The North American Late Wisconsin ice sheet and mantle viscosity from glacial rebound analyses
The North American Late Wisconsin ice sheet and mantle viscosity from glacial rebound analyses
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DOI：
10.1016/j.quascirev.2016.11.033
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发表时间：
2017-02-15
影响因子：
4
通讯作者：
Zhao, S.
中科院分区：
文献类型：
--
作者：
Lambeck, Kurt;Purcell, Anthony;Zhao, S.
观测海平面和地壳对冰川加载周期的响应提供了对地幔流变学函数E和冰负载I的限制，后者在很大程度上不受先验的冰川或气候假设，因此适合于检验任何此类假设。本文利用相对海平面变化(RSL)和古湖岸线倾斜的地质证据，并辅以北美现今地壳径向位移观测的松散约束，对威斯康星晚期冰盖的大陆-地幔E和I给出了新的结果。重点是以前的最大冰缘附近或内部的证据，由此产生的地球响应代表了次大陆地幔条件。海平面信息的反演对于地球流变学的分辨率有限，简单的三层模型，以深度平均的有效岩石圈厚度(H)和上、下地幔粘性(分别为Eta(Num)和Eta(Um))为特征，充分描述了响应函数，得到的参数(地球模型E-6)为H=102(85-120)km，Eta(Um)=5.1×10(20)(3.5-7.5)x10(20)，ETA(Lm)=1.3 x 10(22)(0.8-2.8)x10(22)，其中括号中的数字是95%的置信限。除了一个例外，冰盖的细节并不强烈地依赖于这个范围内的流变学假设。例外的是较低的地幔粘度，它仍然与冰盖的大小比例相关：这种联系在很大程度上被冰川负载对地球自转和动态扁平的影响所打破。大陆ETA(Num)与海洋地幔(1-2.5)×10(20)的可比估计值之间的差异在统计上是显著的。来自麦康奈尔、阿加西、阿尔冈昆和奥吉布韦冰川湖泊的海岸线坡度信息对冰盖内部的响应提供了强烈的约束，由此产生的冰盖模型(LW-6)的特征是至少从1718Ica开始有多个冰穹。早期的分辨率在很大程度上受到全球冰量考虑的限制。两个主穹顶位于努纳武特南部(Keewatin穹顶)和魁北克拉布拉多上空，厚度均为3500米，由比穹顶低约1500米的冰脊隔开，后者是梯度信息要求的。在加拿大西部，科迪勒拉斯山脉以东的冰层厚度梯度部分受到前麦康奈尔湖海岸线数据的限制(基瓦丁穹顶的冰层厚度也是如此)，并表明劳伦斯和科迪勒拉组件之间的任何无冰走廊在类似13ka之前不太可能存在。冰川湖泊的重建与四个主要湖泊系统的观测证据的位置和时间一致，并确定了可能的排水路线。LW-6冰量函数的演变以相当海平面表示，其特征是冰量迅速减少，从相似的15减少到14.51。
Observations of sea level and crustal response to glacial loading cycles provide constraints on the mantle rheology function, E, and as well as on the ice load, I, with the latter being largely free from a-priori glaciological or climate assumptions and appropriate, therefore, for testing any such hypotheses. This paper presents new results for both continental-mantle E and I for the Late Wisconsin ice sheet, using geological evidence for relative sea-level change (rsl) and tilting of palaeo-lake shorelines, complemented with loose constraints from observations of present-day radial crustal displacement across North America. The focus is on evidence from near or within the former maximum ice margins and the resulting earth response is representative of sub-continental mantle conditions. The inversion of the sea level information has limited resolution for earth rheology and simple three-layer models, characterized by depth-averaged effective lithospheric thickness (H) and upper-and lower-mantle viscosities (eta(num) and eta(um) respectively) adequately describe the response function, yielding parameters (earth model E-6) of H = 102 (85-120) km, eta(um) = 5.1 x 10(20) (3.5-7.5)x10(20), eta(lm) = 1.3 x 10(22) (0.8-2.8)x10(22) where the numbers in parenthesis are 95% confidence limits. The details of the ice sheet, with one exception, are not strongly dependent on the rheological assumptions within this range. The exception is the lower mantle viscosity that remains correlated with the magnitude scaling of the ice sheet: a link that is largely broken by introducing constraints from glacial loading effects on the Earth's rotation and dynamic flattening. The difference between the continental eta(num) and the comparable estimate of (1-2.5)x10(20) for ocean mantle is statistically significant. Shoreline gradient information from Glacial Lakes McConnell, Agassiz, Algonquin and Ojibway provide strong constraints on the response within the interior of the ice sheet and the resulting ice sheet model (LW-6) is characterized by multiple ice domes from at least 17 18 Ica onwards. The resolution for earlier periods is largely constrained by global ice volume considerations. The two principal domes are over southern Nunavut (the Keewatin Dome) and over Quebec Labrador, both of >= 3500 m thickness, separated by an ice ridge some 1500 m lower than the domes, the latter a requirement imposed by the gradient information. Over western Canada ice thickness gradients east of the Cordilleras are partly constrained by the shoreline data for former Lake McConnell (as is the lateglacial ice thickness of the Keewatin dome) and indicate that any ice-free corridor between the Laurentian and Cordilleran components is unlikely to have existed before similar to 13ka. Reconstructions of the glacial lakes are consistent with the locations and timing of the observational evidence for the four major lake systems with the likely drainage routes identified. The evolution of the LW-6 ice-volume function, expressed as equivalent sea level, is characterized by a rapid decrease in ice volume from similar to 15 to 14.5 1