As part of Ocean Drilling Program Leg 204 at southern Hydrate Ridge off Oregon we have monitored changes in sediment electrical resistivity during controlled gas hydrate dissociation experiments. Two ...cores were used, each filled with gas hydrate bearing sediments (predominantly mud/silty mud). One core was from Site 1249 (1249F-9H3), 42.1 m below seafloor (mbsf) and the other from Site 1248 (1248C-4X1), 28.8 mbsf. At Site 1247, a third experiment was conducted on a core without gas hydrate (1247B-2H1, 3.6 mbsf). First, the cores were imaged using an infra-red (IR) camera upon recovery to map the gas hydrate occurrence through dissociation cooling. Over a period of several hours, successive runs on the multi-sensor track (includes sensors for P-wave velocity, resistivity, magnetic susceptibility and gamma-ray density) were carried out complemented by X-ray imaging on core 1249F-9H3. After complete equilibration to room temperature (17–18 °C) and complete gas hydrate dissociation, the final measurement of electrical resistivity was used to calculate pore-water resistivity and salinities. The calculated pore-water freshening after dissociation is equivalent to a gas hydrate concentration in situ of 35–70% along core 1249F-9H3 and 20–35% for core 1248C-4X1 assuming seawater salinity of in situ pore fluid. Detailed analysis of the IR scan, X-ray images and split-core photographs showed the hydrate mainly occurred disseminated throughout the core. Additionally, in core 1249F-9H3, a single hydrate filled vein, approximately 10 cm long and dipping at about 65°, was identified. Analyses of the logging-while-drilling (LWD) resistivity data revealed a structural dip of 40–80° in the interval between 40 and 44 mbsf.
We further analyzed all resistivity data measured on the recovered core during Leg 204. Generally poor data quality due to gas cracks allowed analyses to be carried out only at selected intervals at Sites 1244, 1245, 1246, 1247, 1248, 1249, and 1252. With a few exceptions, data from these intervals yield low to no gas hydrate concentration, which corresponds to estimates from downhole resistivity logs. However, since the gas cracking may be the result of gas hydrate dissociation, this is a biased sampling. Cores that had contained some gas hydrate may have been excluded.
Bees seem to use landmarks to segment familiar routes. They can associate, with a landmark, a memory that encodes the direction and distance of the path segment between that landmark and the next. ...The expression of the memory results in the performance of a local vector matching the distance and direction of the path segment. The memories of path segments appear to be 'chained' together, so that the performance of one local vector is sometimes sufficient to elicit the subsequent local vector, even in the absence of the associated landmark. We have investigated the effect of visual panoramic context on the expression of local vectors. Bees were trained to fly along a narrow channel to collect sucrose from a feeder positioned partway along it. Panoramic context was provided by various types of patterning on the walls. The channel was partitioned into different segments using landmarks of two kinds: a boundary landmark that marked a change in the pattern on one or both side-walls of the channel, and an isolated landmark, consisting of a baffle through which the bee passed, for which the wall pattern was the same before as after. In tests, we removed the feeder and analysed the search distribution of the bees for various arrangements of landmarks. Altering the spatial relationship between landmarks has different consequences for the two types of landmark. If the final boundary landmark is shifted, the centre of the search distribution shifts by approximately the same amount. Changes in the position of an isolated landmark have a weaker effect. In the absence of the final context, the search is disrupted. We suggest that for local vectors to be expressed the surrounding panoramic context needs to be appropriate. A comparison of search patterns from two different training configurations of landmarks supports the hypothesis that local vector memories merely encode route segments and that global positional coordinates are not linked to landmark memories.
We have studied the changing use of spatial memories in wood ants by charting how the ants' paths transform when ants are first trained to feed at one site and must then switch to another site. ...Because ants, which are trained to approach a single feeding site from a single starting point, are attracted directly to that goal when started from unfamiliar positions, we describe the ants' paths in terms of the use of two stored snapshots. Each snapshot consists of retinotopic views of the ants' surroundings acquired at one of the two feeding sites. When a snapshot is activated, it draws an ant to the related site from a wide range of directions. Here, we focus on routes that occur before ants have learnt to go directly from the start to the second site. The initial direction of the ant's path is then mostly aimed either at the first site or between the two sites. On 62.2% of all recorded paths, this segment is followed by an abrupt turn, after which the ant often aims directly at the second feeding site. The details of this behaviour suggest that, after the turn, control of the path switches from the snapshot recorded at the first feeding site (or some combination of the two snapshots) to the snapshot recorded at the second feeding site. We discuss different ways in which control might be transferred from one snapshot to the other.
Cataglyphid ants travelling between their nest and feeding site follow familiar routes along which they are guided by views of the surrounding landscape,. On bare terrain, with no landmarks ...available, ants can still navigate using path integration. They continually monitor their net distance and direction from the nest, so that they can return home from any point using their computed 'home vector'. Here we ask whether path integration also provides signals to reinforce the learning of visual landmarks. A fall in the value of the home vector indicates when a homing ant moves in roughly the correct direction, and that it is appropriate to store those views that can guide subsequent trips to the nest. We tested this hypothesis by training the ant Cataglyphis cursor to negotiate a variety of mazes that led from a feeding site back to the nest. Efficient passage of each maze required an ant to discriminate between different pairs of shapes. We show that if the value of the home vector drops while the ant approaches and passes a shape, the shape's appearance is learnt, but if the vector grows, or is absent, no visual learning occurs. Path integration may both help ants navigate through an unfamiliar landscape, and assist them to become familiar with it.
Desert ants (Cataglyphis sp.) monitor their position relative to the nest using a form of dead reckoning 1–3 known as path integration (PI) 4. They do this with a sun compass and an odometer to ...update an accumulator that records their current position 1. Ants can use PI to return to the nest 2,3. Here, we report that desert ants, like honeybees 5 and hamsters 6, can also use PI to approach a previously visited food source. To navigate to a goal using only PI information, a forager must recall a previous state of the accumulator specifying the goal, and compare it with the accumulator's current state 4. The comparison – essentially vector subtraction – gives the direction to the goal. This whole process, which we call vector navigation, was found to be calibrated at recognised sites, such as the nest and a familiar feeder, throughout the life of a forager. If a forager was trained around a one-way circuit in which the result of PI on the return route did not match the result on the outward route, calibration caused the ant's trajectories to be misdirected. We propose a model of vector navigation to suggest how calibration could produce such trajectories.
Landmark Learning and Guidance in Insects Collett, T. S.
Philosophical transactions of the Royal Society of London. Series B. Biological sciences,
09/1992, Volume:
337, Issue:
1281
Journal Article
Peer reviewed
Insects use terrestrial landmarks both for retrieving important places in their environment, like a nest, and for guiding
their way along frequently travelled routes. Places are pinpointed by a form ...of image matching: the insect moves to maximize
the fit between the image on its retina and its memory of surrounding landmarks as viewed from close to the goal. In this
case, the insect's stored representation seems to be a filtered but relatively unprocessed replica of the image falling on
the retina, which is parsed for features like the position and orientation of edges, their speed of motion and their colour.
Routes need not be defined so precisely and landmarks are then employed in less demanding ways.
Bees, wasps and ants learn landmarks as views from particular vantage points, storing the retinal positions of landmark edges. By moving so as to minimise the difference between their stored and ...current view, they can return to the vantage point from which a view was taken. We have examined what wood ants learn about a laterally placed, extended landmark, a wall, while walking parallel to it to reach a feeder and how they use this stored information to guide their path. Manipulation of the height of the wall and the ant's starting distance from it reveals that ants maintain a desired distance from the wall by keeping the image of the top of the wall at a particular retinal elevation. Ants can thus employ image matching both for returning to a place and for following a fixed route. Unlike many flying insects, an ant's direction of motion while walking is always along its longitudinal body axis and, perhaps for this reason, it favours its frontal retina for viewing discrete landmarks. We find that ants also use their frontal retina for viewing a laterally placed wall. On a coarse scale, the ant's path along the wall is straight, but on a finer scale it is roughly sinusoidal, allowing the ant to scan the surrounding landscape with its frontal retina. The ant's side-to-side scanning means that the wall is viewed with its frontal retina for phases of the scanning cycle throughout its trajectory. Details of the scanning pattern depend on the scene. Ants scan further to the side that is empty of the wall than to the side containing the wall, and they scan further into the wall side when the wall is of a lower apparent height. We conclude that frontal retina is employed for image storage and for path control.
TDCOSMO Birrer, S.; Shajib, A. J.; Galan, A. ...
Astronomy and astrophysics (Berlin),
11/2020, Volume:
643
Journal Article, Web Resource
Peer reviewed
Open access
The H0LiCOW collaboration inferred via strong gravitational lensing time delays a Hubble constant value of
H
0
= 73.3
−1.8
+1.7
km s
−1
Mpc
−1
, describing deflector mass density profiles by either a ...power-law or stars (constant mass-to-light ratio) plus standard dark matter halos. The mass-sheet transform (MST) that leaves the lensing observables unchanged is considered the dominant source of residual uncertainty in
H
0
. We quantify any potential effect of the MST with a flexible family of mass models, which directly encodes it, and they are hence maximally degenerate with
H
0
. Our calculation is based on a new hierarchical Bayesian approach in which the MST is only constrained by stellar kinematics. The approach is validated on mock lenses, which are generated from hydrodynamic simulations. We first applied the inference to the TDCOSMO sample of seven lenses, six of which are from H0LiCOW, and measured
H
0
= 74.5
−6.1
+5.6
km s
−1
Mpc
−1
. Secondly, in order to further constrain the deflector mass density profiles, we added imaging and spectroscopy for a set of 33 strong gravitational lenses from the Sloan Lens ACS (SLACS) sample. For nine of the 33 SLAC lenses, we used resolved kinematics to constrain the stellar anisotropy. From the joint hierarchical analysis of the TDCOSMO+SLACS sample, we measured
H
0
= 67.4
−3.2
+4.1
km s
−1
Mpc
−1
. This measurement assumes that the TDCOSMO and SLACS galaxies are drawn from the same parent population. The blind H0LiCOW, TDCOSMO-only and TDCOSMO+SLACS analyses are in mutual statistical agreement. The TDCOSMO+SLACS analysis prefers marginally shallower mass profiles than H0LiCOW or TDCOSMO-only. Without relying on the form of the mass density profile used by H0LiCOW, we achieve a ∼5% measurement of
H
0
. While our new hierarchical analysis does not statistically invalidate the mass profile assumptions by H0LiCOW – and thus the
H
0
measurement relying on them – it demonstrates the importance of understanding the mass density profile of elliptical galaxies. The uncertainties on
H
0
derived in this paper can be reduced by physical or observational priors on the form of the mass profile, or by additional data.
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