Dense Sinking Research Articles

Connection intervals in multi-scale infrastructure-augmented dynamic networks

We consider a hybrid spatial communication system in which mobile nodes can connect to static sinks in a bounded number of intermediate relaying hops. We describe the distribution of the connection intervals of a typical mobile node, i.e., the intervals of uninterrupted connection to the family of sinks. This is achieved in the limit of many hops, sparse sinks and growing time horizons. We identify three regimes reflecting various degrees of sink densities. Namely, (1) a regime of dense sinks, in which the limit is deterministic and given as an expectation with respect to percolation clusters, (2) a regime of sparse sinks, in which the limit depends on a random number of reachable sinks, and (3) an intermediate critical regime.

Marine Debris Polymers on Main Hawaiian Island Beaches, Sea Surface, and Seafloor.

Polymeric differences of plastic debris were assessed across four compartments of the Main Hawaiian Islands (sea surface, windward beaches, leeward beaches, and seafloor) to better describe sources and fate. Plastic debris pieces (n = 4671) were collected from 11 beaches, three sea surface tows, and three seafloor dives. Fourier transform infrared spectroscopy identified the polymers of 3551 pieces. Significant differences (p < 0.05) in concentration, types, polymer composition, and weathering were found among four compartments. Windward beaches had 1-2 orders of magnitude more plastic pollution (g/m2) than leeward beaches, despite smaller human populations on windward sides. Sea surface and windward beaches were dominated by severely weathered, less dense floating polymers (polyethylene and polypropylene comprised 92.7 and 93.5% on average, respectively, of the total debris mass), while leeward beaches and the seafloor debris consisted of less weathered and more dense sinking polymers (e.g., 41.0 and 44.7% of total mass consisted of the sum of polystyrene, nylon, cellulose acetate, polyethylene terephthalate, and additive-masked debris). These results are some of the first to provide evidence of polymeric stratification in the marine environment and emphasize that the majority of marine debris in Hawaii is floating in from distant sources rather than from Hawaii's residents or tourists.

The developing premature infant gut microbiome is a major factor shaping the microbiome of neonatal intensive care unit rooms

BackgroundThe neonatal intensive care unit (NICU) contains a unique cohort of patients with underdeveloped immune systems and nascent microbiome communities. Patients often spend several months in the same room, and it has been previously shown that the gut microbiomes of these infants often resemble the microbes found in the NICU. Little is known, however, about the identity, persistence, and absolute abundance of NICU room-associated bacteria over long stretches of time. Here, we couple droplet digital PCR (ddPCR), 16S rRNA gene surveys, and recently published metagenomics data from infant gut samples to infer the extent to which the NICU microbiome is shaped by its room occupants.ResultsOver 2832 swabs, wipes, and air samples were collected from 16 private-style NICU rooms housing very low birth weight (< 1500 g), premature (< 31 weeks’ gestation) infants. For each infant, room samples were collected daily, Monday through Friday, for 1 month. The first samples from the first infant and the last samples from the last infant were collected 383 days apart. Twenty-two NICU locations spanning room surfaces, hands, electronics, sink basins, and air were collected. Results point to an incredibly simple room community where 5–10 taxa, mostly skin-associated, account for over 50% of the amplicon reads. Biomass estimates reveal four to five orders of magnitude difference between the least to the most dense microbial communities, air, and sink basins, respectively. Biomass trends from bioaerosol samples and petri dish dust collectors suggest occupancy to be a main driver of suspended biological particles within the NICU. Using a machine learning algorithm to classify the origin of room samples, we show that each room has a unique microbial fingerprint. Several important taxa driving this model were dominant gut colonizers of infants housed within each room.ConclusionsDespite regular cleaning of hospital surfaces, bacterial biomass was detectable at varying densities. A room-specific microbiome signature was detected, suggesting microbes seeding NICU surfaces are sourced from reservoirs within the room and that these reservoirs contain actively dividing cells. Collectively, the data suggests that hospitalized infants, in combination with their caregivers, shape the microbiome of NICU rooms.

Crustal structure and gravity anomalies beneath the Rif, northern Morocco: implications for the current tectonics of the Alboran region

We analyse Bouguer anomaly data and previously published Moho depths estimated from receiver functions in order to determine the amount of isostatic compensation or uncompensation of the Rif topography in northern Morocco. We use Moho depth variations extracted from receiver function analyses to predict synthetic Bouguer anomalies that are then compared to observed Bouguer anomaly. We find that Moho depth variations due to isostatic compensation of topographic and/or intracrustal loads do not match Moho depth estimates obtained from receiver function analyses. The isostatic misfit map evidences excess crustal root as large as 10 km in the western part of the study area, whereas a ‘missing’ crustal root of ∼5 km appears east of 4.3°E. This excess root/missing topography correlates with the presence of a dense mantle lid, the noticeable southwestward drift of the Western Rif area, and with a current surface uplift. We propose that a delaminated mantle lid progressively detaching westward or southwestward from the overlying crust is responsible for viscous flow of the ductile lower crust beneath the Rif area. This gives rise to isostatic uplift and westward drift due to viscous coupling at the upper/lower crust boundary. At the same time, the presence of this dense sinking mantle lid causes a negative dynamic topography, which explains why the observed topography is too low compared to the crustal thickness.

The Role of Turbulent Mixing in an Overturning Circulation Maintained by Surface Buoyancy Forcing

Abstract The role of externally imposed rates of small-scale mixing in an overturning circulation forced by differential surface buoyancy fluxes is examined in a laboratory experiment. The circulation occupies the full volume and involves a dense turbulent plume against the endwall and a broad upwelling throughout the interior. For strong externally imposed stirring, turbulent diffusion is the primary means of vertical density transport in the flow, and the dependence of the equilibrated circulation on the mixing rate accords with a theoretical model; the overturning rate increases as the ¼ power of the turbulent diffusivity. For weak externally imposed stirring, advection is the dominant mechanism of vertical density transport, and the circulation is independent of the rate of external stirring. The rate of vertical density transport is parameterized as a bulk diffusivity obtained from different methods, including one from a Munk-like advection–diffusion balance and another from the transport of buoyancy across the surface. For strong stirring, the bulk diffusivities returned by the various methods agree with the externally imposed mixing rate. However, the parameterizations implicitly include a nondiffusive component of vertical transport associated with advection of the density field and it is shown that, for weak stirring, the bulk diffusivities exceed the externally imposed mixing rate. For the oceans, results suggest that the primary effect of mixing (with energy sourced from winds, tides, and convection) is to deepen the thermocline, thereby influencing the entrainment and consequent vertical transport of density in the dense sinking regions. It is concluded that this advective transport of density, and not vertical mixing, is crucial for coupling the surface to the abyss.

Tethyan closure, Andean orogeny, and westward drift of the Pacific Basin

The deep-sea trenches that circumscribe the Pacific basin are moving westward relative to the deep mantle by a few cm/yr, together with the Pacific seafloor. While the westward drift of the Pacific basin dominates the observed net westward rotation of Earth's tectonic plates relative to the deep mantle source of the Pacific hotspots, its cause remains uncertain. We demonstrate that the source of this westward drift is tectonic in origin: the westward push of the high Andean topography along the eastern Pacific margin is large enough to shear the entire Pacific upper mantle with a surface velocity of ~ 30 mm/yr. In addition, because the Pacific upper mantle is confined between vertically coherent slabs of subducted lithosphere on both sides of the Pacific, slabs in the western Pacific are forced to advance toward upper plates because the eastern Pacific slabs are pushed westward. This cross-Pacific interaction provides an explanation for trench advance, which is observed in the western Pacific and is not easily explained by the local dynamics of subduction in which dense sinking slabs essentially tend to retreat from overriding plates. Thus, the basic asymmetry of the Pacific system, which has been noted for almost all tectonic features including slab dips, trench motions, and upper plate tectonics, is the result of one-sided Andean forcing on the volume of Pacific upper mantle. The net rotation of the lithosphere is then the result of surface tectonics: westward motion is an accident resulting from the current distribution of tectonic plates, and was triggered by the mid-Cenozoic closure of the Tethys Ocean, via a westward expansion of the Atlantic and Andean cordilleran building.

A simple convective model of the global overturning circulation, including effects of entrainment into sinking regions

We use a simple conceptual model to examine the roles of vertical mixing and surface buoyancy fluxes in the dynamics of the global overturning circulation that ventilates the deep oceans. In addition to using the Munk [Munk, W.H., 1966. Abyssal recipes. Deep-Sea Research 13, 707–730] advection–diffusion balance in the ocean interior, we close the circulation by including the small high-latitude sinking regions, which are assumed to be turbulent geostrophic gravity currents on gentle topographic slopes. The interior and sinking regions are coupled by turbulent entrainment into the sinking regions and we examine the global influence of this entrainment. An important realization is that the rates of mixing into slope currents, predicted to be very small as a function of along-flow distance, imply rates of entrainment per unit depth of fall that are comparable to the entrainment rate for vertical plumes. The overturning mass flux and ocean density stratification are found as a function of the vertical diffusivity and total heat transport. Given a realistic heat transport and the measured average mixing rate of order 10 −5 m 2/s, this simple model yields predictions consistent with data for the increase in volume flux with depth in slope currents, the magnitude of the global overturning circulation, and the averaged top-to-bottom density difference. Despite the absence of other mechanisms thought to be important in the thermocline, the model also gives a realistic thermal boundary layer thickness. As a consequence of entrainment into the dense sinking currents, the linking of abyssal densities to surface fluxes and the assumption of a uniform diffusivity, the model convective flow requires much less energy than the Munk (1966) prediction. The results indicate that the ocean overturning is feasibly a convective one and we suggest there might be no need to search for ‘missing’ mixing.

Stresses and plate boundary forces associated with subduction plate margins

Primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the Earth and associated topographical loading. When such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere, plate boundary forces develop and modify the plate interior stresses. In this paper, unite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries, with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked. In such regions, several types of horizontal deviatoric stress may occur, including (1) local compression in the trench‐arc region caused by the dense sinking slab and the associated surface downflexure; (2) plate interior tension which occurs when this compression is intersected by a weak subduction fault; (3) local tension associated with thickening of the crust at the arc and elsewhere; (4) local tension in the back arc region produced by the underlying low density upper mantle; and (5) downbending stresses in the subducting slab, thermal stresses, and transmitted ridge push, which are not included in the modelling here. A gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked. It is, however, the intersection of the local compression (1) by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces. The state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs. Plate boundary forces have been evaluated for each of the models. It is shown that slab pull and trench suction may be of comparable magnitude.

Asthenosphere flow and plate motions

Summary. A dynamical theory of flow in the asthenosphere is developed by treating it as a thin layer of Newtonian viscous fluid, pushed horizontally by the dense sinking slabs attached to some plates and by the differences in heating under continental and oceanic crust. Two-dimensional lubrication theory is adapted to give the world-wide effects of inflow to the asthenosphere at the slabs, outflow to the plates at mid-ocean ridges, and the horizontal forces applied at the slabs. The resulting torque balance for the plates is better than any previously published, and leads one to conclude that the asthenosphere's viscosity is least at the top.

On the Driving Forces of Plate Tectonics

Sumnznry We suppose that the plates are pulled along on top of an effectively viscous asthenosphere by their cold dense sinking leading edges, and that they also tend to slide down the flanks of ocean ridge systems. Using reasonable literature values of density, viscosity and thickness, we find that a typical strong subduction zone pulls about seven times as hard as a typical mid-ocean ridge pushes. With the simplifying assumptions that other driving forces are much smaller, and the return current in the asthenosphere is everywhere (anti)parallel to the plate velocity, we perform a torque balance for each major plate in order to find its angular velocity, thus finding a set of relative angular velocities to compare with observations. The directions fit quite well but not the magnitudes. On the additional hypothesis that oceanic lithosphere thickens with age, owing to heat diffusion, the driving forces will be greater for large old plates. The fit is thereby greatly improved: predicted speeds are from 0.81 to 1.38 times the observed and the COCOS plate is not anomalous. The model predicts tension in plates near their subduction zones. The Red Sea and African rift appear to have begun spreading as a crack moved into the former African +Arabian plate from a re-entrant corner off the Gulf of Aden which would have concentrated the stress. Gondwanaland may possibly have been split in a similar way.