Shallow-marine carbonate cementation in Holocene segments of the calcifying green alga Halimeda

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, many early-diagenetic cements that developed in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. 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 thin sections, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant Mg calcite (3.9 – 7.2 Mol% Mg), bladed low Mg calcite (0.4 – 1.0 Mol%) and mini-micritic Mg calcite (crystal size < 0.1 µm; 3.2 – 3.3 Mol% Mg). The consecutive development of (1) fibrous aragonite, (2) equant Mg calcite and (3) bladed low Mg calcite is explained by shifts in pore water pH and alkalinity through fluid kinetics and microbial sulfate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Mg calcite, as inferred from the presumable detection of an extracellular polymeric matrix. Radiocarbon analyses of five Halimeda segments furthermore indicate that intragranular aragonite and Mg calcite cementation of microstructurally complex carbonate constituents in the shallow marine phreatic environment is a slower process than intergranular ooid cementation, characterized by relatively smooth surfaces.