TESTING THE BIOGENICITY OF MESOPROTEROZOIC LACUSTRINE STROMATOLITES FROM THE COPPER HARBOR CONGLOMERATE, UPPER PENINSULA OF MICHIGAN

Stromatolites can form from either biogenic or abiogenic accretionary processes. Mesoproterozoic (1.1 Ga) carbonate stromatolites from the nonmarine Copper Harbor Conglomerate in the upper peninsula of Michigan have been described as biogenic although they lack internal microfossil evidence and exhibit a problematic radial-fibrous calcite that may indicate direct abiogenic precipitation. If shown to be biogenic, they will represent one of the earliest examples of terrestrial microbial activity. Therefore, it is important to determine the biogenicity of these stromatolites and refine a methodology for testing stromatolite biogenicity in the absence of microfossil evidence. Stromatolite samples were collected from four localities within the formation that represent abandoned fluvial channels within an alluvial fan system. The stromatolites are often found in thin (< 0.5 m) oolitic grainstone beds that drape clastic cobbles. They are also found in mudstone lenses that display soft-sediment deformational structures. These stromatolites contain mudstone laminations and are found draping shale rip-up clasts. One method of determining biogenicity of the stromatolites will be to measure growth angles of cobble-draping stromatolites to determine if a preferential growth direction towards sunlight occurred. A new biosignature method using magnetic susceptibility will also be used to test the biogenicity of these stromatolites. The magnetic susceptibility is based on the distribution of detrital material across single laminae. The theory behind this methodology is that biogenic stromatolites are better at trapping sediment (including magnetic minerals) than abiogenic stromatolites because of the adhesive nature of microbial mats. Petryshyn et al. (2011) found support for this in both laboratory experiments and the measured susceptibility in ancient stromatolites. If the susceptibility is similar regardless of growth angle then the hypothesis that the Copper Harbor stromatolites are biogenic will be supported. If the susceptibility varies with different growth angles then the hypothesis that the stromatolites are biogenic will not be supported. The results of this study will have important implications for interpreting the early history of life on Earth, particularly in terrestrial settings.