A 1.5, 14.5 +/- A 6.2 and 89 +/- A 27, respectively), but Fe was slightly impoverished (average EFFe of 0.49 +/- A 0.13 and 0.50 +/- A 0.27 in both mat and sediment). Organic carbon to pyrite-sulfur (C/S) molar ratios measured in the mat (2.9 x 10(2)-27 x 10(2)) and sediment (0.81 x 10(2)-6.6 x 10(2)) were, on average, approximately 77 times higher than those typically found in marine sediments (7.5 +/- A 2.1). These results may indicate that ancient evaporation basins or hypersaline sedimentary environments could be identified on the basis of extremely high C/S ratios (e.g., > 100) and low reactive Fe.