南澳地理作业代写 接地带楔(GZW)

2020-04-09 00:09

通过ArcGIS识别和分析的最突出的特征是接地带楔(GZW),人们普遍认为它是在21000年前的最后一次冰期后冰原消退时形成的。当冰原消退时,次冰期的冰碛物可以堆积成2个不同的GZW,分别位于A区和B区,如图4.1所示。当你离开南极海岸时,这两个GZW是向上的,推断这些特征是通过冰的流动形成的。这一点在图4.2中得到了证明,图中GZW 5被认为比GZW4更深(振幅更大)。我们可以推断GZW5比GZW4更古老,这是由于人们普遍认为冰的后退是向南极海岸线的后退,因此由于GZW5的振幅更大,它有更多的时间进行沉积物的积累。这些GZW是由几个冰下机制形成的,当冰静止时,这些机制允许沉积物堆积,从而形成了这些独特的特征。Jakobsson等人(2012)利用沉积物通量估计,一个楔形物的形成需要超过1000年的时间,这表明冰在相当长的一段时间内都是静止的。当冰静止一段时间(也称为冰停滞),冰下的冰碛会帮助冰的流动。当某些冰下过程发生时,冰的冻结就会发生。首先,地形梯度的变化会导致冰停止后退(Graham等,2010)。这可能是海底梯度的增加,导致冰需要更多的能量来上升梯度,导致冰流停止,直到有足够的能量来克服这一点。造成冰速显著下降的另一个因素是水压力的下降,导致冰流和沉积物的重合,导致冰流停滞(Bennet, 2003)。形成的楔形体是由于在地面线缺乏可容纳的空间,使沉积物在冰后退时移出。冰川的退缩也会导致侵蚀作用,比如在河床上的拔毛和磨蚀,从而增加了在GZW中沉积的沉积物的数量。
南澳地理作业代写 接地带楔(GZW)
The most prominent feature identified and analysed through ArcGIS, was the Grounding Zone Wedges (GZW) which are widely assumed to have been formed as the ice sheet retreated after the Last Glacial Maximum 21,000 years ago. When the ice sheet retreated, sub glacial till was allowed to accumulate into 2 distinctive GZW located in Area A and Area B as shown in Figure 4.1. The two GZW grade upwards as you go away from the Antarctic coast, inferring that these features were formed through ice flow as the ice retreated. This is proven in Fig 4.2 where the GZW 5 is seen as deeper (has a greater amplitude) than GZW4. We can infer that GZW5 is older than GZW4 due to the ice retreat accepted as retreating towards the Antarctic coastline, and therefore as the GZW5 has a greater amplitude, it has had more time for sediment to accumulate. These GZW have formed due to several subglacial mechanisms that allowed sediment to build up when the ice became stationary creating these distinctive features. Jakobsson et al. (2012) used sediment fluxes to estimate that it takes over 1000 years for a wedge to form, indicating that the ice must have been stationary for a significant period of time. Subglacial till underlying the ice that assists ice flow becomes pushed out when the ice is stationary for a while (also known as ice stagnation). Ice stagnation can occur when certain subglacial processes taking place. The first of these is where changes in topographical gradient can cause the ice to stop retreating (Graham et al., 2010). This could be an increase in the gradient of the sea bed, resulting in the ice needing more energy to go up the gradient, causing the ice flow to stop until enough energy is provided to overcome this.  Another of these factors that causes a significant decrease in ice velocity is a decrease in water pressure causing the ice stream and sediment coincide causing stagnation of the ice stream (Bennet, 2003). The wedge geometry that is formed is due to the lack of accommodation space at the ground line for sediment to move out as the ice retreats. The retreat of the glacier also leads to erosion through mechanisms such as plucking and abrasion at the bed leading to an increase in sediment being available to be deposited in a GZW.
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