[M-GI28] 地球掘削科学

[MGI28-07] Changes in fossil coral assemblages spanning the last 500 kyr around the island of Hawaii from IODP Exp. 389 cores

*Marc Humblet¹、Donald Potts³、Juan Carlos Braga¹⁰、Jody M. Webster²、Ana Christina Ravelo³、Hannah L.J. Grant⁴、Margaret Stewart⁴、Marisa Rydzy⁵、Erwan Le Ber⁶、Nicola Allison⁷、Ryuji Asami⁸、Brian Boston⁹、Logan Brenner¹¹、Xuefei Chen¹²、Peter Chutcharavan¹³、Andrea Dutton¹⁴、Thomas Felis¹⁵、Naoto Fukuyo¹⁶、Eberhard Gischler¹⁷、Sahra Greve¹⁸、Amy Hagen¹⁹、Youri Hamon²⁰、Ed Hathorne²¹、Stephan Jorry²²、Pankaj Khanna²³、Helen McGregor³²、Richard Mortlock²⁴、Theresa Nohl²⁵、Ana Prohaska²⁶、Nancy Prouty²⁷、Willem Renema²⁸、Kenna H. Rubin²⁹、Hildegard Westphal³⁰、Yusuke Yokoyama³¹ (1.Nagoya University、2.University of Sydney、3.University of California Santa Cruz、4.British Geological Survey、5.University of Leicester、6.Université de Montpellier、7.University of St. Andrews、8.Tohoku University、9.Auburn University、10.Universidad de Granada、11.Barnard College、12.Guangzhou Institute of Geochemistry、13.University of Minnesota 、14.University of Wisconsin、15.MARUM, University of Bremen、16.Geological Survey of Japan、17.Goethe-University Frankfurt、18.University of Heidelberg、19.Virginia Tech、20.IFP Energies Nouvelles、21.GEOMAR Helmholtz Centre for Ocean Research、22.IFREMER、23.IIT Gandhinagar、24.Rutgers University、25.University of Vienna、26.University of Copenhagen、27.U.S. Geological Survey、28.Naturalis Biodiversity Center、29.University of Rhode Island、30.University of Bremen、31.The University of Tokyo、32.University of Wollongong)

キーワード：Quaternary, Glacial-Interglacial cycles, Hawaii, Drowned fossil coral reefs

IODP Expedition (Exp.) 389 (Hawaiian Drowned Reefs) cored 35 holes at 16 sites, in depths ranging from 132 to 1242 mbsl, in drowned fossil coral reef terraces around the island of Hawaii. Nine sites were on the leeward side of the island, five were on the windward side, and one site was off the South Point. A total of 426 m of core material was collected with an average recovery of 66%, consisting of 76% reef boundstone and 15% basalt. The maximum thickness of recovered fossil reef section is 73 m (Site 103), and 10 holes penetrated the boundary between reef limestone and the underlying basalt. The boundstone was classified by the relative proportions of corals, coralline algal crusts, and microbialite into five distinct lithofacies: coralgal boundstone, coralgal-microbialite boundstone, microbialite boundstone, microbialite-algal boundstone, and algal boundstone. Preliminary radiometric dating indicates the cored material spans 10 to 500 kyr. The IODP Exp. 389 cores provide a unique opportunity to explore the responses of coral reefs and reef communities to sea-level and climate changes over multiple glacial-interglacial cycles, as well as the impact of local climatic conditions, e.g., wet windward vs. dry leeward margin. We examined the fossil coral content of IODP Exp. 389 cores and report major trends in taxonomic and morphologic compositions within and between fossil reef terraces.
Unlike the high-diversity coral communities of the western Pacific, where Acropora is the dominant shallow reef-building coral, Hawaiian reefs today have lower taxonomic diversity dominated by Porites and Montipora. Analysis of the fossil corals in IODP Exp. 389 cores shows that Porites has been a major reef-building coral in Hawaii for at least the past 500 kyr. The Porites colonies have a variety of morphologies: laminar, (sub)massive, branching, and columnar. Associated taxa are Cyphastrea, Pocillopora, Montipora, and Agariciidae. On the leeward side, thick successions of branching and columnar Porites were recovered from two distinct terraces at depths of 400 m (Site 97) and 1200 m (Site 108) that span 34 and 28 m of core sections respectively, and suggest responses to sustained relative sea level rise during climate cycles 200 kyr apart were similar. Laminar corals are relatively more abundant on the windward side, which may reflect a higher energy setting or greater paleowater depths. Small encrusting Cyphastrea colonies are common in both leeward and windward sites and consistently occur within thick coralline algal crusts. We are not aware of this association having been reported previously from modern or fossil reefs. Acropora was present at only one site on the windward margin, which suggests this genus has been rare in the main Hawaiian Islands over the last 500 kyr. Trends in lithology and fossil content at the top of reef sequences are complex and no single drowning signature dominates. However, several holes on the windward margin and South Point are capped by rhodolith-rich sections up to 22 m in thick.