Ancient Reefs Research Articles

Geoheritage and geoconservation in the American Midwest: Silurian reefs of the Milwaukee–Chicago region

Abstract Paleozoic reefs were first identified by James Hall (1862) in the Silurian rocks of the American Midwest. Scientifically, they were crucial in defining ancient reefs, documenting the early history of this marine ecosystem, and interpreting regional stratigraphy, palaeogeography and palaeoecology. Schoonmaker Reef in Wauwatosa, Wisconsin, is historically the most significant of these structures, as the first to be correctly identified as a ‘coral’ reef by exhibiting the transition between a reef mound and the surrounding seafloor. After nearly two hundred years of quarrying, Thornton Reef in Thornton, Illinois, is next in importance as it displays an unequalled three-dimensional exposure, demonstrating the impact of Silurian climate change on reef development. Silurian reefs were discovered and studied only through stone industry quarrying. Extensive nineteenth-century fossil collecting by local naturalists was also a key component in reef studies. These reefs are also an important part of the region's geoheritage, comprising elements of its economy, history, education and scenic landscape. Recognizing their scientific and historical importance, some reefs were part of geoconservation projects and are protected sites. However, other reefs and irreplaceable reef fossil collections need further initiatives to preserve the scientific value and historical legacy of these relatively small geological features.

Fossil cold-water corals (Scleractinia and Gorgonacea) from the Burdwood Reef, Argentina Republic

The Burdwood Bank is a rocky platform emerging from the abyssal plain to depths of 50-100 m between the South Atlantic Ocean and the Drake Passage. Regarding its geological affinity, it has a composition similar to the Staten Island (Isla de los Estados), therefore aligned to the Andes-Darwin Cordillera. This bank is crossed from south to north by the Antarctic Bottom Water; very cold and rich in nutrients. Rocks dredged from the shallower portions at its western portion are composed of carbonatic blocks. The analyses indicate fossil specimens of Desmophyllum sp. (Order Scleractinia) and Gorgonacea palmatum (Order Gorgonacea), genera living today in the region. In this sense, sectors of this western portion should be considered as an ancient reef composed mostly of cold-water corals. The major implications are assigned to oceanographic and climatic issues. During the Upper-Pleistocene lowstand, sunlight was more available at shallower depths and therefore corals were very frequent at wave-dominated areas. Sea-level variations have therefore strong influence on cold-water coral growths in the sense that nutrient availability by currents can significantly changed between glacial and interglacial periods.

In the shadow of a giant reef: Palaeoecology of mesophotic coral communities from the Givetian of Anti-Atlas (Morocco)

One of the keys to the success of modern and ancient reef ecosystems is the phenomenon of photosymbiosis. Photosymbiotic corals can thrive down to the limits of the photic zone, and the coral communities living in the lower part of the photic zone are known as mesophotic coral reef ecosystems (MCE). Recent studies have shown that MCEs were already present in the middle Palaeozoic. The reefs of that time reached the peak of their distribution during the Givetian, when they were present even as far as 50°S. One of the southernmost Givetian reefs is the Aferdou el Mrakib reef, exposed in the eastern Anti-Atlas (Morocco). At the base of this reef, numerous small carbonate bioherms are exposed. These structures formed low-relief banks that developed probably at moderate water depths of several tens of meters. In the present contribution, we analyse the paleoecology of these peculiar communities. The assemblage is dominated by platy and encrusting coenitid (Roseoporella) and alveolitid (Alveolites) tabulate corals, as well as abundant platy stromatoporoids and solitary cystyphillid rugose corals. The platy morphology of corals is widely considered as an adaptation related to photosymbiosis, and is especially common in MCEs. In combination with palaeobathymetric and sedimentological data, this characteristic suggests that the studied banks developed at the bottom of the euphotic zone. The MCEs of the Aferdou el Mrakib reef share major taxonomic and ecological similarities with the previously described MCEs from the southern shelf of Laurussia and a reef community from southern China, emphasizing a high degree of cosmopolitanism of the Middle Devonian mesophotic ecosystems. Communities studied here are also the most poleward MCEs described to date.

Ancient Reef Traits, a database of trait information for reef-building organisms over the Phanerozoic

Trait-based approaches are increasingly relevant to understand ecological and evolutionary patterns. A comprehensive trait database for extant reef corals is already available and widely used to reveal vulnerabilities to environmental disturbances including climate change. However, the lack of similar trait compilations for extinct reef builders prevents the derivation of generalities from the fossil record and to address similar questions. Here we present the Ancient Reef Traits Database (ARTD), which aims to compile trait information of various reef-building organisms in one single repository. ARTD contains specimen-level data from both published and unpublished resources. In this first version, we release 15 traits for 505 genera and 1129 species, comprising a dataset of 17,841 trait values of Triassic to mid-Holocene scleractinian corals, the dominant reef-builders in the modern ocean. Other trait data, including for other reef-building organisms, are currently being collated.

Skin patterning and internal anatomy in a fossil moonfish from the Eocene Bolca Lagerstätte illuminate the ecology of ancient reef fish communities.

Colour patterning in extant animals can be used as a reliable indicator of their biology and, in extant fish, can inform on feeding strategy. Fossil fish with preserved colour patterns may thus illuminate the evolution of fish behaviour and community structure, but are understudied. Here we report preserved melanin‐based integumentary colour patterning and internal anatomy of the fossil moonfish Mene rhombea (Menidae) from the Bolca Lagerstätte (Eocene (Ypresian), north‐east Italy). The melanosome‐based longitudinal stripes of M. rhombea differ from the dorsal rows of black spots in its extant relative M. maculata, suggesting that the ecology of moonfish has changed during the Cenozoic. Extant moonfish are coastal schooling fish that feed on benthic invertebrates, but the longitudinal stripes and stomach contents with fish remains in M. rhombea suggest unstructured open marine ecologies and a piscivorous diet. The localized distribution of extant moonfish species in the Indo‐Pacific Ocean may reflect, at least in part, tectonically‐driven reorganization of global oceanographic patterns during the Cenozoic. It is likely that shifts in habitat and colour patterning genes promoted colour pattern evolution in the menid lineage.

Early Miocene coral reef-associated bryozoans from Colombia. Part II: “Ascophora” Cheilostomatida

AbstractBryozoans are common and diverse in fossil and modern coral reefs. However, studies of bryozoans in ancient reefs are generally limited, and even less is known about fossil bryozoan faunas associated with coral reefs in the Caribbean region. This is the second contribution describing the bryozoan assemblage from the early Miocene (Aquitanian) reefs of the Siamaná Formation in the La Guajira Peninsula, southern Caribbean. Here, we describe and illustrate 17 species of ascophoran-grade cheilostomes, including one new genus and three new species,Ditaxiporina colombianan. sp.,Poricella paulaen. sp., andCycloavicularia parvan. gen. n. sp. Of the remaining fourteen taxa left in open nomenclature, one is considered confer and three affinis to species previously described, one is identified at family level, and nine at genus level. The Siamaná bryozoan fauna differs in species and colony-form composition from those associated with other paleoenvironments from Oligocene and Miocene localities of North America, the Caribbean, and Brazil.UUID:http://zoobank.org/043eadcf-0e39-4c1b-b207-f7628d6b5c84

A possible link between coral reef success, crustose coralline algae and the evolution of herbivory

Crustose coralline red algae (CCA) play a key role in the consolidation of many modern tropical coral reefs. It is unclear, however, if their function as reef consolidators was equally pronounced in the geological past. Using a comprehensive database on ancient reefs, we show a strong correlation between the presence of CCA and the formation of true coral reefs throughout the last 150 Ma. We investigated if repeated breakdowns in the potential capacity of CCA to spur reef development were associated with sea level, ocean temperature, CO2 concentration, CCA species diversity, and/or the evolution of major herbivore groups. Model results show that the correlation between the occurrence of CCA and the development of true coral reefs increased with CCA diversity and cooler ocean temperatures while the diversification of herbivores had a transient negative effect. The evolution of novel herbivore groups compromised the interaction between CCA and true reef growth at least three times in the investigated time interval. These crises have been overcome by morphological adaptations of CCA.

Spatial variations in the stable isotope composition of the benthic algae, Halimeda tuna, and implications for paleothermometry

On Conch Reef, Florida Keys, USA we examined the effects of reef hydrography and topography on the patterns of stable isotope values (δ18O and δ13C) in the benthic green alga, Halimeda tuna. During the summer, benthic temperatures show high-frequency fluctuations (2 to 8 °C) associated with internal waves that advected cool, nutrient-rich water across the reef. The interaction between local water flow and reef morphology resulted in a highly heterogenous physical environment even within isobaths that likely influenced the growth regime of H. tuna. Variability in H. tuna isotopic values even among closely located individuals suggest biological responses to the observed environmental heterogeneity. Although isotopic composition of reef carbonate material can be used to reconstruct past temperatures (T(°C) = 14.2–3.6 (δ18OHalimeda − δ18Oseawater); r2 = 0.92), comparing the temperatures measured across the reef with that predicted by an isotopic thermometer suggests complex interactions between the environment and Halimeda carbonate formation at temporal and spatial scales not normally considered in mixed sediment samples. The divergence in estimated range between measured and predicted temperatures demonstrates the existence of species- and location-specific isotopic relationships with physical and environmental factors that should be considered in contemporary as well as ancient reef settings.

The Future of Reef Ecosystems in the Gulf of Mexico: Insights From Coupled Climate Model Simulations and Ancient Hot-House Reefs

Shallow water coral reefs and deep sea coral communities are sensitive to current and future environmental stresses, such as changes in sea surface temperatures (SST), salinity, carbonate chemistry, and acidity. Over the last half-century, some reef communities have been disappearing at an alarming pace. This study focuses on the Gulf of Mexico, where the majority of shallow coral reefs are reported to be in poor or fair condition. We analyze the RCP8.5 ensemble of the Community Earth System Model v1.2 to identify monthly-to-decadal trends in mean Gulf of Mexico SST. Secondly, we examine projected changes in ocean pH, carbonate saturation state, and salinity in the same coupled model simulations. We find that the joint impacts of predicted higher temperatures and changes in ocean acidification will severely degrade Gulf of Mexico reef systems by the end of the 21st century. SSTs are likely to warm by 2.5 to 3C; while corals do show signs of an ability to evolve toward higher temperatures, current coral species and reef systems are likely to suffer major bleaching events in coming years. We contextualize future changes with ancient reefs from paleoclimate analogues, periods of Earth's past that were also exceptionally warm, specifically rapid ``hyperthermal events. Ancient analogue events are often associated with extinctions, reef collapse, and significant ecological changes, yet reef communities managed to survive these events on evolutionary timescales. Finally, we review research which discusses the adaptive potential of the Gulf of Mexico's coral reefs, meccas of biodiversity and oceanic health. We assert that the only guaranteed solution for long-term conservation and recovery is substantial, rapid reduction of anthropogenic greenhouse gas emissions.

Partitioning and Chemical Environments of Minor Elements in Individual Large Benthic Foraminifera Cultured in Temperature-Controlled Tanks

Because reef-dwelling large benthic foraminifera (LBF) tests, which is composed of high-magnesium calcite, are the common components of modern reef sediments, and ancient reef carbonate rocks, they can provide a continuous record of the paleoenvironment of reef sediments. Evaluation of the responses of the minor element concentrations (Mg, Sr, Na, P, and S) of individual tests to ambient temperature and LBF growth modes was conducted with two cultured species, Calcarina gaudichaudii and Amphisorus kudakajimensis. The elements were assessed as tracers of temperature, salinity, nutrient concentration, and calcification rate. The Mg content of C. gaudichaudii and the Sr content of both species were controlled by temperature. It can be confirmed that the Mg of individual tests of C. gaudichaudii is a useful paleothermometer. There was a large biological modulation of Na, P, and S incorporation. In-situ XANES analysis revealed that both inorganic and organic species contributed to the P and S content of LBF tests. We observed five sulfur components with variable oxidation states.

Geotouristic aspects of the Geopark Cabo de Gata-Níjar (Andalusia, Spain)

The Cabo de Gata-Nijar Geopark is one of the most remarkable examples of the geological heritage in Spain, and it comprises the largest volcanic massif in Andalusia. It is built of andesites and dacites with such typical volcanic forms like lava flows, volcanic calderas, volcanic domes and columnar joints. The ancient beaches and reefs demonstrate the presence of climate change as well as of a subtropical sea. The tourism in this region is mainly focused on its beaches and some remains of the cultural heritage, like salt works or the Rodalquilar gold mine. This study pretends to analyze the reasons and consequences of this matter. Highlighting the good state of preservation of this area and the possibility of enjoyment of its beauty, more attention should be paid to the popularization of the local geological aspects, which would significantly increase the awareness and interest among the tourists.

Reef Sedimentary Accretion Model (ReefSAM): Understanding coral reef evolution on Holocene time scales using 3D stratigraphic forward modelling

A detailed understanding of the development of coral reefs and their internal structure has important applications for predicting future reef trajectories in light of climate changes, petroleum exploration and reconstructing past environmental changes such as sea level. Numerical computer modelling provides a quantitative means of testing and understanding reef processes and better predicting past and future reef development. However, most existing carbonate stratigraphic forward models (SFMs) focus on the platform scale and several limitations in their simulation of important reef processes and reef-scale features (e.g. patch reefs, sand aprons) have been previously identified. We present the Reef Sedimentary Accretion Model (ReefSAM) – a new carbonate SFM designed to test new approaches to modelling processes important at the reef scale, with the goal of better simulating reef features and evolution at higher temporal and spatial resolutions. Specific developments include a sediment transport model which calculates deposition based on a range of grain sizes, additional controls on coralgal growth including wave energy and surface substrate specificity, and model outputs showing categorized paleo-conditions (composition, paleo-depth and paleo-energy) along with numerous quantitative measures for objective comparison of model results to real-world observational data, which is often more abundant for modern/Holocene reefs than buried ancient reefs. Simulations of the well-studied Holocene One Tree Reef (OTR) in the southern Great Barrier Reef (GBR) showed improvements upon previous modelling attempts in both quantitative measures (e.g. lagoon size or reef sea level catch-up time), and the ability to produce more realistic reef-scale morphological features (e.g. lagoonal patch-reefs, mostly sand-filled lagoon) and reef development characteristics (e.g. transition from mostly vertical aggradation during sea level catch-up to horizontal progradation afterwards). Controls on reef maturity/lagoonal filling identified in previous modelling experiments were corroborated. Sensitivity testing also revealed a very high sensitivity to reef drowning. These results indicate the new modelling approaches are viable and provide a way forward for future Holocene reef evolution model development.

Evolution of body size, vision, and biodiversity of coral-associated organisms: evidence from fossil crustaceans in cold-water coral and tropical coral ecosystems.

BackgroundModern cold-water coral and tropical coral environments harbor a highly diverse and ecologically important macrofauna of crustaceans that face elevated extinction risks due to reef decline. The effect of environmental conditions acting on decapod crustaceans comparing these two habitats is poorly understood today and in deep time. Here, we compare the biodiversity, eye socket height as a proxy for eye size, and body size of decapods in fossil cold-water and tropical reefs that formed prior to human disturbance.ResultsWe show that decapod biodiversity is higher in fossil tropical reefs from The Netherlands, Italy, and Spain compared to that of the exceptionally well-preserved Paleocene (Danian) cold-water reef/mound ecosystem from Faxe (Denmark), where decapod diversity is highest in a more heterogeneous, mixed bryozoan-coral habitat instead of in coral and bryozoan-dominated facies. The relatively low diversity at Faxe was not influenced substantially by the preceding Cretaceous/Paleogene extinction event that is not apparent in the standing diversity of decapods in our analyses, or by sampling, preservation, and/or a latitudinal diversity gradient. Instead, the lower availability of food and fewer hiding places for decapods may explain this low diversity. Furthermore, decapods from Faxe are larger than those from tropical waters for half of the comparisons, which may be caused by a lower number of predators, the delayed maturity, and the increased life span of crustaceans in deeper, colder waters. Finally, deep-water specimens of the benthic crab Caloxanthus from Faxe exhibit a larger eye socket size compared to congeneric specimens from tropical reefs, suggesting that dim light conditions favored the evolution of relatively large eyes.ConclusionsThe results suggest a strong habitat control on the biodiversity of crustaceans in coral-associated environments and that the diversity difference between deep, cold-water reefs and tropical reefs evolved at least ~63 million years ago. Futhermore, body size and vision in crustaceans evolved in response to environmental conditions in the deep sea. We highlight the usefulness of ancient reefs to study organismal evolution and ecology.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0694-0) contains supplementary material, which is available to authorized users.

Habitat heterogeneity reflected in mesophotic reef sediments

Modern reef sediments reflect the physical and chemical characteristics of the environment as well as the local reef fauna. Analysis of sedimentary reef facies can thus provide a powerful tool in interpreting ancient reef deposits. However, few studies have attempted to differentiate sedimentary facies in mesophotic coral ecosystems, low light habitats defined as residing 30–150m below sea level. The low-angle shelf mesophotic coral ecosystem south of the northern U.S. Virgin Islands (USVI) consists of reefs with different structural characteristics ideal for studying the relationship between habitat variability and sedimentary facies. Textural, compositional, and geochemical analyses of surface sediments were used to identify mesophotic reef subfacies associated with distinct benthic communities and structural habitats. Sediment grain composition and bulk geochemistry were found to broadly record the distribution and abundance of coral and macroalgae communities, foundational mesophotic reef benthic organisms. Overall, sediment composition was found to be a good indicator of specific reef environments in low-angle mesophotic reef habitats. Sedimentological analyses indicate that hydrodynamic forces do not transport a significant amount of allochthonous sediment or potentially harmful terrigenous material to USVI mesophotic reefs. Episodic, maximum current velocities prevented deposition of most silt-size grains and smaller, but biological processes were found to have a greater influence on subfacies partitioning than hydrodynamic processes. Results provide a new analog for studies of ancient mesophotic coral ecosystem geological history and document the relationship between mesophotic reef subfacies, structural complexity, and habitat heterogeneity. They also demonstrate how mesophotic reefs along the same shelf system do not always share similar sedimentary characteristics and thus record a diverse set of ecological and environmental conditions.

14C Dating of the Lime Burial of Cova de Na Dent (Mallorca, Spain): Optimization of the Sample Preparation and Limitations of the Method

Lime burials are a characteristic phenomenon of the protohistoric funerary tradition on the Balearic Islands. At Cova de Na Dent, six samples, representing the entire stratigraphy of the lime burial, were taken for analysis. The radiocarbon dates suggested that the lowest levels of the burial were Late Bronze Age. This is in contradiction with the general belief that the lime burials are a late Iron Age phenomenon. Therefore, a new analysis strategy is put forward, focusing on the so-called 1st fraction, the first CO2released during the acid lime reaction, which is supposed to be free of fossil carbon. The analysis demonstrates the impossibility to eliminate the fossil carbon fraction completely. This is probably due to the different geological formation of the local limestone deposits (ancient reef barriers) compared to the previous lime burials of Mallorca all coming from mountain areas.14C analysis from a cremation layer without lime at the onset of the lime burial reveals an Iron Age origin of the Cova de Na Dent lime burial.

TALES FROM THE CRYPT: EARLY ADAPTATION OF CRYPTOBIONTIC SESSILE METAZOANS

Cryptic ecosystems of modern and ancient reefs contain substantial amounts of biodiversity. It remains uncertain, however, when and how metazoans adapted to such space. Early Cambrian reef systems witnessed the rise and fall of the earliest known cryptic sessile metazoans. Subsequent Middle Cambrian to Early Ordovician microbial-dominated reefs were generally devoid of true frame-building metazoans, as well as cryptic sessile fauna. The Early Ordovician microbial-siliceous sponge patch reefs of the Dumugol Formation, Korea represent one of the oldest in situ spiculate sponge-bearing cryptic communities exploiting intraskeletal cryptic environments. Less than half of these small millimeter- to centimeter-scale crypts contain low-diversity sessile cryptic assemblages of spiculate sponges and microbialites. The cryptic sponges that attach to the walls of the cavities or on top of internal sediments do not show any skeletal distortion at their contacts with host organisms. The spiculate sponges occur both in open spaces as well as in crypts, and are interpreted to be pioneers of intraskeletal crypts after the death of the cavity-providing organisms. The behavior of occupying transient cryptic habitats is interpreted as an incipient stage of sessile metazoan adaptation to a cryptic space by an opportunistic member of the epibenthic community. This resulted in the extension of the open surface community into crypts which occurred far in advance of the eventual establishment of obligate cryptic forms. The present study provides a critical link for establishing the origin and evolutionary history of early cryptic sessile metazoan adaptation.

Ancient reefs provided fishy refuges

Ancient reefs provided fishy refuges

Quaternary coral reef refugia preserved fish diversity.

The most prominent pattern in global marine biogeography is the biodiversity peak in the Indo-Australian Archipelago. Yet the processes that underpin this pattern are still actively debated. By reconstructing global marine paleoenvironments over the past 3 million years on the basis of sediment cores, we assessed the extent to which Quaternary climate fluctuations can explain global variation in current reef fish richness. Comparing global historical coral reef habitat availability with the present-day distribution of 6316 reef fish species, we find that distance from stable coral reef habitats during historical periods of habitat loss explains 62% of the variation in fish richness, outweighing present-day environmental factors. Our results highlight the importance of habitat persistence during periods of climate change for preserving marine biodiversity.

Environmentally controlled succession in a late Pleistocene coral reef (Sinai, Egypt)

The concept of ecological succession has been frequently applied in the study of ancient reefs. Whereas Paleozoic and Mesozoic reefs are commonly thought to reveal an autogenic primary—climax zonation, patterns in Neogene and Quaternary reefs are much more diverse. Here, we describe a well-preserved late Pleistocene coral reef from Dahab on Sinai Peninsula (Egypt), which shows a distinct zonation that resembles an ecological succession. In contrast to classical examples of ecological successions, species composition, paleoenvironmental conditions, and coral biodiversity of the Dahab reef indicate an allogenic, sea-level controlled community change, from marginal marine to reef slope and back reef. A review of the literature confirms that autogenic, short-term successions are virtually absent in Quaternary reefs. We predict that long generation times of corals make it unlikely that classical autogenic successions develop in reefs at all, unless environmental conditions are unusually stable.

Visualizing lateral anisotropy in modern carbonates

Statistical stationarity is a key assumption for the many modeling techniques based on variograms and transiograms used for geostatistical reconstruction of the subsurface. Stationarity expresses the property that the rules of geometry and neighborhood in the model are translation invariant, that is, no directional change in either mean or variance is observed. These criteria are met when the lateral arrangement of lithologic elements into a facies mosaic is isotropic. The balance between isotropy and anisotropy is a defining statistic in the configuration of both real and modeled carbonate landscapes. Even a cursory look at a satellite image of a modern carbonate platform shows that gradients in environment and hydrodynamics cause radical departures from isotropy. Although reef-forming organisms have changed through time, we do not expect that ancient reef systems behaved any differently than today. Hence, significant anisotropy should also be anticipated in the vertical and lateral arrangements of lithologies in the subsurface. To maintain sufficient geologic realism, it is paramount that process-imitating and pattern-replicating models alike be capable of honoring an expected degree of nonstationarity. Despite this need, few studies exist that provide quantitative information to the reach and location of zones of geometric isotropy and anisotropy in carbonate systems, let alone methods with which this property can be assessed. In an effort to close this disjoint, we develop a method for evaluating a modern Pacific depositional system, the Saipan Lagoon, for which we have created a geographic information system stack consisting of mapped facies distributions and a seabed topographic model, both at meter-scale resolution. By developing a lagged spatial metric based on the Markov property of facies transitions, we demonstrate that the degree of anisotropy is influenced by water depth; the shallowest areas (5 m [16 ft]) of the platform interior tend to be anisotropic whereas areas at greater depth are isotropic. This behavior suggests a possible extension to a genetic rule set that could be imparted to subsurface models based on the environment of deposition. This marks an advance in the understanding and, ultimately, handling of geometric nonstationarity in models of carbonate depositional systems.