‘Trapping and binding’: A review of the factors controlling the development of fossil agglutinated microbialites and their distribution in space and time

Abstract

Trapping and binding of allochthonous grains by benthic microbial communities has been considered a fundamental process of microbialite accretion since its discovery in popular shallow-marine modern examples (Bahamas and Shark Bay). However, agglutinated textures are rare in fossil microbialites and, thus, the role of trapping and binding has been debated in the last four decades. Recently, renewed attention on this subject has produced new findings of fossil agglutinated microbialites (those mainly formed by ‘trapping and binding’ and analogous to modern examples), but they are still few and geologically recent (mainly post-Paleozoic) when compared to the 3.5 Gyr long record of microbialites. In order to better understand this discrepancy between modern and fossil examples, an extensive literature review is presented here, providing the first thorough database of agglutinated microbialites, which shows that all of them are formed in shallow-marine environments and most under tidal influence. In addition, a Lower Cretaceous example is described, including very diverse microbialites, each of them formed in a particular paleoenvironment. Some of these microbialites developed in grainy settings, but only those formed in marginal-marine tide-influenced environments accreted mainly by trapping and binding the surrounding grains, being analogous of modern agglutinated microbialites, and matching the environmental pattern observed in the literature database. The combination of the literature review with the case study allows to discuss the factors that control and enhance ‘trapping and binding’: a) occurrence of grains in the microbialite environment; b) frequent currents that mobilize the grains and supply them onto the microbialite surface; c) high concentration and diversity of electrolytes in the water to increase the adhesiveness of the extracellular polymeric substances (EPS) of the microbialite surface; and d) a CaCO3 saturation state not high enough to promote early and strong carbonate precipitation within EPS, which would eventually decrease its availability to adhere grains. Therefore, this review shows that the keys to solve the ‘trapping and binding’ debate may be environmental, because the conjuction of these hydrodynamic and hydrochemical parameters is preferentially achieved in shallow-marine settings and especially in those influenced by tides, at least since Mesozoic times. This explains the limited environmental and stratigraphic distribution of microbialites mainly formed by ‘trapping and binding’, and opens new ways to look, geologically and microbiologically, at this process, so often cited and yet so rare.