Calcified rivulariaceans from the Ordovician of the Tarim Basin, Northwest China, Phanerozoic lagoonal examples, and possible controlling factors

A distinctive association of rivulariacean-like calcified microfossils is recognized in back-reef lagoon facies on theBachu-Tazhong Platform in the Lianglitag Formation (Katian Stage, Upper Ordovician) of the Tarim Basin, basedon investigation of 4500 thin sections from 35 well drill cores. The genera includeHedstroemia,Ortonella,Zonotrichites,Cayeuxia, andGarwoodia, most of which have features comparable with present-day calcifiedcyanobacteria such asRivularia,CalothrixandDichothrix(Rivulariaceae, Nostocales). A similar association is pres-ent in lagoonal and other restricted nearshore shallow-marine carbonate environments during much of the Pa-leozoic and Mesozoic. This suggests the sustained presence of a rivulariacean-dominated cyanobacterialassociation characteristic of back-reef/lagoonal environments. At the present-day, uncalcifiedRivularia,CalothrixandDichothrixremain common in back-reef, lagoon, mangrove-swamp, rocky shore, salt-marsh, and saline lakeenvironments. The ability of these cyanobacteriato grow in environmentslow in inorganic nitrate and phosphatecould help to explain this distribution. Cenozoic decline in marine calcified rivulariaceans is attributed to globalreduction of seawater carbonate saturation state. The Phanerozoic record of calcified rivulariacean cyanobacteriaappears to sensitively reflect long-term variations in the carbonate and nutrient chemistry of marineenvironments