Hiroya Yamano

Holocene sea-level changes and barrier reef formation on an oceanic island, Palau Islands, western Pacific

Abstract Internal facies and development of an oceanic islands barrier reef were revealed by the stratigraphical study of six drill cores in Palau Islands, western Pacific. The Holocene reef development is primarily constrained at its foundation by the antecedent topography of Pleistocene substratum. Holocene barrier reef is an increment on the Pleistocene barrier reef, which had been subaerially exposed during glacial stages. About 8300 cal. year BP (calibrated calendar years B.P.), branching Acropora facies initially formed a bank on the seaward side of a Pleistocene limestone surface with a vertical accumulation rate as high as 30 m/ka (ka=1000 years). After 7200 cal. year BP, when the sea-level rise rate decreased, reef crest facies caught up with the sea surface with an accumulation rate of less than 2.2 m/ka. Corals found in the reef crest facies are similar to the present-day reef crest corals dominated by Acropora digitifera and A. humilis. After the reef crest was formed, bioclastic sand and gravel facies prograded lagoonward of the reef crest and consisted mostly of reef derived materials. The construction of patch reefs post-dated the barrier reef formation. The mature barrier reef provided calm conditions inside the lagoon, which then led to the construction of patch reefs and fringing reefs. Sea-level changes deduced from the accumulation curves show rapid rise before 7200 cal. year BP followed by a slight rise of 4 m at its maximum. This change in sea-level rise rate inspired the change in reef facies from branching Acropora to reef crest.

Anatomy of a Modern Coral Reef Flat: A Recorder of Storms and Uplift in the Late Holocene

ABSTRACT Cores from Kabira Reef, Ishigaki Island, southwest Japan, reveal the internal structure and temporal changes in sedimentary process of a complete coral reef flat. The reef crest caught up with sea level at about 4000 yr BP. Since then, it expanded oceanward and the reef pavement has grown landward. The backreef structure is composed of bioclasts derived from the reef framework. Of all the bioclasts, corals and coralline algae are most abundant. Coral fragments coarser than -3.0 o have dominated the backreef sediments since about 2000 yr BP. Benthic foraminiferal tests first occurred at about 4000 yr BP, and their abundance increased significantly starting around 2000 yr BP. Shallowing of the reef crest is also indicated by the presence of the shallow-water benthic Foraminifera Baculogypsina sphaerulata tests after 2000 yr BP. The date 2000 yr BP is coincident with tectonic uplift at Kabira Reef (Kawana 1989). We consider this uplift to have caused a relative sea-level fall that aided the deposition of coral fragments transported from the reef pavement by storms. The relative sea-level fall also caused subaerial exposure of the reef crest during low tides and the transition of reef-building organisms from corals to shallow-water species of benthic Foraminifera, resulting in a change noticeable in the constituents of backreef sediments. The results of this study suggest that the coral reef flat, and especially the backreef, can be a faithful recorder of relative sea-level changes.

δ15N of seagrass leaves for monitoring anthropogenic nutrient increases in coral reef ecosystems

In a coral reef environment, a slight increase in dissolved inorganic nitrogen (DIN;> or =1.0 micro M) can alter the ecosystem via macroalgal blooms. We collected seagrass leaves from the tropical and subtropical Pacific Ocean in five countries and examined the interactions between nutrient concentrations (C, N, P), molar ratios of nutrients, and delta15N to find a possible indicator of the DIN conditions. Within most sites, the concentrations of nutrients and their molar ratios showed large variations owing to species-specific values. On the other hand, almost identical delta15N values were found in seagrass leaves of several species at each site. The correlations between delta15N and nutrient concentrations and between delta15N and molar ratios of nutrients suggested that nutrient availability did not affect the delta15N value of seagrass leaves by altering the physiological condition of the plants. Increases in delta15N of seagrass leaves mostly matched increases in DIN concentrations in the bottom water. We suggest that delta15N in seagrass leaves can be a good tool to monitor time-integrated decrease/increase of DIN concentrations at a site, both in the water column and the interstitial water.

Lagoonal facies, ages, and sedimentation in three atolls in the Pacific

Abstract In order to investigate Pacific atoll lagoonal facies and temporal evolution, we collected and analyzed surface and core sediments from three Pacific atoll lagoons of various sizes and depths (Kayangel of Palau Islands, Enewetak and Majuro of the Marshall Islands). The lagoons were divided into three facies: Calcarina facies, Calcarina–Heterostegina mixed facies and Heterostegina facies. These were based on the presence or absence of the tests of two larger benthic foraminifera, Calcarina and Heterostegina , as indicators of reef flat and deep lagoon environments, respectively. The Calcarina facies is composed of allochthonous reef-derived materials, the Calcarina–Heterostegina mixed facies is a mixture of reef-derived and in situ lagoonal materials, and the Heterostegina facies is composed mainly of in situ lagoonal materials. As both foraminifera are widely distributed in the Western to Central Pacific region, the facies classification presented in this study can be applicable to other Pacific atolls. Based on the results of radiocarbon dating on samples obtained from cores at Majuro lagoon, the accumulation rates of the lagoonal sediments were estimated as 0.65 m kyr −1 . Calcium carbonate production was calculated on the basis of the accumulation rates and density of sediments to be 710 g CaCO 3 m −2 yr −1 .

Geographic variability in organic carbon stock and accumulation rate in sediments of East and Southeast Asian seagrass meadows

Organic carbon (OC) stored in the sediments of seagrass meadows has been considered a globally significant OC reservoir. However, the sparsity and regional bias of studies on long-term OC accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global OC sink. We evaluated the amount and accumulation rate of OC in sediment of seagrass meadows and adjacent areas in East and Southeast Asia. In temperate sites, the average OC concentration in the top 30 cm of sediment was higher in seagrass meadows (780–1080 μmol g−1) than in sediments without seagrass cover (52–430 μmol g−1). The average OC in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140 to 440 μmol g−1. Carbon isotope mass balancing suggested that the contribution of seagrass-derived carbon to OC stored in sediments was often relatively minor (temperate: 10–40%; subtropical: 35–82%; tropical: 4–34%) and correlated to the habitat type, being particularly low in estuarine habitats. Stock of OC in the top meter of sediment of all the studied meadows ranged from 38 to 120 Mg ha−1. The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32–1.34 mm yr−1). The long-term OC accumulation rates calculated from the sediment accumulation rate and the top 30 cm average OC concentration for the seagrass meadows (24–101 kg ha−1 yr−1) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha−1 yr−1 on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.

Species-Specific Responses of Corals to Bleaching Events on Anthropogenically Turbid Reefs on Okinawa Island, Japan, over a 15-year Period (1995–2009)

Coral bleaching, triggered by elevated sea-surface temperatures (SSTs) has caused a decline in coral cover and changes in the abundances of corals on reefs worldwide. Coral decline can be exacerbated by the effects of local stressors like turbidity, yet some reefs with a natural history of turbidity can support healthy and resilient coral communities. However, little is known about responses of coral communities to bleaching events on anthropogenically turbid reefs as a result of recent (post World War II) terrestrial runoff. Analysis of region-scale coral cover and species abundance at 17–20 sites on the turbid reefs of Okinawa Island (total of 79 species, 30 genera, and 13 families) from 1995 to 2009 indicates that coral cover decreased drastically, from 24.4% to 7.5% (1.1%/year), subsequent to bleaching events in 1998 and 2001. This dramatic decrease in coral cover corresponded to the demise of Acropora species (e.g., A. digitifera) by 2009, when Acropora had mostly disappeared from turbid reefs on Okinawa Island. In contrast, Merulinidae species (e.g., Dipsastraea pallida/speciosa/favus) and Porites species (e.g., P. lutea/australiensis), which are characterized by tolerance to thermal stress, survived on turbid reefs of Okinawa Island throughout the period. Our results suggest that high turbidity, influenced by recent terrestrial runoff, could have caused a reduction in resilience of Acropora species to severe thermal stress events, because the corals could not have adapted to a relatively recent decline in water quality. The coral reef ecosystems of Okinawa Island will be severely impoverished if Acropora species fail to recover.

Influence of wave energy on Holocene coral reef development: an example from Ishigaki Island, Ryukyu Islands, Japan

Abstract Wave energy induced by ocean swell and local winds is an important factor controlling the coral reef development. Here we compare and contrast facies, ages, and accretion rates of reefs that developed independently along windward and leeward margins of the same island. As both reefs face the open ocean, swells provide a background of low- to medium-energy condition with additional energy along windward margins due to prevailing winds. This similarity of the wave-energy regimes (medium to high energy) produced a similar framework facies that was constructed by robust-branching Acropora. However, much greater lateral accretion rates were observed in the high-energy reef than in the medium-energy reef. Also, coral cobble facies were only found in the medium-energy reef, indicating that the reef framework was more susceptible to storm breakage than that in the high-energy reef. These differences are attributable to the differing biological and ecological responses of corals to wave energy and water motion.

A multidisciplinary approach for dating human colonization of Pacific atolls

ABSTRACT The timing of reef platform emergence and the detailed chronology of reef island development provides a powerful backdrop for constraining the earliest period possible for prehistoric human colonization of low-lying atolls. Since Pacific atolls consist of biogenetic sediments, we dated foraminifera sands composed of well-preserved shallow-water species that are reliable indicators of facies formation. From transect excavations across the largest islet of Utrōk Atoll (11°13’N, 169°50’E) and Maloelap Atoll (8°47’N, 171°05’E), Marshall Islands, we selected nine foraminifera dating samples and five charcoal samples from prehistoric ovens in well-defined cultural layers and charcoal from buried A horizons. We document that: 1) the largest islets of Utrōk and Maloelap atolls expanded towards the lagoon shore at a rate of ∼70 m/kyr and ∼200m/kyr, respectively; 2) foraminifera sands immediately below buried A horizons in the islets “core” areas represents the timing of islet development at ∼2750 and ∼2400 cal BP, respectively; and 3) the oldest cultural dates (1850 and 1790 cal BP, ∼900–600 years younger than islet development) indicates that occupation much older than 2000 cal BP is unlikely, which is supported by sea level falling from its high stands to the present levels around 2000 cal BP for the northwestern Pacific.

Human impacts on large benthic foraminifers near a densely populated area of Majuro Atoll, Marshall Islands

Human impacts on sand-producing, large benthic foraminifers were investigated on ocean reef flats at the northeast Majuro Atoll, Marshall Islands, along a human population gradient. The densities of dominant foraminifers Calcarina and Amphistegina declined with distance from densely populated islands. Macrophyte composition on ocean reef flats differed between locations near sparsely or densely populated islands. Nutrient concentrations in reef-flat seawater and groundwater were high near or on densely populated islands. delta(15)N values in macroalgal tissues indicated that macroalgae in nearshore lagoons assimilate wastewater-derived nitrogen, whereas those on nearshore ocean reef flats assimilate nitrogen from other sources. These results suggest that increases in the human population result in high nutrient loading in groundwater and possibly into nearshore waters. High nutrient inputs into ambient seawater may have both direct and indirect negative effects on sand-producing foraminifers through habitat changes and/or the collapse of algal symbiosis.

Conservation of low-islands: high priority despite sea-level rise. A comment on Courchamp et al.

Assuming that a simple drowning model is applicable to all islands facing future climate change and sea-level rise (SLR), the future existence of up to 12% of islands is said to be compromised – and therefore these should not be considered for active management and protection [1,2]. This includes tropical atolls and their low-lying islands. However, we reject the triage strategy elaborated by Courchamp et al. [1]. Evidence from geology, sedimentology, and oceanography, and from the ecology of invasive species, shows that island conservation, especially of low islands, should remain a priority.