Palaeoecology of enigmatic tube microfossils forming “cryptalgal” fabrics (Late Quaternary, Red Sea)

Abstract

Tubular microfossils are important sediment producers in the aphotic zone of the Red Sea. They occur on forereef slopes and as incrustations on hardgrounds of basinal carbonates as well as encrusting drowned reefs. The tubular microfossils (mean inner diameter 10 μm, outer diameter 50 μm) are straight to slightly curved and commonly branched. Because of their size and mode of calcification, they may represent remnants of fungi or cyanobacteria. Combined growth of the microfossils on protected upper surfaces of substrates and little sedimentation produce strictly geofugal cryptalgal fabrics. Although this fabric suggests phototactic growth, the tubular microfossils are eurytopic. In the Red Sea they occur between 63 and 765 m below present sea level. Within the basin the cryptalgal fabrics formed during last glacial lowstand of sea level and along the slope during Holocene rise of sea level. Limiting factors recognized so far seem to be space competition with faster growing encrusters, mainly the corallinaceans, and burial by sediments. During glacial lowstand of sea level salinity exceeded the tolerance levels of most planktonic taxa resulting in low rates of sedimentation in the basin. Along the cliffed slope sedimentation remained low during Holocene transgression. Before the mid-Mesozoic calcified cyanophyceans were very common. It is proposed that, as a consequence of the radiation of calcareous plankton during that time, suitable conditions of low sedimentation in deep water became less common.