Christmas Island lagoonal lakes, models for the deposition of carbonate–evaporite–organic laminated sediments
✍ Scribed by J Trichet; C Défarge; J Tribble; G Tribble; F Sansone
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 842 KB
- Volume
- 140
- Category
- Article
- ISSN
- 0037-0738
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✦ Synopsis
The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Republic (Central Paci®c) lies at about 28N in the intertropical convergence zone. Much of the surface area of the atoll (ca. 360 km 2 ) 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 Nin Äo-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 re¯ected in low d 13 C values of sedimentary organic carbon (214 to 217½), interpreted as being the result of high atmospheric CO 2 demand (Geochim. Cosmochim. Acta, 56 (1992) 335). The well-laminated organic layers present in the sediment pro®le 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 1 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).