Christmas Island lagoonal lakes, models for the deposition of carbonate-evaporite-organic laminated sediments

The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Rep ublic (Central Pacific) Lies at about 2 degreesN in the intertropical conve rgence zone. Much of the surface area of the atoll (ca. 360 km(2)) is occup ied by numerous lakes in which carbonate, evaporite (calcium sulfate, halit e) and organic layers are deposited. Observations suggest that deposition o f these different laminae is controlled by climatic and biologic factors. I t is thought that periodic climatic variations, such as El Nino-Southern Os cillations (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 evaporiti c minerals (carbonate, calcium sulfate, halite) when salinity increases thr ough evaporation. Thick (up to 5 cm) microbial (essentially cyanobacterial) mars develop continuously on the lake bottom surfaces providing the sedime nt with an important (total organic carbon 2-5%) organic contribution in th e form of an internal, geometrically structured, network in which the authi genic minerals precipitate. The high bioproductivity of these microbial pop ulations is reflected in low delta C-13 values of sedimentary organic carbo n (-14 to -17 parts per thousand), 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 de ath and burial of microbial populations at the time of severe climatic even ts (storms, heavy rainfall). These lagoonal lakes provide a model for the deposition of carbonate and or ganic matter in an evaporitic environment. The high ratio of deposited carb onate vs. sulfate + chloride, when compared to low ratio in evaporitic sali nas, results from both a lack of limitation of calcium, magnesium and carbo nate ions tin a carbonate reef environment) and active processes of high-Mg calcite precipitation (organomineralization). (C) 2001 Elsevier Science B. V, All rights reserved.