Early-diagenetic cementation of tropical carbonates results from the combination of numerous physico-chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High-Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low-Mg calcite (0.4 to 1.0 Mol% Mg) and mini-micritic Low-Mg calcite (3.2 to 3.3 Mol% Mg). The co-existence and consecutive development of fibrous aragonite and equant High-Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High-Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low-Mg calcite cements on top of etched substrates of equant High-Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Low-Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini-micrite Low-Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High-Mg calcite takes place in about 100 years. Collectively,
Many lagoons surrounded by reefs are partially or completely infilled with reef-derived detrital carbonate sediment. Sediment deposits in such restricted environments are archives of prevailing environmental conditions during lagoon infill. For Indonesia, no paleoenvironmental reconstructions based on Holocene lagoon sediments exist. Here we analyze the sedimentary record obtained from five percussion cores penetrating 10 m into the unconsolidated subsurface of a reef island in the Spermonde Archipelago, Indonesia. The combined compositional, textural and chronostratigraphic analyses reveal that the sedimentary infill of the lagoon underlying the island, starting 6900 years cal BP, was interrupted between 5800 and 4400 years cal BP, when sea level was ~ 0.5 m higher than at present, and monsoon intensity was lower. After the intensity of the monsoons increased to modern levels, and sea level dropped to its present position, lagoonal sedimentation was re-initiated and created the foundation for an island that built up since 3000 years cal BP. Our study provides the first geological evidence for the strong sensitivity of detrital carbonate systems in Indonesia to fluctuations in sea level and dominant wind direction. It thus sheds light on how changing environmental conditions in the context of global warming could affect the morphological development of reef systems, and thereby also habitable coastal areas.
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