Key species of hermatypic coral for reef formation in the northwest Pacific during Holocene sea-level change
Introduction
Knowledge of coral reef growth patterns and internal facies during the Holocene has increased greatly in the last 40 years. Reefs from the Indo-Pacific region and the Caribbean have been studied so that it is now possible to determine general and regional patterns of formation, and the relationship between reef growth and corals (e.g., Cabioch et al., 1995, Macintyre and Glynn, 1976, Marshall and Davies, 1982, Montaggioni and Faure, 1997). Previous geological studies have revealed that some genera are important in terms of understanding the above topics. Although more than 200 scleractinian genera are currently found throughout the world (Veron, 2000), few played a part in reef formation during the Quaternary. For example, post-glacial reefs in the Pacific were composed mainly of corymbose and arborescent Acropora (e.g., Cabioch et al., 1995, Hopley and Barnes, 1985, Kayanne et al., 1993, Kennedy and Woodroffe, 2000, Marshall and Davies, 1982, Takahashi et al., 1988). Holocene reefs in the Indian Ocean also comprised mainly corymbose and arborescent Acropora (e.g., Camoin et al., 1997, Gischler et al., 2008, Montaggioni and Faure, 1997). Moreover, the genera Porites and Pocillopora were the main reef builders in the Hawaiian Islands and eastern Pacific (e.g., Cortés et al., 1994, Grossman and Fletcher, 2004, Macintyre et al., 1992). In the Caribbean, reefs were also composed predominantly of Acropora (e.g., Aronson et al., 2005, Blanchon et al., 2002, Gischler and Hudson, 2004, Hubbard et al., 2005, Macintyre, 2007, Macintyre and Glynn, 1976). Other genera (e.g., Favia, Favites, Goniastrea, Platygyra, and Montastrea) were observed in the Indo-Pacific region during the reef-formation stage (Cabioch et al., 1995, Camoin et al., 2004, Marshall and Davies, 1982, Montaggioni and Faure, 1997).
The genus Acropora is an especially important scleractinian taxa on reefs throughout much of the Pacific Ocean and the Caribbean. Although the genera Pocillopora and Porites are represented by fewer than 10 species and approximately 80 species, respectively, Acropora is notable as the most diverse coral, with over 200 species (Veron, 2000). In the Caribbean, the present day and the Holocene reefs were dominated by Acropora palmata and Acropora cervicornis (e.g., Aronson et al., 2005, Blanchon et al., 2002, Gischler and Hudson, 2004, Hubbard et al., 2005). In the Indo-Pacific region, the distribution of Acropora species in the present ocean has been intensely studied (e.g., Veron, 1986, Veron, 2000, Wallace, 1999), but reconstructions of reef growth history are usually based on data derived from growth forms, genera, and combinations of certain species. For example, the Holocene reef at One Tree Reef, Great Barrier Reef, was composed of corymbose and arborescent Acropora (Marshall and Davies, 1982), but species-level identification remains to be performed. In Vanuatu, reefs were dominated by several Acropora species, among other corals (Porites, Astreopora, and Montipora), during the past 23,000 years (Cabioch, 2003), although the relationship between species patterns and reef growth remains unknown. Such results have been reported throughout the Indo-Pacific region (e.g., Cabioch et al., 1995, Camoin et al., 1997, Kan and Kawana, 2006); consequently, the distribution patterns of species during past reef formation remains poorly understood.
Johnson et al. (2008) proposed that species diversity was not an important control on reef growth in the Caribbean. Hongo and Kayanne (2010b) also showed that species diversity is not a prerequisite in terms of Holocene reef growth in the Indo-Pacific and the Caribbean and this study indicated that few species are important for reef growth. For example, at Ishigaki Island in the Ryukyu Islands, the reef consisted mainly of several Acropora species (A. digitifera and A. hyacinthus) and other species during the period of Holocene sea-level change (Hongo and Kayanne, 2009). At Kikai Island, the Holocene reef also comprised corymbose and tabular Acropora (A. digitifera, A. hyacinthus, and A. gemmifera) and Isopora palifera (Ota et al., 2000, Webster et al., 1998). In the Caribbean reefs, reefs were composed of robust and arborescent Acropora (A. palmata and A. cervicornis) during the Holocene (Blanchon et al., 2002, Gischler and Hudson, 2004, Hubbard et al., 2005).
Moreover, the species-level records from these fossil corals suggest that the above corals are likely to be key species for reef formation in the near future. These records are valuable in terms of coral preservation, including the designation of marine protected areas (MPAs) and the selection of appropriate species for transplantation (Dizon and Yap, 2006, Edwards and Clark, 1998, Lindahl, 2003, Soong and Chen, 2003) to address the increasing decline in coral reefs caused by climate change and human impacts (Bellwood et al., 2004, Gardner et al., 2003, Hughes et al., 2003). For example, Acropora has been severely affected by coral bleaching events (Loya et al., 2001) and this genus shows a high degree of endemism; consequently, the loss of Acropora species would cause a decline in reef development in the Indo-Pacific and the Caribbean.
In reefs of the northwest Pacific, bio-lithological descriptions and radiometric dates have been obtained from numerous drilling cores recovered since the 1970s and observations of submarine-trench walls have revealed the detailed internal structure of reefs (Table 1). Data on reefs of the northwest Pacific are available from the Palau Islands (Kayanne et al., 2002), Ishigaki Island (Hongo and Kayanne, 2009, Yamano et al., 2001, Yamano et al., 2003), Kikai Island (Konishi et al., 1978, Konishi et al., 1983, Ota et al., 2000, Webster et al., 1998), Yoron Island (Yonekura et al., 1994), and other reefs (e.g., Hamanaka et al., 2008, Kan et al., 1991, Kan et al., 1995, Kan et al., 1997, Takahashi et al., 1988). However, little is known of the reef builder in other reefs during the Holocene because these data are restricted to genera-level observations, with the exception of Kikai Island (28°20′N, 130°00′E) and Ishigaki Island (24°25′N, 124°10′E). The purpose of the present study is to identify the Holocene species-level record from the Palau Islands (7°24′N, 134°21′E) and Yoron Island (27°02′N, 128°26′E) in the Ryukyu Islands, northwest Pacific and to identify the key species for reef formation and for preservation plans in the near future.
Section snippets
Palau Islands
The Palau Islands consist of a number of volcanic and limestone islands (Fig. 1). The Ngemelis barrier reef (7°24′N, 134°21′E) is located west of Babeldaob Island, which is subjected to seasonal winds, including a northeast trade wind from November to May and south to southwest wind from June to October (Wolanski and Furukawa, 2007). The island is exposed to wintertime swell; however, the reef is protected by a northeasterly wind during this period, meaning that it is considered a moderate- to
Bio-sedimentological units of the Palau Islands and Yoron Island
Twelve biological and sedimentological units (Units 1–12) were observed in the analyzed cores from Ngemelis reef in Palau Islands. In Yoron Island, seventeen biological and sedimentological units (Units 13–29) were observed in the cores. The detailed description and the internal structures are shown in Table 4 and Fig. 2, respectively. Coral species list in the cores is shown in Table 5. Below, each of the cores is summarized in turn.
PL-I core: this reef-crest core is composed mainly of in situ
Holocene sea-level change and reef formation
Holocene reef growth is generally influenced by some controlling factors (e.g., sea-surface temperature, sea level, wave energy, and turbidity). Sea-surface temperature (SST) is one of the most important controls on corals. The distribution is very roughly limited by the 18 °C minimum monthly isotherm because low temperature is directly related to coral physiology and survival (Sheppard et al., 2009). For Great Barrier Reef corals, all metabolic activities (including photosynthesis and
Conclusions
The present analysis of Holocene cores recovered from the Palau Islands and Yoron Island provide species-level records for periods of reef growth, leading to the following conclusions.
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At the Palau Islands, 12 biological and sedimentological units (Units 1–12) were identified from three cores, whereas at the Yoron Islands, 17 units were identified (Units 13–29). Based on the occurrence of common corals and sediment among these units, four major facies were identified in the study sites. The
Acknowledgements
The cores recovered from the Palau Islands and Yoron Island was kindly provided by The University Museum (The University of Tokyo). John T. Wells and two anonymous reviewers are thanked for their constructive criticism of the manuscript.
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2015, Marine GeologyCitation Excerpt :Similarly, Hongo and Kayanne (2011) found comparable assemblages of Isopora and Acropora in water depths of 0–7 m on the high energy, low turbidity reef crests and upper reef slopes of the Palau and Ryukyu Islands. Both Pocillopora sp. and Stylophora sp. have been identified on reef flat environments at less than 7 m in the north-west Pacific (Hamanaka et al., 2012; Hongo and Kayanne, 2011; Sagawa et al., 2001), less than 6 m in Tahiti (Montaggioni et al., 1997b) and between 0–5 m in the South-West Indian Ocean (Camoin et al., 1997). Therefore assemblage A2 most likely represents a high energy, wave exposed environment similar to modern day upper reef slopes and outer reef flats, occurring in water depths as deep as 10 m but more likely < 7 m.