Carbonate sedimentation on Pacific coral reefs

Abstract

The contributions in this special issue represent recent advances in carbonate sediment research in and around the coral reefs of the Pacific regions of Japan, New Caledonia, and Tahiti. The issue contains five articles covering a range of subjects including fundamental research on the use corals and large-sized benthic foraminifers in paleoenvironmental reconstructions, facies analyses of modern off-reef sediments, and isotopic analyses of tricanid shells. The issue opens with a classical sedimentary facies description by Yamano et al. (2015), who examine surface carbonate sediments on the outer shelf and slope around New Caledonia. Facies showed significant variations in composition, from benthos- to plankton-dominated, according to water depth, currents, and local seafloor geomorphology. Their results contribute baseline data for examinations of carbonate sedimentary facies in tropical to subtropical environments. Following the description of general facies patterns, the issue contains further investigations on the use of specific organisms (including corals and benthic foraminifers) in reconstructions of facies and paleoenvironments. Humblet et al. (2015) present identification keys for four widespread genera of fossil corals (Acropora, Isopora, Montipora, and Porites) that are commonly recovered from outcrops and drilled cores. They argue that fossil corals should generally be identified from sectioned material, as coral surfaces are usually obscured by encrusters or sediments, and coral specimens are often broken or eroded. Further work extending this type of investigation to all coral genera would greatly contribute to a better understanding of coral reef development and paleoenvironmental assessments. Hongo and Wirrmann (2015) examine cores from Pleistocene and Holocene barrier reefs in New Caledonia and identify several key species for reef formation and maintenance. They emphasize the need for further research on the restoration and conservation of key species, as some of these species are presently in decline. Fujita and Omori (2015) examine large-sized benthic foraminifers from Recent surface sediments and Pleistocene subsurface deposits obtained during Integrated Ocean Drilling Program (IODP) Expedition 310, off Tahiti. By referring to modern foraminifer assemblages, they show that the Pleistocene sequence off Tahiti was deposited in a fore-reef slope environment. As with corals, the authors propose that an examination of modern assemblages of foraminifers would contribute to a better understanding of facies formation and improved paleoenvironmental reconstructions. In addition to their role in sedimentary facies analysis, carbonate skeletons and shells are reliable recorders of paleoenvironments at high temporal resolutions. Asami et al. (2015) present seasonally resolved records of carbon and oxygen isotopic compositions (δ18O) in the fossil shells of giant clams recovered from archeological sites in Okinawa, Japan. Results demonstrate that the annual mean seawater temperature and salinity at Okinawa were approximately 1–3°C higher and 1–2 lower, respectively, at 4000 yr cal. BP and 1900–1700 yr cal. BP than at present; such data provide insights into patterns of human settlement and modes of life in the past. An understanding of facies formation and the reconstruction of paleoenvironments both require process-based analyses. In addition to the five articles mentioned above, Fujita et al. (2015), whose work is to be included in an extended virtual special issue on this topic, examine processes of carbonate sedimentation in seagrass beds in the Ryukyu Islands, Japan. Based on sedimentation rates estimated from coring and sediment influx rates estimated from sediment traps, they identified a sediment-trapping role of seagrass, and characterized the seagrass beds in the Ryukyu Islands as having high sediment fluxes and extremely low sedimentation rates. Collectively, the special issue presents diverse and insightful analyses of Pacific coral reefs. We thank the authors and reviewers who contributed to this issue. Finally, we dedicate this special issue to the memory of our friend and colleague Guy Cabioch (Fig. 1), who sadly passed away in October 2011. All of the senior or corresponding authors, as well as the guest editors, of this special issue collaborated with him. He led many research cruises and drilling programs, and some of the outcomes of his collaborations appear in this special issue. We sincerely thank Guy and pray from the bottom of our hearts that his soul may rest in peace. Guy Cabioch in a research cruise (previous page) and in the field (this page). The photos are provided by Jean-Michel Boré and Denis Wirrmann, respectively.