Biosignatures of microbial mats in Pleistocene coral reef cores from IODP Expedition 389 (Hawaiian Drowned Reefs)

Abstract. We systematically document surfaces and biosignatures of Pleistocene reefal microbialites (Marine Isotope Stages 7–6) recovered during IODP Expedition 389 (Hawai’ian Drowned Reefs). Microbialites are abundant within Pleistocene coral reef successions and offer valuable archives of environmental information under Quaternary climate variability. However, relatively little is known about biofilm-forming microbial consortia, because biosignature preservation is usually very poor. The microbial crusts studied here form encrustations as much as 20 cm thick, ranging from laminated to thrombolitic, within the coral reef framework. Scanning electron microscopy (SEM) of samples from the windward (humid) Hilo and Kohala and the leeward (arid) Kawaihae sides of the Island of Hawai‘i reveals exceptionally well-preserved microbial fabrics that developed during Marine Isotope Stages (MIS) 7–6. Humid side samples exhibit abundant preserved putative exopolymeric substance (EPS) matrices, mineralized filaments, and near spherical, multilobate aggregates that resemble protodolomite spherules formed by modern cyanobacteria or extant coccoid cyanobacteria (e.g., Gloeocapsa-type). In either case, the surfaces appear to have been formed with significant aid of cyanobacteria suggesting formation in a euphotic setting. The microbialites from the arid side display peloidal microfabrics with fewer preserved physical biosignatures, typical of cryptic reefal microbialites, but the surfaces suggest confinement by an organic biofilm. The occurrence of pyrite framboids and huntite-like crystals in the wet-side samples suggests local redox gradients consistent with both sulphate reducing bacteria and cyanobacteria mediating carbonate precipitation. These findings provide the first direct evidence for euphotic microbial mat communities contributing to microbialite formation in Pleistocene drowned Hawai’ian reefs, and, to our knowledge, in Indopacific reefs and beyond. The outstanding preservation of mineralized EPS and microbial morphotypes highlights the potential of these Pleistocene reefal microbialites as sensitive archives of palaeoenvironmental conditions and microbial diversity under glacio-eustatic forcing and associated environmental changes.