HELIOMORPHIC GROWTH OF BIWABIK (1.85 GA) STROMATOLITES, MINNESOTA: CONSEQUENCES OF STRONG DEPENDENCE OF GROWTH RATE ON SOLAR IRRADIANCE

It is well established that the growth rate of cyanobacteria is a function of incident solar irradiance, with a broad maximum beyond which growth decreases. A simple model of surface-normal stromatolite morphogenesis is presented which exhibits such sensitivity, in order to explore whether such a model can explain observed features of the 1.85 Ga Biwabik stromatolites (northern Minnesota). The model is equivalent to that of a Kardar–Parisi–Zhang equation where vertical growth, lateral diffusion, and noise are ignored. Synthetic stromatolites are generated for a chosen paleo-latitude and paleo-obliquity starting from selected substrate shapes. Resulting forms are “heliomorphic”, and exhibit numerous features of the Biwabik stromatolites, including (1) elongation of column cross sections in the direction perpendicular to the vertical plane in which they incline,(2) a tendency for the columns to take on a “kidney bean” shape in horizontal cross section, with indentation in the direction of column tilt, (3) a tendency to quasi-periodically exhibit flat, uniformly inclined upper sides, (4) a tendency for the apical radius of curvature to smoothly vary between small values (conophyton-like) and much larger values (club-like) in vertical cross section, and (5) strong anti-correlation between column inclination and thickness. Linear stability analysis reveals that waves should propagate upward along the flanks of the model stromatolites. Such waves are indeed seen in the simulations, and similar waves are observed in cross sections of the Biwabik specimens as well. The wavelength and propagation rate of these waves may yield estimates of the dependence of growth rate on irradiance levels for ancient cyanobacteria. Flat tops always occur on the sides opposite to the direction of tilt, suggesting a possible large value of paleo-obliquity and a paleo-latitude of northern Minnesota less than the paleo-obliquity. The azimuth of inclination of the columns is uniform at 45+-3 degrees W of N in the specimens, which under this model is the direction of the paleo-equator at 1.85 Ga. This result is nearly identical to values determined using paleomagnetic data (Martanen and Pesonen, 2012).

Counts of “dark bands” after Mohr (1975) per interpreted seasonal cycle may place bounds on the lunar semi-major axis at 1.85 Ga.