Descent of dense water masses along continental slopes

The formation of dense water over the continental shelf and its descent along the continental slope have been investigated both theoretically and experimentally. Models have been developed for slope fronts and dense filaments, with emphasis on the role of the bottom boundary layer. An analytical, two-layer, two-dimensional model is first presented for the development of dense slope fronts near the shelf-break. The effects of vertical viscosity are explored and two behavioral regimes identified. The most relevant regime is determined by the parameter F = (UQ/gεs)(f3/v)1/2 where UQ is the flux of newly created dense water per unit length of coastline, g is the gravitational acceleration, ε is the density anomaly, s is the bottom slope, f is the Coriolis parameter and v is the vertical viscosity. In both cases, the alongslope velocity in the lower layer increases away from the coast during geostrophic adjustment, with an accompanying growth in the downslop Ekman flux. When F is small, dense water production near the coast can be balanced by transport within the boundary layer, which extends down the slope as a shallow intrusion with an alongslope speed of gεs/f. However, when F is large this type of flow cannot provide sufficient downslope transport. Dense water then accumulates, causing the front to steepen while diminishing the influence of the bottom slope. There is a corresponding increase in alongslope speed, which eventually plateaus at (2f/v)1/2/UQ where the Ekman flux balances the production of new dense water. These behaviors are strongly supported by results from laboratory experiments and are consistent with the limited available observations of the Antarctic Slope Front. After moving off the shelf, the dense water mass may continue to move down the slope within the bottom boundary layer, or alternatively, form an isolated filament with a front on both sides. Theoretical solutions are developed for dense filaments both with and without an active upper layer. In the latter case, the influence of dissipation is investigated beginning with a simple bulk parameterization. This produces a filament which broadens as it moves down the slope, while its mean alongslope velocity increases with bottom slope and its horizontal shear decreases. More realistic boundary layer dynamics have also been incorporated using a similar approach to that described for slope fronts. The solutions compare well with results from laboratory experiments on relatively stable filaments. Implications of the study for deep water formation around Antarctica are discussed briefly

Separation and recirculation of the North Brazil Current

The North Brazil Current separates from the French Guiana coast in the western tropical North Atlantic and a portion of the flow retroflects to form a recirculation zone known as the Demerara Anticyclone. The remainder of the North Brazil Current continues northwestward to join the Guiana Current. This part of the flow may be particularly significant, since it carries a cross-gyre transport of heat and mass from the tropical Atlantic into the subtropical gyre. An extension of the recirculation model of Cessi (1988), in which a quasi-geostrophic flow was driven by potential vorticity anomalies along a boundary, has been used to investigate dynamics relevant to the separation and recirculation of the North Brazil Current. Unlike that of Cessi, the present model is two dimensional and consists of two recirculating gyres driven only by the western boundary current potential vorticity distribution. The recirculation zones were much smaller than those predicted by one dimensional models and since most of the streamlines passed through a diffusive boundary layer there was no evidence of homogenization of the potential vorticity field. The influence of diffusivity, boundary forcing and boundary orientation were considered. The western boundary layer was similar to a Munk layer over most of the parameter range investigated. Since the boundary forcing diffused into the interior, the meridional and zonal dimensions of the inertial part of the recirculation both had a weak power law dependency on the boundary forcing and diffusivity. When the western boundary was not aligned with the planetary vorticity gradient, westward drift distorted the shape of the recirculation

Models of abyssal flow in basins separated by amid-ocean ridge

Numerical and laboratory models have been used to study source-driven flows in a system consisting of two basins (with sloping sidewalls) separated by a mid-ocean ridge. Numerical spin-up occurs via topographically modified Kelvin waves which propagate away from the source region around the outer perimeter of the model ocean. Energy is then carried along the ridge by topographic waves and westward by planetary waves. The resulting flow eventually concentrates in strong cyclonic circulation patterns, defined by regions of closed geostrophic contours in the lower latitude portion of each basin. When the deep water source is located at the latitude of closed geostrophic contours, there is no significant flow outside the closed contours. However, when it is located further toward polar regions, strong flow is evident up to the source latitude. There is a close correspondence with the laboratory model when similarity conditions are satisfied. One notable difference was a higher level of wave and eddy activity in the laboratory, particularly near the border between closed and blocked contour regions

Convectively driven coastal currents in a rotating basin

Density driven coastal currents were produced in the laboratory by differentially heating and cooling the end walls of a rotating rectangular cavity. After turning on the heat flux, intrusions propagated along the side walls of the cavity under an inertial buoyancy balance, with a geostrophic cross-stream balance. These boundary currents were internally stratified in temperature, while the environment during the early stages of development of the flow was isothermal. Rotational instabilities developed on the edge of the currents and broke to form cyclone-anticyclone eddy pairs. Measurements were made of the intrusion velocity, the temporal development of the width of the boundary currents, their internal thermal structure, and the characteristics of the unstable waves, including their growth rates, wavelengths, and phase speeds. Comparisons are made with previous field observations of the Leeuwin Current off Western and Southern Australia

Convection in a rotating cavity

Many large scale flows in the ocean are driven by an imposed longitudinal density gradient and the resulting buoyancy-driven flow is both influenced by the Earth's rotation and has a low aspect ratio (i.e. the characteristic vertical scale of the motion is small compared to the characteristic horizontal scale of the motion). The essential features of such flows were incorporated into a laboratory model, by differentially heating and cooling the vertical end walls of a low aspect ratio, rectangular cavity rotating about a vertical axis through its centre. When heating and cooling were initiated at the respective vertical end walls of the cavity, a hot current formed along the surface and a cold current along the bottom. These moved out from each end wall into the interior of the tank, but were confined to the sidewalls (model coastlines) by the effects of rotation. Initially the currents propagated under a balance between buoyancy and inertial forces, with an unstable balance between buoyancy and Coriolis forces in the cross-stream direction. Drag forces eventually slowed the the propagation speeds. The currents were internally stratified in temperature, and became unstable as a result of a rotationally dominated instability, driven by both the potential energy associated with the temperature difference between the currents and the isothermal environment and the velocity shear across the current. The flows were analogous to buoyancy driven coastal currents such as the East Greenland Current, the Norwegian Coastal Current and the Leeuwin Current off Western Australia. As an experiment progressed, the instabilities on the currents grew and broke to produce eddies which eventually filled the cavity. The timescales for development of the stratification within the cavity were found to be dependent on the end wall temperatures, but independent of rotation. In its statistically steady state the mean circulation consisted of baroclinic boundary currents superimposed on two basin-scale counter-rotating gyres and a nearly linear vertical temperature gradient. These observations can be explained in terms of potential vorticity dynamics in the presence of a relative slope between isopotential surfaces and horizontal boundaries. Measurements of the potential vorticity were made in the laboratory flow and the quantity proved to be a very effective dynamical tracer. The steady state flow may have interesting implications for the large scale circulation of the oceans

Palaeogeography and voyage modeling indicates early human colonization of Australia was likely from Timor-Roti

Anatomically Modern Humans (AMHs) dispersed rapidly through island southeast Asia (Sunda and Wallacea) and into Sahul (Australia, New Guinea and the Aru Islands), before 50,000 years ago. Multiple routes have been proposed for this dispersal and all involve at least one multi-day maritime voyage approaching 100 km. Here we use new regional-scale bathymetry data, palaeoenvironmental reconstruction, an assessment of vertical land movements and drift modeling to assess the potential for an initial entry into northwest Australia from southern Wallacea (Timor-Roti). From ∼70,000 until ∼10,000 years ago, a chain of habitable, resource-rich islands were emergent off the coast of northwest Australia (now mostly submerged). These were visible from high points close to the coast on Timor-Roti and as close as 87 km. Drift models suggest the probability of accidental arrival on these islands from Timor-Roti was low at any time. However, purposeful voyages in the summer monsoon season were very likely to be successful over 4–7 days. Genomic data suggests the colonizing population size was >72–100 individuals, thereby indicating deliberate colonization. This is arguably the most dramatic early demonstration of the advanced cognitive abilities and technological capabilities of AMHs, but one that could leave little material imprint in the archaeological record beyond the evidence that colonization occurred

Early human settlement of Sahul was not an accident

The first peopling of Sahul (Australia, New Guinea and the Aru Islands joined at lower sea levels) by anatomically modern humans required multiple maritime crossings through Wallacea, with at least one approaching 100 km. Whether these crossings were accidental or intentional is unknown. Using coastal-viewshed analysis and ocean drift modelling combined with population projections, we show that the probability of randomly reaching Sahul by any route is <5% until ≥40 adults are ‘washed off’ an island at least once every 20 years. We then demonstrate that choosing a time of departure and making minimal headway (0.5 knots) toward a destination greatly increases the likelihood of arrival. While drift modelling demonstrates the existence of ‘bottleneck’ crossings on all routes, arrival via New Guinea is more likely than via northwestern Australia. We conclude that anatomically modern humans had the capacity to plan and make open-sea voyages lasting several days by at least 50,000 years ago

Genome-Wide Profiling of Histone H3 Lysine 4 and Lysine 27 Trimethylation Reveals an Epigenetic Signature in Prostate Carcinogenesis

BACKGROUND: Increasing evidence implicates the critical roles of epigenetic regulation in cancer. Very recent reports indicate that global gene silencing in cancer is associated with specific epigenetic modifications. However, the relationship between epigenetic switches and more dynamic patterns of gene activation and repression has remained largely unknown. METHODOLOGY/PRINCIPAL FINDINGS: Genome-wide profiling of the trimethylation of histone H3 lysine 4 (H3K4me3) and lysine 27 (H3K27me3) was performed using chromatin immunoprecipitation coupled with whole genome promoter microarray (ChIP-chip) techniques. Comparison of the ChIP-chip data and microarray gene expression data revealed that loss and/or gain of H3K4me3 and/or H3K27me3 were strongly associated with differential gene expression, including microRNA expression, between prostate cancer and primary cells. The most common switches were gain or loss of H3K27me3 coupled with low effect on gene expression. The least prevalent switches were between H3K4me3 and H3K27me3 coupled with much higher fractions of activated and silenced genes. Promoter patterns of H3K4me3 and H3K27me3 corresponded strongly with coordinated expression changes of regulatory gene modules, such as HOX and microRNA genes, and structural gene modules, such as desmosome and gap junction genes. A number of epigenetically switched oncogenes and tumor suppressor genes were found overexpressed and underexpressed accordingly in prostate cancer cells. CONCLUSIONS/SIGNIFICANCE: This work offers a dynamic picture of epigenetic switches in carcinogenesis and contributes to an overall understanding of coordinated regulation of gene expression in cancer. Our data indicate an H3K4me3/H3K27me3 epigenetic signature of prostate carcinogenesis

Connectivity and systemic resilience of the Great Barrier Reef

Australia’s iconic Great Barrier Reef (GBR) continues to suffer from repeated impacts of cyclones, coral bleaching, and outbreaks of the coral-eating crown-of-thorns starfish (COTS), losing much of its coral cover in the process. This raises the question of the ecosystem’s systemic resilience and its ability to rebound after large-scale population loss. Here, we reveal that around 100 reefs of the GBR, or around 3%, have the ideal properties to facilitate recovery of disturbed areas, thereby imparting a level of systemic resilience and aiding its continued recovery. These reefs (1) are highly connected by ocean currents to the wider reef network, (2) have a relatively low risk of exposure to disturbances so that they are likely to provide replenishment when other reefs are depleted, and (3) have an ability to promote recovery of desirable species but are unlikely to either experience or spread COTS outbreaks. The great replenishment potential of these ‘robust source reefs’, which may supply 47% of the ecosystem in a single dispersal event, emerges from the interaction between oceanographic conditions and geographic location, a process that is likely to be repeated in other reef systems. Such natural resilience of reef systems will become increasingly important as the frequency of disturbances accelerates under climate change

Trace elements at the intersection of marine biological and geochemical evolution

Life requires a wide variety of bioessential trace elements to act as structural components and reactive centers in metalloenzymes. These requirements differ between organisms and have evolved over geological time, likely guided in some part by environmental conditions. Until recently, most of what was understood regarding trace element concentrations in the Precambrian oceans was inferred by extrapolation, geochemical modeling, and/or genomic studies. However, in the past decade, the increasing availability of trace element and isotopic data for sedimentary rocks of all ages has yielded new, and potentially more direct, insights into secular changes in seawater composition – and ultimately the evolution of the marine biosphere. Compiled records of many bioessential trace elements (including Ni, Mo, P, Zn, Co, Cr, Se, and I) provide new insight into how trace element abundance in Earth's ancient oceans may have been linked to biological evolution. Several of these trace elements display redox-sensitive behavior, while others are redox-sensitive but not bioessential (e.g., Cr, U). Their temporal trends in sedimentary archives provide useful constraints on changes in atmosphere-ocean redox conditions that are linked to biological evolution, for example, the activity of oxygen-producing, photosynthetic cyanobacteria. In this review, we summarize available Precambrian trace element proxy data, and discuss how temporal trends in the seawater concentrations of specific trace elements may be linked to the evolution of both simple and complex life. We also examine several biologically relevant and/or redox-sensitive trace elements that have yet to be fully examined in the sedimentary rock record (e.g., Cu, Cd, W) and suggest several directions for future studies