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Later he also worked as a writer and producer for his own JustKiddingFilms. As grounded ice retreated during the subsequent deglaciation, the paleo-ice-stream troughs were partly backfilled with trough-confined GZWs. These wedges are subaqueous sediment accumulations formed at the mouth of ice streams when the grounding line is stationary or slightly advancing. In dip view, these deposits are asymmetrical low-relief features with a gentle landward dipping topset and a more steeply basinward dipping foreset. These GZWs are important because they provide unequivocal evidence as to the former locations at which the ice sheet paused during its overall retreat.
In addition, GZWs may help protect grounded ice from retreat forced by sea level rise 6. Sediment from shallow-penetration piston and kasten cores recovered from the Ross Sea continental shelf grade upwards from tills deposited by or in proximity to grounded ice to an overlying condensed section a few tens of centimeters thickness of post-glacial sediment deposited after the retreat of grounded ice 5.
Precisely dating ice sheet advance and retreat is challenging because the proximal glacial settings contain very little carbonate and total organic content is low 4 , 5. The existing strategy is to date the resumption of marine sedimentation that follows the retreat of grounded ice 4 , 5.
Despite the consensus concerning this general chronology 3 — 5 , it has not yet been possible to constrain the duration of individual groundings required by stratigraphic superposition of backstepped GZWs because the current number of core sites with good-quality radiocarbon dates is too coarsely spaced along the axes of the paleo-trough systems 3 — 5. Hence, the interval of time when ice sheet paused at any one location during the ongoing post-LGM retreat remains unknown.
Here, we used a new approach to estimate the Whales Deep outer-shelf grounding duration Fig. The new strategy improves upon a previous approach 12 , which was used to estimate the duration of a grounding event on the middle shelf of the Glomar Challenger Basin paleo-ice-stream trough Fig. Our study of the Whales Deep outer-shelf grounding duration involved three distinct objectives.
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The first objective concerned mapping the CGZW extent by seismic correlation. We used high-resolution single- and multi-channel seismic profiles from six surveys that include strike and dip profiles Fig. The interpreted seismic profiles were used to create an isopach contour map showing the CGZW thickness in milliseconds of two-way travel time.
The second objective involved generating an empirical estimate of paleo sediment yield S p for the Whales Deep Basin paleo-ice-stream trough. The estimate of paleo sediment yield was applied to the larger drainage basin area that existed when the WAIS was grounded on the outer continental shelf Fig. Data summarized by Elverhoi et al.
Hence, our upper bound estimate for paleo sediment yield was applied to the larger drainage basin area that existed when the WAIS was grounded on the outer continental shelf Fig. Core locations and corrected radiocarbon dates from Mosola and Anderson 4 are from acid insoluble organic matter within sediments deposited since the retreat of grounded ice. The gray-shaded area shows dimensions of the composite GZW corresponding to the location of the bathymetric saddle at the outer shelf. The GZWs are labeled 1—7 from oldest to youngest but it is not possible to map the individual lobes because the top and base reflectors are not continuous across the study area.
In the third part of our study, we used the paleo sediment flux Q Paleo to estimate the duration of the outer-shelf grounding events using Equation Stratigraphic superposition via seismic correlations show that the Whales Deep CGZW overlies a regional unconformity that can be projected as a seafloor reflection at the shelf edge into the subsurface across the outer- and the middle-shelf portions of the Whales Deep trough Fig.
The individual wedges are labeled GZW 1 through 7 from oldest to youngest. Seismic correlations show that there are no seismically resolvable GZWs in the area south of bathymetric saddle and north of the modern Ross Ice Shelf calving front Fig. The isopach map Fig. Volumetric analysis of the isopach map reveals that the CGZW contains a sediment volume of 5. Isopach map of the composite GZW in meters. The light-shaded rectilinear grid shows the location of seismic profiles used to map the distribution of the unit. A Low and high modern Antarctic sediment yield estimates from Alley et al.
Detailed studies of other glaciated and formerly glaciated margins show that erosion rates i. The nature of the substrate is also important The highest erosion rates are associated with fast flowing Alaskan valley glaciers flowing over fractured bedrock and the lowest erosion rates are associated with slow-flowing dry polar glaciers Several studies have strongly suggested that paleo sediment yield is also dependent upon whether grounded ice is in the advance-mode or retreat-mode of a large-scale grounding-line-translation cycle 16 — Lower yield during gradual glacial advance is attributed to the slow flow and comparatively little meltwater whereas higher sediment yield during deglaciation is associated with faster ice motion.
Unfortunately, there are no data-based estimates of early-deglacial paleo sediment yield and flux for WAIS ice streams catchments. For this reason, we turn to the modern ice streams. Whereas more recent observations indicate that these older estimates of till flux and subglacial erosion may err on the high side 21 , we still use these potentially too high erosion rates because our primary goal is to produce a lower bound duration of CGZW formation. Ice flow during the early deglacial may have been faster and more continuous as the WAIS experienced a prolonged negative mass balance during this timeframe.
Modeling studies also use the assumption that paleo ice-stream sediment flux and erosion rates were higher during the early interglacial To account for the possibility that sediment yield during the early deglacial was higher than the average modern yield, we utilized an upper bound paleo sediment yield estimate of 0.
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Extrapolation of this upper-bound paleo sediment yield i. The lower reach of the drainage basin is underlain by unconsolidated sedimentary strata of upper Cenozoic age The upper-most reaches are deeply excavated and exhibit high relief Fig. This large interior region and the trough axis was undoubtedly the source material for the outer-shelf GZWs. Reflections that bound the GZWs are not regional in extent, and hence, the volumes of the individual GZWs cannot be estimated.
The stratal stacking of the CGZW permits that the wedges may have formed during discrete cycles of WAIS advance and retreat that so happened to reach a similar grounding line position during successive glacial periods. Stratigraphic superposition requires that GZW5b is younger than GZW5a, the wedge of sediment that underlies the outer shelf and upper slope Fig. Cores collected from the outer shelf by Mosola and Anderson 4 penetrated subglacial till from GZW5a at core station 13 Fig.
At this station, four radiocarbon dates were obtained from the post-glacial sediment that overly the subglacial tills. These and the other radiocarbon dates from the basin Fig. The ages are considered suspect because bulk AIOM contains some unknown fraction of older carbon reworked from strata exposed in the Whales Deep paleo drainage basin mixed with the in situ organic carbon that was contemporaneously deposited with the deglacial marine sediment. Hence, the mixing of recycled and contemporaneously produced carbon yields ages that are older than actual. The age offset depends upon the relative proportions of recycled and in situ carbon and unfortunately, the mix ratios are likely to have been variable thru time Within the context of these limitations, the actual dates at which marine sedimentation resumed had to have occurred after the time indicated by the oldest bulk AIOM radiocarbon date from deglacial sediment.
This stratal arrangement precludes the possibility that the outer shelf CGZW represents the deposits associated with glacial cycles that predate the LGM. More recent studies suggest that that fast flow is confined to a narrow zone. Decreasing porosity with depth in the till layer suggested to Kamb 2 that only the upper few centimeters of the subglacial till layer may experience fast flow. Thus, borehole observations of till velocity for the Bindschadler ice core, pertain only to the upper few centimeters of till flux into which a tethered stake was set 2.
In addition to sediment transport in a deforming basal till layer, recent studies indicate that a significant volume of sediment could also be transported to the grounding line in basal ice layers Melting at the grounding line demonstrates that a significant volume of sediment transported in basal ice layers could be deposited at the grounding line Hence, the paleo sediment flux across the grounding line depends on the combined contribution of deformation till and sediment concentrated in basal ice layers.
We assume that the relatively high erosion rate of 0. A paleo sediment flux of 1. If sediment deposited in the CGZW were exclusively derived from a deforming basal till layer, than only fast-flowing thin sediment layers would be consistent with borehole observational data 2. C—D Possible pairs of effective sediment layer thickness from both deforming basal till and sediment-rich basal ice layer and sediment-layer velocity, respectively.
Our upper bound estimate for the paleo sediment yield 0. Our use of a 0. Our paleo flux estimate for the paleo Bindschadler Ice Stream 1. At that time, grounded ice advanced to the North Sea shelf edge 38 , the Norwegian Channel drained an area from 6.
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Hence, the greater sediment flux values for the Norwegian Channel ice stream was due to a combination of a larger drainage area and a higher average yield i. Even higher yields of 2. Those high yields were attributed to the onset of large amplitude oscillations of grounding line across previously unglaciated landscapes mantled by unconsolidated sediment cover.
Relatively low long-term rates of sub-ice stream erosion in the Ross Sea sector of WAIS are also supported by the fact that these ice streams are associated with troughs that have relatively low relief. In contrast, the very high erosion rates beneath temperate mountain glaciers produce valleys with topographic relief that is one to two orders of magnitude greater than observed in our study region. Our calculation provides a reasonable estimate of grounding duration. However, it is important to note that previous studies suggest that the vertical thickness and horizontal width of the deformation till layer are likely to vary over time and from ice stream to ice stream 2 , Moreover, our paleo sediment flux estimate assumed continuous ice streaming but fast flow may also have intermittently been dominated by basal sliding Intervals of ice-stream deceleration, stagnation or shift to streaming by basal sliding would all cause sediment flux to diminish or cease.
In this sense, our sediment flux might be considered a maximum and hence the estimated duration a minimum. However, even this minimum is a relatively long period grounding, as ca. However, neither the duration nor the available age control is sufficiently resolved to establish a correlation to post-LGM sea level changes or other possible mechanisms Bathymetric controls, such as constrictions in troughs and bathymetric highs, have been proposed a mechanism for stabilizing grounding zones 43 — In the case of Whales Deep, there are no nearby shallow pinning points or a significant bottleneck in the trough at the level of the unconformity underlying the CGZW created by the orientation of the Hayes and Houtz banks.
Hence, the CGZW may be an example of a situation where such depositional processes created and maintained a pinning point, which stabilized the regional grounding line position for thousands of years.
A year outer shelf grounding is significant because it provides perspective as to the possible durations of past and modern stationary grounding line positions and the presumably stable ice sheet mass balance. In comparison, GZWs elsewhere on the Antarctic margins with smaller volumes have been inferred to represent significantly shorter duration grounding events ranging from a few decades to slightly longer than a millennium 31 , 43 , The long grounding is also significant with respect to how such a long interval with a stationary grounding line was followed by geologically instantaneous long distance retreat of grounded ice from the outer shelf to Roosevelt Island.
The rapid retreat phase may represent compensation for the fact that GZW growth phase makes an ice stream grounding line stable despite climate and sea level forcings. Once those factors make grounding at the GZW untenable, then the resulting retreat to a new stable position is large and rapid.
Although the exact radiometric timing and duration is yet to be determined, the data clearly highlight an important paradoxical duality to the WAIS dynamics with long intervals of relatively stationary grounding lines followed by rapid return of grounded ice to the global ocean. Our estimate of sediment flux for the paleo Bindschadler Ice Stream predicts that the CGZW would have taken on the order of years to deposit. The absence of any seismically resolvable GZWs south of the bathymetric saddle indicates that a large distance retreat of the grounding line from the outer shelf to the latitude of Roosevelt Island occurred in a geologic instant.
The paradox of a long grounding during West Antarctic Ice Sheet retreat in Ross Sea
The long-duration grounding followed by an abrupt large-distance grounding line retreat may have resulted from a highly non-linear ice sheet response to relatively continuous external forcing such as gradual climate warming or sea level rise. Our findings indicate that reliable predictions of future WAIS retreat may require incorporation of realistic calculations of sediment erosion, transport and deposition. We thank the captain and crew of the Nathaniel B.
We also thank the members of NBPA shipboard party for assisting our efforts in the field.