Paleoenvironmental significance of growth story of long-living deep-water acervulinid macroids from Kikai-jima shelf, Central Ryukyu Islands, Japan

Abstract

Macroids are unattached centimetre-sized nodules built by encrusting invertebrates. Encrusting foraminifera (Acervulina inhaerens) and subordinate thin coralline algae form extensive macroid beds on sandy and gravelly bioclastic carbonates off Kikai-jima, on a coral-reef-related island shelf, in the Central Ryukyu Islands, Japan. At water depths from 75 to 100 m, the beds consist of spheroidal and sub-spheroidal macroids, c. 6 cm in mean diameter, with asymmetric concentric inner arrangement. The macroids are bioeroded by Entobia, Maeandropolydora, Trypanites, Gastrochaenolites, and microborings. They generally show two distinct growth stages separated by an abraded rugged surface deeply colonized by borers, mainly Entobia. Radiocarbon dating yielded an oldest age of c. 4400 cal yr BP for the earliest acervulinid growth, whereas the second stages were much younger, ranging in age from c. 1500 cal yr BP to present day. Datings of the two growth stages in five specimens indicate that active growth and growth interruption were not synchronous in the different nodules. For c. 4400 years the macroids grew within an estimated maximum range of palaeotemperature changes of c. 4.7 °C, under chronic oligotrophic to mesotrophic conditions, low-level hydrodynamism and low sedimentation rates. The lack of synchroneity among individual macroids rules out catastrophic events and ecosystem-wide environmental changes as possible causes of growth interruption. Random biogenic mobilization and temporal occupation of the macroid surface by organisms with no rigid skeleton and/or biofilms likely interrupted acervulinid growth at individual macroid scale. The environmental conditions in which Kikai-jima macroid beds develop do not support interpretations of acervulinid macroid accumulations during Paleocene−Eocene Thermal Maximum (PETM) and Middle Eocene Climatic Optimum (MECO) events in the Western Tethys as indicators of eutrophic conditions.