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

Abstract

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)

Identification of arid phases during the last 50 cal. ka BP from the Fuentillejo maar‐lacustrine record (Campo de Calatrava Volcanic Field, Spain)

“…In fact, the formation of dolomite in this closed lacustrine environment would have required episodic flooding by fresh water (Folk and Land, 1975;Trichet et al, 2001), allowing saline brines to be periodically mixed with fresh water in a low lake system. The salinity drops drastically, but the Mg/ Ca ratio remains almost as high as it was initially.…”

Cyanobacterial populations that build ‘kopara’ microbial mats in Rangiroa, Tuamotu Archipelago, French Polynesia

“…The biogeochemical properties of 'kopara' have been extensively studied with respect to relationships between organic compounds and mineralization processes (Trichet, 1967;De´farge et al, 1985De´farge et al, , 1996De´farge & Trichet, 1990, 1993Trichet & De´farge, 1997;Trichet et al, 2001). The role of proteinaceous compounds and amino acid composition in controlling calcification was studied in deeper layers of 'kopara' mats (Gautret & Trichet, 2005) and other microbialites (Gautret et al, 2004).…”

Composition and function of mucilage macroaggregates in the northern Adriatic

“…Heissenberger et al (1996) observed that particle-associated bacteria exhibit larger envelopes than free-living bacteria, but the fate of these structures was not studied further. The microbial species that may form regular structures, including 'honeycombs' or 'veils,' have been studied mainly in biofilms from cultures or aquatic environments (Dalton et al 1996, Trichet et al 2001, Schaudinn et al 2007 and references therein). Mucus aggregates, comparable to biofilms, could be defined as assemblages of microorganisms and their associated extracellular products attached to an abiotic or biotic surface (Davey & O'Toole 2000).…”