SOME SPECULATIONS ON THE ORIGIN OF LATER PRECAMBRIAN "MOLAR-TOOTH" STRUCTURES

Molar tooth structures form “curious” and “enigmatic” networks of interconnected vertical and horizontal calcite ribbons and occasional spheroidal objects (“blobs”) found in fine-grained, probably marine sediments spanning the late Archean to late Neoproterozoic interval, a duration of ~1900 m.y., or nearly half the Earth’s stratigraphic record.

Vertical ribbons, averaging 5 mm in thickness, are generally intricately folded or fragmented by compaction. Decompaction shows that some ribbons may have been up to a meter in length perpendicular to bedding, forming in sediments that originally contained ~70% of water, implying formation in sediment depths of ~1.7 m or less. Though at least nine origins have been suggested for the blobs and ribbons, experiments suggest that most began as coalesced calcite-filled voids created by the rise of gas bubbles from the decay of organic material. Horizontal ribbons probably formed under conditions of higher than hydrostatic pore-fluid pressure.

Many blobs and ribbons are now recrystallized, equigranular calcite with a grain size of 4-12 µm. Some molar-tooth fill from widely separated areas displays earlier sub-spherical cores embedded in a later, commonly polygonal, overgrowth. In some cases cores have aggregated into cross-laminated units forming fine-grained, well-sorted silt transported by gentle currents flowing between the walls of the fill.

Similar spheroidal, hemi-spheroidal and rhombohedral crystals in the size range 10-50 μm have been made experimentally from calcifying bacteria associated with sticky extracellular polymeric substances (EPS). If EPS were produced by later Precambrian bacteria, their presence could have acted as a seal, greatly reducing the permeability of the sediments, preventing the ready escape of any gases present and allowing the generation of higher than hydrostatic pore-fluid pressures. EPS and bacteria may have veneered the muddy carbonates where molar-tooth structure was initiated with “microbial scum” (i.e. microbial mats). The longevity of molar-tooth structures and the quiet conditions in which they accumulated shows that large areas of the Proterozoic Earth were stable and, to speculate, these structures may provide another template for early life on some terrestrial planets.