Shelf Depths Research Articles

Unexpected Source and Transport of Iron from the Deep Peru Margin

Iron is the most important micronutrient in the ocean, but the nature and magnitude of its sources and sinks to the ocean are poorly constrained. Here we assess our understanding of the sources and sinks of iron in margin environments by synthesizing observations from the U.S. GEOTRACES GP16 Eastern Tropical Pacific Zonal Transect (EPZT) cruise near the Peru margin. GP16 observations showed elevated dissolved iron (dFe) concentrations along the margin, but a larger westward plume of dFe at slope depths (1000–3000 m) in oxygenated waters, rather than at shelf depths (100–300 m) in oxygen deficient waters. We examine the potential explanations for this unexpected observation. Multiple tracers from GP16 suggest that sediment resuspension was important at slope depths, which would lead to enhanced benthic flux of dFe above what was previously measured. The difference in the apparent persistence and penetration of shelf versus slope plumes of dFe into the interior of the ocean likely results from faster removal rates of the shelf dFe compared to slope dFe. The dFe sourced from the shelf was almost entirely in the dFe(II) form, whereas dFe sourced from the slope was almost entirely in the dFe(III) form. Although benthic dFe(II) diffuses into oxygen deficient overlying waters, there is still oxidation of dFe(II), which precipitates to particulate Fe(III). In contrast, the slope plume appears to persist in a stabilized dFe(III) form. We hypothesize that sediment porewaters with moderate organic carbon delivery to sediments and shallow oxygen penetration are especially good sources of persistent dFe to the water column.

Fin whale 40‐Hz calling behavior studied with an acoustic tracking array

Fin whale 40‐Hz calling behavior studied with an acoustic tracking array

Heterogeneity of rhodolith beds expressed in backscatter data

Rhodoliths play a major role as benthic habitats and are important calcium carbonate bio-factories. Several studies have acoustically mapped rhodolith beds as a homogeneous habitat but spatial changes in nodule coverage, or density, commonly occur. Here, the objective is to acoustically recognize heterogeneities in rhodolith bed structure using the response of backscatter to map changes in nodule coverage and density. High-resolution multibeam sonar (MBSS) data were acquired on April 2018, in the Marine Protection Area (MPA) Costas das Algas, located in the Espírito Santo continental shelf, Eastern Brazilian coast. Three areas were selected based on available data, totalizing 73km2 of data acquisition. Ground truth was obtained at 80 stations, using a drop camera system. Seabed classification was determined by applying an image segmentation process on multibeam sonar backscatter mosaic. Ground truth data was used to statistically show the relation between backscatter and seabed types. Results identified three classes related to rhodoliths in different coverage densities: low rhodolith coverage (inferior to 25% of rhodoliths over a determined area), moderate rhodolith coverage (between 25% and 35% of rhodoliths), and high rhodolith coverage (>35%). Classes associated with sediment, bioconcretions and red algae genus Peyssonelias were also identified. A map showing changes in rhodolith coverage across the shelf depth gradient were produced. The seabed classes had a statistically significant correlation with the segmented backscatter classes. In general, higher rhodolith coverage correlates with higher backscatter. The backscatter mosaic over the area is marked by an extensive presence of the red algae Peyssonelias, resulting in a lower correlation index, when compared to the areas with no Peyssonelia. The seabed mapping approach used in this work provided an efficacious tool to map benthic habitats and to detail nodules distribution across a rhodolith bed. Moreover, the recognition of heterogeneities in rhodolith bed structure provides important information for monitoring and protecting these vulnerable marine ecosystems. The main driver of the coralligenous algae nodule coverage, in the research area, seems to be the morphology of the area and the influence of mesoscale circulation.

Fine-scale vertical structure of sound-scattering layers over an east border upwelling system and its relationship to pelagic habitat characteristics

Abstract. Understanding the relationship between sound-scattering layers (SSLs) and pelagic habitat characteristics is a substantial step to apprehend ecosystem dynamics. SSLs are detected on echo sounders representing aggregated marine pelagic organisms. In this study, SSL characteristics of zooplankton and micronekton were identified during an upwelling event in two contrasting areas of the Senegalese continental shelf. Here a cold upwelling-influenced inshore area was sharply separated by a strong thermal boundary from a deeper, warmer, stratified offshore area. Mean SSL thickness and SSL vertical depth increased with the shelf depth. The thickest and deepest SSLs were observed in the offshore part of the shelf. Hence, zooplankton and micronekton seem to occur more frequently in stratified water conditions rather than in fresh upwelled water. Diel vertical and horizontal migrations of SSLs were observed in the study area. Diel period and physicochemical water characteristics influenced SSL depth and SSL thickness. Although chlorophyll-a concentration insignificantly affected SSL characteristics, the peak of chlorophyll a was always located above or in the middle of the SSLs, regularly matching with the peak of SSL biomass. Such observations indicate trophic relationships, suggesting SSLs to be mainly composed of phytoplanktivorous zooplankton and micronekton. Despite local hypoxia, below 30 m depth, distribution patterns of SSLs indicate no vertical migration boundary. The results increase the understanding of the spatial organization of mid-trophic species and migration patterns of zooplankton and micronekton, and they will also improve dispersal models for organisms in upwelling regions.

STRATIGRAPHY AND SEDIMENTARY ENVIRONMENTS OF THE MIOCENE DEPOSITS OF THE NORTH-WESTERN UKRAINIAN PRECARPATHIAN BASED ON FORAMINIFERAL STUDY

The Miocene deposits of the north-western part of the Ukrainian Fore-Carpathians (PreCarpathians), which are part of the Boryslav-Pokuttya and Sambir advanced tectonic nappes, are considered. An attempt was to reconstruct the sedimentation conditions of the Miocene molasses of this region was made. On the basis of the environmental analysis of foraminiferal associations of the Vorotyshcha and Stebnik formations of the Boryslav-Pokuttya and Sambir nappes, conclusions were made on the conditions of sedimentation of the region in the Carpathian time at the boundary between of the early and middle Miocene and on the conditions oa accumulation of the Berezhnytsa Formation of the Sambir Nappe at the end of the middle – at the beginning of the late Miocene. Deposits were formed in marine basin at the shelf depths with normal salinity or close to it.

A late-surviving Triassic protomonaxonid sponge from the Paris Biota (Bear Lake County, Idaho, USA)

Protomonaxonid sponges are a major group of Cambrian and Ordovician fossils in exceptionally preserved (especially Burgess Shale-type) faunas, but are rare thereafter. Rare examples of apparent surviving lineages are known from the late Palaeozoic and Mesozoic, but by this time more derived groups of sponges have generally displaced them in at least shallow-water (shelf depth) ecosystems. The early Spathian (Early Triassic) Paris Biota includes abundant material of a new leptomitid protomonaxonid, Pseudoleptomitus advenus Botting nov. gen., nov. sp., distinguished by having an unbundled longitudinal skeleton and very weak transverse component. This is the first post-Ordovician leptomitid known, and indicates long-term survival of the group in unknown environments. Its occurrence near storm wave base is similar to the preferred environment of earlier examples of the family, suggesting either ecological rarity or taphonomic reasons for their ∼200-million-year absence from later Palaeozoic rocks.

Brief communication: PICOP, a new ocean melt parameterization under ice shelves combining PICO and a plume model

Abstract. Basal melting at the bottom of Antarctic ice shelves is a major control on glacier dynamics, as it modulates the amount of buttressing that floating ice shelves exert onto the ice streams feeding them. Three-dimensional ocean circulation numerical models provide reliable estimates of basal melt rates but remain too computationally expensive for century-scale projections. Ice sheet modelers therefore routinely rely on simplified parameterizations based on either ice shelf depth or more sophisticated box models. However, existing parameterizations do not accurately resolve the complex spatial patterns of sub-shelf melt rates that have been observed over Antarctica's ice shelves, especially in the vicinity of the grounding line, where basal melting is one of the primary drivers of grounding line migration. In this study, we couple the Potsdam Ice-shelf Cavity mOdel (PICO, Reese et al., 2018) to a buoyant plume melt rate parameterization (Lazeroms et al., 2018) to create PICOP, a novel basal melt rate parameterization that is easy to implement in transient ice sheet numerical models and produces a melt rate field that is in excellent agreement with the spatial distribution and magnitude of observations for several ocean basins. We test PICOP on the Amundsen Sea sector of West Antarctica, Totten, and Moscow University ice shelves in East Antarctica and the Filchner-Ronne Ice Shelf and compare the results to PICO. We find that PICOP is able to reproduce inferred high melt rates beneath Pine Island, Thwaites, and Totten glaciers (on the order of 100 m yr−1) and removes the “banding” pattern observed in melt rates produced by PICO over the Filchner-Ronne Ice Shelf. PICOP resolves many of the issues contemporary basal melt rate parameterizations face and is therefore a valuable tool for those looking to make future projections of Antarctic glaciers.

The Role of Maximum Shelf Depth versus Distance from Shore in Explaining a Diversity Gradient of Mushroom Corals (Fungiidae) off Jakarta

Many coral reef systems are shelf-based and consist of reefs that are arranged in rows parallel to the coastline. They usually show an increase in species richness in the offshore direction, coinciding with decreasing terrigenous impact and a deeper seafloor. These two conditions usually concur, which makes it less easy to distinguish how each of them influences coral diversity separately. Since reefs off Jakarta (in the Thousand Islands archipelago) are arranged in an 80 km long string perpendicular to the coastline in south-to-north direction, with a maximum shelf depth halfway along (instead of at the end of) the string, this archipelago is very suitable for studies on inshore–offshore gradients. In the present study, mushroom corals (Fungiidae; n = 31) were used to examine diversity patterns on 38 reef sites along such a gradient, involving species richness over their entire depth range from reef flat to reef base (2–30 m) and separately at shallow depths (2–6 m). Total species diversity was highest in the central part of the archipelago, with unique species occurring in deep habitats. Diversity at shallow depths was only slightly higher here than at reefs located more nearshore and offshore, which both had less clear water. Therefore, shelf depth and distance from the mainland can be considered separate determinants of coral diversity off Jakarta.

Cross and long-shore variations in reef fish assemblage structure and implications for biodiversity management

Fish communities are an important cultural, recreational and commercial resource that also have an important role in the functioning of marine ecosystems. Around the world fish assemblages are experiencing pressures from anthropogenic activities, and marine spatial planning is being established to mitigate these impacts and assist with biodiversity conservation. Information about how fish assemblages are structured across a range of spatial scales which encompass variations in physical, biotic and environmental parameters will assist marine spatial planning and management. We investigated differences in reef fish assemblage composition over three reef lines across an inshore to offshore gradient (3–23 m depth) at two marine reserves (70 km apart) in the Perth metropolitan region, Western Australia. There were significant increases in the number of individuals, species richness, and relative abundance of fish species across the shallow shelf depth gradient in the two locations. There were distinct fish assemblages associated with each reef line, correlated to depth and distance from shore. The differences across the shelf gradient, even over this small depth range, were greater than the differences between the two locations. These findings have implications for marine spatial management and the design of marine reserves that aim to conserve biodiversity. It may be most appropriate for such marine reserves to encompass a wide depth gradient, rather than a large longshore area. At the very least, cross and longshore patterns in fish assemblages should be taken into consideration and used to guide spatial management plans for biodiversity conservation.

Changing depositional environments reflected by foraminifera in a transgressive-regressive sequence of the Lower Cretaceous on Spitsbergen

The Boreal Valanginian to Barremian foraminiferal succession of the Rurikfjellet Formation in central Spitsbergen is applied to stratigraphic zonation, analyses of depositional conditions and developing a sequence stratigraphic model. The study is based on the Janusfjellet section where the formation is 204 m thick, and represents a depth range from upper bathyal to prodelta top. It contains a continuous benthic foraminiferal succession permitting distinction of five assemblage zones. The assemblages are almost entirely agglutinated and reflect calcium carbonate depletion, which appears as an overall feature of Mesozoic Boreal seas superimposed on other, but more local restricting factors such as salinity and food supply. Close taxonomic similarities within the Boreal Realm are revealed by comparison of the Janusfjellet foraminiferal faunas to those of other pan-Arctic sites. The lower half of the Janusfjellet section contains a significant faunal component comparable to modern bathyal faunas and Late Cretaceous to Paleogene flysch-type assemblages indicating upper bathyal to offshore shelf depths. In the upper half of the section increased diversities are created by enhanced food supply in middle shelf setting. This is succeeded by increasing deltaic impact leading to hyposaline environments reflected by low species diversities. Changing foraminiferal biofacies trends through the Janusfjellet section are portrayed in the framework of the third order transgressive–regressive Wimanfjellet Sequence. Its transgressive systems tract was deposited mainly at upper bathyal depths with maximum flooding surface in the lower (Valanginian) part of the Rurikfjellet Formation. The regressive systems tract reveals an upward shoaling succession from upper bathyal conditions, through marine shelf to hyposaline prodelta setting.

New species and records of Uncispionidae and Pygospiopsis (Polychaeta, Spionida) from deep water off the east and west coasts of North America, the Gulf of Mexico, the Antarctic Peninsula, and Southeast Asia.

Five new species and one new genus of the obscure spioniform family Uncispionidae are described together with three new species of the rare and unusual spionid genus Pygospiopsis Blake, 1983. All species are from offshore habitats with most from deep-sea continental slope depths. Among the Uncispionidae are the second and third species of the genus Uncopherusa Fauchald Hancock, 1981, collected from off Brunei in the South China Sea and off Louisiana in the Gulf of Mexico; two new species of Uncispio Green, 1982, the third and fourth to be described, from deep water off the U.S. Atlantic coast and the Gulf of Mexico; and a new species of a new genus, Rhamphispio n. gen., from off the U.S. Atlantic coast. All species of Uncispionidae are compared and a key to the known species is presented. The genus Pygospiopsis Blake, 1983, is currently known for only two species: P. dubia (Monro, 1930) from Antarctic and sub-Antarctic waters and P. occipitalis Blake, 1996, from shelf depths off southern California. In the present study, new collections of the type-species P. dubia from the Antarctic Peninsula include post-larvae and juveniles as well as adults, thus permitting documentation of the development of some key adult morphology. Three new species of Pygospiopsis are described from deep water off the U.S. Atlantic and Pacific coasts and from the Antarctic Peninsula. A review of all species of Pygospiopsis suggests that, based on branchial distribution patterns, the closely related Pseudatherospio fauchaldi Lovell, 1994, should be referred to Pygospiopsis, bringing the total known species to six. All of these are compared and contrasted and the generic definition of Pygospiopsis updated. The status of Pygospiopsis within the Spionidae relative to the closely related genus Atherospio Mackie Duff, 1986, is discussed.

The Cenomanian–Turonian transition in the carbonate platform facies of the Western Saharan Atlas (Rhoundjaïa Formation, Algeria)

Microfacies, fossil macroinvertebrates and microfossil assemblages (foraminifera and ostracods) and δ13C from the Rhoundjaia Formation (Ksour Mountains, Western Saharan Atlas, Algeria) were studied. During the Late Cenomanian–Early Turonian, the Ksour Basin was an isolated platform at mid to outer shelf depths. The neritic conditions were interrupted by pelagic conditions in transgressive intervals. This deepening of the palaeoenvironment is reflected by changes in the foraminiferal assemblages. The composition of benthic and planktic foraminiferal assemblages, characterized by low diversity and abundance of low oxygen tolerant species, reflects a carbonate platform with biotic stress conditions (oxygen and nutrient fluctuations and sea-level changes). Planktic foraminiferal assemblages are dominated by opportunists such as Muricohedbergella and Planoheterohelix, which colonized new ecospace on the shelf during the transgression. The Oceanic Anoxic Event 2 (OAE2) is not represented by organic rich facies but does coincide with the successive proliferation of different opportunist organisms such as microgastropods and small echinoid (hemiasterids) and the heterohelicid shift. The continuous presence of trace fossils (Thalassinoides and Planolites-like) and benthic macroinvertebrates allows us to exclude anoxic conditions during the Whiteinella archaeocretacea Zone. The proliferation of Planoheterohelix indicates the record of eutrophic conditions and the transition from oxygenated to poorly oxygenated waters during stratified open marine settings at the time of maximum flooding. The subsequent increase in thin-shelled bivalves and cytherellid ostracods would confirm the stress conditions at the time of the δ13C isotopic excursion in the upper part of the W. archaeocretacea Zone. The Lower Turonian is represented by a shallowing upward trend in the Ksour Basin. Comparison of the taxonomic composition of the planktic foraminiferal assemblages (phenogram and class diagram) indicates the high degree of relationship of the Ksour Basin with the Northeast Sicily Basin, Central Tunisia Basin and Egypt. This work sheds light on the impact of the OAE2-related environmental perturbation in environments where anoxic conditions were not reached, as well as the response of the fossil assemblages to the biotic crisis.

Effect of continental and nearshore slopes on tsunami height

In the present study, an attempt is made to understand the effect of continental and nearshore slopes on the transmission, propagation and run-up of tsunami. A numerical model study is carried out using shallow water equations that are solved using Crank-Nicolson finite difference approximation method on a staggered grid. A rectangular solitary wave is considered to propagate over typical continental slopes and nearshore profiles available along the Indian coast. The amplification or attenuation of tsunami characteristics over these cross-sections was studied. It was observed that the tsunami run-up is altered with varying continental slope, water depth on continental shelf and nearshore slope. The study suggests that tsunami height on the continental shelf increases for shallow continental shelf depths. The percentage increase in tsunami height with decrease in continental shelf depth from 200 m to 50 m is found to be 31%. Higher tsunami run-up was observed for deeper continental shelf and steeper continental slope, whereas lower tsunami run-up for deeper continental shelf and flatter continental slope. It was observed that for a steep continental slope (1:0.1), a reduction in nearshore slope (steep to flat) increases tsunami run-up up to a slope of 1:632 and then decreases.

Light Shelf as a Daylighting System in a Tropical Climate Office Space

Daylighting is a passive design strategy for lighting office spaces given the fact that offices operate within daytime, usually between 8:00 am to 5:00 pm which corresponds to the period of daylight availability. Daylight is abundant in the tropics, free and has no negative impact on the environment. It has positive physiological and psychological effect on the man as well as good colour rendering. Ironically, despite the abundant availability and the merits of daylighting especially in the tropics, it is not fully utilized. A light shelf is one of the innovative daylighting systems that can be employed to improve office spaces daylighting performance. Therefore, the focus of this study is the application of light shelf as a daylighting system in government offices in Malaysia. This study was carried out through literature search on thirteen disciplines. The disciplines were building and environment, energy, and building, renewable and sustainable energy, energy conservation in Building, lighting research and technology, energy conservation and Management, Solar energy, renewable energy, indoor and built environment, energy policy, energy and buildings, renewable energy, and applied energy. Data were obtained from Science Direct, Web of Science, Scopus and Conference Proceedings. This was done by typing in keywords such as daylighting, light shelf, office spaces, visual performance, tropical climate, and Malaysia. The review was from 1986 to 2016 to cover the length and breadth of the study area for the past 30 years. This work includes previous review and empirical works. Attention was equally given to the methodologies and context of the work. The result of this study showed the poor daylighting performance of government office spaces in Malaysia. The daylighting in these offices is either too low as a result of the use of excessive external shading devices to reduce interior heat gain or too high due to over glazed wall for aesthetics and daylighting. Consequent upon these is inadequate daylighting illuminance, non-uniform illuminance distribution, and glare. This brings about visual discomfort. Also, the study revealed that light shelf as an innovative daylighting lighting system could improve the daylighting performance of these office spaces. The effectiveness of light shelf depends on the latitude, reflectance of light shelf material, the angle of inclination, its depth, mounting position among others. However, there exists little work on the effect of the depth of light shelf and the variation of its distance from the ceiling on the daylight performance of government office spaces in Malaysia. This paper, therefore, recommends investigation on the effect of the depth of light shelf and the variation of its distance from the ceiling on daylighting performance of government office spaces in Malaysia.

ОПРЕДЕЛЕНИЕ ДАВЛЕНИЯ ГИДРОРАЗРЫВА ПЛАСТА ПРИ БУРЕНИИ СКВАЖИН НА ШЕЛЬФЕ

The purpose of the article is to focus on the breakdown pressure of the well bore when drilling offshore for oil and gas. It is much lower than the one when drilling on shore and can vary significantly. Domestic articles on this topic are little to choose. One of the reasons of this situation is that drilling on the already explored offshore deposits is not performed now as the oil prices are low, and drilling from the floating drilling rigs is too expensive. Moreover, severe ice conditions and imperfection of drilling equipment for offshore drilling hinder the development of drilling works in the Northern and Eastern seas. We should also note the lack of experience of Russian drillers in the development of offshore oil and gas fields. Methods. The study uses a refined analytical calculation of the breakdown pressure of the well bore and its gradient. Results. To determine the breakdown pressure it is proposed to allow for the shelf depth and rotary table elevation above the average level of the shelf surface. The depth of the shelf is often not taken into account allegedly because of its small depths. But the described factor is not the only one that causes a short-term increase in the hydrostatic pressure. It could also be caused by daily tides, swabbing under RIH/POOH operations and etc. All together they can cause the loss of circulation. Besides, the article presents the average overburden and formation pressure gradients in tabular variants instead of graphical ones in order to facilitate the calculations. Conclusions. Analytical determination of the fracture pressure when drilling offshore wells (water holes) is an important task as it gives the real values of the fracture pressure. This can help to prevent the loss of circulation.

Macroalgal diversity along an inshore-offshore environmental gradient in the Jakarta Bay – Thousand Islands reef complex, Indonesia

The Jakarta Bay – Thousand Islands reef complex extends to more than 80 km in northwest direction from the major conurbation Jakarta (Indonesia) along a pronounced inshore to offshore environmental gradient. The present study aims to determine to what extent environmental factors can explain the composition of macroalgal communities on the reefs off Jakarta. Therefore, the presence-absence of 67 macroalgal taxa was recorded for 27 sampling sites along the inshore-offshore disturbance gradient and analysed with substrate variables and water quality variables. The macroalgal richness pattern matches the pattern of other reef taxa. The 27 sites could be assigned to one of four geographical zones with 85% certainty based on their macroalgal taxon assemblages. These four zones (i.e., Jakarta Bay and, respectively, South, Central, and North Thousand Islands) had significantly different macroalgal assemblages, except for the North and South zones. Along the nearshore gradient there was a greater shift in taxon composition than within the central Thousand Islands. The patterns of ten habitat and water quality variables resembled the macroalgal diversity patterns by 56%. All ten variables together explained 69% of the variation in macroalgal composition. Shelf depth, % sand cover, gelbstoff/detrital material, chlorophyll a concentration, seawater surface temperature, and % dead coral cover were the best predictors of seaweed flora composition. Furthermore, 44 macroalgal species represented new records for the area. The present study provides important baseline data of macroalgae in the area for comparison in future biodiversity assessments in the area and elsewhere in the region.

Cool eastern rim of the North Pacific during Late Cretaceous time: A seep-carbonate paleothermometry from the Nanaimo Group, British Columbia, Canada

A crater-shaped carbonate body newly recognized in the uppermost Northumberland Formation (Nanaimo Group) of the Canadian Pacific coast, is identified as a remnant of methane seepage during latest Campanian time in the northeastern Pacific region. A two meter-long elliptical mound with central concavity is found on a Cretaceous bedding plane observed at Hornby Island, British Columbia, exposed on an expansive intertidal platform. Microstructures characteristic of carbonate associated with methane seepage are readily observed within the rock body and indicate it was formed near the sediment-water interface. Carbon isotope values of the carbonate range between −45.5 and +4.9‰, where the lowermost values characterize anaerobic oxidation of biogenic methane as the primary process that controlled the carbonate precipitation. Oxygen isotope values also exhibit a wide range of values, from −9.8 to +0.6‰. Areas preserving features of earliest precipitation (i.e., radiaxial bladed calcite and micrite with clotted fabric) on cross-sections exhibit the highest δ18O values and indicate a paleotemperature 8.0±0.7°C for seawater at the seafloor. Development of cool water on uppermost bathyal or outer shelf depths in the northeastern North Pacific is thus implied during the Campanian-Maastrichtian transition. This paleotemperature determination shows a significant variance with those obtained from similar paleo-water depths of the northwestern North Pacific.

The role of heat transfer time scale in the evolution of the subsea permafrost and associated methane hydrates stability zone during glacial cycles

Climate warming may lead to degradation of the subsea permafrost developed during Pleistocene glaciations and release methane from the hydrates, which are stored in this permafrost. It is important to quantify time scales at which this release is plausible. While, in principle, such time scale might be inferred from paleoarchives, this is hampered by considerable uncertainty associated with paleodata. In the present paper, to reduce such uncertainty, one-dimensional simulations with a model for thermal state of subsea sediments forced by the data obtained from the ice core reconstructions are performed. It is shown that heat propagates in the sediments with a time scale of ∼ 10–20kyr. This time scale is longer than the present interglacial and is determined by the time needed for heat penetration in the unfrozen part of thick sediments. We highlight also that timings of shelf exposure during oceanic regressions and flooding during transgressions are important for simulating thermal state of the sediments and methane hydrates stability zone (HSZ). These timings should be resolved with respect to the contemporary shelf depth (SD). During glacial cycles, the temperature at the top of the sediments is a major driver for moving the HSZ vertical boundaries irrespective of SD. In turn, pressure due to oceanic water is additionally important for SD ≥ 50m. Thus, oceanic transgressions and regressions do not instantly determine onsets of HSZ and/or its disappearance. Finally, impact of initial conditions in the subsea sediments is lost after ∼ 100kyr. Our results are moderately sensitive to intensity of geothermal heat flux.

Polar zoobenthos blue carbon storage increases with sea ice losses, because across-shelf growth gains from longer algal blooms outweigh ice scour mortality in the shallows.

One of the major climate-forced global changes has been white to blue to green; losses of sea ice extent in time and space around Arctic and West Antarctic seas has increased open water and the duration (though not magnitude) of phytoplankton blooms. Blueing of the poles has increases potential for heat absorption for positive feedback but conversely the longer phytoplankton blooms have increased carbon export to storage and sequestration by shelf benthos. However, ice shelf collapses and glacier retreat can calve more icebergs, and the increased open water allows icebergs more opportunities to scour the seabed, reducing zoobenthic blue carbon capture and storage. Here the size and variability in benthic blue carbon in mega and macrobenthos was assessed in time and space at Ryder and Marguerite bays of the West Antarctic Peninsula (WAP). In particular the influence of the duration of primary productivity and ice scour are investigated from the shallows to typical shelf depths of 500m. Ice scour frequency dominated influence on benthic blue carbon at 5m, to comparable with phytoplankton duration by 25m depth. At 500m only phytoplankton duration was significant and influential. WAP zoobenthos was calculated to generate ~107 , 4.5×106 and 1.6×106 tonnes per year (between 2002 and 2015) in terms of production, immobilization and sequestration of carbon respectively. Thus about 1% of annual primary productivity has sequestration potential at the end of the trophic cascade. Polar zoobenthic blue carbon capture and storage responses to sea ice losses, the largest negative feedback on climate change, has been underestimated despite some offsetting of gain by increased ice scouring with more open water. Equivalent survey of Arctic and sub-Antarctic shelves, for which new projects have started, should reveal the true extent of this feedback and how much its variability contributes to uncertainty in climate models.

Bone and cortical bone thickness of mandibular buccal shelf for mini-screw insertion in adults.

To analyze the buccal bone thickness, bone depth, and cortical bone depth of the mandibular buccal shelf (MBS) to determine the most suitable sites of the MBS for mini-screw insertion. The sample included cone-beam computed tomographic (CBCT) records of 30 adult subjects (mean age 30.9 ± 7.0 years) evaluated retrospectively. All CBCT examinations were performed with the i-CAT CBCT scanner. Each exam was converted into DICOM format and processed with OsiriX Medical Imaging software. Proper view sections of the MBS were obtained for quantitative and qualitative evaluation of bone characteristics. Mesial and distal second molar root scan sections showed enough buccal bone for mini-screw insertion. The evaluation of bone depth was performed at 4 and 6 mm buccally to the cementoenamel junction. The mesial root of the mandibular second molar at 4 and 6 mm showed average bone depths of 18.51 mm and 14.14 mm, respectively. The distal root of the mandibular second molar showed average bone depths of 19.91 mm and 16.5 mm, respectively. All sites showed cortical bone depth thickness greater than 2 mm. Specific sites of the MBS offer enough bone quantity and adequate bone quality for mini-screw insertion. The insertion site with the optimal anatomic characteristics is the buccal bone corresponding to the distal root of second molar, with screw insertion 4 mm buccal to the cementoenamel junction. Considering the cortical bone thickness of optimal insertion sites, pre-drilling is always recommended in order to avoid high insertion torque.