Facies variations in response to Holocene sea-level and climate change on Bora Bora, French Polynesia: Unravelling the role of synsedimentary siderite in a tropical marine, mixed carbonate-siliciclastic lagoon
Résumé
Five mixed carbonate-siliciclastic sedimentary facies were identified in the barrier-reef lagoon of Bora Bora using microfacies and statistical analyses of 70 sediment samples taken at high resolution from two vibrocores. Fades and fades successions were interpreted with respect to Holocene sea-level and climate changes. The windward lagoon core is characterized by sideritic marly wackestones and foraminifera-sideritic wackestones, deposited around 7700 years BP (years before present) during the early-mid Holocene transgression. At that time, extensive weathering and erosion of iron-bearing minerals from the volcanic island, due to a wetter climate, were expressed in the formation of synsedimentary siderite in lagoonal sediments. The enrichment in delta O-18 (+ 032 to + 0.54 parts per thousand) in the siderite grains indicates marine to mixed marine-meteoric conditions during precipitation. Siderite formation resulted from microbial degradation of organic material, indicated by depleted delta C-13 values (- 13.61 to - 14.48160) that led to reducing conditions in lagoonal sediments, and resulted in iron reduction in the presence of dissolved bicarbonate. The chemical compositions of the siderites changes upcore, from relatively high Fe (91-95 mol%) and low Mn (5-6 mol%) at the core base to relatively low Fe (83-88 mol%) and high Mn (11-16 mol%) at the core top. The substitutions of Fe by Mn, Ca and Mg at grain margins illustrate changes in pore-water chemistry towards more oxygenated conditions and reflect sea-level rise and elevated rainfall during the early-mid Holocene. A drier climate during the mid-late Holocene was accompanied by reduced iron input and the proportion of siderite decreased, approaching zero in the upper section of the core. In the leeward lagoon core, siderite is again common in the lower section, and decreases in abundance upcore. Mollusc-foraminifera many packstones and mollusc wackestones accumulated similar to 5400-3500 years BP during the mid-Holocene in the windward core. Early in this period rotalid and miliolid foraminifera dominated. These are tolerant of environmental stress such as changes in water quality, nutrients or salinity. From the mid-Holocene to the present, textularid foraminifera are common in both cores, and indicate normal marine lagoonal conditions. Since the mid-late Holocene sea-level highstand and fall to modern level, mudstones have dominated in both cores. During the last 1000 years coral fragments have increased in abundance in the windward lagoon, presumably as a result of lagoonward progradation of fringing reefs in the mid-late Holocene. Since the late Holocene, motus on the windward side of Bora Bora have hampered sediment transport and lagoonward pro gradation of sand aprons. Increasing numbers of peloids, largely hardened faecal pellets, in the windward core in the last 1000 years may reflect early submarine lithification within the lagoon. Our study shows that during the Holocene, sea-level and climate change have influenced sediment import, composition and distribution in the Bora Bora lagoon. The sensitive response of the environment to external changes demonstrates the potential of tropical reef lagoons as archives of climate and sea-level changes.