9315056 Gaffey Cementation in modern shallow-water marine carbonate sediments is believed to be confined to the intertidal zone, restricted environments with high salinity, and reefs and sand shoals at platform margins. Boring by microbial endoliths, resulting in micritization or destruction of grains, is thought to be the dominant diagenetic process in other shallow marine environments. However, published studies by various workers, and preliminary work by the investigator, indicate that significant volumes of cement are precipitating in low to moderate energy, normal marine phreatic environments, a fact with important implications for cycling and storage of carbonate within protected settings. It is also widely believed that marine cementation is largely a physicochemical process, and that the role of microorganisms in carbonate cementation is restricted to inducing precipitation of carbonate by changing pH and Eh of pore waters. However, sample collection and preparation methods commonly used by sedimentologists degrade or destroy organic structures, so that their role in cementation often goes unrecognized. The proposed work will assess the role of chasmolithic bacteria, cyanobacteria, and microalgae in cementation by using techniques designed to preserve the microbial component of samples intact, and will develop new criteria for recognition of calcified chasmoliths in ancient sediments. In contrast with high energy, platform margin settings, calcification of chasmoliths rather than physicochemical precipitation of carbonate appears to be the dominant cement- forming process in sands from low energy marine phreatic environments. There also appear to be variations in cement types and abundance between different subenvironments within low energy settings. the proposed work will characterize the types and amounts of cements, and their distribution with bioclastic sands in a low, an intermediate, and a high energy lagoon on San Salvador Is land. Bahamas, and relate types and distributions of cements in these three settings to environmental factors such as sea grass cover, wave and current energy, and degree of bioturbation. Characterization of cement types and of their distributions in relation to environmental factors will provide useful criteria for interpreting early diagenetic environments of ancient shallow marine carbonate sediments.
