Fine-grained carbonates in nearly closed reef environments: Mataiva and Takapoto atolls, Central Pacific Ocean

Abstract

The nature, origin and mode of accumulation of fine-grained carbonate sediments have been investigated in the nearly enclosed lagoons of two French Polynesian atolls (Mataiva and Takapoto). The fine-sized fraction (< 63 μm) averages 50% of the total sediment on the lagoon floors, whereas it is replaced by sand-grained material at the vicinity of the lagoonal buildups, and along the peripheral inner talus. The silt fraction ( > 20 μm ) is substantially higher in amount (70% of the total fine sediments on average) than the mud portion and may increase volumetrically (90–99%) in some localities. Aragonite is the predominant mineral present (90% of the total calcium carbonate), the fines appear to be mainly skeletally-derived. The silt fraction includes coral and molluscan detritus, sponge chips, juvenile benthic foraminiferans and other biogenic debris. The mud is chiefly composed of aggregated nanograins, ascidian spicules, skeletal nanograins (coral-generated, mainly) and coccoliths. A minor part of the mud is certainly of non-skeletal origin. Some portion of freely deposited fines fill cavities or are attached upon sand-grain surfaces. They are associated with scattered inorganic precipitates, and contribute to the formation of micrite envelopes and aggradation of host grains. Such a process is highly significant in the total carbonate budget of the reef. The fine carbonate sediments result directly from the biological disintegration of nearby buildups, mainly through the boring activity of sponges and lithophagid molluscs. We suggest that the authigenic mud sized crystallites are controlled in their precipitation both by carbonate supersaturation and anoxic conditions seasonally affecting the interstitial sediment waters. Enclosed atoll lagoons are relatively self-governing environments of deposition which are influenced by long-term, quiescent water situations rather than by rare, catastrophic events.