The long‐wavelength negative gravity anomaly over Hudson Bay coincides with the area depressed by the Laurentide Ice Sheet during the Last Glacial Maximum, suggesting that it is, at least partly, caused by glacial isostatic adjustment (GIA). Additional contributions to the static gravity field stem from surface dynamic topography, core‐mantle boundary (CMB) topography, and density anomalies in the subsurface. Previous estimates of the contribution of GIA to the gravity anomaly range from 25% to more than 80%. However, these estimates did not include uncertainties in all components that contribute to the gravity field. In this study, we develop a forward model for the gravity anomaly based on density models and dynamic models, investigating uncertainty in all components. We derive lithospheric densities from equilibrium constraints but extend the concept of lithospheric isostasy to a force balance that includes the dynamic models. The largest uncertainty in the predicted gravity anomaly is due to the lower mantle viscosity, uncertainties in the ice history, the crustal model, the lithosphere‐asthenosphere boundary, and the conversion from seismic velocities to density have a smaller effect. A preference for lower mantle viscosities >10²² Pa s is found, in which case at least 60% of the observed long‐wavelength gravity anomaly can be attributed to GIA. This lower bound on the lower mantle viscosity has implications for inferences based on models for mantle convection and GIA.

中文翻译：

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北美下地幔粘度的长波重力场约束

哈德逊湾上空的长波负重力异常与上次冰川最大爆发期间劳伦特德冰盖所压低的区域相吻合，表明这至少部分是由于冰川等静压调整（GIA）引起的。静态重力场的其他贡献来自表面动态地形，岩心-边界边界（CMB）地形和地下密度异常。先前对GIA对重力异常的贡献的估计范围从25％到80％以上。但是，这些估计值并未包括影响重力场的所有分量的不确定性。在这项研究中，我们基于密度模型和动态模型开发了重力异常的正向模型，调查了所有组件的不确定性。我们从平衡约束中得出岩石圈密度，但将岩石圈等静观的概念扩展到包括动力学模型在内的力平衡。预测重力异常的最大不确定性是由于较低的地幔粘度，冰史的不确定性，地壳模型，岩石圈-软流圈边界以及从地震速度到密度的转换影响较小。较低的地幔粘度> 10 发现²² Pa s，在这种情况下，观察到的长波重力异常至少有60％可以归因于GIA。下地幔粘度的这个下限对基于地幔对流和GIA模型的推论有影响。