The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Republic (Central Pacific) lies at about 2°N in the intertropical convergence zone. Much of the surface area of the atoll (ca. 360 km2) is occupied by numerous lakes in which carbonate, evaporite (calcium sulfate, halite) and organic layers are deposited. Observations suggest that deposition of these different laminae is controlled by climatic and biologic factors. It is thought that periodic climatic variations, such as El Niño-Southern Oscillations (ENSO) events which bring heavy rainfall to the atoll, result in the succession of the precipitation of carbonate minerals (during periods after dilution of hypersaline waters by heavy rains), followed by evaporitic minerals (carbonate, calcium sulfate, halite) when salinity increases through evaporation. Thick (up to 5 cm) microbial (essentially cyanobacterial) mats develop continuously on the lake bottom surfaces providing the sediment with an important (total organic carbon 2–5%) organic contribution in the form of an internal, geometrically structured, network in which the authigenic minerals precipitate. The high bioproductivity of these microbial populations is reflected in low δ13C values of sedimentary organic carbon (−14 to −17‰), interpreted as being the result of high atmospheric CO2 demand (Geochim. Cosmochim. Acta, 56 (1992) 335). The well-laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events (storms, heavy rainfall). These lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment. The high ratio of deposited carbonate vs. sulfate+chloride, when compared to low ratio in evaporitic salinas, results from both a lack of limitation of calcium, magnesium and carbonate ions (in a carbonate reef environment) and active processes of high-Mg calcite precipitation (organomineralization)

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Abstract--Samples of ash layers and associated background sediments from Site 808 of ODP Leg 131 in the Nankai Trough accretionary prism were analyzed for changes in mineralogy, porosity and microfabric associated with alteration of volcanic ash. Ash layers range from incipient stages of alteration and dissolution to complete alteration to clay minerals and clinoptilolite. Ash layers contain greater abundances of total clay minerals and lower percentages of quartz than do surrounding background hemipelagic sediments. The clay-sized fraction of ash layers is dominated by pure dioctahedral smectite, whereas the background sediments contain primarily illite and chlorite with minor amounts of smectite. Analysis of microfabric revealed dramatic changes in the distributions and abundances of grains and pores during ash alteration. The relative abundances of large pores, grains, and matrix material were quantified on digital back-scattered electron images (BSEI) of ash layer and background sediment samples. During burial, the abundant glass shards of shallow ash layers are initially altered, presumably to smectite. Subsequent dissolution of the glass leaves open, shard-shaped pores, resulting in increased porosities. With greater burial, these pores are filled with elinoptilolite. Although the presence of ash and its alteration products clearly influences sediment physical properties, there is no apparent correlation of the abundance of ash or its alteration products with the formation of thrust faults or other structures within the Nankai Trough accretionary prism.

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