Biogenic structure of early sediment fabric visualized by low-temperature scanning electron microscopy

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Abstract
The accurate visualization of early biogenic sediment fabric is important in the search for modern analogues to relict fossil structure and, therefore, for the interpretation of past environmental conditions. Water may have a significant structural role within hydrated material and sample dehydration may be detrimental to the structural interpretation of natural sediments and their associated microbiota. Water is present in sediments as free water or in the form of bound or hydration water. Extracellular polymeric substances associated with microbial assemblages have a high content of hydration water and are distorted by dehydration. The use of low-temperature scanning electron microscopy for the visualization of water-saturated sediments and associated biogenic structures is examined. Low-temperature SEM allows samples to be examined without the removal of water that is normally associated with the preparatory procedures for traditional ambient SEM. Samples for low-temperature SEM are first frozen (cryofixed) and then examined, fully-hydrated and still frozen, at low temperature (cf.–180°C) on the stage of a specially adapted SEM. The process of low-temperature SEM for the examination of hydrated sediments is described and several examples of cohesive and mixed-flat sediments and their associated biogenic fabrics are given. Detail of the internal fabric and associated microbiota can be examined by freeze-fracture techniques to preserve the natural condition of the sediments.