Article: Solar cyclicity in the Precambrian microfossil record

The first detailed investigation of a probable noctidiurnal growth rhythm of the tubular oscillatoriacean cyanophyte Siphonophycus inornatum Y. Zhang, in stratiform stromatolites of the mid-Proterozoic Gaoyuzhuang Formation (c. 1400-1500 ma), Hebei Province, North China, is presented herein. The sequence consists of light, thick, silica-filled layers alternating with dark, thinner, algal-rich layers. The filaments exhibit a distinct, layered pattern of horizontally orientated populations alternating with vertically orientated populations. The light layers are composed of erect filaments and are interpreted as recording phototactic, daytime algal growth; the dark layers are composed of prostrate filaments and reflect nocturnal growth. This layered, cryptalgal microfabric is quite similar to the noctidiurnal growth patterns of Phormidium hendersonii Howe filaments in modern stromatolites from the Caribbean area. In favourable conditions the daily increments recorded by fossil biolamination doublets reached as high as 600-700 ^,m. Such biolamination confirms a solar cyclicity 1400-1500 ma ago.