Mound Growth Research Articles

The Moira Mounds, small cold-water coral banks in the Porcupine Seabight, NE Atlantic: Part A—an early stage growth phase for future coral carbonate mounds?

This paper presents a comprehensive overview of the Moira Mounds in the Porcupine Seabight, combining a review of currently published material with new data and interpretations. The Moira Mounds are circular to ovoid-shaped individual or clustered coral banks (referred to as “scleractinian spaced cluster macroreefs”) located in the Belgica (carbonate) mound province, Porcupine Seabight, west of Ireland. They are up to 10m high and 20 to 35m in diameter. For the first time, a map showing the complete extent and location of the Moira Mounds is published, revealing 256 examples grouped into four areas mainly between 800 and 1100m water depth. ROV groundtruthing revealed the importance of bedload sand transport in Moira Mound formation and development processes through sediment entrapment by coral frameworks (Lophelia pertusa and Madrepora oculata). New estimated growth rates for the Moira Mounds are comparable to those measured for much larger Irish coral carbonate mounds but an order of magnitude lower than rates from comparably sized reefs offshore Norway. As well as the framework corals, 26 macrobenthic invertebrate species were identified from a single box core that were similar to those communities found in adjacent off-mound habitats. It is proposed that the Moira Mounds are an example of the early stage growth phase of nearby giant coral carbonate mounds and represent a westward extension of the zone of active cold-water coral mound growth and therefore a deepening of the zone of cold-water coral growth over recent climate cycles.

The importance of the permanent thermocline to the cold water coral carbonate mound distribution in the NE Atlantic

A prominent feature of the NW European continental slope is the presence of numerous cold water coral carbonate mounds that are clustered in a number of provinces. These provinces occupy a relatively narrow depth range along the continental slope: 95% of all coral carbonate mounds identified on the Irish seabed have their mound bases between 500 and 1000 m water depths, with a peak in distribution at ∼ 650 m water depth. The distribution in mound base depths is skewed with a tail extending from the maximum at 650 m to deeper depths. This distribution brackets the depth of the permanent thermocline in the NE Atlantic (600–1000 m) formed below the base of the winter mixed layer. It is shown that the permanent thermocline is associated with the strongest residual near seabed current flow, with typical residual current speeds up to 2–3 times larger at the thermocline depth compared to other depths. The strong vertical density gradient associated with the permanent thermocline, together with the steep continental slope at those depths, also enhances the energy of certain periodic motions such as internal waves and baroclinic tidal currents. These dynamic conditions favour mound growth through the promotion of significant along-slope sediment transport and also provide large across-slope sediment movement and organic matter fluxes. The stability of the thermocline structure is likely the key in providing favourable conditions over long time scales that allow mound growth through sediment baffling processes.

The importance of the terrigenous fraction within a cold-water coral mound: A case study

In the nineties, cold-water coral mounds were discovered in the Porcupine Seabight (NE Atlantic, west of Ireland). A decade later, this discovery led to the drilling of the entire Challenger cold-water coral mound (Eastern slope, Porcupine Seabight) during IODP Expedition 307. As more than 50% of the sediment within Challenger Mound consists of terrigenous material, the terrigenous component is equally important for the build-up of the mound as the framework-building corals. Moreover, the terrigenous fraction contains important information on the dynamics and the conditions of the depositional environment during mound development. In this study, the first in-depth investigation of the terrigenous sediment fraction of a cold-water coral mound is performed, combining clay mineralogy, sedimentology, petrography and Sr–Nd-isotopic analysis on a gravity core (MD01-2451G) collected at the top of Challenger Mound.Sr- and Nd-isotopic fingerprinting identifies Ireland as the main contributor of terrigenous material in Challenger Mound. Besides this, a variable input of volcanic material from the northern volcanic provinces (Iceland and/or the NW British Isles) is recognized in most of the samples. This volcanic material was most likely transported to Challenger Mound during cold climatic stages. In three samples, the isotopic ratios indicate a minor contribution of sediment deriving from the old cratons on Greenland, Scandinavia or Canada. The grain-size distributions of glacial sediments demonstrate that ice-rafted debris was deposited with little or no sorting, indicating a slow bottom-current regime. In contrast, interglacial intervals contain strongly current-sorted sediments, including reworked glacio-marine grains. The micro textures of the quartz-sand grains confirm the presence of grains transported by icebergs in interglacial intervals. These observations highlight the role of ice-rafting as an important transport mechanism of terrigenous material towards the mound during the Late Quaternary.Furthermore, elevated smectite content in the siliciclastic, glaciomarine sediment intervals is linked to the deglaciation history of the British-Irish Ice Sheet (BIIS). The increase of smectite is attributed to the initial stage of chemical weathering processes, which became activated following glacial retreat and the onset of warmer climatic conditions. During these deglaciations a significant change in the signature of the detrital fraction and a lack of coral growth is observed. Therefore, we postulate that the deglaciation of the BIIS has an important effect on mound growth. It can seriously alter the hydrography, nutrient supply and sedimentation processes, thereby affecting both sediment input and coral growth and hence, coral mound development.

Surface mound formation during epitaxial growth of CrN(001)

Single crystal CrN(001) layers, 10 to 160 nm thick, were grown on MgO(001) by reactive magnetron sputtering at growth temperatures T s = 600 and 800 °C. In situ scanning tunneling microscopy shows that all layer surfaces exhibit mounds with atomically smooth terraces that are separated by monolayer-high step edges aligned along ( 110) directions, indicating N-rich surface islands. For T s = 600 °C, the root mean square surface roughness σ initially increases sharply from 0.7 ± 0.2 for a thickness t = 10 nm to 2.4 ± 0.5 nm for t = 20 nm, but then remains constant at σ = 2.43 ± 0.13 nm for t = 40, 80 and 160 nm. The mounds exhibit square shapes with edges along ( 110) directions for t ≤ 40 nm, but develop dendritic shapes at t = 80 nm which revert back to squares at t = 160 nm. This is associated with a lateral mound growth that is followed by coarsening, yielding a decrease in the mound density from 5700 to 700 µm − 2 and an initial increase in the lateral coherence length ξ from 7.2 ± 0.6 to 16.3 ± 0.8 to 24 ± 3 nm for t = 10, 20, and 40 nm, respectively, followed by a drop in ξ to 22 ± 2 and 16 ± 2 nm for t = 80 and 160 nm, respectively. Growth at T s = 800 °C results in opposite trends: σ and ξ decrease by a factor of 2, from 2.0 ± 0.4 and 20 ± 4 nm for t = 10 nm to 0.92 ± 0.07 and 10.3 ± 0.4 nm for t = 20 nm, respectively, while the mound density remains approximately constant at 900 μm − 2. This unexpected trend is associated with mounds that elongate and join along ( 100) directions, yielding long chains of interconnected square mounds for t = 40 nm. However, coalescence during continued growth to t = 160 nm reduces the mound density to 100 µm − 2 and increases σ and ξ to 2.5 ± 0.1 and 40 ± 2 nm, respectively.

The Holocene occurrence of cold water corals in the NE Atlantic: Implications for coral carbonate mound evolution

U-series dating of constructional cold-water corals is a powerful tool to reconstruct the evolution of corals on carbonate mounds. Here we have investigated the time framework of corals such as Lophelia pertusa and Madrepora oculata on five different mound settings of the eastern North Atlantic (on Rockall Bank and in Porcupine Seabight), sampled at variable depth and location (610–880 m water depth). We have found that the past 11 ka reflect a period generally favourable for coral development. We further determined local mound growth rates and identified mound surface erosion (framework collapse) during times of active coral framework construction. “Local” vertical mound growth rates vary between less than 5 cm ka − 1 and up to 220 cm ka − 1 . We interpret rates exceeding 15 cm ka − 1 as representative of densely populated coral reefs. During times of reduced or absent coral development, mound evolution rates are by far smaller (0 to < 5 cm ka − 1 ). The time resolution achieved here furthermore provides first evidence for reduced coral (ecosystem) activity at 1.8–2.0 ka, 4.2–4.8 ka and between 6 and 8.2 ka within the Holocene that may be related to climate driven changes of the coral growth environments. During Glacial periods coral growth in those areas seems apparently extremely reduced or is even absent on mounds.

Effects of Middle–Late Permian sea-level changes and mass extinction on the formation of the Tieqiao skeletal mound in the Laibin area, South China

A skeletal mound is described for first time from a Middle–Late Permian succession that contains the Guadalupian–Lopingian (G/L) boundary at the Tieqiao section in the Laibin area, Guangxi Province, South China. The Tieqiao mound grew on a deep-water, south-facing carbonate ramp during the late Middle Permian. The mound features a range of grainstone–packstone to rudstone–floatstone facies together with sponge boundstone facies. Both sponges and algae acted as frame builders which colonised skeletal debris on the ramp and initiated mound growth. Diverse benthic biota comprising mound dwellers and constructors proliferated in the Tieqiao mound complex. The water depth changes recorded in the Tieqiao mound successions reflect the refined trajectory of global sea-level changes during the great regression at the end of the Middle Permian. The timing of sea-level lowstand coincides with disappearance of the Jinogondella granti conodont zone at Tieqiao. The change to sea-level rise pre-dated the G/L boundary. Growth and demise of the mound is interpreted to have been controlled by changes in sea-level. Abrupt disappearance of both body fossils and fossil fragments of leading fossil groups at Tieqiao is interpreted as the result of the G/L mass extinction rather than environmental change. The potential extinction horizon is ∼30 cm above the G/L boundary at Tieqiao and also records a rise in sea-level. The regional fall in sea-level destroyed the Tieqiao mound, but the G/L crisis halted redevelopment of the mound in the earliest Late Permian. Most mound builders suffered severely the Lazarus effect of the G/L mass extinction.

Rain splash of soil grains as a stochastic advection‐dispersion process, with implications for desert plant‐soil interactions and land‐surface evolution

We formulate soil grain transport by rain splash as a stochastic advection‐dispersion process. By taking into account the intermittency of grain motions activated by raindrop impacts, the formulation indicates that gradients in raindrop intensity, and thus grain activity (the volume of grains in motion per unit area) can be as important as gradients in grain concentration and surface slope in effecting transport. This idea is confirmed by rain splash experiments and manifest in topographic roughening via mound growth beneath desert shrubs. The formulation provides a framework for describing transport and dispersal of any soil material moveable by rain splash, including soil grains, soil‐borne pathogens and nutrients, seeds, or debitage. As such it shows how classic models of topographic “diffusion” reflect effects of slope‐dependent grain drift, not diffusion, and it highlights the role of rain splash in the ecological behavior of desert shrubs as “resource islands.” Specifically, the growth of mounds beneath shrub canopies, where differential rain splash initially causes more grains to be splashed inward beneath the protective canopy than outward, involves the “harvesting” of nearby soil material, including nutrients. Mounds thus represent temporary storage of soil derived from areas surrounding the shrubs. As the inward grain flux associated with differential rain splash is sustained over the shrub lifetime, mound material is effectively sequestered from erosional processes that might otherwise move this material downslope. With shrub death and loss of the protective canopy, differential rain splash vanishes and the mound material is dispersed to the surrounding area, again subject to downslope movement.

Cold-water coral mounds in an erosive environmental setting: TOBI side-scan sonar data and ROV video footage from the northwest Porcupine Bank, NE Atlantic

Cold-water coral mounds are common features in certain regions along the Atlantic margin. They occur in mound provinces in various settings, characterised by specific environmental conditions that steer and influence coral mound initiation, growth and demise (e.g. bottom current intensity, sediment input and food supply). In order to add detail to the diversity of environmental conditions described in relation to these structures, this study investigates mounds in a generally erosive setting on the northwest Porcupine Bank (NE Atlantic)—in contrast to previous studies in less hydrodynamically active settings. TOBI (Towed Ocean Bottom Instrument) side-scan sonar data revealed abundant erosive features in the study area. They occur in the form of erosional scarps, erosional pits and locally scoured seabed. Furthermore, two large (several hundred metres high) and 101 small (tens of metres high) cold-water coral mounds were identified on the TOBI images. Most of the mounds are located on top of erosional scarps pointing to the cold-water corals' preference for areas with enhanced current intensities. Within the study area, a general trend of northward decreasing mound heights and increasing abundance of erosive features can be observed. High-resolution ROV (Remotely Operated Vehicle) video observations at four sites within the TOBI coverage illustrate that only the southernmost mound shows signs of active mound growth while the other mounds represent relict structures with limited coral cover opportunistically taking advantage of available hard substrate in the form of consolidated/lithified sediments. The distribution of erosion features, seabed features and seabed facies on the northwest Porcupine Bank indicates an environmental window in which the interplay of coral growth, sediment input and sediment preservation leads to the formation of mound sediments and therefore mound growth. Whereas too weak bottom currents do not support thriving coral thickets, too strong bottom currents hamper any deposition and, thus, mound growth. As this environmental window varies over time, mound sizes on the Porcupine Bank (and probably also in other mound settings) most likely reflect the duration in time of such an environmental window of optimal growth conditions around a given mound, rather than an indication of the overall age of the mound.

Three‐dimensional architecture and development of Danian bryozoan mounds at Limhamn, south‐west Sweden, using ground‐penetrating radar

AbstractBryozoan mounds from the middle Danian (Lower Palaeocene) of the Danish Basin represent a possibly new class of non‐cemented skeletal mounds. The sedimentology and palaeoecology of the mounds have recently been studied in detail. Three‐dimensional images of middle Danian bryozoan mound structures in the Limhamn limestone quarry, south‐west Sweden, obtained from combined reflected ground‐penetrating radar signals and outcrop analysis provide new information about the architecture and growth development of such mounds. The mounds are composed of bryozoan limestone and dark‐grey to black flint bands which outline mound geometries. Ground‐penetrating radar data sections are collected over a 120 m by 60 m grid of data lines with trace spacing of 0·25 m, providing a depth penetration of 7 to 12 m and a vertical resolution of ca 0·30 m. The ground‐penetrating radar images outline the geometry of the internal layering of the mounds which, typically, have widths and lengths of 30 to 60 m and heights of 5 to 10 m. Mound architecture and growth show great variability in the ground‐penetrating radar images. Small‐scale mound structures with a palaeorelief of only a few metres may constitute the basis for growth of larger mounds. The outermost beds of the individual mounds are commonly characterized by sub‐parallel to parallel reflections which have a circular to slightly oval appearance in map view. The mounds are mainly aggrading and do not show clear signs of pronounced lateral migration during growth, although some mound structures indicate a preferential growth direction towards the south. Growth patterns interpreted from the ground‐penetrating radar images suggest that the palaeocurrents in the study area may have shown great variability, even on a small scale. This observation is in contrast to results from studies of extensive, slightly older early Danian mound complexes exposed in coastal cliffs at Stevns Klint and Karlby Klint located 50 and 200 km away from the study area, respectively. At these locations the mounds show a remarkably uniform development and typically are asymmetrical, clearly showing migration directions towards the south. These differences in mound geometry may be the result of differences in the current systems and water depths that existed during formation of the early and middle Danian mounds, respectively. The mounds at Limhamn were located closer to the basin margin in shallower water than those at Stevns Klint and Karlby Klint. In addition, the difference in mound architecture may be due to the occurrence of non‐layered, irregular coral mounds intercalated with the bryozoan mounds at Limhamn.

A new, Middle Cambrian, Burgess Shale-type biota, Bolaspidella Zone, Chancellor Basin, southeastern British Columbia

A newly discovered Burgess Shale-type (BST) biota occurs in southeastern British Columbia on Haiduk and Tangle peaks. The fossiliferous rocks of the informally named Vermilion sub-unit and Duchesnay unit occur in the Bolaspidella Zone, one trilobite biozone younger than the Burgess Shale Formation. The younger rocks abut the Eldon Escarpment in a stratigraphic and depositional relationship that mirrors that of the Burgess Shale Formation and Cathedral Escarpment. The biota, concentrated at the Eldon Escarpment, is hosted predominantly in the Duchesnay unit, and includes Burgess Shale genera of sponges, worms, some arthropods and brachiopods. New taxa include an arborescent, benthic graptolite with preserved fuselli. Microbial fossils, including vendotaenid bacteria, are locally abundant and range up to 750 m basinward of the Escarpment in both the Vermilion sub-unit and Duchesnay unit, while concentrations of animal fossils are restricted to within 100 m. In the Vermilion sub-unit, large autochthonous carbonate mud mounds, previously interpreted as platform-derived olistoliths, originated at the base of the Escarpment, probably over fluid conduits, and then slid downslope, bulldozing and entraining sediments in their paths. Local magnesian- and barian-rich brine pool deposits occur up to 750 m distal of the Escarpment; fractionation of the fluids may explain the differing geochemistries. At least one MgO-rich brine pool deposit on Haiduk Peak is fringed by abundant animal fossils, including concentrations of priapulid worms. This assemblage, together with high-density, low-diversity assemblages on Tangle Peak likely represent “fringing fauna facies” around brine seeps, a model erected for similar fossil concentrations and distributions in the Burgess Shale. Intrabasinal comparison of the Vermilion sub-unit and Duchesnay unit with the Burgess Shale Formation reveals an identical stratigraphic ordering of carbonates and shales, brine pool deposits, major debris flow deposits, carbonate mud mound growth and fossiliferous deposits. The Eldon and Cathedral platform formations are known to share complimentary brine-derived characteristics as well. The stratigraphic patterns signify that the depositional processes producing the suite of older lithofacies recurred during deposition of the younger units. Interbasinal correlation with the Wheeler and Marjum formations, Utah, identifies the Duchesnay unit deposit as the youngest basinal Burgess Shale-type deposit in North America. Decreasing abundance of animal fossils away from the Eldon and Cathedral escarpments is generally attributed to increasing development of tectonic fabrics. However, on Haiduk Peak, rocks hundreds of metres basinward of the Escarpment contact are essentially undeformed and contain rare Planolites, brachiopods and trilobites as the only macrofauna; locally abundant vendotaenids and Morania are evidence that BST preservation occurred at these distances. The concentration of animal fossils at the Escarpment is therefore a primary ecological feature, not due to a preservational or rheological bias. Communities were clustered at the face of the Escarpment where brines were seeping.

The first microbialite - coral mounds in the Cenozoic (Uppermost Paleocene) from the Northern Tethys (Slovenia): Environmentally-triggered phase shifts preceding the PETM?

Upper Thanetian microbialite-coral mounds from the Adriatic Carbonate Platform (SW Slovenia) are described herein for the first time, representing an important case study of extensively microbially-cemented boundstones in the Early Paleogene. The mounds are constructed primarily by microbialites associated to small-sized coral colonies, forming metric bioconstructions in a mid-ramp setting. Detailed macroscopic and microscopic studies show that microbes are the major framework builders, playing a prominent role in the stabilization and growth of the mounds, with corals being the second most important component. Microbial carbonates represent up to 70% of the mounds, forming centimetric-thick crusts alternating with coral colonies. The microbial nature of the crusts is demonstrated by their growth form and internal microfabrics, showing accretionary, binding, and encrusting growth fabrics, often with gravity-defying geometries. Thin sections and polished slabs reveal a broad range of mesofabrics, with dense, structureless micrite (leiolite), laminated crusts (stromatolites), and clotted micritic masses (thrombolites). A first layer of micro-encrusters, including leiolites and thrombolites, occurs in cryptic habitats, whereas discontinuous stromatolites encrust the upper surface of corals. A second encrustation, the major mound construction phase, follows and is dominated by thrombolites, encrusting corals and other micro-encrusters. This sequence represents the basic constructional unit horizontally and vertically interlocked, in an irregular pattern, to form the mounds. The processes, which favored the deposition of these microbial carbonates, were mainly related to in situ precipitation, with minor evidences for grain agglutination and trapping processes. Scleractinian corals comprise moderately diversified community of small (centimetric) colonial, massive, platy encrusting, and branching forms. Coral colonies are distributed uniformly throughout the mounds without developing any ecological zonation. These features indicate that coral development remained at the pioneer stage throughout the mound growth. The spatial relationships between corals and microbialites, as well as the characteristics of microbial crusts and coral colonies, indicate a strong ecological competition between corals and microbes. A model for the evolution of the trophic structures during the mound growth is proposed, with changes in the paleoecology of the main bioconstructors triggered by frequent environmental perturbations. Turbidity and nutrient pressure, interpreted here as related to frequent recurrences of wet phases during the warm, humid climate of the Uppermost Thanetian, might have promoted temporary dominance of microbes over corals, causing rapid environmentally-driven “phase shifts” in the dominant biota.

Three‐dimensional modelling and sequence stratigraphy of a carbonate ramp‐to‐shelf transition, Permian Upper San Andres Formation

AbstractThis study highlights three‐dimensional variability of stratigraphic geometries in the ramp crest to basin of mixed carbonate–siliciclastic clinoforms in the Permian San Andres Formation. Standard field techniques and mapping using ground‐based lidar reveal a high degree of architectural complexity in channellized, scoured and mounded outer ramp stratigraphy. Development of these features was a function of location along the ramp profile and fluctuations in relative sea‐level. Deposition of coarse‐grained and fine‐grained turbidites in the distal outer ramp occurred through dilute and high‐density turbidity flows and was the result of highstand carbonate shedding within individual cycles. In this setting, high‐frequency cycles of relative sea‐level are interpreted on the basis of turbidite frequency, lateral extent and composition. Submarine siliciclastic sediment bypass during lowstand cycles resulted in variable degrees of siliciclastic preservation. Abundant siliciclastic material is preserved in the basin and distal outer ramp as point‐sourced lowstand wedges and line‐sourced early transgressive blankets. In mounded topography of the outer ramp, siliciclastic preservation is minimal to absent, and rare incised channels offer the best opportunity for recognition of a sequence boundary. Growth of mounded topography in the outer ramp began with scouring, followed by a combination of bioherm construction, fusulinid mound construction and isopachous draping. Intermound areas were then filled with sediment and continued mound growth was prevented by an accommodation limit. Mound growth was independent of high‐frequency cycles in relative sea‐level but was dependent on available accommodation dictated by low‐frequency cyclicity. Low‐angle ramp clinoforms with mounded topography in the outer ramp developed during the transgressive part of a composite sequence. Mound growth terminated as the ramp transformed into a shelf with oblique clinoform geometries during the highstand of the composite sequence. This example represents a ramp‐to‐shelf transition that is the result of forcing by relative sea‐level fluctuations rather than ecologic or tectonic controls.

Influence of benthic sediment transport on cold‐water coral bank morphology and growth: the example of the Darwin Mounds, north‐east Atlantic

AbstractThe Darwin Mounds are small (up to 70 m in diameter), discrete cold‐water coral banks found at c. 950 m water depth in the northern Rockall Trough, north‐east Atlantic. Formerly described in terms of their genesis, the Darwin Mounds are re‐evaluated here in terms of mound growth processes based on 100 and 410 kHz side‐scan sonar data. The side‐scan sonar coverage is divided into a series of acoustic facies representing increasing current speed and sediment transport/erosion from south to north: pockmark facies, ‘mounds within depressions’ facies, Darwin Mound facies, stippled seabed facies and sand wave facies. Mound morphometric changes are quantified and show a south‐to‐north divergence from an inherited morphology, reflecting the outline of coral‐colonized fluid escape structures, to developed, downstream elongated, elevated mound forms. It is postulated that increasing current speeds and bedload sand transport favour mound growth and development by a process of enhanced sand sedimentation within mounds due to current deceleration by frictional drag around coral colonies. Comparisons are made with similar growth processes attributed to comparably sized cold‐water coral mounds in the Porcupine Seabight, offshore Ireland.

Morphology and sedimentology of (clustered) cold-water coral mounds at the south Rockall Trough margins, NE Atlantic Ocean

Cold-water coral mounds on both margins of the Rockall Trough (NE Atlantic Ocean) have a strongly different morphology. Single, isolated mounds occur on the SE margin and are mainly found on the upper slope between 900 and 650 m water depth, while large mound clusters are found on the SW margin in water depths between 600 and 1,000 m, in a narrow zone almost parallel to the slope. Sedimentation rates on the mounds are higher than on the surrounding seabed as a result of baffling of biogenic carbonate debris and siliciclastic particles by the coral framework covering the mounds. This is confirmed by 210Pb measurements. The individual coral growth rate can be three times higher then the vertical growth rate of the coral cover (±10 mm year−1) which in turn is more than an order of magnitude higher then the present-day overall mound growth rate (±0.25 mm year−1). The presence of extensive hardgrounds and firmgrounds and the three-dimensional coral framework are considered to be responsible for the stability of the relatively steep slopes of the mounds. High current velocities in the intramound areas result in local non-sedimentation and erosion, as is shown by the presence of IRD (ice-rafted debris) lag deposits on the seabed and moats around some of the mounds. The morphology and sedimentology of cold-water coral-covered (mainly Lophelia pertusa and Madrepora oculata) mounds on the southern Rockall Trough margins (NE Atlantic Ocean) is discussed and a model describing the development of these mounds is presented.

Mathematical models for Isoptera (Insecta) mound growth

In this research we proposed two mathematical models for Isoptera mound growth derived from the Von Bertalanffy growth curve, one appropriated for Nasutitermes coxipoensis, and a more general formulation. The mean height and the mean diameter of ten small colonies were measured each month for twelve months, from April, 1995 to April, 1996. Through these data, the monthly volumes were calculated for each of them. Then the growth in height and in volume was estimated and the models proposed.

Spiral Growth and Step Edge Barriers

The growth of spiral mounds containing a screw dislocation is compared to the growth of wedding cakes by two-dimensional nucleation. Using phase field simulations and homoepitaxial growth experiments on the Pt(111) surface we show that both structures attain the same large scale shape when a significant step-edge barrier suppresses interlayer transport. The higher vertical growth rate of the spiral mounds on Pt(111) reflects the different incorporation mechanisms for atoms in the top region and can be formally represented by an enhanced apparent step-edge barrier.

Carbonate mounds in a mud volcano province off north-west Morocco: Key to processes and controls

This paper presents a new cluster of carbonate mounds discovered in 2002 in the Gulf of Cadiz off Morocco ( R/V Belgica 2002) in water depths of 500 to 600 m amidst a field of giant mud volcanoes. Multibeam bathymetry, side scan sonar imagery and 2D seismics are analyzed to present four mound provinces: (1) the Pen Duick Mound Province on the Pen Duick Escarpment, (2) the Renard Mound Province on the Renard Ridge, (3) the Vernadsky Mound Province on the Vernadsky Ridge and the Al Idrisi Mound Province on the gas-blanked sediments above the buried Al Idrisi Ridge. Video imagery and surface samples are described to ground-truth the different mound areas. The paradox is that nearly no live corals are presently being observed at the surface of the mounds, while the mound cores display throughout a high number of reef-forming cold-water coral fragments (scleractinians) in association with numerous associated fauna formerly inhabiting the econiches provided by the coral framework. Environmental and oceanographic conditions during the recent past (glacials/stadials) were probably more favourable for cold-water coral growth. Pore water analyses in on-mound cores at the south-eastern edge of Pen Duick Escarpment give evidence of focused, higher methane fluxes and sulphate reduction rates on mounds than in the surrounding sediments. Cores from several mounds display horizons of strong corrosion and dissolution of the coral fragments. A three-phase model for carbonate mound evolution in these settings is proposed. (1) In a first stage external controls (positive oceanographic and environmental conditions, the presence of an active planktonic food chain, based on a high primary production, and a suitable substrate) are responsible for the initiation of cold-water coral growth. (2) Once the cold-water corals established an initial framework, sedimentation becomes an important factor controlling mound growth: the cold-water corals baffle the sediments. (3) Throughout mound growth, the mound may episodically be affected by diagenetic processes responsible for aragonite dissolution and probably carbonate precipitation.

Franken Mound: facies and biocoenoses on a newly-discovered “carbonate mound” on the western Rockall Bank, NE Atlantic

Cold-water coral carbonate mounds are widespread along the Irish continental margin. Whereas the Porcupine Seabight and the Rockall Trough are relatively well studied with regard to mound topography, coral coverage, and benthic life diversity, the situation on the western Rockall Bank is rather unknown. Detailed facies and biocoenoses mapping based on video footage analyses was conducted on the newly-discovered Franken Mound. Facies were identified ranging between cliff-like to planar hardgrounds and soft sediments that are partly rippled. A variety of biocoenoses are associated with these facies comprising discrete live coral colonies, dense live and dead coral framework coverage, abundant to scattered coral debris, and a soft sediment faunal community, whereas the latter is three times less speciose as biocoenoses containing live framework-building corals. The facies and biocoenosis classes are supplemented by exposed dropstones, lost fishery nets, and rubbish. The distribution of the classes clearly indicates a close relationship with local current effects and current intensification. Due to the dominance of dead coral framework and the partially exposed internal sediment sequences on the mound flanks, it is assumed that Franken Mound is approaching the “mound retirement” mound growth state.

Discussion on Pleistocene calcified cyanobacterial mounds, Perachora peninsula, central Greece: a controversy of growth and history Geological Society, London, Special Publications , Vol. 255 , 2006, 53–69

J. E. Andrews, M. R. Leeder, C. Portman, P. J. Rowe & J. Smith write: we were very interested to read Kershaw & Guo's (2006) analysis of the conflicting views on the growth and history of Pleistocene cyanobacterial mounds (bioherms) from the Perachora peninsula in the eastern Gulf of Corinth rift, central Greece. A new field guide to this classic geological area and these spectacular exposures will soon be available (Leeder et al . 2007) and the issues we debate below have an important bearing on regional uplift rates, the role of faulting, and the environmental conditions required for cyanobacterial calcification. We welcome Kershaw & Guo's (2006) fair and balanced account of competing hypotheses for bioherm evolution, and particularly commend and accept their new interpretation of the cavity-dwelling, pendant coralline algae. However, we dispute the more fundamental basis of their interpretations and take this opportunity to discuss them further. In the vicinity of Cape Heraion (Perachora peninsula) uplifted Pleistocene shorelines of marine isotope stage (MIS) 5a/c, MIS 5e, MIS 7a/c and MIS 7e age have been mapped (Leeder et al . 2005), and where possible U/Th dated using coral aragonite (Vita-Finzi 1993; Dia et al . 1997; Leeder et al . 2005). The upper part of a prominent terrace, at c . +25 to +30 m above modern sea level, is composed of marine bioclastic marls and carbonates of proven MIS 5e age (Vita-Finzi 1993; Leeder et al . 2005). However, directly below these sediments is a horizon of spectacular bioherms, up to 10 m high (Fig. 1) constructed by calcified cyanobacteria ( Rivularia haematites ) and coralline algae ( Lithophyllum pustulatum ). There is controversy regarding the age, stratigraphic relations and environment of formation of these bioherms. They have variously been interpreted as brackish lake deposits forming immediately before the …

Danian Cool-Water Bryozoan Mounds at Stevns Klint, Denmark--A New Class of Non-Cemented Skeletal Mounds

Abstract A laterally extensive Danian (early Paleogene) bryozoan mound complex is well exposed in 14 km of nearly continuous outcrop in the coastal cliff Stevns Klint in eastern Denmark. A NW–SE oriented seaway covered the Danish Basin in Danian time. It widened towards the NW, where it passed into the deeper North Sea Basin and narrowed towards the SE, where it was connected with the Polish Trough. The exposed mound succession is up to 20 m thick. Individual mounds are oval in plan view, internally asymmetrical, 5–11 m high, 45–110 m long in NNE–SSW oriented dip sections parallel to the direction of mound migration, and 50–300 m in strike sections. The southern flanks are steep and dip 15–30° mainly towards the SSW, whereas northern flanks have gentle dips of 0–15° towards the NNE. Four facies associations are distinguished: (1) the initial mound association, consisting mainly of bryozoan packstone–rudstone; (2) the steep flank-mound crest association, consisting of bryozoan rudstone with subsidiary octocoral and encrusting bryozoan rudstone and bryozoan grainstone; (3) the gentle-flank association, characterized by bryozoan packstone, and incipient to fully developed hardgrounds; and (4) the intermound association, consisting of echinoid rudstone, packstone, and minor occurrences of bioclastic grainstone and argillaceous wackestone–packstone beds and laminae. Facies analysis and mesoscale mapping indicate that the bryozoan mound complexes were formed in relatively deep, cool water below the photic zone and swell wave base. Deposition was influenced by along-slope currents, with prominent seasonal and long-term variations. Mound growth was governed by primarily in-place benthic carbonate production of mainly delicate branching bryozoans which baffled and trapped pelagic and detrital benthic carbonate mud. Gentle currents rich in particulate nutrients flowing towards the WNW deflected by the Coriolis force, and possibly a tidally or wind-induced upslope component towards the NNE, resulted in mound growth mainly towards the SSW. High production in the surface waters combined with an effective tidally induced mixing of the water column was important for transporting particulate nutrients to the bottom waters. Periodic intensification of the mean WNW-ward flowing bottom current system combined with a storm-associated downslope component towards the SSW resulted in winnowing and erosion of the northern mound flanks. Long-term periodic waning or cessation of bottom currents and possibly stratification of the water column resulted in very limited benthic carbonate production and mound growth associated with formation of extensive Thalassinoides burrow galleries mirrored by the rhythmic occurrences of flint nodule bands. Mound development thus reflects the delicate balance between biogenic growth towards particulate nutrient carrying currents, current winnowing, and erosion. The lack of framework buildup and the poorly cemented nature of the mounds is remarkable. In this respect they deviate from most other mound types known from the geological record, and may—possibly together with the sub-recent analogues on the Great Australian Bight—represent a new class of noncemented skeletal mounds.