A new global GPS dataset for testing and improving modelled GIA uplift rates

We have produced a global dataset of ~4000 GPS vertical velocities that can be used as observational estimates of glacial isostatic adjustment (GIA) uplift rates. GIA is the response of the solid Earth to past ice loading, primarily, since the Last Glacial Maximum, about 20 K yrs BP. Modelling GIA is challenging because of large uncertainties in ice loading history and also the viscosity of the upper and lower mantle. GPS data contain the signature of GIA in their uplift rates but these also contain other sources of vertical land motion (VLM) such as tectonics, human and natural influences on water storage that can mask the underlying GIA signal.

A novel fully-automatic strategy was developed to post-process the GPS time series and to correct for non-GIA artefacts. Before estimating vertical velocities and uncertainties, we detected outliers and jumps and corrected for atmospheric mass loading displacements. We corrected the resulting velocities for the elastic response of the solid Earth to global changes in ice sheets, glaciers, and ocean loading, as well as for changes in the Earth's rotational pole relative to the 20th century average. We then applied a spatial median filter to remove sites where local effects were dominant to leave approximately 4000 GPS sites.

The resulting novel global GPS dataset shows a clean GIA signal at all post-processed stations and is suitable to investigate the behaviour of global GIA forward models. The results are transformed from a frame with its origin in the centre of mass of the total Earth's system (CM) into a frame with its origin in the centre of mass of the solid Earth (CE) before comparison with 13 global GIA forward model solutions, with best fits with Pur-6-VM5 and ICE-6G predictions. The largest discrepancies for all models were identified for Antarctica and Greenland, which may be due to either uncertain mantle rheology, ice loading history/magnitude and/or GPS errors.

我们构建了一套包含约4000个全球定位系统（Global Positioning System, GPS）垂直速率的全球数据集，可作为冰川均衡调整（Glacial Isostatic Adjustment, GIA）抬升速率的观测估算依据。冰川均衡调整是固态地球对过往冰荷载的响应，其主要活跃时期为约2万年前的末次冰盛期（Last Glacial Maximum, LGM）。冰川均衡调整的建模工作颇具挑战性，因为冰荷载历史以及上下地幔的粘度均存在较大不确定性。GPS观测数据的抬升速率中虽包含冰川均衡调整的信号，但同时也夹杂了其他垂直陆地运动（Vertical Land Motion, VLM）来源，例如构造运动、人类与自然活动对水储量的影响，这些因素可能掩盖真实的冰川均衡调整信号。

我们开发了一种全新的全自动策略，用于对GPS时间序列进行后处理并校正非冰川均衡调整的伪影。在估算垂直速率及其不确定度之前，我们先检测并剔除了异常值与跳变，并校正了大气质量负荷位移。随后，我们针对固态地球对冰盖、冰川及海洋负荷的全球变化的弹性响应，以及相对于20世纪平均值的地球旋转极变化，对所得速率进行了校正。之后我们应用空间中值滤波器，去除受局部效应主导的测站，最终保留了约4000个GPS测站。

最终得到的新型全球GPS数据集在所有经过后处理的测站上均呈现出清晰的冰川均衡调整信号，适用于探究全球冰川均衡调整正演模型的表现。在与13套全球冰川均衡调整正演模型解进行对比前，我们将数据从以整个地球系统质心（Centre of Mass of the total Earth's system, CM）为原点的参考框架，转换至以固态地球质心（Centre of Mass of the solid Earth, CE）为原点的参考框架。本次对比结果与Pur-6-VM5及ICE-6G预测值的拟合度最优。所有模型与观测数据的最大偏差均出现在南极洲与格陵兰地区，这可能源于不确定的地幔流变学、冰荷载历史/规模，以及/或者GPS测量误差。