Holocene Reef Development Research Articles

Antecedent topography and active tectonic controls on Holocene reef geomorphology in the Great Barrier Reef

The Great Barrier Reef is generally considered a passive tectonic setting, however the effect of antecedent topography and local geological structures on Holocene reef development is poorly understood. Offshore reefs along the central shelf were recently hypothesized to have grown continuously through a possible small Holocene sea level fall, in response to greater local subsidence, in contrast to reefs immediately to the north and south, which experienced synchronous turnoff phases. We tested this hypothesis by isolating four map zones along the GBR shelf and using: (1) a geomorphic reef type classification applying perceived evolutionary “age” as juvenile, mature, and senile domains based on geomorphology; (2) analysis of bathymetry data to understand the role of antecedent topography on the spatial distribution of reef type domains and general sea floor depths; and (3) the distribution of earthquake epicentres as an indication of possible active geological structures (i.e., faults). The results reveal that juvenile reefs (42.25 %) are more abundant in the central shelf, whereas mature-2 (40 %) and senile (31 %) reefs are more abundant in the northern zone. In the south-central zone, mature-2 reefs are prevalent, while the southern zone includes a mixture of juvenile and mature to senile domains with a sharp internal boundary. These results support the hypothesis that the central zone may have experienced greater active subsidence during the Holocene. Bathymetric data support greater regional subsidence in both central and south-central zones, but reefs are less abundant and cover a lesser percentage area of their antecedent platforms in the central zone, consistent with greater subsidence there. The boundaries between the central zone and adjacent zones are the sites of clusters of recent earthquakes, consistent with the occurrence of active geological faults bounding the zone of greater Holocene subsidence. Combined, our data support the occurrence of a tectonically defined region of active greater subsidence in the central GBR shelf that has affected the geomorphology and growth history of reefs through the Holocene. Knowledge of such spatial partitioning of reef behaviour may allow reef managers to better suite their efforts to local conditions, especially in regard to predicted sea level rise, while highlighting potential seismic risks over longer time frames.

Environmental controls on holocene reef development along the eastern brazilian margin

Environmental controls on holocene reef development along the eastern brazilian margin

Climate and the latitudinal limits of subtropical reef development

Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida’s southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida’s reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5–26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.

A new model of Holocene reef initiation and growth in response to sea-level rise on the Southern Great Barrier Reef

The fossil record provides valuable data for improving our understanding of both past and future reef resilience and vulnerability to environmental change. The spatial and temporal pattern of the initiation of the Holocene Great Barrier Reef presents a case study of reef response to rapid sea-level rise. Past studies have been limited by the lack of well-dated and closely spaced reef core transects and have not closely examined the composition of the reef-building communities through time. This study presents 80 new high precision UTh and 5 radiocarbon ages from twelve new cores located along three transects across different geomorphic and hydrodynamic settings of One Tree Reef, southern Great Barrier Reef, to document three distinct stages of Holocene reef development in unprecedented detail. Temporal constraints on changing paleoecological assemblages of coral, coralline algae and associated biota revealed three distinct phases of reef development, consisting of: 1)a fast, shallow and clear-water reef initiation from 8.2 until 8 ka; 2)a shift to slower, deeper and more turbid-water reef growth from 8 to 7 ka; and 3)a return to shallow and rapid branching coral growth in clear-water conditions as the reef “catches up” to sea-level. A minimum lag prior to reef initiation of 700 years was identified, which differs in length depending on reef environment and Pleistocene substrate height. In this new model, reef growth initiated on the topographically lower leeward margin and patch reef, prior to the start of windward margin development, contrary to the traditional reef growth model. While there was a shift to conditions less favorable for reef growth at 8 ka, this did not prevent the slow accretion of more sediment-tolerant coral communities. The majority of the reef reached sea level by ~6 ka. This new conceptual model of Holocene reef growth provides new constraints on changes in paleoenvironment that controlled reef community composition and growth trajectories through sea-level rise following inundation.

The Variable Influences of Sea Level, Sedimentation and Exposure on Holocene Reef Development over a Cross-Shelf Transect, Central Great Barrier Reef

Coral reefs globally are impacted by natural and anthropogenic stressors that are compounded by climate change. Understanding past reef responses to natural stressors (cyclones, sea-level change, freshwater inputs, and sedimentation) can provide important insights to further understand recent (within the past century) trends in coral cover and diversity. Here we use a compilation of recently published data to investigate the Holocene development of five fringing reefs that are located on a cross-shelf transect on the central Great Barrier Reef, and that are exposed to varying degrees of natural and anthropogenic sedimentation, storm exposure, and Holocene sea-level change. Forty-two reef cores collected using a combination of manual percussion coring and hydraulic drilling techniques, were analysed and dated using uranium-thorium methods. The chronostratigraphic records of reef development established using 105 recently published radiometric ages and seven new uranium-thorium ages from the reef cores and fossil microatolls preserved across the reef flats were compared to investigate cross-shelf variations in reef development. This is the first study to conduct an internal investigation of reef framework across an inshore–offshore gradient to examine the varying levels of influence of sedimentation, sea level and cyclones. Our observations from the central Great Barrier Reef show that reefs furthest offshore from the mainland coast were typically initiated earliest after the post-glacial marine transgression. Reef flat size, morphology, and growth style varied according to constraints placed on reef development by the composition, depth, shape, and relief of the underlying substrate. We establish that terrigenous sedimentation had a marked effect on the development of inshore reefs closest to the mainland (within 10 km of the mainland coast). Periods of relatively high terrigenous sedimentation correspond with enhanced reef accretion rates, and also resulted in a superior record of palaeo-ecological coral composition (i.e., better preservation) at inshore sites. In contrast, mid-Holocene cyclones played a seemingly more important role in the development of reefs >10 km from the mainland; although cyclones clearly affect reefs closer inshore, their geomorphology is affected by a range of controlling factors. Insights provided by these five Holocene reef chronostratigraphies provide useful baseline understanding of reef condition and growth along a cross-shelf transect where the reefs are exposed to variable stressors.

Holocene reef growth over irregular Pleistocene karst confirms major influence of hydrodynamic factors on Holocene reef development

Many factors govern reef growth through time, but their relative contributions are commonly poorly known. A prime example is the degree to which modern reef morphology is controlled by contemporary hydrodynamic settings or antecedent topography. Fortunately, reefs record essential information for interpreting palaeoclimate and palaeoenvironment within their structure as they accrete in response to environmental change. Five new cores recovered from the margin of Heron Reef, southern Great Barrier Reef (GBR), provide new insights into Holocene reef development and relationships between Holocene reefs and Pleistocene antecedent topography, suggesting much more irregular underlying topography than expected based on the configuration of the overlying modern reef margin. Cores were recovered to depths of 30 m and 94 new 230Th ages document growth between 8408 ± 24 and 2222 ± 16 yrs. BP. One core penetrated Pleistocene basement at ∼15.3 m with Holocene reef growth initiated by ∼8.4 ka BP. However, 1.83 km west along the same smooth margin, four cores failed to penetrate Pleistocene basement at depths between 20 and 30 m, suggesting that the margin at this location overlies a karst valley, or alternatively, the antecedent platform does not extend there. A 48 m-long margin-perpendicular transect of three cores documents the filling of this topographic low, at least 30 m beneath the current reef top, with seaward lateral accretion at a rate of 34.3 m/ka. Cores indicate steady vertical and lateral accretion between 3.2 and 1.8 ka BP with no evidence of the hiatus in reef flat progradation seen in most other offshore reefs of the GBR at that time. These cores suggest that the relative protection afforded by the valley allowed for unconsolidated sediment to accumulate, enabling continuous progradation even when other areas of the reef flat appear to have ‘turned off’. Additionally, the cores suggest that although reefs in the southern GBR clearly owe their location to Pleistocene antecedent topography, modern reef morphology at sea level primarily reflects the interaction of Holocene reef communities with contemporary hydrodynamics.

Late Quaternary barrier and fringing reef development of Bora Bora (Society Islands, south Pacific): First subsurface data from the Darwin‐type barrier‐reef system

AbstractThe universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium‐series dates obtained and subsequently analysed. Sea‐level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age–depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 ± 50 yr bp; accretion rates average 5·65 m kyr−1. The barrier reef consists of >30 m thick Holocene coralgal and microbial successions. Holocene barrier‐reef growth began 10 030 ± 50 yr bp and accretion rates average 6·15 m kyr−1. The underlying Pleistocene reef formed 116 900 ± 1100 yr bp, i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0·05 to 0·14 m kyr−1. In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sea‐level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier‐reef systems in Tahiti and Mayotte exhibit more differences than similarities.

Microborer ichnocoenoses in Quaternary corals from New Caledonia: reconstructions of paleo-water depths and reef growth strategies in relation to environmental changes

Coral reef growth and development depend on several environmental factors, including tectonic and climatic parameters and local ecological drivers. Reef growth is especially sensitive to sea-level variations. Paleo-water depth reconstructions are essential tools used to determine reef growth patterns during different periods of reef growth. Assemblages of corals and/or coralline algae have been commonly used in such paleodepth reconstructions. This study shows that using microendolith ichnocoenoses can sometimes provide better accuracy than traditional coralgal analyses, particularly in the depth-range 0–10 m where coralgal assemblages usually show broad distribution ranges. Holocene and Pleistocene cores from two barrier reef sites on the west coast of Grande Terre in New Caledonia are examined here. Holocene reef development at these sites feature examples of microendolith ichnocoenoses that document rapid environmental changes and small sea-level variations of about 2–5 m in amplitude, and record these changes with more accuracy than coral and coralline algae assemblages which are highly dependant on the hydrodynamic energy of the setting. During the Pleistocene, which was less chronologically constrained, the microendolith ichnocoenoses also reflect paleo-water depths and reef-growth patterns at different periods of reef history.

Holocene aggradation of the Dry Tortugas coral reef ecosystem

Radiometric age dating of reef cores acquired at the Dry Tortugas coral reef ecosystem (DTCRE) was merged with lidar topographic mapping to examine Holocene reef development linked to spatial variation in growth and erosion under the control of sea level. Analysis of variance of lidar topography confirmed the presence of three distinct terraces on all three major DTCRE banks (Loggerhead Bank, Garden Bank, and Pulaski Bank). Reef building on the middle terrace (T2) began atop Pleistocene edifices on Loggerhead Bank by 8.0 ka (thousands of years ago) and on Garden Bank by 7.2 ka at elevations of about −16.0 m and −11.9 m, respectively, relative to present mean sea level. Following this initiation at different elevations, T2 aggraded vertically on both banks at different rates during the early Holocene under foundering conditions until a highstand at 5.2 ka, resulting in a 2.21 m offset in present mean T2 elevation between these banks. Initiation of an upper terrace (T1) occurred on both Loggerhead Bank and Garden Bank in association with sea-level fall to a lowstand at near 4.8 ka. This upper terrace initiated on Garden Bank at about 5.0 ka and then grew upward at rate of 2.5 mm year−1 until approximately 3.8 ka. On Loggerhead Bank, the upper T1 terrace formed after 4.5 ka at a higher vertical aggradation rate of 4.1 mm year−1, but at a lower elevation than on Garden Bank. Terrace T1 aggraded on Loggerhead Bank below the elevation of lowstands during late Holocene sea-level oscillation, and consequently erosion on Loggerhead Bank was minimal and likely limited to the crest of the upper terrace. In contrast, after 3.8 ka terrace T1 on Garden Bank likely tracked sea level and consequently underwent erosion when sea level fell to second, third and fourth lowstands at 3.3, 1.1, and 0.3 ka.

Development and demise of a fringing coral reef during Holocene environmental change, eastern Ningaloo Reef, Western Australia

Reefs lining the western Exmouth Gulf, located at the northern limit of the 300 km long Ningaloo Reef in Western Australia, represent modern incipient coral reefs and veneers of non-reef-building coral/algal communities on exposed Pleistocene or ‘give-up’ Holocene reef surfaces. Acquisition of sixteen cores alongside U-series TIMS dates were used to confirm the nature of the Pleistocene foundation and characterise Holocene reef development. Three calcretised Pleistocene units were identified as 1) the Last Interglacial (MIS 5e) reef directly underlying Holocene units, 2) a mid-Pleistocene (MIS 7?) bioclastic conglomerate unit, and 3) a Pleistocene alluvial fanglomerate. Eight Holocene reef facies (total thickness of 1.8–5.3 m) included coral framework facies (domal, arborescent, mixed, tabulate and encrusting) and detrital facies (carbonate sand, skeletal rubble and alluvial fan deposits). Holocene ages range from 7.93 to 5.8 ka BP with vertical accretion ranging from 1.46 to 9.88 mm/year (avg. 4.11 mm/year). Highest rates of accretion and thickest accumulation occurred in the most seaward and deepest cores composed of massive coral framestone and coralline algal crusts. A six stage Holocene chronology is proposed, including 1) coastal inundation from 8 to 8.5 ka BP, 2) initiation ‘start-up’ from 8 to 7.5 ka BP, 3) rapid growth ‘catch-up’ and back-step from 7.5 to 7 ka BP, 4) rapid aggradational growth ‘catch-up’ from 7 to 6.5 ka BP, 5) reef decline ‘give-up’ and detrital buildup from 6.5 to 5.8 ka BP, and 6) detrital buildup and progradation from 5.8 ka BP to present. Changes in reef facies and the ultimate demise of the Holocene reef probably involved a combination of increased sea-level, coastal flooding and erosion during the mid-Holocene highstand, with associated increase in sedimentation, turbidity and decline in water quality; burial by sediment buildup during the mid-Holocene highstand and detrital progradation during the mid- to late-Holocene regression; and, the introduction of alluvial sediment during cyclones and other severe storms to an already stressed community. Modern communities have thus shifted from coral-dominated to bored macroalgal pavements. This study shows that integration of reef development processes with response to environmental change can be used to assess future pressures on coral reef ecosystems globally.

Potential of submarine-cave sediments and oxygen isotope composition of cavernicolous micro-bivalve as a late Holocene paleoenvironmental record

A sediment layer (43 cm thick) and surface sediments (5 cm thick) in a submarine limestone cave (31 m water depth) on the fore-reef slope of Ie Island, off Okinawa mainland, Japan, were examined by visual, mineralogical and geochemical means. Oxygen isotope analysis was performed on the cavernicolous micro-bivalve Carditella iejimensis from both cored sediments and surface sediments, and the water temperature within the cave was recorded for nearly one year. These data show that: (1) water temperature within the cave is equal to that at 30 m deep in the open sea; (2) the biotic and non-biotic environments within the cave have persisted for the past 2000 years; (3) mud-size carbonate detritus is a major constituent of the submarine-cave deposit, and may have come mainly from the suspended carbonate mud produced on the emergent Holocene reef flat over the past two millennia; (4) the δ 18O-derived temperature ( T δ 18 O) of C. iejimensis suggests that the species grows between April and July; (5) the T δ 18 O of C. iejimensis from cored sediments implies that there were two warmer intervals, at AD 340 ± 40 and AD 1000 ± 40, which correspond to the Roman Warm Period and Medieval Warm Period, respectively. These suggest that submarine-cave sediments provide unique information for Holocene reef development. In addition, oxygen isotope records of cavernicolous C. iejimensis are a useful tool to reconstruct century-scale climatic variability for the Okinawa Islands during the Holocene.

Shelf stratigraphy and the influence of antecedent substrate on Holocene reef development, south Oahu, Hawaii

Paired analyses of drill cores and high-resolution seismic reflection data show that development of Holocene framework reefs on the Oahu (Hawaii) shelf is limited to settings of low wave energy and to the period 8000 to 3000 yr BP. A prominent bounding surface that is mapped across much of the Oahu shelf is an erosion surface cut into Marine Isotope Stages 5 and 7 limestones that show extensive loss of primary porosity, aragonite, and MgCO 3 owing to meteoric and vadose-zone diagenesis. This acoustic reflector is found exposed at the surface where wave energy is high or in the shallow subsurface below Holocene reef and sand sheet deposits where energy is low. Ship-towed video along 30 km of the shelf reveals a steady decrease in limestone accumulation from offshore of Honolulu southeast to Koko Head where the seafloor is characterized by volcanic pavement and/or thin sand deposits. This may reflect the build-up of late Pleistocene volcanics associated with the Hanauma Bay eruption (30,000–7000 yr BP) that now comprise the substrate in depths shallow enough to limit reef accretion. The absence of significant Holocene reef build-up on the south Oahu shelf is consistent with observations from north-facing coasts that lack Holocene reefs, indicating that Holocene reef formation in Hawaii is complex and patchy.

Holocene Reef Development Along the Northeastern St. Croix Shelf, Buck Island, U.S. Virgin Islands

Abstract Eight cores were recovered from Buck Island Underwater National Monument (U.S. Virgin Islands). Facies were defined based on recovered coral species, fabrics observed in core slabs and thin sections, and detailed notes on drilling character. Thirty-six radiometric dates constrained the timing of reef accretion. Together, these data provide a detailed history of reef development under varying regimes of sea-level rise and physical oceanography. Holocene reefs around Buck Island initiated atop a broad antecedent bench at 13–16 meters below present sea level. Shelf flooding near Buck Island occurred as early as 9,500 years ago (CalBP), but preserved reefs lagged by as much as 1,800 years. Earliest reef development was dominated by branching Acropora palmata near the shelf edge and massive corals closer to Buck Island. By 7,200 CalBP, A. palmata apparently declined near the platform margin and was absent until ca. 5,200 CalBP throughout the study area. Over time, the reefs closer to Buck Island built upward (ca. 16 m) and seaward (ca. 50 m), as the rate of sea-level rise slowed and carbonate production increasingly exceeded the accommodation space that was being created. Reef topography and zonation became progressively more distinct, with A. palmata dominating the shallow reef crest. Branching coral apparently disappeared again between ca. 3,030 and 2,005 CalBP for reasons that are not clear. This and the previous decline of A. palmata mimic patterns seen around St. Croix and throughout the Caribbean. By 1,000 CalBP, the reefs close to Buck Island had largely assumed their present character and continued to track slowly rising sea level until the present. Around 1,200 CalBP, vertical accretion along Buck Island Bar apparently ceased. Paradoxically, the surface of this outer reef has historically been dominated by large stands of A. palmata since the area was first described, but rapid coral growth has not resulted in preservation of this species over the last millennium. Modern community structure mimics facies patterns seen in cores. Over the past 7,700 years, the southern reef crest appears to have remained slightly shallower than its northern counterpart, a condition that persists today. Observations after Hurricane Hugo in 1989 suggest that this difference in elevation is related to the piling up of debris on the broader, southern reef crest by high waves from storms passing south of St. Croix. Also, facies along the southern reef are more variable in species composition than their northern counterparts, a condition that is exhibited by the modern reef community. Coral abundance and diversity in the cores (total coral = 20–30%; dominated by A. palmata) are comparable to the community structure present in the late 1970s (Bythell et al. 1993; Hubbard et al. 1993). In contrast, fossil-coral abundance and diversity are consistently higher than what was measured in the 1980s and early 1990s (total coral = 7–14%; A. palmata ≤ 2%), after the onset of White Band Disease, a putative pathogen, which has recently decimated branching acroporids throughout the region. The dominance of branching A. palmata in the cores would seemingly reflect an absence of disease or other factors that would discourage its continued abundance. In apparent contrast, two lengthy gaps in the A. palmata record reflect previous disappearances that roughly correspond to similar lapses elsewhere in the Caribbean. Thus, the spatial persistence of a species in the fossil record cannot necessarily be equated with its temporal continuity. Comparisons between changes in modern reefs on a time scale of decades and their fossil forebears must be made with great care. Understanding the role of short-term changes and how they are reflected in the preserved record is thus critical to relating the late Holocene A. palmata gaps to the recent decline of the species. This has important implications for our understanding of how preserved community structure relates to what actually existed in the past, and could limit our ability to use the recent geologic record as a proxy to short-term, future changes in coral reefs.

Late Quaternary structure and development of the northern Ningaloo Reef, Australia

Ningaloo Reef, situated on the central west coast, is Australia's largest fringing reef system extending southward from 22°S for over 200 km. Its narrow lagoon is backed by a coastal plain, which is largely composed of an emergent Last Interglacial reef on the flank of folded Tertiary limestones. The west-facing reef is exposed to strong oceanic swells across a narrow (8 km) continental shelf. Climatic aridity, cyclones, tsunamis, and the poleward flowing Leeuwin Current all influence the reef system. Seismic profiling and a coring and dating program along a transect through a reef pass indicate two periods of reef development in the northern part of the reef: Holocene and Last Interglacial. Seaward of the crest, the Holocene reef forms either a prominent 500 m-wide bulge with 10 m of relief and an abrupt seaward slope, or a series of discrete patch reefs. Holocene reef development is limited to depths of less than 30 m and reaches a maximum thickness of ca. 10–15 m below the reef crest. U/Th TIMS dates from distal parts of the Last Interglacial section between −18 and −36 m give ages toward the end of the high stand (120–115 ka). Last Interglacial reef growth was more extensive of the two, filling much of the available accommodation space, perhaps as a result of a stronger Leeuwin Current. This substrate subsequently provided an antecedent foundation for Holocene reef development.

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

Abstract Internal facies and development of an oceanic island's 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.

Coral assemblages and physico-chemical characteristics of the Whitsunday Islands: evidence of recent community changes

Benthic communities were assessed and 22 environmental variables were monitored at seven leeward localities (L1ŒL7) in the Whitsunday Islands, Queensland, Australia. L1 was near the Proserpine and O’Connell river mouths and L7 ~80 km north of the river mouths. Distinct physico Œchemical and biological gradients were evident. Sparse scleractinian coral communities, dominated by faviids, Montipora spp. and encrusting Porites colonies, were present at L1, L2 and L3, whereas diverse reef-building communities, dominated by Acropora spp., were more common at and beyond L4. The number of coral recruits (age <6 months) did not differ significantly among localities, suggesting that coral recruitment was near random and that the environment shapes the adult community from those recruits. The study demonstrates strong negative relationships between chlorophyll a and the following: percentage coral cover, coral species richness and coral abundance. The reef-building capacities of the coral communities and the extent of Holocene reef development were inconsistent at L2 and L3, which is interpreted as a sign of anthropogenic effects.

Holocene development of three isolated carbonate platforms, Belize, Central America

Locally operating factors such as topography of the reef basement and exposure to waves and currents rather than regionally effective factors such as the post-glacial sea level rise in the western Atlantic explain the different Holocene developments of the three isolated carbonate platforms Glovers Reef, Lighthouse Reef, and Turneffe Islands offshore Belize. A series of NNE-striking tilted fault-blocks at the passive continental margin forms the deep basement of the Belize reefs. Glovers and Lighthouse Reefs are located on the same fault-block, while Turneffe Islands is situated west of Lighthouse Reef on an adjacent fault-block. The three platforms are surrounded by deep water and have surfacebreaking reef rims. Significant differences exist between platform interiors. Glovers Reef has only 0.2% of land and an 18 m deep, well-circulated lagoon with over 800 patch reefs. Lighthouse Reef has 3% of land and a well-circulated lagoon area. Patch reefs are aligned along a NNE-striking trend that separates a shallow western (3 m) and a deeper eastern (8 m) lagoon. Turneffe Islands has 22% of land that is mainly red mangrove. Interior lagoons are up to 8 m deep and most have restricted circulation and no patch reefs. Surface sediments are rich in organic matter. In contrast, the northernmost part of Turneffe Islands has no extensive mangrove development and the well-circulated lagoon area has abundant patch reefs. Holocene reef development was investigated by means of 9 rotary core holes that all reached Pleistocene reef limestones, and by radiometric dating of corals. Maximal Holocene reef thickness reaches 11.7 m on Glovers Reef, 7.9 m on Lighthouse Reef, and 3.8 m on Turneffe Islands. Factors that controlled Holocene reef development include the following. (1) Holocene sea level. The margin of Glovers Reef was flooded by the rising Holocene sea ca. 7500 YBP, that of Lighthouse Reef ca. 6500 YBP, and that of Turneffe Islands between 5400 and 4750 YBP. All investigated Holocene reefs belong to the keep-up type, even though the three platforms were flooded successively and, hence, the reefs had to keep pace with different rates of sea level rise. (2) Pre-Holocene topography. Pleistocene elevation and relief are different on the three platforms. This is the consequence of both tectonics and karst. Different elevations caused successive reef initiation and they also resulted in differences in lagoon depths. Variations in Pleistocene topography also explain the different facies distribution patterns on the windward platforms that are located on the same fault-block. On Lighthouse Reef tectonic structures are clearly visible such as the linear patch reef trend that is aligned along a Pleistocene fault. On Glovers Reef only short linear trends of patch reefs can be detected because the Pleistocene tectonic structures are presumably masked by the higher Holocene thickness. The lower Pleistocene elevation on Glovers Reef is probably a consequence of both a southward tectonic tilt, and stronger karstification towards the south related to higher rainfall. (3) Exposure to waves and currents. Glovers Reef, Lighthouse Reef, and the northernmost part of Turneffe Islands receive the maximum wave force as they are open to the Caribbean Sea. Adjacent lagoons are well-circulated and have luxuriant patch reef growth and no extensive mangrove development. By contrast, most of Turneffe Islands is protected from the open Caribbean Sea by Lighthouse Reef to the east and is only exposed to reduced wave forces, allowing extensive mangrove growth in these protected areas.

The evolution of a Holocene fringing reef and island: reefal environmental sequence and sea level change in Tonaki Island, the Central Ryukyus

Within the lndo-Pacific region Holocene reef development over tlie last 6000 yBP has occurred during a near stable sea level period. In particular, development of reef flat and related features have been associated with the stillstand. This is illustrated by the Holocene evolution of a reef in the Japanese Ryukyu Islands. A continuous reef structure, 8 m in thickness and 630 n~ in length, was observed from a fresh excavation in a modern fringing reef in the western part of Tonaki Island of the central Ryukyu Islands. Documented by 34 radiocarbon ages, the reef first reached a relative sea level of ca. 1 m above the present level at about 5200 yBP by growth of branching Acropora thickets, and by accumulation of angular clasts of tabular Acropora at the landward side. Storm features occur within all the observed reef structure and suggest that the reef has been continuously affected by high-energy events for at least 5500 yBP. The topographic outline of the modern fringing reef was formed at an early stage of sea level stillstands affected by wind, climate, and substrate topography. Delayed closure of the 'Holocene high energy window' resulted in vigorous reef growth on the landward side. The reef flat accreted seaward about 400 m during the last 4500 yBP with the development of spurs and grooves. 'The growth environment shifted from a sheltered to a wave-affected condition during the seaward accretion of the reef flat. Reduced colony sizes of tabular Acropora and decreased upward reef growth rates also occurred at about this time. With the seaward accretion, the shoreward grooves became isolated and infilled by rounded clasts. Radiocarbon age from an early archeological feature ('Touma and Oshiro 1979) indicates that the formation of a Holocene tombolo is closely tied to sea level fall around 3500 yBP. This sea level fall and sufficient development of wave resistant structures at the reef edge, provided shore protection and contributed to tombolo stability. Department of Physical Geography, Faculty of Education, Okayama University, Okayama 700 Japan. Department of Geography, Faculty of Science, Tokyo Metropolitan University, Minan~i-Osawa, Ilachioji 192-03 Japan. Laboratory of Geography, College of Education, University of the Ryukyus, Okinawa 903-01 Japan. Department of Geography, Kansai Gaikokugo liniversity, Makikata, Osaka 573 Japan. Department of Geography, Komazawa University, Setagaya 154 Japan.

The nature of the pre-Holocene surface, John Brewer Reef, with implications for the interpretation of Holocene reef development

Seismic reflection profiles trace out a steep-sided antecedent platform, assumed to be the Holocene/Pleistocene interface, beneath the post-glacial (Holocene) coralline and sediment veneer of John Brewer Reef, a small mid-shelf reef within the central Great Barrier Reef Province. This platform, defined by a semi-continuous seismic reflector, is characterised by three essentially planar surfaces, at −20m, −32 m and −40 m depth with respect to present low water datum. The −20 m surface, which exhibits a maximum relief of ±2 m, is extensive and is postulated to represent either a senile reef which grew to maturity most probably during the 50 kyr interstadial, or the erosion and concurrent infilling, during this phase, of a near — flat reefal structure which dates from an earlier −20 m interstadial. The deeper surfaces are less extensive. That at −32 m is considered to represent a fringing reef which became established on the flanks of the antecedent platform during a brief late-Pleistocene sea-level stillstand. The −40 m surface appears to be predominantly erosional in origin and also of late-Pleistocene age. Whilst these surfaces are sites of major coral growth, Holocene reef development has occurred largely independent of antecedent platform morphology. Present reef morphology reflects the interaction between the rate of post-glacial sea-level rise, prevailing hydrodynamic conditions and biological activity, and bears no resemblance to that of the underlying antecedent surface. The present morphology of John Brewer Reef is consistent with a continuous post-glacial sea-level rise.

Production and Cycling of Calcium Carbonate in a Shelf-Edge Reef System (St. Croix, U.S. Virgin Islands): Applications to the Nature of Reef Systems in the Fossil Record

ABSTRACT Geologic reefs are the end-products of not only constructive processes that produce calcium carbonate, but also of destructive processes (e.g., bioerosion and wave action) that reduce solid substrate to sediment, and of physical processes that rework the reef fabric and transport sediment. A study was conducted at Cane Bay on the island of St. Croix, U.S. Virgin Islands to quantify the suite of processes that have influenced Holocene reef development. Data on production, reef accretion, bioerosion, and sediment transport were used to construct a detailed carbonate budget and to apply the findings of this study to the character of reefs observed in the fossil record. Total production on the reef ranged from 0 to 5.78 kg/m2 per year, with a reef-wide average of 1.21 kg/m2 per year (1.13 by corals; 0.02 by coralline algae; 0.06 by primarily molluscs, forams and echinoderms). Based on 7 cores, only 0.91 kg/m2 per year of this have been retained within the reef interior. The remaining 0.24 kg/m2 per year of sediment, along with the 0.06 kg/m2 per year contributed directly by molluscs, etc., are deposited within reef channels and are probably flushed from the reef by major storms. Of the material recovered in the cores, 58% was either loose sediment and rubble or open void space. Much of the recognizable coral material was demonstrably reworked and out of place. Accretion rates across the shelf generally increased with water depth, as a result of active slumping along the steepening reef face over the past 2,000-3,000 years. The importance of detrital material in the reef fabric and the major role played by secondary processes that constantly rework the substrate have resulted in a reef whose interior is more of a garbage pile than an in-place assemblage of corals cemented together into a rigid framework. The physical similarities between the detrital fabric of the reef at Cane Bay and those found in many ancient deposits imply that the general processes operating along the Cane Bay shelf have persisted during much of the evolution of ancient reefs. It is proposed that our modern models overemphasize the importance of in-place framework as a criterion for an ecologic reef.