MINERALS/Carbonates 531 constant Precipitation under conditions of rapid CO2 degassing and/or evaporation commonly produces bizarre and unusual crystals Trigonal, dendritic, skeletal (i.e., hollow), or platy calcite crystals, for example, are found in many deposits that have formed around the vents of hot springs and geysers Many of these crystals are composite crystals, with the constituent subcrystals being readily apparent in high-magnification, high-resolution scanning electron microscope images Compared to the role of plants, animals, and other eukaryotes in forming marine carbonates, their role in forming non-marine carbonates is relatively minor The distribution of the biota in these settings is controlled primarily by water temperature, water acidity, and, in some spring systems, the presence of elements that may be toxic One of the main controls over the distribution of the biota is the maximum water temperature that each taxon of can tolerate (e.g., 45– 50 C for fungi) In the simplest sense, however, the diversity of plants and other organisms tends to decrease with increasing temperature In some coldwater streams, lakes, and spring systems, however, calcite is commonly precipitated around charophytes (freshwater green algae) and bryophytes (mosses, liverworts, and hornworts) On the sides of valleys or tufa dams in some cold-water spring systems, calcite precipitation around these species can lead to the construction of large fan-shaped deposits that stretch from one side of a valley to another and cause ponding on the upstream side Microbial communities formed of cyanobacteria, bacteria, and/or fungi are common in many freshwater lakes, streams, and spring systems As in the marine environments, the microbes can play a major role in CaCO3 accumulation by providing nucleation sites for crystal growth, by inducing precipitation through modification of the physiochemical conditions in their surrounding microenvironment, and/or by trapping and binding sediment to the substrate Such activity commonly leads to the formation of microbial mats and/ or stromatolites that are akin to those found in marine environments Diagenesis Carbonate diagenesis is primarily driven by the fact that carbonate minerals are highly reactive to changes in temperature and pressure conditions, especially in the presence of vast quantities of water Dissolution, cementation, inversion, recrystallization, and replacement, which are commonly triggered by pressure and temperature changes, may lead to the complete transformation of carbonate sediments and limestones In many cases, the original components of the limestones will be obliterated, and porosity and permeability are either created or destroyed Dolomitization, which is still poorly understood, is commonly pervasive, with thick successions of limestone being replaced by dolomite Carbonate diagenesis will start on the seafloor and continue until the rocks have been buried to depths at which metamorphism takes place In general, the factors that control diagenesis are poorly understood because they operate on a microscale in settings that cannot be directly observed or monitored Much more is known about diagenesis on the seafloor and in nearsurface settings than is known about diagenesis that takes place at depth, simply because the former settings are much more amenable to observation and monitoring Diagenesis on the seafloor (e.g., hardground formation, reef lithification) and along the shoreline (e.g., beachrock) typically involves the precipitation of aragonite and/or calcite cements directly from seawater Once exposed in the vadose zone (i.e., above the water table), carbonate sequences are prone to significant changes Climate plays a critical role because maximum carbonate diagenesis takes place where vast quantities of water flow through the rocks under high-temperature conditions Thus, carbonate rocks located in hot, humid climates tend to undergo more rapid diagenesis than those in cool, dry climates The surface and subsurface landforms associated with karst terrains provide clear evidence of the effects that surface and near-surface diagenesis has on carbonates Dissolution is mediated by the weak carbonic acid that is formed as rainwater absorbs CO2 from the atmosphere and from decaying vegetation on the ground Aragonitic components are either dissolved, producing fossil-mouldic porosity (Figure 3C), or are transformed to calcite Dissolution of aragonite and calcite means that the groundwaters commonly become supersaturated with respect to CaCO3, allowing precipitation of aragonite and/or calcite cements to take place in cavities in which suitable physiochemical conditions exist Diagenesis in the freshwater phreatic zone, which depends on water chemistry, may involve dissolution or cementation Under conditions of deep burial, the rocks are subjected to considerable overburden pressures that may lead to dissolution and the formation of stylolites Many ancient carbonate successions have been pervasively dolomitized (Figure 3F) by processes that are still not well understood All dolomite models, irrespective of their mechanics, require a source of Mg, a mechanism for transporting the Mg to the dolomitization site, and a dolomitization site that is physiochemically conducive to dolomite formation Some models, such as the reflux model, involve near-surface