530 MINERALS/Carbonates volcano The construction and rigidity of a reef, irrespective of its size and morphology, are controlled by the growth of the animals/plants and the deposition of sediment and/or precipitation of cements in cavities between those formative organisms Destructive processes, which offset the constructive processes, include bioerosion and physical damage caused by highenergy events such as storms and hurricanes Not all reefs have been formed by corals Archaeocyathids (Cambrian), sponges (Ordovician), stromatoporoids (Silurian, Devonian), tabulate corals (Ordovician, Silurian), phylloid algae (Pennsylvanian), and rudist bivalves (Cretaceous), for example, have all been responsible for the construction of large reefal structures at different times throughout the Phanerozoic Mounds, which vary from low-relief lenses to mounds with slopes up to 40 , are generally categorized as microbial mounds, skeletal mounds, or mud mounds Their genesis is poorly understood because the origin of the constituent mud is poorly constrained, the organisms that mediated their formation are typically poorly preserved or absent, and there are no modern examples of mud mounds Growth and development of microbial mounds were typically mediated by cyanobacteria, algae, diatoms, and other micro-organisms that can become calcified, trap sediment, and/or induce CaCO3 precipitation Skeletal mounds formed where organisms such as bryozoa, corals, stromatoporoids, sponges, and rudist bivalves induced local sedimentation through current baffling, sediment trapping and binding, and sediment stabilization Marine Sedimentary Processes The formation of carbonate deposits through deposition and/or precipitation depends on the depositional environment and, in particular, whether animals and/ or plants are present Plants play many different roles in the formation of marine carbonate sediment Calcareous algae, seagrasses, and mangroves, for example, play important roles in carbonate sedimentation Carbonate sedimentation is influenced by the binding action of their roots, the baffling action of their leaves, and the sediment production by the epibionts that live on their leaves and/or roots The roles played by each plant vary Calcareous algae, with their calcareous skeletons, are major sediment producers but play a minimal role in terms of current baffling or sediment binding Conversely, Thalassia (‘turtle grass’) has long-bladed leaves that reduce current strengths and thereby cause deposition of any sediment suspended in that current, as well as long complex rhizomes that bind the sediment in place and prevent erosion except by the strongest of storm-driven currents Thalassia is also a major sediment source because the calcareous skeletons of the small animals and plants that live on their leaves become part of the sediment load following the death and decay of the leaves (Figure 2F) Mangroves (Figure 2B), found in coastal regions, are characterized by extensive prop-root systems that bind sediment in place and are extremely effective at reducing currents, thereby causing sediment deposition Sediment production, however, is generally limited to those plants and animals that attach themselves to the roots of trees Micro-organisms commonly play a critical role in carbonate sedimentation Various consortia of microbes can form microbial mats that cover vast areas of the seafloor and the intertidal flats In many areas, these microbial communities mediate the growth and construction of stromatolites (Figure 2G) by trapping and binding sediment to the seafloor or by acting as nucleation sites for aragonite and/or calcite precipitation Removal of the mats commonly leads to erosion and transportation of the underlying sediment In many areas, the microbial communities mediate the growth and construction of stromatolites, which are highly variable in terms of their size and morphology Non-Marine Carbonates In non-marine settings, carbonate deposits will form from any water that is supersaturated with respect to CaCO3 Rapid degassing of CO2 and/or evaporation commonly trigger such precipitation (Figure 1B and E) In most situations, abiotic precipitation, in contrast to biologically influenced precipitation, is more important Nevertheless, coated grains (Figure 1C), bioclasts, peloids, and lithoclasts can form in many of these settings Indeed, it is commonly difficult to distinguish between allochems that form in marine and non-marine settings if their depositional context is unknown The precipitation of aragonite, as opposed to calcite, in non-marine settings has commonly been attributed to water temperature, with calcite being precipitated from water that is cooler than 40 C and aragonite being precipitated from water that is warmer than 40 C This relationship, however, is not universally true Exceptions are found when extreme CO2 degassing leads to aragonite precipitation or when slow ion delivery, caused by microbial biofilms or high viscosity, leads to calcite precipitation In the vents of some hot springs (T > 90 C), alternating aragonite and calcite precipitation can take place in response to variations in the rate of CO2 degassing, even though the water chemistry remains essentially