Article: Periodicities in stromatolite lamination from the early Proterozoic Hearne Formation, Great Slave Lake, Canada
Stromatolites from the Hearne Formation exhibit several orders of finely preserved laminae. This lamination has previously been interpreted in terms of daily, fortnightly, monthly, and annual periodicities, the ratios of which have been used to estimate a past rate of the Earth's rotation. Parallels between Hearne Formation stromatolites and certain Recent algal mats suggest, however, that the finest Hearne Formation laminae may reflect episodes of storm sedimentation, while the second order of lamination represents seasonal maxima in algal productivity. Visual measurements of periodicity are generally unreliable, but as adjacent laminae are mineralogically differentiated, the sequence of accretion can be recorded quantitatively by electron microprobe analysis. The chemical measurements demonstrate the antithetic relationship of Mg to Ca, Si and Al. Mg is interpreted as representing originally organic (blue-green algal) rich laminae, whereas the other three elements represent laminae originally dominated by detrital matter. Fourier spectral analysis reveals a strong second-order periodicity in Mg and Al, though with opposite phase, indicating that Al is broadly representative of the inorganic fraction of lamination. Selective filtering of the Mg time series allows first- and second-order variations to be highlighted separately, and the average ratio of the two orders, counted as peaks in the filtered time series, is found to be about seven.