Holocene coral reef accretion in Hawaii: a function of wave exposure and sea level history

 In the high Hawaiian Islands, significant accretion due to coral reef growth is limited by wave exposure and sea level. Holocene coral growth and reef accretion was measured at four stations off Oahu, Hawaii, chosen along a gradient in wave energy from minimum to maximum exposures. The results show that coral growth of living colonies (linear extension) at optimal depths is comparable at all stations (7.7–10.1 mm/y), but significant reef accretion occurs only at wave sheltered stations. At wave sheltered stations in Hanauma Bay and Kaneohe Bay, rates of long term reef accretion are about 2.0 mm/y. At wave exposed stations, off Mamala Bay and Sunset Beach, reef accretion rates are virtually zero in both shallow (1 m) and deeper (optimal) depths (12 m). At wave sheltered stations, such as Kaneohe Bay and Hanauma Bay, Holocene reef accretion is on the order of 10–15 m thick. At wave exposed stations, Holocene accretion is represented by only a thin veneer of living corals resting on antecedent Pleistocene limestone foundations. Modern coral communities in wave exposed environments undergo constant turnover associated with mortality and recruitment or re-growth of fragmented colonies and are rarely thicker than a single living colony. Breakage, scour, and abrasion of living corals during high wave events appears to be the major source of mortality and ultimately limits accretion to wave sheltered environments. Depth is particularly important as a modulator of wave energy. The lack of coral reef accretion along shallow open ocean coastlines may explain the absence of mature barrier reefs in the high Hawaiian Islands. Accepted: 14 May 1998     

Pleistocene mollusc assemblage in the slightly raised atoll lagoon of Niau (Tuamotu,French Polynesia)

Surveys conducted at Niau, a slightly raised atoll in the Tuamotu Archipelago, revealed a brackish community with only a few extant species of fishes, crustaceans and molluscs in the residual semi-enclosed lagoon. Dredging of superficial sediments near the Niau lagoon resulted in a rich shell mollusc assemblage with 120 species, dating from the Pleistocene Last Interglacial Maximum (120,000 years BP). The assemblage contains 3 previously described new species, collected nowhere else since, and 12 species locally extinct but extant in the Indo-Pacific. Results are compared with previous studies on Pleistocene reefs of the Indo-Pacific.     

A geological perspective of the upland biota of Laysan atoll (Hawaiian Islands)

Laysan Island, a low atoll in the Leeward Hawaiian Islands, exhibits a biota that contains nine genera of upland and montane lineages of animals and plants usually of restricted range and occurrence and adapted to high island habitats. The geological history of Laysan Island, interpreted in terms of plate tectonic theory and recent ideas on the formation of linear island chains, shows that Laysan existed as an active high volcanic island approximately 15 million years ago. Since that time subaerial erosion and tectonic subsidence have combined to reduce the height of the island. During the past 250,000 years glacioeustatic sea level changes have resulted in Laysan Island fluctuating, geomorphologically, between a high limestone island and an atoll status. Laysan is viewed as a refugium for upland and montane lineages able to keep pace, via great adaptive flexibility, with drastic habitat changes. The thesis that plants of Laysan are recent arrivals is considered unlikely, as regards the upland contingent, in view of the geological history of the island.     

Model of fringing reef development in response to progressive sea level fall over the last 7000 years – (Kikai-jima, Ryukyu Islands, Japan)

 Kikai-jima in the central Ryukyu Islands of Japan is fringed by exposed terraces of Holocene reefs, which formed as a result of periodic local tectonic uplift associated with subduction/collision. The terraces form four topographically distinct features (TI-IV) around the island and represent reefs that grew to sea level at 9000–6065 y BP, 6065–3390 y BP, 3790–2630 y BP, and 2870 to 1550 y BP. The modern reef terrace has been growing since approximately 1550 y BP. The reef terraces were uplifted sequentially around 6050 y BP (4 m), 3390–3790 y BP (2.5 m), 2630–2870 y BP (1 m) and 1550 y BP (2.5 m). Five sites were studied to define reef development in response to periodic relative sea level fall and different stillstand recovery periods. Thirty coral genera and 70 species were recorded from four distinct shallow reef flat to upper reef slope and one deeper reef slope coral assemblage. Significant lateral variations in total coral abundance, genera number, diversity, and the coverage density of Acropora spp. and Faviids occur both within and between the terraces. Stratigraphically, drill core and outcrop data recorded shallowing upward sequences characterised by tabulate Acropora spp. overlying massive Porites sp. and Faviids. The biological variations may represent growth strategies responding to initial colonisation, episodic perturbation (relative sea level fall) and differing recovery times during stillstands, and indicate a reef ecosystem stable and strong enough to recover after substantial perturbations. However, this study suggests that relatively small geological changes have had substantial biological effects, and modelling indicates that such changes would have been more profound had a third factor, such as substrate angle, varied more dramatically. In such a case, the drowning growth strategy exhibited in the drill core transect may have been more prevalent, and reefs would be struggling to grow around Kikai-jima today. Accepted: 27 May 1998     

Geomorphology of the uplifted Pleistocene atoll at Henderson Island, Pitcairn Group

Henderson Island, in the Pitcairn Group, preserves a Pleistocene atoll physiography with the rim of the raised reef structure, supporting spur and groove topography, enclosing a central lagoon. Excellent preservation of coral reef communities occurs along the ancient atoll rim and within the central lagoon. The previously interpreted depositional nature of the fossil atoll structure is herein corroborated with geomorphologic and stratigraphic evidence from previously un-visited portions of the island. Stratigraphic and lateral facies relationships indicate a physiographic zonation which includes spur and grooves, outer reef flat, lagoon margin, and an interior lagoon with patch reefs. The in situ occurrence and zonation of reef coral communities around the periphery and within the interior of the island appear to reflect the original physiography of the atoll lagoon, with the most pronounced reef development on the SE side of the original atoll. Stratigraphic units which comprise the raised atoll lagoon structure represent different time intervals, so the atoll lagoon structure formed during various sea level fluctuations. The modern atolls of the Pitcairn Group, Oeno and Ducie, provide some comparisons (similarities and differences) with the fossil lagoon on top of Henderson Island.     

Extrinsic control of species replacement on a Holocene reef in Belize: the role of coral disease

 Well-preserved, Holocene coral reefs provide the opportunity to discriminate between models of intrinsically driven succession and extrinsically driven species replacement, especially when paleontological patterns can be combined with ecological observations of the underlying mechanisms. Rhomboid shoals in the central shelf lagoon of the Belizean Barrier Reef experienced a recent and dramatic change in community composition. Agaricia tenuifolia replaced Acropora cervicornis as the dominant coral species at 3–15 m depth along the flanks of the reefs. We tested the hypothesis that shallowing upward caused this shift in dominance. A core extracted from 0.5 m water depth on one of the shoals, Channel Cay, revealed a shallowing-upward shift in dominance from Acropora to Porites divaricata. This successional sequence was quite different from the Acropora-to-Agaricia transition observed in four cores from 6–11 m water depth. Ecological observations showed that Agaricia became the dominant at ≥3 m depth after Acropora populations were decimated by a regional outbreak of white-band disease. The Acropora-to-Agaricia transition was clearly a case of extrinsically driven species replacement rather than an intrinsically driven, successional, shallowing-upward sequence. Accepted: 14 May 1998     

Coral growth in subtropical eastern Australia

 Extension rates of corals at two sites in subtropical eastern Australia (Solitary Islands and Lord Howe Island) were measured to determine whether growth was low relative to tropical locations. Growth was measured using alizarin staining of skeletons and X-radiographic analysis, and was compared between colonies, species, and sites. Linear extension of individual Pocillopora damicornis colonies averaged 12.4 to 16.1 mm per year at Solitary Islands and Lord Howe Island respectively, which is 50% to 80% of published values for this species at tropical sites. Similarly, average extension of most massive faviid species examined at these sites was between 2.6 mm and 4.6 mm per year, considerably lower than most values reported from lower latitudes (generally 6 mm to 10 mm per year). However, growth rates of Acropora yongei, Turbinaria frondens, and Porites heronensis were close to those of closely-related taxa from the tropics. Causal links between latitude, growth rates of coral colonies, and the potential for reef accretion remain unclear. Accepted: 22 April 1999     

Shifting ecological baselines and the demise of Acropora cervicornis in the western North Atlantic and Caribbean Province: a Pleistocene perspective

 In recent years, marine scientists have become increasingly alarmed over the decline of live coral cover throughout the Caribbean and tropical western Atlantic region. The Holocene and Pleistocene fossil record of coral reefs from this region potentially provides a wealth of long-term ecologic information with which to assess the historical record of changes in shallow water coral reef communities. Before fossil data can be applied to the modern reef system, critical problems involving fossil preservation must be addressed. Moreover, it must be demonstrated that the classic reef coral zonation patterns described in the early days of coral reef ecology, and upon which “healthy” versus “unhealthy” reefs are determined, are themselves representative of reefs that existed prior to any human influence. To address these issues, we have conducted systematic censuses of life and death assemblages on modern “healthy” patch reefs in the Florida reef tract that conform to the classic Caribbean model of reef coral zonation, and a patch reef in the Bahamas that is currently undergoing a transition in coral dominance that is part of a greater Caribbean-wide phenomenon. Results were compared to censuses of ancient reef assemblages preserved in Pleistocene limestones in close proximity to each modern reef. We have determined that the Pleistocene fossil record of coral reefs may be used to calibrate an ecological baseline with which to compare modern reef assemblages, and suggest that the current and rapid decline of Acropora cervicornis observed on a Bahamian patch reef may be a unique event that contrasts with the long-term persistence of this taxon during Pleistocene and Holocene time. Accepted: 19 May 1998     

Holocene uplifted coral reefs in Lanyu and Lutao Islands to the southeast of Taiwan

Lanyu and Lutao Islands to the southeast of Taiwan are located in the northern extension of the Luzon Arc. Crustal deformation of these islands provides a key to understand the collision of the Luzon Arc against Taiwan. To clarify the style and the rate of vertical movement during the Holocene, uplifted coral reefs fringing these two islands were investigated. Living corals were also investigated for comparison with fossil corals. It was found that Isopora palifera lives dominantly in the upper slope of the present-day fringing coral reefs in Lanyu Island at an average depth of 101 ± 46 cm (one standard deviation) below mean sea level. Using I. palifera as an accurate indicator of paleo-sea levels, Holocene relative sea-level changes were reconstructed. Lanyu Island has been uplifted continuously at a rate of 2.0 mm yr⁻¹, at least during the late Holocene from 2,269 cal. yr BP to the present. Lutao Island has been uplifted at an average rate of 1.2 mm yr⁻¹, since at least 5,749 cal. yr BP, although it is unclear whether the uplift was continuous. The present observations, combined with the GPS displacement field and deep crustal structure, suggest that the continuous uplift is related to aseismic slip on the Longitudinal Valley Fault.     

Holocene growth of a mid-Pacific atoll: Tarawa,Kiribati

Cores from ten holes, drilled to a maximum depth of 30 m, on Tarawa atoll in the central Pacific have been utilised in a study of the Holocene development of the atoll. Four dominant lithologies, in descending order, are cay rock, unconsolidated sediment, corals and leached limestone. Petrographic and radiometric age analyses indicate that the Holocene reef has developed on a previous (last interglacial) reef; the latter shows the effects of both vadose and phreatic freshwater diagenesis. Hydrological investigations beneath the present islands indicate the presence of freshwater lenses up to 29 m thick; the modern lenses are unrelated to freshwater diagenetic imprints preserved within the limestones. Vertical accretion rates of 5–8 m/1000 years for the Holocene reef section on Tarawa are significantly higher than rates measured for other Pacific atolls. The dated coral sequences suggest a more rapid rate of sea level rise during the early Holocene, and a relatively earlier stabilisation of sea level than has been suggested previously.     

U-series evidence for widespread reef development in Shark Bay during the last interglacial

Field observations and U-series ages reveal that Shark Bay, Western Australia (WA) has been inundated by the sea on at least three occasions during the Late Pleistocene/Holocene, resulting in a succession of marine deposits around the Bay. The exact age of these deposits has until now been problematic due to a lack of reliable and accurate age data. This study reports 16 new U-series coral dates from emergent reef deposits around Shark Bay, and point to an extended period of coral reef development during the peak of the last interglacial, marine isotope stage (MIS) 5e. This is attributed to enhancement of marine circulation within the reaches and basins, a result of higher sea levels and an absence of major sill and bank structures. Stromatolites are absent from the geological record within Shark Bay until the late Holocene, suggesting that sea levels and marine sedimentary processes that have operated during the present sea-level highstand are unique to this period. There is little direct evidence of fossil reef development occurring during interglacials of the middle/late Pleistocene (MIS 9/11).     

Holocene sea level changes and reef development in the southwestern Indian Ocean

 The sedimentological and chronological study of Holocene reef sequences recovered in drill cores through modern reefs of Mauritius, Réunion Island and Mayotte allows the reconstruction of sea level changes and reef growth patterns during the Holocene. The branching-coral facies systematically predominates over coral head facies throughout the Holocene reef sequences, and Acropora is the main frame builder among the branching forms. The reconstructed sea level curves, based both on identification of coral assemblages and on radiometric U/Th ages, are characterized by a rapid rise between 10 and 7.5 ky BP, followed by a clear inflection between 7.5 and 7 ky BP. The stabilization of sea level at its present level occurred between 2000 and 3000 years ago, probably without a higher sea level stand. Rates of vertical reef accretion range between 0.9 and 7 mm. y^-1. In Mauritius, and also probably in Réunion Island, the reef first tracked, then caught-up to sea level to reach an equilibrium position (“catch-up” growth), while the barrier reef margin off Mayotte has been able to keep pace with rising sea level (“keep-up” growth). Accepted: 1 March 1997     

Pleistocene facies of Belize barrier and atoll reefs

The knowledge of Pleistocene reef facies of Belize, Central America, is largely limited to outcrops in the northernmost part of the country. Otherwise, Pleistocene limestone, which forms the basement of the modern barrier and atoll reefs, occurs in the subsurface and is to a major extent unstudied. Based on the study of 40 m of core from 25 rotary core holes collected on central and southern Belize barrier and on atoll reefs, five Pleistocene reef facies are distinguished in the present study. They include (1) Acropora palmata grainstone, (2) Acropora cervicornis grainstone, (3) biogenic grainstone, (4) mollusk packstone, and (5) mollusk-foram wackestone. Facies 1 and 3 occur on marginal reefs, facies 2 is found on marginal and lagoonal reefs, and facies 4 and 5 mark lagoon shoals and lagoons, respectively. Most of the facies have equivalents in the Pleistocene of the wider Caribbean and also in the modern of the study area. Diagenetic features include dissolution, caliche formation, laminated blocky low-magnesium-calcite and dogtooth spars. Age data from Pleistocene corals obtained during earlier studies are discussed, and indicate deposition during marine isotope stage 5, between 140–80 ka bp.     

Porites growth characteristics in a changed environment: Misima Island, Papua New Guinea

 The construction in 1988 of an open-cut gold mine and ore-processing facility at Misima Island, Papua New Guinea, resulted in disturbance of the adjacent fringing coral reef, mostly because of large increases in sedimentation. This provided an opportunity to examine whether growth characteristics of the major reef-building coral, Porites, changed in response to this sudden and sustained increase in sedimentation. Annual variation in skeletal density was measured in 93 colonies variously affected by sedimentation. The colonies provided data for average annual density, annual extension and annual calcification covering the periods 5 y before and 5 y after mining operations began. The average depth of skeleton occupied by tissue (tissue layer thickness) was also measured for most colonies. There was high mortality of Porites in regions strongly affected by increased sedimentation. In colonies that survived, density, extension and calcification tended to be less (in some cases significantly) in the period after mining operations began compared with pre-construction levels. However, these decreases were not linked with proximity to the mine site and probably reflect a regional-scale response of Porites growth to some other environmental change. This suggests that periods of high sedimentation may not be recorded by the growth characteristics of massive Porites. Average growth characteristics of surviving Porites from Misima Island were similar to those from inshore reefs of the northern Great Barrier Reef (GBR). Tissue layer thickness in Porites from the control areas at Misima Island were also similar to colonies from the northern inshore GBR reefs. However, tissue layer thickness significantly decreased with increased proximity to the mine site at Misima Island. Accepted: 15 May 1999     

U-series and radiocarbon analyses of human and faunal remains from Wajak,Indonesia

Laser ablation U-series dating results on human and faunal bone fragments from Wajak, Indonesia, indicate a minimum age of between 37.4 and 28.5 ka (thousands of years ago) for the whole assemblage. These are significantly older than previously published radiocarbon estimates on bone carbonate, which suggested a Holocene age for a human bone fragment and a late Pleistocene age for a faunal bone. The analysis of the organic components in the faunal material show severe degradation and a positive δ¹³C ratio indicate a high degree of secondary carbonatisation. This may explain why the thermal release method used for the original age assessments yielded such young ages. While the older U-series ages are not in contradiction with the morphology of the Wajak human fossils or Javanese biostratigraphy, they will require a reassessment of the evolutionary relationships of modern human remains in Southeast Asia and Oceania. It can be expected that systematic direct dating of human fossils from this area will lead to further revisions of our understanding of modern human evolution.     

Phylogeography of West Indies Coral snakes (Micrurus): Island colonisation and banding patterns

In this study, we analyse New World coral snakes in a phylogenetic framework based upon an increased molecular data set, including novel sequences for the only two sympatric species known from an island (Trinidad, West Indies). Their presence in Trinidad and absence in Tobago offers a unique system to study the phylogeography of the region. We assess the tempo and mode of colonisation of Micrurus on the island, in addition to discussing the phylogenetic relationships for the genus Micrurus concerning two phenotypic traits, body and tail banding patterns. These relationships are analysed for the first time on statistical coalescent phylogeographic discrete ancestral reconstruction. We find a robust phylogenetic component in these characteristics, where strongly supported clades are recovered: prior to the onset of the Early Miocene, a triadal and tricolour tail clade composed of species from South America, and a second clade dating to the Middle‐Late‐ Miocene with monadal and bicolour tails widely distributed from North to South America. The divergence between clades dates to the Oligocene and suggests an ancient pre‐isthmus divergence supporting the arrival of the triadal clade into South America, before the connection between Central and South America was established. We find the two coral snakes present in the West Indies, M. diutius and M. circinalis, belong to the triadal and monadal clades, respectively. Guyana and Trinidad Micrurus diutius share the same haplotypes suggesting a Late Pleistocene–Holocene vicariance when sea level rises separated Trinidad from the mainland. A second lineage of diutius‐like snakes is present in Guyana and is confirmed as M. lemniscatus which is assigned as a voucher and restricts the type locality for M. lemniscatus.     

Climate‐driven shifts in the distribution of koala‐browse species from the Last Interglacial to the near future

The koala's Phascolarctos cinereus distribution is currently restricted to eastern and south‐eastern Australia. However, fossil records dating from 70 ± 4 ka (ka = 10³ yr) from south‐western Australia and the Nullarbor Plain are evidence of subpopulation extinctions in the southwest at least after the Last Interglacial (~128–116 ka). We hypothesize that koala sub‐population extinctions resulted from the eastward retraction of the koala's main browse species in response to unsuitable climatic conditions. We further posit a general reduction in the distribution of main koala‐browse trees in the near future in response climate change. We modelled 60 koala‐browse species and constructed a set of correlative species distribution models for five time periods: Last Interglacial (~128–116 ka), Last Glacial Maximum (~23–19 ka), Mid‐Holocene (~7–5 ka), present (interpolations of observed data, representative of 1960–1990), and 2070. We based our projections on five hindcasts and one forecast of climatic variables extracted from WorldClim based on two general circulation models (considering the most pessimistic scenario of high greenhouse‐gas emissions) and topsoil clay fraction. We used 17 dates of koala fossil specimens identified as reliable from 70 (± 4) to 535 (± 49) ka, with the last appearance of koalas at 70 ka in the southwest. The main simulated koala‐browse species were at their greatest modelled extent of suitability during the Last Glacial Maximum, with the greatest loss of koala habitat occurring between the Mid‐Holocene and the present. We predict a similar habitat loss between the present and 2070. The spatial patterns of habitat change support our hypothesis that koala extinctions in the southwest, Nullarbor Plain and central South Australia resulted from the eastward retraction of the dominant koala‐browse species in response to long‐term climate changes. Future climate patterns will likely increase the extinction risk of koalas in their remaining eastern ranges.     

A unique coral reef formation discovered on the Great Astrolabe Reef, Fiji

 A spectacular mound-like reef formation (126 m in circumference, 10 m high) dominated by highly arched and record-size colonies of the unattached mushroom coral Halomitra pileus, along with 17 other species of the family Fungiidae, occurs in 31 m of water on the sedimentary lagoon floor of the Great Astrolabe Reef, Fiji. Core samples show radiocarbon dates which indicate that the formation hypothetically began building ∼4500 y ago, with a calculated mean accretion rate of 2.2 mm ⋅ y^-1. The majority of fossil and living material is contributed by H. pileus colonies between 40–70 cm mean diameter, with some individuals up to 1.5 m in diameter. The size, fungiid diversity, and geological history of the bioherm is unprecedented and represents the first example of a coral reef constructed almost entirely by Fungiidae. Accepted: 29 July 1996     

Supercolony mosaics: two different invasions by the yellow crazy ant, Anoplolepis gracilipes, on Christmas Island, Indian Ocean

Invasive species are one of the main reasons for the ongoing global loss of biodiversity. Anoplolepis gracilipes is an invasive ant that has recently received significant attention due to its negative effect on the native fauna and flora of Christmas Island, Indian Ocean. This species has contributed to a drastic change in the structure of the Christmas Island rainforest through its negative impact on the island’s endemic red land crab, the dominant consumer on the islands forest floor. In this study, we investigate the population structure of A. gracilipes on Christmas Island in order to determine whether multiple introductions occurred on the island and how they correspond to known infestations. We genotyped 578 individuals collected from 50 nests across the Island. We identify two distinct subgroups in the population that represent two different supercolonies. These supercolonies are interspersed across the island, however both nuclear (microsatellites) and mitochondrial markers strongly suggest that there is no gene flow between the two colonies. Significant heterozygote excess within the entire sampling area, with all but one worker examined being heterozygous for all seven microsatellite loci, suggests an unusual reproductive system in these ants. Our results are consistent with recent sociogenetic findings in a population of A. gracilipes in Northern Borneo.     

Implementation of a small-scale “no-use zone” policy in a reef ecosystem: Eilat’s reef-lagoon six years later

 A small-scale, “no-use zone policy” has been implemented since 1992 at Eilat’s Coral Nature Reserve (Northern Red Sea). Six years later, the status of this closed-to-the-public reef area was compared to two nearby open-to-the-public sites, by evaluating populations of the scleractinian coral Stylophora pistillata in the strolling zone (0.5–1.5 m depth). Results from the open sites show that: (1) Live coral cover was three times lower than at the closed site; (2) numbers of small colonies (recruits) were significantly higher than in the closed site, while numbers of medium and large size colonies (geometric mean radius, rˉ>4.1 cm) per m² were significantly lower; (3) maximum rˉ was almost half than that in the closed site (9.6 cm versus 16.7 cm); (4) average number of broken colonies was three times higher than in the closed site; (5) significantly fewer colonies were partially dead. The latter result may reflect senescence processes in the large colonies of the closed site. Although colony breakage is reduced, it appears that the “no-use zone” policy is not sufficient for protecting small reef areas. The intense exploitation of Eilat’s coral reef by the tourist industry requires’ in addition to the conventional protective measures, the initiation of novel management solutions such as reef restoration by sexual and asexual recruits. Accepted: 11 August 1999