Rhodoliths from deep fore-reef to shelf areas around Okinawa-jima, Ryukyu Islands, Japan Shinya Matsuda a, ⁎, Yasufumi Iryu b a Faculty of Education, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan b Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan abstract article info Article history: Received 2 June 2010 Received in revised form 9 February 2011 Accepted 27 February 2011 Available online 11 March 2011 Communicated by J.T. Wells Keywords: rhodolith nongeniculate coralline alga encrusting foraminifer fore-reef Ryukyu Islands The distribution, abundance, composition, and growth history of rhodoliths were investigated based on 222 grab samples and 202 submarine photographs taken from 223 sites arranged at regular intervals and on 13 additional samples (5 dredge and 8 grab samples). These samples were collected at water depths ranging from 15 to 970 m around Okinawa-jima, Ryukyu Islands, Japan. The rhodoliths grow in deep fore-reef to shelf areas at water depths of 50 to 135 m. Where rhodoliths occur, they cover 45% of the sea bottom. The rhodoliths are primarily spheroidal to ellipsoidal in shape (with mean diameters usually less than 8 cm); internally they are primarily composed of nongeniculate coralline algae and an encrusting foraminifer Acervulina inhaerens. The rhodoliths have envelopes of well-preserved, concentric to irregular laminations or, much more commonly, are bored and display various degrees of bioerosion. Constructional voids (primary spaces between encrusters) and borings range from empty to completely filled with unlithified and lithified mixtures of micrites and bioclasts. The bioerosion is more extensive with increasing water depth and is progressively much more pervasive at water depths greater than 90 m. The rhodoliths are covered with nongeniculate coralline algae and A. inhaerens associated with other epilithic skeletal and nonskeletal organisms. The living biotic cover on rhodoliths is relatively great down to water depths ~100 m; below this, the cover decreases rapidly with increasing water depth. Rhodoliths with similar size, shape, and composing organisms to those in the Ryukyu Islands are commonly found on deep fore-reef to shelf areas or on the banks and seamounts of tropical reef regions, likely as the combined result of ecological degradation (=decreased number and coverage) of hermatypic corals and the relative predominance of nongeniculate coralline algae and encrusting foraminifers in such areas. The slow accretion rates of rhodoliths (b 0.1 mm/year) indicate that their formation is commonly to frequently intermittent, probably because of their burial in the surrounding sediment. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Rhodoliths are algal nodules or unattached algal growths with a nodular form, consisting principally of nongeniculate coralline red algae. Rhodoliths are present in tropical to boreal regions and found from the intertidal zone to water depths exceeding 200 m in modern marine environments (Bosellini and Ginsburg, 1971; Bosence, 1983; Iryu, 1985; Foster, 2001); they are common constituents of fossil, especially Cenozoic, reefs and carbonate platforms. Many studies showed that rhodoliths are sensitive recorders of some environmental parameters (e.g., Bosellini and Ginsburg, 1971; Bosence, 1976; Foster, 2001; Bassi et al., 2009) and that their characteristics, such as shape, coralline algal growth form, coralline taxonomic assemblages, and diagenetic features, can be used as relatively significant paleoenvironmental indicators (e.g., Studencki, 1979; Bosence and Pedley, 1982; Braga and Martìn, 1988; Braga and Aguirre, 2001; Payri and Cabioch, 2004; Bassi, 2005; Checconi et al., 2010; Iryu et al., 2010). In contrast, Reid and MacIntyre (1988) expressed doubt that those features have a predicable relationship with particular environmental conditions. Rhodoliths are abundant in the Ryukyu Group, Pleistocene reef- complex limestones with associated siliciclastic marine and terrestrial deposits that crop out throughout the islands of the Central and South Ryukyus (=Ryukyu Islands) (Fig. 1). Rhodolith limestone is one of the major lithofacies of the group (MacNeil, 1960; Nakagawa, 1972; Nakamori et al., 1995a; Iryu et al., 1998; Iryu et al., 2006; Humblet et al., 2009). Minoura and Nakamori (1982) interpreted the depositional environment of the rhodolith limestone based on its stratigraphic position within the group and the rhodoliths' shape. They presumed that these rhodoliths originated in a shallow lagoon, grew while they rolled along the bottom of a sandy channel, and finally accumulated on a gentle fore-reef slope. An alternative interpretation was suggested by Iryu (1984, 1985), who concluded that the rhodoliths formed and accumulated in deep fore-reef to shelf areas at water depths exceeding 50 m. This interpretation was based on the stratigraphic position of rhodolith limestones that are widespread, encircling proximal coral limestones, and grade laterally into distal, poorly sorted detrital limestones, as well as the fact that the maximum elevation of rhodolith Marine Geology 282 (2011) 215–230 ⁎ Corresponding author. Tel.: +81 98 895 8363; fax: +81 98 895 8316. E-mail addresses: smatsuda@edu.u-ryukyu.ac.jp (S. Matsuda), iryu.yasufumi@a.mbox.nagoya-u.ac.jp (Y. Iryu). 0025-3227/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2011.02.013 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo