Ultra-high-resolution stable isotope sampling of slow-growing and fragile coral skeletons

The calcareous (CaCO3) skeleton of massive scleractinian corals represents a reliable archive for environmental proxy data. Stable carbon and oxygen isotope records built from series of powder samples drilled along the axis of maximum growth represent the backbone of most coral sclerochronologies. Because isotope analyses require a certain minimum amount of sample powder, commonly used drill bit diameters are >= 400 mu m. In order to achieve monthly (or higher) sampling resolution, annual upward growth of the corals must therefore exceed 5 mm. While this threshold represents no limitation for most coral sclerochronologies of massive corals from the tropics, it hampers the preparation of significant datasets from slow-growing and delicate corals from subtropical and temperate regions and the fossil record. Here, we present a modified milling technique, which allows sampling carbonate powder from slow-growing, very porous and delicate corals with a minimum spatial resolution of 50 mu m. By using dental composite resin as a stabilising medium in combination with a newly developed mounting apparatus, accurate high-resolution sampling along straight growth axes becomes feasible. With two case studies, we demonstrate applications of this method to two different kinds of slow-growing corals: A recent phaceloid Cladocora caespitosa from the Columbretes Islands (Spain, Mediterranean Sea) and a middle Miocene massive Porites sp. from the Vienna Basin (Austria, Central Paratethys Sea). Compared to conventional milling-based methods, the modified sampling design allows minimizing averaging effects caused by the integration over skeletal elements formed at different times, and enhancing the highest spatial resolution hitherto achieved by factor 1.5 for Porites and even factor 4 for Cladocora. In this respect, the herein presented method represents a significant step forward for exploring also sub-annually resolved proxy time-series from slow-growing and delicate corals (extant and fossil), thus allowing the broadening of sclerochronology to so far underexplored environmental coral archives.