Quaternary development of resilient reefs on the subsiding kimberley continental margin, Northwest Australia

The Kimberley region in remote northwest Australia has poorly known reef systems of two types; coastal fringing reefs and atoll-like shelf-edge reefs. As a major geomorphic feature (from 12ºS to 18ºS) situated along a subsiding continental margin, the shelf edge reefs are in a tropical realm with warm temperatures, relatively low salinity, clear low nutrient waters lacking sediment input, and Indo-West Pacific corals of moderate diversity. Seismic architecture of the Rowley Shoals reveals that differential pre-Holocene subsidence and relative elevation of the pre-Holocene substrate have controlled lagoon sediment infill and reef morphology, forming an evolutionary series reflecting differential accommodation in three otherwise similar reef systems.The Holocene core described for North Scott Reef confirms previous seismic interpretations, and provides a rare ocean-facing reef record. It demonstrates that the Indo-Pacific reef growth phase (RG111) developed during moderate rates of sea level rise of 10 mm/year from 11 to about 7-6.5 ka BP until sea level stabilization, filling the available 27 m of pre-Holocene accommodation. Despite the medium to high hydrodynamic energy imposed by the 4m tides, swell waves and cyclones the reef-building communities represent relatively low-wave energy settings due to their southeast facing and protection afforded by the proximity of the South Reef platform. This study demonstrates the resilience of reefs on the subsiding margin whilst linking Holocene reef morphology to the relative amount of pre-Holocene subsidence

Climate-driven range extension of Amphistegina (protista, foraminiferida) : models of current and predicted future ranges

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in PLoS ONE 8 (2013): e54443, doi:10.1371/journal.pone.0054443.Species-range expansions are a predicted and realized consequence of global climate change. Climate warming and the poleward widening of the tropical belt have induced range shifts in a variety of marine and terrestrial species. Range expansions may have broad implications on native biota and ecosystem functioning as shifting species may perturb recipient communities. Larger symbiont-bearing foraminifera constitute ubiquitous and prominent components of shallow water ecosystems, and range shifts of these important protists are likely to trigger changes in ecosystem functioning. We have used historical and newly acquired occurrence records to compute current range shifts of Amphistegina spp., a larger symbiont-bearing foraminifera, along the eastern coastline of Africa and compare them to analogous range shifts currently observed in the Mediterranean Sea. The study provides new evidence that amphisteginid foraminifera are rapidly progressing southwestward, closely approaching Port Edward (South Africa) at 31°S. To project future species distributions, we applied a species distribution model (SDM) based on ecological niche constraints of current distribution ranges. Our model indicates that further warming is likely to cause a continued range extension, and predicts dispersal along nearly the entire southeastern coast of Africa. The average rates of amphisteginid range shift were computed between 8 and 2.7 km year−1, and are projected to lead to a total southward range expansion of 267 km, or 2.4° latitude, in the year 2100. Our results corroborate findings from the fossil record that some larger symbiont-bearing foraminifera cope well with rising water temperatures and are beneficiaries of global climate change.This work was supported by grants from the German Science Foundation (DFG; www.dfg.de) to ML and SL (LA 884/10-1, LA 884/5-1)

Seismic-based paleoenvironmental reconstructions of carbonate systems:Pitfalls, limitations and perspectives - Reply to the comments on"Development patterns of an isolated oligo-mesophotic carbonate buildup,early Miocene, Yadana field, offshore Myanmar" by Teillet et al. (2020),authored by Paumard et al. (2020)

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The mode and timing of windward reef-island accretion in relation with Holocene sea-level change: A case study from Takapoto Atoll, French Polynesia

Takapoto Atoll (northern Tuamotu Islands, French Polynesia, Central Pacific) was selected as a test area to clarify the conditions of atoll island accretion in relation to mid- to late-Holocene sea-level changes. Surveys were conducted along two distinct cross-island profiles, on the windward coast of the atoll. In addition, the stratigraphy of an ocean-facing islet was described from an excavation in order to reconstruct the successive island accretionary stages. At both sites, the basement of the atoll-rims consists of conglomerate pavements on which lie shingle ridges, reaching 4 m in elevation. Stratigraphic analysis of the excavated ridge reveals alternation of gravelly sand-supported to gravel-dominated sediments. The chronology of island accretion is based on dating of 41 U/Th surface and excavated coral specimens. Ridge initiation occurred from about 1000 yr BP when sea level was close to its present position, shingle deposits progressively prograded from the lagoon margins oceanwards and were partially cemented at their bases. Cementation may have increased the resistance of the islets to erosion. As a result, some island lands accumulated and have persisted over the last millennium. The modern gross island morphology was acquired during the last 500 years. This model can be considered to be of regional value for the northern Tuamotu islands, adjusted for local thermal subsidence, hydroisostasy and/or lithospheric flexure. Compared to some other Indo-Pacific reef islands, island initiation at Takapoto appears to be have been delayed by 2 to 4 millennia, probably in response to retardation in the reef catching-up with mid-Holocene sea level. Dating of individual coarse-grained coral clasts allowed the major wave-surge events that have hit Takapoto to be identified for the last millennium. The use of gravels results in the identification of a greater number of medium-energy surge impacts, when compared with megaclast-based records. The frequency of storm events identified is consistent with that derived from historical observations; severe storms have a very low frequency of occurrence – one to two events per century on average

Origins and development of Holocene reefs: a revisited model based on boreholes in the Seychelles, Western Indian Ocean

Until recently, concepts of coral reef growth and accumulation have been predominantly based on a Darwinian model. In this, the upwards and outwards growth of a reef core (a coral framework) takes place over a foreslope consisting of reef talus, with the simultaneous filling of the back-reef lagoon by reef-derived debris. The principal adaptations of this pattern relate to the influence of relative changes in sea level and commonly ignore oceanographic factors such as storm frequency and severity. Boreholes through the outer edge of a fringing reef in the Seychelles, western Indian Ocean, reveal a record of Holocene sediment accumulation first established approximately 8 ka ago. Faunal and floral associations show that growth of this body began in relatively deep water but that this shallowed to <5 m within 1 ka. Subsequent accumulation was of “keep-up” style but, as the rate of sea-level rise slowed, shoaling became more frequent and aggradation was limited by reducing accommodation space. Constructional facies are characterised either by massive corals, including Leptastrea, Porites and faviids, or by branching corals, typically Acropora of the danai-robusta group. Coral surfaces may be encrusted by red algae, foraminifera and vermetids, and are commonly bored by filamentous algae, clionids and molluscs. However, detrital facies are volumetrically dominant, and the paucity of a constructional framework requires a re-evaluation of models of reef accretion. New models relate the geometry of accretion to the interplay between extreme storm events and fairweather hydrodynamic conditions. These suggest that a contiguous framework forms in areas of moderate fairweather energy without extreme storm events. Severe storms destroy the continuity of reef structures and generate increasing volumes of coarse detritus. Low storm severity, coupled with low fairweather hydrodynamic energy, may promote the accumulation of fine-grained reef-derived sediments that inhibit framework growth. While ecology reflects year-by-year sea conditions, lithology and structure are controlled by exceptional storms, with the effects of changing sea level superimposed

Significance of relic carbonate deposits along the central and Southwestern margin of India for late Quaternary environmental and sea level changes

Environmental and sea level indicators were investigated using dredge samples from late Quaternary carbonate deposits along the shelf break between Goa and Cape Comorin, India. Geomorphic features in the area were identified from sonar profiles and included isolated patch reefs with a relief of up to 10 m, and linear reefs with reliefs between 2 and 15 m. The main clast types recovered from these features include fragmented corals and carbonate nodules dominated by either encrusted foraminifera or coralline algae. Some of these clast types are clearly of shallow-water origin. Fragments of reef-forming Poritid corals, for example, were collected off Mangalore at depths of 110-105 m and dated between 11,520 and 12,610 14C years BP (13.42-14.77 ka). Nodules of similar age dominated by Lithothamnium and capped by foraminiferal veneers were also collected at -90 m off Cape Comorin. Their altered algal tissues are consistent with formation in shallow water, high-energy conditions. In contrast, nodules recovered off Kochi and Mangalore-Goa are of deeper water origin, younger in age (10,980-7350 14C years BP), and are dominated by Gypsina encrustations with volumetrically less algal encrustation. They show cyclic succession of foraminiferal-algal, or foraminiferal-algal-coral laminations in which the algal species are typical of deeper waters. The age and elevation of corals and shallow-water nodules are both consistent with published glacio-eustatic sea-level curves. In addition, the alternate micro-encrustations of foraminifera, algae and encrusting corals could indicate changing conditions from nutrient-rich and turbid to nutrient-poor and clear water that may be attributable to seasonal variations in sediment flux caused by monsoons

Coral Reef sedimentation on Rodrigues and the Western Indian Ocean and its impact on the carbon cycle

Coral reefs in the southwest Indian Ocean cover an area of ca. 18530 km2 compared with a global reef area of nearly 300000 km2. These regions are important as fishing grounds, tourist attractions and as a significant component of the global carbon cycle. The mass of calcium carbonate stored within Holocene neritic sediments is a number that we are only now beginning to quantify with any confidence, in stark contrast to the mass and sedimentation rates associated with pelagic calcium carbonate, which have been relatively well defined for decades. We report new data that demonstrate that the reefs at Rodrigues, like those at Réunion and Mauritius, only reached a mature state (reached sea level) by 2-3 ka: thousands of years later than most of the reefs in the Australasian region. Yet field observations show that the large lagoon at Rodrigues is already completely full of carbonate detritus (typical lagoon depth less than 1 m at low spring tide). The presence of aeolian dunes at Rodrigues indicates periodic exposure of past lagoons throughout the Pleistocene. The absence of elevated Pleistocene reef deposits on the island indicates that the island has not been uplifted. Most Holocene reefs are between 15 and 20 m in thickness and those in the southwest Indian Ocean appear to be consistent with this observation. We support the view that the CO2 flux associated with coral-reef growth acts as a climate change amplifier during deglaciation, adding CO2 to a warming world. southwest Indian Ocean reefs could have added 7-10% to this global flux during the Holocene. <br/

Post-obduction carbonate system development in New Caledonia (Nepoui, Lower Miocene)

For the first time, depositional models of Lower Miocene carbonate systems from New Caledonia (Southwest Pacific) are proposed, on the basis of a sedimentological and paleoenvironmental study of both cores and outcrops. In the Nepoui area, two distinct stages of carbonate ramp development (Aquitanian Lower Nepoui and Burdigalian Upper Nepoui carbonate systems), separated by a phase of siliciclastic deltaic deposition, are evidenced. The post-obduction marine transgression of the Western New Caledonian margin occurred at approximately 24 Ma and is characterized by the development of an aggrading foraminiferal-coralline algal-scleractinian ramp system ("Chapeau Chinois Limestone") during the early Aquitanian (24-23 Ma). A retrogradational event is evidenced at approximately 23 Ma followed by the development of a shallowing upward carbonate unit (Operculina "Green Sands" and Xuudhen Limestone) during the late Aquitanian. This unit is topped by a major erosional unconformity overlain by conglomeratic deposits ("Pindai conglomerates"), and interpreted to record a significant uplift at around 21-19 Ma. During the Burdigalian, a marine transgression occurred at around 19 Ma, followed by the development of a low-angle carbonate ramp or open platform ("Nepil Limestone") up to the late Burdigalian (19-17 Ma). In both Aquitanian and Burdigalian carbonate ramps, extensive sea-grass meadows are shown to have colonized the proximal ramp environments within the euphotic zone. In the Aquitanian carbonate ramp (Lower Nepoui Formation), carbonate production within sea-grass meadows is dominated by large benthic foraminifera, together with red algae and sparse scleractinians. Mesophotic environments are characterized by large and flat lepidocyclinids, rhodoliths and platy corals whereas in deeper oligophotic settings significant carbonate producers consist mainly of large and flat benthic foraminifera. In the Burdigalian carbonate ramp (Upper Nepoui Formation), porcellaneous foraminifera thriving in sea-grass meadows together with red algae and scattered coral colonies characterize the carbonate production in the euphotic zone. Antecedent topography is regarded as a major factor controlling the extension of carbonate systems at regional and local scale. The thickness and development pattern of Lower Miocene deposits from Nepoui are dominantly controlled by tectonic subsidence. Finally, extensive sea-grass development promoted the dominance of foralgal carbonate production within the euphotic zone