Carbonate diagenesis of the Late Pleistocene limestone of the Faaa M0020A core: IODP Expedition 310, Tahiti Sea Level Change

Abstract

The diagenetic history of the pre-LGM (Last Glacial Maximum) limestone from the core by IODP Expedition 310 was investigated texturally and geochemically. The limestone is mostly deposited in reef front and fore-reef settings and mostly composed of corals, coralline algae, and microbialites with a minor contribution of green algae, mollusks, and echinoderm fragments. Relatively high abundance of microbialites is the quite distinctive feature, compared to other reefal sediments described elsewhere. Texturally, shallow marine cements are characterized by acicular and botryoidal aragonite and druse HMC, and equant LMC cements of meteoric origin are relatively rare. Presence of LMC equant cements appears to be facies-controlled, thus is dependant upon the nature of substrates. It appears that the limestone was mostly subjected to vadose meteoric diagenesis during the last glacial period. The fibrous HMC cements in vuggy pores together with their oxygen isotopic compositions and textural evidence suggest that the limestone was subaerially exposed during the last glacial period and resubmerged during deglaciation. Stable isotopic and trace elemental analyses were carried out for microbialites that were originally composed of HMC. Oxygen isotopic compositions and Fe and Sr contents of pre-LGM microbialites are clearly distinguished from those of post-LGM limestone. This suggests that the limestone had undergone meteoric diagenesis in the different diagenetic system, depending upon the magnitude of the partition coefficients of trace elements. No clear difference between unaltered (post-LGM) and altered (pre-LGM) limestone was observed for Mn, Mg, and carbon isotope, indicating a closed diagenetic system. However, the diagenetic system was semi-open with respect to Sr and oxygen isotope. In addition, enriched carbon isotopic compositions of the pre-LGM limestone may imply that vegetation cover was negligible for the formation of paleosol layers. This is also supported by poorly preserved unconformity surface with an absence of paleosol layers within the core.