Living forms, especially vegetal, can contribute to sedimentation by acting as traps for sedimentary particles; this usually happens in shallow water, intertidal and swamp zones. Among the examples sketched in the inset, algal mats have a fly-paper effect on passing sediment; they capture it with their mucilaginous filaments. Actually, they should be called bacterial mats, as the constituent micro-organisms, which were regarded as unicellular algae, have been reclassified as cyanobacteria. Cyanobacteria represent one of the oldest form of life on Earth; the diffusion of their mats is restricted today by grazing animals, but was widespread during the early history of our planet. The bacteria rarely fossilize, but their mats do, and build a structure that is called stromatolite. It must be clear that a stromatolite is not, strictly speaking, a fossil, which means the mineralized remain of a living organism, but a biogenic structure, i.e., the fossil evidence of the past activity of organisms.
A stromatolite is a lithified bacterial mat; lithification is due to recrystallization of fine carbonate matrix and precipitation of cement in pores. Most stromatolites are part of carbonate calcareous and dolomitic formations, where they have fixed enormous quantities of calcium carbonate during the first three billion years of Earth history. In so doing, bacteria have sequestered CO2 from the atmosphere, thus contributing not only to sedimentation but also to a decreasing of the greenhouse effect and of the Earth's temperature. In other terms, the proliferation of primeval forms of life populated the planet in a way that favored the further development of life by keeping the temperature lower than the boiling point of water. Life seems thus to have set up a global self-regulated system apt to its survival. Today, the occurrence of bacterial mats is limited to the carbonate environments of the intertropical zone (Bahamas, Persian Gulf, Australia, etc.). Except in Shark Bay, Australia, they form no longer domal (plate 141 A) and columnar structures as in primeval seas, where grazing animals were lacking.
Stromatolites are less frequently fossilized in noncarbonate rocks; we see here two examples (B and C) of stromatolitic gypsum. In this case, the bacterial mats trapped a peculiar type of detritus, made of small gypsum crystals or fragments of larger crystals. These crystals of salt precipitated from evaporitic brines, to be later on resuspended by waves and storms. It has been observed in Modern environments that gypsum crystals can also precipitate within the mats, as bacteria can tolerate a high salinity. Crystals form in this case the seeds for cementing the mat (see plate 142).
As is obvious from these images, stromatolites are laminated structures; laminae are commonly wavy and crisped. Take care not to confuse their geometry with that of tractive (ripples) or convolute lamination. An indicative characteristic is the nonuniform curvature, with rounded convexities and acute, pointed concavities (in ripples and convolutions, it is just the opposite). Stromatolitic laminae reveal that the accretion of the mat is not continuous; there are, first, daily pauses (bacteria are photosynthetic organisms and interrupt their activity during the night: see inset), then seasonal or sporadic erosional events.
A: Paleozoic carbonate of Petrified Sea Gardens, N.Y.; B and C: Messinian gypsum beds of maritime Tuscany, Italy.